Ash slagging caused by pyrite (FeS2) transformation in coal-fired boilers is accompanied by sulfur evolution from pyrite oxidation during oxy-fuel coal combustion. Theoretical studies based on density functional theory (DFT) were performed to systematically understand the temperature-dependent sulfur evolution chemistry of CO2-assisted pyrite oxidation. The results show that there is a strong interaction between CO2 and FeS2 surface. The interaction intensity is promoted by the presence of surface O atom. The adsorbed CO2 molecule dissociates into CO molecule and active surface O atom, which can supply oxygen source for SO2 formation. CO molecule strips a lattice sulfur atom from FeS2 surface to generate COS species. CO2 decomposition on FeS2 surface at 1073 K is activated by 677.73 kJ/mol, and exothermic by 405.69 kJ/mol. SO2 evolution from CO2-assisted pyrite oxidation includes different elementary reaction steps: CO2 adsorption, CO2 decomposition, CO desorption, SO2 formation, SO2 desorption, and O replenishment. CO2 decomposition is regarded as the rate-limiting step of SO2 evolution. S vacancy produced from surface lattice sulfur consumption can be replenished by the active oxygen atoms from CO2 decomposition. Compared with COS formation pathway, sulfur evolution of CO2-assisted pyrite oxidation prefers SO2 formation pathway. The proposed reaction network can be used to better understand the CO2-mediated sulfur evolution chemistry of pyrite oxidation. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.