Heat transfer in circulating fluidized beds

Heat transfer in circulating fluidized beds was studied in both a 7.3 m high, 152 x 152 mm square, pilot-scale combustor and a 9.3 m high, 152 mm ID transparent cold model unit. Results were obtained for particles of mean size 171-299 µm at superficial gas velocities from 4 to 9.5 m/s and for solids circulation rates up to 70 kg/m².s. For the combustor, results obtained by using membrane walls and a vertical tube as heat transfer surfaces show a strong influence of the cross-sectional area-averaged suspension density on time-averaged, length-averaged suspension-to-surface heat transfer coefficient. The influence of superficial gas velocity is found to be small. Radiation becomes significant at suspension temperatures higher than 400 C and at low suspension densities. Heat transfer coefficients were also found to vary with the lateral position of the tube. The vertical length of heat transfer surface is shown to be an important parameter, allowing seemingly discrepant published results to be reconciled. For the cold model unit, sudden and dramatic peaks in instantaneous heat transfer coefficients were measured using an instantaneous heat transfer probe. Simultaneous heat transfer and capacitance measurements suggest that these peaks are caused by the arrivals of particle strands at the heat transfer surface. Two-probe heat transfer measurements suggest the existence of a characteristic residence length for the strands at the wall in this column. A proposed heat transfer model, based on an overall core-annulus flow structure in the riser, and periodic formation, movement along the wall, and disintegration of strands in the annulus, gives reasonable agreement with a wide range of published data. It accounts successfully for the effects of heat transfer surface length and particle sizes. However, the effect of the heat transfer surface configuration on the flow pattern of particles must also be taken into account to give improved agreement with experimental data.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat