ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
otary
tion
nstru
W. E. DIXON, JR. Arnold 0.Beckman, lnc., 1020 Mission Sf., S o u f h Pasadena, Calif.
IK
THE past 1.5 years instrumentation and automatic control have become indispensable to the continuous process industries. Some processes possible in the lahoratoty for yeais became practical in large scale production n ith the development of present-dav methods of automatic control. These developments have provided instruments u hich are being successfullv applied to rotary kilns for the reduction of cost, maintenance of product quality, and increased production per capital dollar. This paper outlines the means of measurement and control of variables related to efficient rotary kiln operation. Because rotary kilns are used for processing so many different materials, any one kiln may have markedly different instrument requirements from any other kiln. For example, a 50-foot kiln designed for expanding perlite will represent much less investment, will operate a t lower temperatures, and has less critical operating limits than a 400-foot kiln designed to burn clinker. In addition, special purpose kilns may have unique problems unrelated to the operation of the kiln itself. These include kilns for drying inflammable materials, expanding aggregate, and decarbonizing catalyst. I n this paper, the emphasis is placed on those variables common to most kilns. !There possible, these variables are discussed separately so that the instrumentation related to each variable can be considered in the light of the requirements for a particular kiln. Interrelationship of Ternperutures, Feed Rate, and Kiln Speed Directly Affect Quality of Product
The processing of materials in a rotary kiln involves subjecting the material to a time-temperature cycle. The temperature, temperature distribution along the kiln, kiln speed, and feed rate must all be in proper relation to ensure high quality and maximum production.
KILN LINING
Figure 1.
High Temperature Fire Zone Measurement
Temperature. On kilns used for calcining anti other high temperature processes, fire-zone temperatures are currently measured with recording radiation pyrometers sighted either on the kiln lining or on the product ( 2 - 4 ) . Figure 1 shows schematically the sighting paths of pyrometers for these two measurements. Records of these measurements reflect differences in kiln lining, product movement, flame encroachment on the sighting paths of the pyrometer, and variations in dusting conditions. As a result, the recording pen wipes a relativelv wide band on the chart and a reading a t any given moment may not be indicative of actual temperature, An average of the band ie, however, indicative of trends in the kiln operation. This is illustrated in
1436
Figure 2 When the pyrometer is used x i t h electronic-type recorders, the band can be narrowed by introducing electrical capacity in the recorder input circuit. This damps the fluctuations and gives a close indication of the temperature trend. Figure 3 s h o w an actual installation of a radiation pyrometer on a cement kiln. The air-purged fitting on the front of the pyrometer ensures against deposits of dust on the lens The pyrometer is suspended from above so that it does not interfere with the operator's Tvork.
AVERAGE READlNO
Figure 2.
Chart of Fire-Zone Temperature
On kilns used for drying and other loser temperature operations, the product discharge temperature is freqnently measured with a thermocouple. When the quantity and consistency are such that the thermocouple is continuously immersed, this measurement is satisfactory. Recently, lower temperature radiation pyrometers have become available and are probably more generally applicable to lower temperature kiln operation. Exhaust Gas Temperature. Exhaust gas temperature measurements have been standard practice for many years. Thie measurement, although not specifically related to a process variable, can be used to indicate variations in fuel rate, deficiency or excess of combustion air, ring formation, or other causes of feed rate variation inside the kiln ( 2 , 4 ) . Variations in exhaust gas temperature generally precede variations a t the firing end. For this reason, corrective action may be indicated by exhaust gas temperature measurements prior to serious deviations. Temperature Distribution. Temperature distribution measurements are not in general practice but have been used primarily in investigat'ions of kiln characteristics and other processes and equipment development work. Methods have been worked out on a few kilns, Figure 4 shows one set of collector rings for taking thermocouple measurements on a rotating kiln. Alternately, a series of thermocouples may be located around the periphery of the kiln, all connected to the same set of collector rings to average the readings at any one location in the kiln (2). I n practice, much maintenance is generally needed on thiv type of equipment. Collector rings become dirty and need cleaning; it is generally difficult to replace thermocouples which receive severe mechanical abrasion and need frequent replacement.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 46,No. 1
