Molten salt bubble columns for low-carbon hydrogen from CH4 pyrolysis: Mass transfer and carbon formation mechanisms

Methane pyrolysis in molten salts could potentially provide a large-scale, low-cost and low-CO2 production route of H2 from CH4. The interplay of reaction and transport phenomena in molten pyrolysis reactors are currently poorly characterised and further understanding of the mass transfer and mechanistic pathways elucidated here may prove key to designing separation strategies that favour minimum carbon contamination and maximised production rates. Kinetic experiments were carried out at 850–1000 °C in a 590 mm bubble column of a eutectic mixture of molten NaBr and KBr. The use of four different injector sizes allowed for the variation of the mean surface area of the CH4 bubbles in the range of ~600–1200 m2 m−3 (0.77–2.9 cm2 per bubble). For the first time, a clear decoupling of bubble specific surface area and volume-based CH4 reaction rates in molten pyrolysis systems was demonstrated. These phenomena become more complex with catalyst particles present. Tests with γ-Al2O3 particles of varying sizes and with varying particle loadings were undertaken. The carbon depositing on the particles has a high catalytic activity and the material is thus a promising material for structured packings. Two clearly distinguishable kinetic regimes were identified, which transitioned at a γ-Al2O3 loading of ~1.25 wt%. We propose that the reaction is primarily enhanced by a shuttling mechanism of γ-Al2O3 adsorbing hydrocarbons (HC) in the HC-rich liquid-side film surrounding the bubble and desorbing it in the HC-lean liquid bulk. The findings provide a basis for (i) further models to extract kinetic parameters from CH4 pyrolysis in molten salt particle suspensions and (ii) for reaction rate modelling of industrial-scale pyrolysis reactors.