Measurement of Heat Transfer in a 465t/h Circulating Fluidized Bed Boiler

The study of heat transfer and thermal boundary layer in the combustor of a circulating fluidized bed (CFB) is important to the boiler design and operation. Both heat transfer coefficient between the solid-gas flow and the water-wall and the thickness of thermal boundary layer are key data to determine the amount and layout of the tube walls in a CFB furnace. A series of experiments was conducted on a 465t/h commercial CFB boiler, which operated at bed temperature between 850 and 900°C, and at superficial gas velocity between 5.2 to 5.9m/s. Local bed to water wall heat transfer coefficients and temperature profiles near the wall were measured at a set of test ports at different heights of the sidewall. In the same time, the local solid bulk density near the wall was also measured. Special tools such as heat flux probe, solid bulk density sampling probe and temperature probe were developed for the experiments and their structures were introduced. The experimental results were compared with the data from previous studies. Theoretical analysis of the factors that play important role in heat transfer in a CFB boiler was also performed. The relationship between heat transfer and thermal boundary layer was also discussed. Furthermore, a simple model correlating the local heat transfer coefficients with bulk density was developed.