The Effect of Bed Particle Inventories with Different Particle Sizes in a Dual Fluidized Bed Pilot Plant for Biomass Steam Gasification

The paper reviews recent results obtained at the dual fluidized bed pilot plant for steam gasification of biomass at the Vienna University of Technology. The dual fluidized bed reactor system involves the combination of a steam fluidized bubbling bed and an air blown fast fluidized bed. Bed particles of olivine with different mean particle sizes are applied as solid inventory in the fluidized bed system. The applied mean particle sizes are 520 and 260 mu m, and therefore differ in coarse and fine solid particle inventories. Increased conversion of higher hydrocarbons is assumed for the fine particle inventory due to higher turbulence (improved gas-solid contact), higher gas residence times in the bubbling bed, and the higher specific particle surface which is exposed to the gas phase. Experimental test runs with conventional wood pellets were conducted by varying the fuel feed, gasification temperature, and the steam-to-fuel ratio. The results are presented according to hydrodynamic considerations, gas composition, tar content, and tar composition, and further specific data (e.g., gas yields, gas residence times, water conversion, and deviation from equilibrium state). The effect of the fine particle inventory is found to significantly influence the tar content of the GC-MS detectable tar fraction; it is reduced by about 50%. Furthermore, it is found that the main gas composition is broadly independent of the particle inventory.