Patent Description:
In fluidized bed boilers, bed material, which typically includes sand, is in fluidized state. Typically walls of the fluidized bed boiler comprise heat exchanger pipes limiting the furnace to recover heat from the furnace. The heat exchanger pipes are connected to each other with fins or similar connectors to form a gas-tight wall structure. As a result of the fluidizing, typically in the furnace near the walls of the boiler the bed material flows downwards. If the wall comprises protrusions or bends, the flowing bed material may easily erode the heat exchanger pipes. It is known e.g. from <CIT> that in such a solution, the erosion of the heat exchanger pipes can be reduced by providing the heat exchanger pipes with a circumferentially extending metal coating. A thickness of the coating, and a diameter of the coated part of the pipe, is selected such that a diameter of the coated part is at most equal a diameter of an uncoated part. Fins are continuously welded to such heat exchanger pipes to form the wall. Part of such a wall may be covered with refractory.

It has been realized that near the interface of the refractory and the heat exchanger pipes, the refractory interferes the flow of the bed material, which, as discussed above, takes place near the walls. Thus, the downwards flowing bed material coincides a surface of the refractory, which interferes the bed material flow. It has been found that near the refractory, highly localized erosion takes place. Unless maintained, such erosion may cause small apertures or holes to the heat exchanger pipes, and thus malfunction of the whole boiler. A size of these holes or apertures may be e.g. in the range of e.g. <NUM>-<NUM> by <NUM>-<NUM>.

There is thus a need to reduce the erosion of heat exchanger pipes of such a wall structure in particular near the refractory.

<CIT> and <CIT> show further examples of fluidized bed boilers having a wall of water tubes and a refractory arranged on an inner side of the wall.

Without being bound to theory, the inventors consider that the highly localized erosion is a result of a vortex of the bed material being formed near the interface of the refractory and the heat exchanger pipe of the wall. The formation of the vortex is depicted as a comparative example in <FIG>. In typical use, the direction towards the reader in <FIG> is upwards vertical (denoted by Sz in <FIG>). Thus, the downwards flowing bed material, flowing along the wall formed by the pipes, when hitting the surface of the refractory, forms vortexes near the refractory, on the side of the furnace, as shown in <FIG>. It seems that these vortexes can be responsible of highly localized erosion of the heat exchanger pipes of the wall. As a result, the holes or apertures, as discussed above, may erode to the pipes.

The inventors have found that such erosion of heat exchanger pipes can be reduced by (i) increasing the size of the vortex or (ii) by forming such a wall structure that does not generate such vortexes. In line with this finding, when the wall does not comprise the fin near the interface between the refractory and the heat exchanger pipes, the bed material and gases can propagate deeper in between the heat exchanger tubes, thereby providing substantially more space for the vortex. Such a wall structure is disclosed in more specific terms in the independent claim <NUM>.

Moreover, the inventors have found that the vortex-induced erosion of heat exchanger pipes of such a wall structure can be reduced by reducing the tendency of forming any vortexes. In line with this finding, when the wall does not comprise the fin near the interface between the refractory and the heat exchanger pipes, and furthermore the bed material and gases can propagate through the wall to another side of the wall, the tendency of forming any vortexes becomes reduced. Such a wall structure is disclosed in more specific terms in the dependent claim <NUM>.

Other preferable embodiments of the wall are disclosed in the dependent claims <NUM> to <NUM>.

The wall is preferably a wall of a circulating fluidized bed boiler, as detailed in claims <NUM> to <NUM> and <NUM>. The wall is preferably a division wall of a furnace of a circulating fluidized bed boiler, as detailed in claims <NUM> to <NUM>.

These and other embodiments are further explained in the description and the drawings.

Several of the drawings show different views of the embodiments. The different views/details have been indicated close to the number of the figure. Thus, e.g. the drawing <FIG> shows the view/detail la of <FIG>. This is indicated by the reference "(Ia)" near the figure number "<FIG>". In line with this, one can see the text "<FIG> (Ia)" on the figure page <NUM>/<NUM>. Similar notation is used in other figures, too, as detailed below.

In the Figures, the directions Sx, Sy, and Sz are three mutually orthogonal directions. In a preferable use, the direction Sz is reverse to gravity (i.e. Sz is upwards and vertical).

The invention relates to a fluidized bed boiler <NUM>. A fluidized bed boiler <NUM> is shown in <FIG> in various views. The fluidized bed boiler <NUM> of <FIG> is a circulating fluidized bed boiler. Another fluidized bed boiler <NUM> is shown in <FIG>. The fluidized bed boiler <NUM> comprises a furnace <NUM>. The fluidized bed boiler <NUM> comprises walls. Some of the walls limit the furnace <NUM>. A division wall may divide the furnace <NUM> to two parts such that combustion takes place on both sides. The invention relates to a wall <NUM> of the fluidized bed boiler <NUM>. The wall <NUM> may be a division wall, as depicted in <FIG>. In the alternative, the wall <NUM> may be a side wall of the furnace <NUM> as shown in <FIG>. Naturally, a fluidized bed boiler may comprise both a wall <NUM> having the structure disclosed in detail below as a division wall and another wall <NUM> have the having the structure disclosed in detail below as a side wall.

<FIG> show, in various views, a wall <NUM> for a fluidized bed boiler. Referring to <FIG> the wall <NUM> comprises a first heat exchanger pipe <NUM> and a second heat exchanger pipe <NUM>. A purpose of the heat exchanger pipes <NUM>, <NUM> is to recover heat from the furnace <NUM>. Another purpose of the heat exchanger pipes <NUM>, <NUM> is, in an embodiment, to carry mechanical load. Another purpose of the heat exchanger pipes <NUM>, <NUM> is, in an embodiment, to prevent material from escaping the furnace. The first heat exchanger pipe <NUM> comprises a first primary portion <NUM> and a first secondary portion <NUM>. The second heat exchanger pipe <NUM> is arranged in parallel with the first heat exchanger pipe <NUM> and comprises a second primary portion <NUM> and a second secondary portion <NUM>.

