EP2257379A1

EP2257379A1 - Device and method for cleaning a flue-gas flow of particles and system for fuel combustion

Info

Publication number: EP2257379A1
Application number: EP09708612A
Authority: EP
Inventors: Ismo Talka
Original assignee: Jyvaskylan Yliopisto
Current assignee: Jyvaskylan Yliopisto
Priority date: 2008-02-05
Filing date: 2009-02-05
Publication date: 2010-12-08
Also published as: FI20085099A0; FI20085099A; WO2009098356A1; FI120620B

Abstract

The invention relates to a device for cleaning a flue-gas flow of particles, which device (10) can be fitted to a flue-gas flow channel (11.1, 11.2), and which device includes collector means (12) for collecting charged particles (18) with the aid of an electrical field (17), and through which collector means the flue-gas flow (19) is arranged to be led through the device. The collector means include a collector spiral element (12), which is arranged to form a helical through-flow channel (13) for the flue gas. In addition, the invention also relates to a system for fuel combustion and a method for cleaning a flue-gas flow of particles.

Description

DEVICE AND METHOD FOR CLEANING A FLUE-GAS FLOW OF PARTICLES AND SYSTEM FOR FUEL COMBUSTION

The present invention relates to a device for cleaning a flue- gas flow of particles, which device can be fitted to a flue-gas flow channel, and which device includes collector means for collecting charged particles with the aid of an electrical field, and through which collector means the flue-gas flow is arranged to be led through the device. In addition, the inven- tion also relates to a system for fuel combustion and a method for cleaning a flue-gas flow of particles.

During the combustion of different types of fuel, a large number of small particles are formed. Larger power plants have well-managed electrostatic precipitation (ESP) of flue gases and relatively small emissions, due to this cleaning. In a power-plant environment, the cleaning of particles from flue gases is based on massive plate-electrode precipitators. In these, the electrodes are in a pack, with a large number of electrodes parallel to each other but with a space between them, the flue-gas flow between the electrodes travelling in an essentially straight path through the pack.

However, the small-scale combustion class, with an output of, for example, less than 300 kW, from fireplaces and boilers, lacks a reliable and easy-to-use flue-gas cleaning technology.

In some countries, for example Finland, as much as a quarter of small-particle emissions come from the small-scale combustion of wood, requiring even more attention to be paid to it in the future. Small particles are a significant health risk. For example, in Europe, they are estimated as causing 350 000 premature deaths.

However, flue-gas electro filters known from the prior art, the collector components of which have traditionally a plate-like shape, demand a particular division of the flue-gas flow inside the collector component, to be able to effectively clean particles from the flue gas. For the plate-like collector components to operate when the flue-gas flow in the flue flows into the collector, the flow must be expanded into a larger cross-sec- tional area and volume than that in the flue. This is because the flow divides evenly into the space filled by the collector plates and the plates can thus collect with the necessary- efficiency. This considerably increases the size of the electro-filter apparatus.

Because the collector components known from the prior art contain several separate plate electrodes, a high voltage must be brought to each of them. These numerous voltage-input points are also continuously in the flue-gas flow and thus gradually dirty, so that finally the high voltages in the collector component decrease and the particle removal efficiency of the entire electro filter diminishes substantially.

Thus, there are several unresolved drawbacks relating to clean- ers according to the prior art, which are therefore unsuitable for use with small-scale combustion. One bottleneck in the continuous and reliable collection from a small-scale combustion flue-gas flue has been the arrangement of the cleaning of the collector component of the particle filter. This is because the particulate material collected in the filter must be removed at regular intervals, to permit continuous and effective operation. The particulate material accumulating on the collector plates weakens the operation of an electro filter, as it weakens the electrical field, so that counter-corona phenomena, i.e. sparking, may arise in the particulate material in a high- tension electrical field..

In the collectors, there are several components that become dirty and require cleaning, as well as components, from which the collected material does not detach easily. In the case of a 20-kW wood-pellet boiler (an average device size in, for example, a Finnish detached house) , a collector component operating with a high small-particle collection efficiency can annually collect several kilograms of small-particle mass, which due to its lightness can take up a volume of about 5 litres. Such an amount of collected particles must be removed from the electrodes of the collector at regular intervals, so that the device will not become blocked and its collection efficiency will not diminish.

Also the insulation points, from which the electrically charged components of the cleaning device are supplied with the required charging and collection potential, dirty easily and must be cleaned. In known constructions, it is difficult, if not impossible to distribute the flue-gas flow evenly over the collector plates. One way to even the flow is to arrange a perforated plate transversely to the flow before the collector plate. The perforated plate too will become dirty and its holes will become blocked, so that it too must be cleaned regularly. In addition, large amounts of the particle material collected on the perforated plate may detach during combustion, in which case the concentrated material may travel uncontrollably past the collector plates and thus out of the flue.