ENGINEERINGr DESIGN, AND PROCESS DEVELOPMENT Because of the long time lag from feed to discharge, temperature distribution measurements are significant in maintaining temperature stability. Trends within the kiln can be maintained or corrected to give desired results a t product discharge. On wet-process kilns with chains, a measurement after the load leaves the chains can indicate removal of moisture, thus aiding in the elimination or control of mud rings (4). Time. Another function as important as maintaining proper temperatures is control of the time the product is exposed to these temperatures. These functions are interrelated, of course, because the feed rate and kiln speed will affect the temperatures as well as the retention time. Generally, there are two basic philosophies employed in regulating this interrelationship ( 3 ) . Some operators set the fuel rate and vary the kiln speed to maintain desired firing-zone temperature. Others hold the kiln speed constant and vary the fuel to obtain the desired temperature. I n practice, it becomes necessary to change both variables 60 that, practically, there is little difference between the two approaches. I n addition, it becomes necessary to vary feed rate which further complicates the regulation. Measurements of kiln speed and feed rate are important if kiln stability and maximum production rates are to be maintained. Fortunately, kiln speed and feed rates are relatively simple to measure. Currently, many kilns use digital counters to indicate total revolutions of feed screws and kiln. More modern kilns, however, have recording tachometers which record these variables as rates rather than revolutions. Figure 5 indicates how tachometer generators can be applied to make these meaaurements. The electrical output of these generators is proportional to the speed and can be recorded on standard potentiometers. Since the ratio of feed rate to kiln speed is of importance, this ratio may also be recorded as an operating criteria. The advantage of recording these measurements as rates along with the temperature records is that continuous data are provided on the interrelation of feed rate, kiln speed, temperature distribution, and high fie-zone temperature. Instability of any of these variables will affect the stability of the others with consequent loss of product quality or production. Corrective action to stabilize any one of the variables must take into account its effect on all the other variables. This is extremely difficult if data on the interrelation are not available. Proper Instrumentation Ensures Combustion Efflcieney and lowers Product Cost
Fuel costs are a major expense in operating rotary kilns and as fuel prices continue to rise savings through improved com-
Figure
3.
Radiation Pyrometer Measuring Kiln Lining Temperature Riverside Cement Co., Ore Grande, Calif.
July 1954
COURTESY MINNEAPOLIS-HONEYWELL REQULATOR CO.
Figure 4.
Collector Ring Pickup
Penn-Dixie Cement Co., Kingsport, Tenn.
bustion efficiency are becoming more and more important. -4s indicated in Figure 6, for any given combustion process there is an optimum air-to-fuel ratio for maximum efficiency. If less than the optimum amount of air is supplied, unburned fuel will be wasted. If more than enough air is supplied, heat will be lost in raising the temperature of the unneeded air. I n general, a t the optimum point more air (excess air) is needed than will supply the theoretical requirement of oxygen to completely burn the fuel. The reasons for this lie in the difficulty of obtaining complete mixing of the fuel and air. I n addition, in some combustion processes, the fiame is chilled by the heat-receiving material before complete combustion takes place. I n high temperature rotary kiln operation, particularly cement and lime burning, the kiln represents a voluminous combustion chamber a t a very high temperature so that the fiame is not immediately chilled. Provided the burning equipment gives good mixing, the optimum excess air under these circumstances may be as loiv as 0.5% ouygen, while in a boiler it may be between 3 and 6% oxygen. One company operates a gas fired rotary kiln for burning dolomite a t no exces air and has not detected any carbon mono.uide in the exhaust gases. It is difficult to maintain high combustion efficiency without continuous exhaust gas analysis equipment. Frequently, changing operating conditions, particularly fuel rate and draft, make it impractical to maintain optimum conditions by periodic Orsat measurements. On coal fired kilns continuous oxygen measurement is of particular importance in indicating blockage of coal feeders as well as needed changes in fuel rates as the quality of the coal changes ( 4 ) . Historically, carbon dioxide measurements have been used as a combustion efficiency criterion. Figure 7 shows that the carbon dioxide value for any given excess air factor is dependent upon the fuel being burned. There is also a a i d e band of carbon dioxide values a t a given excess air for any one type of fuel. These considerations, along with the fact that most materials processed in rotary kilns evolve carbon dioxide, make this measurement impractical for the great majority of installations. Figure 7 also shows that, in the range of interest, oxygen is directly proportional to excess air for all fuels. It is true that variations in carbon dioxide released by the product will have second-order effects on the oxygen percentage, These variations are insignificant, however,
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1437
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT in the range of oxygen percentages in the exhaust gases from rotary kilns. It has been demonstrated that combustion efficiency is improved by making continuous oxygen measurement. One report ( 1 ) shows a drop in fuel requirement from 140 to 120 pounds of coal per barrel of clinker during 1 year by the use of an ovygen analyzer.
