Abstract:
A method is disclosed for heat treating granular or pulverulent raw material wherein the raw material is directed to a heat exchanger having at least two preheating units operating in parallel, each preheating unit having at least a first preheating stage and at least a lowermost calcination stage to effect a preheating and at least partial calcination of the material. The preheated, at least partially calcined material is fed from the heat exchanger into a kiln and undergoes a sintering process in the kiln. The sintered material is directed from the kiln into a cooling means in which cooling air is directed in a manner to effect a heat exchange between the material and the cooling air. The method further comprises dividing the heated cooling air exiting the cooling means and directing a portion of the heated cooling air to at least one of the calcination stages of the heat exchanger and directing a portion thereof to the kiln in excess of that required to support combustion of fuel in the kiln for the sintering process thereby intensifying the effectiveness of a material cooling zone within the kiln proper. Hot kiln exit gases are directed to at least one of the calcination stages and fuel is directed to at least one of the calcination stages to support combustion with preheated cooling air to provide supplementary heating of the material and at least partially calcining of the preheated material. An apparatus for practicing the inventive method is also disclosed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a plant and method for preheating, calcining, and sintering granular or pulverulent raw materials, such as cement raw meal. 
     2. Description of the Prior Art 
     In the technology pertaining to the heat treatment of granular or pulverulent raw materials such as cement raw meal, it is always desirable to utilize heat energy as much as possible. My, commonly assigned U.S. Pat. No. 3,864,075 dated Feb. 4, 1975, discloses a plant which is characterized by the division of the hot waste cooling air leaving the cooler so that a portion is directed as combustion air to the burner of the kiln, and a portion is directed to the air inlet of one heat exchanger unit. The gas inlet of another heat exchanger unit is connected to an exhaust gas outlet of the kiln and the heat exchanger units each have at or near its gas (or air) outlet end, means associated therewith for controlling the air or gas flow through that unit and hence the division of the heated cooling air. 
     In a preferred embodiment of my earlier development, one heat exchanger unit has an air inlet to which hot waste cooling air is directed, with means being provided for increasing the heat content of the air. The other heat exchanger unit--the gas inlet of which, exhaust gas from the kiln is directed--has means for discharging the preheated material into the first mentioned heat exchanger unit at or near its inlet end. 
     German Publication No. 2,262,213 to Rohrbach (laid open for inspection Dec. 12, 1974) relates to a method of heat treating such raw materials by directing preheated raw materials from several strings of cyclone-type suspension preheaters to a single calcinator for clacining material prior to feeding it into a kiln. Waste gases from a clinker cooler are directed to the preheaters to utilize the heat therefrom in preheating the raw material. 
     Commonly assigned copending application Ser. No. 603,867, filed Aug. 11, 1975 relates to a method for heat treating such pulverous raw materials prior to subjecting them to further heat treatment in a kiln by dividing the gas flow and the raw meal flow into substantially equal divisional flows, with each flow being associated with a sub-stage of a final stage of a preheater. Other related patents--which were considered in the examination of my U.S. Pat. No. 3,864,075 --are as follows: U.S. Pat. No. 1,817,048 to Washburn; U.S. Pat. No. 3,037,757 to Deussner; U.S. Pat. No. 3,452,968 to Shimizu et al.; and U.S. Pat. No. 3,664,650 to Weber et al. 
     If the granular or pulverulent material such as cement raw meal is to be treated according to prior art methods, at least the greater part of the calcination process will take place almost exclusively near the air inlet end of the heat exchanger unit which is fed with hot waste cooling air. Also, the combustion of the fuel--nourished by the wate cooling waste required for performing the calcination process will take place in that locality. No worthwhile calcination will take place in the kiln nor in the heat exchanger unit which is fed with exhaust gas from the kiln. 
