Background of the Invention A continuous production process of board according to the dry and wet method, based on material containing lignocellulose such as wood, straw, bagasse <BR> <BR> 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 of fibre from the raw material is today preferably carried out in a so-called thermo- mechanical process in one step or in a thermal 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, and then in a pressurized chip preheater at a temperature 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 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- 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

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 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 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 WO 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 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 the wood chips are separated and disposed of through the outlet into a scrubber. By 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.

Summary of the Invention 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. Simula- neously, the capacity in the production plant should be maintained and so also the 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.

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- 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 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 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.

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 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.

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 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 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.

Thus, by means of the present invention, reduced discharges of foremost VOC and formaldehyde are obtained during the entire production process of board.

This results in important environmental improvements, both internally at the produc- <BR> <BR> 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 less discharge in the environment where it is used.

Brief Description of the 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: Figure 1 shows schematically a plant for production and treatment of wood fi- bres, 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 Figure 3 shows an embodiment example of a second part of the plant according to the present invention.

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.

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 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 scale in fig. 2. Via an inlet 6, wood chips A are fed into the preheater 1, usually from an alkalinizing 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 8. 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 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 such as VOC and formaldehyde, are then separated at a high concentration in the top of the preheater via the top outlet 8. Next, 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 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

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 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 the 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 that part of the screw conveyor 12 that is closest to the beater 2, having an increasing diame- 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 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- nected 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 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 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

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 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 °C.

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. Water sprayers 22 may advantageously also be positioned in the inlet line 26 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 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 NaHSO3 and NaOH in order to optimise the function of the emission reduction. Thus, the 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 ways, as is realized by a person skilled in the art, within the scope of the accompa- nying claims.