(99f) Soil Carbon: An Overlook Component of Biofuel Life Cycle Assessment

Converting renewable biomass to biofuel has great potential to enhance energy security and decarbonize the transportation sector. Forest residues are abundant biomass sources in the United States. As overstocked forest residues are often linked to wildfire risks, they emerge as viable feedstocks for biofuel production, offering benefits of energy generation and fire hazard reduction. While numerous life cycle assessments (LCA) have explored the environmental and decarbonization benefits of biofuels from forest residues, many overlook or minimize the impacts of residue removal on the soil carbon pool, assuming such effects to be negligible when considering the overall life cycle greenhouse gas (GHG) emissions of biofuels.

This study addresses a critical question: Does the alteration in soil organic carbon (SOC) due to the removal of forest residues significantly affect the life cycle GHG emissions of biofuels? To answer this question, we utilize an innovative LCA framework that dynamically links process-based simulations, soil carbon modeling, and forest growth modeling. Different climate conditions and forest productivity are simulated to understand whether variations in environmental conditions would affect the conclusion. The analysis encompasses current practices, including the natural decay of forest residues, and innovative biofuel production scenarios, such as those incorporating carbon capture and storage (BECCS) and the co-production of biofuel and biochar, given the strong interest in using BECCS and biochar for carbon dioxide removal.

Our results show the considerable impact that residue removal has on SOC levels, which in turn contributes to significant GHG emissions in the life cycle carbon footprint of forest residue-derived biofuels. The impact of SOC change is particularly larger for emerging systems like BECCS and the co-production of biofuels and biochar. These insights underscore the necessity of incorporating soil carbon assessments into future LCA studies, biofuel policies, and forest management strategies, even when utilizing waste biomass without land-use change. Our integrated LCA framework also offers a more holistic approach to estimating the carbon footprint of biofuels derived from residue biomass to inform decision-making towards sustainable energy and forest management practices.