A gas flare, alternatively known as a flare stack, is a gas combustion device 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, flare stacks are primarily used for burning off flammable gas released by pressure relief valves during unplanned over-pressuring of plant equipment. During plant or partial plant startups and shutdowns, flare stacks are also often used for the planned combustion of gases over relatively short periods.
A great deal of gas flaring at many oil and gas production sites has nothing to do with protection against the dangers of over-pressuring industrial plant equipment. When petroleum crude oil is extracted and produced from onshore or offshore oil wells, raw natural gas associated with the oil is produced 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 (as in the adjacent photo) or it may occur in a ground-level flare in an earthen pit.
Overall flare system in industrial plants
Whenever industrial plant equipment items are over-pressured, the pressure relief valves provided as essential safety devices on the equipment automatically release gases and sometimes liquids as well. 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 terms of 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 to the flame, a condition known as "over steaming" can occur resulting in reduced combustion efficiency and higher emissions. In order 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 and/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/or 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 whenever needed.
- The flare stack, including a flashback prevention section at the upper part of the flare stack.
There is also safe method to divert the flare gas is insertion of Liquid U seal with Liquid Hold up vessel. The Liquid U seal is designed to take pressure up to permitted back pressure of the system. This helps to diver the flare gas to recovery system.In case of plant upset, pressure rises & liquid in the U seal will move into Liquid Hold up vessel. On normalization of plant, Liquid U seal will come into picture & start diverting the gas again.(www.kpec.in)
Impacts of waste flaring associated gas from oil drilling sites and other facilities
Flaring is a contributor to the worldwide anthropogenic emissions of carbon dioxide. 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 benzapyrene, which are known to be carcinogenic.
Flaring can impact 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".
As of the end of 2011, 150 × 109 cubic meters (5.3 × 1012 cubic feet) of associated gas are flared annually. That is equivalent to about 25 per cent of the annual natural gas consumption in the United States or about 30 per cent of the annual gas consumption in the European Union. If it were to reach market, this quantity of gas (at a nominal value of $5.62 per 1000 cubic feet) would be worth $29.8 billion USD.
Also as of the end of 2011, 10 countries accounted for 72 per cent of the flaring, and twenty for 86 per cent. The top ten leading contributors to world gas flaring at the end of 2011, were (in declining order): Russia (27%), Nigeria (11%), Iran (8%), Iraq (7%), USA (5%), Algeria (4%), Kazakhstan (3%), Angola (3%), Saudi Arabia (3%) and Venezuela (3%).
That amount of flaring and burning of associated gas from oil drilling sites is a significant source of carbon dioxide (CO2) emissions. Some 400 × 106 tons of carbon dioxide are emitted annually in this way and it amounts to about 1.2 per cent of the worldwide emissions of carbon dioxide. That may seem to be insignificant, but in perspective it is more than half of the Certified Emissions Reductions (a type of carbon credits) that have been issued under the rules and mechanisms of the Kyoto Protocol as of June 2011.
- EPA/452/B-02-001, Section 3.0: VOC Controls, Section 3.2: VOC Destruction Controls, Chapter 1: Flares. (A U.S. Environmental Protection Agency report, dated September 2000.)
- A. Kayode Coker (2007). Ludwig's Applied Process Design for Chemical And Petrochemical Plants, Volume 1 (4th ed ed.). Gulf Professional Publishing. pp. 732–737. ISBN 0-7506-7766-X.
- Sam Mannan (Editor) (2005). Lee's Loss Prevention in the Process Industries: Hazard Identification, Assessment and Control, Volume 1 (3rd ed ed.). Elsevier Butterworth-Heinemann. pp. 12/67–12/71. ISBN 978-0-7506-7857-5.
- Milton R. Beychok (2005). Fundamentals of Stack Gas Dispersion (Fourth ed ed.). self-published. ISBN 0-9644588-0-2. (See Chapter 11, Flare Stack Plume Rise).
- "A Proposed Comprehensive Model for Elevated Flare Flames and Plumes", David Shore, Flaregas Corporation, AIChE 40th Loss Prevention Symposium, April 2006.
- Product Overview Ignition Systems, Excellent source of information about flare stack pilot flames and their ignition systems.
- 7,500 songbirds killed at Canaport gas plant in Saint John (online CBC News, September 17, 2013).
- Seabirds at Risk around Offshore Oil Platforms in the North-west Atlantic, Marine Pollution Bulletin, Vol. 42, No. 12, pp. 1,285-1,290, 2001.
- The Global Taxonomy Iniative - The Response to a Problem (scroll down to the section entitled "Pollinating moths")
- Global Gas Flaring Reduction Partnership (GGFR), World Bank, October 2011 Brochure.
- Annual Energy Review, Table 6.7 Natural Gas Wellhead, Citygate, and Imports Prices, 1949-2011 (Dollars per Thousand Cubic Feet), United States Energy Information Administration, September 2012.
- Estimated Flared Volumes from Satellite Data, 2007-2011. From the website of the World Bank.
- Global Gas Flaring Reduction. From the website of the World Bank.
- Estimation of Gas Flaring Volumes Using NASA MODIS Fire Detection Products (alternative). Christopher Elvidge et al, NOAA's National Geophysical Data Center (NGDC) annual report, February 8, 2011.
- Flare Gas Recovery - An option to stop flare gas being burned to atmosphere.
- Flare Stack sizing calculator
- Modelling industrial flares environmental impacts
- Flare Stack marking (aircraft warning lights)
- Flare Gas Electricity Generator
|World Bank video about reducing flaring|
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