WO2012079580A1

WO2012079580A1 - Method and plant for heating raw materials

Info

Publication number: WO2012079580A1
Application number: PCT/DK2011/050477
Authority: WO
Inventors: Morten Boberg Larsen; Kim Allan Dam-Johansen; Anders Rooma Nielsen
Original assignee: Flsmidth A/S; Glarborg, Peter
Priority date: 2010-12-14
Filing date: 2011-12-13
Publication date: 2012-06-21

Abstract

Described is a method for heating raw materials (1), such as cement raw materials, limestone or other mineral-containing raw materials, by which method the raw materials and a secondary fuel (5) are introduced in a material inlet end (6) of one and the same rotary kiln (3) through which the raw materials (1) as well as the secondary fuel (5) are transported while being heated by gases formed by burning of a primary fuel. The method is characterized in that the secondary fuel (5) and the raw materials (1) are introduced in batches one after another in the material inlet end (6) of the rotary kiln (3).

Description

Method and plant for heating raw materials

The invention relates to a method for heating raw materials, such as cement raw materials, limestone or other mineral-containing raw materials, by which method the raw materials and a secondary fuel are introduced in a material inlet end of one and the same rotary kiln through which the raw materials as well as the secondary fuel are transported while being heated by gases formed by burning a primary fuel. The invention also relates to a plant for carrying out the method.

The method can be utilized in various industries where it is required to heat raw materials in rotary kilns, for example in the cement or minerals industry. Cement production is energy intensive with an energy usage of approximately 3 MJ per kg cement clinker produced. Fuel consumption accounts for about 30- 40% of the total cement clinker production costs. Traditionally, cement production has mainly depended on fossil fuels as coal, oil and natural gas. Due to fierce competition in the cement market, increasing fossil fuel prices as well as environmental concerns, cement producers have increased the utilization of secondary fuels as a substitute for fossil fuels in order to achieve the most economical fuel mix. In this context, secondary fuels cover all non-fossil fuels and waste from other industries. Examples of secondary fuels that can be utilized in the cement industry are tyre-derived fuels, railway sleepers, furniture, carpets, wooden waste, garden waste, kitchen waste, paper sludge, biomass, pet coke, wastewater sludge, meat and bone meal, fuller's earth and different commercial and industrial wastes. It is advantageous to utilize coarse, solid secondary fuels in the material inlet end of cement rotary kilns in order to save expenses for shredding the fuels to smaller particles and to increase fuel flexibility of the system. High temperatures in the rotary kiln and a retention time for the secondary fuel of several minutes provide favourable conditions for fuel burnout.

The main challenge with respect to secondary fuel utilization in the material inlet end of cement rotary kilns is that the solid fuel particles will be in physical contact with the raw materials. For example the problem with physical contact exists in

US4913742A where a continuous flow of raw materials and secondary fuels at the same time are introduced to an inlet zone of a rotary kiln. During the fuel devolatilization, reducing agents such as CO, H₂ and/or hydrocarbons are formed. These reducing agents may react with minor elements in the calcined raw materials before they are oxidized to their ultimate combustion products, H₂O and CO2. In addition, if the fuel particles are fully or partly covered by calcined raw materials, mass transfer of oxygen to the fuel char will be hindered. Sub- stoichiometric amounts of oxygen will lead to incomplete oxidation of the fuel char, forming reducing agents in the form of CO, H₂ and/or hydrocarbons. It is widely recognized that Portland cement should be burnt under oxidizing combustion conditions. The reason is that the existence of local reducing conditions in the calcined raw materials charge may affect the product quality and process stability of the kiln system. The product quality can be influenced by calcined raw materials components such as Fe(lll) being reduced to Fe(ll). Fe(ll) catalyzes alite (3CaO SiO2) decomposition, the main strength-giving component in cement. The process stability is affected by increased release of sulphur from the calcined raw materials. The sulphur release is mainly due to reductive decomposition of CaSO₄ and other sulphates present in the calcined raw materials. Increasing amounts of SO2 in cement kiln systems are problematic because SO2 promotes formation of sulphospurrite (2(2CaO SiO2) CaSO₄) and calcium sulphoaluminate (3CaO-3AI₂O3-CaSO₄), some of the principal constituents of deposit build-ups found in rotary kilns and kiln riser ducts. These sulphur-containing deposit build-ups can lead to blockages that need to be removed, sometimes necessitating a temporary plant shutdown. In recent research work conducted at the Technical University of Denmark, release of sulphur from calcined raw materials under both oxidizing and reducing conditions was investigated. The investigations included thermodynamic equilibrium calculations in the temperature interval 800-1 , 500°C and experiments in a high temperature tube furnace reactor within the temperature range 900- 1 , 100°C. The investigated conditions resembled actual conditions in the material inlet end of rotary kilns. It was found that the sulphates CaSO₄, K₂SO₄ and Na₂SO₄ were all stable under oxidizing conditions, but began to decompose under reducing conditions. Particularly CaSO₄ was sensitive to reducing conditions. The experiments also showed that the sulphur release was most significant if the gas atmosphere frequently shifted between oxidizing and reducing conditions which resulted in significantly higher sulphur release than under either constant oxidizing or constant reducing gas atmospheres.