As for the terminology of these portions and with reference to <FIG>, an outer diameter d12 of the first secondary portion <NUM> is less than or equal to an outer diameter d11 of the first primary portion <NUM>. Moreover, an outer diameter d22 of the second secondary portion <NUM> is less than or equal to an outer diameter d21 of the secondary primary portion <NUM>. In use, the first primary portion <NUM> is arranged above the first secondary portion <NUM>, and the secondary primary portion <NUM> is arranged above the second secondary portion <NUM>, as shown in <FIG>, <FIG>. For definition of the direction Sz, see above. These features have the effect that the bed material, flowing along the wall <NUM> downwards (in the negative Sz direction) flows without forming vortexes at a boundary between the primary portions (<NUM>, <NUM>) and the secondary portions (<NUM>, <NUM>) as there are not protrusions when moving from a side of the primary portions (<NUM>, <NUM>) to a side of the secondary portions (<NUM>, <NUM>). It is also noted that in an embodiment, the secondary portions (<NUM>, <NUM>) are provided with an overlay. Herein the outer diameter refers to an outer diameter of the overlay of the secondary portion, when an overlay has been provided, as shown in <FIG>. If an overlay is not provided, the outer diameter refers to an outer diameter of the plain secondary portion, as shown in <FIG>. This definition ensures that even if the overlay is provided, the vortex is not generated, as discussed above. Preferably, the first secondary portion <NUM> is co-axial with the first primary portion <NUM>; and the second secondary portion <NUM> is co-axial with the second primary portion <NUM>. This in connection with the outer diameters thereof ensure that the secondary portions (<NUM>, <NUM>) do not protrude from the from a tubular curtain defined by the primary positions (<NUM>, <NUM>).

Referring to <FIG>, the wall <NUM> comprises a first connector, e.g. first fin <NUM>. In <FIG> a first fin <NUM> serves as the first connector. However, as will be detailed below, in the alternative, an auxiliary heat transfer surface <NUM> may serve as the first connector. In what follows, the fist fin <NUM> is used as an example of the first connector, unless otherwise indicated.

The first fin <NUM> is fixed to the first primary portion <NUM> and to the second primary portion <NUM>. A purpose of the first fin <NUM> is to tie the first heat exchanger pipe <NUM> and the second heat exchanger pipe <NUM> so as to improve the integrity of the wall <NUM>. Moreover, when the wall <NUM> is used as a side wall of a furnace <NUM>, the first fin <NUM> prevents gases and bed material from escaping from the furnace <NUM> between the first primary portion <NUM> the second primary portion <NUM>.

In the more general case, the first connector (e.g. the first fin <NUM> or the first auxiliary heat transfer surface <NUM>) is fixed to the first primary portion <NUM> and to the second primary portion <NUM>. A purpose of the first connector is to tie the first heat exchanger pipe <NUM> and the second heat exchanger pipe <NUM> so as to improve the integrity of the wall <NUM>. Moreover, when the wall <NUM> is used as a side wall of a furnace <NUM>, the first connector prevents gases and bed material from escaping from the furnace <NUM> between the first primary portion <NUM> the second primary portion <NUM>.

Referring to <FIG>, in the wall <NUM>, at least a part of the first secondary portion <NUM> and at least a part of the second secondary portion <NUM> extend straight, in parallel, and within an imaginable plane P. The plane P is imaginable, i.e. it is not a physical object, but a plane as well known in the field of geometry. In an embodiment, at least the part of the first secondary portion <NUM> and at least the part of the second secondary portion <NUM> extend straight and in parallel so that the central axes of these parts belong to the plane P.

Referring to <FIG>, the wall <NUM> comprises a first gap <NUM>. The first gap <NUM> is arranged between the first secondary portion <NUM> and the second secondary portion <NUM>. In <FIG>, the first gap <NUM> is arranged in front of the second secondary portion <NUM> (i.e. the first gap <NUM> is towards the reader from the second secondary portion <NUM>).

The wall <NUM> is configured such that at least a part of the first gap <NUM> penetrates through the plane P, whereby gases and bed material can propagate through the first gap <NUM>. Thus, the wall <NUM> is configured such that at least a part of the first gap <NUM> penetrates through the plane P such that particulate material (i.e. bed material, and thus also gas) can propagate (i.e. the particulate material is able to propagate) through the first gap <NUM> from a first side S1 of the plane P to the opposite second side S2 of the plane P. Naturally this feature is not a feature concerning the particulate material as such, which is arranged in the furnace <NUM> in use. In other words, the wall <NUM> is free from such parts that would prevent the particulate material or the gases from propagating through at least a part of the first gap <NUM> and through the plane P from a first side S1 of the plane P to the opposite second side S2 of the plane P.

Propagation of the particulate material (i.e. bed material) through at least a part of the first gap <NUM> and through the plane P from a first side S1 of the plane P to the opposite second side S2 of the plane P is shown by the arrow AR1 in <FIG>.

In has been found that when the particulate material (i.e. bed material) can propagate in such a way, if a vortex is formed within the first gap <NUM>, the vortex spreads to a much larger area and generates less erosion to the heat exchanger pipes. Such a vortex may form e.g. if the secondary parts <NUM>, <NUM> of the heat exchanger pipes are connected by a plate on the second side S2 (not shown) instead of the fin of <FIG>. Naturally, the fin of <FIG> prevents the particulate material (i.e. bed material) from propagating from a first side of the wall to an opposite second side of the wall, because this is one of the main tasks of the fins.

