Life cycle assessment of adipic acid production from lignin

Abstract

Lignin is a heterogeneous, aromatic polymer and one of the main components of plant biomass. Current lignocellulosic biorefineries primarily focus on polysaccharide conversion from biomass, and separate and combust the residual lignin for heat and power. By using lignin only as a fuel, this polysaccharide-centric approach potentially limits the valorization potential of biomass feedstocks. In this study, we performed a life cycle assessment (LCA) on an emerging lignin upgrading process, namely the production of bio-based adipic acid from lignin sourced from bioethanol production, relative to the conventional petrochemical production pathway. The LCA predicts an overall lower environmental impact for the bio-based route, primarily due to the utilization of a biorefinery side-stream as feedstock material and in the avoidance of nitrous oxide emissions. Bio-based adipic acid is predicted to lead to 4.87 kg CO_{2 eq.} per kg_AA for greenhouse gas emissions, which is a reduction of −62% to −78% compared to conventional adipic acid. Furthermore, results from the sensitivity analysis identify sodium hydroxide utilization and heating needs as the inputs that contribute the largest environmental burden in the bio-based process. Alternative lignin depolymerization processes and development of microbial strains that can tolerate low pH are possible optimization strategies to further improve the environmental profile of bio-based adipic acid. The effects of the LCA modeling assumptions on the environmental profile of bio-based adipic acid are also examined, demonstrating that the electricity footprint and the assumptions made to estimate the effects of diverting lignin from energy to material production play an important role in the model predictions. More broadly, this study highlights that partial lignin conversion to select chemicals in biorefineries may be more environmentally beneficial than solely producing bio-power through combustion, which is the current biorefinery paradigm for lignin utilization.