Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Arrangement in rotary kiln and method for guiding air to said
rotary kiln
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
ac-
cording 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 capa-
ble 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 of-
ten 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
burn-
ing 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 combus-
tion bed, and in corresponding problems. The weakened air flow into the com-
bustion 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 irri-
tating, even detrimental to the environment.
Brief description of the invention
[0005] An object of the present invention is thus to provide a meth-
od and an apparatus implementing the method so as to enable a completely
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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 character-
istics 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
roll-
ing 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
with-
out sticking thereto.
[0009] A non-rotatable guide plate settling against an end in the ro-
tary 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 ad-
vantages 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.
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Therefore, a thermal power plant equipped with the present rotary kiln and a
so-called post-combustion chamber provides, in addition to heat energy pro-
duced 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, expand-
ing stepwise towards the discharge opening, provides the rotary kiln with a
naturally sloping rolling surface. Therefore, it is not necessary to install
the ro-
tary kiln, as in prior art, in an inclined position in order to roll the fuel
in a direc-
tion of the discharge opening, but the kiln may be installed substantially
hori-
zontally.
[0014] Other advantages provided by the invention are disclosed in
the following more detailed description of specific embodiments of the inven-
tion.
Brief description of the figures
[0015] In the following, some preferred embodiments of the inven-
tion 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 ro-
tary kiln and a combustion air blower provided in a guide plate therein.
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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
em-
bodiments. 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
us 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
longitudi-
nal 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 rota-
ry 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 chan-
nels 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, depend-
ing on the quality of the fuel 5, for instance at 5 to 40 rounds per hour. Be-
cause of the rotating and the inclined position of the drum, the fuel rolls
for-
wards 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,
im-
proving 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 11 of the circular
cylinder.
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[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 guid-
ed to the present rotary kiln 20 according to Figure 2, for instance, along an
in-
ner surface 24 of a shell 23 surrounding a combustion chamber 22 in particu-
lar, 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
accord-
ing 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 cir-
cumference of the circular cylinder. The tyres form the rotating rails of the
rota-
ry 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 combus-
tion to the combustion chamber 22.
[0024] As described above, the rotary kiln 20 has a first end 28 pro-
vided 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
dis-
charge 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
cen-
tral 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-
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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 neces-
sary 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
sub-
stantially 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
insu-
lation 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
lead-
ing 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. Up-
on 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 di-
rection 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
sev-
eral 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.
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[0029] The rotary kiln 20 may also be manufactured from prefabri-
cated 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 in-
terconnecting 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 ro-
tary 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 cylin-
der 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 pro-
tective 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 ac-
count 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 cham-
ber. 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 effi-
ciently 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
oth-
er waste materials generated during combustion to be adjusted efficiently. By
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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
em-
bodiment, 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 mo-
tion, 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
on-
ly 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
ad-
justable in size. The adjustment may be implemented for instance by a struc-
tural 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, ra-
ther than at the aforementioned air supply inlet only, the shell 23 of the
rotary
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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 re-
lated 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 accom-
panying claims.
