Note: Descriptions are shown in the official language in which they were submitted.
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OPPOSED FIRED ROTARY KILN
_~s~Lnical Field
This invention relates generally to rotary
kilns and is particularly useful with mobile rotary
kilns.
Backaround Art
A rotary kiln is a refractory-lined cylin-
drical vessel commonly used7 for example, in the
incineration of waste, in the calcining of cement,
coke or other materials, in the firing of ceramic,
and in many other uses. In the incineration of
15 waste, the waste is provided into the kiln and is
combusted while passing through the kiln by the
combustion fuel and oxidant which is injected into
the rotary kiln at one end of the kiln. The
injection of the fuel and oxidant into the kiln may
20 be either concurrent with the flow of waste or other
material through the kiln, or it may be
countercurrent to the flow of waste or other material
through the kiln. Gases from within the kiln are
removed through a flue located at one end of the
25 kiln. After the waste has passed through the kiln,
ash rom the combusted waste is removed from the kiln.
In a countercurrent kiln the hot combus~ion
gases and excess air are carried through the kiln
first volatizing combustibles from the waste. These
30 combustibles are combusted generatillcl additional heat
Elowing countercurrently to tlle flowinc~ waste whicl
further dries the waste. It is imperative that the
furnace gases contain sufficient mass to absorb the
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heat release without overheating which can cause
refractory damage to the kiln or kinetically favor
the generation of nitrogen oxides (NOX). Accordingl~
the throughput of material, such as waste, through
5 the kiln is limited by the quantity of furnace gases
generated within the kiln by the injected fuel and
oxidant, and by the combusting volatiles if volatiles
are present, and also by the rate at which heat can
~be transferred to wet material or to other heat sinks
10 by the furnace gases.
In a concurrent kiln another problem arises
in that the heat released from volatile combustibles
is passing away from the wet material heat sink. An
auxiliary burner is generally required to provide
15 extra heat to the drying zone to dry the material so
as to enable volatization oE the volatile
combustibles. This increases the volumetric flowrate
o~ the gases passing out the flue increasing
particulate carryover and burden on the air pollution
20 devices thus limiting the throu~hput through the kiln.
The mismatch of heat source and heat sink
which creates throughput limitations for both
countercurrent and concurrent rotary kilns is more
severe for long rotary kilns, such as kilns having a
25 length to diameter (L/D) ratio exceedin~ 4.
A recent use for rokary kilns which has been
gainin~ wide acceptance has been in the incineration
oE hazardous waste. A particlllarly aavallta~eous
rotary kiln for thls applicatioll :is a mobile Ol
30 transportable rotary kiln which can be transported to
the hazardous waste site and then removed when the
hazardous waske site has been cleaned. Unfortunately
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a mobile rotary kiln is by necessity smaller than a
stationary rotary kiln in order to enable
transportability. Thus the throughput limitations
discussed above are even rnore acute in the case of a .
5 mobile rotary kiln.
According].y it is an object of this
invention to provide a rotary kiln having increased
throughput over conventional rotary kilns without
~causing high potential for refractory damage or
10 creating conditions highly favorable for NOX
formation.
It is another object of this invention to
provide a method for operating a rotary kiln so as to
increase throughput over that obtainable with
15 conventional rotary kiln operating methods without
causing high potential for refractory damage or
creating conditions highly favorable for NOX
formation.
20 Summary Of The Invent on
The above and other objects which will
become apparent to one skilled in the art upon a
reading of this disclosure are attained by the
present invention one aspect of which is:
A rotary kiln comprising:
(A) a rotatable cylindrical body havlng an
internal diameter;
(B) a non.rotatable wall at each end of the
rotatable cylindrical body;
3U (C) flue means at one end of the rotatable
cylindrical body;
(D) a first o~idant injection means
positioned within the nonrotatable wall at the end
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opposite to the flue end, said first oxidant
injection means oriented to inject oxidant into the
rotatable cylindrical body toward the flue end; and
(E) a second oxidant injection means
5 positioned within the nonrotatable wall at the flue
end, said second oxidant injection means oriented to
inject oxidant into the rotatable cylindrical body
toward the end opposite the flue end and adapted to
~inject the oxidant with a momentum sufficient to
10 pass through a length equal to at least two times
the internal diameter of the rotatable cylindrical
body.
