WO2013075719A1

WO2013075719A1 - Method and plant for manufacturing cement clinker

Info

Publication number: WO2013075719A1
Application number: PCT/DK2012/050424
Authority: WO
Inventors: Søren Hundebøl
Original assignee: FLSmidth AS
Current assignee: FLSmidth AS
Priority date: 2011-11-23
Filing date: 2012-11-20
Publication date: 2013-05-30
Anticipated expiration: 2014-05-23

Abstract

Described is a method and an apparatus for manufacturing cement clinker by which method cement raw materials are heat treated in a kiln system comprising a suspension preheater (1) and a kiln (5), where the cement raw materials are introduced and preheated in the preheater (1) by heat exchange with hot exhaust gases and burned into cement clinker in the kiln (5), where the exhaust gases from the preheater (1) are optionally fed to a cement raw material milling department (9) or to a gas treatment system (11), respectively, for capturing of harmful component in the exhaust gases on dust contained in the exhaust gases, which is then fed to a main filter (15) for removal of the dust carrying the captured harmful components. The method and apparatus are peculiar in that the exhaust gases from the preheater (1) in two different operating modes A and B alternately is fed to the cement raw material milling department (9), operating mode A, and to the gas treatment system (11),operation mode B, and in that the dust removed from the exhaust gases in the main filter (15) and carrying the captured harmful components during operation mode A is returned to the preheater (1) and during operation mode B is extracted for further treatment. It is hereby obtained that the amount of harmful components that are circulating between the preheater of the kiln system and the cement raw material milling department may be removed by disposal of far less dust than by the known method. The reason for this is that the concentration of harmful components in the precipitated dust which is finally disposed is much higher. This higher concentration of the harmful components in the finally disposed dust is obtained by operating the plant in two different modes A and B, where during operating mode A the concentration of harmful components circulating between the preheater of the kiln system and the cement raw milling department is gradually increased by returning the main filter dust to the preheater until a certain concentration depending on the kind of harmful components is reached, and during operating mode B the dust when having such a certain concentration of harmful components is removed for further treatment.

Description

Method and plant for manufacturing cement clinker

The present invention relates to a method for manufacturing cement clinker by which method cement raw materials are heat treated in a kiln system comprising a suspension preheater and a kiln, where the cement raw materials are introduced and preheated in the preheater by heat exchange with hot exhaust gases and burned into cement clinker in the kiln, where the exhaust gases from the preheater are optionally fed to a cement raw material milling department or to a gas treatment system, respectively, for capturing of harmful component in the exhaust gases on dust contained in the exhaust gases, which is then fed to a main filter for removal of the dust carrying the captured harmful components. The invention also relates to a plant for carrying out the inventive method.

A method as well as a plant of the aforementioned kind is commonly known, e.g. from EP 462305. By this known method the reduction of harmful components in the exhaust gases is of the kind of the so-called end-of-pipe treatment. The emphasis is on reducing such harmful components that are circulating between the preheater of the kiln system and the cement raw milling department, e.g. by being evaporated in the relatively hot preheater and condensed again in the relatively cold cement raw milling department resulting in the component being returned with the raw meal to the kiln system. In EP 462305 the exhaust gases from the preheater is fed to both the milling department and the cooling device in a mutually adjustable ratio, where after dust with a relatively low concentration of harmful components is precipitated from both these exhaust gas streams simultaneously in a filter installation, and is either used in the finished cement or disposed. A major disadvantage by this known method is that a relatively large amount of dust is either disposed or mixed into the finished cement where it because of lack of hydraulic properties has no more effect on the strength properties of the cement than ordinary filler materials, thus most of the energy used for milling and preheating said dust is lost. The reason for the amount of dust gets so high by this known method is that the concentration of harmful components in the precipitated dust is very low, meaning that the amount needs to be high in order to have a balanced amount of harmful components circulating in the plant between the preheater and the milling department.

It is the object of the present invention to provide a method and a plant for manufacturing cement clinker by means of which the aforementioned disadvantages are eliminated or significantly reduced.

According to the invention this is achieved by a method of the kind mentioned in the introduction, and being characterized in that the exhaust gases from the preheater in two different operating modes A and B alternately is fed to the cement raw material milling department, operating mode A, and to the gas treatment system, operation mode B, and in that the dust removed from the exhaust gases in the main filter and carrying the captured harmful components during operation mode A is returned to the preheater and during operation mode B is extracted for further treatment.

