Research On Mechanism Of Calcium Sulfate Reductive Decomposition By Density Functional Theory

Chemical looping combustion is a new concept of combustion of fossil fuel. Different from the traditional combustion with one step, in the chemical looping combustion there are two steps. In the first step, fossil fuel is converted to carbon oxide (CO2) and water (H2O) after oxidation and reduction; in the second step, the oxygen transfer is achieved by oxygen carrier which was recycled in the system. CO2 is enriched in the combustion process and separated inherently.Most of present investigations are focus on the chemical looping combustion using calcium sulfate non- metallic oxygen carrier. The mechanism of CaSO4 reductive decomposition in the chemical looping combustion is difficult to obtain by experiment, because of the reductive process of CaSO4 more complex. Therefore, in this paper, the mechanism of CaSO4 reductive decomposition is studied by Density Functional Theory, and the optimal reaction path and condition are determined by comparisons of the activation energies for each reaction. Firstly, to test the adequacy of the developed model, the activation energy of similar simple reaction, the structure parameters of CaSO4 crystal and CO, CO2 molecules are calculated and compared with the experimental data in literatures. The calculated results are close to the experimental data, which shows that the calculation algorithm is available. Then the mechanism of CaSO4 reductive decomposition by CO and CH4 are studied using the developed model, Density Functional Theory.σandÏ€bonds are formed simultaneously when CO interacts with CaSO4. The formation of C-O bond and the breaking of S-O bond are achieved by electrons transition. Electrons transfer from 5a orbital (CO) to O (CaSO4), while partial electrons of O (CaSO4) are reversed back to 2Ï€* orbital (CO). The most likely pathway for CO reducing CaSO4 to CaS and CO2 is 4CO+CaSO4â†'3CO+CO2+CaSO3â†'2CO+2CO2+CaSO2â†'3CO+CO2+CaSOâ†'4CO2+CaS. The activation energies for each reaction are 191.19 kJ/mol,178.14 kJ/mol,90.76 kJ/mol and 59.54 kJ/mol, respectively. The decomposition course of CaSO4 into CaSO3 is the rate-limiting step. With the ratio of CO and CaSO4 as 4:1, the objective product CaS can be obtained if CO concertration is sufficient.Three pathways of CaO (by-product) formation are investigated. The activation energies of CaSO4 decomposition into CaO and SO3, and CaSO3 decomposition into CaO and SO2 are 448.92 kJ/mol and 318.28 kJ/mol, respectively. The activation energies of three paths are compared with each other, and experimental data in literature show that the molten liquid intermediate model is more suitable to explain the mechanism of CaO formation if the diffusion-controlled is considered. The reaction temperature of CaO formation is higher than that of CaS formation and the ratio of CO and CaSO4 is 1:1, which means that the formation of CaO can be controlled by the high reductant ratio and low temperature.The reaction pathways and activation energies of CH4 reducing CaSO4 to CO2, H2O and CaS are calculated. The calculated results show that electrons are provided to O(CaSO4) by the side group of CH4(HOMO) when CH4 interacted with CaSO4. The possible pathway of CaS formation is CH4+CaSO4â†'CH3OH+CaSO3â†'CH2OOH+CaSO2Hâ†'HCHO+H2O+CaSO2â†'COOH+CaSO+H2Oâ†'2CO3+CaS+H2Oâ†'CaS+CO2+2H2O. The activation energies for each reaction are 340.99 kJ/mol,262.42 kJ/mol,60.01 kJ/mol,295.88 kJ/mol,170.78 kJ/mol and 95.64 kJ/mol, respectively. The CaSO4 decomposition course is the rate-limiting step.