Go look at the photos of the molten steel flowing down the girders, it was from the nano thermite. A kilm effect could hold some level heat, it would not create it, cannot maintain it for any length of time without a source.

The initial heat came from the molten steel, it was maintained by additional molten steel flowing into the cauldron created in the basement of the tower,which was open in large measure. Again, go look at the photos, the reports of firemen and construction workers.