【作者】 施军；

【导师】 冉景煜；

【作者基本信息】 重庆大学 ， 动力工程及工程热物理， 2016， 博士

【摘要】 由于化石燃料的紧缺和严重的环境污染,甲醇混合燃料作为一种清洁、可再生能源受到广泛研究与应用。由于该燃料主要由甲醇和丙烷混合而成,具有热值低,汽化潜热大的特点,所以燃烧效率不高,使用专用的气化燃烧器才能实现稳定燃烧,而且该种燃料在热解以及欠氧燃烧过程中容易产生积碳,造成气化器和喷嘴被堵塞,影响燃烧器正常使用,严重阻碍该燃料的应用推广。因此,本文首先通过实验方法研究了甲醇、丙烷以及甲醇丙烷混合热解的产气特性以及积碳生成特性,然后以实验为基础,通过数值计算的方法,优化了现有的甲醇热解反应机理,构建并优化了甲醇丙烷混合热解及积碳生成反应机理,最后通过构建的反应机理,分析了甲醇对丙烷热解气相反应,苯的形成,多环芳香烃(PAHs)及积碳形成过程的影响,最终揭示了甲醇抑制丙烷热解过程中积碳形成的作用机理。本文研究为提高甲醇混合燃料的燃烧效率,减少热解与燃烧过程中的积碳形成提供理论依据。(1)首次研究获得了甲醇丙烷混合热解的产气特性和积碳生成特性,发现在甲醇丙烷混热解过程中,甲醇会抑制乙烯向乙炔的转化过程,同时明显的减少了丙烷热解积碳生成速率以及生成积碳颗粒大小。甲醇在910 K左右开始分解,首先产生甲醛和氢气,然后甲醛再分解产生一氧化碳和氢气,当温度高于1100 K时,开始大量生成甲烷。丙烷在973 K左右开始分解,首先大量生成乙烯和氢气,反应后期乙烯再转化成乙炔,并且积碳的生成速率和形成积碳直径随温度和反应时间的增加而增加。反应温度较低时,甲醇对丙烷热解反应过程影响较小,而反应温度较高时,甲醇会抑制乙烯向乙炔的转化过程。甲醇丙烷混合热解时明显减小了积碳生成速率以及生成积碳颗粒大小。(2)进一步揭示了甲醇热解化学反应机理,并构建了甲醇丙烷混合热解及积碳生成化学反应机理。该机理包括117种化学反应物质,613步气相基元反应和17步表面反应,能够准确地计算丙烷热解以及甲醇丙烷混合热解各产气的摩尔分数和积碳生成速率,能准确分析积碳生成反应路基及关键的控制反应。(3)研究获得了丙烷热解过程中的苯、多环芳香烃以及积碳形成反应路径和关键的控制反应。丙烷热解过程中C3H3的聚合反应是生成苯的主要反应通道,而C3H4脱氢形成C3H3是苯形成的控制步骤。苯脱氢后再与乙烯和乙炔反应,进而生成多环芳香烃。苯的脱氢反应是形成大分子多环芳香烃的控制反应。大分子多环芳香烃以及乙炔发生脱氢反应是积碳形成和增长的主要反应路径。(4)基于化学反应动力学理论,首次探索获得了甲醇对丙烷热解的气相反应、苯及多环芳香烃形成过程的作用规律,揭示了甲醇抑制丙烷热解积碳形成的作用机理。由于甲醇丙烷混合热解产生了大量的氢气以及氢原子,改变了反应系统的化学反应平衡,减少了甲基的摩尔量,并使丙烯分解为乙烯的反应速率加快,从而减少了反应系统中C3和C4碳氢化合物的形成量。特别是由于减少了C3H3的形成量,所以直接减少了苯的形成量,从而大量地减少了混合热解过程中生成的多环芳香烃摩尔量,最终减少了生成的积碳量。另外,由于甲醇丙烷混合热解抑制了苯的脱氢反应,从而推迟了多环芳香烃的形成过程,并且抑制了多环芳香烃和乙炔脱氢生成积碳的反应过程,最终导致了混合热解过程中积碳生成量的减少和生成时间的延迟。更多还原

