Mechanisms of heat transfer between a surface and a gas-fluidized bed for combustor application

There is currently considerable interest in the development of fluidized-bed boilers for efficient and environmentally clean combustion of coal. Fluidized beds have the advantages of high heat transfer rates and intimate mixing of additives such as limestone or dolomite for sulfur dioxide absorption produced as a result of combustion of coal containing sulfur. However, design for optimum heat transfer remains uncertain and essentially empirical. The mechanisms of heat transfer are complicated because of the many variables in a commercial combustion operation such as particle size distribution, particle shape, particle and gas thermal properties, reactor geometry and boiler tube design.An understanding of the mechanisms of bed to tube heat transfer is essential to sound design and interpretation of empirically derived correlations. Here we will review and criticize the major mechanisms of heat transfer that have been proposed. These mechanisms are proposed and developed from two schools of thought: (a) The principal resistance to heat transfer is a fluid film, and the moving fluidized particles scour the film to reduce the resistance to heat transfer; (b) Heat is absorbed by the fluidized particles and the rate of heat transfer depends on the rate of heat absorption.Radiant heat transfer is also discussed in this review in detail. Heat transfer by radiation is an important consideration in combustors but has received limited attention. The results of theoretical calculations are given which have been recently reported on the basis of the alternate-slab model of Gabor.The review will predominantly deal with the mechanistic models of heat transfer and various correlations developed over years will not be covered as this topic is dealt with in another review article by Saxena, Grewal, Gabor, Zabrodsky and Galershtein.