Exhaust gas treatment device for exhaust gas from a small combustion plant and method for treating exhaust gas from a small combustion plant

Description

Technical field

[0001] The application concerns an exhaust gas treatment device for exhaust gas from a small combustion plant and an associated process. This involves combining an agglomerator, in particular an ionization chamber, and a centrifugal separator. State of the art

[0002] Small combustion plants are mostly fueled by biomass, especially wood. For the sake of a favorable carbon dioxide balance, these are often desirable. However, the remaining emissions, particularly dust emissions, are considered harmful to health and should therefore be prevented. Consequently, flue gas cleaning is becoming increasingly important.

[0003] Electrostatic precipitators are well-known for removing particles. In electrostatic precipitators, a high-voltage field is generated between a wire-shaped spray electrode and a flat collecting electrode. This field electrically charges the dust particles in the exhaust gas flow and simultaneously deflects them from the current. For this purpose, a spray electrode is supplied with a negative DC voltage (from 30 to 80 kV), which emits electrons that ionize the surrounding gas molecules. The dust particles are negatively charged by ion bombardment (for dust particles > 1 µm) or ion diffusion (for dust particles < 0.5 µm) and move in the applied electric field perpendicular to the exhaust gas flow towards the positively charged collecting electrode.

[0004] Wet scrubbers are also known. In these, the dust particles are trapped in water droplets and can then be removed more effectively from the exhaust gas flow due to mass-proportional forces resulting from the increase in mass.

[0005] Finally, centrifugal separators are a well-known type. The gas / solid mixture to be separated, i.e., the exhaust gas containing particles, is fed tangentially or axially into a cylindrical container, usually with a conical lower section. The swirl is generated either by the tangential entry of the gas or by guide vanes attached to the circumference of the cyclone housing. The resulting rotational flow in the separation chamber exerts centrifugal forces on the solid particles, flinging them outwards. From the wall of the centrifugal separator, these solids slide downwards into a solids collection container. The gas rotating inside the separator is discharged upwards through a cylindrical or conical dip tube.

[0006] This means that high-performance separators are generally available. However, especially for small combustion plants, the costs must not be too high. Furthermore, installation space is limited. The object of the invention is therefore to create an exhaust gas treatment device that is technically simple and thus inexpensive to implement and requires little installation space. A corresponding method is also to be provided.

[0007] From EP 2 868 880 A1, an exhaust gas treatment device is known in which an ionization chamber and a centrifugal separator are provided. Separation takes place in the ionization chamber. Only a partial flow, which has a particularly high concentration of separated particles, is directed into the centrifugal separator.

[0008] US Patent 5,695,549 A discloses an exhaust gas treatment system for coal-fired power plants. This system could also be used to treat exhaust gas from a small combustion plant. The exhaust gas treatment system comprises an agglomerator and a centrifugal separator downstream of the agglomerator.

[0009] DE 38 11 400 A1 discloses that dust agglomeration can improve separation in a centrifugal separator. DE 38 11 400 A1 also outlines the limitations of separation. To further improve separation, it is proposed to divert a partial gas stream with the highest possible dust content from one cyclone and feed it into another.

[0010] US Patent 4,246,013 A discloses a centrifugal separator in which a portion of the gas flow, after passing through the separator, can be recirculated through the separator by means of an exhaust gas recirculation device. For this purpose, a separator plate is provided which can be moved by a spindle to adjust the proportion of gas recirculated through the separator. A handle is attached to the spindle for this purpose.

[0011] The solution to this problem is found particularly in the independent claims. The dependent claims provide advantageous further developments. Further information can be found in the description and the drawing.

[0012] It was determined that an exhaust gas treatment device for exhaust gas from a small combustion plant, according to claim 1, comprising an agglomerator and a centrifugal separator downstream of the agglomerator, must be provided. The centrifugal separator is positioned downstream such that the exhaust gas flowing through the agglomerator flows completely into the centrifugal separator. The agglomerator is designed to improve the agglomeration behavior of particles. The centrifugal separator has an exhaust gas recirculation device with which, after passing through the centrifugal separator, a portion of the exhaust gas can be returned to the inlet area of ​​the centrifugal separator to enable a desired volume flow rate within the centrifugal separator. Where particles are mentioned, these are generally dust particles.

