Chapter 2

The Building Blocks of CDR Systems

Authors

Mineralization

Greg Dipple, Peter Kelemen, and Caleb M. Woodall

Ocean Alkalinity Enhancement

Phil Renforth and Ben Kolosz

Soil Carbon

Keith Paustian, Pete Smith, Rory Jacobson, and Margaret Torn

Forest Carbon

Bill Anderegg, Jeremy Freeman, Rory Jacobson, and Margaret Torn

Coastal Blue Carbon

Tiffany Troxler

Biochar

Erica Belmont, Daniel L. Sanchez, Pete Smith, and Margaret Torn

BECCS

Erica Belmont, Rory Jacobson, and Daniel L. Sanchez

DAC

Noah McQueen and Jennifer Wilcox

Geological Sequestration

Susan Hovorka and Peter Kelemen

 

The previous chapter explained why carbon dioxide removal is necessary to combat global temperature increases, what constitutes negative emissions, and the connection of CDR to the global carbon cycle. It also discussed the potential benefits and costs of negative emissions, especially in the context of social justice. This chapter provides an overview of the types of CDR approaches that have been developed or are being developed today. Together, they comprise a portfolio of approaches, or “building blocks,” for CDR systems.

As introduced in Section 1.2, the concept of “potential CDR” is critical when evaluating specific CDR approaches. While all the approaches discussed in this chapter have the capacity to achieve net-negative emissions, they will not do so under all conditions. Life cycle analysis, discussed in detail in Chapter 4, provides one tool for evaluating whether net negativity can be achieved under different deployment assumptions.

This chapter introduces each of the potential CDR approaches in Figure 2.1. The approaches this book explores all have unique dynamics with respect to the stocks and flows of carbon, and the resulting form of carbon storage. These characteristics put key constraints on large-scale CDR deployment, suggest the need for a strong portfolio of multiple CDR approaches and inform policy and governance frameworks. The CDR approaches discussed in this chapter include:

  1. CO2 Mineralization: processes by which certain minerals react and form a bond with CO2, removing it from the atmosphere and resulting in inert carbonate rock.

  2. Ocean Alkalinity Enhancement: increasing the charge balance of ions in the ocean to enhance its natural ability to remove CO2 from the air.

  3. Soil Carbon Sequestration: the use of land or agricultural practices to increase the storage of carbon in soils.

  4. Improved Forest Management: land management practices designed to increase the quantity of carbon stored in forests relative to baseline conditions (e.g., by modifying harvest schedules).

  5. Afforestation and Reforestation: These strategies involve growing new forests in places where they did not exist before (afforestation) or restoring forests in areas where they used to grow (reforestation).

  6. Coastal Blue Carbon: techniques that utilize mangroves, tidal marshes, seagrass meadows, and other coastal habitat to increase carbon-removing biomass and, in particular, soil carbon.

  7. Biomass Storage: the conversion of biomass into derived materials with more durable storage than the biomass source, including using pyrolysis to convert biomass into bio-oil (fast pyrolysis) or biochar (slow pyrolysis).

  8. Biomass Energy with Carbon Capture and Storage (BECCS): a form of energy production that utilizes plant biomass to create electricity, hydrogen, heat, and/or liquid fuel. This process simultaneously captures and sequesters some portion of the carbon from the biomass for storage.

  9. Direct Air Capture (DAC): a process that removes CO2 from ambient air and concentrates it for storage deep underground or use in a wide variety of products.

  10. Geological Storage: the injection of CO2 into a geologic formation deep underground for essentially permanent timescales. This activity is not considered a CDR approach by itself, but rather, is a way of safely storing carbon removed from the atmosphere through DAC and BECCS.

Figure

2.1

CDR approaches considered within the primer (NASEM, 2019)

cdr primer figure 2 1

CDR approaches considered within the primer (NASEM, 2019)

These approaches vary in their capacity, cost, permanence, and storage dynamics, and thus may play complementary roles. For example, DACCS provides a flux of CO2 from the atmosphere and, through geological storage, provides an effectively permanent form of storage (secure carbon storage deep underground over a period of thousands of years or more). Forest growth increases the flux of CO2 from the atmosphere into tree biomass, but it remains prone to decomposition, fire, and other loss on decadal/century timescales. As such, afforestation and reforestation provide a moderate level of permanence, often have a lower cost, and are generally constrained by available land.

In summary, each approach functions as a complete system, is organized into individual component parts, and contributes to a broad portfolio of CDR approaches. These dynamics form the fundamental understanding of each potential CDR system and frame the rest of this section. 

This chapter examines each of the CDR approaches described above, their current research, development, and deployment status, and the predicted scale of their potential for removing CO2. This chapter also explores significant considerations for scale-up and the aforementioned duration of storage potential – including land constraints, environmental constraints, energy requirements, supply chain limitations, costs, physical barriers, the permanence (or reversibility) of CO2 storage, monitoring, verification, and governance. Each section then discusses the costs of each option, including current and projected costs and financial incentives for deployment. Finally, for each option, this chapter offers a perspective on existing “landmark” projects, or in some cases, potential future projects, to inspire new thinking within the field, in addition to assisting in addressing some current challenges.

2.1

2.1 —

CO2 mineralization

 

2.1.1

Introduction

Mineralization of CO2 is a process that reacts alkaline material with CO2 to form solid carbonate minerals, for CO2 removal from air, for stable and permanent carbon storage, or for post-processing, where the alkaline agents are separated and the CO2 is stored elsewhere. (Jorat et al., 2015a; Jorat et al., 2015b; McQueen et al., 2020a). Sources of alkalinity (i.e., Mg2+- and Ca2+-rich materials) can be naturally occurring silicate minerals (such as olivine or wollastonite and serpentine group minerals) or waste material from industrial or mining operations (e.g., steelmaking slags and nickel mine tailings) (Mayes et al., 2018; Renforth, 2019). See Section 2.1.3 to learn about challenges specific to scaling carbon mineralization.

Carbon mineralization occurs naturally in alkaline environments. Atmospheric CO2 reacts with rocks (particularly mafic and ultramafic rocks rich in magnesium and calcium, such as basalts, peridotites and serpentinites) to precipitate carbonates. The rate of mineralization is fast on a geological timescale and could be enhanced for CDR for climate change mitigation (Kelemen et al., 2011; Kelemen and Matter, 2008). As recently reviewed by Kelemen et al. (Kelemen et al., 2019, 2020 and NASEM 2019b), engineered mineralization of CO2 may be applied to geologic formations rich in alkalinity (e.g., basalt and peridotite) and to alkaline industrial wastes by reaction with CO2 via (a) ex-situ processes (in high pressure and/or high-temperature reactors), (b) surficial processes (ambient weathering or sparging of CO2-enriched gas or fluid through ground materials), or (c) in-situ processes (sending CO2 -rich waters or fluids underground to react with alkaline minerals below the surface). These methods, visually represented in Figure 2.2, may be applied to achieve:

  • CO2 storage via reaction of solid feedstock with fluids or gases already enriched in CO2 by some other process; 

  • CDR from air (i.e., DAC); or 

  • Combined CDR and storage.

In the latter two cases, CO2 would be sourced from air in ambient weathering, or from CO2-bearing surface waters.

Figure

2.2

Visual representation of in-situ, ex-situ, and surficial processes

cdr primer figure 2 3

Visual representation of in-situ, ex-situ, and surficial processes

Ex-situ opportunities involve extracting and grinding minerals for reaction with CO2. The CO2 source to which mineralization is paired determines whether the process achieves negative emissions. Mineralization can be configured to react with pure CO2, or instead with flue gas as a method of post-combustion capture. This results in reduced CO2 emissions and CO2 storage, but not in negative emissions – i.e., it prevents new CO2 from entering the atmosphere but does not remove CO2 already present.

To achieve negative emissions, alkalinity must be reacted directly with CO2 from ambient air or CO2 separated from the air, for example, by using synthetic sorbents or solvents. One can view some proposed DACCS processes as ex-situ mineralization methods, but conventionally-proposed methods termed ex-situ mineralization involve the extraction of the mineral and reaction off-site and at times with the use of reactors capable of achieving engineered pressure and temperature conditions for enhanced reactivity.

Surficial methods for negative emissions are generally considered in terms of ambient weathering, since sparging of ground or pulverized materials using CO2-enriched fluids or gases might be subject to leakage and CO2 loss. Past studies of surficial methods have commonly focused on existing mine tailings of mafic and ultramafic rocks (mined for commodities such as nickel, platinum group elements (PGEs), and diamonds) (Harrison et al., 2013; Mervine., 2018; Power et al., 2013; 2014; Wilson et al., 2009; Wilson et al., 2011), and on alkaline industrial wastes, since these materials are already present at the surface with an appropriate grain size or particle size of the mineral feedstock. Experiments have shown that reacting ultramafic mine tailings with a 10 percent CO2 gas mixture and aerating the tailings can increase the mine tailings’ rate of capturing CO2 by roughly an order of magnitude (Hamilton et al., 2010; Power et al., 2020). More recent work extends ideas on surficial mineralization to processes that mine and grind rocks for the purpose of DACCS – possibly with nickel or PGEs as a byproduct – and to processes that recycle alkalinity by heating weathered, carbonated materials to produce CO2 for offsite storage or use, and then returning metal oxides back to the surface for another cycle of DACCS via ambient weathering (Kelemen et al., 2020; McQueen et al., 2020a).

Early investigations of in-situ, subsurface mineralization focused on CO2 storage via circulation of CO2-rich fluids through reactive rocks, such as what is practiced at the CarbFix project (Aradóttir et al., 2011). More recently, a few studies have provided rate, cost, and area estimates for in-situ DACCS via mineralization, circulating surface waters through reactive aquifers to achieve capture and permanent storage of dissolved CO2, and then producing CO2-depleted waters at the surface, where they will draw CO2 from the air.

In the context of in-situ mineralization, investigators are just beginning to consider hybrid methods involving the concentration of CO2 up to 10 – 20 percent purity via DACCS, dissolution of CO2 into fluids, and further CO2 capture and storage via subsurface circulation of fluids through reactive rock formations. Optimal combinations of DACCS and in-situ mineralization may exist because – for example – enrichment of air to 5 wt%CO2 is significantly less energy-intensive (Wilcox et al., 2017) and subsequently less costly, compared to enrichment to >95 percent CO2 (Kelemen et  al., 2020).

 

2.1.2

Current status

As outlined in the previous section, there are three primary ways by which negative emissions can be achieved from mineralization: ex-situ methods, surficial methods and in-situ subsurface methods.

The general mechanism of carbon mineralization involves a pH-swing in which alkaline cations (i.e., Mg2+ and/or Ca2+) are released from their silicate structure by dissolution in aqueous fluids at low pH and subsequently react with aqueous CO2, forming carbonates at high pH. Such a swing can be engineered in ex-situ systems (e.g., Park and Fan, 2004), but also arises spontaneously in natural weathering of peridotite (Bruni et al., 2002; Canovas III et al., 2017; Paukert et al., 2012; Vankeuren et al., 2019).

Two primary schemes have been proposed to achieve ex-situ CO2 mineralization for storage. Direct mineralization is a one-step process that reacts CO2 with the alkaline feed. While direct mineralization is a simpler method, the full extent of the reaction may be difficult to achieve on the timescales for optimal ex-situ processes, depending on the nature of the feedstock. Some alkalinity sources contain rigid silicate structures, which cause transport limitations associated with the dissolution of calcium or magnesium cations into solution. Given that these reactive ions are the largest atoms in the silicate mineral structure, their release into solution requires distortion of the crystal lattice, which is more difficult in rigid structures. For this reason, the surface cations are often the most available for reaction, after which a passivation layer is created, which limits further transport of the underlying cations (NASEM, 2019). Further, the presence of other species such as iron oxides or silicates can decrease final product purity, which is important in some further uses of the product, such as use as synthetic aggregate. To mitigate these factors, indirect mineralization extracts alkalinity from the feed and converts it into a form that is more reactive with CO2, thus indirectly reacting the alkaline source with CO2. Separation of the alkalinity is also commonly invoked in proposed, indirect mineralization processes, to remove undesired species initially present in the feed and to increase product purity. Currently, ex-situ mineralization methods are much more expensive than storage of supercritical CO2 in subsurface pore space (space between the grains of rock), and, for both surficial and in-situ, than subsurface mineralization. Thus, over the past couple of decades, ex-situ applications have focused on the possibility of selling mineralized material as value-added products, such as building materials (Huang et al., 2019; Ostovari et al., 2020; Pan et al., 2020; Woodall et al., 2019).

Because this primer concerns carbon dioxide removal, whereas ex-situ methods are generally designed to store emissions captured elsewhere, below we focus more on surficial and in-situ systems in which the pH swing arises from fluid-rock reactions rather than use of low- and high-pH fluid reagents.

Coupling of DAC and mineralization is being explored in the CarbFix2 project in Iceland, where a DACCS unit has been installed at Hellisheiði Geothermal Power Station (Snæbjörnsdóttir et al., 2020). However, this could potentially have less impact due to the extensive energy required for DACCS with synthetic sorbents or solvents, and subsequent compression of CO2.

Another option is to allow alkaline materials to passively react with atmospheric CO2 via ambient weathering at the surface (Harrison et al., 2013; McQueen et al.; 2020a; Mervine et al., 2018; Wilson et al., 2009). Pilot experiments on one kind of surficial method are being carried out at several mines by the De Beers Group of Companies, which aim to make ten of their mines carbon-neutral within the next five years (Mervine et al., 2018). Here, the mineralization process’ overall carbon footprint can be perceived as negative or neutral depending on how the system boundaries are framed. Example system boundaries include: whether the tailings require preprocessing, or stirring, or the type of energy required for potentially concentrating and mixing CO2 in air with the tailings. In addition, the mine tailings are drawing CO2 from ambient air, thus removing the CO2 from the atmosphere. However, there are additional emissions associated with power generation for all the mine’s normal operations. De Beers frames its boundaries in this light, aiming for its mineralization system to counteract day-to-day emissions. A large, UK-funded project is evaluating advanced weathering via addition of mafic rock material (basalt) to agricultural soils, with the hope that this might also increase crop productivity (Beerling et al., 2018).

More ambitious systems intended to reach the scale necessary to remove billions of tonnes of CO2 from air each year, via surficial weathering of alkaline industrial wastes and mined rock material, are under consideration at the lab and theoretical level, but they have not yet been implemented at pilot scale. Similarly, ideas about in-situ mineralization for DACCS have been evaluated by lab and modeling studies but have not yet been tried at the field pilot scale.

 

2.1.3

Predicted scale and storage potential

Solid industrial alkaline wastes could effectively store up to 1.5 GtCO2 per year based on current production and possibly more than 3 GtCO2 per year based on future forecasts (Kelemen et al., 2019b; Renforth, 2019). Note that these are estimates of storage capacity; reaction rates could limit these values significantly. Mineralization for storage alone, using high concentrations of CO2 captured elsewhere, would be much faster than mineralization via ambient weathering of most alkaline industrial wastes. Opportunities to use wastes for storage or DACCS vary geographically, as they are widely dispersed among different mines and industrial facilities, and some are already sold, for example, to be used in concrete mixtures as supplementary cementitious materials. These materials exhibit properties similar to ordinary Portland cement, but because their production does not result in CO2 emissions, their use in the production of concrete leads to a reduced carbon footprint. Alkaline industrial wastes sometimes contain hazardous asbestiform chrysotile or heavy metals. In many cases, the associated environmental hazards can be ameliorated through carbon mineralization. Natural sources of alkalinity (e.g., serpentine group minerals, basalt, or peridotite) are abundant in Earth’s crust, and could store on the order of 10 GtCO2 per year, as shown in Figure 2.3. In this case, storage capacity estimates include both the overall size of the potential CO2 reservoir and achievable rates, using reactants with both high CO2 concentrations (ex-situ processes, in-situ basalt, and peridotite storage) and ambient air (enhanced weathering processes).

Figure

2.3

Summary of annual storage potential versus cost in US$/tCO2 for proposed solid storage using fluids enriched in CO2. NOTE: Costs should be compared to the cost of storage of supercritical CO2 in subsurface pore spaces, ~$10-20/tCO2 (NASEM, 2019).

cdr primer figure 2 4

Summary of annual storage potential versus cost in US$/tCO2 for proposed solid storage using fluids enriched in CO2. NOTE: Costs should be compared to the cost of storage of supercritical CO2 in subsurface pore spaces, ~$10-20/tCO2 (NASEM, 2019).

The CarbFix and Wallula CO2 storage projects (reviewed in Snæbjörnsdóttir et al., 2020 and Kelemen et al., 2020) have demonstrated the potential of CO2 storage via in-situ mineralization by injecting CO2-enriched, aqueous fluids into underground basaltic reservoirs, where it is rapidly precipitated in carbonate minerals (Matter et al., 2016). This provides a safe, permanent storage solution for the captured carbon where the caprock integrity might be less important over the long time frame than in sedimentary formations (Sigfusson et al., 2015). (See Section 3.9 on geological storage.) The caprock is a primary trapping mechanism by which the CO2 is physically trapped in the subsurface due to low-permeability overlying rock. It is important to note, however, that the CO2 injection process at CarbFix is “piggybacked” on the reinjection of spent geothermal fluid, which is required to not impact freshwater. To ensure the protection of freshwater, monitoring is required. During the injection process, the pressure is increased, and if the caprock is not effective, the elevated pressure may lead to leakage, either of the re-injected geothermal fluid or CO2. At CarbFix, condensation of natural steam in the geothermal power generation cycle produces “non-condensable gases,” mainly CO2 and H2S. The CarbFix project currently captures and stores about 12,000 tonnes of CO2 annually, with the aim to increase injection by a factor of about three, to about 90 percent of the CO2 emissions from the Hellisheiði geothermal power plant before 2030. In addition to enriched CO2, injected fluids in CarbFix2 contain an equivalent enrichment of H2S, which reacts with basalt to form sulfide minerals, thus mitigating the environmental impact of geothermal H2S as well as CO2 emissions.

The total amount of CO2 that could be stored globally in mafic and ultramafic rocks has been estimated to be around 60,000,000 GtCO2 (Kelemen et al., 2019b), about half in onshore reservoirs within a few kilometers of the Earth’s surface. Other estimates focused on the upper-bound capacity of submarine lavas in oceanic crust suggest that 8,400 – 42,000 GtCO2 could be stored in seafloor aquifers along mid-oceanic ridges, and 21,600 – 108,000 GtCO2 in basalt aquifers associated with intra-plate volcanism (Goldberg and Slagle, 2008).

 

2.1.4

Current challenges to deployment

Limitations of this technology are related to accessibility, reaction kinetics, and diffusion. While it is estimated that the magnitude of alkalinity sources available for CO2 mineralization is sufficient for significant CO2 storage, complications arise with the location of such sources. Most basalts and ultramafic rocks are on the ocean floor. However, significant volumes of continental flood basalts and ultramafic rocks are on land. (See Chapter 3, Figure 3.5.) For natural silicates, it is estimated that hundreds of trillions of tonnes of rock are available (NASEM, 2019). For instance, the largest ophiolite in the world is the Samail ophiolite. It is a 350 km x 15 km peridotite massif in Oman. Considering accessible depths down to 3 km, and that 1 m3 of rock weighs about three tonnes, about 47,000 Gt of peridotites are accessible for carbon mineralization in the Samail ophiolite. The magnesium atoms in the ophiolite represent roughly 40 – 45 percent of the rock. If all magnesium atoms are reacting with CO2 to form carbonates, this ophiolite alone could sequester about 20,000 GtCO2. Other ophiolites worldwide could increase that number by a factor of five or more, raising the sequestration potential of peridotites to more than 100,000 GtCO2. This estimate does not include seafloor peridotites near oceanic ridges, which have an even greater potential than onland massifs (Kelemen et al., 2011). Locating and mining so much feedstock would be expensive both economically and energetically and would potentially require single-purpose use of huge areas. Alternatively, some proposed technologies would circulate CO2-bearing fluids and/or gases into subsurface rock formations, rather than mining and grinding gigatonnes of rock (Kelemen and Matter, 2008).

Laboratory-scale studies have shown that mafic and ultramafic materials in general react much more quickly with CO2 than sedimentary rocks in proposed reservoirs for storage of supercritical CO2 in the pore space of the rocks (void space between the mineral grains) (Kelemen et al., 2019; NASEM, 2019), dramatically increasing the permanence of CO2 storage in the pore space of these formations. Supercritical CO2 is a fluid in which CO2 exhibits both gas and liquid properties, and in this phase makes it less susceptible to leaking from the reservoir. In particular, pilot-scale studies have shown that CO2 injected into basalt formations can be mineralized in about a year. (See Section 3.1.1.) While natural systems are supply-limited and record CO2 mineralization over millennia, the results of the CarbFix experiment confirm theoretical estimates that – if rapid fluid circulation can be achieved and sustained in the target rock formations – all CO2 will be removed from circulating surface waters to form solid carbonate minerals on a timescale of years.

An important parameter for ex-situ and surficial mineralization is the grain (particle) size of the feedstock, where the optimal size for some feedstocks has been estimated to be ~100 µm (Sanna et al., 2013) for ex-situ processes. Crushing and grinding of mined minerals is both energetically and economically intensive, so it is advantageous to use an alkalinity source that is already of sufficient particle size/reactivity, like some industrial and mining wastes (Kelemen et al., 2020).

