Modeling of sulfur retention by limestone in coal briquette

摘要

A detailed simulation model was developed for sulfur retention by limestone in a coal briquette. Four submodels, i.e., coal briquette combustion, volatile and sulfur evolution, H_2S retention, and SO_2 retention, were included in the simulation. In the model, the coal briquette combustion was divided into two successive stages: volatile ignition and char burnout. The temperature profile and its time variation, sulfur release and retention behavior within the burning coal briquette for the two stages were simulated separately. In the stage of volatile evolution and ignition, a part of coal sulfur is released as H_2S, and reacted with the calcined limestone in the area near the particle surface, where the local temperature is increased along with the volatile ignition, and the limestone in the coal briquette is partially calcined. H_2S retention in the coal briquette was thus simulated as a result of the competition among its transportation by the bulk flow of evolved volatile, its molecular diffusion due to the concentration gradient within the coal briquette, and its capture by the calcined limestone. In the stage of char burnout, the remaining part of coal sulfur is released as SO_2 with char burning which is simulated by the shrinking core model. SO_2 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. SO_2 retention was thus simulated as a result of the competition among the sulfate formation with the calcined limestone, SO_2 diffusion in the ash layer and its emission from the briquette surface. The sulfur retention by limestone in a spherical centimeter sized coal briquette was simulated by the model. The effects of heating rate, briquette size, calcium to sulfur ratio (Ca/S), and volatile matter of coal on the sulfur retention were predicted. The simulation results showed that rapid heating condition was good for both the H_2S retention in the volatile and the SO_2 retention in the combustion gas. The simulation also predicted a higher SO_2 retention for a larger sized coal briquette. The coals of higher rank and lower organic sulfur contents also showed a higher sulfur retention.