I'm not really sure where you are right now, but I assume it's the part where you have to start a fire to open the security door?
Spoiler below!
Take the barrel, place it under the electrical panel, then pull the lever on the wall, it will then spark in the panel, and the content in the barrel will start burning. Place the barrel under one of the smoke sensors, you can see those on the ceiling, I think there are one in the middle of the room.