柴油机颗粒物热重参数优化选择及其氧化特性研究

Optimization of Thermogravimetric Parameters and Oxidation Characteristics of Diesel Particulate Matter

探究了柴油机颗粒物中可溶有机成分(soluble organic fraction,SOF)及热重参数对碳烟氧化特性的影响,并基于Coats-Redfern算法得到不同升温速率下碳烟的氧化动力学参数。研究结果表明:在碳烟缓慢氧化阶段,柴油机颗粒中SOF组分阻碍了碳烟氧化反应的进行,而脱SOF后碳烟比表面积增大,更利于氧化反应的进行。与脱SOF前相比,脱SOF后相应的氧化特征温度均有所降低,因而颗粒物去除SOF后的热重试验更能反应碳烟的氧化动力学特性。通过比较试验,确定进样量、工作气流量和升温速率等热重参数的优化取值,其中升温速率对碳烟氧化燃烧反应的影响显著。随着升温速率增大,碳烟氧化反应出现滞后现象,其特征温度升高且最大失重率峰值降低,而升温速率对碳烟热解程度和氧化反应温度区间大小无明显影响。通过对比分析,低升温速率(3℃/min)条件下扩散控制可以忽略不计,氧化特征温度重复性最好。不同升温速率下碳烟燃烧反应动力学参数活化能E和指前因子A之间存在动力学补偿效应,E与A均随升温速率的降低而减小。

The effects of SOF components in diesel particulate matter （PM） and different thermogravimetric parameters on soot oxidation characteristics were investigated. Thereafter the kinetic parameters of soot combustion characteristics were calculated based on Coats-Redfern method. The results reveal that the SOF components in PM hinder the soot oxidation reaction in slow oxidation stage, while the increase in the specific surface area of the soot after devolatilization increases accelerates the oxidation reaction. Compared with the soot as-received, the corresponding oxidation characteristic temperatures of the soot after devolatilization have been decreased. Through comparative test, the optimized thermogravimetric parameters such as the initial sample mass, working gas flow rate and heating rate can be determined, among which the heating rate has vital effects on soot combustion reaction. With the increase in heating rate, the soot oxidation behaves thermal decomposition hysteresis. Therefore, the soot oxidation characteristic temperatures would be risen and the peak weight loss rate would be descended at the same time. However, the heating rate has no effects on the final soot pyrolysis degree and the temperature width of oxidation reaction window. The comparative analysis shows that the diffusion effect under the condition of low heating rate （3 ℃/min） is negligible, and the oxidation characteristic temperatures can be determined with best repeatability. There is an obvious compensation effect between activation energy （E） and pre-exponential factor （A） under different heating rates, and E and A will tend to fall with the decrease in heating rate.