Methane pyrolysis in monovalent alkali halide salts: Kinetics and pyrolytic carbon properties

Abstract

Methane pyrolysis in molten salts has the potential to provide low-cost, low-CO2 emission H2 on an industrial scale. The alkali halides (NaBr, KBr, KCl, NaCl, (Na,K)Br) are inexpensive and environmental benign salts, which may facilitate sequestration or sales of the produced carbon even if low-to-moderate amounts of salt remain trapped in the carbon. In this novel work, alkali halides have been tested as the liquid reaction media, and the results of kinetic measurements and carbon characterisation are reported. The observed activation energies were found to be in the range 223.5–277.6 kJ mol−1, which is significantly lower than those measured during gas-phase methane pyrolysis (~422 kJ mol−1). After washing procedures with deionised water, the purity of the produced carbon was in the range 91.7–97.4 atom% or 55.0–91.6 wt%, with the carbon purities correlating well with the salt compounds size and salt-carbon wettability. The carbon samples generated in each salt are all low density (<1 g cm−3), highly porous (30.1–75.2%), low surface area (1.84–3.14 m2 g−1) and have relatively low-levels of structural order. The suggested relationships between the salt selection and the carbon purity, the mass fraction and degree of well-ordered carbon as well as surface and pore morphologies can be used to optimise process designs. Furthermore, the relationships could be used to tune the carbon properties to tailor it to the carbon market needs if the carbon is considered as a co-product.