Heat transfer characteristics in a small-scale fluidized bed boiler

Abstract

SUMMARYHeat transfer characteristics in a small-scale fluidized bed boiler (2 MW, ) were studied using lignite and corn cob as fuels. Depending on air velocity, the heat transfer rates from bed to water membrane wall and from hot flue gas to convective tube bank were in the ranges 7 5 5 5 % and 25-45% of the total heat absorbed by the boiler, respectively. At designed capacity, the heat transfer flux based on bed cross sectional area and on water membrane wall area were about 045 and 015MWm-2, respectively. Under the conditions studied, it was found that the overall heat transfer coefficient between bed and water membrane wall was 100-300Wm-2K-1, whereas that between flue gas and convective tube bank was 10-30Wm-* K-l. The study of heat transfer to a horizontal tube immersed in the bed as well as placed in the freeboard region were also studied. The effective heat transfer coefficients were found to be 300-800Wm-2K-' for in-bed tube and 30-150Wm-2 K-' for the freeboard region, depending on air velocity. Comparison of these data with those predicted by both modelling and correlation reported in the literature was also made. For the immersed tube, good agreement was observed for low air velocity, while at high air velocity the experiment produced results twice those estimated from modelling and correlation. For the freeboard region, the model gave a fair prediction.KEY WORDS: fluidized bed heat transfer combustion boiler 1. INTRODUCTION One of the features of fluidized beds that make this technology attractive for use in industrial coal/biomass fired boilers is the high heat transfer capability between bed and surface. Enhancement of heat transfer characteristics not only significantly reduces the heat transfer area but also allows the transfer process to occur at low bed temperature (1000-1200K), thereby decreasing NO, emission. The effective heat transfer coefficient for tubes immersed in the bed is an order of magnitude higher than that for tubes placed on the free board region or in the fixed bed. The difference can be utilized to control the boiler output by lowering and raising the bed level to vary the portions of the exchanger tubes exposed in the freeboard or by stopping the air supply to the individual cells of the bed.Studies of the heat transfer characteristics of fluidized beds, both experimental and theoretical models, have recently been reviewed in the literature (Botterill, 1986;Xavier and Davidson, 1985;Mathur and Saxena, 1987; Kunii and Levenspiel, 1991). It was found that the heat transfer between the surface immersed in the bed and that placed in the freeboard region is affected by solid and gas properties, fluidization conditions, types of air distributor, bed dimensions, temperature and pressure. The develop ment of a heat transfer model from first principles has achieved limited success owing to the chaotic behaviour of the system. For high temperature operation such as in the combustor, the contribution of radiation to the total heat transfer cannot be ignored, which increases the degree of ...