小型非道路柴油机排气管内颗粒的粒径分布与氧化特性

Characteristics of particle coagulation and oxidation in exhaust pipe of diesel engine

为了深入了解柴油机颗粒在排气管内的变化过程,分析柴油机颗粒粒径分布和氧化特性的变化趋势,采用废气排放颗粒物粒径谱仪(engine exhaust particle Sizer,EEPS),测量了柴油机沿排气气流运动方向,不同位置的颗粒粒径,研究了不同位置颗粒粒径的分布规律;应用热重/差热同步分析仪,分析了沿排气气流方向不同位置颗粒的表观活化能、着火温度、燃尽特性指数等特征参数的变化规律。结果表明:沿柴油机排气气流方向,核态颗粒数量减小,聚集态颗粒数量增加;聚集态颗粒的粒径增大。热重试验结果表明:沿柴油机排气气流方向,在SOF挥发阶段和soot氧化过程中,颗粒失质量速率增加,失质量峰值对应温度降低;颗粒中H2O质量由1.3%增加到2.46%,SOF质量由38.21%增加到42.05%,soot质量由57.84%减少到52.24%,灰分质量由3.01%增加到3.25%;颗粒的指前因子由7.93×1012减小到4.67×1012,反应活化能由157.3降低到127.9,着火温度由546℃降低到501℃,降低了约8.2%,燃烧特性指数由8.16×10-9增加到1.62×10-8,燃尽特性指数上升了14.5%。该研究是对颗粒衍化机理的完善,可为颗粒捕集器的优化设计提供参考。

Diesel particulate is one of the main sources of atmospheric particulate matter PM2.5, which seriously harms human health. With the emission regulations become increasingly stringent, higher requirements have been put forward, not only on the quality of particulate emissions but also on the particle number. The change trend of particle size distribution and oxidation characteristics of the diesel particulate are analyzed, in order to understand the change process of the diesel engine particles in the exhaust pipe. For further reduction of the diesel engine exhaust particulate, the post processing technologies have been widely used, such as diesel particulate filter（DPF）. A lot of carbon dioxide（CO2）, water vapor,hydrocarbon（ HC） and particulate matter and other components are contained in the diesel engine exhaust, and the temperature, pressure and other parameters of the exhaust gas flow changed in the exhaust pipe of the diesel engine. The exhaust particulate matter was also accompanied by condensation and oxidation reduction and other physical and chemical processes, and as a result the physical and chemical characteristics of particles also changed. The particle size distribution and oxidation characteristics of the particles were important for the particle capture efficiency and regeneration. The particle size distribution along the direction of movement of the exhaust gas flow and different positions was measured by using the engine exhaust particle size spectrometer（EEPS）. Along the direction of the exhaust gas particles at different sites during the pyrolysis process, pyrolysis kinetic parameters, ignition temperature and burnout characteristics index were studied, and particle pyrolysis reaction rate and degree of difficulty were analyzed by using the device of thermogravimetry/differential simultaneous thermal analyzer. According to the experiment data, along the direction of the exhaust gas flow, the number of nucleation particles decreased, the number of aggregated particles increased, and the particle size of aggregated particles became larger. It showed that in SOF（soluble organic fraction） evaporation stage and soot oxidation process, the particle weight loss rate increased, and the weight loss peak temperature decreased; along with the diesel engine exhaust gas flow direction, when reducing the temperature of the gas, the content of H2 O in particles increased from 1.3% to 2.46%,the content of SOF increased from 38.21% to 42.05%, the soot percent content decreased from 57.84% to 52.24%, and the ash content increased from 3.01% to 3.25%; the pre-exponential factor of particle was reduced from 7.93×1012to 4.67×1012and the activation energy could be reduced from 157.3 to 127.9, the ignition temperature decreased from 546 to 501 ℃,which was reduced by about 8.2%, the combustion characteristic index increased from 8.16 ×10-9to 1.62 ×10-8, and the burnout characteristics index was risen by 14.5%. Therefore, based on the study of the particle size distribution and oxidation characteristics which came from the small non-road diesel engine exhaust pipe of particles, it can be concluded that along the exhaust gas flow direction, with lower exhaust temperature, the burnout performance of the particle gets improved and effectively raises the regeneration of particle collector.