U.S. patent number 3,578,299 [Application Number 04/861,425] was granted by the patent office on 1971-05-11 for method and apparatus for cement kiln control.
This patent grant is currently assigned to General Electric Company. Invention is credited to Myron R. Hurlbut.
United States Patent |
3,578,299 |
Hurlbut |
May 11, 1971 |
METHOD AND APPARATUS FOR CEMENT KILN CONTROL
Abstract
A method and apparatus for controlling the operation of a rotary
cement kiln which is fed with an incoming slurry. The fuel rate set
point and the exit gas rate set point are controlled. The control
of fuel rate set point is based upon kiln drive motor torque
measurements in conjunction with feedback signals generated by a
dynamic kiln model which stores a record of past control actions
and a calculated effect of the slurry drying characteristics in the
kiln. Control of the exit gas rate set point is based upon
measurements of gas temperature at the feed end of the kiln. Oxygen
content of the exit gas is monitored and employed to exercise
overriding control to insure that no combustibles which might cause
an explosion appear in the exit gas.
Inventors: |
Hurlbut; Myron R. (Peabody,
MA) |
Assignee: |
General Electric Company
(N/A)
|
Family
ID: |
25335758 |
Appl.
No.: |
04/861,425 |
Filed: |
September 26, 1969 |
Current U.S.
Class: |
432/17; 34/539;
432/45; 432/37; 432/49 |
Current CPC
Class: |
F27B
7/42 (20130101) |
Current International
Class: |
F27B
7/42 (20060101); F27B 7/20 (20060101); F27b
007/20 () |
Field of
Search: |
;263/32 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Camby; John J.
Claims
I claim:
1. In a rotary cement wet kiln control system including chain means
for contacting wet feed entering the kiln and driving moisture
therefrom, drive means for rotating the kiln at a constant speed,
heating means for causing a material transformation in the kiln at
a burning zone, sensing means responsive to total torque developed
by said drive means to rotate the kiln for providing a
corresponding output, the total torque representing, in part,
conditions in the chain means, means for determining feed
temperature as the feed leaves the chain means, means responsive to
said feed temperature determining means for providing an output
variable in accordance with torque changes resulting from
conditions in the chain means and control means including means
adapted to be responsive to said torque sensing means and said
means for indicating chain means torque changes for controlling the
heat input to the kiln to maintain the remainder of the total
torque constant.
2. A rotary cement wet kiln control system as recited in claim 1
wherein said feed temperature determining means comprises means for
measuring conditions at the chain means and means for performing a
heat balance for the chain means to thereby indicate the
temperature of feed leaving the chains.
3. A rotary cement wet kiln control system as recited in claim 2
wherein said means for measuring conditions at the chain means
comprises second and third sensing means for measuring gas
temperature entering the chain means and leaving the chain means,
respectively, fourth sensing means for measuring the rate of
material feed into the kiln, fifth sensing means for measuring the
rate dust is introduced into the kiln and sixth sensing means for
measuring the rate of flow gas through the kiln and wherein said
feed temperature determining means additionally comprises means
responsive to said second through sixth sensing means for
determining the feed temperature.
4. A rotary cement wet kiln as recited in claim 3 wherein said
control system additionally comprises means for storing information
concerning past control actions initiated by said control means for
inhibiting variations of heat input to the kiln by said control
means in response to variations in the torque developed by the
drive means directly caused by such past control actions, said
information storage means also being responsive to said means for
indicating chain means torque changes to directly and immediately
inhibit variations of heat input in response to variations in the
torque developed by the drive means.
5. A rotary cement wet kiln as recited in claim 4 wherein said
means for indicating chain means torque includes means responsive
to successive feed temperatures from said feed temperature
determining means for generating a trended feed temperature, means
responsive to said feed temperature determining means and said
trended feed temperature generating means for generating a feed
temperature error and filter means responsive to said feed
temperature error generating means for generating the chain means
torque indication.
6. A method for controlling a rotary cement wet kiln including
chain means for contacting wet feed entering the kiln and driving
moisture therefrom, drive means for rotating the kiln at a constant
speed and heating means for causing a material transformation in
the kiln at a burning zone including the steps of measuring the
total torque developed by the drive means to rotate the kiln, said
total torque measurement representing, in part, conditions in the
chain means, determining the temperature of feed leaving the chain
means, generating a signal variable in accordance with torque
changes resulting from conditions in the chain means in response to
said determination of feed temperature and controlling the heat
input to the kiln so as to maintain the total torque minus the
chain means torque at a constant value.
7. A method for controlling a rotary cement wet kiln as recited in
claim 6 wherein said determination of feed temperature is made by
measuring conditions at the chain means and performing a heat
balance for the chain means based upon those measured conditions to
thereby obtain the feed temperature.
8. A method for controlling a rotary cement wet kiln as recited in
claim 7 wherein said chain means condition measurement includes
measuring the gas temperature entering the chain means, the
temperature of gas leaving the chain means, the rate of material
fed into the kiln, the rate at which dust is introduced into the
kiln, and in response to said measurements determining the feed
temperature.
9. A method for controlling a rotary cement kiln as recited in
claim 8 additionally comprising the step of storing information
concerning past control actions initiated by said control method
for inhibiting variations of heat input to the kiln by said control
method in response to said measured torque variations developed by
the drive means directly caused by past actions of said control
method, said information storage step additionally including step
of directly and immediately inhibiting variations of input in
response to variations of the torque developed by the drive means
in response to said chain means torque determination.
10. A method for controlling a rotary cement wet kiln as recited in
claim 9 wherein said determination of chain means torque includes
the steps of generating a trended feed temperature based on said
feed temperature determination, generating an error function based
on said feed temperature determination and said trended feed
temperature determination and filtering the error from said error
generation to obtain the chain means torque indication.
Description
BACKGROUND OF THE INVENTION
This invention relates to the production of cement in rotary kilns
and, in particular, to an improved method and apparatus for
controlling and regulating the operation of a group of rotary
cement kilns known as wet kilns to provide stable kiln operation
with resulting uniformity of product quality and improved fuel
efficiency.
Typical rotary kilns employed in the production of portland cement
are steel cylinders 8 to 25 feet in diameter and between 100 and
700 feet long. The cylinders are lined with refractory brick and
inclined 2.degree. to 3.degree. from the feed end to the discharge
end. The steel cylinder is supported at spaced points and rotated
through a gear drive by an electrical motor at speeds in the order
of 20 to 120 revolutions per hour. Cement raw material such as
finely ground limestone, clay or shale intermixed in the desired
proportions and either in the form of a finely ground slurry or a
dry pulverized, intermixed material are fed into the upper or feed
end of the rotary kiln.
In wet kilns, to which this invention is adapted, the raw, input
materials are in a slurry. As the raw materials move slowly down
the kiln at a rate which is a function of the kiln rotational
speed, they engage chains suspended from the kiln through
approximately 25 percent of the kiln length. The chains generally
comprise the drying zone of the kiln and may comprise a portion of
a preheating zone. After the feed passes through the chains, it
progresses through successive zones including the remainder of the
preheating zone, the calcining zone and the clinkering or burning
zone. The chains are suspended from the kiln to contact the slurry
and serve as a heat exchanger to drive off moisture. As the
materials move down the kiln, they are slowly heated by a stream of
hot gases which are produced by a burner positioned at the lower or
discharge end of the kiln and which flow counter to the direction
of material movement in the kiln. A fan at the feed end of the kiln
creates a slightly negative pressure in the kiln and draws the hot
combustion gases produced by the burner through the kiln to heat
the raw materials moving in the opposite direction, causing the raw
materials to undergo successive changes due to the steadily
increasing temperature of the materials.
The temperature of the dried raw materials increases until the
calcining temperature is reached at which time carbon dioxide is
liberated from the raw materials, changing the carbonates to
oxides. The calcining zone occupies the major portion of the kiln
length. The temperature of the material changes little within the
calcining zone since the calcining reaction is endothermic and
requires heat. A measurement of the material temperature within
this zone gives little indication of the degree of calcination. At
a point down the kiln where calcination is complete, a large
temperature difference exists between the solid materials and the
counter-flowing hot gases. Thus, when calcination is complete, the
temperature of the solid material begins to increase rapidly to the
point where the exothermic clinkering reactions are initiated. The
heat generated by these chemical reactions causes the solid
material temperature to rapidly increase 700--800.degree. F. The
clinkering or burning zone is near the discharge end of the kiln
and the material remains at or near the high temperature until it
leaves the kiln and is thereafter cooled. The degree of completion
of the chemical reaction in the clinkering or burning zone depends
upon the feed composition, the temperature in this zone and the
residence time of an increment of feed within the zone.
