WO2025078165A1 - Method for producing a wood-fiber material, and wood-fiber material production device - Google Patents

Abstract

The invention relates to a method for producing a wood-fiber material (32), comprising the steps: providing fibrous material (30); combusting a fuel (24) in a firing system (16) so that waste gas (27) is produced; drying fibrous material (30) in a drier (28) by means of the waste gas (27) so that dried fibrous material (32) and vapors (34) are produced; and discharging vapors (34) into the surroundings (44), recirculating a portion of the vapors (34) into the firing system (16) so that volatile organic substances contained in the recirculated vapors (34) are combusted by the steps: measuring a total concentration (cvoc) of volatile organic substances in the vapors (34) and changing the recirculated portion (R) of recirculated vapors (34) according to the total concentration (cvoc).

Description

Method for producing a wood pulp and wood pulp producing device

The invention relates to a method for producing a wood fiber material, comprising the steps of (a) preferably providing fiber material and/or burning a fuel in a furnace to produce exhaust gas, (b) drying fiber material in a dryer using the exhaust gas to produce dried fiber material and vapors, and (c) discharging vapors into the environment and preferably (e) returning a portion of the vapors to the furnace to burn volatile organic substances contained in the returned vapors. The invention also relates to a method for producing a wood-based panel, which comprises the method according to the invention for producing the wood fiber material.

According to a second aspect, the invention relates to a wood fiber production device for producing a wood fiber material, comprising (a) a furnace for burning a fuel to produce exhaust gas, (b) a refiner designed to defiberize wood chips to produce fiber material, (c) a dryer arranged downstream of the refiner in the direction of wood material flow for drying the fiber material using the exhaust gas to produce vapors, and (d) an exhaust air system for discharging vapors generated in the dryer during drying of the fiber material into the environment, and preferably (e) a branch connected to the dryer for returning a portion of the vapors to the furnace. The invention also relates to a wood-based panel production device, which is designed in particular for producing a medium-density or high-density fiberboard, which comprises a wood fiber production device according to the invention.

When producing wood pulp, especially as in a preferred embodiment from a softwood, volatile organic substances, especially terpenes and aldehydes, are released during the drying of the fiber material. Emissions of volatile organic substances into the environment must be limited to comply with legal requirements.

Limiting the emission of volatile organic compounds (VOCs) typically requires the use of energy and/or chemicals, which is undesirable. For example, attempts can be made to wash out volatile organic compounds with water. However, this is ineffective because terpenes are almost insoluble in water, so additional measures must be taken.

WO 2018/157945 A1 describes a generic method and a generic wood pulp production device in which drying vapors from the dryer are fed back into the furnace via a cyclone separator and a heat exchanger so that volatile organic components contained in the drying vapors are burned.

US 2011/0056090 A1 describes a process in which the drying vapors are also partially recirculated to the furnace. The volatile organic components present in the drying vapors are additionally removed by thermal oxidation before entering an exhaust stack.

The invention is based on the object of reducing the emission of volatile organic substances.

The invention solves the problem by a method having the features of claim 1. According to a second aspect, the invention solves the problem by a generic wood fiber production device having the features of claim 10.

The invention further solves the problem by a generic method comprising the steps of (a) measuring a nitrogen oxide concentration CNOX of nitrogen oxides (i) in the vapors, in particular in the material flow direction downstream of the dryer and/or before the vapors are discharged into the environment and/or (ii) the exhaust gases, in particular in the material flow direction downstream of the furnace and upstream of the dryer and (b) changing the recirculation proportion R of recirculated vapors as a function of the nitrogen oxide concentration CNOX. This method preferably comprises the steps of Characteristic of claim 1 . The preferred embodiments described below also relate to this invention.

The invention further solves the problem by a generic wood fiber production device which comprises (a) a nitrogen sensor for measuring a nitrogen oxide concentration of nitrogen oxides (i) in the vapors (34) in the material flow direction downstream of the dryer and/or before the vapors are released into the environment, or (ii) the exhaust gases in the material flow direction downstream of the furnace and upstream of the dryer, wherein the branch is designed to automatically change the recirculation proportion R of recirculated vapors as a function of the nitrogen oxide concentration CNOX. Preferably, the wood fiber production device comprises a concentration meter for measuring a total concentration cvoc of volatile organic substances, wherein the branch is designed to automatically change the recirculation proportion of recirculated vapors as a function of the total concentration cvoc. The preferred embodiments described below also relate to this invention.

The advantage of the invention is that the amount of volatile organic substances released into the environment can be controlled using comparatively simple technical means.

