生物柴油发动机细颗粒物特征及可检出性研究

Accurate measurement of particulate matters (PM) emitted from engines has been extremely important for emissions control. As compared with conventional diesel, the organic carbon (OC) components in biodiesel particulate matters were increased, while the element carbon (EC) components were decreased. Meanwhile, the particulate numbers (PN) of biodiesel PM and its distribution are also different. Due to different optical absorptivity between various PM sources, a deviation may come out while the optical method is adopted to measure PM. At the same time, the impact of the PN of very fine particle matters can not be well considered when using a filtration method. Therefore, the detection for biodiesel fine PM emission owns its particularity. Also the authenticity and objectivity of emission test results are worth deep exploring. Research on the characteristic of carbonaceous components and the difference of surface adsorption and morphology between diesel PM and biodiesel PM will be carried out in this project. The influence of the carbonaceous components, size distribution and PN concentration of PM on the optical absorption will be comprehensively analyzed. PM growing and developing in the exhaust pipe, which has great effects on the final sampling results with raw or dilute methods, will also be simulated and analyzed. In the meantime, the detected values for biodiesel PM will be effectively compared with that for diesel PM at the same operating modes, which will assist to find the correlation between PM concentration and PN and explore the detectability of biodiesel PM. This project will provide a favorable reference for the improvement in detecting technology and testing standard for biodiesel PM and also will promote the further study on the optimal regeneration of diesel particulate filter (DPF) for biodiesel engines and the formation mechanism of the secondary organic carbon (SOC).

发动机燃烧颗粒物(PM)的准确计量对污染物控制极其重要。生物柴油燃烧PM中有机碳较多而元素碳较少，颗粒数目(PN)及其分布特性也有别于普通柴油，当采用光学法测量PM时，会由于吸光性而产生差异，当采用过滤法测量时，则无法考虑细颗粒PN的影响，因此，生物柴油发动机PM排放检测有其特殊性，检测结果的真实性和客观性也值得探讨。本项拟开展生物柴油和普通柴油PM碳质组分特性、表面吸附和颗粒形貌的差异性的研究，分析PM碳质组分、粒径分布和PN浓度对PM吸光性的影响，模拟分析排气过程中PM的生长演化历程对PM直接采样和稀释采样的影响，进行同工况下柴油机燃用生物柴油和普通柴油PM排放检测比对试验，分析PM浓度与PN的关联，探讨生物柴油发动机PM排放的可检出性与检出规律。项目开展为生物柴油发动机PM检测方法的完善、检测标准的制定等提供参考，对生物柴油发动机DPF再生及二次有机碳形成机理拓展研究也具有借鉴意义。

发展生物柴油是推动发动机实现“碳中和”的重要手段，准确检测生物柴油燃烧颗粒物对发动机污染物控制非常重要。然而燃用不同原料生物柴油会直接影响发动机排放颗粒物的光学特性，造成生物柴油燃烧颗粒物光学可检出性和排放控制有其特殊性。为此本课题开展了发动机燃用柴油和典型原料生物柴油颗粒物理化特性评价表征和光学特性研究。根据生物柴油脂肪酸甲酯的差异主要由双键数量导致的特点，选择三种典型原料生物柴油为对象，利用气相色谱-质谱联用仪、液体密度计、旋转粘度计等仪器测量分析燃油的密度、粘度和碘值，用粒径谱仪在线测量发动机燃用柴油和生物柴油的颗粒物。研究表明：生物柴油的理化性质与碘值具有明显的关联性，颗粒物数量随燃油碘值升高而增加，生物柴油燃烧颗粒物粒径向小方向偏移，柴油颗粒物数量浓度大于生物柴油颗粒物。改进设计了颗粒物热泳采样、探针采样和快拆采样装置，并采集发动机燃用柴油和生物柴油颗粒物。利用扫描电镜、透射电镜、比表面积和孔径分布分析仪、拉曼光谱、红外光谱仪、X射线衍射仪、热重分析仪和碳质分析仪等仪器，研究了柴油和生物柴油燃烧颗粒的微观结构、表面官能团、孔隙率、氧化特性和光学特性。研究表明：生物柴油燃烧颗粒物粒径偏小，颗粒物团聚集程度随碘值的增大而减少；柴油燃烧颗粒物石墨化程度明显高于生物柴油颗粒；生物柴油颗粒物有更加开放的孔隙特征、更小的平均孔半径、更大的比表面积以及更粗糙复杂的表面特征；生物柴油颗粒氧化活性高于柴油颗粒；生物柴油颗粒物OC/EC的值和燃油碘值正相关，颗粒物的吸收系数在消光中的占比随着粒径的增加而减小，随着生物柴油碘值的降低，消光效果越弱；颗粒物EC的光吸收增强作用越随OC含量升高而增强，散射截面、吸收截面和消光截面均随着OC涂层厚度的增加而增加。.本课题对完善生物柴油燃烧颗粒物的检测方法研究具有重要科学意义，为生物柴油发动机的颗粒排放控制提供了理论及技术支持。