A method and an apparatus for removing lime mud depositions from internal surfaces of a rotary lime kiln

Abstract

The invention relates to a method for removing lime mud depositions (30) from internal surfaces (10a) of a rotary lime kiln (10), wherein the lime mud is lime mud formed in pulp production and in which rotary lime kiln (10) the lime mud is heated in the interior space of the rotary lime kiln (10) with an internal heater to calcinate the lime mud, and wherein the method comprises method steps of -providing an external temperature control arrangement (20) being capable to change the internal surface temperature of the rotary lime kiln (10) at least in one location from outside the interior space (12) of the rotary lime kiln (10), -changing the internal surface temperature of the rotary lime kiln (10) at least in one location by the external temperature control arrangement (20) to remove lime mud depositions (30) from the internal surface (10a) at the at least one location, -increasing the temperature at the internal surface at least to 800°C, preferably at least to 900 °C. The invention relates also to an apparatus for removing lime mud depositions (30) from internal surfaces (10a) of a rotary lime kiln (10).

Description

[0001] A METHOD AND AN APPARATUS FOR REMOVING LIME MUD DEPOSITIONS FROM INTERNAL SURFACES OF A ROTARY LIME KILN

[0002] Field of the invention

[0003] The present invention relates to a method and an apparatus for removing lime mud depositions from internal surfaces of a rotary lime kiln.

[0004] Background of the invention

[0005] The composition of lime mud calcinated in lime mud heat treating devices, such as in stationary or rotary lime kilns depends on many factors: wood species, the impurities in the make-up lime and refractory bricks used in the lime kiln, the efficiencies of slakers, causticizers, clarifiers and mud washers, and the burning conditions in the lime kiln. On a dry basis, lime mud typically contains about 95 wt% CaCOs and 5 wt% of impurities.

[0006] It is generally known that heated rotating lime kilns have tendency to build up so called “ring formation” or other kind of sticky depositions which grows on the internal surfaces of the rotary lime kiln over the time and thereby hindering the operation of the rotary lime kiln (several published patent applications / patents for instance US 3,578,299 A reveal such phenomena, and this issue has been also discussed in scientific non-patent literature, such as e.g., Tran Honghi: “Lime kiln chemistry and effects on kiln operations", Conference Paper January 2007, TAPPI Kraft Recovery Course, Atlanta). According to present knowledge and theories, ring formation and lime mud build up on the rotary lime kiln internal surfaces are caused by temperature fluctuation and sodium content changes occurring in the rotary lime kiln during its use. Development of such deposition build-up on the internal surfaces of a rotary lime kiln is described e.g., in the above- mentioned Tran Honghi’s conference paper.

[0007] Removing of ring formation or other kind of lime / lime mud formation from a rotary lime kiln or from flue gas channel internal surfaces leads typically shutdown and production lost at the rotary lime kiln. Usually, there is need for total shutdown of the lime kiln process to clean the formation inside of the rotary lime kiln and / or from its flue gas channels. Naturally this will lead to loss of production reducing utilization degree of such rotary lime kiln. Ring formation usually needs to be cleaned more often than in annual or biannual maintenance shutdowns.

[0008] Summary of the invention

[0009] It has been observed based on knowledge on general behavior of the lime deposition as well as laboratory scale tests that lime mud depositions are possible to remove during operation of a rotary lime kiln or other kind of lime mud heat treating device by rapid temperature change (preferably heating) of the respective internal surface. Temperature change needs to be done directly at the location where the lime mud is stuck on the internal surface at typical temperature area based normal operation. Temperature shock for surface can be done by increasing (or decreasing) temperature of the surface rapidly and lime formation that is in contact with the internal surface cracks and drops off. The respective internal surface can be heated up for example by using electric heater or other external heating source. Cooling can be arranged, for example, by means of electric cooler, circulation of suitable cooling medium with suitable heat exchanger arrangement or by spraying device arranged to spray some suitable cooling medium on the respective interior surfaces.

[0010] Another method that may be applied is chemical reaction by calcinating the lime mud particle on the surfaces by increasing temperature higher than calcinating temperature, typically 900-1100°C to make formed deposition more fragile which removes it from the internal surface e.g., in rotary lime kilns due to its rotation.

[0011] Furthermore, lime mud depositions formed at the internal surfaces in feed head area of a rotary lime kiln (i.e. in the region of the first end of the rotary lime kiln) can be mechanically detached by applying suitable cleaning device arrangements and by increasing flue gas flow velocity locally at critical positions of the feed head, especially at the region of inlet opening of the flue gas riser duct through which the flue gas is lead out from the rotary lime kiln to a separator device being arranged to separate lime mud from the flue gas stream. However, this issue is not discussed herein since it has been already dealt with applicant’s other patent application.

