Research on the dynamics and mechanism of thermal cracking of coal-based endothermic hydrocarbon fuels

Abstract

Thermal cracking of endothermic hydrocarbon fuels (EHF) plays an important role in the endothermic process when the fuel temperature exceeds the supercritical temperature. To gain a deeper understanding of the thermal cracking behavior of EHF, this work discusses the thermal cracking law and reaction mechanism of coal-based EHF at different temperatures through static thermal cracking experiments, and establishes a total molecular reaction dynamics model containing 34 species and 24-step reactions based on product distribution. The results show that in the temperature range of 450℃ to 500℃, the gas phase yield of the fuel increases linearly from 8 % to 56 %, and the conversion rate reaches up to 96.1 %. The kinetic reaction of thermal cracking conforms to the primary kinetic equation, the cracking rate constant k is between 1.08×10⁻⁴∼7.92×10⁻⁴ s⁻¹, the activation energy E_a=184.47±11.5 kJ∙mol⁻¹, and the precursor factor lnA=21.61±3.0. Gas phase products include hydrogen, methane, ethane, ethylene, propane and butane. Liquid phase products include paraffins, alkyldecahydronaphthalenes and aromatic hydrocarbons. As the cracking depth increases, the degree of fuel branching first increases and then decreases, up to 0.33. Paraffins and alkyldecahydronaphthalenes are gradually converted into olefins, cyclic olefins, benzene, naphthalene, indene, fluorene and pyrene and other compounds. Based on the product distribution, the reaction mechanism of thermal cracking of coal-based EHF is speculated to include single-molecule β-cracking, bimolecular F-S-S, intramolecular H-transfer, cyclization and isomerization mechanisms.