WO2008120109A1

WO2008120109A1 - Method and plant for manufacturing cement clinker

Info

Publication number: WO2008120109A1
Application number: PCT/IB2008/050290
Authority: WO
Inventors: Morten Boberg Larsen
Original assignee: Flsmidth A/S
Priority date: 2007-03-30
Filing date: 2008-01-28
Publication date: 2008-10-09
Also published as: DK176553B1

Abstract

Described is a method as well as a plant for manufacturing cement clinker whereby cement raw meal is preheated and burned into clinker in a kiln system (1, 3, 5) and subjected to cooling in a subsequent clinker cooler (7), by which method waste is heated in a separate compartment (9) where the gas generated by the waste heating process is fed to the kiln system for heating the cement raw meal and where the solids content is diverted from the compartment (9). The method and plant is peculiar in that the waste is heated to a temperature of at least 150° C in the separate compartment (9) and in that the diverted solids content from the compartment is broken into smaller particle sizes in a comminution means (10).

Description

METHOD AND PLANT FOR MANUFACTURING CEMENT CLINKER

The present invention relates to a method for manufacturing cement clinker whereby cement raw meal is preheated and burned into clinker in a kiln system and subjected to cooling in a subsequent clinker cooler, by which method waste is heated in a separate compartment where the gas generated by the waste heating process is fed to the kiln system for heating the cement raw meal and where the solids content is diverted from the compartment.

The invention also relates to a plant for carrying out the method.

Examples of combustible waste include automobile tyres, railway sleepers, furniture, carpets, wood refuse, garden waste, kitchen waste, paper sludge, biomass, petcoke, sewage sludge and bleaching earth.

From EP-1200778, the contents of which is hereby invoked as being part of the present application, is known a method as well as a plant of the aforementioned kind according to which the waste is burned in the separate compartment subject to simultaneous supply of hot air coming preferably from the clinker cooler. The exhaust gases generated during the combustion process are led from the compartment into the preheater of the kiln system where they are used for heating the cement raw meal. In actual practice, this method and plant have proved to be particularly suitable for waste incineration and provides a significant heat input contribution which is beneficial for the total operating economics of the plant. However, it has also ascertained that the NOx emission from this known plant may be quite high due to the fact that there is no reduction of the NOx which is formed in the kiln in connection with the burning of the cement clinker.

In EP-1756502, the contents of which is also invoked as being part of the present application, it is proposed that this problem is solved by introducing NO_x containing exhaust gases, preferentially from the kiln of the kiln system, into the compartment thereby significantly reducing the NO_x content of the exhaust gases by various NO_x reducing reactions. For both plants described above, the amount of waste which can be treated in the separate compartment is limited by the heat requirement in the kiln system preheater to which the combustion gases are fed from the compartment. Furthermore, the waste incineration in the compartment cannot fully substitute the fuel which is required in the preheater since, in actual practice, it has proved to be difficult to control the temperature in the waste incineration compartment, hence necessitating firing of at least a small amount of fuel directly in the preheater. For the known plants the size of the lumps of waste being treated in the separate compartment is also limited by the given retention time in the compartment since any unburned waste which is directed from the compartment into the preheater system may cause operational problems due to the formation of coatings and increased emission rates of CO and SO₂. Traditionally, this problem is solved by ensuring that the waste has a sufficiently small particle size which will ensure complete incineration in the compartment during the given retention time. However, much of the waste which may be used as fuel in connection with the cement manufacturing process is available in relatively large unit sizes, such as railway sleepers, furniture and car tyres, entailing need for a relatively expensive comminution process before the waste can advantageously be used as fuel.

It is the object of the present invention to provide a method as well as a plant for manufacturing cement clinker according to which the amount and size of waste being treated in the separate compartment is independent of the specific cement production of the plant, and so that the comminution of waste will require significantly less resources.

