Process for producing fiberboards with reduced VOC emissions

Description

[0001] The invention relates to a method for producing fiberboards with reduced VOC emissions, in particular for producing HDF boards or MDF boards.

[0002] A continuous production process for wood fibers using the dry and wet method, based on lignocellulosic materials such as wood, straw, or bagasse, includes, among other things, comminution of the raw material into free fibers or fiber aggregates, which are subsequently coated with adhesive, dried, shaped, and pressed into a final product, the so-called board or wood fiberboard. Today, the release of fibers from the raw material is preferably carried out in a so-called thermomechanical process in one step or in a thermal and mechanical process in at least two separate steps.

[0003] Before the first thermal treatment, the wood chips are usually washed to remove contaminants such as soil or stones. The thermal treatment, i.e. the heating of the raw material, takes place, among other things, in a first thermal treatment device at a preferred temperature of up to approximately 100 degrees Celsius, in particular under atmospheric pressure, and then in a preferably pressurized second thermal treatment device at a temperature of, for example, approximately 150 to 190 degrees Celsius, in particular under a pressure of approximately 4 to 13 bar. The residence time of the wood chips in the thermal treatment devices can be adjusted depending on the prevailing process conditions and can be, for example, between approximately 1 and 10 minutes. According to the prior art, the thermal heating in the second thermal treatment device is preferably carried out using steam.Mechanical processing then takes place in a refiner, also known as a fiberizer. The residence time of the wood chip raw material in the refiner is short. The energy converted into mechanical energy during mechanical processing is converted into heat in the reduction zone and released into the processing system as exhaust gas, particularly steam, generated from the moisture in the raw material.

[0004] Typically, after fiberization in the refiner, the wood fibers are pneumatically transported to a fiber dryer, where the drying process is carried out with a large air volume and a controlled inlet air temperature of approximately 140 to 200 degrees Celsius, depending on the current fiber moisture content. The fibers are mechanically separated from the drying air. The dried fibers are then transported further for forming, pre-pressing, and finally, final pressing of the board. The drying air is subjected to exhaust gas scrubbing. For this purpose, wet scrubbing, wet electrostatic precipitators or biofilters, and biological wastewater treatment are used.

[0005] According to the state of the art, the wood emissions released during fiber extraction and drying, especially in the second thermal treatment device, are transported from the first thermal treatment device through the refiner along with the bulk fiber to the dryer, where the majority of the fiber is separated and, finally, moist drying air from the dryer is released into the atmosphere after exhaust gas scrubbing. These wood emissions primarily contain volatile organic substances, so-called volatile organic compounds (VOCs).

[0006] Among VOCs, there are substances whose solubility in water during wet exhaust scrubbing is so low that their capture efficiency is only a small percentage or even close to zero. This particularly applies to terpenes. It is therefore known that wet capture processes only achieve an emission reduction efficiency of 10-30%. The low solubility is due, on the one hand, to the substances' inherently low solubility in water and, on the other, to their strong dilution in the drying air, which drastically reduces the partial pressure and thus the thermodynamic driving force. The terpenes originate from the resin of the wood used. They are highly volatile oils and are also known as turpentine.

[0007] The remaining quantities that do not leave the dryer follow the fiber flow to the subsequent processing units, where they are gradually released into the surrounding atmosphere or appear as residual products in the final product, the board. Thus, emissions can also be released into the atmosphere from the final product.

[0008] From WO 99 / 10594, it is previously known that the second thermal treatment device is equipped with an upper outlet for degassing the organic emissions released there. Here, the steam is introduced into the lower part of the first thermal treatment device, and the wood chips entering the upper part of the first thermal treatment device are washed in the counterflowing steam while the steam condenses. This is achieved by the steam moving upward through the wood chip column to the cooler wood chips in the upper part of the first thermal treatment device. The released emissions, exhaust air, and steam resulting from the evaporation of the moisture in the wood chips, are separated and disposed of through the outlet in a suitable device.This publication also discloses that the wood chips from the first thermal treatment device are transported to the refiner by means of a screw conveyor, which compresses and dewaters the wood chips during transport.

[0009] According to published patent application EP 1597427 A1, a process is known in which the exhaust gases generated during compaction are disposed of in a screw conveyor via an outlet located in the compaction zone. The system according to this document is characterized in that an exhaust gas outlet is arranged in the compression zone to remove evaporated moisture that is generated during compaction of the wood chips and contains VOC-containing exhaust gases.

[0010] EP 2 573 258 A1 describes a method and apparatus for processing wood chips for the production of wood-containing pulp. Regarding the washing of the wood chips, it is stated that this is done by heating them to approximately 90°C with water heated to up to 98°C.

