Ammonium nitrate has a few added quirks that make it more unpredictable, too. Pure ammonium nitrate is neither flammable nor explosive. If it gets heated enough, though, it will decompose into things that are not ammonium nitrate, and that's where the situation gets weird. Each ammonium nitrate molecule has a choice of five different decomposition pathways, each ending in a different set of products. Only one of those pathways, the one that ends up producing oxygen gas, is a serious fire or explosive risk. The other four can produce some unpleasant stuff, but not stuff that's quite so lively.Dave wrote: ↑Thu Jun 20, 2019 5:46 pmJust plain ammonium nitrate (without an added hydrocarbon fuel) can explode, if sufficiently mistreated. If it's heated enough, it can decompose explosively... that's what happened in the Texas City disaster I was alluding to (a ship loaded with AN caught fire, burned hot enough to boil the water around itself, and eventually exploded and set fire to a lot of the city).
The big risk in the situation AnotherFairport fan was referring to, would have been the possibility of a fire. Oil-soaked AN fertilizer which caught fire (for any reason) might well have burned hot enough to trigger that sort of explosive decomposition... it might not have been as potent an explosion as a triggered detonation but it could still have completely ruined the day (and all subsequent ones) for anyone nearby.
The problem is that nobody's quite clear on just what influences ammonium nitrate to "choose" a particular decomposition pathway, so we can't really predict whether it will take the earth-shattering kaboom path or not. And since different molecules of ammonium nitrate in the same batch might take totally different pathways, sometimes the products of one molecule's decomposition will interfere unpredictably with the products of another molecule's oxygen-producing pathway and make it less likely to blow up. . .or not.
Even if enough of the molecules in a pile of ammonium nitrate do take the oxygen-producing pathway, it's usually more of a fire hazard than an explosive hazard. You'll only get the explosions when the stuff is heated up in a confined space (of course, a big enough pile of ammonium nitrate can generate its own confined space by the mass of the outer layers of the pile confining the middle layers). When you mix other materials in, then things get even more confused and possibly lively. You really don't want to be around ammonium nitrate if it has been contaminated with sulfides, for example.
It doesn't need to be, but it makes it more effective.