Direct Air Capture of CO2 with Chemicals: A Technology Assessment for the APS Panel on Public Affairs

R. Socolow, M. Desmond, R. Aines, J. Blackstock, O. Bolland, T. Kaarsberg, N. Lewis, M. Mazzotti, A. Pfeffer, K. Sawyer, J. Siirola, B. Smit, and J. Wilcox, Direct Air Capture of CO2 with Chemicals: A Technology Assessment for the APS Panel on Public Affairs. (American Physical Society, 2011).http://www.aps.org/policy/reports/assessments/upload/dac2011.pdf

Executive Summary

This report explores direct air capture (DAC) of carbon dioxide (CO2) from the atmosphere with chemicals. DAC involves a system in which ambient air flows over a chemical sorbent that selectively removes the CO2. The CO2 is then released as a concentrated stream for disposal or reuse, while the sorbent is regenerated and the CO2-depleted air is returned to the atmosphere.

To guide the reader to an understanding of the factors affecting costs, a benchmark system is introduced that could be built today. With optimistic assumptions about some important technical parameters, the cost of this system is estimated to be of the order of $600 or more per metric ton of CO2. Significant uncertainties in the process parameters result in a wide, asymmetric range associated with this estimate, with higher values being more likely than lower ones. Thus, DAC is not currently an economically viable approach to mitigating climate change. Any commercially interesting DAC system would require significantly lower avoided CO2 costs, and thus would likely have a design very different from the benchmark system investigated in this report. This report identifies some of the key issues that need to be addressed in alternative designs.

The physical scale of the air contactor in any DAC system is a formidable challenge. A typical contactor will capture about 20 tons of CO2 per year for each square meter of area through which the air flows. Since a 1000-megawatt coal power plant emits about six million metric tons of CO2 per year, a DAC system consisting of structures 10-meters high that removes CO2 from the atmosphere as fast as this coal plant emits CO2 would require structures whose total length would be about 30 kilometers. Large quantities of construction materials and chemicals would be required. It is likely that the full cost of the benchmark DAC system scaled to capture six million metric tons of CO2 per year would be much higher than alternative strategies providing equivalent decarbonized electricity. As a result, even if costs fall significantly, coherent CO2 mitigation would result in the deployment of DAC only after nearly all significant point sources of fossil CO2 emissions are eliminated, either by substitution of non-fossil alternatives or by capture of nearly all of their CO2 emissions.

Nonetheless, DAC is one of a small number of strategies that might allow the world someday to lower the atmospheric concentration of CO2. The wide-open science and engineering issues that will determine ultimate feasibility and competitiveness involve alternative strategies for moving the air and alternative chemical routes to sorption and regeneration.

Ultimate judgments about the future role for DAC and its future cost are necessarily constrained by the scarcity of experimental results for DAC systems. No demonstration or pilot-scale DAC system has yet been deployed anywhere on earth, and it is entirely possible that no DAC concept under discussion today or yet to be invented will actually succeed in practice. Nonetheless, DAC has entered policy discussions and deserves close analysis. This report provides insights into how DAC relates to greenhouse gas emissions.

This report was prepared for the APS Panel on Public Affairs (POPA). POPA routinely produces reports on timely topics so as to inform the debate with the perspectives of physicists and other scientists working in the relevant issue areas, including energy and the environment. Most reports prepared for POPA are policy studies, often making policy recommendations and suggesting priorities for research support. This report, by contrast, is a technology assessment and contains no policy or funding recommendations. The analysis is the outcome of a two-year study conducted by a 13-member committee whose members work in industry, academia, and national and government laboratories.

Context: Global net-negative CO2 emissions and the potential role of DAC

CO2 removal strategies such as DAC might allow the world to pursue a strict stabilization target for the CO2 concentration, by first overshooting the target and later approaching the target from above via net negative global emissions. The latter part of an overshoot requires a sustained period of net-negative global CO2 emissions. To contribute, DAC would need to be applied on a large scale, and to be accompanied by a reliable system for long-term storage of the captured CO2. Some century-scale economic models of global CO2 emissions feature such overshoot trajectories. Given the large uncertainties in the future cost of DAC and other CO2 removal strategies, such approaches should be viewed with extreme caution.

