Selenium emission control at high temperatures with mineral sorbents, Final report, September 1, 1993--August 13, 1994

The focus of this project is on toxic heavy metal removal by sorbent injection in the upper-furnace region of a coal-fired boiler. Selenium is chosen as the candidate heavy metal to be studied because of its high volatility and associated difficulty in removal. The preliminary sorbent screening experiments were performed in a differential reactor, built in the first year of this project. A host of sorbents, such as, alumina, kaotinite, limestone and also hydrated lime were tested at a reaction temperature of 900{degrees}C, and for reaction duration of 4 hrs. The reason for choosing these minerals was because of their proven moderate to high capability of lead and cadmium capture, and also for moderate selenium capture at high temperatures, reported by recent studies. The sorbent screening experiments have used selenium dioxide as the Se source, since in the oxidizing atmosphere of the furnace, that is reported to be the existing form of selenium species. The preliminary sorbent screening experiments have shown that Ca(OH){sub 2} is the most promising sorbent for selenium capture out of all the sorbents tested. A careful review of the sorption results for Ca(OH){sub 2} has also revealed the strong possibility for occurrence of a chemical reaction. Since Se belongs to group VI of the periodic table along with sulfur, and shares many common properties with the latter, formation of a calcium selenite (CaSeO{sub 3}) or selenate (CaSeO{sub 4}) compound is likely by the reaction of CaO with SeO{sub 2}. The captured selenium has exhibited poor leachability in water, a property which is also shared by CaSeO{sub 4}. The presence of CaSeO{sub 4} is confirmed by the X-ray diffraction analysis of the sorbent sample. Preliminary studies for investigating the effect of temperature on SeO{sub 2}/Ca(OH){sub 2} reaction have shown that the percent of water-leachable selenium increases with decreasing reaction temperature