WO2009030700A2

WO2009030700A2 - Method and plant for the simultaneous production of electricity and cement clinker

Info

Publication number: WO2009030700A2
Application number: PCT/EP2008/061602
Authority: WO
Inventors: Sören HUNDEBÖL
Original assignee: Flsmidth A/S
Priority date: 2007-09-07
Filing date: 2008-09-03
Publication date: 2009-03-12
Also published as: WO2009030700A3; WO2009030700A9

Abstract

The present invention relates to a method for simultaneous generation of electricity and cement clinker in a cement plant comprising a kiln system (1 ), a drying means (2), separation means (3) and means (4) for generation of electricity, according to which cement raw materials are dried in the drying means (2) subject to simultaneous supply of hot gases and subsequently separated from the hot gases in the separation means (3) and directed to the kiln system via a feed duct (5). The method is characterized in that hot exhaust gases from a thermodynamic machine (6) are utilized for drying the cement raw materials. It is hereby obtained that the thermal energy in the hot exhaust gases from thermodynamic machine is utilized in expedient manner. When a generator is connected to the thermodynamic machine and the generated electricity is utilized in the cement plant, the energy consumption per tonne of cement clinker will, for raw materials with a high moisture content, be reduced relative to a situation where the cement raw materials are dried by different methods. Hence the emission of CO2 from the cement plant per tonne of cement clinker will also be reduced.

Description

METHOD AND PLANT FOR THE SIMULTANEOUS PRODUCTION OF ELECTRICITY AND CEMENT CLINKER

The present invention relates to a method for simultaneous generation of electricity and cement clinker in a cement plant comprising a kiln system, a drying means, separation means and means for generation of electricity, according to which cement raw materials are dried in the drying means subject to simultaneous supply of hot gases and subsequently separated from the hot gases in the separation means and directed to the kiln system via a feed duct. The invention further relates to a plant for carrying out the method.

A substantial amount of energy is normally used in connection with the manufacture of cement, typically being of the order of 100-1 10 kWh/ton finished cement in modern plants. Patent literature contains several suggestions of ways to generate this electricity at the cement plant.

From GB1483166A is known a cement manufacturing plant in which the thermal energy in exhaust gases from the kiln system of the plant is used for generation of electricity. It is described how steam produced in a boiler is led to a turbine which is connected to a generator, whereby electricity is generated. It is a well- known fact that the efficiency with which thermal energy can be converted into electrical energy is substantially increased in step with the inlet temperature of the process gases being introduced into a boiler section. Two major problems encountered when increasing the inlet temperature relate to contamination and erosion of the boiler tubes. For known plants it is a proven fact that the heat transfer decreases substantially over time because coatings are formed on the outer side of the boiler tubes. The aforementioned problem in the form of coating formations on the boiler tubes can be avoided by placing the boiler section at a location where the temperature of the exhaust gases is less than 500 °C, which is the case in GB1483166A, whereby most of the alkali and chloride will be condensed. However, the disadvantage of this method is that the efficiency will be so low as to render it a less attractive option from an economic viewpoint. From DE10062066A1 is known a cement plant in which a gas turbine with internal combustion connected to a generator is used to generate electricity. Here exhaust gases from the gas turbine are directed to the gas turbine in the rotary kiln, thereby utilizing the thermal energy in the exhaust gases. However, in a modern kiln system only a small amount of air is introduced through the main burner, e.g. 3-10% of the total amount of air required for the burning processes in the kiln system due to the fact that there a sufficient amount of hot air can be sourced from the cooler, which cools the produced clinker. Furthermore the main burner will not be able to handle too hot gases since the burner lance, which protrudes in the very hot rotary kiln must be kept cool to maintain its mechanical stability. Therefore, a gas turbine, which is designed with capability to deliver its full amount of exhaust gases to the main burner in the rotary kiln, would be of a modest size and may in the best-case scenario be able to produce around 2 kWh/ton clinker. Also in DE10062066A1 use is not made of the fact that the gas turbine could produce a significant pressure of the exhaust gases whereby the electricity consumption of the primary air blower could be significantly reduced. On the contrary the blower in DE10062066A1 must be capable of taking in cold air on the suction side, which demands an under-pressure. The function of the stack in DE10062066A1 is exactly to avoid a pressure build-up. This design, where the exhaust gases from a gas turbine are used as combustion air in a main burner in a rotary kiln, must, from an energy viewpoint, be considered to be an inappropriate solution.

It is the objective of the present invention to provide a method as well as a plant for simultaneous generation of electricity and cement clinker by means of which the aforementioned disadvantages are eliminated. The invention is mainly concerned with plants using materials with a high moisture content for the production.