DAC could at best be deployed slowly. Therefore, it is not at all matched to the task of reacting quickly to an abrupt climate emergency, for which the required rates of construction of facilities above and below ground are implausible. If humanity someday chooses to reduce the atmospheric CO2 concentration gradually, DAC would compete with two terrestrial biological strategies: 1) afforestation, reforestation, and other measures that store additional carbon on the land, and 2) capture of CO2 from bioenergy facilities, such as biomass power plants. DAC might well be deployed in parallel with these biocapture strategies and still other strategies for removing CO2 from air. This report focuses on only the DAC alternative, with the expectation that other alternatives will eventually receive comparable critical attention.

DAC costing

To evaluate a large DAC facility that could conceivably be built today, this report uses a simplified costing methodology applied in industry to early-stage projects. The benchmark DAC system is assumed to have a capacity of 1 MtCO2/yr and to absorb CO2 by passing air over a solution of sodium hydroxide in a counter-current, closed system. The sodium hydroxide solution containing sodium carbonate is then cross-reacted with calcium hydroxide to form calcium carbonate as a precipitate. The solid calcium carbonate is decomposed in a natural-gas-fueled, oxygen-fired kiln, with capture of the released CO2. The capital cost is estimated to be 2.2 billion dollars, a normalized cost of $2200/(tCO2/yr). Capital recovery contributes 60% of the $600/tCO2 estimated avoided cost.

For the sake of comparison, using the same methodology the avoided cost for “post-combustion capture” (PCC) of CO2 from the flue gas of a reference coal power plant is estimated. In the reference PCC system the CO2 is 300 times more concentrated and the CO2 removal rate is about three times larger than in for the benchmark DAC system. Relative to the benchmark DAC system, the normalized capital cost for the reference PCC system is estimated to be $180/(tCO2/ yr), twelve times smaller, and the total avoided cost for capture is estimated to be about $80/tCO2, about eight times smaller. Since the total cost includes both operating and capital costs, evidently the operating cost ratio for the two systems is less disadvantageous to the benchmark DAC system than the capital cost ratio. One reason the ratio of operating costs is smaller than the ratio of capital costs is that the assumed energy requirements for the DAC system are optimistic. For example, DAC electricity demand includes fan power to move the air, which is proportional to the pressure drop through the contactor, and the pressure drop assumed in the cost calculation is at the very low end of a credible range for the benchmark system.

The capacity to estimate future DAC costs is limited. Costs could fall as a result of technological learning and with the introduction of fundamentally new ideas. On the other hand, industry experience suggests that cost estimates for any system rise after the completion of pilot plant operations, when the necessary compromises in materials choices, process conditions, component efficiencies, and component lifetimes are taken into account.

The cost estimates in this report are capture costs. They do not include the cost of dealing with CO2 beyond the boundary of the capture facility. Specifically, the costs of sequestering the captured CO2 from the atmosphere have not been estimated. The principal sequestration strategy under discussion today is injection of CO2 in geological formations for multi-hundred-year storage. The cost of geological storage is expected to be smaller than the capture cost even for capture from flue gas, but its commercialization at very large scale will require the resolution of formidable reservoir-engineering, regulatory, and public acceptance challenges. It was beyond the scope of this report to investigate post-capture management of CO2 in any detail.

Net-carbon considerations and centralized emissions of CO2

All air capture strategies are strongly constrained by the need to remove more CO2 from the atmosphere than one emits to the atmosphere during the capture process—the “net-carbon” problem. The benchmark DAC system studied in this report is seen to be tightly constrained by net-carbon considerations. For illustrative purposes, the benchmark system assumes that the natural-gas-derived CO2 emissions are captured at the kiln and then combined with the CO2 removed from the air. For each ten CO2 molecules removed from the atmosphere by the sodium hydroxide, about four molecules are released by combustion of natural gas at the kiln, so that 14 CO2 molecules need to be sequestered.

The benchmark system also assumes that the electricity required for fans, pumps, compressors, and other devices is provided from the average US power grid, which has substantial carbon intensity. As a result, for each ten CO2 molecules removed from the atmosphere by the DAC system, three CO2 molecules are emitted to the atmosphere at distant power plants, so that total capture costs are spread over seven-tenths as much captured CO2 as would have been the case if the electricity had been produced without CO2 emissions. Indeed, if the power required for the fans were at the high end of the credible range and the power were provided by the same grid, fossil emissions for that power would offset the full amount captured, driving the cost of avoided CO2 emissions to infinity. Only stringent combinations of a small pressure drop through the contactor and low-carbon power are consistent with a viable DAC system, from the perspective of net carbon.

