4/15 Miscellaneous Sources 13.5-1

13.5 Industrial Flares

13.5.1 General

Flaring is a high-temperature oxidation process used to burn combustible components, mostly 

hydrocarbons, of waste gases from industrial operations. Natural gas, propane, ethylene, propylene, 

butadiene and butane constitute over 95 percent of the waste gases flared. In combustion, gaseous 

hydrocarbons react with atmospheric oxygen to form carbon dioxide (CO2) and water. In some waste 

gases, carbon monoxide (CO) is the major combustible component. Presented below, as an example, is 

the combustion reaction of propane.

C3H8 + 5 O2 > 3 CO2 + 4 H2O

During a combustion reaction, several intermediate products are formed, and eventually, most are 

converted to CO2 and water. Some quantities of stable intermediate products such as carbon monoxide, 

hydrogen, and hydrocarbons will escape as emissions. 

Flares are used extensively to dispose of (1) purged and wasted products from refineries, (2) 

unrecoverable gases emerging with oil from oil wells, (3) vented gases from blast furnaces, (4) unused 

gases from coke ovens, and (5) gaseous wastes from chemical industries. Gases flared from refineries, 

petroleum production, chemical industries, and to some extent, from coke ovens, are composed largely of 

low molecular weight hydrocarbons with high heating value. Blast furnace flare gases are largely of inert 

species and CO, with low heating value. Flares are also used for burning waste gases generated by 

sewage digesters, coal gasification, rocket engine testing, nuclear power plants with sodium/water heat 

exchangers, heavy water plants, and ammonia fertilizer plants.

There are two types of flares, elevated and ground flares. Elevated flares, the more common type, 

have larger capacities than ground flares. In elevated flares, a waste gas stream is fed through a stack 

anywhere from 10 to over 100 meters tall and is combusted at the tip of the stack. The flame is exposed 

to atmospheric disturbances such as wind and precipitation. In ground flares, combustion takes place at 

ground level. Ground flares vary in complexity, and they may consist either of conventional flare burners 

with no enclosures or of multiple burners in refractory-lined steel enclosures.

The typical flare system consists of (1) a gas collection header and piping for collecting gases 

from processing units, (2) a knockout drum (disentrainment drum) to remove and store condensables and 

entrained liquids, (3) a proprietary seal, water seal, or purge gas supply to prevent flash-back, (4) a singleor multiple-burner unit and a flare stack, (5) gas pilots and an ignitor to ignite the mixture of waste gas 

and air, and, if required, (6) a provision for external momentum force (steam injection or forced air) for 

smokeless flaring. Natural gas, fuel gas, inert gas, or nitrogen can be used as purge gas. Figure 13.5-1 is 

a diagram of a typical steam-assisted elevated smokeless flare system.

Combustion requires three ingredients: fuel, an oxidizing agent (typically oxygen in air), and heat 

(or ignition source). Flares typically operate with pilot flames to provide the ignition source, and they use 

ambient air as the oxidizing agent. The waste gases to be flared typically provide the fuel necessary for 

combustion. Combustible gases generally have an upper and lower flammability limit. The upper 

flammability limit (UFL) is the highest concentration of a gas in air that is capable of burning. Above 

this flammability limit, the fuel is too rich to burn. The lower flammability limit (LFL) is the lowest 

concentration of the gas in air that is capable of burning. Below the LFL, the fuel is too lean to burn. 

Between the upper and lower flammability limits, combustion can occur. Flare waste gases with 
13.5-2 EMISSIONS FACTORS 4/15

concentrations above the UFL will become more dilute as the waste gas mixes with ambient air above the 

flare tip. As this dilution occurs, the air-waste gas mixture will pass through the flammability region, and 

combustion will occur. However, if flare waste gas concentrations are near the LFL prior to mixing with 

air, the air-waste gas mixture can fall below the flammability region, and reduced combustion efficiencies 

can occur. If steam is added to the flare waste gas at or prior to the flare tip (i.e., prior to the “combustion 

zone” where the mixing with air occurs), the steam will act to dilute the waste gas. Thus, even if there are 

adequate concentrations of combustibles in the waste gas, if too much steam is added to the waste gas so 

that the combustibles concentration becomes diluted to near the LFL as the steam-waste gas mixture 

enters the combustion zone, reduced combustion efficiencies will result. Consequently, critical 

considerations of flare combustion include the net heating value and the combustibles concentration in the 

flare gas and in the combustion zone (e.g., accounting for the amount of dilution by steam or other assist 

gas that occurs to the waste gas prior to the combustion zone). 

