TECHNOLOGY FOCUS - MECHANICAL ENGINEERING - Preferred Processing Subsystem: The fuel processing subsystem further comprises (1) a hydrogen sulfide scrubber fluidly connected and located downstream of the autothermal reformer for receiving the reformate stream, (2) a shift reactor fluidly connected and located down stream of the hydrogen sulfide scrubber to receive the reformate stream and (3) a decomposer for converting the hydrogen peroxide to pure oxygen to supply to the stack.Preferred System: The hydrogen separator is plate-and-frame, spiral wound or a hollow fiber module and can tolerate transmembrane pressure differentials of at least 2750 (preferably at least 3450) kPa. The fuel cell stack comprises at least one solid polymer electrolyte fuel cell and an array of fuel cell stacks. The burner is a catalytic burner, which is fluidly connected to receive the cathode exhaust stream from the fuel cell stack. The system comprises a turbo-compressor for compressing air to produce a compressed air stream and is connected to a turbo-expander, which is fluidly connected to receive the burner outlet stream from the burner. The system further comprises (1) an oxidant and water supply for supplying oxidant and water respectively to the fuel processing subsystem, (2) a compressor fluidly connected to the carbon dioxide scrubber for pressurizing the oxygen-containing stream supplied to the burner and (3) a fuel supply for supplying the fuel to the fuel processing subsystem.Preferred Method: The method further comprises (1) supplying the hydrogen peroxide stream to the decomposer, (2) regulating the pressure of the reformate stream so that the pressure is greater than the external water pressure adjacent the hull of the submarine and not less than the minimum operating pressure of the separator and (3) supplying the cathode exhaust to the burner. The air stream in the turbo-compressor is compressed to produce the oxidant stream. The burner outlet stream is supplied to the turbo-expander.Preferred Stream: The reformate stream further comprises hydrogen sulfide. The fuel stream comprises a fuel selected from diesel, gasoline or liquid synthetic hydrocarbon fuel (preferably No. 2 diesel fuel). The reformate and the burner outlet streams are supplied to the hydrogen separator and the carbon dioxide scrubber respectively at a pressure of 2750 - 4150 (preferably 3450 - 4150) kPa. The hydrogen and the oxidant streams are supplied to the stack at a pressure of at most 700 (preferably 120 - 550) kPa.INORGANIC CHEMISTRYPreferred System: The oxidant supply comprises liquid oxygen, hydrogen peroxide or air. The hydrogen sulfide scrubber is a metal oxide (preferably zinc oxide) absorbent bed or a hot carbonate scrubber. The membrane is supported and is selected from palladium membrane, palladium alloy membrane, platinum membrane, platinum alloy membrane, titanium alloy membrane, ceramic membrane, zeolite molecular sieve membrane, carbon molecular sieve membrane and/or inorganic poly-acid membrane (preferably palladium membrane or palladium alloy membrane). EXAMPLE - None given.