This is obtained according to a first aspect of the present invention by means of a method of the kind mentioned in the introduction, characterized in that the waste is heated to a temperature of at least 150°C in the separate compartment and in that the diverted solids content from the compartment is broken into smaller particle sizes in a comminution means. According to a second aspect of the invention a plant is provided for manufacturing cement clinker comprising a kiln system, a clinker cooler and a separate compartment for heating of waste, said compartment comprises an inlet for introducing waste into the compartment and outlets, respectively, for transporting the waste generated during the waste heating process to the kiln system and for diverting the solids content, characterized in that it comprises means for heating the waste to a temperature of at least 150°C in the separate compartment and a comminution means for breaking the solids content diverted from the compartment into smaller particle sizes.

Hence it will be possible to treat waste in the separate compartment in much larger unit sizes and also in greater quantities than previously possible due to the fact that the temperature to which the waste is heated in the separate compartment can be adjusted so that the amount of fuel contained in the gases which are directed from the separate compartment to the kiln system preheater corresponds essentially to the heat requirement in the preheater, and due to the fact that the solids content of the heated waste which contains the remaining amount of fuel in the waste is diverted from the compartment and subjected to comminution in a comminution means. By heating the waste to a temperature of at least 150° C it is further obtained that substantially less resources will be required for the subsequent comminution process, which is primarily ascribable to the fact that the transformation which takes place during the heating process will substantially increase the brittleness of the heated waste, making it easier to break down than the untreated waste, and the fact that the absolute mass of the heated waste is smaller due to the release of volatile components in the waste during the heating process. The heated and comminuted waste can subsequently be used as fuel at any location in the cement manufacturing plant where introduction of fuel will be necessary, such as in the kiln or calciner. It will thus be possible to substitute the entire fuel requirement in the cement manufacturing plant for relatively low-cost waste fuels.

In principle, the waste may be heated in any suitable manner in the separate compartment, e.g. by means of electricity, hot gases, by firing of fuel or by introducing preheated, calcined or burned raw meal. However, it is preferred that the waste is heated by means of process gas from the plant per se, in which case the process gas may take the form of hot air from the clinker cooler or hot exhaust gases from the kiln system. Heating of the waste in the separate compartment may take place by direct as well as indirect contact between the gases and the waste. Heating of the waste in the separate compartment may further take place in concurrent flow, counter flow or in cross-flow relative to the supplied gas depending on which method is deemed to provide optimum performance.

The heated and comminuted solids may be introduced directly from the comminution means or from an intermediate store at a location which is deemed to be optimum for the subsequent combustion process. If, for example, the waste is heated to such a temperature, typically higher than 300° C, in the separate compartment that the solids content consists mainly of coke, such coke may, according to the invention, be advantageously introduced to the kiln system at a location where the oxygen content is high, for example at a level of 10% or higher, in order to ensure swift burnout of the coke. In connection with the subsequent detailed description of the different examples of embodiments of the plant according to the invention, this aspect of the invention will be described in further details.

In order to optimize the capacity of the plant and to control the temperature in the separate compartment, it is further preferred that cement raw meal can be introduced into the compartment via an inlet for cement raw meal. The input raw meal will serve as a heat reservoir which will contribute towards maintaining the temperature at the desired level even subject to fluctuating process conditions. The compartment may comprise an inlet for introducing raw meal from the preheater, the calciner and/or a raw meal store.

In one embodiment the plant comprises means for heating the waste in the separate compartment in the form of a duct for introducing process gases into the compartment, said duct is either connected to the clinker cooler or to the kiln system.

The compartment for heating the waste may be configured in any suitable manner. In one embodiment it may be configured as a housing containing a substantially horizontal rotary disc which supports and transports the waste through the compartment from its inlet to its outlet, where the compartment comprises a suitable arrangement for diverting the solids content of the heated waste.

In an alternative embodiment of the invention the compartment may consist of a drum, the centre axis of which being preferentially fitted at a small angle relatively to horizontal, and being configured for rotation about its centre axis where the waste is introduced at one end of the drum and discharged at the other end. The advantage of utilizing a drum is that, because of the resulting impacting and pounding, the surface of the waste is thus continuously eroded and will gradually fall off as the waste from the outside and inwards towards the core is embrittled.

