1 October 1997
temperatures desired in the respective zones of the furnace. For this reason, a primary chamber
designed to operate in the starved-air mode will use a different configuration and type of insulation
than a unit designed for operation in the excess-air mode
g. Chamber Volume/Gas Residence Time.
(1) The combustion chamber must be sized to provide adequate residence time for
complete destruction. The time required in the primary chamber is a function of the characteristics
of the waste, the type of charging used, and the mode of operation (e.g., the starved-air mode
requires more time than does the excess-air mode).
(a) Of principal interest to the furnace designer is the projected hourly throughput
required, the average moisture in the waste, and the major constituents, including the percentage of
inerts and highly combustible materials.
(b) Best performance is obtained by providing as uniform a feeding of the waste to the
primary combustion chamber as possible. Similarly, allocation of space in the secondary chamber is
a function of the volume of the total gas throughput and the need to provide the necessary 1.0 to
2.5 seconds of residence time, at temperature, before the gases are discharged to the downstream
h. Turbulence. Turbulence takes on two forms in the waste combustion process, slow
agitation of the solid waste and turbulent mixing of gases and air in the gas combustion zones.
(1) Continuous and/or routine agitation of the solid waste during the drying and volatilization
phases assures that material in the lower part of the bed will not be insulated from the heat and
gases sweeping over the top of the bed. By providing some physical means (usually a patented
grate system) for the continual turning of the waste, the drying process will proceed at a more
(2) Excessive and violent agitation of the bed by vigorous turning or by high-velocity flow of
air up through the bed can be detrimental. Although drying may be more rapid and the waste is
more frequently exposed to the radiated heat from the chamber walls, the lightweight ash and
partially burned material on the surface could be excessively agitated.
(a) Vigorous agitation will result in excessive amounts of solids being carried up with the
gases. Some of these solids (e.g., flakes of paper and partially burned lightweight material) will
burn with the gases as they enter the secondary zone and end up as fly ash. However, some of
this solid material will continue to burn long after it leaves the secondary zone simply because it
takes much longer to oxidize the solid material than it does to oxidize the gases. Unless special
equipment is provided, the burning material may cause fires in the bag house.
(b) Aside from the fire potential, the excess solid particulate creates problems by fouling
the heat transfer surfaces and by producing additional loading to the gas cleanup system.
(c) Release of these unburned artifacts can also contribute to the adverse emissions
from waste incinerators. The release of dibenzo-p-dioxins (dioxins) and dibenzofurans (furans) has
been associated with this condition; it has been postulated that the airborne unburned carbon
provides the sites for reaction of the chlorine gas released from the oxidized, chlorine-containing