Device for purifying the flue gases of a wood-fuel fireplace

Abstract

The invention relates to a device for purifying the flue gases of a wood-fuel fireplace (1), having a catalytic purification device (6) arranged in the flue gas duct (5) of the wood-fuel fireplace (1) for the oxidation of carbon monoxide and / or nitrogen oxides and for the catalytically-induced combustion of soot and / or carbonization products, and having a fine particles trap (13), arranged in the flue gas duct (5) downstream of the catalytic purification device (6) in the flow-through direction, for trapping fine particles that remain in the flue gas. In order to ensure, in such a purification device, that the catalytic purification device (6) is brought, starting from any operating state of the wood-fuel fireplace (1), as quickly as possible to the operating temperature required for it to be effective without the need for additional electrical heating, the invention proposes providing, in the flue gas duct (5) in the region between the catalytic purification device (6) and the fine particles trap (13), a throttle valve arrangement (7) that is actuated on the basis of the temperature of the flue gas.

Description

[0001] The invention relates to a device for cleaning the flue gases of a wood-fuel fireplace, comprising a catalytically active cleaning device arranged in the flue gas duct of the wood-fuel fireplace for the oxidation of carbon monoxide and / or nitrogen oxides and for the catalytically stimulated combustion of soot and / or carbonization products, and comprising a fine dust separator arranged in the flue gas duct downstream of the catalytically active cleaning device for the separation of fine dust remaining in the flue gas. Such a device is known, for example, from DE 10 2007 027 136 A1. Furthermore, WO 2019 / 193084 A1 discloses a device for cleaning flue gases with the features of the preamble of claim 1.

[0002] The combustion of wood in the form of logs or wood pellets is becoming increasingly important for the provision of heat from renewable energies. A long-known problem with the combustion of wood in wood fuel fireplaces is the high pollutant content of the flue gases produced during wood combustion. The pollutants contained in the flue gases consist partly of solid particles, namely soot (i.e. unburned carbon particles) and dust with a high proportion of fine dust containing a complex mixture of solid and liquid particles, i.e. evaporation and carbonization products that are produced during the drying and carbonization (pyrolysis) of the wood fuel that initiates the combustion process. The flue gases also contain gaseous pollutants, including highly toxic carbon monoxide (CO), volatile organic hydrocarbons and nitrogen oxides (NOx).Due to the dangers and environmental damage caused by these pollutants, strict legal regulations have been in place for some time to minimize these pollutant emissions while adhering to specified limit values.

[0003] Prompted by these legal regulations, a number of proposals for devices to clean the flue gases from wood-fuel fireplaces have also been on the agenda for some time, including the proposal of the type mentioned above. The device operating according to this proposal has the fundamental advantage that the catalytically effective cleaning device mentioned there is installed upstream of the fine dust separator located in the flue gas duct. This catalytically effective cleaning device ensures that the flue gases escaping from the wood-fuel fireplace are first largely freed of both gaseous pollutants and soot particles or smoldering products through catalytically stimulated oxidation (i.e., combustion) before being fed to the fine dust separator.As a result, only dry dust needs to be separated in the fine dust separator, which has great advantages for the separation itself and especially for the cleaning of the fine dust separator that is required from time to time.

[0004] One problem with the catalytic cleaning devices mentioned above is that they only work effectively once the correct operating temperature has been reached. They only become effective at temperatures between 150°C and 200°C. The optimal operating temperature is between 300°C and 400°C. A maximum of 800°C must not be exceeded. Maintaining an optimal temperature window when using the catalytic cleaning device is particularly difficult with wood-fuel fireplaces because wood fuel combustion is very uneven.

[0005] According to the prior art (DE 696 04 027 T2), it is known in the context of a heavy fuel oil combustion plant to provide a catalytically active purification device in the exhaust gas stream to remove as much nitrogen oxides (NOx) as possible from the exhaust gas. To maintain this catalytically active purification device at the optimal operating temperature, a preheating system consisting of a superheater, a reheater, a preheater, and a flow control valve is installed upstream of it in the flow direction, and a low-temperature preheater is installed downstream.