Furthermore, the wall <NUM> comprises a refractory <NUM>. The refractory <NUM> is arranged on at least one side of both a part of the first secondary portion <NUM> and a part of the second secondary portion <NUM>. Preferably, the refractory <NUM> is arranged on both sides of a part of the first secondary portion <NUM> and a part of the second secondary portion <NUM>. Thus, in an embodiment, a part of the refractory <NUM> laterally surrounds both a part of the first secondary portion <NUM> and a part of the second secondary portion <NUM>. A function of the refractory <NUM> is to protect the part of the first secondary portion <NUM> and the part of the second secondary portion <NUM>. For protecting the secondary portions <NUM> and <NUM> a thickness of the refractory is preferably <NUM> to <NUM>. Herein the thickness refers to a thickness of the refractory on the first secondary portion <NUM>; i.e. a thickness of refractory between the open space of the furnace <NUM> and the outer surface of the first secondary portion <NUM>. A top surface of the refractory <NUM> may be planar and horizontal as shown in <FIG>. However, to ease flow of the particulate material, the refractory may have a shape that tapers upwards, as shown in <FIG>, and also in <FIG>. In these figures, a top surface of the refractory <NUM> comprises an inclined part.

In the wall <NUM>, a part of the first secondary portion <NUM> extends from the refractory <NUM> to a first direction Dir1, a part of the second secondary portion <NUM> extends from the refractory <NUM> to the first direction Dir1, and the first gap <NUM> extends from the refractory <NUM> to the first direction Dir1. The first direction Dir1 is a direction of the plane P and perpendicular to a direction that is directed from a first point on a central axis of the first secondary portion <NUM> to a second point on a central axis of the second secondary portion <NUM>, wherein the second point is on the central axis of the second secondary portion <NUM> and the point closest to the first point. The first direction Dir1 is preferably more or less upwards vertical, as more specifically detailed below. Thus, the first gap <NUM> is arranged to such a location, at which, without the first gap <NUM>, the highly localized erosion would take place. A possible reason could be the formation of a highly localized vortex. Such a location is depicted in <FIG>. However, providing more space for such vortex will result is less localized erosion. Thus, as shown in <FIG> according to the embodiment, the first gap <NUM> is arranged at a similar location to prevent the formation of the vortex or to at least increase a size of the vortex.

In line with what has been said above, in an embodiment the first secondary portion <NUM> is not provided with a fin or fins, whereby a shape of an outer contour of a cross section of a body of the first secondary portion <NUM> is circular throughout a length of the first secondary portion <NUM>. Moreover, in an embodiment, the second secondary portion <NUM> is not provided with a fin or fins, whereby a shape of an outer contour of a cross section of a body of the second secondary <NUM> portion is circular throughout a length of the second secondary portion <NUM>. Herein the term "body" refers to the (first, second,. ) secondary portion (<NUM>, <NUM>) if the (first, second,. ) secondary portion (<NUM>, <NUM>) has not been provided with a (first, second,. ) overlay <NUM>, <NUM>. However, if the (first, second,. ) secondary portion (<NUM>, <NUM>) comprises the (first, second,. ) overlay (<NUM>, <NUM>), the term "body" refers to (first, second,. ) secondary portion (<NUM>, <NUM>) without the (first, second,. ) overlay (<NUM>, <NUM>). In general, the overlay, if used, is made from different material than the body.

Referring to <FIG>, it has also been found that when the particulate material can return to the first side S1 through another gap, a vortex is not generated at all, or at least the tendency of forming such a vortex is significantly reduced.

Thus, a more preferable embodiment of the wall comprises a third heat exchanger pipe <NUM>, which limits a second gap <NUM>, through which the particulate material can return, as detailed below. A path for the bed material through two gaps is shown by the arrow AR2 in <FIG>.

In this embodiment, the third heat exchanger pipe <NUM> is arranged in parallel with the second heat exchanger pipe <NUM> and comprises a third primary portion <NUM> and a third secondary portion <NUM>. Reference is made to <FIG>. A second fin <NUM> (or second connector, e.g. second auxiliary heat transfer surface) is fixed to the second primary portion <NUM> and to the third primary portion <NUM> for similar reasons as the first fin <NUM> (or first connector) is fixed to the heat exchanger pipes <NUM>, <NUM>. Moreover, at least a part of the first secondary portion <NUM>, at least a part of the second secondary portion <NUM>, and at least a part of the third secondary portion <NUM> extend straight, in parallel, and within the imaginable plane P. In an embodiment, at least the part of the first secondary portion <NUM>, at least the part of the second secondary portion <NUM> and at least the part of the third secondary portion <NUM> extend straight and in parallel so that the central axes of these parts belong to the plane P.

In this embodiment, the wall <NUM> comprises a second gap <NUM>. The second gap <NUM> is arranged between the second secondary portion <NUM> and the third secondary portion <NUM> (see <FIG>). Moreover, the wall <NUM> is configured such that at least a part of the second gap <NUM> penetrates through the plane P. Moreover, the particulate material can propagate (i.e. is able to propagate) through the first gap <NUM> from the first side S1 of the plane P to the opposite second side S2 of the plane P (as detailed above), and the particulate material can return (i.e. is able to return) through the second gap <NUM> from the second side S2 of the plane P to the first side S1 of the plane P. The particulate material may, e.g., go round a side of the second secondary portion <NUM> on the second side S2 of the plane P the first gap <NUM> to the second gap <NUM>. Such propagation of the particulate material is shown by the arrow AR2 in <FIG>. Naturally this feature is not a feature concerning the particulate material as such, which is arranged in the furnace <NUM> in use. In other words, in this embodiment, the wall <NUM> is free from such parts that would prevent the particulate material from.

Preferably, the wall is also free from such parts that would prevent the particulate material from going round a side of the second secondary portion <NUM> on the second side S2 of the plane P from the first gap <NUM> to the second gap <NUM>. For example, if the wall <NUM> is used as a division wall (see <FIG>), bed material is able to propagate from the first side S1 to the second side S2 and from the second side to the first side S2 through the gaps <NUM>, <NUM>. However, in use, the bed material does not immediately return to the first side, nor does it typically go round the pipe along a shortest possible route. Instead, the bed material may circulate some time on the second side S2 before returning back to the first side S1.

