2700 tonnes of ammonium nitrate, apparently (about 1.1 kilotons of TNT)
That's a lot of fertilizer and having previously worked on smallholdings where they have stored it. It doesn't take much to set that baby off when coming into contact with any potential ignition, considering it's got a fairly low activation rate (around 200c from memory).
It isn't as powerful as TNT though but it is certainly more unstable. Unless by TNT you mean dynamite? But those are also two different things.
Anyhow, 2700 tonnes is a hell of a huge stockpile. That must've been targeted at a supposed, specialised warehouse or similar. When I saw the explosion, my first thought was that it was one hell of a gas leak (seriously, it may have started via steam pipes). For me the second explosion has to be a chain reaction, in relation to any decomposition.
One big question that may be asked (regardless of terror attack or not) would be why was so much stored in just one place? If so, was it stored in the correct way, as a preventative measure, for damage limitation? Often these reactionary types are stored in concrete bunds and/or have an effective deluge system. So if there is a breach the material limits the chance to create a chain reaction.
Now we might think that such a breach or preventative measure wouldn't be overlooked but it would be very interesting to find out if negligence in the storage, is a reason why this building (assuming warehouse/facility) was then subsequently targeted (if terror related).
Of course bloody awful for the citizens of Beirut and Lebanon, with all that have been lost. A country once full of so much beauty and architecture. It's cliche but this is why we can't have nice things. Too many cunts in the world wanting to destroy stuffs.