Energy & Fuels, Vol.34, No.7, 8600-8607, 2020
Understanding the Interaction Mechanism of Char and CaSO4 Oxygen Carrier in Chemical Looping Combustion: Semi-empirical Tight-Binding Method Calculation and Grand Canonical Monte Carlo Simulation
Controlling carbon emissions is an effective way to mitigate the greenhouse effect. According to the primary energy consumption of China, chemical looping combustion (CLC) using coal as the fuel has great development prospects. In the solid-fuel CLC process, the interaction of char and oxygen carrier cannot be ignored. The CaSO4 oxygen carrier is cheap and oxygen-rich. Thus, the interaction of char and CaSO4 was investigated in this paper to reveal the reason for the low reaction rate for the char- CaSO4 reaction from the perspective of the binding process. It could offer the guide for improving the binding ability of the CaSO4 oxygen carrier. The binding process of the char model composed of five benzene rings and the CaSO4 cluster was calculated by the semi-empirical tight-binding quantum chemistry method, and the influence of the temperature and pressure was simulated by the grand canonical Monte Carlo (GCMC) method. The interaction of char and CaSO4 is the exothermic reaction, and the dispersion contributes the most energy to attractive interaction. The binding system is the typical weak interaction system. From the property analysis, the H1-H2-O ring causes the system to stay stable, and there are hydrogen bonds that appear in the interaction area between the two fragments. The results of the independent gradient model verify the position of the hydrogen bond interaction, and the atom contributions of char and CaSO4 to the interaction are 0.1564-0.5958 and 0.2347-0.4854, respectively. The results of the electron density difference demonstrate that most atoms in the interaction area (z = 5.5-9.5 angstrom) get electrons to cause the positive electrostatic energy. According to the GCMC simulation, the binding process changes from type 2 binding to type 3 binding with the temperature increasing and the binding energy decreases. Meanwhile, the fitted energy curves change from the polynomial function to the exponentiation. To improve the interaction ability of the two fragments, it would be a choice to increase the electron transfer ability between char and the CaSO4 surface.