Effects of design features on combustion efficiency and emission performance of a biomass-fuelled fluidized-bed combustor

This paper presents an analysis of some measures leading to intensification of the combustion process in a biomass-fuelled fluidized-bed combustor with a cone-shape bed (or ‘conical FBC’). Two combustors firing rice husks with elevated fuel-ash content were the focus of this study. Compared to the pilot 350-kW_th conical FBC exhibiting combustion efficiency of up to 96%, the newly constructed 400-kW_th combustor included geometrical and design modifications aimed at improving the combustion efficiency and emission performance of the reactor. Differences between the air distributors and Δp–u diagrams (accounting for the total pressure drop across the air distributor and gas–solid fluidized bed) for the two reactors are discussed. Axial temperature and gas concentration (O₂, CO and NO_x) profiles in the combustors were compared for similar operating conditions (excess air and heat release rate per unit cross-sectional area). At excess air of 40–60%, the bed temperature in the advanced conical FBC was substantially, by about 180 °C, higher than that in the pilot combustor, mainly, due to better fuel–air mixing and higher residence time of reactants. The formation and decomposition of CO and NO in the bed region as well as in the freeboard of these two combustors showed quite different trends under similar operating conditions. At excess air of 40–60%, the CO emission from the advanced conical FBC was found to be much (7–8 times) lower than that from the pilot combustor, while the NO_x emissions were represented by almost the same values. High (over 99%) combustion efficiency was achieved when firing rice husk in the advanced 400 kW_th conical FBC for the range of excess air.