In this embodiment, the tendency of forming the vortexes is reduced as the particulate material may go through the first gap <NUM> and return to the first side S1 through the second gap <NUM>. Thus the particulate needs not return through the same first gap <NUM>, which would form a vortex of some kind. It is also noted that the bed material needs not go round the side of the second secondary portion <NUM> on the second side S2 of the plane P near the second secondary portion <NUM> on the second side S2. Instead, in particular when the wall <NUM> is used as a division wall, the bed material may circulate some time on the second side S2 before returning back to the first side S1.

In this embodiment, a part of the third secondary portion <NUM> extends from the refractory <NUM> to the first direction Dir1, and the second gap <NUM> extends from the refractory <NUM> to the first direction Dir1. Moreover, for reasons detailed above, preferably, an outer diameter d32 of the third secondary portion <NUM> is less than or equal to an outer diameter d31 of the third primary portion <NUM>.

The wall <NUM> of <FIG> is shown without the refractory <NUM> in <FIG> also show some measures of the heat exchanger pipes <NUM>, <NUM>, <NUM>. Referring to <FIG> and <FIG>, in an embodiment, the first secondary portion <NUM> has a first profile shape extending in a longitudinal direction of the first secondary portion <NUM> such that the first profile shape has a constant outer diameter d12 throughout the length of the first secondary portion <NUM>, the length of the first secondary portion <NUM> being more than zero. In the embodiment, this applies in particular to the body of the first secondary portion <NUM>. This has the benefit that the first secondary portion <NUM> of the first heat exchanger pipe <NUM> can be manufactured by using a pipe having a smaller outer diameter than the first primary portion <NUM> of the first heat exchanger pipe <NUM>. The first secondary portion <NUM> may be e.g. welded to the first primary portion <NUM>. Another suitable manufacturing technique for forming the first secondary portion <NUM> is to turn a pipe, which has reasonable thick wall, with a lathe to reduce the outer diameter of the first secondary portion <NUM>. However, joining two pipes with different outer diameters is oftentimes economically more feasible.

When the heat exchanger pipe has been made by joining different types tubes to form the portions <NUM>, <NUM>, preferably, an inner diameter di<NUM> of the first secondary portion <NUM> is less than an inner diameter di<NUM> of the first primary portion <NUM> and an inner diameter di<NUM> of the second secondary portion <NUM> is less than an inner diameter di<NUM> of the second primary portion <NUM>. This has the effects that, on one hand, the primary portions <NUM>, <NUM> form a smaller flow resistance, because their inner diameter is large, and on the other hand all the portions <NUM>, <NUM>, <NUM>, <NUM> of the pipes may have a sufficient, but not excessive, pressure and temperature resistance.

For similar reasons, in the embodiment, the second secondary portion <NUM> has a second profile shape extending in a longitudinal direction of the second secondary portion <NUM> such that the second profile shape has a constant outer diameter d22 throughout the length of the second secondary portion <NUM>, the length of the second secondary portion being more than zero. In the embodiment, this applies in particular to the body of the second secondary portion <NUM>.

In an embodiment, the secondary portions <NUM>, <NUM> are provided with overlays <NUM>, <NUM>. In an embodiment, the body of first secondary portion <NUM>, i.e. the first secondary portion <NUM> without the overlay <NUM>, has a first profile shape extending in a longitudinal direction of the first secondary portion <NUM> such that the first profile shape has a constant outer diameter d12 throughout the length of the first secondary portion <NUM>. In an embodiment, the body of second secondary portion <NUM>, i.e. the second secondary portion <NUM> without the overlay <NUM>, has a second profile shape extending in a longitudinal direction of the second secondary portion <NUM> such that the second profile shape has a constant outer diameter d22 throughout the length of the second secondary portion <NUM>.

In such a case, preferably, only a part of the first secondary portion <NUM> is covered by the refractory <NUM> and only part of the second secondary portion <NUM> is covered by the refractory <NUM>. In such a case, a part of the first secondary portion <NUM> (which may have the constant outer diameter) limits the first gap <NUM>, and a part of the second secondary portion <NUM> (which may have the constant outer diameter) limits the first gap <NUM>. In such a case, the first gap <NUM> is reasonable large for allowing the particulate material to enter the second side S2 from the first side S1, which ensures the spreading of the vortex and/or the propagation of the bed material from first side to second side and the returning thereof as discussed above.

In an embodiment, the first primary portion <NUM> has a third profile shape extending in a longitudinal direction of the first primary portion <NUM> such that the third profile shape has a constant outer diameter d11 throughout the length of the first primary portion <NUM>, the length of the first primary portion <NUM> being more than zero. In a similar manner, in an embodiment, the second primary portion <NUM> has a fourth profile shape extending in a longitudinal direction of the second primary portion <NUM> such that the fourth profile shape has a constant outer diameter d21 throughout the length of the second primary portion <NUM>, the length of the second primary portion being more than zero.

Moreover, the first secondary portion <NUM> needs not be directly connected to the first primary portion <NUM>. Thus, in an embodiment, the first heat exchanger pipe <NUM> comprises a first primary connecting portion <NUM> connecting the first primary portion <NUM> to the first secondary portion <NUM>; and the second heat exchanger pipe <NUM> comprises a second primary connecting portion <NUM> connecting the second primary portion <NUM> to the second secondary portion <NUM>. Reference is made to <FIG>, <FIG>.

The erosion resistance or maintainability of the (first, second, third) secondary portions <NUM>, <NUM>, <NUM> can further be improved by using an overlay (<NUM>, <NUM>, <NUM>). Such overlays <NUM>, <NUM>, <NUM> are shown in <FIG> and <FIG>. The overlay preferably comprises erosion-resistant metal or ceramic. However, it may comprise sacrificial metal, which need not be erosion-resistant. The overlay <NUM>, <NUM>, <NUM> may be in the form of a tube, into which the (body of the) secondary portion <NUM>, <NUM>, <NUM> is pushed while manufacturing the heat exchanger pipe <NUM>, <NUM>, <NUM>. In the alternative, the overlay may be sprayed onto the body of the secondary portion <NUM>, <NUM>, <NUM>. As a further the alternative, in case the overlay comprises metal, the overlay may be welded onto the a body of the secondary portion <NUM>, <NUM>, <NUM>. Preferably, the overlay <NUM>, <NUM>, <NUM> is an overlay welding comprising suitable alloy. The overlay <NUM>, <NUM>, <NUM> may form a part of the secondary portion <NUM>, <NUM>, <NUM>. It has been noticed that a weld overlay cladding by materials having at least <NUM>% Cr and a low Fe content on the surfaces exposed to furnace gases significantly reduces the wall erosion. Examples of suitable alloys include Alloy <NUM> (Ni-22Cr-9Mo-<NUM>. However, the overlay need not be extremely resistant to abrasion, because a technical function of the overlay may be to serve as a sacrificial layer. Thus, the wall <NUM> may be maintained by adding more sacrificial material after a period of use, if needed, without the need of replacing the tube itself.