Figure 13.5-1. Diagram of a typical steam-assisted smokeless elevated flare.

Combustion efficiency is the percentage of hydrocarbon in the flare vent gas that is completely 

converted to CO2 and water vapor. Destruction efficiency is the percentage of a specific pollutant in the 

flare vent gas that is converted to a different compound (such as CO2, CO or other hydrocarbon 

intermediate). The destruction efficiency of a flare will always be greater than the combustion efficiency 

of a flare. It is generally estimated that a combustion efficiency of 96.5 percent is equivalent to a 

destruction efficiency of 98 percent.10

Smoking may result from combustion, depending upon waste gas components and the quantity 

and distribution of combustion air. Waste gases containing methane, hydrogen, CO, and ammonia 
4/15 Miscellaneous Sources 13.5-3

usually burn without smoke. Waste gases containing heavy hydrocarbons such as paraffins above 

methane, olefins, and aromatics, have a higher tendency to smoke. An external momentum force, such as 

steam injection or blowing air, is used for efficient air/waste gas mixing and turbulence, which promotes 

smokeless flaring of heavy hydrocarbon waste gas. Other external forces may be used for this purpose, 

including water spray, high velocity vortex action, or natural gas. External momentum force is rarely 

required in ground flares.

Steam injection is accomplished either by nozzles on an external ring around the top of the flare 

tip or by a single nozzle located concentrically within the tip. At installations where waste gas flow 

varies, both are used. The internal nozzle provides steam at low waste gas flow rates, and the external jets 

are used with large waste gas flow rates. Several other special-purpose flare tips are commercially 

available, one of which is for injecting both steam and air.

Flares are generally designed to handle large quantities of waste gases that may be intermittently 

generated during plant emergencies, although they may also be used routinely to dispose of low-volume 

continuous or intermittent emissions from various sources at the plant. Flare gas volumes can vary from a 

few cubic meters per hour during regular operations up to several thousand cubic meters per hour during 

major upsets. Flow rates at a refinery could be 45 to 90 kilograms per hour (kg/hr) (100 - 200 pounds per 

hour [lb/hr]) during regular operation but could reach a full plant emergency rate of 700 megagrams per 

hour (Mg/hr) (750 tons/hr). Normal process blowdowns may release 450 to 900 kg/hr (1000 - 2000 

lb/hr), and unit maintenance or minor failures may release 25 to 35 Mg/hr (27 - 39 tons/hr). Thus, the 

required flare turndown ratio can be over 15,000 to 1.

Many plants have 2 or more flares, in parallel or in series. In the former, 1 flare can be shut down 

for maintenance while the other serves the system. In systems of flares in series, 1 flare is intended to 

handle regular gas volumes and the other flare is generally intended to handle excess gas flows from 

emergencies.

13.5.2 Emissions

Noise, heat, and visible flame and/or smoke are the most apparent undesirable effects of flare 

operation. Flares are usually located away from populated areas or are sufficiently isolated, thus 

minimizing their effects on populations.

Emissions from flaring may include carbon particles (soot), unburned hydrocarbons, CO, and 

partially burned and altered hydrocarbons. Also emitted are nitrogen oxides (NOx) and, if sulfurcontaining material such as hydrogen sulfide or mercaptans is flared, sulfur dioxide (SO2). The quantities 

of hydrocarbon emissions generated relate to the degree of combustion. The degree of combustion 

depends largely on the rate and extent of fuel-air mixing and on the flame temperatures achieved and 

maintained. Properly operated flares achieve at least 98 percent destruction efficiency in the flare plume, 

meaning that hydrocarbon emissions amount to less than 2 percent of the hydrocarbons in the gas stream.

