CARBON COMBUSTION EFFICIENCY AND SULFUR DIOXIDE EMISSION IN FLUIDIZED BED COMBUSTION OF TEXAS LIGNITE (LOW-RANK COAL)

There is currently significant interest in the application of fluidized bed combustion (FBC) of coal to exploit the large reserves of low-rank coals in the United States. Lignite is of particular interest in FBC because it has the potential for high carbon efficiency with low emissions of sulfur dioxide. This study is an effort to characterize the relationships between both the carbon efficiency and sulfur dioxide emission and the operating conditions of a pilot plant FBC unit.; The design and instrumentation of the pilot plant FBC facility used in the experimental studies are described. The relevant operating parameters are identified and their influence on carbon efficiency and sulfur dioxide emission is determined experimentally. The effects of the operating parameters are then consolidated through use of the carbon residence time in the so-called emulsion-phase of the reactor as a single correlating parameter for the carbon efficiency. The variation in the sulfur retention by the lignite ash mineral matter with operating conditions is measured and the reduction in sulfur emission obtained with limestone sorbent addition is determined. The effects of staged combustion and recycle of the elutriated material are also studied. High carbon efficiencies are found to be obtainable without the use of recycle. Operation under fuel-rich conditions is found to increase the utilization of calcium, providing more efficient sorbent use.; A model is developed to predict the carbon efficiency based on the residence time of the char in the bed. The char particle size distribution in the bed is calculated by use of a population balance assuming the rate of combustion to be controlled by diffusion of oxygen to the char particle surface. The predictions of the model show good agreement with the experimental pilot plant data. Comparison of the pilot plant results with those of microreactor studies shows the mechanism by which sulfur is retained to be similar for both cases and that the calcium utilization may be predicted from small-scale studies.