A ductwork, uses of a ductwork, and a method for cleaning flue gas

Abstract

A ductwork (100) suitable for leading flue gas from a flue gas outlet (811) of a pulp mill or a power plant to air pollution control equipment (900) The ductwork (100) comprises a first main duct (110) extending in a longitudinal direction (dL) from a first inlet (111) to a first outlet (121) and configured to lead flue gas in the longitudinal direction (dL) from the first inlet (111) to the first outlet (121).The first main duct (110) is equipped with a mechanical conveyor (200) such that the mechanical conveyor (200) is configured to convey ash in the longitudinal direction (dL) or in a direction (-dL) reverse thereto. Use of the ductwork (100) for cleaning flue gas. Use of the ductwork (100) for constructing or upgrading a part of a pulp mill or a power plant.

Description

[0001] A ductwork, uses of a ductwork, and a method for cleaning flue gas

[0002] Technical field

[0003] The invention relates to power plants, pulp mills, and pulp mill processes. The invention relates to cleaning flue gases of a pulp mill or a power plant. The invention relates to flue gas ducts.

[0004] Background

[0005] Flue gas of a pulp mill or a power plant is typically cleaned using air pollution control equipment before expelling the flue gas to atmosphere. Air pollution control equipment comprise one or more air pollution control devices. An electrostatic precipitator (ESP) is an example of an air pollution control device. An ESP or several electrostatic precipitators (ESPs) are commonly used to clean flue gas by removing particulates from boiler flue gas. As an example, the patent US 10,751 ,729 discloses an ESP in connection with a chemical recovery boiler. A chemical recovery boiler is boiler configured to cover cooking chemicals in a pulp mill process, e.g., the sulphite process or the sulphate process, the sulphate process being also known as the Kraft process.

[0006] Such boilers, as well as certain other furnaces of a pulp mill or power plant may produce ash that is sticky, the ash being comprised by the flue gas. In power plants, the stickiness of the ash depends on the fuel used. However, in a pulp mill, the ash contained in the flue gas is typically sticky. This means that, unlike fly ash, the sticky ash relatively easily accumulates on surfaces of ducts leading the flue gas containing the sticky ash. When ash sticks on the surfaces of the ducts, an effective cross-section of the duct becomes smaller, which increases pressure losses in the system; or eventually the ducts may even become clogged by the ash.

[0007] Oftentimes the ESP or other air pollution control device can not be placed in an immediate vicinity of a source of the flue gas. Therefore, a ductwork is used to lead the flue gas to the ESP. For preventing accumulation of the sticky ash to the surfaces of the ductwork, the ductwork nowadays commonly comprises only such parts that form relatively large angles with the horizontal plane. As an example, Fig. 1 shows an economizer section (ECO), an electrostatic precipitator (ESP), and in between them a ductwork for leading flue gas from the ECO to the ESP. As show in Fig. 1 , the ductwork comprises a riser section (close to the ECO) and a downcomer section (close to the ESP). Both these sections are arranged at a relatively large angle, typically at least 55 degrees, to the horizontal plane to prevent ash from accumulating on walls of the ductwork. Because of the large angle, ash collected at the riser section falls back to the ECO and ash collected in the downcomer section falls to the ESP. At least this is the intention of the riser and downcomer sections.

[0008] Even with such sections, there is oftentimes still certain risk of the ash sticking to the walls of the sections. This may happen, e.g., because the boiler is driven at a variable load. Typically, when the load is low, a flow velocity of the flue gas is also low, which increases the risk of ash sticking onto surfaces of ducts. When a boiler is operated in a conventional way, the load is adjusted upon needs, whereby instances of low load, and correspondingly high risk of ash accumulation, can not be avoided. Thus, even if normal design specifications are implemented, some ash may stick on the surfaces of the ductwork. Moreover, the accumulated ash may form avalanches so that every now and then a large amount of ash will fall to the ESP or to the ECO, depending on in which part the avalanche occurs. As the pulp mills are large, the amount of ash also is large. It may be, e.g., several tens of cubic metres. This poses problems to the operation of the plant and poses risks of the ash avalanche damaging the equipment.

[0009] Moreover, in use, the ductwork leading the flue gas and the ash is hot. However, for maintenance the process may be stopped, whereby the ductwork starts to cool down. It has been observed that cooling increases the risk for the ash avalanches. Moreover, during maintenance this poses a high risk for the personnel maintaining the plant. In particular, the personnel may need to enter the ESP and / or the ECO, and an ash avalanche at that time would be hazardous for the personnel.

[0010] Furthermore, assembling such a ductwork is a complex task, particularly when the ESP is located far away from the ECO. When the distance is large, also the height of the structure must be increased because of the required minimum angle. Because the angle is large, a total length of the ductwork is somewhat larger than a distance between the ECO and the ESP, mainly because a straight duct can not be used. In general, the ducts are large in cross section, and as detailed above, also somewhat lengthy. Therefore, the ductwork requires an extremely sturdy body onto which it can be installed. It may sometimes be hard to find proper locations for such a body. Normally, e.g., the walls of an existing boiler are not sufficiently sturdy for fixing such ductwork. Therefore, if the ductwork is supported to the wall of the ECO by a support (see “support” in Figs. 1 ), also the walls of the ECO must typically comprise further support. In case the ductwork is attached to an existing ECO, the existing walls may need to be further strengthened for the purpose. Moreover, the support itself must by sufficiently sturdy. All these imply that in addition to the duct as such, a lot of supporting structures are needed. Because of the complexity of the installation as well as material costs related to the supporting structures, installation of such a ductwork is expensive.

[0011] These problems affect the design of a new pulp mill or power plant. However, the problems are even more prominent when upgrading an existing pulp mill or power plant. As an example, in some cases an operator wishes to equip an existing pulp mill or power plant with new (i.e., unused) ESPs or other air pollution control devices. In such a case, it is not always possible to freely design the locations for the new ESPs or other air pollution control devices with respect to the pre-existing (i.e., used) flue gas outlets. A designer must respect the layout of the pre-existing mill or plant. Therefore, the new or other air pollution control devices may have to be arranged reasonably far away from the existing flue gas outlets. In line with what has been said above, in such a case, the lengthy riser-downcomer sections would imply high material costs and complex installation.

[0012] For these reasons there is a need for an improved device and method for cleaning flue gas of a pulp mill or power plant.

[0013] Summary

[0014] It has been found that the ductwork for leading the flue gas to the air pollution control device can be equipped with a mechanical ash conveyor to enhance the conveying of the ash. The mechanical ash conveyor, or at least that part of the conveyor that conveys the ash, can be arranged at a bottom of the ductwork. Thus, the ash accumulates at the bottom of the ductwork, and the conveyor conveys the ash, e.g., for proper use. Therefore, the ductwork keeps clean by itself and without maintenance; or at least a maintenance interval is substantially increased. Moreover, even if the ash temporarily sticks on the inner walls of the ductwork, avalanches of the ash within the ductwork are much smaller, at least if the ductwork is substantially horizontal. Moreover, because of the mechanical conveyor, the ductwork can be arranged substantially horizontally. This makes installation of the ductwork much easier than installing a ductwork of prior art. Moreover, this makes the ducts shorter than in prior art. Moreover, this facilitates constructing the ductwork, as there is no need for high structures.

[0015] The ductwork is used in a pulp mill or a power plant. The power plant may be a waste to energy plant including a waste incinerator.

[0016] The invention is disclosed in more specific terms in the appended claims.

[0017] Brief description of the drawings

[0018] Fig. 1 shows an economizer, an electrostatic precipitator, and a known ductwork in between them,

[0019] Fig. 2 shows a flue gas outlet, an air pollution control device, and a ductwork in between them,

[0020] Fig. 3 shows a cross-section of a first main duct of a duct work,

[0021] Fig. 4 shows a mechanical conveyor for the ash in detail, the conveyor being arranged in a first main duct of a ductwork,

[0022] Fig. 5 shows a mechanical conveyor for the ash in detail, the conveyor being arranged in a first main duct of a ductwork,

[0023] Fig. 6 shows a mechanical conveyor for the ash in detail, a part of the conveyor being arranged in a first main duct of a ductwork,

[0024] Fig. 7a shows in a side view a first main duct of a ductwork, the first main duct being thermally insulated,

[0025] Fig. 7b shows a cross section of a first main duct of a ductwork, the first main duct being thermally insulated,

[0026] Fig. 7c shows a cross section of a first main duct of a ductwork, the first main duct being thermally insulated by two layers of thermally insulating material, Fig. 8a shows an economizer, an air pollution control device, and a ductwork in between them,

[0027] Fig. 8b shows a part of the ductwork of Fig. 8a in the view depicted by VI I lb in Fig. 8a,

[0028] Fig. 8c shows a part of a ductwork of in a similar view as in Fig. 8b,

[0029] Fig. 9 shows a part of a ductwork for feeding flue gas to two air pollution control devices in a similar view as in Fig. 8b,

[0030] Fig. 10 shows a part of a ductwork for feeding flue gas to two air pollution control devices in a similar view as Fig. 8b,

[0031] Fig. 1 1 shows a part of a ductwork for feeding flue gas to two air pollution control devices,

[0032] Fig. 12a shows a flue gas generating device, four air pollution control devices, and a ductwork for feeding flue gas from the flue gas generating device to each one of the air pollution control devices,

[0033] Fig. 12b shows an arrangement like the one of Fig. 12a in a top view,

[0034] Fig. 12c shows an arrangement like the one of Fig. 12b, but comprising also a second main duct,

[0035] Fig. 12d shows an arrangement like the one of Fig. 12b, but comprising also a second main duct, a third main duct, and a fourth main duct,

[0036] Fig. 13 shows in a top view an arrangement, in which the flue gas duct feeding flue gas to the first main duct of the ductwork comprises an auxiliary duct,

[0037] Fig. 14a shows in a top view an arrangement, in which both ends of the first main duct of the ductwork serve two air pollution control devices,

[0038] Fig. 14b shows in a top view an arrangement, in which an end of two main ducts of the ductwork each serve two air pollution control devices,

[0039] Fig. 15 shows in a top view an arrangement, in which the flue gas duct feeding flue gas to the first main duct of the ductwork comprises an auxiliary duct and both ends of the first main duct serve two air pollution control devices,

[0040] Fig. 16a shows in a side view an arrangement comprising a flue gas generating device, five air pollution control devices, and a ductwork in between them,

[0041] Fig. 16b shows the detail XVIb of Fig. 16a in the same view as Fig. 16a,

[0042] Fig. 16c shows the arrangement of Fig. 16a in a perspective view,

[0043] Fig. 16d shows the detail XVIde of Fig. 16c in the same view as Fig. 16c, and Fig. 16e shows the detail XVIde of Fig. 16c in a front view. In the figures Sz denotes a direction that is parallel to gravity thereby being downward vertical, Sx is a horizontal direction and Sy is another horizontal direction. The directions Sx, Sy, and Sz are mutually perpendicular. A first direction d1 , which is more specifically defined in the description, is preferably substantially parallel to gravity as detailed in the description. A longitudinal direction dL of the first main duct is preferably substantially horizontal, as detailed in the description, and the flue gas is configured to be lead in this direction. The longitudinal direction dL and the first direction d1 are perpendicular to each other. Moreover, the directions d1 , Sy, and dL are mutually perpendicular.

