1 October 1997
particles of waste are continuously injected at a high rate into the bed, and ample oxygen is always
available to all parts of the bed. This allows each distinct piece of waste to undergo its drying,
volatilization, oxidation of the gases and vapors, combustion of organic solids, and complete
burndown of the char in all parts of the bed at the same time. Residence time for destruction of the
small, (-2 inch), sized waste in the 1,5000F bed is of the order of minutes.
(c) Co-firing of Refuse-Derived Fuel (RDF). Co-feeding and co-firing of specially
prepared refuse, RDF, with coal in a coal-fired boiler allows the waste to be destroyed at a rate
comparable with that of burndown of the coal fed to the boiler furnace. This method of destruction
of waste requires that the waste be sized, prepared, and fed into the furnace in a manner that
assures that the 10-20% by weight of waste will burn down at a rate faster than that required for
the 80-90% by weight of coal, for which the boiler was originally designed. Thus, a spreader stoker
furnace burning 2-in. coal is co-fired with 0.75-1.5 in.-diameter pellets or cubes of RDF.
Suspension-fired boiler furnaces firing pulverized coal are co-fired with fluff RDF.
d. Special Design Considerations.
(1) In the case of incinerators used to burn hazardous substances, a minimum residence
time of 2 seconds at a minimum of 1,800oF is used in the design criteria for achieving the 99.99%
destruction efficiency required by law for hazardous waste incinerators. Some states are also
requiring this higher efficiency destruction on municipal waste incinerators in order to assure the
destruction of dioxins.
(2) Toxic materials may be formed during primary combustion by the reaction of partially
burned hydrocarbons with chlorine and must be destroyed in the secondary combustion process.
(3) Figure 3-2 shows the relationship of destruction efficiency of biphenyls and
chlorobenzenes (autogenous ignition temperature of 1,3190F and 1,2450F, respectively), with time
e. Mechanical Features Used to Achieve Process Control. The design of a mass burn furnace
requires special provisions:
(1) Controlled introduction of air at the appropriate locations above and below the bed is
required in order to accomplish the drying, volatilization, and combustion processes in the
respective zones of the combustion chamber.
(2) Incinerators with primary combustion chambers operating in the starved-air mode require
proportionately larger amounts of secondary air. This larger amount of air tends to cool the gases
and requires that an auxiliary burner be provided to heat and maintain the gases at the required
temperature. Typically, the less the amount of air delivered to the primary chamber (i.e., starved-air
mode), the more air and the greater the auxiliary burner (either oil or natural gas fired) input to the
secondary. This requires that the starved-air units (SAU) be provided with a larger volume
secondary chamber than their comparable capacity excess-air unit (EAU). Table 3-13 lists the
typical air distribution for primary and secondary combustion chambers/zones for modular and field-
f. Chamber Geometry and Insulation. Well-designed units make provision for the necessary
features (i.e., insulation, size and shape of the chambers, etc.) for attaining and maintaining the