Background of the invention

[0001] The present invention relates to an arrangement in a rotary kiln according to claim 1 , and to a method for guiding air to said rotary kiln according to claim 13.

[0002] Incineration plants are used in the production of heat and other energy. Incineration plants and power plants using solid fuel - biobased solid fuel in particular - play a more and more important role when trying to meet the ever- increasing energy demand in an economically and ecologically sustainable manner.

[0003] As to economic and ecological benefits, it is advantageous that the availability and transport distance of fuels for the equipment remain reasonable. This can be achieved most reliably by employing incineration equipment capable of utilizing various fuels available in the immediate surroundings, such as tree bark, wood chips, wood dust, peat, sludge from waste water purification plants or the like, manure, municipal waste, etc.

[0004] In incineration plants employing a so-called rotary kiln, it is often a challenging task to provide a sufficient amount of combustion air so as to keep up the combustion process in the kiln. Conventional fixed or moving grate solutions for the combustion bed that guide combustion air into the kiln and their air holes or nozzles often tend to become clogged. Particularly upon burning many different materials, a problem arises when liquefying ash, chemical compounds or the like penetrate during the combustion process into the air holes leading to a combustion chamber. In addition, these components being formed may also result in waste materials becoming sintered to the combustion bed, and in corresponding problems. The weakened air flow into the combustion chamber also contributes to disturbing the combustion process. The disturbance of the combustion process is particularly harmful when different and qualitatively varying fuels are used in the same incineration plant. Indeed, such disturbance may produce additional impurities, such as fine particles, that are irritating, even detrimental to the environment.

Brief description of the invention

[0005] An object of the present invention is thus to provide a method and an apparatus implementing the method so as to enable a completely novel solution to be provided to the aforementioned problems caused by prior- art drawbacks.

[0006] This object is achieved such that the arrangement in a rotary kiln and the method for guiding air to said rotary kiln are, in accordance with the present invention, provided with the characteristics defined in the claims. In particular, the present problems may be solved by combining the characteristics as disclosed in the characterising parts of claims 1 and 13.

[0007] Preferred embodiments of the invention are disclosed in the dependent claims.

[0008] The invention provides considerable advantages. Thus, air pipes supplying combustion air to the rotary kiln are protected by an insulating layer. At the same time, mouths of these pipes opening up into the combustion chamber and air jets flowing from the air pipes are located substantially non- cloggingly in a direction of propagation of the combustion process taking place in the combustion chamber. Consequently, in the rotating rotary kiln, fuel rolling from a feed opening towards a discharge opening and the combustion event thereof do not enter to the air pipes but by-pass the mouths thereof without sticking thereto.

[0009] A non-rotatable guide plate settling against an end in the rotary kiln is provided with one or more air supply inlets. By adjusting this inlet, the air supply of the combustion chamber of the rotary kiln may be guided into desired air pipes.

[0010] An inner surface of the rotary kiln and air pipes formed in a shell of the rotary kiln are protected by insulation casting. In addition, an inner surface of the shell serving as the combustion bed is protected by a glasslike and sintering-repellent fireproof coating. This enables the rotary kiln to clean itself while rotating.

[0011] The insulation casting of the shell and the air to be blown through the air pipes enable the surface temperature of an outer circumference of the rotary kiln to be kept appropriate. In such a case, the bearings of the rollers carrying the rotary kiln do not become too heated, either.

[0012] By ensuring continuously a sufficient supply of combustion air to the combustion chamber, the present invention enables all the advantages and safety of cylinder combustion to be achieved. At the same time it can be ensured that the combustion process can utilize the maximum energy content of the material to be burnt from the burning material being processed. Therefore, a thermal power plant equipped with the present rotary kiln and a so-called post-combustion chamber provides, in addition to heat energy produced by the fuel itself, and in incineration of hazardous waste as well, the same advantages and safety as do previously known cylinder incineration plants intended for hazardous waste in particular. For instance, it is impossible for liquids seeping from hazardous waste or the like to cause harm in any part of the combustion chamber, but they are guided controllably from an end of the rotary kiln to an ash pit.

[0013] The structure of the inner surface of the rotary kiln, expanding stepwise towards the discharge opening, provides the rotary kiln with a naturally sloping rolling surface. Therefore, it is not necessary to install the rotary kiln, as in prior art, in an inclined position in order to roll the fuel in a direction of the discharge opening, but the kiln may be installed substantially horizontally.