[0044] Detailed description

[0045] In this description, a direction means a single direction characterizable by a vector. A direction and the corresponding positive direction are the same. E.g., the directions Sz and +Sz both refer to a downward vertical direction. A negative direction refers to a direction that is opposite, i.e., reverse to the corresponding direction. E.g. -Sz refers to an upward vertical direction. Similar convention applies mutatis mutandis to other directions, such as a longitudinal direction dL. In this description, the acronym ESP stands for an electrostatic precipitator. An ESP is an example of an air pollution control device. The acronym APCD is used for an air pollution control device. Air pollution control equipment 900 of a pulp mill or a power plant comprises an APCD (910, 920, 930, 940, 950), typically several APCDs, and is configured to clean flue gas produced in the pulp mill or in the power plant. In this description, the term “ash” refers all solid material flowing with flue gas. In particular, the origin of such solid material does not matter. Thus, even if in a lime kiln the combustion of fuel as such does not necessarily produce any solid material, heating of the lime mud by said combustion has the effect that solid material of the lime mud becomes mixed with the flue gas. The solid material of this mixture is referred to as ash. Such material could be called dust in the alternative. However, in this description, the term ash is adopted. In boilers, ash typically results directly from burning fuel (e.g. black liquor or biomass).

[0046] Figure 2 shows an arrangement 10, the arrangement 10 comprising a ductwork 100, air pollution control equipment 900, and a flue gas outlet 811. In Fig. 2 the air pollution control equipment 900 comprises a first air pollution control device 910. The flue gas outlet 811 is an outlet for letting out flue gas from a flue gas generating device 810 of a pulp mill or a power plant.

[0047] Within this description, the term pulp mill process refers to the processes carried out in a pulp mill, including separating fibres from wood to produce pulp. The pulp mill process may include manufacturing paper. The pulp mill process may include recycling paper to from pulp. Within this description, the term pulp mill refers to the system used for running the pulp mill process. Sometimes the pulp mill process, as defined above, is referred to as a pulp and paper process. However, in a pulp mill, paper is not necessarily produced, even if it may be produced. Flue gases generated in a pulp mill are generally sticky or extremely sticky. Within this description, the term power plant refers to a plant in which fuel is burnt to produce steam; and the steam may be used to produce electricity. The fuel burnt in the power plant may comprise biomass and / or residue-derived fuel, which both typically produce relatively sticky ash. In the power plant, water is boiled to obtain the steam. Thus, the steam and flue gas are separately produced. Electricity may be produced by means of a generator combined with a steam turbine, which is operated by the steam. The term power generating process refers to the process run in the power plant to produce the steam. An example of a power plant is a waste to energy plant. A waste to energy plant comprises a waste incinerator. A power plant does not need to use waste or residue derived fuel as fuel, whereby a power plant does not need to be a waste to energy plant.

[0048] In such application, the flue gas moving within the ductwork 100 comprises ash that is sticky or extremely sticky. In prior art, the stickiness of the ash poses the problems indicated in background. In particular, the flue gas outlet 811 may be one of the following:

[0049] - an outlet of a flue gas channel of a chemical recovery boiler configured to burn black liquor of a Kraft pulp process,

[0050] - an outlet of a flue gas channel of a chemical recovery boiler configured to burn brown liquor of a sulphite pulp process,

[0051] - an outlet of a flue gas channel of power boiler configured burn fuel to produce steam, the power boiler being a power boiler of a pulp mill, - an outlet of a flue gas channel of power boiler configured burn fuel to produce steam, the power boiler being arranged in a power plant, e.g., waste to energy plant, or

[0052] - an outlet of a flue gas channel of a lime kiln of a pulp mill, the lime kiln being configured to calcinate calcium carbonate.

[0053] Sometimes the flue gas of a lime kiln is referred to as exhaust gas. However, the terms flue gas and exhaust gas are used in this description interchangeably. In the lime kiln, material containing calcium carbonate is thermally dissociated at a temperature of about 900 °C thereby forming burnt lime, unslaked lime, or quicklime. Thus, at least some of the calcium carbonate converts to calcium oxide. In the context of a pulp mill process, the lime kiln is configured to calcinate calcium carbonate of lime mud. It is also noted that in case of a lime kiln, a cyclone is oftentimes used for removing some of the ash (i.e., dust) from the flue gas. The remaining flue gas is typically conveyed to the air pollution control equipment. Thus, in case the flue gas generating device 810 is a lime kiln, the flue gas outlet 811 may be, e.g., an outlet of the cyclone used for de-dusting the flue gas. Thus, the cyclone may be seen as part of the lime kiln. Reference is made to what has been said about the terms ash and dust above.

[0054] The power boiler, if used in a pulp mill, may be configured to burn wood and / or wood residues to produce steam. Wood residues may include such parts of wood that the wood fibres of the residues do not form the pulp produced in pulp mill. Examples include bark, sawdust, stumps, and branches. The power boiler, if used in a more general power plant, may be configured to burn biomass (including wood and wood residues as well as other biomass) and / or residue derived fuel.

[0055] In line with what has been said above, the flue gas outlet 811 is an outlet of a flue gas generating device 810, which may be one of: a chemical recovery boiler (Kraft or sulphite), a power boiler (of a pulp mill or a power plant), and a lime kiln.

[0056] A pulp mill or a power plant may comprise further flue gas outlets. For example, a flue gas outlet for each inlet (111 , 112, 113, 114) of the first main duct 110. Concerning the ductwork 100, the ductwork is thus suitable for leading flue gas that comprises ash from the flue gas outlet 811 of the pulp mill or the power plant to the first air pollution control device 910 of the pulp mill or the power plant (respectively). Referring to Fig. 2, the ductwork 100 comprises a first main duct 110. The first main duct 110 extends in a longitudinal direction dL from a first inlet 111 to a first outlet 121 . The first main duct 110 is configured to lead flue gas in the longitudinal direction dL from the first inlet 111 to the first outlet 121. In an embodiment, the first main duct 110 has a profile shape that extends in the longitudinal direction dL.

[0057] The air pollution control equipment 900 comprises at least the first air pollution control device 910. The first air pollution control device 910 may be one of or comprise at least one of: an electrostatic precipitator (dry or wet), a selective catalytic reaction reactor (SNCR), a selective non-catalytic reaction reactor (SCR), a filter, such as bag house filter, a flue gas scrubber, and a flue gas desulphurization unit. Preferably, the first air pollution control device 910 comprises an electrostatic precipitator or a bag filter.

[0058] Referring to both Figs. 2 and 3, the first main duct 110 is equipped with a mechanical conveyor 200; i.e., the first main duct 110 comprises a mechanical conveyor 200. The mechanical conveyor 200 is configured to convey ash in the longitudinal direction dL of the first main duct 110 or in a direction -dL that is reverse to the longitudinal direction dL. The mechanical conveyor 200 comprises a movable part 210, which is configured to move the ash by its movement. Moreover, at least the part of the mechanical conveyor 200 that is configured to move the ash is arranged inside the first main duct 110. Preferably, at least the part of the mechanical conveyor 200 that is configured to move the ash is arranged in such a region E of the interior of the first main duct 110 that is located farthest in a first direction d1. The first direction is perpendicular to the longitudinal direction dL. This region E constitutes a bottom of the first main duct 110 in a preferable use of the ductwork 100.

[0059] As an example, in preferred use of the ductwork, the first direction d1 is downward vertical (i.e., parallel to Sz), whereby the region E is at a bottom of the interior of the first main duct 110. However, the first main duct 110 does not need to be exactly horizontal whereby the first direction d1 does not need to be exactly downwards. In an embodiment, the first direction d1 forms an angle of at most 50 degrees or at most 25 degrees or at most 10 degrees with a downwards vertical direction Sz.

[0060] Referring to Fig 3, in an embodiment, a cross-section XS of the first main duct 110 has a shape that tapers in the first direction d1 . Herein the cross-section refers to a cross-section of an interior of the first main duct 110. As will become clear, thermal insulation 300 may be provided on the first main duct 110, and if a thickness of the thermal insulation is not constant, a shape of the outer surface of the first main duct 110 may be different from the shape of the interior of the first main duct. Herein the cross-section XS is such a cross-section, of which normal is the longitudinal direction dL. Therefore, by definition, the first direction d1 is perpendicular to the longitudinal direction dL.

[0061] The tapering shape of the first main duct 110 ensures that ash, which will be collected on walls of the first main duct and eventually fall due to gravity to the region E, such as a bottom of the first main duct, will be collected to the mechanical conveyor 200 by the tapering shape. The tapering shape is also preferable, because then the cross-sectional area of the first main duct 110 can be made large without an excessively wide mechanical conveyor 200. Cross-sectional area should be kept large to avoid pressure losses in the first main duct 110. A tapering shape is not essential, even if preferable. Instead, a shape of the cross section of the first main duct may be, e.g., rectangular.

