【作者】 李玉阳；

【导师】 齐飞；

【作者基本信息】 中国科学技术大学 ， 工程热物理， 2010， 博士

【摘要】 化石燃料的燃烧为现代社会提供了约85%的能源,也引发了一系列环境问题和社会问题,如大气污染和能源危机等。芳烃燃料是化石燃料的重要成分,也是航空替代燃料的必要组分,对其燃烧化学动力学的研究有助于理解实用燃料和航空替代燃料的燃烧过程,并帮助预测实际工程燃烧的特性参数。另一方面,芳烃燃料较之其它类型的烃类燃料更易产生碳黑这一重要的大气污染物,因此芳烃燃料的燃烧是研究碳黑形成机理的理想平台。然而以前的芳烃燃烧研究多以苯和甲苯这两种结构最简单的芳烃作为燃料,缺乏对具有复杂支链结构的芳烃燃料的研究,严重影响了航空替代燃料燃烧化学动力学模型的发展和对碳黑形成机理的理解。本论文创新性地将同步辐射真空紫外光电离质谱方法应用于广阔当量条件下芳烃燃料低压层流预混火焰的化学结构研究,并结合燃烧化学动力学模型发展和生成速率分析方法,对四种C0-C2单支链芳烃(苯、甲苯、苯乙烯和乙基苯)与三种C1双支链芳烃(邻二甲苯、间二甲苯和对二甲苯)燃料的进行了系统的研究。实验工作中,通过扫描光电离效率谱对上述七种芳烃燃料共20个贫燃、当量和富燃火焰的中间体构成进行了鉴别,特别着重于对同分异构体的分辨和对自由基的检测。利用高精度或组合的量化计算方法对未知电离能的分子进行计算以获得其计算电离能,从而为中间体鉴别提供了依据。在贫燃火焰中共鉴别出分子量介于15至168之间的八十余种中间体,其中有大量芳烃氧化物；在富燃芳烃火焰中共检测到分子量介于15至240之间的一百余种中间体,其中包括数十种含有2-5个碳环的多环芳烃(PAH)。通过在不同光子能量下扫描燃烧炉位置得到了质谱信号的空间分布,从中推导出各主要火焰物种和中间体的摩尔分数曲线,并对火焰化学结构和燃烧特性随当量变化的规律进行了总结。对于不同芳烃燃料,研究了其火焰中间体构成的异同点,并详细分析了在七种芳烃燃料最富燃的火焰中苯乙烯、苄基和苯等关键中间体以及部分典型PAH的摩尔分数随燃料的变化。此外,利用直径0.100 mm的Pt-6%Rh/Pt-30%Rh热电偶对所有火焰的温度曲线进行了测量,为动力学模拟提供了关键的输入参数。模型工作中,将经典链烃燃烧化学动力学模型(USC MechⅡ模型)与最新动力学研究成果相结合,先易后难地发展了苯、甲苯、苯乙烯、乙基苯和三种二甲苯的详细燃烧化学动力学模型,其中提出了全新的苯、甲苯和乙基苯的子机理,并在燃烧研究领域首次发展了苯乙烯的燃烧化学动力学模型和三种二甲苯的火焰化学动力学模型。利用CHEMKIN 2软件得到了各种芳烃燃烧化学动力学模型对实验火焰的模拟结果,在综合考虑实验误差范围和模拟误差范围的基础上,通过比较模拟结果与实验结果对各种芳烃燃料的燃烧化学动力学模型进行了验证和优化,添加了最新研究的反应路径、剔除了不适当的反应路径并替换了陈旧的速率常数,从而保证了模型的可靠性及对实验结果预测的准确性。利用生成速率分析方法,对燃料分解过程的关键中间体和典型大质量芳烃的生成与消耗路径进行了详尽的分析。在燃料分解过程中,一些分子量小于燃料的中间体,如苯乙烯、苯乙炔、苄基、苯、苯基、环戊二烯基、乙烯基乙炔、炔丙基、乙炔等起到了汇集碳流量的作用,成为燃料分解过程的关键中间体。在贫燃火焰中这些中间体的生成和消耗大多通过氧化反应进行,而在富燃火焰中则主要依赖于热解反应。在芳烃生长过程中,苯基、苄基、苯乙炔、苯乙烯、茚基等中间体是生成茚萘、苊烯、菲等双环和三环PAH所需的主要前驱体,而乙炔基、乙炔、炔丙基、乙烯基乙炔、环戊二烯基等小型中间体则是重要的环增长载体。通过上述分析确定了燃料向最终氧化产物和PAH转化的主要反应路径。最后,深入探讨了不同芳烃燃料燃烧化学动力学之间的相似性和差异之处。对相似性的分析结果表明：芳烃火焰中除燃料初级分解产物外,绝大部分在燃料分解过程中产生的中间体及其主要反应路径均保持一致；HACA机理、共轭稳定自由基加成机理、脱氢机理在芳烃火焰中均共同作用于芳烃生长过程。对差异之处的研究结果显示,燃料分子结构会影响苯基、苯、苄基、苯乙炔、苯乙烯等关键中间体的浓度,使得燃料分解过程和芳烃生长过程中主要反应路径发生变化。特别在芳烃生长过程中,更复杂的燃料支链结构保证了火焰中更全面的主要前驱体构成,是更多碳流量流向大质量芳烃尤其是PAH的前提,也解释了七种芳烃燃料的碳黑形成趋势满足苯<甲苯与苯乙烯<四种CsHio燃料的原因。因此,燃料分子结构对燃烧化学动力学的影响仅限于中间体浓度的区别,而中间体构成与化学反应机理在各芳烃燃料火焰中均应保持一致。根据这一原则,我们将七种芳烃燃料的燃烧化学动力学模型融合成统一芳烃燃烧化学动力学模型。该模型包含186个物种和883个反应,目前能够预测上述七种Co-C2单支链芳烃与C1双支链芳烃的低压层流预混火焰结构,在未来进一步发展和改进的基础上将能够涵盖更多具有复杂支链结构的芳烃燃料并有助于预测实际工程燃烧的关键燃烧参数,如点火延迟、点火温度、火焰传播速度、污染物排放和火焰温度等。更多还原

【Abstract】 Combustion of fossil fuels provides around 85% of total energy supply for modern society, and meanwhile causes many environmental problems and social problems such as air pollution and energy crisis. Aromatic hydrocarbons are major components of fossil fuels, and are commonly used in jet surrogate fuels as well. Therefore investigations on the chemical kinetics of aromatic hydrocarbon combustion will help us understand the combustion behaviors of practical fuels and jet surrogates and predict key parameters of practical combustion processes. On the other hand, aromatic hydrocarbons have greater sooting tendencies than other hydrocarbons, making their combustion an ideal system to study soot formation mechanism. However, most previous stuides of aromatic hydrocarbon combustion focused on benzene and toluene which have the simplest molecular structures among aromatic fuels. The lack of studies on aromatic fuels with