Quantum Chemistry Study On The Reaction Mechanism Of Trace Element Se And Sn In The Coal Combustion Process

Emission of poisonous trace elements from coal combustion, which are serious pollutants considered after SO₂, NOX and CO₂, has attracted worldwide attention. Control of trace elements and their compounds has been an emerging and frontier area in the research fields of pollutants emission and control from coal combustion. As yet, study on trace elements is concentrated on such areas as enrichment, distribution and transformation principle, as well as sampling devices and control techniques. The study about reaction mechanisms of trace elements with coal combustion products and other matters, which is helpful to conceive better control technique with lower cost and higher efficiency, is short. Based on quantum chemistry theory and transition state theory, reaction mechanism of selenium and tin during coal combustion were studied at microcosmic aspect, and thermodynamical and kinetic parameters were calculated and compared with reference data. The main contents are as follows: (1) Reaction mechanism of three reactions in Se/O₂ system has been studied by ab initio calculations of quantum chemistry. The geometry optimizations of reactant, transition state, intermediate and product are made at MP₂/SDD level. Then, the reaction potential barriers are calculated at the same level and the zero-point energies are also corrected. Finally, the reaction heat, reacion entropy, active energy and absolute rate constant are calculated. According to calculated results, isoelectronic density graphs of all molecules were plotted, then population analysis were carried out, so the properties of molecules and reaction mechanism were illuminated at microcosmic aspect. (2) The microcosmic mechanism of reactions between tin and gas was studied by ab initio calculations of quantum chemistry. The geometry optimizations of reactant, transition state, intermediate and product are made at MP₂/SDD level. The reaction potential barriers are calculated at the same level. The zero-point energies are also corrected. The rate constant is derived by application of transition state theory and compared with the data of reference. According to calculated results, isoelectronic density graphs of all molecules were plotted, then population analysis were carried out, so the properties of molecules and reaction mechanism were illuminated at microcosmic aspect. The calculated results of selenium and tin approach reference data, which shows that calculation by quantum chemistry is an effective method to study gas-phase reaction mechanism of trace elements, and to calculate thermodynamical and kinetic parameters. The calculated parameters can provide new foundation for emission model of trace elements during coal combustion.