No abstract

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.

The process by which basic/ultrabasic silicate minerals (e.g., olivine) are reacted with CO2 to produce solid carbonate minerals (“mineral carbonation”) has been suggested as a method to sequester carbon dioxide from point sources into stable carbonate minerals. Alternatively, the addition of lime (produced from calcining carbonate minerals) to the surface ocean (“ocean liming”), which results in an increase in ocean pH and a draw-down of atmospheric CO2 has been proposed as a “geoengineering” technology, which stores carbon as dissolved alkalinity in the surface ocean. Combining these approaches, in which the magnesium carbonate minerals produced from mineral carbonation are used as a feedstock for ocean liming (mineral carbonation-ocean liming; MC–OL), may reduce the limitations of individual technologies while maximizing the benefits. Approximately 1.9 metric tons of magnesium silicate (producing 0.7 ton of magnesium oxide) are required for every net ton of CO2 sequestered. A total of 0.7 ton of CO2 is produced from this activity, 70% of which is high-purity (>98%) from calcining and potentially amenable for geological storage. The technology can be conceptually viewed as an alternative to direct air capture and swaps ambient CO2 for high-purity point source CO2. MC–OL requires approximately 4.9 and 2.2 GJ of thermal and electrical energy ton–1 of CO2 sequestered. MC–OL has less demand for geological storage; only 0.5 ton of CO2 needs to be injected for every ton of CO2 removed from the atmosphere. However, manipulation of ocean chemistry in this way potentially creates an additional environmental impact (localized elevated pH or co-dissolution of trace metals) and requires additional attention.

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.