I'd be interested to know roughly what the empty tank weighs, relative to its approximate capacity. Just your best guess.
People get hurt and killed every year doing this.
You might get awy with it but it is a considerable risk with a serious outcome.
Best steam clean tank then fill it with water before cutting the top off while full. Once the top is off 99% of the danger is gone.
Monkey patching programmer [retired ]
If filling with water doesn't appeal and the area you are working in is reasonably well ventilated, get a CO2 fire extinguisher and squirt(no more than a second blast) some into the tank before cutting, and stop at intervals to squirt more into it until you have cut a large opening. Once there is a large opening there is no risk of explosion(not that there is much risk anyway, but mitigating risk is always a good idea) and only a small risk of fire. Use the opening to mop up any remaining fuel then you can cut the rest up with Gay Abandon, that nice lass from the next narrowboat.
Please have a safety person standing by throughout, preferably equipped with a first aid kit, a charged mobile phone, a bucket of water, a bucket of sand, and fire extinguishers(ideally 1 CO2 and 1 foam).
Grünkraft? - nein danke.
Dont try this at home. As a child (15) I worked with our mechanic who during the war welded bullet holes in petrol tanks. (those that had not exploded) what you did to remove the risk of fire is put some paraffin in, leave overnight, empty, then from a distance throw a lit match in. So when a few years ago I had a BMW with a leaking petrol tank, this is what I did. There was a small "Pouf" (not to be confused with a gay dwarf) and the tank had "Flashed" so I was able to weld away.
This is what I would do with a diesel tank. Mind you instead of using an angle grinder you can get these small saws with metal cutting reciprocating blades which would also cut the tank up.
the HSE publication 'CS15' gives lots of interesting thoughts on decommissioning tanks: see www.hse.gov.uk, as does 'hot work on small drums and tanks'.
The emphasis is on purging all residue to avoid accidents and environmental incidents - how much you want to take on board is, of course, entirely up to you! As others have already suggested, I'd definitely avoid grinding as fires will almost certainly result, even from what looks like fairly clean steel.
You might use a wallpaper steamer to clean it out.
Even then, I think this is an excuse for a tool purchase, get yourself a reciprocating metal saw.
For one thing, grinding it makes a mess. You will have bits of rusting metal appearing for months. You need to catch the sparks in something that does not matter.