Carbon-neutral fuels and chemicals: Economic analysis of renewable syngas pathways via CO2 electrolysis

Producing syngas, a blend of CO and H-2, is the starting point to the large-scale production of valuable chemicals including fuels and methanol. This study evaluates pathways for syngas production from air-captured and tailgas captured CO2 in terms of net CO2 emissions, energy efficiency and associated costs. We analyzed three direct air capture (DAC) to syngas plants (H-2/CO = 2.5, suitable for Fischer Tropsch or methanol synthesis) that perform the CO2-reduction step by either: i) thermocatalytic reverse water gas shift (rWGS); ii) gaseous CO2 electrolysis, and iii) direct (bi)carbonate electrolysis of the carbon capture solution. The gaseous electrolysis plant did not offer any advantage over the thermolytic rWGS because it implied similar net CO2 emissions (equivalent to 50% of the CO2 captured initially), even if renewable electricity is used. The carbon footprint of the gas CO2-electrolysis pathway originates in the calcination step for the CO2 recovery from the air and tail-gas capture solutions. Only a significant improvement in state-of-the-art single pass conversion of CO2 in gas CO2 electrolyzers (10-30%) would decrease the associated CO2 emissions. In contrast, the novel DAC-(bi)carbonate electrolysis offered the best pathway in terms of net CO2 emissions. However, it entailed higher syngas costs (1.90 $ kg(-1)) compared to gaseous electrolysis (1.30 $ kg(-1)) or the conventional rWGS (1.1 $ kg(-1)) pathways. The DAC-carbonate electrolysis plant may become cost-competitive with reasonable improvements in performance, mainly faradaic efficiency to CO and cell voltage, electricity price <= 35 $ MWh(-1) and membrane price < 450 $ m(-2). Even in the best case scenario all the CO2-based alternatives entailed a higher levelized cost of syngas, synfuel or methanol than current retail prices.