A thermodynamic study of sorption-enhanced CO2 methanation at low pressure

Pure carbon dioxide methanation by hydrogen with simultaneous in situ steam removal (sorption-enhanced methanation - SEM) was studied, from a thermodynamic point of view, using the Gibbs free energy minimization method. The focus of the analysis was to describe in detail the carbon deposition boundaries for such a system at low pressure (1-10 at m), and to check the suitability of the equilibrium product gas composition for injection in the natural gas grid. Results show that, while there is an advantage in working under SEM conditions with respect to traditional methanation, steam removal can result in significant carbon generation, especially at high temperature and low pressure. In order to avoid carbon formation, only partial steam capture should be performed if a stoichiometric CO2/H-2 feed is used. On the other hand, total steam capture can be possible without carbon formation if an excess of hydrogen is fed. Limitations regarding the maximum CO and CO2 content in the gas for grid injection could be reasonably overcome. The H-2 content in the product, instead, appears to be the most critical limit to be respected.