
A gas flare, alternatively known as a flare stack, flare boom, ground flare, or flare pit, is a gas
combustion
Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion ...
device used in places such as
petroleum refineries,
chemical plants and
natural gas processing
Nature is an inherent character or constitution, particularly of the ecosphere or the universe as a whole. In this general sense nature refers to the laws, elements and phenomena of the physical world, including life. Although humans are part ...
plants, oil or gas extraction sites having
oil wells,
gas wells,
offshore oil and gas rigs and
landfill
A landfill is a site for the disposal of waste materials. It is the oldest and most common form of waste disposal, although the systematic burial of waste with daily, intermediate and final covers only began in the 1940s. In the past, waste was ...
s.
In industrial plants, flare stacks are primarily used for burning off
flammable
A combustible material is a material that can burn (i.e., sustain a flame) in air under certain conditions. A material is flammable if it ignites easily at ambient temperatures. In other words, a combustible material ignites with some effort ...
gas released by
safety valves during unplanned overpressuring of plant equipment.
[ (See Chapter 11, ''Flare Stack Plume Rise'').] 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.
[Global Gas Flaring Reduction Partnership (GGFR), World Bank]
October 2011 Brochure.
Overall flare system in industrial plants

When industrial plant equipment items are overpressured, the
pressure relief valve is an essential safety device that automatically releases 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
Within industry, piping is a system of pipes used to convey fluids (liquids and gases) from one location to another. The engineering discipline of piping design studies the efficient transport of fluid.
Industrial process piping (and accomp ...
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
joule
The joule ( , or ; symbol: J) is the unit of energy in the International System of Units (SI). In terms of SI base units, one joule corresponds to one kilogram- metre squared per second squared One joule is equal to the amount of work d ...
s 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
Steam is water vapor, often mixed with air or an aerosol of liquid water droplets. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporization. Saturated or superheated steam is inv ...
is very often injected into the flame to reduce the formation of black smoke. When too much steam is added, a condition known as "oversteaming" 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 overpressure safety system.
The adjacent flow diagram depicts the typical components of an overall industrial flare stack system:[
* A knockout drum to remove any oil or water from the relieved gases. There may be several knock out drums: high-pressure and low-pressure drums taking relief flow from high-pressure and low-pressure equipment. A cold relief drum which is segregated from wet relief system because of the risk of freezing.
* A water seal drum to prevent any flashback of the ]flame
A flame () is the visible, gaseous part of a fire. It is caused by a highly exothermic chemical reaction made in a thin zone. When flames are hot enough to have ionized gaseous components of sufficient density, they are then considered plasm ...
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
In Newtonian mechanics, momentum (: momenta or momentums; more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. ...
force used for efficient mixing of air
An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...
with the relieved gas, which promotes smokeless burning.
* A pilot flame (with its ignition system
Ignition systems are used by heat engines to initiate combustion by igniting the fuel-air mixture. In a spark ignition versions of the internal combustion engine (such as petrol engines), the ignition system creates a spark to ignite the fuel-ai ...
) 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.
The schematic shows a pipe flare tip. The flare tip can have several configurations:
* a simple pipe flare
* a sonic tip – upstream pressure > 5 bar
* a multi nozzle tip, sonic or subsonic
* a Coandă tip – a profiled tip using the Coandă effect to entrain air into the gas to improve combustion.
Flare stack height
The height of a flare stack, or the reach of a flare boom, is determined by the thermal radiation
Thermal radiation is electromagnetic radiation emitted by the thermal motion of particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation. The emission of energy arises from a combination of electro ...
that is permissible or tolerable for equipment or personnel to be exposed to. For continuous exposure of personnel wearing appropriate industrial clothing a maximum radiation level of 1.58 kW/m2 (500 Btu/hr.ft²) is recommended. Higher radiation levels are permissible but for reduced exposure times:
* 4.73 kW/m2 (1500 Btu/hr.ft²) would limit exposure to 3 to 4 minutes
* 6.31 kW/m2 (2000 Btu/hr.ft²) would limit exposure to 30 seconds.
