US3469828A - Method and apparatus for cement kiln control - Google Patents

Description

PAW/ 550 Sept. 30, 1969 J. w. LANE 3,469,828

METHOD AND APPARATUS FOR CEMENT KILN CONTROL Filed Oct. 30, 1967 5 Sheets-Sheet l k 2% 2 a Q 1 ATTORNEY J. W. LANE Sept. 30, 1969 METHOD AND APPARATUS FOR CEMENT KILN CONTROL Filed Oct. 30, 1967 5 Sheets-Sheet 2 Sept. 30, 1969 J. w. LANE 3,469,828

METHOD AND APPARATUS FOR CEMENT KILN CONTROL Filed Oct. 30. 19s? s Sheets-Sheet Mf /140,? Y

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METHOD AND APPARATUS FOR CEMENT KILN CONTROL Filed Oct. 30, 1967 s Sheets-Sheet J. W. LANE Sept. 30, 1969 METHOD AND APPARATUS FOR CEMENT KILN CONTROL Filed Oct. 30, 1967 5 Sheets-Sheet a United States Patent 3,469,828 METHOD AND APPARATUS FOR CEMENT KILN CONTROL James W. Laue, Canoga Park, Califi, assiguor to General Electric Company, a corporation of New York Filed Oct. 30, 1967, Ser. No. 678,851

Int. Cl. F27b 7/00 US. Cl. 263-32 20 Claims ABSTRACT OF THE DISCLOSURE A method and apparatus for controlling the operation of a rotary cement kiln by controlling the fuel rate set point and the exit gas rate set point. 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. Control of the exit gas rate set point is based upon measurements of gas temperature at a selected intermediate area in 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.

BACKGROUND OF THE INVENTION Field 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 rotary cement kilns to provide stable kiln operation with resulting uniformity of product quality and improved fuel eliiciency.

Description of the prior art Typical rotary kilns employed in the production of portland cement are steel cylinders eight to twenty-five feet in diameter and between one-hundred and sevenhundred feet long. The cylinders are lined with refractory brick and inclined two to three degrees 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 the dry pulverized intermixed material itself are fed into the upper or feed end of the rotary kiln.

During rotation of the kiln, the raw materials move slowly down the kiln at a rate which is a function of the kiln rotational speed and pass through successive zones known as the drying zone, the pre-heating zone, the calcining zone and the clinkering or burning zone. If the raw materials entering the feed end of the kiln are in the form of a slurry, the heat within the kiln evaporates the water, drying the slurry. 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 pres sure 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 dry 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 calciningzone occupies Patented Sept. 30, 1969 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 counterflowing 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 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. Modern kilns do not depend upon visual observation of the burning zone by the operator but rather employ elaborate instrumentation to sense various parameters during kiln operation. The operator, therefore, has more information of higher accuracy at his disposal in determining proper control action. However, the results obtained are still a function of the operators interpretation of the measure ments and his judgment.

Several important characteristics of rotary cement kilns detract from the effectiveness of operator control:

(1) The product of a cement kiln is a solid of complex chemical-physical composition for which on-line analyzers are not as yet available. In addition, the conditions in the burning zone do not permit direct measurement of the burning zone temperature by means of thermocouples. Radiation pyrometers are often used to obtain burning zone temperature measurements. However, the interpretation of radiation pyrometer measurements is complex and such measurements are consequently not always reliable indicators of burning zone temperature. Consequently, there is a lack of process measurements in the most critical area of the kiln.

(2) The residence time of the solid materials in the kiln is a matter of several hours and the quality of the clinker product is influenced by all the control actions which have taken place during its progression through the kiln. An operator cannot mentally keep track of all of the disturbances and control actions which have influenced an increment of feed material entering the burniCe ing zone for clinkering and therefore essentially only takes control action after past actions have influenced the burning zone conditions, as determined by his observations and the measurements available.

(3) The chemical process which occurs in a rotary cement kiln is sensitive and has inherent stability problems. A control action taken by an operator has various reactions in different sections of the kiln which will ultimately affect the chemical reactions at different times.

In essence, operator control actions are attempts to control the heat input into the kiln such that the calcining reaction ceases and the clinkering reaction commences at the correct point so that the burning zone will be the proper length to permit proper formation of clinkers. If the burning zone occurs too close to the discharge end of the kiln, clinkering will be incomplete with resulting depreciation in the quality of the cement. If the burning zone is too long, the temperature of the burning zone may increase to a degree where the refractory brick lining of the kiln is damaged, requiring kiln shutdown and repair. In addition, the fuel consumption per unit quantity of product increases, decreasing kiln efiiciency.

