Note: Descriptions are shown in the official language in which they were submitted.
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Method and apparatus for drying of fuel
Cross-Reference to Related Applications
This application claims priority under 35 USC ~119 to Finnish Patent
Application No. 20045423 filed on 5 November 2004.
Field of the invention
The invention relates to a method and an apparatus for drying of fuel.
Backgiround of the invention
In power plants, numerous different materials are used as fuels, such
as bark, chips, coal, peat, compressed sludge or municipal waste. Be-
fore the combustion process the fuel is typically crushed, chipped or
cut into particles of suitable size for combustion. Thereafter they are
stored by the power plant and possibly dried before they are fed to the
furnace of the boiler. The properties, composition and particle size of
the fuel affect its moisture content. The above-mentioned storing also
has an effect therein. For example bark, which is a commonly used fuel
in the power boiler of a pulp mill, has naturally a high moisture content.
It is especially high in such pulp mills in which wet-debarking is used.
The moisture content of chips is also relatively high. The moisture
content of bark and chips is also affected by the way in which they are
stored. In stacks arranged outdoors, they are exposed to variations
caused by the change of seasons. Especially in the northern hemis-
phere, sun and rain in the summer and snow and ice in the winter of
fect the moisture content of fuels.
Sludges are produced in various processes as their final products or
waste. For example in paper and pulp mills sludges are produced in a
number of different process stages. At present, excess liquid is re
moved from sludges for example by pressing, and the processed
sludge is fed to waste treatment either to be dumped or burned.
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The moisture in fuels is primarily water bound either in the fuel particles
or on the surface and between the fuel particles. It is possible that
other substances have dissolved in the water from the fuel, or other
liquids have been mixed therein in connection with the treatment of
fuel.
In view of the combustion process of the power plant it is very impor-
tant that the moisture content of the fuel fed to the furnace of the boiler
is optimal for the combustion process used. So-called fluidized bed
combustion is especially suitable for the purpose of burning wet fuel or
fuel with varying moisture content. In fluidized bed combustion, the fuel
is introduced into hot fluidized bed material floating in the furnace by
means of fluidizing air, in which bed material the fuel first dries and
then burns. The moist fuel reduces the temperature of the bed material
and thus helps in keeping the temperature within suitable limits. How-
ever, the fuel must not be too wet so that the efficiency of the furnace
would remain sufficiently high.
Moisture is removed from fuels in a number of different ways before
they are fed for burning. Bark, sludge and municipal waste can be dried
for example by mechanical pressing. The presses to be used are,
however, heavy and large in size, their acquisition costs are high and
they consume great deal of electric energy. Furthermore, when using
them, the sufficiently low moisture content required for the burning is
seldom reached.
It is also known to dry fuels by means of different kinds of driers before
they are fed for burning. There are for example drum or screw type dri
ers in use, in which a drying medium, such as hot air is fed into the fuel
to be dried. Indirect driers based on convection are also known.
US publication 6,163,981 discloses a process of drying wood material
chipped into small particles, such as chips and bark, before they are
fed to a power boiler to be burned. In the solution according to the pub-
lication, the chips to be dried are fed to a drier, in which a bed of chips
is formed on a moving conveyor, said bed of chips moving forward
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along with the movement of the conveyor. Heated drying gas is fed
from below the conveyor, perpendicularly against the movement of the
conveyor. The chips to be dried do not, however, spread evenly on the
conveyor, which results in that the drying result reached by the appa-
ratus is uneven. Furthermore, if the capacity of the power boiler is high,
and it burns a large amount of chips, it is necessary to build the drying
apparatus large enough, which is not economical. Furthermore, it is
almost impossible to insulate the apparatus from outdoor air, wherein
the gas used in the drying can leak into the surrounding air, which is a
work safety and environmental risk.
