1 October 1997
combustion air to be introduced between the pipes and supplied to the bed of waste as it slowly
tumbles with the rotation of the cylinder. All four stages of combustion progress to completion as the
solid waste passes through the chamber. Residence time is determined by the speed of rotation.
The water in the pipes is pumped from manifolds attached to rotary seals that are located some
distance from the actual combustion of the waste. Thus, the seals are not subject to the high gas
temperature environment of the pipes that make up the walls of the rotating cylindrical chamber.
(3) Unique Applications. Rotary kilns may be operated in either a starved-air mode or an
excess-air mode. The mode of operation will dictate the materials of construction. Of concern is
the corrosiveness of the products of partial combustion in the primary chamber. The refractory-
lined kilns have been especially effective for the destruction of difficult to burn, high-moisture-
content materials that require air-rich conditions and high agitation of the solids.
(a) In such applications, the primary source of heat is the auxiliary burner used to
maintain the temperature in the rotating chamber at the desired 1,800 to 2,000F.
(b) The chamber stoichiometry is maintained in the air-rich condition to ensure there is
enough oxygen to destroy the waste. The partially burned gases leave the rotary kiln and enter into
the stationary secondary combustion chamber. Additional air and a supplementary burner is
required in the secondary chamber to maintain optimum temperature. The necessary temperature,
residence time and outlet gas stoichiometry conditions are required to completely destroy unburned
gases and particulates to the same extent as in conventional furnace units.
(c) Since these type units generate large amounts of particulates, even small
packaged-unit installations may require some type of permit that will mandate the unit meet some
set of emissions requirements.
4-6. FLUIDIZED-BED COMBUSTOR (FBC) INCINERATOR.
a. General Description. Figure 4-5 illustrates a typical bubbling-bed FBC unit. Combustion air
enters the lower portion of the combustion chamber and passes through a grid that acts as a floor
for the inert (usually sand) bed. The bed is levitated and kept in constant agitation by the flowing
(vertical) air. Auxiliary fuel burners are used to heat air delivered to the bed to the temperature
required for ignition of the waste fuel. The waste is shredded, sized, and air classified to eliminate
the larger and/or heavier materials before being fed into the FBC unit via a pneumatic feed system.
b. Process Description. Once the bed reaches the fuel-ignition temperature, sized feedstock
is fed into the bed. The "boiling/scrubbing" action of the sand/fuel mix keeps the feed material in
constant contact with the combustion air. As combustion progresses, lighter fuel particles rise to
the top of the bed and are consumed; heavier residue particles settle to the bottom and are
routinely discharged. Since the bed temperature is typically 1,500F, NOx emissions are low and
particulate emissions are high. Heat is recovered by hot water or steam pipes in the wall of the
chamber and in the waste-heat recovery heat exchanger in the path of the discharge gases.
Instead of an external scrubber, specially sized limestone is added to the bed to accomplish acid-
gas control within the unit.
c. Waste Feed. One of the main advantages of using an FBC unit to burn waste (besides
environmental emissions) is the insensitivity of the combustor portion to fuel quality. However,
preparation of the fuel in order to feed it into the bed is a major impediment of FBC. Although