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Effects of Volatile-Char Interaction on the Char Reactivity In the Biomass Gasification

Tomoko Wada, Chihiro Fushimi, and Atsushi Tsutsumi. Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo, 153-8505, Japan

Biomass is becoming increasingly important as a renewable source which can reduce the CO2 emission. Steam gasification is a promising technology for hydrogen production with high energy conversion efficiency. In the initial stage of biomass steam gasification, pyrolysis takes place and volatiles (tar and gases) and solid residue (char) are produced. Subsequently, steam reacts with the residual char and tar, producing gases. It is necessary to realize high conversion of the char and to accelerate the rate of gasification for achieving high overall efficiency of biomass steam gasification. Furthermore, large amount of tar is evolved in biomass gasification. The presence of tar can cause blockages and corrosion of pipes and also reduce overall thermal efficiency of the process. It is, therefore imperative to facilitate steam reforming of tar.

In coal gasification, it has been reported that interaction between the volatiles and char inhibits steam gasification of char. Similarly, it is possible that the inhibition occurs in biomass gasification. For proper design and operation of biomass gasification process, it is necessary to clarify the influence of the volatile-char interaction. However, so far there is few research that quantitatively investigates the interaction in biomass gasification.

In this research, we have developed a two-stage reactor which consists of a drop tube reactor and a thermobalance fixed-bed reactor to investigate the influence of the volatiles-char interaction on gasification rate of char derived from biomass. Initially, in the lower thermobalance fixed-bed reactor char produced by pyrolysis in advance was put into a ceramic basket. Then, after the reactor was heated and biomass sample was dropped into the upper drop tube reactor, pyrolysis started to produce char which was trapped by a filter at the middle of two reactors. Subsequently, the volatiles produced in the upper reactor was fed to the lower thermobalance fixed-bed reactor and contacted with char stocked in advance. After that, char was gasified by feeding steam and the gasification rate of char was measured. We also measured the evolution rates of gaseous products from the lower reactor using a mass spectrometer and a micro chromatograph to examine whether the decomposition of tar is facilitated at the surfaces of char.