Note: Descriptions are shown in the official language in which they were submitted.
TITLE OF THE INVENTION
METHOD FOR CONTROLLING THE PULVERIZATION AND
DRYNESS OF FLAMMABLE MATERIALS PASSING THROUGH
A PULVERIZER, AND METH~D OF CONTROLLING THE
PULVERIZING RATE OF THE PULVERIZER
1 BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a method of
controlling the pulverization and drying of flammable
matèrials, such as coal, in a system including a
pulverizer. The invention also relates to a method of
controlling the pulverizing rate of the pulverizer over
a wide range.
Generally, in the case of pulverizing a flammable
material- such as coal, there is adopted a method
wherein a high temperature dry gas for removing
` moisture contained in the material is fed into a
- pulverizer to preheat and dry the pulverized coal and
~ at the same time the pulverized coal is conveyed
pneumatically to a predetermined place by the dry
- gas. In the case of practicing such a method, there is
a danger o~ the pulverized coal exploding in an -~
atmosphere wherein the oxygen concentration is in the
range of 10 to 13% or higher. Therefore, in order to
~0 avoid such a danger, it is necessary that the oxygen
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1 concentration of the dry gas be held below the
concentration at which the coal dust may explode.
Therefore, an inert gas or an exhaust combustion
gas is usually used as the dry gas, and in many cases,
it has been mixed with a temperature regulating air or
exhaust gas having an oxygen content in a range below
the above-mentioned oxygen concentration.
As possible methods of operation in such a
pulverizing and drylng system, there are the positive
pressure method for maintaining the interior of the
system at a positive pressure and the negative pressure
method for maintaining it at a negative pressure. In
the former, the operation and control are performed by
~P
~ a push-blower mounted at the inlet~system for pushing
dry gas into the system. In the lat~er, a pull-blower
is mounted at the outlet of the system for sucking out
dry gas. However, these methods involve the following
problems, and have been unsatisfactory. That is, -the
positive pressure method creates a positive pressure in
the system such that the dry gas and the pulverized
coal may leak out of the system, resulting not only in
a lack of oxygen in the surrounding environment due to
the dry gas, but also a danger of a secondary explosion
due to the leakage of the pulverized coal. The
negative pressure method is also disadvantageous in
that air is likely to leak inside of the system through
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1 the rotating and piercing portions of the pulverizer, or
the coal supply equipment, thus resulting in increased
oxygen concentration within the system, which may cause an
explosion of coal dust within the pulverizer.
The pulverized and dried coal is typically supplied
to a boiler having a feedback control based upon the
demand of the boiler for the pulverize~ coal as fuel. The
amount of pulverized coal required is not constant over
time and, as the case may be, the minimum demand is as
low as 1/3 or less than the maximum de~and. Therefore, in
operating the pulverized coal preparation system, it is
necessary to construct a system so that the coal feed
rate is automatically controlled by the appropriate
feedback from the boiler, or other fin~l use.
In the conventional system,the pressure drop or
differential pressure across the pulverizer was measured,
and the rate of coal feed into the pulverizer was modified
in accordance with this pressure drop~ Thus, the control
of the feed into the pulverizer in the conventional
system was performed so that the amoun~ of coal remaining
in the pulverizer was always constant, since the pressure
drop across the pulverizer was proportional to the amount
of coal within the pulverizer. During normal operation~
a differential pressure through a loop connected between
the inlet end and the outlet end of the pulverizer was
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1 measurea and its deviation from a preset differential
pressure value was determined. The coal feed rate was
controllea by this deviation. Where it was desired to
positively change the coal pulverizing rate, it was
necessary to change the preset value of the differential
pressure to match a preset value corresponding to the
desirea coal pulverizing rate. The resulting differential
pressure deviation then acted upon the controller for
the coal feed and varied the coal feed rate.
However, in such a conventional system, the
control of the coal pulverizing rate was made, not by
regulating the pulverizing motor speed of the pulverizer,
itself, but by simply ad~usting the coal feed rate in
accordance with the increase or decrease of the am~unt
of raw materials staying in the pulverizer. Therefore,
the controllable range for the coal pulverizing rate
was narrow and the ratio of the maximum pulverizin~
coal rate to the minimum coal pulverizing rate was only
about 1.5 to 2, at most.
