1-1
1.1 Introduction
Flaring is a high-temperature oxidation process used to burn waste gases containing
combustible components such as volatile organic compounds (VOCs), natural gas (or
methane), carbon monoxide (CO), and hydrogen (H
2
). The waste gases are piped to a remote,
usually elevated location, and burned in an open flame in ambient air using a specially
designed burner tip, auxiliary fuel, and, in some cases, assist gases like steam or air to
promote mixing for nearly complete (e.g., ≥ 98%) destruction of the combustible components
in the waste gas. Note that destruction efficiency is the percentage of a specific pollutant in the
flare vent gas that is converted to a different compound (such as carbon dioxide [CO
2
], carbon
monoxide, or another hydrocarbon intermediate), while combustion efficiency is the
percentage of hydrocarbon in the flare vent gas that is completely converted to CO
2
and water
vapor. The destruction efficiency of the gases being combusted in a flare will always be
greater than the combustion efficiency of these same gases in that same flare. It is generally
estimated that a combustion efficiency of 96.5 percent is equivalent to a destruction efficiency
of 98 percent (U.S. EPA, 2015). Gases flared from refineries, petroleum production, chemical
industries, and to some extent, from coke ovens, are composed largely of inerts and low
molecular weight hydrocarbons with high heating value. Blast furnace flare gases are largely
composed 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. (U.S. EPA, 2015)
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 LFL
and UFL, combustion can occur. Completeness of combustion in a flare is governed by flame
temperature, residence time and flammability of the gas in the combustion zone, turbulent
mixing of the components to complete the oxidation reaction, and available oxygen for free
radical formation. Combustion is complete if all hydrocarbons and CO are converted to CO
2
and water. Incomplete combustion results in some hydrocarbons or CO discharged to the flare
being unaltered or converted to other organic compounds such as aldehydes or acids.
The flaring process can produce some undesirable by-products including noise, smoke,
heat radiation, light, sulfur oxides (SO
x
), nitrogen oxides (NO
x
), CO, and can be an
undesirable potential source of ignition. However, by proper design, these can be minimized.
To improve the clarity of this chapter, the following terms are defined:
Assist air means all air that intentionally is introduced prior to or at a flare tip through
nozzles or other hardware conveyance for the purposes including, but not limited to,
protecting the design of the flare tip, promoting turbulence for mixing or inducing air into the
flame. Assist air does not include the surrounding ambient air.