In US publication 4,888,885 warm gas is also fed through a bed of
chips moving forward in a dryer. The bed of chips moves forward on an
inclined transport towards its discharge end. The transport enables the
penetration of gas. It is a problem in this solution that the transport is
easily clogged, because moving of material based merely on gravity is
not sufficient for feeding elongated, viscose material typically used in
power plants. It is also difficult to attain high drying capacity by means
of the apparatus.
As was mentioned above, the high moisture content of the fuel reduces
the efficiency of the power plant. Furthermore, in a fluidized bed boiler
the control system of the boiler tends to compensate the reduction in
the temperature of the fluidized bed resulting from the supply of moist
fuel by reducing the amount of flue-gas containing circulating gas and
increasing the amount of air in the fluidizing air. The increase in the
amount of combustion air produces a larger combustion flame in the
fluidized bed, which would require building the furnace larger in size,
which is an expensive investment. Correspondingly, if the fuel to be
burned in the boiler is dryer, there is a risk that the temperature of the
fluidized bed increases so high that it starts to sinter. The controlling of
the process is easier if the moisture content of the fuel to be fed is as
stable as possible. Thus, the moisture content of the fuel is significant
in view of the combustion process and efficiency of the boiler.
Brief description of the invention
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Consequently, it is an aim of the present invention to provide a method
for drying of fuel, which avoids the above-mentioned problems, and by
means of which it is possible to efficiently dry the fuel to be fed to the
power boiler, and the moisture content of the fuel to be dried can be
stabilized. Furthermore, it is an aim of the invention to provide an appa-
ratus implementing the aforementioned method.
The apparatus according to the invention, in turn, is primarily The
invention is based on the idea that the fuel supplied to the furnace of a
power boiler is dried in a continuous process in such a manner that the
temperature of the fuel is first increased rapidly in a heating phase,
whereafter it is introduced to the drying phase. The heating phase and
the drying phase are arranged successively in such a manner that the
fuel compressed in the heating phase is discharged to the drying
phase, and it is passed therethrough as a bed having a substantially
uniform thickness. In the heating phase the fuel flows through a vertical
heating section, in which hot, gaseous medium is fed to the fuel flow to
increase the temperature of the fuel sufficiently high for the drying
phase.
In the heating phase, the hot, gaseous medium is fed crosswisely in
relation to the fuel flow travelling downward, to the middle of the flow
and to its edges. The fuel heats up rapidly in the heating phase, be-
cause the amount of heating medium is large and its flow rate is high,
and all material travels through the heating medium flow. In the heating
phase the fuel is heated up to the drying temperature or at least close
to it. The hot, gaseous medium fed to the fuel flow in the cross direction
does not hinder the flow of fuel material.
After the heating phase the fuel flow is fed as a dense bed through the
drying phase comprising a horizontal or an oblique drying section. In
the drying phase a hot, gaseous medium is fed through the fuel bed to
maintain the drying temperature. Most of the steam necessary for the
drying is fed to the heating section. The amount of steam necessary in
the drying section is small, because the steam is only used for main-
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taining the drying temperature attained in the heating section. Cor-
respondingly, the delay time of fuel in the drying section is considerably
longer than in the heating section. By means of a long delay time it is
possible to ensure steady drying of fuel throughout the entire fuel bed.
5 The gaseous steam released in the drying section is removed from the
gas space above the fuel bed. By means of removing steam the de-
sired pressure is maintained in the drying section. The produced con-
densate is removed from the bottom of the drying section. The final
moisture of the fuel can be controlled by controlling the amount of fuel,
i.e. the thickness of the fuel bed on the conveyor as well as the delay
time and temperature of the fuel in the drying section.
According to an embodiment of the invention, the moisture content of
the fuel can be controlled and thus fuel with a stable, i.e. the same or
almost the same moisture content can be attained as a result of the
drying process. If necessary, it is possible to spray water or other liquid
into the fuel moving in the drying apparatus to control the moisture
content.
