【作者】 刘鹏；

【导师】 林赫；

【作者基本信息】 上海交通大学 ， 动力机械及工程， 2017， 博士

【摘要】 多环芳烃（PAHs）产生于碳氢燃料的不完全燃烧,被认为是碳烟生成的前驱物,具有强烈的毒性与致癌性。深入理解PAHs在火焰中的演变机制有利于从燃烧源头上降低碳烟颗粒的排放量,从而缓解目前严重的雾霾气候。本文以量子化学理论以及反应速率动力学理论为基础,从分子层面研究燃烧过程中PAHs的生成氧化过程,并应用激光诱导荧光（LIF）技术实验探究主要燃烧参数对PAHs生成的影响。主要内容如下:（1）基于密度泛函理论（DFT）研究了CH₂-C₂H₂与PAHs基之间的加成反应。研究结果表明CH₂可直接参与PAHs生长,主要通过如下5个反应步完成PAHs的生长,1)通过PAH基与C₂H₂之间的反应生成带有乙烯基的PAHs分子结构,2)分子内部H转移反应,3)CH₂吸附到PAHs的活性C原子上,4)PAHs分子上CH₂与C₂H₂之间C-C键的生成,5)脱氢反应生成稳定的局部最小结构。（2）C₄H₄+C₄H₄（+H）反应的势能面和动力学研究结果表明,C₄H₄+C₄H₄（+H）反应可以生成在PAHs生长过程中至关重要的苯乙烯和苯乙炔。在生成苯乙烯的过程中,C-C键的生成与断裂是主旋律。H转移反应与H脱除反应是苯乙炔的生成过程的主要特征。（3）研究了CO₂在并五苯基上的化学吸附机理与随后的CO解离反应机理。CO₂与并五苯基反应可以生成羰基氧化物、杂环氧化物和内酯氧化物产物,它们的生成均为放热过程,所释放的能量大小排序为:羰基氧化物<杂环氧化物<内酯氧化物。在CO解离基元反应中,限速反应步的反应速率系数在1200 K下的值高达4.1×10⁶ s^-1,意味着在燃烧温度下CO的解离反应速率较快。（4）研究了大尺寸1,2-benzanthracene氧自由基I-IV和3H-cydopenta[a]anthracene氧自由基的热解反应机理。结果表明PAHs氧自由基的热解反应速率系数与温度、压力和表面原位类型有关。提出了一条新的PAHs演变路径:6碳环（?）5碳环（?）4碳环（?）带有乙烯基PAHs（?）6碳环或5碳环。（5）通过DFT、含时密度泛函理论和PAHs化学动力学模拟的联合使用,调查了13种气相PAHs的电子发射特性。研究结果表明PAHs的最大发射波长对5碳环结构很敏感。结构中含有5碳环的PAHs最大发射波长位于可见光区域,而且结构中的C原子数目对发射波长没有影响。结构中不含5碳环的PAHs所发射的荧光信号波长要小于450 nm,而且对结构很敏感,其荧光波长会随着C原子数目的增加而红移。（6）基于LIF技术研究了温度、当量比和CO₂含量对PAH生成的影响。研究结果表明由于PAHs氧化反应与生长反应速率对温度敏感度不一致,使得PAHs的生成对火焰温度极其敏感,在1730 K左右其浓度会到达最大值,在更低或者更高的温度会减小。高当量比火焰中含有更多的C₂H₂,会促进PAHs的生成。而CO₂的化学效应会抑制C₂H₂和C₃H₃的生成,导致PAHs的生成量减少。（7）以ABF机理、甲苯和甲基萘的裂解反应机理为骨干构成了LLM机理。新的机理基于文献更新了部分反应的反应速率常数,发展了CH_X、C₂H_X、C₃H₃、C₄H₄、C₅H₅、C₉H₈、c-C₆H₆O和A₁C₂HC₂H₂物质与PAHs的相关反应。与实验数据的对比分析结果表明LLM机理对气相小分子物质的浓度预测有所提升,尤其改善了CH₄和C₂H₂等在PAHs生长过程中起关键作用的物质的浓度预测精度。LLM机理对PAHs浓度的预测性能较文献机理有了明显提升。更多还原