Several control arrangements for rotary cement kilns have been proposed and some successfully employed which control the rate of fuel flow to the kiln based upon measurements of the temperature within the burning zone by means of radiation or two-color pyrometers. This type of control has been successfully used in rotary kilns which employ natural gas as a fuel, since natural gas burns with a clean flame and the flame temperature is not readily influenced by the environment within the kiln. However, only limited success has been attained by this type of control arrangement when employed in kilns utilizing oil or coal for fuel. Coal and oil flames differ radically from the gas flame, being produced by particles of burning fuel which have a much slower burning rate than gas. In addition, coal and oil flames are substantially influenced by the kiln environment, since heat transferred from the environment to the flame sustains the flame combustion. Further, oil and coal fired kilns tend to be more dusty, interfering with burning zone temperature measurements. Therefore, the visibility of the burning zone in oil or coal firing kilns is poor in comparison to gas fired kilns, rendering diflicult the accurate measurement of burning zone temperature by means of pyrometers. Even in gas fired kilns, the burning zone temperature measurements are in some instances not valid because of interference of alkali and sulphur vapors with the pyrometer measurements. Thus, temperature measurements as indicators of burning zone conditions are of relatively poor reliability.

In at least one kiln control arrangement, temperature measurements have been supplemented by measurements of the torque generated by the drive motor to rotate the kiln, the latter parameter being employed as an aid in interpreting th temperature measurements. However, this type of control system is still subject to uncertainties because of the basically unreliable nature of pyrometer temperature measurements in the kiln. In addition, control of kiln operation is still subject to operator interpretation of the temperature and torque measurements. Accordingly, it is desirable to provide a kiln control arrangement for more accurately and reliably controlling rotary cement kiln operation.

It is therefore an object of this invention to provide an improved arrangement for controlling the operation of a rotary kiln.

It is another object of this invention to provide a method and apparatus for controlling rotary cement kiln operation to produce a kiln product of uniform quality.

It is another object of this invention to provide control apparatus and a control method which eliminates cycling in a rotary cement kiln.

It is a further object of this invention to provide a method and apparatus for controlling the operation of a rotary cement kiln independent of measurements of burning zone temperature within the kiln.

It is a further object of this invention to provide a method and apparatus for controlling the operation of a rotary cement kiln which is based primarily upon the torque required to rotate the kiln.

SUMMARY In accordance with one aspect of this invention, cement kiln control is provided by generating a kiln drive motor torque signal. This signal is then used as a variable to control the heat rate to the kiln so as to maintain the torch at a constant value.

In accordance with other aspects of this invention, temperature control apparatus responsive to the temperature of the kiln exit gases is also provided for controlling the exit gas rate to maintain the exit gas temperature at the feed end of the kiln substantially constant. Override logic is provided to check the oxygen content of the exit gases and to maintain the oxygen content at or above a minimum safe level, overriding the control functions of the torque and temperature control apparatus as required.

BRIEF DESCRIPTION OF THE DRAWINGS The subject matter of the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation may best be understood by reference to the following description taken in connection with the accompanying drawings, in which:

FIGURE 1 is a schematic diagram illustrating in longitudinal cross section a rotary cement kiln embodying and utilizing the present invention;

FIGURE 2 is a block diagram illustrating the control system of the invention employed to control the operation of the rotary cement kiln of FIGURE 1;

FIGURE 3 is a block diagram illustrating the organization of the process model in the control system of the invention;

FIGURE 4 is a flow diagram illustrating the operation of the control system of the invention; and

FIGURE 5 is a signal diagram illustrating the operation of the control system of the invention in controlling rotary cement kiln operation.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGURE 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 SiO Fe O MgCO and CaCO plus small amounts of K 0, Na O and sulphur. The blended raw materials or feed may be in the form of a dry powder or a slurry and may be preheated in a heat exchanger utilizing the kiln exit gases. Kiln 10, inclined downward at an angle of approximately 3 degrees 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 or feed to slowly cascade forward, the rate of forward progress of the feed within kiln 10 being approximately proportional to kiln rotational speed. Motor 201 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 is lined with a refractory material (not shown) which is capable of absorbing heat from flame 20 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. The gases emerging from feed end 14 of kiln 10 pass through a series of dust collectors 37 which recover the dust for reintroduction into kiln 10 and also pass through exit gas damper 38.

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. If the feed is a slurry, the slurry is heated to the point where the water is boiled off, the Water vapor joining the other exit gas products. The temperature of the dry feed increases until the calcining temperature is reached. At this point, the calcium carbonate CaCO and the magnesium carbonate MgCO begin to decompose, forming CaO and MgO. The released carbon dioxide CO 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 on C S, C A and C AF 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 of the kiln. While at this temperature, most of the remaining CaO combines with the C 8 to form C 8. 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. 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 sensors 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 FIGURE 1, hopper 11 has a feed rate detector 50 associated therewith which provides a signal to control systern 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 near feed end 14 of the kiln 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 also 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 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. Control system 51 utilizes the information concerning kiln operation provided on lines 52, 54, 56, 58 and 61 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.