The collector device must also be suitable for long-term opera- tion. The following is one example of the operating variables of a particle collector suitable for small-scale combustion: average annual heating requirement in Finland 20 Mwh → 20 kW boiler running for about 1000 h, particles in flue gas 70 mg/Nm3, annual particle emissions 3 kg, of which collectable 50 - 80 % → 1,5 kg - 2 , 4 kg, particle-mass density 0,4 kg/dm3 - volume taken annually by particle mass about 4 - 6 litres

(being l/5th of the volume of a typical rectangular-model collector) . On the basis of the above, it can be said that the collector will have to be cleaned quite frequently, which, with several collector and insulator surfaces, is difficult to perform mechanically. In small-scale combustion and slightly larger plants, the cleaning technology must be guite cheap, for example, at most 10 % of the purchase price of the combustion device, so that the cleaning costs will not become excessively great. Further, the operation of the plant must be reliable. Combustion devices in the size class in question are characterized by having a relatively large amount of particles in their flue gases. In addition, the size of the cleaning apparatus cannot be very large, as is the case with the electro filters in large power plants, as the cleaning apparatus must be able to be installed in relatively small flues, and must not be unreasonably detrimental aesthetically to the dwelling and living environment.

Because small-scale combustion apparatuses are used, for exam- pie, to heat dwellings, their users cannot be required to have thorough expertise in cleaning a particle collector. In fact, the interaction between the collector and the user should be at an absolute minimum, and also so simple as not to detract from either living comfort or cleaning efficiency. Besides the required operating ease, the user will not have any previous routines relating to cleaning a collector, because this is entirely new. Therefore, the cleaning of the collector must be very simple and easy, so that it will perform its task efficiently and will also have a long life.

The present invention is intended to mitigate the problems associated with collectors according to the prior art and thus create an improved device and method for cleaning flue gas of particles. The characteristic features of the device according to the invention are stated in Claim 1. In addition, the device also relates to a system according to Claim 12 and a method according to Claim 15 for cleaning a flue-gas flow of particles .

The collector means of the device include a collector spiral element, which is arranged to form a helical through-flow channel for the flue gas. By means of this, the flue-gas flow is better directed to the device' s collector means and an effective collection result is achieved.

The collector spiral element provides a long interaction length in the flow-through channel for the flue-gas flow and by means of it a significant advantage relating to the cleaning of the device is achieved.

The operational totalities of the particle collector can be implemented in several different ways. According to a first embodiment, the operation of the device can be so-called single-stage, in which the creation of ionization and the collection of the particles are combined to take place in the same device. According to a second embodiment, the operation of the device can be so-called two-stage. The ionization of the particles then takes place either partly or even entirely outside the device while the actual collection of the particles is performed in the device.

The single-stage device according to the first embodiment can be implemented, for example, in such a way that a charger spiral element for charging the particles electrically is also integrated in the device. A charger spiral element can be fitted in between a collector spiral element forming a helical through-flow channel, while being electrically insulated from the collector spiral element. Particle charging can then be performed over the entire length of the collector spiral element. In addition, by being in the middle of the through-flow channel formed by the collector spiral, the collector spiral element will be symmetrical relative to the charger spiral element .

The construction of the helical collector component is very simple and compact. It provides a flue-gas particle-cleaning solution suitable for long-term operation. According to one embodiment, the collector spiral element is arranged principally to fill the device's chamber, so that the tips of the helix lie tightly against the internal wall of the chamber. In addition, the collector spiral element is arranged to rotate around its central axis. Significant advantages are achieved by rotation of the collector spiral element. A first advantage achieved by rotation is that the device can be cleaned by rotating the collector spiral. By rotating the collector spiral, particles that have accumulated in the device can be moved, for example, to a collector arrangement arranged at the end of the collector spiral. In addition, by rotating the collector spiral, it is possible to achieve the agitation impulse necessary to detach the particles from the collector spiral and also detach material that may have become layered on the inner walls of the chamber, which for its part restricts the through-flow channel.

Besides the aforementioned advantages, several important additional advantages are also achieved by means of the invention. The effectiveness of the device corresponds at least to that of known rectangular collectors, but the device is substantially- more compact than them. The device is unaffected by the form of the flue-gas flow, it has fewer dirtying problems and it is easy to clean. The device can be used to separate electrically as much as 80 % of the particles in a mass flow and solve the problems relating to the collection of particles, which have been a bottleneck in the adoption of ESP devices in small-scale combustion. The other characteristic features of the invention will become apparent from the accompanying Claims while other advantages achieved by means of the invention are stated more extensively in the description portion.

The invention, which is in no way restricted by the embodiments described in the following, is described in greater detail with reference to the accompanying figures, in which Figure 1 shows a side view of an embodiment of the device, without a chamber,

Figure 2 shows an embodiment of the device, fitted to a flue- gas flow channel, Figure 3 shows a side view of an example of the collector and charger element of the device,

Figure 4 shows a perspective view of the collector and charger element shown in Figure 3,

Figure 5 shows an exploded view of an embodiment of the de- vice,

Figure 6 shows a schematic diagram of the cleaning of a flue- gas flow of particles with the aid of an electrical field,

Figure 7 shows a cross-section of a second embodiment of the device, and

Figure 8 shows a schematic example of a heating system, to which is fitted an embodiment, without a charging element, of the device.