Figure
5. Control of Feed Rate and Kiln Speed
High dust loading in the exhaust gases presents a major problem in obtaining a continuous gas sample from a rotary kiln. There are a number of sampling systems in use and, although there are mixed reports, it is generally conceded t,hat these require a significant amount of maintenance. The savings n-hich accrue with their use, however, justify the costs involved. In the author's opinion, much of this maintenance can be traced to the attempt to use equipment designed for a general solution of the problem rather than designed to include variations t o meet particular problems. The following cases are act,ual installations n-hich are examples of this type of improper installation. On 10 bauxite kilns the sample cleaning is accomplished by a primary filter inside the kiln. The bauxite is dry, does not cake or flow, and eventually builds up to an equilibrium \There further deposits on the filter are offset by material sloughing off the filter. These filters have operated for more than 2 years without replacement. The same type of system was tried on a cement, kiln where the feed was primarily steel inill wastes. The dust acted like a hydraulic slag, plugging up the filter in a short time, I n another
the difficulties of transporting a dust- and moisture-laden sample through t,he sample lines. The positive pump gives sufficient head to force accumulations through the line. The final separator acts to protect against forcing solids and liquids through the analyzer as well as to bypass a large portion of the sample 60 that response speed mill not be limited by the flow rate required by the analyzer. This system is used Kith a variet,y of sample tubes which may incorporate such things as primary filters, water sprays, and air blowback. I n order to keep sample time lags to a minimum, the oxygen analyzer is normally mounted at the feed end of the kiln. Most of the available analyzers allow for remote location of the oxygen recorder so that it can be installed a t the firing end where the record can be used in kiln operation. Before much progress is made in obtaining efficiency through the use of this equipment, the operators must have confidence in the significance of the readings. For this reason, it is usually wise to have a periodic check of the instrument operation which can be observed by the operator. One convenient method is t o obtain an instrument with an air check range in addit'ion to the combustion range. With a remote operated valve in the sampling system, the operator can introduce air and thus verify the reliability of the analysis equipment.. Because the oxygen percentages are low in kiln operation> it is advantageoug t o obtain an instruinelit with a narrow scale, say 0 to 5 % oxygen, rrhich is accurate in the range of 0 to 1% oxygen.
m
'
i / '
0
E
9 I d 20 k?
0,
EXCESS
E?O
$$ IO
s3 s
V
0
-20 -10 0
IO
20 30 40 50 60 70 80
EXCESS AIR, %
Figure 6.
