Control of Volatile Organic Compounds Emissions from Manufacturing

and burner type but should be kept as low as possible. Using too much
excess air wastes fuel because this air must be raised to the combustion
temperature but does not contribute any heat by participating in the
oxidation reaction. Large amounts of excess air also increase the flue
gas volume and may cause an operator to invest in a larger system than
requi red.
A thermal incinerator usual ly contains a refractory-1 ined chamber
(which may vary in cross-sectional size along its length) containing a
burner at one end. Because of the risk to the refractory, incinerators
are neither brought quickly up to nor cooled down quickly from operating
temperatures. They require a fairly constant fuel input to maintain
combustion temperature. A diagram of a thermal incinerator using discrete
burners is shown in Figure 3-3. (Numbers in parentheses following the
mention of equipment parts or streams denote the numbered items on the
referenced figures.) Discrete dual fuel burners (1 )I and inlets for the
offgas (2) and combustion air (3) are arranged in a premixing chamber
(4) to thoroughly mix the hot products from the burners vith the offgas
air streams. The mixture of hot reacting gases then passes into the
main combustion chamber (5). This section is sized to a1 low the mixture
enough time at the elevated temperature for the oxidation reaction to be
completed (residence times of 0.3 to 1 second are common). Energy can
then be recovered from the hot flue gases with the installation of a
heat recovery section (6). Preheating of combustion air or the process
waste offgas fed to the incinerator by the incinerator exhaust gases
will reduce auxi 1iary fuel usage. In some instances, the inci nerator
exhaust gas may be used in a waste heat boiler to generate steam.
Insurance regulations requi re that ifthe process wisste of fgas is preheated,
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the VOC concentration must be maintained below 25 percent of the lower
explosive limit (LEL) to minimize explosive hazards .16
Thermal incinerators designed specif ical ly for VOC incineration
with natural gas as the auxiliary fuel may use a grid-type (distributed)
gas burner similar to that shown in Figure 3-4. The tiny gas flame jets
(1) on the grid surface (2) ignite the vapors as thley pass through the
grfd. The grid acts as a baffle for mixing the gasles entering the
chamber (3). This arrangement ensures burning of all vapors using less
fuel and a shorter burning length in the duct than conventional forward
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