Figure 6 shows a rough schematic diagram of the basic principle of collecting particles 18, suspended in a flue-gas flow, from the flow. In it, the particles 18 in the flue-gas flow are subjected to gas ions 32 in an electrostatic field 17. As a result, the particles 18 become charged and drift under the influence of the electrical field 17. Taken roughly, the operational mechanisms of electrostatic cleaning can be divided into the main stages of the ionization of the flue gas and the collection of the particles 18. Sub-stages in the cleaning are then the production of the electrostatic field 17 in order to achieve charging and drifting of the small particles 18, sufficient retention of the flue-gas flow, to permit the drift of the particles 18 onto the collector surface 12, the prevention of the re-removal of the particles 18, and the removal of the collected particles 18 from the collection surface 12 and the collector device. In order to achieve gas ionization, the electrical breakdown strength of the gas is exceeded, at least locally. The particles 18 can be charged with the aid of a so-called corona discharge, in which the progress of the discharge remains incomplete. A high positive or negative potential is brought to the charger electrode 16. A sufficiently strong, inhomogeneous electrical field around the charger electrode 16 causes the air to form a conductor, in which ionization of the molecules of the flue gas takes place, when electrons detach from the atoms of the flue gas. The inhomogeneous electrical field around the electrode 16 limits the strength of the electrical field 17 to be strong only in a small area, so that a rapid conducting of current, i.e. a movement of the electrons, to the opposite potential, i.e. spark-over will not take place. At the same time, the powerful electrical field 17 accelerates the positive and negative ions that arise towards the potential with the opposite sign, which is directed at the collector electrode 12.

An electron detaching from an atom during ionization can re- peatedly ionize other atoms of the gas. During continuous maintenance of the electrical field, the positive and negative ions moving in different directions in this ionization area

(corona area) of the electrical field 17 collide with the other neutral molecules and particles of the gas. In the collisions in the ionization area, the neutral gas molecules are in turn ionized, so that a mass drift of the ions, which are either positive or negative depending on the sign of the potential of the charger electrode 16, arises towards the potential 12 with the opposite sign. As such, the operating principles of ESP devices are well known among those versed in the art.

Figure 1 shows a side view of an embodiment of the device 10, without a collector chamber while Figure 2 shows it with a chamber 29 fitted to the flue-gas channel 11.1, 11.2. The device 10 is used to clean a flue-gas flow 19 of the particles 18 that travel with it. For this task, the device 10 can be fitted to a flue-gas channel formed from one or several consecutive parts, at one end of which there can be, for example, a heating device 39, or a corresponding source of flue gas, while at the other end there is an exit opening 33 (Figure 8) .

With reference to the exploded view of Figure 5, the device can be said to have an intake side 20.1, which is equipped with an inlet opening, and an exhaust side 20.2, which is equipped with an outlet opening, with a casing chamber 29 remaining between them, through which the flue-gas flow 19 is led through the device 10. In the casing chamber 29, there can be a chamber component 29.2 with a cylindrical cross-section while its end

29.1 on the intake side 20.1 can be conical. The length of the conical portion 29.1 can be, for example, 1/5 of the length of the cylindrical portion 29.2. The length of the entire chamber 29 in the direction of travel of the gas flow can be, for example, 300 - 800 mm while the internal diameter of the cylindrical portion 29.2 can be, for example, 200 - 400 mm. The chamber 29 can be of, for example, some corrosion-resistant material and there can be insulation in it (not shown) .

At the opposite end of the flue-gas-flow channel 11.1 to the intake side 20.1 of the chamber 29, there is a combustion device, for example, a heating device 39 or similar source of flue gas (Figure 8) while at the opposite end of the flue 11.2 to the exhaust side 20.2 there is an outlet opening 33, from which the cleaned flue gas 19' is led out of the device 10, generally to the outside air. In the case according to the embodiment, the intake side 20.1 channel 11.1 is mainly hori- zontal while the exhaust-side 20.2 channel 11.2 is mainly vertical. Of course, the orientations of the channels 11.1,

11.2 do not affect in any way the basic principle of the invention, but, by arranging the intake channel 11.1 and the device 10 too at a rising slant to the direction of flow of the flue gas, it is possible to affect, for example, the draft. As an example of the cross-sectional diameter of the flues 11.1, 11.2, it is possible to give 50 - 800 mm, more usually 100 - 300 mm, though typically one that is less than that of the cylindrical portion 29.2 of the device 10.

5 As can be seen from Figure 1, the device 10 includes collector means 12 for removing electrically-charged particles 18 from the flow being led through the device 10. By using them, the particles 18 can be collected from the flue gas 19, with the aid of an electrical field. For collecting, the collector means

10 12 are connected to a selected potential, which creates the electrical charging of the collector means 12. The flue-gas flow 19 through the collector means 12 can be lead through the device 10, when the effect of the collector means 12 will be mainly directed to the entire flow.

