Mohit Uberoi

The kinetics and mechanism of alkali removal from flue gases by solid sorbents

Kaolinite, bauxite and emathlite have been found suitable for alkali removal from hot flue gases in coal conversion systems. The effect of temperature on the kinetics and mechanism of alkali adsorption/reaction on these sorbents was studied under a simulated flue gas atmosphere. Kaolinite and emathlite reacted irreversibly with the alkali; however for bauxite, 10% of the total weight gained was due to physisorption. Kaolinite was found to have the highest capacity and the largest activation energy for alkali removal. The overall sorption process is not just physical and non-selective, but rather a combination of physical and chemical processes, which are dependent on the temperature and sorbent chemistry. The reaction product of alkali with emathlite has a melting point of approximately 1270 K, while kaolinite and bauxite form compounds with a melting point of about 1870 K. Consequently, kaolinite and bauxite are more suitable for in situ removal of alkali, while all three can be used for downstream alkali removal.

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.