It is hereby obtained that the amount of harmful components that are circulating between the preheater of the kiln system and the cement raw material milling department may be removed by disposal of far less dust than by the known method. The reason for this is that the concentration of harmful components in the precipitated dust which is finally disposed is much higher. This higher concentration of the harmful components in the finally disposed dust is obtained by operating the plant in two different modes A and B, where during operating mode A the concentration of harmful components circulating between the preheater of the kiln system and the cement raw milling department is gradually increased by returning the main filter dust to the preheater until a certain concentration depending on the kind of harmful components is reached, and during operating mode B the dust when having such a certain concentration of harmful components is removed for further treatment. Due to the fact that the harmful components have attained high concentration in the exhaust gases, the gas treatment system will perform with high degree of capture. During operation mode A it is preferred that the dust removed from the exhaust gases in the main filter and carrying the captured harmful components is introduced into the preheater together with the cement raw materials. In cases during operation mode B where the harmful components captured by the dust being extracted from the main filter for further treatment mainly consist of combustible components, such as oil or other volatile or semi-volatile carbon- rich compounds it is preferred that it is introduced into a combustion zone of the kiln system for destruction of these harmful components. In cases where the harmful components captured by said dust mainly consist of heavy metals and/or CI containing compounds in amounts that may be tolerated in the finished cement it is preferred that the dust is mixed herewith, whereas, if the amount of such compounds is too high, the dust is preferably disposed or used for other purposes. In cases where the harmful component captured by the dust is ammonia or a derivate of it, it is preferred that the dust is introduced into or right after a combustion zone of the kiln system, in which case the ammonia will be decomposed by reacting with NO in the exhaust gases.

During operation mode B additives for reduction/absorption of emission components may be added to the exhaust gases before the main filter, if necessary after the exhaust gases has been cooled in a cooling device, or the mixing with additives and the cooling of the gas may take place simultaneously.

In order to reduce the dust freight in the exhaust gases from the preheater and in this way to reduce the amount of dust containing the harmful components to be handled it may be advantageously to make a pre-separation of dust in said exhaust gases in an auxiliary filter prior to being fed to the cement raw material milling department or the gas treatment system. The dust separated by the auxiliary filter may be returned to the kiln system, preferably the preheater.

For different reasons it may further be advantageously to feed a proportion of the cement raw materials into the exhaust gases from the preheater before the auxiliary filter. One reason is for reducing the temperature of the exhaust gases before being entered into the auxiliary filter. Another reason is if SO₂, which is produced in the preheater of many cement manufacturing plants, has to be captured. During operation mode A a rather high amount of SO₂ can usually be captured in the cement raw material milling department as sulphite. However during operation mode B, it will be necessary to capture the SO₂ for instance by adding calcium hydroxide to the exhaust gases. The optimal temperature for this reagent is probably about 450-500 °C. By adding a proportion of the cold cement raw materials to the exhaust gases from the preheater before returning it to the preheater via the auxiliary filter the temperature of the exhaust gases leaving the preheater will be increased, thereby attaining a more optimum temperature window for SO₂ capture. If the emission of SO₂ is to be reduced, it is further suggested to feed a proper absorbent into the exhaust gases from the preheater upstream to the point where a proportion of the cement raw materials are introduced.

In addition, the auxiliary filter may be adjusted so that a part of the sulphite produced will pass directly to the main filter from where it can be returned to a combustion zone of the kiln system for oxidation to sulphate and become a part of the clinker.

In some cases it may be advantageously to adjust the separation efficiency of the auxiliary filter in order to let a fine dust fraction be entrained into the gas treatment system, which in some cases could facilitate the capturing of the harmful substances in question.

In order to be able to determine when to switch operation mode from A to B and vice versa it is preferred that the amount of harmful components in the exhaust gases from the preheater is measured.

In many plants the moisture content of the raw materials will determine a minimum ratio of the total operation time in operation mode A to the total operation time in operation mode B, respectively. A number of known methods may be used to get more operation time in operation mode B, such as bringing hot excess air from a cement clinker cooler into the cement raw material milling department during operation mode A.

