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 +5O2 > 3 CO2 +4H2O

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 discharging horizontally 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 single- or 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.

Complete combustion requires sufficient combustion air and proper mixing of air and waste

gas. 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

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

methane, olefins, and aromatics, cause 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

9/91 (Reformatted 1/95) Miscellaneous Sources 13.5-1
flaring of heavy hydrocarbon waste gas. Other external forces may be used for this purpose, including

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

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. Typical steam usage ratio

varies from 7:1 to 2:1, by weight.

Waste gases to be flared must have a fuel value of at least 7500 to 9300 kilojoules per cubic

meter kJ/m3 (200 to 250 British thermal units per cubic foot [Btu/ft3]) for complete combustion;

otherwise fuel must be added. Flares providing supplemental fuel to waste gas are known as fired, or

endothermic, flares. In some cases, even flaring waste gases having the necessary heat content

will also require supplemental heat. If fuel-bound nitrogen is present, flaring ammonia with a heating

value of 13,600 kJ/m3 (365 Btu/ft3

) will require higher heat to minimize nitrogen oxides (NOx)

formation.

At many locations, flares normally used to dispose of low-volume continuous emissions are

designed to handle large quantities of waste gases that may be intermittently generated during plant

emergencies. 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

13.5-2 EMISSION FACTORS (Reformatted 1/95) 9/91
from 45 to 90 kilograms per hour (kg/hr) (100 - 200 pounds per hour [lb/hr]) for relief valve leakage

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). A 40 molecular weight gas typically of

0.012 cubic nanometers per second (nm3

/s) (25 standard cubic feet per minute [scfm]) may rise to as

high as 115 nm3

/s (241,000 scfm). The required flare turndown ratio for this typical case is over

15,000 to 1.

Many flare systems have 2 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, usually

a low-level ground flare, is intended to handle regular gas volumes, and the other, an elevated flare, to

handle excess gas flows from emergencies.

13.5.2 Emissions

Noise and heat 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 include carbon particles (soot), unburned hydrocarbons, CO, and other

partially burned and altered hydrocarbons. Also emitted are NOx and, if sulfur-containing 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 combustion efficiency in the flare plume, meaning

that hydrocarbon and CO emmissions amount to less than 2 percent of 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

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 air-assisted. Flare gas composition is a critical factor in determining the amount of steam

necessary.

9/91 (Reformatted 1/95) Miscellaneous Sources 13.5-3
Since flares do not lend themselves to conventional emission testing techniques, only a few

attempts have been made to characterize flare emissions. Recent 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). The tests conducted on steam-assisted flares at velocities as low as

39.6 meters per minute (m/min) (130 ft/min) to 1140 m/min (3750 ft/min), and on air-assisted flares at

velocities of 180 m/min (617 ft/min) to 3960 m/min (13,087 ft/min) indicated that variations in

incoming gas flow rates have no effect on the combustion efficiency. Flare gases with less than

16,770 kJ/m3 (450 Btu/ft3) do not smoke.

Table 13.5-1 presents flare emission factors, and Table 13.5-2 presents emission composition

data obtained from the EPA tests.1 Crude propylene was used as flare gas during the 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

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.

Table 13.5-1 (English Units). EMISSION FACTORS FOR FLARE OPERATIONSa

EMISSION FACTOR RATING: B

Component

Emission Factor

(lb/106 Btu)

Total hydrocarbonsb 0.14

Carbon monoxide 0.37

Nitrogen oxides 0.068

Sootc 0 - 274

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.

13.5-4 EMISSION FACTORS (Reformatted 1/95) 9/91
Table 13.5-2. HYDROCARBON COMPOSITION OF FLARE EMISSIONa

Composition

Volume %

Average Range

Methane 55 14 - 83

Ethane/Ethylene 8 1 - 14

Acetylene 5 0.3 - 23

Propane 7 0 - 16

Propylene 25 1 - 65

a Reference 1. The composition presented is an average of a number of test results obtained under the

following sets of test conditions: steam-assisted flare using high-Btu-content feed; steam-assisted

using low-Btu-content feed; air-assisted flare using high-Btu-content feed; and air-assisted flare using

low-Btu-content feed. In all tests, "waste" gas was a synthetic gas consisting of a mixture of

propylene and propane.

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.

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