1 October 1997
(1) The objectives of combustion in an incinerator are the complete destruction of the
organic constituents to form harmless gases and the prevention of the release of any harmful
material to the environment. Efficient conversion of the heat released into useful energy, though
important, is secondary to safe and efficient destruction of the waste.
(2) The oxidation of the combustible elements requires a temperature high enough to ignite
the constituents, mixing of the material with oxygen, or turbulence and sufficient time for complete
combustion, (i.e., the three "Ts" of combustion). Proper attention to these three factors can
produce destruction/conversion efficiencies of 99.9%-99.95% in well-operated incinerators.
e. Excess Air.
(1) High-efficiency destruction (oxidation) of any combustible material requires that more
oxygen be present than what is required by the chemistry of the process. Since combustion is a
chemical process, the rate of oxidation is contingent upon many factors that can make the reactions
occur at a faster or slower rate. The percent of excess oxygen present and available to the reaction
is one of these factors.
(a) In general, combustible gases and vapors require less excess oxygen to achieve
high-efficiency oxidation than do solid fuels due to the ease of mixing and the nature of the
compounds in the gases and vapors.
(b) Solid fuel materials, because of the more complex processes involved in their
combustion, require more excess air and more time.
(2) Quantities of excess air have been determined empirically for different fuels and are
given in table 3-10.
(3) Increasing the quantity of excess air beyond the percentages indicated does not
benefit the combustion process and lowers the gas temperature thereby reducing the efficiency of
the downstream heat-recovery process.
(a) The best combination of combustion efficiency and energy recovery when mass
burning municipal waste in a large water-wall incinerator has been observed to occur with a system
S.R. of 1.4 to 1.5.
(b) The secondary combustion chambers in modular and packaged incinerators
achieve their highest destruction efficiency at S.R.s of 1.5 to 2.0.
3-5. MECHANISM OF COMBUSTION.
a. Primary Combustion Process. The thermal destruction of waste (or any other solid fuel with
significant moisture content) is accomplished in four phases as described below:
(1) Phase One. The first phase is the drying phase that occurs in the initial heating of the
heterogeneous material. Moisture is driven off as the material is heated past the vaporization
temperature of water. Drying is usually complete by the time the material has reached 300oF.