The plant for carrying out the method according to invention for manufacturing cement clinker comprises a suspension preheater and a kiln for heat treatment of cement raw materials, where the cement raw materials is introduced and preheated in the preheater by heat exchange with hot exhaust gases and burned into cement clinker in the kiln, a cement raw material milling department and a gas treatment system where to the exhaust gases from the preheater is optionally fed for capturing of harmful component in the exhaust gases on dust contained in the exhaust gases, and a main filter for removal of the dust carrying the captured harmful components, and being characterized in that it comprises means for alternately feeding the exhaust gases from the preheater to the cement raw material milling department and the gas treatment system, respectively, and in that it comprises means for returning the dust removed from the exhaust gases in the main filter and carrying the captured harmful components to the preheater and means for extracting said dust for further treatment, respectively.

The plant preferably comprises means for introducing the dust into the preheater together with the cement raw materials. Further, the plant preferably comprises means for introducing the dust into a combustion zone of the kiln system.

Also, the plant may comprise means for mixing the dust with the finished cement and/or for disposing or using the dust for other purposes.

In order to reduce/absorb emission components during operation mode B the plant may comprise means for adding additives into the exhaust gases, said means being positioned before the main filter, preferably after the cooling device, if included. The cooling device can e.g. be a cooling tower with water injection or a heat exchanger arrangement. A gas suspension absorber (GSA) such as described in EP 1 14 477 is a cooling device to which additives may be added simultaneously with the cooling agent water.

The plant preferably comprises an auxiliary filter being positioned upstream the cement raw material milling department and the gas treatment system in order to reduce the dust freight in the exhaust gases from the preheater and in this way to reduce the amount of dust containing the harmful components to be handled. The auxiliary filter is preferable adjustable with respect to separation efficiency.

In cases where the plant comprises an auxiliary filter it may preferably also comprise means for returning the dust separated by said auxiliary filter to the kiln system, preferably to the preheater.

The plant may also comprise means for feeding a proportion of the cement raw materials into the exhaust gases from the preheater before the auxiliary filter. Further, the plant may comprise means for feeding a proper S0₂ absorbent into the exhaust gases from the preheater, said means being position upstream to the point where a proportion of the cement raw materials are introduced.

Further, the plant may advantageously comprise means for measuring the amount of harmful components in the exhaust gases from the preheater.

The invention will now be explained in further details with reference to the drawing, being diagrammatical, and where Fig. 1 shows a sketch of a plant according to the present invention for manufacturing cement clinker, and . 2 shows another embodiment comprising a GSA.

In Fig. 1 is seen a plant for manufacturing cement clinker. The kiln system of the shown plant comprises a suspension preheater 1 , a calciner 3 and a kiln 5 for heat treatment of cement raw materials being feed from a storage F into the preheater 1 via an inlet 6. In the kiln system the cement raw materials is preheated in the preheater 1 by heat exchange with hot exhaust gases coming from the calciner 3, calcined in the calciner 3 and burned into cement clinker in the kiln 5. The cement clinker is afterwards cooled in a clinker cooler 7. The exhaust gases are drawn through the kiln system by means of a fan 8.

The plant further comprises a cement raw material milling department 9 and a gas treatment system 1 1 comprising a cooling tower 12 and a mixing chamber 13 for cooling and mixing additives into the exhaust gases, respectively, where to the exhaust gases from the preheater is optionally fed during operation of the plant for capturing of harmful component in the exhaust gases on dust contained in the exhaust gases. The plant also comprise a main filter 15 for removal of the dust carrying the captured harmful components, a fan 16 for drawing the exhaust gases though the cement raw material milling department 9 and a gas treatment system 1 1 , respectively and a stack 17 for discharging the exhaust gases to the atmosphere. The cement raw material milling department 9 may contain one or more fans, not shown. Connected to the main filter 15 is a discharge duct 18 with a splitter gate 19 for conducting dust containing harmful components from the main filter 15 to a silo 20 for cement raw materials or dust storage 21 , respectively.