FIGURE 2 illustrates the details of control system 51 shown in FIGURE 1. Referring to FIGURE 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 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 14), with constant field and supply voltages. For purposes of this description, motor 20 is assumed to be a DC motor and the signal on line 61 representing the armature current in and the torque developed by motor 20 is termed AMP Sensor 60 therefore comprises an instrument capable of meastiring and providing an output signal proportional to the armature current 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 is a very 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 indicates changing heat conditions within the kiln which are causing the burning zone to lengthen or shorten. Slow changes in 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 therefore represents the instantaneous heat state within the kiln and changes in the value of AMP indicate corresponding changes in the condition of the burning zone. In accordance with the invention, the torque measurement represented by the signal AMP is employed to effect control of kiln operation independent of actual burning zone temperature measurements.

Filtering and smoothing of the AMP 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 St). The output signal of filter 80 is FAMP The filtering action of filter 80 is described in the equation:

Where:

FAMP is the new filtered value, FAMP is the last filtered value, AMP is the present scan value, and K is the filter constant.

The function of filter 80 may conveniently be performed in a digital computer with K FAMP and FAMP being stored in the computer memory. This calculation, in the illustrated embodiment of the control system of the invention, is performed at short intervals, for example every 5 seconds, to insure that signal FAMP represents the current condition of the burning zone, forming an accurate basis for control action. K 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 of filter 80 is applied to check logic 81. Check logic 81 compares the present output signal FAMP of filter 80 with the previous output signal FAMP,, If the present and previous filtered values difier by more than a given amount, it is assumed that some unusual conditions exist within the kiln and output signal FAMP of filter 80 is not used until it returns to within a reasonable range of FAMP,, 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.

Output signal FAMP of filter 80, if within the required range, is applied "to summing amplifier 82. A kiln amp setpoint signal AMP is also applied to summing amplifier 82. The kiln amp setpoint represented by signal AMP 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 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 well known in the art and provides an amp error signal EAMP proportional to the difference between the kiln amp setpoint AMP and the present filtered value of kiln amps represented by signal FAMP as expressed by the equation:

EAMP :AMP. FAMP Amp error signal EAR/1P 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 provided by summing amplifier 82 could be used directly to control the fuel rate setpoint. However, as previously described, the process in the kiln reacts very slowly to a control action 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 in the order of 30 minutes. Because of these characteristics, an analog controller cannot adequately perform the control function.

Effective control of kiln operation is accomplished, in accordance with the invention, by employing a dynamic process model. The process model, identified by reference numeral 83 in FIGURE 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 five 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 five minutes between control value calculations, the fourth storage position down the table will contain the control value calculated twenty minutes earlier. The 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. The delay table of the process model comprises a sufiicient 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 Feedback signal FBAMP is applied to suming amplifier 84 along With amp error signal EAMP from summing amplifier 82. Output signal DELAMP of summing amplifier 84 constitutes a control value and represents the sum of feedback signal FBAMP and amp error signal EAMP, as expressed by the equation:

DELAMP =EAMP +FBAMP The function of summing amplifier 84 may conveniently be performed by a digital computer. Control value signal DELAMP is applied to torque controller 85 which controls the fuel rate setpoint in accordance with the magnitude of DELAMP Signal DELAMP is also applied to process model 83 for storage in the delay table. The delay table of process model 83, assuming a period of five minutes between control value calculations, has stored therein the signals DELAMP DELAMP DELAMP,, DELAMP DELAMP,, where m is a number of minutes equal to or greater than the delay characteristic of the kiln being controlled.

The arithmetic apparatus forming a part of process model 83 periodically, e.g. every five minutes, calculates feedback signal FBAMP in accordance with the followm equation:

FBAMP :FBAMP +K (DELAMP FBAMP where FBAMP is the present feedback signal generated by the process model,

9 FBAMP is the last feedback signal generated by the process model, K is the process model feedback constant, and DELAl\ ll is a selected control value stored in the process model table (x=delay time of kiln in minutes).