[0013] The system therefore combines two known exhaust gas treatment devices: an agglomerator and a centrifugal separator. A high exhaust gas velocity of more than 3 m / s, and even 30 m / s to 40 m / s, has proven effective. The actual particle separation takes place in the centrifugal separator. Due to the agglomeration process, separation can occur there largely independently of particle density.

[0014] The exhaust gas flows from the agglomerator into the centrifugal separator. Various configurations are possible for centrifugal separators. The common configuration features a cone that tapers downwards when installed. A dip tube extends into the cone from above. The gas containing the particles, in this case agglomerated particles, enters laterally from the top and flows downwards in a rotating motion around the dip tube within the tapered cone. Centrifugal force forces the particles against the cone wall. The gas, now cleaned of the particles, exits the centrifugal separator through the dip tube. In addition to centrifugal force, the electrical force generated by the particles' charge can also promote separation.

[0015] As mentioned, a flue gas recirculation system is installed to ensure stable operation. This system allows a portion of the flue gas to be returned to the centrifugal separator's inlet after it has passed through. This enables the desired volume flow rate through the centrifugal separator to be adjusted. Consequently, a desired volume flow rate, and thus a desired flue gas velocity, can be set under varying operating conditions of the small combustion plant. This has proven crucial in the present combination of agglomerator and centrifugal separator.

[0016] Exhaust gas treatment systems for engine technology and internal combustion engines are described in documents DE 32 38 793 C2, DE 35 00 373 C2, and EP 0824376 B1. While these documents did not consistently rely on exhaust gas recirculation, it has become clear that exhaust gas recirculation is crucial, at least for small combustion plants. In this context, it is also noted that combustion in engines occurs under different parameters, particularly at higher pressures, than in small combustion plants. Therefore, exhaust gas treatment systems designed for engine exhaust are not readily suitable for small combustion plants.

[0017] In addition to separating dust, both organic and inorganic, the process has also proven effective for separating gaseous pollutants such as NOₓ, SO₂, and HCl. Based on current knowledge, the exhaust gas treatment system is designed to be eligible for approval under the preliminary DiBt test program for dust separators.

[0018] In centrifugal separators, the immersion tube can be positioned in either the upper or lower section. Furthermore, a cyclone with an axial or tangential inlet can be used. Axial cyclones are particularly well-suited for separating droplets, as the uniform liquid distribution around the circumference of the cyclone largely prevents the formation of thick strands and thus the resuspension of already separated liquid.

[0019] The combination of the agglomerator and centrifugal separator enables exhaust gas velocities of 30 m / s to 40 m / s, whereas conventional electrostatic precipitators operate at exhaust gas velocities of 0.05 m / s to 1 m / s. These significantly higher exhaust gas velocities allow for considerably smaller cross-sections while maintaining the same exhaust gas flow. Thus, despite the combination of two components—the agglomerator and the centrifugal separator—the required installation space can be significantly reduced.

[0020] In one embodiment of the invention, the agglomerator is an ionization chamber. However, this is not intended to be operated in the classical mode of an ionization chamber, in which the dust is separated. Rather, it is sufficient to design the ionization chamber in such a way that the particles are negatively charged in such a manner that they form agglomerates.

[0021] Besides negative charging in an ionization chamber, there are alternative or additional methods to induce agglomeration. For example, sound waves with suitable frequencies can be used. Appropriate turbulence in the flow, at least in certain areas, can also cause agglomeration. This usually involves strong turbulence. Agglomeration can also be induced thermally. For this, appropriate heating elements must be used.

[0022] In one embodiment of the invention, it is provided that liquid can be introduced into the exhaust gas, particularly in the inlet area of ​​the centrifugal separator. This liquid is typically water. Water can be introduced, especially as mist or vapor. This further improves the separation process. This is primarily because the charged particles are bound to and within the water droplets. It is possible to introduce sufficient liquid to effectively flush out the particles, typically agglomerated particles. Within the centrifugal separator, the particles are flung against the separator wall along with the water droplets. The liquid then flows to the bottom of the separator. It is possible to clean and reuse the liquid, i.e., to reintroduce it into the exhaust gas.

