Heat Transfer in Circulating Fluidized Bed Boilers

Doctoral thesis, 2000

Heat transfer to single particles in the transport zone of circulating fluidized beds and heat transfer to membrane tube walls have been investigated. The heat transfer coefficient to spheres in the transport zone of a circulating fluidized bed boiler has been measured with dark and light calorimeters. A model was developed for the heat transfer mechanisms involved: radiation, gas and particle convection. The model predicts the measured data within 30 %. The gas convective constituent is 50% larger than in a single-phase non turbulent flow, which is explained by turbulence induced by bubbles in the bottom bed. The heat transfer model and a correlation for mass transfer were used to calculate the temperature of burning char particles. The calculation confirmed previous measurements, showing particle temperatures of several hundred degrees above the average bed temperature. Heat transfer from the core to the wall-layer was investigated by a heat balance of the wall-layer. It was found that the computation underestimates the measurements with in average 25%. This is explained by a horizontal in-flow and out-flow of particles, instead of the net-flow used in the model. Heat transfer and suspension density data from circulating fluidized bed boilers were averaged over the entire heat transfer surface of the furnace. The averaged data forms the basis for a correlation, which estimates the heat transfer coefficient with a standard deviation of 15 %. An attempt to separate radiation from convection was made, but it did not improve the accuracy of the correlation. Heat transfer experiments in a laboratory unit under thermally scaled conditions has been investigated theoretically and experimentally. Criteria for thermal similarity were derived from the energy equations of gas and particles. It was found that the ratio of the heat capacity of the particles and the gas should be constant in addition to the fluid-dynamic scaling. Measurements in a boiler and a scaled laboratory rig showed that the Nusselt number was not the same in the two units. However, the heat transfer coefficients from the two units coincided if they were compensated for the difference in thermal conductivity of the gas.