Methane cracking: experimental and kinetic study

This work presents a comprehensive and integrated investigation into methane pyrolysis, emphasizing methane conversion and carbon formation across various reactor systems and operating conditions ranging from 875 to 2230 °C and pressures of 1 to 2 atm. The study spans multiple reactor modalities, starting from millisecond-scale shock tube experiments to laboratory-scale tubular void and packed bed reactors, and extending to near-industrial molten media bubble reactors. This experimental approach is combined with advanced in-situ spectroscopy and mass spectrometry techniques, along with ex-situ Raman spectroscopy and electron microscopy (SEM, TEM) for detailed carbon characterization. Through this combination, the research reveals fundamental pathways underlying gas-phase evolution as well as carbon formation under diverse reaction environments, timescales, and surface-to-volume ratios of 2.7, 4, and 10.7 cm⁻¹. From a kinetic perspective, the dissertation employs both detailed chemical reaction mechanisms (using the CRECK mechanism) and simplified global models. Detailed kinetic simulations have been used for the shock tube and tubular/packed bed configurations and incorporate gas-phase pyrolysis chemistry, homogeneous soot particle formation following a sectional size-class approach, and heterogeneous surface deposition mechanism based on graphitic active sites. Simplified global models have been adopted for the molten media case to provide global kinetics insights of the process as methane conversion and C2 formation products. In shock tube experiments, rapid methane consumption and acetylene generation are accompanied by soot formation, with experimental results broadly consistent with gas-phase model predictions, although further investigation is warranted into the initial carbon inception steps. In tubular and packed bed reactors, methane conversion exhibits a pronounced increase with temperature, rising from about 10% at 950°C to approximately 75% at 1150°C. Carbon deposition emerges as a critical process, especially in packed bed reactors where elevated surface-to-volume ratios enhance the role of heterogeneous surface reactions, primarily involving the growth of carbon structures through acetylene and ethylene deposition. While the kinetic models reliably predict overall carbon formation in void reactors, they tend to overpredict nanoparticles generation in packed beds due to the complex surface deposition behavior of carbon particles. Molten media bubble reactors demonstrate significant methane conversions up to around 35% at 1070°C, despite very short residence times of around one second. This enhancement is attributed to heterogeneous surface reactions occurring at the bubble-liquid interface that surpass conversion rates achievable by solely homogeneous gas-phase reactions. Global apparent activation energies have been derived from the experimental description of C₂ intermediates formation, which are available for future implementation in chemical process simulators. The carbon material produced in molten media reactors displays more ordered, sheet-like structures, markedly distinct from the amorphous soot particles obtained in the gas phase and deposited carbons obtained on surfaces. This highlights the profound influence of the reaction environment on the morphology and structural order of carbon products.