Generation with humidity swing direct air capture of CO₂ versus mea-based postcombustion capture

Most carbon capture and storage (CCS) envisions capturing CO₂ from flue gas. Direct air capture (DAC) of CO₂ has hitherto been deemed unviable because of the higher energy associated with capture at low atmospheric concentrations. We present a Life Cycle Assessment of coalfired electricity generation that compares monoethanolamine (MEA)-based postcombustion capture (PCC) of CO₂ with distributed, humidity-swing-based direct air capture (HSDAC). Given suitable temperature, humidity, wind, and water availability, HS-DAC can be largely passive. Comparing energy requirements of HS-DAC and MEA-PCC, we find that the parasitic load of HS-DAC is less than twice that of MEAPCC (60-72 kJ/mol versus 33-46 kJ/mol, respectively). We also compare other environmental impacts as a function of net greenhouse gas (GHG) mitigation: To achieve the same 73% mitigation as MEA-PCC, HS-DAC would increase nine other environmental impacts by on average 38%, whereas MEA-PCC would increase them by 31%. Powering distributed HS-DAC with photovoltaics (instead of coal) while including recapture of all background GHG, reduces this increase to 18%, hypothetically enabling coal-based electricity with net-zero life-cycle GHG. We conclude that, in suitable geographies, HS-DAC can complement MEA-PCC to enable CO₂ capture independent of time and location of emissions and recapture background GHG from fossil-based electricity beyond flue stack emissions.