[0062] Referring to Fig. 3, in an embodiment, a height H of the cross-section XS of the first main duct 110 is greater than a width W of the cross-section XS of the first main duct 110. Herein the cross-section refers to the cross section of the interior of the first main duct 110, as seen best in Fig. 7b. The height H is measured in the first direction d1 and the width W is measured in a second direction Sy, which is perpendicular to the first direction d1 . Naturally, both d1 and Sy are perpendicular to the longitudinal direction dL by the definition of the cross-section. These measures have the effect that a width of the cross- sectional area of the first main duct 110 can be made large without an excessively wide mechanical conveyor 200. The cross-sectional area should be large to avoid pressure losses in the first main duct 110.

[0063] As for other dimensions of the ductwork, in a preferable embodiment, the first main duct 110 extends in the longitudinal direction dL for at least two metres or at least five metres. Benefits of the substantially horizontal first main duct 110 are more prominent in long ducts, the benefits including easy installation, reduced risk of ash avalanches, and reduced need for supporting the ductwork. Typically, a length of one mechanical conveyor is at most 25 metres. Thus, in an embodiment, the first main duct 110 extends in the longitudinal direction dL for at most 25 metres. However, if longer ducts are needed, a second main duct can be arranged next to the first main duct 110 such that in combination they extend a longer distance. In the alternative the first main duct 110 may be equipped with several mechanical conveyors.

[0064] A preferable area of the cross-section of an interior of the first main duct 110 depends on the capacity of the flue gas generating device 110 as well as on the number of ducts used for leading the flue gas. The cross-sectional area of the interior of the first main duct may be selected such that in regular use, a flow velocity of the flue gases within the first main duct 110 is 10 m / s to 50 m / s such as about 20 m / s (one significant digit). In any case, the ductwork 100 is dimensioned such that sufficient air circulation for the flue gas generating device 810 is secured.

[0065] Concerning the number of main ducts comprised by the ductwork, the ductwork comprises at least the first main duct 110 as discussed above. However, the ductwork 100 may comprise further main ducts, such as a second main duct 110b, a third main duct 110c, and a fourth main duct 110d. Reference is made to Figs. 12c, 12d, and 14b. The further main ducts, such as the second main duct 110b, the third main duct 110c, and the fourth main duct 110d comprise a mechanical conveyor as the first main duct 110. Moreover, further main ducts 110b, 110c, 110d are preferably arranged substantially horizontal as disclosed in more specific terms for the first main duct 110 mutatis mutandis.

[0066] The second main duct 110b comprises a first inlet 111 b of the second main duct (see Figs. 12c and 12d and 14b) and may comprise a second inlet 112b of the second main duct (see Fig. 12c). The second main duct 110b comprises a first outlet 121 b of the second main duct (see Figs. 12c and 12d and 14b) and may comprise a second outlet 122b of the second main duct (see Fig. 12c). The third main duct 110c comprises a first inlet 111 c of the third main duct and a first outlet 121 c of the third main duct. The fourth main duct 110d comprises a first inlet 111 d of the fourth main duct and a first outlet 121 d of the fourth main duct. Naturally, also the third and fourth main ducts 110c, 110d may comprise further inlets and / or outlets according to needs. The outlets 811 , 812, 813, 814 of the flue gas generating device 810 of the pulp mill or the power plant may be coupled to various inlets of the main ducts (110, 110b, 110c, 11 Od) according to needs. Examples are shown in Figs. 12b to 12d and 14a and 14b. Furthermore, two main ducts may be arranged subsequently to form a flue gas passage of which length is greater than a length of the mechanical conveyor 200 of the first main duct or a length of a mechanical conveyor of the second main duct 110b. The second main duct 110b, if used, is like the first main duct 110 and comprises a second mechanical conveyor. Also, the third main duct 110c and the fourth main duct 110d are like the fist main duct 110, if used at all. What has been said or will be said about the location of the mechanical conveyor with respect to the first main duct 110 applies to the second mechanical conveyor with respect to the second main duct 110b mutatis mutandis. What has been said or will be said about the structure the mechanical conveyor 200 of the first main duct 110 applies to the second mechanical conveyor of the second main duct 110b mutatis mutandis. This applies to the mechanical conveyor of the third main duct 110c and the mechanical conveyor of the fourth main duct 110d, too, mutatis mutandis.

[0067] The number of main ducts 110, 110b, 110c, 110d may be selected according to needs, e.g., such that the flow velocity of the flue gas is within the limits disclosed above. Factors affecting a suitable number include the number of air pollution control devices and capacity of the flue gas generating device 810 as well as a cross-sectional area of the main ducts.

[0068] The ductwork is usable in a versatile manner. The first main duct 110 may be coupled to several outlets of the flue gas generating device, if needed. Likewise, the first main duct 110 may serve several air pollution control devices through one or several outlets of the first main duct 110. As detailed above, further main ducts can also be applied. What has been said concerning the first main duct 110 applies to the second main duct 110b, the third main duct 110c, and the fourth main duct 110d, mutatis mutandis.

[0069] Therefore, the first main duct 110 may comprise a second inlet 112. Reference is made to Figs. 12b and 14a. Thus, the first main duct 110 may be configured to lead flue gas from at least two inlets (111 , 112) to the first outlet 121 . Thus, the first main duct 110 may be configured to lead flue gas from at least two inlets (111 , 112) to an outlet (as in Fig. 12b), and through the outlet to one or more air pollution control devices. For example, in Fig. 12b flue gas is lead through one outlet to one air pollution control device, whereas in Fig. 14, flue gas is lead through one outlet to two air pollution control devices. In Fig. 12b, a first inlet 111 of the first main duct 110 receives flue as from a first outlet 811 , a second inlet 112 of the first main duct 110 receives flue gas from a second outlet 812, a third inlet 113 of the first main duct 110 receives flue gas from a third outlet 813, and a fourth inlet 114 of the first main duct 110 receives flue gas from a fourth outlet 814. The outlets 811 , 812, 813, 814 are outlets of the flue gas generating device 810 of the pulp mill or the power plant.

[0070] The first main duct 110 may comprise a second outlet 122. Reference is made to Figs. 12b and 14a. Thus, the first main duct 110 may be configured to lead flue gas from one, two, or more than two inlets to the first outlet 121 and the second outlet 122. Thus, the first main duct 110 may be configured to lead flue gas from at least one inlet to at least two outlets, and through the outlets to two or more air pollution control devices.

[0071] In line with what has been said above, the embodiments relate to a use of a ductwork 100 comprising the first main duct 110 and optionally comprising further main ducts 110b, 110c, 110d, as disclosed above. The first main duct 110 is equipped with the mechanical conveyor 200 configured to convey ash as detailed above. The first main duct 110 is used for leading flue gas that comprises ash from a flue gas outlet 811 of a pulp mill or power plant to a first air pollution control device 910 of the pulp mill or the power plant, e.g., to a first electrostatic precipitator of the pulp mill or the power plant as disclosed above. The ductwork 100 is used such that the longitudinal direction dL of the first main duct 110 is horizontal or forms an angle of at most 50 or at most 25 or at most 10 degrees with a horizontal plane.

[0072] In line with what has been said above, the embodiments relate to an arrangement 10 that comprises the ductwork 100. In addition, the arrangement comprises at least one of (i) a chemical recovery boiler configured to burn black liquor or brown liquor of a pulp process, (ii) a power boiler (of pulp mill or power plant), and (iii) a lime kiln configured to calcinate calcium carbonate. What has been said above concerning a lime kiln applies. The chemical recovery boiler, the power boiler, or the lime kiln comprises a flue gas channel having at least the flue gas outlet 811 , from which the ductwork 100 is configured to lead flue gas that comprises ash to the first air pollution control device 910 of the pulp mill or the power plant. The arrangement further comprises the first air pollution control equipment 900, including the first air pollution control device 910. Examples of air pollution control devices have been disclosed above. Preferably, the first air pollution control device 910 comprises an electrostatic precipitator or a bag filter.

[0073] For the reasons disclosed above, in the arrangement 10, the first main duct 110 of the ductwork 100 is arranged substantially horizontal. More specifically, in the arrangement, the longitudinal direction dL is horizontal or forms an angle of at most 50 degrees with a horizontal plane. Preferably, the longitudinal direction dL is horizontal or forms an angle of at most 25 or at most 10 degrees with a horizontal plane. The first direction d1 is perpendicular to the longitudinal direction dL and substantially parallel to gravity in line with what has been said above. More specifically, the first direction d1 is a projection of a downward vertical direction Sz to the plane of the cross-section XS, of which normal is the longitudinal direction dL.

[0074] In the arrangement 10, at least the part of the mechanical conveyor 200 that is configured to convey the ash is arranged at a lower part of the first main duct 110. The lower part comprises the region E that is located farthest in the first direction d1 . For the region E, reference is made to Figs. 3 to 6. The region E is located farthest in the first direction d1 as measured from a longitudinal central line CL of the first main duct 110, the longitudinal central line of the first main duct 110 being parallel to the longitudinal direction dL and intersecting the cross-section XS at a centre of the cross-section. Reference is made to Fig. 3 for the longitudinal central line CL. Thus, the region E constitutes a bottom of the first main duct 110 when the longitudinal direction dL is horizontal.

[0075] In the arrangement 10, the flue gas generating device, i.e. the chemical recovery boiler, the power boiler, or the lime kiln, is arranged to lead flue gas to the first inlet 111 of the first main duct 110 through the flue gas outlet 811 ; optionally via an auxiliary duct 190, which can be seen either as being part of the flue gas channel 810 or as a part of the ductwork 100. Moreover, the first air pollution control device 910 is configured to receive flue gas from the ductwork 100. The first air pollution control device 910 may be configured to receive flue gas, e.g., directly from the first outlet 121 of the first main duct 110, or from the first outlet 121 through a duct, e.g., a duct comprising a first riser duct 911 and a first downcomer duct 912. The first air pollution control device 910 is operated in a conventional manner to clean the flue gas from at least some of the pollutants contained by the flue gas. Herein examples of pollutants include ash, sulphur oxides, and nitrogen oxides.

