The generator has three spaced catalyst-coated honeycomb elements (1,2,3) of which two stand parallel in chamber (4) legs (41,43) as opposed to element (12) which is arranged in the chamber crosspiece (42). The mixture tube (15) should be externally finned (46) and lies in the third element outflow area. The mixture tube passes through a chamber wall which stands parallel to the crosspiece and demarcates an off-gas chamber (48) together with the inner wall (47) of the crosspiece and the walls enclosing the elements. A finned heat exchanger (28) held in this wall (45) is adjoined by a flue (29). Cooling tubes (19) flowed by coolant reach into the chamber in the line of flow downstream of the first and second elements.
