Madhu, Kavya; Pauliuk, Stefan; Dhathri, Sumukha; Creutzig, Felix

The importance of direct air capture (DAC) of carbon dioxide (CO2) to meet the climate targets, as stipulated by the Paris Agreement, has been emphasized by International Panel on Climate Change. However, there is a lack of knowledge on the comparative environmental impact of the current prototypes of the DAC technologies and the environmental implication of their large-scale deployment. The goal of this study is to firstly provide a systematic comparative evaluation of the environmental impacts of current prototypes of the two DAC technologies, namely, Temperature Swing Adsorption (TSA) and High Temperature Aqueous Solution (HT-Aq) DAC, under various scenarios encompassing different technical parameters. Secondly, to investigate the changes in the impacts when these technologies are deployed at a large scale i.e. 1Mt CO2 captured per year. For the analysis, Life Cycle Assessment methodology is used and the technologies are evaluated under three dimensions: climate change, metal depletion and particulate matter formation. Our results show that in all the three dimensions, the life cycle environmental impact of TSA DAC is 2-3 times smaller than HT-Aq DAC, mainly due to lesser energy requirement in the use phase. Under different technological parameters, the Carbon Capture Efficiency (CCE), i.e. net life cycle CO2-eq emissions per ton CO2 captured, of the TSA DAC prototype ranges from 30% to 80% and HT-Aq DAC ranges from -50% to 40%. Moreover, CCE of TSA DAC increases to 85% and HT-Aq to 70% when the DACs were supplied with low-carbon energy. Maximum impact for the impact categories metal depletion and particulate matter formation is due to construction and demolition of the DAC plant for both technologies. Due to the modular size of the TSA DAC, a linear scale-up can be expected with negligible CCE increase apropos its current prototype. Scale-up scenario of HT-Aq DAC reports CCE of 60%, due decrease of material intensity per ton CO2 captured.