Nuclear weapons are not some magic force. If you ignore the radiation aspect, the damage they do is done in the same way and at the same scale as the equivalent amount of TNT. That's why they "rate" them as equivalent.
The advantage of a nuclear weapon is that manufacturing and delivering 40,000 tonnes of TNT is clearly impractical. A nuclear bomb is a far more compact package, making it possible to actually deliver to the target.
The video ChriniC posted above demonstrates this nicely - the explosion there was purely TNT and you can see just how relatively small a pile of 100 tonnes of TNT actually is, and how large that explosion was.
To state that a nuclear blast is capable of xyz damage but a non-nuclear blast of the same size isn't betrays your complete lack of understanding of how things like this work.
There are a great many chemical processes that can happen during a fire like that. Chemical mixtures have been developed to stabilise otherwise extremely sensitive/explosive chemicals that, when set on fire, burn off the stabilising chemicals leaving you with something that is incredibly explosive. Some of these chemicals can be safely transported and stored until something goes wrong, at which point you're best to just run away and leave it to burn. To mitigate this, they have limits on how much of a chemical you can store and for some of the really exotic stuff, how close it can be stored to other chemicals. If you simply ignore these limits in the quest for profit, you'll end up with an accident waiting to happen. In this case, it has happened with a large and regrettable loss of life.
As for the exclusion zone, most of the chemicals I'm talking about here don't burn clean. All sorts of things are easily produced when you get a conflagration like this - Hydrogen Cyanide, Hydrogen Flouride, various Mercury compounds, the list is limited only by your imagination and all of them are impressively good at killing people. Radiation from a nuclear blast, in the main, is delivered over an extremely short period of time during the fission reaction itself (a fraction of a second). People dosed during the explosion would already be dead, you'd be seeing casualties suffering radiation burns from a reasonable distance away too - if you've got the stomach for it you can look up images of radiation burn patterns from Hiroshima and Nagasaki to see what happens. Exclusion zones days after would be only to contain exposure to localised irradiated materials and fallout. There is an extremely impressive number of detectors developed and installed during the Cold War where both Russia and the USA were intent on knowing the composition of each other's devices - they achieved this by measuring the contents of the fallout products. IF this was nuclear (which I am 99.9999% certain it isn't) we would almost certainly have seen all of these factors. Any one of them (Radiation sickness/death, walking wounded with radiation burns, fallout products) would be a tell-tale sign of a nuclear explosion. None of them have happened.
It isn't nuclear. It is a very, very nasty industrial accident.
EDIT:
I just want to add that setting a nuclear bomb on fire won't detonate it either. From publicly available sources and basic physics knowledge of critical mass and so on, there are two main ways that bomb designers go about creating a nuclear detonation. The first is a gun-type bomb. They basically have a uranium slug in what is essentially a regular cannon and a uranium ball at the other end. The cannon fires the slug into the ball and boom, you've got criticality and the resultant nuclear explosion. The second is an implosion type weapon where you have a sphere of uranium inside a bigger sphere of explosives. Explosives go bang, force the uranium inwards to form a ball of uranium at critical mass and ta-da, criticality and a large explosion. Quite aside from the things called Permissive Action Links and other safety devices such as filing the cavity with ball bearings, the explosives they use in these things are not at all sensitive to fire or heat and will simply burn if set on fire. There have been a number of aircraft accidents involving nuclear bombs in the past where the bombs have either burned up or inadvertently detonated the explosives and in every single case this hasn't resulted in a nuclear detonation.
