The chemical (MMA) is a very well-known one in industry. Firefighters aren't familiar with it because it doesn't catch fire that often. Basically, it's a flammable organic liquid, like alcohol, only less volatile. It is more volatile than gasoline, kerosene, diesel, etc. It is also more toxic and irritant than any of those. Certain conditions cause it to polymerize to a solid, which releases a lot of heat. The heat causes a buildup of pressure as more MMA evaporates, and heat can also increase the rate of polymerization -- a vicious cycle known as "thermal runaway". By spraying the tank with hoses, they are helping to carry away heat (mostly by evaporation of water, which is why it's best sprayed as widely dispersed as possible) and prevent thermal runaway. If the tank cools sufficiently, the pressure will drop, and the reaction should also slow down. Disaster averted, if the water spray keeps coming fast enough.
The solid formed on polymerization can actually be de-polymerized by heat, re-forming a volatile liquid (and vapor). This makes it a little hard to predict just what's happening in any particular conditions of heat and pressure.
To quibble, it's more the fire they don't know what to do with, than the chemical involved. Apparently one stuck or broken valve was enough to trigger a situation like this, which shouldn't be the case, if backups are properly planned and maintained. If it turns out that bad design/planning led to the plant not having a backup plan for emptying the tank in an emergency, this could lead to some heads rolling in the aftermath. Unfortunately, at smaller companies less under the public eye, such preventative measures often don't get the attention they would at a major corp. plant.
https://www.democraticunderground.com/100221255572#post7
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