[0011] US 4,925,527 describes a process for obtaining turpentine from a TMP (Thermal Mechanical Pulp) process, in which a gas stream is taken from a refiner and fed to a condenser.

[0012] EP 1 021 612 A1 discloses a plant for producing and treating wood fibers, comprising a fiber-producing section (A) equipped with a chip preheater and a beater, which serve to separate the fibers from the wood chips, and at least one drying stage (C) for drying the fibers. Between the fiber-producing section (A) and the drying stage (C), a steam separation section (B) is provided, which comprises a cyclone separator, the inlet of which is connected to the blower line used for the fibers and the steam obtained from the beating machine. The lower outlet of the cyclone separator is connected via a sluice gate valve to a conveying / drying line for the fibers. The upper outlet of the cyclone separator is connected to devices for separating volatile organic substances and for recovering heat from the steam of the cyclone separator.

[0013] US 2012 / 227918 discloses a steam separation system for refiners according to the present invention, comprising a blow line for transporting a fiber material mixture from a refiner to an inlet of a steam separator. The exhaust steam is discharged from the separator through an exhaust steam outlet. Cleaned fiber material is discharged from the separator through an outlet that prevents a substantial portion of the exhaust steam from passing through the outlet. A relay pipe communicates with the outlet and a dryer duct and transports the cleaned fiber material between the two. A resin inlet communicates with the relay pipe and supplies resin thereto. The resin is mixed with the cleaned fiber material before the cleaned fiber material is dried in the dryer duct. The present invention also relates to a method for reducing VOC emissions generated during the refining of cellulosic fiber material.

[0014] WO 2021 / 063555 A1 discloses a method and a system for applying glue to a fiber stock in a fiber-steam mixture, which is fed via a blow line from a refiner to a gluing device and then to a dryer. The fiber-steam mixture is fed to the gluing device in a circumferential direction, whereby the fiber-steam mixture forms a swirling flow in the gluing device. This allows for efficient distribution of the glue with minimal contamination of the inner wall and minimal energy loss.

[0015] However, in the interest of environmentally friendly wood processing, there is a need to remove VOCs even more efficiently from a process for producing fiberboard, in particular for producing HDF boards or MDF boards, and in particular to enable a resource-saving process.

[0016] The object is achieved according to the invention by a method for producing fiberboards with reduced VOC emissions having the features of claim 1. Preferred embodiments of the invention are specified in the subclaims, which may each individually or in combination represent an aspect of the invention.

[0017] A process for producing fiberboards with reduced VOC emissions is described, the process comprising at least the following process steps: a) Providing wood-containing wood chips; b) Thermally treating the wood chips in a thermal treatment device or in a plurality of thermal treatment devices; c) Shredding, in particular defibrating, the wood chips in a refiner; d) Gluing the shredded, in particular defibrated, wood chips; and e) Pressing the shredded, in particular defibrated and glued wood chips to form the fiberboard, wherein f) steam used or generated in the process is separated from the process in a controlled manner, in particular continuously, at at least one steam emission point, wherein the steam is separated in a predetermined quantity range such that a lower limit and an upper limit of the quantity range of the total separated steam are dependent on at least one specification of the wood chips used in process step a).

[0018] Such a process offers a particularly advantageous way to efficiently reduce environmentally harmful VOC emissions during fiberboard production. It also allows for a resource-saving process.

[0019] For the purposes of the present invention, the term VOC (Volatile Organic Compounds) refers in particular to those volatile compounds that are present in the wood that serves as the starting material for the process described here. In particular, the VOCs described in this process are terpenes that occur in the wood as wood oil. Examples include the following substances, which can occur in the weight percentages stated in parentheses based on the VOCs contained: α-pinene (20-70%), β-pinene (5-20%), limonene (1-5%), camphene (1-5%), phenol (0.2-2%). Other components can include myrcene, α-, β-phellandrene, 3-carene, cymene / cymene, terpinoles, ocimene.

[0020] The term "controlled" in the context of the present invention, with regard to steam separation, means that the quantity and / or flow of the steam to be separated is adjustable, preferably controllable. In this respect, non-adjustable and / or non-controllable steam emission points are not to be understood as a controlled steam separation within the meaning of the invention.

[0021] The process described here is used to produce fiberboard. Fiberboard within the meaning of the present invention is understood to mean boards in a conventional manner which comprise wood fibers in a matrix made of a binder. For example, the fiberboards can comprise so-called medium-density fiberboards (MDF boards, density for example 700-800 kg / m 3< ) or low-density fiberboards (LDF, density for example < 650 kg / m 3< ). Furthermore, so-called high-density fiberboards (HDF boards, density for example > 800 kg / m 3< ) can be produced using the described process. MDF boards or HDF boards can particularly preferably be produced using the described process. Such fiberboards are particularly suitable for interior house construction as underlay panels for roofs or external wall paneling. The boards also have a wide range of uses in furniture construction.It is also suitable for use as floor, ceiling or wall coverings for the interior design of rooms.

