Life cycle assessment of carbon capture and utilisation as a negative emission technology: Recommendations and case study

PurposeSince 2020, three guidelines have been published in order to harmonise life cycle assessment (LCA) studies of carbon capture and utilization (CCU) systems. CCU systems can be designed to produce negative emissions (CCUNET), raising additional challenges for the functional unit definition and solving multifunctionality. The purpose of this article is to assess the relevance of the recommendations for meeting these two challenges proposed within the distinct frameworks of LCA of CCU systems and NET systems.MethodsA comparative LCA is performed without including the 'carbon removal' function in the functional unit. The results are used to discuss the relevance of the existing recommendations on the reference flow to be used to quantify the 'carbon removal' function. A reference flow is relevant if it avoids distorting the comparison. Each recommended method for solving multifunctionality (substitution, allocation and circular footprint formula) is applied to the same case study to calculate its negative emissions potential. A method is relevant if it leads to the same conclusion as the one obtained with system expansion. The case study includes biomass production (maize), CO2 source (ethanol production), CO2 valorisation into a polypropylene bag and the bag incineration with carbon capture and storage. Full accounting of atmospheric carbon is performed.Results and discussionThe studied CCUNET system generated numerous environmental trade-offs due to the consumption of energy as well as catalysts and chemicals. Contrary to using the mass of CO2 treated to quantify the 'carbon removal' function, using the mass of CO2 permanently stored distort the comparison, by leading to the inclusion of a NET system within the system boundaries of the CCUNET system in order to maintain its functional equivalence with its reference system. Regarding multifunctionality, the result from system expansion indicates that the system does not achieve net negative emissions. However, by using allocation on carbon content (or substitution), an LCA practitioner can argue that producing the CO2-based product results in net negative emissions. The circular footprint formula removes the potential for negative emissions.ConclusionThe mass of atmospheric CO2 treated should be used to define the 'carbon removal' function. 'Treatment' encompasses any capture of atmospheric CO2 and its subsequent treatment or direct release. To calculate a net negative emission potential, system expansion should be used to solve multifunctionality.