Chapter 5

The Role of Carbon Utilization

Authors

Pete Psarras, Caleb M. Woodall, and Jennifer Wilcox

CO2 utilization refers to any process that transforms captured CO2 into valuable products. Also known as carbon use, carbon recycling (or upcycling), or carbon tech, these processes strive to permanently store carbon, generate revenue, and/or avoid emissions from conventional products with large amounts of embodied carbon.

While the primary objective of CDR is net removal of atmospheric CO2 through permanent storage, there are many cases where permanence is not feasible or is too expensive. In some cases, carbon utilization may serve as an alternative pathway to permanent storage. However, it is important to note that many carbon utilization pathways do not offer permanent storage and instead may even cycle CO2 back into the atmosphere on short timescales. 

Given the lack of economic incentive today to store CO2 permanently deep underground, utilization of captured CO2 as a feedstock for a product that has value, like fuel, chemicals, plastics, or concrete, could make its capture more economically feasible. Policies such as the 45Q tax credit or LCFS (see glossary for more information and references) can also help incentivize utilization. This chapter provides an overview of how carbon utilization operates today and highlights future utilization opportunities that could play a role in CDR. 

5.1

5.1 —

How CO2 is sourced today

The current scale of global CO2 utilization is very limited – around 180 MtCO2/yr (Zhang et al., 2020). This represents roughly 4.8 percent of 2018 global CO2 emissions (total energy emissions were roughly 33 GtCO2 in 2018). Some projections state that 700 MtCO2/yr can be utilized by 2050 (Mac Dowell et al., 2017), while more optimistic projections envision gigatonne-scale utilization over the same time period (Hepburn et al., 2019). 

The manufacture of urea (a building block of fertilizer) uses more CO2 than any other industry, accounting for nearly 65 percent of all CO2 use (Kleij et al., 2017). However, this CO2 is considered captive; in other words, it is self-supplied from the on-site capture of CO2 generated from hydrogen production, which is a primary feedstock for ammonia production. The major hydrogen production method used globally reforms natural gas with steam (known as steam-methane reforming), which produces CO2 as a byproduct at relatively high purity (Ligouri et al., 2020). This is different from merchant CO2, which is captured for the purpose of off-site distribution. In fact, in most other cases where CO2 is required as a feedstock, it is not available on site and must be purchased from the merchant market. The urea produced releases CO2 back into the atmosphere upon field application, and thus offers no long-term storage. This means that urea production can be, at best, carbon-neutral, if the COis originally sourced from the atmosphere, rather than from the avoided CO2 emissions of hydrogen production from natural gas. Consider the chemical reaction of CO2 and ammonia (NH3) to make urea (CH4N2O):

Equation

5.1

cdr primer equation 5 1

Here, one molecule of CO2 yields one molecule of urea. Upon application, urea breaks down by the reverse reaction:

Equation

5.2

cdr primer equation 5 2

Thus, one molecule of urea yields one molecule of CO2. Such relationships, as determined by chemical stoichiometry, place real-world limitations on the impact of certain utilization pathways on CDR. In order for such pathways to be carbon-neutral, all material and energy flows going into the system must be carbon-neutral. Due to the complexity of ensuring all flows are carbon-neutral, making products with CO2-to-chemicals or CO2-to-fuels processes creates a smaller carbon footprint than making the same products with fossil feedstocks, with the co-benefit of decreasing our reliance on fossil-based products.

Currently, the greatest demand for merchant CO2 comes from enhanced oil recovery (EOR), mainly in the U.S. In EOR, CO2 is used as a physical solvent, which helps increase oil production by pressurizing trapped oil and lowering its viscosity, making it easier to extract. Because CO2 injection is expensive, EOR operators have tried to minimize the amount of CO2 required. To further reduce costs, nearly 80 percent of the CO2 used in EOR is derived from natural reservoirs: deep geological reservoirs called “domes” (Kuuskraa and Wallace., 2014; IEA., 2009; Kallahan et al., 2014). The COis mined and transported through a network of nearly 4,000 miles of pipelines. It is widely used because it is a high-volume, low-cost source of CO2.

 

5.2

5.2 —

Relationship between CO2 utilization and CDR

Chapter 4 presented LCA results of a solvent-based DAC facility configured with various energy sources, with each scenario resulting in a different amount of net-removed CO2. In a similar fashion, the choice of utilization pathway – and management of that pathway – can greatly influence a CDR project’s overall effectiveness at removing CO2 from the air, in addition to the permanence of its removal. In fact, some pathways may never achieve CDR, no matter how well they are managed. This section walks through several examples that illustrate the implications of utilization on overall life cycle emissions and their ability to achieve CDR. 