Another advantage is that the reduction of emitted volatile organic substances can be influenced by the recirculation fraction of recirculated vapors. The recirculation fraction is the percentage by weight that the vapor stream recirculated to the furnace represents of the total vapor stream leaving the dryer. Any disadvantages resulting from recirculating vapors can generally be limited to the necessary minimum.

Another advantage may be that the reduction in volatile organic substances by recirculating the vapors to the furnace reduces the complexity of any subsequent process steps for cleaning the vapors before discharging them into the environment. For example, if volatile organic substances are scrubbed from the vapors in a gas scrubber, as provided in a preferred embodiment, this creates wastewater that must be treated. The lower the content of volatile organic substances in the vapors, the less wastewater is produced and the less effort is required to clean it.

For the purposes of this description, a wood-based panel is understood to mean, in particular, an LDF (low-density fiberboard, 170-250 kg/ ^m³ ), MDF (medium-density fiberboard), HDF (high-density fiberboard), or a wood-fiber insulation board. In particular, the wood-based panel is a wood-fiber board.

Combustion is understood as oxidation with the development of flames.

The total concentration of volatile organic substances in the vapors can, in principle, be measured anywhere. However, it is best to measure immediately before the vapors are discharged into the environment, especially before they enter the exhaust air system. For example, the measurement could be taken before the branch or between the branch and the exhaust air system.

According to a preferred embodiment, the process comprises the step of removing volatile organic substances by spray washing and/or electrofiltration before discharging the vapors into the environment. Spray washing can also be referred to as quenching. Electrofiltration, which can also be called electrostatic filtration, is in particular a wet electrofiltration process. For this purpose, a wet electrostatic precipitator, for example, is used, which removes particles through electrostatic charging. These particles are washed out continuously or discontinuously. The advantage of this is the reduction of particles released into the environment.

It is advantageous if the recirculated vapors are fed to a burner, a mixing chamber and/or a combustion chamber of the furnace. In the combustion chamber, the fuel to be burned is mixed with the air and/or vapors. By feeding the vapors into the combustion chamber, the vapors are heated to particularly high temperatures, so that the volatile organic substances are burned effectively. Alternatively or additionally, the recirculated vapors can be returned to the furnace as by-pass air, which can also be referred to as supply air. By-pass air refers to air or vapors that are not the fuel is introduced into the combustion chamber. In particular, the additional air is introduced into the combustion chamber at a location separate from the air that was mixed with the fuel before being introduced into the combustion chamber.

Preferably, the recirculation proportion of the vapors returned to the furnace is at most 35 percent by weight, in particular at most 30 percent by weight. A higher recirculation proportion may lead to an accumulation of water in the exhaust gases used to dry the fiber material, which is undesirable. Preferably, the recirculation proportion is at least 10 percent by weight. However, it is possible for the recirculation proportion to be temporarily below 10 percent by weight, in particular zero, at most half the time. In other words, the recirculation proportion is preferably at least 10 percent by weight if vapors are recirculated at all.

According to one embodiment, the method comprises the step of measuring a total concentration of volatile organic substances in the vapors. Preferably, the total concentration is measured downstream of the dryer, particularly upstream of the branch.

Preferably, the method includes the step of increasing the recirculation fraction if the total concentration exceeds a predetermined maximum concentration. Increasing the recirculation fraction reduces the concentration of volatile organic substances released into the environment. This ensures that a predetermined limit value above the maximum concentration is not exceeded. The limit value is, for example, a legal limit.

Preferably, the method comprises the step of reducing the recirculation portion when the total concentration falls below a predetermined minimum concentration. This is advantageous when recirculation results in disadvantages, for example, in the form of reduced efficiency of the furnace and/or dryer. In particular, the method preferably comprises the step of controlling the recirculation portion so that the total concentration approaches a target concentration.

According to one embodiment, the method comprises the step of measuring a nitrogen oxide concentration of nitrogen oxides (i) in the vapors, in particular downstream of the dryer and/or upstream of a branch in which a recirculation stream of vapors that are returned to the furnace is separated from a discharge stream of vapors that are released into the environment, and/or (ii) the exhaust gases, in particular downstream of the furnace and upstream of the dryer in the material flow direction. The recirculation of the vapors into the furnace generally leads to a drop in the temperature of the exhaust gas, thereby resulting in lower nitrogen oxide concentrations. If the nitrogen oxide concentration exceeds a predetermined maximum nitrogen oxide concentration, the recirculation proportion is preferably increased. Nitrogen oxides in the exhaust gas can improve the oxidation of volatile organic substances. The method therefore preferably comprises the step of reducing the recirculation proportion if the nitrogen oxide concentration falls below a minimum nitrogen oxide concentration.