Ground flares
Ground flares are designed to hide the flame from sight and to reduce thermal radiation
Thermal radiation is electromagnetic radiation emitted by the thermal motion of particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation. The emission of energy arises from a combination of electro ...
and noise. They comprise a steel box or cylinder lined with refractory
In materials science, a refractory (or refractory material) is a material that is resistant to decomposition by heat or chemical attack and that retains its strength and rigidity at high temperatures. They are inorganic, non-metallic compound ...
material. They are open at the top and have openings around the base to allow combustion air to enter. They may have an array of multiple flare tips to provide turndown capability and to spread the flame across the cross-section of the flare. They are generally used onshore in environmentally sensitive areas and have been used offshore on floating production storage and offloading installations (FPSOs).
Crude oil production flares
When crude oil
Petroleum, also known as crude oil or simply oil, is a naturally occurring, yellowish-black liquid chemical mixture found in geological formations, consisting mainly of hydrocarbons. The term ''petroleum'' refers both to naturally occurring u ...
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. 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
The European Union (EU) is a supranational union, supranational political union, political and economic union of Member state of the European Union, member states that are Geography of the European Union, located primarily in Europe. The u ...
. At market, this quantity of gas—at a nominal value of $5.62 per 1000 cubic feet—would be worth US$29.8 billion.[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.
Additionally, the waste is a significant source of carbon dioxide
Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
(CO2) and other greenhouse gas emissions
Greenhouse gas (GHG) emissions from human activities intensify the greenhouse effect. This contributes to climate change. Carbon dioxide (), from burning fossil fuels such as coal, petroleum, oil, and natural gas, is the main cause of climate chan ...
.
Biogas flares
An important source of anthropogenic
Anthropogenic ("human" + "generating") is an adjective that may refer to:
* Anthropogeny, the study of the origins of humanity
Anthropogenic may also refer to things that have been generated by humans, as follows:
* Human impact on the enviro ...
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
Biogas is a gaseous renewable energy source produced from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste, Wastewater treatment, wastewater, and food waste. Biogas is produced by anaerobic ...
. 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
A circular economy (also referred to as circularity or CE) is a model of resource Production (economics), production and Resource consumption, consumption in any economy that involves sharing, leasing, Reuse, reusing, repairing, refurbishing, and ...
, and that may include the generation of electricity, the production of natural gas
Natural gas (also fossil gas, methane gas, and gas) is a naturally occurring compound of gaseous hydrocarbons, primarily methane (95%), small amounts of higher alkanes, and traces of carbon dioxide and nitrogen, hydrogen sulfide and helium ...
quality biogas for vehicle fuel or for heating in buildings, drying refuse-derived fuel or leachate
A leachate is any liquid that, in the course of passing through matter, extracts soluble or suspended solids, or any other component of the material through which it has passed.
Leachate is a widely used term in the environmental sciences wh ...
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
Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion ...
temperature and are usually supplied to conform to a specific residence time of 0.3s within the chimney to ensure complete destruction of any toxic molecules contained within the biogas. Flare specification usually demands that enclosed flares must operate at >1000 °C and <1200 °C; this in order to ensure a 98% destruction efficiency and avoid the formation of NOx.
Environmental impacts
The natural gas that is not combusted by a flare is vented into the atmosphere as methane. Methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...
's estimated global warming potential
Global warming potential (GWP) is a measure of how much heat a greenhouse gas traps in the atmosphere over a specific time period, relative to carbon dioxide (). It is expressed as a multiple of warming caused by the same mass of carbon dioxide ( ...
is 28-36 times greater than that of CO2 over the course of a century, and 84-87 times greater over two decades. Natural gas flaring produces CO2 and many other compounds, depending on the chemical composition of the natural gas and on how well the natural gas burns in the flare. 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.
Flaring emissions contributed to 270 Mt ( megatonnes) of CO2 in 2017 and reducing flaring emissions is thought to be an important component in curbing global warming. An increasing number of governments and industries have pledged to eliminate or reduce flaring. Th
Global Methane Pledge
signed at COP26, in which 111 nations committed to reducing methane emissions by at least 30 percent from 2020 levels by 2030, is also playing a role in raising the global focus on methane.
