Computational study on the chain cracking mechanisms and rate constants of C2 chain hydrocarbons during pyrolysis/gasification
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摘要: 链烃裂解在热解/气化过程中大量存在，其中轻质链烃反应时间短并且存在多种反应路径，难以通过试验的方式对单一演变路径进行准确的检测和分析。本文选用Gaussian及其配套软件对C2系列链烃（包括乙烷、乙烯和乙炔）的反应位点进行了预测，并对上述链烃在H/OH/O自由基及水分子作用下的碳链裂解机理进行研究。结果表明，自由基对乙烷的C原子及H原子均可发起进攻，而对于乙烯和乙炔的进攻则主要集中在C原子。在上述三种自由基中，OH自由基对不饱和烃的裂解效果最佳，而H自由基对于饱和烃的裂解效果更好，该结果反映了实际过程中水蒸气对于C2系列链烃化合物的碳链断裂具有积极作用。此外，对比乙烯和乙炔与OH自由基作用的最优路径可以发现CH2CH2OH在低于1200 K的环境中比CH2CHO更容易裂解，而在高于1200 K的环境中则是CH2CHO裂解更易，从中可以推断出醛类基团对于温度变化的响应速度优于醇类基团。Abstract: Chain hydrocarbons cracking always occurs in pyrolysis/gasification. Among them, the reaction time of light chain hydrocarbons is short and they have many reaction paths, which make it difficult to accurately detect and analyze each evolution path by experiment. In this study, Gaussian and its related software were employed to predict the reaction sites and to study the chain cracking mechanism of C2 chain hydrocarbons (including ethane, ethylene and acetylene) under the action of H/OH/O radicals or H2O. According to the results, radicals can both attack the C and H atoms of ethane, while the C atoms of ethylene and acetylene are the main attack sites. Among the above radicals, OH radical is the best for unsaturated hydrocarbons cracking, while H radical is the best for saturated hydrocarbons, showing that the steam is conducive to the cracking of C2 chain hydrocarbons in actual process. In addition, comparing the optimal path of the reaction of ethylene or acetylene with OH radical, it can be found that CH2CH2OH radicals are easier to crack than CH2CHO radicals below 1200 K, while CH2CHO radicals are easier to crack above 1200 K, inferring the better response speed of aldehyde groups for temperature than alcohol groups.
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Key words:
- C2 hydrocarbon /
- H2O /
- radical /
- reaction site prediction /
- reaction path /
- rate constant
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图 1 C2链烃化合物在电子密度为0.001a.u.等值面上的静电势（ESP）及平均局部离子化能（ALIE）分布图（(a)、 (b)、 (c)为ESP分布；(d)、 (e)、 (f)为ALIE分布）
Figure 1 The distribution of electrostatic potential and average local ionization energy of C2 chain hydrocarbons on the isosurface of electron density of 0.001a.u. ((a), (b), (c) are ESP；(d), (e), (f) are ALIE)
图 2 乙烷在不同路径下的反应能垒变化图
Figure 2 The change of reaction energy barrier of ethane under different paths
图 3 乙烯在不同路径下的反应能垒变化图
Figure 3 The change of reaction energy barrier of ethylene under different paths
图 4 乙炔在不同路径下的反应能垒变化图
Figure 4 The change of reaction energy barrier of acetylene under different paths
图 5 乙烷在不同路径下的反应速率常数
Figure 5 Reaction rate constants of ethane under different reaction paths
图 6 乙烯在不同路径下的反应速率常数
Figure 6 Reaction rate constants of ethylene under different reaction paths
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