I've debated whether to explain my procedure for cutting up a propane tank. I'll put it down here, but this isn't a recommendation to anyone, just an explanation of what I did, and why I did it that way.
I used to build pulse jet engines, and some of the principles apply, to my way of thinking.
The first principle in avoiding a detonation in a container is to remove all fuel. No fuel, no detonation.
The second is to reduce the volume of potentially combustible mix as much as possible. Reduced volume, reduced energy available.
The third is to provide a relatively large opening in the container. The less containment/restriction the more likely conflagration (rapid combustion) vs detonation (explosion).
Taken all together:
1.) I opened the gas valve on this propane tank and left it outside for 3 years. That still doesn't guarantee no "fuel" since these tanks typically have a tarry residue internally, which if heated will boil and offgas under torch cutting heat and even though it isn't propane itself, it is flammable (and explosive if contained).
2.) Removed the valve entirely with a pipe wrench. This increases the size of the orifice greatly compared to leaving the valve on. This alone reduces the chance of a detonation.
3.) Washed the tank out with hot soap and water. This won't remove most of the tarry residue. So it would be foolish to depend on it. It will remove lighter oils, to some degree.
4.) Filled the tank with water to an inch under the top. This drastically reduces the volume of potential air and fuel mix. The ratio of the current orifice size (about 3/4" diameter) to the contained gas volume (about a pint) has now been increased maybe 1000 times what it was with the open valve attached and the tank empty. There is little volume to burn, and an easy way for hot gasses to exhaust if it does.
5.) With the tank full of water I lit a match and held it over the orifice. No reaction. no pop. Nothing. (Again, I'm not reccomending this to anyone, I'm just repeating what I did and why I did it this way. What I did applies only to this tank, this type of fuel, and this lead-up procedure, not every situation. Make no assumptions about different situations based on this.)
6.) I drained out about a quart of water and repeated. No ignition.
7.) I drained out enough water to lie a little below my cutting line, and repeated. No ignition.
8.) I used a 4-1/2" grinder with a cutting disk in it to score around the tank, still filled with water, and then cut through. The water level was low enough not to wet the disk or grinder, and I wore rubber gloves.
I did NOT use a cutting torch. A cutting torch raises the temperature of the surrounding metal tremendously, and I did not want to vaporize any possible tarry residue in the tank. A torch is in my opinion a bad tool for this kind of thing, and procedures which use inert gasses in the tank are less safe

than using water. If the gas isn't right, you can't tell, and the contained volume is way too big for safety. Leaving the valve on the tank really increases the danger.
That's my personal opinion. Many tanks have been cut with torches, and many swear by inert gas fills to do it with. I prefer my way. It makes sense to me, and results are apparent at every step.
Sorry to go on at length here, but I haven't ever seen this explained this way before and thought it might be useful. Nothing in this guarantees safety, and every person must assume personal responsibility if they try to use a propane tank. Other substances and tanks can behave completely differently.