 

5.2.1

CO2 utilization in EOR 

Throughout the injection and flooding of an oil field, recovery and processing of the oil and gas mixture, and subsequent re-injection of separated and recycled CO2, nearly all of the CO2 used in EOR remains sequestered deep below the Earth’s surface (Núñez-López et al., 2019). There are a number of operating parameters (e.g., project length, the technology used to separate CO2 from produced oil) that may influence how much CO2 can be stored. However, the historical average amount of CO2 permanently stored in the earth per barrel of oil (bbl) produced is roughly 0.5 tonnes. In practice, this storage, termed “associated storage,” is highly variable over the duration of the project, and the highest rate occurs in the first several years. An EOR operator may also choose to maximize oil recovery by sending excess recycled CO2 to an adjacent field, or maximize storage by re-injecting the excess CO2 into a geologic reservoir. 

Globally, an estimated 40 MtCO2 (anthropogenic) are reliably stored in the earth each year, with over 90 percent of storage projects associated with EOR. Most EOR activity takes place in the United States, with other projects in Canada, Brazil, Turkey, China, Norway, Saudi Arabia, UAE, and Malaysia (Verma, 2015; Global CCS Institute, 2019; Sweatman et al., 2011; IEA, 2015; Kuuskraa and Wallace, 2014).

Figure 5.1 shows four scenarios of how CO2 could be used for EOR. Of the 68 MtCO2 used in the U.S. per year for EOR, roughly 60 Mt is sourced naturally from the earth. The remaining 8 MtCO2 are supplied anthropogenically from a mix of sources, primarily natural gas processing and to a lesser extent bioethanol production and the exhaust streams of fossil fuel combustion or gasification (State CO2-EOR Deployment Work Group, 2017; Irlam, 2017; EPA, 2018; Global CCS Institute, 2019). In the case of natural gas processing, the natural gas recovered from an underground geologic formation is often mixed with CO2 and H2S. When the natural gas is recovered at the surface, the acid gases (CO2 and H2S) must be separated before pipeline transport of natural gas. The CO2 separated in the natural gas recovery process is not conventionally included in the carbon footprint of the natural gas, but it is important to note that it would not be produced in the absence of natural gas recovery. Since natural gas requires purification for pipeline transport, the cost of the CO2 separation is embedded in the recovery costs of the natural gas. In both cases (Figures 5.1a and b), the CO2 used for EOR is simply “moved” from one location underground to a different location underground, resulting in oil production, and in some cases combined natural gas and oil production. 

Figure

5.1

An overview of the various sources of CO2 for EOR in the US, with the pie chart representing the distribution of CO2 sources used for EOR today (Global CCS Institute, 2019): a) natural CO2 domes in the earth, b) “natural” CO2 sourced from natural gas recovery, c) avoided CO2 from fossil fuel combustion (EPA, 2018), and d) CO2 separated from the fermentation process of making bioethanol (State CO2-EOR Deployment Work Group, 2017). The bottom image, (e), depicts a pathway that could result in CDR if more CO2 is stored in the earth than produced as a result of the LCA of the recovered oil.

cdr primer figure 5 1

An overview of the various sources of CO2 for EOR in the US, with the pie chart representing the distribution of CO2 sources used for EOR today (Global CCS Institute, 2019): a) natural CO2 domes in the earth, b) “natural” CO2 sourced from natural gas recovery, c) avoided CO2 from fossil fuel combustion (EPA, 2018), and d) CO2 separated from the fermentation process of making bioethanol (State CO2-EOR Deployment Work Group, 2017). The bottom image, (e), depicts a pathway that could result in CDR if more CO2 is stored in the earth than produced as a result of the LCA of the recovered oil.

As shown in Figure 5.1c, when CO2 is captured at a point source (such as a fossil-fueled power plant) and used for EOR, the CO2 emissions associated with the combusted fossil fuel (coal or natural gas) are avoided since they are stored back in the earth via EOR. In this case, the cost of separating CO2 from the power plant may be incorporated into the cost of the electricity generated by the power plant, which would result in the plant producing electricity with a lower carbon intensity. The use of this avoided CO2 for EOR, however, does not impact the carbon intensity of the oil recovered from the enhanced recovery process. Any “credit” associated with avoiding CO2 emissions into the atmosphere can be counted only once, i.e., to offset the carbon intensity of the fossil-based electricity or the crude oil being recovered. 

Figure 5.1d shows the scenario where CO2 is sourced from the fermentation process of converting biomass to ethanol, which produces CO2 at a purity of 98 percent or even more. To accurately determine the reduced carbon footprint of bioethanol compared to fossil-based ethanol, one must account for all of the CO2 emissions associated with the energy required for the fermentation process, in addition to the embodied carbon in the materials and infrastructure of the biomass conversion process. Some of the carbon from the biomass is associated with the bioethanol fuel, and some is associated with the separated CO2 that is available for utilization. The LCA tools outlined in Chapter 4 provide a framework for determining the reduction potential of this route compared to conventional processes. For this route to qualify as CDR, there must be more CO2 stored underground than is generated through the process of biomass conversion through its utilization. 

