Techniques for reducing contaminants in drying equipment

Abstract

This invention relates to a method for reducing contaminants generated during the drying of moist materials in a drying apparatus (100). The method includes the steps of: recirculating at least a portion of the dryer exhaust generated during the drying of the moist material back to a burner (10) of the drying apparatus (100), maintaining the temperature of the portion of the dryer exhaust to be recirculated to the burner (10) above the condensation temperature of the water vapor contained in the dryer exhaust; and burning the contaminants contained in the recirculated dryer exhaust in the burner (10). The invention also relates to a recirculation device (80) and a wood chip drying apparatus (100) including the recirculation device for carrying out the method.

Description

Technical Field

[0001] This invention relates to the field of environmental technology. In particular, this invention relates to a technique for reducing pollutant emissions generated during the drying of wet materials in a drying equipment. Background Technology

[0002] Drying equipment is known to be constructed for drying moist materials, such as sawdust. This drying equipment has a dryer (e.g., a drum dryer) with a drying chamber into which the material to be dried is fed in a controlled manner. Furthermore, a controlled volumetric flow rate of hot drying gas (e.g., hot air) is introduced into the drying chamber, slowly conveying the material through it. The hot drying gas flows throughout the drying chamber, heating the material and absorbing moisture released during the process. At the outlet of the drying chamber, the dried material is separated from the drying gas (e.g., by a cyclone separator). The drying gas at the outlet of the drying chamber is saturated with water vapor and typically contains contaminants generated during the drying process; this is called dryer exhaust. The dryer exhaust can be purified and released into the environment as waste gas.

[0003] To improve the efficiency of drying equipment, it is also known to recirculate at least a portion of the dryer exhaust gas exiting the drying chamber outlet back to a mixing chamber upstream of the drying chamber inlet. The recirculated dryer exhaust gas (also known as recirculated dryer exhaust) is mixed with hot drying gas there. The gas mixture is then fed back into the drying chamber to dry the wet material. By adding recirculated dryer exhaust, less hot drying gas needs to be supplied by the burner. In particular, the waste heat from the recirculated dryer exhaust can be utilized, meaning the drying equipment can operate more efficiently and energy-savingly overall.

[0004] DE 40 23 518 A1 discloses a drying apparatus in which a portion of the dryer exhaust gas discharged from the outlet of the chamber is directly returned to the combustion chamber of the burner via a return gas line. The returned dryer exhaust gas portion (or the recirculated dryer exhaust portion) passes through a tower-shaped condenser scrubber, which cools the dryer exhaust gas portion below the condensation temperature (dew point temperature), thereby dehumidifying the dryer exhaust gas portion. This condensation process also partially purifies the dryer exhaust gas, as contaminants contained in the dryer exhaust gas are also condensed and precipitated. This means that the recirculated dryer exhaust gas portion, having been cooled below the dew point and at least partially dehumidified and purified, is sent back to the burner or combustion chamber. However, using a condenser scrubber is expensive; furthermore, a significant portion of the condensation heat is lost in the condenser scrubber or can only be recovered with considerable technical effort.

[0005] Furthermore, EP 2 230 477 A1 discloses a wood chip drying apparatus for drying wood chips, which has a recirculation device for returning dryer exhaust gas to the drying cycle. The recirculation device has a dryer exhaust gas heater with a regenerative and / or catalytic heat exchanger configured to heat the dryer exhaust gas to a relatively high temperature in the range of 720°C to 900°C, such that the organic matter or solid particles carried in the dryer exhaust gas are primarily oxidized (combusted). A portion of the heat-regenerated dryer exhaust gas is then fed into the burner as combustion air. Another portion of the heat-regenerated dryer exhaust gas can be fed into a mixing chamber located between the burner and the drying chamber. The use of catalytic heat exchangers is complex and costly.

[0006] Furthermore, US 5,983,521A discloses a drying apparatus in which dryer exhaust gas supplied at the outlet of the drying chamber is completely recirculated back into the drying cycle. Here, the dryer exhaust gas, or recirculated dryer exhaust gas, is split into two branch airflows, which are then recirculated back to different regions of the combustion chamber. Each of the two recirculated dryer exhaust gas branch airflows passes through a heat exchanger, where heat from the gas flow originating from the combustion chamber is transferred to the corresponding recirculated dryer exhaust gas branch airflow. One of the two heat exchangers, primarily heated by the burner, is a so-called high-temperature heat exchanger, while the other is a so-called low-temperature heat exchanger. However, this patent does not mention the temperature parameters of the two recirculated dryer exhaust gas branch airflows. This method of dryer exhaust gas recirculation is also technically complex because two heat exchangers and corresponding recirculation lines must be provided for the two branch airflows. Furthermore, this method involves complex control of the two recirculated dryer exhaust gas branch airflows.

