Thermogravimetric characteristics analysis of particulate matter of emission of divided diesel

Abstract: 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 N2 and then O2. 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 N2, 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 N2 atmosphere slowly stopped at various positions, while those were almost overlapping in O2 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 O2 atmosphere, the derivative thermal gravimetry (DTG) curves of different sizes of particles in the SOF volatilation phase were consistent with those in N2 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 O2. 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.