Figure 5.1e shows a scenario where CO2 for EOR comes purely from the atmosphere. Strictly in terms of carbon accounting, this combined scenario can result in CDR if more CO2 is stored in the earth than is produced in the oil recovery process. When considering that fuel combustion releases CO2 at a ratio of approximately 73 grams (g) CO2/MJ fuel lower heating value (LHV) and approximating a barrel of oil at 5.8 GJ fuel LHV (diesel), at least 0.42 tCO2/bbl must be utilized and stored to account for combustion emissions alone. Adding the remaining emissions associated with processing, transport, crude oil refining, and any other upstream or downstream life cycle emissions, even best-practice EOR operations are more carbon-intensive than the aforementioned conventional production of diesel fuel (0.50 – 0.51 tCO2eq/bbl), and under typical conditions, emissions are roughly 0.59 tCO2/bbl. Thus, an operator would need to utilize 0.59 tCO2/bbl from the atmosphere for the EOR to be considered carbon-neutral. Since the average amount of CO2 stored over the life of an EOR project is 0.5 tCO2/bbl, offsetting the emissions associated with the recovery, refining, transport, and oxidation of the fuel would require roughly an additional 0.1 tCO2/bbl for dedicated storage. Also, critically, while this combined scenario can leave more CO2 underground than is emitted in fuel production and consumption, it inevitably leads to the production of “new” carbon dioxide from the burning of fossil fuels, which may not have otherwise entered the atmosphere. The potential systemic impacts of this activity on fossil fuel consumption must be considered alongside the carbon accounting.

In practice, several additional parameters dictate the actual carbon footprint for EOR, including oil field characteristics (e.g., the injection pressure needed to recover oil, because higher injection pressures require more energy, and thus burn more fossil fuels), the carbon intensity of the electrical supply (grid) required to support CO2 compression, separation of the recovered oil and CO2 mixture, upstream and downstream transport, and whether other products are recovered. For instance, gasoline is more carbon-intensive to produce than diesel, and many EOR operations also yield feedstocks for non-fuel-based products like chemicals. While these decisions can have a moderate impact on lifecycle emissions, on the order of  6 – 10 percent (Cooney et al., 2015), the overall carbon intensity is ultimately most sensitive to the CO2 utilization rate. A higher CO2 utilization rate means more CO2 stored per unit of oil recovered. 

Coupling DAC to utilization of any form can assist in financing its deployment today. However, when coupling DAC to EOR, volatility in the price of oil can lead to vulnerabilities in financing DAC projects. Although not a DAC project, the point-source capture project Petra Nova recently shut down due to the precipitous drop in oil prices that made the cost of CO2 separation from the power plant too expensive (International CCS Knowledge Centre, 2020). Sourcing CO2 from DAC and other point-source capture projects can become non-viable when the project costs ($70 – $100/tCO2 for point-source capture and $250 – $600/tCO2 for DAC) exceed those of a facility with CO2 sourced from underground (Rubin et al., 2015; Psarras et al., 2020; NASEM, 2019; McQueen et al., 2020). 

 

5.2.2

Use of CO2 as a feedstock for chemicals and fuels 

Several chemical routes are the basis of profitable industries, including production of fuels, organic carbonates, methanol, urea, and aliphatic/olefinic hydrocarbon chains. Due to the strength of the carbon-oxygen double bond, the thermodynamics of chemical CO2 conversion are unfavorable.  Hence, several different methods can be used to facilitate efficient CO2 conversion, including combination with high-energy reactants such as Hor strained-ring epoxides, shifting the chemical equilibrium by removing water from the products or co-producing water in the reaction, or adding energy to the reaction in the form of electricity or light. The energy source and amount of energy associated with the conversion of CO2 or preparation of high-energy reactants such as carbon monoxide and hydrogen have a significant impact on the carbon footprint of the process. It is important to understand how the CO2-derived product pathway compares to conventional production pathways. Take, for example, the production of methanol from carbon oxides:

Equation

5.3

cdr primer equation 5 3
Equation

5.4

cdr primer equation 5 4

The feedstocks (CO and hydrogen gas, or H2) of Equation 5.3 are collectively termed “synthesis gas,” and are produced during gasification (reacting fossil fuels or biomass at high temperatures without combustion). Methanol (CH3OH) has been produced this way for decades (U.S. EIA, 2019; Hydrocarbon Processing, 2020a, 2020b), and there are many plants coming online globally that can produce thousands of tonnes of methanol per day. 

