Page 323 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

300 10 Post-combustion Air Emission Control

10.6.3 Flaring

Flaring is more often employed for gas flaring in oil and gas refinery plants than the
flue gas or exhaust gas from a combustion source. For gases with VOC concen-
trations above the UFL level, flaring at the stack outlet can burn the VOCs into
other less problematic pollutants. Addition of steam is necessary for many VOCs
flaring, otherwise only a few VOCs can be oxidized without major problems.
There are two types of flares, being elevated flare and ground flare. The most
commonly used type in refineries and chemical plants are elevated flares. They are
characterized with large capacities. The waste gas stream is fed through a stack with
a height of from tens to a hundred meters. Combustion takes place at the top of the
stack.
The elevated flare, can be steam assisted, air assisted or non-assisted. Steam/air
injection promote smokeless combustion; adequately elevated flare has the best
dispersion characteristics for malodorous and toxic combustion products, which are
mainly caused by incomplete combustion due to the cross wind. On the other hand,
steam injection/air injection cause noise pollution.
Ground level flare can achieve smokeless operation without noise, but it is poor
in dispersion of combustion products because its short stack is close to ground. This
may result in severe air pollution or hazard if the combustion products are toxic or
in the event of flame-out. Ground level flares are often used for the destruction of
landfill gases, which contain a large amount of methane and other odorous
compounds.

10.6.4 Thermal Oxidizers

A thermal oxidizer burns VOC-containing gas streams in an enclosed refractory-
lined chamber that contains one or more burners. The design of thermal oxidation
systems for VOCs has been well documented in the literature (e.g., [19]). It is
summarized as follows.
An example of a thermal VOC incinerator is shown in Fig. 10.7. The waste gas
is oxidized before entering the stack, and meanwhile the heat is recovered. The
thermal oxidizer consists of a refractory-brick lined chamber that has one or more
gas- or oil-fired burners. The contaminated gas stream does not usually pass
through the burner itself, unless a portion of the gas stream is used to provide the
oxygen needed to support combustion of the fuel. Instead, the burners are used to
heat the gas stream to the temperature necessary to oxidize the organic contami-
nants. That temperature is based primarily on the auto ignition temperature of the
most difficult to destroy compound in the gas stream. Auto ignition temperatures for
most organic compounds range from 400 to 540 °C. Operation at temperatures near
the auto ignition value will result in destruction of the contaminants; however, the