Feedback constant K may, for example, have a value in the range 0.3 to 0.5. The signal DELAMP 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,, is used by the process model to calculate feedback signal FBAMP Feedback signal 'FBAMPH may be calculated by the arithmetic apparatus of process model 33 and control signal DEL HAP generated at any desired interval, for example every five minutes. After calculation of feedback signal FBAMP the resulting value of control signal DELAMP 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 may conveniently be accomplished in a digital computer. The table of process model 83 may, for example, comprise a selected series of memory locations 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 FIGURE 3. Feedback constant K and the previously calculated feedback signal FBAMP,, 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 in the table and utilizes the contents of the table and the stored values of Kfb and FBAMP to calculate feedback signal FBAMP Output signal DELAMP of summing amplifier 84 is applied to torque controller 85 along with signal FUEL from filter 86 representing the filtered rate of fuel flow to mixing chamber 27 and, thus, the rate of heat input to kiln lit] at the time control of the kiln by control system 51 is commenced. Thereafter FUEL remains constant. Filter 86 receives on line 58 signal FUEL representing the output of fuel rate sensor 59 and filters and smooths signal FUEL in accordance with the following equation:

Where:

FFUEL is the new filtered value, FFUEL is the last filtered value,

K is the filter constant, and

FUEL is the present output of sensor 59.

At the time automatic control of the kiln by control system 51 is initiated, filtering of FUEL in filter 86 is terminated and the value of .FFUEL at that time becomes FUEL which thereafter remains constant. The function of filter 86 may conveniently be performed in a digital computer.

Torque controller 85, in response to signals DELAMP and FUEL generates signal FUEL representing the calculated fuel setpoint required to maintain or reach a stable operating condition in the kiln. Torque controller 85 calculates signal FUEL in accordance with the following equation:

FUEL =FUEL +K DELAMP Where:

FUEL is the calculated fuel rate setpoint, FUEL is the base fuel value represented by output signal FFUEL of filter 86 at the time the kiln is ill placed under control of control system 51 and thereafter remains constant at this base value, and K is the fuel/kiln amp proportionality constant.

Proportionality constant K is a function of the characteristics of the kiln being controlled and may be in the order of 0.1. Torque controller as thus responds to the output of summing amplifier $4 represented by signal DELAMP and to the value of the base fuel rate represented by the signal FUEL, to provide signal FUEL representing the desired fuel setpoint to maintain stable operation of the kiln or to regain stable operation after a disturbance. Fuel setpoint signal FUEL is applied to controller 63 on line 65, as shown in FIG- URE 1. The function of torque controller 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 Ztl to rotate the kiln. For example, a decrease in the value of signal FAMP indicating a shortening of the burning zone, results in an output signal from torque controller 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 FAlr iP Conversely, an increase in the value of signal FAMP indicates a lengthening of a burning zone and the response of torque controller 35, 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 The control arrangement of the invention 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 process model of the invention serves to prevent cycling of the kiln by introducing into the control arrangement the expected future response to each control action taken. 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 the burning zone disturbance. 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, in accordance with the invention, prevents the change in required kiln rotational torque due to the efifects of a control action from again affecting the fuel rate setpoint, thus preventing cycling of the kiln. The process model of the invention 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 torque changes due to kiln disturbances not directly caused by previous control actions serve as a basis for further control action.

Fuel setpoint signal FUEL generated by torque controller 85 is transmitted to controller 63 on line as through logic switch 87, as illustrated in FIGURE 2. Logic switch 87 normally connects the output of torque controller 85 to controller 68 but may serve to interrupt the controlling action of torque controller 85 in response to certain conditions in the kiln, as described hereafter in the specification.

As illustrated in FIGURE 2, a second control loop is provided in the control system of the invention comprising filter 88, check logic 89, summing amplifier 9t temperature controller 91 and logic switch 92. This control loop serves to maintain a relatively constant gas temperature near 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 and of the kiln is thereby decoupled from control actions which vary the rate of fuel fiow and therefore the rate of heat input into the kiln due to control actions initiated in the torque control loop, previously described. Thus, as the fuel rate is increased or decreased to adjust burning zone conditions as refiected 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 drying and/or preheating zones.

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, and 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 previously described. The temperature control loop thus compensates for disturbances and for the effects of control actions taken in the torque control loop so that the effect of these disturbances and control actions do not cause further future disturbances in kiln operation requiring further control actions.

Filter 38 receives on line 54 a signal representing gas temperature at exit end of the kiln, as measured by device 53. The gas temperature information represented by the signal is filtered in filter 3% in accordance with the following equation:

Where: FTIG is the present filtered value,

FTIG is the previous filtered value, 'lIGr is the present measured value, and K is the filter constant.

A typical value for constant K is 0.2 when FTIG is calculated once every minute. Output signal PTlG of filter 83 is applied to check logic 89 where signal F116,, is compared with the previous output signal FTIG of filter 88. If the two signals differ by more than a given amount, it is assumed that the thermocouple has failed and signal FTIG is not used. The functions of filter 88 and check logic may be conveniently performed in a digital computer, with the signals FTiG FTlG and TIG and the constant K being stored in the computer memory.

