Significant changes in global climate patterns and increasing ocean acidities with their negative impacts on the health of our planet have been ascribed to the ever-increasing rise in atmospheric CO2 levels, primarily attributed to anthropogenic sources such as the combustion of fossil fuels. The mitigation of these acid gas emissions is a daunting task, both because of the scale of the problem and because of the economic ramifications associated with the capture of the greenhouse gases and their subsequent utilization or subsurface storage. Effective means for the direct treatment of emissions with CO2 concentrations of 5 to 15% (or higher), from a wide range of sources, such as in the power industries, at industrial facilities and from on-board vehicle exhausts, are sorely needed. Of late, there has also been some interest in the capture of CO2 directly from the atmosphere, at concentrations close to 0.04%, which offers its own challenges for implementation.
The traditional means for CO2 capture and release generally rely on either chemical or physical absorption in solvents at temperatures well below those at which the acid gas is generated, with subsequent heating to release the captured CO2 and regenerate the sorbent. The captured CO2 can then be compressed for injection and sequestration in subsurface geological formations, or used as a feedstock for the synthesis of fuels and chemical products. These capture processes require significant energy integration with the process plant which adds complexity and cost to the overall capture operation.
We will describe a number of approaches for the treatment of gas streams under ambient conditions (isothermal electrochemically mediated capture and release) and at very high temperatures (temperature and pressure swing with solid and molten metal oxides) that cover the spectrum of CO2 capture needs including direct air capture, power generation, and a range of industrial processes. The general principles underlying these acid gas separation processes will be outlined, with an emphasis on the thermodynamic and transport considerations required for their effective implementation in carbon capture.