Metal capture by sorbents in combustion processes

Abstract

The use of sorbents to control trace metal emissions from combustion processes was investigated, and the underlying mechanisms governing the interactions between trace metals and sorbents, were explored. Emphasis was on mechanisms in which the metal vapor was reactively scavenged by simple commercial sorbents, to form water unleachable products, which are easy to collect and isolate from the environment. Results are presented from two different scales of experimentation, involving a bench scale thermo-gravimetric reactor and a 17 kW down-fired laboratory combustor, respectively. Results from the bench scale tests showed that lead and cadmium, vaporized from the chloride salt, could be reactively captured at temperatures above the dew point. Both kaolinite and bauxite were effective sorbents for lead, while bauxite but not kaolinite was effective for cadmium. The primary reaction products, as identified by X-ray diffraction analyses, consisted of lead and cadmium aluminosilicates. Laboratory combustor tests, completed in the absence of coal char or coal ash particles, showed that lead could be effectively reactively scavenged in situ, in a combustor, downstream of the primary flame. Here, the high temperatures of the combustion process were being exploited to promote the reactions between the metal vapor and kaolinite sorbent, that were identified in the bench scale tests.