(17d) Effect of Interaction among Major Components and Alkali and Alkaline Earth Metals on Tar Evolution in Steam Gasification of Biomass

Biomass, a carbon neutral renewable fuel, can contribute to the demand for heat, electricity and synthesis gas. Biomass gasification is a possible alternative to the direct use of fossil fuel energy. However, large amount of condensable organic compounds (tar) is evolved in biomass gasification. The presence of tar causes blockages and corrosion of pipes and also reduces overall thermal efficiency of the system. In gasification, alkali and alkaline earth metal (AAEM) species may be volatilized into the flue gas, causing corrosion of turbine blade of the gas turbine and adhesion to the wall of piping etc. as a result the heat transmission becomes worse. These inefficiencies in the technology render biomass gasification economically unviable. On the other hand, AAEM species can act as an excellent catalyst for the gasification and combustion reactions. Their retention in the char after pyrolysis would not only minimize their adverse impacts on the operation of a gasification/reforming-based power generation system, but also enhance the conversion of the solid char to gaseous products. It is, therefore, necessary to know the mechanism of tar evolution and thermal behavior of AAEM for proper design and operation of biomass gasification system. In this research, we investigated the interactions of the three main biomass components (cellulose, hemicellulose and lignin) during pyrolysis and gasification in detail by using mixtures of the three components as model biomass samples and a continuous cross-flow moving bed type differential reactor, in which tar and gases can be fractioned according to reaction time. We also investigated the effects of AAEM on the interactions among the three components by wet impregnation of AAEM into the mixtures at various temperatures. It is found that time profiles of tar evolution from the model biomass was different from those estimated from superposition of these components. This suggests the interactions between cellulose, hemicellulose and lignin in pyrolysis and gasification. It was also found that the decomposition of levoglucosan, which is the representative component of the water-soluble tar obtained from cellulose, is promoted when cellulose is mixed with lignin.