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The source of air comes from the atmosphere outside the tank. Are you saying it was some kind of vacuum tank? That would be amazing and would go against the common practice of pumping inert gas into fuel tanks to prevent explosions (something TWA 800 didn’t have).

For what it's worth, I believe someone pointed out earlier that the plane was at 13,000 feet. I believe at 10,000 feet a source of pressurized air is required for passengers.

My recollection of fire is that people can breath levels of oxygen which are so low that a fire cannot be sustained.

So how much air would there have been in the tanks at 13,000 feet? Probably not enough.

There is air in the tanks. There’s plenty of evidence for that you can find by doing an Internet search. Here’s an example:

Later this year, the FAA plans to order reductions in the flammability of gases that float in the void above Jet A fuel in airliner tanks. Airlines and manufacturers will be able to meet the requirement by using systems based on a prototype developed by the FAA—an onboard inert-gas generation system (OBIGGS), which replaces much of the air in a fuel tank with nitrogen, a gas that does not support combustion. The technology “will virtually eliminate the possibility of future fuel tank explosions,” FAA Administrator Marion Blakey said in February when announcing the agency’s plans to recommend that 3,800 Boeing and Airbus airliners be fitted with inerting technology.

http://www.airspacemag.com/how-things-work/safer-fuel-tanks-5883916/?no-ist

Go to page 131 of the NTSB report and you’ll find the extensive testing and research done on the flammability of the fuel air mixture in the center tank of TWA 800 at the accident altitude.

http://www.ntsb.gov/investigations/AccidentReports/Reports/AAR0003.pdf

For what it's worth, I believe someone pointed out earlier that the plane was at 13,000 feet. I believe at 10,000 feet a source of pressurized air is required for passengers.

My recollection of fire is that people can breath levels of oxygen which are so low that a fire cannot be sustained.

So how much air would there have been in the tanks at 13,000 feet? Probably not enough.

Aircraft cabin pressure is maintained, depending on the altitude of the plane at between the equivalent of a surface altitude of 6000 to 8000 feet above sea level. Fires often are sustained at those altitudes, so that would not be a problem. However, at 13,800 feet, the flash point mixture for a Jet A/air mixture is considerably different than the flash point at sea level, requiring quite a bit more air and it has to be aerated quite a bit better than just would be achieved by heating the fuel. In addition, the cold air from the exterior of the plane at 13,800 feet would have condensed what fuel there was in the atmosphere in the Center Wing Tank back into the puddle of ullage.

One of the scenario's the NTSB claims was possible was a spark from a fuel pump, but all the wiring is external to the pump's exposure to fuel. . . and the pump would be under the fuel level in any case, unexposed to the fuel/air mixture. In addition, the pump switch for the Center Wing Tank was in the OFF position. The other was a spark in a fuel gauge for the CWT, but again, all the electrical is external to the tank and low voltage to boot. They postulated that the low voltage wiring shared a wire chase with high-voltage wiring, and somehow high-voltage shorted to the low voltage wire. . . but Boeing claims all low-voltage wiring is run separately from high-voltage wires. Oops, that doesn't work either.