Method for reducing contaminants in drying systems for OSB strands

Abstract

The invention relates to a drying system (100) for drying OSB strands, comprising a dryer (20) with a drying chamber (22) which is designed to dry OSB strands, thereby introducing hot drying air into the drying chamber (22); a burner (10) with a burner chamber (12) for providing hot drying air; and a device (80) which is designed to return at least one part of the exhaust vapor exiting the drying chamber (12) to the burner (10) in order to combust contaminants contained in the exhaust vapor part in the burner (10). The device (80) is additionally designed to maintain the temperature of the exhaust vapor part to be returned at a temperature above the condensation temperature of the water vapor entrained in the exhaust vapor part.

Description

[0001] Technology for reducing pollutants in drying plants for OSB strands

[0002] Technical area

[0003] The present invention relates to the field of environmental technology. In particular, the present invention relates to a technique for reducing emissions of pollutants generated during the drying of OSB strands in a drying plant.

[0004] background

[0005] Drying systems designed for drying moist material, particularly wood chips or OSB strands, are known. Such drying systems comprise a dryer (e.g., a drum dryer) with a drying chamber to which the material to be dried is fed in a controlled manner. Furthermore, a controlled volume flow of hot drying gas (e.g., hot air) is fed to the drying chamber, slowly transporting the material to be dried through the drying chamber. The hot drying gas fed to the drying chamber flows around and heats the material to be dried, absorbing the released moisture. At the outlet of the drying chamber, the dried material is separated from the drying gas again (e.g., by a cyclone separator). The drying gas at the outlet of the drying chamber is saturated with water vapor, usually contains pollutants that arise during the drying of the material, and is called vapor.The vapors can be cleaned and released into the environment as exhaust gas.

[0006] To increase the efficiency of drying systems, it is also known to recirculate at least a portion of the vapor exiting the drying chamber into a mixing chamber located upstream of the drying chamber inlet. The recirculated vapor (also called back vapor) is mixed there with hot drying gas. The gas mixture is then fed back into the drying chamber to dry the moist material. By adding back vapor, less hot drying gas needs to be provided by the burner. In particular, the residual heat of the back vapor can be utilized, allowing the drying system to operate more energy-efficiently overall.

[0007] DE 40 23 518 A1 discloses a drying system in which a portion of the vapor exiting the combustion chamber is returned directly to the burner's combustion chamber via a return gas line. The recirculated vapor portion (or return vapor portion) is passed through a tower-shaped condensation scrubber, which cools the vapor portion to below the condensation temperature (dew point temperature), thereby dehumidifying it. This condensation process also partially purifies the vapor, as pollutants contained in the vapor condense out. Thus, a portion of the return vapor cooled to below the dew point, at least partially dehumidified, and purified, is returned to the burner or combustion chamber. However, the use of a condensation scrubber is costly; furthermore, a significant portion of the condensation heat is lost in the condensation scrubber or can only be recovered with considerable technical effort.

[0008] EP 2 230477 A1 further discloses a wood chip drying system for drying wood chips, which has a recirculation device for recirculating vapors into the drying circuit. The recirculation device has a vapor heater that has a regenerative and / or catalytic heat exchanger and is designed to heat the vapors to a relatively high temperature in the range of 720°C to 900°C, so that organic substances or solid particles entrained in the vapors are predominantly oxidized (combusted). A portion of the thermally regeneratively treated vapor is then fed to the burner as combustion air. Another portion of the thermally regeneratively treated vapor can be fed to a mixing chamber arranged between the burner and the drying chamber. The use of a catalytic heat exchanger is complex and cost-intensive.Furthermore, US Pat. No. 5,983,521 A discloses a drying system in which the vapor provided at the outlet of the drying chamber is completely recirculated into the drying circuit. The vapor or back vapor is divided into two partial streams, which are then recirculated to different areas of the combustion chamber. The two back vapor partial streams each pass through a heat exchanger, in which heat is transferred from a gas stream discharged from the combustion chamber to the respective back vapor partial streams. One of the two heat exchangers, which is primarily supplied with heat from the burner, is a so-called high-temperature heat exchanger, while the other is a so-called low-temperature heat exchanger. However, no information is provided about the temperatures of the two back vapor partial streams.This vapor recirculation approach is also technically complex, as two heat exchangers and corresponding return lines must be provided for the two partial streams. Furthermore, this approach involves complex control of the two partial vapor streams.

[0009] Furthermore, WO 01 / 59381 A1 discloses a drying system in which a portion of the vapor is recirculated both to the combustion chamber and to a mixing chamber between the combustion chamber and the drying chamber. For this purpose, the return line is divided into two sub-lines, one leading to the combustion chamber and the other to the mixing chamber. A fan for controlling the return vapor flow is arranged in the common section of the return line, i.e., upstream of the two sub-lines. However, this makes it difficult to control the return vapor flow leading to the combustion chamber. Furthermore, the drying system described in WO 01 / 59381 A1 uses a belt dryer. Such dryers are known for operating at relatively low temperatures (< 100°C), but for requiring high drying gas flows (approximately three times the drying gas flow compared to drum dryers).Since the operating temperature of the drying system is moderate, the temperature of the return vapor stream is also moderate. This creates the risk that moisture contained in the return vapor stream will condense on the combustion chamber wall and thus impair the burner. Although, as described above, numerous drying systems are known that incorporate return devices for recirculating and recycling vapors in order to increase the efficiency of such systems and reduce pollutant emissions, further improvements are still needed.

