Transformation and capture mechanism of selenium in sludge gasification: Modeling and density functional theory study

Abstract

In this work, selenium homogeneous transformation and metal oxide capture mechanism in sludge gasification were investigated based on the combination of density functional theory (DFT) and kinetics calculation. Through DFT calculation, 51 selenium transformation elementary reactions and 15 thermodynamic properties of selenium species were first proposed. The calculation results confirmed that H2Se was the entire selenium species at 700–900 °C, and H2Se was converted to Se0 with temperature increase. The kinetic calculations indicated that the slower rate of SeO2 reduction at 700 °C led to less H2Se content in the system. When the temperature exceeded 1200 °C, SeO2 generated H2Se through two reaction pathways: SeO2→SeO→Se0→H2Se (more) and SeO2→Se0→H2Se (less), in which CO was the main contributor to SeO2→SeO→Se0. Moreover, chlorine had hardly effect on the selenium transformation in gasification. The adsorption calculation results showed that the adsorption of H2Se by metal oxides involved chemical adsorption. Compared to SeO2 and Se0, the adsorption strength was inhibited and the selenium atom was not directly fixed on the metal oxide surfaces. The electronegativity of the selenium atom in the H2Se made it unfavorable for adsorption by active sites on the metal oxides surface. Overall, CaO showed better adsorption performance for selenium species and could be an excellent selenium adsorbent.