低温等离子体对柴油机排放颗粒物组分净化效果的热重分析

Purification effect of non-thermal plasma on particulate matters from diesel exhaust using thermogravimetric analysis

为探究低温等离子体(non-thermal plasma,NTP)对柴油机排气中颗粒物(particulate matter,PM)不同组分的净化效果及作用规律。利用自行设计的NTP发生器以O2为气源生成NTP活性物质并喷射入柴油机排气中,在不同的反应温度下对PM进行净化试验。通过滤膜对反应前后的PM分别进行采样并进行热重分析。热重分析中采用变气氛的控制策略实现将PM中挥发性组分(volatile fraction,VF)和元素碳(elemental carbon,EC)失重过程的区分,并利用阿伦尼乌斯(Arrhenius)法对EC进行了氧化动力学分析。试验结果表明在反应温度为120℃时,NTP对PM的去除量最大,达到了66.79%,对EC的去除量也达到了各反应温度下的最大值。经NTP处理后,VF的挥发起始温度与终止温度无明显变化,PM中VF的质量分数下降了5.86%~13.90%,变化幅度随着反应温度的升高而提高。LVF(low volatile fraction)在VF中所占质量分数明显上升,表明NTP与VF中不同组分的反应速率有明显差异。EC在NTP的作用下氧化起始温度和终止温度降低了30~40℃。EC的表观活化能在NTP处理后从175.97~210.49 k J/mol降低至94.13~109.13 k J/mol。依据EC曲线变化可总结出反应温度的升高对NTP处理EC过程的影响主要体现在处于半氧化态的EC质量分数的上升。该文证实了NTP能够对柴油机排气中PM进行有效去除,为NTP应用于排气处理提供了试验依据。

Much attention has been paid to diesel engines,due to their lower operating cost,higher thermal efficiency and durability.However,diesel engine emits high quantities of particulate matter(PM),which poses a huge threat to human health and environmental protection.Non-thermal plasma(NTP)technology is a promising method to control diesel emission.NTP includes many types of active radicals,such as atoms,ions,electrons,and excited state molecules,which can react with other substances without additional energy.Carbon monoxide,hydrocarbon and PM in diesel engine exhaust gas could be oxidized and eliminated at the same time with NTP injected into it.PM is one of the most obstinate contaminants in exhaust gas,because the structure of PM is intricate,the carbon in PM is difficult to be oxidized and PM contains many kinds of organic matter.To investigate the effect of NTP on multifarious components in PM,a self-designed NTP reactor was used and NTP which was generated from oxygen was injected into exhaust gas.These tests were performed at different reaction temperatures which contained 80,120 and 160℃.Both raw PM and those treated by NTP were collected by filter membranes and detected in thermogravimetric(TG)experiments.The atmosphere was controlled during TG experiments,in which PM was heated to 450℃in nitrogen firstly,after cooled to 250℃,PM was heated to 700℃in oxygen.In this way,the volatilization behavior of volatile fraction(VF)and oxidation behavior of elemental carbon(EC)were monitored respectively in detail.The effect of NTP was evaluated by the change of start or ending moment of mass loss,and the differences among the TG and DTG curves.The Arrhenius equation was also used to extract the apparent activation energies and pre-exponential factors to estimate the change occurring in EC.The results showed that the greatest purification occurred at the reaction temperature of 120℃,and 66.79%of PM was removed.The activity of the active radicals increases with the rise of reaction temperature,facilitating the oxidation of the PM,however,the active radicals in NTP are labile at high temperatures and easy to decompose,with the content decreased,weakening the oxidation of PM.After NTP injection,the mass fraction of VF decreased,and the range increased with the raise of reaction temperature,which indicated that VF is easier to react with NTP than EC.The start moment and ending moment of VF volatilization scarcely changed.The mass fraction of low volatile fraction(LVF)was increased,manifesting there are some distinctions in the reaction between different kinds of VF and NTP.As for EC,the ignition temperature and burn out temperature decreased by 30-40℃after the reaction with NTP.The DTG curves became gentler,and the temperature corresponding to the first peak of DTG curves hardly changed with reaction temperature.The apparent activation energies of EC reduced from 175.97-210.49 to 94.13-109.13 kJ/mol after the reaction.The apparent activation energies were mainly influenced by nanostructure and oxygen functional groups.The nanostructure of PM became more regular and the oxygen content in EC increased after oxidized by NTP.Based on the TG results of EC and the change of apparent activation energies,a conclusion has been drawn that the effect of reaction temperature increasing on the rest of EC is incarnated mainly on the raising of the mass fraction of half oxidized EC.