Filtered gas temperature signal FTlG is applied to summing amplifier along with signal "H6 representing the gas temperature setpoint as determined by the operator. Gas temperature error signal ETIG produced by surnmirn amplifier 9i) is a function of signals FTlG and TlG as expressed in the following equation:

ETZG ITIG -FTIG,

and is applied to gas temperature controller Bl. The function of summing amplifier 9% 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 temperal2 ture controller 91 is represented by the equation for output signal EXIT of controller 91, as follows:

Where: EXIT is the desired exit gas flow rate,

EXlT is the previous exit gas flow rate, ETlG is the present temperature error signal, ETlG is the previous temperature error signal,

and K and K are controller constants.

Typical values of constants K and K are 0.11 and 0.i0 respectively and EXIT may be calculated, for example, every five minutes. Output signal EXIT of gas temperature controller @1 is applied to line 66 for application to contnoller 69 through logic switch 92. In the illustrated embodiment, signal EX lT 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, but may also serve to interrupt the connection, as subsequently described in the specification.

The function of gas temperature controller 91 may be conveniently performed in a digital computer with signals EXIT EXIT ETIG and ETIG and constants K and K 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%, 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 sytsem, as shown in FIGURE 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 sufliciently 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 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 86 and 92 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. 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:

[29.354 FUELsp-1+ 15.91 FEED,,]

Where:

The predicted oxygen content of the exit gas based on the new fuel rate FUEL determined by the torque control loop can then be calculated as follows:

29.354 FUE sp+15-91 FEED 02, -21[1 EXIT Where:

FUEL is the new required fuel rate determined by torque controller 85 in the torque control loop,

FEED is the present feed rate of raw materials into the kiln,

EXIT is the new required exit gas rate determined by temperature controller 91 in the gas temperature control loop, and

02 is the predicted oxygen content.

If the predicted oxygen content 02 is less than the safe minimum, a new overriding exit gas rate OEXIT is calculated as follows:

OEXIT WW Where 02 is the safe minimum oxygen content of the exit gas.

If the recalculated exit gas rate OEXIT is greater than the capacity of fan 31, then a new overriding fuel rate OFUEL which will be less than FUEL determined by torque controller 85 must be calculated using maximum capacity of fan 31 or EXIT as follows:

min. 21

This recalculated fuel rate OFUEL 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 as calculated by oxygen override logic 95, or a signal representing the maximum exit gas rate EXIT if required, is applied to logic switch 92 and takes precedence over the exit gas rate EXIT determined by gas temperature controller 91. Similarly, if a new fuel setpoint OFUEL is calculated by oxygen override logic 95,

EXrT,,,, (1- 15.91 FEED a signal representing this new fuel setpoint is applied to logic switch 87 and takes precedence over the fuel rate setpoint FUEL determined by torque controller 85. The functions of oxygen override logic may be conveniently performed in a digital computer, with the computer memory being employed to store the signals required for the computations.

FIGURE 4 illustrates a flow chart of the operation of the control system of the invention. Referring to FIG- URE 4, signal AMP representing the armature current of kiln drive motor 20 and therefore the torque developed by motor 20 is made continuously avail-able to the torque control loop. The signal is filtered periodically, e.g. every five seconds, to obtain a filtered value FIAMP The signal FAMP is compared to the previous filtered value FAMP,, and if the two values differ by more than a predetermined amount, the previous value FAMP is saved and used in lieu of FAMP Otherwise, the filtered value FAMP is compared to the kiln amp setpoint AMP set by the operator and an error signal EAMP is generated representing the dilference between FAMP and AMP Feedback signal FBAMP provided by the process model and amp error signal EAMP are employed to generate signal DELAMP The torque controller of the torque control loop utilizes signal DELAMP and signal FUEL, representing base fuel rate to calculate a new fuel setpoint FUEL Concurrent with the above-described operations in the torque control loop, the following operations occur in the temperature control loop, as illustrated in FIGURE 4. Signal TIG on line 54 representing the temperature of the exit gases in the kiln is made available to the control system and the signal is filtered periodically, for example every three minutes, to obtain filtered value FTlG Signal FTIG is compared with the previous filtered value represented by signal FTIG and if the two values differ by a predetermined amount, an alarm typeout occurs and previous value FTIG is saved and used in lieu of FTIG Otherwise, filtered value FTIG is compared with the gas temperature setpoint TIG provided by the operator and any difference between the filtered temperature value and the temperature setpoint is represented by gas temperature error signal ETIG The temperature controller in the temperature control loop utilizes error signal ETIG to calculate the new exit gas rate setpoint EXIT Prior to utilizing the newly calculated fuel rate and exit gas rate setpoints FUEL and EXIT respectively, the oxygen override logic determines the present exit gas rate EN and calculates the predicted oxygen content 02 of the exit gas based on the new fuel rate and exit gas rate setpoints and on the rate of feed of raw materials into 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 control 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 is calculated based on the required minimum safe oxygen content. If the recalculated exit gas rate setpoint OEXIT is not greater than the capacity of the exit fan EXIT 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 calculated gas exit rate setpoint OEXIT reguires an exit gas rate greater than the capacity EXIT of the exit fan, the maximum exit gas rate setpoint EXIT is employed and a new fuel rate setpoint OFUEL is calculated by the oxygen override logic. The maximum exit gas rate setpoint EXIT and the new fuel rate setpoint OFUEL as determined by the oxygen override logic are then employed to control kiln operation.