SUMMARY OF THE INVENTION
It is an ob~ect of the present invention to
provide a method for controlling the pulverization and
dryness of a flammable material such as coal, while
overcoming the above shortcomings of the prior art.
--5
1 It is a further object of the invention to provide
a control ~ethod for a pulverizer which is capable of
expanding the control range for the coal pulverizing
rate, to the greatest extent possible.
According to one aspect of the invention, a main
dry gas and a temperature regulating gas are mixed in
advance and the resulting mixed gas is fed under
pressure as a dry gas into a flammable material
pulverizer by means of a push-blower to preheat and dry
the flammable material within the pulverizer. Then,
the dry gas, together with the pulverized flammable
material, is discharged from the pulverizer and -the
pulverized flammable material is conveyed pneumatically
into a dust collector by means of a pull-blower. The
pulverized material is then separated from the dry gas
by means of a dust collector. According to the
invention, there is performed a pressure control step
for the dry gas, comprising measuring the average
pressure of the dry gas in the pulverizer and at an
inlet or outlet portion of the pulverizer and adjusting
A the suction~pr~ssure of the pull-blower to maintain the
average pressure of the dry gas in the pulverizer at
substantially atmospheric pressure. There is also
simultaneously performed a flow rate control for the
dry gas, comprising measuring the flow rate of the dry
gas at the inlet portion of the pulverizer, and
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1 adjusting the forced flow rate of the main dry gas by
means of the push-blower to maintain the flow rate of
the dry gas at a constant value. There is also
simultaneously performed a temperature control for the
dry gas, comprising measuring the temperature of the
dry gas at the outlet portion of the pulverizer and
adjusting the mixing ratio of the temper~ture reyulating
gas to the main dry gas so as to keep the temperature
of the-mixed dry gas at the outlet poLtion of the pulverizer
constant.
. According to another aspect with the present
invention, the feed rate of the flammable material,
such as coal, into the pulverizer is positively
. . increased or decreased without changing a preset
differential pressure measured across the pulverizer.
Since a change in the coal feed rate ~ill change the
pressure drop across the pulverizer, this differential
pressure across the pulverizer is detected by a
pressure difference controller (PdC~, and the speed of
the pulverizing motor is changed so a, to return this
differential pressure to the preset level. Alternatively~
the motor speed of the motor turning -the pulverizer
is changed to vary the differential pressure across
the pulverizer, and then the feed rate.is changed so as
to return the differential pressure.to the preset level.
Thus, not only is it possible to maintain an optimum
differential pressure (pressure
.....
1 drop) across the pulverizer without changing the preset
value of the differential pressure, but since the
control is made by positively changing either the feed
rate or the pulverizing capacity (rotating speed of the
pulverizer motor), the control range for the
pulverizing rate is greatly expanded.
BRIEF DESCRIPTION OF THE DRAWI~IGS
Various other objects, features and attendant
advantages of the present invention will be more fully
appreciated as the same becomes better understood from
the following detailed description when considered in
` connection with the accompanying drawings in which like
reerence characters designate llke or corresponding
parts through~the several views and wherein:
FIGURE l is a schematic illustration of one
: embodiment of a system for controlling the pulverizing,
drying and pneumatic conveying of coal according to the
present invention;
FIGURE 2 is a schematic view of a pulverizer
together with the conventional means for controlling
the pulverizing rate thereof;
- FIGURE 3 is a graph comparing the method of the -
present invention with the conventional method;
FIGURE 4 is a schematic view of one embodiment of
the control system for the pulverizing rate according
to the present invention; and
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.,
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1 FIGURE 5 illustrates a second embodiment of the
method of FIGURE 4.
DETAILED DESCRIPTION OF THE PREFERRED EM~ODIMENTS
The reference numeral 1 designates a coal
~ pulverizer such as a-bal~` mill. Coal is charged into
the pulverizer via a belt conveyor 2 and pulverized to
a desired particle size (e.g. particle sizes with 80%
being 200 mesh or below). The pulverizer has a gas
inlet on one side and a gas outlet on the other side.
