柴油机排放颗粒物的热重特性分析

Thermogravimetric characteristics analysis of particulate matter of emission of divided diesel

为了解不同粒径颗粒物在特定氛围下的氧化特性,该文利用MOUDI采样器收集到的柴油机颗粒物,在纯N2及纯O2环境下,对0.18～0.32、0.32～0.56、0.56～1.00和1.00～1.80μm4个粒径级颗粒分别进行热重分析试验。结果表明,随着粒径级的增大,颗粒物中水分和SOF(solubleorganicfraction)的含量下降,而干碳烟(soot)和无机盐的含量增加。在纯N2氛围下,颗粒在SOF挥发阶段随着颗粒物粒径级减小,其SOF含量和失重峰值速率随之增加;但在soot热解阶段不同粒径级的失重速率趋同。程序升温终了时,各粒径级颗粒的热重曲线在纯N2氛围下缓滞停在不同位置,而在纯O2氛围下则渐趋归一。随着颗粒物粒径级的减小,热重曲线呈下降趋势,颗粒越细则越易升温(氧化)失重;而在纯O2氛围下,各粒径级在SOF挥发阶段表现出与纯N2氛围下一致的规律但失重速率峰值明显增加。在soot热解阶段,随着颗粒物粒度减小,比表面积增加使其吸附氧的能力增加,发生化学反应的活性增大,颗粒氧化的起燃温度降低,且起燃时刻对应的失重速率增加;各粒径级颗粒物的失重速率峰值出现在600～640℃之间,随着颗粒物粒径增大其soot所占含量随之增多,热解失重速率峰值亦显著增加。研究结果可为颗粒物处理的技术措施提供基础物性数据,有助于推动颗粒物处理装置的改进和优化。

Particulate matter （PM）, which contains soluble organic fraction （SOF）, soot and inorganic salt, has been one of the main pollutions from diesel engine. Longtime exposure to the particulate matters especially those smaller than 2.5 micrometers （PM2.5） which can go directly to the alveoli of the lungs, is a major health hazard. In addition, many studies have revealed that the particles of smaller size would bring about greater harm to the human body. Due to the potential health risk in urban areas, the elimination of fine and ultrafine particulates emissions from diesel engine has attracted much attention in recent years. Meanwhile, the strict regulations for PM emission have been enforced in many developed countries. The micro-orifice uniform deposition impactor （MOUDI） is a favorable apparatus for measuring particles size distribution of atmospheric aerosol base on aerodynamic method. It can not only obtain the particle size distribution but also collect the particles in the different size ranges after classification, which extremely facilitates further research for chemical components, microstructure, oxidation characteristic and biological toxicity of the divided particle samples. Thermogravimetric analysis （TGA） has been widely used as an analytical method for kinetic of chemical reactions. Thermal gravimetric analysis means to investigate the relationship between the material weight and temperature under the condition of programming temperature rise. The curve of sample weight then can be obtained with the temperature. Derivative thermal gravimetry （DTG） curve, the first order differential to TG curve, can reveal the features of mass variation with the temperature. To study the ignition and oxidation properties of particulates, thermogravimetric method would be an effectual option. The particulates collected by MOUDI were divided into four size ranges which were 0.18 ~ 0.32μm, 0.32 ~ 0.56μm, 0.56 ~ 1.0μm and 1.0~1.8μm Thermogravimetric experiments on the divided particulates were carried out in the atmosphere of N 2 and then O 2 . The experimental results showed that the moisture and SOF contents in PM increased with the increase of particle size, while the soot and inorganic salt contents decreased conversely. During the SOF volatilation phase in the atmosphere of N 2 , the SOF content and the peak weight loss rate of particles decreased with the decrease of particle size. However, the weight loss rates of the divided particles converged during the soot pyrolysis phase. At the end of programmed temperature rise, the TG curves of particles of each size in N 2 atmosphere slowly stopped at various positions, while those were almost overlapping in O 2 atmosphere. With the decrease of particle size, the TG curves showed an apparent downward trend, which means the smaller the particle size was the easier the losing of weight was due to heating or including oxidation. In O 2 atmosphere, the derivative thermal gravimetry （DTG） curves of different sizes of particles in the SOF volatilation phase were consistent with those in N 2 atmosphere but the peak weight loss rates augmented evidently. In the soot pyrolysis phase, the soot content increased with the increase of particle size, and the peak weight loss rate also boosted significantly by oxidation of O 2 . This study aimed at assessing the oxidation features of different sizes of particles in the specified atmosphere and providing fundamental data for the after-treatment technology of diesel particulate matter.