[0022] In the process described here, wood-containing wood chips are first prepared according to process step a). In this step, any type of wood can be prepared, which is then coarsely chopped so that it can be made available as wood chips.

[0023] The wood used is not fundamentally limited; for example, wood selected from pine, spruce, larch, birch, beech, dead oak, alder, etc. can be used, but is not limited to these.

[0024] The raw wood can be processed into wood chips, for example, by roughly chopping the wood used as the starting material, debarking it, and cleaning it of coarse impurities, such as sand or stones. There is no fundamental limit to the size of the wood chips, as is generally known to those skilled in the art from the production of fiberboard.

[0025] According to process step b), the process comprises thermally treating the wood chips in one or more thermal treatment devices. In this process step, the wood chips can be treated, in particular, with steam or with hot water under pressure, for example, to remove VOCs from the wood. Accordingly, the temperature in this process step can be, at least in part, in a range above 100°C. Furthermore, the thermal treatment(s) can serve to further purify the wood chips.

[0026] According to process step c), the wood chips are shredded in a refiner. In this process step, the previously coarsely shredded wood is further shredded to take on the shape suitable for the panels to be produced. This can be achieved, for example, by adjusting the grinding mechanism or the energy thus introduced into the wood chips and / or the duration of the wood chip treatment, as is generally known to those skilled in the art. In particular, the wood chips can be defibrated in this process step.

[0027] The shredded or fiberized wood chips obtained after process step c) are then glued according to process step d). Gluing refers, in particular, to the incorporation of the wood chips into a matrix of a binder that serves as an adhesive. The binder or glue can be, for example, a urea-formaldehyde resin, reinforced with melamine or phenol. Furthermore, the glue or binder is preferably curable, for example, by applying heat, so that after curing, a stable structure is formed that can serve as a corresponding fiberboard.

[0028] Accordingly, the defibrated and glued wood chips can then be pressed according to process step e) to form a fiberboard, in particular using heat and / or electromagnetic radiation. Understandably, the specific parameters to be applied in this process step depend on the materials to be pressed, in particular on the glue or binder used. In the process described here, it is further provided that, according to process step f), steam used or generated in the process is separated from the process in a controlled manner at at least one steam emission point, wherein the steam is separated in a predetermined quantity range such that a lower limit and an upper limit of the quantity range of the total separated steam depends on at least one specification of the wood chips used in process step a).The steam separation can preferably be carried out continuously. Continuous steam separation can include, for example, uninterrupted separation or continuous periodic separation, i.e., comprehensively definable periodically recurring pauses.

[0029] In particular, by separating steam used or generated in the process from the process in a controlled manner at at least one steam emission point, wherein the steam is separated in a predetermined quantity range such that a lower limit and an upper limit of the quantity range of the total separated steam are determined depending on at least one specification of the wood chips used in process step a), significant advantages can be achieved compared to the solutions from the prior art.

[0030] The invention is based in particular on the fact that by separating steam from the process, VOCs can be separated from the production stream, as they accumulate in the steam. Thus, appropriate steam separation reduces VOC emissions, for example, as exhaust gases or as vapors from the manufactured product, i.e., the fiberboard produced.

[0031] Surprisingly, it has been shown that it is not necessary to continuously remove large amounts of steam from the process to achieve a significant reduction in VOC emissions. Rather, it has been found that even the emission of comparatively small amounts of steam can remove almost the entire amount of VOCs, especially terpenes. This can significantly reduce the amount of steam removed and thus, for example, the amount of steam requiring further processing. This can reduce the effort and costs of the overall process.

[0032] Furthermore, in the production of fiberboard, it is often necessary to generate additional steam in addition to the steam already generated during the process in order to obtain a sufficient amount of steam for the respective processing steps. However, the generation of steam is also associated with effort and costs, which can be significantly reduced with the invention.

[0033] The process described here also takes advantage of the fact that terpenes, the most important VOCs in this process, have a boiling point above 150°C. However, it has been found that even evaporation streams or, in general, steam streams with temperatures below 100°C can contain very significant amounts of volatile organic substances, particularly terpenes. Therefore, it is advantageous in the process described here to focus on the total amount of steam separated, regardless of its origin or the local separation point.