The kiln must be controlled in such a manner as to produce a
clinker product having a satisfactory quality and preferably a
uniform quality. The variables over which a kiln operator has
immediate control and which directly influence the kiln operation
are the kiln feed rate, i.e. the rate at which the raw materials
are fed into the upper end of the kiln, the kiln rotational speed,
the fuel rate, i.e. the rate at which fuel is injected into the
kiln and burned, and the exit gas rate, i.e. the rate at which the
combustion gases and other gaseous kiln products are drawn through
the kiln and exhausted from the feed end into the atmosphere. The
kiln operator attempts to select values for each of these control
variables which will result in stable kiln operation producing a
desirable product at the required product volume.
In early rotary cement kilns, the operator visually observed the
color of the burning zone, the position of the boundary between the
calcining and burning zones and the clinker size and consistency
and took corrective action based upon these observations, using
judgment gained by past experiences. In general, kiln performance
based on this type of control was poor in terms of product quality,
product uniformity and fuel efficiency. More recently, elaborate
instrumentation has been employed to sense various parameters
during kiln operation. This provided the operator with more
information of higher accuracy for determining proper control
action. However, the results obtained are still a function of the
operator's interpretation of the measurements and his judgment.
Still more recently, several automatic control arrangements for
rotary cement kilns have been proposed. One such system which is
effective to control rotary cement kiln operation is described in
U.S. Pat. Ser. No. 678,851) issued (filed Oct. 30, 1967), now U.S.
Pat. No. 3,469,828, to James W. Lane and assigned to the same
assignee as the present invention. As described in that patent,
kiln drive amps are a measure of the torque required to turn the
kiln and accurately indicate the temperature in the burning zone as
the torque increases as the temperature increases. With wet kiln
operation where a slurry is being introduced, the slurry is
converted to a sticky form which rides up on the walls giving a
large angle of repose in the chain section. As the feed dries, the
weight of the water is lost and the powder that remains flows down
the kiln with a small angle of repose requiring very little torque.
The total torque required then is a function of the amount of wet
feed that must be moved. Therefore, as the heat input to the chains
is increased, the amps due to the chain section will decrease. When
the amp controller of the aforesaid Lane patent was implemented, it
was found that this amp change could be interpreted as being due to
the burning zone requiring additional fuel when, in reality, the
fuel rate should have been decreased.
Therefore, it is an object of this invention to provide an improved
method and apparatus for controlling the operation of a wet rotary
cement kiln.
It is another object of this invention to provide a control method
and apparatus for maintaining control of a rotary wet cement kiln
by eliminating control ambiguities which can occur in this
particular type of cement kiln.
It is a further object of this invention to provide a method and
apparatus for maintaining control of burning zone conditions in a
wet rotary cement kiln in conjunction with a control system which
responds to drive torque.
SUMMARY
Generally, a torque control apparatus including a sensor for
measuring the torque developed by the kiln drive motor to rotate
the kiln and a filter for smoothing and filtering the sensor output
are provided. The filtered signal is checked to insure that it
falls within a predetermined range and is then compared to a
setpoint signal. The error signal representing the difference
between the filtered signal and the setpoint signal is applied to a
summing amplifier which also receives the output of a process
model. The output of the summing amplifier is applied to the
process model and to a torque controller which receives a signal
indicating the base rate of fuel flow to the kiln. In response to a
variation in the output of the summing amplifier, the torque
controller calculates a new fuel rate setpoint which tends to
correct the burning zone conditions and return the drive motor
torque to the setpoint value.
Ambiguities in the control of the process are obviated by measuring
the temperature drop across the chain section and heat rates into
and from the chain section. This indicates the condition of the
process materials and permits compensation of the process model
output for the torque required to turn the chains. This torque
bears no specific relationship to the burning zone temperature
which is the primary parameter being observed and controlled.
The subject matter of the invention is particularly pointed out and
distinctly claimed in the concluding portion of the specification.
The above and further objects and advantages of this invention may
be better understood by reference to the following description
taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FiG. 1 is a schematic diagram depicting a rotary cement kiln
embodying and utilizing the present invention;
FIG. 2 is a block diagram illustrating a control system
incorporating the invention and employed to control the operation
of the rotary cement kiln of FIG. 1;
FIG. 3 is a block diagram illustrating the organization of the
process model in the control system of FIG. 2;
FIGS. 4 and 4a are flow diagrams illustrating the operation of the
control system of FIG. 2;
FIG. 5 is a signal diagram illustrating the operation of the
control system of FIG. 2 in controlling rotary cement kiln
operation.
DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT
Referring to FIG. 1, a typical rotary cement kiln with its
associated equipment is schematically illustrated. Rotary cement
kiln 10 has at its upper or feed end a kiln feed hopper 11 and a
kiln feed pipe 12 for feeding blended raw materials 13 into the
upper end of kiln 10. The raw materials normally include A1.sub.2
O.sub.3, SiO.sub.2, Fe.sub.2 O.sub.3, MgCO.sub.3 and CaCO.sub.3
plus small amounts of K.sub.2 O, Na.sub.2 O and sulfur. The blended
raw materials or feed will be in the form of a slurry. Chains 16
would be suspended along the kiln adjacent the feed end to remove
moisture from the slurry. Kiln 10, inclined downward at an angle of
approximately 3.degree. from feed end 14 to discharge end 15, is
rotated by an electric motor 20 shown here driving a pinion gear 21
that engages a ring gear 22 encircling and attached to kiln 10. As
kiln 10 is rotated by kiln drive motor 20 through gears 21 and 22,
the kiln rotation causes the raw materials of feed to slowly
cascade forward, the rate of forward progress of the feed within
kiln 10 being approximately proportional to kiln rotational speed.
Motor 20 is normally controlled to drive kiln 10 at a predetermined
constant rotational speed.
At the discharge end of the kiln, a fuel supply line 25 and a
primary air supply line 26 are connected to a fuel-air mixing
chamber 27 which injects a high-energy flame 30 into kiln 10.
Natural gas, pulverized coal, oil or combinations thereof may be
employed as fuel, the fuel being fed into line 25 from a suitable
source. The primary air is forced through line 26 and into chamber
27 by fan 28.
The interior of kiln 10 is lined with a refractory material (not
shown) which is capable of absorbing heat from flame 30 and
transmitting it to the gases and feed travelling through kiln 10.
The combustion gases and other gaseous kiln products are drawn
through the kiln by an induced draft fan 31 which exhausts the
gases through a dust collector and stack 32. Induced draft fan 31
creates a slightly negative pressure in the kiln drawing secondary
air from clinker cooler 35 through the kiln 10. The gases emerging
from feed end 14 of kiln 10 pass through a series of dust
collectors 37 which recover the dust and an exit gas damper 38. The
dust may be reintroduced to the kiln through a conduit 33 to a dust
feeder 34. A sensor 36 senses the rate of dust reintroduction.
As the feed proceeds slowly down the kiln, it is heated by hot
gases flowing counter to it and also by the heated refractory walls
of the kiln. The temperature of the dry feed increases until the
calcining temperature is reached. At this point, the calcium
carbonate CaCO.sub.3 and the magnesium carbonate MgCO.sub.3 begin
to decompose, forming CaO and MgO. The released carbon dioxide
CO.sub.2 joins the combustion gas and is drawn from kiln 10 by fan
31. The zone of kiln 10 where this reaction occurs is called the
calcining zone. This reaction continues over a major portion of the
kiln length. The temperature of the feed changes very little within
this zone since the calcining reaction is endothermic and requires
heat. A measurement of feed temperature within this zone will not
give a meaningful indication of the degree of calcination of the
feed.
At the point in kiln 10 where calcination of the feed is complete,
a large temperature difference exists between the feed and the
combustion gases and therefore a rapid increase in feed temperature
results. The temperature at which the exothermic clinkering
reaction occurs is reached quickly and the heat generated by the
clinkering reaction causes the temperature of the feed to increase
still further to the point where the solids become partially
liquefied. The clinkering reaction for the formation of (CaO).sub.2
.sup.. (SiO.sup.2), (CaO).sub.3 .sup.. (A1.sub. 2 O.sub.3) and
(CaO).sub.4 .sup.. (A1.sub.2 O.sub.3).sup.. (Fe.sub.2 O.sub.3),
which are the crystalline compounds that determine the physical
properties of the cement, occurs rapidly. The resulting partly
fused mass of varying size continues to move down the burning zone
of the kiln and remains near its maximum temperature until it nears
discharge end 15 of the kiln. While at this temperature, most of
the remaining CaO combines with the (CaO).sub.2 .sup.. (SiO.sub.2)
to form (CaO).sub.3 .sup.. (SiO.sub.2). The degree of completion of
this clinkering reaction depends upon the feed composition, the
temperature in the burning zone and the residence time of an
increment of feed within the zone.
