Life cycle energy and environmental assessment of bio-CNG utilization from cassava starch wastewater treatment plants in Thailand

摘要

Global warming, energy security, and the rising costs of oil have added a greater driving force to the development of feasible alternatives to petroleum-based transportation fuels. In parallel, wastes and wastewater generated from various industries should be avoided or converted to energy more in the future in order to reduce environmental problems and provide additional sources of energy. In this aspect, biogas plant is an effective option where gas is produced biologically by the fermentation of animal dungs, sewage, and agricultural residues. To utilize biogas as a transportation fuel, raw biogas has to undergo two major processes: cleaning and upgrading, to achieve natural gas quality. The upgraded biogas (so called bio-methane or bio-CNG) is considered green fuel with respect to environment, climate, and human health. However, the resulting bio-CNG from the processes still needs to be evaluated in terms of greenhouse gas emissions and energy aspects. This paper presents the integrated life cycle energy and environmental assessment of compressed bio-methane gas (CBG or bio-CNG) generated from cassava starch wastewater treatment plant in Thailand. The functional units were set to be 1 MJ of bio-CNG and 1 km of vehicle driven. The system boundary covered six main steps: digestion, purification and upgrading, compression, distribution, refueling, and combustion. The energy analysis result showed that the net energy ratio was higher than one, indicating a net energy gain. For the greenhouse gases aspect, the results showed that the biogas production and biogas upgrading step had the highest impact due to methane loss and high energy consumption. Comparing with other fuels, the global warming potential of bio-CNG was lower than those of fossil-based CNG and gasoline.