I guess there are a few reasons why a fire would produce thick black smoke. Particularly, as someone pointed out, if it's burning synthetics and hydrocarbons. And also at the beginning of a fire. But, with the exception of the beginnings, we do not see heavy black smoke.
My point about smoke and fire is that, if all you're seeing for the duration of the event is smoke, with little sign of flames, and the activity, especially over a period of several hours, does not appear to be growing or spreading, then it's obviously not a raging fire. It appears to be mostly a smouldering one. This is certainly true for WTC7, and appears to be largely true for the towers.