The tendency of a fuel to smoke or make soot is influenced by fuel characteristics and by the 

amount and distribution of oxygen in the combustion zone. For complete combustion, at least the 

stoichiometric amount of oxygen must be provided in the combustion zone. The theoretical amount of 

oxygen required increases with the molecular weight of the gas burned. The oxygen supplied as air 

ranges from 9.6 units of air per unit of methane to 38.3 units of air per unit of pentane, by volume. Air is 

supplied to the flame as primary air and secondary air. Primary air is mixed with the gas before 

combustion, whereas secondary air is drawn into the flame. For smokeless combustion, sufficient 

primary air must be supplied, this varying from about 20 percent of stoichiometric air for a paraffin to 

about 30 percent for an olefin. If the amount of primary air is insufficient, the gases entering the base of 

the flame are preheated by the combustion zone, and larger hydrocarbon molecules crack to form 
13.5-4 EMISSIONS FACTORS 4/15

hydrogen, unsaturated hydrocarbons, and carbon. The carbon particles may escape further combustion 

and cool down to form soot or smoke. Olefins and other unsaturated hydrocarbons may polymerize to 

form larger molecules which crack, in turn forming more carbon.

The fuel characteristics influencing soot formation include the carbon-to-hydrogen (C-to-H) ratio 

and the molecular structure of the gases to be burned. All hydrocarbons above methane, i. e., those with a 

C-to-H ratio of greater than 0.33, tend to soot. Branched chain paraffins smoke more readily than 

corresponding normal isomers. The more highly branched the paraffin, the greater the tendency to 

smoke. Unsaturated hydrocarbons tend more toward soot formation than do saturated ones. Soot is 

eliminated by adding steam or air; hence, most industrial flares are steam-assisted and some are airassisted. Flare gas composition is a critical factor in determining the amount of steam necessary.

Since flares do not lend themselves to conventional emission testing techniques, until recently 

only a few attempts have been made to characterize flare emissions. Early EPA tests using propylene as

flare gas indicated that efficiencies of 98 percent can be achieved when burning an offgas with at least 

11,200 kJ/m3(300 Btu/ft3).1 However, recent studies on flare performance using passive Fourier 

Transform Infrared (pFTIR) spectroscopy have been performed on a number of different flares.

4-8

 The 

studies cover a number of flares at different industrial sources with different waste gas compositions. The 

pFTIR studies support the conclusion that the combustion zone properties of the steam-waste gas mixture 

are predictive of proper flare combustion.

10

 There have also been recent studies on sources, including 

flares, using differential infrared absorption LIDAR [light detection and ranging] (DIAL). To date, many 

of these studies do not provide the data necessary to isolate the emissions from a particular flare. But 

enough data existed in one study that the emissions measured by DIAL could be attributed to the flare.9

Table 13.5-1 presents flare emissions factors from the EPA tests1; Table 13.5-2 presents flare 

emissions factors from pFTIR and DIAL studies.

4-9

 Crude propylene was used as flare gas during the 

early EPA tests. Methane was a major fraction of hydrocarbons in the flare emissions, and acetylene was 

the dominant intermediate hydrocarbon species. Many other reports on flares indicate that acetylene is 

always formed as a stable intermediate product. The acetylene formed in the combustion reactions may 

react further with hydrocarbon radicals to form polyacetylenes followed by polycyclic hydrocarbons.2 

Typical refinery waste gas feeds were used as flare gas during the pFTIR and DIAL studies. 