[0034] The separation of steam streams can generally be carried out using state-of-the-art methods, and it is advantageous that the steam is treated to collect the VOCs and not released directly into the environment along with the VOCs. For example, the steam can be separated using overpressure or negative pressure.

[0035] The fact that the steam is separated in a predetermined quantity range such that a lower limit and an upper limit of the quantity range of the total separated steam is dependent on at least one specification of the wood chips used in process step a) can be implemented in a variety of ways, as described in more detail below.

[0036] Following the process described here, the fiberboards produced can be further processed, particularly depending on their specific application. For example, the fiberboards produced can be sanded, sawn into smaller sheets, or additional layers can be applied, for example, in lamination processes. Furthermore, it is possible to incorporate specific structures into the sheets, which can serve, for example, to attach them to one another or to other substrates. This allows the fiberboard to be advantageously used for the desired application.

[0037] With regard to the at least one specification of the wood chips, it should be mentioned that only one specification can serve as the basis for determining the amount of steam to be separated, or that preferably a plurality of specifications can serve as the basis for determining the amount of steam to be separated.

[0038] For example, one specification or a plurality of specifications may be selected from the following specifications.

[0039] In particular, a specification can be the quantity of wood chips used in the process. The quantity of both wood chips and steam can be the absolute quantity in a batch process, for example, or the quantity of both wood chips and steam in a continuous process can be the quantity per unit of time. It is understandable that, regardless of the specific design and the components of the wood chips, the quantity of wood chips has a significant influence on the VOCs introduced into the process by the wood and thus equally on the VOCs to be removed. Therefore, the quantity of wood chips should be given priority when determining the amount of steam to be separated.

[0040] Alternatively, or preferably in addition to the quantity of wood chips used, it may be preferable for a lower limit and an upper limit of the quantity range to be determined depending on the quantity of VOCs, in particular the terpenes, contained in the wood chips provided in process step a). Thus, it is possible, in particular, to determine or estimate how many VOCs, and thus in particular how many terpenes found in wood, are contained in the wood chips per quantity of wood chips. In other words, the quantity of terpenes or VOCs present in the wood chips in percent by weight, based on the quantity of wood chips, can be considered.

[0041] This specification can be particularly advantageous since it has been shown that different wood species also contain different amounts of terpenes. Accordingly, the amount of VOCs present in a given quantity of wood chips can depend on the specific wood species used.

[0042] In particular, selecting such specifications allows the amount of steam to be separated to be reduced particularly reliably, as it ensures that fluctuations in the VOCs present during steam separation do not result in too little steam being separated, thus resulting in an undesirably high VOC content. Furthermore, the amount of steam to be separated and possibly produced can still be reduced reliably and without the aforementioned risks.

[0043] It may also be advantageous if the amount of VOCs, in particular terpenes, contained in the wood chips used in process step a) is determined by examining the wood chips used or is estimated based on the type of wood chips used.

[0044] Determining the amount of VOCs by analyzing the wood chips can enable a particularly precise determination of the VOCs contained in the wood chips, thus also allowing the determination of the amount of vapor to be separated to be carried out very precisely. Determining the amount of VOCs can be done in a conventional manner by analyzing the components of the wood chips. This can be advantageous, for example, because the VOC content can simply be reduced due to evaporation during storage, or because fluctuations in the VOC content can occur within the same wood species.

[0045] Estimating the VOCs contained in the wood chips, particularly the amount of terpenes they contain, based on the type of wood chips used, i.e., particularly considering the type of wood the wood chips are made from, can allow for a particularly simple determination of the VOC quantity, while keeping the effort very low. This approach can be based, in particular, on the fact that different types of wood, such as birch or spruce, often have different amounts of VOCs such as terpenes, but the amount of VOCs and, in particular, the amount of terpenes they contain is characteristic of the type of wood. Thus, with knowledge of the wood used, the VOC quantity can be estimated in advance without the need for analysis.

[0046] To ensure that potentially occurring inaccuracies in the VOC content of the respective wood are not critical, the amount of separated vapor can be determined with a definable safety factor, i.e., a definable larger amount of vapor can be separated than necessary according to the terpene content data used. This also allows for a particularly safe and reliable reduction in the amount of VOCs released from the process.

[0047] It has been found that it is sufficient if the total amount of steam separated in process step f) is in a range of 0.5 to 100 times the mass, preferably 0.5 to 50 times the mass, particularly preferably 0.5 to 10 times the mass, based on the amount of terpene in the wood chips provided. This amount is significantly lower than the amount of steam separated in solutions from the prior art, for example in US 4,925,527, but is surprisingly sufficient to essentially remove the entire amount of VOCs from the process and thus significantly reduce VOC emissions in the process for producing fiberboards described here. It has therefore been shown that, for example, if the process described here or the amount of steam separated in the process is based on the amount of VOCs introduced into the process by the wood chips, such asTerpenes, a surprisingly small amount of vapor can be separated, which is sufficient to achieve the object of the invention.