A preheating, drying and pneumatically conveying gas,
i.e., the mixed gas 3 (hereinafter referred to simply
as "dry gas") held at a high temperature is fed from
inlet line 36 through the gas inlet into the pulverizer
to preheat and dry the pulverized coal, thereby
decreasing the moisture content thereof down to a
target value (about 1%) or less. At the same time the
pulverized coal, while being classiEied and separated,
is discharged and pneumatically conveyed, together with
DLJ~
the dry gas from the gas outlet, ~ line 37 to a
dust collector. The dust collector consists of a
cyclone 6 and a bag filter 7. The greater part of the
- pulverized coal which has been conveyed together with -
the dry gas ~ is collected and separated by the cyclone
6 and then Eed to a coal-bin 8 and stored therein. The
; 25 dry gas ~, after further collection of fine coal
4~
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1 particles in the bag filter 7, is discharged as a clean
gas into the a-tmosphere air. The fine coal particles
collected by the bag filter 7 are recovered into the
coal-bin 8.
In the method of the present invention, the dry - -
gas and the pulverized coal are conveyed by two blowers
which are a push-blower 9 and a pull-blower 10 disposed
at inlet and outlet ends, respectively, of a closed
interior of the system.
lO ~ The push-blower 9 functions to feed the dry gas
into the pulverizer 1, while the pull-blower 10
functions to suck the dry gas fed into the pulverizer,
and the pulverized coal conveyed together with the dry
gas, out of the pulverizer into the dust collector and
to discharge a clean gas, obtained after collection of
the coal dust, into the atmosphere. An important
feature of the method of the present invention resides
in controlling the pressure within the pulverizer 1,
from which the intra-system gas may leak out, and into
which the extra-system gas may enter. The pressure
within the pulverizer is maintained substantially at
atmospheric pressure by combining the "push-pull"
functions of both blowers. ~ore specifically, this
control-pressure is set so that the pressure ~over
ambient atmospheric pressure) at the central part of
the pulverizer is in the range of -lOmmH20 to +lOmm
.
~Z~4~4
--10--
1 which corresponds to 0 to +400mmH20 at the gas inlet
portion and -400 to OmmH2O at the gas outlet portion.
The dry gas 3 is obtained by mixing, in an
appropriate ratio, a main gas 4 with a temperature
regulating gas 5. The main dry gas 4 comprises an
inert gas such as preheated N2 and/or CO2, or an
exhaust flue gas discharged from various combustion
furnaces or firing furnaces, and its oxygen
concentration is held below 10 to 13% in order to
prevent the possible explosion of coal dust. The
temperature regulating gas 5 is for regulating the
temperature of the dry gas according to feed rate and
- moisture content of coal, and as the temperature
regulating gas 5 there is utilized an exhaust flue gas
produced by the combustion of a mixture of a fuel gas
5' and combustion air 5".
The pressure control for the interior of the
; pulverizer 1 according to the method of the present
invention is performed in the following manner. A
20 ~ pressure ~ ~ 11 is provided on the inlet or outlet
side (the inlet side in the embodiment of the Figure)
of a gas supply duct of the pulverizer 1 to measure at
- ~ all times the pressure of the dry gas at this portion, -
and a measured pressure val~e signal is transmitted to
a dry gas pressure control section 12. In the control
section 12, the measured value is compared with a
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4~
1 preset value (e.g. 0 -to -~400mmH20) and when it deviates
from the preset value, the opening of a damper 13
disposed before the pull-blower 10 is changed, whereb~
the inlet pressure of the pulverizer is automaticall~
adjusted so that the pressure within the pulverizer
assumes a value in the preset range. More specifically,
when the measured value on the pressure meter 11 is
higher than the preset value, the opening of the damper
13 is increased to increase the gas volume of the pull-
blower 10, whereas when the measured value is smaller than
the preset value, the opening of the damper 13 is
decreased to decrease the gas volume of the pull-blower
10. Conse~uently, by this pressure control, the average
pressure within the pulverizer 1 is always maintained at
the desired level, that is, substantially at atmospheric
pressure, whereby the leak-in of the extra-system air and
the leak-out of the intra-system gas and pulverized coal
can be prever~ted.