[0010] In particular, the object of the present invention is to provide a technology that further reduces pollutant emissions from drying systems, especially from drying systems for OSB strands, while at the same time being cost-effective and simple to implement. Furthermore, the provided technology should be easy to retrofit into existing OSB strand drying systems.

[0011] Brief outline

[0012] To achieve the above-mentioned object, according to a first aspect of the invention, a drying system for drying OSB strands is provided. The drying system comprises a dryer with a drying chamber designed to dry OSB strands by introducing hot drying gas into the drying chamber; a burner with a combustion chamber for providing hot drying gas; and a device designed to recirculate at least a portion of the vapor exiting the drying chamber to the burner in order to combust the pollutants contained in the vapor portion in the burner. The device is further designed to maintain the temperature of the vapor portion to be recirculated at a temperature above the condensation temperature of the water vapor entrained in the vapor portion.

[0013] OSB strands are long, slender chips obtained from debarked roundwood and used to produce OSB (Oriented Strand Board). OSB strands differ from conventional wood chips in their dimensions. They can be 100-200 mm long, 10-50 mm wide, and 0.5-1 mm thick. To produce OSB, OSB strands are glued after drying in the drying plant, arranged in several oriented layers (for example, using a throwing process), and then pressed. Vapors are the steam-saturated drying exhaust gases that arise during the drying of the moist OSB strands and exit at the exit of the drying plant (more precisely, at the exit of the drying chamber). The vapors or vapor portions returned to the burner are also referred to below as return vapors or return vapor portions.

[0014] The vapors or back vapors released during the drying of OSB strands can be contaminated with pollutants. These pollutants can primarily include volatile organic compounds (so-called VOCs - Volatile Organic Compounds) and / or solid particles that pollute the environment. An example of volatile organic compounds are terpenes ((CsHs)n with n > 2), which can be released during the drying of OSB strands. However, these pollutants can also include other compounds, such as nitrogen-based compounds (e.g., nitrous oxide N2O, ammonia NH3, NO X ) that have a significant impact on the environment.

[0015] The burner of the drying system can be designed to generate a burner flame in the combustion chamber having a flame temperature in the temperature range from 600°C to 1200°C, preferably in the temperature range from 600°C to 800°C. For example, a gas, oil, solid fuel (e.g., a wood dust burner), or multi-fuel burner can be used as the burner, which is designed to generate a flame temperature in the above-mentioned temperature range. With the aid of the burner flame, a drying gas supplied to the combustion chamber or a continuously supplied volume flow of drying gas can be heated to a desired temperature in the temperature range from 600°C to 1200°C, preferably in the temperature range from 600°C to 800°C. Drying air, for example, can be used as the drying gas.Ambient air can be supplied (continuously) to the combustion chamber, where it is heated to the desired temperature using the burner flame.

[0016] In addition, the burner flame can be used to heat the vapor returned to the burner to the desired temperature and simultaneously purify it. It has been shown that the pollutants released during the drying of OSB strands and contained in the vapor (primarily VOCs, nitrous oxide, ammonia, and dust particles) can be effectively combusted (oxidized) at burner or flame temperatures in the range of 600 °C to 1200 °C. The heated / hot drying gas can be (re)supplied to the dryer or a mixing chamber located between the burner and the dryer, along with the return vapor heated and purified in the burner.

[0017] Thus, the drying system according to the invention with vapor recirculation to the burner not only recycles a portion of the vapor and makes it available for a subsequent drying process, but also combusts the pollutants entrained in the vapor in the burner flame. This allows pollutant emissions from the drying system to be reduced in a simple design. Additional cleaning devices for cleaning the recirculated vapor, as known from the prior art, are not required in the system according to the invention.

[0018] The dryer can be designed as a drum dryer, which has a rotating drum as the drying chamber. The drying chamber (drum) can be designed to convey the OSB strands continuously fed at the inlet side of the chamber to the opposite outlet side of the drying chamber. The OSB strands moving toward the outlet side are exposed to the hot drying gas / drying air introduced into the drying chamber and are thereby dried. The hot drying gas continuously introduced at the inlet side of the drying chamber flows through the drying chamber, absorbs the moisture released by the OSB strands in the drying chamber, and leaves the chamber at the outlet as vapor. The temperature of the hot drying gas can depend on the nature of the OSB strands and can be adjusted accordingly.The dryer is preferably operated with drying gas at a temperature in the temperature range from 230°C to 500°C, more preferably in a temperature range from 250°C to 400°C. The volume flow of the hot drying gas supplied to the drying chamber can be adjusted accordingly depending on the amount of OSB strands supplied. To return the at least one vapor portion to the burner, the device (hereinafter also referred to as the return device) can comprise at least one return line. This can be coupled at its first end to the outlet of the drying chamber and at its second end to the burner. For example, the at least one return line can be coupled to the combustion chamber and / or the muffle of the burner.