In flaring waste gases containing no nitrogen compounds, NO is formed either by the fixation of 

atmospheric nitrogen (N) with oxygen (O) or by the reaction between the hydrocarbon radicals present in 

the combustion products and atmospheric nitrogen, by way of the intermediate stages, HCN, CN, and 

OCN.2 Sulfur compounds contained in a flare gas stream are converted to SO2 when burned. The amount 

of SO2 emitted depends directly on the quantity of sulfur in the flared gases.
4/15 Miscellaneous Sources 13.5-5

Table 13.5-1 (English Units). THC AND SOOT EMISSIONS FACTORS FOR FLARE OPERATIONSa

EMISSIONS FACTOR RATING: B

Pollutant SCC Emissions Factor

Value

Emissions Factor 

Units

Total hydrocarbonsb 30190099 0.14 lb/106 Btu

Nitrogen oxidesc 30190099 0.068 lb/106 Btu

Sootc 30190099 0 - 274 μg/L

a Reference 1. Based on tests using crude propylene containing 80% propylene and 20% propane.

b Measured as methane equivalent.

c Soot in concentration values: nonsmoking flares, 0 micrograms per liter (μg/L); lightly smoking flares, 

40 μg/L; average smoking flares, 177 μg/L; and heavily smoking flares, 274 μg/L.

Table 13.5-2 (English Units). VOC and CO EMISSIONS FACTORS FOR FLARE OPERATIONSa

Pollutant SCC Emissions Factor

(lb/106 Btu)

Representativeness

Volatile organic compoundsb 30190099; 

30600904

0.57 Poorly

Carbon monoxidec 30190099; 

30600904

0.31 Poorly

a These factors apply to well operated flares achieving at least 98% destruction efficiency and operating in 

compliance with the current General Provisions requirements of 40 CFR Part 60, i.e. >300 btu/scf net 

heating value in the vent gas and less than the specified maximum flare tip velocity. These factors are 

based on steam-assisted and air-assisted flares burning a variety of vent gases.

b References 4-9 and 11. 

c References 1, 4-8 and 11.
13.5-6 EMISSIONS FACTORS 4/15

 

References for Section 13.5

1. Flare Efficiency Study, EPA-600/2-83-052, U. S. Environmental Protection Agency, Cincinnati, 

OH, July 1983.

2. K. D. Siegel, Degree Of Conversion Of Flare Gas In Refinery High Flares, Dissertation, 

University of Karlsruhe, Karlsruhe, Germany, February 1980.

3. Manual On Disposal Of Refinery Wastes, Volume On Atmospheric Emissions, API Publication 

931, American Petroleum Institute, Washington, DC, June 1977.

4. TCEQ 2010 Flare Study Final Report. Texas Commission on Environmental Quality Tracking 

No. 2008-81. Austin, Texas. The University of Texas at Austin Center for Energy and 

Environmental Resources. August 2011. 

5. Performance Test of a Steam-Assisted Elevated Flare with Passive FTIR-Detroit. Marathon 

Petroleum Company, LP. Detroit, Michigan. Clean Air Engineering, Inc. November 2010.

6. PFTIR Test of Steam-Assisted Elevated Flares-Port Arthur. Flint Hills Resources Port Arthur, 

LLC. Port Arthur, Texas. Clean Air Engineering, Inc. June 2011.

7. Deer Park Refinery East Property Flare Performance Test Report. Shell Deer Parking Refining 

LP. Deer Park, Texas. Shell Global Solutions (US) Inc. April 2011.

8. Passive Fourier Transform Infrared Technology (FTIR) Evaluation of P001 Process Control 

Device. INEOS ABS (USA) Corporation. Addyston, Ohio. INEOS ABS (USA) Corporation. July 

2010.

9. Differential Absorption Lidar Study – Final Report. Texas Commission on Environmental 

Quality. Austin, Texas. March 2010.

10. Parameters for Properly Designed and Operated Flares. Prepared for U.S. Environmental 

Protection Agency, Office of Air Quality Planning and Standards, Research Triangle Park, NC. 

April 2012.

11. Performance Test of a Steam-Assisted Elevated Flare with Passive FTIR. Marathon Petroleum 

Company, LLC. Texas City, Texas. Clean Air Engineering, Inc. May 2010.