The overlay <NUM>, <NUM> may be machined, e.g. turned after having been welded onto the secondary portion <NUM>, <NUM>, <NUM>. As an example, a tube intended for use as the secondary portion can be first covered (e.g. welded) with the overlay. Thereafter, particular if the overlay has a variable thickness, the tube can be turned to level out the variance of the thickness of the overlay. This ensures that the secondary portion do comprise protrusions that would cause further generation of vortexes. Moreover, this may help to reduces a difference between the outer diameter of the primary parts <NUM>, <NUM> and the secondary parts <NUM>, <NUM>.

The overlay <NUM>, <NUM>, <NUM>, if used, is preferably provided at such a location wherein the particulate material has a tendency of propagating in the direction of normal of the wall <NUM>. Moreover, it is the refractory <NUM> that causes the tendency of the particulate material to propagate in the direction of normal of the wall <NUM>. Thus, the overlay <NUM>, <NUM> is provided such that the overlay <NUM>, <NUM> limits the first gap <NUM> and extends from the refractory <NUM>. That is, a non-overlaid portion of the heat exchanger pipe is not arranged between the refractory an overlaid part of the heat exchanger pipe in the longitudinal direction of the heat exchanger pipe.

Thus, in an embodiment, the wall <NUM> comprises a first overlay <NUM> arranged on at least a part of the first secondary portion <NUM> and a second overlay <NUM> arranged on at least a part of the second secondary portion <NUM>. Reference is made to <FIG>. Thus, the first overlay <NUM> forms a part of a surface of the first secondary portion <NUM>; and the second overlay <NUM> forms a part of a surface of the second secondary portion <NUM>.

At least a part of the first overlay <NUM> extends from the refractory <NUM> to the first direction Dir1 on the first secondary portion <NUM> and laterally fully encircles at least a part of the first secondary portion <NUM>. Thus, at least a part of the first overlay <NUM> limits the first gap <NUM>. As shown in <FIG>, the first overlay <NUM> needs not cover the whole first secondary portion <NUM> in the longitudinal direction (Sz in <FIG>). However, as indicated above, laterally the first overlay <NUM> fully encircles at least a part the first secondary portion <NUM>.

In a similar manner, at least a part of the second overlay <NUM> extends from the refractory <NUM> to the first direction Dir1 on the second secondary portion <NUM> and laterally fully encircles at least a part of the second secondary portion <NUM>.

In this way, at least a part of the first overlay <NUM> limits the first gap <NUM> and at least a part of the second overlay <NUM> limits the first gap <NUM>. As shown in <FIG>, the first gap may also by limited by non-overlaid parts of the first secondary portion <NUM> and the second secondary portion <NUM>.

In line with what has been said above, the first overlay <NUM> comprises metal or ceramic, and the second overlay <NUM> comprises metal or ceramic. The materials, such as alloys, disclosed above are usable.

Referring to <FIG>, preferably, the first overlay <NUM> extends in the first direction Dir1 to the first primary connecting portion <NUM> (if present) or to the first primary portion <NUM>. In a similar way, preferably, the second overlay <NUM> extends in the first direction Dir1 to the second primary connecting portion <NUM> (if present) or to the second primary portion <NUM>. In this case the overlay <NUM>, <NUM> protects the rest of the secondary portions <NUM>, <NUM>.

However, near the primary portions <NUM>, <NUM>, the bed material tends to fall downwards along the primary portions <NUM>, <NUM>. Thus, the bed material has a tendency of continuing propagating in the same direction (i.e. reverse to the first direction Dir1) also below the primary portions <NUM>, <NUM>. This tendency forms a curtain, along which the bed material normally falls, at least for some distance, but not necessary until the surface of the refractory <NUM>. The areas of the secondary portions <NUM>, <NUM> that remain between this curtain and the imaginable plane P are not exposed to high erosion.

As detailed above, the secondary portions <NUM>, <NUM> do not protrude from the curtain defined by the outer surfaces of the primary portions <NUM>, <NUM>. In particular, in an embodiment, the secondary portions <NUM>, <NUM> are co-axial with the primary portions <NUM>, <NUM> and have a smaller outer diameter, whereby upper parts of the secondary portions <NUM>, <NUM> are arranged in a shadow of the primary parts <NUM>, <NUM> (i.e. between the imaginable plane P and the curtain). Thus, upper parts of the secondary parts <NUM>, <NUM> need not comprise an overlay (see <FIG>). However, because the refractory <NUM> guides the bed material towards the plane P, preferably, the overlays <NUM>, <NUM> are used; and they extend for a certain distance from the refractory <NUM> in the first direction Dir1.

Preferably, the wall <NUM> comprises the third heat exchanger pipe <NUM> as discussed above. Moreover, in such a case, at least a part of the second overlay <NUM> limits also the second gap <NUM>. In an embodiment, the wall <NUM> comprises a third overlay <NUM> arranged on at least a part of the third secondary portion <NUM>. At least a part of the third overlay <NUM> limits the second gap <NUM> in a similar manner as detailed above for the other overlays <NUM>, <NUM>.

To ensure that even if the refractory <NUM> wears, a non-overlaid part is not exposed to erosion, preferably, the overlays <NUM>, <NUM>, <NUM> extend also into the refractory, i.e. the overlays extend from the interface of the refractory to the reverse first direction -Dir1.