Popular alkalinity extraction methods for the first step of indirect, ex-situ CO2 mineralization involve the use of acids, salts, and/or heat. Strong acids extract alkalinity at sufficient rates but present environmental and health hazards. Some weak acids have been explored and have shown some success but are not as effective (Teir et al., 2007). Ammonium-based salts have also been successful on small scales (Zevenhoven et al., 2017), but an economically-integrated process has yet to be proven. This is partly due to the high expense of the salts and the energy-intensive salt regeneration step. This process is also known as the Abo Akademi process, which works by reacting an alkaline silicate mineral with an ammonium sulfate salt at a temperature above 400º Celsius (Zevenhoven et al., 2017). This process produces alkaline sulfates that are water soluble, allowing them to be dissolved and separated from the remaining insoluble waste rock. The produced alkaline solution can then be carbonated by bubbling CO2 through the solution to reach saturation and precipitating the carbonate species.

For the carbonation step of indirect mineralization, limitations are dominated by diffusion. Diffusion of CO2 into ex-situ, aqueous systems can be accelerated by decreasing the diffusion length scale via sparging (e.g., Gunnarsson et al., 2018) and/or stirring (e.g., Gadikota, 2020), and/or by increasing the chemical potential gradient via introduction of CO2 at an elevated partial pressure (Gadikota, 2020).

 

2.1.5

Current and estimated costs

According to NASEM (2019), the cost to store one tonne of CO2 from air via carbon mineralization lies between $20 and 100 per tCO2. Within this range, storage of carbon within mine tailings can be carried out at a relatively low cost, but it provides minimal storage capacity. Figure 2.4 illustrates storage potential versus cost.

Figure

2.4

The mechanism of trapping changes over time from structural to solubility to mineral, leading to increased permanence and decreased risk (Figure 8 in Kelemen et al., 2020; Figure 5.9 in IPCC, 2005a; Figure 6.7 in NASEM, 2019; Figure 9 in Snæbjörnsdóttir et al., 2017)

cdr primer figure 2 2

The mechanism of trapping changes over time from structural to solubility to mineral, leading to increased permanence and decreased risk (Figure 8 in Kelemen et al., 2020; Figure 5.9 in IPCC, 2005a; Figure 6.7 in NASEM, 2019; Figure 9 in Snæbjörnsdóttir et al., 2017)

Most ex-situ mineralization methods for CO2 storage are significantly more expensive than storage of supercritical CO2 fluid in subsurface pore space (~ $10 – 20/tonne CO2, NASEM 2019, Chapter 7). As a result, storage of CO2 via mineralization may not be cost-competitive with injection into unreactive storage rock in many places. However, in places where there is no appropriate reservoir for long-term, subsurface storage of large amounts of supercritical CO2 in pore space, mineralization may be a better storage method. Storage via mineralization may also be advantageous in the long term, as the CO2 is molecularly bound in solid minerals that are inert and pose little risk of groundwater contamination.

It is important to note, again, that some proposed mineralization methods use ambient weathering or subsurface reactions to achieve CDR. As such, their costs should be compared with estimates for more highly engineered methods for DAC with solid sorbents or solvents, which are much higher than cost estimates for storage alone.

Specifically, mining and grinding of rock for the primary purpose of DACCS via ambient weathering has similar costs to engineered DACCS using synthetic sorbents. However, the area requirements for capture and storage of CO2 in ground rock material or alkaline industrial wastes on the surface at the scale of gigatonnes per year are very large—much larger than for DAC using synthetic sorbents coupled with subsurface CO2 storage. Recycling alkalinity may greatly reduce the cost of feedstocks, and the long-term area requirement, per tonne of CO2 net removed from air for DACCS via ambient weathering. As a simple example, this might be achieved by weathering MgO to produce MgCO3; calcining MgCO3 to produce MgO plus CO2 for offsite storage or use; and redistribution of produced MgO for another cycle of weathering. While there are many uncertainties involved, this “MgO looping” method currently has the lowest peer-reviewed cost estimates of any proposed DAC method. Finally, in-situ, subsurface carbon mineralization for DAC coupled with solid storage may be less expensive than many surface approaches, and requires far less land, but there is uncertainty about permeability, surface reaction rates, and potential surface area.

 

2.1.6

Example projects

The following project examples all have the potential to be carbon dioxide removal, provided that the appropriate steps are followed and that the CO2 is sourced from either BECCS (3.7) or DAC (3.8). In 2013, nearly a kilotonne of pure, supercritical CO2 liquid was injected into the Columbia River flood basalts near Wallula, Washington, in the United States, in two brecciated (broken up or fragment rock) basalt zones at a depth of 800 – 900 meters. Subsurface reactions between the CO2, formation waters, and basaltic rocks were monitored. Some solid carbonate reaction products were observed in rock core samples, but the proportion of CO2 that has been mineralized, versus dissolved in aqueous fluids within the basalt rock, is unknown. Extensive monitoring several years after injection confirmed that no substantial leakage of CO2 had occurred from the storage formation since its injection. Injection of CO2 from a second geothermal plant in the area is scheduled to begin in 2021.  

CO2 mineralization can be made profitable, or at least less expensive, by selling the carbonate products for use in the construction industry (Chapter 5). There are several examples of how this is currently being done, some of which are finalists in the Carbon XPRIZE competition (XPRIZE, 2020). CarbonCure, a Carbon XPRIZE finalist based in Nova Scotia, Canada, injects CO2 into concrete during the concrete curing process to enhance cement curing reactions and thus reduce the amount of calcined material used in concrete. The CO2 reacts with calcium to form nanocrystals of calcium carbonate that seed conventional cement hydration reactions (Penn State, College of Engineering, 2020), which take place once cement has been mixed with water and result in the formation of products that contribute to the short- and long-term characteristics of Portland cement concrete.

The process results in enhanced-hydration products and a stronger overall concrete that requires 7 – 8 percent less Portland cement, decreasing concrete’s overall carbon footprint by 4.6 percent (Monkman and MacDonald, 2017). The CO2 used by CarbonCure is sourced from industrial emitters, collected and distributed by gas suppliers, and stored in pressurized vessels at concrete plants. The technology is easily retrofitted at a concrete plant and has already been installed across North America and Asia (CarbonCure Technologies, 2020). Alternatively, Blue Planet Ltd, located in California, dissolves CO2 from flue gas CO2 into an ammonia solution to carbonate a stream of Ca2+ ions (originating from alkaline industrial waste), forming synthetic limestone. Small aggregate substrates present in the reactor are coated with synthetic limestone, creating layered aggregates. The carbonate layers of the aggregate products are 44 wt%CO2 and can be produced in sizes ranging from that of sand to gravel (Blue Planet Ltd., 2019).

As discussed previously for CO2 mineralization, these examples of CarbonCure and Blue Planet Ltd could achieve negative emissions only if using a CO2 stream from a DAC process, which is not currently the case. However, using CO2 mineralization to store CO2 captured from air could broaden the possible opportunities for DAC in the absence of other nearby storage options, while also providing economic value by selling building materials.

Areas for potential improvement of proposed in-situ mineralization methods include a deeper understanding of the characteristics of possible reservoirs (nano-to-kilometer scale), the distribution of the reaction products in such reservoirs, the reaction rate of target minerals, the evolution of permeability and pressure in the reservoir, the large-scale impact of the chemical physics processes leading to clogging or cracking, and the effect of potential geochemical contamination of aquifers and surface waters. The deployment of subsurface CO2 storage, via mineralization or in pore space, would also require a change in regulations to include a larger variability of storage options, with requirements corresponding to the type of rocks (Kelemen et al., 2019; NASEM, 2019). For example, the CarbFix pilot project has demonstrated that an impermeable caprock might not be necessary to ensure the permanence of storage in basalt formations, due to rapid mineralization of CO2 dissolved in injected fluids (solution trapping followed by carbon mineralization) (Matter et al., 2016; Sigfusson et al., 2015). Because CO2-free fluids may then be recycled, initial concerns about water consumption for solution trapping seem to have been addressed (Sigfusson et al., 2015).

2.2

2.2 —

Ocean alkalinity enhancement

 

2.2.1

Introduction

The ocean contains approximately 40 trillion tonnes of dissolved carbon, mostly in bicarbonate ions (HCO3-) with lesser amounts of carbonate ions (CO32-). These anions exist in charge balance with dissolved cations in the ocean (Ca2+, Mg2+, Na+, K+). Natural weathering promotes the release of cations via dissolution of carbonate minerals (Equation 2.1) or silicate minerals (Equation 2.2), and reaction with aqueous CO2. In solutions in contact with air (rivers, lakes, and the surface of the ocean), the required aqueous CO2 is provided by uptake of CO2 from the atmosphere.

Equation

2.1

cdr primer equation 2 1
Equation

2.2

cdr primer equation 2 2

Methods to accelerate this process and capture and store atmospheric CO2 in the ocean as dissolved carbonate and bicarbonate ions have been proposed for more than 25 years (as reviewed in Renforth and Henderson, 2017), and include CDR strategies that propose adding crushed minerals directly to the land surface or oceans, or weathering of minerals in seawater reactors. Like carbon mineralization, these methods accelerate natural weathering mechanisms. However, rather than promoting the formation of carbonate minerals, the carbon dioxide is converted to dissolved bicarbonate (HCO3-), which would reside in the ocean. The chemistry of the surface ocean is a strong inhibitor of solid carbonate formation (thus preventing the reversal of reaction in Equation 2.1). As a result, every mole of Ca2+ or Mg2+ dissolved is charge-balanced at near-neutral pH by almost two moles of CO2 dissolved as HCO3-. In detail, there are between 1.5 and 1.8 moles of CO2 dissolved, rather than 1:1 in solid CaCO3 or MgCO3. (As an aside, note that looping of alkalinity, repeatedly recycling CaO or MgO to capture CO2 from air via enhanced weathering as described in Section 2.1, could greatly increase the number of moles of CO2 removed from air, per mole of initial feedstock.)

In addition to CO2 capture from air, ocean alkalinity enhancement could be an important part of an environmental management strategy designed to ameliorate the impacts of ocean acidification (in addition to the impacts of CO2 reduction from the atmosphere, Rau et al., 2012). However, the technologies to do this remain largely untested beyond the laboratory, and the environmental consequences of elevated ocean alkalinity are poorly understood.

 

2.2.2

Current status

Currently, ocean alkalinity enhancement is practiced on a small scale by commercial seafood companies and aquaria to mitigate the effects of ocean acidification on shell-forming organisms. Sodium hydroxide, sodium carbonate or limestone are the sources of increased alkalinity, and the net carbon balance – CO2 uptake from air versus input of CO2 from carbonate feedstock – has not been considered.

 

2.2.3

Predicted scale and storage potential

If the impacts were limited only to increasing ocean alkalinity, and if those impacts were instantaneously distributed throughout the global surface ocean, then it could be possible to store trillions of tonnes’ worth of CO2 without surpassing carbonate saturation states that have been present in the open oceans over the past ~800,000 years, as shown in Figure 2.5. (Hönisch et al., 2009). However, the total storage potential constrained in this way depends on atmospheric CO2 concentration. A larger atmospheric CO2 level corresponds to a greater capacity for increasing ocean alkalinity (Renforth and Henderson, 2017). In practice, the storage potential may be limited by the capacity to create technologies at scale, for example, to distribute metal oxides at low concentration over a large area, or the environmental impact around an area where a large amount of metal oxides is introduced.

Figure

2.5

a) Calcite saturation reconstruction of the surface ocean over the last 800,000 years (Hönisch et al., 2009); b) projection of future surface ocean saturation states as a consequence of an RCP8.5 emission scenario (red line) and the change in saturation state if that emission scenario was wholly mitigated by increasing ocean alkalinity (blue line, equivalent of ~5,000 GtCO2 storage, adapted from Renforth & Henderson (2017)

cdr primer figure 2 5

a) Calcite saturation reconstruction of the surface ocean over the last 800,000 years (Hönisch et al., 2009); b) projection of future surface ocean saturation states as a consequence of an RCP8.5 emission scenario (red line) and the change in saturation state if that emission scenario was wholly mitigated by increasing ocean alkalinity (blue line, equivalent of ~5,000 GtCO2 storage, adapted from Renforth & Henderson (2017)

 

2.2.4

Current challenges to deployment

Technical challenges for upscaling alkalinity extraction, transport, and treatment are likely similar to those of carbon mineralization (Section 3.1.1). The environmental impacts of these approaches will not be instantaneously distributed across the global surface ocean and depend on the distribution method. One option is to apply crushed minerals or alkaline solutions to smaller parts of the surface ocean and then rely on currents to distribute the impact. As such, there will always be a temporal and spatial gradient of impact between the point of addition and the wider surface ocean. The nature of this gradient is important if the goal is to protect ecosystems from the impacts of ocean acidification and would be a key design parameter in such a scheme. However, excessively localized alkalinity may have undesirable impacts on ecosystems (Bach et al., 2019; Fukumizu et al., 2009). Elevated alkalinity may also promote biological carbonate formation, which would release CO2 (for example via Equation 2.1, read from right to left) and decrease the effectiveness of the storage. (Conceptually, this is similar to leakage from storage of supercritical CO2 in subsurface pore space, in which CO2 is returned to the atmosphere, yet the magnitude and rate of carbonate formation remain poorly understood by both models and experimentation. In addition, some potential mineral feedstocks contain elements that can impact ecosystems (e.g., iron, silicon, nickel), the implications of which are reviewed by Bach et al. (2019). The consequence of elevated alkalinity within a range of minerals may be broad ecosystem shifts to favor mollusks, sponges, diatoms and other marine organisms that use silicate or calcium to grow and develop, which in turn might lead to other ecological impacts.

 

2.2.5

Proposed methods and estimated costs

Proposals include spreading crushed minerals on the land surface, after which dissolved cations and bicarbonate are transported to the oceans via runoff, rivers, and groundwater (Beerling et al., 2018; Hartmann et al., 2013), adding minerals directly to the ocean (Harvey, 2008; Köhler et al., 2013) or coastal environments (Meysman and Montserrat, 2017; Montserrat et al., 2017), creating more reactive materials for addition to the ocean  (Renforth et al., 2013; Renforth and Kruger, 2013), using a reactor to dissolve limestone (Rau, 2011; Rau and Caldeira, 1999) and using electrolysis to create alkaline solutions and neutralizing the produced acidity through weathering (House et al., 2007; Rau et al., 2013). All of these proposals require research to assess their technical feasibility (laboratory and demonstration scale), cost, environmental impact, and social acceptability (Project VESTA, 2020).

Kheshgi (1995) proposed adding lime (CaO) or portlandite (CaOH2) to the surface ocean through the calcination of limestone, combined with flue gas CO2 capture and sequestration. Lime added to the ocean would dissolve and result in an increase in alkalinity. For this process to be carbon-negative, it is essential to curtail the release of CO2 produced during calcination. The techno-economics of this approach were assessed for a range of mineral feedstocks (Renforth et al., 2013). They suggested that lime or hydrated lime production in a kiln with CCS, together with the associated energy costs of raw material preparation and ocean disposal, would require between 2 and 10 GJ per net tonne of CO2 sequestered.

While it is currently not permitted to add materials to the ocean (IMO, 2008, 2003), large parts of the supply chain (mineral extraction, transport, calcination) to do so already operate at global scale. If these could be adapted and expanded, it may be possible to achieve GtCO2/yr CDR relatively quickly. As such, the net negative balance of ocean liming may be limited only by the ability to deploy CCS on existing and new facilities. The engineering challenges associated with ocean alkalinity-based CDR approaches may, in the long term, constrain the scalability of this method of carbon storage. In the near term, it is important to understand the environmental impacts and beneficial effects of mitigating ocean acidification, together with their potential for social acceptance, and political and regulatory implementation.

There have been limited techno-economic assessments of ocean alkalinity enhancement proposals, and they are exclusively based on theoretical flow sheets (although many involve existing components or supply chains). However, these assessments provide approximate costs and energy requirements (Table 2.1).

Table

2.1

Comparison of electrical and thermal energy requirements and financial costs of ocean alkalinity carbon storage technologies (adapted from Renforth & Henderson, 2017)

cdr primer table 2 1

Comparison of electrical and thermal energy requirements and financial costs of ocean alkalinity carbon storage technologies (adapted from Renforth & Henderson, 2017)

2.3

2.3 —

Soil carbon sequestration

 

2.3.1

Introduction

Soil carbon sequestration for CDR involves making changes to land management practices that increase the carbon content of soil, resulting in a net removal of CO2 from the atmosphere (Paustian et al., 2016; Kolosz et al., 2019; Sanderman and Baldock, 2010). The stock of carbon in the soil over time is determined by the balance between carbon inputs from litter, residues, roots or manure, and losses of carbon, mostly through microbial respiration and decomposition, which is increased by soil disturbance (Coleman and Jenkinson, 1996).

Globally, terrestrial biotic carbon stocks include around 600 Gt carbon in plant biomass (mainly forest) and ca. 1,500 Gt carbon as organic matter within the soil to a total depth of around 1 meter (ca. 2,600 Gt C to 2m). The total annual fluxes moving between the atmosphere and land-based ecosystems (i.e., net primary productivity by plants and respiration by the soil biota) each equate to around 60 Gt C/yr (Le Quéré et al., 2016). These fluxes are mostly within balance; however, it is estimated that there is currently a net uptake (or “sink”) of carbon in biomass and soils of land-based ecosystems of approximately 1 – 2 Gt carbon (Le Quéré et al., 2016). It is thought that this “unmanaged” carbon sink is largely due to greater carbon uptake by forests and grasslands due to CO2 fertilization, increased atmospheric nitrogen deposition, and recovery of previously logged forests in parts of the Northern Hemisphere (Houghton et al., 1998). In contrast, historic land use change involving clearing of forests and plowing of prairies for new cropland is thought to have resulted in a total loss of 145 Gt C from the woody-based biomass and soils between 1850 and 2015 (Houghton and Nassikas, 2017). Over the past 12,000 years, land use and land cover change has resulted in an estimated loss of 133 Gt C from soils alone (Sanderman et al., 2017). Hence, most managed agricultural soils are depleted in carbon relative to the native ecosystems from which they were derived. The basis of soil carbon sequestration approaches is to reverse this historical trend and rebuild carbon stocks on managed lands.

In general, management practices that increase the inputs of carbon to soil, or reduce losses of carbon, promote soil carbon sequestration. There are many land management practices that can promote soil carbon sequestration (such as no-till agriculture, planting cover crops, and compost application) (Lal, 2013, 2011; Smith et al., 2014, 2008), some of which can also promote carbon sequestration in above-ground biomass (e.g., agroforestry practices). The goal of all of these approaches is to increase and maintain carbon stocks in the form of soil organic matter, which is derived from the photosynthetic uptake of carbon dioxide by plants. A key quantity of interest is the mean residence time, or how long organic matter remains in the soil. While small fractions of organic matter can have mean residence on the order of centuries, much of the organic matter in soils is relatively labile (prone to decomposition or transformation). Thus, the key to maintaining greater carbon storage in soils is to maintain long-term conservation practices (Sierra et al., 2018).

 

2.3.2

Current status

Several crop and soil management practices that can potentially increase soil carbon sequestration have been explored on a limited scale. For example, there is considerable research on the use of cover crops (plants grown after the primary food or fiber crops have been harvested, instead of leaving the soil bare), and many farmers are successfully using them, but currently less than 5 percent of the land used for annual crops in the U.S. includes cover crops in the planting rotation (Jian et al., 2020; Wade et al., 2020). Like cover crops, which prevent erosion and can build soil fertility by enhancing carbon and nitrogen stocks, many soil carbon sequestration activities are related to so-called conservation practices that increase or maintain soil health more generally. But various barriers limit the adoption rate of these methods. Farming is inherently risky and instituting major changes in management increases those risks. Strategies to reduce the risks of adopting conservation practices include improved technology, education, financial incentives to overcome the costs of management changes, and reform of crop insurance regulations (Paustian et al., 2016). Ideally, farmers and ranchers can be incentivized to transition to practices that create healthier soils, increase the stability of year-to-year crop yields, and reduce the need for purchased inputs like fertilizer. Incentives to maintain such practices are vital, either through increased profitability or with subsidies for improved environmental performance – or both. If, instead, a farmer reverts to conventional (non-conservation) practices, then much of the previously sequestered carbon may be lost back to the atmosphere, with minimal (or no) net climate benefit. Thus, to a large degree, the permanence of CDR through soil carbon sequestration is limited by socioeconomic and behavioral factors.

Rates for soil carbon sequestration vary considerably, depending on the climate, soil type, land use history, and management practices employed (Ogle et al., 2005; Paustian et al., 1997; Smith, 2012). As a rough approximation, best practices on land growing annual crops (such as barley and corn) can yield annual carbon sequestration rates up to 0.6 tC/ha/yr (around 2.2 tCO2/ha/yr), whereas conversion of tilled annual cropland to pastures, conservation buffers, or grassland set-asides can yield increases of 1 tC/ha/yr or more (NASEM 2019). Considerable care should be taken when extrapolating these example data points to larger-scale estimates due to variability across space, time, and depth.

 

2.3.3

Predicted scale and storage potential

The technical potential for soil carbon sequestration globally, assuming widespread adoption of certain practices, could be as high as 5 GtCO2eq per year, without considering economic constraints. (Note that mass of CO2 rather than of C is used when reporting effects on atmospheric CO2 stocks; Paustian et al., 2020; Smith, 2016.) According to the NASEM (2019) report, the conservative (lower) potential rate for CO2 removal given the current state of the technology, at a cost below $100 tCO2/yr, is around 3 GtCO2/yr, globally. New technologies, such as perennial grain crops and annual crop phenotypes with larger, deeper root systems, that lead to greater belowground carbon inputs (e.g., via increased mycorrhizal colonization, increased root sloughing and exudation, and/or roots with more recalcitrant tissues) could yield removal potentials as high as 8 GtCO2eq/yr globally (Paustian et al., 2016). For the U.S. alone, estimates of technical potentials are 240 – 800 Mt/y CO2eq, with the lower number representing widespread deployment of current conservation management practices while the higher level represents deployment of new technologies, such as enhanced root phenotypes for annual crops that have not yet been commercially developed. These upper bounds, especially, may not be achievable given socioeconomic constraints.