FIGURE 5 is a signal diagram graphically illustrating the operation of the torque control loop in the control system of the invention in maintaining stable kiln operation. Referring to FIGURE 5, assuming a decrease in signal FAMP indicating a reduction in the torque developed by kiln drive motor 20 and thus shortening of the burning zone, amp error signal EAMP increases accordingly. Assuming that this variation in signal FAMP is not due to a previous control action, feedback signal ".FBAMP generated by the process model remains unchanged. Signal DELAMP applied to torque controller 85 thus increases in proportion to amp error signal EAMP Torque controller 85, in response to signal DELAMP increases the fuel rate setpoint, represented by signal FUEL to increase the rate of heat input to the kiln to compensate for the shortening of the burning zone.

Assuming that the time required for the kiln to react to a control action in thirty minutes, the value DELAMPIHO stored in the table of the process model is utilized in calculating output signal FBAMP of the process model. Thirty minutes after the control action, i.e. the change in signal FUEL in response to the change in signal FAMP the increase in the rate of heat input to the kiln is reflected in the lengthening of the burning zone with a corresponding increase in the torque developed by kiln drive motor 20, as represented by signal FAMP As the value of signal FAMP approaches the kiln tam-p setpoint AMP amp error signal EAMP falls to zero. However, output signal FBAMP of process model 83 increases due to signal DELAMP stored in the process model, as illustrated in FIGURE 5. The dashed portion of the curve of signal FBAMP illustrate the effect of coefficient K on curve shape. Because the decrease in amp error signal EAMP is offset by the increase in output signal FBAMP of process model 83, signal DELAMIP applied to torque control 85 remains constant, maintaining the fuel rate setpoint at the new value. The curves of FIGURE 5 thus illustrate the effect of the process model of the invention in maintaining kiln stability and in preventing kiln cycling.

Assuming that filtered amp signal FAMP subsequently decreases, as shown in the middle portion of FIGURE 5, the torque control loop again reacts by increasing fuel setpoint FUEL further increasing the rate of heat input into the kiln. Process model 83, remembering the expected response to the control action, viz. an increase in torque developed by drive motor 20 approximately thirty minutes after the control action, maintains fuel setpoint FUEL at its new value.

The right-hand portion of FIGURE 5 illustrates the response of the torque control loop to an increase in signal FAMP due to a kiln disturbance, indicating lengthening of the burning zone. In response to the increase in signal FAMP beyond the kiln amp setpoint, amp error signal EAMP decreases causing signal DELAMP to decrease with a resulting decrease in the fuel setpoint FUEL :as calculated by torque controller 85. The rate of heat input into the kiln is reduced accordingly. After a time period equal to the process delay has passed, the effect of the reduction in the rate of heat input to the kiln is detected as a decrease in signal FAMP indicating a shortening of the burning zone. Amp error signal EAMP returns to zero as signal FAMP approaches the kiln amp setpoint AMP Output signal FBAMP of process model 83 reflects the anticipated decrease in signal FAMP and maintains input signal DELAMP to torque controller 85 constant so that the fuel setpoint FUEL remains at the new value determined when the disturbance was detected. FIGURE 5 therefore illustrates the function of process model 83 in decoupling and isolating fuel rate setpoint control from the expected effects of past control actions.

In the description, the desirability of employing a digital computer to perform the functions of the torque control apparatus, temperature control apparatus and the override logic has been indicated. In such an implementation of the method and apparatus of the invention, analogto-digital and digital-to-analog converters would be employed to convert analog signals into digital quantities and digital representations 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.

I claim:

1. In a rotary cement kiln control system, the combination comprising: drive means for rotating the kiln, sensing means responsive to the torque developed by said drive means to rotate the kiln for providing a corresponding output, and control means directly and primarily responsive to variations in the output of said sensing means for varying the rate of heat input into the kiln so as to maintain relatively constant the torque developed by said drive means to rotate the kiln.

2. In a rotary cement kiln control system, the combination comprising: drive means for rotating the kiln, sensing means responsive to the torque developed by said drive means to rotate the kiln for providing a corresponding output, and control means responsive to variations in the output of said sensing means not directly due to previous control actions initiated by said control means for varying the rate of heat input into the kiln to maintain relatively constant the torque developed by said drive means to rotate the kiln.

