GB2126692A - Method and apparatus for burning or roasting fine-grained material, particularly cement raw meal - Google Patents

Abstract

A method of burning or roasting fine grain material comprising preheating the material in stages (1), largely calcining (5) the heated material, heating the largely calcined material to a temperature necessary for clinker reaction in the burning or roasting zone of a burner unit (8). At least part of the largely calcined material is supplied in suspension (28, to one side of the burning zone of the unit (8) and is guided through the burning zone in parallel flow with the combustible gases of the burner unit. After at least partial sintering, the material is conveyed back (30, 31) through the burner unit as a bed of material in counter current to the combustible gases for completion of sintering. A separating unit (7) for separating material and gas may be associated with the burner unit at the waste gas side. <IMAGE>

Description

SPECIFICATION Method and apparatus for burning or roasting fine-grained material, particularly cement raw meal The invention relates to a method and an apparatus for burning or roasting fine-grained material, particularly raw cement meal, in a heat treatment system, wherein the material is preheated in stages, then largely calcined and subsequently heated to the temperature necessary for the clinker reaction in the burning zone of a burning unit and remains in the burning unit until the termination of the clinker reaction.

In the usual modern burning or roasting methods and apparatuses for the production of cement used industriaily, raw meal is preheated with hot gases in a suspension-type heat exchanger and subsequently largely calcined with the addition of fuel whereupon the material thus pretreated is fed to a rotary kiln and heated relatively slowly to sintering temperature in a bed of material and sintered up to the finishing burn.

The finishing burn usually takes place in the rotary kiln with the material and gas in counter current.

It was recognised early (DE-PS 337 312 of the 27th May 1921) that in order to bring about a better and more rapid utilization of the flame gases, it was necessary to ensure an intimate contact between flame and material not during the sintering but previously, namely before the entry into the kiln until nearly the beginning of the sintering.

As a solution, a divided rotary kiln was proposed, the sintering portion of which rotates more slowly than the other portion, in which case the kiln serving for the heating up to close to the sintering point rotates at such a high speed that the material is lifted close to the apex of the kiln and from there falls freely down through the crosssection of the kiln. This invention was overtaken by technology, however, as a result of the introduction of the method and apparatuses mentioned at the beginning for heating raw meal in a suspension-type heat exchanger and particularly in a calcinator.

Recognition of the fact that it is a great advantage, for energetic and kinetic reaction reasons, to accomplish the heating-up interval between the calcining of the raw meal at about 9000C and the finishing burn or roast at about 1 3500C with as high a heating-up gradient as possible, has since led to a number of further proposals.

For example, a further possibility for the socalled rapid burning of raw material is mentioned in the DL-PS 97 409 and this consists in that the mixture, in the form of dust or agglomerated, is rapidly heated in a fluidised bed in a reactor and sintered up to the finishing burn. Furthermore, according to that, it is known that material in the form of dust is heated up very rapidly to finishing burn temperatures in reactors.

Reheating at the maximum temperatures reached up to the full finishing burn is not possible in the course of this, however. This is particularly because the temperatures close to the alite formation in the material cause very severe adhesive phenomena as melt formation begins and so cause crust formations which do not permit operation of the reactors without disturbance.

The prior publication proposes working with extremely high heating-up gradients in the temperature range between about 1100 and 13500C. A reduction in the finishing burn time of by about 70% is given as an advantage with positive effects resulting therefrom, for example by possible reduction in size of the finishing-burn reactor or an increase in its throughput or carrying out the finishing burn at lower temperatures with the usual dwell times. This advantage results with extremely high heating-up gradients preferably by avoiding a deactivation of the material to be burned. How such extremely high heating-up gradients are to be achieved is not given by any concrete teaching in the said document, however.

The present invention seeks to achieve an improvement in the clinker quality and reduction of the power requirements as a result of the fact that the heating up, following on the calcination, to sintering temperature is carried out as quickly as possible so that the active state of the material, following on the calcination can be used for a rapid, far-reaching and homogeneously distributed formation of alite nuclei, so as to achieve a significant shortening of the finishing burn time and possibly also a reduction in size of the finishing burn reactor as well as a reduction in the primary energy used.

