Hydrogen reactivity factor and effects of oxygen on methane conversion rate by chemical equilibrium calculation

Abstract

To solve the global warming problem, it is necessary to minimize the use of fossil fuels such as coal and natural gas. As a power-to-gas technology, CO2 methanation has been focused on, which can produce the synthetic methane through the catalytic conversion of carbon dioxide and hydrogen, called methanation. However, it is better not to use catalysts, because the catalyst could be damaged due to the thermal degradation due to the exothermic reaction. In the present study, we have investigated the characteristics of the methanation reaction based on the chemical equilibrium calculations, NASA-CEA. We changed the reaction temperature, the ambient pressure and the ratio of carbon dioxide and hydrogen. Six various pressures of 1, 2, 5, 10, 15, 20 atm were applied, with the temperature range of 500 to 1500 K. The methane conversion rate is defined by the deficient reactant of carbon dioxide or hydrogen. Especially, a new parameter of the hydrogen reactivity factor (H.R.F.) is proposed to characterize the initial components in the mixture with the methane conversion rate. To promote the methanation reaction, we have tested to add oxygen as the secondary oxidizer. Results show that, in the case of no oxygen, the methane conversion rate is almost 100% for the excess of hydrogen (H.R.F. < 1). As more carbon dioxide is supplied at (H.R.F. > 1), the methane conversion rate is decreased. The methane conversion rate is reduced at higher reaction temperature or lower ambient pressure, which can be explained by the so-called Le Chatelier's principle. However, the methane conversion rate is enlarged by adding oxygen. This is the case when carbon dioxide is the excess species, and coking is observed with production of graphite through the pyrolysis of methane. Since the mass fraction of graphite is smaller when oxygen is added, oxygen can improve the methane conversion rate due to less coking. For developing the non-catalytic system, the methanation reactor with circulation (MeRCi) has been proposed, where the reactants in the flow are circulated to ensure the enough residence time and utilize the exothermic heat of the methanation reaction.