[0019] If the at least one return line is coupled to both the combustion chamber and the burner muffle, the at least one return line can split into two sub-lines at its second end. A first sub-line can lead directly to the combustion chamber; a second sub-line can lead (directly) to the burner muffle. A first portion of the vapor portion returned to the burner can be supplied to the combustion chamber (the high-temperature zone of the combustion chamber) as combustion air or main air via the first sub-line. A second portion of the vapor portion returned to the burner can be supplied to the burner muffle as cooling air via the second sub-line. After the burner muffle has been cooled, the second portion can reach the high-temperature zone of the combustion chamber, where it can be further heated and cleaned. Thus, both parts of the return vapor are heated to a high temperature in the combustion chamber, and the pollutants entrained are burned.The vapor part thus cleaned can be made available for further drying of the drying chamber.

[0020] Alternatively, the at least one return line can comprise two return lines, with a first return line being coupled to the combustion chamber of the burner and the second return line being coupled to the burner muffle. A first portion of the return vapor (as combustion air) can be returned to the combustion chamber via the first return line. A second portion of the return vapor (as cooling air) can be returned to the burner muffle via the second return line.

[0021] The vapor leaving the drying chamber has a temperature that is above the condensation temperature (dew point temperature) of the water vapor entrained in the vapor. Preferably, the temperature of the vapor portion returned to the burner is kept above the condensation temperature (dew point temperature) at all times during the recirculation. In other words, the recirculation device is designed to keep the temperature of the recirculated vapor portion at a temperature above the condensation temperature (dew point temperature) during the recirculation process. This can be achieved, for example, by thermally insulating the at least one return line. The thermal insulation of the at least one return line can contribute to the return vapor portion conducted in the return line losing little heat and thus being able to be kept above the condensation temperature.

[0022] Additionally or alternatively, the recirculation device can further comprise a preheating device or be coupled to an (external) preheating device. The preheating device can be provided to preheat the at least one return line as needed. This can be necessary, for example, when the drying system is started up and the at least one return line has "cooled down" (despite thermal insulation) due to low ambient temperatures. To prevent the temperature of the vapor portion returned to the burner from falling below the condensation temperature, the preheating device can be activated to preheat the at least one return line to a desired temperature (preheating temperature). The preheating temperature can be less than or equal to the temperature of the return vapor.

[0023] According to one implementation, the preheating device can comprise a preheating gas source that can be selectively coupled to the return line of the return device to introduce preheating gas into the return line. The preheating gas flowing through the return line can bring the return line to the desired preheating temperature. Dehumidified preheating air, in particular, can be used as the preheating gas.

[0024] By keeping the temperature of the return vapor section above the condensation temperature at all times, the water vapor contained in the return vapor section is prevented from partially condensing in the return line provided for recirculation, on the combustion chamber walls, or burner muffle. This prevents valuable condensation heat from being lost during recirculation; on the other hand, it also prevents moisture from condensing in the return line or on the combustion chamber walls, which can lead to long-term damage to the burner.

[0025] The recirculation device can further comprise a control device for controlling the volume flow of the vapor portion to be recirculated. For this purpose, the control device can comprise at least one conveying fan arranged in the at least one return line. The at least one conveying fan can be designed to move the return vapor portion in the direction of the burner. The volume flow of the return vapor portion flowing in the at least one return line can thus be (actively) adjusted or controlled via the at least one conveying fan. The return vapor volume flow can be adjusted depending on the burner output or depending on the drying gas volume flow required in the drying chamber.If the return device has two partial lines or at least two return lines for returning a first vapor part to the combustion chamber and a second vapor part to the burner muffle, at least one conveying fan can be provided in each of the at least two return lines or partial lines in order to actively adjust the volume flows of the first and second vapor part.

[0026] Furthermore, the control device can comprise a valve unit arranged in the at least one return line. The valve unit can further regulate the volume flow of the vapor portion to be recirculated, which is to be fed to the combustion chamber and / or the muffle of the burner.

[0027] The drying system can further comprise a mixing chamber arranged between the burner and the dryer. This mixing chamber can be designed to mix hot drying gas provided by the burner with a vapor portion returned to the mixing chamber and / or with an external drying gas and then feed it to the drying chamber. By admixing vapors and / or external drying gas to the drying gas provided by the burner, the resulting drying gas mixture can be brought to a temperature tailored to the operation of the dryer, which temperature can be significantly lower than the temperature of the drying gas provided by the burner. The drying gas (drying gas mixture) provided to the dryer by the mixing chamber preferably has a temperature in the range of 230°C to 500°C, more preferably a temperature in the range of 250°C to 400°C.The volume flow of the hot drying gas supplied through the mixing chamber of the drying chamber can be adjusted accordingly depending on the amount of OSB strands supplied.

[0028] To return at least a portion of the vapor to the mixing chamber, the return device can further be coupled to the mixing chamber. According to one variant, the at least one return line can additionally be coupled to the mixing chamber at its second end. This can be achieved by the return line splitting into at least two sub-lines at its second end, with a first sub-line leading to the burner and a second sub-line leading to the mixing chamber. According to one variant, the return line can have at least three sub-lines at its second end, with a first sub-line leading to the combustion chamber, a second sub-line leading to the burner muffle, and a third sub-line leading to the mixing chamber. The volume flow in the return line and in particular the volume flow proportion of each sub-line can be actively controlled via the at least one conveying fan and / or the valve unit of the control device described above.

