F

CO₂ Capture and Purification Technology Research, Development, and Demonstration Needs

This appendix describes research, development, and demonstration (RD&D) needs for the crosscutting technologies of CO₂ capture and CO₂ purification. Additional crosscutting research needs for markets, life cycle assessment and techno-economic assessment, policy, and infrastructure are covered in Chapters 2, 3, 4, and 10, respectively.

CO₂ CAPTURE

Common to all CO₂ utilization processes is the need first to capture CO₂ from a point source, the air, or the ocean.¹ As discussed in the committee’s first report, there are a variety of carbon capture technologies at different technology readiness levels (TRLs) (see NASEM 2023, Table 4.1), and the choice of capture technology will depend on “the initial and final desired CO₂ concentration (i.e., the percentage of CO₂ to be removed), scale of CO₂ capture, operating pressure and temperature, composition and flow rate of the gas stream, integration with the original facility, and cost considerations” (NASEM 2023, p. 75). The cost and energy requirements for CO₂ pressurization are important considerations for the viability of a CO₂ utilization project, and capture systems that release CO₂ at the pressures required for transportation or downstream processing (i.e., conversion) are preferred to reduce or eliminate compression costs. Table F-1 shows the approximate concentration of CO₂ from different sources, the total annual U.S. emissions from each source type (where applicable), and estimated capture costs.

Government-supported demonstration of CO₂ capture projects, integrated with transport and storage technologies, can further the deployment and replication of CO₂ capture technologies. These demonstrations enable modularization of equipment at scale that may reduce construction costs, increase capture efficiency as improved technologies become available to substitute for current ones, and provide operational data that can build confidence with project investors and reduce financing risks. Beyond cost reductions achieved through learnings from technology scale up, additional RD&D on capture technologies will also be important for decreasing CO₂ capture costs. Table 4.2 from the committee’s first report (NASEM 2023), which is reproduced and expanded upon in Table F-2 below, outlines RD&D targets to reduce costs from different classes of capture technologies. RD&D needs for enabling technologies, which could benefit multiple capture systems, include mitigating aerosol emis-

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¹ In some systems, capture and conversion of CO₂ are integrated, as described in Chapter 7, Section 7.2.5.

TABLE F-1 CO₂ Concentration, Annual U.S. Emissions, and Estimated Capture Costs from Different Sources

^a NETL (n.d.(a)).

^b Claros (2023).

^c GAO (2022).

^d Measured in $/ton of CO₂.

^e Bennett et al. (2023).

^f Moniz et al. (2023).

^g Estimated for first-of-a-kind facility.

^h Estimated for nth-of-a-kind facility.

ⁱ Measured in $/tonne of CO₂.

^j Table 2-7 of NPC (2019).

^k DOE (2023a).

^l Hughes et al. (2022).

^m Emission streams in this range are blast furnace stove, power plant stack, and coke oven gas.

ⁿ DOE (2023b).

^o Table 3-73 of EPA (2023).

^p Capture costs for electrochemical processes; NASEM (2022).

^q Budinis and Lo Re (2023).

^r NASEM (2019).

^s Cost of net CO₂ removed, accounting for any CO₂ emissions from powering the direct air capture system.

TABLE F-2 Research, Development, and Demonstration Targets to Improve Carbon Capture Systems

SOURCES: NASEM (2023, Table 4-2); NETL (2020, n.d.(b)).

sions; improving stability, compatibility, and corrosion resistance; and reducing viscosity and degradation products (Bostick et al. 2021; NETL n.d.(b)). Basic science and applied research needs to advance solvent- and sorbent-based direct air capture (DAC) technologies, including synthesizing and testing new materials; designing and testing new equipment and system concepts; performing independent materials testing, characterization, and validation; and establishing and managing a public materials database (NASEM 2019b). For electrochemical direct ocean capture (DOC) technologies, research needs include new designs for electrochemical reactors; novel electrode and membrane materials; systems integration with rock dissolution; and demonstration projects to verify carbon removal, monitor environmental impacts, and investigate scale-up strategies (NASEM 2022).

Direct conversion of impure CO₂ streams to products can reduce the net energetic and capital costs of CO₂ utilization, as an intermediary purification step is not required. This process, termed “integrated CO₂ capture and conversion” in this report, is described in detail in Chapter 7. Primary research needs for integrated capture and conversion include discovery of relevant molecules and materials, understanding of catalytic mechanisms, process optimization, and reactor design.

CO₂ PURIFICATION

Most CO₂ transport, utilization, and storage applications require the removal of at least some impurities present in the CO₂ gas streams. The committee’s first report (NASEM 2023) contained a robust discussion of typical impurities present in CO₂ gas streams (Tables 4.3 and 4.4), impurity thresholds for different CO₂ transport modes (Table 4.5), and impurities of concern for CO₂ utilization routes (Table 4.6). Appendix H of this report reproduces Tables 4.3 and 4.4 (as Tables H-1 and H-2, respectively) and updates Table 4.5 (as Table H-3) based on more recent information. Table 4.6 is reproduced as Table 3-1 in Chapter 3. From this assessment, the committee concluded that the impurities present in CO₂ streams may influence the viability of utilization processes, with mineralization and biological conversion being the most resilient to impurities² and electro- and thermochemical conversions being the most sensitive (Finding 4.3, NASEM 2023). The lack of standard specifications for CO₂ purity across capture, transport, utilization, and storage could result in increased energy and operational costs for some stakeholders, as well as efficiency losses throughout the value chain (Neerup et al. 2022). Common separation technologies used to reach very pure streams (akin to food-grade CO₂) are capital- and energy-intensive. Examples are cryogenic distillation and pressure-swing adsorption. These types of purification technologies, therefore, can become a key bottleneck to achieve cost-effective CO₂ utilization, let alone to run such CO₂ utilization processes flexibly. Ongoing research is investigating CO₂ conversion catalysts and technologies that can tolerate impurities (see, e.g., Harmon and Wang 2022; Ho et al. 2019), which might improve the economic feasibility of some CO₂ utilization systems. As discussed in Chapter 7, more research is needed to develop CO₂ conversion catalysts that are impervious to impurities. Additionally, developing standards and methodologies for measuring ppm or ppb levels of impurities in CO₂ streams will help to inform RD&D on CO₂ capture and utilization technologies.

The specific separation technology to be used must be selected with the contaminants and desired CO₂ purity in mind. For example, membranes today would be exceptionally effective at dehydrating CO₂ streams and could remove contaminants such as hydrogen sulfide (H₂S), but they still require significant development to increase their cost-effectiveness and performance in harsh feed conditions. Furthermore, there is a need to develop performance-tuned membrane materials that can handle gas flow rates and compositions that change with time. This will especially be important for CO₂ utilization technologies that target CO₂ from (bio)waste feedstocks. Membranes tuned to separate H₂S would not remove contaminants such as N₂, O₂, or argon. Monoethanolamine-based approaches necessarily leave the purified CO₂ stream saturated with water. Sorbents could be rapidly saturated with even low levels of condensable components. Developing more efficient CO₂ separation methods and hybrid separation technologies, rather than relying on a single method or approach, will likely yield the most promising separation results.

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² Notably, however, anaerobic biological conversion is sensitive to oxygen, and purification to remove oxygen can be energy-intensive.

REFERENCES