Effect of Air-to-Fuel Ratio on Operating Efficiency
cabe, on a wet-process cement kiln, the sample 11-aspulled through a water spray a t the mouth of the sample tube and the system operated with low maintenance. When the sample system was applied to a dry process kiln where the dust loading was higher, the tube could not be kept unplugged for more than a fev hours. One sample cleaning system is shown in Figure 8. The sample is washed immediately after it is removed from the kiln. The condensibles and solids are discharged at this point, thus avoiding
1438
A R-PER
CENT
Figure 7. Variation of Oxygen and Carbon Dioxide in Flue Gas with Fuel and Excess Air
Kiln Draft. Draft conditions have direct influence on flame configuration, location, stability, and luminosity, as ~vcllas total air volume through the kiln. In turn, flame configuration, location, and luminosity have direct bearing on the tempera ture and temperature distribution. These variables relate directly t o quality and quantity of product. In addition, the draft determines the total air volume and thus relate? directly to combustion efficiency. Indications are that flame stability, which is dependent on stable draft conditions, influences ring formation. Flames of varying configuration and location tcnd to promote ring build-up in the kiln. Draft measurement and control are, therefore, extremely important to efficient kiln operation. Kiln pressure (draft) is relatively simple t o measure and because it has a short time lag is generally easy t o control. Figure 9 indicates the arrangement of components in a hood pressure control
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 7
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
DOUBLE S & M P L l N O T L B E
P R I M A S " rlLrCR
*
'd
SAMPLE CLEANER
Figure 8.
Combustion Sampling System
system. I t is generally best practice to use a differential measurement at the point of measurement to avoid the influence of varying differentials between the location of the indication or recorder and the draft tap. Location of the pressure measurement is important since it is desired to reflect conditions a t the flame. For control purposes, a measurement a t the firing hood is preferable because it is not influenced by variations in kiln resistance and thus reflects conditions a t the flame. Feed end draft determination is an important secondary measurement to indicate ring formation, but is not generally the best location for control purposes. The draft control should preferably be of the thrott.ling type which makes corrections immediately in proportion to the deviation from the desired setting. I n conjunction with automatic reset, this type of control can be effective regardless of damper charact,eristics and differences in response a t various damper settings. This type of control is also compatible with automatic kiln firing.
difficult because a thermocouple cannot be located where it will not be affected by radiation from the product, refractories, or flame. Shielding the thermocouple or installing it in a recess in the refractories is common practice to avoid direct radiation effect. This also cuts down the convective heat transfer to the thermocouple so that, a t best, a relative measurement is obtained. Pulling a sample of this air over a completely shielded thermocouple may give more reliable results. In one commonly used recuperator the product is moved by reciprocating rakes through which the secondary air is blown. The secondary air temperature is usually controlled by varying the rake speed. This system transports the product a t varying speeds regardless of production rates. .4t times the product builds up on the grate; a t other times it is a thin scattered layer. This results in variations in pressure drop across the bed and consequent varying air volume through the bed. The air temperature changes are, therefore, not directly proportional to rake speed. I n addition, the time required to change the bed depth is appreciable. This combination of factors makes it difficult to stabilize secondary air temperature with this method of control and often leads to cycling conditions in the kiln.
1
1
Secondary Air Temperature Control Is Important Part of Economical Process
Figure 10. Clinker Cooler Control
Recuperation of heat from discharged product adds appreciably to the economy of kiln operation. Most of the larger kilns have recuperators either of the grate or rotary type in which the secondary air contacts the hot product prior to entry into the firing zone of the kiln. The temperature a t which this air enters the kiln affects the flame temperature and temperature distribution and, consequently product quality and quantity a t any one fuel rate. Therefore, it is generally desirable to maintain this temperature as high as possible in order to maintain high production rates. Some regulation or limiting of this temperature is needed, however, because of the interrelations of this variable with the major operating variables of the kiln. If this air temperature is not adequately regulated, cycling conditions are likely to result. Measurement of the secondary air temperature is extremely
Figure 10 illustrates another system which has been used TTith generally good results. The rake speed is set a t its maximum and secondary air temperature is controlled by operating a damper in the supply blower duct. Because of the short and constant time lags, the control is readily adjusted to give stable conditions. Maintenance of a neutral or slightly negative pressure above the rakes will protect workmen cleaning product discharge grates from blasts of excessively hot gases. In addition, this tends to hold a constant secondary air volume and thus has a stabilizing effect on other kiln operations. h draft controller operating a LINING TEMPERATURE
FUEL RATE CONTROL
-- I
-----
1
1 Figure 9.
July 1954
Draft Control
AIR
I
Figure 1 1.