[0076] The arrangement 10 is used to clean the flue gas. Thus, the embodiments relate to a method for cleaning flue gas. The method comprises burning material in a pulp mill or in a power plant to produce the flue gas. As detailed above, materials burned include at least the following:

[0077] - black liquor, which is burnt in a chemical (Kraft., i.e., sulphate) recovery boiler,

[0078] - brown liquor, which is burnt in a chemical (sulphite) recovery boiler,

[0079] - biomass, including wood and wood residues, which is burnt in a power boiler,

[0080] - residue derived fuel, which is burnt in a power boiler (e.g., waste incinerator),

[0081] - fuel oil or gas, which is burnt in a lime kiln.

[0082] The ash contained by the flue gas produced by said burning is sticky. In case of a chemical recovery boiler and power boiler, the stickiness is a result of the content of the burnt material. In case of a lime kiln, the stickiness is a result of the calcium carbonate containing material treated in the lime kiln, even if the fuel oil or gas as such is relatively clean.

[0083] The method comprises leading the flue gas in the first main duct 110 of the ductwork 100 to the first air pollution control device 910. The flue gas is lead in the longitudinal direction dL or in a direction -dL reverse thereto. As detailed above, the longitudinal direction dL is horizontal or forms an angle of at most 50 degrees or at most 25 degrees or at most 10 degrees with a horizontal plane. Also, the direction -dL that is reverse to the longitudinal direction dL is horizontal or forms an angle of at most 50 degrees or at most 25 degrees or at most 10 degrees with a horizontal plane. The method comprises collecting ash to a bottom of the first main duct 110 and conveying the ash at the bottom of the first main duct 110 in the longitudinal direction dL or in the direction -dL reverse thereto by using the mechanical conveyor 200. Notable, the ash can be conveyed in either direction irrespective of the direction in which the flue gas is lead. They can be lead or conveyed in the same direction or in opposite directions.

[0084] The gas may be lead in the same direction as the ash is conveyed, e.g., both in the longitudinal direction dL, which, in the context of the ductwork 100, is defined as being from the first inlet 111 to the first outlet 121 . However, in the method, as defined above, only a more general longitudinal direction dL is defined, whereby the flue gas, in the thus defined method, can be lead in the reverse direction -dL. In addition, in contexts of the method, arrangement, and the ductwork, the flue gas may be lead in the longitudinal direction dL directed from the fist inlet 111 to the first outlet 121 , and the ash may be conveyed in the same longitudinal direction dL and / or in a direction reverse thereto.

[0085] Concerning the expression “and / or” in the former expression, reference is made to Figs. 12a and 12b as well as Figs. 16a to 16e. Therein the first main duct 110 is configured to lead flue gas from the first inlet 111 to a first outlet 121 in the longitudinal direction dL. The first outlet 121 is for letting out flue gas for the first air pollution control device 910. In addition, the first main duct 110 is configured to lead flue gas from the first inlet 111 to a fourth outlet 124 (see Fig. 12b) for letting out flue gas for a fourth air pollution control device 940, whereby the first main duct 110 is configured to also lead flue gas from the first inlet 111 to the fourth outlet 124 in a direction -dL that is reverse to the longitudinal direction dL as defined as above. The mechanical conveyor 200 may convey ash at a bottom of the first main duct 110, e.g. to a first end of the of the first main duct 110 only in the direction +dL, to a second end of the of the first main duct 110 only in the direction -dL, from both ends of the first main duct 110 to a centre of the first main duct 110 in both the directions +dL and -dL, or from a centre of the first main duct 110 to both ends of the first main duct in the directions +dL and -dL. In the arrangement of Fig. 12a, the air pollution control equipment 900 further comprises a second APCD 920, a third APCD 930 and a fourth APCD 940. The embodiment of Figs. 16a to 16e comprises a fifth APCD 950. In the embodiment of Figs. 16a to 16e, the first main duct 110 is equipped with the mechanical conveyor 200 and is also equipped with a second mechanical conveyor 200b of the first main duct 110. The conveyors 200, 200b may convey ash to either or both directions (+dL, -dL) according to needs. It is noted that also in the context of the embodiment of Fig. 2, the ash can be conveyed, upon needs, to either end of the first main duct, from a centre to both ends of the first main duct, or from both ends to a centre of the first main duct 110.

[0086] Moreover, the method comprises cleaning the flue gas in the first air pollution control device 910. The first air pollution control device 910 is used in a conventional manner.

[0087] In line with the arrangement, an embodiment of the method comprises producing the flue gas in a chemical recovery boiler of a pulp mill (sulphite or sulphate pulp mill), in a lime kiln of a pulp mill, or in a power boiler (of a pulp mill or a power plant).

[0088] When the first APCD 910 is used in a conventional manner, the flue gas to be cleaned in the first APCD 910 is preferably reasonably hot. This helps the process of removing the pollutants from the flue gas within the first APCD 910. For example, when the first APCD 910 is an ESP, this helps the process of electrically capturing the ash particles from the flue gas within the first electrostatic precipitator. This applies also when the APCD is or comprises a filter. Naturally this applies to a second APCD 920, a third APCD 930, a fourth APCD 940, and a fifth APCD 950, if used; as well as any further APCDs, which may also be used. The number of air pollution control devices for one flue gas generating device 810 is preferably at least two. This allows for a flexible use of the APCDs, as detailed below. A suitable number of APCDs depends on the volume of the flue gas to be handled as well as the capacity of each APCD, and the number may be selected based on needs.

[0089] Flue gas is naturally hot. However, when the ductwork 100 is reasonably long, the flue gas could cool down before the APCDs, if the ductwork 100 was not thermally insulated. This cooling would also imply thermal losses in the system thereby decreasing efficiency. For these reasons, in an embodiment of the ductwork 100 and / or the arrangement 10, the first main duct 110 is thermally insulated. Thus, in an embodiment, the ductwork 100 or the arrangement 10 comprises thermal insulation 300 configured to thermally insulate the interior of the first main duct 110 from an exterior the first main duct 110. Reference is made to Figs. 7a to 7c showing thermal insulation 300 on the first main duct 110. Thermal insulation can be applied also in other embodiments. Thermal insulation can be applied also on the second main duct 110b, the third main duct 110c, the fourth main duct 110d , and the auxiliary duct 190, if any of them is used.

[0090] As for the term thermal insulation 300, the thermal insulation 300 comprises thermally insulating material having a thickness T of at least 50 mm. Herein the thickness T of the thermal insulation 300 is measured in a direction that is perpendicular to the longitudinal direction dL. The thermal insulation 300 may laterally surround the first main duct 110 as depicted in Figs. 7a to 7c; and the thickness T is measured in a lateral direction (see Figs. 7a to 7c). As for the definition of “thermally insulating material”, a material is considered to be thermally insulating, if

[0091] - a thermal conductivity of the material is at most 0.1 W / mxK, preferably at most 0.05 W / mxK and / or

[0092] - a density of the material is at most 500 kg / m3.

[0093] Naturally, both the thermal conductivity and the density are at least zero.

[0094] Moreover, in view of the present application, the material of the thermal insulation 300 is selected such that it withstands a temperature of 250 °C without degrading. Naturally, when the thermal insulation 300 withstands a temperature of 250 °C without degrading, it may withstand higher temperatures without degrading.

[0095] Thermal insulation’s capability of insulating is commonly also characterized by an R-value, which is defined as the thickness of the material divided by the thermal conductivity. The greater R-value, the better insulation. Preferably, the thickness T and the material of the thermal insulation 300 are selected such that the R-value of the thermal insulation is at least 1 m2K / W, more preferably at least 2 m2K / W. In theory, the is no upper limit for the R-value. However, in practice the R-value may be, e.g., at most 10 m2K / W. An example of a suitable thermal insulation is mineral wool, a thickness of which being at least 100 mm, such as 100 to 400 mm. However, a suitable thickness depends on the environment in which the pulp mill or the power plant is arranged. Thicker insulation may be needed near north and south poles than near the equator.

[0096] Preferably the thermal insulation comprises at least two layers of thermally insulating material. Two layers has the benefit that a seam of a layer can be covered by a seamless part of another layer. For example, an inner layer 311 (see Fig. 7c) of the thermal insulation may comprise a seam S1 , and the seam

[0097] 51 is covered by an outer layer 312 of the thermal insulation such that a seam

[0098] 52 of the outer layer 312 does not overlap with a seam S1 of the inner layer 311 . In other words, the seam S1 of the inner layer 311 is covered by seamless part of the outer layer 312. This prevents heat from escaping the fist main duct 110 through seams S1 , S2 between parts of the thermal insulation. Preferably, the inner layer 311 of the thermal insulation is substantially as thick as the outer layer 312 of the thermal insulation. The values for the thickness T as disclosed above relate to total thickness, i.e., sum of thicknesses of different layers, as shown in Fig. 7c.

[0099] Concerning the method and the use of the ductwork, an embodiment comprises leading the flue gas in the first main duct 110 such that a temperature in the first main duct 110 is at least 130 °C. The temperature of the flue gas may be, e.g., at most 400 °C. Typically, a temperature of flue gas of a lime kiln is less than 300 °C, and in case of recovery boilers even lower. In this embodiment, preferably, at least the first main duct 110 is thermally insulated as detailed above. Thus, preferably in this embodiment of the method, an interior of the first main duct 110 is thermally insulated from an environment in which the first main duct 110 is arranged. The material of the thermal insulation 300 should be selected according to the use so that it withstands the needed temperature. Likewise, preferably in this embodiment of the use, an interior of the first main duct 110 is thermally insulated from an environment in which the first main duct 110 is arranged. The material of the thermal insulation 300 should be selected according to the use so that it withstands the needed temperature. The thermal insulation 300 preferably comprises at least the two layers discussed above.