15

The collector spiral element 12 fitted inside the chamber 29 of the device 10 forms a surface 14, transversely to the flow direction of the flue-gas flow 19, guiding the flue-gas flow 19. This achieves several important advantages. Due to the

20 transverse surface, it is not essential to arrange special flue-gas flow 19 dispersing and/or spreading structures, which have a tendency to collect dirt and would be difficult to clean, for the intake side of the device 10. In addition, thanks to the transverse surface 14 of the spiral 12, the flow

25 19 makes better contact with the collector element 12 and it can also be used to retain the flue-gas flow sufficiently to permit the drift of the particles 18 onto the collector surface 12. The through-flow channel 13 formed inside the chamber 29 is closed over the axial cross-section of the chamber 29. In other

30 words, when examined from the intake-side 20.1 end of the chamber 29, the flue-gas flow 19 does not have a direct unobstructed path to the end of the exhaust side 20.2 of the chamber 29, but instead the transverse helical surface 14 located in the chamber 29 cuts off the direct flow connection between

35 the ends 20.1, 20.2 principally over the whole area of the chamber cross-section. As can be seen from Figure 1 and further from Figures 3, 4, and 7, the transverse surface 14 guiding the flue-gas flow 19 can be formed by means of an elongated collector spiral element 12 belonging to the collector means, by means of the electrical field created by which the charged particles 18 are collected on the collector surfaces 14 of the spiral. The collector spiral 12 can be, for example, electrolytically polished, thus improving its collecting capability. The helix element 12 can form a helical through-flow channel 13 inside the chamber 29 for the flue gas 19. So that most of the flow 19 will travel along the helical channel 13 and not, for example, through the edges of the chamber 29, the radial cross-section of the of the collector spiral element 12 relative to the internal cross- section of the chamber 29 is arranged to be such that the helix element 12 is arranged to mainly fill the chamber 29, in the direction of its cross-section. For example, there can be a Teflon seal (not shown) at the edges of the tips of the helix element 12. The internal surface of the chamber 29 will then form a wall delimiting the flow channel 13. Correspondingly, at the inner farthest end of the collector spiral element 12, the helical tip surface 14 always extends to the central shaft 23 of the helix element 12.

When the collector surface 14 is in an auger-like form, it forms a helical elongated structure, by means of which the collection efficiency of a traditional plate-like collector model is achieved, but without requiring expensive, dirtying, and space-taking flue-gas baffles, which require separate cleaning. In the embodiment of Figures 1 - 4, there are six turns of the helix, the gap between them, i.e. the through-flow channel 13, between them having a width of, for example, 30 -

120 mm.

The velocity of the flue-gas flow 19 in a small-scale combus- tion flue-gas-flow channel 11.1, 11.2 is generally about 1,5 m/s. In a device 10 of the order of size according to the invention, the distance travelled by the particles 18 is about 1 - 6 m, more generally 3 - 5 m, for example, 4 m, the entire collection surface area of the device 10 being 0,5 - 2 m², more generally 0,5 - 1,5 m², for example 1 m², and the delay about 1,5 - 4 s, more generally 2 - 3 s.

Further, the device 10 can additionally include a elongated charger spiral element 16 fitted to the through-flow channel 13 for charging the particles 18, which is also shown in Figures 1, 3, 4, and 7. The spiral 16, by means of which the particles 18 in the flue-gas flow 19 are charged electrically, can be in between the collector spiral element 12 forming the through- flow channel 13 and be electrically insulated from the collector spiral element 12. In that case, the electrodes are heli- cally wound in between each other through the channel 13. The charger spiral element 16 can be used to produce a charge in a controlled manner to the particles 18, because the surfaces 14 of the collector channel 13, i.e. the helix of the collector spiral 12, are symmetrical relative to the helix of the charger spiral 16. Thus, there are now six rounds in the charger helix 16 too and, because the charger spiral 16 is a double spiral ying within the collector spiral 12, the gap between the rounds of the charger spiral 16 can also be 30 - 120 mm. In the radial direction, the charger spiral 16 can be closer to the outer jacket 29.2 of the chamber 29 than to the centre shaft 23 of the collector spiral 12.

In addition, the device 10 includes high-voltage feed-throughs 21.1, 21.2, for connecting the charger sprial element 16 and the collector spiral element 12 to the selected opposing voltage potentials. At least one of the feed-throughs 21.1, 21.2 can be fitted entirely to the centre shaft 23 of the helix element 12. It should be noted that the charger element 16 and the collector element 12 are insulated from each other. Thus, the collector electrode 12 can, as the larger element, be eguipped with its own centre shaft 23 and the charger element 16 can be implemented as simply a helix without an actual axial cent re . Wi th s ui t abl e s uppo rt , t he cha rge r spi ra l 1 6 wi l l remain a l ight structure separate from the col lector element 12 .