It is an object of the present invention to provide a method as well as a plant for heating raw materials, where both raw materials and a solid secondary fuel is introduced to a rotary kiln, by means of which method the release of sulphur from the heated raw materials is 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 secondary fuel and the raw materials are introduced in batches one after another in the material inlet end of the rotary kiln.

Hereby it is obtained that the release of sulphur from the raw materials is significantly reduced resulting in fewer deposit build-ups in the rotary kiln and in the kiln riser duct which lead to a better process stability. This is due to the batch- wise introduction which ensures that the raw materials are not exposed to frequent shifts in oxidizing and reducing conditions but subjected to constant oxidizing gas atmospheres.

It is preferred that the raw materials at least are heated to the calcination temperature in order to provide the materials with the necessary thermal treatment. For example clay raw materials, if intended to be used as supplementary material in blended cement, only need to be heated to the calcination temperature. If the raw materials are cement raw materials the temperature must be higher than the calcination temperature in order to burn the cement raw materials into clinker.

Furthermore it is preferred that each batch initially after the introduction in the rotary kiln, when it rests on the inner side of the rotary kiln, is kept separate from the prior introduced batch. The initial separation of an introduced batch to the prior introduced batch may be obtained by use of restrictions on the inside of the rotary kiln or just due to the distance the prior batch has travelled towards the outlet end of the rotary kiln. Such a distance is preferred to exceed at least 0, 1 meter in the transport direction of the materials. The distance must be greater when the introduced batches are made up of small particles as they to a certain extent will flow out to the sides. The distance between the batches can be controlled by the period of time between the feeding of the batches. It is preferred that the period of time is between ten seconds and five minutes, more preferably between thirty seconds and two minutes.

The plant for carrying out the method according to the invention comprises a rotary kiln with a material inlet end wherein the raw materials and a secondary fuel are introduced for subsequently being transported towards a material outlet end while being heated by gases formed by burning of a primary fuel. The plant is characterised in that it comprises means for introducing the raw materials and the secondary fuel in batches one after another in the material inlet end of the rotary kiln.