3. In a rotary cement kiln system having drive means for rotating the kiln at a predetermined rate, feed means for supplying raw materials to the kiln and a high energy heat source for heating the materials in the kiln, a control system comprising: sensing means responsive to the torque developed by said drive means to rotate the kiln for providing a corresponding output, and control means directly and primarily responsive to variations in the output of said sensing means not directly due to previous control actions initiated by said control means for varying the rate of heat input from the heat source into the kiln to maintain relatively stable conditions in the kiln and a relatively constant torque developed by said drive means to rotate the kiln.

4. In a rotary cement kiln system having drive means for rotating the kiln, feed means for supplying raw materials to the kiln and a high energy heat source for heating the materials in the kiln, a control system comprising: sensing means responsive to the torque developed by said drive means to rotate the kiln for providing a corresponding output, and control means for receiving the output of said sensing means and responsive to variations in the output of said sensing means for varying the rate of heat input to the kiln so as to maintain relatively constant the torque developed by said drive means to rotate the kiln, said control means including means for storing information concerning past control actions initiated by said control means for inhibiting variations of heat input to the kilnby said control means in response to variations in the torque developed by said drive means directly caused by such past control actions.

5. The control system of claim 4 including temperature control means responsive to variations in the gas temperature at a predetermined area within the kiln for controlling the rate of flow of gases through the kiln to maintain the gas temperature at the predetermined area substantially constant.

6. The control system of claim which further includes override logic means responsive to the oxygen content of the gases flowing through the kiln for adjusting the rate of flow of gases through the kiln and the rate of heat input to the kiln from the high energy heat source as required independent of the gas temperature and the torque developed by said drive means to maintain the oxygen content of the gases above a predetermined level.

7. In a rotary cement kiln system having drive means for rotating the kiln, feed means for supplying raw materials to the kiln and a high energy heat source for heating the materials in the kiln, a control system comprising: sensing means responsive to the torque developed by said drive means to rotate the kiln for providing a corresponding output, control means responsive to variations in the output of said sensing means from a setpoint for providing a control signal, said control means including means for storing control signals previously produced by said control means and responsive to a selected one of said stored control signals for generating a feedback signal representing an expected variation in the output of said sensing means due to a previous control signal produced by said control means, said control means including further means responsive to the feedback signal for preventing variations in the control signal produced by said control means in response to variations in the output of said sensing means due to previous control signals produced by said control means, and means responsive to variations in the control signal produced by said control means for varying the rate of heat input to the kiln from said high energy heat source so as to maintain relatively constant the conditions Within the kiln and the torque developed by the drive means to rotate the kiln.

8. In a rotary cement kiln system having a drive means for rotating the kiln at a predetermined rate, feed means for supplying raw materials to the kiln and a high energy heat source for heating the materials in the kiln, a control system comprising: sensing means responsive to the torque developed by said drive means to rotate the kiln for providing a corresponding out-put, control means for receiving the output of said sensing means and for generating a control signal, said control means including a process model having storage means for storing past control signals generated by said control means and arithmetic means for generating a feedback signal in response to a selected one of the past control signals stored in said storage means, said control means further including means responsive to the feedback signal generated by said process model and to variations in the output of said sensing means from a selected set-point for causing the control signal generated by said control means to vary only in response to variations in the output of said sensing means which are not directly due to previous variations in said control signal, and means responsive to variations in the control signal generated by said control means for varying the rate of heat input to the kiln from said high energy source means, whereby the conditions in the burning zone of the kiln and the torque developed by the drive means are maintained relatively constant.

9. In a rotary cement kiln system having drive means for rotating the kiln, feed means for supplying raw materials to the kiln and a high energy heat source for heating the materials in the kiln, a control system comprising: sensing means responsive to the torque developed by said drive means to rotate the kiln for providing a corresponding output, filter means for filtering the output of said sensing means, means for providin a setpoint value, comparison means for comparing the output of said filter means to said set-point value to generate an error signal proportional to the difference between the output of said filter means and said setpoint value, control signal generating means responsive to the output of said comparison means and to a feedback signal for providing an output signal equal to the sum of the feedback signal and the error signal generated by said comparison means, process model means including storage means for periodically storing successive output signals of said control signal generating means and including arithmetic means responsive to a selected signal stored in said storage means for periodically generating a feedback signal for application to said control signal generating means which is proportional to the expected variation in the output of said filter means from the setpoint value due to the output signal of said control signal generating means represented by the selected signal in said storage means, and means responsive to the output signal of said control signal generating means for controlling the heat input to the kiln from the high energy heat source whereby the kiln burning zone conditions and the torque developed by said drive means are maintained substantially stable.

