Mechanism and model of n-decane pyrolytic coking based on the coupling of surface mass transfer and chemical reactions

The pyrolytic coking of supercritical hydrocarbon fuels is a complex procedure involving various flow, mass transfer, and reaction processes, in which surface mass transfer plays a significant role. Pyrolytic coking experiments were conducted with n-decane in a pressure range of 3–5 MPa, a fuel conversion range of 0–0.62, and an operating time range of 5–40 min. The mass, morphology and graphitization degree of coke were studied using several characterization methods: temperature-programmed oxidation, scanning electron microscopy and Raman spectrometry. The coking mechanism based on the coupling of surface mass transfer and chemical reactions was revealed, and the cause of the coking mode transition was clarified. When the catalytic reaction rate was lower than the bulk diffusion rate, the coking mode was catalytic coking, and when the catalytic reaction rate was higher than the bulk diffusion rate, the coking mode was free radical growth coking. A coking model was established to calculate the transition diameter of the metal carbide particles corresponding to the coking mode transition and the proportion of the catalytic coking mode. The coke mass in the catalytic coking region can be calculated using this model, and the model results were consistent with the morphology and Raman spectroscopy characteristics. The mechanism and model will help in describing the coking process over a wide range of operating conditions and conducting more accurate simulations of regenerative cooling systems.