Bed-to-wall heat transfer coefficient in a supercritical CFB boiler at different bed particle sizes

The role of bed particle size in the heat transfer to membrane walls of a supercritical circulating fluidized bed (CFB) combustion system was studied. In this work, values of the heat transfer coefficient between the membrane walls and the bed include contributions of particle convection, gas convection, cluster convection, gas conduction and also radiation. The heat transfer conditions in the CFB combustor were analyzed for five sizes of bed inventory, with Sauter mean particle diameters of 0.219, 0.232, 0.246, 0.365 and 0.411 mm (Geldart group B). The operating parameters of a circulating fluidized bed combustor covered a range of 3.13-5.11 m s(-1) for superficial gas velocity, 22.3-26.2 kg m(-2) s(-1) for the circulation rate of solids, 0.11-0.33 for the secondary to primary air ratio and 7.16-8.44 kPa pressure drop. Furthermore, the bed temperature, suspension density and the main parameters of cluster renewal approach were treated as experimental variables along the furnace height. To estimate the local bed to wall heat transfer coefficient, some experimental data from CFB boiler and some simple correlations were used. A simple semi-empirical method was proposed to estimate the overall heat transfer coefficient inside the furnace as a function of particle size and suspension density with an accuracy of 21%. Computationally obtained results were compared with the experimental data for CFB unit in a large-scale. It was observed that both bed particle sizes as well as the suspension density significantly influence heat transfer conditions. (C) 2014 Elsevier Ltd. All rights reserved.