In such a case and in a preferable embodiment, a part of the first overlay <NUM> is arranged laterally between the first secondary portion <NUM> and the refractory <NUM>. More specifically, and considering that the first overlay <NUM> forms a part of the first secondary portion <NUM>, a part of the first overlay <NUM> is arranged laterally between an inner surface of the first secondary portion <NUM> and the refractory <NUM>. In a similar way, a part of the second overlay <NUM> is arranged laterally between the second secondary portion <NUM> and the refractory <NUM>. More specifically, and considering that the second overlay <NUM> forms a part of the second secondary portion <NUM>, a part of the second overlay <NUM> is arranged laterally between an inner surface of the second secondary portion <NUM> and the refractory <NUM>. Reference is made to <FIG>.

Preferably, the first overlay <NUM> comprises metal alloy and is an overlay welding, and the second overlay <NUM> comprises metal alloy and is an overlay welding. Preferably, a thickness of the first overlay <NUM> is <NUM> to <NUM>, more preferably <NUM> to <NUM>. Preferably, a thickness of the second overlay <NUM> is <NUM> to <NUM>, more preferably <NUM> to <NUM>.

Particularly preferably the wall <NUM> is used as a division wall of furnace of a fluidized bed boiler, such as a circulating fluidized bed boiler (<FIG>) and the secondary portions <NUM>, <NUM>, <NUM> are provided with the overlays <NUM>, <NUM>, <NUM>. In addition, when the wall <NUM> is used as a division wall, preferably, the refractory <NUM> is arranged between two planes Pa and Pb that are parallel to the plane P defined above, as show in <FIG>. Preferably, in this embodiment, the refractory <NUM> is arranged between a first plane Pa and a second plane Pb that are parallel to the imaginable plane P, in which at least a part of the first secondary portion <NUM> and at least a part of the second secondary portion <NUM> extend straight; and the wall <NUM> is configured such that particulate material (i.e. bed material) can propagate (i.e. the particulate material is able to propagate) through the first gap <NUM> from a first side S1 of the first plane Pa to an opposite second side S2 of the second plane Pb. Such propagation of bed material is shown in <FIG> by the arrow AR3.

This has the benefit that bed material, once having propagated from the first side S1 of the first plane Pa through the first gap <NUM> to the opposite second side S2 of the second plane Pb, is able to fall downwards on the second side S2, which further reduces the tendency of forming vortexes near the refractory <NUM>.

As discussed, the outer diameter of the secondary portions <NUM>, <NUM>, <NUM> may be substantially constant, and the outer diameter may include an overlay <NUM>, <NUM>, <NUM>, as detailed above. To clarify the definitions, reference is made to <FIG>. As shown therein in the longitudinal direction (Sz in <FIG>), an outer diameter of the secondary portions <NUM>, <NUM>, <NUM> is constant only as long as the secondary portions <NUM>, <NUM>, <NUM> are provided with the overlay <NUM>, <NUM>, <NUM>, respectively. However, when the overlay ends, the outer diameter reduces by twice the thickness of the overlay <NUM>, <NUM>, <NUM>. In line with these definitions, in the embodiment of <FIG>, the first primary connecting portion <NUM>, which connects the first primary portion <NUM> to the first secondary portion <NUM> comprises a taper and a part having a constant outer diameter, the constant outer diameter being equal to a non-overlaid part of the first secondary portion <NUM>. This applies, mutatis mutandis, to the second primary connecting portion <NUM> and the second secondary portion <NUM> shown in <FIG>.

In an embodiment (to be discussed in detail), the wall <NUM> is substantially planar and extends downwards to a bearing structure. In such a case, the refractory <NUM> may extend downwards to a grate or to the bearing structure. Referring to <FIG>, the heat exchanger pipes may comprise tertiary portions (<NUM>, <NUM>, <NUM>) which are wider than the secondary portions (<NUM>, <NUM>, <NUM>). However, this is not mandatory. Instead, the secondary portions (<NUM>, <NUM>, <NUM>) may extend within the refractory throughout the needed length and only having the outer diameter d12, d22, d32. Referring to <FIG>, it may be that only a part of each of the heat exchanger pipes <NUM>, <NUM>, <NUM> extending within the refractory <NUM> is covered with the overlay <NUM>, <NUM>, <NUM>. Thus, the outer diameter of the heat exchanger pipes extending within the refractory <NUM> may even decrease downwards. However, with reference to <FIG>, when the heat exchanger pipes <NUM>, <NUM>, <NUM> are provided with the overlays <NUM>, <NUM>, <NUM>, preferably, the heat exchanger pipes <NUM>, <NUM>, <NUM> comprise tertiary portions <NUM>, <NUM>, <NUM> which are wider than the secondary portions (<NUM>, <NUM>, <NUM>). In such a case, a first secondary connecting portion <NUM> may connect the first secondary portion <NUM> to the first tertiary portion <NUM>. What has been said about the outer diameter of the first secondary portion <NUM> and the extension of the first primary connecting portion <NUM> applies, mutatis mutandis, to the first secondary connecting portion <NUM>.

Preferably and with reference to <FIG>, <FIG>, and <FIG>, the first heat exchanger pipe <NUM> comprises a first tertiary portion <NUM> and the second heat exchanger pipe <NUM> comprises a second tertiary portion <NUM>. The first secondary portion <NUM> is arranged (in the longitudinal direction of the first heat exchanger pipe <NUM>) between the first primary portion <NUM> and the first tertiary portion <NUM>. The second secondary portion <NUM> is arranged (in the longitudinal direction of the second heat exchanger pipe <NUM>) between the second primary portion <NUM> and the second tertiary portion <NUM>. To reduce flow resistance generated by the first and second heat exchanger pipes <NUM>, <NUM>, an inner diameter di<NUM> of the first tertiary portion <NUM> equals an inner diameter di<NUM> of the first primary portion <NUM> and an inner diameter di<NUM> of the second tertiary portion <NUM> equals an inner diameter di<NUM> of the second primary portion <NUM>. Preferably also the inner diameter di<NUM> of the first tertiary portion <NUM> is greater than an inner diameter di<NUM> of the first secondary portion <NUM> and the inner diameter di<NUM> of the second tertiary portion <NUM> is greater than an inner diameter di<NUM> of the second secondary portion <NUM>.