Sequestering soil organic carbon (SOC) can be an efficient and stable means of CDR because it has very long residence times, with bulk SOC residence times ranging from hundreds to thousands of years (Torn et al., 2009). It is also less vulnerable to ecosystem disturbances like wildfire and disease than forest biomass, and soil carbon release to the atmosphere, should it occur, is more gradual. Nevertheless, storage of soil organic carbon is at risk of reversibility. Concerns about permanence can be separated into biogeochemical (climate impacts and biological feedbacks) and socioeconomic factors (changes in land ownership and practices), either of which could impact the integrity of the soil carbon sink. For example, increases in temperature due to climate change can stimulate soil respiration (the release of carbon dioxide from soils) (Luo et al., 2001; Hicks Pries et al., 2017), counteracting practices that aim to reduce soil respiration rates as a means to increase soil carbon stocks. Socioeconomic factors, such as changes in land ownership, could cause the abandonment of soil carbon sequestering management practices – for example, if a new landowner chooses to revert to annual crops or conventional tillage – resulting in losses of previously stored soil organic carbon.

 

2.3.4

Current challenges to deployment

Barriers to implementation include:

  • Lack of implementation support and education among farmers about new practices;

  • Limited demonstration projects;

  • Lack of policy and financial incentives to help de-risk practice changes that may require several years to fully take effect;

  • Reliably attributing the incremental (or additional) benefit of specific actions when efforts involve changes to existing land management practices;

  • Incompletely defined and demonstrated monitoring and verification methods and costs (Smith et al., 2020); and

  • Difficulty guaranteeing the long-term (e.g., 100-year time horizon) integrity of stored soil carbon (Smith, 2012).

As discussed in Section 1.5, another critical challenge for soil projects is additionality: evaluating the degree to which sequestration occurred because of some intervention above and beyond what would have happened in a no-intervention baseline scenario. As many efforts involving soil carbon sequestration involve changes to existing practices, accurately accounting for the potential CDR benefit requires comparison to a counterfactual scenario – what would have happened otherwise – that can only be estimated, not observed (Haya et al., 2020). 

Dedicated pilot projects and demonstration programs could help identify the measures required to overcome these barriers, with an emphasis on learning by doing and resolving key uncertainties through data acquisition and optimization of methods (Paustian et al., 2019; Vermeulen et al., 2019). Since soils have been managed for millennia, there is a high level of knowledge and readiness, with the potential to contribute to other global sustainability goals such as improved water quality, ecosystem restoration, biodiversity preservation, job creation, and increased yields/food security (Smith et al., 2019).

 

2.3.5

Current and estimated costs

The economic costs of establishing and maintaining large-scale soil carbon sequestration projects are uncertain, given that market experience consists mainly of a few pilot projects and academic studies. Tan et al. (2016) reviewed a number of economic analyses and pilot projects and, in 20 of the 21 studies reviewed, costs were less than $50/tCO2eq. By combining an economic model with empirical estimates of soil carbon sequestration rates, Smith et al. (2008) estimated an economic potential of between 1.5 and 2.6 GtCO2eq per year at carbon prices between 20 and 100/tCO2eq (Smith, 2016; Smith et al., 2008). Marginal costs ranged from negative to positive. Smith (2016) estimated that global soil carbon sequestration at a rate of 2.6 GtCO2eq per year would save a net $7.7 billion: $16.9 billion in savings minus costs of $9.2 billion.

Because costs vary widely across practices, geographies, and cropping systems, cost estimates typically assess total sequestration potential using marginal abatement cost curves from the literature bounded by a maximum average cost per tonne. Figure 2.6 shows Griscom et al.’s estimates for additional sequestration potential through soil carbon management practices such as improved grazing management and conservation agriculture in comparison to other land-based CDR approaches, bounded by average costs of $10 per tonne CO2eq (low cost) and $100 per tonne CO2eq (cost-effective) (Griscom et al., 2017; Bossio et al., 2020). This analysis also incorporates safeguards for fiber security, food security, and biodiversity conservation, which constrain the potential of sequestration approaches. While these estimates account for the costs of practice conversion and implementation, they do not account for the costs of other social, political, and educational programs or measurement and monitoring efforts that will likely be needed to scale these solutions (Schlesinger & Amundson, 2019).  Additionally, much of the literature on the costs and potential of soil carbon approaches evaluates total mitigation potential (which includes avoided emissions) rather than additional sequestration exclusively. Using the Griscom et al. methodology, Bossio et al. estimate that CDR represents 60 percent (3.3 GtCO2eq/yr) of the total mitigation potential of soil carbon management (Bossio et al., 2020).

Currently there are a few soil carbon projects in the portfolios of voluntary emissions reduction or offset registries, such as VCS (Verra, 2020), and there are no soil carbon offsets being included in mandatory emission-reduction cap-and-trade programs (such as in California or the EU). Therefore, the per-tonne carbon value of soil sequestration remains low in the context of environmental markets. However, there has been some direct financing within voluntary emission-reduction markets: For example, 2016 yielded a total volume of 13.1 Mt CO2eq ($5.10/tCO2), approximating to $67 million. However, more than 95 percent of the emission reduction was from forestry biomass, with soils playing a very minor role. In the U.S., the federal government subsidizes soil conservation practices through the USDA, mainly on the basis of reductions in soil erosion and improved water quality. However, cost sharing and other subsidies indirectly benefit carbon sequestration, and Chambers et al. (2016) estimated that these programs resulted in increased storage of between 13 and 43 Mt C on U.S. agricultural lands at a cost for the assistance programs of approximately $60 million per year (2005-2014).

Figure

2.6

Scale and cost comparison for land-based carbon dioxide removal approaches. Removal potential is estimated for low-cost (< $10 MgCO2e−1 yr−1), cost-effective assuming a global ambition to hold warming to <2° C (<$100  MgCO2e−1 y−1), and maximum deployment with safeguards. Modified from Griscom et al., 2017.

cdr primer figure 2 6

Scale and cost comparison for land-based carbon dioxide removal approaches. Removal potential is estimated for low-cost (< $10 MgCO2e−1 yr−1), cost-effective assuming a global ambition to hold warming to <2° C (<$100  MgCO2e−1 y−1), and maximum deployment with safeguards. Modified from Griscom et al., 2017.

 

2.3.6

Example projects

As previously discussed, only a few projects are currently included in the portfolios of registries for voluntary crediting, and protocol systems thus far lack robust third-party verification and monitoring from financially disinterested parties. Evaluating the impact of pilot projects is challenging, given significant variability in outcomes, and will require careful sampling methods, baseline estimation, and interpretation that is guided, but not replaced, by models (Paustian et al., 2017; Campbell and Paustian et al., 2015). Some companies are also attempting to create financial incentives for soil carbon sequestration, but these efforts are at an early stage with uncertain futures, and thus far lack rigorous and transparent verification or validation.

2.4

2.4 —

Improved forest management, afforestation, and reforestation

 

2.4.1

Introduction

Terrestrial ecosystems remove around 30 percent of human CO2 emissions annually (~9.5 Gt CO2eq/yr over 2000 – 2007), and Earth’s forests accounted for the vast majority of this carbon uptake (~8.8 Gt CO2eq/yr over the same period) (Friedlingstein et al., 2014; Pan et al., 2011). Thus, forests may hold substantial potential for further carbon dioxide removal, particularly if actively managed with CDR in mind (Anderegg et al., 2020; Griscom et al., 2017b). We emphasize that preventing emissions by slowing or stopping deforestation (often referred to as “avoided conversion”) is another crucial climate change mitigation strategy, has generally much larger per-unit-of-land-area climate benefits than forest-based CDR approaches, and will have a much more rapid positive climate impact than forest-based CDR approaches. All forest-based CDR approaches take decades, and often more than 100 years, to have substantial radiative effects, whereas preventing deforestation starts reducing climate change immediately and maintains the co-benefits (e.g., biodiversity) of old-growth forests. Although discussed briefly in Section 3.2.1, avoided conversion is otherwise not a focus area for this primer.

Improved forest management (IFM) for CDR refers to active modification of forestry practices to promote greater forest biomass and carbon storage (Putz et al., 2008). Common IFM strategies include lengthening harvest schedules, thereby generally increasing the age and carbon storage of the forest on average; improved fire management; thinning and understory management; and improved tree plantation management (Griscom et al., 2017b; Griscom and Cortez, 2013; Putz et al., 2008). Afforestation refers to the establishment of new trees and forest cover (often monoculture plantations) in an area where forests have not existed recently, while reforestation refers to the replanting of trees on recently deforested land (Hamilton et al., 2010). Some ecologists question the feasibility or utility of large-scale afforestation (Lewis et al., 2019). The regeneration of a damaged or harvested forest is typically considered reforestation, but can also co-occur alongside other forms of improved forest management. Agroforestry practices entail the integration of trees into agricultural systems, in combination with crops, livestock, or both. Improved forest management, afforestation, reforestation, and agroforestry projects form part of several voluntary and mandatory carbon-offset trading schemes worldwide (Diaz et al., 2011; Miles et al., 2015).

From a carbon cycle perspective, reforestation and afforestation are forms of CDR in so far as new growth sequesters CO2 and accumulates new growth in the form of biomass. IFM projects are more complex because they not only include continued (or accelerated) sequestration from existing or new vegetation, but also claim to prevent (or decrease) emissions relative to what would otherwise have occured (CarbonPlan, 2020).

 

2.4.2

Current status

Improved forest management, afforestation, and reforestation could play a role in a near-term CDR portfolio given that these approaches do not rely on any future technological developments (Griscom et al., 2017b; NASEM, 2019b). Forest-based CDR projects totaled an estimated 90 MtCO2eq per year in 2015 and 2016; (the most recent years for which global data are available) and are a major component of California’s cap-and-trade system, making up the majority of carbon offsets as of 2019 (Anderegg et al., 2020; State of Forest Carbon Finance, 2017). Hundreds of CDR forestry projects have been deployed globally since 2000 through both voluntary and compliance markets, according to an analysis of 14 major registries and emissions trading schemes (State of Forest Carbon Finance 2017), although a global database of projects is not currently available.

 

2.4.3

Predicted scale and storage potential

Globally, the CDR potential for IFM, afforestation and reforestation has been estimated at between 4 and 12 GtCO2/yr (State of Forest Carbon Finance, 2017) and up to roughly 12.5 GtCO2/yr by 2030 at a carbon price of $100/tCO2eq/yr (Griscom et al., 2017a). One recent study (Fuss et al., 2018) reported cumulative potentials, with estimates for the year 2100 ranging from 80 to 260 GtCO2, although such a high-end scenario would require vast areas of land and could conflict with other uses, such as agriculture. In the U.S., the potential increase in carbon uptake ranges from 0.7 to 6.4 tC/ha/yr between a period of 50 and 100 years, as illustrated in Figure 2.6. While the global potential of reforestation alone has recently been estimated as very high (Bastin et al., 2019), widespread criticism of that work revealed fundamental methodological flaws in forest area and carbon calculations and a lack of accounting for biophysical feedbacks that could cancel out climate benefits, indicating that such high estimates are likely not credible (Veldmen et al., 2019; Lewis et al., 2019; Skidmore et al., 2019; Friedlingstein et al., 2019).

Figure

2.7

Changes in soil carbon stock from afforestation – shortleaf pine example from U.S. data in Smith et al., (2006)

cdr primer figure 2 7

Changes in soil carbon stock from afforestation – shortleaf pine example from U.S. data in Smith et al., (2006)

As is also discussed in Section 3.3.3, carbon sequestered from IFM, afforestation, and reforestation practices may be disrupted by socioeconomic and environmental risk factors that reduce permanence. Three further elements of IFM, afforestation, and reforestation that differ from most other CDR approaches are:

  1. Tree growth takes a long time, usually decades, to sequester large amounts of CO(Zomer et al., 2017);

  2. In the best-case scenario, with rigorous monitoring and strong contractual agreements around land use, the maximum duration of durable storage is likely to be around 100 years which is still orders of magnitude less than what’s offered by geological or mineral storage; and

  3. Forest projects often have immense co-benefits beyond carbon storage, including benefits for biodiversity and conservation, ecosystem goods and services like water purification and pollination, and local and indigenous communities’ livelihoods (Anderegg et al., 2020).

 

2.4.4

Current challenges

Widespread deployment of IFM, afforestation and reforestation must respond to six important challenges: land competition, permanence risks, biophysical feedback to the climate, additionality, leakage, and ethically and socially responsible deployment. Many of these challenges are particularly acute for afforestation because it involves expanding forests into non-forest lands, which intensifies competition with other land uses, and such lands may not be climatically suitable for long-term forest stability, indicating much greater risks to the permanence of carbon storage. In organic soils, it may lead to soil carbon losses that cancel out carbon gains in biomass (Friggens et al., 2020). Many recent studies (Fargione et al., 2008; Griscom et al., 2017b, 2019) have explicitly avoided assessment and quantification of afforestation approaches, as these issues are particularly problematic and pose a challenge to implementation of forest-based CDR in mitigation strategies.

As with soils and as discussed in Section 1.4, attributing the incremental (or additional) benefit of specific actions, i.e., additionality, is another vexing problem for forest-based CDR, because so many efforts, including any IFM projects and some reforestation projects, involve changes to existing land management practices. Accounting for the potential CDR benefits of such changes requires comparison to a counterfactual scenario of no management change, which can only be estimated at regional scales, not directly observed. While afforestation may be more straightforward to demonstrate as new and additional, the feasibility and ecological suitability of this project category may be limited.

Competition for land and water used for food, fuel, and other natural resource production is an important concern for forest-based CDR approaches. With IFM, land competition is rarely an issue, as the land is already used for forestry. Achieving large carbon dioxide removal rates and volumes with reforestation and afforestation would require very large tracts of land – approximately 27.5 million ha for 1 Gt of CO2 removed (Houghton et al., 2015; NASEM, 2019) – and potentially large volumes of water (Smith et al., 2016a; Smith and Torn 2013; Trabucco et al., 2008). Land constraints could be reduced through agroforestry approaches with suitable crops like coffee and cacao; the challenges would be greater with modern staple crops like wheat, maize, soy, and rice. Many of the studies cited above used reasonable safeguards for food, textiles, and biodiversity. But competition for resources and the economic value of other land uses remain major constraints on the amount of land available for afforestation and reforestation (Lewis et al., 2019).

Climate change poses significant hazards to forest stability and permanence, which could substantially undermine their effectiveness in removing carbon (Anderegg et al., 2020). Ecological disturbances such as fire, hurricanes, droughts, and outbreaks of biotic agents (e.g., pests and pathogens) are a natural part of many ecosystems and should be factored into sequestration projections when disturbance regimes (probability and severity of disturbances) are constant over time (Pugh et al., 2019). Unfortunately, climate change is also greatly altering and increasing disturbances, particularly of wildfire, drought, and biotic agents (Dai, 2013; Williams & Abatzoglou, 2016; Williams et al., 2020; Anderegg et al., 2020). Increasing climate-driven disturbances will decrease the carbon-storage potential of forests and can even drive forests to become a net carbon source to the atmosphere (Kurz et al., 2008). These increasing risks to permanence must be accounted for in policy and project design, and more research is needed to quantify, forecast, and assess risks and how to mitigate them (Kurz et al., 2008b).

Beyond simply storing carbon, forests have other major impacts on global water and energy cycles, termed “biophysical feedbacks,” which mediate their net effect on the climate. In particular, changes in albedo – the degree to which Earth’s surface reflects solar energy –  is considered one of the most prominent issues from a climate perspective and has been overlooked in some recent, exaggerated estimates of afforestation and reforestation potential (IPCC, 2019). Indeed, several studies with Earth system models have shown that an expansion of forest in the tropics would result in cooling, while afforestation in the boreal zone might have only a limited effect or might even result in global warming (Kreidenweis et al., 2016; Laguë et al., 2019; Jones et al., 2013a; Jones et al., 2013b). There are also significant uncertainties about the impacts on non-carbon dioxide greenhouse gases, emissions of volatile organic compounds, evapotranspiration (the combination of evaporation from land and transpiration from plants), and other issues (Anderegg et al., 2020; Benanti et al., 2014; Bright et al., 2015; Kirschbaum et al., 2011; Zhao and Jackson, 2014) that can influence the net climate effects of forestry projects.

Finally, obstacles may arise regarding monitoring and sustaining sequestered carbon in the long term due to carbon sink saturation, changing practices among forest managers and farmers, and creation of market and policy contingencies.

At least some of these six challenges may be tackled and minimized with improved science and an appropriate policy and regulatory framework. As previously mentioned, it is crucial to remember that forests also have the potential to contribute substantially to other global sustainability goals, particularly if co-benefits, such as biodiversity and managing for diverse and native forests, are included along with societal goals in project design and policies.

 

2.4.5

Current costs and estimated costs

Maximal costs of afforestation and reforestation have been estimated at $100 per tonne of sequestered CO2, though there is less agreement on the lowest potential cost, with the National Academy of Sciences (NASEM, 2015) quoting $1 and others citing a range of $18 – $20 per tonne of CO2 (Fuss et al., 2018). Crucially, most estimates indicate that it will be more costly to restore forests than to preserve existing ones, emphasizing the critical role of reducing deforestation compared to planting new forests (Reid et al., 2019).

 

2.4.6

Example projects

As discussed above, projects and policies involving IFM, reforestation and afforestation are in development around the world. Forest projects, and in particular IFM projects, make up a large fraction of compliance with California’s cap-and-trade system (California Air Resources Board, 2020), though as discussed above, significant challenges and concerns have been raised around how these programs address additionality, leakage, and permanence risks (Anderegg et al., 2020). The United Nations’ Trillion Tree Campaign (“The Trillion Tree Campaign,” 2020) aims to support tree-planting efforts around the world and claims that 13.6 billion trees have already been planted under its auspices (Goymer, 2018). Other IFM, afforestation and reforestation projects are ongoing through the UN REDD+ program, the Bonn Challenge, and other mechanisms (State of Carbon Finance 2017; Angelsen et al., 2018; Roopsind et al., 2019) but these programs, too, have related concerns (West et al., 2020).

2.5

2.5 —

Coastal blue carbon

 

2.5.1

Introduction

Coastal blue carbon refers to land use and management practices that increase the organic carbon stored in living plants or soils in vegetated, tidally-influenced coastal ecosystems such as marshes, mangroves, seagrasses, and other wetlands. These approaches are sometimes called “blue carbon” or “blue carbon ecosystems” but refer to coastal ecosystems instead of the open ocean (Crooks et al., 2019). Restoration of high-carbon-density, anaerobic ecosystems, including “inland organic soils and wetlands on mineral soils, coastal wetlands including mangrove forests, tidal marshes and seagrass meadows and constructed wetlands for wastewater treatment” can provide another form of biological CDR (IPCC, 2014). It is increasingly critical to not only preserve existing wetlands, but also to restore and construct these ecosystems for CDR given other co-benefits, including coastal adaptation and other ecosystem services (Barbier et al., 2011; Vegh et al., 2019). Although these ecosystems are very efficient and have high productivity rates, large uncertainties persist in the fraction of organic material that must be buried (sunk to the sea floor so that its carbon is sequestered) to ensure reliable CDR for macroalgal systems such as kelp (NASEM, 2019).

 

2.5.2

Current status

Global wetlands have been reported to store at least 44 percent of the world’s terrestrial biological carbon in vegetation, but primarily in deep stocks of soil organic carbon (Zedler and Kercher, 2005). These carbon stocks in peatlands and coastal wetlands are also vulnerable to reversal due to climate change and human activities (Parish et al., 2008). In fact, roughly one-third of global wetland ecosystems had been lost by 2009 (Hu et al., 2017), with coastal blue carbon ecosystems releasing on the order of 150 – 1050 MtCO2/yr globally due to drainage and excavation (Pendleton et al., 2012). These ecosystems also share significant carbon sequestration capacity under appropriate management and adaptation measures (Page and Hooijer, 2016). As more information about vegetation and soil organic carbon in wetlands has become available, this method has received more attention as a land mitigation option (IPCC 2014).

 

2.5.3

Potential scale and storage potential

The total carbon flux per year, and potential carbon impact of coastal blue carbon, is most influenced by the total area of coastal carbon ecosystems, the rate at which they bury organic carbon, and the capacity to implement approaches given potential barriers of managing, creating, and restoring areas for CDR (NASEM, 2019). Long-term sequestration rates in coastal wetlands are estimated from 1 – 8 tCO2/ha/yr (IPCC, 2014), a rate that significantly increases when emissions avoided from (previously degraded) restored wetlands are counted (Mitsch, 2012; Parish et al., 2008; Smith et al., 2008). The focus on coastal blue carbon reduces the potential for unintended and adverse emissions of non-carbon dioxide greenhouse gases (e.g., methane), as salinity of less than 18 psu (practical salinity units) is shown to significantly reduce or inhibit methane production (Poffenbarger et al., 2011). However, determining the limits of this salinity boundary can be challenging in scaling CDR. While freshwater wetlands also store significant amounts of carbon in above- and below-ground biomass, and in soil, they are estimated to be the source of 20 to 25 percent of global methane emissions (Mitsch et al., 2012). As a result, restoring some wetlands could induce a short-term net warming effect (Mitsch et al., 2012) due to increased emissions of methane and nitrous oxide, whereas restoring tidally-restricted wetlands would significantly decrease methane emissions (Kroeger et al., 2017). 

