Why do coal-burning power plants and oil refineries have tall smokestacks?

#1) Why do coal-burning power plants and oil refineries have tall smokestacks?
They are gas combustion devices used in industrial plants such as petroleum refineries, chemical plants, natural gas processing plants as well as at oil or gas production sites having oil wells, gas wells, offshore oil and gas rigs and landfills. In industrial plants, these are primarily used for burning off flammable gas released by pressure relief valves during unplanned over-pressuring of plant equipment.

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