As the hot clinker material approaches the end of the kiln, it
begins to lose some of its heat to the incoming secondary air. At
the exit end of the kiln, the clinker drops onto the travelling
grate 40 usually reciprocated by a motor 40a. Air is blown through
grate 40 by fan 41 to cool the clinker. Part of the resulting
heated air becomes secondary air which is drawn through kiln 10 by
fan 31, the remainder being exhausted by fan 42 through dust
cyclone 43 to the atmosphere. The cooled clinker is transported by
conveyor 45 to grinding apparatus (not shown) which pulverizes the
clinker to form cement.
A number of sensor are provided to monitor various parameters of
kiln operation and to generate electrical signals representing the
values of these parameters. These signals are employed by the
control system of the invention to direct the operation of the
kiln. As illustrated in FIG. 1, hopper 11 has a feed rate detector
50 associated therewith which provides a signal to control system
51 over line 52 indicating the rate at which feed is being supplied
to the kiln. A temperature measuring device 53, for example a
thermocouple, is provided at the lower end of the chain section 16,
of the kiln 10 to provide a signal transmitted to control system 51
on line 54 indicating the temperature of the gases flowing through
the kiln at that point. Analyzer 55 is provided near the feed end
14 of the kiln to measure the oxygen content of the gases being
exhausted from the kiln, the signal representative of the oxygen
content being applied to control system 51 on line 56. A signal
from the dust feed rate sensor 36 is provided to the control system
51 on a line 57.
A signal representing the rate of fuel flow to mixing chamber 27 is
provided to control system 51 on line 58 by sensor 59 associated
with fuel supply line 25. Sensor 60 associated with kiln drive
motor 20 provides a signal to control system 51 on line 61
representing the torque developed by motor 20 as required to rotate
kiln 10 at the predetermined rotational speed. A temperature
measuring device 62, for example a thermocouple, is provided near
feed end 14 of the kiln to provide a signal transmitted to control
system 51 on line 63 indicating the feed end temperature of the
gases flowing through the kiln at that point after they exit from
the chain section 16. A signal representing the kiln speed is
provided to control system 51 by a kiln speed sensor 64 coupled to
the control system 51 by line 67.
Control system 51 utilizes the information concerning kiln
operation provided on lines 52, 54, 56, 57, 58, 61, 63 and 67 to
produce kiln control signals on lines 65 and 66. The control signal
on line 65 represents a fuel rate setpoint and is applied to
controller 68 to control the rate of flow of fuel into mixing
chamber 27 and therefore the heat input to kiln 10. The control
signal on line 66 represents an exit gas rate setpoint and is
applied to controller 69 to control the speed of induced draft fan
31 and therefore the exit gas flow rate. The signal on line 66
representing an exit gas rate setpoint may, alternatively, be
employed to control the position of damper 38, thereby adjusting
the exit gas flow rate. Controllers 68 and 69 are standard analog
controllers as known in the art and will not be described in
detail.
FIG. 2 illustrates the details of control system 51 shown in FIG.
1. Referring to FIG. 2, the signal on line 61 representing the
torque developed by kiln drive motor 20 to rotate kiln 10 is
applied to filter 80. If kiln drive motor 20 is an AC motor,
assuming a constant speed of rotation of kiln 10, the signal on
line 61 is a measure of the kilowatt power input to motor 20 which
represents the torque developed by motor 20 to rotate kiln 10. If
kiln drive motor 20 is a constant field, DC motor, the signal on
line 61 is a measure of the armature current of kiln drive motor 20
which represents the torque developed by motor 20 to rotate kiln
10, with constant field and supply voltages. For purposes of this
description, motor 20 is assumed to be a constant field, DC motor;
and the signal on line 61 representing the armature current in and
the torque developed by motor 20 is termed AMP.sub.scan. Various
means for obtaining a torque signal from other motors are known in
the art. Sensor 60 (FIG. 1) comprises an instrument capable of
measuring and providing an output signal proportional to the
armature current or torque of motor 20.
Experience has shown that when a signal directly proportional to
the torque required to turn the kiln is filtered and smoothed, it
can be reliable and sensitive indicator of the heat state within
the kiln, in particular the condition of the burning zone, and of
the relative amount of dense clinker material in the burning zone.
The material in the burning zone undergoing the clinkering reaction
is much denser than the remainder of the feed in the kiln. In
addition, since normally 10 to 30 percent of the material in the
burning zone is in a liquid state, the flow characteristics of this
material differ drastically from the remainder of material in the
kiln. The liquefied material in the burning zone is much stickier
and tends to form a mass which adheres to the refractory surface.
The material thus rides much higher on the kiln wall as the kiln
rotates and requires more torque to carry the material along the
kiln.
As the kiln temperature increases, the burning zone lengthens and
the amount of this dense liquefied material increases, requiring an
increase in the rotational torque supplied by kiln drive motor 20.
As the kiln temperature decreases, the burning zone shortens and
the amount of this dense liquefied material decreases, decreasing
the torque developed by drive motor 20. Thus, changes in the torque
developed by drive motor 20 in rotating the kiln indicate changing
heat conditions within the kiln which are causing the burning zone
to lengthen or shorten. Slow changes in the torque over a period of
time indicate very small imbalances in heat input, normally
undetected by an operator, which, if corrected immediately, will
prevent larger upsets and more drastic corrective action at a later
time. The signal AMP.sub.scan therefore represents the
instantaneous heat state within the kiln and changes in the value
of AMP.sub.scan indicate corresponding changes in the condition of
the burning zone. The use of torque measurements represented by the
signal AMP.sub.scan to effect control of kiln operation independent
of actual burning zone temperature measurements are claimed in the
aforesaid U.S. Patent. Filtering and smoothing of the AMP.sub.scan
signal to remove noise and other signal variations unrelated to the
condition of the burning zone, for example the effect of kiln
rotation on the signal, is performed in filter 80. The output
signal of filter 80 is FAMP.sub.n. The filtering action of filter
80 is described in the equation:
FAMP.sub.n =FAMP.sub.n.sub.-1 +K.sub.amp (AMP.sub.scan
-FAMP.sub.n.sub.-1)
Where FAMP.sub.n is the new filtered value,
Famp.sub.n.sub.-1 is the last filtered value,
Amp.sub.scan is the present scan value, and
K.sub.amp is the filter constant.
The function of filter 80 may conveniently be performed in a
digital computer with K.sub.amp, FAMP.sub.n and FAMP.sub.n.sub.-1
being stored in the computer memory. This calculation is performed
at short intervals, for example every 5 seconds, to insure that
signal FAMP.sub.n represents the current condition of the burning
zone, forming an accurate basis for control action. K.sub.amp is
selected to be small enough to eliminate noise and the effect of
kiln rotation on the signal but not so small as to damp out the
signal and may be, for example .005.
Output signal FAMP.sub.n of filter 80 is applied to check logic 81.
Check logic 81 compares the present output signal FAMP.sub.n of
filter 80 with the previous output signal FAMP.sub.n.sub.-1. If the
present and previous filtered values differ by more than a given
amount, it is assumed that some unusual conditions exist within the
kiln and output signal FAMP.sub.n of filter 80 is not used until it
returns to within a reasonable range of FAMP.sub.n.sub.-1. Check
logic 81 serves to mask momentary or short term disturbances. The
function performed by check logic 81 may be conveniently
implemented in a digital computer; it may be implemented either on
FAMP.sub.n or on AMP.sub.scan.
Output signal FAMP.sub.n of filter 80, if within the required
range, is applied to summing amplifier 82. A kiln amp setpoint
signal AMP.sub.sp is also applied to summing amplifier 82. The kiln
amp setpoint represented by signal AMP.sub.sp is controlled by the
operator by means of a potentiometer or a value stored in a digital
computer and will normally be based upon the chemical analysis of
the kiln product periodically reported to the operator. For
example, if the free lime (uncombined CaO) content of the kiln
product is too low, the operator will lower the kiln amp setpoint,
whereas if the free lime content is too high, the operator will
raise the kiln amp setpoint. Typical kiln amp setpoint values for a
particular type and rate of feed and for a particular quality kiln
product based on past experience will be employed by the operator
to select an initial kiln amp setpoint.
