Method for reducing contaminants in drying systems

Abstract

The invention relates to a method for reducing contaminants in a drying system (100), said contaminants arising during the drying process of humid material in the drying system (100). The method has the steps of returning at least one part of the exhaust vapor produced during the process of drying the humid material to a burner (10) of the drying system (100), maintaining the temperature of the exhaust vapor part to be returned to the burner (10) at a temperature above the condensation temperature of the water vapor contained in the exhaust vapor part, and combusting the contaminants contained in the returned exhaust vapor part in the burner (10). The invention additionally relates to a return device (80) and to a wood chip drying system (100), which has the return device, for carrying out the method.

Description

[0001] Technology for reducing pollutants in drying plants

[0002] Technical area

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

[0004] background

[0005] Drying systems designed for drying moist material, such as moist wood chips, 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, a large number of drying systems are known that have recirculation devices for recirculating and recycling vapors in order to increase the efficiency of such systems on the one hand and reduce pollutant emissions on the other, further improvement is still required. In particular, the object of the present invention is to provide a technology that further reduces pollutant emissions from drying systems, in particular from wood chip drying systems, and on the other hand is cost-effective and simple. Furthermore, the provided technology should be easy to retrofit into existing wood chip drying systems.

[0010] Brief outline

[0011] To achieve the above-mentioned object, according to a first aspect of the invention, a method for reducing pollutants produced during the drying of moist material in a drying plant is provided. The method comprises the following steps: recirculating at least a portion of the vapor produced during the drying of the moist material 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.

[0012] The moist material to be dried may preferably be wood chips; accordingly, the drying plant may be a wood chip drying plant.

[0013] Vapor refers to the steam-saturated drying exhaust gas that is produced during the drying of the moist material and exits the drying system (more precisely, the drying chamber). The vapor or vapor fraction returned to the burner is also referred to as the return vapor or return vapor fraction.

[0014] Depending on the material being dried, the vapor or back vapor can be contaminated with pollutants. These pollutants can primarily include volatile organic / carbon-based compounds (so-called VOCs - Volatile Organic Compounds) and / or solid particles that pollute the environment. However, these pollutants can also include other compounds, such as nitrogen-based compounds (e.g., nitrous oxide N2O or ammonia NH3), which also have a significant impact on the environment. If the material being dried is wood chips, the pollutants carried in the vapor or back vapor can include, for example, terpenes (CsHsIn with n > 2).

[0015] Drying air is the preferred drying gas. Other drying gases are also conceivable.

[0016] 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. Keeping the temperature of the recirculated vapor portion above the condensation temperature at all times prevents the water vapor contained in the recirculated vapor portion from partially condensing in the recirculation line provided for recirculation or in the burner. This prevents valuable condensation heat from being lost during recirculation; and it also prevents moisture from condensing in the recirculation line or on the combustion chamber walls, which could lead to long-term damage to the burner.

[0017] Furthermore, the process can be operated such that the holding temperature of the vapor portion to be recirculated (return vapor portion) is at least 100°C. Preferably, the holding temperature of the return vapor portion can be at least 105°C. Even more preferably, the holding temperature of the return vapor portion can be at least 110°C or even higher. In a particularly simple implementation of the process, the return vapor portion can be maintained at a temperature that is higher than the condensation temperature, but not higher than the temperature of the vapor upon leaving the drying chamber of the drying plant. This makes the process particularly simple to implement, since no additional heat exchangers are required to increase the temperature of the return vapor.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.

[0018] For preheating, for example, a preheating gas (e.g., dehumidified preheating air) can be introduced into the at least one return line or before the vapor portion is returned. The preheating gas can have a temperature that is lower than the temperature of the recirculating vapor, but does not deviate too significantly from it. The preheating gas can preferably have a temperature in the range of 70°C to 100°C, more preferably in the range of 80°C to 90°C. This allows the return line to be brought to a desired preheating temperature. This enables the recirculating vapor or recirculating vapor portion to maintain a predetermined temperature above the condensation temperature over the entire return path.