At the national (U.S.) scale, NASEM (2019) highlighted several coastal blue carbon approaches for tidal wetland and seagrass ecosystem management that could contribute to net carbon dioxide removal and reliable sequestration of 5.4 Gt CO2 by 2100. These included restorations of former wetlands, use and creation of nature-based features in coastal resilience projects, managing the natural development of new wetlands as sea levels rise (migration), augmentation of engineered projects with carbon-rich materials, and management to prevent potential future losses and enhance gains in carbon capacity. At a global scale, it has been estimated that avoided coastal wetland and peatland impacts combined with coastal wetland and peat restoration could deliver 2.4 – 4.5 Gt/yr globally by 2030 (Griscom et al., 2017b). There is also the added potential to contribute to other global sustainability goals, such as improved water quality, ecosystem restoration, biodiversity preservation, job creation, and climate resilience (Barbier et al., 2011).

 

2.5.4

Current challenges

The impacts of changing ecosystem drivers that affect the rate and future of CO2 removal determine coastal blue carbon’s reliability as a long-term CDR approach. Ecosystem drivers include relative sea-level rise, temperature, light availability and watershed management, and coastal development activities that affect sediment availability, salinity, nutrient inputs, and available area for wetlands to migrate inland as sea level rises (NASEM, 2019). While integrated approaches that couple experiments, hierarchical approaches to scaling, and field-validated remote sensing have greatly improved our understanding of organic carbon accumulation and landscape-scale estimation of CO2 removal, research gaps persist. For example, the fate of organic carbon eroded from coastal wetlands and the effect of warming on plant production and decomposition for different types of coastal blue carbon ecosystems remain challenges to our understanding of future CDR capacity. Available lands that support migration of wetlands inland, but could be used for other purposes (e.g., agriculture, ports, industrial sites, and other high-value capital assets) may pose social and economic barriers and limit the extent to which these lands can be used for CDR (NASEM, 2019). Researchers are exploring other coastal blue carbon CDR approaches, such as expanding coastal wetland areas by beneficial use of carbon-rich materials, but they are in either the research or small-scale demonstration phase.

A combination of management activities has the potential to preserve and enhance the high rates of organic carbon sequestration that wetlands already provide and to expand the area covered by coastal blue carbon ecosystems. These activities, supported by monitoring and research, can:

  1. Increase the organic carbon density in soils of coastal systems;

  2. Retard edge erosion of existing wetlands;

  3. Expand wetlands through transgression into upland areas as these areas become flooded by the ocean; and

  4. Augment mineral sediment availability to ensure wetland elevation remains in balance with increasing rates of sea-level rise.

Finally, as discussed in the context of additionality for soil and forest projects, the climate benefits and CDR potential of coastal blue carbon projects typically involve changes to existing management practices, and thus the impacts must be considered relative to baseline, business-as-usual practices.

 

2.5.5

Current costs and estimated costs

The costs of coastal blue carbon projects and restoration of other wetland ecosystems vary widely, resulting in an equally large range in costs for carbon dioxide removal (King and Bohlen, 1994; Turner and Boyer, 1997; Bridges et al., 2015).  If, however, the projects carry a degree of multi-functionality rather than being designed simply for carbon capture, these costs can be reduced to the basic monitoring of coastal blue carbon, estimated to be less than or equal to $100/t CO2 (NASEM 2019). One example is the Coastwide Reference Monitoring System (CRMS), designed to monitor and determine the effectiveness of Louisiana projects within the federal Coastal Wetlands Planning, Protection and Restoration Act (CWPPRA) within multiple levels of geography. CRMS offers different types of data and research for a variety of user groups. Total estimated costs of research and monitoring are $80/ha or $6/ha/yr based on projects funded thus far (Steyer et al., 2003). Total system costs for national-level programs (e.g., REDD) were reported to be between $0.50 and $5.50/ha (Böttcher et al., 2009).

 

2.5.6

Example projects

Several coastal blue carbon CDR approaches are being implemented around the U.S. and worldwide. In the Sacramento-San Joaquin Delta in California, the area of completed and planned restoration projects was expected to be nearly 8,000 ha by 2020 (Drexler et al., 2019). Nearly all restoration projects were completed for purposes other than CDR, including connectivity, fish and wildlife habitat, levee improvement, recreation, ecosystem function, and flood control. Notably, rough estimates of the CDR impact of these projects represents, over a 100-year period, just 1 percent (or 3.3 Mt CO2) of the 83 – 100 Mt of carbon lost from the Delta, with a significant increase in restored area and management strategies to enhance CDR rates needed (Drexler et al., 2019). In the Tampa Bay, Florida, estuary, restoration sites are estimated to have accumulated 0.2 Mt CO2 over 10 years (2006 – 2016; ESA, 2016), with cumulative CO2 sequestration of 7.3 – 7.4 Mt CO2, and net CDR of 1.5 – 2.7 Mt CO2 estimated over 100 years, considering impacts of sea-level rise and maintaining current areas of developed lands (Sherwood et al., 2019). In submerged Louisiana wetlands where sediment management is required to raise the wetland elevation for restoration and creation, dredged material has proved to be a valuable sediment source (CPRA 2017). Projects that employ natural and nature-based features are increasing around the world (Bridges et al., 2015). Case studies of demonstration projects converting hardened and eroding shorelines to natural and nature-based shorelines are being established in China, Europe, Mexico, and the United States. (See reviews in Bilkovic et al., 2017; Saleh and Weinstein, 2016; and Zanuttigh and Nicholls, 2015.) Monitoring is required to verify and maintain projected gains against a baseline using an adaptive management approach, and applied research is needed to reduce uncertainties in how capacities can be increased.

2.6

2.6 —

Biochar

 

2.6.1

Introduction

Biochar is a carbon-rich product created through pyrolysis or gasification of biomass that is more durable against biological degradation than the biomass from which it is derived. The carbon removed from the atmosphere via photosynthesis in plants is returned to the atmosphere much more slowly if biomass is converted to biochar (Lehmann et al., 2006; Lehmann, 2007). Biochar production can be a standalone operation or can be combined with a bioenergy generation pathway (e.g., BECCS, see Section 3.7) that produces energy. Biochar is more resistant to decomposition than untransformed plant material (Lehmann et al., 2015), though the degree of durability depends on the chemical composition of the biochar and the conditions under which it is stored (Campbell et al., 2018; Spokas, 2010). However, it is not necessarily more stable than microbial byproducts in soil or bulk soil organic carbon (Hammes et al., 2008; Singh et al., 2012; Santos et al., 2012).

From a carbon-cycle perspective, the production of biochar can be considered either an avoided emission (because it prevents biomass from decomposing) or CDR (if including the biomass growth and biochar production all in the same system) (Campbell et al., 2018).

Biochar can also stabilize other organic matter added to soil, such as compost (Weng et al., 2017). When added to agricultural or forested lands, it may form long-term carbon pools in the soil with the possible addition of soil fertility and soil-quality co-benefits. Although not available in all cases, when applied as a soil amendment, biochar can stimulate microbial benefits (Lehmann et al., 2011), increase the soil’s water-holding capacity (Masiello et al., 2015), improve nutrient availability (Liang et al., 2006; Laird et al., 2010), decrease susceptibility to plant disease (Elad et al., 2010), and remediate contaminated soils (Beesley et al., 2011; Hale et al., 2011). By enhancing soil quality, such as by raising pH, biochar application can increase crop yields (Spokas et al., 2012; Jeffery et al., 2017; Crane et al., 2013) and carbon return to soil, thereby further increasing soil carbon storage (Whitman et al., 2011). In this way, biochar can potentially form a positive feedback loop: increasing biomass growth and further increasing carbon sequestration.

 

2.6.2

Current status

Further research is needed on biochar to better understand mechanisms and timescales of oxidation (NASEM, 2019). More than half of the studies on biochar as a soil amendment have been conducted under greenhouse conditions (Jeffery et al., 2017), with research still needed on its efficacy across soil types and over time. Similarly, more field and long-term data are needed for persistence of biochar carbon in soil (Wang et al., 2016). To maximize crop yield responses and the overall impact of biochar applications on soil carbon sequestration, a much better understanding is needed of the complex interactions among biochar, soil, crops, climate, management factors and non-CO2 greenhouse emissions.

 

2.6.3

Potential scale and storage potential

Recent estimates of biochar’s climate change mitigation potential range from 1.1 to 3.3 GtCO2eq/yr by 2030 (Griscom et al., 2017b; Paustian et al., 2016; Woolf et al., 2010). However, this range is uncertain. New generations of models are needed to predict the net impact of biochar applications on soil carbon sequestration and net greenhouse gas emissions, which requires mechanistic models that account for complex interactions between biochar and the conditions in which it is introduced. Process-based models are essential to understand these interactions, which can influence crop yield and environmental responses to biochar applications, and they must be calibrated and validated using data from long-term field trials across diverse soils, climates, and management systems.

 

2.6.4

Current challenges

Several gaps in research remain for biochar. Biochar life cycle analyses must account for the persistence and associated carbon storage value of the biochar itself, the proportion of feedstock released as CO2 during pyrolysis, and the sequestration potential and agronomic benefits of the feedstock in the absence of pyrolysis (e.g., as organic amendment to soil),  as well as any effects of biochar on agricultural productivity and soil trace gas emissions of nitrous oxide or methane (Laird 2008; Cayuela et al., 2014). Lifecycle analyses of biochar should continue to evolve as field trials yield relevant results on the impacts of biochar production and application across a range of sites.

For biochar to provide a long-term climate change mitigation tool, a significant proportion should remain in durable form for hundreds to thousands of years. However, most estimates are based either on short-term laboratory incubation (Spokas, 2010) or short-term mineralization and crop yield studies in greenhouses (Wang et al., 2016). Field experiments to date suggest that wood- and grass-derived pyrolyzed carbon has similar residence times in soil as the bulk soil organic matter (Singh et al., 2012). Longer-term studies that track soil carbon transformations and stabilization are needed to optimize biochar application for systems where it can achieve the desired effect of increasing long-term soil carbon storage.

As with any soil amendment, different biochars applied to different soils and crops generate different yield responses (Jeffrey et al., 2017). Heterogeneity of biochar quality and type can add further variation when added as a soil amendment across different geographies, climates, and soil types. To broadly deploy biochar and other long-lived bioenergy co-products as soil amendments, we must conduct field trials that span climates, soil types, and agricultural and forestry practices where biochar could be used, while tracking soil carbon storage, broader biogeochemical and hydrologic cycles, and agricultural yields.

Biomass resources are widely distributed geographically, and variable feedstocks produce variable biochars with distinct characteristics. To optimize these geographically distinct systems, hybrid systems that produce biochar can be tested on agricultural and forest feedstocks that vary in the quality of energy produced and the biochar co-products they can create. These systems would likely need to be modular to address the distributed nature of the resources and the size of the biochar production opportunity in any one location.

 

2.6.5

Current and estimated costs

While biochar production is a mature technology, it is commercially immature. Markets, buyers, and products must be developed. Near-future cost estimates vary significantly and are strongly influenced by production method and application rate (Williams and Arnott, 2010). Indeed, some studies suggest that for economically viable biochar application, CO2 prices between $30 and $50/tCO2 are sufficient (Lomax et al., 2015a; Roberts et al., 2010), while other estimates reach $60 – $120/tCO2 or more by considering dedicated biomass feedstocks (plants grown specifically to convert to biochar), rather than wastes and residues such as plant material left over from crop harvests (McGlashan et al., 2012; Shackley et al., 2011; Smith, 2016). The cost of abatement is not necessarily equal to the market price of biochar. Cost estimates of biochar production range widely depending on the feedstock, production process, scale, and end use. Industry analysis and techno-economic assessment of current production models indicate that current mean costs range from approximately $96 – $1,834/t of biochar (Campbell et al., 2018). Additionally, the lifecycle abatement or removal potential of biochar depends on process and feedstock. For example, Roberts et al. estimate that slow pyrolysis of stover (leaves and stalks of field crops) and yard wastes could yield an abatement of 864 and 885 kg CO2eq/t of dry feedstock, respectively.

 

2.6.6

Example projects

The biochar industry is relatively immature, despite very high interest amongst producers. A 2018 survey of biochar producers by the U.S. Biochar Initiative estimated current production around 45,000 tonnes per year (range: 35,000 – 70,000 tonnes per year) from 135 biochar producers in the United States. It noted, however, that the overall market size could potentially be more than 3 billion tonnes. This is a tremendous range – it is clear that the biochar industry is still in its infancy.

2.7

2.7 —

Biomass energy with carbon capture and storage (BECCS)

 

2.7.1

Introduction

Biomass energy with carbon capture and sequestration (BECCS) couples the natural photosynthetic growth of plants with the engineered production of bioenergy, with an associated carbon-containing product (often CO2, but alternatively long-lived carbon products) that are utilized or stored for long periods. The term “bioenergy” denotes the conversion of biomass into energy or energy carriers, including electricity, heat, and solid, liquid, or gaseous fuels. Traditional biomass use – the combustion of wood or dung for cooking and heating – has been ubiquitous in human history. The last several decades have seen large-scale production of ethanol and biodiesel fuels from food crops, particularly in the U.S. (primarily from maize and soy) and Brazil (sugarcane). However, most decarbonization plans now envision significant scale-up of the production of liquid transportation fuels and other modern energy products from non-consumable cellulosic biomass feedstocks (Fulton et al., 2015). BECCS is considered a CDR system when bioenergy is provided alongside capture and storage of carbon (e.g., resultant CO2 emissions), such that the net balance of carbon (again, e.g., CO2) released during biomass production, transport, conversion, and utilization is negative (Fuss et al., 2018; NASEM, 2019).

 

2.7.2

Current status

A wide range of conversion technologies have been developed or proposed to produce biomass energy, products, and services. Certain conversion technologies are more appropriate for certain feedstocks and require additional processing to sequester carbon, including co-product and byproduct management. These technologies range from combustion, which produces electricity and/or heat alongside CO2 for capture and sequestration, to biochemical or thermochemical conversion methods that create energy products, such as hydrogen and liquid transportation fuels, alongside CO2 and/or biochar for carbon sequestration. Biochemical pathways rely on living microorganisms, often yeast or bacteria, to process biomass into more useful forms. Much research and engineering has focused on the biochemical conversion of cellulose to fuels (Lynd 2017), and most of the pioneering commercial-scale cellulosic biofuel production facilities built to date are based on fermentation (Lynd et al., 2017). While technical and policy barriers prevent widespread production of cellulosic biofuels today, fermentation remains a key technology both in current biofuel production and in production of carbon-negative fuels. As a potentially carbon-negative pathway, fermentation produces both biofuel and a high-purity stream of CO2 for carbon sequestration or utilization (Sanchez et al., 2018) and has been demonstrated by retrofitting existing first-generation corn ethanol facilities with CCS.

In contrast, thermochemical conversion involves the controlled heating and decomposition of biomass into liquid, gaseous, and solid products, and may entail upgrading liquid and gaseous intermediates into finished liquid or gaseous fuels (Tanger et al., 2013). While thermochemical conversion technologies, including gasification and pyrolysis, have not yet achieved the same deployment scale as biochemical technologies, they are highly amenable to carbon-negative configurations, and thus are prime candidates for additional targeted research and deployment support. Gasification coverts biomass to a hydrogen- and carbon monoxide-rich synthesis gas (“syngas”) product. Syngas can then be burned to produce electricity or catalytically or biologically upgraded to liquid fuels, of which jet fuels are an important end product. Alternatively, the hydrogen content can be separated from the syngas, after possibly being enhanced via a water-gas shift reaction (where carbon monoxide and water vapor react to form carbon dioxide and hydrogen), leaving a CO2 stream for capture and sequestration. Pyrolysis, in contrast, entails the thermal decomposition of biomass in the absence of oxygen, producing liquid (bio-oil), gaseous, and solid (biochar) products. Bio-oils are turned into liquid fuels, such as by catalytic hydrogenation, deoxygenation, and cracking, and the solid biochar byproduct provides a sequestration pathway as described in Section 3.6.

 

2.7.3

Current challenges

While many biomass conversion technologies are ready for commercial deployment, a major limiting factor for large-scale BECCS is the availability of biomass and land requirements to increase biomass availability. Large-scale expansion of lignocellulosic crops (such as trees, straw, and grasses) for BECCS may put pressure on food security, forest conservation, and other uses of productive land. Other limiting factors for biomass production can be nutrient limitations, albedo effects, water availability, and biodiversity (Smith and Torn, 2013).

Two significant CCS knowledge gaps need to be addressed to advance BECCS. First, CCS must be evaluated in the context of a distributed bioenergy production industry. Important engineering and societal questions must be answered before we build systems to accumulate CO2 from many distributed small sources, such as: Do we attempt to design BECCS facilities to capture and store CO2 near the site of production, despite a potential lack of co-location with biomass resources? Or do we build BECCS plants near biomass resources and collect CO2 into a network of pipelines and move it to geological sequestration sites? Each of these approaches has different carbon intensities and challenges.

Second, it is essential to evaluate the relative competitiveness of alternative BECCS-based carbon capture strategies (Woolf et al., 2016) – CCS, CCU, long-lived products, and biochar or bio-oil sequestration systems – across scales, carbon price scenarios, geographic regions, and policy scenarios.

 

2.7.4

Current and estimated costs

Cost estimates in the literature range from $15 – $400/tCO2 (Fuss et al., 2018). This wide range depends on the sector and on the specific source for CO2 capture. Increasingly sophisticated modeling has helped identify particularly low-cost or profitable implementations of BECCS systems. In particular, high-purity CO2 streams produced as a byproduct of fermentation or fuels production is a near-term opportunity for BECCS deployment (Sanchez et al., 2018). These sources have lower costs associated with CO2 capture, which is typically the largest portion of CCS system costs. Feedstock availability, system integration, and CO2 transportation infrastructure are critical components of the BECCS scale-up challenge. In this regard, geospatial, techno-economic, and life cycle analyses of BECCS mitigation potential are key tools to assess future deployment.

 

2.7.5

Potential scale and storage potential

Estimates for BECCS range from 1.2 – 5.2 GtCO2/yr of carbon dioxide removal to 31 – 77 GtCO2/yr (Fuss et al., 2018). These values are derived from assessments of biomass availability, identified as a major limiting factor for BECCS at scale, with total bioenergy potential estimated in 2050 to range from 60 – 1,548 exajoules (EJ) per year, and by considering that 1 EJ of biomass typically yields around 0.02 – 0.05 GtCO2 worth of negative emissions (Fuss et al., 2018). These bioenergy estimates depend on available land and biomass, which in turn are driven by assumptions regarding future population and diet, biodiversity and conservation restrictions, or land quality and technology improvements. Lower-bound estimates of BECCS potential using waste biomass alone have estimated up to 0.5 GtCO2/yr may be achievable in the U.S., and up to 5.2 GtCO2/yr could be achieved globally with BECCS fueled by biomass waste exclusively (NASEM, 2019).

 

2.7.6

Example projects

Currently, five facilities around the world are deploying BECCS, and these facilities cumulatively collect approximately 1.5 MtCO2/yr (Global CCS Institute, 2019). Notable among these is a full-scale BECCS demonstration plant in Illinois that captures up to 1 MtCO2/year from the fermentation process of a corn ethanol plant. The captured CO2 goes into in a geological storage site (Section 3.9) underneath the facility. The BECCS facilities operating today are small-scale ethanol production plants that use most of the captured CO2 for enhanced oil recovery (EOR), or CCS demonstrations at waste-to-energy facilities. It should be noted that the net CO2 emissions of corn ethanol production with CCS are still positive, meaning there is net CO2 release from their operation, owing to CO2 emissions from the production process.

2.8

2.8 —

Direct air capture (DAC)

 

2.8.1

Introduction

Direct air capture (DAC) refers to technologies that use a chemical approach to capture CO2 from ambient air. Today’s leading technologies capture CO2 using either synthetic solid sorbents or water-based solvents (Heidel et al., 2011; Keith et al., 2018b; Kumar et al., 2015; NASEM, 2019). Fans push air through large contactors, where it is met with CO2-reactive chemicals (e.g., amines, hydroxides). As the air passes through the contactor, CO2 collides and binds with the capture agent while the primary components of air (nitrogen and oxygen) continue to move through. The contactors moderate air flow via structured materials to allow adequate time for CO2 to move through the liquid (solvent) or micro and mesoporous channels (solid sorbent) and chemically react with amines to form carbamate bonds, or with hydroxide to form carbonate bonds. Ultimately, high-purity CO2 is recovered by breaking the chemical bonds (carbamate or carbonate) using heat, pressure differential, or chemical-displacement reactions (e.g., potassium carbonate may be displaced by calcium carbonate, which easily precipitates out of solution, aiding in the CO2 separation and purification process). The CO2 is then compressed for transportation in a truck or pipeline (e.g., for storage deep underground).

 

2.8.2

Current status

Many different technologies are in development for DAC at varying levels of research and deployment. Solid sorbents for DAC are micro- and mesoporous materials (e.g., silica, activated carbon, cellulose, alumina, and metal-organic frameworks) to which amines or amino groups (nitrogen-based molecules) are added. The solid materials are typically manufactured as beads or sheets and configured to form a sorbent bed, or are embedded in a structured contactor, not unlike the catalytic converter in an automobile. Air is blown through the contactor, which allows for the selective removal of CO2. The porous solid sorbent materials have large surface areas to maximize the mass of chemicals on the surface for chemically binding CO2. As an example, a microporous activated carbon may have a surface area of approximately 2,200 m2/g, while a single gram of metal organic framework sorbent may have a surface area of approximately 6,000 m2/g – just over the size of a football field (Wilcox, 2012). It is important to note that with these materials, depending on the relative humidity, both water and CO2 may be adsorbed. Hence, upon regeneration, a co-benefit could be the production of water in addition to high-purity CO2 after a condensing step.

