Understanding of gas-phase methane pyrolysis towards hydrogen and solid carbon with detailed kinetic simulations and experiments

Methane (CH4) pyrolysis is studied in a tubular flow reactor for the production of hydrogen and solid carbon by combining kinetic modelling, numerical simulation and experiments in a high-temperature flow reactor. Operating conditions are varied such as temperature in the range of 1273-1873 K, hydrogen addition to the feed with a molar H2:CH4 ratio between 0 and 4, and residence time in the range of 3-7 s. Elementary-step reaction mechanisms consisting of pyrolysis and carbon coupling reactions among C1, C2, aromatic and polycyclic aromatic hydrocarbon species are used in the numerical simulations of the species profiles in flow direction. A thermodynamic analysis is performed to acquire the boundary conditions for operation as well to estimate probable by-products (C2H2, C2H4, C2H6, C6H6, C16H10 etc.) over the temperature range. Gas-phase kinetic modelling is performed revealing that CH4 conversion starts at temperatures above 1273 K. Higher temperatures increase CH4 conversion and H2 yield peaking at 1573 K. The cooling temperature gradient downstream of the hot zone in the reactor causes reverse reactions in gas-phase suppressing CH4 conversion. H2 addition to the feed is found to be a crucial parameter for controlling the by-product formation. In the experimental study, a solid carbon yield of 84 % is achieved while gaseous by-products remain less than 1 mol-% at 1673 K, H2:CH4 ratio of 2, and residence time of 5 s. Gas-phase reactions are found to be coupled to surface reactions at higher temperatures.