Deputy Chief Charles Blaich of the New York City Fire Department would not predict when the last fire might be extinguished. But compared to the situation at the end of September, when aerial thermal images showed the whole of Ground Zero to be a hot spot, conditions today are much safer for the workers clearing the rubble.
This is in part due to the use of a special foaming agent called Pyrocool FEF. On 27 September, the officials ordered 2000 gallons of the liquid, which when added to water produces a slippery, low-viscosity foam.
When untreated water meets a greasy or painted surface it forms beads. "But FEF is a blend of surfactants that reduce the surface tension of water," explains Robert Tinsley of Pyrocool.
This makes the water in the foam much "wetter" so that it flows over and coats surfaces.. Paul Berger, a chemist at Pyrocool told New Scientist: "Pyrocool-treated water is able to develop a high surface area relative to total mass, permitting a very rapid heat transfer from the hot object to the water."
And while normal detergents create foams with long-lasting bubbles, FEF foam collapses in seconds or minutes. This allows the liquid to coat and quench burning surfaces.
Berger adds that "Pyrocool also contains two powerful ultra-violet absorbers." These chemicals absorb the high-energy emissions from the fire, which are most able to spread the fire to other materials, and re-emit the energy at a longer, lower-energy wavelength.
The use of FEF foam began on 28 September, with thousands of gallons being pumped into the rubble. One target was the large Freon tanks that had served the WTC air-conditioning system and might have exploded. Blaich told New Scientist: "The foam also extinguished the fires in World Trade Center No 7, the wreckage of a 40-story office tower."
Dec 3 2001: http://www.newscientist.com/article/dn1634
http://www.pyrocooltech.com/PA020.pdf Pyrocool Effective In Extinguishing World Trade Center Fires (someone please transcribe this)