Experiments and Predictions of Heat Transfer Characteristics in Circulating Fluidized Bed Boilders

Doktorsavhandling, 1994

This thesis presents experiments and predictions of heat transfer characteristics in circulating fluidized bed boilers. The boiler used for the experiments has a maximum load of 12 MWth, and the dimensions of the combustion chamber are 1.4 m x 1.7 m x 1 3. 5 m. The equipment is located at Chalmers University of Technology in Goteborg. A method for measuring the local suspension temperature with a shielded thermocouple in the combustion chamber of a fluidized bed combustor is described. Two major sets of heat transfer experiments were conducted. The first concerned temperature measure me nts close to the wall. In these experiments, temperature distributions were measured at different heights above the distributor plate for different operating conditions. It was found that the thermal boundary layer thickness decreases with increasing su pe rficial gas velocity, but is less affected by the bulk bed temperature. The aim of the second set of experiments was to measure the local time-averaged heat transfer coefficients at the membrane wall of the combustor. It was found that the heat transfer c oefficient increases slightly with decreasing height, superficial gas velocity, suspension density, and with bulk bed temperature. It was also found that heat transfer coefficients at the membrane wall were, lower than those at the refractory wall (flat w all). Finally, on basis of the current understanding gained from experiments, an empirical-analytical model is proposed. The fluid is assumed to be composed of a dispersed phase and clusters of particles. Several parameters are present in the model, and metho ds for estimation of these are discussed. The model was able to predict heat transfer coefficients in decent agreement with corresponding experiments.