Analysis of the SO2-lime reaction system: mathematical modeling and experimental studies with emphasis on stoker applications

The growing public concern over the effects of acid rain has resulted in renewed interest in the S02-lime reaction as a means of capturing gaseous SO2 using calcium-based sorbents. In particular, the injection of dry, pulverized limestone directly into coal-fired boilers is an attractive possibility due to the ease of retrofitting existing boilers and the low cost of limestone. This study examines the application of the pulverized, dry limestone injection process to stoker-fired boilers using a comprehensive mathematical model of the S02-lime reaction in conjunction with a series of bench-scale experiments. The mathematical model was also used to assess the application of limestone injection for pulverized, coal-fired boilers. A comparison of model predictions and SO2 capture data obtained as part of this study, and from studies by other researchers, led to the conclusion that the best description of the reaction process was one incorporating an intrinsically zero-order chemical sulfation reaction serially with pore and product layer diffusion steps. Model predictions showed that sulfur capture increased linearly within creasing Ca/S molar ratio. The predicted apparent dependence on SO2 concentration varied from zero-to-first-order depending on particle size and reaction conditions. For time-temperature histories typical of boilers, the model demonstrated that the optimum limestone injection location is at the highest temperature at which sulfation is thermodynamically possible (ca. 2250°F at 3000 ppm SO2). Bench-scale experiments using a pulverized limestone sorbent showed that SO2 capture increased roughly linearly with increasing molar Ca/S ratio. The effects of changes in SO2 partial pressure were small; a six-fold increase in SO2 partial pressure resulted only in a doubling in capture. The introduction of cooling coils in the bed region of the test facility had no significant impact on SO2 capture. Capture levels obtained with natural gas doped to 3000 ppm of SO2 at Ca/S=2 ranged from 25 t o 30 percent. With high sulfur Illinois coal at Ca/S = 2 the capture was 35 percent. Comparisons of the model to the data showed good agreement at all conditions.