Study On DeNO_x Technology By Selective Catalytic Reduction (SCR) For Heavy-Duty Diesel Engines

With the increasing shortage of fossile oil,aggravation of environmental pollution and human health,as well as stringent exhaust emission standards,it is the best method to develop the advanced technology about diesel engines and exhaust aflertreatments for reducing the pollutants from vehicles.In recent years,the practical application of diesel engines has become more and more important in medium/heavy trucks and passenger carriages,because they operate with a high air-fuel ratio,thus allowing fuel-economy improvements.Under many circumstances,it is possible to reduce the NO emission from automotive diesel engines using primary measures.However, because of more and more stringent emission regulations,it is harder to remove both PM and NOx simultaneously to meet the regulation limits by primary measures,and whilst being relatively inexpensive,primary measures are usually insufficient to meet the increasingly strict emission standards set by the legislation.It is now generally assumed that the Chinaâ…£emission standards for heavy-duty diesel engines proposed for the year 2010 will no longer be feasible by primary measures alone but will require additional aftertreatment techniques.The TWC(Three-Way Catalysts)technology can not be applied to the aftertreatment of exhaust from lean burn engines due to the low NO_x conversion in the presence of oxygen.Thus,it is the time for diesel engines to develop appropriate exhaust aflertreatments.The Selective Catalytic Reduction(SCR),using urea as reducing agent,is presently considered the most promising technique for the removal of nitrogen oxides from the exhaust of heavy-duty diesel vehicles.It has contributed to control the emissions from vehicles in Europe.However,the study about Urea-SCR technology is not self-dependent currently in our country although the native manufacturers have been starting the application in SCR technology on heavy-duty diesel engines.Due to the key technology and patents for SCR system which are held by manufacturers abroad,the price of this system is very high,so this system has been becoming the bottle-neck of the SCR techonogy extending and application in China.With the diesel vehicles are increasing significantly in our country,it is very urgent to reduce the pollutants from vehicles,and it has a rapidly increasing demand of the advanved SCR catalysts.The catalysts as one of the key parts for SCR technology for the removal of NO_x emissions from diesel vehicles,has a great gap to that of the abroad,and is lack of information and experiences in components,preparation,influence factors and optimizing target about SCR catalysts.It is dificult for us to obtain the information about their technics from the pubilic literatures,which is very strict to keep secret because of the demand of protecting their patents.Therefore,it is very significant to develop our own catalyst products to break the monopolization abroad and decrease the price of SCR system.This paper aims at developing the V₂O₅-WO₃/TiO₂ catalysts equipped with a 12-liter diesel engine through the measures of experimentation,theoretics and numerical simulation.The work can be divided into the following main topic:(1)In order to improve the intrinsic activity and widen the temperature window of the nanometre-class V₂O₅-WO₃/TiO₂ catalysts,the concentration of components and preparation technics is investigated.Through testing in series,the catalyst exhibits the optimum performance in the removal of NO,and restrains the oxidation of ammonia at high temperature.(2)During praparing the catalysts process,the activity and stabilization of the catalysts are not sufficient for the NO removal in the several tests primarily. Based on the sequential experiments,this paper shows a proposal of enhancing the activity of the catalysts by using vapor in the high temperature conditions,and that has proved correctly.There are two possible reasons for this phenomenon. On the one hand,the dusts on the catalyst surface can be cleared by the vapor with the help of the flowing gas,and more active sites will be exposed;on the other hand,based on Topsφe's mechanism which combines two cycles,involving the acid-base and the redox-base about the V/Ti-based catalysts,and the acid-base cycle involves V⁺⁵-OH species,whereas the redox cycle is associated with V⁺⁵=O species.The SCR reaction can happen quickly when the quantity of redox sites V⁵⁺=O and acid sites V⁵⁺-OH keeping in balance.The fraction of the acid sites V⁵⁺-OH decreases significantly when the hydrogen is removed from the catalyst due to being decomposed at high temperature and in the presence the oxygen in the calcining process,and interrupts the reaction path,so it exhibits insufficient activity in SCR process.Through several tests,there will be water appeared in the SCR process and the activity will be resumed slowly by the vapour under high temperature conditions,and it is not efficient apparently.Thus,the activity can be resumed more rapidly by vapor in high temperature conditions through adding vapor.This method not only saves the experiment time,but also reduces the false results.