Effects of Flue Gas Components on the Reaction of Ca(OH)2 with SO2

Abstract

A differential fixed-bed reactor was employed to study the effects of the flue gas components, H2O, CO2, NOX, and O2, on the reaction between Ca(OH)2 and SO2 under conditions similar to those in the bag filters of a spray-drying flue gas desulfurization (FGD) system. The presence of CO2 with SO2 in the gas phase enhanced the sulfation of Ca(OH)2 only when NOX was also present. When either NOX (mainly NO) or O2 was present with SO2, the enhancement effect was slight, but became great when both NOX and O2 were present, and was even greater when CO2 was also present. The great enhancement effect exerted by the presence of NOX/O2 resulted from the rise in the NO2 concentration, which enhanced the oxidation of HSO3- and SO32- to SO42- in the water layer adsorbed on Ca(OH)2 surface and the formation of deliquescent salts of calcium nitrite and nitrate. The enhancement effect due to the presence of NOX/O2 was more pronounced when the relative humidity was above that at which the salts deliquesced; the extent of sulfation was more than twice that obtained when SO2 alone was present. The presence of H2O, CO2, NOX, and O2 in the flue gas is beneficial to the SO2 capture in the low-temperature dry and semidry FGD processes. The presence of NOX/O2 also enhanced CO2 removal when SO2 was absent.