Page 624 - Materials Chemistry, Second Edition

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Flue

Secondary chamber

Secondary burner
Secondary combustion
air blower

Mechanical charge
system
Viewport

Ash removal
door

Primary
burner
Primary chamber
Primary combustion
air blower

FIGURE 20.6 Controlled-air incinerator (U.S. EPA, 1989b).

the volatiles and other hydrocarbons emitted from the primary chamber. Turbulence is provided to
promote mixing of the air and combustion gases. The gas/air mixture is combusted at relatively high
temperatures.
Controlled-air incinerators possess several advantages over the existing multiple-chamber
incineration technology. The starved-air environment of the primary chamber allows for slow, non-
turbulent combustion, which minimizes the entrainment of particulates in combustion gases and
thus reduces particulate emissions to the atmosphere. The lower temperatures achieved in this
chamber avoid the melting and fusion temperatures of most metals, glass, and other noncom-
bustibles, thus minimizing slagging and the formation of clinker. The high temperatures and
excess air environment of the secondary chamber help to ensure more complete combustion of
volatile gases, thus reducing hydrocarbon emissions. Controlled-air incinerators are comparatively
low in cost and carry out clean combustion, making them popular in the hospital industry (U.S.
EPA, 1991).

20.7 MODES OF INCINERATOR OPERATION

Medical waste incinerators can be operated in one of three modes: batch, intermittent duty, and con-
tinuous duty. As the name implies, batch incineration involves burning a single batch of waste, often
only once per day. Waste is often loaded manually, combusted, and cooled, after which the ash is
manually removed. Intermittent duty incinerators are loaded continuously and frequently with small
waste batches and operate less than 24 h per day. A typical operating cycle for an intermittent duty
system includes a 15- to 30-min period of cleanout of ash from the previous day, a 15- to 60-min
preheat, a 12- to 14-h waste combustion period, a 2- to 4-h burndown period, and a 5- to 8-h
cooldown period. Continuous duty incinerators operate 24 h per day and use automatic charging
units, such as a ram-feed system (Figure 20.7), to input waste into the firebox in small, frequent
batches. A mechanism will automatically remove the ash from the incinerator (U.S. EPA, 1991).