Investigation On NO_X Conversion Of Diesel Engine Based On The Coupled System Of NTP-SCR

The main emissions of diesel engines are NO_X and PM.With the increasingly stringent emission regulations of diesel engine,the way to reduce NO_X emissions of diesel engines has become a focus in the research field of engine emissions.Relying on the internal purification of engine solely could not meet the requirements of the increasingly stringent emissions regulations,SCR is one of the most popular technologies being used to reduce NO_X.The urea-SCR system has been widely used for taking advantage of urea aqueous solution as the carrier of NH₃,which has good chemical properties,convenience of storage and transportation.However,there still exist some problems in this kind of technology such as relatively low conversion efficiency under the condition of low temperature,urea crystallization,etc.At present,the main way to increase the NO_X conversion efficiency in the range of low temperature is to add DOC in the front end of SCR system so as to realize the fast SCR reaction.More than that,DOC has high requirement of sulphur content in diesel so it could lead to the failure of noble metal catalyst in DOC due to the sulfur poisoning.In this thesis,in order to solve the scientific problem of low conversion efficiency under the condition of low temperature for SCR system,the coupled system of NTP-SCR has been designed and manufactured.Firstly,the performance of NTP reactor was tested by air discharge experiment and the working parameters of NTP reactor were optimized.Then the feasibility of NO oxidation in diesel engine by NTP reactor was studied through the experiment of simulative diesel engine exhaust.Finally,the experimental research of NO_X reduction through simulative diesel engine exhaust based on the coupled system of NTP-SCR was conducted.The main contents of this study are as follows:（1）A set of air discharge experiment system of double-dielectric NTP reactor was designed on the basis of the principle of dielectric barrier discharge and harmful gas conversion in non-thermal plasma.The effects of air flow、applied voltage peak-peak value（VP-P）and discharge frequency on discharge characteristics were studied.Besides,the effects of air flow,VP-P and discharge frequency on the volume fractions of NO and NO₂ on discharge region were analyzed and some spectral information of discharge region was collected.（2）Based on the principle of dielectric barrier discharge and conversion of harmful gas in NTP,a set of experimental system was designed to use double-dielectric for NO oxidation in NO/O₂/N₂ mixture gas.Moreover,the mixture gas flow,the initial concentration of NO and the influence it has brought to NO_X volume fractions in discharge region were analyzed.The spectral information of discharge region was also collected.（3）In order to solve the problem of low conversion efficiency of NH₃-SCR system in diesel engine under the condition of low temperature,a set of simulated exhaust testing system based on the coupled system of NTP-SCR was designed to reduce the NO_X emission of diesel engine.Except that,the effect of SCR temperature,airflow,NO₂/NO and initial concentration of NO and O₂ on the volume fraction of NO_X in simulated exhaust were analyzed.And comparing to the experiment of SCR system,it can be found that the coupled system of NTP-SCR will increase the NO_X conversion effectively under the condition of low temperature.