[0007] Furthermore, WO 01 / 59381 A1 discloses a drying apparatus in which a portion of the dryer exhaust gas is recirculated back to the combustion chamber and a mixing chamber between the combustion chamber and the drying chamber. For this purpose, the recirculation line is divided into two branch lines, one leading to the combustion chamber and the other to the mixing chamber. A fan for controlling the recirculation dryer exhaust gas flow is located in the common section of the recirculation line, i.e., upstream of the two branch lines. However, this makes it difficult to control the flow of the recirculation dryer exhaust gas towards the combustion chamber. Additionally, the drying apparatus described in WO 01 / 59381 A1 uses a belt dryer. Such dryers are known to operate at relatively low temperatures (<100°C) but require a large drying gas flow (approximately three times the drying gas flow required by a drum dryer). Because the operating temperature of the drying apparatus is moderate, the temperature of the recirculation dryer exhaust gas flow is also moderate; this creates a risk that moisture in the recirculation dryer exhaust gas flow may condense on the combustion chamber walls, thereby damaging the burner.

[0008] Although, as mentioned above, many drying devices are known to have recirculation mechanisms for recycling and recovering dryer exhaust gas to improve the efficiency of such devices and reduce pollutant emissions, improvements are still needed. In particular, the present invention aims to provide a technology that further reduces pollutant emissions from drying systems (especially wood chip drying systems) and is also cost-effective and simple to design. Furthermore, the provided technology should be easily integrated into existing wood chip drying equipment. Summary of the Invention

[0009] To address the aforementioned problems, a first aspect of the present invention provides a method for reducing contaminants generated during the drying of moist materials in a drying apparatus. The method includes the steps of: recirculating at least a portion of the dryer exhaust generated during the drying of the moist material back to the burner of the dryer; maintaining the temperature of the recirculated dryer exhaust portion in the burner above the condensation temperature of the water vapor contained in the dryer exhaust portion; and burning the contaminants contained in the recirculated dryer exhaust portion in the burner.

[0010] The moist material to be dried is preferably wood chips; therefore, the drying equipment can be a wood chip drying equipment.

[0011] The dryer exhaust saturated with water vapor, generated during the drying of damp materials and discharged at the outlet of the drying equipment (more precisely, at the outlet of the drying chamber), is called dryer exhaust. Dryer exhaust recirculated back to the burner, or the dryer exhaust section, is subsequently also referred to as recirculated dryer exhaust or recirculated dryer exhaust section.

[0012] Dryer exhaust or recirculation dryer exhaust can be contaminated with pollutants, depending on the material being dried. These pollutants can primarily consist of environmentally polluting volatile organic compounds (VOCs) and / or particulate matter. However, these pollutants can also include other compounds, such as nitrogen-based compounds (e.g., nitrous oxide N₂O or ammonia NH₃), which also have significant negative environmental impacts. If the material being dried is sawdust, the pollutants carried in the dryer exhaust or recirculation dryer exhaust may include terpenes ((C₅H₈)n, where n ≥ 2).

[0013] Dry air can be used as the preferred drying gas. Other drying gases are also conceivable.

[0014] Preferably, the temperature of the recirculating dryer exhaust section is maintained above the condensation temperature / dew point temperature throughout the recirculation process. In other words, the method according to the invention operates in such a way that the temperature of the recirculating dryer exhaust section does not fall below the condensation temperature throughout the entire recirculation process. By consistently maintaining the temperature of the recirculating dryer exhaust section above the condensation temperature, water vapor contained in the recirculating dryer exhaust section is prevented from condensing in the recirculation line or burner used for recirculation. This prevents valuable condensation heat from being lost during recirculation and also prevents moisture from condensing in the recirculation line or on the combustion chamber walls, which could cause long-term damage to the burner.

[0015] Furthermore, the process can be operated in such a way that the temperature of the recirculated dryer exhaust section (recirculated dryer exhaust section) is maintained at at least 100°C. Preferably, the temperature of the recirculated dryer exhaust section can be maintained at at least 105°C. More preferably, the temperature of the recirculated dryer exhaust section can be maintained at at least 110°C or higher. In a particularly simple embodiment of the method, the temperature of the recirculated dryer exhaust section can be maintained at a temperature above the condensation temperature, but not higher than the temperature at which the dryer exhaust leaves the drying chamber of the drying equipment. This makes the method particularly easy to implement because no additional heat exchanger is required to increase the temperature of the recirculated dryer exhaust.