Summing amplifier 82 is of the type will known in the art and
provides an amp error signal EAMP.sub.n proportional to the
difference between the kiln amp setpoint AMP.sub.sp and the present
filtered value of kiln amps represented by signal FAMP.sub.n, as
expressed by the equation:
EAMP.sub.n =AMP.sub.sp -FAMP.sub.n
Amp error signal EAMP.sub.n is positive if the present filtered
value of kiln amps is less than the kiln amp setpoint and is
negative if the present filtered value of kiln amps exceeds the
kiln amp setpoint. The function of summing amplifier 82 may
conveniently be performed in a digital computer.
If the process being performed in kiln 10 responded quickly to
changes, for example a change in the rate of fuel combustion, the
amp error signal EAMP.sub.n provided by summing amplifier 82 could
be used directly to control the fuel rate setpoint. However, as
previously described, the precess in the kiln reacts very slowly to
control action on fuel rate setpoint and normally the reaction to a
control action may not be detected for a long period and then will
normally continue for a long time thereafter. In responding to a
control action, the process therefore has a long lag time plus a
long time constant. The reaction time of a kiln, i.e. the time
period between initiation of a control action and the resulting
change in burning zone condition may be up to 30 minutes or more.
Because of these characteristics, an analog controller cannot
adequately perform the control function.
Effective control of kiln operation in dry kiln operation can be
accomplished solely by employing a dynamic process model in
accordance with the above-identified U.S. patent. The process
model, identified by reference numeral 83 in FIG. 2, comprises a
delay table in which control values are stored each time a control
action is taken. If, for example, a control value is calculated
every 5 minutes to initiate a control action, if required, this
control value is stored in the process model and the control values
previously stored are shifted through one storage position in the
process model each time a new value is entered. Assuming an
interval of 5 minutes between control value calculations, the
fourth storage position down the table will contain the control
value calculated 20 minutes earlier. The actual delay incorporated
in the process model is the time period which elapses between
initiation of a control action and the kiln response to this
control action as reflected in change of burning zone condition,
this delay being a function of the characteristics of a particular
kiln. In a typical kiln, the delay between a control action, e.g. a
fuel setpoint change, and the response thereto in the burning zone
of the kiln may be of the order of 30--35 minutes or more. The
delay table of the process model comprises a sufficient number of
storage positions so that the delay range available in the delay
table encompasses the delay characteristic of the kiln being
controlled.
Process model 83 also includes arithmetic apparatus for providing
feedback signal FBAMP.sub.n. Feedback signal FBAMP.sub.n is applied
to summing amplifier 84 along with amp error signal EAMP.sub.n from
summing amplifier 82. Output signal DELAMP.sub.n of summing
amplifier 84 constitutes a control value and represents the sum of
feedback signal FBAMP.sub.n and amp error signal EAMP.sub.n as
expressed by the equation:
DELAMP.sub.n =EAMP.sub.n +FBAMP.sub.n
The function of Summing amplifier 84 may conveniently be performed
by a digital computer. Control value signal DELAMP.sub.n is applied
to torque controller 85 which controls the fuel rate setpoint in
accordance with the magnitude of DELAMP.sub.n. Signal DELAMP.sub.n
is also applied to process model 83 for storage in the delay table.
The delay table of process model 83, assuming a period of 5 minutes
between control value calculations, has stored therein the signals
DELAMP.sub.n, DELAMP.sub.n.sub.-5, DELAMP.sub.n.sub.-10,
DELAMP.sub.n.sub.-15, ... DELAMP.sub.n.sub.-m where m is the number
of minutes equal to or greater than the delay characteristic of the
kiln being controlled.
The arithmetic apparatus forming part of process model 83
periodically, e.g. every 5 minutes, calculates feedback signal
FBAMP.sub.n in accordance with the following equation:
FBAMP.sub.n =FBAMP.sub.n.sub.-1 +K.sub.fb (DELAMP.sub.n.sub.-x
-FBAMP.sub.n.sub.-1)
Where FBAMP.sub.n is the present feedback signal generated by the
process model,
Fbamp.sub.n.sub.-1 is the last feedback signal generated by the
process model,
K.sub.fb is the process model feedback constant, and
Delamp.sub.n.sub.-x is a selected control value stored in the
process model table (x=delay time of kiln in minutes)
Feedback constant K.sub.fb may, for example, have a value in the
range 0.005. The signal DELAMP.sub.n.sub.-x may be any of the
stored control values in the process model table corresponding to
the delay between a control action and the reaction in the burning
zone which is characteristic of the particular kiln. If the delay
characteristic of the kiln is 35 minutes, stored control value
DELAMP.sub.n.sub.-35 is used by the process model to calculate
feedback signal FBAMP.sub.n. Feedback signal FBAMP.sub.n may be
calculated by the arithmetic apparatus of process model 83 and
control signal DELAMP.sub. n generated at any desired interval, for
example every 5 minutes. After calculation of feedback signal
FBAMP.sub.n, the resulting value of control signal DELAMP.sub.n
furnished by summing amplifier 84 is stored in the process model
table and the previously stored values of signal DELAMP are shifted
down the table in the process model.
The storage of successive control signals DELAMP and the
calculation of feedback signal FBAMP.sub.n may conveniently be
accomplished in a digital computer. The table of process model 83
may, for example, comprise a selected series of memory locations
within the stored control values with the stored control values
being shifted through the series of memory locations as the control
values are entered into the table, as illustrated diagrammatically
in FIG. 3. Feedback constant K.sub.fb and the previously calculated
feedback signal FBAMP.sub.n.sub.-1 may also be stored in the
computer memory. The arithmetic unit of the digital computer serves
to control the storage of successive control values DELAMP.sub.n in
the table and utilizes the contents of the table and the stored
values of K.sub.fb and FBAMP.sub.n.sub.-1 to calculate feedback
signal FBAMP.sub.n.
In wet kilns, significant portions of the overall torque changes
sensed by torque sensor 60 are due to changes in the feed material
condition in the chain section 16. It has been found that such
torque changes can be closely approximated with knowledge of the
feed rate and moisture content, the temperature across the chain
section 15 and the gas flow rate through the chain section 16. One
of the parameters used to determine or calculate the effect of the
chains is the intermediate gas temperature measured by the
intermediate gas temperature sensor 53 which generates a signal
TIG.sub.scan on line 54. Check logic 85 compares successive signals
TIG.sub.scan. If two successive signals differ by more than a given
amount, it is assumed that the sensor 53 has failed and the
previous value of TIG.sub.scan is used. Filter 87 receives the
signal TIG.sub.scan and produces intermediate gas temperature
information represented by the signal filter 87 in accordance with
the following equation:
FTIG.sub.n =FTIG.sub.n.sub.-1 +K.sub.tig (TIG.sub.scan
-FTIG.sub.n.sub.-1)
Where FTIG.sub.n is the present filtered value,
Ftig.sub.n.sub.-1 is the previous filtered value,
Tig.sub.scan is the present measured value, and
K.sub.tig is the filter constant.
A typical value for constant K.sub.tig is 0.2 when FTIG.sub.n is
calculated once every minute. The functions of check logic 86 and
filter 87 may be conveniently performed in a digital computer with
the signals FTIG.sub.n, FTIG.sub.n.sub.-1, successive values of
TIG.sub.scan and the constant K.sub.tig being stored in the
computer memory. Filtered gas temperature signal FTIG.sub.n is used
in conjunction with other parameters to determine the signal
CHAMP.sub.n.
Check logic 88 receives on a line 63 a signal representing the feed
end gas temperature at exit end 14 of the kiln, as measured by
device 62. The gas temperature information represented by the
signal is applied to check logic 88 to be compared with the
previous value of the signal FET.sub.scan. If the two successive
signals differ by more than a given amount, it is assumed that the
device 62 has failed and the first signal FET.sub.scan is used.
Filter 89 receives the signal FET.sub.scan and filters the value in
accordance with the following equation:
FFET.sub.n =FFET.sub.n.sub.-1 +K.sub.fet (FET.sub.scan
-FFET.sub.n.sub.-1)
Where FFET.sub.n is the present filtered value,
Ffet.sub.n.sub.-1 is the previous filtered value,
Fet.sub.scan is the present measured value, and
K.sub.fet is the filtered constant.
A typical value for constant K.sub.fet is 0.2 when FFET.sub.n is
calculated once every minute.
