Highly Efficient Biomass Utilization with Solid Oxide Fuel Cell Technology

Abstract

Mankind has been consuming plants, i.e. biomass, as an energy source for living and developing on earth from the paleolithic period to early the modern period. Consumption of bio-energy does not change the atmospheric environment because carbon dioxide emitted by the use of bio-energies will be used by plants through the photosynthesis (Zuttel, 2008). Since 1769 James Watt significantly improved the steam engine, invented by Thomas Savery in 1698. The steam engine was widely introduced for producing mechanical work from chemical energy of fuels, i.e. mineral coal and wood. More practical heat engines, external and internal combustion engines, have served for developing of human society for almost two and a half centuries. Since the Otto-Langen engine was first introduced in 1867, human society has developed using the internal combustion engines (IC engines), which nowadays are used worldwide for transportation, manufacture, power generation, construction and farming. However, large consumption of fossil fuels may bring about environmental pollution and climate change. Fuel cells are electrochemical devices that convert chemical energy of fuels directly into electrical energy without the Carnot limitation that limit IC engines. Even in the smallest power range of less than 10 kW, fuel cells exhibit electrical efficiencies of 35-50 %LHV (lower heating value), while being silent, whereas engines and microturbines show low electrical efficiency of 25-30%LHV and high levels of noise. Therefore, the fuel cell which can be operated with very low environmental emission levels, is regarded as a promising candidate for a distributed power source in the next generation. Although most fuel cells operate with hydrogen as a fuel, solid oxide fuel cells (SOFCs) operated in a high temperature range between 600 and 900 oC accept the direct use of hydrocarbon fuels. Hydrocarbon fuels directly supplied to SOFCs are reformed in the porous anode materials producing H2-rich syngas, which is subsequently used to generate electricity and heat through electrochemical oxidation (Steele & Heinzel, 2001; Sasaki & Teraoka, 2003). This type of SOFC, so called internal reforming SOFC (IRSOFC), enables us to simplify the SOFC system. Electrochemical performances of IRSOFC have been reported for gaseous and liquid fossil fuels such as methane (Park et al., 1999), propane (Iida et al., 2007), n-dodecane