1 October 1997
(2) Phase Two. The second phase is the volatilization of vapors and gases which occurs as
the temperature of the waste continues to rise. Vapors and gases diffuse out as their respective
volatilization temperatures are attained. Those vapors and gases having low flash points (i.e., the
temperature at which a specific gas or vapor will ignite) may react with primary combustion air to
burn at the surface of the bed of waste. If excess oxygen is not available, as in the case of starved-
air incinerators, the low-temperature volatilization of vapors and gases may react to form other
hydrocarbons and/or partially oxidized compounds (i.e., carbon monoxide, etc.). These compounds
must be burned later in the secondary combustion process where there is sufficient oxygen for
complete combustion. The higher flash point gases and vapors will most likely burn only after they
have been swept up in the gas flow and subsequently ignite when they are exposed to their
respective ignition temperatures. How well they are destroyed will depend upon their being
subjected to their requisite "three T" conditions in the higher temperature zones of the furnace. The
flash point for the gases and vapors driven off in this phase of the primary combustion process
ranges from approximately 500 to 1,300oF, which is usually several hundred degrees higher than
their respective volatilization temperatures. Consequently, combustion of the gases and vapors
occurs some distance above the bed in a zone where there is sufficient temperature and oxygen for
them to be oxidized. If either or both conditions are not met, the partially oxidized vapors and gases
will be carried through the system until the right conditions for completion of the oxidation process
are met. Table 3-11 shows the ratios of air to weight of solids to burn different types of solid waste.
(3) Phase Three. The third phase in the burndown of solids is the in-place oxidation of the
burnable solids left after the vapors and gases have been volatilized. The remaining, partially
oxidized cellulose, lignins, and other hydrocarbon solids, when further heated, oxidize to form
carbon dioxide and water vapor. This portion of the combustion process occurs in or on the bed in
a fairly violent manner. In excess-air systems, the residues from this phase are incompletely
burned carbon (char) and inert noncombustibles. Starved-air systems will also have some
(4) Phase Four. The fourth phase in the process involves the final burndown of char and
the consolidation and cooling of the inert residues, known as bottom ash (metals and ceramic
oxides; primarily alumina, silica and calcia, plus lesser amounts of other oxides; see table 3-12).
This material is the end product, which, after a short period of cooling on the hearth/grate, is
dumped into the ash-receiving system. In small units, the ash may be dumped directly into a dry
collection hopper. In large units, the grate continually dumps the ash into the ash quench pit where
it is cooled by water.
b. Secondary Combustion.
(1) The final destruction process requires specific conditions. The secondary combustion
zone (i.e., secondary combustion chamber in packaged and modular units and the high-
temperature secondary combustion zone in large field-erected units) must provide the desired
temperature, turbulence, and excess air required to achieve complete destruction of all the
unburned gases, vapors, and particulates released from the primary combustion process.
(a) The complete destruction of high-flash-point, low-heat-content vapors and
particulates requires more time and greater turbulence than does the complete destruction of the
more easily burned materials.