Another aspect of this invention comprises:
A method for operating a rotary kiln
15 comprising:
(A) providing feed comprising volatile
material into a rotatable cylindrical body;
(B) removing gas from the rotatable
cylindrical body through a flue at one end of the
20 rotatable cylindrical body;
(C) injecting oxidant into the rotatable
cylindrical body at the end opposite the flue end in
the direction of the flue end to create a flow o
gas toward the flue end;
(D) injecting oxidant into the rotatable
cylindrical body at the flue end in the direction of
the end opposite the flue end having a momentum at
least e~ual to that of gas flowing toward the flue
end; and
(E) volatizin~ material f.rom the feed
within the rotatable cylindrical hody.
As used herein the term "cylindrical" means
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tubular, generally but not necessarily having a
circular radial cross-section.
As used herein, the term "waste" means any
material intended for partial or total combustion
5 within a combustion zone.
As used herein the term "burner" means a
device through which both oxidant and combustible
matter are provided into a combustion 7one either
~ separately or as a mixture.
As used herein the term "lance" means a
device through which either oxidant or combustible
matter but not both is provided into a combustion
zone.
As used herein the term "recirculation
15 ratio" means the ratio of the mass flowrate o~
material recirculated back toward the periphery of a
jet to the mass flowrate of the total fluid injected
into a combustion zone.
As used herein the term "combustible" means
20 a substance that will burn under combustion zone
conditions.
As used herein the term "incombustible"
means a substance that will not burn under
combustion zone conditions.
As used herein the term "volatile" means a
rnaterial which will pass into the vapor state under
combustion zone conditions such as, for example, the
vapor materials resulting from drying, or from the
decomposition or thermal dissociation of solid or
30 liquid materials.
As used herein the term "equivalent
diameter" means that diameter of a single circular
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orifice which would provide the same total area as
the sum of the areas of a multi-orifice injection
means.
5 Brief Description ~f The Drawinqs
Fig~re 1 is a schematic representation of
one embodiment of the inven~ion carried out in
conjunction with waste incineration within a
countercurrent kiln.
Figure 2 is a schematic representation of
another embodiment of the invention carried out in
conjunction with waste incineration within a
concurrent kiln.
Figure 3 is a schematic representation of
15 another embodiment of the invention illustrating the
invention carried out with a plug flow zone.
Figure 4 is an illustration of a single
oriEice oxidant injection means for injecting
oxidant with a high momentum into a kiln at the flue
20 end.
Figure 5 is an illustration of a
multi-orifice oxidant injection means for injecting
oxidant with a high momentum into a kiln at the flue
end.
Figure 6 is an illustration of a burner
which may be used in the practice of this invent:ion.
Pigure 7 is an illustration of a means to
react fuel and oxidant in a recessed cavity prior to
injection into the kiln.
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Detailed DescriPtion
The invention enables a significant
5 increase in rotary kiln throughput by maintaining a
desirable temperature profile throughout the kiln.
This reduces large temperature gradients through the
kiln reducing the need for a high temperature in one
part of the kiln in order to provide heat to another
10 part of the kiln. In addition the need for
auxiliary fuel combustion to provide heat to a
dr~ing zone within the kiln is reduced. Thus
throughput limitations caused by localized hot
temperatures or flue gas flowrates are relaxed.
The invention will be described in detail
with reference to the Drawin~s.
Referring now to Figure 1, there is
illustrated rotary kiln 1 having a rotatable
cylindrical body 2, and nonrotatable walls 3 and 4
20 at each axial end of the rotatable cylindrical body
to define a combustion zone 5. Preferably the kiln
has a length to diameter ratio exceedin~ 4 but less
than 8.
