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. 

The current scale of global CO₂ utilization is very limited – around 180 MtCO₂/yr (Zhang et al., 2020). This represents roughly 0.5 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 CO₂ is 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):

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

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 CO₂ is 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.

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. 

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. 

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). 

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 H₂ or 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:

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 1, 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. 2

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 3 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.

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.  

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 CO₂ – 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. 4

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 CO₂ from 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 1.2 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 CO₂ emitted 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).

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.

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 CO₂ that 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 CO₂ that 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. 

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 5 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 CO₂ due 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). 

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., t > 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). 

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.

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.