Jim Hoffman tells us that a gravitational collapse of the WTC couldn't release enough energy to both pulverise the concrete, and heat it up into the expanding dust clouds observed.
Hoffman boosts the energy required to pulverise the concrete by assuming, in his calculations, that it was all turned to dust. Actually in his text he admits it was "nearly all", which leads to the question: how much is that? And how do we know? Whatever the answer, this makes his first figure potentially an overestimate.
And second, he treats the tasks of pulverising the concrete, then heating it up, as entirely separate. So, for instance, we read that crushing the concrete may have required 135,000 KWH, while heating it could have required 1,145,000 (low estimate).
Of course this is missing a point, because the kinetic energy used to pulverise the concrete didn't simply disappear. Much of it became heat, and so crushing the concrete would also increase its temperature.
What difference would this make? We don't know, but it's another flaw in the calculations. Heating the concrete dust need not require as much additional energy as Hoffman claims.