It is an advantage of the invention that the particle-like fuel supplied to
the power boiler can be dried rapidly and in a simple manner. The
feeding of heating medium to the middle and edges of the fuel in the
heating section results in that the fuel heats up rapidly to the drying
temperature. Thus, the variations in temperature between the fuel par-
ticles caused by the storage of fuel can be stabilized, for example
frozen lumps of fuel can be defrosted. The transverse steam flow used
in the heating section does not cause disturbances in the fuel flow in
the heating section, wherein the entire amount of fuel heats up to the
desired drying temperature. The drying temperature maintained in the
drying section ensures efficient drying of the entire fuel flow. The drying
of fuel can also be intensified by sucking steam containing moisture
released from the fuel from the drying apparatus. The possibility of
controlling the moisture of fuel as suggested by an embodiment of the
invention ensures that fuel having precisely the right moisture level is
attained in the power boiler in use. The invention can be utilized espe-
cially in connection with power boilers in pulp and paper mills, because
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they usually use as a fuel bark waste and chips produced in pulp and
paper making. Furthermore, the sludges produced in pulp and paper
mills can be dried by means of the invention and fed to be burned in a
power boiler.
Brief descri~~tion of the drawings
In the following, the invention will be described in more detail with ref-
erence to the appended drawings, in which
Fig. 1 shows a drying apparatus according to the invention in a
side-view and in a cross-section,
Fig. 2 shows a side-view of a second drying apparatus according
to the invention, and
Fig. 3 shows section A-A marked in Fig. 1.
Detailed description of the invention
In this context the term fuel refers to any burning substance, which has
a sufficient thermal value after the drying, so that it can be utilized as a
fuel for the power plant. Such fuels include for example chips, bark,
coal, peat, sludges from process industry or municipal waste.
Fig. 1 shows a fuel drying apparatus 1 according to the invention. The
apparatus comprises a vertical heating section 2 having the shape of a
downward expanding cone. Feeding means (not shown in the figure),
such as a bin or a screw are connected to the upper part of the heating
section and they feed the fuel to be dried to the heating section 2 from
its upper part in accordance with the arrow marked in the figure. The
fuel fed to the heating section 2 moves by means of gravity vertically
downward as shown by arrow A. The fuel is fed to the drying apparatus
constantly in such a manner that the fuel fills substantially the volume
of the heating section from the fuel surface 10 formed in the heating
section 2. The heating section comprises a feeding pipe 4 for feeding
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hot, gaseous medium, such as steam into the middle of the heating
section 2. At the lower end of the feeding pipe 4 there is a steam distri-
bution member 5 extending around the feeding pipe, which can be a
perforation extending around the feeding pipe, or some suitable means
attached to the feeding pipe, such as, for example, a screen, which
distributes the steam from the steam feeding pipe 4 evenly to the fuel
horizontally in an angle of 360 degrees. In the embodiment of Fig. 1, a
screen 5 is arranged in the feeding pipe. In the shell of the heating
section, on the same height with the screen 5, there is a steam distri-
bution member 6, such as a screen, extending around the entire shell
for feeding steam horizontally to the fuel within the perimeter of the en-
tire heating section. A distribution chamber 8 placed on the perimeter
of the heating section, substantially on the same height with the screen
6, divides the steam evenly to the screen 6 surrounding the entire peri-
meter of the heating section. To the distribution chamber, one or
several ducts 7 are connected, said duct/ducts feeding steam to the
distribution channel 8. The heating section also contains at least one
duct 9 for removing the exhaust steam from the gas space 11 above
the fuel surface 10.
The moisture and initial temperature of the fuel to be heated affect the
energy demand of the heating. The physical and geometrical properties
of the fuel to be heated as well as possible treatment preceding the
drying, for example removing of moisture by means of pressing, affect
the warming up rate of the fuel. The temperature of the steam to be
used also affects the heating result. Rapid heating of fuel in the heating
section requires that a sufficient amount of steam is fed as a steady
flow to the fuel. In the drying phase the amount of steam required for
maintaining the temperature of the fuel is small. The delay time of fuel
in the heating section is short, considerably shorter than the delay time
of fuel in the drying section.
