Studies On The Oxidation Purification Of Particles And The Change Of Physicochemical Properties Caused By NTP

Particulate matter（PM）in the exhaust gas of diesel engine has become one of the main constituent part of airpollutant,and poses great threats to human health.In recently years,non-thermal plasma（NTP）technology has been a research hotspot in the field of diesel engine after-treatment.In this paper,NTP was generated from O₂ by using self-designed NTP reactor and was injected into the exhaust gas to eliminate PM.The experiments were carried out under various temperatures of exhaust gas and engine loads,and the PM samples were collected with and without NTP injection.To evaluate the purification effect,the change of nanostructure and oxidation reactivity of PM after the reaction with NTP,multiple measurement equipments and analysis methods were used,such as engine exhaust particle sizer（EEPS）,thermo-gravimetric analyzer（TGA）,transmission electric microscope（TEM）,Raman spectroscopy and X-ray photoelectron spectrometer（XPS）.The major research work and achievements of this dissertation are listed as follows:1.The size distributions of the PM concentration with and without NTP injection were measured by EEPS under various temperatures of exhaust gas and engine loads,and the purification efficiency of the particles in various modes was analyzed.The results showed that the PM concentration reduced obviously after NTP injection,and the maximum of purification efficiency fell at the reaction temperature of 120 ^oC under every engine loads.The purification efficiency was 63.72%when the engine load was 25%.However,the purification efficiencies of the particle in different modes showed significant difference.It reached 80.12%when we focused on the purification of nuclei mode particle,the efficiency of ultrafine particle was lower than that of nuclei mode particle,and the efficiency of accumulation mode particle was minimum among these modes.2.The purification effect was also evaluated based on the change of the weight of different components in PM,which mainly refer to volatile fraction（VF）and soot.The PM samples were tested in TGA,and the volatilization behavior of VF and the oxidation behavior of soot were monitored separately by the strategy that the sample was heated twice in pure N₂ and oxidizing atmosphere in sequence.Based on the oxidation behavior of soot,the oxidation kinetic parameters of soot were calculated through Arrhenius equation.The results indicated that 66.12%of PM was removed by NTP which is the maximum when the temperature of exhaust gas was 120 ^oC.The proportion of VF in PM reduced remarkably after the reaction with NTP.The ignition temperature and final oxidation temperature of soot decreased after being oxidated by NTP,and the apparent activation energy of soot showed the same trend.However,there is no apparent relationship between the falling range and the temperature of exhaust gas.3.The microstructure of PM was observed by TEM.To describe the microstructure accurately,the graphene layer length（La）and separation distance of graphene layer（d）were defined,and these parameters were extracted by Matlab.Based on the change of microstructure of PM,the oxidation process was discussed.The results showed that the structure of PM was disintegrated during the oxidation.When the temperature of exhaust gas was 160 ^oC,the maximum of thefalling ranges of mean La and the mean d appear.The purification process may divide into two parts.One is the excited species diffused into the inside of particles,and these particles separated into several smaller ones.The other is the reaction occurred on the surface of primary carbon particles,the graphene layers in primary carbon particle were broken and the diameter of these particles reducescontinuously until they were totally removed.4.The curves of first-order Raman spectra and XPS spectra of PM were fitted to analyze the change of graphitic lattice and the content of chemical element.The results indicated that the graphitic lattice in PM became more regular,and the amorphous carbon in PM reduces after the oxidation by NTP.However,the change showed no obvious relationship with the temperature of exhaust gas.The content of oxygen in PM increased after the oxidation by NTP.With the increase in the temperature of exhaust gas,the content of oxygen increased,and the way which oxygen connected with carbon changed from C-O to C=O.This phenomenon indicates that the increase of reaction temperature may enhance the reactivity of excited species in NTP.