     With the earlier arrangements, the amount of hot waste cooling air drawn through the kiln per time unit is small and amounts only to that which contains enough oxygen to nourish the combustion of fuel at the burner in the kiln for carrying out the sintering. The remaining part of the hot waste cooling air will by-pass the kiln. and be led to the heat exchanger unit in which the calcination process is effected. This may result in the so-called pre-cooling zone located in the rotary kiln betweeen the mouth of its burner pipe and the clinker discharger outlet or outlets (as the case may be) not functioning properly, i.e. the clinker will not be precooled to the desired extent before leaving the kiln, because the amount of cooling air passing through the pre-cooling zone is too small, and the clinker cooler may be damaged. 
     Another drawback arising from the reduced amount of gas drawn through the kiln and originating from the hot waste cooling air passed into the kiln is that the alkalis--which almost invariably will be released near the exhaust gas outlet end of a kiln for burning cement clinker--will be concentrated in a comparatively slight amount of combustion gas with the result that there will be a tendency to encrustration in the lower part of the riser pipe leading from the material inlet end of the kiln to the adjacent heat exchanger unit. I have developed a unique method and plant which avoids these drawbacks while providing a new and improved approach to heat treating such raw materials as cement raw meal. 
     Summary of the Invention 
     In accordance with my invention, a method of heat treating granular or pulverulent raw material comprises directing the raw material to a heat exchanger having at least two preheating units operating in parallel, each unit having at least a first preheating stage and at least a lowermost clacination stage to effect a preheating and at least partial calcination of the material. The method further comprises feeding preheated, at least partially calcined raw material from the heat exchanger into a kiln, and sintering the preheated, at least partially clacined material in the kiln. The method further comprises directing the sintered material from the kiln into a cooling means, and directing cooling air into the cooling means in a manner to effect a heat exchange between the sintered material and the cooling air. The heated cooling air exiting the cooling means is divided and directed as follows. A portion of the heated cooling air is directed to at least one of the calcination stages, and a portion is directed to the kiln. The portion directed to the kiln is in excess of the amount of air required to support combustion of fuel in the kiln for the sintering process thereby intensifying the effectiveness of a material cooling zone within a portion of the kiln so as to make it early cooling of the sintered material. The method further comprises directing hot kiln exit gases to at least one of the calcination stages, and directing fuel to at least one of the calcination stages thereby supporting combustion with said preheated cooling air to provide supplementary heating of the material and at least partially calcining the preheated material. In the preferred embodiment the method comprises controlling the division of waste cooling air between the kiln and the heat exchanger unit or units. 
     A plant is disclosed for heat treating granular or pulverulent raw material according to the inventive method, which comprises a heat exchanger having at least two preheating units operating in parallel relation, each preheating unit having at least a first preheater unit for preheating raw material, and at least a lowermost calcination chamber for at least partially calcining the preheated material. A kiln communicates with the heat exchanger and is adapted to receive preheated, at least partially calcined material from the heat exchanger for sintering, with means being provided in communicating relation with the material discharge end portion of the kiln to receive sintered material from the kiln for cooling the sintered material. The plant further comprises means for dividing waste heated cooling air leaving the cooling means and for directing at least one of the calcination chambers of the heat exchanger and a portion of the cooling air to the kiln in excess of the air required to support combustion in the kiln for the sintering process, the excess cooling air thereby intensifying the effectiveness of the cooling zone within the kiln for cooling the material burnt therein. The invention also comprises means to direct hot kiln exit gases to at least one calcination chamber of the heat exchanger, and means to supply fuel to at leat one of said calcination chambers to support combustion least with said preheated cooling air to thereby provide supplementary heat for the material and to at least partially calcine the preheated material. 
     A significant distinction between the plant according to the present invention and the prior art developments, particularly my preferred arrangement shown in my U.S. Pat. No. 3,864,075, is that in my earlier development, a calcination chamber is provided only in the heat exchanger unit fed with waste cooling air whereas according to my present invention, a calcination chamber is provided in all of the heat exchanger units. With this feature, the amount of cooling air drawn into the rotary kiln per time unit will be increased by the amount required for carrying through the calcination process taking place in the heat exchanger unit or units connected to the kiln exhaust gas outlet. 
