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Coking is the most common accident that occurs during the conversion process and poses the greatest threat to the conversion catalyst. Carbon deposition covers the surface of the conversion catalyst and blocks its micropores, resulting in a deterioration of the conversion process or the occurrence of local overheating, hot spots, thermal zones, and heat pipes, along with an increase in the methane and aromatic content in the converted gas. Coking not only shortens the service life of the conversion tubes and causes the catalyst to break down into powder, thereby increasing the resistance in the catalyst bed, but it also often forces the system to shut down due to accidents, resulting in severe losses. 1. Carbon deposition reactions and reaction equilibrium: During the steam methane reforming process, carbon deposition reactions can occur under conditions such as an imbalance in the water-to-carbon ratio or catalyst poisoning: 2CO = CO2 + C, ΔH298 = -171 kJ; ① CO + H2 = C + H2O, ΔH298 = -122 kJ; ② CH4 = C + 2H2, ΔH298 = +82.3 kJ. (1) As the temperature increases, the likelihood of carbon deposition via reactions ① and ② decreases, while the likelihood of carbon deposition via reaction ③ increases. (2) As the pressure increases, the likelihood of carbon deposition via reaction ③ decreases, while the likelihood of carbon deposition via reactions ① and ② increases. (3) Under normal operating conditions in ammonia synthesis plants where syngas is produced from natural gas, there is no possibility of carbon deposition due to reactions ① and ② anywhere in the conversion tubes (except in emergency situations) ； At the upper part of the conversion tube, carbon deposition may occur due to reaction ③. (4) When producing syngas for synthetic methanol or other carbonyl synthesis processes from natural gas, in addition to carbon deposition resulting from reaction ③, carbon deposition due to reaction ① may also occur. 2. Carbon deposition kinetics: When the water-to-carbon ratio during methane conversion is lower than the theoretical minimum value, carbon deposition may occur based on the equilibrium conditions of reaction ③. Under the operating conditions in the inlet section of the conversion tube, most cases fall within the thermodynamic region where carbon deposition is possible; however, due to the low temperature in this section, the rate of methane cracking is lower than the rate of carbon removal, so no carbon deposition actually occurs despite being in the thermodynamic region for carbon deposition according to reaction ③. As the conversion temperature increases, the reaction rate of methane cracking reaction ③ keeps rising, but the corresponding methane concentration decreases due to the simultaneous steam conversion reaction. Calculations show that in the section of the conversion tube where the temperature ranges from 650 to 700°C, the relationship between the carbon deposition rate and the carbon removal rate is closely related to the activity of the conversion catalyst. Catalysts with high conversion activity exhibit a carbon removal rate that exceeds the carbon deposition rate in the temperature range of 650–700°C, making carbon deposition less likely to occur ； For catalysts with low conversion activity, in the temperature range of 650–700°C, the rate of carbon removal is lower than the rate of carbon formation, leading to easy carbon deposition. Generally, this occurs around 1/3 of the length of the conversion tube measured from the inlet, which is the area where carbon deposition tends to occur most frequently.