The method preferably comprises the steps of (a) heating wood chips in a pre-cooker using water or steam, wherein the thermal and/or mechanical energy required for this is preferably provided by the combustion of the fuel. The method preferably comprises the step (b), in particular after heating the wood chips, cooking the wood chips using steam in a cooker, wherein the steam is preferably generated by the combustion of the fuel. The method preferably comprises defibrating the cooked wood chips in a refiner to produce fiber material. Preferably, the fiber material is glued to produce glued fiber material, wherein the glued fiber material is dried. The pre-cooker can also be designed as a washer, be part of a washer, or be a unit independent of the washer.

Preferably, the method comprises the step of scattering the dried fiber material to form a fiber cake and preferably the step of pressing the fiber cake to form a wood-based panel. Preferably, the inlet temperature at which the exhaust gas enters the dryer is at least 300°C, in particular at least 350°C. This achieves high drying performance and partially oxidizes volatile organic substances. Alternatively or additionally, the inlet temperature is at most 450°C, in particular at most 450°C. This prevents thermal damage to the fiber material.

According to one embodiment, the exit temperature at which the vapors leave the dryer is at least 50°C, in particular at least 50°C. This prevents water condensation. Preferably, the exit temperature is at most 90°C, in particular at most 80°C. This achieves the highest possible drying efficiency.

A wood pulp production device according to the invention preferably has a concentration meter for measuring a total concentration of volatile organic substances in the vapors. This is preferably arranged to measure the total concentration downstream of the dryer in the material flow direction. Preferably, the branch is designed to automatically change the recirculation proportion of recirculated vapors as a function of the total concentration cvoc. Preferably, the branch is designed to automatically (i) increase a recirculation proportion of recirculated vapors if the total concentration cvoc.st exceeds a predetermined maximum concentration cvoc.max and/or (ii) decrease the recirculation proportion if the total concentration cvoc.st falls below a predetermined minimum concentration cvoc.soii. Alternatively or additionally, the branch is designed to automatically regulate the recirculation proportion so that the total concentration cvoc.st approaches a target concentration cvoc.son. Preferably, the branch has a control, for example, an electronic, electrical, or analogue control.

According to a preferred embodiment, the method comprises the step of introducing an oxidizing agent into the vapors so that volatile organic substances contained in the vapors are oxidized and purified vapors are produced. Instead of the term "vapors", the term "exhaust air" can also be used. It is advantageous if the oxidizing agent is introduced into those vapors that are not returned to the furnace, but their material flow does not pass through the furnace into the environment.

According to a preferred embodiment, a portion of the steam is diverted after defibration in the refiner and used to heat the wood chips in the precooker. This reduces energy consumption during production.

Preferably, the method comprises the step of introducing an oxidizing agent into the diverted steam so that volatile organic substances contained in the steam are oxidized and purified steam is produced, wherein the purified steam is used at least partially to heat the wood chips. Preferably, the oxidizing agent is a flameless oxidizing agent. According to one embodiment, the oxidizing agent contains oxygen.

The wood pulp production device preferably has (a) a pre-cooker for heating wood chips using water or steam, (b) a cooker, which is arranged downstream of the pre-cooker in the direction of wood material flow, for cooking the wood chips using steam to produce cooked wood chips, and (c) a refiner, which is arranged downstream of the cooker in the direction of wood material flow, for defibrating the cooked wood chips to produce fiber material. The wood pulp production device preferably has (d) a steam branch, arranged downstream of the refiner in the direction of wood material flow, for branching off a portion of the steam after defibringing, and (e) an exhaust steam line that connects the steam branch to the pre-cooker or cooker for supplying steam. The wood pulp production device preferably comprises a steam cleaner designed to introduce an oxidizing agent into the steam so that volatile organic substances contained in the steam are oxidized and purified steam is produced.

A steam cleaner is understood to be a device by means of which a VOC concentration (volatile organic compounds, volatile organic substances), in particular terpenes and/or aldehydes, can be reduced by at least 70% by chemical reaction of the volatile organic compounds with an oxidizing agent. It is advantageous if the oxidizing agent releases elemental oxygen upon reaction with terpenes and/or aldehydes. For example, the oxidizing agent is hydrogen peroxide or ozone. Hydrogen peroxide also includes an aqueous solution of hydrogen peroxide. The hydrogen peroxide may contain an Fe(II) salt, ammonium persulfate, cytochrome P450, monooxygenases, or ammonium peroxide. In particular, the oxidizing agent is not molecular oxygen or air.