complex side-chain structures limits the development of kinetic models of jet surrogate fuels and the understanding of soot formation mechanism.In this dissertation, low-pressure laminar premixed aromatic hydrocarbon flames at broad ranges of equivalence ratio (lean, stoichiometric, and rich flames) were experimentally studied using synchrotron vacuum ultraviolet photoionization mass spectrometry. The fuels studied includes four C0-C2 mono substituted aromatic hydrocarbons (benzene, toluene, styrene, and ethylbenzene) and three C1 bisubstituted aromatic hydrocarbons (o-xylene, m-xylene, and p-xylene). Kinetic models of these fuels were developed to reproduce the experimental results. Rates of production (ROP) analysis were performed for the deep insight of the chemical kinetics of aromatic hydrocarbon combustion.Experimentally, photoionization efficiency spectra of all observed mass peaks were measured to identify the intermediate pools of 20 flames studied, with special interest on isomers and radicals. Quantum chemical calculations on molecules with unknown ionization energies (IEs) were performed to get the calculated IEs for intermediate identification. More than 80 intermediates with molecular weights between 15 and 168, including many oxygenated aromatic hydrocarbons, were detected in the lean aromatic hydrocarbon flames, while more than 100 intermediates with molecular weights between 15 and 240, including dozens of polycyclic aromatic hydrocarbons (PAHs), were identified in the rich aromatic hydrocarbon flames.Mole fraction profiles of major flame species and intermediates were evaluated from the spatial distributions of signals measured by scanning burner position at several photon energies, from which the variation trends of mole fractions with varying equivalence ratio were concluded. Comparing mole fractions of intermediates indicated the similarities and differences among the flames of different fuels, particularly showing the varying mole fractions of styrene, benzyl radical, benzene, and some typical PAHs with different fuels. Furthermore, temperature profiles of all flames studied were measured using a Pt-6%Rh/Pt-30%Rh thermocouple with 0.100 mm in diameter.In the modeling work, kinetic models of seven aromatic hydrocarbons were developed in the order of structural complexity of fuels from a classical kinetic model for hydrocarbon combustion (USC MechⅡmodel), which were updated with many recently studied reaction pathways and newly reported rate coefficients of some key reactions. In particular, new fuel submechanisms of benzene, toluene, and ethylbenzene were constructed in this work; and it is the first time to report the flame models of styrene, o-xylene, m-xylene, and p-xylene among the combustion community. Validation of all models was performed by comparing experimental and modeling results in order to provide the accuracy of these models and the performance of model predictions.ROP analysis was the main approach to analyze the major reaction pathways in fuel decomposition and aromatic hydrocarbon growth