The heated fuel is discharged from the lower part of the heating section
to the drying section 3. The heating section 2 and the drying section 3
are arranged successively on top of each other in such a manner that
the heating section is positioned in an angle of 90 degrees with respect
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to the drying section 3. The heating section is arranged in connection
with the drying section, to one of its ends in such a manner that the
heated fuel discharged from the heating section 2 travels substantially
through the horizontal drying section 3, fed by a conveyor 12 on the
bottom of said drying section. In the heating section 2 and below the
same the fuel flow becomes dense and the fuel is fed through the
drying section 3 as a dense fuel bed FB.
The drying section 3 is an elongated vessel, whose length is greater
than its cross-section, and whose ends are closed. On the bottom of
the drying section a conveyor 12 is arranged, said conveyor carrying
the fuel from the heating section 2 end of the drying section to the other
end of the drying section 3, which comprises members 13 for removing
dried fuel from the drying apparatus 1 to a feed screw 14 that feeds the
fuel either to an intermediate storage or to the furnace of the power
boiler. At the initial end of the drying section 3, i.e. at the heating sec-
tion end, there is an adjustment plate 16 attached gas-tightly to the roof
15 the drying section. The adjustment plate 16 is a plate-like element
extending across the width of the drying section 3 that protrudes a dis-
tance inside the drying section 3 and determines the height and shape
of the upper edge of the fuel bed FB formed in the drying section 3. By
adjusting the height and shape of the adjustment plate 16 it is possible
to adjust the height and shape of the upper edge of the fuel bed FB.
The adjustment plate 16 seals the gas space 17 of the drying section 3
remaining between the upper edge of the fuel bed of and the upper
surface of the drying section, i.e. the roof 15 in such a manner that the
pressure therein may be higher than the pressure in the heating
section 2. The upper part of the drying section comprises a duct 18 for
removing exhaust steam.
Hot, gaseous medium, such as steam is fed through the fuel bed via
openings 19 arranged on the conveyor, said fuel bed travelling by
means of the conveyor 12 in the drying section 13. Steam is brought
via one or several ducts 20 attached to the bottom of the drying section
3. The purpose of feeding steam is to maintain a temperature
favourable for the drying of fuel, which together with the delay time ad-
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justed by the speed of the conveyor 12 brings about the drying of the
fuel. The condensate formed in the drying section is removed via a
duct 21 attached to the bottom of the drying section. The condensate is
fed to the waste water system of the plant.
The conveyor 12 arranged on the bottom of the drying section 3 can be
any type of a conveyor suitable for feeding fine-grained material, for
example a scraper conveyor, a belt conveyor or a slat conveyor. The
essential aspect is that the conveyor is formed in such a manner that it
is possible to feed hot, gaseous medium through the fuel bed located
on top of the conveyor and moving by means of the same. Thus, the
conveyor is provided with a set of openings for the passage of the
medium. One possible conveyor is shown in Fig. 3 which will be de-
scribed later.
The hot, gaseous medium used in the heating and drying phase can be
for example hot air, steam, dry steam or flue gases from the power
boiler. It is also possible to use the primary or secondary steam from a
steam-generating process, such as a turbine located in the vicinity of
the drying device. The exhaust steam produced in the heating phase
and in the drying phase can be recovered and compressed and used
again in heating or it can be led, for example, elsewhere in the power
boiler process. The exhaust steam can also be fed to the flue gas
processing system of the power boiler or it can be condensed and re-
moved. The exhaust steam produced in the drying device can also be
circulated again to the drying of fuel. Thus, the exhaust steam removed
both from the heating section 2 and the drying section 3 is fed back to
the heating section 2 and to the drying section 3, which is illustrated by
means of broken lines in Fig. 1. If desired, the exhaust steam produced
in the heating section 2 and in the drying section 3 can be heated in a
heat exchanger 24 illustrated by means of broken lines in Fig. 1, before
it is fed back to the heating section and to the drying section.