[0012] It has been also found that internal surfaces of rotary lime kilns having temperature higher than calcinating temperature stay clean during operation. A type of lime kiln using such hot surfaces can be applied in producing lime pellets or pebbles, without any lime mud sticking. Temperature controllable internal surfaces are possible to use in all such lime kiln internal surfaces during calcinating process where the lime / lime mud / non process elements touch the lime kiln internal surfaces during the operation. All lime kiln internal surfaces are meant herein internal surfaces of the rotary kiln shell, the flue gas channel and the other possible inside the rotary lime kiln with their immediate vicinity.

[0013] Thus, the above-described problem is solved because in the method according to invention because temperature of at least at the most critical rotary lime kiln or other type of lime mud heat treating device internal surfaces, and also at the flue gas channels (in all areas where the harmful lime mud depositions exist) are controlled in above-described manner to remove lime mud depositions from these internal surfaces based on thermal expansion and / or based on chemical reactions weakening and fracturing the deposited lime mud formations at the temperature being higher than 800 ° C More specifically the method according to the present invention is characterized by what have been described in the independent claim 1 and the apparatus what have been described in the independent claim 8. Dependent claims 2-7 describe advantageous embodiments of the method according to the invention and the dependent claims 9-14 advantageous embodiments of the apparatus according to the invention.

[0014] An advantage of the present invention is that productivity of the rotary lime kiln is increased because shutdowns caused by the problems relating to lime mud depositions are eliminated, or at least remarkably reduced. This naturally has positive effect on profitability of the whole pulp production at the pulp mill, due to higher utilization rate being reachable in calcination process with the rotary lime kiln.

[0015] Brief description of the drawings

[0016] In the following, the invention will be described in more detail with reference to the appended drawing in which:

[0017] Fig. 1 shows a rotary lime kiln having an embodiment of the apparatus according to the invention seen from a side, and

[0018] Fig. 2 shows the transversal cross-section A-A of the rotary lime kiln of the figure 1 .

[0019] Detailed description of some advantageous embodiments

[0020] Figure 1 shows a rotary lime kiln comprising an apparatus according to the invention. The rotary lime kiln in figure 1 is a type of rotary lime kiln being used to reburn lime mud formed in causticizing plant at a pulp mill to produce white liquor which is then used at fiber production line for disintegrating lignin from the wood fibers. Principally, the rotary lime kiln could be any kind of a rotary lime kiln being used in pulp mills for production of the white liquor..

[0021] The rotary lime kiln 10 shown in the figures 1 and 2 has a rotary kiln shell 11 having an interior space 12 between a first end wall 13 and a second end wall 14 at the ends of the rotary kiln shell 11 . The rotary lime kiln 10 also has a flue gas riser duct 15 at the first end wall 13 extending out from interior space 12 of the rotary kiln shell 11 through the first end wall 13. These parts of the rotary lime kiln are those on to which internal surfaces the lime mud tends to grow up due to the exposure of hot flue gases comprising sticky lime mud particles. The rotary lime kiln shown in the figures l and 2 includes also other necessary components of a conventional rotary lime kiln, such as feeding device 16 to feed the lime mud, separator device (not shown in the figure) for separating the lime particles from flue gas stream at the end of riser duct 15, and burner to heat the lime mud in to calcinating temperature (i.e., to temperature of about 700°C) to reburn the lime mud during its travel through the interior space 12 of the rotary kiln shell 11 . The rotary lime kiln 10 shown in the figures 1 and 2 comprises further an apparatus for removing lime mud depositions from internal surfaces of the rotary lime kiln 10 at least one location within the above-mentioned portions of the rotary lime kiln 10. Such an apparatus comprises an external temperature control arrangement 20 being capable to change the internal surface temperature at least in one location from outside the interior space of the rotary lime kiln 10. This means that temperature control arrangement changes the temperature of the rotary lime kiln 10 internal surfaces 10a at the said at least in one location but the aim of the invention is not to change the temperature of the atmosphere inside the rotary lime kiln 10. The purpose of such temperature control (i.e., heating and / or cooling) is to cause thermal expansion to the internal structures of the rotary lime kiln 10 which detaches the deposited lime mud, e.g., a lime mud ring 30 from the internal surface of the rotary kiln shell 11 .

[0022] The external temperature control arrangement 20 comprises in the embodiment shown in the figures 1 and 2 an electric heater 21 . Such electric heater comprises typically a heating resistor 22. In the embodiment shown in the figures 1 and 2 is arranged in the electric heater 21 that is placed on the rotary lime kiln wall. However, the heating resistor 22 may be alternatively or in addition arranged inside the rotary lime kiln wall and / or other structures of the rotary lime kiln 10 having internal surfaces in the rotary lime kiln 10 than to the rotary kiln shell 1 1 , riser duct 15 and the first end wall 13 as in the embodiment shown in the figures 1 and 2.