In order to decrease the amount of dust containing harmful components which dust has to be removed from the plant for disposal, so that a balanced and allowable amount of harmful components are circulating between the preheater 1 of the kiln system and the cement raw material milling department 9, it is suggested according to the invention to operate the plant in two different operating modes A and B. Thus, during operation mode A the exhaust gases from the preheater 1 is fed to the cement raw material milling department 9 and the dust removed from the exhaust gases in the main filter 15 and carrying the captured harmful components is returned to the preheater, in the shown embodiment via duct 18, splitter gate 19 and silo 20. During operation mode B the exhaust gases from the preheater 1 is fed to the gas treatment system 1 1 and the dust removed from the exhaust gases in the main filter 15 and carrying the captured harmful components is extracted for further treatment, in the shown embodiment via duct 18, splitter gate 19 and storage 21 .

By returning the dust from the main filter 15 to the preheater 1 , as suggested during operation mode A, the concentration of harmful components circulating between the preheater 1 and the cement raw milling department 9 is gradually increased. This operation mode is continued until a certain, preferably predetermined, concentration depending on the kind of harmful components is reached, where after the operation mode is switched to operating mode B. Then, during operation mode B, the harmful components are extracted out of this circulating system by conducting the exhaust gases through the gas treatment system 1 1 and removing the dust from the main filter 15 during this operation mode for further treatment. Due to the fact that the harmful components have attained high concentration in the exhaust gases during operation mode A, the gas treatment system will perform with high degree of capture during operation mode B.

The main filter 15 is preferably a bag-house filter; the cleaning sequence of which is adjustable, so that the cleaning frequency can be selected according to the switching sequence between the two operating modes. One advantage of using the same bag-house filter as main filter 15 for both de-dusting of the exhaust gases from the cement raw material milling department 9 and for de- dusting of the exhaust gases from the gas treatment system, is the fact that the bags are pre-coated with dust during operating mode A and therefore being more capable of catching many small particles that could be produced during operating mode B. Prior to going into operating mode B, the pre-coating on the bags could be preserved by decreasing the cleaning frequency. Likewise the bag-house filter could be emptied for dust containing the harmful components by increasing the cleaning frequency right before starting operation mode A. The shown plant also comprises an auxiliary filter 22, such as an electrostatic precipitator (ESP) or a cyclone battery with mechanical means for adjusting the separation efficiency. By means of the auxiliary filter 22 the dust freight in the exhaust gases from the preheater 1 and thus the amount of dust containing the harmful components to be handled may be reduced prior to being fed to the cement raw material milling department 9 or the gas treatment system 1 1 . The dust separated by the auxiliary filter 22 is as shown returned to the preheater 1 . The separation efficiency of the auxiliary filter 22 is adjustable.

The shown plant further comprises a duct 6a for feeding a proportion of the cement raw materials from storage F into the exhaust gases from the preheater 1 before the auxiliary filter 22. In this way the temperature of the exhaust gases may be lowered to a temperature the auxiliary filter 22 can cope with before being entered into said filter 22. Further, the temperature of the exhaust gases leaving the preheater 1 will in this way be increased, thereby attaining a more optimum temperature window within the range from 450-500 °C for S0₂ capture by adding calcium hydroxide to the exhaust gases upstream to the point where a proportion of the cement raw materials is introduced. During operation mode A a rather high amount of S0₂ can usually be captured in the cement raw material milling department 9 as sulphite.

In the shown plant, it is probably advantageous to operate the duct 6a for feeding a proportion of the cement raw materials from storage F into the exhaust gases from the preheater 1 and the auxiliary filter 22 in the same mode for both operating modes, if there is enough heat in the exhaust gases for drying of the cement raw materials in the milling department 9. However, should this not be the case, the exhaust gases from the preheater 1 should preferably not be cooled by introducing raw materials via duct 6a and should preferably be bypassed the auxiliary filter 22, as indicated by the dotted line 29.

The plant also comprises a measuring device 23 for measuring the amount of harmful components in the exhaust gases from the preheater 1 . In this way the plant operator is able to determine when to switch operation mode from A to B and vice versa.

