Experimental investigation of the effect of physical pre-treatment on air-blown fluidized bed biomass gasification

Abstract

The effect of comminution, drying, and densification on bubbling fluidized bed gasification was investigated by fractionating a forestry residue into a feedstock consisting of different particle sizes, moisture levels, and by densifying to pellets. The gasification performance was evaluated at nominal average bed temperatures of 725°, 800° and 875 °C at a constant fluidizing velocity (0.91 m s−1) with feed input rates between 9 and 24 kg h−1.The gas composition was observed to be influenced by both the particle size and form. Smaller particles led to a gas richer in carbon monoxide and depleted in hydrogen. The gasification of pellets led to a gas with the greatest hydrogen to carbon monoxide ratio. The smallest particles tested resulted in the worst gasification performance, as defined by cold gas efficiency, carbon conversion, and tar production. Despite differences in the gas composition among the larger particles and the pellets, similar carbon conversion and cold gas efficiency was observed.Relative to comparable test conditions with dry feed fractions (having a moisture mass fraction of 7–12%), an average 11% increase in carbon conversion was observed for the wetter feed fractions containing a moisture mass fraction of 24–31%. This increase in carbon conversion offset much of the expected decrease in cold gas efficiency by using a wetter feed material. A slight increase in hydrogen production and negligible change in tar production was observed for the wetter feed fractions relative to the dry feed fraction.