Info

H2 + C02

AH298 =

+3

kJ

(4)

The overall methanol-reforming reaction in the APOR may be written as:

CH3OH + x(02 + 3.76^ + (1-2*)H20 (3-2x)H2 + C02 + 3.76jcN2 (5)

where x is the oxygen-to-methanol molar ratio, and (l-2x) is the theoretical amount of water required to completely convert CO to C02. The energy released (or absorbed) by reaction (5) depends on the value of x. At x = 0, reaction (5) becomes the endothermic steam-reforming reaction (3); at x = 0.5, reaction (5) becomes the exothermic partial-oxidation reaction (1). Reaction (5) becomes thermally neutral at x ~ 0.23. To provide for the sensible heat in the reformate and the heat loss from the reactor, the operating oxygen-to-methanol ratio is a little higher than that needed for thermal neutrality.

The bench-scale APOR built and tested in our laboratory is shown schematically in Fig. 3 [4], It consists of a cylindrical reactor packed with a copper-zinc oxide catalyst (both pellet and honeycomb catalyst structures have been tested). Methanol and water are injected as a fine spray (by using an ultrasonic nozzle) into a down-flowing air stream. The fuel-water-air mixture flows past a nichrome wire "igniter," which vaporizes a small amount of the methanol. The methanol is oxidized on the surface of the catalyst, and the heat generated rapidly raises the temperature near the inlet end of the catalyst bed to =500°C; methanol decomposition, steam-reforming, and the water-gas shift reactions then decrease the temperature to =200°C at the reactor exit. The reformate contains -50% H2, -1% CO, and no CH4 (see Fig. 4). The APOR needs no external heating or cooling. The reformate from the APOR can be fed to a phosphoric acid fuel cell as is, but it must be conditioned (e.g., preferential oxidation to reduce CO plus water injection to cool and humidify) before being fed to a polymer electrolyte fuel cell. Discussion

A big advantage of the APOR over the more conventional steam reformer arises from the absence of indirect heat transfer, thus avoiding the weight and volume of the heat exchange components in a steam reformer. For example, the weight and volume of the methanol steam reformer in the transit bus powered by a 50-kW phosphoric acid fuel cell are 266 kg and 415 L, respectively [5]; the corresponding values for the APOR are less than 35 kg and 25 L.

Because the mass (and the corresponding thermal mass) of the APOR is lower than that of the steam reformer, the time and fuel consumed during reformer start-up are reduced by at least one order of magnitude. Figure 5 shows that in the APOR, significant hydrogen is produced in less than two minutes; the steam reformer on the fuel cell bus requires at least 30 minutes.

The reformate from the APOR does have a lower H2 concentration than that from a steam reformer (50% rather than 70-75%), leading to a small decrease (-10 mV) in cell voltage. Another disadvantage of the APOR is that the fuel cell anode must accommodate a 50% greater flow rate, requiring wider flow passages and leading to a decreased power density of the fuel cell stack. The reduced power density is offset, however, by the comparative simplicity of the fuel cell system using the APOR instead of a steam reformer. Simplifications include elimination of the recycle loop and the reformer burner, as well as the air and fuel preheater and/or vaporizer.

The APOR uses the copper-zinc oxide catalyst in the oxidized form, which does not sinter easily and can withstand high temperatures without degradation. The catalyst needs no activation before use, nor sequestration between uses, and may routinely be heated to 500°C or greater. The steam reformer uses a similar catalyst, but in a reduced form; the catalyst must be kept isolated from air, which would teoxidize it and render it ineffective. Also, the reduced catalyst sinters readily, and temperatures above ~280°C must be avoided. Hydrocarbon Reforming

There is a great deal of interest in operating fuel cell vehicles on conventional gasoline and diesel fuels. The APOR concept has been used to reform the simple hydrocarbons octane and pentane as surrogates for such fuels. Preliminary tests with selected catalysts have yielded H2 concentrations >40%. Research to develop improved catalysts is continuing.

Conclusion

Argonne's partial-oxidation reformer is a compact, lightweight, rapid-start, and dynamically responsive device to convert liquid fuels to H2 for use in automotive fuel cells. An APOR catalyst for methanol has been developed and tested; catalysts for other fuels are being evaluated. Simple in design, operation, and control, the APOR can help develop efficient fuel cell propulsion systems. Acknowledgment

This research was supported by the U.S. Department of Energy, Office of Advanced Automotive Technologies in the Office of Transportation Technologies, under contract number W-31-109-Eng-38. References

1. "Reformers for the Production of Hydrogen from Methanol and Alternative Fuels for Fuel Cell Powered Vehicles," R. Kumar, S. Ahmed, M. Krumpelt, and K. M. Myles, Argonne National Laboratory Report ANL-92/31, 1992.

2. "Fundamentals of Fuel Cell System Integration," M. Krumpelt, R. Kumar, and K. M. Myles, Journal of Power Sources, 49, 37-51 (1994).

3. "Dynamic Response of Steam-Reformed, Methanol-Fueled, Polymer Electrolyte Fuel Cell Systems," H. K. Geyer, R. K. Ahluwalia, and R. Kumar, Proceedings of the 31st Intersociety Energy Conversion Engineering Conference IECEC 96, August 11-16, 1996, Washington, DC, Vol. 2, pp. 1101-1106.

4. "Fuels Processing for Transportation Fuel Cell Systems," R. Kumar and S. Ahmed, Proceedings of the First international Symposium on New Materials for Fuel Cell Systems, July 9-13, 1995, Montreal, Canada, pp. 224-238.

5. "Research and Development of a Phosphoric Acid Fuel Cell/Battery Power Source Integrated in A Test-Bed Bus," Phase II Final Report HPC-0144, H-Power Corp., Belleville, NJ, May 30, 1996, p. 24.

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