Experimental Study On NO Reduction By Methane Over Iron And Its Oxides

Coal is the major energy resource in China. Nitrogen oxides emitted from coal combustion process is one of the major pollutants from the thermal power plants, 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.This thesis presents the results of expeimental stduy on NO reduction by methane over iron and its oxides in a one-dimensional electrically heated ceramic tubular reactor at300-1100℃with simulated flue gas of0.05%NO. Based on the results of NO reduction by metalic iron, the NO reduction by methane over iron/iron oxides was further experimentally studied and the effefct of SO2was carefully tested by a series of experiments. The iron samples were anylyzed by SEM, XRD and EDS to investigate their surface micro-structure and components changes after the reaction in order to study the mechanism. The following main conclusions could be drawn from this thesis:(1) The influence of O2, CO2and NO concentration on catalytic reduction of NO by iron was firstly investigated. The results showed that oxygen preferentially reacted with iron in the competition reaction with NO on the surface of iron, leading to a significant decrease in NO reduction. The NO reduction efficiency exceeded98%when the temperature was above700℃in N2atmosphere, while the efficiency was lower than24%when O2was added in. There was a relatively small impact of CO2on NO reduction because its oxidizing resistance was weaker than that of oxygen. The effect of NO concentration on NO reduction was determined by temperature, which disappeared when the temperature exceeded700℃.(2) The evolution of the iron oxides at different temperatures during the reduction of NO was analyzed using XRD. Results showed that iron was oxidized to Fe3O4Fe and FeO at600℃, and Fe2O3generated with the disappearance of FeO at700-800℃.When the temperature rised to900-1100℃, FeO appeared again and the major oxides were FeO and Fe2O3.(3) In N2atomsphere and at800℃, the NO reduction efficiency was still as high as96%after a continuously test during28.6h. When the durable test was beyond68.2h, the NO reduction efficiency decreased to below27%. Temperature influenced the durableness of NO reduction by iron. The higher the reaction temperature was, the shorter the durableness of NO reduction by iron became. SEM results showed that the higher the temperature was, the coarser the iron oxide particles were. CO could improve NO reduction efficiency by reducing iron oxides into metallic iron, but CO could not continuously reducing iron oxides into metallic iron at a high level in simulated flue gas atmosphere, resulting in a poor persistence of NO reduction in the flue gas. NO reduction efficiency decreased to18.3%after only5.05h continuously reaction with iron in simulated flue gas atmosphere.(4) NO reduction by methane over iron oxides in N2atomsphere was experimental tested. The iron oxides was the iron sample after acontinuously reaction with NO at800℃during68.2h. Results showed that NO reduction efficiency was greatly improved when methane was added into the reactor. NO reduction efficiency was52%at700℃and exceeded95%above800℃. Fe2O3showed better performance of complete oxidation and partical oxidation of methane at low temperature conditions (570℃) and high temperature conditions, respectively. Iron oxides coule be reduced in the order of Fe2O3-Fe3O4-FeO-Fe. The test results of SEM and EDS demonstrated that temperature greatly affected the deposition of carbon and the formation of carbon nanotubues.(5) Temperature and SR(stoichiometric ratio) demermined the efficiency of the NO reduction by methane over iron and its oxides in the simulated flue gas containing O2, CO2, and CH4with N2balanced, and the efficiency remained at a high level after the burnout zone in the presence of iron and its oxides. NO could be reduced through reburning and direct catalysis by the excessed methane when SR<1.0, and iron oxides could be reduced into metallic iron by methane at high temperatures, which further improved the NO reduction efficiency. While the efficiency decreased significantly when SR=1.0. In this case the methane was completely oxidized into CO2and H2O, which might compete with NO in reactions of iron in the iron surface, thus the efficiency decreased. The NO reduction efficiency were less than20.0%in the presence or absence of iron and its oxides when SR>1.0, and the efficiency was slightly higher with the presence of iron than that of iron oxides. We believed that the excessed oxygen inhibited the reaction of iron and NO.(6) The durable experiment results of NO reduction by methane over iron showed that, with respect to CO, methane could provide a better NO reduction under the same conditions. The XRD results showed that the iron was oxidized into Fe3O4and FeO. Oxygen atoms were found more than iron and carbon atoms according to the EDS results, which indicated that the iron samples were oxidized completely.(7) When the temperature was above700℃, SO2had very little inhibitation on NO reduction.0.05%NO in N2base was completely reduced by iron and1000ppm SO2could be removed at the same time when the temperature exceeded700℃. SO2was consumed by physical adsorption, weak chemisorptions and strong chemical adsorption. The influence of SO2on NO reduction by methane over iron in the simulated flue gas was found when the temperature was below800℃, while when temperatuure ws above800℃, SO2improved NO reduction. SO2reduced the persistence of NO reduction by iron, and the XRD results showed that the iron was oxidized to Fe2O3after the durable test when there was SO2in the reactor.The iron mesh roll could efficiently reduce NO in N2atmosphere, while in the simulated flue gas containing O2and CO2, the iron alone was unable to provide a high NO reduction efficiency. The addition of methane could significantly improve the NO reduction efficiency at appropriate temperature and stoichiometric ratio. Experiment results showed that the iron samples’ resistance to SO2poisoning characteristics were also excellent, which indicated that it is a new way to remove NO cheaply and efficiently.