In the rt.ethod of the present invention, moreover, there
is provided a control for the flow rate of gas a~ the
inlet side of the pulverizer 1 in.addition to the above-
mentioned pressure control. More specifically, a flow
meter 14 is also provided in the gas supply duct on the
inlet.side of the pulverizer 1, whereby the flow rate of
the dry gas being continuously fed in~o the pulverizer 1
is measured, and the measured signal is
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1 transmitted to a flow rate control section lS. In the
flow control section 15, the measured value on the flow
meter 14 is compared with a preset value of the gas
flow rate which is set in advance in conformity with
the capacity of the pulverizer 1 and the desired
particle size of the pulverized coal taken out of the
pulverizer 1 together with the dry gas. When the
measured value is ou~side the preset range, the opening
o~ a damper 16 disposed before the push-blower 9 is
changed to adjust the flow rate of the main dry gas 4,
whereby the flow rate of the dry gas 3 fed to the
pulverizer 1 is con-trolled to become constant within
the range of the preset value. In this case, when the
measured value is larger than the preset value, the
opening of the damper 16 is decreased, whereas when it
is smaller than the preset value, the opening of the
damper 16 is increased. In this way, the flow rate of
the dry gas 3 fed to and discharged from the pulverizer
1 is maintained constant. As a result, the classifying
performance in the pulverizer is stabilized, thus
permitting pulverized coal of the desired particle size
to be obtained with a high accuracy.
- In the method of the present invention, moreover, --
in addition to the described pressure control and flow
rate control, there is simultaneously performed a
temperature control for the dry gas. More
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1 specifically, a thermometer 17 is provided in a
dlscharge duct on the outlet side of the pulverizer 1
to measure the temperature of the dry gas after
fulfilling its preheating and drying function within
the pulverizer, and the measured temeprature value is
transmitted to a temperature control section 18. In
the temperature control section 18, the measured value
on the thermometer is compared with a preset value of
the gas temperature (e.g. 70 to 90C) which is set in
advance in conformity with a desired moisture content
of the pulverized coal to be stored as product in the
coal bin ~. When the measured value deviates from the
preset value, the mixing ratio of the temperature
regulating gas 5 to the main dry gas 4 is changed to
adjust the temperature of the dry gas 3, and on the
basis of this adjustment the gas temperature at the
outlet side of the pulverizer 1 is controlled to be
held in the preset range. ~ore particularly, when the
measured value on the thermometer 17 is larger than the
preset value, the opening of flow control valves 19 and
20 for the fuel 5' and combus-tion air 5" is decreased
; to decrease the mixing ratio of the temperature
regulating gas 5 to thereby lower the temperature of
the dry gas. When such measured value is smaller than
the preset value, the opening of the flow control
valves 19 and 20 is in~reased, whereby the mixing ratio
, ~ ,
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1 of the temperature regulating gas 5 is increased to
raise the temperature of the dry gas. By such a
temperature control for the dry gas, even when the
moisture content oE coal fed to the pulverizer changes,
it is possible to lower the moisture content of the
pulverized coal as product to a desired value stably
and efficiently.
- In performing the pressure control for the
pulverizer, the pressure loss of the bag filter 7
varies periodically from the minimum pressure loss j~st
after sweeping-away the pulverized coal adhered and
accumulated onto the fil-ter (by an non-illustrated back
flow system) up to the maximum pressure loss just
before the sweeping-away of the pulverized coal, and
this gives rise to a disturbance in the pressure
control systern. In order to remove such a distubrance
and to effect a more efficient pressure control, it is
preferable that there be performed the following
differential pressure compensating control steps for ~
the bag filter.
