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

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 degrees C, and H2Se was converted to Se0 with temperature increase. The kinetic calculations indicated that the slower rate of SeO2 reduction at 700 degrees C led to less H2Se content in the system. When the temperature exceeded 1200 degrees 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 electro-negativity 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.

Automated summary

In this study, the selenium homogeneous transformation and metal oxide capture mechanism in sludge gasification were investigated using density functional theory (DFT) and kinetics calculation. DFT calculation proposed 51 selenium transformation elementary reactions and 15 thermodynamic properties of selenium species. The results showed that H2Se was the dominant selenium species at temperatures between 700-900 degrees C, which then converted to Se0 as the temperature increased. Kinetic calculations revealed that the slower reduction rate of SeO2 at 700 degrees C resulted in a lower H2Se content in the system. At temperatures above 1200 degrees C, SeO2 generated H2Se through two reaction pathways, with CO being the main contributor. Additionally, chlorine had minimal effect on the selenium transformation. Adsorption calculations showed that the adsorption of H2Se by metal oxides involved chemical adsorption, with CaO demonstrating better adsorption performance for selenium species.