[0023] The introduction of liquid also allows aggressive exhaust gases such as HCl, HF, NOx, and SOx to be removed by washing. The excess washing liquid can be circulated in a closed water circuit.

[0024] In one embodiment, a mist and / or vapor nozzle is provided for introducing liquid into the exhaust gas. This allows the liquid to be well distributed in the exhaust gas.

[0025] Regardless of whether liquid is introduced into the exhaust gas, it can also be used to clean the centrifugal separator of separated particles. It is also not necessary to discharge the liquid into the sewer system. If water is used, water vapor can be released. The separated particles form a sludge mass that can be removed through an opening.

[0026] In one embodiment, the exhaust gas recirculation device is suitable for maintaining a constant volume flow in the centrifugal separator. As already mentioned, adjusting the volume flow in the centrifugal separator has proven important. Often, there is an optimal volume flow. In such cases, it makes sense to consistently select this volume flow, i.e., to set a constant volume flow. If more exhaust gas is produced by the small combustion plant, then correspondingly less or even no exhaust gas should be routed through the recirculation device.

[0027] In one embodiment, an insulator is arranged between the agglomerator and the centrifugal separator. This is advantageous given the high electrical voltages when using an ionization chamber as an agglomerator.

[0028] In one embodiment, an induced draft fan is provided for extracting the exhaust gas from the centrifugal separator. This allows the exhaust gas to be selectively extracted through the dip tube. This contributes to the stable operation of the exhaust gas treatment system. It can also affect the operation of the small combustion plant. Often, the operation of the small combustion plant is significantly controlled by the induced draft fan of the exhaust gas treatment system anyway. In another embodiment, a droplet separator is provided for separating water droplets from the exhaust gas exiting the centrifugal separator. Despite the separation process in the centrifugal separator, the exhaust gas may still contain droplets after leaving the separator. These droplets should not be released into the environment. The droplet separator serves this purpose.

[0029] In one embodiment, a spray electrode is provided in the agglomerator. A spray electrode is particularly suitable for charging the particles to such an extent that agglomeration occurs. This applies especially in an agglomerator designed as an ionization chamber.

[0030] The invention also relates to a method for treating exhaust gas from a small combustion plant, according to claim 10, in which particles in the exhaust gas are first at least partially agglomerated in an agglomerator and the exhaust gas is then completely fed to a centrifugal separator. Particles are separated in the centrifugal separator. A desired volume flow rate is set in the centrifugal separator by feeding a portion of the volume flow exiting the centrifugal separator at its inlet, as needed.

[0031] In one embodiment, the exhaust gas is at least partially ionized in the agglomerator, which is designed as an ionization chamber, to agglomerate the particles. This allows agglomeration to be effected in a suitable manner. As explained above, other methods for agglomeration are also available.

[0032] In one embodiment, liquid, usually water, is added to the exhaust gas before or upon entering the centrifugal separator. Although operation without liquid is also possible, the separation rate can generally be further improved by adding liquid. The wet operating mode thus offers advantages over the dry operating mode. An operating mode, referred to as quasi-dry, is also conceivable. In this mode, only enough liquid is added to moisten the particles and improve separation. However, not so much liquid is added that it runs down the wall of the centrifugal separator. This initially appears efficient. However, there is a risk that the moist particles will adhere to the wall of the centrifugal separator, requiring cleaning after a reasonable operating period.

[0033] In one embodiment, the supplied liquid contains calcium hydroxide. This promotes the binding of the particles separated in the centrifugal separator, especially the ash particles. This allows the particles separated by the liquid to be easily removed. The liquid flowing to the bottom of the centrifugal separator, typically water, can then be more easily cleaned. During cleaning, the agglomerated particles, which are well bound by calcium hydroxide, are removed from the liquid. The liquid can then be stored, for example, in bags. A screw conveyor can be used to transport the cleaned liquid from the small-scale incineration plant, for example, to a waste collection site. As already mentioned, the cleaned liquid can be reused.

[0034] In one embodiment, the method is carried out using the exhaust gas treatment device described above.