[0019] The recirculation step can involve recirculating a first vapor portion as combustion air or main air directly into the burner's combustion chamber. The recirculated first vapor portion (first return vapor portion) can be passed directly through the high-temperature zone in the combustion chamber. Due to the high temperature of the burner flame or the combustion chamber, which is preferably in the temperature range of 700°C to 1200°C, the pollutants (VOCs, N2O, NH3, etc.) or solid particles contained in the first return vapor portion are largely combusted / oxidized. It has been shown that the pollutants contained in the return vapor can only be effectively combusted at temperatures above 700°C. The first vapor portion, cleaned and heated in the combustion chamber, can be made available again for the drying of moist material in the drying chamber, together with additional combustion air heated in the combustion chamber.The recirculation step may alternatively or additionally comprise returning at least a second vapor portion as cooling air to the muffle of the combustor. The second vapor portion supplied to the muffle initially acts as a cooling gas for the muffle. The second vapor portion can then also enter the combustion chamber, where it is further heated by the burner flame. The pollutants or particles entrained in the second vapor portion are also combusted / oxidized in the hot burner flame at temperatures in the range of 700 °C to 1200 °C, as described above.

[0020] Through the combustion of pollutants in the burner flame described here at a temperature in the range of 700 °C to 1200 °C, the pollutant content in the vapor / return vapor and thus the overall emission balance of drying plants can be significantly improved. Additional cleaning devices, such as a condensation scrubber or catalytic heat exchanger, as proposed in the prior art, are not necessary in the process according to the invention. However, the direct recirculation of at least a portion of the vapor into the combustion chamber described here also saves energy, since the recirculated vapor portion already has a temperature greater than 100 °C. Thus, the burner needs to consume significantly less heat energy than if only cold drying gas (drying air) is supplied to the burner, which then has to be heated to the desired operating temperature.

[0021] The volume flow of the first vapor portion fed to the combustion chamber and / or the volume flow of the second vapor portion fed to the muffle can preferably be regulated depending on the burner output and / or the volume flow required in the drying chamber. The volume flow of the vapor portion can be regulated by at least one conveying fan arranged in the return line. According to a second aspect of the invention, the method described above is used to reduce pollutants in a wood chip drying system. In particular, the method described above is suitable for effectively combusting pollutants released during the drying of wood chips, such as terpenes (in particular α-, β-pinenes), which have a low boiling point and are found in the vapors from wood chip drying systems.According to a third aspect of the invention, a device is provided for reducing pollutants that arise in a drying system during the drying of moist material, in particular moist wood chips. The device is designed to recirculate at least a portion of the vapor exiting a drying chamber of the drying system to a burner of the drying system in order to combust the pollutants contained in the vapor section. The device is further designed to maintain the temperature of the return vapor section at a temperature above the condensation temperature of the water vapor entrained in the vapor section.

[0022] To return the return vapor portion to the burner (or into the burner's combustion chamber), the device can comprise at least one return line. This can be coupled at its first end to the drying chamber outlet and at its second end to the burner or the burner's combustion chamber. If a return vapor channel is already provided in the drying system, which, for example, directs part of the return vapor to a mixing chamber upstream of the drying chamber, the return line can alternatively be coupled at its first end to the return vapor channel. This makes the return device particularly simple and cost-effective to implement, since existing lines are also utilized.

[0023] According to a further development, the at least one return line can split into two sub-lines at its second end, with a first sub-line leading to the combustion chamber of the burner in order to direct a first return vapor portion directly into the combustion chamber (high-temperature zone of the combustion chamber). A second sub-line can lead to the muffle of the burner and be used to cool the muffle. The second return vapor portion is then also directed into the high-temperature zone of the combustion chamber. In both cases, the two return vapor portions are heated to a high temperature in the combustion chamber, and the pollutants entrained are burned. The vapor portion thus purified can be made available for further drying of the drying chamber. The at least one return line can be thermally insulated.The thermal insulation helps to ensure that the return vapor part of the burner in the return line loses little heat and can be kept above the condensation temperature at all times.

