Note: Descriptions are shown in the official language in which they were submitted.
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A METHOD AND A PLANT FOR PRODUCING AND TREATING WOOD FIBRES
Technical Field of the Invention
The present invention relates to a method for producing and treating wood fi-
s byes in a plant comprising a fibre-producing part having a chip preheater
and a
beater used to free fibre from wood chips, according to the preamble of claim
1, as
well as a corresponding plant according to the preamble of claim 9.
Background of the Invention
io A continuous production process of board according to the dry and wet
method, based on material containing lignocellulose such as wood, straw,
bagasse
etc., comprises, among other things, a disintegration of the raw material to
free fibre
or fibre aggregate, which in the subsequent steps is coated with glue, dried,
formed
and pressed to a finished product, so-called board or wood fibre board. The
freeing
is of fibre from the raw material is today preferably carried out in a so-
called thermo-
mechanical process in one step or in a fihermal and mechanical processing step
in
two separate phases.
The thermal part, heating of the raw material, takes place inter alia in an
alka
linizing device at a temperature of up to approx. 100 °C, under
atmospheric pressure,
2o and then in a pressurized chip preheater at a fiemperature of approx. 150-
190 °C un
der a pressure of approx. 4-13 bar(e). The dwell time in the preheater may be
be-
tween 1-10 minutes depending on prevailing process conditions and may be ad-
justed. The thermal heating in the chip preheater preferably takes place by
means of
steam. The mechanical processing takes place in a beater between beater discs
2s where the final fibre freeing takes place in a state where temperature and
pressure is
higher than in adjacent process stages. The dwell time of the wood chips raw
mate-
rial in the beater zone is very short. The power that is converted to
mechanical en-
ergy in connection with the mechanical processing transforms into heat in the
beater
zone and occurs as steam in the system, generated from the moisture in the raw
ma-
3o terial.
After the defibering in the beater, the fibre is transported to a pneumatic
fibre
drier where the drying process is performed by means of a large amount of air
and a
controlled entering air temperature of approx. 140-200 °C, depending on
the current
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fibre moisture included. The dried fibre is then transported further to
forming, pre-
pressing and finally to final pressing of the board.
According to older prior art, the wood emissions released during the process,
foremost in the chip preheater, are transported all the way from preheater via
beater
s together with the fibre bulk to the drier, where the majority is separated
from the fibre
and finally accompanies humid drying air leaving the drier and out into the
atmos-
phere. These emissions contain, above all, volatile organic substances, so-
called
VOC (Volatile Organic Compounds), as well as formaldehyde. The remaining
amounts, which are not leaving the drier, follows the fibre flow to subsequent
process
to units where they successively are released to surrounding atmosphere or
appear as
residual product in the finished product, the board. Thus, also from the
finished prod-
uct, discharge of emissions to the atmosphere takes place.
From !NO 99/10594 it is, however, previously known to provide the chip pre
heater with a top outlet for degassing of organic emissions released there.
Here, the
is steam is introduced in the lower part of the preheater and the wood chips,
which en
ter the upper part of the preheater, are washed in the counterflowing steam
during
condensation. This is achieved by means of the steam moving upwards through
the
wood chips column towards the colder wood chips in the top of the preheater,
and re-
leased emissions, air and steam being generated by evaporation of the moisture
in
2o the wood chips are separated and disposed of through the outlet into a
scrubber. Sy
this publication, it is also known to transport the wood chips from the
preheater into
the beater by means of a screw conveyor, which also compresses and dewaters
the
wood chips during the transportation.
2s Summary ~t'the Inventi~n
The object of the present invention is to provide a method and a plant that re-
duce the emission discharges, foremost of VOC and formaldehyde, to the
atmosphere,
from the production process as well as from the finished product, the boards.
Simulta-
neously, the capacity in the production plant should be maintained and so also
the
3o quality of the final product.
This object is attained by a method having the new features that are defined
in the characterizing clause of claim 1, as well as by a plant having the new
features
that are defined in the characterizing clause of claim 9.
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Thus, the method according to the present invention is characterized in that
flue gases generated during the compression are disposed of through an outlet
ar-
ranged in the compression zone. The plant according to the present invention
is
characterized in that in the compression zone a flue gas outlet is arranged
for dis-
c posal of evaporated moisture, which is generated upon the compression of the
wood
chips and which contains flue gases.
In the compression of the wood chips in the screw conveyor, water is
pressed out, which contains a great deal of remaining wood emissions. By
providing
an outlet in the compression zone, the water can evaporate and the released
emis-
sions can be discharged through the outlet. By catching and diverting the
emissions
that are released in the compression zone directly and on the spot, a
considerably
higher concentration of the emissions is obtained than if they instead had to
be
transported further with the fibre bulk and be mixed with the drying air,
which repre-
sents a very large flow of gas, and thereby provides low concentration,
according to
is prior art. Thus, a considerably more efficient handling of the flue gases
from the pre-
heating is obtained by means of the present invention.
