Page 27 - Materials Chemistry, Second Edition

Ch001-P373623.qxd 3/22/07 5:25 PM Page 8
Sustainable Industrial Design and Waste Management
8
Sufficient time must be provided to permit complete burnout of
the solid particles in the liquid suspension. Inorganic particles carried in the
liquid waste stream may become molten and agglomerate into molten ash. The
combustor must be designed to collect the molten ash without plugging
the flow passages of the incinerator. Primary and secondary combustion cham-
bers are used in liquid feed incinerators. Primary combustion chambers are
used to burn wastes, which have sufficient heating value to burn without
auxiliary fuel. Secondary combustion chambers require auxiliary fuel to
control the temperature and destroy the toxic emission. Sufficient air must
be provided at all times to oxidize the organics in the combustion chamber.
Incinerators can produce soot when burning under insufficient oxygen and
poor air mixing conditions. Soot can clog up nozzles, and accumulate in the
chamber, impairing burning conditions. The physical, chemical, and ther-
modynamic properties of the waste must be considered in the design of the
incinerator.

Rotary kiln incinerator
The rotary kiln is often used in solid/liquid waste incineration because of its
versatility in processing solid, liquid, and containerized wastes. The kiln is
refractory lined. The shell is mounted at a 5 degree incline from the hori-
zontal plane to facilitate mixing the waste materials. A conveyor system or
a ram usually feeds solid wastes and drummed wastes. Liquid hazardous
wastes are injected through a nozzle(s). Non-combustible metal and other
residues are discharged as ash at the end of the kiln. Rotary kilns are also fre-
quently used to burn hazardous wastes.
Rotary kiln incinerators are cylindrical, refractory-lined steel shells
supported by two or more steel trundles that ride on rollers, allowing the
kiln to rotate on its horizontal axis. The refractory lining is resistant to cor-
rosion from the acid gases generated during the incineration process. Rotary
kiln incinerators usually have a length-to-diameter (L/D) ratio between 2
and 8. Rotational speeds range between 0.5 and 2.5 cm/s, depending on the
kiln periphery. High L/D ratios and slower rotational speeds are used for
wastes requiring longer residence times. The kilns range from 2 to 5 meters
in diameter and 8 to 40 meters in length. Rotation rate of the kiln and resi-
dence time for solids are inversely related; as the rotation rate increases, res-
idence time for solids decreases. Residence time for the waste feeds varied
from 30 to 80 minutes, and the kiln rotation rate ranges from 30 to 120 rev-
olutions per hour. Another factor that has an effect on residence time is the
orientation of the kiln. Kilns are oriented on a slight incline, a position
referred to as the rake. The rake typically is inclined 5° from the horizontal.
Hazardous or non-hazardous wastes are fed directly into the rotary
kiln, either continuously or semi-continuously through arm feeders, auger
screw feeders, or belt feeders to feed solid wastes. Hazardous liquid wastes
can also be injected by a waste lance or mixed with solid wastes. Rotary kiln