San Shwe Hla, Lu Aye, Stewart, DF, Connor, MA, Fung, PYH & Ng, HS 2004 (in review) 'Tar levels in a stratified downdraft gasifier', Science in Thermal and Chemical Biomass Conversion, Victoria, Vancouver island, Canada, 30 August-2 September 2004. pp. 1-6.
Abstract
A major problem limiting the usefulness of biomass gasifiers is the high tar levels in the gases they generate. Gas cleaning systems can be employed but it would be much more helpful if the tars produced during gasification could be broken down within the gasifier itself. Stratified downdraft gasifiers would appear to offer the best prospects for achieving high rates of internal tar breakdown. Accordingly a project was initiated to learn more about tar formation and tar destruction rates within a stratified downdraft wood gasifier. Firstly a dynamic model was developed that could predict primary and secondary tar levels at different points along the gasifier. Then, in order to verify these predictions, a pilot scale experimental downdraft gasifier was designed, fabricated and tested using Eucalyptus regnans (river red gum) wood blocks of two different sizes (side lengths 20mm and 30mm respectively). Provision was made in the design for gas samples to be withdrawn at five different points along the gasifier; the tar content of these samples was determined using recently developed protocols. Provided the gasifier was filled with charcoal to the height of the ignition port prior to start up, exit gas tar contents of 20 to 200 mg/Nm3 were predicted for the range of input parameters employed. However, if the initial fuel bed was composed entirely of wood, the predicted exit gas tar content was 20 times higher. Experimentally measured tar levels were reasonably consistent with those predicted using the model. Measured exit gas compositions and temperature profiles along the gasifier axis were also in reasonably close agreement with those predicted by the model. The model was used to explore the influence on gas composition and tar content of changes in various design and operating parameters.