     With the present development the objects of my earlier invention are clearly preserved. In particular, it can be seen that my present invention preserves my earlier concept and enables an easy and effective control of the gas flow through the various heat exchanger units in combination with a combustion chamber associated with at least one of the preheater units. This control is enhanced if each unit has its own means --such as an adjustable fan or valve --for controlling the flow of gas or air through that unit. Alternatively if there is more than one of each kind of unit (i.e. fed with air from the cooler or gas from the kiln) they may have a common control. 
     In one mode of operation, at least partly calcined raw material from the calcination chamber of each heat exchanger unit is fed directly into the kiln. With this arrangement it will be seen that the principle of the invention is fully untilized. 
     In an alternative mode of operation, the raw material treated in each heat exchanger unit fed with kiln exhaust gases is incompletely calcined in that unit and is fed from the calcination stage of that unit to the calcination stage of at least one heat exchanger unit, fed directly with waste cooling air from the cooler, for further calcination before being fed into the kiln. Such a mode of operation may be regarded as a hybrid of the method to be carried out in the preferred plant shown in my U.S. Pat. No. 3,864,075 and of the mode of operation described above in the preceding paragraph. This hybrid mode of operation may be found useful only when it is considered sufficient to increase the amount of gas drawn through the kiln to a limited extent. In such case raw material which has been preheated in a heat exchanger unit fed with exhaust gas from the kiln and has been slightly calcined in the calciner forming the last stage of that heat exchanger unit, will be directed into a claciner forming the last stage of a heat exchanger unit fed with waste cooling air from the clinker cooler so as to join raw material which has been preheated in the preceding stages of that heat exchanger unit. In the last mentioned calciner all the material entering it, whether orginating from one source or the other, and whether being slightly calcined or only preheated, may be substantially fully calcined. 
     The calcination stage of each heat exchanger unit fed with kiln exhaust gases may be heated at least partly by the combustion of fuel in the kiln in excess of that required for carrying through the sintering process, or at least partly by the local combustion of fuel; the latter being preferred to avoid overheating of the kiln gas outlet if the calcination is to approach completion in that unit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention are described hereinbelow with reference to the drawings wherein: 
     FIG. 1 is a side elevation of a plant embodying the principles of the present invention having two separate heat exchanger units cooperating in parallel; 
     FIG. 2 is a side elevation of an alternate embodiment of the invention; 
     FIG. 3 is a side elevation of an alternate embodiment of the invention utilizing four separate heat exchanger units cooperating in parallel; and 
     FIG. 4 is a side elevation of a plant similar to the plant of FIG. 2 having three heat exchanger units cooperating in parallel. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a rotary kiln 1, the supporting and driving means of which are not illustrated, and a clinker cooler 2 which may be of any known type, one end of which receives hot clinker from the kiln 1 through guiding means, such as a hood 3. The hot clinker is advanced as a layer through the cooler while being traversed or swept by atmospheric cooling air with the result that the hot clinker is cooled and the cooling air is heated. 
     Part of the hot waste cooling air leaving the hood 3 is passed into the kiln 1 where the oxygen contained therein serves to nourish the combustion of the fuel blown into the kiln through a burner pipe 4 provided at the material outlet end of the kiln. In these circumstances a flame will be formed at the mouth of the burner pipe 4 and hot exhaust gas will pass up through the kiln countercurrent to the preheated, substantially completely calcined raw material which is fed into the kiln at its material inlet end. The material moves down through the kiln so as gradually to be chemically and physically changed under the influence of the heat in the kiln. The exhaust gases leave the kiln through a stationary hood 5 and a conduit 6. 
