Patent Application
20240151468
The invention relates to a rotary kiln for burning carbonate-containing material, in particular limestone or dolomite, comprising a rotary tube having an inlet end for feeding the material to be burned and an outlet end for discharging the burnt material, and a burner unit arranged in the region of the outlet end, the rotary tube having an inlet zone at its inlet end and an outlet zone at its outlet end, a preheating zone and a combustion zone being arranged between the inlet zone and outlet zone in the direction of transport of the material. Furthermore, the present invention relates to a method for burning carbonate-containing material, in particular limestone and dolomite, in such a rotary kiln.
Rotary kilns for burning carbonate-containing material have been known from the prior art for decades and have proven themselves as an efficient and reliable type of furnace for continuously calcining various types of materials. The published prior art is found, for example, in EP 0 674 145 A1, U.S. Pat. No. 1,544,504 A, or U.S. Pat. No. 3,124,338 A.
A central component of a rotary kiln is a long cylindrical rotary tube (often 100 m or longer), which is typically inclined at about 2 to 7% with respect to the horizontal. The rotary cylinder rotates slowly at 0.5 to 1.5 rpm around its axis, and the material to be burned which is introduced at its inlet end by means of a screw conveyor or the like slowly moves by gravitational force through the rotary cylinder in the direction of a flame generated by a burner arranged in the region of the kiln outlet. The kiln is typically filled with the material to be burned up to about 8 to 20% of the kiln diameter. The material to be burned migrates from the inlet zone, which is usually a few meters long, to the preheating zone (sometimes also referred to as the transition zone), in which there are already temperatures of approx. 1100-1200° C., and from there into an extensive combustion zone, which is characterized by maximum temperatures of 1500-1600° C. Due to the temperature, the material to be burned is actually calcinated to a very large extent. The combustion zone is in turn followed by the outlet zone of the rotary tube, which is several meters long and in which the burnt material begins to cool down. From there, the burnt material usually falls through a shaft onto a cooling unit, for example a grate cooler. The specific lengths of the individual zones also depend on the overall length of the rotary tube and the relative position of the burner lance and the flame length.
In order to operate the kiln efficiently, high throughput rates are always sought, while at the same time it must be ensured that the calcination of the material to be burned is complete and all the carbon dioxide is removed from the material. Thorough intermixing of the material to be burned is crucial for a rapid and complete calcination.
Another problem in practical operation is the uneven temperature distribution in the burnt material, which is a direct result of the mixing characteristics of the material in a rotary kiln. Thus, an active portion of the surface of the available kiln volume forms in the material bed, which portion is actively moving during the rotation of the rotary tube and is in intensive temperature exchange with the kiln atmosphere, while a passive part underneath tends to form clusters and does not reach the surface. Accordingly, the active portion in the material bed can cool down rapidly in the outlet zone, while the passive part, due to a lack of exchange with the kiln atmosphere cooling in the outlet zone, maintains a high temperature level for a long time. This can lead to very hot burned material from the passive portion of the material bed damaging the cooling device downstream of the rotary cylinder due to overheating.
It is therefore an object of the present invention to provide a rotary kiln for burning carbonate-containing material, in particular limestone or dolomite, of the type mentioned at the outset which ensures a high material throughput and as complete a calcination as possible of the material to be burned. In particular, damage to components downstream of the rotary cylinder, in particular those for cooling the burnt material, that results from the temperature of the burnt material being too high should be avoided.
According to a first aspect of the present invention, the aforementioned object is achieved with a rotary kiln in that at least one projection is arranged in the outlet zone of the rotary tube, the at least one projection having a contour that tapers in the direction of rotation of the rotary tube.
