Hazardous Waste 307sufficient calorific value to support combustion in a conventional combustor or burner.
Noncombustible liquids cannot be treated by incineration and include materials that
would not support combustion without the addition of auxiliary fuel and would have a
high percentage of noncombustible constituents such as water. To support combustion
in air without the assistance of an auxiliary fuel, the waste must generally have a heat
content of 18,500 to 23,000 kJkg (800Crl0,OOO Btu/lb) or higher. Liquid waste having
a heating value below 18,500 kJkg (8000 Btu/lb) is considered apartially combustible
material and requires special treatment.
When starting with a waste in liquid form, it is necessary to supply sufficient heat
for vaporization in addition to raising it to its ignition temperature. For a waste to be
considered combustible, several rules of thumb should be used. The waste should be
plumbable at ambient temperature or capable of being pumped after heating to some
reasonable temperature level. Since liquids vaporize and react more rapidly when finely
divided in the form of a spray, atomizing nozzles are usually used to inject waste liquids
into incineration equipment whenever the viscosity of the waste permits atomization.
If the waste cannot be pumped or atomized, it cannot be burned as a liquid but must
be handled as a sludge or solid.
Several basic considerations are important in the design of an incinerator for a
partially combustible waste. First, the waste material must be atomized as finely as
possible to present the greatest surface area for mixing with combustion air. Second,
adequate combustion air must be provided to supply all the oxygen required for oxida-
tion or incineration of the organic present. Third, the heat from the auxiliary fuel must
be sufficient to raise the temperature of the waste and the combustion air to a point
above the ignition temperature of the organic material in the waste.
Incineration of wastes that are not pure liquids but that might be considered sludge
or slurries is also an important waste disposal problem. Incinerator types applicable
for this kind of waste would be fluidized bed incinerators, rotary kiln incinerators, and
multiplehearth incinerators,l all of which increase incineration efficiency.
Incineration is not a total disposal method for many solids and sludge because
most of these materials contain noncombustibles and have residual ash. Complications
develop with the wide variety of materials that must be burned. Controlling the proper
amount of air to give combustion of both solids and sludge is difficult, and with most
currently available incinerator designs this is impossible.
Closed incinerators such as rotary kilns and multiple-hearth incinerators are also
used to burn solid wastes. Generally, the incinerator design does not have to be limited
to a single combustible or partially combustible waste. Often it is both economical
and feasible to use a combustible waste, either liquid or gas, as the heat source for the
incineration of a partially combustible waste that may be either liquid or gas.
Experience indicates that wastes that contain only carbon, hydrogen, and oxygen
and that may be handled in power generation systems may be destroyed in a way that
'Fluidized bed incinerators bum waste that has been treated to flow almost like a liquid. Rotary kiln
incinerators burn waste in a rotating chamber, which exposes new surfaces to the burners as it rotates.
Multiple-hearth incinerators have more than one burning chamber.