Comparative thermodynamic sustainability assessment of lignocellulosic pretreatment methods for bioethanol production via exergy analysis

The pretreatment of lignocellulosic biomass for cellulosic ethanol production involves the heavy use of chemicals and energy, which eventually affect the thermodynamic efficiency of the process. This study investigates into the thermodynamic sustainability of three pretreatment methods for oil palm fronds (OPFs) cellulosic ethanol production using exergy analysis. The initial exergy content of OPFs of 19,930 MJ/tonne was found to degrade by 42-49% during steam explosion, organosolv and microbial pretreatment methods. Microbial pretreatment of OPFs was found to be the most exergetically efficient (90.93%) method with the highest thermodynamic sustainability index (TSI) of 9.32 compared to steam explosion (66.65% exergy efficiency and TSI of 1.56) and organosolv pretreatment (90.30% exergy efficiency and TSI of 7.43) methods. Heat transfer and separation equipment like dryers and centrifuges were the major contributors to the internal exergy destruction during the pretreatment of OPFs for bioethanol production. Exergy efficiency improvement methods during process and equipment designs are elaborated in this work.