The particular advantage of the rotary kiln according to the invention is that by arranging at least one protrusion in the outlet zone the burnt material is effectively intermixed along the entire depth of the material bed, so that an intensive temperature exchange with the kiln atmosphere which is cooling down in the outlet zone occurs not only in the active portion of the material but also in the passive portion. This in turn means that no temperature spikes occur in the material, so that temperature-related damage to the components downstream of the rotary cylinder, in particular the cooling units, is minimized by the burnt material. Because the at least one projection has a contour that tapers in the direction of rotation of the rotary tube, it moves through the material in the manner of a plow and intermixes and combs through it intensively. As a result, maximum intermixing is achieved and the division of the material bed into active and passive components is completely eliminated.
As a result of the improved cooling capacity in the outlet zone, the material throughput through the rotary kiln and thus the process efficiency can overall be increased.
According to a first advantageous embodiment of the invention, it is provided that the at least one projection contains a fireproof material. This means that it can be used even at very high temperatures without any problems. Concrete, in particular refractory concrete, as is known per se to a person skilled in the art, is particularly suitable as a refractory material.
According to the invention, the at least one projection has a contour that tapers in the direction of rotation of the rotary tube. It is preferably V-shaped, the tip of the “V” being oriented in the direction of rotation of the rotary tube and thus plowing through the burnt material to be intermixed.
The tapering contour can also be embodied by a prismatic geometry, in particular the geometry of a triangular prism, preferably an isosceles triangular prism. The geometry of a truncated pyramid is also possible, an edge of the truncated pyramid preferably being arranged in the direction of rotation of the rotary tube for the purpose of forming the contour tapering in the direction of rotation of the rotary tube. The edges of the triangular prism or the truncated pyramid can be blunted or rounded to prevent crumbling of sensitive edges in kiln operation.
To dimension the at least one projection, the applicant has carried out intensive experimental investigations. Accordingly, for the purpose of an especially thorough intermixing of the burnt material in the outlet zone of the rotary tube, the at least one projection advantageously has a height in the radial direction with respect to the rotary cylinder of between 50 mm and 500 mm, preferably between 100 mm and 300 mm and particularly preferably of approx. 200 mm, on.
Furthermore, the at least one projection can have a length in the circumferential direction of the rotary tube between 50 mm and 2000 mm, in particular between 100 mm and 1000 mm, preferably between 300 mm and 500 mm and very particularly preferably of approximately 400 mm. The maximum width of the at least one projection in the longitudinal direction of the rotary tube can be between 50 mm and 600 mm, preferably between 300 mm and 500 mm and particularly preferably approximately 400 mm.
The at least one projection provided according to the invention in the outlet zone can be permanently connected to the inner lining of the rotary tube. In order to ensure a particularly secure connection between the inner lining of the rotary tube and the projection, it is provided according to a further advantageous embodiment of the invention that the at least one projection is attached by means of a metallic anchor, in particular a welded metallic anchor, to the inner rotary kiln casing.
According to a particularly advantageous embodiment of the invention, it is provided that a plurality of projections is provided in the outlet zone, the projections being arranged in groups of 1 to 10, especially 3 to 7, preferably 4 to 6, distributed over the circumference of the rotary tube, depending on the diameter of the rotary tube. As a result, the intermixing performance and the associated more uniform cooling of the burnt material are significantly increased already in the outlet zone.
It proves to be advantageous if the projections of a group of projections are alternately offset in the longitudinal direction of the rotary tube, for example in the manner of a zigzag line. Furthermore, a plurality of groups of projections can be arranged one behind the other in the longitudinal direction of the rotary tube. Both measures further increase the effectiveness of the intermixing of the burnt material in the outlet zone.
It is further preferred that at the outlet end of the rotary tube an edge section is provided in which no projections are arranged. As a result, the intended ejection position of the burnt material in the cooler adjoining the rotary tube is not changed with respect to a rotary tube without projections, so that subsequent mounting of the projections is easily possible. Whatever the case, a uniform material bed is ensured in the cooler.
According to a further particularly advantageous embodiment of the invention, it is provided that a plurality of further projections is arranged in the preheating zone of the rotary tube, the further projections being arranged one behind the other as groups essentially parallel to the longitudinal axis of the rotary tube. This results in a greatly improved mixing of the material to be burned and thus a faster calcination with the same time intervals, since due to the thorough mixing not just material at the surface but also material located in the interior of the material bed is exposed to high calcination temperatures and thus can be calcined faster.