[0006] In contrast to heavy oil firing, wood firing distinguishes between the combustion phases of initial combustion, main combustion, and burnout. Pollutant emissions are usually relatively high during the initial combustion phase and often also during the burnout phase due to the low flame temperature, especially during the burnout phase when fuel is added.

[0007] In order to make the best possible use of the energy content of the wood fuel used and at the same time keep pollutant emissions as low as possible, it is well known and common practice to control the chimney draught in the flue gas duct using throttle valves so that combustion takes place in the wood fuel fireplace that is optimized both in terms of heat yield and pollutant emissions.

[0008] If a catalytic cleaning device of the type mentioned above is installed in the flue gas duct of a wood-fuel fireplace, the chimney draft naturally also influences its function, because the temperature of the passing flue gases and their flow velocity depend on the chimney draft. As a result, the temperature and flow velocity of the passing flue gases should be adjusted so that, on the one hand, the temperature window discussed above is maintained for as long as possible, and, on the other hand, the contact between the flue gas and the catalytically active surfaces of the cleaning device is as intensive as possible.

[0009] To achieve this, it has already been proposed according to the prior art (cf. DE 10 2006 021 133 A1) to arrange a lens-shaped catalyst cartridge filled with catalytically active metal chips in the flue gas duct, which catalyst cartridge can be pivoted between a blocking position and a passing position. This catalyst cartridge should also take on the function of a throttle valve, i.e. in the blocking position it should reduce the chimney draft to a minimum and in the passing position it should increase it to a maximum. This dual function gives rise to a conflict of objectives in that in the passing position, i.e. when a rapid temperature increase and intensive contact with the catalytically active surfaces is required, this intensive contact is not present because the catalyst cartridge is in the passing position.For this reason, this prior art requires the catalyst cartridge to be equipped with an additional electric heater, which brings the catalytically active metal chips to the temperature required for continuous operation as quickly as possible. However, such an additional electric heater is complex and maintenance-intensive. A further disadvantage is that such a pivoting catalyst cartridge does not allow for the fine adjustment of the chimney draft required for sufficiently precise control of the combustion conditions in the wood-fuel fireplace.

[0010] The object of the invention is therefore to further develop the device of the type mentioned above such that the catalytically active cleaning device can be brought to the operating temperature required for its effectiveness as quickly as possible, starting from any operating state of the wood-fuel fireplace, without the need for additional electrical heating. Furthermore, the chimney draft in the wood-fuel fireplace should be adjustable as precisely as possible.

[0011] To achieve this object, the invention, based on the device of the type mentioned above, proposes that a throttle valve arrangement actuated as a function of the temperature of the flue gas be provided in the flue gas duct in the area between the catalytically active cleaning device and the fine dust separator. Such a throttle valve arrangement, separate from the catalytically active cleaning device and arranged downstream of it in the flow direction, makes it possible for the first time to finely control the chimney draft prevailing in the flue gas duct without impairing the flow conditions of the catalytically active cleaning device. Consequently, it is also possible for the first time to quickly bring the catalytically active cleaning device to the required working temperature without external additional heating and to maintain this working temperature with sensitive control.

[0012] A useful development of the invention provides that the throttle valve arrangement comprises at least two flap discs arranged one behind the other in the flow direction, which can be pivoted independently of one another between a blocking position running transversely to the flow direction and a through position running parallel to the flow direction. This use of two or more flap discs arranged one behind the other in the flow direction and operable independently of one another enables particularly precise adjustment of the chimney draught prevailing in the flue gas duct over a wide adjustment range. In this way, it is also easily possible to use the throttle valve arrangement as a combustion controller for the furnace under natural draught conditions, because the throttling function results in temperature-dependent chimney draught limitation.

[0013] In order to avoid the chimney draught being completely blocked either accidentally or due to a control error, it is further provided that the edges of each flap disc of the throttle valve arrangement each have a passage opening extending over part of the circumference of the flap disc, which remains open even when the flap disc is in the blocking position.