According to a first aspect of the invention, there is provided a method of burning or roasting fine grain material in a heat treatment system comprising preheating the material in stages, largely calcining the preheated material, heating the largely calcined material to a temperature necessary for clinker reaction in the burning or roasting zone of a burning or roasting unit and maintaining it in the burning or roasting unit until completion wherein at least part of the largely calcined material is supplied, in suspension, to the burning or roasting unit to one side of the burning zone, the suspended material is guided through the burning zone in parallel flow with the combustible gases and heated rapidly so that the sintering reaction starts and is at least partially complete, the material is then separated from the combustible'gas and is conveyed back through the burning unit in a bed of material in counter current to the combustible gases for completion of the sintering.

As a result, in a very advantageous manner, the heating up to sintering temperature, following on the calcination, that is to say in the temperature range between about 900 and 13000C may take place as quickly as possible with very high heating-up gradients.

By this means, the active state of the material may be used for a rapid, far-reaching and uniform formation of alite nuclei. Consequently, recrystallisations can be avoided and short diffusion paths can be obtained for the alite formation.

The extremely rapid heating up is successful because radiation and convection act on the material finely divided in the suspension with a maximum active surface and therefore the heat transfers are more favourable by a multiple than in a loose bed of material. On the other hand, the risk of caking as a result of melt phase formation can be avoided by the fact that this is prevented from developing permanently in the rotary kiln, as is known, by the abrasive action of the rotating circulated bed of material.

In the present method the heated material, which is partially already completely burnt, falls down out of the suspension at relatively low gas speeds, at the end of the rotary kiln, or is separated from the gas in a separating device and collects in a bed of material and is conveyed back counter to the flame in the rotary kiln by its inclination and rotary movement, the material being conveyed in counter current to the flame gases in known manner, and at the end is sintered or completely burnt and is discharged at the outlet from the kiln. Subsequently it falls into a clinker cooler of known construction.

It is a great advantage that, with this method, the rotary kiln may be substantially shortened in length, with the same performance, as a result of the more favourable heat transfer. In addition, the formation of solid clod-like agglomerates in the completely burnt clinker is considerably reduced which leads to a further advantage, namely that the grindability of the finished clinker is considerably better so that a considerable proportion of the power necessary for the clinker grinding is likewise saved.

Because the deactivation of the components of the material being burnt is only inconsiderable, a higher reaction activity results and hence better reaction kinetics which lead to a shortening of the complete burn time and/or lowering of the complete burn temperature and so results in a distinct reduction in the specific primary energy requirements per unit of weight or clinker.

Also, as a result of the uniform formation of alite nuclei, a very advantageous homogenization may take place in the distribution of any free lime which may still be present, the disadvantageous swelling of which is therefore suppressed.

The component amount of largely calcined material may amount to at least 50% of the total amount. Nevertheless other quantitative distributions should not be exercised when carrying out the method according to the invention.

A great advantage results with the invention from the fact that this may be carried out using a conventional rotary kiln as a burning unit, and this may comprise a burner, the flame of which penetrates at least partially through the burning zone.

Because the rotary kiln is used both as a flyingdust heating and burning unit with parallel flow and at the same time as a conventional burning unit for burning in a bed of material in counter current, the use of additional heating units can be avoided and at the same time the associated risk of caking in the region of local overheating can also be avoided. With the double heat transfer through radiation and convection in parallel flow and counter current, the maximum possible utilization of heat and hence economy can be achieved.

The material may be blown into the burning unit with carrier gas in a particle jet substantially parallel to the flame, preferably above the burner.

In this case there is the further advantage that raw materials with a high proportion of chlorine or alkalis can also be used. For, as a result of the heating up the material substantially in suspension, the harmful substances are rapidly released and appear in a high concentration in the kiln waste gas where they cannot condense. In addition, because only a very small proportion of the total fuel requirements is burnt in the rotary kiln when carrying out the above method, therefore, if necessary, up to 100% of the kiln waste gases can be carried off in a by-pass without the heat consumption of the whole system being inadmissibly increased or the method becoming uneconomical.

In the normal case, however, the heat of the waste gases is used either for the process or for the production of flow.

A component amount of material, particularly material which has been largely calcined, may be fed to the burning unit from the gas outlet side also.

By this means, the separation of the material burned in suspension from the waste gas may be facilitated.

If component amounts of material are fed to the burning unit at both sides, uses may advantageously be made of the measure that types of material with a different material component composition may be fed separately from one another, one at each side.