Flue 6 is positioned at one axial end o
25 rotatable cylindrical body 2. Although shown in
Figure 1 as having a horizontal orientation, the
flue may have a vertical or any other suitable
orientation. A first oxidant injection means such
as first burner 7 is positioned within norlrotatable
30 wall 4 opposite the end hav;rltl flue 6. First burneL
7 is oriented to inject fuel alld oxidant into
combustion zone 5 within rotatable cylindrical body
2 in a direction toward the flue end. Second
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oxidant injection means such as second burner 8 is
positioned within nor~rotatable wall 3 at the flue
end and is oriented to inject fuel and oxidant into
combustion zone 5 in a direction toward the end
5 opposite the flue end. Alternatively either or both
of the first and second oxidant injection means may
be a lance, such as lance 12. In such a case only
oxidant is provided into the combustion zone from a
~lance.
The second oxidant injection means which
injects oxidant into the kiln in the direction away
from the flue end is adapted to inject the oxidant
with a momentum sufficient to pass through the kiln
a length equal to at least two times the internal
15 diameter of, and preferably at least 50 percent of
the length of, the rotatable cylindrical body. One
means of accomplishing this high momentum is by the
injection of the oxidant through a restricted
orifice having a diameter, or multiple orifices
20 having an equivalent diameter, not exceeding 1~30 of
the kiln internal diameter and preferably not
exceeding 1/100 of the kiln internal diameter. The
restricted orifice imparts a high velocity to the
oxidant as defined by Bernoulli's equation, and the
25 high velocity causes the momentum to increase since
momentum is the product of mass and velocity.
Another means oE accomp]ishing hi~h momentum is by
increasin~ the mass o the second oxidant. However,
this is not preferable because this simultalleously
30 increases the mass and the momentum of the gas
flowing toward the flue end.
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Figures 4, 5 and 6 illustrate such second
oxidant injection means. Referring to Figure 4
there is illustrated a single orifice nozzle having
a restricted diameter for the injection of oxidant.
5 Figure 5 illustrates a multiple orifice nozzle
having an equivalent diameter of the defined
restriction to enable the attainment of the required
high momentum. Figure 6 illustrates a burner
,wherein oxidant and fuel may be injected through
10 concentric tubes to produce oxidizing gas. Oxidant
may be fed through the center tube and fuel may be
fed through the outer annular passage or vice
versa. The center tube may be fitted with a single
or a multiple orifice nozzle.
In anther embodient illustrated in Fiyure
7, one can cause oxidant and some fuel to react and
expand within a cavity recessed within the kiln
wall. The cavity provides a restriction so that the
hot combustion products at near the adiabatic flame
20 temperature of the mixture leave the cavity at a
high velocity. In this case the cavity would have a
diameter at the point of communication with the kiln
of less than 1/10 of the kiln internal diameter.
In operation, feed, such as waste 9,
25 comprising volatile material is provided into
combustion zone 5, such as throu~h ram eeder 10, to
form a bed which 10ws through the combustion zone.
Other feeds which be used with this invention
include cemen-t, coke, ceramic and ether materials
30 which include a volatile compollellt such as water.
The method of this invention will be described in
detail with waste as the feed which may include
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volatile combustible and volatile incombustible
matter. Waste may be liquid and/or solid waste such
as is defined in the Resource Conservation Recovery
Act ~RCRA) or the Toxic Substances Control Act
5 (TSCA). The waste passes sequentially through a
drying zone 13 wherein it is dried of volatile
incombustible matter such as water and some of the
lighter volatile combustible matter, a pyrolysis
zone 14 wherein additional combustible matter is
10 volatized out, and a char burnout zone 15 wherein
the residual solids are combusted. Resulting ash is
removed from combustion zone 5 through ash removal
door 11. As is appreciated by one skilled in the
art, there is not a clear demarcation between these
15 zones. In Figure 1 the arrows indicate the
volatization of incombustible and combustible matter
ln zones 13 and 14 respectively.
Fuel and oxidant are injected through
burner 7 into combustion zone 5 wherein they are
20 combusted to provide heat to the combustion zone to
carry out the drying, pyrolyzing and burning of the
waste discussed above. The oxidant may be air,
technically pure oxygen havincl an o~ycJen
concentration greater than 99.5 percent, or
25 oxygen-enriched air having an oxygen concentration
of at least 25 percent and preferably greater than
30 percent. The uel may be any suitable f luid fuel
such as natural gas, propane, fuel oil, or liquid
waste.