[0029] The drying system may further comprise a separation device arranged at the outlet of the drying chamber and designed to separate OSB strands from the vapor. For example, the separation device may comprise one or more cyclone separators designed to separate OSB strands.

[0030] The drying system may further comprise a filter device designed to filter out pollutants from the vapor portion that is released into the environment and thus is not recirculated to the burner via the aforementioned device and made available again for the drying process. According to a second aspect of the invention, a system for producing OSB is provided, the system comprising: a device for producing OSB strands; the drying system according to the first aspect of the invention for drying the OSB strands; a device for applying glue to the dried OSB strands; a device for oriented stratification of the glued OSB strands into multiple layers; and a pressing device for pressing the layers into an OSB (Oriented Strand Board).

[0031] According to a third aspect of the invention, a method is provided for reducing pollutants produced during the drying of OSB strands in an OSB strand drying plant. The method comprises the following steps: recirculating at least a portion of the vapor produced during the drying of OSB strands to a burner of the drying plant; maintaining the vapor portion to be returned to the burner at a temperature above the condensation temperature of the water vapor contained in the vapor portion; and combusting the pollutants contained in the recirculated vapor portion in the burner.

[0032] Preferably, the temperature of the recirculated vapor portion is kept above the condensation temperature (dew point temperature) at all times during recirculation. In other words, the process according to the invention is operated such that the temperature of the recirculated vapor portion does not fall below the condensation temperature during the entire recirculation process. For example, the process is operated such that the temperature of the recirculated vapor portion (return vapor portion) is maintained at at least 100°C. Preferably, the temperature of the return vapor portion is maintained in a temperature range from 105°C to 135°C, more preferably in a temperature range from 115°C to 135°C.

[0033] In a particularly simple implementation of the process, the return vapor portion can be maintained at a temperature higher than the condensation temperature, but not higher than the temperature of the vapor upon leaving the drying chamber of the drying plant. Typical temperatures of the vapor leaving the drying chamber are in the temperature range of 115 °C to 135 °C; accordingly, the recirculated vapor portion can be maintained at a temperature in the temperature range of 115 °C to 135 °C (e.g., 125 °C) or at slightly lower temperatures. This makes the process particularly simple to implement, as no additional heat exchangers and / or heating devices are required to increase the temperature of the return vapor.

[0034] By keeping the temperature of the return vapor section above the condensation temperature at all times, the water vapor contained in the return vapor section is prevented from partially condensing in the return line provided for recirculation or in the burner. This prevents valuable condensation heat from being lost during recirculation; on the other hand, it also prevents moisture from condensing in the return line or on the combustion chamber walls, which can lead to long-term damage to the burner.

[0035] To keep the return vapor above the condensation temperature of the steam at all times, the return line intended for the return vapor, or at least a portion of the return line intended for this purpose, can be preheated as needed. This preheating step is optional and is performed as needed, for example, during start-up of the drying plant when the return line has cooled down due to cold outside temperatures and there is a risk that the return vapor portion will fall below the condensation temperature when returning to the burner.

[0036] For preheating, for example, a preheating gas (e.g., dehumidified preheating air) can be introduced into the at least one return line or introduced before the vapor portion is returned. The preheating gas can have a temperature that is less than or equal to the temperature of the recirculating vapor. The preheating gas can preferably have a temperature in the range from 70°C to 110°C, more preferably in the range from 80°C to 100°C. This allows the return line to be brought to a desired preheating temperature. This enables the recirculating vapor or recirculating vapor portion to have a predetermined holding temperature above the condensation temperature over the entire return path. The recirculating step can comprise recirculating a first vapor portion to the burner and a second vapor portion to a mixing chamber.The vapor part returned to the mixing chamber can be mixed there with drying gas / drying air provided by the burner and then made available to the dryer.

[0037] The vapor portion returned to the burner can be returned directly to the combustion chamber. In an alternative variant, the vapor portion returned to the burner can first be fed to the burner muffle for cooling and then conducted into the combustion chamber. In another alternative variant, part of the vapor portion returned to the burner can be fed directly to the combustion chamber (as combustion air) and another part (the remaining part) of the vapor portion returned to the burner can be fed to the burner muffle. In all variants, the vapor portion returned to the burner is passed through the high-temperature zone of the combustion chamber, whereby the vapor is heated to temperatures in the range of 600 °C to 1200 °C, preferably to temperatures in the range of 600 °C to 800 °C, and is thereby purified.In this temperature range, pollutants contained in the back vapor, such as VOCs, nitrogen-based pollutants such as ammonia or nitrous oxide, as well as solid particles (e.g. dust particles) are burned.

