Transformation of selenium during coal thermal conversion: Effects of atmosphere and inorganic content

Study on selenium (Se) transformation during coal combustion will facilitate the development of emission-control technologies. A fixed-bed reactor was used to conduct pyrolysis and combustion experiments involving three types of coal at 600-1000 degrees C. In an inert atmosphere, the release ratios of Se increased slowly as temperature increased, varying between 56% and 74% for different coals at 1000 degrees C. All forms of Se in coal were thermally unstable and the conversion of Se in other forms into residual and organic-bound Se occurred. In an oxidizing atmosphere, the release ratios of Se differed significantly for different coals at low temperatures, but approached 100% at 1000 degrees C. Each form of Se in coal was released in different ratios. The water-soluble and ion-exchangeable Se, and carbonate- and oxide-bound Se were produced in the solid products. The oxidizing atmosphere was more conducive to the release of Se because the sulfide- and organic-bound Se was fully oxidized. Se could be converted into water-soluble and ion-exchangeable form and released less in Ca-rich coal. Sulfur (S) prohibited Ca from adsorbing Se by forming CaSO4, promoting the release of Se in S-rich coal.