In Fig. 2 the auxiliary filter 22 is situated after the fan 8. In operation mode A the gases from the filter 22 are conducted into the cement raw material milling department 9, whereas in operation mode B the gases are conducted into a gas treatment system 1 1 . In the embodiment shown in Fig. 2, the gas treatment system 1 1 comprises a gas suspension absorber 25 (GSA) with injection of absorbent and cooling water near a bottom inlet 26 and a cyclone 27. A proportion of the solids separated by the cyclone 27 is returned to the GSA. The proportion not returned is extracted from the plant for further treatment via storage 21 a. Fig. 2 further shows a duct 28 for bringing hot excess air from the clinker cooler 7 into the cement raw material milling department 9 in case the plant is running in operation mode A and an additional heat supply is needed. The hot air is mixed into the exhaust gas flow from the preheater 1 after fan 8 in front of the auxiliary filter 22.

Claims

1 . A method for manufacturing cement clinker by which method cement raw materials are heat treated in a kiln system comprising a suspension preheater (1 ) and a kiln (5), where the cement raw materials are introduced and preheated in the preheater (1 ) by heat exchange with hot exhaust gases and burned into cement clinker in the kiln (5), where the exhaust gases from the preheater (1 ) are optionally fed to a cement raw material milling department (9) or to a gas treatment system (1 1 ), respectively, for capturing of harmful component in the exhaust gases on dust contained in the exhaust gases, which is then fed to a main filter (15) for removal of the dust carrying the captured harmful components, characterized in that the exhaust gases from the preheater (1 ) in two different operating modes A and B alternately is fed to the cement raw material milling department (9), operating mode A, and to the gas treatment system (1 1 ), operation mode B, and in that the dust removed from the exhaust gases in the main filter (15) and carrying the captured harmful components during operation mode A is returned to the preheater (1 ) and during operation mode B is extracted for further treatment.

2. A method according to claim 1 , characterized in that the dust removed from the exhaust gases in the main filter (15) and carrying the captured harmful components during operation mode A is introduced into the preheater (1 ) together with the cement raw materials.

3. A method according to claim 1 , characterized in that the dust being extracted from the main filter (15) during operation mode B is introduced into a combustion zone (3, 5) of the kiln system in cases where the harmful components captured by the dust for further treatment mainly consist of combustible components, such as oil or other volatile or semi-volatile carbon-rich compounds.

4. A method according to claim 1 , characterized in that the dust being extracted from the main filter (15) during operation mode B is mixed with the finished cement in cases where the harmful components captured by the dust for further treatment mainly consist of heavy metals and/or CI containing compounds in amounts that may be tolerated, and if the amount of such compounds is too high, the dust is disposed or used for other purposes.

5. A method according to claim 1 , characterized in that a pre-separation of dust in the exhaust gases from the preheater (1 ) is made in an auxiliary filter (22) prior to being fed to the cement raw material milling department (9) or the gas treatment system (1 1 ).

6. A method according to claim 5, characterized in that the dust separated by the auxiliary filter (22) is returned to the kiln system, preferably the preheater (1 ).

7. A method according to claim 1 , characterized in that a proportion of the cement raw materials is fed into the exhaust gases from the preheater (1 ) before the auxiliary filter (22).

8. A plant for manufacturing cement clinker comprising a suspension preheater (1 ) and a kiln (5) for heat treatment of cement raw materials, where the cement raw materials are introduced and preheated in the preheater (1 ) by heat exchange with hot exhaust gases and burned into cement clinker in the kiln (5), a cement raw material milling department (9) and a gas treatment system (1 1 ), where to the exhaust gases from the preheater (1 ) are optionally fed for capturing of harmful component in the exhaust gases on dust contained in the exhaust gases, and a main filter (15) for removal of the dust carrying the captured harmful components, characterized in that it comprises means (10) for alternately feeding the exhaust gases from the preheater (1 ) to the cement raw material milling department (9) and the gas treatment system (1 1 ), respectively, and in that it comprises means (18, 19, 20) for returning the dust removed from the exhaust gases in the main filter (15) and carrying the captured harmful components to the preheater (1 ) and means (18, 19, 21 ) for extracting said dust for further treatment, respectively.

9. A plant according to claim 8, characterized in that it comprises an auxiliary filter (22) being positioned upstream the cement raw material milling department (9) and the gas treatment system (1 1 ), and means for returning the dust separated by said auxiliary filter (22) to the kiln system, preferably to the preheater (1 ).

10. A plant according to claim 9, characterized in that it comprises means (6a) for feeding a proportion of the cement raw materials into the exhaust gases from the preheater (1 ) before the auxiliary filter (22).