A gas flare, alternatively known as a flare stack, is a gas combustion device used in industrial plants such as petroleum refineries, chemical plants and natural gas processing plants. They are also common at oil or gas extraction sites having oil wells, gas wells, offshore oil and gas rigs and landfills.
In industrial plants, flare stacks are primarily used for burning off flammable gas released by safety valves during unplanned over-pressuring of plant equipment. During plant or partial plant startups and shutdowns, they are also often used for the planned combustion of gases over relatively short periods.
At oil and gas extraction sites, gas flares are similarly used for a variety of startup, maintenance, testing, safety, and emergency purposes. In a practice known as production flaring, they may also be used to dispose of large amounts of unwanted associated petroleum gas, possibly throughout the life of an oil well.
Overall flare system in industrial plants
When industrial plant equipment items are over-pressured, the pressure relief valve is an essential safety device that automatically release gases and sometimes liquids. Those pressure relief valves are required by industrial design codes and standards as well as by law.
The released gases and liquids are routed through large piping systems called flare headers to a vertical elevated flare. The released gases are burned as they exit the flare stacks. The size and brightness of the resulting flame depends upon the flammable material's flow rate in joules per hour (or btu per hour).
Most industrial plant flares have a vapor-liquid separator (also known as a knockout drum) upstream of the flare to remove any large amounts of liquid that may accompany the relieved gases.
Steam is very often injected into the flame to reduce the formation of black smoke. When too much steam is added, a condition known as "over steaming" can occur resulting in reduced combustion efficiency and higher emissions. To keep the flare system functional, a small amount of gas is continuously burned, like a pilot light, so that the system is always ready for its primary purpose as an over-pressure safety system.
- A knockout drum to remove any oil or water from the relieved gases.
- A water seal drum to prevent any flashback of the flame from the top of the flare stack.
- An alternative gas recovery system for use during partial plant startups and shutdowns as well as other times when required. The recovered gas is routed into the fuel gas system of the overall industrial plant.
- A steam injection system to provide an external momentum force used for efficient mixing of air with the relieved gas, which promotes smokeless burning.
- A pilot flame (with its ignition system) that burns all the time so that it is available to ignite relieved gases when needed.
- The flare stack, including a flashback prevention section at the upper part of the stack.
Crude oil production flares
When petroleum crude oil is extracted and produced from oil wells, raw natural gas associated with the oil is brought to the surface as well. Especially in areas of the world lacking pipelines and other gas transportation infrastructure, vast amounts of such associated gas are commonly flared as waste or unusable gas. The flaring of associated gas may occur at the top of a vertical flare stack or it may occur in a ground-level flare in an earthen pit (as in the adjacent photo). Preferably, associated gas is reinjected into the reservoir, which saves it for future use while maintaining higher well pressure and crude oil producibility.
Advances in satellite monitoring, along with voluntary reporting, have revealed that about 150 × 109 cubic meters (5.3 × 1012 cubic feet) of associated gas have been flared globally each year since at least the mid 1990s until 2020. In 2011, that was equivalent to about 25 percent of the annual natural gas consumption in the United States or about 30 per cent of the annual gas consumption in the European Union. At market, this quantity of gas—at a nominal value of $5.62 per 1000 cubic feet—would be worth US$29.8 billion. Also, the waste is a significant source of carbon dioxide (CO2) and other greenhouse gas emissions.
An important source of anthropogenic methane comes from the treatment and storage of organic waste material including waste water, animal waste and landfill. Gas flares are used in any process that results in the generation and collection of biogas. As a result, gas flares are a standard component of an installation for controlling the production of biogas. They are installed on landfill sites, waste water treatment plant and anaerobic digestion plant that use agriculturally or domestically produced organic waste to produce methane for use as a fuel or for heating.
Gas flares on biogas collection systems are used if the gas production rates are not sufficient to warrant use in any industrial process. However, on a plant where the gas production rate is sufficient for direct use in an industrial process that could be classified as part of the circular economy, and that may include the generation of electricity, the production of natural gas quality biogas for vehicle fuel or for heating in buildings, drying Refuse Derived Fuel or leachate treatment, gas flares are used as a back-up system during down-time for maintenance or breakdown of generation equipment. In this latter case, generation of biogas cannot normally be interrupted and a gas flare is employed to maintain the internal pressure on the biological process.
There are two types of gas flare used for controlling biogas, open or enclosed. Open flares burn at a lower temperature, less than 1000 °C and are generally cheaper than enclosed flares that burn at a higher combustion temperature and are usually supplied to conform to a specific residence time of 0.3s within the chimney to ensure complete destruction of the toxic elements contained within the biogas. Flare specification usually demands that enclosed flares must operate at >1000 °C and <1000 °C; this in order to ensure a 98% destruction efficient and avoid the formation of NOx.
Methane's estimated global warming potential is 34 times greater than that of CO2. Therefore, to the extent that gas flares convert methane to CO2 before it is released into the atmosphere, they reduce the amount of global warming that would otherwise occur. However, flaring emissions contributed to 270 Mt CO2 in 2017 and reducing flaring emissions is key to avoid dangerous global warming. An increasing number of governments and industries are committing to eliminate flaring by 2030.
Improperly operated flares may emit methane and other volatile organic compounds as well as sulfur dioxide and other sulfur compounds, which are known to exacerbate asthma and other respiratory problems. Other emissions from improperly operated flares may include, aromatic hydrocarbons (benzene, toluene, xylenes) and benzo(a)pyrene, which are known to be carcinogenic. A 2013 study found that gas flares contributed over 40% of the black carbon deposited in the Arctic, thus further increasing the rates of snow and ice melt.
Flaring can affect wildlife by attracting birds and insects to the flame. Approximately 7,500 migrating songbirds were attracted to and killed by the flare at the liquefied natural gas terminal in Saint John, New Brunswick, Canada on September 13, 2013. Similar incidents have occurred at flares on offshore oil and gas installations. Moths are known to be attracted to lights. A brochure published by the Secretariat of the Convention on Biological Diversity describing the Global Taxonomy Initiative describes a situation where "a taxonomist working in a tropical forest noticed that a gas flare at an oil refinery was attracting and killing hundreds of these [hawk or sphinx] moths. Over the course of the months and years that the refinery was running a vast number of moths must have been killed, suggesting that plants could not be pollinated over a large area of forest".
The Bhopal disaster exemplifies the consequences of a failure to flare escaping methyl isocyanate gas. The gas was released from an over-pressured tank to a detached flare tower by a safety valve, and flooded the surrounding area.
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- 7,500 songbirds killed at Canaport gas plant in Saint John (online CBC News, September 17, 2013).
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- The Global Taxonomy Initiative - The Response to a Problem (scroll down to the section entitled "Pollinating moths")
|World Bank video about reducing flaring|
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