[0048] Alternatively or additionally, the dry mass of the provided wood chips can also be a good indicator for determining the amount of steam to be separated. It can be advantageous if the total amount of steam separated in process step f) is in a range of 0.001 to 0.2 times the mass, preferably 0.001 to 0.1 times the mass, particularly preferably 0.001 to 0.02 times the mass, based on the dry mass of the provided wood chips.

[0049] Even with such a correlation, the amount of steam to be separated is significantly lower than the amount separated in prior art solutions, for example, in US Pat. No. 4,925,527, but is also surprisingly sufficient to remove almost the entire amount of VOCs from the process and thus significantly reduce VOC emissions in the process for producing fiberboard described here. Thus, it has been shown that, for example, even if the process described here, or the amount of steam separated in the process, is based on the dry mass of the wood chips provided, a surprisingly small amount of steam can be separated that is sufficient to achieve the inventive object.

[0050] The dry mass of the wood or wood chips refers in particular to absolutely dry wood (dry dry), as is common in wood processing. The dry mass of the wood used can be determined analytically or estimated based on known data for the type of wood used. Furthermore, the mass can be easily determined in continuous or batch processes, either as a quantity per unit of time or as an absolute quantity, as described above.

[0051] It has also been shown that it is advantageous to position at least one steam emission point upstream, i.e., before the refiner in terms of process technology. It has been shown that a significant amount of VOCs are already released from the wood before the refiner, and it is therefore advantageous to remove the VOCs from the process before the refiner by steam separation. This allows for effective VOC removal. Furthermore, it can prevent the VOCs from being entrained in the process, which can potentially make removal more difficult.

[0052] With regard to positioning upstream of the refiner, it may be particularly advantageous if the steam emission point positioned upstream of the refiner comprises a thermal treatment device or is positioned between the refiner and a thermal treatment device, such as a digester. It has been shown that, particularly at these positions, VOCs, such as terpenes, can be effectively removed from the process by steam separation, so that the method can be implemented particularly effectively in this embodiment.

[0053] Accordingly, it may also be advantageous for the steam emission point to be a steam treatment device upstream of a wood chip digester, the wood chip digester itself, or located between the steam treatment device and the digester. These emission points have also been shown to effectively remove VOCs, and especially terpenes, from the process stream.

[0054] A steam separation upstream of a position, such as the refiner, can be a position on the main material flow of the wood chips, or a steam return, which runs in the opposite direction to the main material flow but is nevertheless considered upstream due to its position adjacent to the corresponding position of the main material flow or due to the path of the steam return. Thus, for example, a steam return from the refiner to a thermal treatment device is considered upstream of the refiner.

[0055] As described above, it can be very effective to separate the steam at one or more steam emission points upstream of the refiner. However, since the process described here is characterized by the fact that only a very reduced amount of steam is separated, it can be advantageous to position at least one steam emission point downstream of the refiner to enable particularly effective steam separation and to remove the VOCs as completely as possible from the process. This configuration thus makes it possible to expel all terpenes from the process downstream of the refiner, even if not all terpenes are removed upstream of the refiner.

[0056] In this design in particular, VOC emissions can be reduced particularly effectively.

[0057] In terms of effective VOC emission reduction, it can be particularly advantageous for the steam emission point located downstream of the refiner to be a steam separator. A steam separator is a device designed to remove steam from the process. In addition to effective VOC reduction, this design can also be implemented without significant equipment complexity.

[0058] It may further be preferred that at least one vapor emission point is generated from a liquid stream. In this embodiment, vapor can thus escape from a correspondingly hot liquid stream, which is then separated, or a cooler liquid stream from which no vapor escapes can be heated until vapor escapes in order to separate the vapor streams thus generated.

[0059] This design takes into account the fact that VOCs or terpenes released from the wood do not only accumulate in the vapor, but are also present in liquid streams, at least in small amounts. Vapor emission from these liquid streams can then effectively remove such VOCs from the process, further enhancing the overall VOC reduction.

[0060] Examples of liquid streams in which VOCs have been found include a squeeze water stream directly from a plug screw or a liquid stream resulting from a squeeze water stream from a plug screw.

[0061] It may also be advantageous to collect the VOC-containing vapor removed in process step g) and, if necessary, further treat one or more components. In this embodiment, the process can thus not only serve to reduce VOC emissions, but the process can also be carried out significantly more economically due to the possibility of collecting separated vapor streams and, if necessary, further treating them. This is because the materials contained in the vapor stream or other properties of the vapor stream, such as its heat, can be used in the process or other processes, thus saving costs and resources.