[0014] Other advantages provided by the invention are disclosed in the following more detailed description of specific embodiments of the invention.

Brief description of the figures

[0015] In the following, some preferred embodiments of the invention will be explained in closer detail and with reference to the accompanying drawing, in which

Figure 1 schematically shows a previously known rotary kiln, Figure 2 is an axonometric view showing a lengthwise section of a rotary kiln according to the invention,

Figure 3 is an axonometric view showing a lengthwise section of the rotary kiln according to Figure 2, with a guide plate installed,

Figure 4 is a detailed view of the rotary kiln according to Figure 3, Figure 5 is a detailed view showing a structure of a shell at A-A of

Figure 4,

Figure 6 is a detailed view showing the structure of the shell at B of

Figure 4,

Figure 7 is a schematic sectional view showing a first end of the rotary kiln and a combustion air blower provided in a guide plate therein. Detailed description of preferred embodiments

[0016] The present figures do not show the arrangement in a rotary kiln and the method for guiding air to said rotary kiln in scale but the figures are schematic, illustrating the general structure and operation of the preferred embodiments. The structural parts shown by reference numbers in the attached figures then correspond to the structural parts marked by reference numbers in this specification.

[0017] Figure 1 shows a solution known per se, wherein a rotary kiln 1 is utilized as part of a thermal plant. Being suitable for burning solid, liquid, pasty as well as gaseous materials, rotary kilns are also used inter alia in the production of cement clinkers and in incineration of waste. The structure of such a kiln is a straight circular cylinder which is fire and heat resistant and in which the material to be treated is burnt in a drum rotating around its longitudinal axis. However, in order to keep up the combustion, it has been necessary to feed the kiln with combustion energy from outside, for instance oil or gas, since it has not been previously possible to provide the kilns in question with hinderless air feed required by independent combustion of the material to be burnt.

[0018] The rotary kiln 1 is ordinarily connected to a separate post- combustion chamber 2, where gases formed in the kiln are burnt out. The rotary kiln is most often installed in a slightly inclined position. In such a case, it has a first end 3, located higher and provided with a specific feed opening 4 for guiding fuel 5 to a combustion chamber 6 surrounded by a circular cylinder. The combustion air is conventionally fed to the combustion chamber via channels 8 penetrating through a shell 7 of the rotary kiln, substantially in a radial direction of the circular cylinder.

[0019] During combustion, the rotary kiln 1 rotates slowly, depending on the quality of the fuel 5, for instance at 5 to 40 rounds per hour. Because of the rotating and the inclined position of the drum, the fuel rolls forwards in the circular cylinder, towards a discharge opening 10 located at a second, lower end 9 thereof. At the same time, the fuel is efficiently mixed, improving the contact of the combustion air with the fuel and enabling the fuel to burn thoroughly.

[0020] Since the combustion takes place against the shell 7 of the rotary kiln 1 , it has to be constantly cooled. This is carried out in the prior art either with air or water circulating an outer surface 1 1 of the circular cylinder. [0021] Combustion gases and ash generated during the combustion are discharged via the discharge opening 10 in the lower end 9 of the rotary kiln to the post-combustion chamber 2.

[0022] In contrast to the known solutions, combustion air 21 is guided to the present rotary kiln 20 according to Figure 2, for instance, along an inner surface 24 of a shell 23 surrounding a combustion chamber 22 in particular, under the rolling material to be burnt. For this purpose, the circular cylinder, i.e. drum, rotating around its central axis 25, is provided with specific air pipes 26 in a longitudinal direction of the rotary kiln. It is preferable to arrange these air pipes over the entire circumference of the rotary kiln. The structure of such a rotary kiln is shown in closer detail in the cross section of the shell 23 according to Figure 5. Thus, an outer surface 27 of the shell is preferably formed by a circular cylinder made of steel. In a known manner, two tyres surround the circumference of the circular cylinder. The tyres form the rotating rails of the rotary kiln. The kiln rests on these on a bearing. If desired, also the rotating of the rotary kiln may be produced by said tyres. This structure known per se and its operation is not discussed in any further detail herein.