Another approach to DAC uses solvents, or aqueous alkaline solutions, to react with CO2. In this case, the chemical is dissolved in an aqueous solution instead of being bound to porous solid materials. The solution may be pumped over structured packing with large surface area. The solution has optimized density, viscosity, and surface tension properties so that it may optimally coat the packing material to provide maximum potential for CO2 to react and ultimately be removed from the air stream flowing through the contactor. First-generation packing materials for absorption processes were invented in the 1940s. The packing material allows the solution to uniformly and thinly distribute throughout the contactor to maximize the surface area between the gas containing CO2 and the chemical in the solution, like the solid sorbent method. An advantage of the liquid solution approach is that the solvent is inexpensive and easy to make in large quantities.

Both the sorbent and solvent technologies require thermal energy and electricity to power the process. The systems require electricity for fans used to move air through the contactors and pumping equipment to move fluids through the system, as well as to power other process equipment. The thermal energy requirements arise from regeneration of the solid sorbents or solvent-based solutions. Each process has an energy mix of roughly 80 percent thermal energy and 20 percent electricity, with the energy requirements totaling 300 – 500 megawatts (MW) (NASEM, 2019). Although this total appears high, much of the energy requirements are associated with thermal energy, which can be achieved in many ways and need not tie directly to the grid. There are emerging technologies based on the direct use of electrons for sorbent regeneration. These approaches are still being researched at the laboratory scale (Voskian and Hatton, 2019). While using electrons for sorbent regeneration yields high energy efficiencies, it also presents challenges with respect to sorbent stability in the presence of high oxygen concentrations.

A major difference between solvent and sorbent technologies is the CO2 regeneration temperature. The solvent-based system requires temperatures on the order of 900º Celsius (Keith et al., 2018; NASEM, 2019b) because of the formation of precipitated calcium carbonate (CaCO3). Calcium carbonate requires calcination to produce high-purity CO2 and lime (CaO), which is re-used throughout the capture process. The high temperature requirements limit the solvent-based system’s available heat sources. In contrast, the solid sorbent-based systems require regeneration temperatures near 100º Celsius (NASEM, 2019). This heat is required to break the carbon-bonding to the sorbents, thereby releasing CO2 and regenerating the solid sorbent. The lower temperature requirements allow the solid sorbent-based technology to couple directly to low-carbon energy resources such as geothermal, concentrated solar, and even low-grade waste heat (Wevers et al., 2020; McQueen et al., 2020b; McQueen et al., 2021). An added reason for coupling solid sorbent technologies with renewables is that they have the potential to have shorter thermal cycles, versus a high-temperature decomposition process, which requires continuous operation.

 

2.8.3

Current challenges

In 2018, the concentration of CO2 in the atmosphere was 407 ppm (NOAA), or roughly 300 times less than CO2 in the exhaust of a coal-fired power plant and 100 times less than in the exhaust of a natural gas-fired power plant. The minimum amount of thermodynamic energy required to capture CO2 from air is three times greater than that required for CO2 capture from a coal-fired power plant. The greater dilution of CO2 in air translates to DAC requiring 300 times more contact area than coal-fired power plants to capture the equivalent CO2 (Wilcox, 2012). These differences translate directly to greater energy and capital costs for DAC compared to more concentrated sources. For instance, using detailed numerical simulations, Stampi-Bombelli et al. (2020) showed that the minimum thermal regeneration energy for DAC with an amine-appended cellulose adsorbent is approximately 440 kilojoules/mol CO2 (kJ/molCO2), which translates to a 2nd-law thermodynamic efficiency of approximately 5 percent, assuming the thermodynamic minimum work is about 20 kJ/molCO2. This efficiency can be calculated by dividing the minimum work of CO2 separation by the actual work of a defined separation process (Wilcox., 2012).

The energy required to carry out DAC on a scale of millions of tonnes of removal per year should not be underestimated. Depending on the energy resource, capturing 1 MtCO2/yr requires 180 – 500 MW of power (Baker et al., 2020; NASEM, 2019). Therefore, the design of a DAC plant must also include the design of an energy plant coupled to it, to maximize the net removal of CO2 from air. According to the NASEM report (2019) a solid sorbent energy system requires three steps to capture CO2: heat for the CO2 desorption process (removing the CO2 from the sorbent in the contactor), electricity for the contactor fans, and electricity for the vacuum pumps.

These energy requirements are similar to those for liquid solvents, where the primary energy components include thermal energy for high-temperature regeneration (calcination), electricity for contactor fans, steam requirements for the regeneration of calcium hydroxide from lime, and electricity for the pellet reactor (Keith et al., 2018).

Care should be taken to ensure that CO2 is not emitted by the power source, which means that either renewable power or natural gas power with additional capture from a natural gas power plant flue gas should be used. Thus, maximizing the potential of DAC requires coupling capture plants with carbon-free energy.

Another challenge for solid sorbent DAC in particular, is developing sorbents with high CO2 uptake and long lifetimes to avoid frequent sorbent replacement. Similarly, recycling and reuse of solid sorbents presents opportunities for future research.

Generally, the potential impact of DAC is limited by financial constraints, not technical ones. Despite being able to theoretically locate DAC anywhere, the reality might be different. The DAC plant is part of a CDR system that also relies on the availability of energy supply infrastructure (such as renewable energy sources) and geological sequestration opportunities for storing the carbon that DAC plants capture. The geographical proximity of the components of this CDR system largely determines the net amount of CO2 that is captured and permanently removed from the atmosphere, with each energy type possessing its own constraints.

 

2.8.4

Current costs and estimated costs

Climeworks is a Swiss company developing solid sorbent-based DAC units. Their experts have claimed through multiple deployments that the current cost of DAC can equal $600/tCO2 (Evans, 2017; Gertner, 2019). Since the power source coupled to the DAC plants operated by Climeworks is low- to zero-carbon, the cost of removal roughly equates to the net removed cost. Climeworks’ experts have publicly stated that they anticipate these costs decreasing down to $200 – $300/tCO2 by 2024 (Gertner, 2019).

The capital expense of the solid sorbent DAC approach is dominated by the cost of manufacturing the solid sorbents used in the process (NASEM, 2019). While sorbent manufacturing does not benefit from economies of scale today, increased deployment could drive up demand and capitalize on increased production. Solid sorbent systems benefit from their repetitive contactor geometry, which allows them to repeatedly manufacture the same contactor unit to create larger contactors. With this approach there is reduced investment on account of smaller DAC units, and the DAC units can feed into smaller, niche markets for CO2 supply. Conversely, the smaller contactor units may also be a constraint of the sorbent system, which uses a vacuum to remove residual air from the contactor, requiring thicker process equipment with larger cross-sectional areas. While the repeated geometry may be a constraint, the smaller scale, more repetitive contactor geometry for the sorbent DAC units may allow the technology to achieve higher  learning rates typically associated with manufactured products. Learning rates for manufactured products are usually above 15 percent, where large processing equipment often sees learning rates around 10 percent (Azarabadi and Lackner, 2019; Rubin, 2019; van der Spek et al., 2017).

For the solvent-based approaches to DAC, the capital expense is dominated by the large equipment required for the separation, such as the air separation unit, oxygen-fired calciner, and pellet reactor, all which would benefit from economies of scale. For these applications, larger-scale plants on the order of 1 MtCO2/yr removal are more cost-effective than smaller plants capturing less than 100,000 tCO2/yr.

Several studies maintain that with mass deployment of DAC plants, the cost per tonne of CO2 removed will drop significantly. These studies estimate that future DAC plants that separate high-purity (i.e., more than 97 percent) CO2 from the air suitable for transportation and geological storage will operate on the order of $100/tCO2 (Keith et al., 2018; NASEM, 2019). Climeworks has demonstrated DAC deployment at thousands of tonnes (or kilotonnes) of CO2 removed per year (ktCO2/yr). If this can increase to millions of tonnes per year over the next decade or two as hoped, these lower costs may be realized. Deploying DAC on a significant scale (i.e., millions of tonnes of CO2 removed per year) requires a significant support infrastructure for both energy and – depending on the technological configuration – land. At $100/tCO2 across the full lifecycle of separation, the transport infrastructure, and sequestration using appropriate carbon storage, the overall cost would equal $100 billion per year (0.5% U.S. GDP) at 1 GtCO2/yr.

Figure

2.8

A visual representation of an air contactor and a few examples of possible storage solutions. The fire and lightning bolt icons represent the heat and electricity the air contactor uses.

cdr primer figure 2 8

A visual representation of an air contactor and a few examples of possible storage solutions. The fire and lightning bolt icons represent the heat and electricity the air contactor uses.

 

2.8.5

Potential scale and storage potential

A plant designed to remove 1 MtCO2/yr from air may require up to 2 km2 of land for the DAC plant and. energy source (Beuttler et al., 2019a; NASEM, 2019b). The land area associated with the energy to run the plant can vary significantly depending on the energy source. For example, the electric component of solid sorbent-based DAC may be entirely powered by renewable electricity coupled with battery storage. In addition, solar energy may be used to provide heat for the low-temperature (solid sorbent) approach, through the incorporation of heat pumps, resistive heating, or concentrated solar power (CSP) (as investigated in Section 3). Exemplary land area requirements are provided below for three different electricity generation technologies: natural gas power plant, solar (photovoltaic, or PV) energy, and wind electricity (Baker et al., 2020). Depending on the power required of the DAC plant, the following land areas may be used for a given energy resource, assuming a solar and wind capacity factor of 28 percent:

  • Natural Gas: 1,400 m2/MW (0.34 acres/MW) (Stevens et al., 2017)

  • Solar (PV) Energy: 116,550 m2/MW (28.8 acres/MW) (Stevens et al., 2017)

  • Wind Energy: 242,811 m2/MW (60 acres/MW) (Stevens et al., 2017)

The land required for the energy plant coupled to the DAC plant may vary significantly since it depends on the DAC technology being used and whether continuous operation is required. Maximizing the operation of the DAC plant will allow for maximal capture of CO2, which is desired for minimizing costs and maximizing net removal of CO2 from air. However, some of the energy resources, such as wind and solar, are intermittently available, which requires potentially overbuilding or including energy storage in the system design. These system optimization parameters will influence the energy required to run a given DAC plant and will increase the total land area required for the entire system for energy generation and CO2 capture.

In terms of capture and storage, DAC offers the potential for high levels of permanence when coupled with geological storage, particularly sedimentary storage (see Section 2.9).

 

2.8.6

Example projects

Siting DAC plants where they can achieve maximum potential for carbon dioxide removal requires co-locating DAC with permanent storage. As previously discussed, the availability of low-carbon energy is another siting consideration. How to best determine the availability of this valuable resource is a central question, as low-carbon energy should always be prioritized for displacing fossil energy, with CDR being a secondary consideration.

Determining how much land is required for a given scale of DAC is still a somewhat open question since DAC has yet to scale beyond thousands of tonnes per year. For instance, uncertainty exists surrounding the spacing of the contactors, where spacing is dependent upon the inter-contactor mixing time – or the period of time it takes for the CO2-depleted stream leaving the contactor to mix with ambient air and achieve atmospheric CO2 concentrations. This will affect the spacing of each subsequent contactor, and therefore will affect the total footprint of the DAC plant. In addition, increasing the scale of DAC will require significant amounts of energy and materials. The energy required for the leading technologies described is dominated by low-carbon thermal energy, which can be sourced through a variety of approaches, including geothermal energy, concentrated solar power, nuclear power, and leak-tight natural gas with carbon capture. Independent investigation of these supply chains through dedicated techno-economic analyses, as well as the possible release of a public materials database to provide a blueprint for manufacturing guidelines, would be helpful in understanding the full potential for using these energy sources for DAC (NASEM, 2019).

An excellent example that reflects all of these siting considerations is the CarbFix project in Iceland. CarbFix collaborates with Climeworks to store CO2 in basalt rock through carbon mineralization reactions. Ultimately, CO2 is sequestered underground by injecting a mixture of CO2 and water into the subsurface of basalt formations. A basalt formation is a good candidate for CO2 storage if it is sealed by layers of impermeable rock from above but demonstrates a certain porosity in layers of the basalt that allows for carbon mineralization. The CarbFix project has three major natural resource requirements: basalt formations with these characteristics, often found on the continental surface and the ocean floor; CO2 sourced from DAC; and a water source. For heat, CarbFix uses geothermal energy as a low-carbon option.

Chapter 3 dives further into the considerations involved in selecting geographic sites for CO2 injection. Carbon storage opportunities exist in myriad geologic areas, including both sedimentary basins and mineral formations. Decision makers need to review the physical characteristics of their location to assess which storage options are best suited for their situation.

2.9

2.9 —

Geological sequestration

 

2.9.1

Introduction

Geological sequestration of CO2, although not a CDR approach in and of itself, is an essential building block of CDR systems such as DACCS and BECCS and allows these approaches to achieve negative emissions. In this section, the potential, requirements, and constraints of geological sequestration are examined more closely.

Sedimentary formations are made of an accumulation of layers of sediments that are compacted over time and transformed into rocks via the diagenesis process. Overall, this transformation reduces the porosity and the permeability of the sedimentary layers. However, each type of sediment reacts differently to this transformation; some become impermeable, while others keep a high porosity (approximately 30%), with all porosity levels in between. This porosity is filled with either brines (salt water) or hydrocarbons, resulting either in saline aquifers or oil and natural gas reservoirs. Most prospective sedimentary formations for CO2 sequestration are located in less deformed areas and are made of a vertical succession of porous and impermeable layers. CO2 is injected in porous layers (reservoir rock) deep underground, while impermeable layers act as a structural trap (cap rock), preventing it from rising back to the surface.

Current sequestration techniques that are known or under development involve either injection of supercritical CO2 in deep sedimentary formations or CO2 mineralization into carbonate rocks by interaction with alkaline material (Section 2.2). When gases such as CO2 are in supercritical states in most basins at depths of more than 800 meters, their density is much higher (approximately 600 kg/m3, as opposed to about 2 kg/m3 in gaseous phase). U.S. environmental regulations prohibit CO2 injection in or near freshwater and typical injection depths are isolated well below drinkable groundwater sources. Ideal geologic reservoirs are thick, high-porosity and high-permeability formations more than 1,000 meters below the surface (NASEM, 2019). Confidence in the concept of geological storage in deep saline formations is based on analogs to oil and gas fields, where large volumes of buoyant fluids have been trapped in the subsurface for tens of millions of years.

With this technique, CO2 remains supercritical at depth, which increases both the amount of CO2 that can be stored and the permanence of the storage (NASEM, 2019). Characteristics of the reservoir and its operation during injection monitoring and closure procedures that ensure permanent storage have been defined and are currently used in regulations (EPA, 2010; Parliament, 2009). A confining system that effectively isolates injected fluids from the near surface is required to demonstrate that freshwater resources are not damaged. The required properties of the confining system depend on the pressure increase in the injection reservoir that will result from injecting the planned volume at the planned rate. Additionally, the CO2 at reservoir conditions is buoyant compared to brine, necessitating a confining system to limit upward migration and escape over time (IPCC, 2005a, 2005b). Additional trapping mechanisms include solubility trapping (dissolution of CO2 into brine contained in the porosity of the reservoir) (Emami-Meybodi et al., 2015), residual trapping (CO2 trapped in rock porosity by capillary forces associated with the channels between the grains within the mineral) (Krevor et al., 2015), and mineralization (chemical reaction between the CO2, pore fluid, and rock) (Kelemen et al., 2019; Matter et al., 2016; Zhang and DePaolo, 2017). Multiple trapping mechanisms ensure long-term isolation from the atmosphere.

The succession of porous and impermeable layers enhances the structural trapping mechanism. Stacked storage (Hill et al., 2013) might be suitable to inject CO2 in several porous layers located at different depths, increasing the capacity of the basin. This explains the superposition of sedimentary reservoirs suitable for CO2 sequestration when mapping the reservoirs. Each layer of rock is characterized by its physical characteristics, such as the porosity and permeability, and chemical characteristics, such as the chemical composition and mineralogy. These parameters are used to identify and characterize suitable sites for CO2 injection and sequestration. This includes calculating the capacity of the formation, the maximum injection rate (or maximum injectivity), and ideal locations of injection wells.

 

2.9.2

Current status

Large volumes of fluids have been injected into the subsurface for more than a century. Initial volumes were oil-field brines that were diverted from hazardous surface disposal for reinjection into deep geologic formations. Reinjection occurred in the same reservoir from which the brines were produced or deeper or shallower zones. Other poor-quality fluids from industrial and municipal sources are also injected. The environmental standards for such injection were developed by the U.S. EPA as required by the Safe Drinking Water Act (SDWA) of 1974 (EPA, 1996). Other countries developed similar standards. Since 1972, large volumes of CO2 have been injected into oilfields for Enhanced Oil Recovery (EOR), which is also regulated under the SDWA. 

EOR as a greenhouse gas mitigation activity has been extensively discussed elsewhere (Azzolina et al., 2016; Núñez-López et al., 2019). CO2 injection into deep geologic formations is a newer technology developed in response to incentives to reduce emissions to the atmosphere. The initial large-volume (1MtCO2/year) injection project designed to avoid releasing CO2 back into the atmosphere is operated by Equinor and started in 1996. Additional project growth is inventoried at 51 major projects injecting 94 MtCO2/yr (GCCSI, 2019). The rate of expansion has been greatly limited by funding, which is insufficient to capture additional large volumes of CO2 from most sources. Storage projects have generally been successful, however use of the CO2 for EOR has been a dominant part of the mechanism driving project financing.

 

2.9.3

Potential scale and storage potential

The global sequestration capacity of saline aquifers and hydrocarbon reservoirs totals 5,000 – 25,000 GtCO2 (de Coninck and Benson, 2014). However, present rates of storage are only on the order of tens of MtCO2/yr. To achieve ambitious CDR targets at the scale of gigatonnes of removal and storage per year, the CO2 storage industry will need to scale up fast, and that will likely involve enormous costs, on the order of $1 billion per year over 10 – 20 years.

 

2.9.4

Current challenges

Decades of CO2 injection in sedimentary basins show that in the rare instances of measurable leakage, the major cause is poor well construction. Regulations require that wells isolate subsurface fluids (brine, oil, gas) from underground sources of drinking water and the surface. Previous studies indicate that a single leakage incident takes place out of every 1000 operations in a given year and that they are often associated with well operations. (Jordan and Benson, 2009; Porse et al., 2014; Skinner, 2003). However, some leakage may occur during injection, when wells that were poorly constructed, improperly plugged, damaged during operation, or constructed prior to regulation are exposed to increased pressure. Preparation and permitting of a storage site that contains existing wells therefore has a strong focus on well identification, qualification for service, or remediation. Surveillance of wells remains the key element during operation as pressure increases in the reservoir. A few famous well failures at non-CO2 storage sites (Macondo and Aliso Canyon) highlight how a mature practice should experience few high-impact events (Pan et al., 2018). Recent experience with large-volume water injection has also shown that pressure management is essential to avoid unacceptable frequency and magnitudes of induced seismicity (Lemons et al., 2019). Applying pressure management to geological storage will be important to future regulation.

To ensure that the well infrastructure is sound, in many sedimentary basins with existing well penetrations, good information is needed to predict the area where CO2 will migrate during and after injection. In addition, the area of pressure elevation must be predicted accurately (known as Area of Review), and any wells or other features that might provide a migration risk in this area must be inspected and documented as properly designed to isolate fluids in the subsurface. This requires both good characterization of the subsurface and suitable fluid flow modeling. Improvements to reduce cost and increase confidence are needed, as are targeted and cost-effective monitoring to provide data to feed back into models and provide sufficient assurance that the project can continue over its planned lifetime and be closed without undue concerns about future liabilities.

 

2.9.5

Current costs and estimated costs

Costs and energy penalties for storage in deep sedimentary formations are relatively low. Estimates of geological storage can range from $1 – $18/tCO2. An economic analysis from Rubin et al. indicates a range of $7 – $13/tCO2 (2015). However, the need to construct pipelines to transport CO2 from areas where it is captured to suitable storage sites can increase capital costs. Additional costs may apply in countries where the subsurface does not belong to the government but to private owners who expect to be compensated for use of the subsurface.

The cost of storage and the cost of capture are sometimes co-mingled in financial incentives and technology growth curves. To avoid a tax on carbon emissions from the Norway government, the Sleipner project was launched in 1996 (Torp and Brown, 2005). In the U.S., tax credits for large-scale CO2 storage were developed in 2008 and increased in 2019 (The National Law Review, 2019). While information on use of this credit is sparse because of the confidential nature of tax reporting, it appears it has been effective in incentivizing storage of CO2 that is captured at low cost, in particular the nearly pure CO2 stream removed from produced methane to purify it to meet pipeline standards. However, a detailed analysis of the cost of integrated capture, transport, and storage of CO2 shows that to help generate more than the current few low-cost facilities, incentives must be doubled or tripled. However, in this analysis, carbon dioxide removal approaches were not considered (NPC, 2019). Additional incentives for carbon capture and storage have been announced as part of the California Low Carbon Fuel Standard (California Air Resources Board, 2020).

 

2.9.6

Example projects

Examples of integrated CCS projects that include capture, transportation, and storage are inventoried by the Global CCS Institute. Cases include the previously mentioned Sleipner project, which has been operating at 1Mt/year injecting CO2 stripped from natural gas processing since 1996 (Furre et al., 2017), and large volumes of CO2 captured from coal-fired power plants and sent for EOR and storage at the Saskpower (International CCS Knowledge Centre, 2020) and PetraNova plants (NRG, 2020).  The storage project most closely related to CDR is the CO2 capture from corn ethanol production at the Archer Daniels Midland plant in Decatur, IL (Bioenergy International, 2017). This project has advanced in scale and shows promise to be replicated. A design study has been announced to match a DAC facility to EOR in the Permian Basin (Bioenergy International, 2017). Carbon Engineering and OXY Low Carbon Ventures are the project proponents, and the announced motivation is to earn Low Carbon Fuel Standard (LCFS) and 45Q tax credits.