The combustion of the fuel and oxidant
injected lnto combustion zone S throuc~ll first burner
7, and the combustion of the volatile combustibles
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evaporated Erorn the waste, create a flow of gas
toward the flue end. Gas is removed from combustion
zone 5 through flue 6.
Fuel and oxidant are injected into
5 combustion zone 5 through second burner 8 and can be
defined the same as the fuel and oxidant injected
through first burner 7. The fuel and oxidant
injected through burner 8 is injected having a
momentum at least equal to, and preferably greater
10 than 200 percent of, the momentum of the gas flowing
toward the flue end. The gas flowing toward the
flue end may include fuel and oxidant injected
through the first burner and the combustion products
thereof, water vapor, combustion products from the
15 material injected through the second burner, and
combustion products from the combustion of volatized
combustible material. As is known, momentum is
equal to the mass times the velocity of the fluid.
In this way the combustion reaction stream injected
20 through burner 8 penetrates a significant distance
into combustion zone 5, preferably at least two kiln
diameters. Heat released from the combustion of the
fuel and oxidant injected into combustion zone 5
through burner 8 serves to provide heat for the
25 aforedescribed drying, pyrolyzing and hurning of the
waste.
The arrangement of the invention wherein
burners fire opposed to one allother causes the
temperature within the cornbustiorl Z.Olle to be mucl
30 more uniform than with converltional rotary kiln
arrangements because the two injected combustion
streams tend to cause each other to recirculate
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through the combustion zone as indicated ky the
reversing ~low arrows 16 in Figure l, although only
the recirculation of the gas flowing from the flue
end is necessary for the successful operation of the
5 invention. Furthermore, the high momentum of the
flue end combustion stream causes enhanced recircula-
tion as shown by arrows 17. In this way temperature
gradients within the kiln are better controlled so
~throughput li~itations based on heat transfer rate
lO considerations or flue gas flowrate considerations
are relaxed. In addition, the high momentum flame
may be manipulated to enhance local radiative and
convective heat transfer to the load when desired.
In a preferred operating method either or
15 both of the oxidant streams injected through oxidant
injection means 7 and 3 are injected at a hi~h
velocity so as to provide a recirculation of gases
within the combustion zone, preferably to pro~ide a
recirculation ratio exceeding 4. Preferably the
20 oxidant stream velocity exceeds 150 feet per second.
In this way the ternperature uniformity within
combustion zone 5 is enhanced. This is particularly
the case for the oxidant strearn injected throu~h
first burner 7 so that, as illustrated in Fi~ure l,
25 the ~ases do not merely pass throu~h combustion zone
5, b~lt rather recirculate one or more times within
combustion zone 5 so as to enhance mixing and
combustion efficiency within combustion zone 5 and
thus further enhance temperatu}e uniformity withirl
30 ~ach oE the two recirculatlon zolles at the two parts
of the combustion zone.
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In an alternative arrangement, the
injection end of the second oxidant injection means
located at the flue end protrudes a distance into
the combustion zone as illustrated in Figure 3
5 rather than having its injection end ~lush with the
wall within which it is positioned as is illustrated
in Figures 1 and 2. The numerals in Figure 3
correspond to those of Figure 1 for the common
elements. In this way a plug flow zone is establish
10 immediately before the flue. In a plug flow zone -
there is very little backmixing or recirculation of
gases. In the more quiescent plug flow region, the
gas velocity is reduced due to the lack of
recirculation flow. Therefore, air borne
15 particulates have the opportunity to settle down
from the gas stream. ~lso the gas is allowed to
cool down somewhat, resulting in reduced gas
velocity. The protrusion can be as long as
practical and typically is about one kiln diameter.
In a countercurrent kiln it may be desirable
to inject additional oxidant, such as technically
pure oxygen, into the cornhustion zone at the flue
end in order to carry out further combustion in the
drying zone. This is particularly the case where a
25 large amount of combustibles are volatized rom the
eed and are carried into the drying zorte by the
flowing yases resulting in pyrolytic or fuel-rich
conditions in the drying zone. The additional
oxidant may be injected through burner 8 or througl
30 lance 12 depending on which is used as the second
oxidant injection rneans.