5

In the tip of the charger spiral 16 there can be a tubular centre 37 (Figure 7), which can be bent from the same material as the tips 38.1, 38.2 of the charger spiral 16. The centre 37, for its part, gives rigidity to the charger spiral structure 16

10 too. Such support can also be implemented in, for example, the insulation disc 34 at the end of the exhaust side 26.2, on which the charger element 16 is supported by means of a rod 35 (Figures 3 and 7) . Of course, there can also be insulated supports on the shaft 23 of the collector 12. In connection

15 with the chamber 29, there can be only a single, even common feed-through to both electrodes 12, 16, thus minimizing the number of dirtying high-voltage feed-throughs . In the feed- throughs 21.1, 21.2, there can be a non-melting insulation, or a corresponding location for the connection points. One possi-

20 bility for a feed-through is described in greater detail slightly later in Figure 7.

The tip 24 of the spiral of the charger spiral element 16 can be sharp-edged, in order to concentrate the charge on the tip

25 24. Thus, the sharp edges 24 of the charger surface 16 of the collector device create ion discharges in the through-flow channel 13, in which the charging of the flue-gas particles 18 takes place. The diameter Dl of the charger spiral element 16 can be, for example, 20 - 70% of the diameter D2 of the collec-

30 tor spiral element 12 (Figure 4) . The height of the tip 24 of the charger spiral 16 can be, for example, 30 mm.

An elongated and sharp corona wire 25 or similar charger element for charging the particles 18 by means of an ion discharge 35 already on the intake 20.1 in the flue-gas-flow channel 11.1 can be fitted to the intake side 20.1 end of the hub of the charger spiral element 16. The possible corona wire 25 will improve the controlled charging of the particles 18, which will begin even before the collector device 10, but in which the charging of the particles 18 will nevertheless still take place using structures integrated in the device 10. The length of the charger wire 25 can be, for example, 30 - 600 mm and it can be in the centre of the flue channel 11.1. The charger wire 25 can be created by suitably bending the end of the charger helix 16 and cutting the charger helix material.

By equipping the device 10 with a charger spiral element 16 and a possible corona wire 25, the device 10 will operate as an independent electro-filter package. If the edges 24 of the charger spiral 16 are rounded and the optional corona needle 25 is removed, a separate particle charger, such as a blast charger, can be arranged, for example, in front of the collector device 10. A schematic embodiment of this is shown in Figure 8.

At least the collector spiral element 12, or alternatively the entire electrode package 12, 16 can be rotated. The rotation can take place, for example, through the centre shaft 23 of the collector spiral element 12. This feature permits the cleaning of the electrodes 12, 16, and the chamber 29, more generally the through-flow channel 13, by means of a rotation movement of the collector spiral element 12. By rotating, the collector spiral element 12 is able to move the particles 18' that have accumulated inside the device 10 in the chamber 29 to a collector arrangement 27, which can be, for example, on the exhaust side 20.2 of the device 10, and out of the flue 11.2. The rotation allows the continuous and effective operation of the electro-filter 10, i.e. the removal of the accumulated particle layer from the collection surfaces 14, as well as the removal of the collected particles 18' from the chamber 29 of the device 10. According to one embodiment, the collector arrange- ment 27 can be equipped with an emptying hatch and thus able to be opened and closed again. The collector arrangement 27 can be formed by a casing chamber 36 delimiting the ash space, which is a saddle-like semicircle open at the top, to the exhaust side of which 20.2 the collection chamber 29 can be fitted. The casing chamber 36 can have a rectangular cross-section.

Figure 7 shows a cross-section of a second embodiment of the device 10, which does not have a corona wire on the charger element. In this case, tips 38.1, 38.2, with a height of, for example, 30 mm, are formed, for example, of metal strips, are on both side of the tube hub 37 of the charger spiral 16. In addition, the cone 29.1 on the intake side 20.1 of the device 10 is steeper than in the previously described embodiment. The steepness of the cone 29.1 can be, for example, 35 - 75 de- grees, more particularly 45 - 60 degrees. The steeper cone 29.1 gives the device 10 a shorter construction while the cone 29.1 will remain cleaner better, due to the greater drop.

Figure 7 also shows the voltage feed to the charger and collec- tor elements 12, 16 in greater detail. The collector element 12 can be earthed through its centre shaft 23, by means of which it can be rotated. The voltage-feed cable 40 of the charger element 16 can be brought into the device 10 through the centre shaft 23 of the collector spiral element 12, from where it diverts to the charger spiral 16.

In the centre shaft 23, there can be a radial feed-through arm 41, inside which the cable 40 is fitted. The feed-through arm 41 can extend in the radial direction of the cylinder 29.2 to the level of the charger spiral 16, from where the cable 40 can be connected to the charger spiral 16, for example, by means of a conductor rod 35, which can be formed, for example, by an extension of the hub 37 of the charger spiral 16. The length of the feed-through arm 41 can be used to affect the spark-over sensitivity in such a way that a longer arm 41 will reduce the spark-over sensitivity. The feed-through arm 41 can be mainly of a ceramic 42, for example. It will then remain cleaner better. In addition, the arm 41 can also be vitrified.

