MODELING OF SULFUR RETENTION BY LIMESTONE IN COAL BRIQUETTE

A detailed simulation model was developed for sulfur retention by lime stone in a coal briquette. Four submodels, i.e., coal briquette combus tion, volatile and sulfur evolution, H2S retention, and SO2 retention, were included in the simulation. In the model, the coal briquette com bustion was divided into two successive stages: volatile ignition and char burnout. The temperature profile and its time variation, sulfur r elease and retention behavior within the burning coal briquette for th e two stages were simulated separately. In the stage of volatile evolu tion and ignition, a part of coal sulfur is released as H2S, and react ed with the calcined limestone in the area near the particle surface, where the local temperature is increased along with the volatile ignit ion, and the limestone in the coal briquette is partially calcined. H2 S retention in the coal briquette was thus simulated as a result of th e competition among its transportation by the bulk flow of evolved vol atile, its molecular diffusion due to the concentration gradient withi n the coal briquette, and its capture by the calcined limestone. In th e stage of char burnout, the remaining part of coal sulfur is released as SO2 with char burning which is simulated by the shrinking core mod el. SO2 release rate is assumed to be proportional to the char burning rate and controlled by oxygen diffusion in the ash layer. The sulfate formation occurs in the ash layer within which oxygen exists. SO2 ret ention was thus simulated as a result of the competition among the sul fate formation with the calcined limestone, SO2 diffusion in the ash l ayer and its emission from the briquette surface. The sulfur retention by limestone in a spherical centimeter sized coal briquette was simul ated by the model. The effects of heating rate, briquette size, calciu m to sulfur ratio (Ca/S), and volatile matter of coal on the sulfur re tention were predicted. The simulation results showed that rapid heati ng condition was good for both the H2S retention in the volatile and t he SO2 retention in the combustion gas. The simulation also predicted a higher SO2 retention for a larger sized coal briquette. The coals of higher rank and lower organic sulfur contents also showed a higher su lfur retention. (C) 1997 Elsevier Science B.V.