Fossil fuel combustion system is one of the major sources of carbon dioxide (CO2) emissions, which is considered to be the main greenhouse gas contribution to the global warming. Chemical looping combustion (CLC) is a promising combustion technology with inherent separation of CO2. Calcium sulfate (CaSO4) can be used as a new oxygen carrier in this system. In CLC, CaSO4 is reduced to calcium sulfide (CaS) in the fuel reactor firstly. Then CaS is oxygenated to CaSO4 back in the air reactor. The decomposition of CaSO4 is usually accompanied by some side reactions, and the products depended on the experimental conditions. In this paper, the decomposition mechanism of CaSO4 reduced by methane (CH4) was investigated in the frame of density functional theory (DFT). In the calculation, the exchange-correlation term was approximated by Perdew-Wang (PW91) equation, a function within the generalized gradient approximation (GGA) family. To understand the interaction between CH4 and CaSO4, the reaction pathways and TSs of CaSO4 decomposition were investigated and identified. The calculated results showed that, in the interaction of CaSO4 and CH4, electrons were transferred from sp(3) hybrid orbital to O p state and S s state when CH4 was activated. C-H bond and S-O bond were activated and stretched with the electron transfer, which resulted in the dissociation of S-O bond finally. The objective product CaS could be obtained with the reductive decomposition of CaSO4 by CH4. A probably mechanism was proposed, that is, the reaction process of CaSO4 and CH4 is CH4 + CaSO4 -> CH3OH + CaSO3 -> CH2OOH + CaSO2H -> HCHO + H2O + CaSO2 -> HCOOH + H2O + CaSO -> 2H(2)O + CO2 + CaS. (C) 2012 Elsevier Ltd. All rights reserved.