Heat transfer to walls of a circulating fluidized-bed furnace

A critical review of information on heat transfer between the furnace and enclosing walls of a circulating fluidized-bed boiler is presented. A good understanding of the heat transfer process was impeded for some time by a lack of detailed information about the hydrodynamics of fast fluidization. With improvement in the understanding of the furnace hydrodynamics a clearer picture of the heat transfer process is also emerging. Several mechanistic models for the heat transfer process exist and the surface renewal model explains the observed phenomenon most faithfully. Efforts to calculate heat transfer coefficients from first principles, have been frustrated by a lack of data on residence time and surface coverage of particle strands on the wall. However, a mechanistic model is still useful in scale up of data and in the assessment of the impact of changes in the design or operating variables. Unlike in bubbling fluidized beds, the particle size has a minor effect on the heat transfer, while the average bed (suspension) density shows a major effect. A large variation in reported data between laboratory and industrial scale units is noted. Uncertainty in the measurement of suspension densities in large CFB furnaces may be responsible for this difference. Empirical correlation based on measurements in large commercial units are proposed for design calculations.