[0035] Here are a few details regarding wet operation. In this process, water is added as a scrubbing agent throughout the entire operation via a mist, steam, or spray nozzle into the exhaust gas flow at the inlet of the centrifugal separator. The scrubbing water must be added to the exhaust gas flow in such a way as to create a large interface between the exhaust gas and the scrubbing water. Inertial forces, diffusion forces, and condensation forces all contribute to transporting the particles to the liquid surface. When the dew point is reached, the finest particles act as condensation nuclei, increasing the mass increase and thus promoting particle separation. The rotation within the centrifugal separator facilitates the transport of particles to the liquid surface. During the pre-ionization of the dust particles, electrical forces further contribute to optimal transport and improved binding of the fine dust particles to the water droplets.Water droplets typically have no inherent electrical charge, but negatively charged dust particles induce a complementary charge in the water droplets. The electrons within the water droplet can move slightly. When the negatively charged particles approach the water droplets, they repel the electrons, causing the water droplets near the particles to become positively charged. The attraction between this positive charge and the negatively charged particles results in a force that acts on the water droplets, attracting them and allowing the particles to bind more effectively to and within the water droplet.

[0036] The negatively charged particles in the exhaust gas stream are more easily trapped in water droplets and then removed from the exhaust gas flow in the centrifugal separator by mass-proportional forces or centrifugal force due to the increase in mass. The negative charge of the particles ensures that the dust particles bind better to and within the water droplet, thus achieving a higher separation efficiency, especially for fine organic dust particles.

[0037] In wet processing, no mechanical cleaning devices such as beaters are required to clean the spray / precipitation systems. A liquid film forms on the precipitation electrodes under the influence of the electric field, which flows continuously. Any particles present are thereby carried away as a suspension. If necessary, an additional system of continuously operating mist nozzles can be provided for gas saturation. This is particularly advantageous with higher solids content. Sludge deposits on the precipitation electrodes are thus prevented. The additional mist nozzles intensify the liquid film on the electrode system and reduce its solids concentration. The filters are cleaned at predetermined intervals by flushing nozzles. The achievable clean gas dust concentrations are < 1 mg / m³.

[0038] The figures will be used to explain further details of the invention.Fig. 1 schematically shows an exhaust gas treatment system. Figure 2 This is a perspective schematic view that shows further details.

[0039] The exhaust gas 1, originating from the small combustion plant (not shown) and also referred to here as raw gas, first enters the agglomerator 2, which is designed as an ionization chamber. A spray electrode 3 is located in the ionization chamber 2. The spray electrode 3 is supplied by a high-voltage supply 4, allowing high voltage to be applied. The ionization chamber 2 is separated from the centrifugal separator 6 by an insulator. A mist nozzle 7 is located in the inlet area of ​​the centrifugal separator 6, through which water, serving as the liquid, is sprayed into the exhaust gas 1. The exhaust gas 1 then flows further in the centrifugal separator and, rotating around a dip tube 8 that projects into the centrifugal separator 6 from above, flows into the lower section of the centrifugal separator 6.Droplets, particles, agglomerated particles, and droplets containing particles and agglomerated particles contained in the exhaust gas 1 are forced by centrifugal force against the wall 9 of the centrifugal separator 6 and flow along the wall 9 into the sump 10 of the centrifugal separator 6. Water located there is pumped by a water pump 11 through a wastewater filter 12, which includes a neutralizer, to the misting nozzle 7 and sprayed back into the exhaust gas 1. Fresh water 13 can be supplied via a supply line. The supply is controlled by a float valve 14. The exhaust gas 1 is discharged by an induced draft fan 15. The exhaust gas 1 is guided through a droplet separator 16, in which any remaining droplets in the exhaust gas 1 are separated. Exhaust gas 1 exiting the induced draft fan 15 is released into the environment as so-called clean gas.

[0040] Part of the exhaust gas can be recirculated through a line 17, which serves as an exhaust gas recirculation device and in which a recirculation fan 18 is arranged, back into the inlet area of ​​the centrifugal separator 6. This allows an optimal exhaust gas flow for the operation of the centrifugal separator 6 to be set, independent of the exhaust gas flow coming from the small combustion plant.

