The flame often seen on many oil refineries is the "burnoff flare". Gases may come out of the petroleum as it gets processed, and one refinery I once toured told us "We don't burn off anything- we concentrate it and refine it and sell it to someone who has a use for it. (Another refinery just burns it off because it's cheaper than paying the fines for the vapors that get emitted.) Sometimes you see the flare flaming up very high and then going dim again, in response to some internal industrial rhythm I don't understand. Maybe I should have beeen a chemist.
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