By contrast, the reaction pathway shown in Equation 5.4 uses CO2 as a feedstock. The primary difference between these two methanol production pathways is the relative amount of CO2 and H2 required for the reaction, based on their differing stoichiometry. An additional 0.06 tonnes of hydrogen gas is required for the direct conversion of CO2 to 1 tonne of CH3OH (Equation 5.4) when compared to the conventional route (Equation 5.3). Today, 95 percent of the H2 produced globally comes from steam methane reformation (SMR) (Rapier, 2020), where roughly 9 tonnes of CO2 are generated for every tonne of H2 produced (Sun and Elgowainy, 2019). Thus, the direct conversion of CO2 to CH3OH yields an additional 0.54 tonnes of CO2 in emissions for every tonne of CO2 utilized. Lowering the carbon intensity of hydrogen production is thus vital to reducing methanol’s carbon footprint. 

“Blue” hydrogen generation refers to steam methane reforming (reacting methane under pressure with steam to produce hydrogen) with carbon capture to reduce direct CO2 emissions. Steam methane reforming with carbon capture can theoretically reduce emissions by up to 100 percent, but is closer to 70 percent in practice (Ligouri et al., 2020). “Green” hydrogen is produced through the electrolysis of water, preferably powered by renewable, low-carbon energy. The choice of energy source is even more important here, as electrolysis is highly energy-intensive (between 50 and 60 MWh/tH2) (National Renewable Energy Laboratory [NREL], et al., 2018). Understanding how CO2 utilization impacts production is particularly important for inputs with sizable carbon footprints. It is equally vital to carefully consider how low- or zero-carbon energy sources are used, and to prioritize them for purposes that maximize carbon reduction, at least while such energy sources are in short supply. With respect to the stoichiometry of carbon dioxide, 1.38 tonnes of CO2 is “fixed” in each tonne of CH3OH produced. However, any CO2 fixed within methanol is released back into the atmosphere on a short timescale (days to weeks after its combustion as a fuel, or up to decades if used as a precursor to form chemicals like formaldehyde, acetic acid, and methyl tert-butyl ether). Using a standard emission factor associated with methanol as a fuel , operational use of one tonne of methanol results in 1.55 tonnes CO2eq released into the atmosphere (Argonne National Laboratory, 2020). If this carbon came from a fossil fuel (i.e., the CO2 utilized was captured from a fossil fuel-fired power plant or industrial facility), these emissions result in a 1.55-tonne increase in atmospheric CO2 per tonne CH3OH produced. If, however, the CO2 came from the atmosphere, the net emissions are lowered to 0.17 tonnes CO2 per tonne CH3OH produced.

The total effects of these considerations are outlined in Table 5.1 using “gray” hydrogen derived from SMR without carbon capture as an example of a poorly managed process, and green hydrogen derived from solar-powered electrolysis  as an example of an optimal process. Note that the carbon intensity of solar electricity will continue to decline with increased use of carbon-free energy in its manufacturing process.

Table

5.1

cdr primer table 5 1

The best-case scenario presented is direct hydrogenation of ambient CO2 (from the atmosphere) coupled with green hydrogen, yielding 0.42 tonnes CO2eq per tonne of methanol produced.  However, this scenario still results in net positive emissions to the atmosphere. At best, the amount of CO2 fixated in a product can exactly offset the amount released upon consumption or breakdown, leading to net-neutral emissions. In reality, there are several additional steps (e.g., processing, transport) that lead to incremental emissions and net emissions to the atmosphere. However, replacing fossil fuel-sourced carbon with atmospheric carbon can still result in significant emission reductions. While these reductions are not negative emissions, they can play an important role in mitigating climate change. An additional route, still nascent in this field, is sourcing hydrogen from biomass waste gasification coupled with reliable storage of the generated CO2. This pathway has the potential to produce “negative hydrogen,” which when coupled with CO2 sourced from air has the potential to produce fuel with a negative carbon footprint and thus result in CDR. 

Methanol is a flexible intermediate in that it can be used to produce ethylene and propylene, which in turn are feedstocks for plastics, coolants, and resins. Methanol can also be used as an intermediate in the production of gasoline, diesel, soaps, and cleaning fluids (Olah, 2005). Furthermore, since methanol is a liquid at room temperature, its transport is less energy-intensive than compressing and transporting CO2, and it is safer to handle than pressurized hydrogen. Several approaches exist today at varying stages of development (Opus 12, 2020; Prometheus, 2020; De Luna et al., 2019) that use low-carbon energy (or in some cases photons, or light energy) to catalytically react CO2 and hydrogen together to create synthetic fuels and chemicals (Lewis, 2016; Mckone et al., 2014; Walter et al., 2010; Wilcox, 2012). Advancing the production of synthetic fuels, chemicals, and products using CO2 as a feedstock helps create an alternate pathway for meeting the demand for these consumer products, without the need for crude oil.  

