Current Carbon Capture and Storage (CCS) technologies focus on large, stationary sources that produce approximately 50% of global CO 2 emissions. We propose an industrial technology that captures CO 2 directly from ambient air to target the remaining emissions. First, a wet scrubbing technique absorbs CO 2 into a sodium hydroxide solution. The resultant carbonate is transferred from sodium ions to calcium ions via causticization. The captured CO 2 is released from the calcium carbonate through thermal calcination in a modified kiln. The energy consumption is calculated as 350 kJ/mol of CO 2 captured. It is dominated by the thermal energy demand of the kiln and the mechanical power required for air movement. The low concentration of CO 2 in air requires a throughput of 3 million cubic meters of air per ton of CO 2 removed, which could result in significant water losses. Electricity consumption in the process results in CO 2 emissions and the use of coal power would significantly reduce to net amount captured. The thermodynamic efficiency of this process is low but comparable to other "end of pipe" capture technologies. As another carbon mitigation technology, air capture could allow for the continued use of liquid hydrocarbon fuels in the transportation sector.

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Reducing greenhouse gas emissions from the transportation sector may be the most difficult aspect of climate change mitigation. We suggest that carbon neutral hydrocarbons (CNHCs) offer an alternative pathway for deep emission cuts that complement the use of decarbonized energy carriers. Such fuels are synthesized from atmospheric carbon dioxide (CO2) and carbon neutral hydrogen. The result is a liquid fuel compatible with the existing transportation infrastructure and therefore capable of a gradual deployment with minimum supply disruption. Capturing the atmospheric CO2 can be accomplished using biomass or industrial methods referred to as air capture. The viability of biomass fuels is strongly dependent on the environmental impacts of biomass production. Strong constraints on land use may favour the use of air capture. We conclude that CNHCs may be a viable alternative to hydrogen or conventional biofuels and warrant a comparable level of research effort and support.

Direct air capture, the chemical removal of CO2 directly from the atmosphere, may play a role in mitigating future climate risk or form the basis of a sustainable transportation infrastructure. The current discussion is centered on the estimated cost of the technology and its link to "overshoot" trajectories, where atmospheric CO2 levels are actively reduced later in the century. The American Physical Society (APS) published a report, later updated, estimating the cost of a one million tonne CO2 per year air capture facility constructed today that highlights several fundamental concepts of chemical air capture. These fundamentals are viewed through the lens of a chemical process that cycles between removing CO2 from the air and releasing the absorbed CO2 in concentrated form. This work builds on the APS report to investigate the effect of modifications to the air capture system based on suggestions in the report and subsequent publications. The work shows that reduced carbon electricity and plastic packing materials (for the contactor) may have significant effects on the overall price, reducing the APS estimate from $610 to $309/tCO2 avoided. Such a reduction does not challenge postcombustion capture from point sources, estimated at $80/tCO2, but does make air capture a feasible alternative for the transportation sector and a potential negative emissions technology. Furthermore, air capture represents atmospheric reductions rather than simply avoided emissions.

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