NOVELTY - Combustion preheater (H1) or (H2) to transfer heat from post water-gas shift reformate flow (a) to combustor feed (b) comprises: housing with first and second axially extending, concentric annular passages in heat transfer relation; and convoluted fin in them to direct (a) and (b) respectively, or cylindrical wall; with first and second convoluted fin with axial convolutions bonded to inward and outward wall surface, respectively, to direct (a) and (b) respectively.    USE - As a combustion preheater for fuel processing system to preheat combustor feed by transferring heat from post water-gas shift reformate flow to combustor feed (claimed), useful for production of small scale hydrogen production for refueling of fuel cells.    ADVANTAGE - The combustor preheater transfers heat from post water-gas shift reformate flow to combustor feed and transfer the heat required for the steam reforming reaction so as to result in a smaller scale distributed production of hydrogen at high level of product efficiency required for transportation fuel cell infrastructure in refueling of fuel cell vehicles; without extremely high metal temperatures. The bonding of the fins in the preheater allows for thermal expansion relative to other portions of the housing, and improved heat transfer and even distribution of the combustion reaction over the length of the reactor, that results in a highly thermally integrated and volumetrically compact high-pressure steam reforming fuel processor. This results in maximized conversion of methane to hydrogen with minimization of the required effectiveness of a recuperator; and further avoids the need for additional natural gas, which then simplifies the control of the fuel processor.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a fuel processing system comprising: a water-gas shift reactor; a combustor; and (H1) or (H2) connected downstream from the water-gas shift reactor with respect to (a) provided by the reactor and upstream from the combustor to direct (b) to the combustor after transferring heat from (a) to (b).