In addition to being used with the signal FTIG.sub.n to determine
the value of the signal CHAMP.sub.n, filtered feed end temperature
FFET.sub.n is applied to summing amplifier 90 along with signal
FET.sub.sp representing the gas temperature setpoint as determined
by the operator. The summing amplifier 90 together with the
temperature controller 91 and logic switch 92 constitute a control
loop for maintaining a relatively constant gas temperature near the
feed end 14 of the kiln to provide a relatively constant source of
heat for the feed entering the kiln and a relatively constant
temperature profile from the discharge end to the feed end of the
kiln. The gas temperature at the feed end of the kiln is thereby
decoupled from control actions which vary the rate of fuel flow and
therefore the rate of heat input into the kiln due to control
actions initiated in a torque control loop to be described
hereinafter. As the fuel rate is increased or decreased to adjust
burning zone conditions as reflected in required kiln drive torque,
the temperature control loop adjusts the exit gas flow rate to
maintain sufficient heat availability in the feed preparation
section of the kiln comprising the chain section 16 and preheating
zone.
Further, if heat requirements change due to changes in the
characteristics of the raw materials entering the feed end of the
kiln or due to changes in the feed rate, exit gas flow rate changes
may become necessary. For example, if the raw materials require a
greater quantity of heat, decreasing gas temperature at the kiln
feed end an increase in exit gas flow is required to carry more
heat to the feed end of the kiln, thus maintaining the desired
temperature profile in the kiln. If such action is not taken, the
resulting decrease in gas temperature at the kiln feed end would
eventually affect burning zone conditions and appear as a
disturbance which would require more drastic corrective action to
be taken in the torque control loop. The temperature control loop
thus compensates for disturbances and for effects of control
actions taken in the torque control loop so that the effect of
these disturbances and control actions do not cause further
disturbances in kiln operation requiring further control
actions.
Gas temperature error signal EFET.sub.n produced by summing
amplifier 90 is a function of signals FFET.sub.n and FET.sub.sp, as
expressed in the following equation:
EFET.sub.n =FET.sub.sp -FFET.sub.n
and is applied to gas temperature controller 91. The function of
summing amplifier 90 may conveniently be performed in a digital
computer.
Gas temperature controller 91 determines a desired exit gas flow
rate in the kiln and includes both proportional and integral modes.
The function of gas temperature controller 91 is represented by the
equation for output signal EXIT.sub.n of controller 91, as
follows:
EXIT.sub.n =EXIT.sub.n.sub.-1 +K.sub.1 EFET.sub.n +K.sub.2
EFET.sub.n.sub.-1
Where EXIT.sub.n is the desired exit gas flow rate,
Exit.sub.n.sub.-1 is the previous exit gas flow rate,
Efet.sub.n is the present temperature error signal,
Efet.sub.n.sub.-1 is the previous temperature error signal, and
K.sub.1 and K.sub.2 are controller constants.
Typical values of constants K.sub.1 and K.sub.2 are 0.11 and -0.10,
respectively, and EXIT.sub.n may be calculated, for example, every
five minutes. Output signal EXIT.sub.n of gas temperature
controller 91 is applied to line 66 for application to controller
69 through logic switch 92. In the illustrated embodiment, signal
EXIT.sub.n is employed to control the speed of fan 31 but may, as
an alternative, serve to control the position of damper 38. Logic
switch 92 normally connects temperature controller 91 to controller
69, as illustrated, by may also serve to interrupt the connection,
as subsequently described.
The function of gas temperature controller 91 may be conveniently
performed in a digital computer with signals EXIT.sub.n,
EXIT.sub.n.sub.-1, EFET.sub.n and EFET.sub.n.sub.-1 and constants
K.sub.1 and K.sub.2 being stored in the computer memory.
A major safety consideration in the operation of a cement kiln is
the oxygen content of the exit gases. The oxygen content must be
above a minimum safe level, usually 0.5 percent, to be assured that
no combustibles, or carbon monoxide, appear in the exit gases which
might cause an explosion in the dust collection system. The oxygen
content of the exit gases depends upon the exit gas rate and the
fuel rate. If the oxygen content of the exit gases falls below the
minimum safe level, the exit gas rate determined in the gas
temperature control loop, and possibly the fuel rate determined in
the torque control loop, must be altered to maintain safe kiln
operation. For example, if the fuel rate required by the torque
control loop will result in an oxygen content below the minimum
safe level at the exit gas rate determined by the gas temperature
control loop, the exit gas rate determined by the gas temperature
control loop must be overruled and a safe rate set. If the exit gas
rate is already at a maximum, as limited by the position of damper
38 or by the speed of fan 31, and if the oxygen content is still
below the minimum safe level, the fuel rate determined by the
torque control loop must be overruled and a new fuel rate
determined to insure safe operation.
Oxygen override logic 95 of the control system, as shown in FIG. 2,
monitors the oxygen content of the exit gas and determines what the
new oxygen content will be after the contemplated control actions
are taken. If the predicted oxygen content is less than the
prescribed minimum safe level, logic 95 takes overriding action.
The priority of the overriding logic is such that the exit gas rate
calculated by gas temperature controller 91 is sacrificed first to
permit the desired fuel rate determined by torque controller 85,
override logic 95 calculating a new exit gas rate setpoint which
will result in a predicted oxygen content at the minimum safe level
for the desired fuel rate. However, if the exit gas rate cannot be
adjusted sufficiently to provide the required minimum oxygen
content, the fuel rate is also adjusted by override logic 95 to
produce the safe minimum oxygen content at the maximum exit gas
rate. Oxygen override logic 95 thus prevents dangerous conditions
from occurring by preventing the selection of fuel and exit gas
flow rates which reduce the oxygen content of the exit gas below a
minimum safe level.
Oxygen override logic 95 receives the outputs of torque controller
85 and gas temperature controller 91 in addition to the signal on
line 56 from analyzer 55 representing exit gas oxygen content and
the signal on line 52 from sensor 50 representing feed rate of raw
materials into feed end 14 of the kiln. The output signals of
oxygen override logic 95 are applied to logic switches 92 and 93
respectively to override, as required, the fuel rate setpoint and
exit gas rate as determined by torque controller 85 and gas
temperature controller 91 respectively, when such action is
necessary to maintain a minimum safe level of oxygen in the exit
gases. Normally, no action is taken for high oxygen content in the
exit gas although such action can be taken with similar logic
considerations. Oxygen override logic 95 has the capability of
calculating predicted oxygen content and employing this as a
substitute for measured oxygen content when analyzer 55 is
unavailable due to, for example, operating problems.
Oxygen override logic 95 calculates the present exit gas rate as
follows:
Where EN is the present exit gas rate in moles per hour,
Fuel.sub.sp.sub.-1 is the old fuel rate setpoint in KSCF (thousands
of standard cubic feet) per hour presently being used,
Feed.sub.n is the present feed rate in tons per hour, as measured
by feed rate detector 50 and as represented by the signal on line
52,
02.sub.n is the present oxygen content of the exit gas as measured
by analyzer 55 and represented by the signal on line 56,
21 represents the normal percent oxygen in the atmosphere, and
29.354 and 15.91 are specific constants for a given fuel and feed
composition.
The predicted oxygen content of the exit gas based on the new fuel
rate FUEL.sub. sp determined by the torque control loop can then be
calculated as follows:
Where FUEL.sub.sp is the new required fuel rate determined by
torque controller 85 in the torque control loop,
Exit.sub.n is the new required exit gas rate determined by
temperature controller 91 in the gas temperature control loop,
and
02.sub.n.sub.+1 is the predicted oxygen content.
If the recalculated exit gas rate OEXIT.sub.n is greater than the
capacity of fan 31, then a new overriding fuel rate OFUEL.sub.sp
which will be less than FUEL.sub.sp determined by torque controller
85 must be calculated using maximum capacity of fan 31 or
EXIT.sub.max as follows:
This recalculated fuel rate OFUEL.sub.sp will result in the minimum
safe exit gas oxygen content when the exit gas rate is at the
maximum. A signal representing exit gas rate OEXIT.sub.n, as
calculated by oxygen override logic 95, or a signal representing
the maximum exit gas rate EXIT.sub.max, if required, is applied to
logic switch 92 and takes precedence over the exit gas rate
EXIT.sub.n determined by gas temperature controller 91. Similarly,
if a new fuel setpoint OFUEL.sub. sp is calculated by oxygen
override logic 95, a signal representing this new fuel setpoint is
applied to logic switch 93 and takes precedence over the fuel rate
setpoint FUEL.sub.sp determined by torque controller 85. The
functions of oxygen override logic 95 may be conveniently performed
in a digital computer, with the computer memory being employed to
store the signals required for the computations.
