Abstract:
A method and apparatus for controlling the temperature in a kiln in which cement clinker is manufactured. The temperature is controlled by controlling the fuel rate to the kiln. The fuel rate is adjusted according to a calculation that is made of the sulphur evaporation in the kiln, thereby obtaining a measure of the instantaneous temperature in the burning zone. The evaporation factor is calculated on the basis of measurements of the sulphur content in the cyclone material which passes to the kiln or a measurement of the sulphur content in the exit gases out of the kiln.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method for controlling the temperature in a kiln for manufacturing cement clinker. 
     It is common knowledge that cement is manufactured by a number of raw materials, particularly lime (CaCO 3 ), clay, sand, pyrite ash, fly ash and other materials being mixed and interground into a raw meal in which the content of the oxides CaO, SiO 2 , Al 2  O 3 , Fe 2  O 3  must be specifically adapted within relatively narrow boundaries. The raw meal is subsequently preheated and calcined, during which process H 2  O and CO 2  are driven off. A range of reactions will then occur between the oxides, first and foremost the following reactions: 
     2 CaO+SiO 2  →(CaO) 2  (SiO 2 ) (belite) (CaO) 2  (SiO 2 )+CaO→(CaO) 3  (SiO 2 ) (alite) 
     These reactions between solid substances take place in a molten mass, with the aluminium and iron oxides which are necessary for forming the molten mass. 
     For every burning process the overall objective is always to ensure that the sufficient amount of alite is formed and that, simultaneously, the volume of free, not yet consumed CaO is reduced to an acceptably low level. Traditionally the burning process takes place in a rotary kiln and the final reaction occurs in the burning zone of the rotary kiln, with the reaction process being controlled by a regulation of the temperature by adjusting the firing rate and the flame setting. The temperature during the burning process approaches the 1400°-1450° C. range for the common types of clinker. 
     Accordingly, it will be necessary to perform a measurement providing continuous indication of the temperature in the kiln, hence indicating also the composition and quality of the clinker, but so far it has not been possible to measure the interesting temperature directly. 
     Thermocouples (Pt-PtRh) are impossible to install in a manner ensuring that they are not destroyed within a short period of time as a result of the contact with the hard clinker. 
     Radiation pyrometers can be used, but only if there is good visibility in the burning zone, which is rarely the case since a certain dust load will inevitably be generated during the burning process. 
     An indirect signal, which is extensively used to indicate the temperature, is a measurement of the force which is used to rotate the kiln. The reason why this signal can be used is that the higher the temperature of the clinker, the greater the amount of molten mass being formed, hence causing more of the charge to be drawn higher upwards along the side of the rotary kiln during rotation. As a result, the moment of force (force x arm) will be increased, hence increasing also the power required to rotate the kiln. However, the moment of force is a relative signal which is affected by a multitude of factors: arbitrary skewness in the crust formation, the adhesive properties of the raw materials along the entire length of kiln etc. Consequently, it is impossible to indicate exactly what the moment should be to ensure sufficient burning. 
     Another method for measuring the temperature in the burning zone involves measurement of the NO x  -emission from the kiln. The NO x  formation in the burning zone relates specifically to the temperature level in the flame, and it is influenced, at constant production and unaltered burner setting, only by the surplus air required for the burning process, and, since the overall aim is to keep the surplus air constant, the NO x  emission is a direct measure of the burning temperature. As it is, kilns have been operating for many years on the basis of NO x  -measurements, being controlled both manually and automatically, e.g. by means of Fuzzy logic. 
     However, it is a recognized fact that the emission of NO x  is detrimental to the environment, and, therefore, many efforts are mainly concentrated on reducing the emission of NO x , including the emission of the rotary kiln, of a cement kiln plant. 
     These measures severely reduce the possibilities of controlling the kiln. This is best understood by considering a curve of the NO x  formation as a function of the temperature T (° C.) (see FIG. 1). The curve is found by measuring the NO x  formation as a function of the final temperature of the clinker subject to a specific flame setting. 
     When burning common clinker, one has an operating point which is situated in much the same position as A. Here the formed NO x  is essentially of thermal origin, i.e. the nitrogen atom in NO x  originates from the N 2  of the air, and high/low deviations from the target temperature are markedly reflected in a significant change in the amount of formed NO x . 