According to one embodiment, the disposal of the flue gases that are gener-
ated during the compression is achieved by conveying them back to the upper
part of
the chip preheater from which they then are disposed of together with other
flue
2o gases. The flue gases may advantageously be conveyed to destruction, e.g.,
to in-
cineration in a boiler, where the emissions transform to carbon dioxide and
water.
According to another advantageous feature, upon the disposal of flue gases,
the flow of flue gases can be controlled, and in this way, also the steam
losses in the
chip preheater can be controlled.
2s According to a further embodiment, a so-called catcher agent may be added
to the wood chips upon the feeding thereof into the beater. A catcher agent is
a
chemical that is injected in order to bind some substance, in this case
formaldehyde.
It is previously known to add catcher agents in other places in the process.
However,
it is not previously known to add a catcher agent directly into the beater and
at tests it
3o has turned out that this is particularly efficient as for binding
formaldehyde. Test re-
sults with a catcher agent added into the beater have demonstrated a reduction
of
formaldehyde content by 8-15 % and also a clear improvement of perforator
values
in the final board.
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According to a further embodiment, the wood fibres produced and treated in
the beater are sent to a drier part and then to a scrubber, and air leaving
the drier
part, on the way to the scrubber, is conveyed through a heat exchanger for air
enter-
ing the drier part so that the air to the drier part is heated and the air to
the scrubber
s is cooled. Thereby, the advantage is gained of a certain heat recovery
taking place
for the air to the drier. Another important advantage is, however, that the
air to the
scrubber is cooled. Since the condensable hydrocarbons in the emissions
originating
from the previous stages in the process tend to be aerosols, this makes the
purifica-
tion effect of the scrubber more difficult. The equilibrium relationship
between air and
io water solution of said hydrocarbons is temperature dependent so that at
higher tem-
perature in the scrubber, it is difficult to bring the emissions into water
phase. For this
reason, it is advantageous to cool the air entering the scrubber.
Furthermore, water drops may advantageously be separated in the scrubber
by a demister device. Additional cooling by supply of water is also possible.
is Thus, by means of the present invention, reduced discharges ofi foremost
V~C and formaldehyde are obtained during the entire production process of
board.
This results in important environmental improvements, both internally at the
produc-
tion line and externally, i.e., in the environment outside the plant.
Furthermore, the
advantage is obtained of an improved final product in the form of a board that
causes
20 less discharge in the environment where it is used.
Brief Description of tMe Drawings
The invention will now be described in more detail in the form of embodiment
examples and reference being made to the appended drawings, in which:
2s Figure 1 shows schematically a plant for production and treatment of wood
fi-
bras, comprising an embodiment example according to the present in-
vention,
Figure 2 shows an embodiment example of a first part of the plant according to
the present invention, and
3o Figure 3 shows an embodiment example of a second part of the plant
according
to the present invention.
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Detailed Description of the Invention
The plant for producing and treating wood fibres, illustrated schematically in
fig. 1, is included in the illustrated example as a part of a continuous
process for pro-
ducing board of material containing lignocellulose.
s Such a plant comprises a plurality of stages. The stages that are
illustrated
schematically in fig. 1 includes a first stage having a fibre-producing part
comprising
a chip preheater 1 and a beater 2 for freeing fibres of wood chips, a second
stage
having a drier part 3 comprising, for instance, a cyclone, and a third stage
in connec-
tion with the drier part and comprising a scrubber 4, foremost intended for
efficient
io particle separation in connection with the drying. It should be
particularly mentioned
that the drier part in itself may comprise a plurality of drying steps having
a plurality of
similar apparatuses and the plant may moreover also comprise additional
intermedi-
ate steps and details that have not been shown for reasons of clarity.
The fibre-producing part of the plant in fig. 1 is shown on a somewhat larger
m scale in fig. ~. Via an inlet 6, wood chips A are fed into the preheater 1,
usually from
an alltalini~ing bin, which is not shown. In the lower part of the chip
preheater, steam
B is supplied via steam inlets 7. The chip preheater is also provided with a
top outlet
~. In the chip preheater, the wood chips are heated by means of condensing
heat
from the steam, which is supplied at pressure, temperature and time having
been
2o pre-set and adapted to the wood chips raw material. By the fact that the
steam B is
supplied into the lower part of the chip preheater, it will move up through
the wood
chips column C towards the colder entering wood chips in the top, and in this
way the
moisture in the wood chips is evaporated and forms steam containing released
or-
ganic emissions from the wood. Said emissions, foremost organic volatile
substances
as such as V~C and formaldehyde, are then separated at a high concentration in
the
top of the preheater via the top outlet 5. fVext, from the top outlet, the
flue gases
travel to some kind of destruction device, which is not shown. The destruction
may,
for instance, be effected by incineration or by evaporation.