[0014] Furthermore, the apertures of the damper discs of successive damper discs in the flow direction are advantageously positioned facing opposite wall sections of the flue gas duct, thus forcing the flue gas flow to meander. This special arrangement of the apertures has the primary advantage that the chimney draft in the flue gas duct can be adjusted particularly precisely and that any dust that may fall back into the flue gas pipe cannot easily penetrate the damper arrangement.

[0015] To operate the damper discs of the throttle valve assembly, bimetallic spring actuators can be assigned to each damper disc. These actuators are in thermally conductive contact with the flue gas passing through and pivot the respective damper disc depending on the flue gas temperature. This has the advantage that the damper adjustment operates completely independently depending on the temperature and does not require the supply of auxiliary energy for adjustment. Alternatively, one or more motorized actuators can be assigned to the damper discs of the throttle valve assembly, which adjust the pivot angle of the damper discs depending on the flue gas temperature measured in the flue gas.

[0016] A particularly preferred embodiment of the cleaning device according to the invention provides that the catalytically active cleaning device comprises one or more catalyst elements arranged stationary in the flue gas duct and around which the flue gas flows. The use of stationary catalyst elements has the advantage that the flow conditions in the area of the catalytically active cleaning device remain constantly unchanged regardless of the operating state of the wood-fuel fireplace and can be optimally adapted to the respective local conditions from the outset.

[0017] According to the invention, several catalyst elements are mounted at a distance from one another in the flow direction on opposite wall areas of the flue gas duct, forcing the passing flue gas stream to meander in the area of the catalytically active purification device. This ensures that, through constant flow redirection, a very large portion of the passing flue gas volume comes into contact with the catalytically active surfaces of the catalyst elements.

[0018] A particularly advantageous embodiment of the invention provides that the fine dust separator arranged in the flue gas duct is designed as an electrostatically operating fine dust separator. The combination of such an electrostatically operating fine dust separator with an upstream and precisely controlled catalytically active cleaning device has the advantage that all electrically conductive dust components, i.e., in particular, soot and sticky hydrocarbon particles, are kept away from the electrostatic fine dust separator, which often lead to arcing and hinder the cleaning of the separator electrodes.

[0019] It is also advantageous if the catalytic cleaning device, the throttle valve assembly, and the electrostatically operating fine dust separator are arranged vertically one above the other in the flue gas duct, which runs vertically in the direction of gravity, and if cleaning openings are provided in the wall of the flue gas duct in the area of the catalytic cleaning device, the throttle valve assembly, and the electrostatically operating fine dust separator. This special arrangement of the listed components has the advantage that the cleaning of the system, which is particularly necessary after the wood-fuel fireplace is decommissioned, is particularly easy to carry out. The dust remaining in the system falls down due to gravity and largely remains on the flap discs in the area of the throttle valve assembly, where it can therefore be easily removed.If necessary, all adhering dust particles can be loosened and removed using a suitable shaking mechanism and / or suitable cleaning tools.

[0020] Alternatively, it may also be advantageous if the catalytically active cleaning device and the throttle valve assembly are arranged vertically one above the other in a section of the flue gas duct running vertically in the direction of gravity, and the electrostatically active fine dust separator is arranged in an adjoining, horizontally running section of the flue gas duct, with the fine dust separator being assigned a separate cleaning opening in the wall of the flue gas duct. This measure prevents the fine dust separated by the electrostatically active fine dust separator from falling back into the area of the throttle valve assembly and the catalytically active cleaning device.

[0021] Further advantages arise if a temperature sensor is installed in the flue gas duct downstream of the electrostatic particulate separator. This sensor measures the temperature of the flue gas flow and, depending on the temperature of the flue gas flow, uses a control device to regulate the feed of additional primary air into the wood-fuel fireplace. This additional stabilization of the combustion temperatures in the wood-fuel fireplace further improves the desired catalytic afterburning of pollutants contained in the flue gas.