[0016] To ensure that the exhaust gas from the recirculation dryer is always above the condensation temperature of water vapor, the recirculation line provided for recirculating the dryer exhaust gas, or at least a portion of the recirculation line provided for this purpose, can be preheated as needed. This preheating step is optional and performed as required; for example, if the recirculation line has already "cooled down" due to the colder external temperature when starting the dryer, there is a risk that the recirculation dryer exhaust gas section will be below the condensation temperature when recirculating to the burner.

[0017] For preheating, preheated gas (e.g., dehumidified preheated air) can be introduced into at least one return line or before the dryer exhaust section recirculates. The temperature of the preheated gas can be lower than the temperature of the recirculated dryer exhaust, but cannot deviate too much from it. Preferably, the temperature of the preheated gas can be in the range of 70°C to 100°C, and even more preferably in the range of 80°C to 90°C. This allows the return line to reach the desired preheating temperature. This allows the recirculated dryer exhaust or the recirculated dryer exhaust section to have a specified holding temperature higher than the condensation temperature over the entire recirculation path.

[0018] The recirculation step may include directly returning a first portion of the dryer exhaust as combustion air or main air to the combustion chamber of the burner. This first recirculated exhaust (first recirculated dryer exhaust) can pass directly through the high-temperature zone of the combustion chamber. Due to the high temperature of the burner flame or combustion chamber, preferably in the temperature range of 700°C to 1200°C, most of the contaminants (VOCs, N2O, NH3, etc.) or solid particles contained in the first recirculated dryer exhaust are burned / oxidized. This is because it has been shown that contaminants contained in the recirculated dryer exhaust can only be effectively burned at temperatures above 700°C. The first dryer exhaust, already purified and heated in the combustion chamber, can be reused, along with other combustion air heated in the combustion chamber, to dry the moist material in the drying chamber.

[0019] The recirculation step may alternatively or additionally include recirculating at least a second dryer exhaust portion as cooling air back to the muffle of the burner. The second recirculated dryer exhaust portion fed into the muffle initially serves as cooling gas for the muffle. It may then also enter the combustion chamber, where it is further heated by the burner flame. Contaminants or particles carried by the second dryer exhaust are also burned / oxidized in the hot burner flame at temperatures ranging from 700°C to 1200°C, as described above.

[0020] As described herein, by burning pollutants in a burner flame at temperatures ranging from 700°C to 1200°C, the pollutant content in dryer exhaust / recirculated dryer exhaust can be significantly improved, thereby significantly improving the overall pollutant balance of the drying equipment. In the method according to the invention, no additional purification devices are required, such as condenser scrubbers or catalytic heat exchangers proposed in the prior art. However, as described herein, the direct recirculation of at least a portion of the dryer exhaust to the combustion chamber also saves energy, since the recirculated portion of the dryer exhaust already has a temperature greater than 100°C. This means that the burner must consume significantly less thermal energy than if only cold dryer gas (dry air) were fed into the burner and then had to be heated to the required operating temperature.

[0021] The volumetric flow rate of the exhaust gas from the first dryer entering the combustion chamber and / or the exhaust gas from the second dryer entering the muffle sleeve can preferably be adjusted according to the required volumetric flow rate from the burner output and / or the drying chamber. The volumetric flow rate of the dryer exhaust gas can be controlled by at least one conveying fan located on the return line. According to a second aspect of the invention, the above-described method is used to reduce contaminants in the wood chip drying equipment. In particular, the above-described method is suitable for effectively burning contaminants released during the drying of wood chips, such as terpenes (especially α-, β-pinene), which have low boiling points and are found in the dryer exhaust gas of the wood chip drying equipment.

[0022] According to a third aspect of the invention, an apparatus is provided for reducing contaminants generated in a drying apparatus during the drying of damp materials (particularly damp sawdust). The apparatus is configured to recirculate at least a portion of the dryer exhaust gas discharged from the drying chamber of the drying apparatus back to the burner of the drying apparatus to burn off contaminants contained in the dryer exhaust gas portion. The apparatus is also configured to maintain the temperature of the recirculated dryer exhaust gas portion above the condensation temperature of the water vapor contained in the dryer exhaust gas portion.

[0023] The device may include at least one recirculation line for returning a portion of the recirculation dryer exhaust to the burner (or the burner's combustion chamber). A first end of the recirculation line may be connected to the outlet of the drying chamber, and a second end to the burner or the burner's combustion chamber. Alternatively, if a recirculation dryer exhaust passage is already provided in the drying system, for example, a recirculation dryer exhaust passage that directs a portion of the recirculation dryer exhaust to a mixing chamber upstream of the drying chamber, the first end of the recirculation line may be connected to the recirculation dryer exhaust passage. This makes the recirculation device particularly simple and cost-effective to implement, as existing piping can also be used.

