The plume model for large utility scale atmospheric fluidized bed combustors

Graduation date: 1981The atmospheric fluidized bed combustor (AFBC) is an alternative to\ud the pulverized coal burner. The size of a utility scale AFBC would be\ud enormous so that the use of large particles is necessary to reduce its\ud size. In modeling the combustion of coal and capture of sulfur released\ud from the coal two physical factors must be accounted for in such a bed.\ud First, because large particles must be used the minimum fluidizing gas\ud velocity will be so high that gas will rise as slow bubbles or in a\ud churning turbulent manner, but never as fast bubbles surrounded by clouds.\ud Secondly, devolatilization of coal in a hot fluidized bed is so fast that\ud volatiles are all released very near the feed ports to rise as plumes of\ud unburned volatiles, much like gas flames. Since these plumes can contain\ud nearly 50% of the heating value of the coal and more than 50% of sulfur\ud in the coal, this is not a factor to be ignored.\ud Most models in the literature have not considered either of these\ud phenomena. The plume model, proposed here, is the simplest model which\ud accounts for the phenomena mentioned above. It visualizes plumes of\ud volatiles, combustion of volatiles at the plume boundaries by diffusion\ud flame and devolatilized coal evenly distributed throughout the bed and\ud burning outside the plumes. Sulfur dioxide forms in the oxygen rich\ud region outside the plume as well as at the plume boundaries by the\ud reaction of gaseous sulfur compounds with oxygen. Sulfur capture occurs\ud inside and outside the plumes as limestone particles react with sulfur\ud dioxide outside the plumes and with hydrogen sulfide inside the plumes.\ud Calculation results are presented and compared with those of the\ud plumeless model which is also developed. The plumeless model has the\ud same assumptions as the plume model except that it assumes that volatiles\ud are released uniformly throughout the bed. Calculations show that the\ud plumeless model, which is much simpler than the plume model, closely\ud approximates the plume model for the prediction of carbon efficiency.\ud On the other hand, the plume model must be used for the prediction of\ud the temperature jump above the bed, to represent the variation of gas\ud concentration across the bed, and to properly account for sulfur retention\ud by the limestone.\ud Finally, the model predicts that the spacing of the coal feed\ud ports in the fluidized bed combustor is of primary importance when\ud considering the conversion of volatiles in the bed. This suggests that\ud the kinetics of devolatilization of coal deserves more study. A comprehensive\ud example is given to show how to use the design charts provided.\ud The last chapter extends the RDT expressions for the convective\ud model to Bingham plastic flowing through tubular pipes and some cases\ud of improper tracer injection and/or measurement