In principle the means for introducing the raw materials and the secondary fuel in batches could be constituted by any suitable means as long as they are capable of feeding the required size of batches and controlling the time between the introduction of each batch. In one embodiment the means for introducing the raw materials in batches in the material inlet end comprises a preheating arrangement wherein the raw materials are preheated prior to the introduction in the rotary kiln and means for periodic recirculation of the raw materials in the preheating arrangement. The preheating arrangement comprises a number of cyclones and a calciner and the means for periodic recirculation of the raw materials comprise a duct connecting a cyclone, preferably the lowermost cyclone in the preheating arrangement, to the calciner. The recirculation means also comprise regulation means, such as a flap damper, preferably mounted in a duct leading the raw materials from the lowermost cyclone to the rotary kiln. The regulation means should be capable of having at least two positions. In one position the raw materials will be transported from the lowermost cyclone to the rotary kiln in known manner. In the other position the raw materials from the lowermost cyclone will be transported to the calciner thus creating a recirculation of raw materials in the preheating arrangement. The regulation means may be electronically or manually controlled. In another embodiment the means for introducing the raw materials in batches in the material inlet end comprises a silo with regulation means, such as a slide gate. In this case the silo may be located between the lowermost cyclone in the preheating arrangement and the rotary kiln so the continuous flow of raw materials from the preheater is stored in the silo until a batch of raw materials is needed. The means for introducing the secondary fuel may comprise a silo but could also comprise a conveyor system which delivers the secondary fuel in batches.

In a preferred embodiment the first portion of the rotary kiln, which comprises the material inlet end, has a decreasing inner diameter in the transport direction of the raw materials. The material residence time and inner kiln diameter are inversely proportional, which means that the raw material gradually will be transported slower and slower through the first portion of the rotary kiln. In this way the gaps between the raw materials batches are closed as they are transported through the first portion of the rotary kiln resulting in a continuous flow of heat treated raw materials leaving the rotary kiln. The continuous flow out of the rotary kiln may also be ensured by other means, e.g. by adjusting the rotary kiln inclination angle or by use of internal restrictions. The rotary kiln may not only have a variable inner diameter, but possibly also a variable outer diameter.

In a further embodiment the first portion of the rotary kiln, which comprises the material inlet end through which the batches are introduced, comprises internal restrictions affecting the retention time of the raw materials and the secondary fuel in the rotary kiln. In principle the restrictions may have all kind of shapes as long as they are capable of affection the retention time. In yet another embodiment the first portion of the rotary kiln, which comprises the material inlet end through which the batches are introduced, comprises means to optimize the gas/solid heat transfer. The means may be lifters which mechanically influence the mixing and/or transport of the raw materials and the solid secondary fuel and they may be manufactured from steel, stone or heat- resistant material.

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

Fig. 1 shows a plant for carrying out the method according to the invention, and

Fig. 2 shows another embodiment of a plant for carrying out the method according to the invention.

In Fig. 1 is illustrated a plant for manufacturing cement clinker where cement raw materials 1 are preheated in a preheating arrangement 2 and burned into clinker in a rotary kiln 3 and subsequently cooled in a clinker cooler 4. The cement raw materials 1 and a secondary fuel 5 are through separate inlets 8, 9 introduced in the material inlet end 6 of the rotary kiln 3 for subsequently being transported towards an outlet end 10 while being heated by gases formed by burning of a primary fuel in a main burner 1 1 . During the transport the secondary fuel 5 is converted to gases and solid matter in the form of combustion residues whereas the cement raw materials 1 gradually will be heated to form cement clinker. The process gases are drawn in known manner through the rotary kiln 3 and onward through the preheating arrangement 2 by means of a fan (not shown). The gases formed during the heating of the secondary fuel 5, inclusive of solid matter entrained in the gases, may be utilized for additional process stages, such as in a calciner 7. Hot air from the clinker cooler 4 is directed to the calciner 7 via a duct

16. The secondary fuel 5 and the raw materials 1 are introduced in batches one after another in the material inlet end 6 of the rotary kiln 3. The batch-wise introduction ensures that the raw materials 1 are not exposed to frequent shifts in oxidizing and reducing conditions but subjected to constant oxidizing gas atmospheres. In this way the release of sulphur from the raw materials 1 is significantly reduced which results in fewer build-ups in the rotary kiln 3 and in the kiln riser duct 17. This leads to a better process stability. A silo 12 is located between a lowermost cyclone 14 in the preheating arrangement 2 and the kiln 3 to store the continuous flow of raw materials 1 from the preheating arrangement 2. An electronically controlled slide gate 13 in the bottom of the cyclone 14 opens when a batch of raw materials 1 is required. When the batch has been transported a distance towards the outlet end 10, due to the rotation and inclination of the rotary kiln 3, a batch of secondary fuel 5 is introduced through the inlet 9. The distance between the batches is controlled by the time between the introduction of the batches. The optimal distance depends on the kind of materials in the batches. A first portion 15 of the rotary kiln 3 has a decreasing inner diameter in the transport direction of the raw materials 1 and because the residence time and the inner kiln diameter are inversely proportional the raw materials 1 gradually will be transported slower and slower through the first portion 15 of the rotary kiln. In this way the gaps between the batches of raw materials 1 are closed as they are transported through this first portion 15 resulting in a continuous flow of heat treated raw materials 1 leaving the rotary kiln 3.