The most mature storage technique today with ongoing commercial-scale projects is the injection of supercritical CO2 into sedimentary formations. However, integration of this storage with capture via BECCS and DACCS remains to be accomplished. New work is needed to optimize the co-location of these new and potentially very large sources of CO2 with suitable geological storage. This may resemble the source size matching done for industrial and power plant sources but also may require different optimizations and could open new opportunities. For example, some of the largest and best-quality geologic formations globally are near offshore (Ringrose and Meckel, 2019). The feasibility of developing DACCS and BECCS to use this resource has not been widely considered.

The amount of net CO2 removed could also be optimized by improving the decision-making process for CO2 sequestration site selection. Considering the entire CDR system (energy source, CO2 capture, and CO2 sequestration), as opposed to screening for sites based solely on the reservoir characteristics, would help minimize the transportation distances between the steps of the process and increase the amount of net CO2 removed per unit of CO2 captured. Finally, the improvement of monitoring and cost reduction during this process will enable more positive techno-economic analyses. Opening dialogues and sharing information with local communities can help improve public understanding of CO2 sequestration. The most mature storage technique today with ongoing commercial-scale projects is the injection of supercritical CO2 into sedimentary formations.

1

Permanence here relates to the duration that the CO2 can be stored when using the proposed CDR system. Each technology has a unique set of individual requirements in terms of relative permanence.

2

This optimal number is an approximate value that ensures energy and economic burdens are at their lowest.

3

This more conservative (safe) estimate is with the assumption that there would be no major conversion of arable cropland (e.g., to conservation reserves) and thus no decrease in crop production that could impact food security.

4

Although vegetation density is positively correlated with the strength of precipitation sheds and has a moderating effect on the volatility of water availability. Also, forests that have been seeded in areas or regions with very little rainfall can pose serious threats to groundwater, irrigation, and streamflow.

5

Distinct from ocean alkalinity covered in Section 3.1.2, coastal blue carbon is another component of coastal and marine management (Fig. 3.1) that refers to the stocks and fluxes of organic carbon and greenhouse gases in tidally influenced coastal ecosystems such as marshes, mangroves, seagrasses, and other wetlands (Crooks et al. 2019)

1

Permanence here relates to the duration that the CO2 can be stored when using the proposed CDR system. Each technology has a unique set of individual requirements in terms of relative permanence.

2

This optimal number is an approximate value that ensures energy and economic burdens are at their lowest.

3

This more conservative (safe) estimate is with the assumption that there would be no major conversion of arable cropland (e.g., to conservation reserves) and thus no decrease in crop production that could impact food security.

4

Although vegetation density is positively correlated with the strength of precipitation sheds and has a moderating effect on the volatility of water availability. Also, forests that have been seeded in areas or regions with very little rainfall can pose serious threats to groundwater, irrigation, and streamflow.

5

Distinct from ocean alkalinity covered in Section 3.1.2, coastal blue carbon is another component of coastal and marine management (Fig. 3.1) that refers to the stocks and fluxes of organic carbon and greenhouse gases in tidally influenced coastal ecosystems such as marshes, mangroves, seagrasses, and other wetlands (Crooks et al. 2019)

45Q

45Q tax credit

AFOLU

agriculture, forestry, and other land use

ARPA-E

Advanced Research Projects Agency-Energy

ASU

air separation unit

bbl

barrel of oil

BECCS

bioenergy with carbon capture and sequestration/storage

BTU/mmBTU

British thermal unit/one million British thermal units

C

carbon

CAPEX

capital expenditures

CCS

carbon capture and storage

CCU

carbon capture and utilization

CCUS

carbon capture, utilization, and storage

CDL

cropland data layer

CDR

carbon dioxide removal

CI

carbon intensity

CKD

cement kiln dust

CLT

cross-laminated timber

CO2

carbon dioxide

CO2eq

CO2 equivalent

CRF

capital recovery factor

CRMS

Coastwide Reference Monitoring System

CSP

concentrated solar power

CWPPRA

Coastal Wetlands Planning, Protection and Restoration Act

DAC

direct air capture

DOE

United States Department of Energy

EIA

United States Energy Information Administration

EJ

exajoule

EOR

enhanced oil recovery

EPA

Environmental Protection Agency

EU

European Union

FOAK/Nth-OAK

first-of-a-kind (FOAK) or nth-of-a-kind

GCCC

Gulf Coast Carbon Center

GHG

greenhouse gas

GIS

geographic information system

Gt

gigatonne

GTM

global timber model

GWP

global warming potential

ha

hectare

HE

hard-to-avoid emissions

HHV

higher heating value

IAM

integrated assessment model

IEAGHG

International Energy Agency Greenhouse Gas Research and Development Programme

IFM

improved forest management

IPCC

Intergovernmental Panel on Climate Change

ISBL

inside battery limits

ISO

International Standards Organization

J

Joule

kJ

kilojoule

kt

kilotonne

kW

kilowatt

kWh

kilowatt-hour

LCA

life cycle analysis

LCFS

low-carbon fuel standard

LED

low energy demand scenario

LHV

lower heating value

LR

learning rate

MJ

megajoule

Mt

megatonne

MW

megawatt

NASEM

National Academies of Science, Engineering, and Medicine

NET

negative emissions technology

OSBL

outside battery limits

PGE

platinum group elements

pH

potential of hydrogen

ppm

parts per million

PSU

practical salinity units

PV

photovoltaics

RCP

representative concentration pathway

RD&D

research, development, and demonstration

SAU

storage assessment unit

SBSTA

Subsidiary Body for Scientific and Technological Advice

SDWA

Safe Drinking Water Act

SLCP

short-lived climate pollutant

SMR

steam methane reformation

SOC

soil organic carbon

SRM

solar radiation management

t

tonne

TEA

techno-economic analysis

TRL

technology readiness level

UN

United Nations

UNEP

United Nations Environment Programme

UNFCCC

United Nations Framework Convention on Climate Change

USDA

United States Department of Agriculture

USGS

United States Geological Survey

WACC

weighted average cost of capital

WBCSD

World Business Council for Sustainable Development

WGSR

water-gas shift reaction

WRI

World Resources Institute

wt

weight

Greenhouse gas (GHG)

Those gaseous constituents of the atmosphere, both natural and anthropogenic (human-caused), that absorb and emit radiation at specific wavelengths within the spectrum of terrestrial radiation emitted by the Earth’s surface, by the atmosphere itself, and by clouds. This property causes the greenhouse effect, whereby heat is trapped in Earth’s atmosphere. Water vapor (H2O), carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4), and ozone (O3) are the primary GHGs in the Earth’s atmosphere. There are also a number of entirely human-made GHGs in the atmosphere, such as the halocarbons and other chlorine- and bromine-containing substances, managed under the Montreal Protocol on Substances that Deplete the Ozone Layer (Montreal Protocol). Besides CO2, N2O and CH4, the Kyoto Protocol deals with the GHGs sulphur hexafluoride (SF6), hydrofluorocarbons (HFCs), and perfluorocarbons (PFCs).

Carbon dioxide equivalent (CO2eq)

Describes the impact of a given GHG (CO2, CO, CH4, N2O, etc.) by converting its mass to the equivalent mass of CO2 that would have the same global warming effect. The mass of a GHG is converted to the mass of CO2eq based on the GHG molecule’s potential to affect global warming, or its global warming potential (GWP). The GWP takes into account both the radiative forcing effect of the GHG and the gas’s lifetime in the atmosphere, and is dependent on the time horizon, which is most commonly 20 years (GWP20) or 100 years (GWP100). These values are different because the GWP is time-integrated and the GWP of CO2 is always 1, regardless of the time horizon. (See Chapter 4, Supplement 4.1.)

Montreal Protocol on Substances that Deplete the Ozone Layer

A 1987 international treaty that regulates the production and consumption of nearly 100 man-made chemicals referred to as ozone-depleting substances. (UNEP)

Kyoto Protocol

A 1997 agreement that operationalizes the United Nations Framework Convention on Climate Change (UNFCCC) committing industrialized countries and economies in transition to limit and reduce greenhouse gas emissions in accordance with agreed-upon individual targets. (UNFCCC)

Paris Agreement

A 2016 agreement formed by Parties to the UNFCCC to combat climate change and to accelerate and intensify the actions and investments needed for a sustainable low-carbon future. (UNFCCC)

Carbon dioxide removal (CDR)

Activities that remove CO2 from the atmosphere and durably store it in geological, terrestrial, or ocean reservoirs, or in products. CDR includes enhancement of biological or geochemical sinks and direct air capture (DAC) and storage, but excludes natural CO2 uptake not directly caused by human intervention.

CDR approach

Refers to methods, which may not be complete CDR systems, that can achieve carbon dioxide removal. For example, DAC in the absence of permanent storage is a CDR approach, but is not on its own a CDR system.

(Potential) CDR system

A system with the capacity to generate net-negative emissions, but which will not necessarily do so under all conditions. A comprehensive life cycle analysis (LCA) and other analyses are required to conclusively demonstrate that a “potential” CDR system is, indeed, a CDR system. For example, in the absence of an LCA, a CDR system that barely achieves net-negative emissions could look the same as one that achieves far greater net climate benefits per tonne of CO2 removed from the atmosphere.

Large-scale CDR

Carbon dioxide removal on the order of or approaching gigatonnes of CO2 removed per year.

Gigatonne of carbon dioxide (GtCO2)

Refers to a billion metric tonnes (metric tons) of CO2, which is equivalent to 1015 g. It is also helpful to know that 1 GtCO2 is equivalent to 0.273 gigatonnes of carbon (GtC). This unit of measurement is used most frequently when discussing the scale of CDR required to prevent the worst impacts of climate change and to keep warming below 1.5° C (i.e., gigatonne-scale CDR).

Sink

Any process, activity or mechanism that removes a greenhouse gas, a precursor of a greenhouse gas, or an aerosol from the atmosphere.

Source

Any process, activity, or mechanism that creates a greenhouse gas, a precursor of a greenhouse gas, or an aerosol.

Positive emissions

Occur when a  particular source — created or enhanced by human activity — adds greenhouse gases to the atmosphere.

Negative emissions

Occur when a sink — created or enhanced by human activity — removes greenhouse gases from the atmosphere. (In this primer, only CO2 is considered.)

Net-negative emissions

Achieved when more greenhouse gases are removed from the atmosphere than are emitted into it.

Net-zero emissions

Achieved when the total emissions entering the atmosphere are balanced by the total removal of emissions from the atmosphere. It is sometimes used interchangeably with the term carbon-neutral.

Life cycle analysis (LCA)

An analysis of the balance of positive and negative emissions associated with a certain process, which includes all of the flows of CO2 and other greenhouse gases, along with impacts on other environmental or social impacts of concern. This analysis also includes greenhouse gas emissions that result from the materials used to construct a given process (commonly referred to as embodied emissions), as well as from the energy resources used to meet the energy demands of the process.

Embodied emissions

Emissions that result from the production or use of any good, or the provision of any service. For example, the embodied emissions of steel used for a reactor will include emissions associated with the acquisition of raw materials, processing, manufacturing, transportation, and the energy used in steel production.

Hard-to-avoid emissions

Emissions that are either physically extremely difficult to eliminate within a certain timeframe (e.g., because of dependence on a particular infrastructure with a long lead time for carbon-free substitution, or because avoidance would require a technology that relies on a scarce resource) or which would be unacceptable to avoid from a social justice perspective (e.g., if mitigation would deprive people of the means to satisfy their basic needs, like food security).

Avoided emissions

Permanently storing what would have been a CO2 emission in order to avoid an increase in atmospheric CO2. Note that this is distinct from CDR because the source of CO2 is a potential emission rather than the atmosphere. The climate benefit of such an intervention depends on the counterfactual scenario – what would have happened without our action? – which can only be estimated, not observed. If a project claims to avoid emissions that weren’t going to happen anyway, and other emissions are allowed to continue in exchange, then an avoided emission project can do more damage to the climate than doing nothing at all.

Emissions mitigation

A human intervention to reduce or avoid greenhouse gas emissions. Some publications have used the broad term “mitigation” to refer exclusively to emissions mitigation, whereas others have used mitigation (or “climate change mitigation”) to include emissions mitigation, CDR, and any other strategies for combating climate change.

Reservoir

Refers to the place where a greenhouse gas is stored. Examples include geological formations, alkaline-containing minerals, forest biomass, and soils.

Stock

A carbon reservoir that exchanges carbon with the atmosphere over relatively short timescales (e.g., less than 100 years), including the terrestrial biosphere and shallow oceans. By comparison, other reservoirs, such as geologic formations and the deep ocean, exchange carbon over much longer timescales (e.g., 1,000 years).

Storage (or Sequestration)

Two terms that can be used interchangeably to describe the addition of CO2 removed from the atmosphere into a reservoir, which serves as its ultimate destination. For example, some CDR strategies store carbon in biological systems, such as forests or soil ecosystems, whereas others inject CO2 deep underground or chemically transform CO2 into stable, mineral forms.

Permanence (or Durability)

The duration for which CO2 can be stored in a stable and safe manner. Storage duration can differ significantly, depending on the type of  reservoir. For example, concentrated CO2 stored in geologic formations deep underground is effectively permanent (thousands of years), whereas forest carbon stocks can release carbon back into the atmosphere due to wildfire or tree harvesting.

Leakage (physical)

Refers to initially-stored CO2 (or other GHG) that has left its storage state and returned to the atmosphere. Examples include combustion of a fuel made from CO2, burning of biomass, and migration of CO2 from underground storage.

Flux

The rate of movement of greenhouse gases between reservoirs (e.g., capturing CO2 through the chemical reactions of photosynthesis, resulting in carbon storage within a plant’s biomass.

Carbon cycle

The residence time and flux of carbon — in various chemical states — between the ocean, land, terrestrial biosphere, atmosphere, and geological formations in the Earth.

Additionality

Evaluates the degree to which an intervention (e.g., a CDR project) causes a climate benefit above and beyond what would have happened in a no-intervention baseline scenario. By definition, this counterfactual baseline scenario cannot be directly observed (because it did not happen), so can only be estimated or inferred based on contextual information. Additionality can be assessed at the level of individual projects or protocols that define categories of projects. In policy regimes such as cap-and-trade programs, where emissions are permitted in exchange for reduction or storage elsewhere, failures of additionality result in increased emissions.

Carbon offsets

Programs or policy regimes in which companies or individuals pay for activities that result in emissions reductions or CDR. In voluntary offset programs, individuals or companies pay project developers (or similar) directly to implement some activity that results in emissions reductions or CDR. In compliance offset programs, such as cap-and-trade programs, companies that are responsible for large amounts of emissions are allowed to continue to emit above a certain cap in exchange for projects taking place elsewhere that reduce emissions or remove carbon. In a compliance regime, an offset has no effect on total emissions in the best-case scenario and will result in more emissions than would have occurred otherwise if the project is ineffective in any way (e.g., due to failures of additionality or permanence).

Climate tipping points

Abrupt and irreversible climate events, such as ice sheet loss and ecosystem collapse.

Leakage (socioeconomic)

Occurs when CDR activities displace emissions to other locations, times, or forms. For example, leakage occurs in forest carbon offset credit programs when a reduction in timber harvesting at a project site, incentivized due to its potential for emissions reductions and/or CDR, causes timber harvesting to increase somewhere else to meet demand. Similarly, if an engineered CDR approach coupled to CO2 utilization results in higher costs, alternatives that emit more CO2 may become economically favorable.

Overshoot

Climate stabilization scenarios in which emissions trajectories exceed their concentration or temperature targets (e.g., 1.5º C or 2º C) early in the century, but use substantial amounts of CDR later to reduce atmospheric CO2 levels and achieve the original targets. See Chapter 1 for a discussion of moral hazards and other challenges associated with these scenarios.

Equity

The principle of fairness in access to opportunities, power-sharing, and burden-sharing. Equity is crucial to determining how to deploy strategies to address climate change, including CDR, that minimize harm to marginalized people and frontline communities. From the World Resources Institute: “Climate change poses the greatest threat to those that are the least responsible – generally people that are already vulnerable to deep-rooted challenges such as poverty. Conversely, those who have contributed the most to climate change have much better capacity to protect themselves from its impacts. As the effects of climate change mount, so does the urgency of addressing this equity challenge.”

Social justice

Just or fair relations within society that seek to address the inequitable distribution of wealth, access to resources, opportunity, and support and remove discriminatory systems and structures that block marginalized groups from accessing these benefits on the basis of race, gender, economic status, or any other factor.

Frontline communities

Some climate justice advocates have described these as communities that experience the “first and worst” consequences of climate change. These are often communities of color and those with low income levels that have insufficient infrastructure (such as well-maintained roads and up-to-date flood protection) and that will be increasingly harmed as our climate deteriorates. These may include (for example) Native communities whose resources have been exploited, or communities of laborers whose daily work or living environments are polluted or toxic.

Moral hazard

An ethical concern whereby skeptics of potential CDR systems worry that emission reduction will be reduced or delayed on the promise of future CDR deployment. Experts agree that most temperature-stabilization scenarios require ambitious efforts both on greenhouse gas mitigation and CDR, and that the potential for large-scale deployment of CDR does not justify slowing down the pace of mitigation.

Negative-emissions technologies

This term has been used as an alternative to CDR approaches in other publications.

Geoengineering

This term refers to the concept of manipulating the climate system in order to reduce the impacts of climate change. It usually refers to solar radiation management (SRM), a controversial strategy that involves the injection of material into the atmosphere, which directly affects radiative forcing to reduce warming, but does not change concentrations of greenhouse gases. CDR has been inaccurately conflated with geoengineering in some literature.

Storing C vs. storing CO2

Some CDR methods capture and store CO2 (carbon dioxide) directly, whereas others remove CO2 from the atmosphere but store only C (carbon). For example, in biological CDR approaches involving plants and trees, CO2 is fixed through the chemical reactions of photosynthesis, and is then broken down into C stored in the plant body and roots and O2 released back into the atmosphere. The relationship between stored C and stored CO2 is given as a ratio of the molecular weights (MW), i.e., GtCO2 = GtC×(MWCO2/MWC) = GtC×(44/12) = 3.67×GtC.

Active emissions vs. legacy emissions

The first term refers to the GHG being actively emitted by an entity/technology, while legacy emissions are emissions associated with previous emissions that contribute to the accumulated pool in the atmosphere.

CDR vs. geoengineering

Unlike geoengineering, CDR removes excess CO2 from the atmosphere, thereby reducing CO2’s warming effect on the Earth. Geoengineering such as SRM focuses only on the symptoms of climate change, without changing concentrations of greenhouse gases.

CDR vs. carbon capture

Unlike carbon capture, the siting of CDR is not fixed to a point source. Point-source carbon capture targets higher-purity streams of CO2, capturing them from emissions sources like smokestacks before they reach the atmosphere. By contrast, CDR approaches target the CO2 that is already in our atmosphere. While removing this diluted CO2 from the atmosphere is more challenging,  CDR has the potential to compensate for emissions from non-point sources (e.g., transportation) and hard-to-abate sectors like heavy industry and agriculture.

45Q tax credit (45Q)

Provides a progressive production tax credit for CCS (including DACCS, BECCS, and CCUS) reaching a maximum value of $50 USD per ton CO2 storage and $35 per ton for beneficial reuse in 2026 and adjusted for inflation thereafter. Under current provisions, qualified facilities can claim the credit for up to 12 years after being in service.

absorber

A unit operation used in the chemical industry to separate gases. In an absorber, the gas comes in contact with a specific solvent that will selectively scrub (remove) the target contaminant from the gas stream. For absorbers used in carbon dioxide (CO2) avoidance or removal, the target contaminant is CO2. Absorbers themselves can be configured in many ways, including spray columns, packed columns, or bubble columns. (See also: solvent)

absorption

When atoms, molecules, or ions enter the liquid or solid phase of a sorbent. (See also: solvent)

adsorption

The adhesion of atoms, molecules, or ions to a surface, creating a thin film of the adsorbate (the chemical species that is doing the adhering) on the surface of the adsorbent (the species that is being adhered to).

Advanced Research Projects Agency-Energy (ARPA-E)

United States federal agency which advances high-potential, high-impact energy technologies that are not yet ready for private sector investment. ARPA-E awardees are unique in that they are developing novel ways to generate, store, and use energy.

aerobic ecosystem

An aerobic environment is one characterized by the presence of free oxygen (O2), in contrast to an anaerobic environment, which is one devoid of O2. (Elgamal, 2016).

