An oxidative methane coupling (OCM) reactor has a reactor vessel that defines a reactor vessel interior. The reactor vessel having opposite ends and a central longitudinal axis that extends between the opposite ends. A catalyst bed assembly is positioned within the reactor vessel interior having a catalyst bed containing a layer of OCM catalyst of a uniform thickness. The catalyst bed assembly divides the reactor vessel interior into an upstream zone and a downstream zone. The catalyst bed has an upstream face for receiving the one or more flowing feed gases of methane and oxygen gas as a flowing mixture from the upstream zone. The upstream face of the catalyst bed is configured to have a total area that exceeds the largest transverse cross-sectional area of the interior of the reactor vessel that is perpendicular to the central longitudinal axis of the reactor vessel. The catalyst bed assembly is configured so that any portion of the flowing mixture passes from the upstream zone to the downstream zone through a single catalyst bed of the catalyst bed assembly.

A process for operating a fuel fired reactor includes introducing fuel into the reactor and burning the fuel in the reactor by means of at least one main burner. Relevant parameters of the process are monitored. Within a predetermined critical operating range for an enforced shut down, a secondary, more stringent operating range is implemented as shut down criteria. The main burner is shut down upon one or more of the relevant parameters leaving the secondary operating range while at least one pilot burner continues to operate as long as the relevant parameters are maintained within the critical operating range.