Selenium removal using Ca-based sorbents: Reaction kinetics

Selenium, along with mercury and halides, represents one of the most volatile trace metallic emissions from coal-fired combustors and utility boilers. Calcium-based sorbents are shown to capture selenium oxide via a chemical reaction leading to the formation of calcium selenite. In this study, intrinsic kinetics of this chemical interaction is investigated. Experiments are conducted to obtain initial reaction rate and to determine basic kinetic parameters such as activation energy of the reaction, order of reaction with respect to the gas-phase selenium concentration, and influence of sorbent initial surface area. Experimental investigations are conducted in a differential-bed flow-through reactor system at 400-600 degrees C temperature conditions with SeO2 partial pressure in the range of 5-30 Pa. Activation energy of the reaction of 4.03 kcal/gmol is obtained, and the reaction is found to have an order of 0.67, with respect to SeO2 partial pressure. Rate of reaction is determined to vary with the initial sorbent surface area. Sorption of selenium oxide in the presence of SO2 in the gas phase suggests that the two reactions are competitive, and the formation of CaSeO3 is considerably slower than the sulfation of the sorbent when the concentration of the two species in the gas phase is comparable. This suggests that the reaction between sorbent particles and SeO2 is insignificant in the presence of SO2.