[0024] The device can further comprise a preheating device designed to preheat the return device as needed. This may be necessary, for example, when the drying system is started up and the at least one return line has cooled down due to low ambient temperatures despite thermal insulation. To prevent the temperature of the return vapor portion directed to the burner from falling below the condensation temperature, the preheating device can be activated. This can preheat the at least one return line to a desired temperature, as described above in connection with the method.

[0025] 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 temperature. Dehumidified preheating air can be used as the preheating gas, in particular.

[0026] The recirculation device can further comprise at least one conveying fan arranged in the at least one return line. The at least one conveying fan can be designed to convey (move) the return vapor portion toward the burner. The at least one fan can thus (actively) adjust or control the volume flow of the return vapor portion flowing in the at least one return line. 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.

[0027] According to a fourth aspect of the invention, a wood chip drying system for drying moist wood chips is provided. The wood chip drying system comprises a dryer with a drying chamber for drying moist wood chips while introducing hot drying gas into the drying chamber, a burner with a combustion chamber for providing the hot drying gas, and the device according to the third aspect for recirculating at least a portion of the vapor exiting the drying chamber to the burner in order to combust the pollutants contained in the recirculated vapor portion in the burner.

[0028] By recirculating at least part of the vapors escaping from the drying chamber to the burner, the pollutant load of the wood chip drying plant can be significantly reduced, since the high flame temperature in the burner can efficiently combust volatile organic substances and solid particles.

[0029] The dryer can, for example, be a drum dryer through whose drying chamber the drying gas provided by the burner flows and which transports the moist material introduced into the drying chamber through the drying chamber and dries it in the process.

[0030] The burner can be a gas burner, oil burner, solid fuel burner, or a multi-fuel burner designed to generate burner or flame temperatures in the range of 700 °C to 1200 °C in the combustion chamber. Due to the high flame temperature, the burner is suitable for the oxidation / combustion of pollutants contained in the recirculated vapor.

[0031] The wood chip drying system may further comprise a mixing chamber arranged between the burner and the dryer. This mixing chamber is designed to mix the hot drying gas provided by the burner with a return vapor and / or external drying gas supplied to the mixing chamber and then feed it to the drying chamber.

[0032] The wood chip drying system may further comprise a separation device located at the outlet of the drying chamber and designed to separate the dried material from the vapors. For this purpose, the separation device may comprise one or more cyclone separators.

[0033] The wood chip drying plant may further comprise a filter device designed to filter out pollutants from the residual vapor that is released into the environment and is therefore not returned to the burner via the above-mentioned device and made available again to the drying process.

[0034] Short description of the drawings

[0035] Further details and advantages of the invention will be further described with reference to the non-limiting embodiments illustrated in the drawings. They show:

[0036] Figure 1 is a schematic representation of a wood chip drying plant according to the present invention; and

[0037] Figure 2 is a flow diagram illustrating a method according to the invention for reducing pollutants in vapors.

[0038] Detailed description

[0039] Figure 1 shows a schematic representation of a drying system 100 according to the present invention. The drying system 100 is a wood chip drying system designed for drying moist wood chips.

[0040] The wood chip drying system 100 comprises a burner 10 with a combustion chamber 12, a dryer 20 with a drying chamber 22 and a mixing chamber 30 arranged between the combustion chamber 12 and the drying chamber 22. The wood chip drying system 100 further comprises a separating device 40 arranged at the outlet of the drying chamber 12 for separating vapors and dried wood chips, a return vapor line 50 coupled to the mixing chamber 30 and a vapor discharge line 60 with a filter device 62 for filtering pollutants from that part of the vapor which is released into the environment.

