306 ENVIRONMENTAL ENGINEERING
- Unless controlled by applications of air pollution control technology, the gaseous
and particulate products of combustion may be hazardous to health or damaging to
ProPeY
The decision to incinerate a specific waste will therefore depend first on the environ-
mental adequacy of incineration as compared with other alternatives, and second on
the relative costs of incineration and the environmentally sound alternatives.
The variables that have the greatest effect on the completion of the oxidation of
wastes are waste combustibility, residence time in the combustor, flame temperature,
and the turbulence present in the reaction zone of the incinerator. The combustibility is
a measure of the ease with which a material may be oxidized in a combustion environ-
ment. Materials with a low flammability limit, low flash point, and low ignition and
autoignition temperatures may be combusted in a less severe oxidation environment,
i.e., at a lower temperature and with less excess oxygen.
Of the three “T’S~’ of good combustion, time, temperature, and turbulence, only
the temperature may be readily controlled after the incinerator unit is constructed. This
may be done by varying the air-to-fuel ratio. If solid carbonaceous waste is to be burned
without smoke, a minimum temperature of 760°C (1400°F) must be maintained in the
combustion chamber. Upper temperature limits in the incinerator are dictated by the
refractory materials available to line the inner wall of the burn chamber. Above 1300°C
(2400°F) special refractories are needed.
The degree of turbulence of the air for oxidation with the waste fuel will affect the
incinerator performance significantly. In general, both mechanical and aerodynamic
means are utilized to achieve mixing of the air and fuel. The completeness of combus-
tion and the time required for complete combustion are significantly affected by the
amount and the effectiveness of the turbulence.
The third major requirement for good combustion is fime. Sufficient time must be
provided to the combustion process to allow slow-burning particles or droplets to burn
completely before they are chilled by contact with cold surfaces or the atmosphere.
The amount of time required depends on the temperature, fuel size, and degree of
turbulence achieved.
If the waste gas contains organic materials that are combustible, then incineration
should be considered as a final method of disposal. When the amount of combustible
material in the mixture is below the lower flammable limit, it may be necessary to
add small quantities of natural gas or other auxiliary fuel to sustain combustion in
the burner. Thus economic considerations are critical in the selection of incinerator
systems because of the high costs of these additional fuels.
Boilers for some high-temperature industrial processes may serve as incinerators
for toxic or hazardous carbonaceous waste. Cement kilns, which must operate at tem-
peratures in excess of 1400°C (2500°F) to produce cement clinker, can use organic
solvents as fuel, and this provides an acceptable method of waste solvent and waste
oil disposal.
Incineration is also apossibility for the destruction of liquid wastes. Liquid wastes
may be classified into two types from a combustion standpoint: combustible liquids
and partially combustible liquids. Combustible liquids include all materials having