Investigation On NOx Emission Of Coal-fired Boilers And Manufacture Of Catalyst For Flue Gas Denitrification

Concentrating on the higher emission of pollutant in the gas exhausted from power station,the NOx emission characteristics,the effects of low NOx combustion technology and low NOx burner were investigated.With theory analyse and experiment, the influence of compositions,additives and the molding technologies on the catalytic activity and molding performance were first investigated in laboratory.The molding honeycomb catalyst was obtained by extruding method.The influences of gas compositions and experimental parameters on catalytic performance of honeycomb catalyst were investigated by simulated SCR reaction on middle-sized combustion test-bed.All these analysis and tests lead to a kind of honeycomb catalyst with excellent molding and catalytic performances,which can be used for SCR reaction.1.The investigation of NOx emission of boilers with different types,capacities, coal types,low NOx technologies and burners was systematically done in Shandong electric network.As the low NOx technologies early used were in low level,the low capacity boilers all have high NOx emission from 800 to 1200mg/m³.The 300MW units put into production early have NOx emission from 600 to 1100mg/m³,and the value increases to 800-1100mg/m³ for boilers burning lean coal.The NOx emission of the boilers with big volume installed recently was reduced to 300-700mg/m³,due to the new low NOx technologies and burners in these units.However the NOx emission value would still reach to 800-1000mg/m³ if low NOx technologies and burners could not work sufficiently due to the delay of combustion regulation for boilers burning lean coal or bituminous coal.2.The influences of coal type,boiler configuration,burner mode,boiler operational conditions and parameters on NOx emission were studied.Among all the influencing parameters,coal type is the most important factor.Among all the operational parameters,the oxygen content and distribution in furnace have the most obvious influence on NOx emission.W-flame boiler burning lean coal has the highest NOx emission from 1000 to 1500mg/m³.Boiler with quadrilateral tangential straight-flow burner comes next.The emission of boiler with low swirl-flow burner is lower than that of quadrilateral tangential straight-flow burner when the same coal is burnt.CFB boiler has the lowest NOx emission no more than 200mg/m³.Boiler with quadrilateral tangential straight-flow burner burning bituminous coal has about 50% NOx emission lower than that of boiler burning lean coal.With excess air coefficient and boiler load decreasing,the reduction rate of NOx emission of boiler burning bituminous coal is larger than that of boiler burning lean coal.The result shows that the NOx emission is largely impacted by boiler operational conditions and parameters,and it can be reduced about 10-20%by adjusting combustion condition with boiler operational conditions and parameters changing.3.Much regularity was obtained by investigation on the catalytic performance of V₂O₅-WO₃-MoO₃/TiO₂ powder catalyst:the reaction rate and activity of SCR increases with the content of primary ingredient V₂O₅ growing.When the V₂O₅ content grows to 1%,there is high activity and a wide active temperature range from 200℃to 300℃. However,with the content of V₂O₅ increasing further,the catalytic activity decreases, its active temperature range becomes cabined.The W in catalyst mainly increases the activity at high temperature.But WO₃ would take the place of V₂O₅ active sites when the W content is higher than 7%,which will reduce the V₂O₅ active sites and consequently decrease the catalytic activity.The Mo in catalyst also improves the catalytic activity at high temperature,meanwhile it increase the surface acidity and NH₃ adsorption,which improves the selectivity of the reaction.But these benefits would not be remarkable when the Mo content is higher than 7%.Finally a kind of catalyst composing of 1%V,5%W,5%Mo was obtained,having the highest activity above 95%.4.The influences of active substances,additive species and their contents on molding performance,as well as the activity of catalyst have been investigated.The molding honeycomb catalyst was obtained by extruding method.The result of research shows that additives have some influences on the molding performances and activity of the catalyst.Both additive species and their contents have large impacts on the molding performances and activity of the catalyst.Through adjusting additives and their contents, changing the molding technologies,a recipe of one honeycomb catalyst with good molding performance and high activity was obtained finally.5.The characterizations of the catalyst and the influences factors on them were investigated by MPI(mercury intrusion porosimetry) and SEM(scanning electron microscope).The pore characterizations are largely impacted by active compositions, carrier characterizations,additive species and molding technologies.Catalysts made by mechanical blend method have bigger pore volume and pore area than those of catalysts made by dipping method.Appropriate additives can benefit molding performance and pore structure of the catalyst.Extruding process has slight negative impacts on pore structure for honeycomb catalyst resulting in the decrease of pore volumn and pore area. With the calcining temperature increasing,the decreasing pore area induces the drop of activity for honeycomb catalyst.The catalyst calcined at 350℃has the biggest pore volume and pore area among all of the honeycomb catalyst,so it has the highest activity. The investigations by SEM show that:For catalyst with good molding performance and high acitivity,calcined at 350℃,additives and active substances and TiO₂ carrier bond well on its surface,which form regular granules and honeycomb pores on surface of granules.Catalysts with different pore structures represent different catalytic performances.6.Tests on catalytic performance of the honeycomb catalyst were done by simulation SCR reaction on middle-sized combustion test-bed.The results show that: The NOx conversion is impacted by input ratio of NH₃ and NO,initial NO concentration,reaction temperature and space velocity.When the input ratio of NH₃ and NO is higher than 0.85 while lower than 1.1,the NOx conversion increases with the ratio growing,but this effect would not be remarkable if the ratio is above 1.1.For consideration of NH₃ escape,the ration is preferably fixed at 1.Within the NO concentration between 250 and 830ppm,the NOx conversion is not affected obviously by the initial NO concentration.The NOx conversion decreases with space velocity growing.For middle-sized combustion test-bed,the space velocity is 2000/h.Reaction temperature has large impact on NOx conversion,the NOx conversion is less than 70% when the temperature is between 100℃and 200℃.When the reaction temperature is above 200℃,the NOx conversion rapidly increases with temperature growing.The NOx conversion is above 90%at 315℃,and exceeds 95%around 375℃,then it slightly decreases with temperature growing.Therefore the active reaction temperature ranges from 300℃to 425℃,which is qualified for SCR reaction.This project is co-sponsored by International Science Cooperation Major Project (Technology for the control of nitrogen oxides emissions,NO.2004DFA7700) and Shandong Power Group Science Project(Experimental study on NOx emission and control technology of high-capacity boilers,NO.2008ZB-19).