[0024] With further development, at least one return line can split into two branches at its second end. The first branch leads to the combustion chamber of the burner, directly guiding the first portion of the return dryer exhaust into the combustion chamber (the high-temperature zone). The second branch can lead to the muffler of the burner and be used to cool the muffler. The second portion of the return dryer exhaust is then also guided into the high-temperature zone of the combustion chamber. In both cases, the two portions of the return dryer exhaust are heated to high temperatures in the combustion chamber, thus burning off the contaminants they carry. The dryer exhaust purified in this way can then be used in the drying chamber for re-drying.

[0025] At least one return line can be insulated. Insulation helps ensure that the return dryer exhaust section, which is guided to the burner in the return line, loses very little heat and can always be kept above the condensation temperature.

[0026] The apparatus may also include a preheating device configured to preheat the recirculation line as needed. This may be necessary, for example, when the dryer is started up and at least one recirculation line is “cooled” despite being insulated due to the low ambient temperature. The preheating device can be activated to prevent the temperature of the recirculation dryer exhaust section directed to the burner from dropping below the condensation temperature. This can preheat at least one recirculation line to the desired temperature, as described in the related methods above.

[0027] According to one embodiment, the preheating device may include a preheating gas source, which can be selectively connected to the return line of the return device to introduce preheating gas into the return line. The flow of preheating gas through the return line can bring the return line to the desired temperature. Specifically, dehumidified preheated air can be used as the preheating gas.

[0028] The recirculation device may also include at least one delivery fan disposed in at least one recirculation line. The at least one delivery fan may be configured to deliver (move) the recirculation dryer exhaust portion to the burner. Therefore, the volumetric flow rate of the recirculation dryer exhaust portion flowing in the at least one recirculation line can be (actively) regulated or controlled by at least one fan. The recirculation dryer exhaust flow rate can be adjusted based on the burner output or based on the required volumetric flow rate of the drying gas in the drying chamber.

[0029] According to a fourth aspect of the invention, a wood chip drying apparatus is provided for drying damp wood chips. The wood chip drying apparatus includes: a dryer having a drying chamber for drying damp wood chips by introducing hot drying gas into the drying chamber; a burner having a combustion chamber for providing the hot drying gas; and a device according to a third aspect for recirculating at least a portion of the dryer exhaust gas discharged from the drying chamber back to the burner so as to burn contaminants contained in the recirculated dryer exhaust gas portion in the burner.

[0030] By recirculating at least a portion of the dryer exhaust from the drying chamber back to the burner, the pollutant load on the wood chip drying equipment can be significantly reduced, as the high flame temperature in the burner efficiently combusts volatile organic compounds and solid particles.

[0031] For example, the dryer can be a drum dryer, in which the drying gas supplied by the burner flows through the drying chamber of the dryer, and the dryer conveys the wet material introduced into the drying chamber through the drying chamber, thereby drying the wet material.

[0032] The burner can be a gas burner, oil burner, solid fuel burner, or multi-fuel burner, configured to produce a burner temperature or flame temperature in the combustion chamber ranging from 700°C to 1200°C. Due to the high flame temperature, the burner is suitable for use in dryer exhaust sections containing contaminants in oxidation / combustion recirculation processes.

[0033] Wood chip drying equipment may also have a mixing chamber located between the burner and the dryer. The mixing chamber is configured to mix the hot drying gas supplied by the burner with the return dryer exhaust gas and / or external drying gas that is fed into the mixing chamber before sending it into the drying chamber.

[0034] Wood chip drying equipment may also include a separation device located at the outlet of the drying chamber and configured to separate the dried material from the dryer exhaust. For this purpose, the separation device may include one or more cyclone separators.

[0035] Wood chip drying equipment may also include a filtration device configured to filter out contaminants from the exhaust portion of the recirculating dryer, which is released into the environment and therefore does not flow back to the burner through the aforementioned device, and can be reused in the drying process. Attached Figure Description

[0036] Further details and advantages of the invention are described below with reference to the non-limiting embodiments shown in the accompanying drawings.

[0037] Figure 1 A schematic diagram of a wood chip drying apparatus according to the present invention is shown; and Figure 2 A flowchart illustrating a method for reducing contaminants in dryer exhaust gas according to the present invention is shown. Detailed Implementation

[0038] Figure 1 A schematic diagram of a drying apparatus 100 according to the present invention is shown. The drying apparatus 100 is a wood chip drying apparatus configured for drying moist wood chips.

[0039] The wood chip drying equipment 100 includes a burner 10 with a combustion chamber 12, a dryer 20 with a drying chamber 22, and a mixing chamber 30 disposed between the combustion chamber 12 and the drying chamber 22. The wood chip drying equipment 100 also includes a separation device 40 disposed at the outlet of the drying chamber 22 for separating dryer exhaust and dried wood chips, a return dryer exhaust pipeline 50 connected to the mixing chamber 30, and a dryer exhaust discharge device 60 with a filter device 62 for filtering pollutants contained in the dryer exhaust discharged into the environment.

