My understanding is that early reports determined the type of explosive by comparing the blast expansion rate against the shutter speed of the cameras which captured the explosion, which provided the detonation velocity. That detonation velocity is specific to explosive type, and therefore the explosive was known to be AN.
I'm not sure where they got the number of 2750 tons - presumably from the size of the blast once the explosive type was known, or from paperwork detailing what was being stored at that location?
In any case, as others have mentioned, AN has a relative yield of .42 compared to TNT. Assuming the amount of 2750 tons is correct, and that nothing else was contaminating the AN, that produces an explosion 1.155kt in size. As to whether that was the actual size of the explosion, I'd say it's a fair estimate, but who knows what else may have exploded along with the AN (fireworks, etc.).
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