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Release Profile of Volatiles in Fluidised Bed Combustion of Biomass

Jaakko Saastamoinen

A simplified model for the release profile of volatiles in fluidised and circulating fluidised beds is presented. The location of the release of volatiles in the furnace depends on the interplay of flow of fuel particles affecting their location and heating of fuel particles affecting rates of drying and pyrolysis and consequently the mass of fuel particles. The coupled equations describing heating, de-volatilization including drying, elutriation and movement of particles are solved to find the release profile. Large heavy particles remain in the dense bed whereas light particles are entrained by the gas and elutriated from the bed to the riser. The situation is dynamic, since fuel particles lose weight due to release of moisture and volatiles and decrease in size due to shrinkage affecting their movement and escape from the lower dense bed to the riser. The model is based on the following the fate of a small batch of fuel particles undergoing de-volatilization and elutriation from the dense bed. The model predicts the location of the release of volatiles and the profile of the release of volatiles in the riser. Part of volatiles is released in the dense bed and the rest above the bed in the riser.