[0062] For example, it may be advantageous to isolate a mixture of terpenes or turpentine oil as a further treatment step. While such substances should be reduced as emissions from the fiberboard manufacturing process to prevent their release into the environment, these substances can be valuable products for other processes or applications. This embodiment can therefore be particularly advantageous with regard to the economics of the process described here and with regard to the added value of the wood used. The same applies if, for example, a hydrolate is isolated as a further treatment step. For the purposes of the invention, a hydrolate is generally understood to be the aqueous phase obtained after condensation of the vapor, which may contain correspondingly water-soluble components, such as formaldehyde.

[0063] It may also be advantageous for the separated vapor or one or more components to be further treated by combustion or exposure to high temperatures, adsorption, absorption, membrane technology methods, condensation, crystallization or other suitable process engineering methods.

[0064] Combustion or exposure to high temperatures, for example, enables thermal afterburning and, in doing so, the energetic utilization of the VOCs contained in the separated vapor stream. The other methods mentioned can all relate to the isolation or separation of individual substances.

[0065] It may also be advantageous for the heat of a material stream occurring in the process, for example, a separated steam stream, to be reused as energy in the process. In this embodiment, the energy inherent in the material stream can thus be reused in the form of heat, in particular to heat other material streams. This step can also improve economic aspects of the process according to the invention and thus conserve costs and resources.

[0066] A simple method for further processing the VOC-containing vapor removed in process step g), or a portion of it, is thermal treatment, e.g., by introducing the vapor into an incinerator. In integrated systems, for example, the VOC-containing vapor can be introduced into the process heat recovery system. This allows the energy released during VOC combustion to make a positive contribution to process heat recovery, while simultaneously thermally converting the VOCs into CO2, which is less harmful from an ecological perspective.

[0067] The invention will now be explained by way of example with reference to the accompanying drawings using preferred embodiments, wherein the features presented below may represent an aspect of the invention both individually and in combination. It shows: Fig. 1 is a schematic representation of a method according to the present invention.

[0068] In Figure 1 A process according to the present invention is shown schematically. Solid arrows indicate the main material flow, and dotted arrows indicate vapor recirculation.

[0069] In step 10, wood chips are prepared. These are made from a generally selectable wood and are produced by coarse reduction of the wood and, in particular, by coarse washing.

[0070] The wood chips are then treated in a plurality of thermal treatment devices. This is achieved in a first thermal treatment step 20, a second thermal treatment step 40, and a third thermal treatment step 50.

[0071] In principle, the invention provides for VOC-containing exhaust gases to have a high temperature. This means, in particular, a temperature greater than the boiling point of water, i.e., 100 °C, so that this temperature can also be present, at least in part, during the thermal treatment.

[0072] The first thermal treatment step 20 takes place in a so-called pre-steam vessel at a preferred temperature of up to approximately 100°C, in particular under atmospheric pressure. This is a first thermal treatment of the wood chips and is carried out using steam, preferably water vapor. In the process, some of the VOCs can be transferred from the wood chips to the steam. This VOC-containing steam can be discharged from the first thermal treatment device at a steam emission point, preferably from its upper section and, for example, via a pipe arranged on the roof.

[0073] Subsequently, before the second thermal treatment step 40, the wood chips are washed or cleaned in a cleaning step 30. The cleaning of the wood chips in a washing device takes place in particular at a temperature above room temperature and less than or equal to the boiling point of water, in particular between 80°C and 100°C. An elevated temperature enables a better separation of wood chips and foreign matter. Thus, foreign matter that is not wood chips is filtered out of the processing system and removed. The wood chips are preferably cleaned with a water-containing, in particular water-based, medium.

[0074] It is an advantageous option for the washing device to collect the aforementioned VOC-containing condensate from the first thermal treatment device. This VOC-containing condensate can be discharged from the processing system together with the VOC released during washing.

[0075] The second thermal treatment step 40 of the wood chips takes place in the second thermal treatment device, also called a pre-steam vessel, which is designed, for example, to receive and separate VOC-containing exhaust gases, in particular back into the first thermal treatment device. The second thermal treatment takes place, for example, without pressure at a temperature above room temperature, in particular at a temperature less than or equal to the boiling point of water, i.e., less than or equal to 100°C. An elevated temperature enables a better release of VOCs from the wood chips. The VOC-containing exhaust gases are, in particular, discharged from the second thermal treatment device and / or forwarded to the first thermal treatment device.Furthermore, VOC-containing exhaust gases from a device subsequently used to carry out the process can be fed into the second thermal treatment device in order to further heat the wood chips or to release VOCs.