For manufacturing reasons it is preferable that the tertiary portions <NUM>, <NUM> are similar to the primary portions <NUM>, <NUM>. Thus, preferably also an outer diameter d13 of the first tertiary portion <NUM> equals an outer diameter d11 of the first primary portion <NUM>, and an outer diameter d23 of the second tertiary portion <NUM> equals an outer diameter d21 of the second primary portion <NUM>.

To protect the tertiary portions <NUM>, <NUM> from erosion, in an embodiment, at least a part of the first tertiary portion <NUM> is covered by the refractory <NUM> and at least a part of the second tertiary portion <NUM> is covered by the refractory <NUM>.

Any embodiment of the wall <NUM> detailed above is particularly usable as a wall of a fluidized bed boiler <NUM>. A fluidized bed boiler <NUM> is shown in <FIG> and <FIG> as well as in <FIG>. The fluidized bed boiler <NUM> shown in the figures is of the circulating type. Any embodiment of the wall <NUM> detailed above is usable as a wall of a bubbling fluidized bed boiler <NUM>.

Thus, the fluidized bed boiler <NUM> comprises a furnace <NUM>, air nozzles <NUM> for letting combustion air into the furnace <NUM>, and a fuel inlet <NUM> for letting fuel into the furnace <NUM>. Reference is made to <FIG> and <FIG> as well as in <FIG>. If the fluidized bed boiler is a circulating fluidized boiler, it comprises a particle separator <NUM> for separating particulate material from a stream received from the furnace <NUM> for returning at least some of the separated particulate material back to the furnace <NUM>. In a circulating fluidized bed boiler, the air nozzles <NUM> are operated so that such an amount of air is fed to the furnace <NUM> that a mixture of bed material, fuels, ash, air, and flue gas mainly travels upwards in the furnace <NUM> and further propagates to the particle separator <NUM>. The particle separator <NUM> preferably comprises a cyclone for separating at least most of the solid materials from the mixture and for returning them back to the furnace <NUM>. The flue gas (i.e. the remaining part of the mixture) is expelled to a flue gas channel <NUM>. The fluidized bed boiler <NUM> is also provided with a heat exchanger <NUM> for recovering heat from the furnace <NUM> and/or from the flue gases. As an example, <FIG> shows a heat exchanger <NUM> arranged in the flue gas channel <NUM>.

The fluidized bed boiler <NUM> further comprises a least one wall <NUM> according any embodiment of the wall <NUM> as detailed above. As detailed in background, even if most of the material flows upwards in the furnace <NUM>, in the vicinity of the wall <NUM> of the furnace, the material flow may be substantially downwards.

In an embodiment, the wall <NUM> is a division wall dividing the furnace <NUM> to two parts that function in a substantially similar way, as detailed in <FIG> and <FIG>. In an embodiment, the wall <NUM> is a side wall of the furnace <NUM>, as detailed in <FIG>. In an embodiment, the fluidized bed boiler <NUM> comprises a first wall according any embodiment of the wall <NUM> as detailed above and a second wall according any embodiment of the wall <NUM> as detailed above (not shown). In an embodiment the first wall is a division wall of the furnace and the second wall is a side wall of the furnace. In an embodiment the first wall is a division wall of the furnace and the second wall is another division wall of the furnace. In an embodiment the first wall is a side wall of the furnace and the second wall is a side wall of the furnace.

In what follows, the wall <NUM> refers to the (sole) wall of the type disclosed above, the first wall, the second wall, or both the first and the second wall.

In the circulating fluidized bed boiler <NUM>, the wall <NUM> limits the furnace <NUM> such that at least the first side S1 of the plane P is exposed to a part of the furnace <NUM>. In other words, a part of the furnace <NUM> is arranged on the first side S1 of the plane. Moreover, in this embodiment at least a part of the refractory <NUM> is arranged on the first side S1 of the plane P. When arranged in such a way, the gaps <NUM>, <NUM> as detailed above function in the way discussed above. Moreover, the wall is arranged such that the first direction Dir1 forms an angle of at most <NUM> degrees with an upward vertical direction Sz. In this way, the first direction Dir1 is, in this embodiment, more or less upwards vertical, as indicated above. This angle also ensures that the gaps <NUM>, <NUM> function as intended; and that the secondary portions <NUM>, <NUM> of the heat exchanger pipes <NUM>, <NUM>, which are smaller in their outer diameter than the primary portions <NUM>, <NUM>, have the technical effect as detailed above.

Preferably, the wall <NUM> is a division wall of the furnace <NUM>. In such a case, on both sides of the wall <NUM> the bed material has a tendency of flowing downwards along the wall until it hits the refractory <NUM> (or bed material accumulated thereon). And, at that, point, the bed material is guided in a transverse direction, one of which transverse directions is towards the wall <NUM>. Thus, in this case, the gaps <NUM>, <NUM> provide for bed material and gas passage from either side of the wall <NUM> to the opposite side of the wall <NUM>. Thus, the wall <NUM> has been found particularly effective as a division wall of a fluidized bed boiler <NUM>. Therefore, in an embodiment, a first part P1 of the furnace <NUM> is arranged on a first side of the wall <NUM> and a second part P2 of the furnace <NUM> is arranged on a second, opposite side of the wall <NUM>. In other words, in an embodiment, the first part P1 of the furnace <NUM> is arranged on a first side of the imaginable plane P defined by the wall and a second part P2 of the furnace <NUM> is arranged on a second, opposite side of the imaginable plane P. Thus the second side S2 of the plane P is exposed to the second part P2 of the furnace <NUM>. Reference is made to <FIG>. Also in <FIG> the plane P divides the furnace into two parts. Moreover, for dividing the furnace <NUM> to these two parts (P1, P2) such that they function in a substantially similar manner, the first part P1 of the furnace <NUM> is provided with a first set 330A of air nozzles <NUM> for fluidizing bed material in the first part P1 of the furnace <NUM>, and the second part P2 of the furnace <NUM> is provided with a second set 330B of air nozzles <NUM> for fluidizing bed material in the second part P2 of the furnace <NUM> (see <FIG>). In this embodiment, preferably, the first and second primary portions (<NUM>, <NUM>) and first and second secondary portions (<NUM>, <NUM>) of the heat exchanger pipes (<NUM>, <NUM>) of the wall <NUM> extend in the plane P. Thus there is no bend point between the primary portions (<NUM>, <NUM>) and the secondary portions (<NUM>, <NUM>), which also ensures that both sides of the wall <NUM> function in a substantially same manner.

