Oxidation Property And Nanostructure Of PM Emitted By A Non-road Diesel Engine With The Effect Of NTP

With the more stringent emission regulations on diesel engines,effective after-treatment systems are necessary to satisfy the regulations.Also,the emissions emitted by non-road diesel engines account for a huge percent of the total emissions.As an alternative after-treatment technology,non-thermal plasma（NTP）presents excellent removal performance of particulate matter（PM）emitted by diesel engines.When diesel exhaust flows through the NTP reactor,part of PM is captured on the collection plate of NTP reactor under the influence of electric field force.With the accumulation of PM captured on the collection plate,the back corona phenomenon happenes that causes the re-entrainment.Re-entrainment leads to the suddenly drop of PM removal efficiency.To keep the excellent performance of the NTP reactor,the regeneration problem must be resolved.Detailed analysis of the oxidation behaviours and thermal kinetics of PM captured on the collection plate（PM aggregation）conduces to the investigation and design of the NTP regeneration device.The microstructures and Raman spectra of diesel PM,also,their correlations with oxidation behaviours and thermal kinetics were investigated.The oxidation behaviours and thermal kinetics of raw PM and PM escaped from NTP reactor（PM-NTP）were compared with those of PM aggregation.Oxidation behaviours and thermal kinetics of diesel PM both before and after pre-treatment in N₂ atmosphere were investigated.The results showed that the thermogravimetry analysis（TGA）curves of PM without pre-treatment were divided into two parts:the volatilization and oxidation of organic components,and oxidation of carbon black.Raw PM sampled at 60%engine load showed the largest content of the volatile organic compounds（VOC）,and the final oxidation temperature was the lowest.The final oxidation temperatures of PM aggregation sampled at different engine loads were affected by the engine loads slightly.The VOC content of PM obtained using TAG experiments performed in an air atmosphere was higher than that in a N₂ atmosphere.When PM was pre-treated in a N₂ atmosphere,the final oxidation temperature of PM aggregation sampled at 80%engine load was lower compared with it without pre-treatment,while it was higher for PM aggregation sampled at 100%engine load.The durations of PM oxidation in an isothermal condition were greatly dependent on the engine loads.The mean activation energy of PM calculated using Flynn-Wall-Ozawa（FWO）method was 85.18¹59.77kJ/mol and 113.89¹83.98 kJ/mol respectively for PM before and after pre-treatment in a N₂ atmosphere.The apparent activation energy increased greatly after PM being pre-treated in a N₂ atmosphere.The temperature zones of normalized differential scanning calorimetry（DSC）curves of raw PM sampled at 60%engine load was divided into two parts:200⁴00 ^oC and 400⁵80 ^oC.The temperature of the maximum heat release rate in DSC experiments was consistent with the temperature of the maximum mass loss rate in TGA experiments.The differences in oxidation kinetics calculated using TGA method with pre-treated PM and DSC-based method with raw PM were small.The microstructures of PM sampled at different engine loads and their correlations with oxidation kinetics were investigated.The results showed that raw PM presented onion-like structures with small and randomly arranged crystallites.PM-NTP and PM aggregation sampled at 60%engine load showed core-shell like structures with several void cores and orderly arranged shells.The accumulation degree during PM aggregation oxidation decreased,and the morphology went through onion-like structures,then,core-shell like structures,lastly,the orderly arranged band-like structures.Microstructure parameters（crystalline separation distance,crystalline length,and crystalline tortuosity）were greatly influenced by the engine loads,and the microstructure parameter changes after PM flowing through the NTP reactor were greatly dependent on the engine loads.The mean activation energy of PM increased with increasing crystallite length.The instantaneous activation energy and oxidation rate constant of PM aggregation sampled at 100%engine load showed excellent correlations with microstructure parameters:E=a·d^-3.75·L^5.71·C^-3.88;k=b·d^-21.02·L^6.93·C^-49.18.The dimensionless parameter A=d²·C/L² was put forward to evaluate the PM oxidation activity after being pre-treated in a N₂ atmosphere that higher A value meant higher oxidation activity.The oxidation activity dropped after being pre-treated in a N₂ atmosphere.Raman spectra of PM sampled at different engine loads were observed,and the correlations of Raman parameters and thermal kinetics were analyzed.The intensity ratio of Raman parameters I_D/I_G of PM changed greatly with the influence of plasma and the peak positions shifted slightly.I_D/I_G increased during the oxidation process for PM aggregation sampled at 60%engine loads while it increased and then dropped for that sampled at 100%engine load.The Raman parameters of I_D1/I_G,I_D3/I_G,A_D1/A_G,A_D3/A_G,and full width at half maxium（FWHM）showed no evident tendency with engine load.The I_D1/I_G increased and then dropped with increasing oxidation temperature,while it decreased continuously for I_D3/I_G.The average activation energy increased with increasing G band FWHM.The temperature at 50%mass loss decreased with increasing D₃ band FWHM,and the oxidation rate constant increased with increasing D₁ band FWHM.The crystallite size calculated using I_D1/I_G was similar to the results of HRTEM experiments.