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Coal combustion in spouted and spout-fluid beds Zhao, Jiansheng


This study of coal combustion was carried out in a half-column spout-fluid bed combustor, in which Forestberg coal, a sub-bituminous Alberta coal, was burned in inert beds of sand. The combustor consists of a half cylindrical stainless steel column of 0.152 m I. D. and 1.06 m long, a half cylindrical cone fitted with an inlet orifice of 15.9 mm and a perforated plate as distributor surrounded by a plenum chamber, and a flat stainless steel panel with quartz glass windows. Aspects studied included hydrodynamic and combustion patterns, axial and radial temperature profiles, axial oxygen concentration profiles and burnout times of coal particles. Depending on operating conditions and properties of bed materials, four different flow patterns were established: Stable spouting, pulsatory spouting, jet-in-fluidized-bed and slugging. It was found that the maximum stable spouting height decreased as the bed temperature increased. Axial temperature profiles in the spout and annulus were found to be uniform for both spouted and spout-fluid beds except for a short distance above the inlet orifice. However, a temperature increase was found in the fountain above the spout when finer coal particles were employed. Above the annulus the temperatures increased substantially. More uniform axial temperature profiles could be achieved by introducing auxiliary air to create a spout-fluid bed. Radial temperature profiles were uniform both in the annulus and in the fountain region. Axial oxygen concentration profiles were found to be closely related to the flow patterns and solids properties. When larger coal particles (1mm) were used the oxygen concentration profiles were uniform within and above the spout, but a decrease of concentration was observed when fine coal particles (0.6 mm) were used. In the annulus a sharp decline of concentration started near the bed surface, and a minimum was reached in the fountain region. Concentration profiles became more uniform when auxiliary air was introduced. Compared with data reported in literature for fluidized bed combustion, the burnout times of coal particles in spouted and spout-fluid beds were found to be significantly shorter. A model for estimating the burnout times was proposed and tested against the experimental data. For the particle size and temperature range tested, the combustion was mainly controlled by chemical reaction.

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