Heliostat-powered batch calcination for direct air capture of CO2

Current heliostat thinking is largely about renewable electricity generation. But heliostats could play a much bigger role in reducing global warming by powering capture of the existing trillion-ton atmospheric excess of CO2. The most developed capture process uses energy-intensive calcination of CaCO3 at > 840°C. This reaction releases concentrated CO2 for burial while the remaining CaO is reused to capture CO2 from the atmosphere, forming CaCO3 for a new cycle. A heliostat field could provide the heat for the reaction by focusing intense sunlight through a window onto a sealed calcination chamber with a black surface, converting sunlight energy directly into heat with 75% efficiency, compared to 25% efficiency if heliostats first generate electricity to provide heat. Our example unit calcinator uses a 100 m field of twisting heliostats, with 3,000 m² reflector area. This field concentrates typically 1.3 MW through a 0.8 m diameter window onto the black lid of a 6 m diameter calcination chamber that contains a 2-ton batch of CaCO3 particles. The chamber is rotated and the bed fluidized to ensure uniform particle heating from lid radiation, so that all the batch completes calcination at the same time, independent of cloud breaks. Then the rotation is stopped and the CaO particles fall to exit from the paraboloidal dish center. 15 million such units would capture 20 billion tons of CO2 each year, enough to halve the present excess in 25 years. At $50/m², the amortized cost for installed heliostats would then amount to $4.50/ton captured, a small fraction of the $100/ton DOE DAC target.