Page 522 - Materials Chemistry, Second Edition

CAT3525_C15.qxd 1/27/2005 12:40 PM Page 493
Incineration of Hazardous Wastes 493
SOLUTION
Rearrange the equation to solve for r/min:

N 0.19L/(θ DS)
N 0.19 5/(7.5 2.2 0.1)
0.58 r/min

The gas retention time for 99.99% destruction of a compound is given by (Kiely, 1996)

ln t (ln 9.21/A) (E/RT) (15.7)
g
1
where A is the Arrhenius preexponent frequency (s ), E the energy of activation (J/kg mol), R the
universal gas constant 8314 J/kg mol, and T the absolute temperature (K).
The variables A and E are typically known for a specific compound.
A source of heat is required to bring the system to operating temperatures and to maintain the
desired combustion temperature. Supplemental fuel (e.g., natural gas) is injected into the kiln through
a nozzle. Excess air is also provided to promote combustion. A negative pressure (i.e., suction) is
applied to the kiln via an induced draft fan to remove particulate matter and noncombusted vapors.
There are two modes of kiln gas flow. In the co-current mode, the burner is installed at the
entrance to the kiln, and the gas flow is in the same direction as waste flow. In the counter-current
mode, the burner is placed near the kiln exit, and the gas flow is opposite to the direction of waste
flow. A counter-current system has been demonstrated to be effective for the combustion of aque-
ous wastes. The gases exiting the kiln serve to dry the incoming aqueous waste. The entrance region
is sufficiently hot for the heating and combustion of wastes entering the kiln.
Organic gases and particulates may be drawn into a second, stationary chamber, labeled an
afterburner. Here temperatures are higher compared with the kiln and permit a more complete
destruction of any remaining organic particles or vapors. The gases discharged from the afterburner
are subsequently directed to an air pollution control system such as an electrostatic precipitator,
baghouse, or scrubber.
Rotary kilns experience a high entrainment of particulate matter. Entrainment occurs because
solids are continuously tumbling within the kiln and are reintroduced to the gas stream. Particle size
ranges are similar for both rotary kiln incinerators and liquid injectors. In rotary kilns, entrained
particles tend to be larger than 10 µm.
In some cases, it may be desirable to operate a kiln in the so-called “slagging” mode. At temper-
atures of approximately 1090 to 1200°C (2000–2200°F), ash will liquefy. The ash fusion temperature
is a function of the composition of the waste as well as incinerator conditions (e.g., oxygen concen-
tration). When the ash is in molten form, salt-laden wastes and metal drums are more easily incor-
porated into the kiln. The production of particulates is also minimized. However, the temperature
range must be maintained in a higher range in a slagging kiln, resulting in higher energy costs and
possible accelerated wear of components; temperatures may average 1425 to 1540°C (2600–2800°F),
compared with less than 2000°F in a nonslagging kiln. The construction of a slagging kiln is more
complex than for a nonslagging kiln. Finally, maintenance tends to be more frequent with a slagging
kiln (Brunner, 1993).
There are several sites within the kiln in which leakage of gases may occur. Critical points
include the inlet ports and the kiln seals. Efficient kiln operation requires a limited introduction of
unwanted airflow into the system. During a phenomenon known as “puffing,” the introduction of a
volatile organic waste will result in the instantaneous production of gases with consequent expan-
sion and a very rapid pressure increase at the inlet end of the kiln. This pressure may be sufficient
to weaken kiln seals. In order to limit such a pressure increase the kiln atmosphere is maintained
with a negative pressure draft.