[0041] According to the invention, the wood chip drying system 100 further comprises a recirculation device 80, which is or can be coupled to the burner 10. It is intended to direct at least a portion of the vapor (also called return vapor) recirculated in the return vapor line 50 to the hot burner flame of the burner 10. As a result, pollutants contained in the return vapor (in particular VOCs and / or solid particles, which occur more frequently during the drying of wood chips) can be oxidized / combusted in the burner 10 or in the hot combustion flame, thereby significantly reducing the overall pollutant emissions of the wood chip drying system 100. The structure and operation of the recirculation device 80 are described in more detail below in connection with Figures 1 and 2.

[0042] The burner 10 is designed to provide a hot drying gas. Hot drying air is preferably used as the drying gas. For this purpose, ambient air is supplied to the combustion chamber 12, where it is heated to a desired temperature by the burner flame. The burner can be a gas burner, oil burner, dust burner, or a multi-fuel burner designed to generate flame temperatures in the range of 700°C to 1200°C in the combustion chamber.

[0043] The heated drying gas leaving the combustion chamber 12 is fed to the mixing chamber 30, where it can be mixed with a vapor portion (return vapor portion) originating from the drying chamber 22 and recirculated via the return vapor line 50. By recirculating and reusing at least a portion of the vapor flowing out of the drying chamber 22, the energy efficiency of the wood chip 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 wood chip drying system 100 (e.g., a wood processing system). This can further improve the energy balance of the drying system 100.

[0044] By mixing the heated drying gas provided by the combustion chamber 12 with the vapor portion returned to the mixing chamber 30 via the return vapor line 50 (and optionally additional supplied gas streams), the temperature of the drying gas resulting in the mixing chamber 30 and provided to the drying chamber 22 (drying gas stream) decreases. However, the temperature is adjusted by appropriate mixing and gas supply from the combustion chamber 12 such that the resulting drying gas in the mixing chamber 30 has a desired temperature value that is approximately 20°C to 40°C above the temperature at the dryer inlet. This excess temperature in the mixing chamber 30 is necessary to compensate for heat losses due to convection and radiation.

[0045] The dryer 20 is preferably a drum dryer, which, depending on the quantity, quality, and composition of the wood chips, operates in the temperature range of 230°C to 500°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 wood chips.

[0046] The moist wood chips are fed to the drying chamber 22 in a controlled manner (not shown in Figure 1). Furthermore, the mixing chamber 30 provides a drying gas volume flow for the drying chamber 22, depending on the quantity and nature of the wood chips to be dried. The hot drying gas supplied to the drying chamber 22 flows around and heats the wood chips to be dried, absorbing the moisture released in the process. The gas flow slowly transports the wood chips through the drying chamber 22.

[0047] The above-mentioned operating temperature, which ranges from 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 set at all times such that the moisture and pollutants released during the drying process cannot recondense within the drying chamber 22.

[0048] Accordingly, the vapor flowing out of the drying chamber 22 at the outlet has a relatively high temperature, which is above the condensation temperature of water vapor. Typical vapor temperature values ​​at the outlet of the drying chamber 22 are above 100°C, preferably in the temperature range from greater than 100°C to 125°C. After the wood chips entrained in the vapor have been separated in the separation device 40, at least a portion of the vapor can be returned to the mixing chamber 30 via the return vapor line 50. This recirculated vapor portion (also called the return vapor portion) is thus returned to the drying process and, due to its high energy content (no further heat of vaporization needs to be generated), improves the energy efficiency of the wood chip drying system 100.

[0049] However, the problem is that the pollutants (VOCs, N2O, NH3, solid particles) entrained in the return vapor section cannot be oxidized / combusted in the mixing chamber 30 due to the low temperatures (maximum 500 °C to 550 °C). This creates the risk that more and more pollutants will accumulate in the return vapor section, which is recirculated to the mixing chamber 30 and made available again to the drying chamber 22. This can lead to an undesirable accumulation of pollutants in the wood chips to be dried.