[0023] In some other embodiment the external temperature control arrangement may comprise instead or in addition to the heating resistor based electric heater, for instance, a burner, an induction heater, microwave heater or other type of heater being suitable for heating the rotary lime kiln interior surfaces in corresponding manner. Induction heater may comprise e.g., a coil arranged inside the wall of the rotary kiln shell such that electro-magnetic field formed to the coil would induce vortex to the heating material (such as Alnico) rotary kiln shell which heats it up. Of course, such vortex may be produced also the other portions of the rotary lime kiln, e.g., to the riser duct and / or to the first end wall to also heat them.

[0024] Furthermore, the external temperature control arrangement may comprise a cooling device. In an embodiment the cooling device comprises a cooling medium circulation arrangement to circulate cooling medium inside or on the rotary lime kiln the wall or other structures which inner surfaces need to be cooled. For providing cooled medium such a cooling device may comprise a heat exchanger to dissipate heat from the cooling medium. Heat transferred to the cooling medium may be also recovered and utilized in some suitable phase of the pulp production. Alternatively, the cooling device may be formed of sprinkler spraying cooling fluid which evaporates in hot surfaces of the rotary lime kiln and thereby cools the internal surface accordingly through a phase change. It may be also used to cool down the deposition instead of the internal surfaces of the rotary lime kiln. In such case the corresponding internal surface may be heated and thereby greater temperature difference may be formed between the deposition layer and the internal surface. In some embodiment such sprinkler type cooling device may comprise a specific spraying device (e.g., such as SNCR injection device) which has spraying lances arranged in suitable positions inside the rotary lime kiln. The cooling medium used with such cooling devices may be suitable cooling gas or liquid. Cooling gas may be e.g., nitric oxide. The cooling liquid may be water, aqueous ammonia, or aqueous urea (preferably containing at most 25% ammonia). The advantage of the latter is that in addition of the cooling effect it will reduce NOx emissions of the rotary lime kiln. Furthermore, with this type of cooling device it is possible to enhance the cooling effect by turning the burner off just before start of the spraying phase which, of course, leads to still faster temperature reduction.

[0025] When using the embodiment of the apparatus shown in the figures 1 and 2 the internal surface temperature of the rotary lime kiln 10 is changed at least in one location by the external temperature control arrangement 20 to remove lime mud depositions from internal surface at the at least one location. Controlling the temperature means most typically heating the internal surface 10a at the respective location. However, it may also mean cooling, or both heating and then cooling or vice versa. The purpose which is to cause appropriate amount of thermal expansion (i.e., expansion and / or shrinking) to the wall having lime mud deposition. Such heating shock should quick enough so that the thermal expansion happens quicker in the respective rotary lime kiln’s 10 walls than in the lime mud deposition, and thereby the thermal expansion detaches the deposition from the inner surface 10 of the rotary lime kiln 10 at least in the said at least one location where the external temperature control arrangement 20 effects to the walls (or other structures having internal surfaces) to cause appropriate thermal expansion.

[0026] Preferably, the temperature at the internal surface 10a of the rotary lime kiln 10 is changed by the temperature control apparatus at least 10°C within 1 minute. In the embodiment shown in the figures 1 and 2 the internal surface temperature is raised preferably at least to 800°C, and more preferably at least to 900°C. However, in some other embodiments the temperature may be raised higher than calcinating temperature 900-1100°C to make the formation of lime mud fragile. Such weakened formation breaks off from surfaces during kiln rotation or by some external impulse device which causes suitable bang to the walls to drop off the weakened formation.

[0027] When applying cooling in the apparatus according to the invention it should be considered that cooling causes that calcinated lime mud hardens i.e., returns back to calcium carbonate. Thus, when applying cooling, it should be ensured that it will occur quick enough and use appropriate temperature changes to successfully break the depositions.

[0028] Temperature at the different locations of the rotary lime kiln 10 can be controlled by the apparatus. Thus, it may preferably comprise controlling (heating or cooling) of at least internal surface temperature of the rotary kiln shell 11 , the riser duct 15 and / or the first end wall 13. This is because these are the portions of the rotary lime kiln 10 which internal surfaces are the most critical in view of deposited lime mud. Thus, it is advantageous have the temperature control arrangement which effects at least on the internal surfaces of these areas.

[0029] In case of the riser duct 15 the internal surface, temperature may be controlled in any position within the whole riser duct 15. However, preferably, the controlled regions locate close to the inlet opening 15a (as it is shown in the figure 1 ) where the lime mud particles tend to aggregate mostly and where the depositions are the most harmful.