 

5.2.3

CO2 utilization for concrete coupled with low-carbon cement

The cement sector is responsible for roughly 7 percent of global CO2 emissions, and cement production is anticipated to grow 12 – 23 percent by 2050 (Fernandez pales and Leung, 2018). Cement is a major component of concrete, the most widely used man-made material in the world; thus, reducing emissions in the cement sector might be achieved through reduced concrete use and/or incorporation of captured CO2 directly into the concrete mixture (Huang et al., 2019, Woodall et al., 2019). Typical concrete has a volumetric composition of 60 – 75 percent aggregate, 7 – 15 percent cement, 14 – 18 percent water, and up to 8 percent air (Huang et al., 2019). Synthetic aggregate can be formed from the direct reaction between carbon dioxide and an alkaline source to yield solid carbonates (Kurda et al., 2018). This process has three advantages over conventional concrete production. First, synthetic aggregate can replace a portion of other coarse and fine aggregates (e.g., sand and dolomitic limestone), potentially reducing emissions associated with material transport and handling. Second, synthetic aggregate is often less dense than non-synthetic aggregates, which can make the resulting concrete blocks less dense, as well. This can reduce costs by lowering the amount of cement required to make the equivalent number of concrete blocks for a building project (assuming blocks built with synthetic aggregate have the same mechanical strength). Finally, carbonates formed from atmospheric carbon have the potential to achieve carbon dioxide removal, contingent on a full LCA (Huang et al., 2019). A rough breakdown of the carbon footprint of conventional concrete is 113, 13, 0.6, 0.01, and 4.65 kgCO2/t concrete for cement, gravel, sand, water, and mixing, respectively, totaling a conventional concrete carbon intensity of roughly 131 kgCO2/t concrete (Kurda et al., 2018).

Several companies in the building materials sector are actively pursuing low-carbon cement and/or concrete production (Blue Planet, 2019; CarbiCrete, 2020; CarbonCure, 2020; Solidia, 2020). Critically, their sources of CO2 today are often from industrial waste streams, and thus count as avoided emissions; any “credit” associated with these efforts cannot be counted twice. For instance, if a cement producer obtains CO2 from the exhaust of a natural gas-fired power plant, the plant could claim credit for selling low-carbon electricity, or the cement producer could claim credit for utilizing and storing the CO– but not both! The CO2 avoided can be counted only once. 

To illustrate this concept, we will describe several potential approaches for producing carbon-negative concrete. Each approach is effective, but would be especially powerful in combination with the others, illustrating how a combination of approaches may be required for utilization to result in CDR. The three approaches are: 

  1. Capturing and storing carbon from the exhaust of the cement kiln;

  2. Replacing gravel and sand with synthetic aggregate that stores CO2 from DAC; and 

  3. Replacing fossil fuels used in the kiln for clinker production with biomass, coupled with carbon capture and storage.

Here we describe each approach in more detail. Figure 5.3 demonstrates the carbon intensity of concrete and how each approach can reduce concrete’s carbon footprint and improve the potential of carbon production. Figure 5.3a shows the conventional carbon intensity of 131 kgCO2/t concrete, compared to 5.3b, where, when coupled with point-source capture at the kiln of the cement plant, the carbon intensity is reduced to 29 kgCO2/t concrete. Finally, Figure 5.3c shows several removal approaches being applied, to increase the net removal of CO2 from the atmosphere to 116 kgCO2/t concrete. This analysis serves as a best-case scenario since it assumes that all energy used within each process is carbon-free, from the process itself to the fuels used in the transportation of feedstocks or products.  

1.   Carbon capture and storage from the cement kiln

Conventional cement (e.g., ordinary Portland Cement) has a carbon intensity of about 850 kgCO2/t cement, most of which results from calcining limestone to produce clinker  (approximately 750 kgCO2/t of cement). The remaining 100 kgCO2/t of cement has a mixture of sources, including indirect emissions associated with the extraction of raw materials (such as the limestone) and the fuels burned in the kiln. Retrofitting the kiln with carbon capture technology at 90 percent reduction results in a carbon intensity of 675 kgCO2/t cement. This is a roughly 79 percent reduction of the carbon footprint of conventional cement. Since cement comprises about 15 percent of the concrete, this technology could reduce CO2 emissions by roughly 101 kg/t of concrete.