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The invention enables the kiln operator to
operate the combustion zone of the rotary kiln with
two separate combustion control zones at each end of
the kiln. In addition to stoichiometric operation,
5 the combustion control zone at each end of the kiln
may be operated with pyrolytic (fuel-rich) or
oxidating (oxygen-rich) conditions thus adding
flexibility to the kiln design and to the combustion
~process control. Fo~ example, especially with the
10 processing of high-BTU waste, the combustion control
20ne at the flue end of a countercurrent rotary kiln
can be run in the pyrolytic mode so that combustible
gases released from the waste are recirculated and
entrained into the high momentum stream from the
15 flue end burner thus consuming the oxidant. Residue
char in the combustion control zone at the other end
of the kiln can be exposed to oxidating conditions
to complete the burnout.
In the method of this invention the use of
20 oxygen enrichment serves to decrease the momentum of
the gases flowing toward the flue thus enabling
easier flue end injection into the kiln, and also
serves to decrease the volumetric flowrate of gases
flowing through the flue thus increasing
25 throughput. Accordingly the lower the percentage of
inert nitro~en introduced into the combustion zone
with the o$idant, the more advantageous will be the
operation of the method of this invention, Thus, to
achieve ma~imutn throughput, the most preferred
30 oxidant is technically pure oxygen, air inleakage
notwithstanding.
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Figure 2 illustrates the rotary kiln and
operating method of this invention carried out with
the incineration of waste in a concurrent kiln. The
numerals in ~igure 2 correspond to those of Figure 1
5 for the common elements. In the embodiment
illustrated in Figure 2, flue 20 is located at the
end opposite the end at which waste is provided into
the kiln. First oxidant injection means such as a
lance or burner 21 is positioned within nonrotatable
10 wall 3 at the end opposite the flue end and second
oxidant injection means such as a lance or burner 22
is positioned within nonrotatable wall ~ at the flue
end. Oxidant injection means 21 and 22 inject
oxidant toward the wall opposite from where they are
15 positioned. The operation of the rotary kiln
illustrated in Fiyure 2 is similar to that of the
kiln illustrated in Figure 1 except that the flow of
gases toward the flue end is concurrent with, not
countercurrent to, the flow of waste sequentially
20 through the drying, pyrolyzing and char burning
zones.
In a conventional rotary kiln used to
incinerate hazardous waste, hazardous fumes released
from the waste may not always pass throu~h the flarne
25 region of the combustion zone. For a conventional
countercurrent rotary kiln the ~umes may not even be
exposed to a high temperature wi.thin the kiln.
Accordingly conventional incineration systems employ-
ing rotary kilns depend in great measure on a
30 secondary combustion chamber for the destruction of
hazardous constituents. ~lowever with the system of
this invention wherein opposed fired burners cause
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extensive gas recirculation within the combustion
zone, fumes volatized from the waste pass several
times through the flame region thus increasing the
destructio~ efficiency of the hazardous constituents.
5 This may, in some cases, eliminate the need ~or a
secondary combustion chamber in the incineration of
hazardous waste.
The invention enables the operation of a
rotary kiln with improved control by enabling
lO independent or separate adjustment of the oxidant and
fluid fuel injected at the flue end and at the end
opposite the flue end. This is particularly advan-
tageous when these two o~iclants have differing oxygen
concentrations, e.g. air and technically pure oxygen.
For example, one may determine the
volumetric flowrate of the gas being removed through
the flue. As used herein the term "determine" means
any way of arriving at a value including measuring,
calculating or estimating the value. The flowrate
20 may then be compared with a predetermined desired
flowrate and the flowrate ratio of the oxidants may
then be adjusted, i.e. changed, so that the
determined flowrate changes in the direction toward
the desired flowrate. Because of the hi~h momentum
25 of the oxidant injected at the 1ue end which passes
significant gas 10w away from the Elue into the
kiln, as opposed to prior art processes, chan~es in
1ue gas 10wrate can be accornplished witll changes
in the 10wrate ratio of the in~ected oxic1allts while
30 being able to maintain a desirable temperature
profile and furnace atmosphere.
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In another e~arnple, one may determine the
pressure within the rotatable cylindrical body.
