How (Carbon) Negative Is Direct Air Capture? Life Cycle Assessment of an Industrial Temperature-Vacuum Swing Adsorption Process

Abstract

Current climate targets require negative emissions. Direct air capture (DAC) is a promising negative emission technology, but energy and materials demands lead to trade-offs with indirect emissions and other environmental impacts. Here, we show by Life Cycle Assessment (LCA) that the first commercial DAC plants in Hinwil and Hellisheiði can achieve negative emissions already today with carbon capture efficiencies of 85.4 % and 93.1 %. Climate benefits of DAC, however, depend strongly on the energy source. When using low-carbon energy, as in Hellisheiði, adsorbent choice and plant construction become important with up to 45 and 15 gCO<sub>2e</sub> per kg CO<sub>2</sub> captured, respectively. Large-scale deployment of DAC for<br>1 % of the global annual CO<sub>2</sub> emissions would not be limited by material and energy availability. Other environmental impacts would increase by less than 0.057 %. Energy source and efficiency are essential for DAC to enable both negative emissions and low-carbon fuels.<br>

Potential and risks of hydrogen-based e-fuels in climate change mitigation

“…We optimistically assume that the heat demand of DAC (1500 kwh/t CO 2 44 ), comprising ~15-20% of overall energy input, is met by waste heat from other processes and thus excluded from the calculation.…”

“…Such full recycling of CO 2 could become a pillar of a circular climate-neutral economy. However, capturing atmospheric carbon requires either significant land (in case of using biogenic CO 2 ) or energy resources (in case of DAC), which have to be low carbon to minimize indirect GHG emissions 44 . E-fuels cannot reduce emissions, if the heat supply of DAC is met by natural gas or an average mix of heat sources used by the petro-chemical industry in the EU today (Extended Data Figure 4 shows a sensitivity analysis on heat supply assumptions).…”