In this case, it may further be provided, with advantage, that additional fuel, preferably lowgrade or substitute fuel, may be fed to the burning unit, likewise from the gas outlet side, together with the component amount of calcined material.

According to a second aspect of the invention, there is provided a plant for burning or roasting fine grain material comprising a preheater for preheating the material in stages, a calciner for largely calcining material from the preheater, a burning or roasting unit for heating the largely calcined material to a temperature necessary for clinker reaction wherein means are provided feeding at least part of the largely calcined material in suspension through the burning zone of the burning or roasting unit in parallel flow with the combustible gases for rapid heating and at least partial sintering and means for separating the material from the combustible gases and means for conveying the material back through the burning unit in a bed of material in counter current to the combustible gases.

Preferably a device for producing and introducing a suspension of calcined material and carrier gas is provided at the burner side of the rotary kiln, and the rotary kiln, which is operated both in parallel flow and in counter current, preferably has an inclination with a drop in the direction of the burner side.

This inclination of the rotary kiln, in cooperation with the introduction of the material in suspension, is an important feature of the plant which is in contrast to the usual arrangement of a rotary kiln operated in parallel flow. Such a kiln, for example according to the DE-OS 27 38 987, has a distinct inclination with a drop in the direction of the parallel flow of material and gas.As a result of the fact that the rotary kiln of the present plant may rise in the direction of the parallel flow, it not only renders possible the double operation in parallel flow and counter current in a very advantageous manner, but also this situation also encourages the operationally important sedimentation of the material out of the carrier gas at the end of the burning zone and may according to the dimensioning and gas velocity also render possible a falling out of the solid particles to the necessary extent even without a separate separating unit.

Nevertheless, a separating unit for separating material and gas may be associated with the rotary kiln at the waste-gas side.

The invention will now be described in greater detail, by way of example with reference to the drawings, in which: Figure 1 is a block schematic diagram of a known cement burning plant; Figure 2 is a block schematic diagram of one form of burning plant according to the invention; and Figure 3 is a block schematic diagram of a modified burning plant according to the invention with alternative combinations of individual operational elements.

The cement burning plant according to the prior art as shown in Figure 1 comprises a suspensiontype heat exchanger 1 with heat-exchanger cyclones 2, 3, 4 and with a calciner system consisting of a reaction section 5 with additional firing 6 and a separator 7. The burning plant further comprises a rotary kiln 8 with a main burner 9 and a following grate cooler 10 from which a hot air pipe, the so-called tertiary air pipe 11, ends immediately beside the additional firing 6 at the foot of the calcination reaction section 5.

Raw meal, indicated by the arrow 12, is fed to the suspension-type heat exchanger 1 while finished clinker is discharged from the cooler 10 as shown by arrow 1 3. The inclination of the rotary kiln extends from the inlet head 14 of the kiln with a slight drop of about 3.5 in the direction of the burner 9.

The operation of the burning plant according to the prior art is as follows: Raw meal is fed into the gas pipe 15 at 12, which gas pipe connects the preheater cyclone 3 to the separator cyclone group 4. In this, the raw meal is heated to about 300 to 350O and after extraction from the cyclone group 4 is introduced into the connecting pipe 16 between cyclones 2 and 3 and is heated therein to about 450 to 5000C. Then the raw meal from the cyclone 3 is separated from the stream of hot gas and introduced into the connecting pipe 1 7 between cyclone 2 and separator 7. In this, the material is further heated to about 6500C.At this temperature it is introduced, from the cyclone 2, through the fall pipe 18, into the lower region of the calcination reaction section 5 wherein, mixed with hot air from the tertiary air pipe 11 and fuel from the secondary firing 6 as well as with hot kiln gases, it is subjected to flameless combustion with far-reaching calcination. The material, largely calcined in this manner, enters the separator 7 at a temperature of about 9000C. The separator separates the solid particles from the stream of gas and introduces them, by means of the fall pipe 19, into the inlet chamber 14 of the rotary kiln 8.