This is obtained according to the present invention by a method of the kind mentioned in the introduction and being characterized in that hot gases from a thermodynamic machine are used for drying the cement raw materials. Hence it is possible to utilize in appropriate manner the thermal energy in the hot exhaust gases of the thermodynamic machine. When a generator is connected to the thermodynamic machine and the generated electricity is utilized in the cement plant, the energy consumption per tonne of cement clinker is reduced compared to a situation where the electricity for the production is produced in a power plant. This is due to the avoidance of the conversion loss in the power plant and the transmission loss from power plant to cement plant. Hence, the total emission of CO₂ from the cement plant manufacture per tonne of cement clinker will be reduced.

The thermodynamic engine may be a gas turbine with internal combustion, defined as a unit in which the entire thermodynamic process with compression, combustion, etc. is taking place or a diesel engine, possibly with a turbocharger, where the fuel for the mentioned thermodynamic engines for example may be natural gas or light/heavy fuel oil. Common to these thermodynamic engines is that they suck in cold air for compression, inject fuel which, by its combustion, causes an expansion of the gas whereafter the gas on its way out to a lower pressure operates a turbine wheel or a piston which transfers the power to a rotating shaft. Since no thermodynamic engine can transform the calorific value of the fuel completely into mechanical energy, there will always be hot exhaust gases left over from any production of electricity using a thermodynamic engine. In addition to the use of the thermal energy in the hot combustion gases, it is also possible to use the pressure in the exhaust gases from the thermodynamic engine directly to transport the raw materials through the system.

Furthermore the thermodynamic engine may be a steam turbine, defined as a unit in which the entire thermodynamic process with incineration, evaporation etc. takes place, where firing of coal or other combustible material causes water to evaporate. In such a plant it is liquid water which is put under pressure and by taking up heat from the boiler room the water is converted to steam during expansion, after which the steam on its way to a lower pressure in a condenser operates a turbine wheel. In such a plant clean air is directed to the boiler room, and not exhaust gases from the kiln system, whereby problems with coating formations due to alkali and chlorides on the boiler tubes is avoided without entailing a loss of efficiency. Furthermore, it is an advantage that ashes, besides the hot gases, from the incineration continue into the drying means and form a part of the raw materials in the cement production. The exhaust gases from the boiler room do not in this case have a positive pressure, but this is relatively easy to generate by using a fan to handle the cold air to the boiler room.

In addition to drying the cement raw materials, it will be possible to crush the cement raw materials in the drying means and, therefore, the latter may be arranged as a grinding apparatus, for example a vertical roller mill, a ball mill or a drier crusher. The latter option is particularly advantageous when the raw materials have a high moisture content. Also, the drying means may have a different static design, where crushing takes place in external located grinding equipment which is not swept by hot gases.

Hot exhaust gases can be used both from the thermodynamic engine and from the kiln system so as to ensure sufficient amount of hot combustion gases to dry the raw materials to extent required. In this situation it will be possible in a quite simple manner to use the exhaust gases of the thermodynamic engine to lift the raw materials against the gravity. This is achieved by introducing hot gases from the thermodynamic engine to the drying means, where a relative high pressure is necessary, while the exhaust gases from the kiln system are introduced into the drying means through a second channel to an area where a relatively low pressure is required. It is preferred that the exhaust gases from the kiln system are introduced in the drying means at a position, which is located higher and closer to the outlet from the drying means than the position for the inlet of the exhaust gases from the thermodynamic engine. In this way the thermodynamic engine will execute the major part of the transportation of the raw materials and not the fans. The size of the thermodynamic engine can be selected depending on the volume and temperature of the exhaust gases coming from the kiln system.

Since the kiln system does not supply the entire thermal energy for drying, it is not necessary to have a high temperature of the exhaust gases from the kiln system. This means that the number of cyclone steps in a cyclone preheater in the kiln system can be selected without regard to whether the outlet temperature from the preheater is high enough to allow their sole use for drying the raw materials. It is preferred to use four, five or six cyclone steps in the preheater. A low outlet temperature from the preheater increases the efficiency in the kiln system and has the additional advantage that the fan the of the kiln system can be placed immediately after the preheater, which means that only a moderate underpressure will be present in the drying means. With the known techniques it can be necessary, when the raw materials have a very high moisture content, to reduce the number of cyclone steps in the preheater to three, two, or in worst case to one, to carry out the drying, but then the outlet temperature from the preheater will be so high that the fan of the kiln system must be placed after the drying means, which means that a significant underpressure will be present in the drying means.