Time-Temperature Function Control
INDUSTRIAL AND ENGINEERING CHEMISTRY
1439
ENGINEERING. DESIGN. AND PROCESS DEVELOPMENT damper in the recuperator exhaust stack accomplishes this ,Kith ease. Special Instrumentation. The instrumeiit,ation TThich has been discussed is generally applicable to kiln operation arid is more or less independent of local conditions such as type of fuel, burner, and auxiliaries such as dust collection equipment. There are a number of important points of measurement and control lvhich vary with individual plant equipment. Fuel and primary air flow recording are important not only for accounting purposes but also as an aid to kiln operation. These records are important as an indication of proper burner functioning, and for operating purposes they can be correlated with kiln responses to prior fuel rate changes. The actual measurement may iitiiize orifice plates and Venturis for air, gas, and oil flow, a r e a - t j p meters for oil flow, and wind box differentials or primary air flow measurements for coal feed rate. On oil fired kilns, fuel temperature control is of importance to proper burner operation. Oil pressure measurement at the burner can be used to indicate coking or other burner obstruction. T o protect against costly damage to dust collection and elcctrostatic precipitation equipment,, temperature control of gases entering this equipment is important, This regulation can be accomplished Tithi a temperature cont,roller operating teniperiiig dampers, A second controller of the on-off r!-pe with manual reset can be used t o protect against malfunction of the primary controller.
which sets the control point of a draft controller on the hood pressure. 4. Control of the secocclary air temperature by one of the systems described.
A simplification of thia system would be to set manually the kiln speed and feed rate while automatically controlling the kiln lining t'emperature through automatic regulation of fuel rate (3). It is felt that the manual control of kiln speed and feed rate would be facilitated if the lining temperature \vas fixed automatically. Although necessary control equipment varies with the type of fuel, firing equipment, and kiln drive, this equipment is available. Some refinements in the application 1%-ill enhance the stability and accuracy of coiitrol. The temperature measurements should be damped to minimize the spurious effects of variation in kiln lining, product movement', stingouts, and variations in dusting conditions. T o further avoid upsets by spurious signals, wide range thrott,ling type control should be used. This type of control will give only small corrections for deviations from thc control point. In conjunction x i t h automatic reset, it can be turned t o control from the trend rather than from immediate measurement. Further stability can be obtained by ha.ving the primary temperature controls sct the indexes of a f l o ~controller on the fuel and a tachometer controller on the kiln speed. This is illustrated in Figure 11. This type of control has been found desirable in the petroleum and chemical fields where long time lags are involved betmen corrective action and sensing of the result of corrective action. I n l d n operations, for instance, a fuel rate controller on oil will maintain fuel flov as set by the temperature Automatic Operation Offers controller regardless of variations in burner resistance. If the M a x i m u m Efficiency and Production temperature controller Fere to vary fuel rate by directly operating a. valve, variations in fuel rate due to coking n-ould not be deI n most of this discussion, it, as assumed that variabli-1: n-ould tected until the temperature trend had changed some t,inie later. be recorded for use by the operator in manual adjustment of the Thus, stability can be maintained and a t the same time corrective kiln. Moat of the variables are interrelated and variations in action can be taken in synchronism with the process responses. any one directly affect others or make it necessary to adjust other The objective of this control system is to maintain maximum provariables to remain rvithin the operating limits of the equipment. duction rate and, at t'he same time, high product qua1it)y and liih These variables include high fire-zone temperature, product tern.