[0100] As for the type of the mechanical conveyor 200, in principle any conveyor is usable. In line with what has been said above, preferably the mechanical conveyor 200 comprises a movable part 210, the movable part 210 being configured to move the ash by its movement.

[0101] For example, the movable part 210 could be a screw of a screw conveyor (not shown). For example, the movable part 210 could be a belt of a belt conveyor (not shown).

[0102] Referring to Figs. 4 to 6, in a preferable embodiment, the mechanical conveyor 200 comprises scrapers 221 , 222. The scrapers 221 , 222 are movable, whereby also they are examples of the more general movable part 210. When the scrapers 221 , 222 move, they push ash that has been accumulated in front of them by their movement.

[0103] Referring to Figs. 4 and 6, the scrapers 221 , 222 may be fixed to an endless chain 231 , which goes round a first wheel 232 and a second wheel 233. By rotating the wheels 232, 233 the chain 231 as well as the scrapers 221 , 222 move. Thus, the chain 231 is configured to move the scrapers (221 , 222). As an example, when the second wheel 233 rotates in the direction shown by the arrow close to the wheel, the scraper 231 arranged in the region E moves in the longitudinal direction dL and pushes ash towards an ash outlet 129 of the first main duct 110. Depending on the width of the scrapers 221 , 222, one, two, or more chains may be used in parallel. The scrapers 221 , 222 are configured to push ash along a bottom of the ductwork 100 in the longitudinal direction +dL (as in Figs. 4 and 6) or in a direction -dL reverse thereto (not shown). The flue gas flows from the first inlet 111 to the first outlet 121 in the longitudinal direction dL.

[0104] Thus, in an embodiment the ductwork 100 comprises an ash outlet 129 configured to let out ash from the first main duct 110. Moreover, the ash outlet 129 is arranged at a location to which the mechanical conveyor 200 is configured to convey the ash. The ash outlet 129, if present, is preferably arranged at a bottom of the first main duct 110. The first main duct may be provided with other outlets for the ash, e.g., when ash is conveyed to both ends of the first main duct 110 (not shown).

[0105] However, an ash outlet 129 is not needed. Instead, the ash may be conveyed with the mechanical conveyor 200 through the flue gas outlet 81 1 back to the flue gas generating device 810, from which ash may be collected. Likewise, the ash may be conveyed with the mechanical conveyor 200 to the first APCD 910, from which ash is collected. However, preferably the ductwork 100 comprises the ash outlet 129, which is separate from the first outlet 121 (and other flue gas outlets of the first main duct 110) and separate from the first inlet 111 (and other flue gas inlets of the first main duct 110). Then the ash can be withdrawn from the ash outlet 129, which decreases the need for ash removal from other parts and in this way improves process control and reduces maintenance needs.

[0106] Irrespective of whether the ash is collected from the fist APCD 910, the ash outlet 129, or the flue gas generating device 810, or some or all of them, preferably the ash is utilized. Especially when the flue gas is produced by burning black liquor or brown liquor in a chemical recovery boiler, the ash comprises cooking chemicals, which can be recovered by the chemical recovery boiler. Therefore, an embodiment of the method comprises producing the flue gas and recovering cooking chemicals in a chemical recovery boiler of the pulp mill by burning black liquor or brown liquor. The embodiment further comprises collecting ash from at least one of the first air pollution control device 910, the first main duct 110, and the chemical recovery boiler (e.g., a flue gas channel thereof). The ash can be collected from the first main duct 110 through the ash outlet 129. The ash may be collected form the first APCD 910 via an APCD outlet 919 shown at least in Fig. 2. The embodiment further comprises burning at least some of the collected ash in the recovery boiler to recover cooking chemicals from the ash. For burning the ash, the ash can be mixed with the liquor to be burnt in the chemical recovery boiler.

[0107] For reasons detailed above, preferably the ash that is conveyed by the mechanical conveyor 200 is let out via the ash outlet 129. Thus, a preferable embodiment comprises collecting ash from the first main duct 1 10 through an ash outlet 129 of the first main duct and burning at least some of the collected ash in the recovery boiler to recover cooking chemicals from the ash. As detailed above, in this embodiment, flue gas is let out to the first APCD 910 via the first outlet 121 .

[0108] In a preferable embodiment, the air pollution control equipment 900 comprises an electrostatic precipitator or a filter, more preferably an electrostatic precipitator or a bag filter. In the embodiment, first ash is preferably collected from the electrostatic precipitator or the filter, such as bag filter. The first ash may be collected through the APCD outlet 919. In the embodiment, second ash is preferably collected from the first main duct 110 through the ash outlet 129 of the first main duct 110. The first and second ashes are reasonably similar in chemical composition, whereby they can be utilized in the same way, e.g., in the way disclosed above in the context of a chemical recovery boiler.

[0109] However, because the ashes can be utilized in the same way, a secondary conveyor (not shown) can be used to convey both first ash and the second ash. Thus, a preferable embodiment comprises conveying both the first ash and the second ash using a secondary conveyor for utilization. The ashes may be mixed with each other. Thus, an embodiment comprises mixing the first ash with the second ash. The embodiment also comprises utilizing the mixture of the ashes.

[0110] In case of a chemical recovery boiler, the utilization preferably involves burning the ash in the chemical recovery boiler as detailed above. In case of a lime kiln, the ash can be conveyed back to the lime kiln for calcination or used as burnt lime in the pulp mill, depending on the degree of calcination of the ash. In general, the utilization may involve recycling the ash back to the main process.

[0111] In the embodiment of Fig. 4, the whole mechanical conveyor 200 is arranged within the first main duct 110. This is also preferable in view of thermal insulation of the first main duct 110. In this embodiment, the scrapers 221 , 222 push ash along a bottom of the first main duct 110 in the longitudinal direction +dL and the scrapers 221 , 222 are configured to return in the direction -dL reverse to the longitudinal direction inside the first main duct 110. Likewise, in a preferable embodiment (not shown), the scrapers 221 , 222 push ash along a bottom of the first main duct 110 in a direction -dL that is reverse to the longitudinal direction and the scrapers 221 , 222 are configured to return in the longitudinal direction +dL inside the first main duct 1 10.

[0112] However, another alternative is presented in Fig. 6, wherein only a part of the mechanical conveyor 200 is arranged within the first main duct 110. The movable part 210, i.e., the scraper 221 , is arranged within the first main duct 110 as depicted in Fig. 6. However, in that embodiment, the scrapers return outside the first main duct 110. As an example, the scraper 222 is arranged outside the first main duct 110. Also, parts of the wheels 232, 233 are arranged outside the first main duct 110.

[0113] As another example of a mechanical conveyor using scrapers 221 , 222, reference is made to Fig. 5. Therein the scrapers 221 , 222 are fixed to a rod 235. The rod 235 is configured to move the scrapers 221 , 222, and to move back and fort in the longitudinal direction dL and the opposite direction -dL. Moreover, the shape of the scrapers 221 , 222 is made antisymmetric so that when moving in the positive longitudinal direction dL, the scrapers 221 , 222 push the ash in this direction; and when moving in the negative longitudinal direction -dL, the scrapers 221 , 222 move below the ash without significantly moving the ash (or moving the ash mainly in the first direction d1 , but not in the negative longitudinal direction -dL). The scrapers 221 , 222 are thus configured to push ash along a bottom of the first main duct 110in the longitudinal direction +dL (as in Fig. 5). Naturally, the shape of the scrapers 221 , 222 can be designed so that the back-and-forth movement thereof moves ash mainly in a direction -dL that is opposite to the longitudinal direction (not shown).

[0114] A width of the scrapers 221 , 222, as measured in the second direction Sy, is preferably at least 200 mm, such as 200-1500 mm or 300-900 mm.

[0115] To control the flow of flue gas into the first APCD 910, in an embodiment, the ductwork 100 or the arrangement 10 comprises a first damper 913 configured to limit a flow of the flue gas to the first APCD 910. The first damper 913 is shown in figs. 8b, 8c, 9, 10, 11 , 12a and 16a-e. The first damper 913 may comprise a slidable baffle plate and / or a baffle plate pivotable about an axle. The first damper 913 can be used for limiting the flow of flue gas through the first APCD 910, e.g., for not exceeding the capacity of the first APCD 910. The first damper 913 can be used for completely stopping the flow of flue gas to the first APCD 910, e.g., for maintaining the first APCD 910. Using the first damper 913 is particularly beneficial, when the ductwork 100 is configured to lead a part of the flue gas to a second APCD 920 as shown in Figs. 9 to 16e in addition to being configured to lead a part of the flue gas to the first APCD 910. However, the inventor has found that when the first damper 913 is arranged at such a location wherein the flow of flue gas is directed or is configured to be directed downwards, most often ash starts to accumulate on the first damper 913 when the first damper is at least partly closed. As for the location, if the first damper 913 is completely closed, there is no flow of the flue gas, but since at least occasionally the first damper 913 is configured to be open, a direction for the flow of the flue gas can be defined for the location of the damper 913. Typically, the part of the duct, in which the first damper 913 is arranged, has a profile shape extending in the same direction as the flue gas flows or is configured to flow in that part of the duct.

[0116] Thus, preferably, the first damper 913 is arranged in such a location of the ductwork 100 that an average direction of the flow of the flue gas at that location forms or is configured to form an angle of at most 90 degrees with an upward vertical direction -Sz. Examples of such locations are shown in Figs. 8b, 8c, and 9 to 12a as well as Figs 16a to 16e. In figs 8b, 8c, 9, 10, and 11 the arrow dF1 denotes the direction of flow of flue gas at the location of the first damper 913 and the arrow dF2 denotes the direction of flow of flue gas at the location of a second damper 923. In case the damper is completely closed, the average direction of the flow of the flue gas that is considered to be configured to form at that location in the above meaning refers to the average direction of the flow of the flue gas at that location if the first damper 913 was open.