The torque control loop responds not only to the difference between
the kiln amp setpoint AMP.sub.sp and the filtered signal indicating
motor torque FAMP.sub.sp to produce an error signal which is then
added to a feedback signal FBAMP.sub.n, it is also responsive to a
signal indicating the effect of the chains on the total torque,
CHAMP.sub.n. The effect of the chains is initially calculated in a
temperature computer which responds to several signals. The
filtered feed end and intermediate gas temperatures, FFET.sub.n and
FTIG.sub.n together with the exit gas flow rate signal EXIT.sub.n
on the line 66 and dry feed rate signal on line 52 FEED.sub.n are
applied to the temperature computer 100. A dust feed rate signal
TSPD.sub.n is applied on line 57. These signals may be either
constant values or they may be and preferably are represented by
constantly scanned variables which are treated by filtering and
check logic in the same manner as the signal AMP.sub.scan is
converted to FAMP.sub.n by filter 80 and check logic 81. A kiln
speed signal KSPD.sub.n can similarly be provided on line 67 from
the kiln speed sensor 64 shown in FIG. 1. Operator inputs to the
temperature computer 100 include a feed temperature FTMP and a
signal representing the percent moisture in the feed PMIF which are
normally relatively constant for a given process.
With these inputs, it is possible for the temperature computer 100
to calculate the temperature of the feed at the termination of the
chain section 16, the signal being designated TSCF.sub.n. This
value is determined by performing a heat balance around the chain
section 16. Initially, it can be assumed that the temperature of
the dust carried back into the chain section 16 from the dust
feeder 34 is a linear function of the intermediate gas temperature
FTIG.sub.n in accordance with:
TID.sub.n =K.sub.id FTIG.sub.n +K.sub.dt
Where TID.sub.n is the temperature of the dust at the chain
section,
Ftig.sub.n is the present desired intermediate gas temperature,
K.sub.id is a proportionality constant, and
K.sub.dt is a dust temperature calculation constant.
If the feed rate sensor 50 generates a signal which is variable in
accordance with the wet feed rate, then the signal on line 52 can
be used directly. If, on the other hand, the dry feed rate is
provided by sensor 50, the wet feed rate can be calculated by:
FEEDW.sub.n =FEED.sub.n /(1-PMIF)
Where FEEDW.sub.n is the calculated wet feed rate,
Feed.sub.n is the dry feed rate, and
Pmif is the per unit moisture in the feed as specified by the
operator.
With these values, it is possible to analyze the heat inputs
according to:
Heat input (feed) = FEEDW.sub.n .times.C.sub.p-feed FTMP
Where C.sub.p-feed is the specific heat of the raw feed, and
Ftmp is the temperature of the slurry as entered by the
operator.
The heat input from the dry gas coming into the chain section 16
is:
Heat input (intermediate gas) = EXIT.sub.n.sub.-1 C.sub.p.sub.-g
FTIG.sub.n
Where EXIT.sub.n.sub.-1 is the previous value of exit gas rate,
C.sub.p.sub.-g is the specific heat of the gas passing into the
chain section, and
Ftig.sub.n is the present value of intermediate gas
temperature.
The heat input from the dust is given by:
Heat input (dust) = TSPD.sub.n TID C.sub.p-dust
Where TSPD.sub.n is the measured dust rate,
Tid.sub.n is the calculated dust temperature, and
C.sub.p-dust is the specific heat for the dust.
This equation assumes that dust going into chain section 16 is
equal to the dust feed rate; but it has been found that a
circulating dust load exists which is greater than the dust input.
The effect of this higher circulating load, which varies from kiln,
can be factored into the equations by increasing K.sub.id.
Additional variations from actual conditions can be corrected
somewhat by the use of a radiation constant K.sub.rad in the
equation representing the heat balance.
Heat losses occur as the dry gas and moisture exit, through
evaporation and in the dust leaving the chains. The heat loss due
to dry exit gas is:
Heat loss (dry gas = EXIT.sub.n.sub.-1 C.sub.p-chgas FFET.sub.n
Where EXIT.sub.n.sub.-1 is the previous value of the exit gas
rate,
C.sub.p-chgas is the specific heat of the gas exiting the chain
section, and
Ffet.sub.n is the feed end temperature. The heat loss due to
moisture from the feed that leaves as steam can be approximated
by:
Heat loss (steam) = PMIF FEEDW.sub.n FFET.sub.n (1000)
Where PMIF is the percent moisture content of the feed,
Feedw.sub.n.sub.-1 is the previous value of the wet feed rate,
and
1000 is the specific heat of one-half B.t.u. per pound .times.
2,000 pounds per ton.
The heat loss to vaporize water in the feed can be approximated
by:
Heatout (vaporization) = PMIF FEEDW.sub.n C.sub.p-vap
Where C.sub.p-vap is the heat of vaporization of water plus a
constant.
With these variables, it is possible to calculate the temperature
of the heat lost in the dust leaving the chains which is
represented by:
Heat loss (dust) = TSPD.sub.n FFET.sub.n C.sub.p-dust
Finally, the heat loss of the feed leaving the chains is:
Heat loss (Feed) = FEED.sub.n C.sub.p-feed TSC.sub.n
Where FFED.sub.n is the dry feed rate,
C.sub.p-feed is the specific heat for the dry feed and,
Tsc.sub.n represents a function of the temperature of dry feed
leaving the chain section.
By balancing these equations, it is possible to obtain the
temperature function TSC.sub.n of the feed:
TSC.sub.n =[K.sub.rad +FEEDW.sub.n.sub.-1 C.sub.p-feed FTMP+
EXIT.sub.n.sub.-1 (C.sub.p.sub.-g FTIG.sub.n -C.sub.p-chgas
FFET.sub.n)+
TSPD.sub.n (TID-FFET.sub.n) C.sub. p-dust -
PMIF FEEDW.sub.n (C.sub.p-vap +1000 FFET.sub.n)]/(FEED.sub.n
C.sub.p-feed)
The instantaneous value for the feed temperature as it leaves the
chain section is then converted and filtered to account for the
thermal time constant of the chain system in accordance with:
TSCF.sub.n =TSCF.sub.n.sub.-1 +K.sub.tsc (TSC.sub.n
-TSCF.sub.n.sub.-1)
Where TSCF.sub.n is the present filtered value of the feed
temperature,
Tscf.sub.n.sub.-1 is the previous filtered value of the feed
temperature,
K.sub.tsc is a filter constant, and
Tsc.sub.n is the present instantaneous value of the feed
temperature.
The output of the temperature computer, TSCF.sub.n is then applied
to a trending filter 101 which responds to the input to generate an
output signal TSCT.sub.n in accordance with:
TSCT.sub.n =TSCT.sub.n.sub.-1 +K.sub.tr (TSCF.sub.n
-TSCT.sub.n.sub.-1)
Where
Tsct.sub.n is the present trended value of the feed
temperature,
Tsct.sub.n.sub.-1 is the previous trended value of the feed
temperature, and
K.sub.tr is a trend constant for the filter.
The output of the filter 101 is applied to a positive input of a
summing amplifier 102 while the input to the filter 101 is applied
to a negative input of the summing amplifier 102. The output
DTSC.sub.n from the summing amplifier 102 is:
DTSC.sub.n =TSCT.sub.n -TSCF.sub.n
Where DTSC.sub.n is the change in the temperature of the feed.
This signal is then applied to filter 103 having an output signal
CHAMP.sub.n generated in accordance with:
CHAMP.sub.n =K.sub.ch DTSC.sub.n
Where CHAMP.sub.n is the calculated effect of the change on the
total torque,
K.sub.ch is a filter constant.
The functions of the temperature computer 100, the filter 101, the
summing amplifier 102 and the filter 103 can all be accomplished
conveniently by a digital computer.
Hence, as indicated earlier, the output of the summing amplifier 84
DELAMP.sub.n varies in accordance with:
DELAMP.sub.n =EAMP.sub.n +FBAMP.sub.n +CHAMP.sub.n
Output signal DELAMP.sub.n of summing amplifier 84 is applied to
torque controller 85 along with signal FUEL.sub.base from filter 96
representing the filtered rate of fuel flow to mixing chamber 27
and, thus, the rate of input to kiln 10 at the time control of the
kiln by control system 51 is commenced. Thereafter, FUEL.sub.base
remains constant. Filter 96 receives on line 58 signal
FUEL.sub.scan representing the output of fuel rate sensor 59 and
filters and smooths signal FUEL.sub. scan in accordance with the
following equation:
FFUEL.sub.n =FFUEL.sub.n.sub.-1 +K.sub.fuel (FFUEL.sub.scan
-FFUEL.sub.n.sub.- 1)
Where FFUEL.sub.n is the new filtered value,
FFUEL.sub.n.sub.-1 is the last filtered value,
K.sub.fuel is the filter constant, and
Ffuel.sub.scan is the present output of sensor 59.
