The Study On Removal Of Diesel NOx And PM Emission By NTP-DPF Assisted With NH₃-SCR Catalytic Technology

Diesel engine is widely used because of its good dynamic performance and economic performance,but the environmental pollution caused by particle matter(PM)and nitric oxides(NOx)in diesel exhaust is attracting more and more attention.And the international diesel engine emissions control standards are increasingly stringent.There is a'trade-off'effect on PM and NOx emissions of diesel engines.It is difficult to significantly reduce NOx and PM emissions simultaneously by relying on traditional pre-treatment and in-machine purification technologies.However,in order to meet strict emission regulations,after-treatment technologies of diesel engines must be applied.In recent years,non-thermal plasma(NTP)technology has been applied in the field of diesel engine emission control due to its advantages of high efficiency,no secondary pollution and low dependence on oil.Selective Catalytic Reduction(SCR)using ammonia water as reducing agent can reduce NOx in diesel exhaust by more than 90%.However,the application of SCR technology requires low sulfur content in diesel and complex injection system.In the present study,NTP technology and SCR technology are combined to explore the mechanism of purification of PM and NOx simultaneously in diesel exhaust,so as to achieve complementary advantages.The main research work is as follows:(1)The structure of NTP generator is designed based on the principle of Dielectric Barrier Discharge(DBD).A static test system was set up to analyze and optimize the performance of coaxial NTP reactor.The effects of grid electrode length,discharge voltage,discharge frequency,gas type,gas flow and other factors on generator power consumption,single-period charge transmission and ozone(O₃)production were explored based on Monte Carlo calculation method and Lissajous graph method.As the electrode length increases,the discharge power and single-period charge transmission increase,and O₃ concentration increases gradually,while O₃production rate decreases gradually.The optimized generator can achieve dynamic matching of different energy powers and engine emission conditions,and then cooperate with DPF and NH₃-SCR catalytic system to simultaneously remove diesel PM and NOx emissions.(2)A series NTP-DPF test system for diesel engine PM purification was constructed.The variation of diesel engine particle emission before and after NTP was used under different working conditions was studied by using engine bench test platform.The real-time measurement of PM emission was carried out by particle size analyzer under constant speed variable load and constant load variable speed test conditions respectively.Taking the total quantity concentration,total mass concentration,particle size distribution,modal proportion and geometric mean particle size of PM emission of diesel engine as indexes,the influence law of engine load and speed change on PM emission of diesel engine was analyzed.The results show that the variation of working conditions has little influence on the particle size distribution of PM,which is unimodal.The geometric mean particle size of particle quantity concentration is obviously smaller than that of mass concentration,and the particle size distribution of mass concentration lags behind that of quantity concentration.Under the action of NTP,the quantity concentration of nuclear particles and ultrafine particles discharged from the front and back ends of DPF increased,and the quantity concentration and the proportion of the mass concentration increased,while the quantity concentration of polymerized particles discharged from the front and back ends of DPF decreased,and both the quantity concentration and the proportion of the mass concentration decreased.(3)Cu/SSZ-13 and Fe-?catalysts with better catalytic activity were selected and their physical and chemical properties and NOx catalytic conversion mechanism were investigated.Cu/SSZ-13 and Fe-?catalysts with good activity were prepared.The effects of active components and their proportion on the activity and selectivity of the synthesized catalysts were studied by temperature programmed reaction method.X-ray diffraction,specific surface area,inductively coupled plasma emission spectroscopy,X-ray photoelectron spectroscopy,hydrogen temperature-programmed reaction,temperature-programmed desorption,transmission electron microscopy,thermogravimetric analysis and UV-VIS diffuse reflectance spectroscopy were used to characterize the catalysts.The crystal phase structure,specific surface area,active component content,surface microstructure,elemental valence,reduction performance and ammonia adsorption performance of the catalyst were analyzed,so as to select the best experimental catalyst for active component.The addition of Ce on the surface can effectively improve the low temperature activity of the catalyst and widen the reaction temperature window of SCR,but the high Ce loading will lead to the reduction of the active site and decrease the activity of the catalyst.When Cu loading is 4%,the catalyst has the optimum low temperature activity and the widest active temperature window,and SCR has the best selective catalytic ability for NOx.Fe-?catalysts supported with 1.0wt%Ce have the best NOx removal performance and better N₂selectivity.(4)The catalytic removal of NOx from diesel engine by NTP and NH₃-SCR was studied.When the exhaust temperature is lower than 250?,the NOx conversion efficiency is low due to the reaction temperature window of SCR catalyst.However,the NOx conversion capacity can be improved to some extent by properly adjusting the ratio of NO to NO₂.When the ratio of NO to NO₂ is close to 1:1,the rapid SCR reaction can be achieved,and the NOx conversion rate is the highest.The combination of NTP and HC was used to promote the rapid transformation of NO to NO₂,and the influence law of the combination of NTP and SCR on the efficient transformation of NOx was explored.A Cu/SSZ-13 catalyst with 4%Cu content(Ce content of 1 wt%)and1.0Ce-1.0Fe-?catalyst were used in the SCR system.The diesel engine bench test system was established,and the comprehensive emission performance of SCR catalyst was tested based on THE WHTC and WHSC cycles of the national sixth standard.The influence of the SCR system on the removal of diesel engine NOx emission and the influencing factors of NH₃ leakage were explored.In addition,the durability and aging performance of SCR catalyst were evaluated.The results show that the actual conversion efficiency and ideal conversion efficiency of SCR catalyst are basically close to each other under different urea injection rates,and the maximum conversion deviation is only 1.46%.SCR catalyst has excellent stability and reliability.