NOVELTY - The warming up time of water-gas shift reactor (28) is reduced by injecting oxygen to catalyst bed (38) when the reactor is heated. The oxygen reacts exothermically with carbon monoxide (CO) in the feed gas of the reactor and the heat generated supplements the heating the reactor.    USE - For supplementing energy required for heating the water-gas shift reactor in PEM fuel cell.    ADVANTAGE - The exothermic heating is effectively used and the damage of catalyst is prevented as the high concentration CO feed gas is reduced to low concentration CO mixture by reacting the feed gas with oxygen. The time required to heat the water-gas shift reactor is minimized.    DETAILED DESCRIPTION - The fuel system comprises cell for electrochemically reacting hydrogen rich stream with oxygen to produce electricity. The hydrogen stream with concentration of carbon monoxide (A) is reacted in the first reactor upstream for producing hydrogen stream with CO concentration (B) greater than (A). The fuel reactor comprises a catalytic reactor for reducing (B) to concentration (C) equal (A). The reactor (28) has a housing (34) to enclose the catalyst bed (38) for reacting CO with water at super ambient temperature. An improvement unit comprises an oxygen distributor with several outlets in catalyst bed for admitting oxygen at several sites throughout bed during system warming. Oxygen and carbon monoxide are exothermically reacted during which the catalyst bed is heated.    INDEPENDENT CLAIMS are also included for:    (i) process of accelerating the warm-up of gas reactor in fuel cell system, the system comprising a reformer upstream for generating hydrogen rich stream contaminated with CO for fueling the cell, the catalyst operating at super ambient temperature reacting with CO and stream to form carbon dioxide and hydrogen and oxygen being injected into the catalyst to perform exothermic reaction with oxygen and carbon dioxide; heat generated heats the catalyst to attain super ambient temperature and carbon monoxide concentration in the reformate is reduced; and    (ii) process of warming up of a fuel cell system from its normal operating temperature whereby the catalytic reactor is heated to normal temperature to generate stream containing hydrogen contaminated with carbon monoxide of higher concentration; the gaseous stream being passed from reformer to the catalyst bed by heating the bed to super ambient temperature and oxygen being injected to the bed for performing exothermic reaction with oxygen and carbon dioxide thereby heating the bed.    DESCRIPTION OF DRAWING(S) - The figure shows the sectional view of water-gas shift reactor.    Water-gas shift reactor (28)    Housing (34)    Catalyst (38)