Additionality

Evaluates the degree to which an intervention – for example, a CDR project –  causes a climate benefit above and beyond what would have happened in a no-intervention baseline scenario. By definition, this counterfactual baseline scenario cannot be directly observed (because it did not happen), so can only be estimated or inferred based on contextual information. Additionality can be assessed at the level of individual projects or protocols that define categories of projects. In policy regimes such as cap-and-trade programs, where emissions are permitted in exchange for reduction or storage elsewhere, failures of additionality result in increased emissions.

afforestation

The creation of a new forest in an area that was not previously forested. (IPCC SR 1.5 Glossary).

agricultural byproduct/agricultural residues

Low-value biomass resources that are derived from the cultivation of primary cash crops and do not typically have widespread commercial applications. Examples include rice straw, wheat straw, rice husks, and corn stover, which in many places are left on the fields after harvests, used for fodder and landfill material, or burnt. (See also: stover). (Adhikari et al., 2018).

agriculture, forestry and other land use (AFOLU)

A term used in the 2006 Intergovernmental Panel on Climate Change (IPCC) guidelines, which describes the anthropogenic greenhouse gas emissions from two distinct sectors: Agriculture and LULUCF (Land Use, Land Use Change and Forestry).

agroforestry

The intentional integration of trees and shrubs into crop and animal farming systems to create environmental, economic, and social benefits. Agroforestry has been practiced in the United States and around the world for centuries. (USDA, 2019).

air contactor (contactor)

A structural component of direct air capture systems that provides a means for the capture media, e.g. basic solvents or sorbents, to make contact with and chemically bind carbon dioxide from ambient air.

air separation unit

Technology used to isolate and produce nitrogen or oxygen, and often co-produce argon

albedo

The proportion of the incident light or radiation that is reflected at the Earth's surface.

alkalinity

The capacity of a solution to resist acidification. Differs from basicity, which is an absolute measurement on the pH scale. For carbon mineralization, alkalinity (or alkaline potential) is measured by the presence of alkaline divalent cations (e.g. calcium and magnesium ions). 

allocation method

A collection of methods used in life cycle analysis that partition the input and/or output flows of a process to the product system that is under study. Allocation methods are used when a system is multifunctional (or produces more than one product). These methods provide a framework to divide the impacts of a product system across multiple co-products. (See also: co-product)

ambient weathering

Weathering that occurs naturally under normal environmental conditions. 

anaerobic ecosystem

An ecosystem or environment characterized by its lack of free oxygen (O2) but, which may contain atomic oxygen bound in compounds such as nitrate (NO3), nitrite (NO2), and sulfites (SO3). (Antonietti, 1999)

annualized cost

The cost paid annually on a capital cost loan, determined by the loan amount, loan period, and interest rate. 

anthropogenic emissions (anthropogenic carbon emissions)

Human activities, such as burning fossil fuel, deforestation, and livestock, that result in an overall increase in carbon dioxide emissions. 

artificial upwelling

Pumping nutrient-rich water to the surface to increase phytoplankton activity. This increases the uptake of carbon dioxide into the ocean. 

asbestiform

Fibrous, crystalline, durable material that can be toxic and highly dangerous to the human body. Asbestos is the common name for asbestiform minerals.

avoided emissions

Permanently storing what would have been a CO2 emission in order to avoid an increase in atmospheric CO2. Note that this is distinct from CDR because the source of CO2 is an existing emission rather than the atmosphere. The climate benefit of such an intervention depends on the counterfactual scenario — what would have happened without our action? — which can only be estimated, not observed. If a project claims to avoid emissions that weren’t going to happen anyway, and other emissions are allowed to continue in exchange, then an avoided emission project can do more damage to the climate than doing nothing at all.

bioenergy with carbon capture and storage (or sequestration) (BECCS)

The process of extracting bioenergy from biomass and capturing and storing the emitted carbon, thereby removing it from the atmosphere.

biochar

A stable solid rich in carbon that is made from organic waste material or biomass that is partially combusted in the presence of limited oxygen. The qualities that make up biochar vary depending upon the material from which it is made (feedstocks, i.e., timber slash, corn stalks, manure, etc.) and the temperature at which combustion occurs. (USDA, 2020)

biological carbon stock (biological stock)

Carbon that is fixed and mediated through biological processes, such as photosynthesis or respiration. For example, these stocks would include soil organic carbon and carbon stored in biomass. 

bio-oil

Pyrolysis oil, sometimes known as bio-crude or bio-oil, is a synthetic fuel under investigation as a substitute for petroleum. It is obtained by heating dried biomass without oxygen in a reactor at a temperature of about 500° C with subsequent cooling. (David et al., 2010)

biomass

Plant (or animal) material used as fuel. Examples include wood, agricultural residues such as stover, or crops grown for the purpose of bioenergy production, such as switchgrass.

biomass storage (or sequestration)

The conversion of biomass into derived materials with more durable storage than the source biomass, including the use of pyrolysis to convert biomass into bio-oil (fast pyrolysis) or biochar (slow pyrolysis).

blue hydrogen

Hydrogen produced from natural gas through steam methane reforming coupled to carbon capture and storage.

brecciated rock

Rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, which can be similar to or different from the composition of the fragments.

British thermal units (BTU)

A measure of the energy content in fuel, used in the power, steam generation, heating, and air conditioning industries. 1 mmBTU refers to one million British thermal units.

burial/carbon burial

Burial in sediments removes organic carbon from the short-term biosphere-atmosphere carbon cycle, and therefore prevents greenhouse gas production in natural systems.

calciner

A reaction vessel in which calcination occurs. Calcination is a process by which ore is heated to  temperatures high enough to decompose, but not melt, the ore. For example, calcium carbonate is calcined near 900º C to produce carbon dioxide and calcium oxide. 

capacity factor

The ratio of the actual operating capacity of a facility divided by the maximum operating capacity of the facility over a given period. for example, if a facility operates 24 hours a day, 7 days a week for 47 weeks out of a year, the capacity factor is (24x7x47)/(24x7x52.14) = 90 percent.

cap-and-trade-system

A market-based policy which establishes a ‘cap’ on the total allowable (permitted) emissions for entities covered under the policy. The policy allows covered entities to either reduce emissions or purchase permits (through auctions or from other covered entities) to achieve compliance. The cap declines over time to reduce emissions from covered entities. 

capillary trapping (residual gas trapping)

A secondary carbon dioxide trapping mechanism in subsurface reservoirs that renders the carbon dioxide gas immobile in the pore space via capillary forces. These forces are governed by fluid and interfacial physics and act at the spatial scale of rock pores. (See also: pore space)

capital expenditures

Monetary start-up costs used to implement an asset.

capital recovery factor

The ratio of a constant annuity to the present value of receiving that annuity for a given length of time.

caprock

An impermeable rock type overlying a porous rock type that acts as a reservoir. The pores of the porous rock are commonly filled with brine or hydrocarbons, and the caprock prevents fluids and gas from rising to the surface.

captive carbon dioxide (captive CO2)

CO2 that has been captured as a byproduct of industrial production for the purpose of internal reuse.

carbon accounting

The process of quantifying carbon dioxide and other greenhouse gas emissions throughout a system or product's lifecycle.

carbon capture and storage (CCS)

The process of capturing carbon dioxide formed during power generation and industrial processes and storing it so that it is not emitted into the atmosphere. Carbon capture and storage technologies have significant potential to reduce carbon dioxide emissions in energy systems.

carbon capture and utilization (CCU)

The process of capturing carbon dioxide to be recycled for further usage.

carbon capture, utilization and storage (CCUS)

A term encompassing methods and technologies to remove carbon dioxide from flue gas and from the atmosphere, followed by recycling the carbon dioxide for utilization and determining safe and permanent storage options.

carbon dioxide removal (CDR)

Activities that remove CO2 from the atmosphere and durably store it in geological, terrestrial, or ocean reservoirs, or in products. CDR includes enhancement of biological or geochemical sinks and direct air capture (DAC) and storage, but excludes natural CO2 uptake not directly caused by human intervention.

CDR approach

Refers to methods, which may not necessarily be complete CDR systems, that can achieve carbon dioxide removal. For example, DAC in the absence of permanent storage is a CDR approach, but is not on its own a CDR system.

carbon cycle

The carbon cycle concerns the residence time and flux of carbon – in various chemical states – between the ocean, land, terrestrial biosphere, atmosphere, and geological formations in the Earth.

carbon dioxide equivalent (CO2eq)

Describes the impact of a given GHG (CO2, CO, CH4, N2O, etc.) by converting its mass to the equivalent mass of CO2 that would have the same global warming effect. The mass of a GHG is converted to the mass of CO2eq based on the GHG molecule’s potential to affect global warming, or its global warming potential (GWP). The GWP takes into account both the radiative forcing effect of the GHG and the gas’ lifetime in the atmosphere, and is dependent on the time horizon, which is most commonly 20 years (GWP20) or 100 years (GWP100). These values are different because the GWP is time-integrated and the GWP of CO2 is always 1, regardless of the time horizon. (See Chapter 4, Supplement 4.1.)

carbon flux

The amount of carbon exchanged between the land, ocean, living things (i.e. plants, animals, fungi, bacteria, etc.), and the atmosphere. 

carbon intensity

The amount of carbon or carbon equivalent emissions – typically measured over the entire lifecycle – related to the unit production of a service or good.

carbon offsets

Programs or policy regimes in which companies or individuals pay for activities that result in emissions reductions or CDR. In voluntary offset programs, individuals or companies pay project developers (or similar) directly to implement some activity that results in emissions reductions or CDR. In compliance offset programs, such as cap-and-trade programs, companies that are responsible for large amounts of emissions are allowed to continue to emit above a certain cap in exchange for projects taking place elsewhere that reduce emissions or remove carbon. In a compliance regime, an offset has no effect on total emissions in the best-case scenario, and will result in more emissions than would have occurred otherwise if the project is ineffective in any way (e.g. due to failures of additionality, permanence, etc.).

carbon sink

A reservoir, natural or engineered, that has the capacity to store more carbon than it releases over a certain period of time.

carbon uptake

The flux of carbon from the atmosphere to other terrestrial subsystems.

catalytic hydrogenation

The addition of hydrogen over a reactive pi-bond (double or triple bond) in hydrocarbons. Typically performed over a transition metal catalyst to improve kinetics. 

cement hydration

A chemical reaction in which the major compounds in cement form chemical bonds with water molecules and become hydrates or hydration products.

chemical species

Describes a group of atoms or molecules which are identical to one another. For example, a cylinder of pure carbon dioxide only contains molecules of the same species. 

chemiophysical

Both chemical and physical; relating to both the chemistry and the physics of a substance or organism.

clarifier

Settling tank used to continuously remove solids from a liquid.The collected solids are either discharged from the bottom of the tank (if they are concentrated and denser than the liquid) or removed from the top (if the particles float on the surface). 

climate tipping points

abrupt and irreversible climate events, such as ice sheet loss and ecosystem collapse.

clinker

A solid material produced in the manufacture of Portland cement as an intermediary product. It is the primary contributor to carbon dioxide emissions in concrete production.

Coastal Wetlands Planning, Protection and Restoration Act (CWPPRA)

United States federal legislation  passed by Congress in 1990 to fund wetland enhancement. 

Coastwide Reference Monitoring System (CRMS)

A system designed to monitor the effectiveness of restoration actions at multiple spatial scales – from individual projects to the influence of projects on the entire coastal landscape. 

co-product

In life cycle analysis (LCA), a co-product is sometimes produced alongside the primary product of investigation. Co-product allocation is performed to distribute emission burden across each co-product based upon the scope of the LCA. (See also: allocation method)

carbon mineralization (carbon dioxide mineralization, CO2 mineralization, mineral carbonation or enhanced weathering)

A process in which minerals in rocks react with carbon dioxide, resulting in the formation of new carbonate minerals and permanent storage of carbon dioxide.

coastal blue carbon

The carbon captured by living coastal and marine organisms and stored in coastal ecosystems ((such as mangroves, tidal marshes, seagrass meadows, and other coastal habitat) that facilitate the long-term confinement of carbon in plant materials or sediment. Also refers to carbon dioxide removal techniques that utilize such ecosystems to increase carbon-removing biomass and, in particular, soil carbon. (NOAA, 2020)

compliance carbon market

Compliance carbon markets are marketplaces through which regulated entities obtain and surrender emissions permits (allowances) or offsets in order to meet predetermined regulatory targets.

concentrated solar power (CSP)

Concentrated solar power (also known as concentrating solar power or  concentrated solar thermal) systems generate solar power by using mirrors or lenses to concentrate a large area of sunlight onto a receiver.

concentrated solar power on demand

A system is designed to absorb concentrated solar radiation in a molten eutectic salt.

converted lands

Terrestrial landscapes or freshwater systems already impacted by human activity (e.g. human settlements, agricultural lands, roads, and dams). (Baruch-Mordo et al (2019))

cracking

A catalytic process whereby large, complex organic molecules such as kerogens or long-chain hydrocarbons are broken down into simpler molecules such as light hydrocarbons through the addition of hydrogen at high temperature and pressure.

cradle-to-gate

A life cycle assessment boundary describing those emissions generated as a result of upstream extraction and transport of raw materials through direct processing. This does not include contributions from downstream transport, consumption and/or end-of-life. (See also: life cycle analysis, cradle-to-grave, gate-to-gate)

cradle-to-grave

A life cycle assessment boundary describing all emissions and impacts that result over the entire system or product life cycle. This includes upstream extraction and transport of raw materials, direct processing, downstream transport, consumption, and/or end-of-life. (See also: life cycle analysis, cradle-to-gate, gate-to-gate)

cropland data layer (CDL)

A raster, geo-referenced, crop-specific land cover map for the continental United States, hosted on CropScape.

cross-laminated timber (CLT)

A wood panel product made from gluing together layers of solid-sawn lumber, i.e., lumber cut from a single log.

deep saline formations

Porous rock formations located from near the surface to several kilometers in depth, whose porosity is filled with brine.

degraded land

Land negatively impacted (e.g. by loss of biodiversity or the decrease of organic carbon in soils) by human activity and natural processes (e.g. fires, drought, land use changes, or peatland drainage).

deoxygenation

A chemical reaction where oxygen atoms are removed from a molecule. This can also refer to the removal of molecular oxygen from a gas or liquid solvent stream. 

United States Department of Agriculture (USDA)

The United States federal executive department responsible for developing and executing federal laws related to farming, forestry, rural economic development, and food.

United States Department of Energy (DOE)

A cabinet-level department of the United States government concerned with the United States' policies regarding energy and safety in handling nuclear material. 

direct air capture (DAC)

A chemical process that removes carbon dioxide from ambient air. When paired with carbon storage strategies, sometimes referred to as direct air carbon capture and storage (DACCS).

direct emissions

Emissions that are directly controllable and produced from within the system boundary. 

discount rate (bank rate)

Rate of interest charged by banks on loans.

electrolysis

Using an input of electric current to drive an otherwise non-spontaneous redox reaction (the transfer of electrons between chemical species).

embodied emissions

Emissions that result from the production or use of any good, or the provision of any service. For example, the embodied emissions of steel used for a reactor will include emissions associated with the acquisition of raw materials, processing, manufacturing, transportation, and the energy used in steel production.

emissions mitigation

A human intervention to reduce or avoid greenhouse gas emissions. Some publications have used the broad term “mitigation” to refer exclusively to emissions mitigation (CITE), whereas others have used mitigation (or “climate change mitigation”) to include missions mitigation, CDR, and any other strategies for combating climate change. (IPCC).

energy crop

Crops that are cultivated primarily for the purpose of producing energy feedstocks. 

United States Energy Information Administration (EIA)

A principal agency of the U.S. Federal Statistical System responsible for collecting, analyzing, and disseminating energy information to promote sound policymaking, efficient markets, and public understanding of energy and its interaction with the economy and the environment.

enhanced oil recovery (EOR)

Injection of supercritical carbon dioxide into oil reservoirs for the purpose of increasing the amount of oil recovered beyond primary (conventional) extraction.

United States Environmental Protection Agency (EPA)

An independent executive agency of the United States federal government tasked with environmental protection matters. 

equity

The principle of fairness in access to opportunities, power-sharing, and burden-sharing. Equity is crucial to determining how to deploy strategies to address climate change – including CDR – that minimize harm to marginalized people and frontline communities. From the World Resources Institute: “Climate change poses the greatest threat to those that are the least responsible – generally people that are already vulnerable to deep-rooted challenges such as poverty. Conversely, those who have contributed the most to climate change have much better capacity to protect themselves from its impacts. As the effects of climate change mount, so does the urgency of addressing this equity challenge.” (IPCC).

evapotranspiration

The process by which water is transferred from land to the atmosphere by evaporation from soil and other surfaces and by transpiration from plants. (USGS, 2020).

European Union (EU)

A political and economic union of 27 member states that are located primarily in Europe. 

ex-situ carbon mineralization

Carbon mineralization performed with alkaline feedstock that has been removed from its original location and state (i.e., the Earth). 

exergy

The measure of useful work that can be extracted from a system as it comes into equilibrium with its surroundings. 

fault

A crack in the Earth’s crust.

faulting

Creation of a fault or a system of faults due to mechanical stresses in the Earth's crust. Faulting in geologic formations is observed at several spatial scales, from micrometric to pluri-kilometric. (See also: fault)

feedstock

A raw material that can be converted to a good of higher value (i.e. fuel).

first-of-a-kind (FOAK)

A set of acronyms used in engineering economics where the first item or generation of items using a new technology or design can cost significantly more than later items or generations.

fold

A geological term referring to undulation or waves in a stack of originally planar surfaces of the Earth, such as sedimentary strata.

fold belts

A series of mountainous foothills made of a series of folds and adjacent to an orogenic belt (or mountain range). (See also: fold, folding)

folding

Creation of one or several folds due to mechanical stresses in the Earth's crust. Like faulthing, folding in geologic formations is observed at a variety of spatial scales.

flux

the rate of movement of greenhouse gases between reservoirs – for example, capturing CO2 through the chemical reactions of photosynthesis, resulting in carbon storage within a plant’s biomass.

frontline communities

some climate justice advocates have described these as communities that experience the “first and worst” consequences of climate change. These are often communities of color and those with low income levels that have insufficient infrastructure (such as well-maintained roads and up-to-date flood protection), and which will be increasingly impacted as our climate deteriorates. These may include Native communities whose resources have been exploited, or communities of laborers whose daily work or living environments are polluted or toxic.

gasification

The conversion of any carbon-based feedstock into synthesis gas or syngas (such as carbon monoxide and hydrogen gas) through reaction with a controlled amount of oxygen, typically at high temperature and pressure.

gate-to-gate

A life cycle assessment boundary describing only those emissions and impacts generated as a result of direct processing and excluding contributions from the upstream extraction and transport of raw materials as well as downstream transport, consumption, and/or end-of-life. (See also: life cycle analysis, cradle-to-gate, cradle-to-grave)

geoengineering

This term refers to the concept of deliberately manipulating the climate system in order to reduce the impacts of climate change. It is usually used in reference to solar radiation management (SRM), a controversial strategy that involves the injection of  material into the atmosphere that directly affects radiative forcing to reduce warming, but does not change concentrations of greenhouse gases. CDR has been inaccurately conflated with geoengineering in some literature.

geologic formation 

A body of rock having a consistent set of physical characteristics that distinguish it from adjacent bodies of rock. This body of rock must be distinctive enough to be identifiable, and thick and extensive enough to be plotted on geological maps.

geological sequestration (or storage) of carbon dioxide

Injection of carbon dioxide into porous geologic formations deeper than 800 meters (and preferably 1000 meters) for long-time storage (>1000 years). The carbon dioxide is kept at depth by various trapping mechanisms: mechanical trapping (impermeable caprock over the reservoir), residual gas trapping (capillary forces), solubility trapping (dissolution in brines or hydrocarbons), and mineralization (formation of carbonates). It is the last step of engineered carbon dioxide removal approaches that capture carbon from flue gas (bioenergy with carbon capture and storage) or from the air (direct air capture and storage).

Gigatonne of carbon dioxide

(GtCO2) – refers to a billion metric tonnes (metric tons) of CO2, which is equivalent to 1015 g. 1 GtCO2 is equivalent to 0.273 gigatonnes of carbon (GtC). This unit of measurement is used most frequently when discussing the scale of CDR required to prevent the worst impacts of climate change and keep warming below 1.5° C (i.e., gigatonne-scale CDR).

United States Geological Survey (USGS)

The nation's largest water, earth, and biological science and civilian mapping agency. It collects, monitors, analyzes, and provides scientific understanding of natural resource conditions, issues, and problems.

grain size

The diameter of individual grains of sediment. Grain size can vary within a sample of sediment, and can be expressed as d(0.8), or the diameter which is larger than 80 percent of the grains in a sample. 

greenhouse gas (GHG)

Those gaseous constituents of the atmosphere, both natural and anthropogenic (human-caused), that absorb and emit radiation at specific wavelengths within the spectrum of terrestrial radiation emitted by the Earth’s surface, by the atmosphere itself, and by clouds. This property causes the greenhouse effect, whereby heat is trapped in Earth’s atmosphere. Water vapor (H2O), carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4) and ozone (O3) are the primary GHGs in the Earth’s atmosphere. There are also a number of entirely human-made GHGs in the atmosphere, such as the halocarbons and other chlorine- and bromine-containing substances, managed under the Montreal Protocol on Substances that Deplete the Ozone Layer (Montreal Protocol). Besides CO2, N2O and CH4, the Kyoto Protocol deals with the GHGs sulphur hexafluoride (SF6), hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs).

geographical information system

A computer system for capturing, storing, checking, and displaying geographic data related to positions on the Earth's surface.

global timber model (GTM)

An economic model capable of examining global forestry land-use, management, and trade responses to policies.

global warming potential (GWP)

The heat absorbed by any greenhouse gas in the atmosphere, as a multiple of the heat that would be absorbed by the same mass of carbon dioxide.

Gulf Coast Carbon Center (GCCC)

A research center associated with the University of Texas that studies geological sequestration of carbon dioxide.

hard-to-avoid emissions

Emissions which are either physically extremely difficult to eliminate within a certain timeframe (e.g., because of dependence on a particular infrastructure with a long lead time for carbon-free substitution, or because avoidance would require a technology that relies on a scarce resource) or which would be unacceptable to avoid from a social justice perspective (e.g., if mitigation would deprive people of the means to satisfy their basic needs, like food security).

higher heating value (HHV)

A measure of the total energy released upon combustion of a material plus that which is released through the heat of condensation of any combustion byproducts. Also known as gross energy, upper heating value, gross calorific value, or higher calorific value.

highly-deformed sedimentary basin

Sedimentary rock layers deformed by numerous faults and folds.

hydrocarbon field

Porous rock (reservoir rock) containing oil and/or natural gas that can be commercially recovered, with at least one common reservoir for the entire area.

hydrologic cycle (or water cycle)

The continuous circulation of water in the Earth-atmosphere system.

hydrology

Branch of science that studies the movement, distribution, and management of water in the atmosphere, on the surface, and in the subsurface of the Earth and other planets. This includes the water cycle, water resources, and watersheds.

igneous rocks

Any rock that has resulted from the solidification of a molten or partly molten material (i.e. magma).(Blatt et al., 2006)

improved forest management (IFM)

An agriculture, forestry and land use project category in the Verified Carbon Standard (VCS), improved forest management (IFM) refers to land management practices designed to increase the quantity of carbon stored in forests relative to baseline conditions (e.g., by modifying harvest schedules).

in-situ carbon mineralization

Carbon mineralization performed with feedstock that remains in its original place (i.e., the Earth). Typically, aqueous carbon dioxide is circulated into a deep geologic formation, where it mineralizes to form solid carbonate minerals.

indirect emissions

Emissions that occur as a result of the system activity, but do not occur within the system boundary.

injection

Physical means by which concentrated carbon dioxide (usually supercritical CO2) is transported deep into the Earth's subsurface for the purpose of storage or EOR.

inside battery limits (ISBL)

Includes all equipment and associated components and instrumentation of a given operation. The battery limit outlines the limit of responsibility for the plant operators. (See also: outside battery limits).

integrated assessment model (IAM)

An approach that integrates social, economic, and physical models for the purpose of understanding how different decisions drive emissions and emission mitigation

Intergovernmental Panel on Climate Change (IPCC)

An intergovernmental body of the United Nations that is dedicated to providing the world with objective scientific information relevant to understanding the scientific basis of the risk of human-induced climate change, its natural, political, and economic impacts and risks, and possible response options.