【Abstract】 Polycyclic aromatic hydrocarbons（PAHs）produced in incomplete combustion of hydrocarbon fuels are highly carcinogenic and mutagenic,and are considered as the precursor of soot.Understanding the mechanism of PAHs evolution in flame is beneficial to reduce the generation of soot particles from the combustion,and alleviate the severe fog and haze.Based on the quantum chemistry theory and reaction rate kinetics theory,the formation and oxidation process of PAHs in combustion process was studied from the molecular level.The effect of main combustion parameters on PAHs formation was investigated by laser induced fluorescence（LIF）technique.The main contents are as follows.（1）The addition reaction between CH₂-C₂H₂ and PAHs were studied based on density functional theory（DFT）.The results showed that CH₂ can be directly involved in PAHs growth by the following five reaction steps:1)the PAHs with vinyl group are formed by the reaction between PAH radical and C₂H₂,2)the internal H-transfer reaction,3)the adsorption of CH₂ to active C atom of PAH,4)the formation of C-C bond between CH₂ and C₂H₂ in PAHs,and 5)formation of stable local minimum structures by dehydrogenation.（2）The potential energy surface and kinetic results of the reaction of C₄H₄+C₄H₄（+H）showed that C₄H₄+C₄H₄（+H）reaction can give birth to styrene and phenylacetylene,which are important in the growth of PAHs.In the formation of styrene,C-C bond formation and fracture are the main themes.The H-transfer and the H-elimination reactions are the main features in the formation of phenylacetylene.（3）The chemisorption mechanism of CO₂ on pentacene and subsequent CO dissociation mechanism were studied.Carbonyl oxide,heterocyclic oxide and lactone oxide can be formed in the reaction of CO₂ and pentacene,their formation are all exothermic process.The order of released energy is carbonyl oxide<heterocyclic oxide<lactone oxide.Among CO dissociation elementary reactions,the rate coefficient of the rate-limiting reaction step is as high as 4.1×10⁶ s-1 at 1200 K,which means that the dissociation reaction rate of CO is fast at combustion temperature.（4）The thermal decomposition mechanism of 1,2-benzanthracene oxygen radical I-IV and 3H-cydopenta[a]anthracene oxygen radical were studied.The results showed that the decomposition rate coefficients are sensitive to temperature,pressure and surface type.A new PAHs evolution pathway is proposed:6-membered ring（?） 5-membered ring（?） 4-membered ring（?） PAHs with vinyl （?） 6-membered ring or 5-membered ring.（5）The electron emission properties of 13 PAHs were investigated based on DFT,time-dependent density functional theory and PAHs chemical kinetics simulation.The results showed that the maximum emission wavelength of PAHs is sensitive to the presence of 5-membered ring structures.The maximum emission wavelength of PAHs with 5-membered ring locates in the visible region,and the number of C atoms in the structure has no effect on the emission wavelength.The fluorescence signal wavelength emitted by the PAHs without 5-membered ring is less than 450 nm,and sensitive to the structure.Further,its fluorescence wavelength increases with the number of C atoms.（6）The effect of temperature,equivalence ratio and CO₂ concentration on PAHs formation was studied based on LIF technique.The results showed that the PAHs are sensitive to the flame temperature.The concentrations of PAHs will reach the maximum at about 1730 K and decrease at the lower or higher temperature,resulting from the rate competition bewteen the PAHs oxidation reaction and the PAHs growth reaction.The flame with higher equivalence ratio contains more C₂H₂,promoting the formation of PAHs.While the chemical effect of CO₂ can inhibit the formation of C₂H₂ and C₃H₃,resulting in the reduction of PAHs production.（7）The LLM mechanism is developed on the basis of the ABF,toluene and methylnaphthalene reaction mechanism.The reactions of PAHs and CH_X,C₂H_X,C₃H₃,C₄H₄,C₅H₅,C₉H₈,c-C₆H₆O and A₁C₂HC₂H₂ were added into LLM mechanism based on the our previous study and literatures.The results showed that deviation between the predicted concentration of gas-phase small molecule species with LLM mechanism and corresponding experimental data is within 2 times,especially the concentration of CH₄ and C₂H₂,which plays a key role in the growth of PAHs.The deviation is within one magnitude when comes to PAHs concentration.It also showed excellent adaptability to different flame temperature and different CO₂ content.更多还原