BED-TO-WALL HEAT TRANSFER IN A LARGE-SCALE SUPERCRITICAL CFB COMBUSTOR UNDER FLUE GAS RECIRCULATION CONDITIONS

摘要

In the present work the effect of flue gas recirculation on bed-to-wall heat transfer in a large-scale CFB combustor is investigated.A mechanistic model based on a cluster renewal approach is used in this investigation.The heat transfer mechanism in a CFB combustor has been analysed for a dilute phase and a dense phase conditions above the secondary air injection, where membrane wall surfaces and radiant superheaters are located.Experimental heat transfer studies were conducted on a 1296 [t·h-1] supercritical CFB combustor at the high bed pressure (ca.7.7 [kPa]), the ratio of secondary air to primary air SA/PA=0.33 and also under flue gas recirculation rate 6.9％ using the bed material of mean size in the range of 219 to 246 [μm] (group B).The fluidizing gas velocity and the solid circulation flux varied in the range of 3.89-4.27 [m·s-1] and 23.7-25.6 [kg·(m2·s-1)], respectively.Furthermore, the bed temperature and the suspension density were regarded as experimental variables along furnace height.The percentage contributions of convection and radiation heat transfer components were estimated.The variation in contributions was depended on the operating conditions i.e.both solid suspension density and bed temperature.During all tests, the average contribution of convection and radiation heat transfer components varied between 29％-45％ and 55％-71％, respectively.The results confirmed an increasing trend of the heat transfer coefficient (110-241 [W· (m2·K-1)] with an increase in average suspension density (1.93-1200 [kg·m-3]).