     When measures are introduced to reduce the NO x  level, corresponding to a lower temperature in the kiln, one will approach the operating point B, where the formed NO x  originates mainly from the fuel. In the proximity of point B, temperature dependence of the NO x  formation is negligible and, in actual practice, the NO x  measurement cannot be used as a control parameter in such a low-NO x  operating mode. 
     In addition to lowering the NO x  emission, energy savings are also achieved when the flame temperature is lowered, which makes these conditions particularly desirable during the operation of the kiln. 
     In order to obtain operating conditions with a low degree of NO x  emission, the option of extending the flame in the kiln may be used, for example by reducing the primary airflow or primary velocity of the burner. By this method the clinker will have a lower final temperature, but, on the other hand, the clinker requires a longer retention time at temperatures above the minimum limit where the alite formation may take place. 
     Another method for obtaining operating conditions with a low degree of NO x  emission and a lower necessary flame temperature involves addition of a mineralizer to the raw meal, thereby lowering the temperature required for the alite formation to take place. Addition of sulphur and fluoride involves, for example, that the clinker burning process may occur at a temperature which is approximately 125° lower than normal, i.e. at 1275°-1325° C. 
     SUMMARY OF THE INVENTION 
     So, the objective of the invention is to ensure capability of controlling the temperature in the kiln, and hence the manufacture of the clinker which is produced in the cement kiln, achieving, simultaneously, a reduction of the NO x  emission from kiln to an absolute minimum. 
     This is obtained by a method according to the invention where the sulphur evaporation in the kiln is calculated, thereby obtaining a measure of the instantaneous temperature in the burning zone. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described in further details in the following with reference being made to the drawings, where 
     FIG. 1 shows the amount of formed NO x  as a function of the temperature, 
     FIG. 2 shows the evaporation factor E as a function of the temperature, 
     FIG. 3 shows an elementary sketch of the mass flow in a plant for manufacturing cement. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Here, evaporation is used to designate the decomposition reactions which basically occur at temperatures above 1100° C.: 
     CaSO 4  →CaO+SO 2  +1/2 O 2  K 2  SO 4  →K 2  O+SO 2  +1/2O 2   
     The evaporation factor E is defined as the fraction of the sulphur S which is fed to the burning zone of the kiln together with the calcined raw materials, and which is subject to evaporation. This factor is very interesting because it increases substantially when the temperature is lying within the range 1100°-1500° C. (see FIG. 2) and because this very temperature range is used for the clinker burning process. 
     An expression of the evaporation factor E can be found by making two mass balances for a plant for manufacturing cement clinker (see FIG. 3). 
     Such a plant consists of a system I where the raw materials are being prepared for burning, and a system II in which burning is taking place. System I may advantageously incorporate a cyclone preheater and possibly a calciner, and system II may advantageously incorporate a rotary kiln. 
     The sulphur content of the different flows, i.e. the amount of sulphur S x  that passes in and out of the two systems I and II, can be measured as kg sulphur/hour or as kg sulphur/kg clinker produced at the plant. 
     Raw materials 1 with a sulphur content S feed  and exit gases 2 from the rotary kiln with a sulphur content S kiln  gas gas are fed to system I. 
     Sulphur, S sec .fuel, may also be supplied at 3, via secondary fuel to calciner or riser duct. 
     A flow of cooled exit gases 4 with a sulphur content S exit  gas is discharged from system I, and the precalcined or merely preheated material flows at 5, normally from a cyclone, down into the kiln with the sulphur content S cyc . 
     The input flows to system II consist of the preheated or precalcined material S cyc  5 and kiln fuel 6, i.e. primary fuel, with the sulphur content S prim .fuel. The output flows consist of the exit gases S kiln  gas 2 from the kiln and of the finished clinker 7 with the sulphur content S clink . 
     The evaporation factor E represents, at any particular time, the ratio between the amount of sulphur which evaporates in the burning zone and the amount of solid, combined sulphur which passes into the burning zone. However, the possibility of measuring the sulphur content of these two interesting flows in the burning zone does not exist. 