The comminution means may be comprised by any suitable means for comminution of brittle material, such as a tube mill, a roller mill or a roller press.

The plant according to the invention may further advantageously comprise one or several means for removing metal parts and other undesirable components from the solids content. These means may comprise magnetic separators and may be installed in front of as well as after the comminution means.

Given that the amount of waste which is treated in the separate compartment may be independent of the specific output of cement of the plant, there may be periods when the production of waste fuel far exceeds that required in the cement manufacturing plant. It is therefore preferred that the plant comprises a silo for storing waste fuel which is heated and comminuted as described in the foregoing text.

The invention will now be explained in further details with reference to the drawing, being diagrammatical, and where

Fig. 1 shows a general process diagram for a plant for carrying out the method according to the invention,

Fig. 2 shows a preferred embodiment of a plant for carrying out the method according to the invention, Fig. 3 shows a modified embodiment of the plant shown in Fig. 2, and

Figs. 4 and 5 show alternative embodiments of plants for carrying out the method according to the invention.

In the process diagram shown in Fig. 1 is seen a plant for manufacturing cement clinker. The plant comprises a kiln system with a preheater system which comprises a preheater 1 and a calciner 3, and a kiln 5, a clinker cooler 7 and a separate compartment 9 for heating waste which is introduced via an opening 1 1 in the compartment 9. During the operation of the plant, cement raw meal from a not shown raw meal store is directed to the raw meal inlet F of the preheater 1. From here the raw meal is directed towards the kiln 5 through the preheater 1 and the calciner 3, and during the process it is heated and calcined by means of hot exhaust gases coming from the kiln 5 via ducts A and/or B. In the kiln 5, the calcined raw meal is burned to cement clinker which is cooled in the subsequent clinker cooler 7 by means of atmospheric air. Some of the air thus heated is directed from the clinker cooler 7 via a duct 15 to the calciner 3. As is apparent from the process diagram, the separate compartment 9 for supplying the energy required for heating the waste may via ducts 2 be connected to one or several sources, such as the process units of the plant, comprising the clinker cooler 7, the kiln 5, the calciner 3 or the preheater 1 , or a separate energy unit 6 in the form of, for example, an electrically driven or fuel-driven heat generator. For transporting the gases generated during the waste heating process to the kiln system, the separate compartment 9 may also via ducts 4 be connected to the kiln 5, the calciner 3 and the preheater 1 , respectively. The separate compartment 9 further comprises a suitably configured solids outlet 8 for diverting the heated solids content.

According to the invention, the waste is heated to a temperature of at least 150° C in the separate compartment 9 so that the solids content is embrittled, hence requiring less resources to effect comminution. The heated solids content is subsequently extracted via the solids outlet 8 of the compartment from the compartment 9 and directed to a comminution means 10 in which comminution of the solids content to a smaller particle size takes place, preferentially to a particle size which is suitable for injection via traditional burners. From the comminution means 10 the comminuted solids having a high fuel value can be directed via the ducts 12 directly to the kiln 5 for firing via the kiln main burner or to the calciner 3 for firing herein. Alternatively the comminuted solids can be directed to a silo 16 for storage for later use. A magnetic separator or a similar device may be fitted immediately before or after the comminution means to separate off any undesirable material components.

The temperature to which the waste is heated in the separate compartment 9 can be adjusted in different ways, inter alia by adjusting the volume of gases which are introduced into the compartment 9 and by using gases having different temperatures from the process units of the plant. Hence it will be possible to adapt the amount of fuel which is entrained in the gas stream flowing from the separate compartment to the preheater of the kiln system in accordance with the heat requirement in the preheater, while ensuring at the same time that the solids content of the heated waste contains the remaining amount of fuel in the waste. In the separate compartment 9 it will thus be possible to treat waste in much larger unit sizes and also in greater quantities than hitherto feasible.

