The relationship between the level of atmospheric CO 2 and the impacts of climate change are uncertain, but a safe concentration may be surpassed this century. Therefore, it is necessary to develop technologies that can accelerate CO 2 removal from the atmosphere. This paper explores the engineering challenges of a technology that manipulates the carbonate system in seawater by the addition of calcium oxide powder (CaO; lime), resulting in a net sequestration of atmospheric CO 2 into the ocean (ocean liming; OL). Every tonne of CO 2 sequestered requires between 1.4 and 1.7 tonnes of limestone to be crushed, calcined, and distributed. Approximately 1 tonne of CO 2 would be created from this activity, of which >80% is a high purity gas (pCO 2 >98%) amenable to geological storage. It is estimated that the thermal and electrical energy requirements for OL would be 0.6 to 5.6 GJ and 0.1 to 1.2 GJ per net tonne of CO 2 captured respectively. A preliminary economic assessment suggests that OL could cost approximately US$72-159 per tonne of CO 2 . The additional CO 2 burden of OL makes it a poor alternative to point source mitigation. However, it may provide a means to mitigate some diffuse emissions and reduce atmospheric concentrations.

Direct air capture of CO2 has the potential to help meet the ambitious environmental targets established by the Paris Agreement. This study assessed the techno-economic feasibility of a process for simultaneous power generation and CO2 removal from the air using solid sorbents. The process uses a solid-oxide fuel cell to convert the chemical energy of fuel to electricity and high-grade heat, the latter of which can be utilised to calcine a carbonate material that, in turn, can remove CO2 from the air. The proposed process was shown to operate with a net thermal efficiency of 43.7-47.7%LHV and to have the potential to remove 463.5-882.3 gCO2/kWelh, depending on the fresh material used in the calciner. Importantly, the estimated capital cost of the proposed process (1397.9-1740.5 £/kWel,gross) was found to be lower than that for other low-carbon emission power generation systems using fossil fuels. The proposed process was also shown to achieve a levelised cost of electricity of 50 £/MWelh, which is competitive with other low-carbon power generation technologies, for a carbon tax varying between 39.2 and 74.9 £/tCO2. Such figure associated with the levelised cost of CO2 capture from air is lower than for other direct air concepts.

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