Tar Formation and Destruction in a Fixed Bed Reactor Simulating Downdraft Gasification: Optimization of Conditions

Abstract

There are concerns about the impact of low residual tar concentrations on the long-term operational reliability of downdraft, biomass/waste fired gasifiers. A two-stage laboratory scale fixed-bed reactor has been constructed for simulating the release and destruction of tars in this type of gasifier. The commissioning and preliminary results from the reactor have already been reported. The experimental program has now been extended to investigate the effects of variation in the operating conditions in the second stage of the reactor on the emitted residual tar concentrations. The effects of temperature, char type, char particle size, residence time, and effect of addition of diluted air to the throat have been studied. The tar concentration decreased with increasing temperature, decreasing bed particle size, and increasing residence time. Addition of a limited amount of air also reduced the tar content. Tests with several different feedstocks have been done and these have suggested that the final tar emission does depend on the initial feedstock. There is clearly scope to minimize the tar emissions from different feedstocks, by optimizing the conditions in the second stage. The tars from the first and second stages, operated under different conditions, have been characterized by gas chromatography/mass spectrometry (GC/MS) and size exclusion chromatography (SEC). The results show increasing polynuclear aromatic content with increasing second stage temperature. Overall, the work has shown that the tar emissions from downdraft gasifiers can be reduced to low levels with relative ease but that complete removal will require careful manipulation of reaction parameters, dependent to some degree on feedstock properties.

Nascent Biomass Tar Evolution Properties under Homogeneous/Heterogeneous Decomposition Conditions in a Two-Stage Reactor

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An economic and environmental comparison of a biochemical and a thermochemical lignocellulosic ethanol conversion processes

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The effect of utilizing homogeneous conversion to control the formation of chlorinated hydrocarbons during PVC pyrolysis

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