Studies Of The Wet Additive-Enhanced Limestone/Lime Flue Gas Desulfurization

The two main absorbents in wet flue gas desulfurization(FGD) process are limestone and lime because of their cheapness. The S02 removal efficiency and limestone/lime utilization can be increased by using adequate additives. In this dissertation, some inorganic and organic additives were experimentally selected. The selected effective additives were further tested in detail and the enhancement mechanism was analyzed theoretically. The tests were conducted on two kinds of rotating-stream tray(RST) scrubbers and a stirred tank.Experiments demonstrated that magnesium sulfate, sodium sulfate and sodium humate could effectively increase SO2 removal efficiency, while the promoting effect of black liquor was relatively low. The adequate additive concentrations were determined. The effects of magnesium sulfate mixed with sodium humate were also tested. The enhancement mechanism was analyzed based on the experiment results.For the limestone/lime FGD process enhanced by the three selected additives, effects of pH value, liquid-to-gas ratio, inlet SO2 concentrations on removal efficiency were investigated, and the regression equations were obtained. Meanwhile, the operating factors' affecting mechanism was analyzed.For the magnesium-enhanced limestone/lime FGD process, changes of SO2 removal efficiency and pH value with time were experimentally studied. The reaction mechanism at different pH range was analyzed and the adequate operating pH range was determined. According to the experimental data, the regression equations of *(pH0) and 77(pH0) were obtained and the limestone/lime utilization was calculated. Experiments and analyses showed that the reaction mechanism was different at different pH range; and that adding magnesium sulfate could promote the dissolution rate of limestone/lime, slow down the descent of pH value, achieve higher SO2 removal efficiency at the same pH value and therefor increase limestone/lime utilization. Furthermore, Magnesium-enhanced limestone FGD process was compared with that of lime.Sodium-enhanced limestone FGD tests were conducted on normal RST scrubber. Changes of SO2 removal efficiency and pH value with time were measured and the effects of pH value on S02 removal efficiency and dissolution rate of limestone were analyzed. The effects of plate number on SO2 removal efficiency and pressure drop were tested at temperature approximated to industrial operating value. The relation between the average plate efficiency and plate number was obtained.It was shown that magnesium sulfate, sodium sulfate and sodium humate could all increase limestone dissolution rate and therefor slow down the descent of pH value. Based on theoretical analysis, a mathematical model of limestone dissolution enhanced by additives was developed. The model could correlate test data with good results.The liquid-phase mass-transfer coefficient, enhancement factor and over-all gas-phase mass-transfer coefficient of limestone FGD processes with and without additives were obtained by experiments conducted on stirred tank. The results were compared and analyzed.To understand the additive-enhanced limestone/lime FGD process all-sidedly, the slurry setting rate and setting time, indissoluble solid content of the filter cake, and density of the slurry supemate were measured. The results were compared with that of non-enhanced process.