[0040] According to the invention, the wood chip drying equipment 100 also includes a recirculation device 80 that can be coupled to or connected to the burner 10. The recirculation device 80 is designed to direct at least a portion of the recirculated dryer exhaust (also referred to as recirculated dryer exhaust) from the recirculated dryer exhaust line 50 to the hot burner flame of the burner 10. This allows contaminants contained in the recirculated dryer exhaust (particularly VOCs and / or solid particles, which increase in quantity during wood chip drying) to be oxidized / burned in the burner 10 or the hot burner flame, thereby significantly reducing the overall pollutant emissions of the wood chip drying equipment 100. The following is in conjunction with... Figure 1 and Figure 2 The structure and operation of the reflux device 80 are described in more detail.

[0041] Burner 10 is designed to provide hot, dry gas. Hot, dry air is preferred. For this purpose, ambient air is supplied to combustion chamber 12, where it is heated to the desired temperature by the burner flame. Gas burners, oil burners, dust burners, or multi-fuel burners can be used as burners, configured to produce flame temperatures in the combustion chamber ranging from 700°C to 1200°C.

[0042] The heated, dry gas exiting combustion chamber 12 is fed into mixing chamber 30, where it can be mixed with a portion of the dryer exhaust gas (return dryer exhaust portion) from drying chamber 22 and returning via return dryer exhaust line 50. By recirculating and reusing at least a portion of the dryer exhaust gas exiting drying chamber 22, the energy efficiency of the wood chip drying equipment 100 can be significantly improved. Optionally, another gas stream (air stream) containing preheated gas (air) can be fed into mixing chamber 30. Figure 1 (Not shown in the image), the gas flow is released, for example, in an external device (e.g., wood processing equipment) operating outside the sawdust drying equipment 100. This can further improve the energy balance of the drying equipment 100.

[0043] By mixing the heated dry gas supplied from combustion chamber 12 with the dryer exhaust gas (and optionally additional airflow) that flows back to mixing chamber 30 via return dryer exhaust line 50, the temperature of the dry gas (dry gas stream) generated in mixing chamber 30 and supplied to drying chamber 22 is reduced. However, by properly mixing and adjusting the gas supply from combustion chamber 12, the temperature is regulated so that the dry gas generated in mixing chamber 30 has a desired temperature value approximately 20°C to 40°C higher than the temperature at the dryer inlet. This higher temperature in mixing chamber 30 is necessary to compensate for heat losses due to convection and radiation.

[0044] The dryer 20 is preferably a drum dryer, which operates in a temperature range of 230°C to 500°C depending on the quantity, properties, and composition of the wood chips. Therefore, the temperature of the drying gas supplied to the mixing chamber 30 is in the range of 20°C to 40°C above this temperature. Such a temperature is sufficient to dry the wood chips.

[0045] Moist sawdust is fed into drying chamber 22 in a controlled manner. Figure 1 (Not shown in the image). Furthermore, depending on the quantity and properties of the wood chips to be dried, a volumetric flow of drying gas is supplied from the mixing chamber 30 to the drying chamber 22. The hot drying gas supplied to the drying chamber 22 flows throughout, heating the wood chips and absorbing moisture released during the process. The gas flow slowly transports the wood chips through the drying chamber 22.

[0046] The operating temperature, which is in the range of 230°C to 500°C, is essentially controlled by the burner 10 (e.g., by controlling the fuel supply and thus the burner output). In any case, the temperature in the drying chamber 22 is always set in such a way that the moisture and contaminants released during the drying process cannot be recondensed in the drying chamber 22.

[0047] Therefore, the dryer exhaust gas flowing out of the outlet of drying chamber 22 has a relatively high temperature, higher than the water vapor condensation temperature. Typical temperatures of the dryer exhaust gas at the outlet of drying chamber 22 are above 100°C, preferably in the temperature range of greater than 100°C to 125°C. After the sawdust carried in the dryer exhaust gas is separated in the separation device 40, at least a portion of the dryer exhaust gas can be returned to the mixing chamber 30 via the return dryer exhaust gas line 50. This returned dryer exhaust gas section (also referred to as the return dryer exhaust section) is thus returned to the drying process, and due to its high energy content (no additional evaporative heat is required), the energy efficiency of the sawdust drying equipment 100 is improved.

