Page 519 - Materials Chemistry, Second Edition

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490 Waste Management Practices: Municipal, Hazardous, and Industrial
them cooler than the center of the furnace. The primary and secondary airflows are also utilized to
assist in fuel atomization and prevent unburned materials from contacting furnace linings
(Brunner, 1993).
As the droplets vaporize and combust, any inorganics present in the waste remain in the gas
stream and are carried to the air pollution control equipment. The sizes of the particles are a func-
tion of the size distribution of the original droplets. Larger droplets usually result in larger particles
and vice versa. Smaller particles are less likely to be captured by air pollution control equipment
and are thus more likely to escape to the atmosphere. Particle size depends on other variables,
including the form of the waste burned (i.e., solid, liquid, gas) and factors specific to the combus-
tion device (e.g., temperature, turbulence, and air flow). Only small quantities of bottom ash typi-
cally occur in the liquid injection incinerator.
As for all incinerator types, the firebox temperature, waste residence time, and overall turbulence
are adjusted to optimize destruction efficiencies. The liquid injector operates within a range of tem-
peratures, depending on the waste type. Typical combustion chamber residence times and tempera-
ture ranges are 0.5 to 2 s and 700 to 1650°C (1300 to 3000°F), respectively. Feed rates measure up
to 5000 to 6000 L/h of organic wastes.

15.6.3 OVERVIEW OF LIQUID INJECTION

The type, size, and shape of a furnace are a function of waste characteristics, burner design, air dis-
tribution, and furnace wall design. The furnace can be simple in design as in a vertical, refractory-
lined chamber or it can be relatively complex, involving the preheating of combustion air and the
firing of multiple fuels. Liquid injection systems are capable of burning virtually any combustible
waste that can be pumped. They are usually designed to burn specific waste streams and conse-
quently are not used for multipurpose facilities. Liquid injection facilities routinely destroy a vari-
ety of wastes including phenols, PCBs, solvents, polymer wastes, herbicides, and pesticides.
The advantages in using a liquid injection incinerator include (Freeman et al., 1987):

● Fewer moving parts results in less downtime and less maintenance
● Capability to incinerate a wide range of wastes
● Low maintenance costs due to the few moving parts in the system

Disadvantages include:

● Only capable of combusting pumpable liquids and slurries
● Feed nozzles tend to clog resulting in downtime

15.6.4 ROTARY KILNS
The key component of the rotary kiln incinerator (Figure 15.7) consists of a refractory-lined rotating
cylinder mounted at a slight incline from ground level. Wastes in the form of liquids, slurries, or bulk
solids are fed into the entry port(s) (Figure 15.8) and agitated under elevated temperatures for a pre-
determined length of time depending on waste and kiln characteristics. Waste liquids may be pumped
in through a nozzle, thus atomizing the charge. A screw feed mechanism may be used to inject slur-
ries, and bulk solids may enter by a ram feed or similar mechanical system. Many rotary kilns are
charged discretely; often, entire drums are fed into a kiln in a single charge (Figure 15.9). Such
charging results in a cyclical temperature distribution inside the kiln.
The waste is expected to burn to ash by the time it reaches the kiln exit. Kiln rotation speed varies
in the range of ¾ to 4 r/min (Brunner, 1993). Most kilns possess a smooth inner surface; however, some
are equipped with internal baffles to promote turbulence of the feed. The residence time for solid wastes
in the kiln is at least 30 min and is based on the rotational speed of the kiln and its angle (Wentz, 1995).