Creative Speculation

The debris pile at Ground Zero was always tremendously hot. Thermal measurements taken by helicopter each day showed underground temperatures ranging from 400ºF to more than 2,800ºF. The surface was so hot that standing too long in one spot softened (and even melted) the soles of our safety shoes.

Professional Safety The American Society of Safety Engineers 2002-05-01 - SH&E at Ground Zero Disaster Response

also this...
1. Researchers have found iron spheres in the WTC dust. Note emphasis on iron.

2. Spheres in fly ash that have some form of iron in them are extremely rare, and fly ash "iron" spheres are actually iron-oxide spheres. Note emphasis on oxide. http://www.sciencedirect.com/science/article/pii/S0016236106001906

Therefore, fly ash does not explain the iron spheres found in the WTC dust. Carbon-fueled furnaces leave oxides, in this case iron-oxide. Fire is a form of oxidation = There must have been some other source for the iron spheres.

3. The red-gray chips ignite and leave iron spheres. Again, there is elemental iron in those spheres.

Thermite leaves elemental iron. Researchers have found red-gray chips in the dust that ignite and form iron spheres, this is in fact why those people claim the chips are a form of thermite! So-far, this is the only explanation for the observed iron spheres in the dust.