In Fig. 2 is seen a preferred embodiment of the plant for carrying out the method according to the invention. The shown plant comprises a cyclone preheater 1 , a calciner 3, a rotary kiln 5, a clinker cooler 7 and a compartment 9 for heating waste which is introduced into the compartment 9 via an inlet opening 1 1. During the operation of the plant, cement raw meal is introduced into the preheater 1 via a raw meal inlet F and from here it is directed towards the rotary kiln 5 through the cyclones of the preheater 1 and the calciner 3, and it is thus heated and calcined by means of hot exhaust gases. In the rotary kiln 5 the calcined raw meal is burned into cement clinker which is cooled in the subsequent clinker cooler 7 by means of atmospheric air. Some of the air thus heated is directed from the clinker cooler 7 via a duct 15 to the bottom of the calciner 3. In this embodiment the compartment 9 is shown as a stationary compartment which is located between the rotary kiln 5 and the calciner 3. The compartment 9 is supplied with hot exhaust gases from the rotary kiln 5 via a duct 2. From the compartment 9 the exhaust gases generated in connection with the heating of the waste are directed into the calciner 3 via an opening/duct 4 which are located on the side of the calciner 3. In the shown embodiment the waste is transported during the heating process through the compartment 9 in direction towards the solids outlet 8 of the compartment by means of a rotary disc 20. The transport of the waste through the compartment 9 can, however, be effected according to a number of other conceivable methods, and the compartment 9 per se may also be otherwise configured, for example as a drum.

The exhaust gases which are introduced via the duct 2 into the separate compartment 9 from the rotary kiln 5 will typically have a temperature within the range of 1000 and 1200° C and since all exhaust gases from the rotary kiln 5 are directed through the compartment 9, the waste in this embodiment will typically be heated to a level above its ignition temperature. In this embodiment it will therefore be possible to attain an almost complete burnout of the smaller particles of the waste, so that the solids content which is extracted via the outlet 8 will substantially consist only of coke and larger, brittle units of waste.

A significant advantage of the plant shown in Fig. 2 is that the calciner 3 is configured with a lower section which is fed with preheated air with a high oxygen content from the clinker cooler 7. Hence firing of the heated and comminuted solids content, which comprises coke and coke-like constituents requiring a relatively long retention time in a oxygen-rich environment in order to ensure effective burnout, may take place at the bottom of the calciner 3 where these conditions are fulfilled. Another benefit is a significant NO_x reduction in the exhaust gases from the kiln 5 in the separate compartment 9 as a result of the reducing conditions prevailing here.

The plant shown in Fig. 3 essentially corresponds to that shown in Fig. 2 except for the fact that the compartment 9 has been moved away from its location on the side of the calciner 3. Hence the compartment 9 may in principle be placed at any location, for example directly at ground level. Such a location of the compartment 9 will be particularly advantageous in cases where the amount of waste targeted for treatment in the compartment 9 contains more energy than immediately required for the cement manufacturing process, and where the excess fuel in the form of heated and comminuted waste is either intended for storage in the silo 16 or for sale. Furthermore, for this embodiment it will not be necessary to provide space in the preheater tower per se for the separate compartment, which will often entail difficulties, and furthermore the requirement for transport equipment for transporting the waste to the separate compartment will be reduced. As attained for the embodiment shown in Fig. 2, there will be a significant NO_x reduction in the exhaust gases from the kiln 5 in the separate compartment 9 as a result of the reducing conditions prevailing here.

The plant shown in Fig. 4 deviates mainly from that shown in Fig. 2 in that the hot air from the clinker cooler 7 is directed into the compartment 9 instead of into the calciner 3. Hence the air stream in the compartment 9 will contain a significantly higher level of oxygen, thereby making it possible to achieve burnout not only of the volatile, combustible components of the waste but also of a substantial part of the solid, combustible components. However, the disadvantage of the embodiment is its absence of a NOx reducing zone and the fact that the NOx content in the exhaust gases which is led up through the preheater will be probably be at a relatively high level.