2.   Synthetic aggregate coupled to DAC

Technologies that can make synthetic sand and gravel for concrete production, using an alkalinity source and mineralizing with CO2, already exist (Ando, 2020; Huang et al., 2019; Blue Planet, 2019). Related technologies can also produce concrete using less clinker and cure it with CO2 (CarbonCure Technologies, 2020; Solidia, 2020). These technologies vary in their ability to store CO2 in concrete. It was recently reported that up to 100 kgCO2 could be stored per tonne of concrete (Blue Planet, 2019). Sourcing COfrom the air (DAC) is costly today, but carbon-negative concrete approaches could provide the scale (MtCO2 removal per year) for increased deployment in DAC, facilitating “learning by doing” and driving down costs. On average, a cement plant with a carbon footprint of roughly 1 MtCO2/yr produces roughly 2 Mt cement per year. As mentioned above, cement represents 15 percent of the concrete by weight, so this equates to making roughly 13 Mt of concrete per year, assuming all of the cement goes to making concrete. Using the estimate of 100 kgCO2 stored per tonne of concrete, this indicates that 1.3 MtCO2/yr from DAC is required to produce synthetic aggregate to replace sand and gravel in the concrete formulation. This scale of CO2 demand would couple especially well with DAC technologies that benefit from economies of scale, such as Carbon Engineering’s solvent-based approach (Keith et al., 2018). 

3.   Displacement of fossil fuels with waste biomass-based fuel

Not all biomass waste is created equal. For instance, shells from almonds and pistachios or fruit pits can be fired with other fuels in cement kilns and are usually sourced as a local waste. These particular biomass wastes are very carbon-dense and can serve as a drop-in fuel for coal, which currently represents 60 percent of the fuel burned in cement kilns in the U.S. Since the cement kiln exhaust would be retrofitted with carbon capture, this would count as avoided emissions. However, burning the biomass waste would be considered carbon dioxide removal since it is not a fossil fuel and the organic material itself has taken CO2 from the air. The net removal associated with use of biomass depends on the COemitted from transport and any required pre-processing of the biomass to serve as a drop-in fuel, whether it is natural gas, coal, or diesel. 

Co-firing with nut shells is current practice in cement facilities in Arizona (pecan shells), Southern California (pistachio shells), Florida (peanut shells), and Texas (pecan shells) (Demirbas, 2006; Erol et al., 2010). The heating value of the raw unprocessed shells ranges from 17 to 21 MJ/kg, which is in line with lignite and some sub-bituminous coal. Minimal pyrolysis achieves increased heating values that range from 28 to 31 MJ/kg, which is in line with higher-ranked coals such as bituminous and anthracite (Edgar, 1983). The fuels burned in U.S. cement kilns today are a mix, but 60 percent rely on coal with roughly 50 percent lignite and sub-bituminous and the other 50 percent bituminous, indicating that this high-quality biomass could be used as a drop-in fuel for coal.

Assuming a 50/50 split in weight between coal and diesel co-fired in the cement kiln, and further assuming a carbon footprint for clinker production of 750 kgCO2/t clinker (with calcining comprising ~ 55 percent of the CO2 exhaust and the fuel), the remaining 45 percent leads to a carbon footprint of ~ 202.5 kgCO2/t clinker for the coal. This also assumes that the carbon intensity of the diesel is roughly 60 percent that of coal. (Natural gas would be roughly half.)

If the coal were displaced with high-quality waste biomass, such as nut shells with an equivalent heat value and carbon density, this could result in ~ 27 kgCO2 avoided and removed per tonne of concrete since using the biomass waste as a drop-in fuel for coal coupled to carbon capture is CDR.

Currently, several companies use biomass waste to produce diesel oil. These include NuFuels, Clean Energy Systems, and Charm. Again, assuming that 50 percent of the fuel burned in the kiln is diesel, this would result in 135 kgCO2/t clinker. Replacement of this conventional diesel with biodiesel would result in 18.2 kgCO2 avoided and removed per tonne of concrete since the biodiesel is CDR.

Combining all three approaches could yield a total carbon dioxide removal potential of 116 kgCO2/t of concrete produced. This is almost equal and opposite the conventional approach of producing concrete today, which has an estimated footprint of 131 kgCO2/t of concrete (Figure 5.2).

Figure

5.2

Combining CCS, synthetic aggregate using DAC, and fuel replacement with hiqh-quality biomass waste and biodiesel results in a maximum CO2 removal of 116 kgCO2/t concrete. This is nearly equal and opposite to the conventional approach (above left), which emits approximately 131 kgCO2/t concrete.

cdr primer figure 5 2

Combining CCS, synthetic aggregate using DAC, and fuel replacement with hiqh-quality biomass waste and biodiesel results in a maximum CO2 removal of 116 kgCO2/t concrete. This is nearly equal and opposite to the conventional approach (above left), which emits approximately 131 kgCO2/t concrete.

This example remains hypothetical – any actual implementation would require many more details and would likely include several additional caveats beyond the carbon accounting. However, the example illustrates the potential scale of CDR that can be achieved through a practical, real-world utilization pathway.

