NOVELTY - Once activation energy has been introduced into the ignition zone (3), equipment (V1, 5) supplies at least one, but not the entirety of the media flows (M1, M2) to the reaction zone (2), bypassing the ignition zone.    USE - A reformer reactor producing hydrogen-containing gases from initial materials containing hydrocarbon compound(s) (claimed application). The reformer is suitable for supplying e.g. vehicular fuel cells (especially), domestic fuel cells and power plant fuel cells.    ADVANTAGE - Construction and maintenance are simple. Efficient thermal activation is achieved. The ignition and reaction zones are partially integrated in a convenient arrangement for through-flow. The resulting design is compact and suitable for mobile application. Flow is controlled and diverted to cool the ignition zone. This prevents over-heating of this zone, and of the catalytic material contained. The more typical welded construction is avoided, simplifying maintenance.    DETAILED DESCRIPTION - Lines supplying reagents are interference-fitted to the ignition region. Ignition- and reaction zones are interference-fitted together. The ignition zone includes an igniter. This too, is interference-fitted. The reaction zone includes a mixing region for reagents. It includes catalytically-active material, especially as a surface coating. It includes massive fittings for thermal energy storage. It is double-walled. Reaction occurs internally, with a medium flowing through the interspace. In the two regions arising (internal and interspace) positive and negative reaction enthalpy transfer takes place. A valve controls the flows (M1, M2). It diverts them to the ignition- and/or reaction zone in accordance with control magnitudes. The valve is constructed to supply suitably-matched volumetric flows. Flows are distributed by time-modulated, pulsed opening and closure of the media flow paths, using the valve.    DESCRIPTION OF DRAWING(S) - A simplified flow diagram is presented.    reaction zone (2)    ignition zone (3)    media flows (M1, M2)    valve and bypass system (V1, 5)