Typically when waste is being incinerated the
pressure within the kiln is desired to be a negative
5 pressure. The determined pressure may then be
compared with a predetermined desired pressure and
the flowrate ratio of the oxidants may then be
adjusted so that the determined pressure changes in
the direction of the desired pressure while
10 maintaining a desirable temperature profile and
furnace atmosphere.
In another method for improving the control
of the operation of the rotary kiln, one may
determine the heat demancl at both the flue end zone
15 and at the end zone opposite the flue end and adjust
the flow of one or both of the oxidants and fluid
fuel, if necessary, to accommodate the heat demancds
sirnultaneously.
As can be seen any operating parameter may
20 he determined, compared with a predetermined desired
value for that parameter, and the total flowrate and
the flowrate ratio of the oxidants may be adjusted
so that the determined value of the parameter
changes in the direction toward the desired value
25 for that parameter. As indicated earlier this
advantageous control based on chanc~in~ the total
flowrate and the ratio of the oxidants is d~ to the
high mornenturn o the flu0 encl injected oxidant WhiC
doesn't merely affect the proximity of the flue en~
30 as in conventional processes, but rather has a
marked effect on the ~as flow pattern within the
kiln. A signiicant advantage o the invention is
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the ability to independently control temperature or
heat release and atmosphere at each end of the kiln
while simultaneously controlling gas flowrate or
pressure in the kiln.
Temperature within the kiln may also be
controlled or moderated by the injection of water,
especially as an atomized stream, into the kiln.
The following examples are provided for
illustrative purposes and are not intended to
10 limiting.
EXAMPLE 1
A scaled-down cold flow model of a rotary
kiln similar to that illustrated in Figure 3 was
15 employed. The kiln model had a length of 3.S feet
and an L./D ratio of 7. A nozzle injected gas toward
the flue end at a volumetric flowrate of 7380 cubic
feet per hour (CFH) and a burner fired away rom the
flue end with a high velocity jet injected at a
20 volumetric flowrate of up to 670 CFH wherein the
initial velocity of the jet was about 1000 feet per
second. The momentum of the flow from the burner
ranged between 100 to 500 percent of the momentum of
the gases flowing toward the flue. The flow from
25 the flue end jet penetrated up to 63.3 percent of
the length of the kiln. Recirculation gas flow
within the ki.ln flue end was vigorous.
E~ LE, 2
A countercurrent rotary kiln similar to that
illustrated in Figure 3 is employed having a length
of 95 eet and an interncll diameter of 6.5 feet.
Oxygen at a flowrate of 4092 lb~hr and natural gas at
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a flowrate of 1066 lb/hr, having a heat value of
22,991 BTU/lb, are injected at a high momentum into
the kiln at the flue end through a burner extending 5
feet into the kiln. Air at a flowrate of 11,090
5 lb/hr and natural gas at a flowrate of 613 lb/hr are
injected into the kiln through a burner at the end
opposite the flue end. The kiln is operated at
negative pressure and ambient air leaks into the kiln
~at a flowrate of 5500 lb/hr. Soil comprising
10 hazardous waste and having a water content of 15
percent but no heating value is passed into the kiln
at the flue end at the rate of 25 tons per hour. Ash
is removed from the kiln at a temperature of 900DF at
a flowrate of 42,49~ lb/hr and gas is passed out of
15 the kiln through the flue at the rate of 29,777 lb/hr
(30,630 actual cubic feed per minute) at a
temperature of 1600F and having an oxygen
concentration of 3.1 percent.
With the air fired burner firing alone, the
20 maximum soil processing rate is only 16 tons per hour
while meeting the required ash temperature of 900F.
Moreover with oxygen enrichment at the discharge end
and without the oxygen burner firing toward the
discharge end, the flame is shortened and the
25 combustion gas temperature gradient is significantly
increased so that, at an increased throughput, the
soil does not under~o suEficient residence t:inle at
the elevated temperat.ule to undergo a detoxification
reaction.
Although the inventioll has heen described in
detail with reference to certain embodiments those
skilled in the art will recogllize that there are
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other embodiments of the invention within the spirit
and scope of the claims.
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