[0023] Even if the pipes 26 are herein called air pipes, this does not restrict their use to the supply of air exclusively. Thus, these pipes may also be utilized for feeding other gases or compounds otherwise enhancing combustion to the combustion chamber 22.

[0024] As described above, the rotary kiln 20 has a first end 28 provided with a feed opening 29 for guiding fuel in manners known per se to the combustion chamber 22 of the rotary kiln. This first end is preferably made of a steel plate 30 connected tightly and rigidly to the outer surface 27 of the shell, cf. Figure 5. A second, opposite end 31 of the rotary kiln is provided with a discharge opening 32 through which combustion waste is discharged in a manner known per se and as described above to the post-combustion chamber and further to subsequent treatment.

[0025] The first end 28 is provided with air holes 33 surrounding the feed opening 29 substantially concentrically. The air holes connect to the air pipes 26 provided in the shell and are substantially co-directional with the central axis 25 of the rotary kiln and parallel with the outer surface 27 of the shell. These air pipes are thus to guide combustion air from the first end of the rotary kiln to the combustion chamber of the rotary kiln. Preferably, air holes and air pipes connected thereto are then arranged in the end and the shell in a plurali- ty of concentric annular layers. The air holes may be arranged according to Figure 2 radially in line, but such a radial arrangement is by no means necessary for the operation of the rotary kiln.

[0026] In a first preferred embodiment of the rotary kiln, and in the embodiment according to Figure 5, the manufacture of the shell 23 proceeds as follows. A first insulating layer is cast on a steel cylinder forming the outer surface 27 of the shell. After this has hardened sufficiently, air pipes 26 substantially co-directional with the central axis 25 of the rotary kiln and parallel with the outer surface 27 of the shell are installed against this insulating layer of the combustion chamber. At their outer end, the air pipes are connected to the air holes 33 provided in the steel plate 30 or a corresponding structure in the first end 28. At their second end, these air pipes of the first circular layer are extended at their inner end close to the discharge opening 32 of the rotary kiln. Next, these air pipes of the first circular layer are coated by a second insulation casting, on top of which, again, air pipes 26 that are slightly shorter than the previous ones are installed. The installation of the air pipes and the casting of the insulating layers are continued until a desired number of air pipes leading to the combustion chamber 22 is achieved. At the same time, a somewhat conical expansion designed for the combustion chamber and proceeding from the feed opening 29 to the discharge opening 32 is achieved, cf. Figure 2. Upon completion of the last insulation casting, it is preferable to finish off the inner surface 24 of the combustion chamber by means of a fireproof surface casting or surface treatment covering the surface.

[0027] The inner surface 24 of the rotary kiln 20 manufactured in such a manner looks, for instance as shown in Figure 2, in cross section like a staircase with a long step but a small rise. As viewed from the feed opening, the inner circumference looks like a descending staircase. As seen from the direction of the discharge opening, in turn, mouths 35 of the air pipes 26 located at the base of a front surface 34 of the steps and arranged in the shell in several nested layers can be seen. Confer also detailed view B of Figure 6. Thus, preferably, air pipes are arranged in the shell substantially equally spaced over the entire circumference.

[0028] The stepwise conical expansion of the combustion chamber 22 may equally well be implemented in a spirally expanding manner as in segmental steps of different lengths according to the present embodiment. In terms of manufacturing technology, the latter is simpler to implement. [0029] The rotary kiln 20 may also be manufactured from prefabricated elements. In such a case, the rotary kiln is assembled for instance from cylinder parts having the same outer diameter to be arranged successively. The parts are made such that their inner diameter grows part by part. Upon interconnecting the parts, the above-described conical combustion chamber 22 expanding in a stepwise manner is formed. The cylinder parts are provided with the necessary air pipes 26 during manufacture. While assembling the rotary kiln, these air pipes are aligned with respect to one another in order to produce uniform pipelines. A completed rotary kiln is finished off preferably by means of a fireproof surface casting or surface treatment covering the inner surface of the combustion chamber.

[0030] The rotary kiln may also be assembled from nestable cylinder elements having cross sections that become smaller and smaller. In such a case, the outer surface of the cylinder elements to be installed inside the outermost element is preferably already provided with the necessary air pipes or grooves forming the same. Thus, each air pipe layer remains under a protective structural layer. It is also preferable to finish off such a completed rotary kiln by means of a surface casting or surface treatment.

