NOVELTY - The hydrogen-generation assembly (46) useful in electrochemical fuel cells, comprises a fuel processing system having a hydrogen-producing region (70) to receive a feed stream and to produce a mixed gas stream (74) containing hydrogen gas and other gases, a pressure swing adsorption (PSA) assembly (73) adapted to receive the mixed gas stream, to produce a product hydrogen stream and to produce a byproduct stream (76) containing a substantial portion of the other gases, and a heating assembly to receive and combust the byproduct stream to produce a heated exhaust stream.    USE - The assembly is useful for generating pure hydrogen gas, which is useful in electrochemical fuel cells to convert hydrogen and oxygen into water and electricity, and useful in fuel processing system to produce electrical power, components of the energy-producing system such as feedstock delivery systems, heating assemblies, separation regions or devices, air delivery systems, fuel delivery systems, fluid conduits, heat exchangers, cooling assemblies, sensor assemblies, flow regulators and controllers, a fuel cell stack and an energy-consuming device, where the energy-consuming device is useful in motor vehicles, recreational vehicles, boats and other sea craft and also in residences, commercial offices or buildings, neighborhoods, tools, lights and lighting assemblies, appliances, computers, industrial equipment, signaling and communications equipment, radios, electrically powered components on boats, recreational vehicles or other vehicles and battery chargers.    ADVANTAGE - The hydrogen-generation assembly is capable of effectively removing impurities from the impure hydrogen stream or mixed gas stream produced in the fuel processing system with high efficiency.    DETAILED DESCRIPTION - The hydrogen-generation assembly (46) useful in electrochemical fuel cells, comprises a fuel processing system having a hydrogen-producing region (70) to receive a feed stream and to produce a mixed gas stream (74) containing hydrogen gas and other gases, a pressure swing adsorption (PSA) assembly (73) adapted to receive the mixed gas stream, to produce a product hydrogen stream and to produce a byproduct stream (76) containing a substantial portion of the other gases, a heating assembly to receive and combust the byproduct stream to produce a heated exhaust stream, which is adapted to heat the hydrogen-producing region, and a unit for regulating the flow rate of the byproduct stream to maintain the temperature of the hydrogen-producing region within a determined temperature range for producing the mixed gas stream. The hydrogen stream contains substantially pure hydrogen gas and has a reduced concentration of the other gases than the mixed gas stream. The pressure swing adsorption assembly includes adsorbent beds to separate the mixed gas stream into streams forming the product hydrogen stream and the byproduct stream. The hydrogen-producing region includes a steam reforming region to produce the mixed gas stream from water (80) and a carbon-containing feedstock (82), which includes a hydrocarbon, methanol and the determined temperature range of 375-450 degrees C. The unit regulates the flow rate of purge gas to the adsorbent beds of a PSA assembly to maintain the concentration of carbon monoxide in the heated exhaust stream below 50 ppm. The regulating unit is adapted to selectively increasing and decreasing over a time period the flow rate of the purge gas, and selectively delivering a purge stream to the adsorbent beds according to a variable-flow profile. The variable-flow profile includes an initial portion in which the purge gas is delivered at a flow rate that is less than an average flow rate for the purge gas through the adsorbent beds, and a subsequent portion in which the purge gas is delivered at a flow rate that is greater than the average flow rate for the purge gas through the adsorbent beds. The regulating unit includes a first means for maintaining the fuel value of the byproduct stream within a predetermined range of a determined fuel value. The regulating unit ramps the flow rate of the purge gas to the adsorbent beds. The regulating unit delivers the flow rate of purge gas at an initial flow rate of purge gas that is less than 75% of an average flow rate of purge gas and increases the flow rate of the purge gas. The regulating unit includes second means for selectively distributing a volume of purge gas to the adsorbent beds responsive to a predetermined flow profile, which includes a non-linear profile, an initial flow rate of purge gas and a subsequent flow rate of purge gas that is greater than the initial flow rate. The initial flow rate is less than 50% of the subsequent flow rate. The initial flow rate is 25-75% of an average flow rate of the purge gas during a purge stage of a PSA cycle, and is maintained for 10% of the purge stage. The flow profile includes a portion in which the flow rate of purge gas is decreasing with time. The second means maintains the flow rate of the byproduct stream within a predetermined range of a determined flow rate. The determined flow rate corresponds to a flow rate in which the heated exhaust stream is adapted to maintain the hydrogen-producing region within a selected temperature range for producing the mixed gas stream.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a hydrogen-producing assembly in the form of a steam reformer adapted to produce a reformat stream containing hydrogen gas and other gases from water and carbon-containing feedstock.    Hydrogen-generation assembly (46)    Hydrogen-producing region (70)    Pressure swing adsorption assembly (73)    Mixed gas stream (74)    Byproduct stream (76)    Water (80)    Carbon-containing feedstock. (82)