1 October 1997
(b) The secondary combustion zone or chamber in which this final combustion process
occurs is therefore designed to provide a sufficient volume to achieve the high-temperature
residence times required to complete the oxidation of these harder-to-burn materials.
(c) By maintaining the temperatures and oxygen partial pressure in the secondary
combustion zones well above the requisite minimum conditions, the reactions involved in the
complete destruction of the high-flash-point and/or the low-heat-of-combustion compounds is
allowed to proceed at a rate fast enough to assure a high degree of destruction during the limited
residence time in this zone or chamber.
(2) Common practice in the design of secondary combustion chambers for municipal waste
incinerators is to provide a nominal minimum of 1 to 2.5-seconds gas residence time and nominal
secondary gas temperatures in the range of 1,800 to 2,000oF. Also, since the combustion of these
volatiles will not be complete unless sufficient oxygen is available, additional air is introduced.
(a) For unprocessed municipal solid waste (MSW), the optimal percent excess air
required to achieve high destruction efficiency and high-efficiency energy recovery in a large water-
wall furnace is approximately 40-50% (i.e., a Stoichiometric Ratio of 1.4 to 1.5, which provides an
atmosphere containing 6.6%-7.7% excess oxygen).
(b) The smaller modular and packaged units achieve their highest destruction
efficiencies with 50-100% excess air, or greater (i.e., a stoichiometric ratio of 1.5 to 2.0). They pay
for this higher dilution of exhaust gas by having lower efficiency energy recovery.
(c) Introduction of this amount of excess air has been found to be necessary in order to
supply the necessary partial pressure of oxygen required to achieve the highest destruction
efficiency practical for the conglomeration of materials in municipal waste.
c. Time for Primary Combustion Affected by the Method of Burning.
(1) The time required for complete burndown of municipal solid waste in the primary
combustion chamber is a function of how the solid waste is fed into the system. The time required
varies from six hours to a few minutes, depending upon the design of the furnace and the method
used for feeding and supporting the waste while it is being burned.
(a) Mass Burn Systems. These systems are the dominant type used to burn solid
waste. A mass burn system uses a hearth or a grate to support a large mass of raw or processed
waste as it is progressively burned down. The burndown process typically requires a nominal four to
six hours from the time the waste is introduced into the primary combustion chamber until the ash is
discharged. Incinerators operating under oxygen-deficient conditions (starved-air primary-
combustion mode) require longer burndown times than the furnaces operating in the oxygen-rich
condition (excess-air primary-combustion mode).
(b) Injection-Fed, Dispersed-Bed System. This type of feed and method of distributing
the waste as it is being burned is used in the fluidized-bed incinerator. Because the waste is diluted
as it is rapidly distributed throughout the volume of the fluidized bed, much less time is required for
complete destruction. The thermal destruction of waste in a fluidized-bed incinerator requires
essentially the same sequence for progressive destruction of the material, but instead of occurring
in discrete zones, all of the processes occur simultaneously in a single large bed. Air and small