In the rotary kiln 8, the material which is nearly 900 C hot is heated up to about 1 300 to 1 3500C over a distance of about 3/5ths to 4/5ths of the length of the rotary kiln. This is the temperature limit of incipient alite formation, also called the clinker reaction. The transit time from the kiln inlet 14 to the given temperature level amounts to about 60 min. During this, an average temperature gradient of nearly 7.50/min is reached. The final burn or roast to form clinker is effected in the last part of the rotary kiln 8, in the region of the 4th and 5th parts of its total length, in the region of the flame of the burner 9. In the course of this, the material reaches a final temperature of about 14500C.At this temperature, the finish-burned clinker is dropped into the cooler 10, cooled therein, while surrendering an important proportion of its sensible heat, and finally discharged as shown by the arrow 13.

Figure 2 shows a plant according to the invention in a block schematic diagram. This has largely the same main elements, namely a suspension-type heat exchanger 1, a calciner 5, a rotary kiln 8 and a cooler 10. The tertiary air pipe 11 leads from the cooler 10 to the calciner 5. By the connection 11', hot secondary air is supplied to the flame 29 of the rotary kiln 8, whereas clinker, indicated by the arrow 21, is introduced into the cooler 10 after final burning. The cooler receives cooling air through a feed device 20 and discharges cooled clinker in accordance with arrow 13.

According to the invention, largely calcined material is carried out of the calciner 5 by the pipe 1 9 and, in contrast to the plant according to the prior art (Figure 1), is introduced into the rotary kiln 8, above the burner 9, in accordance with arrow 28, in a particle jet, by an injector 27. A carrier gas for this is indicated by the blower 26.

The material is moved through the rotary kiln 8 in parallel flow with the flame 29 or the hot gases produced by this.

As the velocity of the stream of gas decreases, the solid particles, indicated by the arrow 30, fall out of the stream of gas towards the region of the feed chamber 14 and collect in the bottom region of the rotary kiln 8 where they form a bed of material indicated by the broken line 31. In case portions of dust should still be contained in the waste gas of the rotary kiln 8, the rotary kiln 8 with the waste-gas pipe 24 may be followed by a separator 7 which separates the dust out of the gas and introduces it, by the fall pipe 35, into the bed of material 31 in the rotary kiln 8. In the kiln, the bed of material 31 travels, as known per se, in the direction of the burner, in accordance with arrow 21, to the kiln discharge from which the completely burnt or roasted clinker is dropped into the cooler 10.The waste gas separated from the solid material is discarded in a by-pass according to arrow 25 in the case of a high proportion of harmful substances or, if there is no fear of an enrichment with harmful substances in the circuit, it is introduced into the calciner 5 through the pipe 26 for the purpose of recuperation of the heat contained in the gas. Waste gas from the calciner 5 is supplied to the suspension-type heat exchanger 1 by the pipe 17, while preheated material is introduced into the calciner from the suspension-type heat exchanger 1 by the pipe 1 8.

Also associated with the calciner 5 may be a material divider 19' from which conveyor devices 1 9, 1 9', for calcined material, lead to both ends 8' and 14 of the rotary kiln 8, whereby largely calcined material is fed to the rotary kiln 8 at both ends 8', 14.

Figure 3 shows a modified plant according to the invention with a basic construction similar to Figure 2. In this plant, too, disposed at the side of the burner 9 in the rotary kiln 8, is a heat treatment system I with a suspension-type heat exchanger 1 and a following calciner 5 out of which largely calcined material is introduced into the rotary kiln 8 by a pipe 1 9 via an injector 27 with the help of carrier gas, for example from the blower 26, above the flame 29, in the form of a suspension. The blower 26 may be supplied with warm air from the coller 19 via a branch pipe 11". Supplementing the plant shown in Figure 2, however, the modified plant shown in Figure 3 has a second heat treatment system II, parallel to the first heat treatment system I, with a suspensiontype heat exchanger 1 '.If the system II does not have its own calciner, a pipe 32 may be provided with which preheated raw meal is conveyed out of the heat exchanger 1' into the calciner 5 of the heat treatment system I at the burner side. If the second heat treatment system II has its own calciner 5', this may likewise be equipped with a material divider 36 from which two material conveyor devices 35' and 35" are connected to both sides 14,8' of the rotary kiln 8. Preferably largely calcined material is introduced into the feed chamber 14 of the rotary kiln 8 from the calciner 5' via the pipe 19'. Waste gas from the rotary kiln 8 is drawn off from the inlet chamber 14 by the pipe 24' and introduced into the calciner 5'. A cooler waste-gas pipe 33 may also be provided whereby hot waste gas from the cooler is likewise supplied to the calciner 5'.The device 34 is provided for the feed of additional fuel. Such additional fuel may, for example, be lumpy coal, scrap car tyres, refuse and the like.