[0022] Finally, a lambda probe can be installed in the flue gas duct downstream of the electrostatic particulate matter separator. This probe measures the oxygen content of the flue gas flow and, depending on the oxygen content of the flue gas flow, uses a control device to regulate the feed of additional secondary air into the wood-fuel fireplace. This additional combustion air intensifies the combustion processes in the wood-fuel fireplace if necessary, if the above-mentioned measures to increase temperatures are insufficient and the oxygen content in the escaping flue gas deviates significantly from the required values.

[0023] The use of lambda sensors to control the combustion air supply to a wood-fuel fireplace is known in principle. In known cases, however, the lambda sensors are always installed as close as possible behind the combustion chamber of the fireplace in order to achieve the shortest possible control dead times. This known arrangement of lambda sensors, however, has the disadvantage that the lambda sensor—like the other components of the flue gas cleaning system—quickly becomes clogged with dust, sticky carbon deposits, and soot, resulting in incorrect readings. According to the teachings of the invention, however, the lambda sensor, due to its location behind the fine dust separation system, only comes into contact with the finally cleaned flue gas and consequently remains effective for virtually no time. The extended control dead times can be accepted here because all adjustment processes in such wood-fuel fireplaces are sluggish anyway.

[0024] A particular advantage is that the various control circuits for the functionally important adjustment of the chimney draught on the one hand and for the control of the combustion air supply on the other hand work largely independently of one another and do not impair each other's function.

[0025] An embodiment of the invention is explained in more detail below with reference to the drawings. They show: Figure 1: Schematically a longitudinal section through a wood fuel fireplace with a cleaning device according to the invention; Figure 2: Schematically and enlarged a longitudinal section through the throttle valve arrangement of the Fig.1 illustrated cleaning device according to the invention; Figure 3: Schematic and enlarged view of the bimetallic drive of a flap disc of the throttle valve arrangement according to the invention. Figure 4: Schematic and enlarged view of the motor drive of a flap disc of the throttle valve arrangement according to the invention.

[0026] In the drawing, the wood fuel fireplace is designated in its entirety by reference numeral 1. In the combustion chamber 2 of the wood fuel fireplace 1, there is a combustion grate 3, onto which the wood fuel (logs or pellets) is placed for combustion. The flue gas stream produced during combustion, designated by reference numeral 4, rises upwards and is led via a flue gas duct 5 to a Figure 1 chimney not shown.

[0027] A catalytically active cleaning device 6 is arranged in the initial area of the flue gas duct 5, which serves to oxidize carbon dioxide and / or nitrogen oxides contained in the flue gas stream 4 and to combust soot and / or carbonization products contained in the flue gas stream 4. This catalytically active cleaning device 6 has at least two fixed catalyst elements 6a and 6b, arranged one behind the other in the flow direction and located on opposite wall regions of the flue gas duct 5. These catalyst elements 6a and 6b are more or less gas-permeable themselves, but leave free passage cross-sections to the opposite wall sections of the flue gas duct 5, which force the flue gas stream 4 passing through into a meandering pattern.This ensures that, on the one hand, a sufficiently large passage cross-section always remains free and, on the other hand, the passing flue gas stream 4 comes into intensive contact with the catalytically active surfaces of the catalyst elements 6a and 6b over a relatively long flow path.

[0028] In the flow direction immediately downstream of the catalytically active cleaning device 6, a throttle valve arrangement 7 with a plurality of flap discs 8 is arranged in the flue gas duct 5. In the embodiment shown in the drawing, there are only two flap discs 8. However, the throttle valve arrangement 7 can also have more than just two flap discs 8, which are each arranged at a distance from one another in the flow direction and can be pivoted independently of one another depending on the temperature in the flue gas duct 5 between a blocking position running transversely to the flow direction and a through position running parallel to the flow direction. Between the blocking position and the through position, the flap discs 8 can also assume different throttling positions in which they are pivoted to a greater or lesser extent.

[0029] The edge of each valve disc 8 of the throttle valve assembly 7 has a through-opening 9 extending over a portion of the circumference of the valve disc 8, which remains open even when the valve disc 8 is in the blocking position. This ensures that a minimum flow cross-section always remains open, even in the event of incorrect control or incorrect operation of the throttle valve assembly 7.