[0038] The volume flow of the first vapor portion fed to the burner and / or the volume flow of the second vapor portion fed to the mixing chamber can preferably be controlled depending on the burner output and / or the volume flow required in the drying chamber. The volume flow of the first vapor portion returned to the burner can be controlled depending on whether the vapor is to be fed to the combustion chamber, the burner muffle, or both the combustion chamber and the burner muffle. In particular, the first returned vapor portion can be proportionally distributed between the combustion chamber and the burner muffle.

[0039] The combustion step can involve burning the pollutants contained in the recirculated vapor at temperatures in the range of 750 °C to 1200 °C. Combustion occurs via the burner flame in the high-temperature zone of the burner. This purifies the vapor returned to the burner, significantly improving the overall pollutant balance of drying systems. By returning at least a portion of the vapor to the combustion chamber, energy can also be saved, as the recirculated vapor already has a temperature of greater than 100 °C. Thus, the burner requires significantly less heat energy than if the burner were only supplied with cold drying gas (drying air), which then has to be heated to the desired operating temperature.

[0040] Short description of the drawings

[0041] Further details and advantages of the invention will be further described with reference to the non-limiting embodiments illustrated in the drawings. They show:

[0042] Figure 1 is a schematic representation of a plant comprising a drying plant for OSB strands according to the present invention; and

[0043] Figure 2 is a flow chart illustrating a method according to the invention for reducing pollutants in a drying plant for OSB strands.

[0044] Detailed description

[0045] Figure 1 shows a schematic representation of a plant 1 for the production of OSB (Oriented Strand Board).

[0046] The system 1 comprises a device 2a, 2b for producing OSB strands, a drying system 100 for drying the produced OSB strands, a device 4a, 4b for applying glue to the dried OSB strands, a device 5 for oriented stratification of glued OSB strands in multiple layers, and a pressing device 6 for pressing the layers into an OSB board. The system 1 can further comprise a sifter 3 for separating OSB strands of different sizes. The components of the system 1 are described in more detail below. The device 2a, 2b for producing OSB strands can comprise a debarker 2a, which is designed to debark round wood (with relatively high moisture content). The device can further comprise a chipper 2b, which is designed to chip the debarked round wood. A disc, drum or knife ring chipper can be used as chipper 2b.Using chipper 2b, strands (long, slender chips) are produced from the debarked logs. These strands can range in length from 100 to 200 mm and width from 10 to 50 mm. Their thickness can range from 0.5 to 1 mm.

[0047] The produced (moist) strands are fed to the drying system 100 for drying. More specifically, the strands are fed to a drying chamber 22 of a dryer 20 and dried there with the aid of introduced drying gas or drying air. An embodiment of a drying system 100 according to the invention is described in more detail below.

[0048] The dried strands are fed to a sifter 3. The sifter 3 is designed to classify the strands or separate them according to different sizes. Smaller strands can be used to produce a middle layer of an OSB, while larger strands can be used to produce the outer layers. The strands classified by size are fed to the device 4a, 4b for applying glue to the strands. The larger strands are fed to a gluing device 4a and are glued there, while the finer strands are fed to a gluing device 4b and are glued there.

[0049] The glued strands are then fed to a device 5 for oriented layering of the glued OSB strands into multiple layers. The device 5 is designed to orient the glued larger strands into one or more middle layers (middle layers) and to orient the finer strands into one or more cover layers (cover layers). The layers thus produced with aligned OSB strands are then fed to the pressing device 6. The pressing device 6 is designed to press the layers together to form an OSB. This occurs under the influence of pressure and temperature, at which the glue cures and the layers are pressed together to form a board of a predetermined thickness. A roller press can be used as the pressing device.

[0050] The drying system 100 is further described below.

[0051] The drying system 100 comprises a burner 10 with a combustion chamber 12, the dryer 20 with a drying chamber 22, and a mixing chamber 30 arranged between the combustion chamber 12 and the drying chamber 22. The drying system 100 can further comprise a separation device (not shown) arranged at the outlet of the drying chamber 12 for separating vapors and OSB strands. Furthermore, the drying system 100 can comprise a vapor discharge line 50 coupled to the mixing chamber 30 (or separation device), a device 80 for recirculating at least a portion of the vapor to the burner 10 (and optionally a further portion to the mixing chamber 20), and a filter device 62 for filtering pollutants from that portion of the vapor that is released into the environment. The device 80 for recirculating at least a portion of the vapor is also referred to below as the recirculation device 80.

[0052] The burner 10 is designed to provide a hot drying gas to the mixing chamber 30. For this purpose, the burner 10 has a combustion chamber 10 and a muffle area 11 with a fuel supply 14 (e.g., gas, oil, or wood dust) and a supply 15 for combustion gas / air and / or drying gas / air. The combustion gas / air and / or drying gas / air is supplied to the combustion chamber 12 and heated there to a desired temperature with the aid of the burner flame. A gas burner, oil burner, dust burner (e.g., a wood dust burner), or a multi-fuel burner can be used as the burner, which is designed to generate flame temperatures in the range from 600°C to 1200°C, preferably in the range from 600°C to 800°C, in the combustion chamber. The heated drying gas / air leaving the combustion chamber 12 can thus also have a temperature in the range of 600 °C to 1200 °C, preferably in the range of 600 °C to 800 °C.In the following, the term drying gas or combustion gas is used consistently, although it should be clear that this can also mean drying air or combustion air.