[0117] Thus, preferably, the first damper 913 is arranged in such a location of the ductwork 100 that an average direction of the flow of the flue gas at that location forms, or is configured to form, an angle of at most 90 degrees with an upward vertical direction -Sz. If there is no flow (i.e., the damper 913 completely closes the duct in which it is arranged) the flow is still configured to be formed (by opening the damper), whereby an average direction of the flow is also configured to form, whereby an angle between that and the upward vertical is configured to be formed.

[0118] Referring to Figs. 8a and 8b, in an embodiment, the ductwork 100, which may be comprised by the arrangement 10, comprises a first riser duct 911 and a first downcomer duct 912, as shown in Fig. 8b. To clarify Fib. 8b, it is noted that Fig. 8a shows by the symbols Vlllb the section, which is detailed in Fig. 8b.

[0119] The first riser duct 911 extends from the first outlet 121 of the first main duct 110 and the first downcomer duct 912 extends from the first riser duct 911 to the first APCD 910, as shown in Fig. 8b. More realistic embodiments including a first riser duct 911 and a first downcomer duct 912 is shown in Figs. 11 and 16c.

[0120] As for the term riser duct (the first riser duct 911 , a second riser duct 921 , a third riser duct 931 , or a fourth riser duct 941 ), a riser duct is configured to lead flue gas from the first outlet 121 or another outlet of the first main duct 110 in a direction that forms an angle of at most 35 degrees with an upward vertical direction -Sz. If the damper 913, 923 is arranged in a riser duct 911 , 921 , the directions dF1 and dF2 apply to riser ducts. Thus, the riser duct is configured to lead flue gas a direction shown by the reference dF1 , dF2.

[0121] As shown in Figs. 10 and 11 , a second riser duct 921 may receive the flue gas from the first outlet 121. However, as disclosed in Figs. 12a, 12b, and 16c the second riser duct 921 may receive the flue gas from a second outlet 122 (shown in Fig. 12b), which is different from the first outlet 121 .

[0122] In an embodiment, the first riser duct 911 is configured to lead flue gas from the first outlet 121 of the first main duct 110 in a direction that forms an angle of at most 35 degrees with an upward vertical direction -Sz. Such a small angle with respect to upward vertical direction ensures that ash will not accumulate within the first riser duct 911 .

[0123] Moreover, it has been found that if the first damper 913 is arranged in the first riser duct 911 , substantially no ash will accumulate on the first damper 913 when it is used, e.g., partly closed. Thus, in an embodiment, the first damper 913 is configured to limit a flow of flue gas within the first riser duct 911 .

[0124] As for the term downcomer duct (first downcomer duct 912, second downcomer duct 922, third downcomer duct 932, or fourth downcomer duct 942), a downcomer duct is configured to lead flue gas from receivable from a riser duct (first riser duct 911 , second riser duct 921 , third riser duct 931 , or fourth riser duct 941 , respectively) to an air pollution control device (first APCD 910, second APCD 920, third APCD 930, or fourth APCD 940, respectively) in a direction that forms an angle of at most 35 degrees with a downward vertical direction +Sz. This applies also the Figs. 16a to 16e, even if the fifth riser and downcomer ducts shown therein have not been presented with reference numbers.

[0125] Such a small angle with respect to downward vertical direction ensures that ash will not accumulate within the downcomer duct. Because of the first main duct 110, these riser and downcomer ducts (91 1 , 912) can be made much shorter than the riser and downcomer section of prior art (see Fig. 1 ). Therefore, even if riser and downcomer ducts are used, as in prior art, the problems of prior art are diminished because of the first main duct 110 and the mechanical conveyor 200 therein.

[0126] Moreover, it has been found that the APCD receiving flue gas from the downcomer duct functions better when the downcomer duct is substantially vertical. Using the first APCD 910 as an example, when the first downcomer duct 912 is more vertical, the first APCD 910 functions better. The inventor considers this to relate to the design of the APCD, which is commonly designed to receive the gas to the APCD via an inlet of the APCD that extends in the vertical direction. This applies at least when the first APCD 910 comprises an electrostatic precipitator or a filter (e.g., bag filter). Moreover, when the first downcomer duct 912 is substantially vertical, a substantially laminar flow of the flue gas may form within the first downcomer duct 912 before entering the first APCD 910. It seems that these factors affect the functioning of the first APCD 910 in combination with the first downcomer duct 912. This applies to other APCDs 920, 930, 940, and 950 and downcomer ducts used with them in a similar manner.

[0127] Thus, preferably, the first downcomer duct 912 is configured to lead flue gas receivable from the first riser duct 911 to the first APCD 910 in a direction that forms an angle of at most 10 degrees with the downward vertical direction Sz.

[0128] A preferable embodiment of the ductwork 100 or the arrangement 10 comprises both the first damper 913 and a second damper 923. Reference is made to Figs. 9 to 12a as well as 16a-16e. The second damper 923 is configured to limit a flow of the flue gas to the second APCD 920, which need not be part of the ductwork 100, but which is a comprised by an embodiment of an arrangement 10. Preferably, the first damper 913 and the second damper 923 are operatable independently of each other.

[0129] This allows for even more flexible use of the APCDs 910, 920. For example, the duct for the first APCD 910 can be fully closed with the first damper 913 for maintenance of the first APCD 910 and the system may be operational because a duct for the second APCD 920 can be fully open (or at least partly open) because of the second damper 923 being open, and vice versa. Moreover, by partly opening / closing the first and second dampers 913, 923, an optimal amount of flue gas can be led to each one of the first APCD 910 and the second APCD 920. In case the arrangement comprises a third APCD 930, a third damper 933 can be used in a similar manner mutatis mutandis. In case the arrangement comprises a fourth APCD 940, a fourth damper 943 can be used in a similar manner mutatis mutandis. In case the arrangement comprises a fifth APCD 950, a fifth damper 953 can be used in a similar manner mutatis mutandis. Reference is made to Fig. 12a and Figs. 16a-16e. Even if Figs. 12a and 12b show only four APCDs, the first main duct 110 may serve as many APCDs as required in an obvious manner. By using a longer main duct 110, as many APCDs can be arranged in parallel as needed. In case the first main duct 110 is long, it may be equipped also a second mechanical conveyor 200b as shown in Figs. 16a-16e. In even longer ducts, further mechanical conveyors may be used.

[0130] For the reasons detailed above, preferably, the first damper 913 is arranged in such a first location of the ductwork 100 that an average direction of the flow of the flue gas at the first location forms or is configured to form an angle of at most 90 degrees with an upward vertical direction -Sz. Moreover, the second damper 923 is arranged in such a second location of the ductwork 100 that an average direction of the flow of the flue gas at the second location forms or is configured to form an angle of at most 90 degrees with an upward vertical direction -Sz. This applies to the third damper 933, the fourth damper 934, and the fifth damper 953 mutatis mutandis, if the ductwork or the arrangement comprises such. A preferable embodiment comprises the first riser duct 911 and the first downcomer duct 912 as discussed above. Moreover, the embodiment comprises the first damper 913 configured to limit a flow of the flue gas within the first riser duct 911. Referring to Figs. 10, 11 , 12a-12d, and 16a-16e this preferable embodiment further comprises a second riser duct 921 and a second downcomer duct 922. Moreover, the embodiment comprises a second damper 923 configured to limit a flow of the flue gas within the second riser duct 921 .

[0131] What has been said above concerning the riser and downcomer ducts applies. Thus, in this embodiment, the second riser duct 921 is configured to lead flue gas from the first outlet 121 or second outlet 122 of the first main duct 110 in a direction that forms an angle of at most 35 degrees with an upward vertical direction -Sz. More specifically, in the embodiments of Figs. 10 and 11 , the second riser duct 921 is configured to lead flue gas from the first outlet 121 in said direction. However, in the embodiment of Fig. 12, the second riser duct 921 is configured to lead flue gas from the second outlet 122 of the first main duct 110 in said direction. Moreover, the second downcomer duct 922 is configured to lead flue gas receivable from the second riser duct 921 to the second APCD 920 in a direction that forms an angle of at most 35 degrees with a downward vertical direction +Sz. As for the second outlet 122, reference is made to Fig. 12b, which shows the embodiment of Fig. 12a in a top view.

[0132] Furthermore, the second damper 923 is configured to limit a flow of the flue gas within the second riser duct 921 . For the reasons detailed above, the first damper 913 and the second damper 923 are operatable independently of each other.

[0133] Preferably, to ensure an optimal functioning of the first APCD 910 and the second APCD 920, in an embodiment, the first downcomer duct 912 is configured to lead flue gas receivable from the first riser duct 911 to the first APCD 910 in a direction that forms an angle of at most 10 degrees with the downward vertical direction Sz and the second downcomer duct 922 is configured to lead flue gas receivable from the second riser duct 921 to the second APCD 920 in a direction that forms an angle of at most 10 degrees with the downward vertical direction Sz. As shown in Figs. 12a and 12b, the arrangement may comprise further APCDs, such the third APCD 930, the fourth APCD 940, and the fifth APCD 950. Flue gas to the third APCD 930 may be let out from the first main duct 110 from a third outlet 123 and through a third riser duct 931 and a third downcomer duct 932. Flue gas to the fourth APCD 940 may be let out from the first main duct 110 from a fourth outlet 124 and through a fourth riser duct 941 and a fourth downcomer duct 942. What has been said about the first riser duct 911 applies to the third riser duct 931 and the fourth riser duct 941 mutatis mutandis. What has been said about the first downcomer duct 912 applies to the third downcomer duct 932 and the fourth downcomer duct 942 mutatis mutandis.

