NOVELTY - Burner element used in fuel cell system has parallel plates forming a reaction gap and producing heat as a result of catalytic combustion of a fuel gas/oxygen mixture on a catalytic coating provided on one of the plates.    USE - Used in fuel cell system.    ADVANTAGE - Efficient heat coupling and reduction of material escape are achieved.    DETAILED DESCRIPTION - Burner element has parallel plates (20, 22) forming a reaction gap (24) and producing heat as a result of catalytic combustion of a fuel gas/oxygen mixture on a catalytic coating (19, 21) which is provided on one of the plates facing the reaction gap. The heat is released by radiation, convection and conduction directly through the coated plate on a neighboring burner arrangement. At least one of the plates has structural elements likewise having the catalytic coating and protruding into the reaction gap, the elements extending in the flow direction in rows which are arranged across the flow direction and slanted against each other and consisting of ribs or bars. INDEPENDENT CLAIMS are also included for:    (A) a plate used in the burner element; and    (B) a process for controlling the endothermic reforming reaction in a fuel cell system comprising introducing a fuel/oxygen mixture into a reaction chamber and controlling the reaction by introducing diluent air into the reaction chamber.    Preferred Features: The element is square, rectangular or a trapezium shape with the reaction gap having an inlet and an outlet on the first and second opposite-lying sides of the four-sided element so that the fuel gas /oxygen mixture flow from the inlet (26) on the first side to the outlet (32) on the second side. The plates are corrugated, the longitudinal direction of the troughs and peaks of the corrugations extending in the flow direction of the fuel gases.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-section through the burner element. (Drawing includes non-English language text).    Catalytic coating (19, 21)    Plates (20, 22)    Inlet (26)    Outlet (32)