Dynamics of solar thermochemical methane decomposition for hydrogen and carbon black production

This work reviews various aspects of the methane thermal decomposition process such as the initiation step, production of C2 hydrocarbons/higher hydrocarbons/polyaromatic hydrocarbons (PAHs), mechanism steps for the purpose of kinetic modelling, carbon catalysts to enhance the decomposition process, solar radiation for the purpose of methane pyrolysis, solar reactor configurations and kinetic/mathematical studies concerning the estimation of kinetic parameters. Carbon seeding in reactors demonstrated the advantage of providing a catalytic effect to the decomposition process through surface reactions on active sites and via enhancing heat exchange within the reactor. Mathematical and numerical modelling of the methane decomposition process is important to understanding process aspects which are experimentally challenging such as axial variation of products and intermediates, determination of reactor temperature profiles and identification of reactions or dead zones. Current studies demonstrate the numerical modelling of methane pyrolysis either with autocatalysis from nucleated carbon or with injected seeded catalyst. There are no current CFD-kinetic modelling studies that include both the effects of autocatalysis and injected catalysts. In this study, a combined methodology was employed using the Eularian-Lagrangian framework in ANSYS Fluent to predict gaseous and solid products yields. The particle behaviour included in this study were particle nucleation due to homogenous reactions and growth due to heterogeneous reactions. The model was able to predict hydrogen and other hydrocarbon yields in a fair agreement in comparison to experimental results. The model tested two cases of seeding mass flow rates 1.62 g/h and 3.14 g/h. Simulation results were compared to experimental outcomes, with a focus on H2 yield. The comparison revealed a close correlation between simulation and experiment, with an acceptable error margin of 10.4% for the 1.62 g/h case and 9.5% for the 3.14 g/h case. Concentrations of hydrocarbons such as ethane and ethylene at the outlet were negligible compared to hydrogen and acetylene, making accurate predictions challenging. Seeding was found to increase methane conversion, with a notable effect on the 3.14 g/h case due to enhanced contact between methane molecules and catalyst surfaces. The study concludes by emphasizing the importance of developing a methodology that incorporates simple kinetic mechanisms into 2D or 3D models. This involves reviewing simulation work and creating an iterative method to adjust kinetic rates for accurate correlation with physical testing results under varying conditions.