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Quantum Chemistry Research On Nitrogen Conversion Mechanism During The Combustion Process Of Coal

Posted on:2017-05-08Degree:MasterType:Thesis
Country:ChinaCandidate:H Y ZhuFull Text:PDF
GTID:2272330488485450Subject:Power engineering
Abstract/Summary:PDF Full Text Request
Nitrogen oxide (NOx) is not only one of the major components of air pollution in China, but also the important composition of the haze occuring in China recent years. However, the major sources of the NO^emission are coal-fired power plants in China. Thus, it has important significance to research the formation and transformation mechanism of NOx Fuel-N is the major type of the NOx emission during the coal combustion. The conversion reaction of fuel-N in coal is composed of de volatilization and char reaction. The homophase reaction mechanism between volatiles and NOx is confirmed in previous studies. But the heterophase reaction mechanism between char and NOx has not been reached to an agreement.this paper starts from the geometrical shape of nitrogen in the char. The microscopic path of the NO oxidation with char and the reduction reaction between NO and char with the participation of CO are revealed by constructing the edge model compounds of char and using Mayer bond order theory and transition state theory. The specific research results are as follows:In order to master the rules of char oxidation by O2,and to reveal the microcosmic mechanisms of migration and transformation for char nitrogen,a comparison of two pathways on NO desorption reaction between the nitrogen-containing char edge modelin armchair configuration and O2 was explained in the third chapter based on Mayer bond order theory and transition state theory at the molecular level. To release intermediate HCN firstly in the char combustion processwas the first pathway for the formation of NO. Another pathway was to release NO from the reaction with O2 directly. The reaction energy and energy barrier of every step in the two pathways were calculated in the paper.The results show that the model’s Mayer bond orders of N2-C4 and C1-C3 are minimum so that it can extract HCN when the two bonds break. The energy barrier for this process to overcome is 451.671kJ·mol-1.On the other hand in the process of direct oxidation to produce NO it can be found that the Mayer bond order of C1-N2 is minimum when the model’s Mayer bond order is calculated.NO can be desorpted in the end when the bond is break.The energy barrier from M1 to M2 is 259.81 kJ/mol. The energy barrier from M2 to NO is 133.1 kJ/mol. There fore, the second pathway, which is to generate NO directly by the reaction between O2 and the selected char edge model, is relatively easy.Molecular modeling research based on transition state theory and density functional theory was performed to investigate the elemental reactions of NO heterogeneous reduction by char with the participation of CO under the Oxy-fuel combustion atmosphere in the fouth chapter. The effect of CO in NO heterogeneous reduction by char is studied from the point of energy and thermodynamics.the transition states of every elementary reaction are found. The enthalpy and activation energy of every step are also calculated.The results show that zigzag configuration char has stronger adsorption ability relative to armchair configuration char. Moreover the process of the reaction of NO heterogeneous reduction by zigzag configuration char with the participation of CO needs least energy to react in the four pathways designed in the paper by the contrast of enegy and the Gibbs free energy.The conclusion that CO promotes the NO heterogeneous reduction by zigzag configuration char and does not promote the NO heterogeneous reduction by armchair configuration char is also drew. In this paper, the low NOx emissions characteristics of Oxy-fuel combustion and reducing atmosphere are analyzed theoretically. These results provide a theoretical reference and basis for further controlling the emission of nitrogen oxide.
Keywords/Search Tags:char, heterogeneous oxidation, heterogeneous reduction, nitrogen oxides, char nitrogen, quantum chemistry
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