Bed hydrodynamics and heat transfer to tubes in the freeboard region of a pressurized fluidized-bed coal combustor

Various modes of Pressurized Fluidized Bed Combustor part-load operation are analyzed. Bed change is considered to be the most effective of these methods. The need to understand the variation in heat absorption by exposed "in-bed" tubes immediately above the reducing or increasing bed height has resulted in the pursuit for a clearer understanding of the particle concentration profile and heat transfer mechanisms to the referred tubes.; Bubble characteristics in a PFBC with internals are studied in depth. A model for gas flow through dense and bubble phases is developed. Model results are compared with the limited experimental results available in literature. For both atmospheric and pressurized fluidized bed combustors, the deviation from the two phase theory is highlighted; and this simple model approach has evolved a procedure to quantify gas flows in different streams which was thus far available only qualitatively. The dense phase velocity is found to be much more than the minimum fluidization velocity.; A theoretical model for particle efflux from the bed top surface has been evolved. An empirical model has also been developed for elutriant flux above the Transport Disengagement Height, the concentration of fines in the efflux material, superficial velocity in the freeboard, and mean particle terminal velocity have appeared to be the major factors influencing the carry over. The model developed using the data from a smaller rectangular test rig was tested satisfactorily with the limited data from a larger as well as a similar size circular unit. If the particulate loading profile above the bed surface is known, then the heat transfer to the heat exchanger surfaces may be obtained from the available correlations.