Phenolic-rich bio-oil production by microwave catalytic pyrolysis of switchgrass: Experimental study, life cycle assessment, and economic analysis

This study aims to determine the environmental impacts and feasibility of optimizing the production of phenolic-rich bio-oil, via switchgrass microwave catalytic pyrolysis. K₃PO₄ (Tripotassium phosphate) was used as the catalyst, at different temperatures, throughout this life cycle assessment (LCA) study. Results were compared with non-catalytic microwave pyrolysis (SiC-400) and conventional pyrolysis. K₃PO₄ (KP) was used as the microwave absorber and catalyst to enhance the low microwave absorption of switchgrass during microwave pyrolysis, and to improve the bio-oil quality and selectivity for phenolics production. Pyrolysis temperatures made a considerable difference to the LCA. There was an 86% reduction in the pyrolysis time when heating the sample to 300 °C (KP-300), as compared to 400 °C (KP-400), resulting in a significant reduction of the amount of energy required, and GHG's emitted. The total global warming potential (GWP) for microwave catalytic pyrolysis is observed within 159–223 kg CO₂-eq/1000 kg of dried switchgrass (SG), with the baseline case (SiC-400) being the highest, and KP-300 being the lowest. Using the produced biochar, which is rich in nutrients for soil application, brings the net GWP to negative values through carbon sequestration. KP-300 also showed the highest selectivity for phenol and alkylphenols production, which increased by 252% and 420% respectively, compared to the baseline. The results clearly indicate that introducing K₃PO₄ showed great potential for accelerating microwave heating, and improving bio-oil selectivity towards alkylphenols, which can be used to replace petroleum-based phenol. This in turn can reduce GHG emissions, due to higher conversion efficiencies and lower energy consumption compared with non-catalytic microwave pyrolysis and conventional pyrolysis.