At the time automatic control of the kiln by control system 51 is
initiated, filtering of FUEL.sub.scan in filter 96 is terminated
and the value of FFUEL.sub.n at that time becomes FUEL.sub.base
which thereafter remains constant. The function of filter 96 may
conveniently be performed in a digital computer.
Torque controller 85, in response to signals DELAMP.sub.n and
FUEL.sub.base, generates signal FUEL.sub.sp representing the
calculated fuel setpoint required to maintain or reach a stable
operating condition in the kiln. Torque controller 85 calculates
signal FUEL.sub.sp in accordance with the following equation:
FUEL.sub.sp =FUEL.sub.base +K.sub.fuel DELAMP.sub.n
Where FUEL.sub.sp is the calculated fuel rate setpoint,
Fuel.sub.base is the base fuel value represented by output signal
FFUEl.sub.n of filter 96 at the time the kiln is placed under
control of control system 51 and thereafter remains constant at
this base value, and
K.sub.fuel is the fuel/kiln amp proportionality constant.
Proportionality constant K.sub.fuel is a function of the
characteristics of the fuel and kiln being controlled. Torque
controller 85 thus responds to the output of summing amplifier 84
represented by signal DELAMP.sub.n and to the value of the base
fuel rate represented by the signal FUEL.sub.base to provide signal
FUEL.sub.sp representing the desired fuel setpoint to maintain
stable operation of the kiln or to regain stable operation after a
disturbance. Fuel setpoint signal FUEL.sub.sp is applied to
controller 68 on line 65, as shown in FIG. 1. The function of
torque controller 85 may conveniently be performed in a digital
computer.
Torque controller 85 is so named since changing the rate of flow of
fuel to mixing chamber 27 changes the heat input to the kiln,
eventually effecting the torque required from kiln drive motor 20
to rotate the kiln. For example, a decrease in the value of signal
FAMP.sub.n, indicating a shortening of the burning zone, results in
an output signal from torque controller 85 increasing the fuel
setpoint to cause the temperature in the kiln to increase with a
resulting lengthening of the burning zone which will be reflected
in an increased torque requirement and in an increase in the value
of signal FAMP.sub.n. Conversely, an increase in the value of
signal FAMP.sub.n indicates a lengthening of the burning zone and
the response of torque controller 85, acting under the influence of
process model 83, is to decrease the fuel setpoint, reducing the
heat input to the kiln which will eventually be reflected in the
shortening of the burning zone and the reduction of the torque
required to rotate the kiln, decreasing the value of signal
FAMP.sub.n.
If such a change in the kiln torque signal FAMP.sub.n were caused
by changes in the chain section, however, then the signal
CHAMP.sub.n is applied to the summing amplifier 84 to immediately
increase the value of DELAMP.sub.n and thereby increase the fuel
setpoint level FUEL.sub.sp to the torque controller 85. Such a
disturbance would, without taking into account the effect of the
chains, actually result in a decrease of the fuel setpoint without
this provision. Conversely, changes in the chains which would tend
to decrease the torque and thereby cause a decrease in the fuel
setpoint from the torque controller FUEL.sub.sp actually indicate
that additional fuel is necessary so the signal from the torque
controller 85 FUEL.sub.sp is increased.
This control arrangement thus tends to maintain a constant desired
condition in the burning zone of the kiln, detecting changes in the
condition of the burning zone by sensing drive motor torque and
responding to such burning zone condition changes by varying the
fuel rate setpoint in a direction which tends to return the burning
zone to the desired condition. The effective changes in the chain
section, which can significantly affect the sensed drive motor
torque are thereby compensated. While the process model serves to
prevent cycling of the kiln by introducing into each control
arrangement the expected future response to each control action
taken, the effective changes in the chain section are immediately
inputed to the control. Thus, in response to a change in torque
required to drive the kiln, indicating a change in the condition of
the burning zone, a control action is taken in the form of an
incremental change in fuel rate to compensate for burning zone
disturbances only. Appreciably no effect occurs as a result of a
change in the torque in the chains unless the total effect of the
change of torque is caused by more than that. After an interval of
time determined by the kiln characteristics, the effect of the
incremental change in fuel rate is realized as a corrective change
in the burning zone condition which again affects the torque
required to rotate the kiln. The process model prevents the change
in required kiln rotational torque due to the effects of a control
action from again affecting the fuel rate setpoint, thus preventing
cycling of the kiln. The process model prevents kiln cycling by
remembering changes in kiln torque to be expected due to previous
control actions and by introducing these expected changes in kiln
drive torque into the control loop, so that only kiln drive motor
torque changes due to kiln disturbances not directly caused by
previous control actions or kiln drive torque changes caused by
changes in the condition in the chains serve as a basis for further
control action.
Fuel setpoint signal FUEL.sub.sp generated by torque controller 85
is transmitted to controller 68 on line 65 through logic switch 93,
as illustrated in FIG. 2. Logic switch 93 normally connects the
output of torque controller 85 to controller 68 but may serve to
interrupt the controlling action or torque controller in response
to the oxygen override logic 95.
The torque control loop comprising filters 80, 86, 101 and 103,
check logic 81, summing amplifiers 82 and 84, process model 83,
temperature computer 100 and torque controller 85 is capable of
maintaining burning zone conditions within a desired range by
adjustment of fuel rate setpoint. The torque control loop functions
satisfactorily for variations in kiln torque within a predetermined
range of values centered on the setpoint value. Heat imbalances
reflected in torque variations which are caused by disturbances
such as changes in secondary air temperature, changes in heat loss
rate from the kiln due to change in ambient temperature, changes in
coating thickness, changes in feed water content, changes in feed
composition, etc. occur slowly and usually continuously, requiring
almost constant correction of burning zone conditions which are
well within the effective range of control of the torque control
loop. The torque control loop will normally provide adequate
control of the kiln burning zone for variations in kiln torque over
a range of plus or minus 10 to 20 percent of the setpoint value and
will compensate for changes in the chain amp torque.
FIG. 4 illustrates a flow chart of the operation of the control
system of FIG. 2. Signal AMP.sub.scan representing the armature
current of kiln drive motor 20 and therefore the torque developed
by motor 20 is made continuously available to the torque control
loop. The signal is filtered periodically, e.g. every 5 seconds, to
obtain a filtered value FAMP.sub.n. Signal FAMP.sub.n is compared
with the previous filtered value FAMP.sub.n.sub.-1 and if the two
values differ by more than a predetermined amount, the previous
value FAMP.sub.n.sub.-1 is saved and used in lieu of FAMP.sub.n.
Otherwise, the filtered value FAMP.sub.n is compared to the kiln
amp setpoint AMP.sub.sp set by the operator and an error signal
EAMP.sub.n is generated representing the difference between
FAMP.sub.n and AMP.sub.sp.
Several process related signals are available to temperature
computer 100 and include EXIT.sub.n.sub.-1, the gas rate setpoint,
the feed rate signal FFEED.sub.n.sub.-1, FID.sub.n, the feed rate
for the input dust, KSPD.sub.n, the kiln speed, FFET.sub.n, the
feed end temperature and FTIG.sub.n, the intermediate gas
temperature. In response to these and operator inputs of the feed
temperature, TMPF and the percent moisture in the feed, TMIF, the
temperature computer produces a filtered value TSCF.sub.n
representing the temperature of the feed leaving the chain section.
This is then filtered and summed to produce an output signal which
represents the change in the temperature of the feed DTSC.sub.n
which then indicates the effect of the chain section by the signal
CHAMP.sub.n.
Feedback signal FBAMP.sub.n provided by the process model, amp
error signal EAMP.sub.n and the chain torque signal CHAMP.sub.n are
employed to generate signal DELAMP.sub.n. Torque controller of the
torque control loop utilizes signal DELAMP.sub.n and the signal
FUEL.sub.base representing the base fuel rate to calculating new
fuel setpoint FUEL.sub.sp.