There can be a cleaning element 43 on top of the ceramic insu- lation 42, which sweeps the insulation 42 of the arm 41 clean.

For example, a sleeve 43, which can move backwards and forwards by gravity due to the rotation of the centre shaft 23 of the collector spiral element 12 and thus clean the insulation arm

41 as it moves, can act as this element. Thus, the cleaning of the insulation arm 41 can take place by its own power at the same time as the device 10 is cleaned by rotating the collector spiral 12.

As can be seen from the embodiment of Figure 7, instead of a hatch, the collector arrangement 27 can also be a detachable box 44 with catches. The motor 28 rotating the collector- charger package 12, 16 can be set parallel to the cylindrical component 29.2 behind the exhaust pipe 11.2. The chain transmission between the motor 28 and the centre shaft 23 (not shown) can be in a casing 45. The bearings 46 of the shaft 23 can be inside the device 10, which will further shorten the construction of the device 10. The dust-tightness of the bearings 46 can be taken care of by a labyrinth seal 47 and vacuum. In the voltage-transmission casing 48 there can be axially loaded slider terminals to the voltage cable 40 and the earthing cable of the shaft 23. They can be at different levels in the axial direction of the device 10, in order to prevent spark-over in the air gap.

Figure 8 shows a schematic diagram of an embodiment in principle of a system, in which the device 10' can be one sub-component. The basic components of the system include an apparatus 39 for burning a fuel, a set of flue-gas-flow channels 11.1, 11.2, and a device 10' according to the invention. By means of the flue-gas-flows channels 11.1, 11.2 attached to the combustion apparatus 39, the flue gas 19 produced from the combustion of the fuel is removed from the combustion device 39. The device 10' is arranged in connection with the flue-gas-flow channel 11.1, 11.2 and can be used to clean the flue-gas flow 19 of particles 18. In this embodiment, the device 10' does not include its own charger, but instead before it there is an independent charger unit 16', for example, immediately before the collector device 10'.

In addition to the device 10, the invention also relates to a method for cleaning a flue-gas flow 19 of particles 18. In the method, the particles 18 in the flue-gas flow 19 are charged electrically, removed from the flue-gas flow 19 with the aid of the electrical field of collector means 12, and the flue-gas flow 19 is led out of the electrical field, for example, to the outside air. The flow-gas flow 19 containing the charged particles 18 is led through a through-flow channel 13 formed by helically-shaped collector means 12, in which the charged particles 18 are removed from the flue-gas flow 19. The flue- gas flow 19 is then led towards a helical surface 14 formed by the collector means 12 and transverse to the flue-gas flow 19, by means of which helical surface 14 the flue-gas flow 19 is guided through the device 10.

Once a set criterion has been met, the device 10 can be cleaned of the particles 18' accumulated in it. The criterion can be based, for example, on experiential data, or on primary (e.g., dirtying) or secondary (e.g., heating operating hours) measurements, which correlate with the dirtying of the device 10. The device 10 can, for example, be cleaned at two-weekly intervals, depending of course largely on, for example, the heating system.

The accumulated particle material 18 ' can be cleaned from the collector electrode 12 by shaking, when with the aid of suffi- cient acceleration the particle layer adhering to the collector spiral element 12 will detach in pieces. The particle material 18' shaken onto the bottom of the chamber 29 is removed from the device 10, so that it will not reenter the flue-gas flow, which leaves the device 10 from its exhaust side 20.2 into the outlet channel 11.2.

The removal of the particle material 18' from inside the chamber 29 of the device takes place surprisingly by rotating at least the collector spiral element 12 around its centre shaft 23. According to one embodiment, the rotation can be performed when the heating device is not producing smoke. Of course, the collector spiral 12 can be kept continuously gently rotating. In that case, the collector spiral element 12 will move the particles 18' out of the device 10 like a screw conveyor. More particularly, by rotating the collector component 12 in the correct direction determined by its helix, the removal of the particle material 18' accumulated inside the device 10 can be performed easily in such a way that the spiral 12 and more particularly its tip part will sweep the internal space of the chamber 29 and itself clean and the heavy particle mass 18' will fall into the collection chamber 27' at the exhaust end 20.2 of the spiral 12. The high-voltage feed-throughs 21.1, 21.2 too, of which there is only one per electrode 12, 16, can be cleaned with the aid of mechanical rotation. The contacting of the voltage connections can be arranged, for example, in such a way that the rotation of the spiral 12 is terminated always at the same point, so that the connection contact 21.1 will remain in contact with its counter-contact. Contacting arranged on the same shaft 23 is also possible, as described in connection with the embodiment of Figure 7. The contact 21.2 can be a slider contact at the end of the centre shaft 23 of the spiral 12.

Rotation can be performed either manually by a lever 31 and/or electrically, in which case a motor 28 is connected to the centre shaft 23, for example, through a chain/belt transmission

30. The motor 28 can receive its operating voltage suitably transformed from the same point as the electrodes 12. 16. When cleaning the chamber 29, the rotating speed of the motor 28 need not be high, for example, 8 rpm, if the collector spiral 12 rotates continuously.

