Selenium Removal Using Ca-Based Sorbents:  Reaction Kinetics

Abstract

Selenium, along with mercury and halides, represents one of the most volatile trace metallic emissions from coalfired 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 °C temperature conditions with SeO 2 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 SeO 2 partial pressure. Rate of reaction is determined to vary with the initial sorbent surface area. Sorption of selenium oxide in the presence of SO 2 in the gas phase suggests that the two reactions are competitive, and the formation of CaSeO 3 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 SeO 2 is insignificant in the presence of SO 2 .

Toxic Substances From Coal Combustion-a Comprehensive Assessment

“…Work with other oxy-anions (e.g., arsenic) and the success of lime sorbents to remove Se and SO 2 [145,149] reinforce this hypothesis. Ghosh-Dastidar et al [150] report substantially higher solidphase selenium recoveries from Ca-based sorbents acting on artificially generated Se-rich flue gas compared to kaolinite-, alumina-, or silica-based sorbents.…”

“…Vapor phase selenium is believed to form an oxy-anion of the form SeO 2 [29,35,145,146], although some researchers believe that gaseous selenium will also be present in the elemental state [77] or as seleneous acid [147]. SeO 2 is also predicted by the thermodynamic simulation results presented in Section 3.3.1.…”

Retention of Arsenic and Selenium during Hot Gas Desulfurization Using Metal Oxide Sorbents

“…As with sulphur, some gas-solid adsorption processes that use different solid materials are also effective for arsenic and selenium retention. [17][18][19][20] However, the most simple and costeffective strategy for removing these elements would be their simultaneous capture in sorbents already in use or being developed for sulphur compounds.…”

Treatment of Selenium in Simulated Refinery Effluent Using a Pilot-Scale Constructed Wetland Treatment System

“…Beneficial water use is especially desirable in arid regions and other areas where water is particularly scarce. Several approaches to Se removal have been studied previously, including phytoremediation , biological volatilization (Hansen et al 1998;Zayed et al 1998), chemical treatment (Agnihotri et al 1998), and bacterial/microbial reduction (Zhang et al 2008;Zhang and Frankenberger 2005). Phytoremediation, biological volatilization, and chemical treatment have several disadvantages in contrast to microbial reductive processes in CWTSs.…”