Study On Influencing Factors And Reaction Mechanism Of On-line Degradation Of Diesel Engine Particulate Matter (PM) By Non-thermal Plasma (NTP)

Compared with gasoline engines,diesel engines have higher thermal efficiency,and their superior economy and power make them widely used in industrial and agricultural production and transportation.However,the harm of particulate matter（PM）released by diesel engine exhaust to human health and the environment cannot be ignored.As a new diesel engine post-treatment technology,Non-Thermal Plasma（NTP）technology has the advantages of wide application range,high conversion efficiency,low energy consumption,and no secondary pollution.NTP contains various types of free radicals,such as atoms,ions,electrons,and excited molecules,that can react with other substances without requiring additional energy.At present,the research on the removal of PM from diesel engines by NTP mainly focuses on the performance optimization of NTP generators,and most of the research focuses on simulating diesel engine exhaust.In this paper,a test bench for NTP online degradation of diesel engine PM was built,and the online degradation test of diesel engine PM under different loads and reaction temperatures was carried out through a coaxial cylindrical dielectric barrier discharge NTP generator.The particle size distribution of PM before and after NTP action was monitored by Engine Exhaust Particle Sizer（EEPS）,and the reaction temperature between NTP and PM was optimized by analyzing the oxidation removal rate of NTP on different modal particles.In addition,physical and chemical tests and analyzes were performed on PM before and after NTP,including thermogravimetric analysis（TGA）,transmission electron microscope（TEM）,Raman Spectroscopy（Raman Spectroscopy）,X-ray photoelectron Energy spectrometer（XPS）,etc.,to study the change rules of PM oxidation characteristics,micro-nano structure,graphitization degree of carbon particles,and carbon microcrystalline element composition.Through the study of the evolution of the physical and chemical properties of PM,the reaction mechanism of NTP oxidation to remove PM from diesel engines can be revealed more deeply,and it can provide an important reference for the application of NTP technology in the field of PM degradation.The research results show that NTP technology can effectively reduce the number of PM emissions from diesel engines,and has the advantage of low temperature（<200℃）degradation,which has broad application prospects.The main work and research results of this paper are as follows::（1）A test bench for NTP online oxidative decomposition of diesel engine PM was built,and PM oxidation decomposition tests at different reaction temperatures（50～180℃）were carried out for different engine loads（25%-100%）.EEPS was used to study the particle size distribution of PM before and after NTP reaction,and to analyze the oxidation removal rule of NTP on the concentration of nucleated particles,ultrafine particles and total particle number.The research results show that with the increase of load,the particle size distribution of PM transforms from unimodal to bimodal,the number concentration peak gradually shifts to larger particle size,and the nuclear state changes to the accumulation state.With the increase of reaction temperature,the removal rates of nucleated particles,ultrafine particles and total particle number concentration all showed a trend of first increasing and then decreasing under the same load.With the increase of load,the temperature corresponding to the removal rate of PM total particle number concentration gradually increased,from 90°C（25%load）to 130°C（100%load）.The optimal reaction temperature range for the removal rate of NTP on the number of particulate matter is 90～130℃,lower or higher temperature is not conducive to the degradation of NTP on PM.（2）Sampling the diesel engine PM before and after NTP under different loads（130°C）and different reaction temperatures（75%load）.Thermogravimetric（TGA）tests were performed on PM samples to study the volatility of volatile matter（Volatitle Fraction,VF）and the oxidation characteristics of element carbon（EC）in PM before and after the reaction.The research results show that after NTP action,the temperature corresponding to the maximum weight loss rate of VF decreases under all loads,and VF is more volatile after action.Under the same load,as the reaction temperature increased,the VF maximum weight loss rate shifted to a lower temperature,and the shift was the largest when the reaction temperature was 130℃.After being treated with NTP,the characteristic temperature of EC under different loads showed a downward trend,and the oxidation activity was enhanced.At all reaction temperatures,the light-off temperature T_s of EC is advanced.When the reaction temperature is lower than 130℃,the maximum oxidation rate temperature T_maxis lower than that of the original machine.When the reaction temperature is 130℃,the decrease of T_max is most significant.When the reaction temperature is higher than 130℃,T_max is higher than that of the original machine.（3）The effects of engine load,reaction temperature and other factors on the micro-nano structure of carbon particles,degree of graphitization and relative content of carbon and oxygen elements were analyzed by TEM,Raman spectrometer and XPS.The research results show that with the increase of load,the agglomeration degree of PM gradually increases,and the crystallite length,crystallite spacing and crystallite curvature of carbon particles gradually decrease.After the action of NTP,the floc structure of PM clusters gradually evolved into a chain structure.The long crystallites in the primary carbon particles were oxidized and decomposed to transform into short crystallites,and the crystallite spacing and crystallite curvature gradually decreased.With the increase of reaction temperature,the reaction of NTP and PM is mainly focused on the surface of carbon particles to gradually penetrate into the inner core,and the degree of graphitization of carbon particles as a whole increases.When the reaction temperature was 130℃,the content of amorphous carbon,oxygen-containing organic matter and functional groups in PM was the lowest,and the degree of graphitization was the highest.O is mainly present in PM in the form of C-O combination.With the increase of engine load,the relative content of C-O gradually decreases,and the relative content of C=O gradually increases.The increase of load and the oxidation of NTP can promote the conversion of part of C-O to C=O.