[0048] However, the problem is that, due to the relatively low temperatures (maximum 500°C to 550°C), contaminants (VOCs, N2O, NH3, particulate matter) carried in the recirculating dryer exhaust cannot be oxidized / burned in the mixing chamber 30. This creates the risk that increasing amounts of contaminants will accumulate in the recirculating dryer exhaust, which flows back to the mixing chamber 30 and can be reused in the drying chamber 22. This can lead to undesirable accumulation of contaminants in the sawdust to be dried.

[0049] To avoid this situation, and especially to reduce the contaminant load on the wood chip drying equipment 100 in the long term, the wood chip drying equipment 100 is equipped with a reflux device 80 according to the present invention.

[0050] The recirculation device 80 includes at least one recirculation line 82, with a first end fluidly connected to the recirculation dryer exhaust line 50 and a second end fluidly connected to the burner 10. The at least one recirculation line 82 is configured to recirculate at least a portion of the recirculation dryer exhaust back to the burner 10. The recirculation device 80 also includes at least one conveying fan 84a, 84b, disposed within the at least one recirculation line 82. The volumetric flow rate of the recirculated dryer exhaust portion can be (actively) controlled or regulated by the at least one conveying fan 84a, 84b.

[0051] exist Figure 1In the illustrated embodiment, the reflux device 80 has a reflux line 82, which is divided into two branches 82a and 82b leading to the burner 10. The first branch 82b leads directly to the combustion chamber 12, while the second branch 82a connects to the muffle sleeve of the burner 10. Therefore, a first portion of the reflux dryer exhaust can be directly introduced into the combustion chamber 12 of the burner 10 as combustion gas or combustion air via the first branch 82b. Furthermore, a second portion of the reflux dryer exhaust can be sent into the muffle sleeve of the burner 10 as cooling gas or cooling air via the second branch 82a. Then, the second portion of the reflux dryer exhaust is also sent into the combustion chamber 12, where it is further heated and purified.

[0052] To control the volumetric flow rate of the exhaust gas from the reflux dryer in the reflux device 80, a first conveying fan 84a is located in the common reflux line 82. This conveying fan 84a can be used to regulate the volumetric flow rate of the exhaust gas from the reflux dryer that is fed into the burner 10 as a whole. Furthermore, a second conveying fan 84b can be directly located in the branch line 82b, through which the exhaust gas from the first reflux dryer is directly fed into the combustion chamber 12. The second conveying fan 84b can be used to regulate or control the volumetric flow rate fed into the combustion chamber 12.

[0053] For example, the wood chip dryer 100 can be operated such that 60% of the dryer exhaust volume flow rate discharged from the outlet of the drying chamber 22 is recirculated with the aid of the recirculation device 80. 15% of the dryer exhaust volume flow rate can be directly fed into the combustion chamber 12 as combustion air; while another 15% can be fed into the burner muffle as cooling air. As described above, the volume flow rate ratio is controlled by the delivery fans 84a and 84b. The remaining 30% of the dryer exhaust volume flow rate discharged from the outlet of the drying chamber 22 can be supplied to the mixing chamber 30 via the recirculation dryer exhaust line 50. This means that 60% of the dryer exhaust volume flow rate discharged from the outlet of the drying chamber 22 is recycled, which significantly improves the energy balance of the wood chip dryer 100. 30% of this is supplied to the burner 10 for purification, which also significantly improves the pollutant balance of the wood chip dryer 100, particularly by reducing emissions below legal emission limits (see also the test results in the table below).

[0054] It is obvious that Figure 1The reflux device 80 shown is an example of one possible implementation. Another reflux device 80 is conceivable, in which the reflux line 82 leads only the exhaust portion of the reflux dryer to the combustion chamber 12 of the burner 10. In this case, the reflux line 82 does not have two branch lines 82a, 82b, resulting in a particularly simple design. With this simplified design, a single delivery fan on the reflux line 82 is sufficient to control the volumetric flow rate of the exhaust portion of the reflux dryer to be refluxed.

[0055] The reflux device 80 may also include a preheating device 86. The preheating device 86 is configured to maintain at least one reflux line 82 at a desired temperature level. This prevents the exhaust portion of the reflux dryer from cooling to a temperature below the condensation temperature (dew point temperature) when refluxing through at least one reflux line 82, and prevents water vapor or contaminants carried therewith from condensing uncontrollably along the reflux line 82 or in the combustion chamber 12.

[0056] exist Figure 1 In the illustrated embodiment, the preheating device 86 has a preheating gas source or is connected to an external preheating gas source. The (external) preheating gas source is fluidly connected to the return line 82 (through line section 86a). The preheating gas source 86 is configured to supply warm gas or warm air into the return line 82 for preheating the return line 82. Although the return line 82 is insulated, it may "cool down" at cold ambient temperatures, especially when starting the sawdust drying equipment 100. In this case, the preheating gas source 86 can be activated as needed to supply preheated gas into the return line 82.