Sand and/or shale containing oil may also be supplied to the rotary kiln 8 as additional fuel 34.

Common to the differently modified embodiments of a burning plant according to the invention, as shown in Figures 2 and 3, is the very advantageous characteristic that, as a result of the feed of the possibly largely calcined material in powder form and in the form of a suspension with carrier gas which may be preheated, during the conveying through the burning zone in parallel flow, an extremely rapid heating up with rapid temperature gradients is caused. Starting from an average conveying speed of the cloud of powder of 5 m/sec for example, the dwell time of a solid particle amounts to about 10 sec with a kiln length of about 50 metres. Within this time, the heating up from about 9000 to an average of about 1 3000C takes place.This gives a temperature gradient of 400 C/sec. In contrast, the temperature gradient during heating up in the bed of material in the conventional burning plant for example as shown in Figure 1 amounted possibly to 7.50C/min or 0.1250C/sec. Thus the gradients of the temperature rise are in the ratio of 1:330.

This very fast heating up preserves the active state of the material after the calcination and forms the prerequisites for a rapid and uniform formation of alite nuclei. In addition, recrystallisations are avoided and extremely short diffusion paths are obtained for the alite formation.

In addition, the shock-like heating up causes a multiplication of the lime binding speed.

The method according to the above embodiments of the invention also has the advantage that, as a result of maximum heat transfers, the rotary kiln can be shortened and so the specific output of the plant per unit of volume is increased by the invention. In the course of this, the flame zone of the kiln is used twice, once in parallel flow and once in counter current, and it is superfluous to connect a heating reactor in front of the rotary kiln.

As a result of the arrangement of the second heat treatment system II for the thermal pretreatment of the raw meal for the final burn, it is also possible to improve the total performance of the plant and at the same time to optimize the economy by generally complete utilization of the heat offered. This second heat treatment system II can - as previously shown with reference to the example in Figure 2 - be combined in various ways, with regard to conveying gas and meal, with the first heat treatment system I.

When the partially or largely calcined meal from the heat treatment system II is fed into the upper end 14 of the rotary kiln 8, the kiln region at the waste gas side is first used for the residual calcination. In this case, substitute fuel 34 in the form of waste such as car tyres, low-grade coal, refuse briquettes etc. may be used to great advantage to cover the heat requirements necessary for this, and is rapidly calcined together with the hot meal from the heat treatment system II, possibly by means of flameless combustion. On the other hand, however, the calcination of the material 12' fed to the heat treatment system II can be carried out in an additional, following calciner 5' which is connected between the suspension-type heat exchanger 1' and the kiln 8 and to which fuel is supplied by the burner 6'.The meal heated and precalcined in the heat exchanger 1' may, however, also be introduced into the calciner 5 of the heat treatment system I by the pipe 32 and be completely calcined together with the preheated meal from the heat exchanger 1. It would also be possible to divide the partially or largely calcined stream of meal coming from the heat treatment system II between kiln inlet and kiln outlet. This method of operation enables the heating-up and the sintering speed of a component stream to be adjusted in the required manner, for example. Thus the clinker quality can be adjusted or adhered to in the optimum manner according to the raw materials available. In all cases an oil or coal shale can be used as an additional fuel apart from the usual fuels, its mineral component being used as a raw meal constituent at the same time.

In all these cases, however, the important feature of the invention is the rapid heating up of the partially or largely calcined material in a particle jet and the use of the rotary kiln as a flying-dust reactor in parallel flow while simultaneously retaining the gas burning in counter current with a bed of material formed from the preheated and preliminarily burnt particles.

Claims (24)

1. A method of burning or roasting fine grain material in a heat treatment system comprising preheating the material in stages, largely calcining the preheated material, heating the largely calcined material to a temperature necessary for clinker reaction in the burning or roasting zone of a burning or roasting unit and maintaining it in the burning or roasting unit until completion wherein at least part of the largely calcined material is supplied, in suspension, to the burning or roasting unit to one side of the burning zone, the suspended material is guided through the burning zone in parallel flow with the combustible gases and heated rapidly so that the sintering reaction starts and is at least partially complete, the material is then separated from the combustible gas and is conveyed back through the burning unit in a bed of material in counter current to the combustible gases for completion of the sintering.

