Chemistry: electrical current producing apparatus – product – and – Having magnetic field feature
Reexamination Certificate
2000-05-31
2003-06-17
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Having magnetic field feature
C429S006000
Reexamination Certificate
active
06579637
ABSTRACT:
TECHNICAL FIELD
This invention relates to fuel cell systems, and more particularly to water separators therefor.
BACKGROUND OF THE INVENTION
Fuel cells in general, and PEM fuel cells in particular, have been proposed for use as electrical power plants to replace internal combustion engines, among other applications. PEM fuel cells are well known in the art, and include a “membrane electrode assembly” (a.k.a. MEA) comprising a thin, proton transmissive, solid polymer membrane-electrolyte having an anode on one of its faces and a cathode on the opposite face. The solid polymer electrolyte is typically made from an ion-exchange resin such as perfluoronated sulfonic acid. The anode/cathode typically comprise finely divided catalytic particles (often supported on carbon particles) admixed with proton conductive resin. The MEA is sandwiched between a pair of electrically conductive elements which serve both as current collectors and means for distributing the fuel cell's gaseous reactants over the surfaces of the electrodes. In such PEM fuel cells, hydrogen is the anode reactant (i.e. fuel), oxygen (i.e. from air) is the cathode reactant (i.e. oxidant), and they react together to produce electricity and water. The cathode/air stream (and sometimes the anode/H
2
stream) is typically humidified to keep the ion-exchange membrane from drying out.
Some fuel cell systems use pressurized, or liquid, hydrogen fuel to fuel the fuel cell. Others store the hydrogen chemically as a thermally dissociable hydride, or physiochemically by heat-releasable adsorption on a suitable adsorbent (e.g. carbon nanofibers). For vehicular applications however, it is desirable to dissociate hydrogenous liquids such as gasoline, methanol, or the like to provide the hydrogen used by the fuel cell owing to the ease with which they can be stored on the vehicle. Gasoline is particularly desirable owing to the existence of a nationwide supply infrastructure therefor. To release their hydrogen, hydrogenous liquids are dissociated in a so-called “fuel processor”.
One known fuel processor for dissociating gasoline to form hydrogen is a two stage chemical reactor often referred to as an “autothermal reformer”. In an autothermal reformer, gasoline and water vapor (i.e. steam) are mixed with air and pass sequentially through two reaction sections, i.e. a first “partial oxidation” (POX) section, and a second “steam reforming” (SR) section. In the POX section, the gasoline reacts exothermically with a substoichiometric amount of air to produce carbon monoxide, hydrogen and lower hydrocarbons (e.g. methane). The hot POX reaction products pass into the SR section where the lower hydrocarbons react with the steam to produce a reformate gas comprising principally hydrogen, carbon dioxide, carbon monoxide and nitrogen. The SR reaction is endothermic, but obtains its required heat from the heat that is carried forward into the SR section from the POX section by the POX effluent. One such autothermal reformer is described in International Patent Publication Number WO 98/08771 published Mar. 5, 1998. The process of producing hydrogen from methanol is similar to that used for gasoline except that the POX step is eliminated and the methanol is delivered directly to a steam reformer where it reacts with steam to produce a reformate comprising H
2
, CO
2
and CO. One known fuel processor for dissociating methanol is a steam reformer such as described in U.S. Pat. No. 4,650,727 to Vanderborgh.
The carbon monoxide concentration in the reformate exiting a steam reformer is too high for the reformate to be used in a fuel cell without poisoning it. Accordingly, the CO concentration must be reduced to a very low level that is non-toxic to the fuel cell. It is known to cleanse the reformate of CO by subjecting it to a so-called “water-gas-shift” (WGS) reaction which takes place in a WGS reactor located downstream of the SR reactor. In the WGS reaction, water (i.e. steam) reacts endothermically with the carbon monoxide according to the following ideal shift reaction:
CO+H
2
O→CO
2
+H
2
Some CO survives the water-gas-shift reaction, and must be further reduced (i.e. to below about 20 ppm) before the reformate can be sent to the fuel cell. It is known to further reduce the CO content of H
2
-rich reformate by selectively reacting it with oxygen (i.e. as air) in a so-called PrOx (i.e. preferential oxidation) reaction which is carried out in a catalytic PrOx reactor located downstream of the water-gas-shift reactor. The PrOx reaction is exothermic and proceeds as follows:
CO+1/2O
2
→CO
2
The combination of a WGS reaction followed by a PrOx reaction is usually sufficient to cleanse the reformate enough that it can be then used in the fuel cell.
It is known to burn the cathode and anode tailgases exiting the fuel cell in a combuster to remove any hydrogen from the system's exhaust gasses, and to provide heat for use elsewhere in the system. e.g. (1) to heat a methanol reformer, or (2) to vaporize liquid fuel and water for use in the system. Moreover, it is known that water management of fuel cell systems that are to be used for vehicular applications (i.e. cars, trucks, buses etc.) is an important consideration. In this regard, it is desirable to collect the water generated by the fuel cell and reuse it elsewhere in the system (e.g., in a fuel processor, water-gas shift reactor or humidifier) where it is needed rather than storing an extra supply of water on-board for such system needs. Moreover, it is desirable to minimize the amount of liquid water in the various system streams so as not to detrimentally effect reactors supplied by such streams. Hence for example, liquid water should be eliminated from the fuel cell tailgases, and particularly the cathode tailgas, that are supplied to the combuster so as not to drown the combuster catalyst, or otherwise suppress combustion of the tailgases therein. Similarly, it is desirable to insure that the H
2
-rich fuel gas supplied to the anode and/or cathode sides of the fuel cell contain little or no liquid water that could either drown the catalyst or flood the fuel cell and thereby reduce its effectiveness. It is likewise desirable to recapture water from the exhaust system from the system's combustor. Accordingly, it is known to provide one or more mechanical water separators at various locations within the system to remove liquid water from the various gas streams and direct it to a water collection site. This practice adds additional equipment to the system which is undesirable for vehicular applications particularly since the water separators that have been used heretofore have been large, inefficient and/or have too much pressure drop which is wasteful of system energy.
The present invention mitigates the undesirable impact of mechanical water separators in fuel cell systems for vehicular applications by providing a separator which (1) is compact so as not to consume much space in the vehicle's engine compartment, (2) has a high separating efficiency so as to ensure a high degree of water removal and collection, and (3) has a low pressure drop.
SUMMARY OF THE INVENTION
The present invention relates to a fuel cell system having a compact, efficient, low-pressure-drop water separator for removing liquid water droplets from water-laden system streams. The invention is applicable to all fuel cell systems that comprise a fuel cell, a source of H
2
-rich fuel-gas for fueling the fuel cell, a source of oxygen (e.g. air) for electrochemically reacting with the H
2
in the fuel cell, and a reservoir for collecting water separated from the various system streams for reuse elsewhere in the system (e.g. in a fuel processor or humidifier for the H
2
and/or O
2
streams). Broadly speaking, the invention is applicable to a fuel cell system comprising (1) a fuel cell, (2) a source of H
2
-rich fuel-gas providing a fuel stream for said fuel cell, (3) a source of oxygen providing an oxidant stream for electrochemically reacting with the H
2
-rich fuel-gas in the fuel ce
Forte Jameson R.
Grover Trevor T.
Jensen Eric K.
Savage David R
Brooks Cary W.
Kalafut Stephen
Wills M.
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