稲わらからのバイオエタノール生産システムに関する評価

抄録

Objective. Ethanol production from lignocellulosic biomass such as rice straw and wood are recently getting high attention. In this study, we evaluated energy consumption, CO₂ emissions and cost of the ethanol production from rice straw by concentrated sulfuric acid hydrolysis throughout the life cycle and compared its results with gasoline. For the system boundary of the life cycle of bioethanol it is necessary to consider each process of the bioethanol production system including cultivation, harvesting, collection/transportation, fuel conversion, fuel transportation and fuel use. However, in this study, the cultivation process is not considered because we exclusively evaluate unused rice straw from agricultural residues. We considered harvesting, collection/transportation of rice straw, and conversion of ethanol, transportation of ethanol, lignin boiler, waste disposal, liquid waste treatment process and ethanol use. Beside the operation processes the construction of an ethanol plant, the rice straw collection/transportation vehicles, and the ethanol transportation vehicles were considered. Furthermore, we analyzed the influence of the rice straw cost and capacities of the bioethanol plant on the ethanol production cost.
Results and Discussion. The energy consumption was estimated to be 728MJ, and CO₂ emissions were estimated to be 39kg-CO₂ per production of 1 GJ of ethanol. Energy input and CO₂ emissions of the ethanol conversion process were the most aspects of the life cycle of ethanol. Bioethanol uses about 39% less energy and produces 51% less CO₂ emissions than gasoline. Lignin residue combustion has a high potential for reducing energy consumption and CO₂ emissions. The bioethanol cost was estimated to be 181 JPY/L. The rice straw cost and its conversion cost account for 68% and 30% of the total cost, respectively. In terms of cost, ethanol is not competitive with gasoline. However, if ethanol becomes tax-exempt from the gasoline tax, the ethanol price can become competitive for the end user.
Conclusions. From the viewpoint of energy consumption and CO₂ emissions, bioethanol is likely superior to gasoline. The system which utilizes lignin in a boiler and recovers heat effective increases the energy balance and reduces CO₂ emissions. Since the rice straw collection/transportation cost accounts for a large part of the total cost in the ethanol production system, it is significant to make the price competitive. The development of low cost technologies to collect rice straw and ethanol conversion as well as a financial support system are needed.