Thermodynamic loss analysis of a liquid-sorbent direct air carbon capture plant

Direct air capture of CO2 is often presented as a promising technology to help mitigate climate change, although proposed processes are highly energy intensive. We analyze Carbon Engineering’s 1 Mt-CO2/year natural-gas-powered direct air capture (DAC) process, which requires 273.2 MW per plant, where we find that 252 MW are irreversibly lost, corresponding to a second-law efficiency of 7.8%. Our component-level analysis details the mechanisms by which these losses of thermodynamic work potential occur in the most energy-intensive plant segments. Here, we emphasize the effects of chemical exergy dissipation in the air contactor, where stored chemical exergy is released as low-grade heat into the environment. Other major losses occur in the calciner and its preheat cyclones due to the high temperature demanded by its internal chemical reaction, as well as in the water knockout system, CO2 compression system, and power island. Finally, we illustrate the issues arising from the use of natural gas as a feedstock for heat and power, and suggest directions to pursue for further analysis and process improvements, which we consider imperative to make this DAC process a viable option for large-scale CO2 removal toward IPCC targets.