1 October 1997
(1) Modular incinerators are by far the most prevalent in terms of units built. Multiple, 20-
75-tpd module units, combined to provide a total facility capacity of 50-300 tpd, have been widely
used in small communities and have found wide application at military installations.
(2) Units designed for operation in the starved-air mode (SAU) have historically been most
common because of their ability to achieve sufficiently clean burning of the waste (i.e., relatively low
particulate emission without the need for separate air pollution control equipment. This resulted in
making the modular SAU facility the least expensive facility to construct. New federal regulations
however, have tightened particulate emissions limits, restricted acid gas emissions and subject the
smaller capacity units to federal emissions regulations.
b. System Configuration. Generally, facilities size each unit to carry its share of the daily
waste stream when one unit in the system is down for repair. Thus, a facility consisting of four
primary combustion chambers would have each unit sized to handle one-third of the projected
maximum daily throughput. The operating availability of a modular unit may be approximately 75-
80%. The operator can anticipate that downtime for planned maintenance and repair will amount to
about 10% of the annual operating time, and that forced downtime due to unexpected failures and
malfunctions may account for another 10-15% of the time. If one secondary chamber services two
primary combustion chambers, the ducts between the primary and secondary chambers are
provided with isolation dampers. This allows continued operation of the unaffected primary and
secondary chambers when one primary chamber must be shut down.
c. Typical Design. Figure 4-2 illustrates a typical design of a multiple combustion chamber
arrangement of a modular incinerator. Units designed as either a SAU or an EAU incinerator use
separate primary and secondary combustion chambers. The details of each design differ in order
to accommodate differences in operating conditions. In either mode, measured amounts of waste
are normally batch-fed through a fire door into the primary chamber by using essentially the same
equipment. Beyond this point, their mechanical features vary.
d. Starved-Air Modular Incinerator.
(1) Primary Combustion Chamber. The SAU primary combustion chamber usually has a
solid, stepped-floor, with three or four steps. The vertical riser of each step is connected to a ram
that pushes waste from one level down to the next. The use of slow-moving rams and low flow of
air into the bed result in only enough agitation of the waste to slowly tumble it down over each
succeeding step as it progresses over the full length of the stepped hearth. Minimal agitation and
mixing of the waste helps to minimize particulate generation and carry over in the gases and vapors
leaving the primary chamber.
(a) Longer retention times (i.e. 4-6 s to transverse the length of the primary chamber)
are required to achieve a reasonable burndown of the waste due to the low agitation, reduced rate
of destruction associated with the low stoichiometry (S.R. of 0.6 to 0.9), and the subsequent lower
(b) A minimal flow of air is provided to the bed from overfire air ports along the walls and
underfire air ports below the bed meter. The careful balance of airflow achieves the uniform
substoichiometric conditions required for the partial oxidation of the waste in the front two-thirds of
the furnace and the burndown of the combustible solids and char at the last third of the bed.