Why do they burn off the gas at the stacks of oil refineries?

Whenever we see an oil refinery, they are burning off gas as a huge flame at the stack. I presume that this gas is a by-product of the refining process, but why can't the energy be put to some constructive use instead of just burning it? There is obviously a reason for what they do, but why must that be the only alternative?

Answer:
Usually the gas that you see being flared off is there because there is an upset in the process at some point and they cannot run all areas of the plant at full capacity, so they have to flare the excess gas.
Sometimes they can throttle the flows back, but many times they cannot.
These are usually just temporary operating conditions.
If this was a permanent condition they would install a good waste heat boiler and use it to produce steam or electric power to use elsewhere in the plant.
These plants cannot afford to waste their energy anymore than anyone else.

I was told that it was cheaper to burn the methane than it was to transport it somewhere else for sale. There isn't a market for it.

These are waste gases coming from many different sources and safety systems but are kept to a minimum. Their recovery would be costly.
Some of the flared gas is simply a pilot flame to ensure that, should a safety relief operate, the gas will ignite as required.
They also serve to indicate problems to be investigated in a system according to the size of the flare.

Waste gas is flared

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Why do they burn off the gas at the stacks of oil refineries?

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