Differential pressures on either side of the bag
filter 7 are measured by pressure ~ 21 and 22
which are disposed on the inlet and outlet sides,
respectively, of the filter 7, and the measured
pressure signals are sent to a differential pressure
compensating control section 23. In the control
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l section 23, a differential pressure calculated from
both measured values is compared with a preset value
(e.g. 100 to 150mm~2O), and when it is outside the
range of the preset value, the opening of a damper 2
disposed before the detection port of the pressure
e 22 is adjusted so that the differential pressure
based on the measurement takes a value within the range
of the preset value. When the value of differential
pressure based on the measurement is larger than the
preset value, the opening of the damper 24 ~ ~
increased, whereas when it is smaller than the preset
value, the opening of the same damper is decreased. By
such a differential pressure compensating control, the
foregoing disturbance to the pressure control system
caused by var1ations in the pressure loss of the bag
filter is removed.
Although in the described embodiment, the pull-
blower is disposed downstream of the bag filter 7, the
present invention is not limited thereto. There may be
adopted an arrangement such that the pull-blower is
disposed between the cyclone 6 and the bag filter 7.
FIGURE 2 schematically shows the conventional
pulverizing rate controller for the pulverizer l. It
is conventional to provide a loop 38 between the inlet
end and the outlet end of the pulverizer for measuring
the~pr~ssure ~ r~ thereacross. It is
- 16 -
1 conventional to provide a pressure difference controlle~
~P~C) in loop 38. The PdC is also fed with a prese~
differential pressure from a controller 39. The raw coal
is fed into the pulverizer from a feeding device such
as the conveyor 2. ~he motor 35 of the conveyor 2 i5
controlled by a speed cont:rol SC, whose operation is,
in turn, controlled by a signal from the PdC. The
pulverizer itself is rotat:ed by a motor M. It is possible
for the inlet end of the PdC to serve as the pressure
meter 11 of FIGURE 1.
It is conventional to control the above system so
that the amount of raw mat:erial remaining in the pulverizer
1 is always constant. Since the differential pressure is
proportional to the amount: of material in ~the pulverizer,
during normal operation, t:he pressure drop, or differential
pressure, across the pulverizer 1 is measured and its
deviation from the preset differential pressure from
controller 39 is measured. An~ such dev~ation causes a
signal to be sent from the PdC to the SC which controls
the conveyor motor 35. For examp~e, if the PdC measures
a differential pressure which is too high, as compared to
the preset differentia~ pressure, indicating too much
coal in the pulverizer, the PdC sends a signal to the
SC requiring a reduction in the speed of the motor 35.
This assures a stable operation of the pulverizer.
X '.
4~
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1 If it is desired to positively change the coal
pulverizing rate, the preset value from controller 39
is altered to a l~r~E~ differentia ~corresponding to
the desired coal pulverizing rate. This results in an
initial deviation signal from -the PdC, which, in turn,
modifies the rate of motor 35, through the SC. In such
a conventional system, however, the control of the coal
pulverizing rate is narrow so that the ratio of the
maximum coal pulverizing rate to the minimum coal
pulverizing rate is only 1~5 to 2 at most.
FIGURE 3 graphically compares the principle of the
present lnvention with the conventional method shown in
FIGURE ~, wherein the pulverizing rate and the
pulverizing motor speed are plotted along the axis o
ordinate and the axis of absicissa, respectively, and
each of the straight lines shows the relationship
between the motor speed and the pulverizing rate at a
~ preset differential pressure. As shown therein, the
- motor speed and the pulverizing rate are in a
proportional relation, and if the preset differential
pressures are regarded as parameters, then it is seen
that in the case of larger preset differential pressure
(~Pl~P2>~P3 ), the pulverizing rate is larger even
with the same motor speed, and the pulverized coal is
conveyed away successfully.