[0030] In case of rotary kiln shell 11 preferably temperature of whole circumference of the rotary kiln shell at least in one position in between the first end wall 13 and the second end wall 14 is changed. Preferably, the temperature- controlled areas locate at positions wherein so called “dust ring”, “mid kiln ring”, “firing end ring” and / or “mud ring” is formed. In general, ring formation occurs most frequently within these regions of the rotary kiln shell 11 and hence the most advantageous positions for temperature controlling arrangement locates in these areas.

[0031] It should be also notified that the invention is not limited to the embodiments described above. Thus, the method and apparatus according to the invention can be realized in a manner that differs further from the abovedescribed embodiments. For instance, heated surfaces can be used for pelleting lime mud before calcination reaction. Naturally, heated surfaces can be used to produce heat to the calcination reaction even it is not intended to use the temperature control apparatus for producing heat to the calcination process.

[0032] Furthermore, the cooling can take place simultaneously with heating such that deposited layer is cooled from its internal surface, for instance by shut down the burner / flame heating the interior space, and / or e.g., by providing some cooling gas into the interior space of the rotary lime kiln.

[0033] Thus, the invention is not limited to the embodiments described above but can be varied within the scope of the appended claims.

Claims

Claims1. A method for removing lime mud depositions (30) from internal surfaces (10a) of a rotary lime kiln (10), wherein-the rotary lime kiln (10) comprises a rotary kiln shell (11 ) having an interior space (12) between a first end wall (13) and a second end wall (12),-the rotary lime kiln (10) comprises a flue gas riser duct (15) at the first end wall (13) extending out from interior space (12) of the rotary kiln shell (11 ) through the first end wall (13).-the lime mud is lime mud formed in pulp production and in which rotary lime kiln (10) the lime mud is heated in the interior space (12) with an internal heater to calcinate the lime mud, and wherein the method comprises method steps of-providing an external temperature control arrangement (20) being capable to change internal surface temperature of the rotary lime kiln (10) at least in one location from outside the interior space of the rotary lime kiln (10),-changing the internal surface temperature of the rotary lime kiln (10) at least in one location by the external temperature control arrangement (20) to remove lime mud depositions (30) from the rotary lime kiln (10) internal surface (10a) at the at least one location-increasing the temperature at the internal surface at least to 800°C, preferably at least to 900 °C.

2. The method according to claim 1 wherein internal surface temperature at the said at least one location is changed so quickly that temperature of the internal surface (10a) changes quicker than temperature of the lime mud deposition (30) on the respective internal surface (10a).

3. The method according to claim 2 wherein the temperature is changed at least 10°C in 1 minute.

4. The method according to any of any of preceding claims wherein an internal surface temperature of the rotary kiln shell (11 ) is externally controlled at least in one location.

5. The method according to claim 4, wherein internal surface temperature of whole circumference of the rotary kiln shell (11 ) at least in one position in between the first end wall (13) and the second end wall (14) is changed.

6. The method according to any of any of preceding claims wherein the internal surface temperature of the riser duct (15) is externally controlled at least in one location within the riser duct (15).

7. The method according to any of any of preceding claims, wherein the internal surface temperature of the first end wall (13) is externally controlled at least in one location.

8. An apparatus for removing lime mud depositions (30) from internal surfaces (10a) of a rotary lime kiln (10), wherein-the rotary lime kiln (10) has an interior space (12) inside which the lime mud is heated with an internal heater to calcinate the lime mud,-the rotary lime kiln (10) comprises a rotary kiln shell (11 ) forming the interior space (12) between a first end wall (13) and a second end wall (14),-the rotary lime kiln (10) comprises a flue gas riser duct (15) at the first end wall (13) extending out from interior space (12) of the rotary kiln shell (11 ) through the first end wall (13), and wherein the apparatus comprises an external temperature control arrangement (20) being capable to change the internal surface temperature at least in one location from outside the interior space (12) of the rotary lime kiln (10) and to increase the temperature at the internal surface at least to 800°C, preferably at least to 900 °C.

9. The apparatus according claim 9, wherein the external temperature control arrangement (20) comprises an electric heater (21 ) inside and / or on the rotary lime kiln (10) wall or other structures which internal surface is heated.

10. The apparatus according to claim 9, wherein the electric heater comprises (21 ) a heating resistor (22).

11. The apparatus according to any of claims 8-10, wherein the external temperature control arrangement comprises other heater, such as a burner, an induction heater or microwave heater.

12. The apparatus according to any of claims 8-11 , wherein the external temperature control arrangement comprises a cooling device.

13. The apparatus according to claim 12 wherein the cooling device comprises a cooling medium circulation arrangement to circulate cooling medium inside and / or on the lime kiln wall or other structures which internal surface is to be cooled.

14. The apparatus according to claim 12 or 13 wherein the cooling device comprises a heat exchanger to dissipate heat.

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