Economic and energetic analysis of capturing CO ₂ 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.direct air capture | gas separation economics | separation thermodynamics | concentration factor | Sherwood plot S everal researchers investigating chemical systems for capturing CO 2 from the air* have suggested that air capture could be a viable climate mitigation technology costing no more than a few hundred dollars per tonne of CO 2 avoided (1-3). It has been further argued (4) that air capture may be cost-competitive with more accepted climate change mitigation options like renewable power, nuclear power, and CO 2 capture and storage from large stationary sources (carbon capture and storage, CCS). Indeed, during visits to the Massachusetts Institute of Technology in the spring of 2009, the US President's Science Advisor, John Holdren, and Secretary of Energy, Steven Chu, each mentioned capturing carbon dioxide (CO 2 ) directly from the air as an option that may be needed for stabilizing global CO 2 concentrations and, thereby, global temperatures. To examine these claims, we have undertaken a series of analyses of the costs and energy requirements of air capture.Instead of focusing on any particular proposed air capture system, we analyze the capture of CO 2 from air, where it has a concentration of approximately 0.04%, in the context of analogous industrial separation systems. Although the minimum thermodynamic work to separate CO 2 from air is not a prohibitive burden, separation systems themselves require significantly more energy than the thermodynamic minimum. This approach, which is independent of any particular air capture system or process, is motivated on the one hand by the utility of having a generic analysis, and on the other hand by the lack of literature regarding a detailed design of a particular captu...

Structurally simple complexes of CO2

“…The earliest reported examples have already been mentioned at the beginning of this section. In 2002, an inclusion complex of CO2 and decamethylcucurbit [5]uril (a macrocyclic Nheterocycle-containing compound) was reported, 54 and a few years later so too was a complex of tert-butylcalix [4]arene, tBC, and CO2 (Figure 3). 55 The latter was obtained in two forms with 1:1 and 2:1 CO2:tBC loadings.…”

“…Its increased stability relative to the second CO2 molecule is presumably due to C-H···O hydrogen bonds formed with the tert-butyl groups, however, π-orbital stabilizing interactions of the type previously mentioned might contribute as well. [4]arene with one molecule of CO2 adsorbed in the central cavity [MOVMEQ]. 45 Disorder of the CO2 and one of the t-butyl groups has been removed for clarity by showing only the main contributor.…”

Is atmospheric carbon dioxide removal a game changer for climate change mitigation?

“…It is the dominant factor of BECCS revenues (which include the sale of energy) for prices above $100 per tCO2e and therefore a useful proxy for the marginal cost of carbon dioxide removal via BECCS. This is compared with cost estimates for direct air capture technologies of US$200 to US$1,000 per ton of removed carbon dioxide (House et al 2011;Keith et al 2005;Lackner 2010;Socolow et al 2011). The marginal costs of BECCS are lower than the most optimistic DAC cost estimate up to the removal of 12 GtCO2e per year and then increase rapidly due to bioenergy supply limitations.…”

“…Some authors argue that the cost of large-scale air capture will eventually decrease and become competitive in the long run. Recent publications, however, argue that air capture costs have been underestimated and competitiveness with mitigation technologies is unlikely to be reached even in the long run (House et al 2011;Schellnhuber 2011). …”

Reply to Realff and Eisenberger: Energy requirements of air capture systems

“…By definition, all air capture processes start with ambient air and produce a concentrated stream of CO 2 , as well as a CO 2 -depleted airstream. As shown in our paper (2), one can precisely calculate the difference in exergy between these two end points. This exergy difference is the minimum work required by any air capture process.…”