5.3

5.3 —

The scale of carbon utilization

The scale of CDR required to make an impact on climate change – 10 GtCO2/yr by midcentury and 20 GtCO2/yr by 2100 (Chapter 1) – would result in a supply of COthat could quickly overwhelm CO2 demand, which is roughly 80 Mt/yr in the U.S. and 180 Mt/yr globally (Zhang et al., 2020). Current CO2 markets would need to grow by a factor of 10 to 100 to match that supply. There is competition for carbon utilization opportunities in the form of incumbent CO2 providers, for which CO2 purchasing contracts already exist, as well as from other mitigation strategies, namely point-source CCS. Thus, alternative uses and markets for captured CO2 will likely be necessary. This includes making materials from COthat are currently made with fossil carbon and finding new uses for CO2 in building materials.

Several routes for the transformation of CO2 into a useful product are described in Table 5.2, along with a projected utilization potential by midcentury (Callahan et al., 2014; IEA, 2019; Kuuskraa et al., 2011; Hepburn et al., 2019; IFA, 2017; IEA, 2007; Campbell, 2018; Milani et al., 2015; EIA, 2019; EPA, 2018). In many processes, CO2 is used directly without the need for conversion. For example, in the first commercial DAC facility in Hinwil, Switzerland, Climeworks captures CO2 directly from the air and sends it via pipeline to an adjacent greenhouse, where the CO2 acts as a nutrient, boosting crop yields by more than 20 percent before the CO2 is vented back to the atmosphere (Climeworks, 2020). In other cases, such as synthetic fuels, chemicals, plastics, and carbonates, CO2  is transformed via chemical reactions to form products in which the carbon is embedded. 

Table

5.2

cdr primer table 5 2

Market competition with incumbent processes continues to stall scaling utilization. This challenge motivates understanding where CDR paired with utilization can be profitable and thus attract more investment (Markit, 2016). Industry analysis shows that the market value of delivered CO2 ranges from $44 to $660 per Tonne, with high-purity CO2 delivery at ISBT specifications (99.9 percent CO2) suitable for food and beverage use commanding higher prices. Incentives like the U.S. California LCFS or 45Q tax credit can defray CDR costs. 

Local markets for utilization products could create regional demand for CDR projects. Unlike CCS, CDR projects are not necessarily tethered to point source emissions and/or suitable sinks, making them more flexible in regions where other CO2 supplies are not practically available. Indeed, the existence of nearby utilization markets could serve as a siting consideration when planning CDR deployment strategies. The IHS Markit report (2016) on industrial costs of bulk delivered CO2 notes that buyers in some regions pay more for COdue to high transportation costs. These regions could represent opportunities for local CDR operations to act as CO2 providers and gain access to utilization markets.

Not all utilization pathways are mutually exclusive, and they face market competition. For example, advancement of low-carbon synthetic fuel production could decrease the demand for EOR, and vice versa. Likewise, increased use of cross-laminated timber (prefabricated wood panels) could displace a portion of the market for concrete and steel building materials (Song et al., 2018). Ultimately, the extent of utilization (and the choice of pathway) depends on the agent of financial and other regulatory support (e.g., building codes, government use of fuels, and materials). 

5.4

5.4 —

Timescales of storage in carbon utilization

As described in Chapter 4, CDR requires CO2 to be stored or used in an effectively permanent form. Deep and dedicated geologic storage and proper post-injection monitoring clearly achieves such permanence. The effective permanence of carbon utilization pathways is often less clear, but there are examples of effective permanent storage, such as utilization for manufacture of aggregates and concrete materials. 

In general, we can consider time relative to the moment a CO2 molecule is removed from the atmosphere, which we refer to as t = 0. For a technological system like DAC, t = 0 is the moment ambient CO2 reacts with the basic solvent or sorbent; for a biological system it might be photosynthetic uptake. In either case, at t = 0, the CO2 molecule is in a non-atmospheric subsystem. The purpose of geologic storage is to extend t into meaningful timescales, i.e., > 100 years, according to the IPCC (2014), and perhaps well beyond.

What happens with carbon utilization pathways? If CO2 is used to make short-lived products like industrial chemicals and solvents, then CO2 is released upon degradation or decomposition. This may occur in a few weeks or even sooner, and t rarely lasts longer than a few years. For the case of urea described in Section 5.1, for example, the compound becomes hydrolyzed upon broadcast, releasing CO2 on the order of t = six months. 

On the opposite end of the spectrum, CO2 may be stored in a subsystem for very long periods, either by being absorbed into subsurface pores or through chemical transformation into a stable form from which it cannot be re-released (e.g., carbonate or plastic). 

