A kinetic model for coke formation during steam cracking based on elementary reactions between gas-phase components and the coke surface is described. Hydrogen abstraction by gas-phase radicals results in radical surface species which can add to gas-phase alkenes and alkynes. Cyclization and dehydrogenation leads to the incorporation of carbon atoms into the coke layer. The kinetics of the coke formation reactions are determined from those of corresponding gas-phase reactions provided that the presence of a solid phase is accounted for via a correction factor based on collision theory. The number of required kinetic parameters is substantially reduced by applying the structural contribution technique. Predicted trends and the most important reaction pathways are analyzed at conditions corresponding to ethane cracking with ethene, ethyne, propene, and propyne as coke precursors and H, CH3, C2H5, and C3H5 as gas-phase radicals. Abstraction of hydrogen atoms and the addition of radical surface species to alkenes have the largest effect on the coke formation rate.
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