Tar Formation and Destruction in a Simulated Downdraft, Fixed-Bed Gasifier: Reactor Design and Initial Results

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 CH 4 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.