It is advantageous that the diversion of the steam, in particular the diverted steam mass flow, can be independent of the type of wood chips used. In particular, it is advantageous that a material flow control of the diverted steam, provided according to a preferred embodiment, can be independent of the type and/or properties of the wood chips used. This facilitates the control of the wood fiberboard production device. Instead, the oxidizing agent volume flow can be controlled to always keep the concentration of volatile organic substances below a predetermined maximum.

According to one embodiment, the method comprises the step of irradiating the oxidizing agent with UV light. This results, for example, in the oxidizing agent forming radicals, in particular hydroxyl radicals. Preferably, the oxidizing agent is irradiated with UV light immediately before being introduced into the diverted vapor. In particular, the distance between the point at which the oxidizing agent is irradiated with UV light and the point at which the oxidizing agent first comes into contact with the diverted vapor is at most 10 m, in particular at most 5 m.

Preferably, the branched steam has a steam temperature of at least 110°C when the oxidant is introduced. At higher temperatures, the oxidant reacts more quickly with the volatile organic substances, so that lower concentrations of volatile organic substances can be achieved in the purified steam. It is advantageous if the steam temperature is at most 160°C. At even higher temperatures, the oxidant generally decomposes too quickly. Preferably, the branched steam has a pressure of at least 2 bar and/or at most 5 bar. The more volatile organic substances are broken down by introducing the oxidizing agent into the diverted steam, the lower the VOC concentration in the vapors discharged into the environment. According to a preferred embodiment, the recirculation fraction is selected to be close to the maximum possible recirculation fraction, in particular at least 0.8 times, particularly preferably at least 0.85 times, in particular at least 0.9 times, the maximum possible recirculation fraction. The maximum possible recirculation fraction is the recirculation fraction for which a higher recirculation fraction impairs the functionality of the dryer to such an extent that a predetermined degree of drying can no longer be achieved. In this way, the consumption of oxidizing agent is kept low.

According to a preferred embodiment, the process comprises the step of introducing an oxidizing agent into the vapors before they are released into the environment. Preferably, the oxidizing agent is released into the portion of the vapors that are not fed into the furnace. In this way, predetermined limits for the content of volatile organic substances in the vapors that are released into the environment are reliably maintained.

Preferably, the oxidizing agent is introduced into the vapors or the branched-off steam in such a way that the oxidizing agent oxidizes the volatile organic substances without catalysis. While it is possible and encompassed by the invention for the wood pulp production device to have a catalyst arranged to catalyze the reaction of the oxidizing agent with volatile organic substances, its effects are reduced due to the solids content.

The wood-based panel manufacturing device is preferably designed to produce an MDF or an HDF. The invention is explained in more detail below with reference to the accompanying drawings.

Figure 1 is a flow diagram of a wood fiber production device according to the invention, which is part of a wood-based panel production device according to the invention and is designed to carry out a method according to the invention and

Figure 2 shows a flow diagram of a wood pulp manufacturing apparatus according to a second embodiment of the invention for carrying out a method according to the invention.

Figure 1 shows a wood fiber production device 10 according to the invention, which is part of a wood-based panel production device 12 according to the invention for producing a wood-based panel in the form of a wood fiber board 14. The wood fiber production device 10 has a furnace 16, which has a mixing chamber 18, a burner 20, and a combustion chamber 22. Fuel 24, for example in the form of wood, wood residues, gas and/or oil, and air 26 are supplied to the burner 20. The combustion of the fuel 24 produces exhaust gas 27.

The exhaust gas 27 is fed to a dryer 28, to which fiber material, in particular glued fiber material 30', is also fed. The dryer 28 dries the fiber material 30, producing a wood fiber material 32 in the form of dried fiber material 32 and vapors 34. The vapors 34 are fed to an exhaust air system 36.

The exhaust air system 36 may include a gas scrubber 38 and/or an electrostatic precipitator 40, particularly in the form of a wet electrostatic precipitator. The exhaust air system 36 discharges vapors 34 into the environment 44, particularly via a chimney 42.

By means of a branch 46, a return stream QR of the vapors 34 can be fed into the furnace 16, whereas a QA is fed to the exhaust air system 36. The branch 46 is arranged at a point in the material flow direction M behind a separator 47 by means of which the dried fiber material 32 is separated from the vapors 34 is separated. Preferably, the branch 46 is arranged less than 50 m, in particular less than 30 m, behind the separator.

The currents QR, QA are measured in weight or mass per unit of time, for example, newtons or kilograms per minute. A feedback component

R = QR / (QR + QA) is set by a control unit 48, which may be a control unit.

