Thermal methane pyrolysis in tubular quartz reactor: effect of temperature and surface-to-volume ratio on carbon yields and morphology

This study provides an integrated experimental–modeling assessment of surface and gas-phase carbon formation in methane pyrolysis. Methane pyrolysis was experimentally investigated in a tubular flow reactor under atmospheric pressure to assess the effects of temperature and surface-to-volume (S/V) ratio on gas-phase chemistry, carbon yield and morphology. Two reactor configurations were examined: an empty tube (S/V = 4 cm−1) and a tube packed with ceramic spheres (S/V = 10.7 cm−1), with temperatures ranging from 950 to 1150 °C and deposition times from 30 min to 3 h. Major gas-phase hydrocarbon products were quantified via mass spectrometry, while solid carbon was recovered from hot surfaces for morphological and structural characterization by SEM and Raman spectroscopy. Results showed that temperature significantly increases solid carbon yield, reaching up to 60 wt% at 1150 °C. Morphological analyses revealed a transition from amorphous thin films to structured layers, shaped by deposition time, temperature and gas-phase evolution. Embedded carbon nanoparticles served as nucleation centers for cone-like growth, indicating a tight coupling between homogeneous and heterogeneous processes. Kinetic simulations using the CRECK model, which incorporates gas-phase chemistry for methane pyrolysis coupled with detailed soot and surface deposition mechanisms, reproduced key experimental trends and highlighted the role of C2 species and PAHs in triggering carbon nucleation and deposition. These results underscore the potential of thermal methane pyrolysis for CO2-free hydrogen production, with tuneable carbon coproducts tailored for advanced material applications.