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A fuel cell system having a thermally integrated isothermal CO-cleansing subsystem including a water-gas-shift reactor, a preferential oxidation reactor, at least one system heat exchanger, and a closed-loop heat transfer circuit dedicated to that subsystem. The heat transfer circuit utilizes a heat

A fuel cell system having a thermally integrated isothermal CO-cleansing subsystem including a water-gas-shift reactor, a preferential oxidation reactor, at least one system heat exchanger, and a closed-loop heat transfer circuit dedicated to that subsystem. The heat transfer circuit utilizes a heat transfer medium which has a high specific heat and is circulated through the circuit at a sufficiently high rate as to thermally dominate the gas stream components communicating with the circuit.

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1. In a PEM fuel cell system comprising (i) a primary reactor for converting a carbon-bound hydrogenous fuel into a fuel gas including H2 for said fuel cell, said primary reactor comprising a catalyst for catalytically reacting said fuel with steam to form said fuel gas having a first CO concentrati

1. In a PEM fuel cell system comprising (i) a primary reactor for converting a carbon-bound hydrogenous fuel into a fuel gas including H2 for said fuel cell, said primary reactor comprising a catalyst for catalytically reacting said fuel with steam to form said fuel gas having a first CO concentration and a first H2 concentration, (ii) at least one water-gas-shift reactor for reacting at least a portion of the CO in said fuel gas exiting said catalyst with steam to reduce the CO concentration in said fuel gas to a second CO concentration below said first CO concentration and increase the H2 concentration above said first H2 concentration in said fuel gas, and (iii) a preferential oxidation reactor for selectively reacting at least a portion of the CO in said fuel gas exiting said water-gas-shift reactor with oxygen to reduce the CO concentration in said fuel gas below said second CO concentration and to yield a fuel gas including CO in an amount that is non-toxic to said fuel cell, the improvement comprising:a. said preferential oxidation reactor being an isothermal reactor having a first catalyst selected to effect said selective oxidation of said CO at a first temperature, b. said water-gas-shift reactor being an isothermal reactor having a second catalyst selected to effect the water-gas-shift reaction therein at substantially said first temperature; c. at least one heat exchanger; d. a closed-loop heat transfer circuit communicating with said water-gas-shift reactor, said preferential oxidation reactor, and a heat exchanger, e. a heat transfer liquid circulating in said circuit through said water-gas-shift reactor, said preferential oxidation reactor, and a heat exchanger, and f. a pump adapted to circulate said heat transfer liquid at a rate throughout said circuit as to maintain said water-gas-shift reactor, said preferential oxidation reactor, and said heat exchanger all at substantially said first temperature.