NOVELTY - A microcombustor comprises a first section comprising a combustion fuel channel, and a second section disposed next to the first section. The second section comprises a combustion chamber, and an exhaust channel. The combustion fuel channel and the exhaust channel are arranged on a same side w.r.t the combustion chamber to form first heat exchanger.    USE - Used for a steam reformer used in fuel cell (claimed).    ADVANTAGE - The microcombustor has a very small size, steady performance, and operates at low temperature while maintaining high efficiency levels.    DETAILED DESCRIPTION - A microcombustor comprises a first section comprising a combustion fuel channel having an inlet for connecting the microcombustor to a combustion fuel source and an outlet at a top surface of the first section; and a second section disposed next to the first section. The second section comprises a combustion chamber (402) having an inlet in fluid communication with the outlet (410) of the channel of the first section and an outlet capable of evacuating combustion exhaust products; and an exhaust channel (426) having an inlet in communication with the outlet of the combustion chamber and an outlet at a surface of the second section. The combustion fuel channel and the exhaust channel are arranged on a same side w.r.t the combustion chamber to form a first heat exchanger.    INDEPENDENT CLAIMS are also included for the following:    (a) a steam reformer comprising a microcombustor, and a third section comprising a reformation channel having an inlet for supplying reformation fuel and an outlet for evacuating reformation products;    (b) a fuel cell comprising the steam reformer;    (c) steam reforming comprising passing a fluid through the steam reformer;    (d) a microcombustion process comprising providing a composition comprising combustion fuel and oxidant to a combustion chamber; passing the composition through a combustion catalyst; and reacting the composition in the combustion chamber to produce heat;    (e) a steam reforming process comprising passing a reformation gas through a reforming chamber; and maintaining combustion of a combustion fuel in a chamber to transfer heat from the combustion chamber to the reforming chamber (404); and    (f) the production of hydrogen gas comprising passing a composition comprising water and hydrocarbon into a reforming chamber and reacting the water and hydrocarbon to form a hydrogen rich gas mixture; passing a composition comprising fuel and oxidant into a combustion chamber, and simultaneous to the step of reacting the water and hydrocarbon; reacting the fuel and the oxidant in the combustion chamber to produce heat; transferring heat from the combustion chamber to the reforming chamber, where the average thermal transport distance from the combustion chamber to the reforming chamber is at most 1 mm.    The combustion catalyst (416, 418) comprises porous matrix arranged such that moisture flows through the catalyst to maintain a combustion at at most 500degreesC. The step of reacting fuel with oxidant produces heat that sustains the microcombustion process without energy input. The temperature difference between the combustion chamber and the reforming chamber is at most 100degreesC. At least 80% of the fuel is oxidized in the combustion chamber, and the thermal efficiency is at least 5%. The hydrogen gas production is at least 30, preferably at least 1, standard cubic centimeters per minute hydrogen per cc of steam reformer volume.    DESCRIPTION OF DRAWING(S) - The figure is a cross-sectional front view of a microreformer.    Combustion chamber (402)    Reforming chamber (404)    Outlet channel (410)    Catalyst (416, 418)    Exhaust channel (426)