Experimental Study And Quantum Chemical Modeling On NO Reduction By Iron And Its Oxides

With the development of economy, coal is still the major energy resource in China. Nitrogen oxides emitted from coal combustion process is one of the major pollutants, which places a great harm to human health and the living environment. So the researches of nitrogen oxides controlling technologies have important academic and engineering application values.The paper first introduced NOx control methods. The shortcomings of selective catalysis reduction (SCR) were particularly introduced which is used more widely in current, and NO reduced by iron was discussed in detail. At the same time, the paper presents a short overview on recent quantum chemical calculations in the denitration areas, which detailed mechanism should be further investigated. We propose a research on NO reduction by iron with experiment and quantum chemical calculations.Experimental study on NO reduction by iron and its oxides was investigated in a one-dimensional electrically heated ceramic tubular reactor. Quantum chemical calculations simulate reaction route by molecular level. The following main conclusions could be drawn from this thesis:(1) Based on the experimental results, the oxidation of iron by NO in N2 was discussed:At 600℃, the iron is oxidized to Fe3O4,Fe2O3 and FeO by NO; at 700-800 ℃, Fe2O3 starts generating, while FeO completely disappeared; when the temperature rises to 900-1100℃, FeO began to appear again, its main oxidation product is FeO and Fe2O3. By comparing NO removal efficiency at different temperatures, we found that the optimal temperature about iron reacts with NO is 700℃.According to the Birks mechanism of oxidation of iron, FeO is considered to be the main product of NO oxidation of iron, the establishment of quantum chemical models calculate the main channel of Fe and NO reaction:NO + Feâ†'OFeN, NO+OFeNâ†'FeO+N2O, Fe+N2O-â†' FeO+N2, the activation energy of the reaction is 131.0kJ/mol.(2) The experiments on reaction of COwith NO show that CO can directly reduce NO to N2, but its efficiency is below 20%. Through quantum chemical calculation we obtain the main channels about CO, CH4 reaction with NO respectively, i.e., CO+NOâ†'CO2+N, N+Nâ†'N2, its activation energy is 170.7kJ/mol; CH4 + NOâ†' CH3+HNO, CH4+HNOâ†' CH3+ HNOH, HNOHâ†' N+ H2O, N+Nâ†'N2, its activation energy is 442.7kJ/mol.(3) The XRD results show that the CH4 or CO can reduce iron oxides to metallic iron through partial oxidation over iron oxides. The SEM results show that the surface microstructure of the iron oxides become more porous which promote the adsorption and reduction of NO. Quantum chemical simulation of NO reduction by CO atmosphere and iron was carried out. The results show the activation energy of NO reduced by Fe is smaller than that by CO, so the main reaction channel is:NO+Feâ†' OFeN, NO+OFeNâ†' FeO+N2O, Fe+N2Oâ†' FeO+N2, CO+FeOâ†' CO2+Fe, its activation energy is 131.0kJ/mol.(4) Study results shows that reduction of NO is restrain by Oxygen. By adding CH4 to the experimental system, we found that removal efficiency was maintained 100h and the downward trend does not appear. However, this effect can not be reached by adding CO. Research results on the mechanism showed that NO was reduced with methane via two major routes: one is by reburning while the other is methane reduced iron oxides to metallic iron through redox reactions an then NO was reduced by metallic iron. At the same time, the intermediate products during NO reduction by methane are converted by iron oxides.(5) Through quantum chemical calculations we obtain three channels about generation of HCN:CH3 + NOâ†'HCN + H2O, CH2 + NOâ†'HCN + OH, CH + NOâ†'CN + O, this result is consistent with experimental studies. By calculating the activation energy, we obtain main reaction channel:CH3 + NOâ†'HCN + H2O, and its activation energy is 263.9kJ/mol.(6) Experimental studies have shown that SO2 in the flue gas did not influence NO reduction. NO and SO2 can be reduced by iron at the same time. However, the metallic iron was consumed additionally due to the removel of SO2, so its duration to effectively reduce NO becomes shorter. By adding CH4 to the experimental system, we found that the NO removal efficiency was maintained at 95% during 100 hours, so we can determine the SO2 has no poisoning effect to iron.The present results show that the iron can efficiently reduce NO. Methane can improve the NO reduction efficiency over iron or iron oxides and SO2 in the flue gas did not influence NO reduction. The result of quantum chemical calculations was exactly consistent with the result of experiment, which benefits for the analysis and observations of experimental data, and provides a theoretical basis for the further study of mechanism about NO reduced by iron.