Ground Zero's fires still burning

Almost 12 weeks after the terrorist atrocity at New York's World Trade Center, there is at least one fire still burning in the rubble - it is the longest-burning structural fire in history.

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."

Ultra-violet absorbers

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."

Another strategy that can be used to put out difficult fires is pumping an enclosed area full of the inert gas nitrogen, starving the fire of oxygen. But Ground Zero is thought to be too large and porous for this to be effective.

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Rescue operation

Tinsley says there are several reasons for the longevity of the fire: "First, this is not a typical fire by any means. The combustible debris is mixed with twisted steel in a mass that covers 17 acres, and may be 50 metres deep. This is the one all future fire scenes will be measured against."

The other reasons are human. For nearly three weeks, Tinsley says, city officials insisted that work at Ground Zero was a rescue operation, meaning it would have been inappropriate to flood the rubble with water. As a result, he says, "the fires had a 17-day head start when we arrived."

And there is the issue of human remains. These are still being found and removed and, since the fires are not threatening any property or lives, they are being allowed to burn on.

Jonathan Beard, New York