1 October 1997
supplied above and below the bed in excess of stoichiometric requirements in order to force the
combustion process to progress in a prescribed manner.
(d) Typically, 40-60% of the combustion air is introduced under grate to provide grate
(e) The walls of the primary combustion chamber are lined with refractory insulation,
which covers the water-wall boiler piping to a height that assures that the pipe will not be attacked
by corrosive substoichiometric gases leaving the bed. The roof of the primary combustion chamber
is contoured to force the hot combustion gases to sweep across the top of the bed of waste.
(f) Combustion of the solid waste in these large units is characterized by vigorous,
turbulent burning from the high volume of air used with a consequent high rate of destruction.
Burndown is usually completed in less than four hours.
(2) Secondary Combustion.
(a) Secondary combustion starts as the gases approach the roof of the primary
combustion chamber. Secondary air is injected above the primary combustion overfire air ports; in
addition, air is also injected at the bottom of the secondary combustion area (i.e., at the base of the
vertical radiant boiler) and at one or two elevations along the length of the vertical waterwall boiler.
(b) Air is introduced at strategic positions and in sufficient quantities to impart the
turbulence required to ensure complete combustion and maintain gas temperatures at prescribed
(c) The mean gas temperature in the radiant boiler is approximately 1,600-1800F with
combustion gas hot spots as high as 3,000F. With higher localized temperatures, larger units can
generate significantly higher concentrations of NOx than the modular SAUs and EAUs.
(d) Typically, the efficient field erected incinerators can yield 5,000-6,000 lb of low-
pressure district heating steam, or 350-450 kWh of electricity, per ton of waste destroyed.
4-5. ROTARY KILN INCINERATOR.
a. General Description. The unique feature of the rotary kiln furnace/incinerator is that the
primary chamber rotates in order to agitate the waste and expose it to combustion air and heat
generated by a supplementary burner. Rotary kiln incinerators have found wide application where
the material to be destroyed contains high moisture or is deficient in readily combustible
b. Packaged Rotary Kiln Incinerators. Packaged rotary kiln incinerators have been used
extensively for the destruction of hospital waste and pathological wastes. Typically, they are used
to destroy up to 500 lb/h of these difficult to burn materials.
(1) Primary Combustion. Packaged rotary kilns are very similar to retort-type furnaces,
except that the primary combustion chambers are rotating, inclined, refractory-lined cylinders.
(a) Material to be destroyed is fed into the chamber through a sliding door via a ram in
much the same manner used to feed the retort furnace. The stationary door frame has a sliding
seal that prevents gas leakage between the door frame and the rotating kiln.