(3)The effects of SO₂ contents over the DeNO_x performance have been studied in this paper.The results show that SO₂ can enhance the activity at low/medium temperature and widen the temperature window of the V₂O₅-WO₃/TiO₂ catalysts. It is different between this paper's conclusion and that of others who considered SO₂ contents have negative effects on the catalyst activity in the SCR process. The author consideres that there are two possible reasons for this problem.On the one hand,the objects employed SCR system removing NO_x are various.The objects of this study are for mobile diesel engines removing NO_x,but others are for power plants.Compared to flue gas from power plants,it is tens and hundreds times about SO₂ contents lower than those from the diesel emissions.On the other hand,the contents of catalyst conponents and the preparation technics are different,and the performance of catalysts varies.Moreover,the aging test of 100h proved that the catalyst has the optimum anti-sulphur poisoning performance.The result shows that the SO₂ contents can accelerate the formation of particulate matters in the process of combustion in the cylinders although they can accelerate the removal of NO_x emission from diesel engines.(4)The effects of H₂O contents over the DeNO_x performance have been studied in this paper.The results show that the activity can be inhibited at low/medium temperature and the curves of NO_x conversion efficiency shift to higher temperature range by the effects of H₂O contents.In addition,there is a positive effect in oxidation of NH₃ in the presence of H₂O under high temperature conditions,and enhances the activity of the catalyst indirectly.(5)According to the analysis of the experimental results and the SCR reaction mechanism over vanadium/titanium catalysts,the reaction rate equation has been concluded.On the base of assumption and simplified treatments,the simulation model of SCR has been presented refering to the literatures.The results of the SCR model can offer the reasonable comparison with the 3D simulating results in the CFD simulation process.(6)As the realistic SCR converters,it is unpractical to ensure the uniformity of velocity and flux in the flow domain.In fact,the non-uninformity is controlled blow the acceptable range.Before studying the effect factors about uniformity,it is necessary to establish the rule for evaluating the flow distribution.It is very difficult to setup the CFD mesh for framework containing thin-wall structures, perforated plates and parallelled substrates.Thus,it is shortage of simulation about this type converter in public literatures presently and rules of evaluating the flow uniformity or non-uniformity.On the basis of overcoming these difficulties and succeeding in the mesh setup and simulation,this paper introduces a definition of non-uniformity index M for evaluating the simulation results through comparison one another.(7)The results of several design cases for SCR converters show that there is a rational range for the angle of the holes' distribution on the cylindrical perforated plate in which the flow distribution is optimum.Moreover,it is propitious to uniformity of velocity when there is an edge surrounding every substrate.It is necessary for further uniformity to simulate the detailed design.With the interface supplied by Star-CD codes,the presented submodel about SCR reaction is integrated into CFD software using FORTRAN program.The accuracy and rationality of the submodel are validated combined with experimental results finally.(8)The ESC and ETC tests have shown that the aftertreatment system based on the SCR process can reduce the NO_x emissions of the 12-liter heavy-duty diesel engines to value below the the limits of the forthcoming Chinaâ…£emission standards.Compared with SCR catalysts abroad,the testing catalysts can meet the demand of the removal of NO_x,but the testing catalyst is found to be less active than the catalysts abroad in ETC test result aspects.In a nut shell,the study on the activity and selectivity of catalysts is impotant for the application of SCR system.First,through comparison of the catalysts' perfomance of removing NO_x,the disadvantage of catalyst in praperation of catalyst can be found and will offer the guidance in the optimization catalyst aspects.Second,on the base of information about characteristics of catalyst,the structure of SCR converter can be optimized in the design process.At last,determination of the catalysts' working temperature is important because the reaction kinetics between ammonia and NO_x are sensitive to temperature.Adding insufficient reductant at higher temperatures limits the overall conversion while excess reductant at lower temperatures can lead to un-reacted ammonia slip.The research in this paper has some reference for the application of SCR techniques.