5.5

5.5 —

Assessing risk

That CDR will enable business as usual to continue is a common concern. Carbon utilization specifically may dissuade key actors from pursuing systemic changes to mitigate climate change. As illustrated throughout this chapter, many utilization pathways result in the short-term release of CO2. Use of less effective pathways early on could lead to market-driven technological lock-in, making it difficult and expensive to escape. Locking into pathways that provide minimal or no CDR could be detrimental to broader CDR efforts. 

Another risk associated with carbon utilization is leakage, or the shifting of emissions elsewhere as a result of climate policy. As an example, leakage occurs in forest carbon offset credit programs when a reduction in timber harvesting at a project site, in exchange for emissions reductions or carbon dioxide removal credits, causes timber harvesting to increase somewhere else to meet demand. Another example is leakage in the EU power sector: Emission control drives up the cost of power locally. This, in effect, makes other power generators (those that are not subject to the same constraints) competitive. Likewise, a business may choose to relocate to a region with less stringent climate policies. Either case could result in increased emissions outside of the constrained region. While the EU recognizes both the cement and ammonia sectors as high-risk sectors for leakage, Naegele and Zaklan (2019) found no evidence of carbon leakage in EU manufacturing. Nonetheless, leakage can occur whenever climate policy varies regionally and impacts commodity costs unequally. To maximize CDR potential, it is important to identify at-risk sectors and pathways and provide proper political support, analysis, and oversight to mitigate leakage.

Today, nearly every technological CDR system that captures CO2 from the air has a carbon utilization partner with projects that do not include permanent storage. As mentioned earlier, Climeworks’ first plant in Hinwil removes 900 tonnes of CO2 per year and feeds it via pipeline to an adjacent greenhouse facility. Of their 15 DAC facilities planned or installed, 13 are paired with utilization, including use of carbon dioxide in greenhouses as a fertilizer, for beverage carbonation, or fuel production. Global Thermostat, which employs a DAC technology based on solid sorbent capture, seeks similar carbon customers to purchase captured CO2, and Carbon Engineering (2019) has explored the conversion of its captured CO2 into transportation fuels.

5.6

5.6 —

Conclusions

Carbon utilization may provide economic incentives for CCS or CDR projects while simultaneously storing CO2 on various timescales. Compared to CO2 storage without utilization, several potential benefits of utilization should be clear: the ability to produce valuable products, additional sources of funding or incentives to scale-up removal technologies, and the displacement of fossil fuel-sourced products with ones that use CO2 and carbon-neutral hydrogen as feedstocks (e.g., chemicals, fuels, and plastic). But utilization also creates potential challenges and concerns, including CDR that is less effective when considering the complete life cycle of the utilization pathway, CO2 storage that is not permanent, and the potential to continue enabling systems that contribute to fossil-fuel emissions.

It might appear that increased demand for CO2 utilization could create competition between sources, in particular, between atmospheric CO2 via DAC and point-source capture. However, as we hope this chapter has illustrated, creative industrial design can bring together point-source carbon capture with DAC in the same utilization approach, and avoiding carbon and carbon dioxide removal can work in tandem rather than competitively to achieve maximum impact. 

1

68.09 gCO2eq per MJ CH3OH, and 22.7 MJ per kgCH3OH

2

1.55 tonnes CO2 (emitted in combustion) – 1.38 tonnes CO2 (removed from the atmosphere) = 0.17 tonnes net

3

Assumes 55 MWh/kg H2 produced and 25 kg CO2/MWh carbon intensity of solar electricity

4

In approach (3), the form of biomass used to displace the fuel will depend on the fuel it replaces in the kiln. For instance, three different biomass fuels – H2 sourced from biomass, carbon-dense nut shells, and biodiesel – may be used to replace natural gas, coal, and diesel, respectively.

5

The LCFS awards project-based credits per tonne of CO2 removed for DAC/EOR operations anywhere in the world. Synthetic fuel products made from atmospheric CO2 must be sold in California and are awarded credits against a baseline product carbon intensity.

1

68.09 gCO2eq per MJ CH3OH, and 22.7 MJ per kgCH3OH

2

1.55 tonnes CO2 (emitted in combustion) – 1.38 tonnes CO2 (removed from the atmosphere) = 0.17 tonnes net

3

Assumes 55 MWh/kg H2 produced and 25 kg CO2/MWh carbon intensity of solar electricity

4

In approach (3), the form of biomass used to displace the fuel will depend on the fuel it replaces in the kiln. For instance, three different biomass fuels – H2 sourced from biomass, carbon-dense nut shells, and biodiesel – may be used to replace natural gas, coal, and diesel, respectively.

5

The LCFS awards project-based credits per tonne of CO2 removed for DAC/EOR operations anywhere in the world. Synthetic fuel products made from atmospheric CO2 must be sold in California and are awarded credits against a baseline product carbon intensity.

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.