【Abstract】 As the shortage of fossil fuel and heavy environment pollution, methanol-mixed fuel have been extensively studied and used due to it is a clean and renewable fuel. This fuel mainly consists of methanol and propane, and has Low calorific value and large latent heat of vaporization, so the combustion efficiency is low, and a special combustor is essential for this fuel for stable combustion. Furthermore, it is easy for soot formation in the process of pyrolysis and rich-fuel combustion. The heavy soot block the gasifier and ejector nozzle, and affect the normal use of the burner and hinder the generalization of this fuel. Therefore, an experiment was conducted to study the characteristics of the gas-products and soot in the pyrolysis of methanol, propane and the mixture of them. And then, based on experimental results, the existing pyrolysis mechanism of methanol was improved, and the mechanism for propane and methanol mixture pyrolysis and soot formation was built and optimized using a numerical method. Finally, it was studied that the influence of methanol on the gas reaction, the formation of benzene, polycyclic aromatic hydrocarbons(PAHs) and soot in the pyrolysis of propane using the optimzied mechanism. The principle of soot suppression by methanol in the propane pyrolysis was proposed. This work will be used to improve the combustion efficiency of methanol-mixed fuel, and provide the theory evidence for studying the technique of soot suppression. The major conclusions of this study are shown as follows:The characteristics of the gas-products and soot of the pyrolysis of methanol and propane mixture pyrolysis were revealed firstly in this study. It is that methanol can reduce the acetylene formation from the hydrogen abstraction of ethylene, and also decrease the soot production rate and the diameter of soot particle. Methanol begins decomposing at the temperature of 910 K and forms formaldehyde and hydrogen firstly. And then the formaldehyde decomposes to carbon monoxide and hydrogen. A lot of methane are produced when the temperature is higher than 1100 K. Propane begins decomposing at the temperature of 973 K producing plenty of ethylene and hydrogen, and then the ethylene converts into acetylene.With the increasing in temperature and reaction time, the soot production rate and the soot diameter increases. Methanol does not affect on the propane pyrolysis when reaction temperature is low, but the conversion from ethylene to acetylene is suppressed when reaction temperature is high. The soot production rate and the soot particle diameter of in the pyrolysis of methanol and propane mixture are reduced comparing with that formed in propane pyrolysis.This study improved the calculation accuracy of the existing reaction mechanism of methanol pyrolysis and built the accurate reaction mechanism firstly for calculating the pyrolysis of methanol and propane mixture and soot formation. This reaction mechanism includes 117 chemical substances, 613 gaseous elementary reactions and 17 surface reactions. This reaction mechanism can accurately predict the mole fraction of gas-products and soot production rate, and can be used to analyze the reaction paths of soot formation and key controlling reactions.This study revealed the reaction paths of the formation of benzene and polycyclic aromatic hydrocarbons and soot in the pyrolysis of propane and key controlling reactions. Benzene is produced by the polymerization of C3H3, which is the major reaction path for benzene formation. The hydrogen abstraction reaction of C3H4 to produce C3H3 is the controlling reaction for benzene formation. The controlling reaction for the formation of polycyclic aromatic hydrocarbons is the hydrogen abstraction of benzene forming phenyl. The phenyl reacting with ethylene and acetylene is the major reaction path for the formation of polycyclic aromatic hydrocarbons. The hydrogen abstraction of polycyclic aromatic hydrocarbons and acetylene is the mainly reaction path for producing soot.This study first obtained the impact of methanol on the propane pyrolysis at reaction mechanism level, including the gaseous reaction, the formation of benzene and polycyclic aromatic hydrocarbons, and revealed the principle of soot suppression by methanol in the propane pyrolysis. As plenty of hydrogen and hydrogen atom are produced in the pyrolysis of methanol and propane mixture, the reaction equilibrium is changed, and methyl mole fraction is reduced, and the reaction rate of propylene decomposing to acetylene is increased leading to the reduce of C3 and C4 hydrocarbon, especially for C3H3. The decrease of C3H3 results in benzene formation reduce, thus the formation of polycyclic aromatic hydrocarbons also greatly reduce. Due to the hydrogen abstraction reaction of acetylene and polycyclic aromatic hydrocarbons is suppressed, the soot formation reduce. Besides, as the hydrogen abstraction of benzene is suppressed in the pyrolysis of methanol and propane mixture, the formation of polycyclic aromatic hydrocarbons is delayed, and then the soot formation is also delayed.更多还原