Analysis of chemical mechanisms in methane pyrolysis for carbon product generation*

A compact and accurate chemical mechanism of non-catalytic methane pyrolysis with formation of large polycyclic aromatic hydrocarbon (PAH) molecules has been developed [1]. These PAH molecules form as incipient particles for carbon nanostructures such as carbon black and graphene flakes and soot.

Methane pyrolysis for carbon nanostructure synthesis is a two-stage process, where conversion of CH₄ to C₂ hydrocarbons (C₂H₆, C₂H₄ and C₂H₂) precedes the growth of PAH molecules from C₂H₂. We have developed a single chemical mechanism that accurately describes both stages. It is based on the ABF [2] mechanism for the first stage which was expanded with the most crucial reaction pathways from the mechanism by Tao [3] for small PAH molecules and HACA [4] pathways for larger PAH molecules, up to 37 aromatic rings.

The resulting mechanism was validated through comparison to multiple sets of experimental data for both stages of the process; good agreement was obtained. We show that the inclusion of larger PAH species (up to A37) in the chemical mechanism is crucial for accurate prediction of the fraction of carbon converted to PAH molecules and, correspondingly, the residual fraction of C₂H₂ in the mixture.

The mechanism has been applied to determine optimal conditions of methane pyrolysis in an idealized thermal plasma reactor similar to those used in the experiments [5] and [6]. A Multi-objective Bayesian Optimization framework [7] has been used for an efficient search of the optimal conditions. The focus of the optimization was on increasing the yields of valuable gaseous products (C₂H₂ and C₂H₄) while reducing the formation of soot and energy cost of the process. A Pareto frontier showing a trade-off between these objectives has been found.

[1] A. Khrabry et al, Int. J. Hydrogen Energy 56 (2024) 1340.

[2] J. Appel et al, Combust. Flame 121 (2000) 122.

[3] H. Tao et al, Fuel 255 (2019) 115796.

[4] M. Frenklach et al, Symp. Combust 20 (1984) 887.

[5] J.R. Fincke et al, Plasma Chem. Plasma Process. 22 (2002) 105.

[6] L. Fulcheri et al, Int. J. Hydrogen Energy 48 (2023) 2920.

[7] K. Shao et al, Plasma Sources Sci. Technol. 31 (2022) 055018.