Conventional fires produce low energy infrared radiation. A normal orange flame may burn at 600 K or so. It isn't hot enough to radiate UV rays but it does radiate in the visible and infrared parts of the electromagnetic spectrum. [physlink.com] The following video shows infrared radiation: Ultraviolet radiation is generated by high energy sources. Ultraviolet = high energy. Infrared = low energy.

Click for full size The following indicates the fires in the WTC wreckage were not of a conventional nature: On 27 September, the officials ordered 2000 gallons of [Pyrocool FEF], which when added to water produces a slippery, low-viscosity foam. ... Berger adds that "Pyrocool also contains two powerful ultra-violet absorbers." [New Scientist] Conventional fires cannot generate ultraviolet radiation, but thermite can. A total of 750,000 gallons of the diluted Pyrocool was spread over ground zero in late September and early October, at a cost of about $120,000. When round-the-clock Pyrocool treatment at the trade center was stopped after a week, Chief Blaich said, there was noticeable progress. But the fires were still burning, in large part because of difficulty in getting the substance down through the debris pile and directly onto hot spots. [nytimes.com] This begs a simple question: how could conventional fires in the upper floors of the twin towers create long lasting hot spots deep in the debris pile?