NOVELTY - The apparatus for fuel processing through steam reforming to produce hydrogen to be supplied to fuel cell electric generator, comprises a burner (2), a first steam reforming reactor (3) supplied at the inlet with a flow of fuel to be reformed, first and second vaporizers (6, 7), a second water-gas shift reactor, a third compact preferential oxidation (CO-PROX) reactor (5), and an economizer hydraulically connected in series with the second vaporizer. The first and second vaporizers are sized so as to generate a first- and a second pre-determined flow of steam respectively.    USE - The apparatus is useful for fuel processing through steam reforming to produce hydrogen to be supplied to fuel cell electric generator, where the hydrogen is useful as a carrier gas, as diluent, as a fuel in combustion reactions and in fuel cells and as a reducing agent in many processes and chemical reactions.    ADVANTAGE - The apparatus ensures an efficient nominal operation for fuel processing through steam reforming thus producing high content of hydrogen, enables a significant improvement of the energetic integration of the plant, and enables a reduction of the time necessary to bring the system from idle to the nominal operating conditions.    DETAILED DESCRIPTION - The apparatus for fuel processing through steam reforming to produce hydrogen to be supplied to fuel cell electric generator, comprises a burner (2), a first steam reforming reactor (3) supplied at the inlet with a flow of fuel to be reformed, first and second vaporizers (6, 7), a second water-gas shift reactor, a third compact preferential oxidation (CO-PROX) reactor (5), and an economizer hydraulically connected in series with the second vaporizer. The first and second vaporizers are sized so as to generate a first- and a second pre-determined flow of steam from a predetermined first- and second flow of pre-heated water as a result of a thermal exchange with a flow of flue gases produced by the burner respectively. The first and second vaporizers are hydraulically connected in series with the burner and the first reactor, and in upstream and downstream from the reactor with respect to a flow direction of the flue gases respectively. A unit is arranged for selectively connecting the source of water with the first and second vaporizers respectively in order to selectively supply the first and second flow of water through them. The second water-gas shift reactor and the third CO-PROX reactor are arranged hydraulically in series with first and second vaporizers and in upstream from a selective connection unit of the same to the source of water with respect to the flow of first and second flow of water respectively to preheat the flows of water and cool the second and third reactor. The economizer is arranged in downstream from the second vaporizer with respect to flow of flue gases to heat a flow of air at the expense of the flow of flue gases. The selective connection unit consists of a valve commanded by a control unit. A supply unit of the first and second flow of water is arranged along with the commanded valve. A by-pass unit for the first vaporizer is arranged in parallel with respect to the same in the direction of flow of the flow of flue gases. An INDEPENDENT CLAIM is included for a method of starting up an apparatus for the production of hydrogen to be supplied to a fuel cell electric generator.    DESCRIPTION OF DRAWING(S) - The figure shows a block flow diagram of the fuel processing apparatus.    Burner (2)    First reactor (3)    Second reactor (4)    Third reactor (5)    First vaporizer (6)    Second vaporizer. (7)