Thermally integrated staged methanol reformer and method

Details Thermally integrated staged methanol reformer and method Thermally integrated staged methanol reformer and method

1998-11-05

2001-05-29

Kalafut, Stephen (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Having magnetic field feature

C429S006000, C422S186220, C422S200000, C422S202000

Reexamination Certificate

active

06238815

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a fuel processor for an H
2
-fueled fuel cell and method of operating same in a thermally integrated manner.
BACKGROUND OF THE INVENTION
Fuel cells have been proposed for many applications including electrical vehicular power plants to replace internal combustion engines. Hydrogen is often used as the fuel and is supplied to the fuel cell's anode. Oxygen (as air) is the cell's oxidant and is supplied to the cell's cathode.
The hydrogen used in the fuel cell can be derived from the reforming of methanol in a catalytic reactor known as a reformer. In the methanol reforming process, methanol and water (vapors) are ideally reacted under isothermal conditions to generate hydrogen and carbon dioxide according to the following endothermic reaction:
CH
3
OH+H
2
O→CO
2
+3H
2
This reaction is carried out within a reformer that is heated by exhaust gases from a methanol-fired or hydrogen-fired combuster, and yields a reformate gas comprising hydrogen, carbon dioxide, carbon monoxide, and water. One such reformer is described in U.S. Pat. No. 4,650,727 to Vanderborgh, and one such combuster is described in copending United States patent applications U.S. Ser. No. 08/975,422 (abandoned) and Ser. No. 08/980,087 now U.S. Pat. No. 6,077,620, filed in the name of William Pettit in November 1997, and assigned to General Motors Corporation, assignee of the present invention. Carbon monoxide is contained in the H
2
-rich reformate/effluent exiting the reformer, and must be removed, or reduced to very low concentrations (i.e., less than about 20 ppm), which are nontoxic to the catalyst in the anode of the fuel cell.
It is known that the carbon monoxide, CO, content of the reformate can be reduced by the a so-called “water-gas shift” reaction which can take place within the reformer itself (depending on the operating conditions of the reformer), or in a separate shift reactor downstream from the reformer. In the water-gas shift reaction, water (i.e., steam) reacts with the carbon monoxide according to the following ideal endothermic shift reaction:
CO+H
2
O→CO
2
+H
2
Some CO still survives the water-gas shift reaction and needs to be reduced to below about 20 ppm before the reformate can be supplied to the fuel cell. It is known to further reduce the CO content of H
2
-rich reformate by reacting it with air in a so-called “PrOx” (i.e., preferential oxidation) reaction carried out in a catalytic PrOx reactor. In the PrOx reactor, air preferentially oxidizes the CO in the presence of the H
2
, but without consuming/oxidizing substantial quantities of the H
2
. The PrOx reaction is exothermic and proceeds as follows:
CO+½O
2
→CO
2
The PrOx reactor effluent is then supplied to the fuel cell.
For vehicular power plants, these reactions must be carried out as efficiently as possible and in the most compact space possible. This requires optimal use of available heat to maintain reactor temperatures at their operating temperatures.
SUMMARY OF THE INVENTION
The present invention involves a unique thermally-integrated fuel processor including an isothermal reformer for the two stage conversion of methanol into a H
2
-rich fuel gas for a fuel cell. A gaseous heat transfer medium comprising principally water vapor, methanol vapor, hydrogen and carbon dioxide formed in one stage of the reaction is circulated throughout the fuel processor to transfer heat within the fuel processor. The H
2
O
(v)
—CH
3
OH
(v)
—H
2
—CO
2
medium is an excellent heat transfer medium having a higher heat carrying capacity than just H
2
O
(v)
and/or CH
3
OH
(v)
. A small portion of the circulating medium is drawn off as an input stream to the second stage of the reaction which completes the reforming. More specifically, the invention contemplates a thermally-integrated fuel processor which includes a housing containing a heater, first and second stage catalytic reforming reactors for converting the methanol and water to hydrogen, and a fan that recirculates a gaseous heat transfer medium throughout the housing around and through the heater and first and second stage reforming reactors. During regular operation the medium comprises first concentrations of water vapor, hydrogen, carbon dioxide and methanol vapor formed in the first stage of the conversion reaction. During initial startup, the heat transfer medium comprises a startup gas such as hydrogen, carbon dioxide, or preferably an inert gas such as argon, helium or nitrogen. Water and methanol are injected from separate inlets, or a combined inlet, into the medium as it circulates within the housing. The heater is positioned in the housing downstream of the fan (i.e., in the direction of medium flow in the housing) for heating the circulating heat transfer medium sufficiently for the medium to heat the catalytic reactors which are located downstream of the heater. The first reactor is located downstream from the heater and endothermically converts a small portion of the heat transfer medium which is diverted from the recirculating medium into a reformate gas having second concentrations of hydrogen and carbon dioxide greater than the first concentrations, and concentrations of water and methanol vapors less than the first concentrations.
The first reactor includes a plurality of first and second channels separated and isolated each from the other by a thermally conductive partition defining the channels. The first plurality of channels are adapted to receive the diverted medium portion, and contain a first catalyst for effecting the conversion. The second plurality of channels have an inlet thereto adapted to admit heated medium from the heater into the second plurality of channels for heating the first catalyst in the first plurality of channels. The second plurality of channels also have an outlet for discharging the circulating medium from the second channels after it has heated the catalyst in the first plurality of channels. The first reactor is preferably a cross-flow reactor in which the first plurality of channels cause the diverted medium portion to flow therethrough in one direction, and the second plurality of channels cause the recirculating medium to flow therethrough in another direction transverse (e.g., normal to) the one direction. The general trend for the flow of the diverted medium within the first channels is preferably in the same direction as the flow of recirculating medium in the second channels (i.e., “co-flow” configuration). Most preferably, the first plurality of channels of the first reactor cause the medium portion to flow back and forth therethrough in a serpentine path for maximum residence time and insure substantially complete conversion of the methanol and water to hydrogen while at the same time the co-flow configuration causes the temperature in both the recirculating medium and diverted medium portion to decrease from one end of the reactor to the other thus producing favorable (i.e., cooler) conditions at the outlet of the first reactor for achieving lower CO concentrations.
The housing also encloses a second reactor which is located downstream from the heater, and preferably downstream from the first reactor. The second reactor receives and passes heated medium into direct contact with a second catalyst which promotes the endothermic reaction of some (e.g., about 20% to about 80%, and preferably 50% by vol.) of the water and methanol vapors in the heated medium to form principally hydrogen and carbon dioxide in the medium. Some carbon monoxide is also produced.
A shunt within the housing diverts a small (e.g., up to about 25% by vol.) portion of the circulating medium into the first plurality of channels of the first reactor where the second stage reaction occurs in the formation of the reformate. The diverted portion will preferably comprise about 5% by volume of the recirculating medium and is determined by the rate at which fresh water and methanol are introduced into the housing.
In a preferred embodiment, the heater compris

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