10. The control system of claim 9 which includes temperature control means responsive to variation in the gas temperature in a predetermined area within the kiln from a selected setpoint for controlling the rate of flow of gases through the kiln to maintain the gas temperature in the predetermined area substantially constant.

11. The control system of claim 10 which further includes override logic means responsive to the oxygen content of the gases flowing through the kiln for adjusting the rate of flow of gases through the kiln and the rate of heat input to the kiln from the high energy heat source as required independent of the gas temperature and the torque developed by said drive means to maintain the oxygen content of the gases above a predetermined level.

12. A method of controlling the operation of a rotary cement kiln having a drive motor for rotating the kiln, feed means for supplying raw materials to the kiln and a heat source for heating the materials in the kiln, comprising the steps of: measuring the torque developed by the drive motor to rotate the kiln, and controlling the rate of heat input from the heat source to the kiln in accordance with variations in the measured torque which were not due to earlier adjustments in the rate of heat input from the heat source to the kiln.

13. A method of controlling the operation of a rotary cement kiln having a drive motor for rotating the kiln, feed means for supplying raw materials to the kiln and a heat source for heating the materials in the kiln, comprising the steps of: measuring the torque developed by the drive motor to rotate the kiln, compensating for variations in measured torque directly caused by earlier adjustments in the rate of heat input from the heat source to the kiln, and controlling the rate of heat input from the heat source to the kiln in accordance with the compensated torque measurements.

14. A method for controlling the operation of a rotary cement kiln having drive means for rotating the kiln at a predetermined rate, feed means for supplying raw materials to the kiln and a high energy heat source for heating the materials in the kiln, comprising the steps of: measuring the torque developed by the drive motor to rotate the kiln, comparing the measured torque to a predetermined value, compensating the difference between the measured torque and the predetermined value in accordance with previously initiated control actions, and controlling the rate of heat input to the kiln inversely with variations of the compensated value.

15. A method for controlling the operation of a rotary cement kiln having a drive motor for rotating the kiln, feed means for supplying raw materials to the kiln and a heat source for heating the materials in the kiln, com prising the steps of: measuring the torque developed by the drive motor to rotate the kiln, filtering the torque measurements to provide a filtered torque value, comparing the filtered torque value to a preset value, summing the difference between the filtered torque value and the preset value with a feedback value to develop a control value, periodically storing the control values in a table, periodically calculating the feedback value on the basis of a control value stored in the table a predetermined interval earlier, and controlling the rate of heat input from 19 the heat source to the kiln in accordance with variations in the control value.

16. The method of claim 15 in which the predetermined interval is substantially equal to the delay between adjustment of the rate of heat input from the heat source to the kiln and the effect of the adjustment in burning zone conditions as reflected in measured torque.

17. The method of controlling the operation of a rotary kiln into which raw materials are fed and chemically changed through the application of heat generated by the combustion of fuel with air in the kiln as the materials move through the kiln due to rotation of the kiln by a drive motor, comprising the steps of: measuring the torque developed by the drive motor to rotate the kiln and generating a signal proportional to the measured torque, generating a setpoint signal representing a desired torque, comparing the signal representing measured torque to the signal representing desired torque and generating an error signal proportional to the difference, combining the error signal with a feedback signal to develop a control signal, periodically storing the control signal value in a table, periodically generating the feedback signal as a function of a control signal value stored in the table at a predetermined earlier time, and adjusting kiln operation in accordance with its variations of the control signal in a direction tending to maintain a predetermined relationship between the signal representing measured torque and the signal representing desired torque.

18. The method according to claim 17 in which the last-named step of adjusting kiln operation is accomplished by adjusting the rate of fuel combustion in the kiln in accordance with variations of the control signal in a direction tending to maintain the predetermined relationship between the signal representing measured torque and the signal representing desired torque.

19. The method of controlling the operation of a rotary kiln into which raw materials are fed and chemically changed through the application of heat generated by the combustion of fuel with air in the kiln as the materials move through the kiln due to rotation of the kiln by a drive motor, comprising the steps of: measuring the torque developed by the drive motor to rotate the kiln and generating a signal proportional to the measured torque, generating a control signal as a function of the signal representing measured torque and a feedback signal, periodically storing the control signal value in a table, periodically generating the feedback signal as a function of a control signal value stored in the table at a predetermined earlier time, and adjusting kiln operation in accordance with variations of the control signal in a direction tending to maintain the signal representing measured torque at a predetermined value.

20. The method according to claim 19 in which the last-named step of adjusting kiln operation is accomplished by adjusting the rate of fuel combustion in the kiln in accordance with variations of the control signal in direction tending to maintain the signal representing measured torque at a predetermined value.

References Cited UNITED STATES PATENTS 1,945,652 2/1934 Martin 263-32 3,381,946 5/1968 Ross 26332 JOHN J. CAMBY, Primary Examiner

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