[0031] By arranging the mouth 35 of the air pipe 26 to be orientated towards the discharge opening 32, for instance as shown in Figure 6, the fuel fed to the combustion chamber 22 of the rotary kiln and rolling therein on account of the rotational motion of the drum and the combustion event cannot spread to the mouths of the air pipes. In the present arrangement, the fuel and the combustion event roll continuously away from the mouth, accompanied by an air jet from the air pipes. In such a case, the air pipe is thus arranged to open up into the combustion chamber in the front surface 34 formed by the steps in a flow direction of fuel and hot gases rolling in the combustion chamber. This enables the problem of fuel and waste material collecting around the air nozzles and clogging the mouths peculiar to the known solutions to be efficiently avoided. The structure further makes it possible to ensure that also the incineration of hazardous waste is safe since flowing liquids, ash and other waste materials generated during combustion are guided controllably to the ash pit provided in the post-combustion chamber.

[0032] The structure of the combustion-air-conveying air pipes 26 of the present rotary kiln enables the flow of fuel and flowing liquids, ash and other waste materials generated during combustion to be adjusted efficiently. By adjusting the velocity of the gas or the like flowing from the air pipe, the flow of components surrounding the discharge opening can be influenced.

[0033] In order to guide air to the air pipes 26 co-directional with the longitudinal axis 25 of the rotary kiln 20, the first end 28 of the rotary kiln is provided with specific means 36 for guiding air to the air holes 33 connected to the outer end of the air pipes. Confer Figures 3, 4, and 7. In this preferred embodiment, these means comprise a guide plate to be placed against said end of the rotary kiln and preferably to be arranged to be non-rotatable. This guide plate is provided with at least one air supply inlet 37 for guiding air to the air holes 33 of the air pipes 26 residing at a given moment at the supply inlet. Since the rotary kiln sitting against the guide plate is in constant rotational motion, air is thus guided to some of the air pipes only, i.e. to those ones whose air holes at a given moment happen to reside at the air supply inlet of the guide plate. It is thus particularly preferable to place the guide plate in such a position that the at least one air supply inlet provided therein guides combustion air only to air pipes whose mouth 35 is at a given moment located under and on the sides of the fuel rolling in the combustion chamber.

[0034] In order to guide the air flow, the air supply inlet 37 provided in the guide plate 36 is thus given the shape of a circular sector or that of a sector of a circular ring. Preferably, the air supply inlet is arranged to be adjustable in size. The adjustment may be implemented for instance by a structural solution according to an adjustment principle of a pair of dividers. The size of the supply inlet is thus preferably adjustable automatically by means of the kiln control logic for managing the combustion process.

[0035] In order to produce the necessary air flow to the combustion chamber 22, a pipe fitting 38 to be arranged on top of the air supply inlet 37, schematically shown in Figure 7, may be arranged in the guide plate 36. The pipe fitting may be provided with a separate blower to be used for guiding air forcedly to the air pipes and further to the combustion chamber.

[0036] Preferably, the pipe fitting may also comprise means 39 for guiding air to an outermost air pipe group of the rotary kiln. In such a case, the diameter of the guide plate is reduced enough for the outermost air holes to be released. This enables air to be guided preferably to the outermost air pipes located over the entire circumference of the rotary kiln. By guiding air also to these outermost air pipes over the entire circumference of the rotary kiln, rather than at the aforementioned air supply inlet only, the shell 23 of the rotary kiln may be cooled in a controlled manner. This air flow to the air pipes of the outer circumference may also be arranged to be separately adjustable by means of a blower installed for this purpose.

[0037] By arranging the guide plate 36 planarly against the flat first end 28 of the rotary kiln 20, a sufficient tightness is achieved by a light flexible load for guiding the necessary air flow to the air pipes. Thus, no complete air leakage seal is necessary between the end of the rotating rotary kiln and the fixed guide plate abrasing against it. An air cushion penetrating between the plates at the same time serves to reduce friction and to cool the end.

[0038] It is to be understood that the above description and the related figures are only intended to illustrate the present solution. The solution is thus not restricted to the embodiment described above or defined in the claims, but it will be obvious to a person skilled in the art that different variations and modifications are possible within the scope of the idea defined in the accompanying claims.