Available online at www.sciencedirect.com Biomass and Bioenergy 25 (2003) 257 – 272 Comprehensive comparison of eciency and CO 2 emissions between biomass energy conversion technologies—position of supercritical water gasication in biomass technologies Yoshikuni Yoshida a ; * , Kiyoshi Dowaki b , Yukihiko Matsumura c , Ryuji Matsuhashi d , Dayin Li e , Hisashi Ishitani f , Hiroshi Komiyama e a Department of Geosystem Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan b Department of Industrial Administration, Tokyo University of Science, Yamazaki 2641, Noda-shi, Chiba 278-8510, Japan c Department of Mechanical System Engineering, Hiroshima University, 1-4-1, Kagamiyama, Higashi-hiroshimashi, Hiroshima 739-8527, Japan d Department of Environmental Studies, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan e Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan f Graduate School of Media and Governance, Keio University, 144-8 Ogura, Saiwai-ku, Kawasaki-shi, Kanagawa 212-0054, Japan Received 26 June 2002; received in revised form 23 December 2002; accepted 8 January 2003 Abstract Eciency and CO2 emissions between various methods of biomass energy conversion are compared from the viewpoint of life-cycle evaluation. As for electricity generation, ecient processes are thermal gasication combined cycle, supercritical water gasication combined cycle, and direct combustion in order of eciency for low moisture content biomass. Supercritical water gasication combined cycle is the most ecient for high moisture content biomass. Battery electric vehicle, gasoline hybrid electric vehicle, and gas full cell vehicle (FCV) show high eciency in automobiles. Biomass FCV shows high eciency in the vehicles utilizing biomass. Biogas combustion is the most ecient for heat utilization. Then, the position of supercritical water gasication in various technologies of energy conversion is examined by modeling an overall energy system. The tradeo between CO2 emissions and total cost of technologies is analyzed so that the most cost-eective technology can be determined for dierent CO2 emissions constraints. Computed results show that biomass is mainly consumed for electricity and heat generation so as to utilize nite biomass resources eciently. Transportation fuels are generally made from fossil fuels. Cost-eective processes for CO2 reduction are thermal gasication and reforming when the present eciency and prices are assumed. Supercritical water gasication is also one of the optimal processes when the relative cost to fuel cell decreases. Improving heat exchange eciency also contributes toward enhancing the position of supercritical water gasication in biomass technologies. ? 2003 Elsevier Ltd. All rights reserved. Keywords: Supercritical water gasication; Life cycle; Eciency; CO 2 emissions; Energy system; Pareto optimum ∗ Corresponding author. Tel.: +81-3-5841-7052; fax: +81-3-3818-7492. E-mail address: yoshida@globalenv.t.u-tokyo.ac.jp (Y. Yoshida). 0961-9534/03/$ - see front matter ? 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0961-9534(03)00016-3