The agitation required to detached the particles can also be made by rotating the collector spiral 12, though agitation can also be arranged in other ways. One possibility can be a small axial motion in the collector spiral 12 in the longitudinal direction of the chamber 29, when the ends of the spiral 12 will knock against the ends of the chamber 29, creating suitable agitation.

Aided by the rotation of the collector spiral 12, suction in the flue channel 11.1 can be increased momentarily, for example, when the fireplace 39 is being lit. Thus, in connection with pellet-combustion devices for example, the use of separate flue-gas fans can be replaced by the motorized operation of the device 10. For this purpose, the rotation speed of the motor 28 can be varied, for example, with the aid of a gearbox or frequency converter.

By the continuous rotation of the electrode package 12, 16, or at least of the collector spiral 12, the advantage can also be obtained that the internal wall of the chamber 29 remains continually clean, as does the insulation 41, 42 of the charger electrode 16, which the sleeve 43 cleans with the aid of gravity. In addition, the pressure loss of the device 10 can be reduced by a gentle continuous rotation of the spiral 12. Pressure loss in the device 10 can be greater that in plate- type collectors of the prior art, because the device forces the flow into a tangential motion. The small particles 18 progressing in the through-flow channel 13 collide with each other and thus increase their size. They then drop, for example, towards the exhaust side 20.2 of the device 10, being conveyed by gravity. Though some of the already collected particles may rebound back into the flue-gas flow due to the rotation of the spiral 12, the recharging of particles takes place over the entire length of the collector electrode 12. In pilot-stage tests, it was observed that the accumulated particle mass of a pellet burner typically detached preferably as pieces, rather than as individual particles.

The combustion device 39 for burning a fuel, in connection with which the invention can be applied, can, for example, heat a building or produce domestic hot water or other energy services. Depending on the fuel used, various solutions, such as boilers, fireplaces, or sauna stoves, are used as the combustion device 39. The operation of the heating device 39, which can be a heating boiler, a (heat-storing) fireplace, a wood- pellet stove, a woodchip stoker, a baking oven, or a sauna stove, can be, as such, known, or based on some technology that is still being developed, and based on either manual or automatic (continuous) feed and possibly equipped with a burner. A manually stoked heating boiler can be equipped with a hot-water storage tank or similar arrangement.

Within the context of the invention, the term small-scale combustion refers to combustion on a small scale, in which the fuel used is generally wood, light fuel oil, domestic waste, or a combination of these. Some examples of solid fuels are chopped firewood, wood pellets, wood briquettes, and woodchips. In small-scale combustion, the nominal output of the heating device 39 can be, for example (10 kW) 15 - 300 kW or even larger than that, but nevertheless typically less than 1 MW small-scale combustion, which takes place in a small volume flow. Instead of output class, the combustion apparatus 39 can be said to be more a local source causing a local fallout of small particles than a source causing the long-range transport of airborne pollutants. Small-scale combustion is generally used to heat, for example, detached and terraced houses, out- door and small buildings, agricultural buildings, and larger individual buildings.

In small-scale combustion, the temperatures of the flue gases 19 vary according to the combustion device being used, in such a way that the temperatures with a boiler are 120 - 190⁰C, with a heat-storing fireplace 210⁰C, and a sauna stove 350⁰C. The flue-gas temperatures of pellet-fired boilers are 135°C and of oil-fired boilers 165°C. The vacuum of the flue gas 19 is 18 - 45 Pa with boilers and 12 Pa with fireplaces.

In continuous combustion, the average particle emissions are 10

- 50 mg/MJ, in good batch firing 50 mg/MJ but on average about 100 mg/MJ. The lowest particle emissions can be a few mg/MJ, rising momentarily in poor combustion conditions to as much as 1000 mg/MJ. In batch firing, the particle emissions created in the different stages of combustion are greatest during the ignition stage.

The size distribution of the particles of wood burning are typically two-peak, in the modes of ultra-small particles and of larger particles can be distinguished. The diameter of the ultra-small particles is less than 1 μm and that of the coarse particles greater than this (more than 2.5 μm) , the largest of these being 250 μm. In wood burning, the particle concentration is 100 - 1000 mg/Nm3, depending of the manner of combustion and the device, with a modern wood-pellet burner (300 kW) a particle concentration of 50 mg/Nm3 can be achieved. The particles transferring to the flue-gas flow of combustion and the parti- cles arising in the flue-gas flow typically have a size of 0,02

- 250 μm. The number of flue-gas particles in small-scale combustion is 10¹² - 10¹⁴ particles/MJ. Correspondingly, the size of the flue-gas molecules is 0,1 - 6 μm and the number in the order of 10²⁴ particles/MJ. It must be understood that the above description and the related figures are only intended to illustrate the present invention. The invention is thus in no way restricted to only the embodiments disclosed or stated in the Claims, but many different variations and adaptations of the invention, which are possible within the scope on the inventive idea defined in the accompanying Claims, will be obvious to one versed in the art .

