Why is Flaring done in industries? Why can't the same gases be used for production of energy (Thermal energy)? Or why can't the excess gas be recycled in the previous stream from which it came?
There are 2 general reasons for flaring; In a refinery or chemical plant for safety and at well site for economics - subject to Dept of Interior rules. Refineries and chemical plants have a large number of safety valves , most are automatic . Just like the safety valve you have on your hot water heater : the hot water heater vents hot water outside , no problem . But refinery/chem units release very hot hydrocarbons ( generally) , they must be contained in a piping system and taken to a safe location and ignited/ flared ( during an upset- over-pressure or over-temperature condition). Also, during a start-up or shut-down there may be flaring. This piping system must not have air in it or burning may occur anywhere in the system during an upset: So , the piping is flushed with gas to keep air out. Also there are ignition burners at the flare tip to ignite any emergency release , these are the small flares one routinely sees around the plants... Flares at well sites are to permit oil production . There is essentially always some gas produced with oil , this is separated at the well head . Usually the oil goes into a pipeline ( gathering line) . In a larger field there will be gas lines that take the gas to a compressor . From the compressor, the gas is reinjected into the oil field or it may be combined with other gas and piped to a gas plant where LPG, water and sulfur, etc. are removed and some portion becomes pipeline gas.If there is limited gas from a few wells it is flared according to rules of the Interior Dept; There are time limits and volume limits. The time allows for more wells to be drilled, or pipelines to be built or gas plants to be built. There are various situations.
The usual answer is that nobody in the vicinity of the flared gas wants to buy it, at least not in the quantities in which it is available, and so to not release a flammable gas mixture into the air around the plant, it is ignited and burned off instead. This is a wasteful process, but in the case of a petroleum refinery the energy content of the flared gas is tiny compared to the refinery's total output of useful fuels.
I think, it's because sometimes the flowrate can be very high (at fire blowdown case of light hydrocarbon liquids). At such high flowrate, consumer may not able to absorb thus plant safety is not guaranteed. I think plants and governments are trying to recover the gas for economic and environmental issue, so it has been taken care of.