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 Goals and failings of
flares
- The goal of flare is to
convert, through oxidation, substances in the flare gas stream to their
safest form possible.
- In the case of hydrocarbons, the
most desirable products are carbon dioxide and water vapor.
- Sulpher in compounds like hydrogen sulphide is converted to sulpher
dioxide.
- Other oxides, like the oxides
of nitrogen, or partially oxygenized compounds like carbon monoxide or
formaldehyde are less desirable.
- Toxic compounds like
poly-nuclear aromatic hydrocarbons, aromatics and volatile organic
compounds formed in these diffusion flames may not be fully consumed.
These compounds may be bound
up in the soot that is often emitted from these flames.
 Factors that affect emissions
The efficiency of flares
can be dependent on several factors. Some of these are:
- Composition of the flare
stream,
- Flow rate of flare
gases,
- Wind velocity,
- Ambient turbulence,
- Presence of hydrocarbon
droplets in the flare stream,
- Presence of water droplets in
the flare stream.
All of
these variables can be controlled and their effects analyzed by using the
Combustion Windtunnel facility in the
Department of Mechanical Engineering.
Definitions of Efficiency
- The use of the term “flare
efficiency” can be ambiguous because a wide variety of definitions have be used in the past. The most rigorous and
universal definition of efficiency is the “Carbon Conversion
Efficiency” which, for a hydrocarbon flare, is simply and
often called the "Combustion Efficiency" and is
defined as:
the ratio between the mass of carbon in the
form of carbon dioxide which is produced by the flare and
the mass of carbon in the form of fuel entering the flare.
- The "Hydrocarbon
Destruction Efficiency" can also be useful in characterizing
flare performance. It is defined as the amount of hydrocarbons
entering the flare in the flare gas minus the amount of unburned
hydrocarbons in the products all divided by the amount of hydrocarbons
entering the flare (i.e.: This is the fraction of
hydrocarbons destroyed by the flare).
Formation
rates of toxic compounds
- Some emissions are not
best described by an efficiency.
- These compounds do not
exist in the flare stream but are formed in minor amounts and appear in
the products.
- An alternate method
for quantifying these processes is by measuring the micrograms (µg) of a
substance formed per kilogram of fuel burned.
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