Effect of hydrated lime on the migration behavior of selenium in the MSWI plants: Experimental and theoretical study

Abstract

Selenium emission generated from waste incineration endangers the environment and human health. The injection of lime slurry into the deacidification device is essential for selenium control in municipal solid waste incineration (MSWI) plants. To understand the migration behavior of SeO2 (g) in the semi-dry reactor, this study analyzed the content and speciation of selenium in MSWI fly ashes, and investigated the transformation mechanism of selenium through simulation experiments and theoretical calculations. The results showed that the semi-dry reactor increased the selenium retention and the proportion of selenite in the fly ash significantly. The migration of selenium in the semi-dry reactor was mainly due to the selenium retention on ash particles as well as the reaction between SeO2 (g) and Ca(OH)2. The spraying of lime slurry changed the ash compositions and enhanced the selenium capture by forming selenite. Furthermore, the improvement of the selenium retention was attributed to the excellent adsorption ability of Ca(OH)2 for gaseous selenium. However, HCl, SO2 and H2O inhibited selenium adsorption, which varied with temperature due to their different interactions with Ca(OH)2. Theoretical calculations, based on the density functional theory, demonstrated that the O atom on the surface of Ca(OH)2 was the reaction site for the SeO2 adsorption. Meanwhile, the HCl/SO2 molecule competed with the O site, and the O atom in the SO2 /H2O molecule or the H atom in the H2O molecule could be new site for SeO2, which weakened the adsorption force. These findings provided new ideas for controlling the selenium emission and reducing the environmental impact of selenium in MSWI fly ashes.