Method and a plant for producing and treating wood fibres

Abstract

The present invention relates to a plant and a method for producing and treating wood fibres in a plant comprising a fibre-producing part having a chip preheater (1) and a beater (2) for freeing fibre from wood chips. Such a plant and such a method are included as parts in a continuous production process of board and the object is to reduce the emission discharges of foremost VOC and formaldehyde, during different parts of the process as well as from the finished product. The method comprises feeding of wood chips (A) into an upper part of the chip preheater (1), discharge of wood chips in the bottom of the chip preheater to a screw conveyor (12) for further transportation into the beater (2), compression of the wood chips in a compression zone (15) in the screw conveyor, supply of steam (B) into the lower part of the chip preheater for washing fed chips in the counterflow direction, disposal (8) of flue gases in the form of released organic emissions, steam and air in the upper part of the chip preheater, as well as disposal of flue gases generated during the compression through an outlet (13) arranged in the compression zone. The plant has the corresponding parts.

Description

TECHNICAL FIELD OF THE INVENTION [0001] The present invention relates to a method for producing and treating wood fibres 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 [0002] 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 of fibre from the raw material is today preferably carried out in a so-called thermomechanical process in one step or in a thermal and mechanical processing step in two separate phases. [0003] The thermal p...

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Benefits of technology

[0012] 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.

[0013] 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 results 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.

[0014] 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 entering 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 purification effect of the scrubber more difficult. The equilibrium relationship between air and water solution of said hydrocarbons is temperature dependent so that at higher temperature 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.

[0015] Furthermore, water drops may advantageously be separated in the scrubber by a demister device. Additional cooling by supply of water is also possible.

[0016] 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 production 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.

Problems solved by technology

In the compression of the wood chips in the screw conveyor, water is pressed out, which contains a great deal of remaining wood emissions.

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