The main difference between the plant shown in Fig. 5 and that shown in Fig. 2 is that the compartment 9 is located under the calciner 3 thereby leading all exhaust gases from the rotary kiln 5 and the compartment 9 up through the calciner 3, while the hot air from the clinker cooler 7 is fed into the calciner 3 downstream of the latter viewed in the flow direction of the exhaust gases. As applicable for the embodiments shown in Figs. 2 and 3, there will be a significant NO_x reduction in the exhaust gases from the kiln 5 in the separate compartment 9 as a result of the reducing conditions prevailing here. Furthermore, large parts, such as steel parts, can be fed to the kiln 5 for conversion herein. However, the disadvantage of the embodiment is that the calciner 3 has no zone with a high oxygen content which will ensure effective burnout of the heated and comminuted solids content which comprises coke and coke-like constituents.

In all shown embodiments the temperature in the compartment 9 may also be controlled by introducing cement raw meal from the preheater, the calciner and/or the raw meal store.

The invention is not limited to the shown embodiments but can be used in many alternative configurations employing similar or different process units.

Claims

Patent claims

1. Method for manufacturing cement clinker whereby cement raw meal is preheated and burned into clinker in a kiln system (1 , 3, 5) and subjected to cooling in a subsequent clinker cooler (7), by which method waste is heated in a separate compartment (9) where the gas generated by the waste heating process is fed to the kiln system for heating the cement raw meal and where the solids content is diverted from the compartment (9), characterized in that the waste is heated to a temperature of at least 150°C in the separate compartment (9) and in that the diverted solids content from the compartment is broken into smaller particle sizes in a comminution means (10).

2. Method according to claim 1 , characterized in that the waste is heated in the separate compartment (9) by means of process gas from the plant per se.

3. Method according to claim 2, characterized in that the waste is heated in the separate compartment (9) by means of process gas from the clinker cooler (7) or the kiln system (1 , 3, 5).

4. Method according to any of the claims 1 -3, characterized in that the heating of the waste in the separate compartment (9) takes place by direct contact between the gases and the waste.

5. Method according to any of the claims 1 to 4, characterized in that the heated and comminuted solids from the comminution means (10) or from an intermediate store (16) is introduced to the kiln system at a location where the oxygen content is at a level of 10% or higher.

6. Method according to any of the claims 1 to 5, characterized in that cement raw meal is introduced into the compartment (9) via an inlet for cement raw meal.

7. Plant for manufacturing cement clinker comprising a kiln system (1 , 3, 5), a clinker cooler (7) and a separate compartment (9) for heating of waste, said compartment (9) comprises an inlet (11 ) for introducing waste into the compartment (9) and outlets (4, 8), respectively, for transporting the waste generated during the waste heating process to the kiln system (1 , 3, 5) and for diverting the solids content, characterized in that it comprises means (2, 6) for heating the waste to a temperature of at least 150°C in the separate compartment (9) and a comminution means (10) for breaking the solids content diverted from the compartment (9) into smaller particle sizes.

8. Plant according to claim 7, characterized in that the means for heating the waste in the separate compartment (9) comprises a duct (2) being connected to either the clinker cooler (7) or to the kiln system (1 , 3, 5).

9. Plant according to claim 7, characterized in that the compartment (9) for heating the waste is configured as a housing containing a substantially horizontal rotary disc which supports and transports the waste through the compartment (9) from its inlet (1 1 ) to its outlet (8).

10. Plant according to claim 7, characterized in that the compartment (9) consists of a drum.

11. Plant according to claim 7, characterized in that the comminution means (10) comprises a tube mill, a roller mill or a roller press.

12. Plant according to claim 11 , characterized in that it comprises one or several means for removing metal parts and other undesirable components from the solids content, such as a magnetic separator.

13. Plant according to claim 12, characterized in that it comprises a silo (16) for storing waste fuel which is heated and comminuted.