By means of a VOC concentration meter 50, which is arranged downstream of the dryer 28 in the material flow direction M and is connected to the control unit 48, a total concentration cvoc st of volatile organic substances is measured as a function of time t, so that the respective time-dependent concentration cvoc st is obtained.

If the total concentration cvoc st exceeds a predetermined maximum concentration Cv OC , max , the recirculation portion R is automatically increased by the control unit 48. The maximum concentration is selected such that a predetermined limit value CGrenz of the total concentration of volatile organic substances at an exit point 51 at which they are released into the environment 44 is not exceeded. For example, the limit value CGrenz = 200 mg/m ³ .

If the total concentration cvocjst falls below a predetermined minimum concentration CV0C,min, the feedback component R is automatically reduced. In particular, the control unit 48 can be configured to regulate the feedback component R so that the total concentration approaches a target concentration. For example, the control unit 48 is configured as a PI (proportional integral; amplifying integrating) or PID (proportional integral derivative; amplifying integrating differentiating) controller.

The wood pulp production device can have a washer 52 for washing wood chips 54, which can be made, for example, from fresh wood 56 and/or recycled wood 58. The washer can also be designed as a pre-cooker or have a pre-cooker. A cooker 60 is preferably arranged behind the washer 52 in the material flow direction M, by means of which the wood chips 54 are cooked using steam. In the material flow direction M Behind the digester 60 is a refiner 62 for defibrating the wood chips 54, creating the fiber material 30. The fiber material 30 is glued in a blow line 64.

The dried fiber material 32 is fed to a sifter 66, which removes a fine fraction. The remainder is spread in a spreader 68 to form a fiber cake 70, which is pressed into the wood fiber board 14 by a press, in particular a hot press 72.

The exhaust gas 27 entering the dryer 28 has an inlet temperature TE which can be, for example, between TE = 120°C and TE = 214°C. The vapors 34 have an outlet temperature TA which can be, for example, between TA = 60°C and TA = 85°C. The vapors 34 leave the dryer 28 with a volume flow of Q = 400,000 to 800,000 m ³ /h, for example 600,000 m ³ /h.

Figure 2 shows the flow diagram of a wood fiber board manufacturing device 10 according to the invention for producing a wood fiber board 12 in the form of an MDF board.

Optional components are shown in dashed lines or outlined. The wood-based panel manufacturing device can have a pre-cooker 53, which is arranged upstream of the cooker 60 and downstream of the washer 52 in the material flow direction H. Downstream of the cooker 60 in the wood material flow direction H, the steam-wood chip mixture produced in the cooker 60 enters the refiner 62, where fiber material is produced. An optional steam branch 136 is arranged downstream of the refiner 32 in the wood material flow direction H, by means of which the steam 126 can be branched off. For example, 20 to 30 percent by mass of the steam contained in the steam-fiber material mixture is branched off.

The remaining steam-fiber material mixture is fed to the blow line 64. A steam cleaner 152, arranged in the steam flow direction D behind the steam branch 136, introduces an oxidizing agent 156 into the branched steam 126 at an introduction point 154. In the present case, the oxidizing agent is hydrogen peroxide. The steam cleaner 152 comprises an oxidizing agent source 158, which in the present case has an oxidizing agent container and a metering device 160 in the form of an oxidizing agent pump. Using a VOC concentration meter 162, the steam cleaner 152 measures a first VOC concentration cvoc.i of volatile organic components in the branched steam 126. Depending on the VOC concentration, an oxidant volume flow Qi56 of oxidant 156 is introduced, for example, by injection, into the branched steam 126 by means of an introduction device 157. The oxidant 156 reacts with volatile organic components in the branched steam.

The steam cleaner 152 can have a second VOC concentration meter 164, which is located downstream of the introduction point 154 in the steam flow direction D. The second VOC concentration meter measures a second VOC concentration cvoc,2. In the present case, this is the TOC concentration of the total concentration of organic compounds. If the second VOC concentration cvoc,2 is above a predetermined maximum concentration Cv O C, max, the oxidant volume flow Q156 is increased. The steam cleaner 152 regulates the second VOC concentration cvoc,2 to a VOC target concentration Cvoc, target by increasing or decreasing the oxidant volume flow Q156.

Alternatively, it is possible for the steam cleaner 152 to have an introduction device, for example a nozzle 166, for introducing oxidizing agent at a second introduction point 154' downstream of the second VOC concentration meter 164 in the steam flow direction D. Alternatively, it is possible for the oxidizing agent volume flow Qi56 to be increased if the second VOC concentration cvoc, 2. is above the maximum concentration C\/OC, max. It is possible, but not necessary, for the same oxidizing agent to be introduced at both introduction points 154, 154'. In particular, it is possible for two different oxidizing agents to be used.

