Gas Emission Control - Mathematics and Statistics for the Environment - page 477

degree of oxidation. The reverse is also true—a higher residence time allows the use of a lower tem-
perature. Simply, the residence time needed to complete the oxidation reactions in the incinerator
depends partly on the rate of the reactions at the prevailing temperature and partly on the mixing
of the waste stream and the hot combustion gases from the burner or burners. The residence time of
gases in the incinerator may be calculated from a simple ratio of the volume of the refractory-lined
combustion chamber and the volumetric flow rate of combustion products through the chamber.

t = V / Q (16.16)

where
t = Residence time (sec).
V = Chamber volume (m 3 ).
Q = Gas volumetric flow rate at combustion conditions (m 3 /s).

Q is the total flow of hot gases in the combustion chamber. Adjustments to the flow rate must include
any outside air added for combustion. Example 16.10 below shows the determination of residence
time from the volumetric flow rate of gases.

16.5.1.3 Turbulence
Proper mixing is important in combustion processes for two reasons. First, mixing of the burner
fuel with air is needed to ensure complete combustion of the fuel. If not, unreacted fuel will be
exhausted from the stack. Second, the organic compound-containing waste gases must be thor-
oughly mixed with the burner combustion gases to ensure that the entire waste gas stream reaches
the necessary combustion temperatures. Otherwise, incomplete combustion will occur. A number
of methods are available to improve mixing of the air and combustion streams. Some of these
include the use of refractory baffles, swirl fired burners, or baffle plates. It is not easy to obtain
complete mixing. Unless properly designed, many of these mixing devices may create dead spots
and reduce operating temperatures. Inserting obstructions to increase turbulence may not be suf-
ficient. In afterburner systems, the process of mixing the flame and the fume streams to obtain a
uniform temperature for decomposition of pollutants is the most difficult part in the design of the
afterburner (USEPA, 1973).

16.5.1.4 Oxygen Requirement
Not only is oxygen necessary for combustion to occur, oxygen requirements for the supplemental
heat burner used in thermal incinerators must be taken into account when sizing the burner system
and the combustion chamber. To achieve complete combustion of a compound or the fuel (propane,
No. 2 fuel oil, natural gas, for example), a sufficient supply of oxygen must be present in the burner
flame to convert all of the carbon to CO 2 . This quantity of oxygen is referred to as the stoichiometric
or theoretical amount. The stoichiometric amount of oxygen is determined from a balanced chemi-
cal equation summarizing the oxidation reactions; for example, 1 mole of methane requires 2 moles
of oxygen for complete combustion (USEPA, 1981, p. 3-4):

CH 4 + 2O 2 → CO 2 + 2H 2 O (16.17)

If an insufficient amount of oxygen is supplied, the mixture is referred to as rich. Incomplete
combustion occurs under these conditions. This reduces the peak flame temperature and creates
black smoke emissions. If more than the stoichiometric amount of oxygen is supplied, the mixture is
referred to as lean. The added oxygen plays no part in the oxidation reaction and passes through the
incinerator. To ensure combustion, more than the stoichiometric amount of air is used. This extra
volume is referred to as excess air.