Even low-carbon energy sources for DAC are constrained as long as DAC facilities are located within a regional energy system that is not fully decarbonized. Any low-carbon energy source dedicated to a DAC system could instead be used to displace high-carbon centralized sources in the region. Diversion of low-carbon energy supply into DAC and away from its usual decarbonization assignments will be beneficial from a carbon mitigation perspective only in special circumstances. In general, one should expect coherent CO2 mitigation to produce minimal deployment of DAC until CO2 emissions have been nearly eliminated at all large sources of centralized emissions.

Compensating for decentralized emissions

DAC may have the potential to compensate for some decentralized CO2 emissions. However, for at least the next few decades, unless there are dramatic cost reductions, direct air capture can be expected to be substantially more expensive than many other currently available options for reducing decentralized emissions, including 1) substantial improvement of end-use efficiency in all sectors of the economy, 2) electrification of the present direct uses of fossil fuels, accompanied by decarbonization of electricity, and 3) substitution of low-carbon fuel, biologically derived or produced in some other way. As a result, this report provides no support for arguments in favor of procrastination in dealing with climate change that are based on the imminent availability of DAC as a compensating strategy. The pursuit of many currently promising mitigation options deserves higher priority.

Understanding the costs of direct air capture will illuminate a ceiling on costs for mitigation and adaptation. When the cost of some mitigation or adaptation measure exceeds the cost of CO2 removal from the atmosphere, it will be more cost-effective to remove the carbon from the atmosphere after it has been emitted than to prevent its emission in the first place. It is conceivable that some mitigation options that today appear to be very costly may never be needed if operable DAC systems become available.

Toward lower costs

A substantial portion of this report is devoted to indicating DAC systems that could have the potential to lead to lower costs. Such systems would need to differ radically from the benchmark system. Costs for DAC will not fall substantially through incremental improvements in present-day technology, as improvements in one process step may create additional challenges in other process steps or simply lead to trade-offs between capital and operating cost without reducing the cost of the overall process. A trade-off explored repeatedly in this report, for example, exchanges stronger, more efficient binding of CO2 in the capture reaction for greater energy requirements for regeneration.

Lower costs will require substantial improvements in both components and systems. Systems based on cross-current flow in open systems or based on sorbents chemically bonded to a rigid substrate may have potential. Cycles based on new sorbents could come closer to the thermodynamic minimum than the particular sodium hydroxide cycle studied in the benchmark system. Transformational changes will likely require the integration of achievements in several fields of materials science, as well as in chemical and process engineering.

Ultimately, any full-cycle direct air capture process faces the major challenge of operating effectively and efficiently over hundreds to thousands of consecutive cycles.

Pedagogy

The goal of this report is to enable scientifically literate non-specialists to think independently about DAC, whether they are primarily interested in advancements in DAC technology or in placing DAC in a policy context. Throughout, the report seeks to demystify, to explain unfamiliar vocabulary, and to work through representative calculations.

Key Messages

Implications of direct air capture of CO2 by chemicals (DAC) for climate and energy policy

  • DAC is not currently an economically viable approach to mitigating climate change.
  • In a world that still has centralized sources of carbon emissions, any future deployment that relies on low- carbon energy sources for powering DAC would usually be less cost-effective than simply using the low-carbon energy to displace those centralized carbon sources. Thus, coherent CO2 mitigation postpones deployment of DAC until large, centralized CO2 sources have been nearly eliminated on a global scale.
  • DAC may have a role to play eventually in countering emissions from some decentralized emissions of CO2, such as from buildings and vehicles (ships, planes) that prove expensive to reduce by other means.
  • Given the large uncertainties in estimating the cost of DAC, century-scale economic models of global CO2 emissions that feature “overshoot trajectories” and rely on DAC should be viewed with extreme caution.
  • High-carbon energy sources are not viable options for powering DAC systems, because their CO2 emissions may exceed the CO2 captured.
  • The storage part of CO2 capture and storage (CCS) must be inexpensive and feasible at huge scale for DAC to be economically viable.
  • This report provides no support for arguments in favor of delay in dealing with climate change that are based on the availability of DAC as a compensating strategy.


© Berend Smit 2013