Direct Air Capture of Carbon Dioxide: Process Design and Cost

Abstract

Every credible pathway to net-zero emissions retains a residual of hard-to-abate emissions, and balancing that residual requires removing carbon dioxide from the atmosphere. Direct air capture is the most engineered of the removal options and the most expensive, and this paper examines why. A thermodynamic analysis shows that separating carbon dioxide from air at 420 parts per million requires a minimum work of 438 kJ per kilogram against 119 kJ per kilogram for coal flue gas at 12 percent, a factor of 3.7. That factor is real but modest, and it does not by itself explain the cost gap between air capture and post-combustion capture, which arises instead from the volume of air that must be moved and from the size of the contactor. A levelised cost model built on stated capital, energy and maintenance assumptions gives 211 US dollars per tonne for an aqueous hydroxide plant with high temperature calcination and 252 dollars per tonne for a solid sorbent plant using natural gas heat, both at 60 dollars per MWh of electricity and a 90 percent capacity factor. Supplying the same sorbent plant with an electric heat pump costs 260 dollars per tonne at that electricity price and becomes the cheaper option below about 48 dollars per MWh, while operating the plant at a 50 percent capacity factor raises the cost to 403 dollars per tonne. Heat supply and capacity factor therefore dominate the outcome, which has direct consequences for how such plants should be sited and operated on a renewable-dominated grid.