     The remaining part of the waste cooling air leaving the hood 3 passes into a conduit 7. Each of the conduits 7 and 6 is connected to the bottom stage of its own four-stage suspension heat exchanger unit, marked A and B, respectively. The units are identical, although mounted symmetrically, and are designed to cooperate in parallel in subjecting cement raw material to a heat treatment prior to its entry into the rotary kiln 1. For this purpose each of the units is at its top connected to the suction side of a fan, 8a and 8b, respectively, which draws heated used cooling air and exhaust gas, respectively, through units A and B. To enable an independent control of the amount of air or gas, respectively, sucked through A and B per time-unit, the fans 8a and 8b are either designed for independent variation of their number of revolutions or they are each equipped with a regulable damper. The delivery side of the two fans is connected to a common and conventional dust precipitator not shown. 
     Cement raw material to be heat-treated in the units A and B is introduced in dosed quantities through feed pipes 9a and 9b. The three first stages of units A and B include a three-stage suspension preheater of conventional cyclone type comprising in each stage, a riser pipe in which heat exchange takes place between hot waste cooling air or kiln exhaust gas, as the case may be, and relatively cold granular or pulverous cement raw material. The three first stages also include a cyclone separator in which the preheated raw material is separated from the air or gas in which it was suspended in the riser pipe. 
     The last stage of units A and B constitute a calcination state in which preheated cement raw material discharged from the bottom of the cyclone separator of the last preheater stage is subjected to an almost complete calcination, while suspended in heated used cooling air from conduit 7 or kiln exhaust gas from conduit 6 as the case may be. Each calcination stage consists of the calcination chamber proper, 10a and 10b, respectively, and a cyclone separator, 11a and 11b, respectively, for separating the almost fully calcined raw material from the air or gas in which it was suspended during the calcination process. From the bottom of each of the cyclone separators, 11a and 11b, extends a pipe, 12a and 12b, respectively, the free end of which opens into a common pipe 13 which again, passing through hood 5, extends into the inlet end of rotary kiln 1. 
     Fuel supply pipes 14a and 14b serve to intorduce fuel into the calcination chambers 10a and 10b, respectively, for carrying through the calcination process. The combustion in chamber 10a is nourished by oxygen contained in heated, used cooling air taken from the clinker cooler 2 along the path 2, 3, 7 and the combustion in chamber 10b is nourished by oxygen contained in the exhaust gas taken from the rotary kiln 1 along the path 1, 5, 6. In the latter case the oxygen does, in fact, also originate from the cooler 2, from which it has taken the path 2, 3, 1, 5, 6. 
     A significant feature of the present invention is that the amount of hot waste cooling air (containing about 20 percent oxygen) which is in excess of the amount of oxygen containing gas sufficient to nourish the combustion of the fuel supplied through the burner pipe 4 must actually be passed from the cooler 2 and through the hood 3, the kiln 1, the hood 5 and the conduit 6 into the calcination chamber 10b in order to nourish the combustion of the fuel added through supply pipe 14b. The raw material passed through pipe 13 into the rotary kiln 1 is fully --or almost fully --calcined and will be subjected to a finishing heat treatment in the kiln which transforms the raw material into cement clinker. If the raw material on entering the kiln is not fully calcined, the first step of the heat treatment in the kiln will be a completion of the calcination process. In any case, the main process performed in the kiln is a sintering process, the end product of which is cement clinker. 
     While the calcination process is an endothermic process requiring a considerable amount of heat for its performance (which takes place at about 850° C.), the sintering process is an exothermic process requiring only an amount of heat necessary for achieving the sintering process temperature (about 1450° C.) and for covering the heat losses. The calcination process, therefore, not only requires more fuel for its performance than does the sintering process, but also more combustion air is needed for nourishing the combustion of the requisite amount of fuel. A rule of thumb indicates that twice as much combustion air is required for the calcination process as for the sintering process. In other words, of the amount of oxygen contained in the heated used cooling air originating from the clinker cooler, two thirds is utilized in the calcination process and one third in the sintering process. 
