Release characteristics of chromium and sulfur during cothermal disposal of waste tires and sludge: An experimental and DFT study

Abstract

Chromium (Cr) and sulfur-containing pollutants (S) are released during thermal disposal of municipal sludge (MS) and waste tires (WTs). In this paper, the effects of the mixing ratio, temperature, atmosphere, and CaO on Cr release were determined, and the mechanism for Cr trapping during CaO sulfation was revealed with density functional theory (DFT). The MW (MS and WTs) thermal decomposition behavior, sulfur-containing pollutant release, and kinetic parameters were analyzed. The results showed that the thermal disposal of MW at 850 °C under flue gas immobilized the Cr, and the relative enrichment (RE) for Cr reached 92.66 % when the proportion of WTs was 50 %. DFT calculations revealed the effect of CaO sulfation on Cr adsorption. The energies for CrOOH adsorption on the (001) surface of CaO, and the (001) surface of CaSO4 decreased sequentially. The products of the CrOOH transformations in the flue gas, CrCl3 and CrO2Cl2, also diminished in that order for the corresponding surfaces. Typical decomposition steps were observed, including volatilization of water and small hydrocarbons, cracking of hydrocarbons, and transformations of inorganic materials at higher temperatures. H2S, SO2, CH3SH, and COS were the main sulfur-containing pollutants released during thermal disposal.