Process simulation-based life cycle assessment for fuel-grade bioethanol production from paddy rice straw

Abstract

For the life cycle scenario of bioethanol production from unutilized rice straw, the life cycle stage of paddy rice cultivation can be excluded with a zero-inventory allocation rule, i.e., rice straw with no applied valorization in current practice. This study evaluates the life cycle net energy analysis, greenhouse gas (GHG) assessment, and comparative life cycle environmental impact assessment for nine (09) scenarios of scaled-up bioethanol production process routes using unutilized rice straw as the feedstock. Three different feedstock pretreatment technologies and three different bioethanol dehydration technologies are incorporated to develop the process route scenarios for scaled-up processing plant models. The process simulation technique is integrated to model the scaled-up production plants 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). This research aims to determine the most environmentally benign scenario of the process route to produce fuel-grade bioethanol at an industrial scale from unutilized rice straw. The simulated mass flow and energy flow results are comparable with those of real plants, reported in the published literature, which validates the process simulation results in this study. According to the overall results, fuel-grade bioethanol production using rice straw via adopting dilute acid pretreatment technology for feedstock pretreatment and extractive distillation technology for bioethanol dehydration showcases the most sustainable routine from environmental and energy perspectives. Inclusive of energy generation using the waste flows in the process (i.e., spent wash 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 for the base-case scenario developed for Sri Lankan context with dilute acid pretreatment and extractive distillation. 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. Keywords: Bioethanol production, Unutilized rice straw, Simulation integrated LCA, Net energy analysis, GHG assessment