[0030] As in Figure 1As shown, the through openings 9 of the flap discs 8 which follow one another in the flow direction face diametrically opposite wall sections of the wall of the flue gas duct 5, so that the flue gas flow 4 is forced to follow a meandering course when passing through the throttle valve arrangement 7 - similar to before in the area of the catalytically active cleaning device 6. As a result, the flow path of the passing flue gas flow 4 is extended in the area of the throttle valve arrangement 7 and can be controlled very precisely by a targeted pivoting of individual flap discs 8 of the throttle valve arrangement 7.

[0031] To pivot the damper discs 8 depending on the temperature in the flue gas flow 4, an alternative Figure 3 illustrated bimetal spring drive with a bimetal torsion spring 10 as drive element or a Figure 4illustrated motor actuator11, 12 can be used.

[0032] When in Figure 3 In the bimetallic spring actuator shown, the damper disc 8 is connected to a bimetallic torsion spring 10, which is supported on the wall of the flue gas duct 5 and is in contact with the flue gas flow passing through it. This bimetallic torsion spring 10 automatically pivots the damper disc 8 depending on the temperature in the flue gas flow 4 passing through.

[0033] The Figure 4 The illustrated motorized actuator 11, 12 has a servomotor 11 connected to one or more damper discs 8 for adjusting the damper discs 8. This servomotor 11 is controlled by means of a temperature sensor 12 arranged in the flue gas duct 5.

[0034] An electrostatic fine dust separator 13 is arranged in the flue gas duct 5 downstream of the throttle valve arrangement 7 in the flow direction, with the aid of which the dry fine dust still present in the flue gas stream 4 is separated. This electrostatically active fine dust separator 13, the throttle valve arrangement 7, and the catalytically active cleaning device 6 are arranged vertically one above the other against the direction of gravity in the likewise vertically running flue gas duct, as shown in the drawing. This significantly facilitates the cleaning of the system. For cleaning purposes, cleaning openings are provided at suitable locations in the wall of the flue gas duct 5; these openings are not shown in the drawing.These cleaning openings are located, for example, in the area of the catalytically active cleaning device 6, in the area of the throttle valve arrangement 7 and in the area of the electrostatic fine dust separator 13.

[0035] As an alternative to the embodiment shown in the drawing, the electrostatically acting fine dust separator 13 can also be arranged in a horizontally extending section (not shown) of the flue gas duct 5. In this case, this horizontally extending section (not shown) of the flue gas duct 5 connects via a 90° bend to the Figure 1 shown vertically extending section of the flue gas duct 5 and is provided with a suitable cleaning opening through which the fine dust separated by the electrostatically acting fine dust separator can be discharged.

[0036] Furthermore, the Figure 1The device shown has, for the purpose of optimising the combustion processes in the wood fuel fireplace 1, a temperature sensor 14 arranged in the flue gas stream 4 behind the fine dust separator 13, which, with the aid of a control device 15, controls the feed of additional primary air 16 into the combustion chamber 2 of the wood fuel fireplace 1.

[0037] In addition, a lambda probe 17 is arranged in the flue gas stream 4 downstream of the fine dust separator 13, which determines the oxygen content in the flue gas stream 4 and, with the aid of the control device 15, controls the feed of additional secondary air 18 into the combustion chamber 2 of the wood fuel fireplace. List of reference symbols

[0038] 1Wood fuel fireplace 2Combustion chamber 3Fire grate 4Flue gas flow 5Flue gas duct 6Catalytically effective cleaning device; 6aCatalyst elements; 6bCatalyst element 7Throttle valve arrangement 8Vapour discs 9Passage opening 10Bimetallic torsion spring 11Motorized actuator 12Temperature sensor 13Electrostatic particulate separator 14Temperature sensor 15Control device 16Primary air 17Lambda probe 18Secondary air

