Hydrogen production by methane decomposition: Analysis of thermodynamic carbon properties and process evaluation

Methane decomposition is a promising approach to reduce the carbon footprint of hydrogen production. Solid carbon is one of the products and can improve the economics as a valuable byproduct or it is to be stored. The morphology of the produced carbon affects not only its economic value, but thermodynamic properties as well. Thermodynamic properties of graphite are usually used for modelling and simulation tasks for methane decomposition. In this study, the impact of the produced carbon type on the decomposition of methane is analyzed. Based on experimental data from literature for e.g. amorphous carbon, carbides, carbon filaments and nano tubes, the equilibrium of the methane decomposition reaction in dependence of temperature and carbon type is evaluated. The highest equilibrium methane conversion is observed for multiwall carbon nanotubes and the lowest for coke as carbon product. If another carbon morphology than graphite occurs, the reaction enthalpy is, in average, increased by 17.6%. To evaluate the potential influence on a production process, a methane decomposition process based on an electrically heated moving bed reactor is simulated. The specific electrical energy input and the methane consumption is calculated in dependence of the main process parameters and the produced carbon type. In a base case simulation the specific electrical energy input is 1.13 kWh/Nm(H2)(3) . About 60% of the electrical power is required for heating the reactor. The energy input is highly affected by the amount of recycled bed material, the effectiveness of heat transfer, the methane conversion and by the carbon type. In the simulated temperature range of between 800 and 1300 K, the specific electrical energy input for a produced carbon type that differs from graphite is in average increased by 14.2% and by 58% in maximum. Further research on thermodynamic properties of different carbon types and their relevance for different methane pyrolysis processes is required.