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.