[0076] By way of example and independently of other features, it is possible for the second thermal treatment of the wood chips in the second thermal treatment device to be carried out using steam, preferably water vapor. In this case, a portion of the VOCs can be transferred from the wood chips to the steam. This VOC-containing steam can be discharged from the second thermal treatment device, preferably from its upper section and, for example, via a pipe arranged on the roof. Alternatively or in addition to the separation of the VOC-containing steam, some or all of the steam can condense and release VOCs from the wood chips as condensate. This VOC-containing condensate can, for example, be passed on to a stuffing screw and / or a digester and / or a water treatment plant.

[0077] The third thermal treatment step 50 can in particular take place in a so-called digester. The cooking of the wood chips in the digester, which can be designed to absorb and separate VOC-containing exhaust gases, takes place, for example, at a temperature above room temperature, in particular between 3 bar and 15 bar, preferably between 5 bar and 13 bar, preferably 9 bar, at a temperature greater than the boiling point of water, i.e., 100°C, approximately 90-175°C. An elevated temperature enables a better release of VOCs from the wood chips. The wood chips are preferably cleaned with a water-containing, in particular water-based, medium. The first and second thermal treatments have heated and softened the wood chips in such a way that VOCs contained in the wood chips are efficiently released from the digester.Preferably, a droplet separator is installed downstream of the cooker.

[0078] It is an advantageous option for a screw conveyor upstream of the digester and / or the digester to collect the aforementioned VOC-containing condensate from the second thermal treatment device. This VOC-containing condensate can be discharged from the processing system via the screw conveyor and / or the digester, along with any VOCs released during cooking.

[0079] The wood chips are then shredded in a shredding step or fiberization step in the refiner. The refiner's design and operation can be adapted to the desired application of the board. In principle, the grinding tools, which are part of the refiner and fiberize the wood chips, can generate a grinding energy of 50–200 kWh / t of wood chips. Lower grinding energy, for example, in the range of 50 kWh / t of wood chips, is suitable for flooring, while 150 kWh / t of wood chips is suitable for high-quality furniture.

[0080] From the refiner, the shredded wood chips or the resulting wood fibers are passed through a so-called blowline, and a drying step 70 is carried out in a dryer to dry the wood fibers coming from the refiner. This can again be carried out at elevated temperatures, whereby the resulting humid atmosphere can be removed from the wood fibers in a separation step 80. The exhaust air can, for example, be scrubbed so that the remaining components, especially VOC-containing components, can be washed out and, if necessary, reused or collected.

[0081] The dried, shredded wood chips or the resulting wood fibers are processed into fiberboard in a processing step 90. For this purpose, the shredded wood chips can be glued, and the glued, shredded wood chips can be pressed into a board. The board can then be finished for the specific application.

[0082] In the process described here, steam is generated or additional steam is added. In order to advantageously remove the VOCs escaping from the raw wood during the process, it is provided that steam used or generated in the process is continuously separated from the process at at least one steam emission point. The steam is separated in a predetermined quantity range such that a lower limit and an upper limit of the quantity range of the total separated steam are determined depending on at least one specification of the wood chips used in process step a).

[0083] The separation of steam to remove VOCs thus depends on the VOCs introduced into the process by the wood chips or their wood. This can be done, for example, by taking into account the amount and / or type of wood introduced or specifically via the amount of VOC introduced. In particular, the total amount of steam separated can be in a range of 0.5 to 100 times the mass, preferably 0.5 to 50 times the mass, particularly preferably 0.5 to 10 times the mass, based on the amount of terpenes in the wood chips provided. Alternatively or additionally, the total amount of steam separated can be in a range of 0.001 to 0.2 times the mass, preferably 0.001 to 0.1 times the mass, particularly preferably 0.001 to 0.02 times the mass, based on the dry mass of the wood chips provided.

[0084] Various vapor emission points can be used to separate the vapor and thus remove the VOCs. A vapor emission point is defined as a point where vapor can be separated from the process.

[0085] For example, the following steam emission points are suitable for separating steam: the pre-steam vessel or a first thermal treatment device, the pre-steam vessel or a second thermal treatment device, the digester or the third thermal treatment device, or the refiner. Also suitable are transport units such as a screw or conveyor units, such as downstream of the first thermal treatment device, between the second and third thermal treatment devices, or a transport unit such as a screw between the third thermal treatment device and the refiner. Also suitable are a dewatering unit, such as a dewatering screw, or steam recirculation between individual processing units.