Claims

1. A device for purifying the flue gases of a wood-fuel fireplace (1), - having a catalytic purification device (6) arranged in the flue gas duct (5) of the wood-fuel fireplace (1) for the oxidation of carbon monoxide and / or nitrogen oxides and for the catalytically-induced combustion of soot and / or carbonization products, - and having a fine particles trap (13), arranged in the flue gas duct (5) downstream of the catalytic purification device (6) in the flow-through direction, for trapping fine particles that remain in the flue gas, characterised in that the catalytic purification device (6) has a plurality of catalyst elements (6a, 6b), around which the flue gas flows, which are secured to mutually opposed wall regions of the flue gas duct (5) at a distance from one another in the flow-through direction, and which, in the region of the catalytic purification device (6), impose a meandering course on the flue gas (4) passing through, and in that a throttle valve arrangement (7) that is actuated in dependence on the temperature of the flue gas is provided in the flue gas duct (5) in the region between the catalytic purification device (6) and the fine particles trap (13).

2. The device according to claim 1, characterised in that the throttle valve arrangement (7) has at least two valve discs (8) arranged in succession in the flow-through direction, which are pivotable independently of one another between a blocking position running transverse to the flow-through direction and a passage position running parallel to the flow-through direction.

3. The device according to claims 1 and 2, characterised in that the edge of each valve disc (8) of the throttle valve arrangement (7) in each case has a passage opening (9), which extends over part of the circumference of the valve disc (8) and which remains open also when the valve disc (9) is in the blocking position.

4. The device according to claim 3, characterised in that the passage openings (9) of the valve discs (8) are faced by valve discs (8), arranged successively in the flow-through direction, of diametrically opposed wall regions of the flue gas duct (5) and enforce a meandering course on the flue gas flow passing through.

5. The device according to any one or more of claims 1 to 4, characterised in that the valve discs (8) of the throttle valve arrangement (7) are each assigned bimetal spring drives (10), which are in thermally conductive contact with the flue gas passing through and pivot the assigned valve discs (8) in dependence on the temperature in the flue gas.

6. The device according to any one or more of claims 1-4, characterised in that the valve discs (8) of the throttle valve arrangement (7) are assigned one or more motorised actuators (11), which adjust the pivot angle of the valve discs in dependence on the temperature of the flue gas measured in the flue gas duct (5).

7. The device according to any one or more of claims 1-6, characterised in that the fine particles trap (13) arranged in the flue gas duct (5) is configured as an electrostatically operating fine particles trap (13).

8. The device according to claim 7, characterised in that the catalytic purification device (6), the throttle valve arrangement (7) and the electrostatically operating fine particles trap (13) are arranged vertically one above the other in the flue gas duct (5) running vertically in the direction of the force of gravity, and in that purification openings are arranged in the wall of the flue gas duct (5) in the region of the catalytic purification device (6), the throttle valve arrangement (7) and the electrostatically operating fine particles trap (13).

9. The device according to claim 7, characterised in that the catalytic purification device (6) and the throttle valve arrangement (7) are arranged vertically one above the other in a portion of the flue gas duct (5) running vertically in the direction of the force of gravity, and the electrostatically acting fine particles trap (13) is arranged in an adjoining, horizontally running portion of the flue gas duct (5), wherein the fine particles trap (13) in the wall of the flue duct (5) is assigned a separate purification opening in this horizontally running portion.

10. The device according to any one or more of claims 1-9, characterised in that a temperature sensor (14) is arranged downstream of the electrostatically acting fine particles trap (13) in the flow direction in the flue gas duct (5) and measures the temperature of the flue gas flow (4) passing through and controls the feed of additional primary air (16) into the wood-fuel fireplace (1) with the aid of a control device (15) in dependence on the temperature of the flue gas flow (4) passing through.

11. The device according to any one or more of claims 1-9, characterised in that a lambda probe (17) is arranged downstream of the electrostatically acting fine particles trap (13) in the flow direction in the flue gas duct (5) and measures the oxygen content of the flue gas flow (4) passing through and controls the feed of additional secondary air (18) into the wood-fuel fireplace (1) with the aid of a control device (15) in dependence on the oxygen content of the flue gas flow (4) passing through.

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