     Still, the situation will be such that the amount of sulphur which evaporates in the burning zone will approximately be equal to the amount of sulphur that passes out of the kiln at 2, S kiln  gas, minus the contribution coming from the primary fuel, S prim .fuel, and the amount of solid, combined sulphur which passes into the burning zone will approximately be equal to the amount which passes down from system I at 5, S cyc . This involves that: ##EQU1## 
     The sulphur content in the primary fuel will be constant for a specific type of fuel and, therefore, the S prim .fuel is known if the fuel consumption during the time period t is known. S cyc  will vary with the time period because of fluctuations in the sulphur input to the burning zone and variations in the temperature in the burning zone. In a rotary kiln where the passage of the material from the inlet of the kiln where S cyc  is measured and to the burning zone may take a relatively long time, one has to apply a time delay τ to the measurements, in relation to the measurement of S kiln  gas, which happens almost instantaneously, which means that: ##EQU2## 
     At any particular point in time, the amount of sulphur down into kiln at 5 can be said to be roughly equal to the sum of the amount of sulphur which passes in at 7 (S feed ), 2 (S kiln  gas) and at 3 (S sec .fuel), but minus the amount of sulphur which passes out at 4 (S exit  gas). Still, in most kiln systems, the sulphur content in the last-mentioned stream, S exit  gas, will be equal to 0: 
     S cyc  =S feed  (t)+S kiln  gas (t)+S sec .fuel (t) or S kiln  gas =S cyc  (t)-S feed  (t)-S sec .fuel (t) 
     This entails that E(t) can be calculated both by measuring the sulphur content in the exit gas from the kiln and by measuring the sulphur content in the material being fed to kiln: ##EQU3## 
     These two expressions will only be valid when the kiln is a rotary kiln but similar expressions can be developed for other kilns, e.g. stationary kilns. 
     Generally, there is no point in fitting an SO 2  -meter in the exit gas duct from the rotary kiln. This is because the emission of the kiln system into the atmosphere of this detrimental gas component is not in any way related to the measured value of S kiln  gas due to the almost 100% absorption efficiency of SO 2  in the lower preheater stage or in the calciner where a relatively large air surplus exists. 
     Generally, it is uncomplicated to assess the contributions which the raw materials and fuel give to the amount of sulphur on the basis of the current analyses and the dosage of feed input. 
     E(t) can then be calculated, either on the basis of (A) based on measurements of the SO 2  content in the kiln exit gas, S kiln  gas, or based on (B) if the sulphur content is measured in the stream which passes from the separation cyclone after the calciner and down into the kiln, S cyc . 
     Since SO 2  is the only sulphureous component at a high temperature and air surplus, the easiest method for measuring S kiln  gas is to install an SO 2  -meter, which continuously analyzes the kiln exit gas, in the kiln outlet. 
     A major source of error associated with this method is that calcined raw meal is whirled up in the exit gas so that the sulphur dioxide is chemically combined through the reaction: 
     (C) SO 2  +1/2O 2  +CaO→CaSO 4   
     If the exit gas sample is cleaned and cooled with water, which is the basic operating principle in certain systems, a part of the SO 2  volume may also be bound by the water which is alkaline because of the CaO. As a result hereof, the gas analyzer signal will be too small. 
     However, if due attention is given to these sources of error, it will often be possible to assume that a fixed fraction of the SO 2  volume will disappear, and, accordingly, the signal can still be used for controlling the temperature since the real amount of evaporated SO 2  is proportional to the measured amount of SO 2 , S kiln  gas. measured =constant×S kiln  gas. 
     The method cannot be used for control purposes, if the SO 2  stream out of the kiln (e.g. due to a bypass duct being established to reduce SO 2  and the chloride circulation in the kiln system) is of a size which is so limited that approximately all the SO 2  volume is absorbed by whirled-up CaO. In case of SO 2  presence here, this is more an expression of an air deficit in the kiln, cf. equation (C), than of a high burning zone temperature. 
     The amount of sulphur being fed to the rotary kiln can be determined by several known methods. For example by means of an Outokumpo x-ray analyzer capable of continuously determining the content of the elements Fe, Ca and S in the cyclone material. For this purpose, a substream of raw meal is extracted from the cyclone, cooled down and compacted before it is fed to the analyzer. The signal which is received for the S-content or the S/Ca ratio provides a precise indication of the amount of sulphur that passes down into the rotary kiln.