[0050] In order to avoid this and in particular to sustainably reduce the pollutant load of the wood chip drying plant 100, according to the present invention the wood chip drying plant 100 is provided with the return device 80.

[0051] The recirculation device 80 comprises at least one return line 82, which is fluidically coupled at its first end to the return vapor line 50 and at its second end to the burner 10. The at least one return line 82 is provided to return at least a portion of the return vapor to the burner 10. The recirculation device 80 further comprises at least one conveying fan 84a, 84b arranged in the at least one return line 82. The volume flow of the vapor portion to be returned can be (actively) controlled or adjusted via the at least one conveying fan 84a, 84b.

[0052] In the embodiment shown in Figure 1, the return device 80 has a return line 82, which is divided into two sub-lines 82a, 82b towards the burner 10. The first sub-line 82b opens directly into the combustion chamber 12, while the second sub-line 82a is coupled to the muffle of the burner 10. Thus, a first return vapor portion can be introduced as combustion gas or combustion air directly into the combustion chamber 12 of the burner 10 via the first sub-line 82b. Furthermore, a second return vapor portion can be fed as cooling gas or cooling air to the muffle of the burner 10 via the second sub-line 82a. This second return vapor portion is also subsequently fed into the combustion chamber 12, where it is further heated and purified.

[0053] To control the volume flow of the back vapor in the recirculation device 80, a first conveying fan 84a is located in the common return line 82. This conveying fan 84a can be used to adjust the volume flow of the back vapor portion, which is fed in its entirety to the burner 10. Furthermore, a second conveying fan 84b can be located directly in the partial line 82b, through which the first back vapor portion is fed directly into the combustion chamber 12. The volume flow fed to the combustion chamber 12 can be adjusted or controlled via this second conveying fan 84b.

[0054] For example, the wood chip drying system 100 can be operated such that, with the aid of the recirculation device 80, 60% of the vapor volume flow exiting the drying chamber 22 is recirculated. 15% of the vapor volume flow can be fed directly to the combustion chamber 12 as combustion air; a further 15% can be fed to the burner muffle as cooling air. The volume flow proportions are controlled via the conveying fans 84a, 84b, as described above. A further 30% of the vapor volume flow exiting the drying chamber 22 can be fed to the mixing chamber 30 via the return vapor line 50. Thus, a total of 60% of the vapor volume flow exiting the drying chamber 22 is recycled, which significantly improves the energy balance of the wood chip drying system 100.Of this, 30% is fed to the burner 10 and cleaned there, which also significantly improves the pollutant balance of the wood chip drying plant 100, in particular by reducing it to below the legal emission requirements (see also test results in the table below).

[0055] 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 return line 82 carries only a portion of the return vapor to the combustion chamber 12 of the burner 10. In this case, the return line 82 does not have two sub-lines 82a, 82b, thereby realizing a particularly simple design. In this simplified design, it is sufficient to provide only one conveying fan in the return line 82 to control the volume flow of the portion of the return vapor to be returned.

[0056] The recirculation device 80 may further comprise a preheating device 86. This is designed to maintain the at least one recirculation line 82 at a desired temperature level. This prevents the recirculation vapor portion from cooling to a temperature below the condensation temperature (dew point temperature) during recirculation via the at least one recirculation line 82, and prevents the entrained water vapor or pollutants from condensing uncontrollably along the recirculation line 82 or in the combustion chamber 12.

[0057] In the embodiment shown in Figure 1, the preheating device 86 has a preheating gas source or is coupled to an external preheating gas source. The (external) preheating gas source is fluidically coupled to the return line 82 (via line section 86a). The preheating gas source 86 is designed to feed warm gas or warm air into the return line 82 to preheat the return line 82. Although the return line 82 is thermally insulated, it may happen that the return line 82 is "cooled down" at cold ambient temperatures and in particular during start-up of the wood chip drying system 100. In this case, the preheating gas source 86 can be activated as needed to feed preheating gas into the return line 82.