The exhaust gases from the kiln system will generally be the gases that come out of the cyclone preheater, but it may also include gases which are extracted through a bypass at the rotary kiln, or surplus air from the clinker cooler.

The means for generating electricity may comprise a generator, which is connected to the thermodynamic engine.

The thermodynamic engine may, through reducing means, such as a gear unit, be mechanically linked to crushing means handling raw materials or a cement mill thereby avoiding the energy loss occurring when generating electricity through a generator and re-using it in an engine. Moreover significant costs for electrical equipment used to connect the crushing means or the cement mill to the electricity network will be avoided.

In the following the invention is explained in further details with reference to the drawing, its only figure being diagrammatical, showing a cement plant according to the invention. Figure 1 shows a traditional cement plant with a kiln system 1 comprising a preheater 7, a calciner 8, a rotary kiln 9 and a clinker cooler 10, where cement raw materials in well-known manner are preheated, calcined, burned to cement clinker and cooled. The transportation of the exhaust gases through the system is effected by means of fans 1 1. In addition to the aforementioned features, the plant comprises a drier crusher 2 for drying and crushing the raw materials, which are introduced via a feed inlet 12. The exhaust gases from a thermodynamic engine 6 are directed through a channel 13 to the drier crusher 2. Air is led to the thermodynamic engine 6 through a channel 15. Furthermore hot exhaust gases from the kiln system 1 are directed to the drier crusher 2 via a channel 14. In this way the raw materials are dried by the hot gases. Subsequently, the raw materials are separated from the gases in separations means 3 and directed to the kiln system through a feed inlet 5. Means 4 for generating electricity is connected to the thermodynamic engine 6. It is possible to have several types of treatment of the raw materials, such as additional grinding, screening, intermingling of additional raw materials, homogenization and storage in a silo on the way from the separation means 3 to the kiln system 1. The exhaust gases from the kiln system 1 are via the channel 14 introduced into the drier crusher 2 at a location, which is higher and closer to the outlet of the drier crusher 2 than the location of the inlet of the exhaust gases coming from the thermodynamic engine via the channel 13. In this way it is the thermodynamic engine 6 which executes the majority of the transportation of the raw materials and not the fans 11.

Claims

1. A method for simultaneous generation of electricity and cement clinker in a cement plant comprising a kiln system (1 ), a drying means (2), separation means (3) and means (4) for generation of electricity, according to which method cement raw materials are dried in the drying means (2) subject to simultaneous supply of hot gases and subsequently separated from the hot gases in the separation means (3) and directed to the kiln system (1 ) via a feed duct (5) characterized in that hot exhaust gases from a thermodynamic machine (6) are utilized for drying the cement raw materials.

2. A method according to claim 1 characterized in that the cement raw materials are subjected to comminution in the drying means (2).

3. A method according to any of the preceding claims characterized in that the hot exhaust gases from the thermodynamic machine (6) as well as hot exhaust gases from the kiln system (1 ) are utilized for drying the cement raw materials.

4. A method according to any of the preceding claims characterized in that the means for generating electricity comprises a generator which is connected to the thermodynamic machine (6).

5. A method according to any of the preceding claims characterized in that the kiln system (1 ) comprises a cyclone preheater (7) which comprises at least one cyclone stage.

6. A method according to any of the preceding claims characterized in that the overpressure in the exhaust gases from the thermodynamic machine

(6) is utilized to transport the raw materials through the system.

7. A method according to any of the preceding claims characterized in that the thermodynamic machine (6) is a gas turbine or a diesel engine.

8. A method according to any of the claims 1 -6 characterized in that the thermodynamic machine (6) is a steam turbine with a boiler.

9. A plant for carrying out the method according to claim 1 , comprising a kiln system (1 ), a drying means (2), separation means (3) and means (4) for generation of electricity in which plant cement raw materials are dried in the drying means (2) subject to simultaneous supply of hot gases and subsequently separated from the hot gases in the separation means (3) and directed to the kiln system (1 ) via a feed duct (5) characterized in that hot exhaust gases from a thermodynamic machine (6) are utilized for drying the cement raw materials.

10. A plant for manufacturing cement comprising a kiln system (1 ), a drying means (2) and separation means (3) in which plant cement raw materials are dried in the drying means (2) subject to simultaneous supply of hot gases and subsequently separated from the hot gases in the separation means (3) and directed to the kiln system (1 ) via a feed duct (5) characterized in that hot exhaust gases from a thermodynamic machine (6) are utilized for drying the cement raw materials and in that the thermodynamic machine (6) is mechanically connected to crushing means for the raw materials or a cement mill.