Study On The Characterization And Mechanism Of NO Reduction With Propene And Iron

Nitrogen oxides（NO_x） is one of the main pollutants in the combustion process of fossil fuels, which has a great harm to human health and living environment. Therefore, the effective control of NO_x emissions from coal combustion has become a hot research topic in the academic and industrial fields. In recent years, propene has been widely used in selective catalytic denitrification（SCR） technology. Commonly used catalysts are molecular sieves, supported noble metals and metal oxides and so on. However, the preparation of various catalysts for selective catalytic reduction technology are complex and expensive, and the conversion of NO to N₂ is relatively low at present. Previous studies have indicated that the metal iron with C₁-C₃ has higher efficiency of the NO reduction, and is not affected by SO₂ and H₂ O. On the basis of previous studies,the characteristics of NO reduction by propene over iron was studied in this paper.It was tested in a ceramic tube flow reactor at 300¹100°C in N₂ atmosphere and simulated flue gas conditions,which compared with the characteristics of the NO reduction by propane over iron under the same conditions. Besides,the effect of SO₂ and water vapor on the reduction efficiency of NO at different conditions was also investigated. In order to further analyze the reaction mechanism, XRD, SEM and EDS techniques were used to analyze the composition and surface microstructure of the iron sample. The main results obtained are as follows:（1）In N₂ atmosphere,propene was effective to reduce NO with iron.The reduction efficiency of NO increased with the increase of the temperature.More than 95% of NO was reduced above 800°C. Increasing the volume fraction of propene can enhance the reduction efficiency of NO. In the reaction process, the oxidation of propene will produce a small amount of CO. The more amount of propene, the more amount production of CO. When propene is used as a reducing agent, propene can reduce iron oxide to iron, while propene is oxidized to CO₂ or CO, which can ensure the continuous reduction of NO.（2）In simulated flue gas atmosphere, the reduction efficiency of NO was gradually increased with the increase of the temperature. More than 90% of NO was reduced above 900 °C at fuel-rich conditions by propene with iron respectively. On one hand, NO was directly reduced by iron while propene reduced the iron oxides to iron; on the other hand, propene reduced NO via reburning reaction while the redurning intermediate products were reduced by iron. The reduction efficiency of NO is low at oxygen-rich conditions above 900°C. The acceleration of the combustion reaction of propene leads to the decrease of the reduction efficiency of NO.（3）The reduction efficiency of NO by propene and propane over iron was compared under the same conditions. In N₂ atmosphere, the reduction efficiency of NO by propene is slightly higher than propane blew 700°C. At high temperature, the reduction efficiency of NO has very small difference by propene and propane at fuel-rich conditions. The denitrification efficiency by propane is higher than that by propene at oxygen-rich conditions. The main reason should be attributed to the difference in the combustion characteristics of propene and propane at high temperature and oxygen-rich conditions, that is, propene was more buring reaction than the NO reduction reaction.（4）The effect of SO₂ on NO reduction by propene over iron was experimentally studied. In N₂ atmosphere, SO₂ has slight inhibitory effect on the reduction efficiency of NO blew 900°C. The efficiency of NO reduction with or without SO₂ is very close to 100% above 900°C. In simulated flue gas atmosphere, the reduction efficiency of NO with or without SO₂ is slightly higher than without SO₂.With increase of the volume fraction of SO₂, the reduction efficiency of NO showed a slight decrease trend.（5）The effect of water vapor on NO reduction by propene over iron was experimentally studied. In N₂ atmosphere, after adding water vapor, the reduction efficiency of NO by propene over iron is lower than that by propane under the same conditions. For example, the reduction efficiency of NO by 0.2% propane is higher by about 20% with 7% H₂ O at 900°C,which compared with propene under the same conditions.In simulated flue gas atmosphere, the reduction efficiency of NO is higher with 7% H₂ O than without H₂ O. For example, when the excessive air ratio is 0.9, the reduction efficiency of NO with or without water vapor were 93%, 85.5% at 1000 °C, and increased by about 7.5%.（6）XRD, SEM and EDS techniques were used to analyze the composition and surface microstructure of the iron sample after reaction. Propene can reduce iron oxide to iron, so as to ensure the continuous reduction of NO. In N₂ atmosphere, the main oxidation products were Fe₃O₄, FeO and Fe₂O₃ with 0.1% C₃H₆,which showed that the iron oxide was partly reduced by propene. The products were mainly Fe and a small amount of Fe₃O₄, which showed that the iron oxides were almost reduced to iron.This also showed that more propene can enhance the ability of reducing iron oxide.（7） It showed that propene was adsorbed on the surface of iron oxide and interacted with nitrogen species formed by NO adsorption in the presence of oxygen by situ IR. O_xygen may be involved in the competition reaction, which promoted the formation of R-C=O aldehydes and other active intermediate species.These species will be eventually reducted to N₂.