International Energy Agency Greenhouse Gas Research and Development Programme (IEAGHG)

An international research body established in 1991, evaluating technologies to reduce greenhouse gases from the use of fossil fuels, with a focus on carbon capture and storage.

International Standards Organization (ISO)

An international standard-setting body composed of representatives from various national standards organizations.

kilowatt-hour (kWh)

A standardized measurement of energy over time.

knowledge base uncertainty

A knowledge base is the underlying data or assumptions that make up the knowledge in a certain analysis or, more broadly, a given field of study. Knowledge base uncertainty refers to the uncertainty that is built into these assumptions, on account of lack of knowledge, bias data, or similar causes.

Kyoto Protocol

A 1997 agreement that operationalizes the United Nations Framework Convention on Climate Change (UNFCCC) committing industrialized countries and economies in transition to limit and reduce greenhouse gas emissions in accordance with agreed-upon individual targets. (UNFCCC).

Large-scale CDR

Carbon dioxide removal on the order of or approaching gigatonnes of CO2 removed per year.

latent heat of vaporization

Energy released (upon condensation) or absorbed (upon evaporation) by a body or a thermodynamic system.

leakage (physical)

Refers to initially-stored CO2 (or other GHG) that has left its storage state and returned to the atmosphere. Examples may include combustion of a fuel made from CO2, burning of biomass, or migration of CO2 from underground storage.

leakage (socioeconomic)

occurs when CDR activities displace emissions to other locations, times, or forms. For example, leakage occurs in forest carbon offset credit programs when a reduction in timber harvesting at a project site, incentivized due to its potential for emissions reductions and/or CDR, causes timber harvesting to increase somewhere else to meet demand. Similarly, if an engineered CDR approach coupled to CO2 utilization results in higher costs, alternatives that emit more CO2 may become economically favorable by comparison.

learning rate

The rate at which a technology becomes more efficient in use of resources (both physical and monetary) as the number of produced units, or installed capacity increases. 

life cycle analysis (LCA)

An analysis of the balance of positive and negative emissions associated with a certain process, which includes all of the flows of CO2 and other greenhouse gases, along with impacts to other environmental or social impacts of concern. This analysis also includes greenhouse gas emissions that result from the materials used to construct a given process (commonly referred to as embodied emissions), as well as from the energy resource used to meet the energy demands of the process.

lignin

A complex organic polymer found in the cell walls of many plants, making them rigid and woody.

lignocellulose/lignocellulosic crops

Refers to plant dry matter (biomass) such as wood.  It is the most abundantly available raw material on Earth for the production of biofuels, mainly bioethanol.

low-carbon fuel standard (LCFS)

A rule enacted to reduce carbon intensity in transportation fuels as compared to conventional petroleum fuels, such as gasoline and diesel. 

low energy demand scenario (LED)

Refers to scenarios that limit global warming to 1.5° C, describing major transformations in energy supply and ever-rising energy demand.

lower heating/calorific value (LCV)

A calculation assuming that the water component of a combustion process is in vapor state at the end of combustion.

mafic rock

Igneous rock that contains relatively high amounts of magnesium and iron and is silica-poor (approximately 45 – 52 percent SiO2) – i.e., basalts and gabbros.

megajoule

A unit of energy equal to 1 million joules.

megawatt

A unit for measuring power that is equivalent to one million watts. One megawatt is equivalent to the energy produced by 10 automobile engines.

merchant CO2

carbon dioxide that is captured for the purpose of offsite distribution to other industries. 

metamorphic rock

Rock transformed by metamorphism (i.e. mineralogic or textural change that occurs in a rock in the solid state as a response to changes in environmental variables, especially temperature and pressure). (Blatt et al., 2006).

methanogenesis

Production of methane by archaea (microbial organisms distinct from eukaryotes and bacteria) known as methanogens.

mine tailings

Unwanted rock material that has been processed and rejected by a mining operation. Tailings are typically stored at a mine in ponds or piles, where they may remain for years or decades.

mineral dissolution

The rearrangement or destruction of a mineral's molecular lattice structure upon prolonged contact with a solvent, allowing ions to leave the lattice structure and dissolve into solution.

monoculture

The practice of cultivating only one agricultural crop or livestock at a time in a given area.

Montreal Protocol on Substances that Deplete the Ozone Layer (Montreal Protocol)

A 1987 international treaty that regulates the production and consumption of nearly 100 man-made chemicals referred to as ozone-depleting substances. (UNEP).

moral hazard

An ethical concern whereby skeptics of potential CDR systems worry that emission reduction will be reduced or delayed on the promise of future CDR deployment. Experts agree that most temperature stabilization scenarios require ambitious efforts both on greenhouse gas mitigation and CDR, and that the potential for large-scale deployment of CDR is not an acceptable basis to justify slowing down the pace of mitigation.

National Academies of Science, Engineering and Technology (NASEM)

A United States-based non-governmental organization that provides independent, objective advice to inform policy with evidence, spark progress and innovation, and confront challenging issues for the benefit of society.

Negative emissions

occurs when a sink  – created or enhanced by human activity – removes greenhouse gases from the atmosphere (in this primer, only CO2 is considered).

negative emissions technologies (NETs)

This term is not used in the primer, but has been used as an alternate term for CDR approaches in other publications.

net-carbon balance

The net carbon emissions or reductions resulting from the sum of carbon uptake and emissions.

net-zero emissions

achieved when more greenhouse gases are removed from the atmosphere than are emitted into it.

non-highly-deformed sedimentary basins

An ideally non-deformed sedimentary basin is made of a series of overlapping horizontal layers of sedimentary rocks. In a non-highly-deformed basin, these sedimentary layers underwent slight deformation by faulting and folding.

ocean alkalinity enhancement

Processes that increase the amount of alkalinity (or charge balance of ions) in the ocean, resulting in a carbon dioxide flux from the air to the ocean and an increase in dissolved inorganic carbon concentration.

ocean carbon

Carbon that is in ocean waters, namely dissolved carbon dioxide, bicarbonate and carbonate ions, dissolved or particulate organic carbon, and particulate inorganic carbon.

ocean fertilization 

Promoting the growth of photosynthetic organisms (i.e. macroalgae) for increased carbon uptake  by enhancing ocean conditions for uptake with iron, nitrates or phosphates.

offsets for emissions trading systems

A certifiable reduction, removal, or avoidance of greenhouse gases in one process that can be sold or traded for the purpose of lowering the overall greenhouse gas footprint of another process. 

olivine

A magnesium iron silicate that occurs naturally in basalt, peridotite, and other rocks. It has two endmember compositions, an iron silicate called fayalite and a magnesium silicate forsterite.

outside battery limits (OSBL)

Includes all equipment and associated components and instrumentation of a given operation. The battery limit outlines the limit of responsibility for the plant operators. (See also: inside battery limits)

operating cost

The total cost of operation, including payroll, maintenance, overhead, and other day-to-day costs associated with maintaining a facility or business, including energy costs and chemicals. These are the costs repeatedly incurred by a facility over its lifetime.

ophiolite

Ultramafic rock that originates from the upper mantle of the Earth’s crust, consisting largely of serpentine.

organic carbon

A measure of the carbon content in soils derived from plant and animal matter. Organic carbon is most abundant at the surface of soils and sediments where detritus is deposited, most of which is derived from aerobic photosynthesis.

overshoot

climate stabilization scenarios in which emissions trajectories exceed their concentration or temperature targets (e.g. 1.5º C or 2º C) early in the century, but use substantial amounts of CDR later to reduce atmospheric CO2 levels and achieve the original targets. See Chapter 1 for a discussion of moral hazards and other challenges associated with these scenarios.

oxy-fired kiln or calcination unit

A kiln or calcination unit that is fired with high purity of oxygen instead of air, where oxygen makes up roughly 21 percent of the air stream. 

Paris Agreement

A 2016 agreement formed by Parties to the UNFCCC to combat climate change and to accelerate and intensify the actions and investments needed for a sustainable low carbon future. (UNFCCC)

parts per million (ppm)

The number of units of mass of a contaminant per million units of total mass. As an example, the concentration of carbon dioxide in the atmosphere is approximately 410 ppm.

passivation

A process by which a material becomes more passive or is less prone to corrosion or reaction. For mineral carbonation purposes, this refers to the formation of a silica-rich outer layer as the carbonation reaction progresses. This layer is deficient in alkali cations and is therefore not reactive for the carbonation process and presents a barrier to diffusion. This layer is typically referred to as a "passivation layer."

peatland

A type of wetland that occurs in almost every country on Earth, currently covering 3 percent of the global land surface. The term ‘peatland’ refers to peat soil and the wetland habitat growing on its surface. In these areas, year-round waterlogged conditions slow the process of plant decomposition to such an extent that dead plants accumulate to form peat. (IUCN, 2020)

perfluorocarbons (PFCs)

Human-made chemicals that consist entirely of carbon and fluorine. Perfluorocarbons are powerful greenhouse gases. 

peridotite

A type of ultramafic rock that contains large amounts of olivine. (See also: olivine)

Permanence (or Durability)

The duration for which CO2 can be stored in a stable and safe manner. Storage dDuration can differ significantly between  reservoirs. For example, storage concentrated CO2 stored in geologic formations deep underground is effectively permanent for thousands of years, whereas forest carbon stocks can release carbon back into the atmosphere due to wildfire or tree harvesting.

permeability

The ability of a membrane to allow a certain gas to pass through it.

phenotype

The observable physical properties of an organism; these include the organism's appearance, development, and behavior. An organism's phenotype is determined by its genotype, which is the set of genes the organism carries, as well as by environmental influences upon these genes. (Nature, 2020)

photovoltaics (PV)

Refers to the conversion of light into electricity using semiconducting materials that exhibit the photovoltaic effect, a phenomenon studied in physics, photochemistry, and electrochemistry.

platinum group elements (PGEs)

Six noble, precious metallic elements clustered together in the periodic table. These elements are all transition metals in the d-block (groups 8, 9, and 10, periods 5 and 6). The six platinum-group metals are ruthenium, rhodium, palladium, osmium, iridium, and platinum.

pneumatic controller

A mechanical device that measures temperature or pressure, determines any deviation in temperature or pressure from a given setpoint, and transmits an air signal to a final control element to correct for the deviation.

point source

A localized, stationary and concentrated source of pollution, such as a smokestack.

polygeneration plant

A facility that produces multiple outputs or products from a single source, e.g., a coal plant that yields electricity, heat, syngas, and chemicals.

pore space

A characteristic of the porosity of a material, described as the total amount of empty or void space between material grains or particles.

Positive emissions

Occurs when a  particular source – created or enhanced by human activity – adds greenhouse gases to the atmosphere.

(Potential) CDR system

A system with the capacity to generate net-negative emissions, but which will not necessarily do so under all conditions. A comprehensive LCA and other analyses are required to conclusively demonstrate that a “potential” CDR System is, indeed, a CDR System. For example, in the absence of an life cycle analysis (LCA), a CDR system that barely achieves net negative emissions could look the same as one that achieves far greater net climate benefits per tonne of CO2 removed from the atmosphere.

potential of hydrogen

A scale used to specify the acidity or basicity of an aqueous solution. Acidic solutions (solutions with higher concentrations of H+ ions) are measured to have lower pH values than basic or alkaline solutions.

practical salinity units

The number of parts per thousand of sodium chloride (salt) in a given solution (i.e. 1 PSU = 1 gram per kilogram). This unit is based on the properties of sea water conductivity.

pyrolysis

Thermal decomposition of a material in the absence of oxygen.

radiative forcing/radiative effects

Refers to the difference between solar irradiance (sunlight) absorbed by the Earth and the energy radiated back to space. 

rangeland

Lands on which the indigenous vegetation is predominantly grasses, grass-like plants, forbs, and possibly shrubs or dispersed trees. Existing plant communities can include both native and introduced plants. (USDA)

reforestation

The natural or managed restocking of degraded lands that were or naturally would reach a climax community as a forest ecosystem. 

regional seal

Relatively impermeable rock (e.g. shale, anhydrite, or salt), that forms a barrier or cap above a reservoir rock (e.g. sandstone or limestone) such that fluids cannot migrate beyond the reservoir.

representative concentration pathway (RCP)

A greenhouse gas concentration (not emissions) trajectory adopted by the Intergovernmental Panel on Climate Change.

research, development, and demonstration (RD&D)

The set of innovative activities undertaken by corporations or governments in developing new services or products and improving existing ones. Also referred to as research and development (R&D, R+D), and known in Europe as research and technological development (RTD).

residence time

The average length of time that a substance will remain in a given location, condition, or control volume. For example, in reactor design, the residence time is the average time that a species will spend inside the physical reaction vessel. 

Reservoir

Refers to wherever a greenhouse gas is stored. Examples include geological formations, alkaline-containing minerals, forest biomass, and soils.

Safe Drinking Water Act (SDWA)

The principal federal law in the United States intended to ensure safe drinking water for the public.

second law of thermodynamics

Law of physics which states that the total entropy (disorder) of an isolated system can never decrease over time, and is constant if and only if all processes are reversible. (Wilcox, 2012)

sedimentary formation

Geologic formation made of a succession of layers of sedimentary rocks. Sedimentary formations are mapped by geological age.

sedimentary rock

Any rock that has resulted from the accumulation, induration (hardening), and/or cementation of sediments of any grain size (e.g. sandstones, mud rocks, and carbonate rocks).

sequestration (carbon dioxide sequestration)

See geological storage of carbon dioxide.

serpentine

A mineral subgroup formed when peridotite, dunite, and/or other ultramafic rocks undergo hydrothermal metamorphism. The most common serpentine minerals are magnesium-dominant and comprise three species: lizardite, antigorite, and chrysotile. 

shapefile

A geospatial vector data format or file type for geographic information system (GIS) software.

short-lived climate pollutant (SLCP)

Refers to a greenhouse gas that has a relatively short atmospheric lifetime, including methane and hydrofluorocarbons.

silicate

Refers to a type of mineral which makes up about 90 percent of the Earth's crust. Silicates contain varying amounts of silica tetrahedra, where one silicon atom is surrounded by four oxygen atoms. The tetrahedra can be isolated or combined in chains, double chains, sheets, or frameworks, which defines the different types of silicates.

Sink

Any process, activity or mechanism which removes a greenhouse gas, a precursor of a greenhouse gas, or an aerosol from the atmosphere.

slaker

The equipment in which the slaking reaction occurs. The slaking reaction produces calcium hydroxide by mixing solid lime (i.e., calcium oxide) and water (or steam). 

slurry

A mixture of water and fine particles. 

social justice

just or fair relations within society that seek to address the inequitable distribution of wealth, access to resources, opportunity, and support and remove discriminatory systems and structures that block marginalized groups from accessing these benefits on the basis of race, gender, economic status, or any other factor.

soil amendment

Any material added to a soil to improve its physical properties, such as water retention, permeability, water infiltration, drainage, aeration, and structure.

soil carbon dynamics

Changes in the fluxes, forms, and stocks of carbon within soil pools as a result of natural and anthropogenic inputs. 

soil carbon stock

Carbon trapped within soil.

soil carbon storage (or sequestration)

The accrual of carbon within soils (organic and inorganic forms), either naturally or through improved management.

soil organic carbon (SOC)

A measurable component of soil organic matter. Organic matter makes up 2 – 10 percent of most soil's mass and has an important role in the physical, chemical, and biological function of agricultural soils.

soil respiration

The production of carbon dioxide from the respiration of organisms within the soil.

solar radiation management (SRM)

Strategies that aim to reflect the sun’s rays back into space, characterized as geoengineering.

solvent

A substance that dissolves a solute. For example, potassium hydroxide in water is a solvent that dissolves carbon dioxide, a solute. (See also: absorption)

sorbent

A catch-all term for a solid that is used to absorb (absorbent) or adsorb (adsorbent) a given liquid or gas. For example, solid sorbents for direct air carbon capture are typically amine grafted solids which adsorb carbon dioxide on their surfaces. 

sorption isotherm

Represents the amount of material adsorbed as a function of the relative pressure at a constant temperature. There are six International Union of Pure and Applied Chemistry classifications for adsorption isotherms that can indicate certain pore sizes and adsorption potential in the adsorbent material. These isotherms are typically shown graphically. (Sing et al., 1985)

Source

Any process, activity, or mechanism which creates a greenhouse gas, a precursor of a greenhouse gas, or an aerosol.

sparging

The method of using a diffuser to bubble gas into a liquid solution. This allows for a higher surface area of exposed gas that, dependent on the gas solubility, can increase dissolution into the liquid. 

steady state

An unvarying condition in a physical process.

steam methane reformation (SMR)

A method for producing syngas (hydrogen and carbon monoxide) by reacting hydrocarbons with water. 

Stock

A carbon reservoir that exchanges carbon with the atmosphere over relatively fast timescales (e.g. less than 100 years), including the terrestrial biosphere and shallow oceans. By comparison, other reservoirs, such as geologic formations and the deep ocean, exchange carbon over much slower timescales (e.g. 1,000 years).

Storage (or Sequestration)

Two terms that can be used interchangeably to describe the addition of CO2 removed from the atmosphere to a reservoir, which serves as its ultimate destination. For example, some CDR strategies store carbon in biological systems, such as forests or soil ecosystems, whereas others inject CO2 underground or chemically transform CO2 into stable, mineral forms.

storage assessment unit (SAU)

Mappable volume of rock that consists of a porous storage formation (reservoir rock) and an overlying regional seal formation with low permeability. (USGS)

stover

The leaves and stalks of field crops, such as corn (maize), sorghum, or soybeans, commonly left in a field after harvesting the crop.

Subsidiary Body for Scientific and Technological Advice (SBSTA)

A subsidiary body of the United Nations Framework Convention on Climate Change Conference of the Parties.

supercritical

The state of being beyond supercritical temperature and pressure, i.e. above the thermodynamic critical point. 

surface ocean anoxia

The depletion of oxygen in the ocean.

surficial carbon mineralization

Carbon mineralization performed on the surface of a feedstock with a high reactive surface area. Carbon dioxide-bearing fluid or gas is reacted with mine tailings, alkaline industrial wastes, or sedimentary formations rich in reactive rock fragments.

system boundaries

The divide between what is included in and what is excluded from a system of study.

techno-economic analysis (TEA)

A methodology framework to analyze the technical and economic performance of a process, product or service.

technology readiness level (TRL)

A method developed by the National Aeronautics and Space Administration (NASA) used to determine the level of readiness for deployment of a technology, ranging in scale from 0 – 9.

thermodynamic minimum work of separation

The absolute thermodynamic minimum work or energy that must be provided to a system for a reversible, constant-temperature and constant-pressure separation to occur. The minimum work for a separation system is directly correlated to the inlet and outlet purities of carbon dioxide in addition to the percent capture. (Wilcox, 2012; Wilcox et al., 2017).

turbidity

Measure of the level of particles (e.g. sediment, plankton, or organic by-products), in a body of water. (OSNOAA)

ultramafic rock

Any rock that is composed of ≥90 percent ferromagnesian minerals. They typically have high MgO content (>18 percent), and are ideal feedstock for carbon mineralization. Examples include peridotite, kimberlite, and dunite.

United Nations (UN)

An  intergovernmental organization that aims to maintain international peace and security, develop friendly relations among nations, achieve international cooperation, and be a center for harmonizing the actions of nations.

United Nations Environment Programme (UNEP)

A program responsible for coordinating the United Nations’ environmental activities and assisting developing countries in having environmentally sound policies and practices.

voluntary carbon market

Avoided or removed carbon dioxide emissions (quantified as offset credits) that are exchanged through marketplaces that are not created or utilized for policy compliance. Individuals and companies procure carbon credits through these markets on an entirely voluntary basis.

water-gas shift reaction

Describes the reaction of carbon monoxide and water vapor to form carbon dioxide and hydrogen.

weighted average cost of capital

The projected return rate for investors while financing a company or project.

wetland

Areas where water covers the soil or is present either at or near the surface of the soil either all year or for varying periods of time during the year, including during the growing season. (EPA)

wollastonite

Calcium silicate mineral (CaSiO3) that is a product of a reaction commonly at the boundary of granitic intrusions and limestone.

working fluid

Gas or liquid that primarily transfers force, motion, heat, or mechanical energy.

World Business Council for Sustainable Development (WBCSD)

A CEO-led organization of over 200 international companies that support sustainable business practices.

World Resources Institute (WRI)

A global research non-profit organization established in 1982 with funding from the MacArthur Foundation under the leadership of James Gustave Speth. WRI's activities are focused on seven areas: food, forests, water, energy, cities, climate, and ocean.