     The last process carried through in the rotary kiln (after the sintering) is a pre-cooling of the clinker just produced. This pre-cooling actually takes place in the cooling zone of the kiln extending from the cross-section of the kiln which is flush with the mouth of the burner pipe 4 to the outlet end of the kiln taken in the direction of travel of the material. For example, in FIG. 1, this pre-cooling zone is designated as zone C. The raw material is conveyed down through the kiln because of its inclination and by the time it reaches the point corresponding to a cross-sectional plane aligned with the mouth of the burner pipe, it has been transformed into cement clinker. 
     On advancing further down the kiln, through the pre-cooling zone C, a cooling of the clinker is initiated by means of used cooling air from the clinker cooler 2 passing through the hood 3, into the kiln and inside the kiln in countercurrent to the advancing clinker layer. The pre-cooled clinker will drop from the outlet end of the kiln inside the hood 3 into (or onto, as the case may be, depending upon the type of cooler involved) the clinker cooler so as to be finally cooled in the cooler. 
     The effective pre-cooling thus preformed inside the rotary kiln at its lower outlet end (in the zone C) is very important, primarily because it causes a reduction in the temperature of the clinker before it reaches the clinker cooler 2. Without this fall in temperature of the clinker outside the clinker cooler, the cooler may well be damaged by contact with the hot clinker. 
     The effectiveness of the pre-cooling involved is dependent upon the amount of cooling air passed through the pre-cooling zone C per time unit. Thus a feature of the present invention is to increase this amount of air beyond that applied hitherto, with the result that the gas velocity through the kiln is also increased. This increase in gas velocity is obtained by providing not only the last stage of heat exchanger unit A with a calcination chamber 10a as previously proposed, but by providing such a calcination chamber also as the last stage of heat exchanger unit B. 
     The provision of the calcination chamber 10b will mean that an amount of cooling air being sucked into the kiln and containing sufficient oxygen to nourish the combustion of the fuel introduced through the mouth of the burner pipe 4 will no longer suffice. An extra amount of air containing oxygen enough to nourish the combustion of the fuel added at 14b will also be required, and so the total amount of used cooling air passing from hood 3 into the kiln 1 will be increased as desired. The amount of gas (containing some oxygen) passing through conduit 6 will be increased correspondingly, but the amount of used cooling air passing through conduit 7 will be decreased correspondingly. 
     The increased amount of combustion gas passed through the kiln in addition to improving the efficiency of the pre-cooling of the clinker also causes a useful reduction in the concentration of the alkalis in the kiln combustion gas. 
     Still other advantages are related to the incorporation of a calcination chamber in each heat exchanger unit A and B with the consequent increase in surplus air being passed into the kiln outlet. If the cement raw materials contain sulphur it may be released therefrom during the heat treatment in the kiln and give rise to encrustations in the riser pipe leading the exhaust gas away from the kiln. An increase in the amount of surplus air passed into the kiln will reduce the tendency of the sulphur to be released from the raw material. A greater part of the sulphur will therefore remain in the raw material and be found as sulphur compounds in the clinker, where it will cause no harm. 
     In FIGS. 2, 3 and 4, alternate embodiments are illustrated in which components corresponding to the components of FIG. 1 bear the same reference numerals as in that FIG. The plant shown in FIG. 2 differs primarily from that of FIG. 1 by the fact that the calcination taking place in the calcination chamber 10b is not as extensive as the calcination taking place in the calcination chamber 10b in the plant according to FIG. 1. Further, the calcination taking place in calcination chamber 10b of FIG. 2 is not as extensive as the calcination taking place in calcination chamber 10a of FIG. 2. In the plant according to FIG. 2 the material having been slightly calcined in the calcination chamber 10b provided in the heat exchanger unit B, is directed through a pipe 12b into the calcination chamber 10a provided in the heat exchanger unit A. This slightly calcined material is fully --or almost fully --calcined therein together with the preheated material coming from the first stages of the heat exchanger unit A. (According to FIG. 1 both pipe 12a and pipe 12b open into a pipe 13 leading to the kiln 1.) 