[0134] Referring to Fig. 13, the flue gas channel 810 may be equipped with an auxiliary duct 190, the auxiliary duct 190 being substantially like the first main duct 110. The auxiliary duct 190, too, comprises a mechanical conveyor to move ash in a manner disclosed in the context of the first main duct 110. The auxiliary duct 190 may be substantially horizontal as disclosed in more specific terms for the first main duct 110 mutatis mutandis. Such a configuration may be usable, if several APCDs are needed but they can not be arranged close to a wall 812 of a flue gas generating device 810 having the flue gas outlet 811. Thus, the auxiliary duct 190 may be used to form an extension to the flue gas duct of the device 810. In Fig. 13, an outlet of the auxiliary duct 190 is connected to the first inlet 111 of the first main duct 110. The auxiliary duct 190 may alternatively be considered to form a part of the ductwork 100 or the arrangement 10. Thus, the first inlet 111 of the first main duct 110 does not need to be directly coupled with the flue gas outlet 811.

[0135] Referring to Figs. 14a and 14b, when multiple APCDs are used, a first end of the first main duct 110 may comprise the first outlet 121 serving both the first APCD 910 and the second APCD 920 as shown in Fig. 14a. The more detailed structure of the first end of the first main duct 110 may be identical to that shown in Fig. 11 or Fig. 10. In case riser ducts and downcomer ducts are not used, the more detailed structure of the first end of the first main duct 110 may be identical to that shown in Fig. 9. The second end of the first main duct 1 10 may be used to serve two further APCDs, namely the third APCD 930 and the fourth APCD 940 (see Fig. 14a). The second end may be like the first end. The outlet at the second end of the first main duct 110 may be referred to as a second outlet 122. The second outlet 122 may serve the third APCD 930 through a third riser duct 931 and a third downcomer duct 932 in a similar way as the first outlet 121 serves the first APCD 910 through the first riser duct 911 and the first downcomer duct 912 mutatis mutandis. The second outlet 122 may serve the fourth APCD 940 through a fourth riser duct 941 and a fourth downcomer duct 942 in a similar way as the first outlet 121 serves the second APCD 920 through the second riser duct 921 and the second downcomer duct 922 mutatis mutandis. Thus, a more detailed structure of the second end of the first main duct 110 may be identical to that shown in Fig. 11 or Fig. 10. In case riser ducts and downcomer ducts are not used, the more detailed structure of the second end of the first main duct 110 may be identical to that shown in Fig. 9. Instead of one main duct 110 as in Fig. 14a, two main ducts 110, 110b can be used, as shown in Fig. 14b.

[0136] Depending on needs, an auxiliary duct 190 shown in Fig. 13 can be used also in the context of the embodiment of Fig. 14a. The resulting arrangement is shown in Fig. 15. What has been said in connection with other figures applies mutatis mutandis to Fig. 15.

[0137] Figures 16a to 16e show an embodiment of an arrangement 10, which comprises the flue gas generating device 810, air pollution control equipment 900 having five APCDs 910, 920, 930, 940, 950, and a ductwork 100 in between them. Thus, Figs. 16a-e also show a ductwork 100. The ductwork 100 comprises the first main duct 110 as discussed above. Figures 16a and 16b are presented in a side view and Figures 16c and 16d are presented in a perspective view. Five dampers 913, 923, 933, 943, and 953 are used, each damper configured to limit the flow of flue gas to one of the APCDs, respectively. In addition to the mechanical conveyor 200, the first main duct 110 has been equipped with a second conveyor 200b (see Figs. 16d and 16e). Riser ducts and downcomer ducts are used to lead flue gas from the first main duct 110 to each one of the APCDs as discussed above. Because of the first main duct 110, much shorter riser and downcomer ducts can be used compared to a solution having only the riser and downcomer duct for leading flue gas from the outlet(s) of the flue gas generating device 810 to the APCDs 910, 920, 930, 940, and 950. This reduces the problems of prior art.

[0138] The ductwork 100 as disclosed above can be used when building a new pulp mill or power plant to connect a new (i.e., unused) flue gas generating device 810 to new (i.e., unused) air pollution control devices or an air pollution control device. In such cases there is a lot of freedom is selecting the locations of the APCD or APCDs with respect to the flue gas generating device 810. In such cases, as examples, layouts like shown in Figs. 12a, 12b, 14, or 16a-e can possibly be used.

[0139] Moreover, the ductwork 100 as disclosed above can be used when upgrading an existing pulp mill or power plant. In such a case, the ductwork 100 can be used to connect an old (i.e., used) flue gas generating device 810 to new (i.e., unused) APCDs. In such cases there is only a little or no freedom when selecting the locations of the APCDs with respect to the flue gas generating device 810. In such cases, as examples, layouts like shown in Figs. 2, 8a-8c, 9-11 , 13, or 15 can possibly be used.

[0140] Therefore, the embodiments relate to a use of the ductwork 100 as discussed above. The use of the ductwork 100 is for constructing or upgrading a part of a pulp mill or a power plant. The use comprises connecting the first inlet 111 of the first main duct 110 to a flue gas outlet 811 of a pulp mill or a power plant optionally using an auxiliary duct 190. The use further comprises providing a first air pollution control device 910 and connecting the ductwork 100 to an inlet of the first air pollution control device 910 such that the first air pollution control device 910 is configured to receive flue gas from the ductwork 100. As discussed above, the ductwork 100 can be connected to an inlet of the APCD 910 for example such that the first outlet 121 of the first main duct 110 is connected, directly or through the first riser duct 911 and the first downcomer duct 912, to the inlet of the first APCD 910.

[0141] In line with what has been said, both the flue gas generating device 810 and the first APCD 910 can be new, i.e., unused. Thus, in an embodiment, the first APCD 910 that is provided has not been used for cleaning flue gas received from a pulp mill process or a power generating process or has been used for such a purpose for at most a month when connecting the first inlet 111 of the first main duct 110 to a flue gas outlet 811 of a pulp mill or power plant. Moreover, in this case, the flue gas generating device 810 comprising the flue gas outlet 811 has not been used in a pulp mill process or power generating process or has been used for the purpose for at most a month. However, for reasons detailed in background, the ductwork 100 is particularly suitable when upgrading an existing, i.e., used, pulp mill or power plant. In such a case, typically, the flue gas generating device 810 has been used in a pulp mill process or a power generating process for some time. Using the flue gas generating device 810 in the pulp mill process or the power generating process means, e.g., that the chemical recovery boiler, the power boiler, or the lime kiln comprising the flue gas outlet 811 has been used for burning the corresponding fuel (black or brown liquor in case of chemical recovery boiler, biomass based fuel or RDF in case of power boiler, and suitable fuel, e.g., oil or gas, which may be produced by biomass gasification, in case of a lime kiln) for some time, e.g. at least for months. Thus, in a preferable embodiment of the use, when connecting the first inlet 111 of the first main duct 110 to the flue gas outlet 811 of the pulp mill or the power plant (i.e. the flue gas outlet 811 of the flue gas generating device 810), at least a part of the flue gas generating device 810 comprising the flue gas outlet 811 has been used in a pulp mill process or power generating process for at least several months. However, as detailed in Figs. 13 and 15, an auxiliary duct 190, which is like the first main duct 110, can be used in between the first main duct 1 10 and the old flue gas generating device. The auxiliary duct 190 can be considered to form a part of the new flue gas channel. In the alternative, the auxiliary duct 190 could be considered to form a part of the ductwork 100. However, it is noted that if so considered, then the first inlet 111 of the first main duct 100 is not directly connected to an outlet of the old flue gas channel. Instead, an inlet of the auxiliary duct 190 is connected thereto, and the first inlet 111 of the first main duct 1 10 receives flue gas through the auxiliary duct 190.

[0142] Moreover, preferably, a new APCD, or several new APCDs, are installed to such an existing pulp mill or power plant. Thus, in an embodiment of the use, the first APCD 910 that is provided has not been used for cleaning flue gas received from a pulp mill process or power generating process or has been used for such a purpose for at most a month. Moreover, the ductwork 100 is connected to the first APCD 910 as disclosed above.

[0143] In this way, the invention further relates to a method for upgrading an existing pulp mill or power plant. The method comprises using the ductwork 100 as disclosed above.

Claims

Claims:1 . A ductwork (100) suitable for leading flue gas from a flue gas outlet (811 ) of a pulp mill or a power plant to air pollution control equipment (900), the ductwork (100) comprising- a first main duct (110) extending in a longitudinal direction (dL) from a first inlet (11 1 ) to a first outlet (121 ) and configured to lead flue gas in the longitudinal direction (dL) from the first inlet (11 1 ) to the first outlet (121 ), wherein- the first main duct (110) is equipped with a mechanical conveyor (200) comprising a movable part (210) configured to move the ash by its movement such that- the mechanical conveyor (200) is configured to convey ash in the longitudinal direction (dL) or in a direction (-dL) reverse thereto, and- at least the part of the mechanical conveyor that is configured to move the ash is arranged inside the first main duct (110) and in such a region (E) of an interior of the first main duct (110) that is located farthest in a first direction (d1 ), which is perpendicular to the longitudinal direction (dL) characterized in that- a cross-section (XS) of the first main duct (110), the cross-section (XS) having a normal to the longitudinal direction (dL), has a shape that tapers in the first direction (d1 ).

2. The ductwork (100) of claim 1 , wherein- a height (H) of the cross-section (XS) of the first main duct (110) measured in the first direction (d1 ) is greater than a width (W) of the cross-section (XS) of the first main duct (110) measured in a second direction (Sy), which is perpendicular to the first direction (d1 ).

3. An arrangement (10) for cleaning flue gas, the arrangement (10) comprising,- the ductwork (100) of the claim 1 or 2,- at least one of (i) a lime kiln configured to calcinate calcium carbonate, (ii) a chemical recovery boiler configured to burn black liquor or brown liquor, and (iii) a power boiler and- the air pollution control equipment (900), wherein- the chemical recovery boiler, the power boiler, or the lime kiln comprises the flue gas outlet (811 ),- the longitudinal direction (dL) is horizontal or forms an angle of at most 50 degrees with a horizontal plane,- at least a part of the mechanical conveyor (200) is configured to convey the ash, the part of the mechanical conveyor (200) being arranged at a bottom the first main duct (110),- the chemical recovery boiler, the power boiler, or the lime kiln is arranged to lead flue gas from the chemical recovery boiler, the power boiler, or the lime kiln, respectively, to the first inlet (111 ) of the first main duct (110), and- the air pollution control equipment (900) is configured to receive flue gas from the ductwork (100).