&ability. perature, kiln speed, feed rate, secondary air temperature! fuel Indication of combustion efficiency through oxygen measurerate, draft, and excess air. This is an imposing array of variables ment has been described. Direct operation of damper8 to regto follow, Even with long experience, it is extremely dificult for ulate secondary air volume from an oxygen controller would be an operator to predict accurately the effect of a given variation or the results of a given corrective adjustmcnt. T!ic trend t'o possible. Regulation of secondary air volume by a draft controller would be possible. longer kilns makes manual Regulation of secondary air operat,ion even more diffivolume by a draft concult. The constant prestroller. however, directly flure of higher fuel, labor, compeneates f o r a t m o s and equipment costs depheric changes as well as mands more efficient operavarying operating condition and at the same time tions and thus increases the higher production rates. stability of burner operaThe maximum results cantion. T h e preferable not be attained x i t h method, therefore, is to manual regulation, SO the have the oxygen controller rewards which accrue Tvith automatic operation are set the index of the draft increasing. controller to maintain the A suggested method of excess air at the most. automatic control is as efficient value. follows : On coal fired installations, a possible improve1. Control of the kiln m e n t over t h i s system lining temperature near the maximum safe refractory would be to have the oxygen temperature by automaticontroller set the fuel rate cally varying t h e fuel n M e the lining temperarate. ture controller sets the 2. Control of the product temperature by varyindex of the draft controller. ing the kiln speed and feed In this way, inadvertent COURTESY MINNEAPOLIS-HONEYCIELL REOULAiDR C O , rate to give the desired fuel rate changes would be final product temperature. Figure 12. Instrument Cubical corrected within a minute or 3. Control of the excess Penn-Dixie Cement Co., Kingsport, Tenn. two. Inother words, the linair with an oxygen analyzer
1440
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 46,No. T
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT ingtemperaturecontrollerwouldregulatethesecondary air volume. The oxygen controller would regulate the fuel rate to correspond to the most efficient level for the particular secondary air volume. The temperature controller should thus indirectly regulate the fuel rate and the difficulties of constant coal feed would be avoided. A manual system for this purpose has been demonstrated (4).
are not insurmountable. The rewards are becoming greater as costs continue to rise. These rewards, though perhaps small in per cent of cost, are significant when viewed as a per cent of profit. The objectives of high quality a t higher production rates and lower costs can be attained through better instrumentation.
Trend Is to Use of Miniature Instruments and Graphic Panels
Acknowledgment
Present types of instrument installations are shown in Figures 12 and 13. Figure 12 shows a modern cubical which is pressurized to eliminate dust deposits. The main recording instruments can be seen b y the operator through the windows in the front panel. All manual adjustments are convenient to the operator and a log desk is built in for his use. Figure 13 illustrates another modern installation which includes special lighting for the instrument panel. The trend in process instrumentation is toward miniature instruments which are mounted in a diagram of the process. The instruments are located in the diagram a t the point of measurement of their respective variables. These are called graphic panels. Advantages of this type of instrumentation are reported to be ease of training operators, panel space saving, and ease of discerning sources of upsets in the process. The acceptability of this approach in kiln operations has not been proved, although there seems to be some justificat,ion for its use. Although there are difficult problems in application of instrumentation and automatic control to rotary kilns, these problems
to acknowledge on-the-job training in rotary kiln instrumentation
July 1954
I n addition to specific references given, the author would like over the past few years by John R. Green, manager, Steel and Ceramic Division, Brown Instruments Division, MinneapolieHoneywell Regulator Co., Philadelphia, Pa.; Howard R. Stark, director of engineering, and John M. Sauer, director of research, Riverside Cement Co., Crestmore, Calif.; and W. S. Campbell, chemical engineer, California Portland Cement Co., Colton, Calif. Literature Cited (1) Gray, E. H., Rock Products, 52, 126-6 (1949). ( 2 ) Green, J. R., Am. Znst. Mech. Engrs., Trans., 148, 407 (1942). (3) Green, J. R., Rock Products, 47, 62-9 (1943). (4) Nordberg, B., Zbid., 52, 132 (1949).
RECEIVED for review March
25, 19:3. ACCEPTED March 11, 1954 Presented as part of the Symposium on Calcination, Roasting, and Rotary Kiln Operations before the Division of Industrial and Engineering Chemistry at the 123rd Meeting of the AVERICAN CHEXICAL SOCIETY, Los Angeles, Calif.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1441