[0057] The temperature of the preheated gas supplied by the preheated gas source 86 can be no significantly lower than the temperature of the reflux dryer exhaust gas, preferably in the temperature range of 70°C to 100°C, more preferably in the temperature range of 80°C to 90°C. This temperature is generally sufficient to preheat the reflux line 82, thus preventing the reflux dryer exhaust gas from cooling below the condensation temperature.

[0058] The reflux device 80 described herein is characterized by its simple and compact structure, and is therefore suitable for retrofitting existing wood chip drying equipment to improve the pollutant emission balance of such equipment.

[0059] Combination Figure 2 The method for reducing contaminants in a drying apparatus according to the present invention is described in more detail. This method can be implemented using the aforementioned reflux device 80 and can be used, for example, in a wood chip drying apparatus to reduce contaminant emissions.

[0060] According to step S20, at least a portion of the dryer exhaust generated during the drying of moist materials (e.g., sawdust in a sawdust drying device) is returned to the burner 10 via a return device 80, specifically via at least one return line 82 of the return device 80. With the aid of the return device 80, the temperature of the returned dryer exhaust portion returned to the burner 10 is maintained above the condensation temperature of the water vapor contained in the dryer exhaust (second step S22). This can be achieved, on the one hand, by providing appropriate insulation to at least one return line 82. However, if necessary, a preheating device 86 can also be used to preheat at least one return line 82 to the desired temperature.

[0061] In the third step S24, the recirculated exhaust gas from the reflux dryer is then heated to a high temperature in the burner 10. Specifically, the exhaust gas from the reflux dryer is passed through the high-temperature zone of the combustion chamber 12 (a zone with a temperature in the range of 700°C to 1200°C), whereby the pollutants carried therein are effectively oxidized / burned.

[0062] The purpose of the method and reflux device 80 described herein is to significantly reduce emissions from the drying equipment, particularly bringing them well below legal requirements. The technology (method and reflux device) according to the invention described herein, implemented in experimental wood chip drying equipment in Switzerland, demonstrates that emissions can be significantly reduced in ways that would not be possible without the reflux device 80 (i.e., if only a portion of the reflux dryer exhaust flows back into the mixing chamber 30 via the reflux dryer exhaust line 50).

[0063] The experimental wood chip drying equipment features a dust burner designed for burner / flame temperatures up to 1200°C. A drum dryer is used as the dryer and is designed to dry gas volumetric flow rates up to 290,000 m³ / h and material throughput (wood chip throughput) up to 52,000 kg / h. However, the invention is not dependent on these specific limits. A mixing chamber is provided between the burner and the drum dryer, configured to mix the drying gas heated by the burner with a portion of the dryer exhaust gas exiting the dryer outlet, which is then supplied as drying gas to the drum dryer. Furthermore, the experimental equipment includes the aforementioned recirculation device to allow another portion of the dryer exhaust gas exiting the dryer outlet to be directly recirculated back to the burner for heating and purification.

[0064] The table below shows the reduction of pollutants, particularly VOCs (see total carbon at 18% O2) and nitrogen-based pollutants (see NO at 18% O2), when the test equipment is operated at half load and near full load. x OK).

[0065]

[0066] This table describes the concentrations (in mg / m³) of these contaminants in different throughputs of dried material used in particleboard production (70% softwood and 30% hardwood in this example). 3 (Calculation). As can be seen from the second and third columns of the table, significant reductions in pollutants can be achieved at both moderate sawdust throughput (47% dust burner operating load) and near-full load (85% dust burner operating load). According to the Swiss Air Pollution Control Ordinance (Luftreinhaltungsverordnung; LRVSchweiz), the values ​​of nitrogen-based and carbon-based pollutants are converted to 18 volume percentages (vol.%) of oxygen. For example, the statutory requirement for VOCs is 120 mg / m³. 3 According to the technology of the present invention, the VOC concentration can be reduced to 100 mg / m³. 3 Below (89 mg / m³ at half load) 3 93 mg / m³ at near full load 3 NO x Pollutant levels also saw a similar reduction.

[0067] These values ​​are achieved during the operation of the wood chip drying equipment, where approximately 30% of the volumetric flow rate of the dried gas discharged from the drum dryer outlet (dryer exhaust volumetric flow rate) is returned to the burner for heating and purification with the aid of a recirculation device. The temperature in the high-temperature zone (Zone 1) of the combustion chamber exceeds 700°C (729°C at half load and 849°C at near full load), and the dryer outlet temperature is maintained at 108°C and 111°C respectively (i.e., above the dew point temperature of water vapor). The dryer outlet temperature also corresponds to the temperature recirculation of the dryer exhaust volumetric flow rate (recirculated dryer exhaust volumetric flow rate) to the burner.

