2011

Catalytic Generation of Lift Gases for Balloons

Relatively lightweight, low-power gas generators are based on methanol reforming.

A lift-gas cracker (LGC) is an apparatus that generates a low-molecular-weight gas (mostly hydrogen with smaller amounts of carbon monoxide and/or carbon dioxide) at low gauge pressure by methanol reforming. LGCs are undergoing development for use as sources of buoyant gases for filling zero-gauge-pressure meteorological and scientific balloons in remote locations where heavy, high-pressure helium cylinders are not readily available. LGCs could also be used aboard large, zero-gauge-pressure, stratospheric research balloons to extend the duration of flight.

Selected Parameters are presented in comparison of an LGC capable of generating hydrogen-based lift gas and an electrolytic apparatus capable of generating hydrogen — both at a rate of 100 standard liters per minute." class="caption" align="right">Methanol reforming has been investigated as a means of generating hydrogen for fuel cells. Although the product-gas specifications, process-stream, and control requirements for fuel-cell applications differ from those of lift-gas applications, the underlying methanol-reforming principle is the same for both classes of applications, and some of the heat-exchange and catalyst design requirements from fuel-cell applications are adaptable to lift-gas applications.

In the methanol reforming reactor that lies at the heart of an LGC, methanol is catalytically cracked to carbon monoxide and hydrogen in an endothermic reaction, typically at a temperature in the approximate range of 250 to 350 °C and at a pressure that can lie in a range from somewhat below to somewhat above standard sea-level atmospheric pressure. A small portion of the methanol feed is diverted to a low-pressure combustor to provide the heat for the endothermic reforming reaction and maintain the reactor at the reaction temperature.

When the feedstock is pure methanol, the overall chemical reaction is CH3OH → CO + 2H2. In a steam-reforming variant, the feedstock is a mixture of methanol and steam, typically comprising equal numbers of methanol and water molecules, in which case the overall chemical reaction is CH3OH+H2O→CO2+3H2. The optimum choice of temperature, pressure, and catalyst depends on details of the specific application. The exact formulations of methanol-reforming catalysts are proprietary; what is known is that most of them include copper oxide and zinc oxide on alumina supports.

The only consumables needed for the methanol-reforming process in an LGC, other than methanol, are air and a small amount of electrical power for an air blower and for instrumentation. In principle, an apparatus that generates hydrogen by electrolysis of water could be used as an alternative to an LGC, but an electrolytic apparatus would be less advantageous in several ways: As shown by example in the table, relative to an electrolytic apparatus capable of producing hydrogen at a given rate, an LGC capable of producing hydrogen-based lift gas at the same rate is much less massive and requires much less electrical power and much less fuel. Moreover, the LGC is more reliable and robust.

As contemplated for use in extending the duration of flight of a high-altitude balloon, an LGC would provide the lift gas for an auxiliary buoyancy-control balloon separate from a main lift balloon. The buoyancy-control balloon would be used to compensate for changes in buoyancy associated with diurnal/nocturnal variations in temperature. In this application, the LGC would produce the lift gas by catalytic reforming of methanol at night. During the day, some of the lift gas would be burned with atmospheric air to produce water for use as ballast. At night, the water ballast could be dropped or could be recycled to the LGC for steam reforming of methanol. In this approach, the duration of flight could be extended by a factor of as much as four, relative to a conventional approach in which ballast is dropped at night and gas is vented during the day.

This work was done by Robert Zubrin and Mark Berggren of Pioneer Astronautics for Goddard Space Flight Center. GSC-14792-1

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