The introduction of the oxidizing agent produces purified steam, which is fed to the pre-cooker 53 via an exhaust steam line 169. The steam cleaner 152 can have a light source 174 by means of which the oxidizing agent 156 can be irradiated with UV light. In this way, hydroxyl radicals are formed, which particularly effectively destroy the volatile organic substances in the diverted steam 126.

Optionally, the wood-based material manufacturing device 10 can have an exhaust air cleaner 252, which is preferably arranged behind the branch 46 in the material flow direction M. The exhaust air cleaner 252 could also be called a vapor cleaner. and introduces an oxidizing agent 256 into the vapors 34, which can also be referred to as exhaust air 245, at an introduction point 254. In this case, the oxidizing agent is hydrogen peroxide H2O2.

The exhaust air purifier 252 comprises an oxidant source 258, which in this case has an oxidant container and a dosing device 260 in the form of an oxidant pump. Using a VOC concentration meter 262, the exhaust air purifier 252 measures a first VOC concentration cvoc.i of volatile organic components in the exhaust air 245. Depending on the VOC concentration, an oxidant volume flow Q256 of oxidant 256 is introduced, for example, by injection, into the exhaust air 245 by means of an introduction device 257 at the introduction point 254. The oxidant 256 reacts with volatile organic components in the exhaust air 245.

The exhaust air purifier 252 can have a second VOC concentration meter 264, which is located downstream of the introduction point 254 in the exhaust gas flow direction D. The second VOC concentration meter measures a second VOC concentration cvoc,2. In this case, this is the TOC concentration of the total concentration of organic compounds. If the second VOC concentration cvoc,2 is above a predetermined maximum concentration Cv O C, max , the oxidant volume flow Q256 is increased.

For example, the maximum concentration corresponds to a predetermined exhaust air limit concentration CVOC.BREV, which is, for example, a legal requirement. However, it is also possible that the maximum concentration is lower than the exhaust air limit concentration CVOC.BREV. This ensures that the exhaust air limit concentration CVOC.BREV is definitely not exceeded. For example, cvoc.max = f* CVOC.BREV with a safety factor f e [0.75, ... ,1 ]. The smaller the safety factor, the lower the probability that the exhaust air limit concentration CVOC.BREV will be exceeded at any time, but the higher the consumption of oxidant. The exhaust air cleaner 252 regulates the second VOC concentration cvoc,2 to a VOC target concentration cvoc.soii by increasing or decreasing the oxidant volume flow Q256.

Alternatively, it is possible that the exhaust air cleaner 252 is arranged in the exhaust gas flow direction D behind the second VOC concentration meter 264 has an introduction device, for example a nozzle 266, for introducing oxidizing agent at a second introduction point 254.2. Alternatively, it is possible for the oxidizing agent volume flow 0256 to be increased if the second VOC concentration cvoc,2 is above the maximum concentration CV0C,max. It is possible, but not necessary, for the same oxidizing agent to be introduced at both introduction points 254, 254.2. In particular, it is possible for two different oxidizing agents to be used.

It is also possible for the exhaust air purifier 252 to have only one VOC concentration meter 264, which is arranged downstream of the introduction point 254 in the steam flow direction D, with oxidant being introduced into the exhaust air 245 only at this one introduction point 254. The introduction of the oxidant produces purified exhaust air, which is discharged into the environment via the chimney 42.

The exhaust air purifier 252 can have a light source 274, by means of which the oxidizing agent 256 can be irradiated with UV light. In this way, hydroxyl radicals are formed, which destroy the volatile organic substances particularly effectively. An exhaust air temperature T45 is, for example, 45°C ± 5°C. The first VOC concentration meter 262 measures a first VOC concentration of, for example, cvoc,i = 200 mg/standard cubic meter. The oxidizing agent volume flow is then set to, for example, Q256 = 50 liters/hour. In this case, the oxidizing agent 256 is a 5 percent (by weight) hydrogen peroxide solution. The second VOC concentration meter 264 then measures a second VOC concentration of cvoc,2 = CTOC,2 = 90 mg/standard cubic meter. One standard cubic meter is the amount of gas that occupies 1 cubic meter at standard conditions of 1013 hPa and 23°C.