[0053] The drying gas heated in the combustion chamber 12 is fed to the mixing chamber 30, where it is mixed with a vapor portion (return vapor portion) originating from the drying chamber 22 and recirculated via the recirculation device 80. By recirculating and reusing at least a portion of the vapor flowing out of the drying chamber 22, the energy efficiency of the drying system 100 can be significantly increased. Optionally, a further gas stream (air stream) containing preheated gas (air) (not shown in Figure 1) can be fed to the mixing chamber 30, which is released, for example, in an external system operated outside the drying system 100 (e.g., a wood processing system). This can further improve the energy balance of the drying system 100.

[0054] By mixing the heated drying gas provided by the combustion chamber 12 with the recirculated vapor portion (and optionally additional supplied gas streams), the temperature of the drying gas (drying gas mixture) resulting in the mixing chamber 30 and provided to the drying chamber 22 decreases. However, the temperature is adjusted by appropriate mixing and gas supply from the combustion chamber 12 so that the resulting drying gas in the mixing chamber 30 has a desired temperature value that is approximately 20°C to 40°C higher than the temperature at the dryer inlet. This excess temperature in the mixing chamber 30 may be necessary to compensate for heat losses due to convection and radiation.

[0055] The dryer 20 is preferably a drum dryer, which, depending on the quantity and nature (size, moisture content, wood species) of the OSB strands, is operated in the temperature range from 230°C to 500°C, preferably in the temperature range from 250°C to 400°C (for example, at 350°C). Accordingly, the temperature of the drying gas provided in the mixing chamber 30 is in a temperature range 20°C to 40°C higher. Such temperatures are sufficient to dry OSB strands. The OSB strands to be dried are fed (continuously) at the drying chamber inlet to the drying chamber 22 and are passed (continuously) within the drying chamber 22 until they reach the drying chamber outlet. The process of feeding and conveying the OSB strands within the drying chamber 22 is not shown in the schematic representation in Figure 1.Furthermore, the mixing chamber 30 provides a desired drying gas volume flow for the drying chamber 22, depending on the quantity and nature of the OSB strands to be dried. The hot drying gas supplied to the drying chamber 22 flows around and heats the OSB strands and absorbs the moisture released in the process, thereby drying the OSB strands.

[0056] The above-mentioned operating temperature in the temperature range from 230 °C to 500 °C, preferably in the range from 250 °C to 400 °C, is essentially controlled via the burner 10 (for example, by controlling the fuel supply and thus the burner output). In any case, the temperature in the drying chamber 22 is set at all times such that the moisture and pollutants released during the drying process cannot recondense within the drying chamber 22.

[0057] Accordingly, the vapor flowing out of the drying chamber 22 has a relatively high temperature, which is above the condensation temperature of water vapor. Typical vapor temperatures at the drying chamber 22 outlet are above 100 °C, preferably in the temperature range of 115 °C to 135 °C (for example, approximately 125 °C).

[0058] After separation of OSB strands entrained in the vapor in a separation device provided at the outlet of the drying chamber 22 (not shown in Figure 1), at least a portion of the vapor can be fed back to at least the burner 10 via the recirculation device 80. The remaining portion can be fed to the filter device 62 via the vapor discharge line 50, cleaned / filtered there, and released into the environment. In the drying system 100 shown in Figure 1, the recirculation device 80 comprises at least one return line 82. This is coupled at its first end to the vapor discharge line 50. Alternatively, it is also conceivable for the at least one return line 82 to be coupled (directly) at its first end to the outlet of the drying chamber 22 or to the separation device provided at the outlet for separating vapors and OSB strands.Furthermore, the return line 82 is coupled at its second end to the burner 10 to return a first portion of the vapor to the burner 10. The return line 82 can further be coupled to the mixing chamber 30 to return a second portion of the vapor to the mixing chamber.

[0059] In the embodiment shown in Figure 1, the at least one return line 82 has three sub-lines at its second end, wherein a first sub-line 82a leads (directly) to the burner chamber 12, a second sub-line 82b leads to the burner muffle 11 and a third sub-line 82c leads to the mixing chamber 30.

[0060] Via the first sub-line 82a, a portion of the first return vapor portion returned to the burner 10 can be introduced as combustion gas directly into the combustion chamber 12 of the burner 10 and heated there with the aid of the burner flame to temperatures greater than or equal to 600°C, for example to a temperature in the temperature range from 600°C to 1200°C, preferably to a temperature in the temperature range from 600°C to 800°C. Via the second sub-line 82b, a further portion of the first return vapor portion returned to the burner 10 can also be fed to the burner muffle 11 as cooling gas / air. This return vapor portion is also subsequently fed into the combustion chamber 12 and there further heated and purified to temperatures in the temperature range from 600°C to 1200°C, preferably to temperatures in the temperature range from 600°C to 800°C.

