Competition between reduction reaction and carbon deposition for CaSO4 oxygen carrier in chemical-looping combustion: A first-principle study and reaction equilibrium analysis

To understand the reaction mechanism of chemical looping combustion with calcium-based oxygen carrier, the competition mechanisms between reduction reaction and carbon deposition for CaSO4 oxygen carrier were calculated by density functional theory and simulated by chemical reaction equilibrium analysis. In this paper, the reaction paths of reduction reaction and carbon deposition at different CaSO4(010) surface outer oxygen content (100%, 75%, 50%, 25%) were studied, and the reversibility of the reaction was fully considered. Furthermore, based on the kinetic analysis of the transition state theory, the kinetic parameters of the elementary reaction were obtained. Moreover, the reaction equilibrium simulation revealed the competition of reactions under different conditions. The results show that the energy barriers of forward and reverse reactions decrease with the oxygen content decreasing on the CaSO4 (010) outer layer surface in the reduction reaction. Besides, the forward energy barriers at each stage are lower than the reverse energy. In the carbon deposition reaction, the energy barriers of the forward reaction are higher than the reverse reaction, and the energy barriers of the reverse reaction gradually increase with oxygen content decreasing. The kinetic analysis results show the ratelimiting steps of forward and reverse reaction in reduction reaction are 75% and 25% oxygen content stage. According to the reaction equilibrium, the conversion rate of CO oxidation to CO2 decreases with temperature increasing, and the conversion rate of carbon deposition increases. The rate of CO oxidation to CO2 increase and the occurrence of carbon deposition reaction decrease with oxygen carrier ratio increasing. Thus, when the oxygen carrier is insufficient or most of CaSO4 is reduced, the carbon deposition reaction occurs easily.