According to a particularly advantageous development of the above embodiment, it can be provided that further projections of adjacent groups of further projections arranged adjacent to one another in the circumferential direction of the rotary tube are arranged offset to one another in the longitudinal direction of the rotary tube, such that spiral-shaped transport paths for the material to be burned are formed in the preheating zone. This enables comparatively faster transportation of the material to be burned through the rotary tube and in combination with the improved mixing of the material leads overall to a higher kiln throughput and thus increases the efficiency of the combustion process.
Because of the very high temperatures already prevailing in the preheating zone, it is preferred here that at least some, preferably all, of the further projections each contain a refractory material. These can be various high-temperature resistant materials, as are known per se from the prior art. Concrete, in particular refractory concrete, has proven to be particularly suitable.
In experiments by the applicant, various geometries have proven to be suitable for the further projections provided according to the invention in the preheating zone. Prismatic geometries, in particular in the form of a trapezoidal prism, especially an isosceles trapezoidal prism, are preferred. Such a trapezoidal prism is preferably aligned parallel to the longitudinal extent of the rotary tube and accordingly has two leg surfaces inclined with respect to the radius of the rotary tube. These can include an angle relative to the trapezoidal base between 35° and 75° and preferably between 60° and 70°. The length of such a trapezoidal prism in the longitudinal direction of the rotary tube is preferably approximately 400 mm and the height is preferably approximately 200 mm. In the case of a trapezoidal configuration, the width of the lower trapezoidal base is preferably approximately 200 mm and the width of the upper base is preferably approximately 100 mm.
According to a further advantageous embodiment of the invention, it is provided that 3 to 9, preferably 6 groups of further projections arranged one behind the other in the longitudinal direction are provided over the circumference of the rotary tube.
The groups of further projections arranged one behind the other essentially parallel to the longitudinal axis of the rotary tube provided according to the invention can be arranged over the entire length of the preheating zone. According to a particularly advantageous embodiment of the invention, it is provided that the groups of projections extend over a fifth to a third of the overall length of the rotary tube.
According to a further particularly advantageous embodiment of the invention, it can further be provided that at least one additional projection, preferably a plurality of additional projections, is provided in the inlet zone of the rotary tube, the at least one additional projection having at least one sliding surface inclined with respect to the longitudinal axis of the rotary tube for conveying the material to be burned from the inlet zone to the preheating zone.
This takes advantage of the fact that the material to be burned comes to rest on the at least one sliding surface of the material bed due to the rotation of the at least one additional projection through the material bed and, due to the particular inclination of the sliding surface, slides rapidly towards the preheating zone by gravitational force. It goes without saying that the direction of rotation of the rotary tube and the orientation of the sliding surface in the circumferential direction of the rotary tube must be adapted accordingly to one another. At the same time, this also considerably reduces the formation of dust, since grinding of the material to be burned against the wear rings of the kiln inlet seal as a result of a rearward movement of the material in the inlet zone is effectively prevented.
Different angles of inclination can be provided on the sliding surface of the at least one further projection. According to an advantageous embodiment of the invention, the sliding surface of the at least one further projection has an inclination with respect to the longitudinal axis of the rotary tube of 15° to 70°. Particularly good results were achieved at an angle of inclination between 40° and 50°, especially around 45°.
The at least one additional projection can also contain a refractory material. These can be various high-temperature resistant materials, as are known per se from the prior art. Concrete, in particular refractory concrete, has proven to be particularly suitable.
In experiments by the applicant, different geometries have been found to be suitable for the at least one additional projection. Prismatic geometries are preferred, in particular in the form of a straight prism with a triangular base, especially in the form of a right triangle, in which the at least one sliding surface is formed by the hypotenuse of the right triangle.
