| Particulate matter(PM)emissions from diesel engine is one of the main causes of the haze,which can cause serious damage to the human living environment and health.Non-thermal plasma(NTP)technology has the advantages of safety,high efficiency and environmental protection in removing PM from diesel engine,and has gradually become a research hotspot in the postprocessing direction of diesel engines.In this paper,fiber membranes were used to sample the PM from diesel engine at a specified condition.A self-designed NTP generator was used to generate NTP active substances with oxygen as the air source,and the PM was oxidized in the reaction chamber of a special temperature control system for different durations.Thermogravimetric analyzer(TGA),Raman spectrum analyzer,transmission electron microscope(TEM),X-ray photoelectron spectrometer(XPS)and gas chromatograph-mass spectrometer(GC-MS)were used to analyze the influence of NTP on PM oxidation characteristics and microcrystalline structure,the influence of NTP on microscopic morphology and element content,and the influence of NTP on organic components and content in PM.According to the analysis results of experimental studies,the evolution of PM in the process of NTP oxidation was investigated.The conclusions are listed as follows:(1)TGA was used to separate the volatile fraction(VF)and element carbon(EC)in PM,and the changes of the oxidation characteristics of EC under different NTP action time were analyzed.The relationship between the microcrystalline structure and the oxidative activity of EC was analyzed by combining with Raman spectroscopy.The results showed that NTP had a significant removal effect on PM,and the oxidation removal efficiency of VF was higher than that of EC.When the NTP action time was less than 5 min,the oxidative activity of EC was decreased by NTP;while the NTP action time was more than 5 min,the oxidation activity of EC increased.The Full width at half maximum of peak G and peak D1 showed completely opposite variation trend,indicating that there is a certain mutual conversion relationship between irregular structure and regular structure in PM.With the increase of NTP action time,the degree of graphitization of PM increased first and then decreased,which was completely opposite to the variation trend of the oxidation activity of EC in the thermogravimetric results,which indicates that the oxidation activity of PM was directly related to the degree of graphitization.(2)TEM was used to obtain the microstructure of the PM,and the image of PM microstructure was image-processed to extract the feature parameters.XPS was used to perform full-spectrum scanning and narrow-scanning on the PM samples to analyze the changes in the element content and the morphology of PM.The results showed that with the increase of NTP action time,the microcrystalline structure in the primary carbon particles undergoes the process of reorganization of the short microcrystalline structure into the long microcrystalline structure,and then the decomposition of the long microcrystalline structure into the short microcrystalline structure.When the NTP action time increased to 5 min,the NTP active substances combines with the oxygen-containing functional groups to form COx,which reduces the oxidation activity of the PM.When the NTP reaction time is greater than 5 min,part of the C elements in the PM combine with the O elements of NTP to form the oxygen-containing functional groups and co-exist in the PM,which greatly increases the oxidation activity of the PM.(3)TGA was used to analyze the thermogravimetric curves of organic components in PM to explore the influence of NTP on the volatilization characteristics of VF,and GC-MS was used to analyze the influence of NTP on SOF components to explore the changes of PM organic components under different NTP action time.The results indicated that the volatilization characteristic of original VF of was significantly different from that of VF after NTP oxidation,but the change of NTP action time had little influence on the volatilization characteristics of VF.The physical stability of low volatile fraction(LVF)is higher than that of high volatile fraction(HVF),but the chemical stability is lower than HVF,that is,the original VF is more susceptible to oxidation by NTP.The main components of SOF in the original PM are n-alkanes,branched alkanes and oxygen-containing organic compounds.Alkanes are mostly long-chain and multibranched structures.After NTP oxidation,the long-chain and multi-branched alkanes in SOF will be decomposed into short-chain and less-branched alkanes.NTP has a very high oxidation decomposition efficiency for branched alkanes,and NTP can quickly reduce the alkyl functional group in SOF. |
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