According to an embodiment of the invention, the drying of fuel fed to
the drying device can also be intensified by sucking exhaust steam
containing moisture released from the fuel from the drying apparatus.
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In the heating section exhaust steam is sucked either through the
feeding pipe 4 or through the distribution chamber 8 arranged in the
shell. The sucking can be implemented by any suitable means, such as
a blower or a pump 26, which is connected to the feeding pipe 4 and to
5 the distribution chamber 8. The essential aspect is that exhaust steam
is only sucked from one of them at a time, either through the feeding
pipe 4 or the distribution chamber 8 connected to the heating section,
because at the same time the other, either the feeding pipe 4 or the
distribution chamber 8 that is not connected to the pump 26 feeds hot,
10 gaseous medium to the heating section. In the drying section 3 the
suction of exhaust steam takes place via an exhaust steam removal
duct 18.
According to yet another embodiment of the invention it is possible to
control the drying of fuel by changing the delay time of fuel and the
amount and temperature of the drying gas used in the drying appa-
ratus. if necessary, the final moisture of the fuel discharged from the
drying apparatus 1 can also be adjusted by moistening it. Thus, cooling
water is sprayed from a cooling water line 23 on the fuel bed travelling
in the drying section 3 via nozzles 25 arranged in the top part of the
drying section.
Fig. 2 shows a second drying apparatus 1 according to the invention in
which the drying section 3 is divided into two parts in such a manner
that the final end of the drying section 3b is arranged so that it inclines
downward. The front end 3a of the drying section is horizontal and the
conveyor 12 only extends on the length of the front end. The fuel bed
travels through the final end 3b of the drying section, being pushed by
the conveyor 12 located at the front end 3a. The inclination of the final
end 3b of the drying section downward in the flow direction of fuel re-
duces the force required for feeding the fuel.
Fig. 3 illustrates a cross-section A - A of the apparatus of Fig 1. Fig. 3
shows a way of feeding the material through the drying section by
means of a conveyor according to Finnish patents 83181
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(corresponding to US 5134929) and 109103 (corresponding to EP
1140445).
The feeding apparatus 12 is composed of narrow and long adjacent
lamellae 26, which are positioned on supporting rolls 22. The lamellae
26 extend in the direction of the longitudinal shaft of the drying section
within its entire length. The lamellae are equipped with a drive that
moves the lamellae a predetermined distance back and forth in the
longitudinal direction. The movement of the lamellae in the travel direc-
tion of the fuel is considerably slower than their return movement. By
means of this arrangement a larger number of lamellae move in the
travel direction of the material than in the return direction. The net re-
sult thus attained is the propagation of fuel in the drying section. When
the lamellae are arranged on the bottom of the drying section in the
form of a circular arc in accordance with Fig. 3, the fuel to be dried
tends to pack towards the central area of the drying section. This re-
duces friction on the side walls and helps the material to be treated to
travel forward as an even bed.
The steam is fed from the duct 20 and it travels to the fuel to be dried
from gaps between the lamellae as shown by arrows drawn in dotted
lines. When the heating medium condenses in the heating section and
in the drying section, condensate is produced. The condensate runs to
the bottom part of the fuel space and is discharged via the duct 21 as
shown by the solid arrows.
The invention is not intended to be limited to the embodiments pre-
sented as examples above, but the invention is intended to be applied
widely within the scope of the inventive idea as defined in the ap-
pended claims. The apparatus according to the invention can thus also
be positioned in the immediate vicinity of an outdoor storage of fuel, i.e.
bark or chips stored outdoors, for example in a stack, Thus, the dried
bark or chips can be fed directly to the furnace of a power boiler, and
the drying apparatus can be placed in such a location where it does not
cause lack of space in the immediate vicinity of the power boiler. The
drying apparatus is also suitable for drying of peat, and it can be placed
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for example in a peat site, wherein the dried peat can be fed directly to
the power boiler to be burned, and a separate drying apparatus is not
necessary in the power plant.
