EGR废气组分对柴油机颗粒氧化活性的影响

针对不同EGR废气组分条件下产生的颗粒,采用热重分析的方法,分析了EGR废气组分对颗粒中主要物质含量的影响;考察了颗粒中挥发物析出温度、soot组分着火温度等4个特征温度、燃烧特性指数以及活化能的变化规律。结果表明,与引入废气和N2时相比,只通入CO2时产生颗粒的失质量百分数所占比例最大,颗粒中水分和可溶有机物(soluble organic fraction,SOF)含量增加明显,soot组分含量有较大幅度降低;在颗粒氧化过程中的高温反应阶段,与引入废气和N2时相比,只通入CO2时产生颗粒中soot组分的质量变化率峰值和峰值对应温度均最小;氧化特性参数的计算结果表明,与引入废气和N2时相比,只通入CO2时产生颗粒的挥发物析出温度TSOF1、挥发物起始燃烧温度TSOF2、soot组分着火温度Ti、soot组分燃尽温度Th均最低,燃烧特性指数增加明显,反应活化能最低。说明引入CO2后,颗粒中soot组分在达到相同失质量百分数时,所需的温度较低,反应所需能量较小,EGR废气中的CO2可以显著提高颗粒自身的氧化能力和反应活性,改善颗粒的氧化燃烧性能。

Effect of EGR exhaust gas components on oxidation activity of particle from diesel engine

Abstract: NOX and particles emitted from diesel engine have been the major sources of air pollution in China. Particles which contains soluble organic fraction (SOF), soot and inorganic salt are mainly result from incomplete combustion or pyrolysis of fossil fuels and other organic materials. In diesel engine, the poor mixing of fuel and air creates fuel-rich zones that support particles formation at high temperatures. Particles are, therefore, the major emission of diesel engines. Many particles consist of agglomerated primary particles with diameters on the order of 10-30 nm, chiefly including crystalline and amorphous domains. Numbers studies show that they are harmful to human health when inhaled and may cause cancer, for that reason, it is important to reduce the formation or at least the emission of particles to environment. Meanwhile, the strict regulations for PM emission have been enforced in many developed countries. In order to reduce NOX and particles emission and meet emission regulations, high pressure fuel injection, exhaust gas recirculation (EGR) and particle catalytic oxidation/capture technology (POC/DOC/DPF) are utilized in light vehicle diesel engine. As significant in-cylinder purification action for NOX, EGR reduces the peak combustion temperature by increasing the specific heat value and diluting oxygen concentration of the mixture gas. Nevertheless, the declines of excess air coefficient and oxygen concentration cause the increase of particles. Diesel particulate filter (DPF) is currently the dominant diesel particulate emissions control technology to meet stringent air quality standards, however, the oxidation ability of particles is clearly important to the regeneration of the filter bed in DPF. The micro-orifice uniform deposition impactor (MOUDI) is a favorable apparatus for obtain the particle size distribution and collect the particles in different size ranges after classification. Thermogravimetric analysis (TGA) has been widely used as an analytical method for investigate the relationship between the material weight and temperature under the condition of programming temperature rise. To understand the effects of EGR exhaust gas components on the oxidation activity of particle from diesel engine, thermo gravimetric experiments on the particle collected under different EGR exhaust gas components were carried out in atmosphere of N2 and then O2 by using thermo gravimetric analyzer. The main material content in particle, volatiles precipitation and combustion temperature, combustion characteristic index and activation energy were also analyzed. The results show that the moisture and SOF contents in the particles with application of CO2 are larger than those with application of exhaust gas and N2, but the soot contents have a significant decrease. In the oxidation process of particles, the peak quality rate and corresponding temperature of soot in the particles with application of CO2 is the lowest. In addition, the volatiles precipitation and combustion temperature, ignition and burn out temperature of soot components are all lower than those with the application of exhaust gas and N2. Besides that the particles with application of CO2 have the lowest activation energy. The results indicated that the CO2 in EGR exhaust gas components could increase the oxidation ability of particle and improve the combustion performance.