Regardless of other features of the aforementioned embodiment, the wood fiber production device 10 and/or the wood-based panel production device 12 can have either the branch 46 and the steam cleaner 152 or the branch 46 and the exhaust air cleaner 252 or only the branch 46 or the branch 46, the steam cleaner 152, and the exhaust air cleaner 252. Preferably, the branch 46 is controlled such that the limit value Climit of VOC in the exhaust air 245 that enters the environment is maintained and, in addition, the consumption of oxidizing agent 158, 258 is minimized. By means of a NOx concentration meter 74, which is arranged downstream of the dryer 28 in the material flow direction M and is connected to the control unit 48, a total concentration CNOX of nitrogen oxides is measured, in particular as a function of time t, so that the respective time-dependent nitrogen oxide concentration CNOx st is obtained.

If the nitrogen oxide concentration cvoc st exceeds a predetermined maximum nitrogen concentration CNOx,max, the recirculation portion R is automatically increased by the control unit 48. The maximum nitrogen concentration CNOx,max is selected such that a predetermined limit value CNOx,limit of the nitrogen oxide concentration of nitrogen oxides at the outlet point 51 is not exceeded. For example, the limit value CNOx,limit = 40 pg/m ³ .

If the total concentration CNOx st falls below a predetermined minimum concentration CNOx,min, the recirculation component R is automatically reduced. In particular, the control unit 48 can be configured to regulate the recirculation component R so that the total concentration approaches a target concentration. For example, the control unit 48 is configured as a PI or PID controller.

