NOVELTY - A reforming reactor (1) comprises a reforming section (4) that is contained within a closed volume (8), a boiler section (3), and a combustion section (2). The heat-exchanging medium required for the reforming of the process fluid in the catalyst tubes is a gas-liquid mixture that self-circulates and is encapsulated inside the closed volume, where at least a portion of closed volume protrudes inside the boiler section. The gas-liquid mixture is a saturated steam-water system circulating at a pressure of 55-110 bar g and 270-320degreesC.    USE - For the conversion of a process fluid, e.g. methanol and dimethyl ether (DME) into hydrogen (claimed), used in industrial applications such as in the metallurgical industry, chemical and pharmaceutical industry and fuel cell power plants for the automotive industry.    ADVANTAGE - The reforming reactor exhibits improved temperature distribution across all catalyst tubes; and is compact and free for mechanical devices for circulating a heat exchanging medium from the high temperature section of the reactor to the reforming section of the reactor. The reforming reactor rapidly and simply achieves or maintains its operating temperature after a change in process conditions, such as a change in hydrocarbon feed flow or temperature or a change in burner conditions or during a start-up operation. The reforming reactor is less sensitive to divergent catalyst packing across the catalyst tubes. The reforming reactor is simple in construction, inexpensive and with lower heat loss than in conventional reforming reactors. The reactor is suitable for use in fuel cell plants that are capable of producing 20-50 kW of power. The gas-liquid mixture provides a large heat sink that enables the accumulation and supply of heat for reforming reaction so that all metal parts within the reactor, particularly the catalyst tubes, maintain or rapidly reach the same temperature. The reactor is less sensitive to temporary changes in process conditions, such as changes in burner duty. The process feed tube extends co-axially of the reactor wall inside the common volume from the reforming section of the reactor to the boiler section of the reactor. This enables the provision of a compact reactor as the at least one feed tube is integrated within the reactor where, the preheating or evaporation of the process fluid can advantageously be effected as the gas in the self-circulating gas-liquid mixture outside the tube condenses. Hence, it is possible to integrate the required evaporation stage inside the reactor thus avoiding the inexpedient provision of separate evaporation means outside the reactor. The reactor ensures a better heat transfer for the preheating of the process gas prior to reforming. The reactor enables the reformed gas from the reactor (hydrogen-rich gas) to be cooled from normally about 280degreesC, which is typical for the reforming of methanol to about 150degreesC, thus reducing the effect required in the air cooler downstream and accordingly also reducing its size.    DETAILED DESCRIPTION - A reforming reactor (1) comprises a reforming section (4) that is contained within a closed volume (8), a boiler section (3), and a combustion section (2). The reforming section contains at least one catalyst tube (20) filled with reforming catalyst (5). The boiler section includes at least one flow channel (11) for the passage of flue gas from the combustion section, and the combustion section includes at least one burner (16). The heat-exchanging medium required for the reforming of the process fluid in the catalyst tubes is a gas-liquid mixture that self-circulates and is encapsulated inside the closed volume, where at least a portion of closed volume protrudes inside the boiler section. The gas-liquid mixture is a saturated steam-water system circulating at a pressure of 55-110 bar g and a temperature of 270-320degreesC.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic of the reactor.    reforming reactor (1)    combustion section (2)    boiler section (3)    reforming section (4)    reforming catalyst (5)    closed volume (8)    flow channels (11)    catalytic burner (16)    catalyst tubes (20)