The height of the at least one additional projection, i.e. the extension in the radial direction with respect to the cylindrical rotary tube, is important for a particularly good effectiveness in relation to the conveyance of the material to be burned from the inlet zone in the direction of the preheating zone. This can be 100 mm to 500 mm, preferably 150 mm to 250 mm, typically approximately 200 mm. This ensures that a sufficient amount of material rests on the at least one sliding surface of the at least one additional projection and subsequently slides in the direction of the preheating zone due to the inclination of the sliding surface. Furthermore, it can be provided that the at least one additional projection has a length of 100 mm to 2000 mm, preferably 350 mm to 450 mm, typically approximately 400 mm, which extends substantially parallel to the longitudinal axis of the rotary tube.
The at least one additional projection can be fastened to the inner rotary kiln casing by means of a welded metallic anchor.
According to a particularly advantageous embodiment of the invention, it is provided that a plurality of additional projections is provided in the inlet zone of the rotary tube, the additional projections being grouped in a row such that the respective sliding surfaces of the additional projections form a common sliding surface for the material to be burned. This results in a particularly effective conveyance of the material to be burned from the inlet zone of the rotary tube to its preheating zone.
Because a common sliding surface is provided by a plurality of additional projections lined up as a group, on the one hand a particularly long sliding surface can be provided which extends practically along the entire extent of the inlet zone. At the same time, in the event of damage to, for example, an individual additional projection from the group of additional projections arranged in a row, these can be specifically replaced without having to dismantle the overall construction. The plurality of additional projections are preferably essentially identical to one another.
A further improvement in the conveyance of the material to be burned through the inlet zone of the rotary tube is achieved by arranging 2 to 8, in particular 4 to 6 and very particularly preferably 5 additional projections or groups of additional projections arranged in a row over the circumference of the rotary tube.
According to a further aspect of the present invention, the object mentioned at the outset is achieved with a method for burning carbonate-containing material, in particular limestone or dolomite, which comprises the following steps:
The advantages mentioned above apply accordingly to the method. In particular, an efficient burning process is proposed, which is characterized by a high material throughput through the rotary kiln, a uniformly high product quality for the burnt material as a result of complete calcination, and by being easy to carry out. In particular, thorough intermixing of the burnt material is achieved in the outlet zone of the rotary tube, so that all of the burnt material starts to cool evenly already in the outlet zone. As a result, the following system components, in particular coolers and transport channels, but in particular the downstream conveyor belts, are protected against temperature peaks.
The invention is explained in more detail below in reference to a drawing depicting an exemplary embodiment. Shown are:
In
As shown in the perspective longitudinal sectional view of the rotary tube 1 in
As can be seen in particular in the two-dimensional representation of the “unwound” open inner wall of the rotary tube 1 (
An edge section 4b that is approximately 400 mm wide is provided between the outlet end 4a of the rotary tube 1 and the peripheral group 81 of only three projections 8, in which edge section no projection 8 is arranged, so that the original discharge position into the cooling chute is not changed by the projections 8.
As further shown in
In the longitudinal sectional view of
As shown in
Furthermore, the sliding surfaces 6a of the additional projections 6 and, accordingly, the common sliding surface 6a*, are oriented relative to the direction of rotation D of the rotary tube 1 in such a way that during operation of the kiln the material to be burned comes to rest on the sliding surfaces 6a, 6a* and, on the basis of the selected Inclination of the sliding surfaces 6a, 6a* with respect to the longitudinal axis of the rotary tube 1, slides rapidly in the direction of the preheating zone 3 by gravitational force without any undesirable rearward movement of the material to be burned in the direction of the inlet end 2a.
As shown in particular in
| Number | Date | Country | Kind |
|---|---|---|---|
| 21161137.1 | Mar 2021 | EP | regional |
This application is the United States national phase of International Application No. PCT/EP2022/055693 filed Mar. 7, 2022, and claims priority to European Patent Application No. 21161137.1 filed Mar. 5, 2021, the disclosures of which are hereby incorporated by reference in their entireties.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/055693 | 3/7/2022 | WO |