[0086] However, it has been found that the following gas emission points are particularly suitable.

[0087] For example, at least one gas emission point can be positioned upstream of the refiner. Such positions include, for example, a thermal treatment device or a position in a steam return line between the refiner and a thermal treatment device, a steam treatment device upstream of a wood chip digester or the wood chip digester itself, or a steam return line between the steam treatment device and the digester.

[0088] Alternatively or additionally, it may be advantageous for at least one steam emission point to be positioned downstream of the refiner. In this regard, it is advantageous for the steam emission point positioned downstream of the refiner to be a steam separator positioned downstream of the refiner.

[0089] Furthermore, it may be particularly preferred that at least one vapor emission point is generated from a liquid stream. Examples include a squeeze water stream directly from a plug screw or a liquid stream resulting from a squeeze water stream from a plug screw.

[0090] The process described here provides a cost- and resource-saving way to reduce VOC emissions during the production of fiberboard, especially HDF or MDF boards. List of reference symbols

[0091] 10 Step of preparing wood chips 20 First thermal treatment step 30 Cleaning step 40 Second thermal treatment step 50 Third thermal treatment step 60 Crushing step 70 Drying step 80 Separation step 90 Processing step

Claims

1. A process for manufacturing fiberboards with reduced VOC emissions, wherein the process includes at least the following process steps: a) providing wood-containing wood chips; b) thermally treating the wood chips in a thermal treatment device or in a plurality of thermal treatment devices; c) shredding the wood chips in a refiner; d) gluing the wood chips shredded in step c); and e) pressing the glued wood chips to form the fiberboard, wherein f) vapor used or arising in the process is separated continuously from the process at at least one vapor emission location in a controlled manner, wherein the vapor is separated in a predetermined quantity range such that a lower limit and an upper limit of the quantity range of the total separated vapor depend on at least one specification of the wood chips used in process step a), wherein a lower limit and an upper limit of the quantity range of the separated vapor depend on the quantity of wood chips provided in process step a), or that a lower limit and an upper limit of the quantity range of the separated vapor depend on the quantity of VOCs, especially terpenes, contained in the wood chips provided in process step a).

2. The process according to claim 1, characterized in that the quantity of VOCs, especially terpenes, contained in the wood chips used in process step a) is determined by examining the wood chips used or is estimated based on the type of wood chips used.

3. The process according to any of claims 1 or 2, characterized in that the total separated quantity of vapor in process step f) is within a quantity range from 0.5 to 100 times the mass, preferably from 0.5 to 50 times the mass, more preferably from 0.5 to 10 times the mass, based on the VOC quantity of the wood chips provided.

4. The process according to any of claims 1 to 3, characterized in that the total separated quantity of vapor in process step f) is within a quantity range from 0.001 to 0.2 times the mass, preferably from 0.001 to 0.1 times the mass, more preferably from 0.001 to 0.02 times the mass, based on the dry mass of the wood chips provided.

5. The process according to any of claims 1 to 4, characterized in that at least one vapor emission location is positioned upstream of the refiner.

6. The process according to claim 5, characterized in that the vapor emission location positioned upstream of the refiner includes a thermal treatment device or is located between the refiner and a thermal treatment device.

7. The process according to claim 5 or 6, characterized in that the vapor emission location positioned upstream of the refiner is a vapor treatment device before a wood chip cooker or the wood chip cooker itself, or is located between the vapor treatment device and the cooker.

8. The process according to any of claims 1 to 7, characterized in that at least one vapor emission location is positioned downstream of the refiner.

9. The process according to claim 8, characterized in that the vapor emission location positioned downstream of the refiner is a vapor separator positioned downstream of the refiner.

10. The process according to any of claims 1 to 9, characterized in that at least one vapor emission location is generated from a liquid stream.

11. The process according to claim 10, characterized in that the liquid stream is a squeeze water stream directly from a compaction screw, or a liquid stream emerging from a squeeze water stream from a compaction screw.

12. The process according to any of claims 1 to 11, characterized in that the VOC-containing vapor removed according to process step f) is collected and, if applicable, one or more components are further treated.

13. The process according to claim 12, characterized in that, as further treatment, a mixture of terpenes or turpentine oil is isolated.

14. The process according to claim 12 or 13, characterized in that, as further treatment, a hydrolate is isolated.

15. The process according to any of claims 12 to 14, characterized in that the separated vapor or one or more components are further treated by combustion or exposure to high temperatures, adsorption, absorption, membrane technology techniques, condensation, crystallization, or other suitable process engineering techniques.

16. The process according to any of claims 1 to 15, characterized in that the heat of a material stream occurring in the process is energetically reused in the process.