4. The arrangement of claim 3, wherein- the first direction (d1 ) is a projection of a downward vertical direction (Sz) to the plane of the cross-section (XS).

5. The ductwork (100) or the arrangement (10) of any of the claims 1 to 4, wherein- the mechanical conveyor (200) comprises scrapers (221 , 222) configured to push ash along a bottom of the ductwork (100) in the longitudinal direction (+dL) or in a direction (-dL) reverse thereto, wherein- the scrapers (221 , 222) are attached to a chain (231 ) or a rod (235) configured to move the scrapers (221 , 222), preferably,- a width of the scrapers (221 , 222) is at least 200 mm, more preferably,- the scrapers (221 , 222) are attached to an endless chain (231 ), which goes round a first wheel (232) and a second wheel (233) such that- the scrapers (221 , 222) push ash along a bottom of the ductwork (100) in the longitudinal direction (+dL) or in a direction (-dL) reverse thereto and- the scrapers (221 , 222) are configured to return in the direction (-dL) reverse to the longitudinal direction or in the longitudinal direction (+dL), respectively, inside the ductwork (100).

6. The ductwork (100) or the arrangement (10) of any of the claims 1 to 5, comprising- thermal insulation (300) configured to thermally insulate an interior of the first main duct (110) from an exterior the first main duct (110), wherein- the thermal insulation (300) comprises thermally insulating material having a thickness (T) of at least 50 mm, the thickness (T) of the thermal insulation (300) being measured in a direction that is perpendicular to the longitudinal direction (dL); preferably,- an R-value of the thermal insulation (300) is at least 1 m2K / W, wherein- the R-value is calculated as the thickness (T) of the thermal insulation (300) divided by its thermal conductivity; more preferably,- the thermal insulation (300) comprises an inner layer (31 1 ) of thermally insulating material and an outer layer (312) of thermally insulating material such that-the inner layer (311 ) comprises a seam (S1 ), and- the seam (S1 ) is covered by a seamless part of the outer layer (312).

7. The ductwork (100) or the arrangement (10) of any of the claims 1 to 6, wherein- the first main duct (110) comprises a second inlet (112) for receiving flue gas from another flue gas outlet of the pulp mill or the power plant and- the first main duct (110) is configured to lead flue gas the air pollution control equipment (900);AND / OR- the ductwork comprises a second main duct (110b) equipped with a mechanical conveyor configured to convey ash, and- the second main duct (110b) is configured to lead flue gas from another flue gas outlet of the pulp mill or the power plant to the air pollution control equipment (900).

8. The ductwork (100) or the arrangement (10) of any of the claims 1 to 7, comprising- a first riser duct (91 1 ) configured to lead flue gas from the first outlet (121 ) of the first main duct (110) in a direction that forms an angle of at most 35 degrees with an upward vertical direction (-Sz),- a first downcomer duct (912) configured to lead flue gas receivable from the first riser duct (911 ) to a first air pollution control device (910) of the air pollution control equipment (900) in a direction that forms an angle of at most 35 degrees with a downward vertical direction (+Sz), and- a first damper (913) configured to limit a flow of flue gas within the first riser duct (911 ); preferably,- the first downcomer duct (912) is configured to lead flue gas receivable from the first riser duct (91 1 ) to the first air pollution control device (910) in a direction that forms an angle of at most 10 degrees with the downward vertical direction (+Sz).

9. The ductwork (100) or the arrangement (10) of any of the claims 1 to 8, comprising- a / the first damper (913) configured to limit a flow of the flue gas to a / the first air pollution control device (910) of the air pollution control equipment (900), and- a second damper (923) configured to limit a flow of the flue gas to a second air pollution control device (920) of the air pollution control equipment (900), wherein- the first damper (913) and the second damper (923) are operatable independently of each other; preferably,- the first damper (913) is arranged in such a first location of the ductwork (100) that an average direction of the flow of the flue gas at the first location forms or is configured to form an angle of at most 90 degrees with an upward vertical direction (-Sz) and- the second damper (923) is arranged in such a second location of the ductwork (100) that an average direction of the flow of the flue gas at the second location forms or is configured to form an angle of at most 90 degrees with an upward vertical direction (-Sz); more preferably,- the first and second air pollution control devices (910, 920) are comprised by the arrangement (10) and configured to receive flue gas from the ductwork (100).

10. The ductwork (100) or the arrangement (10) of any of the claims 1 to 9, comprising- a / the first riser duct (91 1 ) configured to lead flue gas from the first outlet (121 ) of the first main duct (110) in a direction that forms an angle of at most 35 degrees with an upward vertical direction (-Sz),- a / the first downcomer duct (912) configured to lead flue gas receivable from the first riser (911 ) duct to a / the first air pollution control device (910) of the air pollution control equipment (900) in a direction that forms an angle of at most 35 degrees with a downward vertical direction (+Sz),- a second riser duct (921 ) configured to lead flue gas from the first outlet (121 ) or another outlet of the first main duct (110) in a direction that forms an angle of at most 35 degrees with an upward vertical direction (-Sz), and- a second downcomer duct (922) configured to lead flue gas receivable from the second riser duct (921 ) to a / the second air pollution control device (920) of the air pollution control equipment (900) in a direction that forms an angle of at most 35 degrees with a downward vertical direction (+Sz),- a / the first damper (913) configured to limit a flow of the flue gas within the first riser duct (911 ) and- a / the second damper (923) is configured to limit a flow of the flue gas within the second riser duct (921 ), wherein- the first damper (913) and the second damper (923) are operatable independently of each other; preferably,- the first downcomer duct (912) is configured to lead flue gas receivable from the first riser duct (91 1 ) to the first air pollution control device (910) in a direction that forms an angle of at most 10 degrees with the downward vertical direction (Sz) and- the second downcomer duct (922) is configured to lead flue gas receivable from the second riser duct (921 ) to the second air pollution control device (920) in a direction that forms an angle of at most 10 degrees with the downward vertical direction (Sz).11 . A method for cleaning flue gas, the method comprising- burning material in a flue gas generating device (810) of a pulp mill or a power plant to produce the flue gas,- leading the flue gas in a first main duct (110) of a ductwork (100) to air pollution control equipment (900) at least in a longitudinal direction (dL) of the first main duct (110) or in a direction (-dL) that is reverse thereto, the longitudinal direction (dL) of the first main duct (110) being horizontal or forming an angle of at most 50 degrees with a horizontal plane,- collecting ash to a bottom of the first main duct (110),- conveying the ash at the bottom of the first main duct (110) using a mechanical conveyor (200) in the longitudinal direction (dL) or in the direction (-dL) reverse thereto irrespective of whether the flue gas is lead in the first main duct (110) in the longitudinal direction (dL) or in the opposite direction (-dL), and- cleaning the flue gas with the air pollution control equipment (900), characterized in that- at least the part of the mechanical conveyor that is configured to move the ash is arranged inside the first main duct (110) and in such a region (E) of an interior of the first main duct (110) that is located farthest in a first direction (d1 ), which is perpendicular to the longitudinal direction (dL) and- a cross-section (XS) of the first main duct (110), the cross-section (XS) having a normal to the longitudinal direction (dL), has a shape that tapers in the first direction (d1 ).

12. Use of a ductwork (100) comprising a first main duct (110), equipped with a mechanical conveyor (200) configured to convey ash in a longitudinal direction (dL) of the first main duct (110) or in a direction (-dL) that is reverse thereto for leading flue gas from a flue gas outlet (81 1 ) of a pulp mill or a power plant to air pollution control equipment (900) such that the longitudinal direction (dL) of the main first duct (110) is horizontal or forms an angle of at most 50 degrees with a horizontal plane, characterized in that at least the part of the mechanical conveyor that is configured to move the ash is arranged inside the first main duct (110) and in such a region (E) of an interior of the first main duct (110) that is located farthest in a first direction (d1 ), which is perpendicular to the longitudinal direction (dL) and a cross-section (XS) of the first main duct (110), the cross-section (XS) having a normal to the longitudinal direction (dL), has a shape that tapers in the first direction (d1 ).

13. The method of claim 11 or the use of claim 12, comprising- producing the flue gas in a chemical recovery boiler, a power boiler, or a lime kiln.

14. The method or the use of any of the claims 11 to 13, comprising- leading the flue gas in the first main duct (110) such that a temperature in the first main duct (110) is at least 130 °C; preferably,- an interior of the first main duct (110) is thermally insulated from an environment in which the main duct (110) is arranged.

15. The arrangement, the method, or the use of any of the claims 4 to 14, wherein- the air pollution control equipment (900) comprises an electrostatic precipitator or a filter, such as a bag filter, preferably,- the arrangement further comprises a secondary conveyor configured to convey first ash from the electrostatic precipitator or the filter and second ash from the first main duct (110) for utilization; or the method or the use comprises- collecting first ash from the electrostatic precipitator or the filter,- collecting second ash from the first main duct (110) through an ash outlet (129) of the first main duct (110), and- conveying the first ash and the second ash using a / the secondary conveyor.

16. Use of the ductwork (100) of any of the claims 1 to 10, the use comprising- connecting the first inlet (111 ) of the first main duct (110) to a flue gas outlet (811 ) of a pulp mill or a power plant,- providing a first air pollution control device (910), and- connecting the ductwork (100) to the first air pollution control device (910) such that the first air pollution control device (910) is configured to receive flue gas from the ductwork (100), preferably, in the use- when connecting the first inlet (111 ) of the first main duct (110) to the flue gas outlet (811 ) of the pulp mill or the power plant, the flue gas outlet (81 1 ) has been used in the pulp mill or the power plant for at least several months,- the first air pollution control device (910) that is provided has not been used for cleaning flue gas received from a pulp mill or a power plant or has been used for such a purpose for at most a month, and- the ductwork (100) is connected to the first air pollution control device (910).