Claims

1. Device for cleaning a flue-gas flow of particles, which device (10) can be fitted to a flue-gas flow channel (11.1, 11.2), and which device (10) includes collector means (12) for collecting charged particles (18) with the aid of an electrical field (17), and through which collector means (12) the flue-gas flow (19) is arranged to be led through the device (10), characterized in that the collector means (12) include a collector spiral element (12) , which is arranged to form a helical through-flow channel (13) for the flue gas (19) .

2. Device according to Claim 1, characterized in that the device (10) includes in addition a charger spiral element (16),, fitted to the through-flow channel (13) , for charging the particles (18), which is fitted in between with the collector spiral element (12) forming the helical through-flow channel (13) .

3. Device according to Claim 1 or 2, characterized in that the device (10) includes a chamber (29) equipped with intake and outlet openings (20.1, 20.2), and which chamber (29) the said collector spiral element (12) is mainly arranged to fill.

4. Device according to any of Claims 1 - 3, characterized in that the device (10) includes high-voltage feed-throughs (21.1, 21.2) for connecting the charger spiral element (16) and the collector spiral element (12) to a selected voltage potential, at least one of which feed-throughs (21.1, 21.2) being fitted to the centre shaft (23) of the helix element (12) .

5. Device according to any of Claims 2 - 4, characterized in that the diameter (Dl) of the charger spiral element (16) is 20 - 70% of the diameter (D2) of the helical collector element (12) .

6. Device according to any of Claims 2 - 5, characterized in that the tip (24) of the charger spiral element (16) is arranged to be sharp, in order to concentrate the charge on the tip (24) .

5

7. Device according to any of Claims 2 - 6, characterized in that an elongated charger element (25) is fitted to the intake end (26.1) of the device (10), in order to charge the particles (18) already in the flue-gas-flow channel (11.1).

10

8. Device according to any of Claims 1 - 7, characterized in that the device (10) includes a collector arrangement (27) fitted to its exhaust side (20.2) for removing the particles (18') collected by the device (10) from the connection of the

15 device (10) .

9. Device according to any of Claims 1 - 8, characterized in that means (23, 28, 30, 31) are fitted to the collector spiral element (12) in order to rotate it.

20

10. Device according to Claim 9, characterized in that the through-flow channel (13) is arranged to be cleaned by means of the rotating motion of the collector spiral element (12) .

25 11. Device according to any of Claims 2 - 10, characterized in that the charger spiral element (16) is arranged to be charged through the centre shaft (23) of the collector spiral element (12) and a feed-through arm (41) fitted radially to it, which arm (41) is arranged to be cleaned by means of the rotating

30 motion of the collector spiral element (12) .

12. System for fuel combustion, which includes

- a combustion apparatus (39),

- a flue-gas-flow channel (11.1, 11.2) connected to 35 the combustion apparatus (39) , for removing the flue gas (19) formed by the combustion of the fuel from the combustion apparatus (39),

- a device (10) fitted in connection with the flue- gas-flow channel (11.1, 11.2) for cleaning the flue- gas flow (19) of particles (18) , which includes charger means (16, 25) fitted in connection with the flue-gas-flow channel (11.1, 11.2) for charging the particles (18) electrically, and collector means (12) for collecting the charged particles (18) with the aid of an electrical field, and through which collector means (12) the flue-gas flow (19) is arranged to be led through the device (10), characterized in that the collector means include a collector spiral element (12) , which is arranged to form a helical through-flow channel (13) for the flue gas (19) .

13. Combustion apparatus (39) for a system according to Claim 12, which can be fitted at the operating location to a flue- gas-flow channel (11.1, 11.2) and in which combustion apparatus (39) a fuel is arranged to be burned, which, when burned, forms a flue-gas flow (19) containing particles (18), characterized in that the particles (18), which are formed when burning the fuel in the said combustion apparatus (39), are arranged to be removed from the flue-gas flow (19) by means of a device (10) according to any of Claims 1 - 11.

14. Combustion apparatus according to Claim 13, characterized in that the combustion apparatus (39) is a device of an order of size, which when installed at the operating location and attached to the exhaust pipe (11.1, 11.2) of the flue gases (19) , is a local small-particle source causing local fallout of the particles (18) .

15. Method for cleaning a flue-gas flow of particles, in which - the particles (18) in the flue-gas flow (19) are charged electrically, - the charged particles (18) are removed from the flue-gas flow (19) with the aid of the electrical field (17) of collector means (12),

- the flue-gas flow (19) if led out of the electrical field (17) , characterized in that

- the flue-gas flow (19) containing the charged particles (18) is guided through a through-flow channel (13) formed as a helix by the collector means (12), - the charged particles (18) are removed from the flue-gas flow (19) in the said through-flow channel (13) .

16. Method according to Claim 15, in which in addition the through-flow channel (13) is cleaned of the particles (18') that have accumulated in it, characterized in that the collector means include a collector spiral element (12), which is rotated, in order to clean the through-flow channel (13) .