One problem encountered in connection with such plants is the emission to the surrounding atmosphere of volatile, organic substances, i. e. Volatile Organic Compounds (VOC), and formaldehyde from the raw wood and from the size used in the process. One purpose of the present invention is to solve this problem. Another purpose is to recover thermal energy in these processes. These purposes are achieved by imparting to a plant designed/constructed in accordance with the invention the characteristics set forth in the claims.

The invention will now be described in greater detail with reference to the attached drawings on which: Figs. la and I b show schematically the process performed in a plant designed/constructed in accordance with the invention.

Fig. 2 shows a part of the plant shown in Fig. I a drawn to a larger scale, but an alternative embodiment is presented here.

Fig. 3 shows the same part as that shown in Fig. 2, but yet another alternative embodiment is shown here.

Fig. 4 shows schematically the process in an alternative embodiment of a plant designed/constructed in accordance with the invention.

The plant shown in Figs. 1 a and 1 b comprises a number of parts: a fibre-production part A, a steam separator part B, two fibre drying stages C I, C2 and an energy subplant D. The plant will be described with regard to its design/construction while simultaneously explaining its operation in order to avoid needless repetition.

In fibre-production part A, the wood chips are preheated in an alkalinizing bin 1 using, if so desired, steam at atmospheric pressure. From bin 1, the chips are fed by plug-type screw conveyor 2 which compresses and dewaters them as they are conveyed to chip preheater 3. Chip preheater 3 heats the chips with heat obtained form condensing steam that is supplied through steam inlet 50 located in the lower part of the preheater, the pressure, temperature and time having been preset and adapted to the raw wood chips. Preheater 3 has a top outlet 56 where released organic emissions are degassed from the wood along with air during heating, thereby improving heat transfer between the steam and chips. The greater part of the emissions, i. e. volatile organic substances (VOC and formaldehyde), is released in the preheater and separated with high concentration in the top of the preheater and conveyed together with steam and air via line 57 to scrubber 11 where solid particles are separated along with certain condensable organic substances and volatile waste gases and where heat is recovered from the steam. Since the steam

is supplied through inlet 50 at a low level the chips, which enter from the top of the preheater, can be washed in counterflowing steam during condensation.

The electrical energy that is added to free fibre from the chips in beater 4 is converted, for the most part, to steam in connection with the mechanical processing of the preheated chips to produce free fibres or fibre bundles. During processing, a certain amount of organic emissions are released from the wood, and they are conveyed forward in blower line 5 in the gaseous state together with the steam. In blower line 5 the fibres, fibre bundles and steam are conveyed at high speed to inlet 52 on cyclone separator 6. If size is to be used, it is added in blower line 5 at 51 thereby sizing the fibre. Emissions of volatile, organic substances are also released from the size and together with the fibre and the steam, they are conveyed to cyclone separator 6 for separation from the fibre. Bottom outlet 53 on cyclone separator 6 is connected, via sluice valve 7, to conveyance line 55 in which the fibre is sent to a fibre drier. However, drying of the fibre can also take place in line 55 due to the fact that the transport medium is drying air (see especially Figs 2 and 4). Consequently, line 55 will hereinafter be called conveyance/drying line 55. Upper connection 54 on the cyclone separator is connected to scrubber 10 that separates the fibres and organic substances from the steam obtained from the cyclone separator.

Cyclone separator 6 can be included in several alternative basic embodiments. In alternative embodiment 1, which is shown in Fig. la, steam ejector 8 is connected downstream from sluice valve 7. Steam ejector 8, which is supplied with steam at 67, handles further conveyance of the fibre together with preheated drying air and flue gases that are sucked from mixing chamber 9. This mixing chamber 9 receives flue gases via line 66, which runs from energy subplant D in the plant and also receives drying air via valve 58. In this alternative embodiment, the fibre is already being dried while it is being conveyed to drier stage C.

In alternative embodiment 2, which is shown in Fig. 2, conveyance/drying line 55 is connected directly to sluice valve 7 wherewith hot air is supplied directly to the drier line at 59.

In alternative embodiment 3, which is shown in Fig. 3, the fibre is transported to the drier stage by means of compressor 31 which is supplied with conveyance air at 60 and feeds injector 32.