[0058] The preheating gas provided by the preheating gas source 86 may have a temperature that is not significantly lower than the temperature of the recirculating vapor, preferably in a temperature range of 70°C to 100°C, more preferably in a temperature range of 80°C to 90°C. This temperature is generally sufficient to preheat the return line 82 sufficiently so that cooling of the recirculating vapor to below the condensation temperature can be avoided.

[0059] The return device 80 described here is characterized by a simple and compact design and is therefore also suitable for retrofitting in existing wood chip drying plants in order to improve the pollutant emission balance of such plants.

[0060] In connection with Figure 2, a method according to the invention for reducing pollutants in a drying system is further described. The method can be implemented using the above-described recirculation device 80 and can be used, for example, in a wood chip drying system to reduce pollutant emissions.

[0061] According to a first step S20, at least a portion of the vapor generated during the drying of moist material (e.g., wood chips in a wood chip drying plant) is returned to the burner 10 via the return device 80, in particular via the at least one return line 82 of the return device 80. This portion of the vapor returned to the burner 10 is maintained at a temperature above the condensation temperature of the water vapor contained in the vapor with the aid of the return device 80 (second step S22). This can be achieved, on the one hand, by suitably stripping the at least one return line 82. If necessary, the preheating device 86 can also be used, which preheats the at least one return line 82 to a desired temperature. In a third step S24, the returned portion of the vapor is then strongly heated in the burner 10.In particular, the return vapor part is passed through the high-temperature zone of the combustion chamber 12 (zone with temperatures in the range of 700 °C to 1200 °C), whereby the pollutants carried along are effectively oxidized / burned.

[0062] The aim of the method described here and the recirculation device 80 is to significantly reduce the pollutant emissions from drying plants, in particular to keep them well below legal requirements. The implementation of the inventive technology described here (method and recirculation device) in a pilot wood chip drying plant in Switzerland has shown that the pollutant emissions can be significantly reduced, which would not be possible without the recirculation device 80 (i.e., if only a portion of the recirculation vapor is recirculated to the mixing chamber 30 via the recirculation vapor line 50).

[0063] The pilot wood chip drying plant features a dust burner designed for burner / flame temperatures of up to 1200 °C. A drum dryer is used as the dryer, which is designed for drying gas flow rates of up to 290,000 m 3 / h and is designed for a dry material throughput (wood chip throughput) of up to 52,000 kg / h. However, the invention is not dependent on these specific limits. A mixing chamber is arranged between the burner and the drum dryer. This mixing chamber is designed to mix the drying gas heated by the burner with a portion of the vapor exiting the dryer outlet and subsequently supply it to the drum dryer as drying gas. Furthermore, the pilot plant has the above-described recirculation device to return another portion of the vapor exiting the dryer outlet directly to the burner, where it is heated and purified.

[0064] The following table shows the reduction of pollutants, especially VOCs (see for example row Total C @18% O2) and nitrogen-based pollutants (see for example row NO X at 18% O2) in half-load and quasi-full-load operation of the pilot plant.

[0065] The table describes the concentration of these pollutants (in mg / m 3) for different throughputs of dry material (in this case 70% softwood and 30% hardwood) used to produce particleboard. As can be seen from the second and third columns of the table, a significant reduction in pollutants can be achieved both at medium wood throughput (operating load of the dust burner at 47%) and at near-full load (operating load of the dust burner at 85%). The values ​​for the nitrogen-based pollutants as well as the carbon-based pollutants are converted to 18 vol.% oxygen, as specified by the Swiss Air Pollution Control Ordinance (LRV Switzerland). While, for example, the legal requirements for VOCs are 120 mg / m 3 the inventive technology was able to reduce VOC concentrations to below 100 mg / m 3 (89 mg / m 3 at half load and 93 mg / m 3 at quasi-full load). Similar reductions also occur for NO x-pollutant load. These values ​​were achieved during operation of the wood chip drying plant in which approximately 30% of the drying gas volume flow (vapor volume flow) exiting the drum dryer outlet was returned to the burner for heating and cleaning with the aid of the recirculation device. The high-temperature zone (zone 1) of the combustion chamber had a temperature of greater than 700 °C (729 °C at half load and 849 °C at near-full load), and the dryer outlet temperature was maintained at 108 °C or 111 °C (i.e., above the dew point temperature of water vapor). The dryer outlet temperature also corresponded to the temperature of the vapor volume flow returned to the burner (return vapor volume flow).

