A suite of improved technologies is being developed to minimize the environmental impact of biomass/waste fired gasification processes. Downdraft, fixed-bed reactors are particularly favored because of their ability to destroy the majority of tars produced from the fuel volatiles. However, there is some concern about the impact of the low residual tar concentration on the long-term operational reliability. A two-stage laboratory scale fixed-bed reactor has been constructed for studying the release and destruction of tars in downdraft gasifiers. The reactor has been commissioned and its performance demonstrated using several biomass feedstocks. Experiments using the first stage only have shown that as the temperature is raised from 250 to 450 °C, the gas and tar yields increase at the expense of the char residue. Four different biomass/waste materials (eucalyptus wood, sludge, plastic waste, and silver birch wood) showed qualitatively similar behavior. Volatile yields appear to stabilize around 450 °C. With silver birch wood, the tar yield reached 47% of the initial fuel. Preliminary tests using a char bed in the second stage have been completed. The presence of the throat and the second-stage char bed results in a substantial reduction in the quantity of tar leaving the reactor. With a hot empty second stage (at 800 °C), the tar content was reduced to 5.3% (by wt of initial fuel charge) in the exit gas from the reactor. Packing the second stage with char (at 800 °C) further decreased the tar content to less than 0.1%. Gas analyses have been performed, showing that some of the initial tar is broken down mainly to CO and CH4 in the second stage of the reactor. Further work is in progress to study the impact of the operating conditions in the second stage on the residual tar concentrations and gas analysis.
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.
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