Additional noxious fumes emitted by flaring may include, aromatic hydrocarbons (benzene
Benzene is an Organic compound, organic chemical compound with the Chemical formula#Molecular formula, molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar hexagonal Ring (chemistry), ring with one hyd ...
, toluene
Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon with the chemical formula , often abbreviated as , where Ph stands for the phenyl group. It is a colorless, water
Water is an inorganic compound with the c ...
, 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
Black carbon (BC) is the light-absorbing refractory form of Chemical_element, elemental carbon remaining after pyrolysis (e.g., charcoal) or produced by incomplete combustion (e.g., soot).
Tihomir Novakov originated the term black carbon in ...
deposited in the Arctic.
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
Liquefied natural gas (LNG) is natural gas (predominantly methane, CH4, with some mixture of ethane, C2H6) that has been cooled to liquid form for ease and safety of non-pressurized storage or transport. It takes up about 1/600th the volume o ...
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
The Convention on Biological Diversity (CBD), known informally as the Biodiversity Convention, is a multilateral treaty. The Convention has three main goals: the conservation of biological diversity (or biodiversity); the sustainable use of its ...
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 awk or sphinxmoths. 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".
Adverse health effects
Flares release several different chemicals including: benzene
Benzene is an Organic compound, organic chemical compound with the Chemical formula#Molecular formula, molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar hexagonal Ring (chemistry), ring with one hyd ...
, particulates
Particulate matter (PM) or particulates are microscopic particles of solid or liquid matter suspension (chemistry), suspended in the atmosphere of Earth, air. An ''aerosol'' is a mixture of particulates and air, as opposed to the particulate ...
, nitrogen oxides, heavy metals
upright=1.2, Crystals of lead.html" ;"title="osmium, a heavy metal nearly twice as dense as lead">osmium, a heavy metal nearly twice as dense as lead
Heavy metals is a controversial and ambiguous term for metallic elements with relatively h ...
, black carbon
Black carbon (BC) is the light-absorbing refractory form of Chemical_element, elemental carbon remaining after pyrolysis (e.g., charcoal) or produced by incomplete combustion (e.g., soot).
Tihomir Novakov originated the term black carbon in ...
, and carbon monoxide
Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...
. Several of these pollutants correlate with preterm birth
Preterm birth, also known as premature birth, is the Childbirth, birth of a baby at fewer than 37 weeks Gestational age (obstetrics), gestational age, as opposed to full-term delivery at approximately 40 weeks. Extreme preterm is less than 28 ...
and reduced newborn birth weight
Birth weight is the body weight of a baby at their birth. The average birth weight in babies of European and African descent is , with the normative range between .
15% of babies born in 2012 had a low birth weight and 14.7% in 2020. It is pro ...
. According to one study from 2020, pregnant women living near flaring natural gas and oil wells have reportedly experienced a 50% greater premature birth rate. Flares may emit methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...
and other volatile organic compounds as well as sulfur dioxide
Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...
and other sulfur
Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...
compounds, which are known to exacerbate asthma
Asthma is a common long-term inflammatory disease of the airways of the lungs. It is characterized by variable and recurring symptoms, reversible airflow obstruction, and easily triggered bronchospasms. Symptoms include episodes of wh ...
and other respiratory disease
Respiratory diseases, or lung diseases, are pathological conditions affecting the organs and tissues that make gas exchange difficult in air-breathing animals. They include conditions of the respiratory tract including the trachea, bronchi, ...
.
A 2021 study found that a 1% increase in flared natural gas increases the respiratory-related hospitalization rate by 0.73%.
See also
* Blowdown stack
* Flue-gas stack
* Gas venting
Gas venting, more specifically known as natural-gas venting or methane venting, is the intentional and controlled release of gases containing alkane hydrocarbons - predominately methane - into Earth's atmosphere.
It is a widely used method for ...
References
Further reading
*
Flare and Vent Disposal Systems on PetroWiki
Media
{{DEFAULTSORT:Gas Flare
Fuels
Oil refining
Air pollution
Air pollution control systems
Volatile organic compound abatement
Gas technologies