Direct catalytic methanation of biogas - Part I: New insights into biomethane production using rate-based modelling and detailed process analysis

Direct methanation of biogas is a promising application of the Power-to-Gas concept, since up to 80% more methane can be produced in comparison to conventional biogas upgrading methods. Six different processes, in which a bubbling fluidized bed or a fixed bed technology serves as the main reactor, were designed, simulated in detail and evaluated in terms of technical feasibility and product gas quality. Both reactor types showed the same chemical performance, since they are both restricted by kinetic and thermodynamic effects. However, the cooled fixed bed reactor requires about three times more catalyst mass than the bubbling fluidized bed. Both methanation technologies did not reach Swiss or German high calorific gas grid requirements in one step. Further upgrading units are necessary which were often not considered in previous literature. Hence, the technological effort for biogas upgrading is higher than often stated in literature. With a subsequent second-stage fixed bed or a gas separation membrane, every process considered reaches the required product gas quality. It is more challenging to fall below the maximum limit of hydrogen (2 vol-%) than to reach the mandatory methane content for grid injection. The electrolysis clearly dominates the power consumption in all processes.