Process Simulation Integrated Life Cycle Net Energy Analysis and GHG Assessment of Fuel-Grade Bioethanol Production from Unutilized Rice Straw

Abstract

The life cycle stage of paddy rice cultivation can be excluded with a zero-inventory allocation rule for the life cycle scenario of bioethanol production from unutilized rice straw, i.e., rice straw with no applied valorization in current practice. Accordingly, this study evaluates the life cycle net energy analysis and greenhouse gas (GHG) assessment for a scaled-up bioethanol production plant using unutilized rice straw as the feedstock. The process simulation technique is integrated to model a scaled-up production plant to produce bioethanol at 99.7 vol% purity from unutilized rice straw, and the simulation results are retrieved to calculate inventory data for life cycle assessment (LCA). The simulated mass flow and energy flow results are comparable with that of real plants, reported in the published literature, which validates the process simulations in this study. Inclusive of energy generation using the waste flows in the process (i.e., wastewater and solid residues), the life cycle net energy analysis results show a net energy gain of 7,804.0 MJ/m3 of bioethanol with a net renewable energy gain of 38,230.9 MJ/m3 of bioethanol that corresponds to a net energy ratio of 1.20 and renewability factor of 5.49. The life cycle GHG assessment exhibits a net global warming potential of 584.8 kg CO2 eq./m3 of bioethanol. The effect of system boundary expansion up to the end-of-life stage as gasohol (E10), the sensitivity of the key process parameters, and the economic benefit via valorization of unutilized rice straw are further analyzed and discussed.