In drier stage Cl, the drying air is heated as indicated by arrows 61 in air/hot-water coil 18 and in air/steam coils 19,20 and also in mixing chamber 21 used for flue gas obtained from energy subplant D. The suspension of steam, air and volatile organic substances (VOC and formaldehyde) that arrives at scrubber 11 from preheater 3 is washed free of solid particles in scrubber 11 using condensate pumped from condensate tank 26 by means of pump 28. Parts of released emissions from the wood are condensed and leave scrubber 11 together with the scrubber water. Steam leaving scrubber 11 is used during condensation to heat the drying air in air/steam coil 20. Condensate from coil 20 leaves separator 14 where volatile organic emissions

proceed via regulator valve 15, suction fan 22 (Fig. I b) and duct 30 to incinerator 62 in energy subplant D.

Steam obtained from cyclone separator 6 that contains organic emissions released in connection with fibre production and sizing is washed free of solid particles in scrubber 10 using condensate from condensate tank 26. Parts of the aforesaid emissions are condensed and leave scrubber 10 together with the scrubber water. The washed steam from scrubber 10 is sent to heating coil 19 where it is used to heat drying air 61. In heating coil 19 the steam is condensed, and the condensate is sent to separator 16 from which volatile non-condensable emissions are sent to incinerator 62 for incineration via regulator valve 17, suction fan 22 and duct 30.

The condensate in condensate tank 26 is transported by pump 27 to heat the drying air in heating coil 18. The condensate leaves coil 18 at a temperature of about 40°C via separator 12 where the remaining emissions of volatile organic gases are sent to incinerator 62 via valve 13, suction fan 22 and duct 30.

Condensate from separator 12 is sent to tank 23, which contains a decanter insert.

Condensate consisting of emission remnants (terpenes) released from the wood substance is decanted and transported by pump 24 and pipe 29 to incinerator 22 where it is used to moisten solid fuel 63 and grindings 64 which are also sent here.

The level in condensate tank 23 is regulated by means of pump 25. Water that proceeds via pump 25 is a) used if so desired to heat drying air, as indicated by arrows 65, in drier stage C2 via heating coil 40, or b) used in the rest of the process wherever needed, or c) sent out directly for purification. Drying air 61 and drying air 65 in drier stages C and C2 are heated to the final temperature together, if so desired, with flue gas from the energy subplant via mixing chambers 21 and 41 or using some other heating medium.

Fig. 4 shows an alternative embodiment of the plant, which is somewhat simplifie relative to the previously described alternative embodiments. Here, fibre-production part A is the same as in the embodiments previously described. In steam separator part B, on the other hand, the emissions released in preheater 3 are sent together with steam and air via line 70 to heat exchanger 73 and condensate tank 76 in order to separate condensable organic substances and volatile waste gases and recover heat from the steam. Bottom outlet 53 on cyclone separator 6 is connected, via sluice valve 7, to injector 32 which is located in conveyance/drying line 55 that runs to the first drier stage Cl. Upper connection 54 on cyclone separator 6 is also connected to heat exchanger 73 in order to heat the drying air.

In drier stage Cl, the drying air is heated, as indicated by arrows 87, in the following: a) heat exchanger 75, b) mixing chamber 74 together with mixed-in drying air leaving cyclone separator 83 in drier stage C2, c) heat exchanger 73 together with steam from cyclone separator 6 and preheater 3 and d) mixing chamber 72 using the necessary supplementary drying energy

introduced at 85. In drier stage C2 the drying air is heated in heat exchanger 78 and also via heating coil 84 using, alternatively, a heating medium supplied at 86.

An alternative to the aforesaid drying air heating arrangement can be provided by eliminating mixing chamber 74 where mixed-in drying air from drier stage C2 is used. Another alternative can be provided by eliminating heat exchangers 75 and 78 which are intended for cooling condensate sent from condensate tank 76 to condensate tank 79 and replacing them with some other form of cooling.

The suspension of steam, air and volatile organic substances (VOC and formaldehyde) which arrives at heat exchanger 73 from preheater 3 and cyclone separator 6 is condensed and sent out as condensate to tank 76 where volatile organic emulsions in the gaseous state are sent to the energy subplant for incineration via 93. Parts of the emulsions mentioned above are condensed and transported together with the condensate by pump 77 to heat exchanger 75 used for drier stage Cl and also to heat exchanger 78 used for drier stage C2, and they transfer parts of their heat content to the drying air. Transport to cyclone separator 83 is provided by fan 82.

Condensate from heat exchanger 78 is sent at a temperature of about 40°C to condensate tank 79 from which remaining emulsions of volatile organic gases are sent via 93 to the energy subplant for incineration. The level in condensate tank 79 is regulated by means of pump 80. If so desired, water from pump 80 is used, via 89, in the rest of the process wherever needed or is sent out directly for purification.