Intermittent Operation of Fixed-Bed Methanation Reactors: A Simple Relation Between Start-Up Time and Idle State Duration

Power to gas is a way to convert surplus electricity available on the grid (that might come from renewable sources like solar or wind) and store it as methane. It is also a way to valorize carbon dioxide and reduce CO 2 emissions into the atmosphere. At the heart of the process, the methanation reaction has to achieve high-performance conversion in a completely dynamic mode of operation. This mode of operation—linked to the fluctuations of available power on the grid, and the possible thermal runaway of the reactor due to the exothermic nature of the methanation reaction—means that special care is needed in the design of the reactor as well as its control. The reactor, which might be catalytic fixed-bed, should also restart quickly after an idle period. Start-up requires the reactor temperature to be sufficiently high to ignite the reaction. In this paper, the intermittent operation of a fixed-bed methanation reactor is studied. A mathematical model is developed in order to simulate successive gas shutdowns and reinjections. It is examined as well if the reactor is able to restart spontaneously (i.e. with no external heating, but utilizing thermal inertia and the exothermicity of the reaction), when reactants are reinjected after an idle phase. A relation is found between the respective durations of spontaneous restart and idle phase, which can be used to evaluate whether using an external heat source is preferable to relying on spontaneous restart.