Reflecting on the environmental impact of the captured carbon feedstock.

Climate change mitigation potentials of carbon capture and utilization (CCU) closely depend on the energy and chemicals used to capture the chemically inert CO. The potential environmental benefits of CCU are typically assessed using Life Cycle Assessment (LCA) methodology. Although LCA is a standardized method, modelling CO as a carbon feedstock instead of an emission introduces an ambiguous "multifunctionality issue". Inconsistent multifunctionality practices have been applied to deal with this methodological complexity in LCAs of CCU technologies. Using one method instead of another can lead to highly positive or negative carbon footprints for the same carbon source and CO capture process. A comprehensive guideline to clarify the best practices to conduct LCAs of CCU technologies was published in 2020 (and updated in March 2022) in a collaborative process involving over 40 experts. In this guideline and linked scientific articles from experts involved in its development, a so-called "substitution method" is recommended to avoid suboptimal choices of CO sources, improve comparability and harmonize decision-making. This article critically reviews the methodological formulation of the recommended method and suggests corrections to possible inaccuracies in a future update of the guideline. Furthermore, various illustrative examples of common CO feedstocks were used to illustrate the meaning of adopting such a method in practice. Economic-based benchmarking of the environmental impacts of CO feedstocks calculated with such a method was also broadly illustrated.