1 In a normal operation of the conventional control
system, for example, at a preset differential pressure
of ~P2, a pulverizing motor speed of S and a
pulverizing rate of Q (the state of point I), if it is
desired to raise the pulverizing rate to Q', a preset
differential pressure ~Pl at which the pulverizing rate
is Q' at the speed S, is calculated and the preset
differential pressure through the nozzle is changed to
~Pl (the state of point II). Since at this initial
time the actual differential pressure remains close to
AP2, there occurs the difference of ~Pl - ~P2 with
respect to the preset differential pressure ~Pl. In
the conventional method, therefore, -the feed rate is
increased until the actual pulverizing rate reaches Q'
~ (until ~P1 = ~P~). That is, control is ~lade by moving
the differential pressure from point I-to point II and
constant speed S. Consequently, an attempt to further
increase the pulverizing rate requires a further
increase of the preset differential pressure ~Pl, thus
leading to deterioration of the control accuracy during
the subsequent interim period as the pulverizing rate
- approaches the desired rate.
-~ - On the other hand, in the present invention, the --
preset differential pressure itself is fixed to a value
which is desirable from the aspect of control accuracy,
e.g. ~P2 (the state of point I). In addition, there is
adopted one of the following:
1 (1) A method wherein the feed rate is increased
to match the desired pulverizing rate without changing
the pulverizing motor speed (the state of point III).
Subsequently, the difference between the measured
differential pressure at that time and the preset
differential pressure ~P2 is detected and the motor
speed is increased until this difference becomes zero
(the state of point IV), or
( 2) A method wherein the pulverizing motor speed
is increased withou~ changing the feed rate (the state
of point V). The difference between the measured
: differential pressure at that time and the preset
differential pressure QP2 is then detected and the feed
rate is increased until this difference becomes zero
(the state of point IV).
In case it is desired to decrease the pulverizing
rate, control may be made in the direction in which the
pulverizing motor speed is decreased, opposite the
above-mentioned control.
Thus, the conventional control system relies
- mainly on adjustment of the feed rate and does not
combine it with adjustment of the pulverizing motor
speed, while in the present invention, the pulverizing
capability itself is controlled by adjustment of the
pulverizing motor speed, wilereby the pulverizing rate
can be controlled oYer a wide range.
-20-
l FIGURES 4 and 5 are schematic illustrations of the
apparatus for practicing the control system of the
present invention. FIGURE 4 shows the first system in
which the feed rate is changed and the pulverizing
motor speed is controlled accordingly. FIGURE 5 shows
the second system in which the pulverizing motor speed
is changed and the feed rate is controlled
accordingly. More specifically, in FIGURE 4 the
control system is constructed in such a manner that the
pressure difference`controller PdC and the speed
controller SC or the motor 35 disposed on the driving
side of the raw material feeding device 2 are made
independent. A signal from the pressure difference
controller PdC is instead fed to the speed controller
SC' of the pulverizing motor M'. In the embodiment o
FIGURE 5, the control system is constructed such that a
rotational speed matching a desired pulverizing rate is
set for the speed controller SC' for the pulverizing
motor M', and a signal from the pressure difference
- 20 controller PdC is fed to the speed controller SC for
the motor 35 on the driving side of the raw material
feeding device 2.
According to this aspect o the present invention, -
as set forth hereinabove, the pressure control for the
dry gas is performed in pulverizing, drying and
pneumatically conveying flammable material such as
-21-
l coal, whereby the leakage of air into the system and
the leakage of the dry gas and pulverized material to
the exterior of the system can be prevented. This
eliminates the danger of explosion of coal dust caused
by an increase of the oxygen concentration within the
system as well as the possibility of a lack of oxygen
and a secondary explosion at the exterior of the
system; that is, a safe operation can be ensured, and
the classifying performance of the pulverizer can be
stabilized by the flow rate control for the dry gas,
and further moisture contained in the pulverized
material can be removed efficiently by the temperature
control for the dry gas. Thus, according to the
present invention there are provided these superior
effects.
The present invention is also constructed as above
- wherein the pulverizing capability of the pulverizer is
adopted directly as one control factor, and
consequently the control range for the pulverizing ra~e
can be expanded to a large extent. Furthermore, the
present invention is applicable not only to the field
of coal pulverizing but also widely to all of the
pulverization systems connnected to pneumatic conveying
systems for powders.
Obviously, numerous modifications and variations
of the present invention are possible in light of -the
3~Z14491~
1 above teachings. It is therefore to be understood that
within the scope of the appended claims, the invention
may be practiced otherwise than as specifically
described herein.
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