List of reference symbols

10 wood pulp production device 28 dryers

12 Wood-based panel manufacturing device 30 fiber material

14 Wood-based panel, wood fiber30' glued fiber material panel 32 Wood fiber, dried

16 Firing fiber material

18 Mixing chamber 34 vapors

36 exhaust air system

20 burners 38 gas scrubbers

22 Combustion chamber

24 Fuel 40 Electrostatic precipitator

26 Air 42 Chimney

27 Exhaust gas 44 Environment Branch 174 Light source

separator

Control unit 245 exhaust air

252 exhaust air purifiers

VOC concentration meter 254 insertion point

Exit point 256 Oxidizer

Scrubber 257 insertion device

Pre-cooker 258 Oxidant source

Wood chips 260 feeder

Fresh wood 262 VOC concentration meter

Recycled wood 264 second VOC concentration meter

Cooker 266 nozzle

Refiner 274 Light Source

Blow-Line, Beieimer

Classifier c Concentration

Spreader CGrenz limit

Fiber cake cvoc total concentration cvoc st total concentration

Press Cv O C , m ax Maximum concentration

NOx concentration meter CNOX nitrogen oxide concentration

M Material flow direction

Steam QA discharge stream

Steam branch QR return flow

Steam cleaner R recirculation share

Place of application t time

Oxidizing agent TA initial temperature

Insertion device TE inlet temperature

Oxidant source

Dosing device

VOC concentration meter second VOC concentration meter

nozzle

Exhaust steam line

Claims

Patent claims 1. A method for producing a wood fiber material (32), comprising the steps (a) providing fiber material (30), (b) burning a fuel (24) in a furnace (16) so that exhaust gas (27) is produced, (c) drying fiber material (30) in a dryer (28) by means of the exhaust gas (27) so that dried fiber material (32) and vapors (34) are produced, and (d) Discharge of vapours (34) into the environment (44), (e) returning a portion of the vapors (34) to the furnace (16) so that volatile organic substances contained in the returned vapors (34) are burned, characterized by the steps (f) measuring a total concentration (cvoc) of volatile organic substances in the vapours (34) and (g) Changing the recirculation proportion (R) of recirculated vapors (34) depending on the total concentration (cvoc) 2. Method according to claim 1, characterized in that the change of the recirculation proportion (R) of recirculated vapors (34) as a function of the total concentration (cvoc) (a) increasing a recirculation proportion (R) of recirculated vapors (34) if the total concentration (cvoc, ist) exceeds a predetermined maximum concentration (cvoc, max) and/or (b) reducing the recirculation fraction (R) if the total concentration (cvoc, ist) falls below a predetermined minimum concentration (cvoc.soii) and/or (c) controlling the recirculation fraction (R) so that the total concentration (cvoc.actual) approaches a target concentration (cvoc.soii). 3. Method according to one of the preceding claims, characterized by the step of removing volatile organic substances by spray washing and/or electrofiltration before discharging the vapors (34) into the environment (44). 4. Method according to one of the preceding claims, characterized by the step Introducing an oxidizing agent into those vapors (34) which are not returned to the furnace, but whose material flow does not pass through the furnace into the environment, so that volatile organic substances contained in the vapors (34) are oxidized and purified vapors are produced. 5. Method according to one of the preceding claims, characterized by the step (a) measuring a nitrogen oxide concentration of nitrogen oxides in the vapors (34) in the material flow direction downstream of the dryer and/or upstream of the branch, or the exhaust gases in the material flow direction downstream of the furnace and upstream of the dryer and (b) Increasing the recirculation proportion if the nitrogen oxide concentration exceeds a predetermined maximum nitrogen oxide concentration and/or reducing the recirculation proportion if the nitrogen oxide concentration falls below a minimum nitrogen oxide concentration. 6. Method according to one of the preceding claims, characterized in that the vapors (34) are fed to a burner (20), a mixing chamber (18) and/or a combustion chamber (22) of the furnace (16). 7. Process according to one of the preceding claims, characterized in that between 10 percent by weight and 35 percent by weight, in particular 30 percent by weight, of the vapors (34) are recycled. 8. Method according to one of the preceding claims, characterized by the steps (a) heating wood chips (54) in a pre-cooker by means of water or steam, (b) then cooking the wood chips (54) by means of steam in a cooker (60), (c) then defibrating wood chips (54) in a refiner (62) to produce fiber material (30), (d) then gluing the fiber material (30) to form glued fiber material (30'), wherein the glued fiber material (30') is dried to form dried fiber material (32), (e) scattering the dried fiber material (32) into a fiber cake (70) and (f) pressing the fiber cake (70) to form a wood-based panel (14). 9. Method according to one of the preceding claims, characterized in that (a) an inlet temperature (TE) at which the exhaust gas (27) enters the dryer (28) is at least 300°C, in particular at least 350°C, and/or at most 480°C, in particular at most 450°C, and/or (b) an exit temperature (TA) at which the vapors (34) leave the dryer (28) is at least 50°C, in particular at least 55°C and/or at most 80°C, in particular at most 75°C. 10. Wood pulp manufacturing device (10) for producing a wood pulp, with (a) a furnace (16) for burning a fuel (24) such that Exhaust gas (27) is generated, (b) a refiner (62) designed to shred wood chips (54) to produce fiber material (30), (c) a dryer (28) arranged downstream of the refiner (62) in the material flow direction (M) for drying the fiber material (30) by means of the exhaust gas (27), so that vapors (34) are formed, (d) an exhaust air system (36) for removing vapours (34) which are produced in the dryer (28) during the drying of the fiber material (30) into the environment, and (e) a branch (46) connected to the dryer (28) for returning a portion of the vapors (34) to the furnace (16), characterized by (f) a concentration meter (50) for measuring a total concentration (cvoc) of volatile organic substances, (g) wherein the branch (46) is designed to automatically change the recirculation proportion (R) of recirculated vapors (34) as a function of the total concentration (cvoc) 11. Wood pulp manufacturing device (10) according to claim 10, characterized in that the branch (46) is designed for automatic (i) Increasing a recirculation proportion (R) of recirculated vapors (34) if the total concentration (cvoc, ist) exceeds a predetermined maximum concentration (cvoc, max) and/or (ii) reducing the recirculation fraction (R) if the total concentration (cvoc, ist) falls below a predetermined minimum concentration (cvoc.min) or (iii) Controlling the recirculation fraction (R) so that the total concentration (cvoc, ist) approaches a target concentration (cvoc,soii). 12. Wood fiber manufacturing device (10) according to claim 10 or 11, characterized by (a) a nitrogen sensor for measuring a nitrogen oxide concentration of nitrogen oxides in the vapors (34) in the material flow direction downstream of the dryer and/or upstream of the branch, or the exhaust gases in the material flow direction downstream of the furnace and upstream of the dryer and (b) the branch (46) is designed for automatic (i) increasing the recirculation rate when the nitrogen oxide concentration exceeds a predetermined maximum nitrogen oxide concentration and/or (ii) Reducing the recirculation rate when the nitrogen oxide concentration falls below a minimum nitrogen oxide concentration. 13. Wood fiber production device (10) according to one of claims 10 to 12, characterized in that the exhaust air system (36) (a) a gas scrubber (38) and/or (b) an electrostatic precipitator (40), in particular a wet electrostatic precipitator. 14. Wood pulp manufacturing device (10) for producing a wood pulp (32) according to one of claims 10 to 13, with (a) a cooker (60) for cooking wood chips (54) by means of steam, the refiner (62) being arranged behind the cooker (60) in the direction of wood material flow, (b) a bucket (64) arranged behind the dryer (28) in the material flow direction (M) for applying glue to the dried fiber material (30) so that glued fiber material (30) is produced, (c) a spreader (68) for spreading a fiber cake (70) of glued fiber material (30) and (d) a press for pressing the fiber cake (70) into a wood-based panel (14).