[0066] The technology described here for reducing pollutants using the recirculation device 80 according to the invention has several advantages over the prior art. It is simple, space-saving, and cost-effective, as it does not require any additional purification systems. Rather, at least a portion of the back vapor is returned directly to the existing burner of the drying system (wood chip drying system) and purified there. This purification process can significantly reduce pollutant emissions from drying systems. Furthermore, it prevents pollutants from accumulating in the material being dried, which can happen, for example, if a portion of the back vapor is simply returned to the mixing chamber and then returned to the drying chamber unpurified.

[0067] However, the technology described here also overcomes a long-held prejudice that vapor saturated with water vapor cannot be directly returned to the burner or combustion chamber, since stable operation of the drying plant would not be possible in this case.

Claims

PATENT CLAIMS 1. A method for reducing pollutants in a drying plant (100) which are produced during the drying of moist material in the drying plant (100), the method comprising: Returning at least part of the vapors produced during drying of the moist material 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 Combustion of the pollutants contained in the recirculated vapor part in the burner (10).

2. The process according to claim 1, wherein the temperature of the recirculated vapor portion is maintained above the condensation temperature at all times during the recirculation.

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

4. A method according to any one of claims 1 to 3, wherein the step of maintaining the temperature above the condensation temperature comprises optionally preheating a return line (82) provided for recirculating the vapor.

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

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

7. The method according to claim 6, wherein the volume flow of the first vapor part supplied to the combustion chamber (12) and / or the volume flow of the second vapor part supplied to the muffle is regulated as a function of the burner output and / or the volume flow required in the drying chamber (22).

8. A process according to any one of claims 1 to 7, 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.

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

10. Device (80) for reducing pollutants that arise in a drying plant (100) during the drying of moist material, in particular moist wood chips, wherein the device (80) is designed to recirculate at least part of the vapor emerging from a drying chamber (12) of the drying plant (100) to a burner (10) of the drying plant (100) in order to burn the pollutants contained in the vapor part, and wherein the device (80) is further designed to keep the temperature of the vapor part to be recirculated at a temperature above the condensation temperature of the water vapor entrained in the vapor part.

11. Device (80) according to claim 10, wherein the device (80) comprises at least one return line (82) which is coupled at its first end to the outlet of the drying chamber (22) or to a return vapor line (50) of the drying plant (100) and at its second end to the burner (10) of the drying plant (100).

12. Device (80) according to claim 11, wherein the at least one return line (82) is thermally insulated, and / or wherein the device (80) further comprises a preheating device (86) which is provided to preheat the at least one return line (82) as required.

13. The device (80) of claim 12, wherein the preheating device (86) comprises a preheating gas source that is selectively connectable to the at least one return line (82) of the return device to introduce preheating gas into the return line (82).

14. Device according to one of claims 11 to 13, wherein the device comprises at least one conveying fan (84a, 84b) which is arranged in the at least one return line (82) and is designed to adjust the volume flow of the vapor part to be returned.

15. A wood chip drying system (100) for drying moist wood chips, comprising: a dryer (20) with a drying chamber (22) for drying moist wood chips while introducing hot drying gas into the drying chamber (22); a burner (10) with a combustion chamber (12) for providing the hot drying gas; and the device (80) according to one of claims 10 to 14 for returning at least a portion of the vapor emerging from the drying chamber (22) to the burner (10) in order to burn the pollutants contained in the vapor portion in the burner (10).

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