The carbon footprint of the carbon feedstock CO2

Abstract

Capturing and utilizing CO₂ as carbon feedstock for chemicals, fuels, or polymers is frequently discussed to replace fossil carbon and thereby help mitigate climate change. Emission reductions by Carbon Capture and Utilization (CCU) depend strongly on the choice of the CO₂ source because CO₂ sources differ in CO₂ concentration and the resulting energy demand for capture. From a climate-change perspective, CO₂ should be captured at the CO₂ source with the lowest CO₂ emissions from capture. However, reported carbon footprints differ widely for CO₂ captured, from strongly negative to strongly positive for the same source. The differences are due to methodological ambiguity in the treatment of multifunctionality in current assessment practice. This paper reviews methodological approaches for determining the carbon footprint of captured CO₂ as carbon feedstock, and shows why some approaches lead to suboptimal choices of CO₂ sources and that increased consistency in life cycle assessment (LCA) studies on CCU is needed. Based on strict application of Life Cycle Assessment (LCA) standards and guidelines, it is shown that substitution should be applied to avoid suboptimal choices of CO₂ sources. The resulting methodological recommendations are applied to estimate the carbon footprint of feedstock CO₂ for current CO₂ sources in Europe and for future CO₂ sources in a scenario for a low carbon economy. For all CO₂ sources, the cradle-to-gate footprint of captured CO₂ is negative ranging from −0.95 to −0.59 kg CO₂ eq. per kg of feedstock CO₂ today and from −0.99 to −0.98 kg CO₂ eq. in a low carbon economy. The carbon footprints of different CO₂ sources differ mainly due to their energy demands. The presented assessment method and the carbon footprints of the CO₂ feedstocks CO₂ provide the basis for future assessments of carbon capture and utilization processes.