The World Trade Center debris pile fire did not have a significant source of air
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The unprecedented structural fire does not have enough oxygen to rapidly devour its enormous fuel supply - desks, carpets, computers, paper, cars and other combustible material contained in and under the 110-story twin towers, experts say. 911ea.org
“When the trade center towers burned and collapsed, tons of concrete, glass, furniture, carpets, insulation, computers and paper were reduced to enormous, oxygen-poor debris piles that slowly burned until Dec. 19, 2001” ucdavis.edu
"Just in the last week the fires have actually been put out," Gov. George Pataki told a group of about 50 upstate elected officials during a tour of the disaster site on Wednesday.
Battalion Chief Brian Dixon confirmed later Wednesday that the main bodies of fire have been extinguished, although he said small pockets or "hot spots" are still being discovered.
"The FDNY has made great progress in putting out the fires at the World Trade Center," Dixon said. But Dixon noted a firetruck remains on standby at the site, and the department considers it an active fire scene. The fires that began with the Sept. 11 attacks had been strong enough that firetrucks had to spray a nearly constant jet of water on them. At times, the flames slowed the work of clearing the site. "You couldn't even begin to imagine how much water was pumped in there," said Tom Manley of the Uniformed Firefighters Association, the largest fire department union. "It was like you were creating a giant lake." The fires were fueled by almost everything inside the towers, from documents to office furniture. As demolition and rescue crews toiled to clear the debris, air pockets would open up, allowing fresh oxygen to cause hot spots to flare up. Manley said at any given time there were at least 10 firefighters working the hose lines — and more when needed. "You always had at least 10, but if you had numerous fires going you brought in more," he said. For the 75 firefighters working at the site daily, knocking the fires down makes the job of finding remains a bit easier — but it does little to help them emotionally.
Thomas A. Cahill, who leads the DELTA (Detection & Evaluation of Long-Range Transport of Aerosols) group at UC Davis, is more concerned about the possible health risks of the plume from WTC. Cahill first started to wonder about the plume after the rainfall of Sept. 14. "The color of the plume was all wrong," he said. "It was a light blue. My background is atmospheric physics, and the color of the plume tells me a lot. A light blue plume means very fine particles. Clearly, the pile was still hot and was giving off very fine particles." Yet very fine particles, he said, are more characteristic of a very high temperature process, such as a coal-fired power plant, a smelter, or a diesel engine. The pile at ground zero wasn't hot enough to generate such fine particles.
Cahill sent one of his instruments to a colleague in New York who started measuring ambient air on Oct. 2. As Cahill looked at the data, he began to think that perhaps the WTC debris pile was acting like an oxygen-poor municipal waste incinerator, "an enormous ground-level waste incinerator that burned for three months." If there is chlorine in an oxygen-poor municipal waste incinerator, the chlorine can combine with metals in the waste and create volatile compounds (for example, VCl4), which leave with the smoke. "Metals that would normally stay, mobilize and come out of the stack," Cahill said. acs.org
"When you have a huge mass of materials deeply buried like this, it's sort of analogous to the Centralia [Coal Fires mine fire] ," said Dr. Thomas J. Ohlemiller, a chemical engineer and fire expert at the National Institute of Standards and Technology in Gaithersburg, Md. "Very little heat is lost, so the reaction can keep going at relatively low temperatures, provided you have a weak supply of oxygen coming through the debris." nytimes.com
Cahill believes that despite the volume of dust, the high particulate counts, and the fineness of the dust, it was the final aerosols in the anaerobic fire under “the pile” that caused the most danger to the public in releasing unprecedented levels of carcinogens.