Combustion of Coal Volatiles in the Freeboard of a Fluidized Bed Combustor: an Experimental Approach

Abstract

Previous attempts to determine the amount of volatile material transferred into and burnt in the freeboard region (FBR) of fluidized bed coal combustors have been few and/or ambiguous. A major reason for this is the difficulty of separating the two combustion processes (volatiles and char) occurring simultaneously within both bed and freeboard sections, and because the instrumentation for measuring these separate phenomena directly is non-existent. Therefore, a calorimetric technique is reported here to produce quantitative information on the fraction of heat released by the volatile component in the FBR. The calorimetric experiments revealed that over one third of the original volatiles may be released and burnt above the bed at low excess air levels ( 850°C) and high excess air levels (up to 40 %). The transfer of coal volatiles in the FBR was found to be influenced by the in-bed operating and fluidizing conditions in the following order: bed temperature (Tb), excess air (XSA), fluidizing velocity (Uo) and coal particle size (dc).