As is seen in fig. 2, the flue gas line in the top of the preheater is
provided
3o with temperature control device, TIC, and the supply line for steam is
provided with a
pressure control device, PIC. By setting a value of the pressure of the steam
that cor-
responds to a certain temperature, e.g., of the order of 170 °C, it is
possible, by
measuring the temperature of the leaving flue gases, to control these so that
unnec-
essary steam and energy losses are prevented. Thus, the temperature of the
flue
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gases should not exceed the temperature of the entering steam. If this would
take
place, the flow of flue gases is automatically reduced, in order to thereby
prevent that
more steam is supplied than what can be utilized in the chip preheater.
In this connection! it may also be mentioned that the degassing in the top of
s the preheater has a favourable impact on the heat transfer between the steam
and
the wood chips in the preheater. Normally, the air that is contained in the
supplied
cold wood chips A reduces said heat transfer, but by means of the present
invention
this air may instead be directly disposed of through the top outlet 8.
In the bottom of the chip preheater, there is a chip outlet 11. In connection
with the same, there is a screw conveyor 12 arranged, which feeds the wood
chips to
the beater 2. The screw conveyor comprises a compression zone 15 where the
water
is pressed out of fibs chip mass and the formed moisture evaporates in the
zone so
that steam is generated. The compression zone 15 is formed by the shaft, of
thafi part
of the screw conveyor 12 fihat is closest to the beater 2, having an
increasing diame-
is ter, so that it obtains a conical shape. This results in the chip material
in said part be-
ing compressed and pushed out against the outer walls of the screw conveyor 12
where a so-called plug flow is formed, which partly plugs up the entrance to
the
beater. The formed plug separates the beater from the screw conveyor so that
steam
from the beater cannot travel rearwards into the compression zone. According
to the
2o preferred embodiment, a flue gas outlet 13 is arranged in the compression
zone so
that it is possible to dispose of said evaporated moisture, which contains
flue gases.
Furthermore, according to the preferred embodiment, said flue gas outlet is
con-
netted to a line 14 that conveys the flue gases back to the upper part of the
chip pre-
heater 1. Here, they can then be disposed of together with other flue gases
via the
2s top outlet 8 and further away for destruction.
In the illustrated example, is also shown how a catcher agent D can be
added into the beater 2. A catcher agent is a chemical, which is injected in
order to
bind some substance, in this case formaldehyde. The member 16 for addition of
a
catcher agent may, for instance, be formed so that the supply of the catcher
agent D
3o takes place via the centre of the screw conveyor 12 into the beater 2. A
suitable
catcher agent in this case may be urea or the like.
In fig. 3, an embodiment example is shown of a scrubber plant 4 according to
the present invention, which is used in connection with a drier stage. Said
scrubber
plant may advantageously be used together with the above-described chip
preheater
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1 and the beater 2, but may also be used together with other types of chip
preheaters
and beaters and also in other types of plants, and may thus be regarded as an
inven-
tion in itself. In fig. 3, a scrubber 4 is shown that receives air E that
travels from the
drier part 3, via a line 28. Before the same air is allowed to pass into the
scrubber, it
s passes a heat exchanger device 20, for instance, a tube heat exchanger, for
the air F
entering the drier. In this way, the heat that the air E leaving the drier
contains is util-
ized in order to preheat the air F, G entering the drier, at the same time as
the air
leaving the drier is cooled down before it enters the scrubber 4. The
equilibrium rela-
tionship between air temperature and the concentration of formaldehyde will
conse-
quently be altered and promotes the transfer of formaldehyde into water phase.
The
air E leaving the drier may have a temperature of the order of +60 °C,
the drying air F
entering the heat exchanger is as a rule taken from outside and may then have
a
temperature of +10 °C, and the preheated air G entering the drier may
then have a
temperature of +30 °G.
is The scrubber is also provided with a number of water sprayers 21, which ac-
cordingly spray out water that cools and absorbs the emissions that are in the
drying
air into water phase. Vl/ater sprayers 22 may advantageously also be
positioned in
the inlet line 25 to the scrubber. In the lower part of the scrubber, there is
a water out-
let 23, where water is drawn off in order to be conveyed to purification. In
the top of
2o the scrubber, there is an outlet 24 for leaving air. The scrubber is
suitably also pro-
vided with a so-called demister or droplet separator 25 in connection with the
upper
air outlet, in order to separate water drops.
The supplied spray water may possibly be chemically treated with NaHS~3
and NaOH in order to optimise the function of the emission reduction. Thus,
the
2s scrubber 4 is provided with a device 27 for supply of said additives to the
water. Test
results have shown that in this way, a 75 °/~ reduction of the
formaldehyde content in
the leaving air is achieved, already without connection of a heat exchanger.
The present invention should not be regarded as limited to the above-
described embodiment examples, but may be modified and varied in a multiple of
3o ways, as is realized by a person skilled in the art, within the scope of
the accompa-
nying claims.