[0041] In Fig. 2 This is a perspective view of the exhaust gas cleaning system, showing further details. The exhaust gas 1, coming from the small combustion plant (not shown), flows into the agglomerator 2. From there, it flows into the centrifugal separator 6, more precisely through a functional unit 19 into the centrifugal separator 6. In the centrifugal separator 6, the rotating flow described above takes place from top to bottom to clean the exhaust gas. Through the in Fig. 2The exhaust gas flows back into the functional unit 19 via the submersible tube 8, which is not visible. The exhaust gas paths in the functional unit 19 are separate; therefore, there is no mixing of the exhaust gas 1 flowing into the centrifugal separator 6 and the exhaust gas 1 flowing out of the centrifugal separator 6 in the functional unit 19. The functional unit 19 also serves as a heat exchanger, as will be explained below.

[0042] From functional unit 19, the exhaust gas 1 is conveyed by the induced draft fan 15 through a pipe 20 to the distribution unit 21. Part of the exhaust gas 1 is returned to the centrifugal separator 6 via the return line 17. The other part, usually the main part, of the exhaust gas flows through a pipe 22 into the environment. Heating can occur in functional unit 21 to prevent excessively low exhaust gas temperatures.

Claims

1. An exhaust-gas treatment device for exhaust gas (1) of a small combustion plant, having an agglomerator (2) and a centrifugal separator (6) arranged downstream of the agglomerator (2) in such a manner that the exhaust gas (1) flowing through the agglomerator (2) flows completely into the centrifugal separator (6), wherein the agglomerator (2) is designed to improve the agglomeration behaviour of particles, characterized in that the centrifugal separator (6) has a line (17) that serves as an exhaust-gas recirculation device (17, 18, 21), in which a recirculation fan (18) is arranged, by means of which, after flowing through the centrifugal separator (6), a part of the exhaust gas (1) can be guided back to the inlet region of the centrifugal separator (6) in order to enable a desired volume flow in the centrifugal separator (6) under different operating conditions of the small combustion plant.

2. The exhaust-gas treatment device according to Claim 1, characterized in that the agglomerator is an ionization chamber (2).

3. The exhaust-gas treatment device according to Claim 1, characterized in that liquid (13), in particular as mist or steam, can be introduced into the exhaust gas (1), in particular in the inlet region of the centrifugal separator (6).

4. The exhaust-gas treatment device according to the preceding claim, characterized in that a mist and / or / or steam nozzle (7) is present for introducing liquid (13) into the exhaust gas (1).

5. The exhaust-gas treatment device according to Claim 1, characterized in that the exhaust-gas recirculation device (17, 18, 21) is suitable for enabling a constant volume flow in the centrifugal separator (6).

6. The exhaust-gas treatment device according to any one of the preceding claims, characterized in that an insulator (5) is arranged between agglomerator (2) and centrifugal separator (6).

7. The exhaust-gas treatment device according to any one of the preceding claims, characterized in that an induced draught fan (15) is present for extracting the exhaust gas (1) from the centrifugal separator (6).

8. The exhaust-gas treatment device according to any one of the preceding claims, characterized in that a droplet separator (16) is present for separating water droplets from exhaust gas (1) emerging from the centrifugal separator.

9. The exhaust-gas treatment device according to any one of the preceding claims, characterized in that a discharge electrode (3) is present in the agglomerator (2).

10. A method for treating exhaust gas (1) from a small combustion plant, wherein an exhaust-gas treatment device according to Claim 1 is used, in which, in the exhaust gas (1), particles are initially at least partially agglomerated in an agglomerator (2) and the exhaust gas is subsequently fed completely to a centrifugal separator (6), wherein particles are separated in the centrifugal separator (6), wherein a desired volume flow is set in the centrifugal separator (6) in that, if required, a part of the volume flow emerging at the outlet of the centrifugal separator (6) is fed to the centrifugal separator (6) at the inlet of the centrifugal separator (6).

11. The method according to Claim 10, characterized in that, for the agglomeration of the particles, the exhaust gas is at least partially ionized in the agglomerator in the form of an ionization chamber (2).

12. The method according to any one of Claims 10 and 11, characterized in that liquid (13) is supplied to the exhaust gas (1) before or when the exhaust gas (1) enters the centrifugal separator.

13. The method according to the preceding claim, characterized in that the liquid (13) supplied comprises calcium hydroxide.

14. The method according to any one of Claims 10 to 13, characterized in that the method is carried out with an exhaust-gas treatment device according to any one of Claims 1 to 9.