[0061] Due to the strong heating of the back vapor to temperatures in the temperature range of

[0062] At temperatures between 600 °C and 1200 °C, in both cases (direct return to the combustion chamber 12 or return via the burner muffle 11), the pollutants entrained in the return vapor (in particular VOCs (e.g., volatile terpenes), nitrogen-based pollutants (e.g., nitrous oxide, ammonia), and / or solid particles, which occur in increased quantities during the drying of OSB strands) can be oxidized / combusted. Thus, effective purification of the returned vapor can be achieved by direct combustion of the pollutants in the burner 10, which can improve the pollutant balance of the drying system 100.

[0063] A second portion of the recirculated vapor can be returned to the mixing chamber 30 via the third sub-line 82c. This recirculated vapor is not significantly purified due to the significantly lower temperatures in the mixing chamber (temperatures in the range of 250°C to 400°C). However, by mixing the second portion of the recirculated vapor with drying gas provided from the combustion chamber 12, the energy balance of the drying system 100 can be improved. This is because, as will be described in more detail below, the recirculation device 80 is designed to keep the recirculated vapor at temperatures greater than 100°C; as a result, the recirculated vapor has a high thermal energy content; in particular, no further heat of vaporization needs to be applied.

[0064] In order to improve or optimize the energy balance and, in particular, the pollutant balance of the drying system 100, it is desirable to recirculate as large a portion as possible of the vapor exiting the drying chamber 22 to the burner 10. The amount (volume flow) of the recirculated vapor that can be recirculated to the burner 10 depends in particular on the operating load of the burner 10, which in turn depends on the nature and quantity of the OSB strands to be dried. Furthermore, care must be taken to ensure that the relatively oxygen-poor vapor portion recirculated to the combustion chamber 12 does not cause the oxygen content of the combustion gas in the combustion chamber to fall below 13 vol.%, which would no longer ensure stable combustion. To avoid this, a volume flow of external combustion gas / air with a significantly higher oxygen content (approx. 20-21 vol.) can be introduced via the feed 15.%) are continuously fed to the combustion chamber 12, which is then mixed with the volume flow of the recirculated vapor in the combustion chamber 12. The recirculation device 80 according to the invention is designed to control the volume flow of the first and second vapor portions returned to the burner 10 or to the mixing chamber as a function of the operating load (and in particular also as a function of the oxygen content in the combustion chamber 12). For this purpose, the recirculation device 80 has a control device which has at least one conveying fan 84 arranged in the at least one return line 82. The volume flow of the vapor conducted through the at least one return line 82 can be actively adjusted via this conveying fan 84.

[0065] The control device may further comprise a valve device designed to divide the volume flow of return vapors returned via the at least one return line 82 into the sub-lines 82a, 82b, 82c leading to the mixing chamber 30 and the burner 10 or to the burner muffle 11 and the combustion chamber 12. The valve device is not shown in Figure 1. According to a simple implementation, the valve device may comprise at least one flap valve designed to divide the returned volume flow into corresponding sub-volume flows for the sub-lines 82a, 82b, 82c.

[0066] It is understood that the return device 80 shown in Figure 1 represents an exemplary implementation. A return device 80 is also conceivable in which the at least one return line 82 leads only to the combustion chamber 12 of the burner 10. In this case, the valve device for dividing the returned volume flow into partial volume flows can also be omitted.

[0067] As already indicated above, the recirculation device 80 is further designed to keep the recirculated vapor (volume flow) at a temperature above the condensation temperature of the water vapor entrained in the vapor part. To achieve this, the recirculation device 80, in particular the at least one return line 82, can be thermally insulated. In addition, the recirculation device 80 can have a preheating device (not shown in Figure 1). This can be designed to keep the at least one return line 82 at a desired temperature level. For example, the preheating device can comprise a preheating gas source or be coupled to an external preheating gas source. The (external) preheating gas source can be optionally coupled to the at least one return line 82. The preheating gas source can be designed to supply warm gas orto feed warm air into the at least one return line 82 to preheat the at least one return line 82. Although the at least one return line 82 is thermally insulated, it may happen that the return line 82 is "cooled down" in cold ambient temperatures and particularly during start-up of the drying system 100. In this case, the preheating gas source can be activated as needed to feed preheating gas into the return line 82.

[0068] As a rule, it is sufficient if the preheating gas provided by the preheating gas source has a temperature that is not significantly lower than the temperature of the recirculation vapor, preferably in a temperature range of 70 °C to 110 °C, more preferably in a temperature range of 80 °C to 100 °C. This temperature is generally sufficient to preheat the at least one return line 82 sufficiently so that cooling of the recirculation vapor to below the condensation temperature can be avoided.

[0069] The thermal insulation and the optional preheating device described here ensure that the return vapor is kept above the condensation temperature at all times during its return. This prevents water vapor or pollutants entrained in the return vapor section from condensing uncontrollably along the at least one return line 82 or in the combustion chamber 10 or the burner muffle 11.

[0070] In connection with Figure 2, a method according to the invention for reducing pollutants in a drying system for OSB strands is further discussed. The method can be implemented using the drying system 100 described in connection with Figure 1, in particular using the recirculation device 80 described therein.

[0071] According to a first step S20, at least a portion of the vapor produced during the drying of OSB strands is returned to the burner 10 via the return device 80, in particular via the at least one return line 82.

