Page 28 - Materials Chemistry, Second Edition

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Current Practice and Future Sustainability
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systems typically include secondary combustion chambers of afterburners
to ensure complete destruction of the hazardous waste. Operating kiln tem-
peratures range from 800°C to 1,300°C in the secondary combustion cham-
ber or afterburner depending on the type of wastes. Liquid wastes are often
injected into the kiln combustion chamber.
The advantages of the rotary kiln include the ability to handle a variety
of wastes, high operating temperature, and continuous mixing of incoming
wastes. The disadvantages are high capital and operating costs and the need
for trained personnel. Maintenance costs can also be high because of the
abrasive characteristics of the waste and exposure of moving parts to high
incineration temperatures.
A cement kiln incinerator is an option that can be used to incinerate
most hazardous and non-hazardous wastes. The rotary kiln type is the typi-
cal furnace used in all cement factories. Rotary kilns used in the cement
industry are much larger in diameter and longer in length than the previously
discussed incinerator.
The manufacture of cement from limestone requires high kiln temper-
atures (1,400°C) and long residence times, creating an excellent opportunity
for hazardous waste destruction. Further, the lime can neutralize the hydro-
gen chloride generated from chlorinated wastes without adversely affecting
the properties of the cement. Liquid hazardous wastes with high heat con-
tents are an ideal supplemental fuel for cement kilns and promote the con-
cept of recycling and recovery. As much as 40% of the fuel requirement of a
well-operated cement kiln can be supplied by hazardous wastes such as sol-
vents, paint thinners, and dry cleaning fluids. The selection of hazardous wastes
to be used in cement kiln incinerators is very important not only to treat the
hazardous wastes but also to reap some benefits as alternative fuel and alter-
native raw material without affecting both the product properties and gas emis-
sions. However, if hazardous waste is burned in a cement kiln, attention has
to be given to the compounds that may be released as air emissions because
of the combustion of the hazardous waste. The savings in fuel cost due to
use of hazardous waste as a fuel may offset the cost of additional air emission
control systems in a cement kiln. Therefore with proper emission control
systems, cement kilns may be an economical option for incineration of haz-
ardous waste.

Grate-type incinerator
Grate-type incinerators are used for incineration of solid wastes. These
types of incinerators are suitable for large irregular-shaped wastes, which
can be supported on a stationary or moving alloy grate, which allows air to
pass through the grate from below into the waste. Generally, grate-type
incinerators have limited application for hazardous waste incineration
because the high temperature required in the chamber may affect the mate-
rial of the grate. The primary furnace is followed by a secondary combustion