A life cycle assessment of greenhouse gas emissions from direct air capture and Fischer–Tropsch fuel production

Abstract

Direct air capture (DAC) separates carbon dioxide (CO 2 ) from ambient air either chemically or physically. As such, it could be a potential climate mitigation tool when paired with geological sequestration of CO 2 or downstream conversion to produce products with low life cycle carbon intensities. Of particular interest is the ability to pair CO 2 from DAC with electrolytic hydrogen powered by renewable electricity to synthesize liquid hydrocarbons that can be used in transportation (often referred to as "e-fuels"). This presents a pathway additional to electric and fuel cell vehicles to harness renewable electricity for use in the transportation sector and may present an attractive opportunity as costs of renewable electricity and electrolysis equipment continue to fall. We conduct a life cycle assessment (LCA) of the greenhouse gas (GHG) emissions of a DAC system paired with Fischer-Tropsch synthesis (FTS) to produce transportation fuel (i.e., diesel). This is the first LCA study of a DAC-to-fuel process based on data from an operating DAC pilot plant. We estimate the system emits 0.51 gCO 2 e per gCO 2 captured from air or 29 gCO 2 e per MJ FTS fuel combusted in the baseline scenario, in which the electricity emissions factor used in the process is relatively low. This carbon intensity (CI) is extremely sensitive to changes in the electricity emissions factor. We find that an electricity emissions factor of less than 139 g CO 2 e per kW h is required for this pathway to provide a climate benefit over conventional diesel fuel. If a low carbon source of electricity is used, this pathway can deliver transport fuels at a CI lower than conventional diesel production and several biofuel pathways. This analysis suggests that fuel synthesis facilities need to be located in regions with very low grid emissions factors, or preferentially, co-located with new-build renewable electricity.

Life Cycle Assessment of Power-to-Syngas: Comparing High Temperature Co-Electrolysis and Steam Methane Reforming

“…A study from van der Giesen et al (2014) examined rWGS process as intermediate step for the production of liquid hydrocarbon fuels (Power-to-Fuel) as an alternative to diesel. In the study from Liu et al (2020) CO 2 emissions from direct air capture (DAC) and Fischer-Tropsch fuel production were assessed by LCA. CO 2 based C1-chemicals (e.g.…”

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

“…13,14 However, DAC not only allows us to remove GHG emissions from our past use of fossil fuels. DAC also enables future fuels with a closed carbon cycle: The captured CO2 could serve as a carbon feedstock for fuels [15][16][17][18][19][20] , and also other value-added products like chemicals [21][22][23][24] and building materials [25][26][27] via carbon capture and utilization (CCU).…”

“…So far, a detailed assessment of this trade-off is only available for GHG emissions for a DAC process with aqueous hydroxy sorbents, where high-temperature heat is usually obtained from natural gas, and the resulting CO2 emissions are re-captured. 20,40 Available assessments for adsorption-based DAC systems consider energy requirements but use proxy data for plant construction and adsorbent. 47,48 Thus, a comprehensive environmental assessment is missing for adsorption-based DAC but urgently needed to establish the role of DAC in climate-change mitigation.…”

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

“…13,14 However, DAC not only allows us to remove GHG emissions from our past use of fossil fuels. DAC also enables future fuels with a closed carbon cycle: The captured CO2 could serve as a carbon feedstock for fuels [15][16][17][18][19][20] and also other value-added products like chemicals [21][22][23][24] and building materials [25][26][27] via carbon capture and utilization (CCU).…”

“…So far, a detailed assessment of this trade-off is only available for GHG emissions for a DAC process with aqueous hydroxy sorbents, where high-temperature heat is usually obtained from natural gas, and the resulting CO2 emissions are re-captured. 15,40 Available assessments for adsorption-based DAC systems consider energy requirements but use proxy data for plant construction and adsorbent. 47,48 Thus, a comprehensive environmental assessment is missing for adsorption-based DAC but urgently needed to establish the role of DAC in climate-change mitigation.…”