Study On The Precipitation Characteristics Of NO_x Precursor And The Heterogeneous Reduction Mechanism Of Char-NO/N₂O During Coal Combustion

Coal plays an important role in the energy consumption structure of our country.More than 70%of NO_xin air pollutants comes from coal combustion.In order to further control NO_xgeneration and achieve ultra-low emission of pollutants,it is necessary to have a comprehensive study of the nitrogen migration characteristics and the heterogeneous removal mechanism of NO_xin coal combustion process.The nitrogen in coal is partially precipitated by the nitrogen-containing small molecular gases such as HCN and NH₃during the initial pyrolysis stage of combustion,and the char nitrogen remaining in the coal is converted into NO_xduring combustion.In the process of coal combustion,factors such as minerals,combustion atmosphere such as oxygen concentration and CO have a great influence on the N migration and NO_xreduction,and their interaction mechanism and synergy mechanism are not clear.In this paper,the TG-MS experiment combined with quantum chemistry are used to deeply study the migration and transformation characteristics of coal N and the char-NO/N₂O heterogeneous reactions under deep air-staged combustion from the molecular level.Firstly,the effects of Ca and Fe in coal on the migration characteristics of nitrogen to NO_xprecursors NH₃and HCN are analyzed in the devolatilization phase.The release characteristics of NO_xprecursors are studied by TG-MS experiments,and density functional theory is used to further reveal the influence mechanism of Ca and Fe on the formation of NH₃and HCN at the micro level.The results shows that the addition of the mineral Ca significantly promotes the NH₃formation and shows an inhibitory effect on HCN.However,Fe inhibits the formation of NH₃and HCN,and the inhibitory effect on HCN is more significant.The migration transformation characteristics of char nitrogen in the reduction zone are thoroughly explored by quantum chemistry.Two transformation paths of char-N are revealed.Path one is that NO firstly undergoes chemical adsorption on the char surface,and then combines with the N atom in the coal to form free N₂molecules and leave the char surface.Path two is that char firstly strips N atoms from the pyridine ring,and then and then recombines with the adsorbed NO to form free N₂molecule and desorb on the char surface.Comparing the results of thermodynamics and kinetics,it is found that path one is more conducive to the precipitation of N on char surface than path two.Under low oxygen conditions,the chemical activity of char surface is improved,and the combination of NO and char-N is promoted to release N₂.With the presence of oxygen,the combustion of C occurs at the same time as the release of N in the char,which is manifested as the combination of NO and N in the char to form N₂or the oxidation of N in the char to N₂O,accompanied by the oxidation of C in the char to CO₂or CO.The effect of the two important parameters,char model size and active site,on NO heterogeneous reduction are thoroughly studied.The reaction paths and energy changes of different sizes of char models and NO are analyzed by quantum chemical method.It is obtained from thermodynamics that the N₂desorption does not depend on char model size.At the same time,the heterogeneous reduction rates of NO on the different model sizes surface are compared using kinetic calculations,and the independent relationship between NO reduction and char model size is clarified at the molecular level.Comparing the char-NO heterogeneous reaction at different active sites,it is found that the active sites of char surface have a great effect on the heterogeneous reduction of NO.The more active sites on the char surface,the better the NO chemisorption on char surface.NO removal has a strong dependence on the active sites.With the increase of active sites,the lower the activation energy required for the reduction products desorption from char surface,the better the NO removal.The heterogeneous reduction mechanism of char-NO under different oxygen concentrations is explored.The results show that the rate-determining step energy barrier of NO heterogeneous reduction by char with one hydroxyl are 57.12 k J/mol and 123.83k J/mol lower than those of NO heterogeneous reduction by char with two hydroxyls.The thermodynamic results show that there is a critical phenomenon in the promotion effect of oxygen on C-NO,and char containing one hydroxyl group is more conducive to the NO reduction.Calculating the rate-limiting step rate constants find that the rate-determining step rate constant for heterogeneous reduction of NO by char with two hydroxyl groups is significantly lower than that of char with one hydroxyl group.The promoting mechanism NO removal by micro-oxygen concentration and inhibiting mechanism of NO heterogeneous reduction by high oxygen concentration in low oxygen atmosphere are revealed from the kinetic level,which provides theoretical basis for deep air-staged combustion with low NO_x.The influence mechanism of oxygen and CO on the heterogeneous reduction of N₂O by char under high-temperature reduction zone of staged combustion are analyzed.By comparing the heterogeneous reactions of N₂O on the char surface without oxygen and containing oxygen,it can be found that the presence of oxygen is beneficial to the N₂O adsorption,but it is not conducive to the N₂O reduction,and is also not conducive to the reduction product N₂desorption on the char surface.The presence of CO can significantly reduce the desorption activation energy of N₂on the char surface,indicating that CO promotes the heterogeneous reduction of N₂O by char.On the other hand,char significantly reduces the rate-determining step energy barrier value of N₂O reduction by CO,which indicates that char has a significant catalytic effect on CO reducing N₂O,provides reactive sites for CO and N₂O,and promotes N₂O reduction.The comprehensive analysis shows that char and CO have a synergistic promotion effect on the N₂O reduction.In addition,CO combines with residual oxygen on the char surface to form CO₂,which significantly reduces the activation energy of oxygen direct desorption and is beneficial to CO₂ release.