NO Reduction By Propane/Methane Over Iron

Energy and environmental problems have become the focus of concern in today’s world. Asthe major primary energy resource in China, coal is still at the dominant position of the energyconsumption. Nitrogen oxides emitted from coal combustion process is one of the major pollutantsfrom the thermal power plants, which places a great harm to human health and the livingenvironment. Strictly control of the NO emission in the process of coal combustion has become avery urgent task, so the researches of nitrogen oxides controlling technologies have importantacademic and engineering application values.Based on the previous study,this thesis presents the results of expeimental stduy on NOreduction by propane over metalic iron in a one-dimensional electrically heated ceramic tubularreactor at300-1100C with simulated flue gas of0.05%NO. The NO reduction efficiencies byC3H8over iron were compared to that by CH4at the same conditions. The further experimentallystudied about the effefct of water vapor and SO2on NO reduction by propane and methane overiron was carefully tested by a series of experiments. At the same time, metallic iron was replacedby industrial waste iron. In order to study the practicability of NO reduction by industrial wasteiron, NO reduction by methane on the surface of industrial waste iron was experimentally studied.The iron samples were anylyzed by SEM, XRD and EDS to investigate their surfacemicro-structure and components changes after the reaction in order to study the mechanism. Andthe In-suit FTIR was used to conduct a preliminary study of the microscopic reaction mechanism.The following main conclusions could be drawn from this thesis:(1) The experimental results showed that propane can effectively reduce NO to N2overmetallic iron. In N2atmosphere, NO reduction efficiency increases with the increase of thereaction temperature. More than95%of NO was reduced by propane over metallic iron when thetemperature was above900C, which created a large amount of CO. The amount of propane hadlittle effect on NO reduction efficiency. Propane can be partial oxidated and dehydrogenatedto propene by lattice oxygen provided by Fe2O3/Fe3O4, while iron oxide was reduced into metalliciron, keeping the sustainable NO reduction capability of iron. Propane was eventually oxidizedto CO2or CO after a series of reactions.(2) In simulated flue gas atmosphere, the NO reduction efficiency by propane over iron is closely related to the excess air coefficient “SR1” during the reaction period. When the excessiveair coefficient was lower than1.0, an excess of propane was able to reduce NO effectivelyby reburning mechanism directly and at the same time, to reduce iron oxide. NO reduction bypropane over iron was significantly high when the temperature was above900C. There was littledifference in NO reduction when there was and was not a burnout process. And whenSR1>1.0, the efficiency of NO reduction decreased significantly. The propane was largelyconsumed by an excess of oxygen, which prevented the effectively reducing of NO and iron oxide.Effect of SO2on NO reduction by propane over iron was rather small and could be ignored.(3) Comparison of the NO reduction by propane and methane showed that NO could bereduced more efficiently by propane than methane over iron. At the same conditions, the NOreduction efficiency by propane over metallic iron was higher than that by methane at700-900C,with less consumption of propane. Propane was dehydrogenated to propene by lattice oxygenduring the reaction. Propene belongs to unsaturated hydrocarbon, whose chemical bond energywas relatively low and easy to be broken. Thus propene was able to reduce NO over the metalliciron surface with higher activity better than methane.(4) The effect of water vapor on NO reduction by C3H8and CH4over iron wereexperimentally investigated. Results demonstrated that water vapor resulted in the oxidation ofiron, leading to the formation of iron oxides such as Fe2O3、Fe3O4, which affected NO reduction toa certain extent. NO reduction efficiency significantly decreased when water vapor was added intothe reactor at600-900℃, as compared to that when vapor was not added. On the other hand, as theoxidation of iron created a loose surface structure, NO could penetrate to the interior and reactedwith fresh iron. So NO reduction efficiency increased as the temperature increased.(5) In simulated flue gas atmosphere, the NO reduction efficiency by propane and methaneover iron under different excessive air ratios increased when water vapor existed. Especiallywhen SR1>1, the NO reduction efficiency improved remarkably after water vapor was added. Inwet flue gas atmosphere, SO2caused changes of NO reduction efficiency by propane and methaneover iron surface and translation of NO reduction curvilinear.(6) The metallic iron was replaced by industrial waste iron. The results indicated thatindustrial waste iron can effectively reduce NO with methane. In N2atmosphere,more than95%ofthe NO was reduced by methane over industrial waste iron at a temperature above1050℃,whichwas very close to that for NO reduction by iron. In the simulated flue gas atmosphere, with anexcessive air ratio being lower than1.0, NO reduced by industrial waste iron was more than NOreduced by metallic iron under the same condition. The existence of water vapor and SO2insimulated flue gas has little restraint on NO reduction by industrial waste iron. Study showed thatreplacement of metallic iron by industrial waste iron in NO reduction has a reliable feasibility and practical application prospect.(7) The results of In situ IR and TPD experiment showed that the capacity of iron oxide toadsorb NO is much higher than that of metallic iron. NO could be adsorbed on the surface ofiron oxide in the form of different bridged nitrates and nitro species groups. The formation ofthese adsorbed species played an important role in further selective catalytic reaction withreductants.(8) In the process of micro reaction of catalytic NO reduction by methane and propane overiron oxide, NO firstly adsorbed on the surface of iron oxide in the form of various nitrates andnitro species. Subsequently, reductants were adsorbed on the surface of iron oxide and reactedwith nitrates and nitro species, creating intermediate species during the reaction. In the presenceof oxygen, O2would participate in the competitive reaction between hydrocarbon reductantand nitro adsorption species, promoting the reaction as wellas resulting in the formationof R-COO-, CH3COO-as active intermediates for the reaction. NO was eventually reduced to N2through constant reaction with these active intermediate species.