processes. In fuel decomposition processes, some intermediates with lighter molecular weights than fuels, such as styrene, phenylacetylene, benzyl radical, benzene, phenyl radical, cyclopentadienyl radical, vinylacetylene, propargyl radical, and acetylene afforded most carbon flux and therefore became the major decomposition products. It was also validated that oxidation reactions and pyrolysis reactions controlled the fuel decomposition processes in the lean and rich aromatic hydrocarbon flames, respectively. In the aromatic hydrocarbon growth processes, phenyl radical, benzyl radical, phenylacetylene, styrene, and indenyl radical were found to be precursors of typical bicyclic and tricyclic PAHs such as indene, naphthalene, acenaphthylene, and phenanthrene, with the ring enlargement driven by addition of small intermediates like ethynyl radical, acetylene, propargyl radical, vinylacetylene, and cyclopentadienyl radical. According to the ROP analysis, major reaction pathways converting fuels to CO and forming PAHs were drawn in schemes of carbon fluxes.Similarities and differences among the chemical kinetics of different aromatic hydrocarbon flames were discussed. It was concluded that after the primary decomposition processes of fuels, most decomposition products and decomposition pathways played similar roles in different aromatic hydrocarbon flames. In the rich flames of most aromatic hydrocarbons, Hydrogen-Abstraction-Carbon-Addition (HACA) mechanism, resonantly stabilized radical addition mechanism, and dehydrogenation mechanism collaborated in the aromatic hydrocarbon growth processes. The predominant differences in chemical kinetics related to fuel structural features were mainly caused by the mole fraction variation of some key intermediates including phenyl radical, benzene, benzyl radical, phenylacetylene, styrene, and so on. Particularly in aromatic hydrocarbon growth processes, more complex side-chain structure the fuel molecule contains, more major PAH precursors and higher concentrations of PAHs its flame has, which interprets the relation of sooting tendencies of all aromatic hydrocarbons studied with the complexities of their molecules (Benzene<toluene & styrene<C8H10 aromatic hydrocarbons)Therefore it was concluded that the intermediate pools and reaction mechanisms of all aromatic hydrocarbons studied should be identical as the fuel changes, and what be actually changed were the contributions of reaction pathways due to the mole fraction variation of their reactants and products (say, the key intermediates). This allows us to develop a uniform kinetic model including 186 species and 883 reactions to predict the low-pressure flame chemistry of all seven aromatic hydrocarbons. In next stage, the uniform kinetic model will be further developed to study the combustion of more aromatic fuels with complex side-chain structure and predict key parameters in practical combustion processes, such as ignition delays, ignition temperatures, laminar flame speeds, pollutant emissions, flame temperatures, etc.更多还原