No abstract

A differential fixed-bed reactor was employed to study the effects of the flue gas components, H 2 O, CO 2 , NO X , and O 2 , on the reaction between Ca(OH) 2 and SO 2 under conditions similar to those in the bag filters of a spray-drying flue gas desulfurization (FGD) system. The presence of CO 2 with SO 2 in the gas phase enhanced the sulfation of Ca(OH) 2 only when NO X was also present. When either NO X (mainly NO) or O 2 was present with SO 2 , the enhancement effect was slight, but became great when both NO X and O 2 were present, and was even greater when CO 2 was also present. The great enhancement effect exerted by the presence of NO X /O 2 resulted from the rise in the NO 2 concentration, which enhanced the oxidation of HSO 3and SO 3 2to SO 4 2in 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 NO X /O 2 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 SO 2 alone was present. The presence of H 2 O, CO 2 , NO X , and O 2 in the flue gas is beneficial to the SO 2 capture in the low-temperature dry and semidry FGD processes. The presence of NO X /O 2 also enhanced CO 2 removal when SO 2 was absent.

Hydrated lime [Ca(OH) 2 ] is frequently used as a sorbent in the spray-drying flue gas desulfurization (FGD) and injection/humidification FGD processes to remove SO 2 from the flue gas. In the present study, the kinetics of the reaction of Ca(OH) 2 with SO 2 has been studied by using a differential reactor and a gas mixture of N 2 , SO 2 , and H 2 O under the conditions of 0-80% relative humidity, 60-90 °C, and 1000-5000 ppm SO 2 . The reaction ceased before 1 h, and Ca(OH) 2 was incompletely converted. The relative humidity of the gas significantly affected the initial reaction rate and the maximum conversion of Ca(OH) 2 . The temperature and SO 2 concentration had mild effects on the reaction. The sulfation of Ca(OH) 2 was well described by the asymptotic equation, which is obtained by assuming that the rate was controlled by the chemical reaction occurring on the water-adsorbed sorbent surface and by considering the surface coverage by the product. The results of this study are useful to the design and efficient operation of the processes using hydrated lime to remove SO 2 from the flue gas.

Ca(OH) 2 /fly ash sorbents were characterized and tested for reactivity toward SO 2 at conditions similar to those in the bag filters in the dry and semidry flue gas desulfurization processes. The sorbents were mesoporous, and their specific surface areas linearly correlated with their specific total pore (1.7-300 nm) or mesopore (2-50 nm) volumes. The 1 h utilization of Ca in a sorbent increased, in general, with increasing fly ash content. Sorbents with Ca(OH) 2 contents larger than 33 wt % had higher weights of SO 2 captured per unit weight of sorbent than Ca(OH) 2 had alone, and the sorbent with 70 wt % Ca(OH) 2 had the maximum capture. The 1 h utilizations of Ca for the sorbents and Ca(OH) 2 increased almost linearly with increasing specific surface area or pore volumes of the sorbent. The SO 2 captured per unit surface area or pore volume of the sorbent decreased almost linearly with smaller Ca(OH) 2 content. The reactivities of the sorbents were enhanced mainly by their large specific surface areas and to a lesser degree by the reactive constituents in fly ash.