2. A method as claimed in Claim 1, wherein the component amount of largely calcined material amounts to at least 50% of the total amount.

3. A method as claimed in Claim 1, wherein a rotary kiln with a burner, the flame of which penetrates at least partially through the burning zone, is used as a burning unit.

4. A method as claimed in any one of claims 1 to 3, wherein the material is blown into the burning unit with carrier gas in a particle jet substantially parallel to the flame.

5. A method as claimed in Claim 4, wherein the particle jet extends above the burner.

6. A method as claimed in any one of the Claims 1 to 5, wherein a component amount of largely calcined material is also fed to the burning unit from the gas outlet side.

7. A method as claimed in any one of the Claims 1 to 6, wherein component amounts of material are fed to the burning unit at both sides and types of material with a different material component composition are fed separately from one another, one at each side.

8. A method as claimed in any one of Claims 1 to 7, wherein additional fuel is fed to the burning unit from the gas outlet side.

9. A method as claimed in Claim 8, wherein the additional fuel is low grade and/or substitute fuel.

10. A method as claimed in any one of Claims 1 to 7, wherein between 0 and 100% of the waste gas from the burning unit is used for preheating and/or calcining of the raw cement meal and/or between 0 and 100% is drawn off in the by-pass.

11. A plant for burning or roasting fine grain material comprising a preheater for preheating the material in stages, a calciner for largely calcining material from the preheater, a burning or roasting unit for heating the largely calcined material to a temperature necessary for clinker reaction wherein means are provided feeding at least part of the largely calcined material in suspension through the burning zone of the burning or roasting unit in parallel flow with the combustible gases for rapid heating and at least partial sintering and means for separating the material from the combustible gases and means for conveying the material back through the burning unit in a bed of material in counter current to the combustible gases.

12. A plant as claimed in Claim 10, wherein a device for producing and introducing a suspension of largely calcined material and carrier gas is provided at the burner side of a rotary kiln and that the rotary kiln which is operated both in parallel flow and in counter current has an inclination with a drop in the direction of the burner side.

13. A plant as claimed in Claim 11, wherein a separating unit for separating material and gas is associated with the rotary kiln at the waste-gas side.

14. A plant as claimed in Claim 11, wherein the device for introducing the material comprises an injector to which material is supplied from the calciner by means of a conveying device and air carrier gas, and the injector is disposed at the end wall of the kiln, at the burner side.

1 5. A plant as claimed in Claim 14, wherein the air is warm air supplied from a cooler following the kiln.

16. A plant as claimed in Claim 15, wherein the pressure of the air is increased by passing it through a blower.

17. A plant as claimed in Claim 14, wherein the injection is disposed above the burner.

18. A plant as claimed in any one of Claims 12 to 14, wherein there is a waste-gas pipe from the rotary kiln with a separator, a pipe to the calciner from which a by-pass pipe may be branched off and a pipe for conveying separated material back into the rotary kiln.

19. A plant as claimed in any one of the Claims 12 to 18, wherein a material divider with two material conveying devices which are connected to both sides of the rotary kiln is disposed at the discharge of the calciner.

20. A plant as claimed in claim 19, wherein a material divider with two material conveying devices which are connected at both sides of the rotary kiln are disposed at the discharge of the calcinator.

21. A plant as claimed in any one of the Claims 12 to 20, wherein a second heat treatment system is provided in addition to the first heat treatment system and comprises a second heat exchanger and a second calciner, through which waste-gases of the rotary kiln flow, and the material discharge of which is in communication with the gas-outlet side end of the rotary kiln through a pipe.

22. A plant as claimed in any one of Claims 12 to 21, wherein two heat treatment systems are provided, one at each side of the rotary kiln, for the thermal preparation of the raw meal for the completion of the burning, and each of which comprises a heat exchanger through which kiln gas or hot cooler waste air flows and one or both of the systems has a calciner.

23. A method of burning or roasting fine grain material substantially as described herein with reference to Figure 2 or Figure 3 of the drawings.

24. A plant for burning or roasting fine grain material substantially as described herein with reference to Figure 2 or Figure 3 of the drawings.