[0068] The technique described herein for reducing contaminants using the recirculation device 80 according to the invention has several advantages over existing technologies. It is simple, space-saving, and inexpensive because it requires no additional cleaning equipment. Instead, at least a portion of the recirculation dryer exhaust is directly returned to the existing burner of the drying equipment (wood chip dryer) and cleaned there. This purification significantly reduces contaminant emissions from the drying equipment. Furthermore, it prevents contaminants from accumulating in the material being dried, which would be possible, for example, if a portion of the recirculation dryer exhaust were simply returned to the mixing chamber and then fed into the drying chamber without purification.

[0069] However, the technology described here also overcomes the long-standing prejudice that water vapor-saturated dryer exhaust cannot be directly returned to the burner or combustion chamber, as this would hinder the stable operation of the drying equipment.

Claims

1. A method for reducing contaminants in a drying apparatus (100), said contaminants being generated during the drying of wet material in said drying apparatus (100), said method comprising: At least a portion of the dryer exhaust generated during the drying of the wet material is returned to the burner (10) of the drying equipment (100). Maintain the temperature of the dryer exhaust section to be returned to the burner (10) above the condensation temperature of the water vapor contained in the dryer exhaust section; and The contaminants contained in the dryer exhaust that is burned back in the burner (10).

2. The method of claim 1, wherein the temperature of the recirculated dryer exhaust section is maintained above the condensation temperature throughout the recirculation period.

3. The method according to claim 1 or 2, wherein the holding temperature is at least 100°C, preferably at least 105°C, and even more preferably at least 110°C.

4. The method according to any one of claims 1 to 3, wherein the step of maintaining the temperature above the condensation temperature includes optionally preheating the return line (82) for returning the exhaust gas from the dryer.

5. The method of claim 4, wherein preheated air is introduced into at least one return line (82) for preheating.

6. The method according to any one of claims 1 to 5, wherein the recirculation step comprises recirculating the first dryer exhaust portion directly as combustion air to the combustion chamber (12) of the burner (10) and / or recirculating the second dryer exhaust portion as cooling air to the muffle of the burner (10).

7. The method according to claim 6, wherein the volumetric flow rate of the first dryer exhaust portion supplied to the combustion chamber (12) and / or the volumetric flow rate of the second dryer exhaust portion supplied to the muffle are controlled according to the burner output and / or the required volumetric flow rate of the drying chamber (22).

8. The method according to any one of claims 1 to 7, wherein the combustion step comprises burning the contaminants contained in the recirculated dryer exhaust at a temperature in the range of 750°C to 1200°C.

9. Use of the method for reducing contaminants in a wood chip drying apparatus (100) according to any one of claims 1 to 8.

10. An apparatus (80) for reducing contaminants generated in a drying apparatus (100) during the drying of wet materials, particularly during the drying of wet sawdust, wherein the apparatus (80) is configured to recirculate at least a portion of the dryer exhaust gas discharged from the drying chamber (22) of the dryer (100) to the burner (10) of the dryer (100) to burn off the contaminants contained in the dryer exhaust gas portion, and wherein the apparatus (80) is further configured to maintain the temperature of the dryer exhaust gas portion to be recirculated above the condensation temperature of the water vapor contained in the dryer exhaust gas portion.

11. The apparatus (80) according to claim 10, wherein the apparatus (80) comprises at least one return line (82), the first end of which is connected to the outlet of the drying chamber (22) or the return dryer exhaust line (50) of the drying equipment (100), and the second end of which is connected to the burner (10) of the drying equipment (100).

12. The apparatus (80) of claim 11, wherein at least one return line (82) is insulated and / or wherein the apparatus (80) further comprises a preheating device (86) configured to preheat the at least one return line (82) as needed.

13. The apparatus (80) according to claim 12, wherein the preheating device (86) includes a preheating source selectively connected to at least one return line (82) of the return device to introduce preheated gas into the return line (82).

14. The apparatus (80) according to any one of claims 11 to 13, wherein the apparatus further comprises at least one conveying fan (84a, 84b) disposed in the at least one return line (82) and configured to regulate the volumetric flow rate of the dryer exhaust portion to be returned.

15. A wood chip drying apparatus (100) for drying moist wood chips, comprising: The dryer (20) has a drying chamber (22) for drying damp sawdust by introducing hot drying gas into the drying chamber (22); The burner (10) has a combustion chamber (12) for supplying the hot drying gas; as well as The apparatus (80) according to any one of claims 10 to 14 is used to recirculate at least a portion of the dryer exhaust gas discharged from the drying chamber (22) back to the burner (10) so as to burn contaminants contained in the dryer exhaust gas portion in the burner (10).

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