In Fig. 2 is shown a plant where the means for the batch-wise introduction of the raw materials 1 from the preheating arrangement 2 differs from the means shown in Fig. 1 . A duct for leading raw materials 1 from the lowermost cyclone 14 to the inlet 8 is provided with a flap damper 18, which is capable of having two positions. In the first position the raw materials 1 from the cyclone 14 are transported to the rotary kiln 3 through the inlet 8. In the second position (the shown position) the raw materials 1 from the lowermost cyclone 14 are transported to the calciner 7 through a duct 19. In this way the raw materials 1 are recirculated in the preheating arrangement 2 until the flap damper returns to the first position. During the recirculation a batch of secondary fuel 5 is introduced through the inlet 9.

Claims

1 . A method for heating raw materials (1 ), such as cement raw materials, limestone or other mineral-containing raw materials, by which method the raw materials and a secondary fuel (5) are introduced in a material inlet end (6) of one and the same rotary kiln (3) through which the raw materials (1 ) as well as the secondary fuel (5) are transported while being heated by gases formed by burning a primary fuel characterized in that the secondary fuel (5) and the raw materials (1 ) are introduced in batches one after another in the material inlet end (6) of the rotary kiln (3).

2. A method according to claim 1 characterized in that the raw materials (1 ) at least are heated to the calcination temperature.

3. A method according to claim 1 or 2 characterized in that each batch initially after the introduction in the rotary kiln (3) is kept separate from the prior introduced batch.

4. A method according to any preceding claim characterized in that each batch initially after the introduction in the rotary kiln (3) is separated from the prior introduced batch by a distance of at least 0,1 meter in the transport direction of the materials.

5. A method according to claim 4 characterized in that the distance is controlled by the period of time between the introduction of the batches.

6. A plant for heating raw materials (1 ), such as cement raw meal, limestone or other mineral-containing raw materials, comprising a rotary kiln (3) with a material inlet end (6) wherein the raw materials (1 ) and a secondary fuel (5) are introduced for subsequently being transported towards a material outlet end (10) while being heated by gases formed by burning of a primary fuel characterized in that the plant comprises means for introducing the raw materials (1 ) and the secondary fuel (5) in batches one after another in the material inlet end (6) of the rotary kiln (3).

7. A plant according to claim 6 characterized in that the means for introducing the raw materials (1 ) and the secondary fuel (5) in batches in the material inlet end (6) comprises a preheating arrangement (2) wherein the raw materials are preheated prior to the introduction in the rotary kiln (3) and means (18, 19) for periodic recirculation of the raw materials (1 ) in the preheating arrangement (2).

8. A plant according to claim 6 characterized in that the means for introducing the raw materials (1 ) and the secondary fuel (5) in batches in the material inlet end (6) comprises a silo (12) with regulation means (13), such as a slide gate.

9. A plant according to claim 6, 7 or 8 characterized in that the first portion (15) of the rotary kiln (3), which comprises the material inlet end (6) through which the batches are introduced, has a decreasing inner diameter in the transport direction of the raw materials (1 ).

10. A plant according to any of the claims 6-9 characterized in that the first portion (15) of the rotary kiln (3), which comprises the material inlet end (6) through which the batches are introduced, comprises internal restrictions affecting the retention time of the raw materials (1 ) and the secondary fuel (5) in the rotary kiln (3).