Concurrent with the above-described operations in the torque
control loop, the following operations occur in the temperature
control loop. Signal FET.sub.scan representing the temperature of
the exit gas from the kiln is made available to the control system
and the signal is filtered periodically, for example every 3
minutes to obtain filtered value FFET.sub.n. Signal FFET.sub.n is
compared with the previous filtered value represented by signal
FFET.sub.n.sub.-1 and if the two values differ by a predetermined
amount, an alarm typeout occurs and previous value
FFET.sub.n.sub.-1 is saved and used in lieu of FFET.sub.n.
Otherwise, filtered value FFET.sub.n is compared with the gas
temperature setpoint FET.sub.sp provided by the operator and any
difference between the filtered temperature value and the
temperature setpoint is represented by gas temperature error signal
EFET.sub.n. The temperature controller in the temperature control
loop utilizes the error signal EFET.sub.n to calculate the new exit
gas rate setpoint EXIT.sub.n. If the exit gas rate setpoint
EXIT.sub.n is less than or equal to maximum fan capacity
represented by signal EXIT.sub.max, that it is used directly in a
comparison with another calculated exit gas rate setpoint described
herewith. Should the signal EXIT.sub.n represent a setpoint having
a greater capacity than the fan is capable of providing, the signal
EXIT.sub.n is set equal to the signal EXIT.sub.max and used in the
comparison.
Prior to utilizing the newly calculated feed rate and exit gas rate
setpoint FUEL.sub.sp and EXIT.sub.n respectively, the oxygen
override logic determines the present exit gas rate and calculates
the predicted oxygen content 02.sub.n.sub.+1 of the exit gas based
on new fuel rate and exit gas rate setpoints and on the rate of
feed of raw materials in the kiln. If the predicted oxygen content
is at least equal to a predetermined safe minimum oxygen content,
the new fuel rate and exit gas rate setpoints determined in the
torque controller and temperature control loops are employed to
control the kiln operation. If, however, the predicted oxygen
content is less than the predetermined safe minimum, the new exit
gas rate setpoint OEXIT.sub.n is calculated based on the required
minimum safe oxygen content. If the recalculated exit gas rate
setpoint OEXIT.sub.n is not greater than the capacity of the
EXIT.sub.n and EXIT.sub.max, the recalculated exit gas rate is used
to control kiln operation in conjunction with the new fuel rate
setpoint determined in the torque control loop. If the recalculated
exit gas rate OEXIT.sub.n required an exit gas rate greater than
the capacity EXIT.sub.max of the exit fan, the maximum exit gas
rate setpoint EXIT.sub.max is employed and a new fuel rate setpoint
OFUEL.sub.sp is calculated by the oxygen override logic. The
maximum exit gas rate setpoint EXIT.sub.max and the new fuel rate
setpoint OFUEL.sub.sp as determined by the oxygen override logic
are then employed to control kiln operation.
FIG. 5 is a signal diagram graphically illustrating the operation
of the torque control loop in the control system of FIG. 2. During
the initial two portions, the system is shown as controlling
disturbances within a normal control range. The illustration of
normal the control operation is described in detail in the
aforementioned U.S. patent. At the end of the second period shown
in FIG. 5, the filtered value representing the torque FAMP.sub.n
increases above the setpoint value. As a result, the error signal
EAMP.sub.n goes negative thereby indicating that fuel should be
increased. However, this change in torque is due primarily to a
change in the conditions within the chain section and the signal
CHAMP also increases.
The effect of utilizing signal CHAMP.sub.n on the process can be
understood best by assuming that the percent moisture in the feed,
PMIF, increases to a new value. When the wet feed contacts the
chain section, it causes the torque required to turn the kiln to
increase during the transport time of the new material through the
chain section. This increases in torque is caused because with the
previous conditions in the kiln, a longer drying period initially
result so that the drying zone within the chain section increases.
Simultaneously with the increase in the torque represented by
FAMP.sub.n, the amp error signal EAMP.sub.n decreases to a new
value. Shortly after the new feed material enters the chain
section, it will also affect the feed end temperature FFET.sub.n
which decreases to a new value causing, through the temperature
control loop an increase in the gas rate through the kiln
EXIT.sub.n. Simultaneously with the increase in motor torque, the
signal CHAMP.sub.n representing the calculated effect of the new
material in the chain section on motor torque will increase. Its
increase normally will equal the decrease in the amp error signal
EAMP.sub.n. Therefore, with the feedback signal from the process
model FBAMP.sub.n remaining constant, the signal DELAMP.sub.n and
the fuel setpoint FUEL.sub.sp remains constant. If the effect of
the increased torque in the chain sections were not utilized, the
torque control loop would respond by decreasing the fuel setpoint,
which is exactly the wrong control response. It will also be
obvious from the discussion hereinafter, that under this condition
with a new feed input, the fuel setpoint FUEL.sub.sp will
eventually have to be increased. It is possible, therefore, to
modify the chain section signal CHAMP.sub.n to more than equal in
magnitude the amp error signal EAMP.sub.n and thereby force the
fuel setpoint FUEL.sub.sp to be increased.
During the time the new material passes from the chains to the
burning zone, the motor torque, amp error signal EAMP.sub.n, and
chain section signal CHAMP.sub.n will decrease slightly as a result
of the action of changing the exit rate EXIT.sub.n and thereby
influencing the feed end temperature FFET.sub.n. However, during
this time, the feedback signal FBAMP.sub.n, the signal DELAMP.sub.n
and the fuel setpoint FUEL.sub.sp remain constant. The new
material, however, will have a lower temperature than the previous
material so as it reaches the burning zone, the motor torque will
decrease substantially. This decrease in motor torque is then
immediately reflected as a decrease in the magnitude of the error
EAMP.sub.n. However, the torque due to the mixture in the chains
will not be altered significantly so the signal DELAMP.sub.n
increases thereby causing the fuel setpoint signal FUEL.sub.sp to
increase to a new value. Immediately after this initial change in
the fuel setpoint, the motor torque will resume a decrease at
approximately the rate caused prior to the fuel setpoint change
along with the amp error signal EAMP.sub.n and the chain section
signal CHAMP.sub.n so that the fuel setpoint signal FUEL.sub.sp
still remains constant at the new value. At some point after the
fuel setpoint is changed, the feed end temperature FET.sub.n will
reach the setpoint and the exit gas rate EXIT.sub.n will stabilize.
Shortly thereafter, the drying through the chain section will also
stabilize and so the chain section signal CHAMP.sub.n will return
to its original value. When the chain section stabilizes, the
torque required to turn the kiln stabilizes as does the error until
the torque loop delay period is completed. When this happens, the
motor torque will increase thereby decreasing the error signal and
the feedback signal FBAMP.sub.n will increase as a result of the
initial increase so that DELAMP.sub.n and the fuel setpoint
FUEL.sub.sp remain constant at their new values. At this point, the
kiln is stabilized and operating at a new set of conditions for the
new moisture content in the feed material.
In the description, the desirability of employing a digital
computer to perform the functions of the torque control apparatus,
temperature control apparatus, override logic and chain section
apparatus has been indicated. In such an implementation of the
method and apparatus of the invention, analog-to-digital and
digital-to-analog converters would be employed to convert analog
signals into digital quantities and digital representation into
analog quantities, as required.
Accordingly, there has been described herein a method and apparatus
for kiln control embodying the instant invention. All the
principles of the invention have now been made clear in the
illustrated embodiment, and there will be immediately obvious to
those skilled in the art many modifications in structure, steps,
arrangement, proportions, elements, materials, and components, used
in the practice of the invention, and otherwise, which are
particularly adapted for specific environments and operating
requirements without departing from those principles. For example,
other measurements might be made in order to determine the
temperature of the feed as it exits the chain section for purposes
of compensating the torque control loop; other parameters or
combinations of parameters described in conjunction with the
temperature computer for calculating the feed temperature as it
leaves the chain section can also be utilized. Certain kilns may be
operated such that the material exiting the chain section is not
completely dry. In such a situation, the same parameters can be
utilized to perform a heat balance. However, the temperature of
feeding exiting the chains would be essentially constant at the
boiling point of water. The variable would be a signal representing
the percent moisture in the exiting feed which would be condition
in the same manner as the feed temperature signal to obtain the
effect on torque of the chain section. It will also be obvious that
the specific implementations of the control system shown in FIGS. 2
and 4 may also be altered without departing from the principles of
the instant invention. The appended claims are therefore intended
to cover and embrace any such modifications, within the limits only
of the true spirit and scope of the invention.
What is claimed as new and desired to be secured by Letters Patent
of the United States is:
* * * * *