     The plant according to FIG. 2 may be used to advantage when the demand for an increased gas velocity in the kiln is not so pronounced as to require a plant according to FIG. 1 as the natural solution. 
     In the plant according to FIG. 2 the fuel supply pipe 14b (but not 14a)may alternatively be dispensed with. In such case the calcination chamber has no fuel supply of its own to produce the heat required for carrying through the calcination. Instead, an amount of fuel in excess of that required for carrying through the sintering process in the rotary kiln is supplied through the burner pipe 4. The total amount of fuel thus supplied is burned off at the mouth of this pipe so as to produce combustion gas having a heat content and temperature sufficient for carrying through both the sintering process in the kiln 1 and the partial calcination process in the calcination chamber 10b. 
     Theoretically, the same alternative procedure might be made use of in the plant shown in FIG. 1, but in practice the temperature of the gas passing out of the kiln 1 and entering the conduit 6 would be higher than the kiln mouth and the riser pipe can withstand. The more extensive the calcination taking place in calcination chamber 10b, the higher the temperature of the gas. 
     The plant shown in FIG. 3 corresponds rather closely to that shown in FIG. 1, the only difference being that each of the heat exchanger units A and B has been split up into two parallel units co-operating in parallel. For example heat exchanger unit A of FIG. 1 corresponds to A&#39; and A&#34; of FIG. 3 on one hand and heat exchanger B of FIG. 1 corresponds to B&#39; and B&#34; of FIG. 3 on the other hand. 
     FIG. 3 has been added to show that both the heat exchanger unit of category A and that of category B may be divided into two units each. When according to FIG. 1 (and to FIG. 2) an extensive calcination is carried through in the calcination chamber 10b belonging to heat exchanger unit B (and in calcination chambers 10b&#39; and 10b&#34; belonging to heat exchanger units B&#39; and B&#34;) almost equal amounts of air or gas will flow through conduit 7 and conduit 6, respectively, per time unit. 
     The plant shown in FIG. 4 corresponds rather closely to that shown in FIG. 2 with the main difference being that the heat exchanger unit A (but not the heat exchanger unit B) has been split up into two separate units, A&#39; and A&#34;, respectively, co-operating in parallel. According to FIG. 2 the raw material is only slightly calcined in the calcination chamber 10b and the slightly calcined material is then conveyed through the pipe 12b to the calcination chamber 10a for further calcination. A corresponding arrangement is employed in FIG. 4, but in this case there is one calcination chamber only (that is to say 10b) of category B, and two calcination chambers (10a&#39; and 10a&#34;, respectively) of category A. Because of this arrangement special measures must be taken which will be described below in greater detail. 
     The additional measures referred to consist in splitting-up the gas discharge pipe 15b from the calcination chamber 10b into two branches 15b&#39; and 15b&#34;, and connecting each branch to a cyclone separator 11b&#39; and 11b&#34;, respectively. The measures further comprise connecting the bottom discharge end of separator 11b&#39; to separator 10a&#34; by means of a pipe 12b&#39; and connecting the bottom discharge end of separator 11b&#34; to separator 10a&#39;. A conduit 16 connects the two cyclone separators 11b&#39; and 11b&#34; with the preheating stage of unit B. At its lower end the conduit 16 is divided into two branches, each of which is connected to the top of their respective cyclone separators 11b&#39; and 11b&#34;. 
     When, as in this case, only a slight calcination is carried out in the B-calcination chamber the amount of air or gas passing per time unit through a B-unit will only comprise about half the amount passed through the A-units. However, as there are two A-units and only one B-unit about one third will pass through each unit. The fans 8a&#39;, 8a&#34; and 8b&#39; will have to be regulated accordingly.