Flue gas desulphurization by spray dry scrubbing

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Koech, Lawrence
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Vaal University of Technology
Post combustion sulphur dioxide (SO2) emission is a major problem facing coal-based power plants. Increased awareness of the detrimental effects of SO2 on human health and the environment has prompted increased pressure to equip power plants with flue gas desulphurization (FGD) systems. FGD is a commercially proven technology for removal of SO2 from flue gas which is considered as a significant pollutant to the environment. Spray dry scrubbing (SDS) process represents a type of semi-dry FGD which is a low-cost retrofit SO2 control technology that could be used in already existing coal-fired power plants. This study explored the experimentation on the performance of a laboratory-scale spray dryer involving flue gas desulphurization (FGD). The experimentation involved test done to investigate the effects of spray characteristics i.e., inlet gas phase temperature, approach to saturation temperature, and calcium to sulphur ratio on SO2 removal efficiency using hydrated lime as a sorbent. Results indicated improved SO2 removal efficiency with increasing the stoichiometric ratio and decreasing the temperature and approach to saturation temperature. An absorption efficiency of SO2 beyond 90% was achieved at a stoichiometric ratio of 2.5. A high degree of conversion of calcium was realized at low stoichiometric ratios with a maximum utilization of 94% obtained at a stoichiometric ratio of 1.5. This study also investigated the interaction effects of independent variables on SO2 absorption using response surface methodology. By analyzing the influence of stoichiometric ratio, inlet gas phase temperature, slurry solid concentration and slurry pH, a predictive quadratic model was developed correlating the independent variables and SO2 removal efficiency. Although all independent variables had impact SO2 removal efficiency, stoichiometric ratio was found to have the largest influence. The recommended optimal conditions for SO2 absorption were inlet gas phase temperature of 140℃, stoichiometric ratio of 2, slurry solid concentration of 8% and slurry pH of 10 to achieve 90% SO2 removal efficiency. An investigation into performance of relevant spray dying FGD sorbents was carried by evaluating different sorbents namely: hydrated lime, limestone and trona. Results show that trona had better performance characteristics with the highest SO2 absorption efficiency when compared to hydrated lime and limestone under the same operating conditions. The analysis of the desulphurization product revealed that trona has better sorbent conversion and utilization in comparison with limestone and hydrated which contained high concentration of unreacted sorbent in the desulphurization product.
PhD. (Department of Chemical Engineering, Faculty of Engineering and Technology), Vaal University of Technology.
Spray drying, Desulphurization, Semi-dry FGD sorbents, Experimentation, Optimization