Economic and energetic analysis of capturing CO2 from ambient air

Abstract

Capturing carbon dioxide from the atmosphere ("air capture") in an industrial process has been proposed as an option for stabilizing global CO(2) concentrations. Published analyses suggest these air capture systems may cost a few hundred dollars per tonne of CO(2), making it cost competitive with mainstream CO(2) mitigation options like renewable energy, nuclear power, and carbon dioxide capture and storage from large CO(2) emitting point sources. We investigate the thermodynamic efficiencies of commercial separation systems as well as trace gas removal systems to better understand and constrain the energy requirements and costs of these air capture systems. Our empirical analyses of operating commercial processes suggest that the energetic and financial costs of capturing CO(2) from the air are likely to have been underestimated. Specifically, our analysis of existing gas separation systems suggests that, unless air capture significantly outperforms these systems, it is likely to require more than 400 kJ of work per mole of CO(2), requiring it to be powered by CO(2)-neutral power sources in order to be CO(2) negative. We estimate that total system costs of an air capture system will be on the order of $1,000 per tonne of CO(2), based on experience with as-built large-scale trace gas removal systems.

Fig. 1.

A Sherwood plot showing the relationship between the concentration of a target material in a feed stream and the cost of removing the target material (6). For a more detailed look at the gas separation processes on this plot, see SI Appendix. [Reproduced from ref.  (Copyright 1998, Cambridge University Press).]

Fig. 2.

Illustration of system under consideration. Mixed stream 1 enters the “black box separation unit,” where work is done to generate two product streams. In the case of air capture, stream 1 is air at ambient conditions; in the case of capture at a power plant, stream 1 is flue gas with approximately 12% CO₂, and so forth. Generally, stream 2 is high purity CO₂, and stream 3 is what is left.

Fig. 3.

Empirical relationship between the concentration factor of industrial separation processes vs. the achieved second-law efficiency of those processes. Processes include separation of impurities from H₂ after steam reforming and CO₂ removal (H₂ cleanup); separation of water from brine (desalination); separation of CO₂ from syngas in an Integrated Gasification Combined Cycle power plant [CO₂ from syngas (Selexol)]; separation of CO₂ from syngas after steam reforming [CO₂ from syngas (PSA)]; production of oxygen from air (N₂/O₂ separation); separation of CO₂ from coal power plant exhaust [CO₂ from flue gas (amine)]; separation of CO₂ from natural gas power plant exhaust [CO₂ from NGCC (amine)]; and separation of ethanol and water (ethanol distillation). Dots and boxes outline scatter in published reports, and lines indicate upper and lower bounds on calculations done on the basis of partial information in published reports. See SI Appendix for a technical explanation.