Electrochemical Oxygen Separation from Aircraft Fuel Tank Ullage

Combustion of oxygen rich fuel vapors in aircraft fuel tanks can lead to explosions and fires. This was demonstrated unfortunately on July 17, 1996 with a Boeing 747 exploding in mid-air due to heated, highly explosive fumes in the center fuel tank. Sloshing of fuel, lightning and static discharge, and, for military aircraft, enemy ground fire can pose significant risks to the fuel tanks when the oxygen content is greater than 9%. Given the importance of fuel safety for both military and commercial aircraft, improved fuel inerting systems are needed that make these fuel tank ullage vapors non-explosive. This is often done by adding a nitrogen-rich gas stream to the fuel tank to reduce the oxygen content to less than 9%, a level that mitigates combustible explosive mixtures. On-board inert gas generating systems (OBIGGS) have been used on aircraft where engine bleed air is used as a pressurized air source flowing through a zeolite adsorption bed to separate oxygen and nitrogen molecules. The resulting enriched nitrogen stream fills the tank ullage lowering the oxygen content to non-explosive limits, however, this occurs at the expense of engine power. For certain military aircraft such as the V-22, the use of compressed engine bleed air to drive the separation process is not desirable, especially in landing zones that throttle back the compressor. For this reason, an electrochemically driven inerting system was developed by Reactive Innovations using a collection of membrane and electrode assemblies to remove the oxygen from the fuel and air mixture. The operating principle behind the separation process is to electrochemically reduce the oxygen in the fuel-air mixture to a water molecule that migrates via diffusion across a polymer ion-exchange membrane. On the opposing anode side, this water molecule is oxidized releasing protons that migrate to the cathode maintaining the separation process. This process is shown in Figure 1. The separator cell consists of a 5 mil thick Nafion membrane that is catalyzed with platinum on the cathode surface and catalyzed with a platinum/iridium oxide mixture on the anode. Air or a mixture of fuel and air flows over the cathode surface while an imposed voltage is applied to drive the process. The resulting current is measured over time while the cathode and anode outlet streams are sent individually to a gas chromatograph to measure the constituent concentrations. To avoid introducing or using external water with this separ