[0072] This return vapor portion returned to the burner 10 is maintained at a temperature above the condensation temperature of the water vapor contained in the vapor by means of the return device 80 (second step S22). This can be achieved, on the one hand, by suitable insulation of the at least one return line 82. If necessary, however, a preheating device can also be used to preheat the at least one return line 82 to a desired temperature.

[0073] In a third step S24, the recirculated vapor portion is then heated to a high temperature in the burner 10. In particular, the vapor portion is passed through the high-temperature zone of the combustion chamber 12 (a zone with temperatures in the range of 600 °C to 1200 °C), effectively oxidizing / burning the entrained pollutants.

[0074] The pollutant reduction technology described here has several advantages over the state of the art. It is simple, space-saving, and cost-effective, as it does not require additional purification systems. Instead, at least a portion of the back vapor is returned directly to the drying plant's existing burner and purified there. This purification process can significantly reduce pollutant emissions from drying plants. The technology described here also overcomes a long-held misconception that vapor saturated with water vapor cannot be returned directly to the burner or combustion chamber, as this would prevent stable operation of the drying plant.

Claims

PATENT CLAIMS 1. A drying system (100) for drying OSB strands, comprising: a dryer (20) with a drying chamber (22) designed to dry OSB strands by introducing hot drying gas into the drying chamber (22); a burner (10) with a combustion chamber (12) for providing hot drying gas; and a device (80) designed to recirculate at least a portion of the vapor emerging from the drying chamber (12) to the burner (10) in order to combust the pollutants contained in the vapor portion in the burner (10), wherein the device (80) is further designed to maintain the temperature of the vapor portion to be recirculated at a temperature above the condensation temperature of the water vapor entrained in the vapor portion.

2. Drying plant (100) according to claim 1, wherein the device (80) comprises at least one return line (82) which is coupled with its first end to the outlet of the drying chamber (22) and with its second end to the burner (10), preferably to the combustion chamber (12) and / or the muffle (11) of the burner (10).

3. Drying system (100) according to claim 2, wherein the at least one return line (82) is thermally insulated, and / or wherein the device (80) further comprises a preheating device or can be coupled to a preheating device which is provided to preheat the at least one return line (82) as required.

4. Drying system (100) according to claim 3, wherein the preheating device comprises a preheating gas source which is selectively coupleable to the at least one return line (82) in order to introduce preheating gas into the return line (82).

5. Drying system (100) according to one of claims 1 to 4, wherein the device (80) further comprises a control device for controlling the volume flow of the vapor part to be recirculated.

6. Drying system (100) according to claim 5, wherein the control device comprises: at least one conveying fan (84) arranged in the at least one return line (82) for adjusting the volume flow of the vapor portion to be recirculated; and / or at least one valve unit arranged in the at least one return line (82) for regulating the volume flow portion of the vapor portion to be recirculated, which is to be fed to the combustion chamber (12) and / or the muffle (11) of the burner (10).

7. Drying system (100) according to one of claims 1 to 6, wherein the device (80) is further designed to return a first vapor part to the burner (10) and a second vapor part to a mixing chamber (30), wherein the mixing chamber (30) is designed to mix drying gas provided by the burner (10) with the vapor part returned by the device (80) and to provide it to the drying chamber (22).

8. Plant (1) for producing OSB, comprising: a device (2a, 2b) for producing OSB strands; the drying plant (100) according to one of claims 1 to 7 for drying the OSB strands; a device (4a, 4b) for applying glue to the dried OSB strands; a device (5) for oriented stratification of the glued OSB strands into several layers; and a pressing device (6) for pressing the layers into an OSB.

9. A method for reducing pollutants produced during the drying of OSB strands in an OSB strand drying plant (100), the method comprising: Returning at least part of the vapor produced during the drying of OSB strands to a burner (10) of the drying plant (100); Maintaining the vapor portion to be returned to the burner (10) at a temperature above the condensation temperature of the water vapor contained in the vapor portion; and burning the pollutants contained in the returned vapor portion in the burner (10).

10. The process according to claim 9, wherein the temperature of the recirculated vapor portion is maintained above the condensation temperature at all times during recirculation.

11. The process according to claim 9 or 10, wherein the temperature of the vapor portion to be recycled is maintained at at least 100°C, preferably at a temperature in the temperature range from 100°C to 135°C, more preferably at a temperature in the temperature range from 115°C to 125°C.

12. The method according to any one of claims 9 to 11, wherein the step of maintaining the temperature above the condensation temperature comprises optionally preheating a return line (82) provided for returning the vapor, and, optionally, wherein the preheating comprises introducing preheating air into the at least one return line (82).

13. The method according to any one of claims 9 to 12, wherein the step of returning comprises returning a first vapor portion to the burner (10), in particular to the combustion chamber (12) and / or to the burner muffle (11), and a second vapor portion to a mixing chamber (30).

14. The method according to claim 13, wherein the step of recirculating comprises controlling the volume flow of the first vapor portion recirculated to the burner (12) and the second vapor portion recirculated to the mixing chamber (30).

15. A process according to any one of claims 9 to 14, wherein the step of burning comprises burning the pollutants contained in the recirculated vapor portion at temperatures in the range of 750°C to 1200°C.

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