NOVELTY - The chemically heated reformer comprises a combustion chamber assembly, a gas chamber filled with the reforming catalyst, a gas chamber designed as a side burner, which is filled with an oxidation catalyst, stands in thermal contact with the reforming side by common walls and comprises a reforming chamber arranged adjacent to a partition wall of the combustion chamber assembly, a flow-guiding structure (1e), and a material-transfer-limiting structure (1g) designed in the form of a punched or a porous element and arranged between the flow-guiding structure and the oxidation catalyst.    USE - The chemically heated reformer is useful in a fuel cell system for generating hydrogen-containing gas by external reformation.    ADVANTAGE - The chemically heated reformer is capable of efficiently generating hydrogen-containing gas in a rapid manner.    DETAILED DESCRIPTION - The chemically heated reformer comprises a combustion chamber assembly, a gas chamber filled with the reforming catalyst, a gas chamber designed as a side burner, which is filled with an oxidation catalyst, stands in thermal contact with the reforming side by common walls and comprises a reforming chamber arranged adjacent to a partition wall of the combustion chamber assembly, a flow-guiding structure (1e), and a material-transfer-limiting structure (1g) designed in the form of a punched or a porous element and arranged between the flow-guiding structure and the oxidation catalyst. The oxidation catalyst is oriented to the partition wall. The combustion chamber has a graduated or graded opening degree of the material-transfer-limiting structure along the flow direction. The material transfer-limiting structure is designed in the form of an optionally compressed metal froth, a porous sintered metal or inorganic structure, a woven structure, a perforated plate and an expanded metal or a metallic or inorganic flies/felt. INDEPENDENT CLAIMS are included for:    (1) a combustion chamber assembly; and    (2) a fuel cell system.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a chemically heated reformer.    Reaction chambers (1a, 1b)    Walls (1c)    Flow-guiding structure (1e)    Oxidation catalyst material (1f)    Material-transfer-limiting structure. (1g)