One problem, particularly in large fluidized bed boilers is bearing of load. The load is caused on one hand by the parts of the boiler as such, and on the other hand by the particulate material arranged within the furnace <NUM>. In particular, typically the air nozzles <NUM> are arranged to a grate <NUM> and the grate bears the load of the particulate material. Thus the grate <NUM> itself needs to be properly supported. While the side walls of the furnace can bear some load, preferably at least the division wall, which is a wall <NUM> according to an embodiment disclosed above, bears load. This has the benefit that a division wall can be substantially planar, whereby the heat exchanger pipes of the wall <NUM> may be straight and substantially vertical. Substantially vertical straight pipes carry load to a much grated extent than e.g. bent pipes or pipes that are not vertical. It is also noted that typically the side walls of a furnace are bent in such a way that their load-bearing capacity is reduced. Reference is made to <FIG> for a bent pipe. Referring to <FIG>, <FIG>, the wall <NUM>, when used as a supporting division wall of the furnace <NUM>, can be suspended from a suspension structure <NUM> and the wall <NUM> can support a bearing structure <NUM>, which supports the grate <NUM>.

For these reasons, with reference to <FIG>, <FIG>, a preferable embodiment of the circulating fluidized bed boiler comprises a suspension structure <NUM> comprising an upper beam, a bearing structure <NUM> comprising a lower beam, and a grate <NUM> arranged above the bearing structure <NUM>. Moreover in this embodiment the wall <NUM> comprises straight heat exchanger pipes and the wall <NUM> is configured to support the bearing structure <NUM>. The grate <NUM> comprises air nozzles <NUM>. When the fluidized bed boiler <NUM> comprises sets of air nozzles, the grate <NUM> comprises a first set 330A of air nozzles and a second set 330B of air nozzles.

More specifically, in this embodiment, the first and second heat exchanger pipes <NUM>, <NUM> of the wall <NUM> extend straight between the suspension structure <NUM> and the bearing structure <NUM>; and the first direction Dir1 forms an angle of at most <NUM> degrees with an upward vertical direction Sz. As detailed above, these features improve the load-bearing capability of the wall <NUM>.

Moreover, to utilize the load bearing capability of the wall, the wall <NUM> is fixed to the suspension structure <NUM> (e.g. the wall <NUM> is suspended from the suspension structure <NUM>), the wall <NUM> supports the bearing structure <NUM> (e.g. the wall <NUM> is fixed to the bearing structure <NUM>), and the bearing structure <NUM> supports the grate <NUM>.

In an embodiment, a lower part of the furnace <NUM> has a downwards tapering shape, while an upper part of the furnace may have a substantially constant cross-section. The upper part may be defined e.g. such that the upper part is arranged above a division plane DP defined either by [A] a bend line BL of a side wall of the furnace <NUM> and a vertical normal or [B] bend lines BL of opposite side walls of the furnace <NUM>. Reference is made to <FIG>, <FIG>, and <FIG>.

Thus, in an embodiment, two opposite sides walls of the furnace <NUM> are each provided with a bend lines BL, as shown in <FIG> and <FIG>. In <FIG> and <FIG>, the bend lines BL extend in a direction Sx, which is perpendicular to the plane of the figure. Such bend lines BL of the side walls define a division plane DP. Thus, the bend lines BL are arranged on the division plane DP. The division plane DP is imaginary in the sense that it is not a material object, but a plane as defined in the field of geometry. Above the division plane DP, a primary part <NUM> of the furnace <NUM> is arranged. Below the division plane DP, a secondary part <NUM> of the furnace <NUM> is arranged. In an embodiment, only one side wall of the furnace <NUM> is provided with a bend line BL, which defines a division plane DP of which normal is vertical. If two bend lines BL define the division plane DP, a normal of the division plane DP need not be vertical.

Because of the at least one bend line BL, the secondary part <NUM> of the furnace <NUM> has a shape that tapers downwards. The primary part <NUM> of the furnace <NUM> may have a shape that has a constant cross-section in a vertical direction, the cross section having a normal in the vertical direction. The secondary part <NUM> extends downwards to the grate <NUM>. The secondary part <NUM> extends upwards to the primary part <NUM>. In <FIG> and <FIG> the division plane DP forms an interface between the primary part <NUM> and the secondary part <NUM>. The primary part <NUM> extends upwards from the secondary part <NUM>.

Because of the downwards tapering shape of the secondary part <NUM> of the furnace <NUM>, in the secondary part <NUM>, the bed material flow is reasonably turbulent. Moreover, a turbulent flow has a tendency of eroding the surfaces. To prevent erosion of the heat exchanger pipes <NUM>, <NUM>, <NUM> within the secondary part <NUM> of the furnace <NUM>, in an embodiment, within the secondary part <NUM> of the furnace, the heat exchanger pipes <NUM>, <NUM>, <NUM> of the wall <NUM> are covered by the refractory <NUM>. More preferably, the refractory <NUM> is provided on the heat exchanger pipes <NUM>, <NUM>, <NUM> of the wall <NUM> throughout the secondary part <NUM> of the furnace <NUM>. As a result, the first and second gaps <NUM>, <NUM> of the wall <NUM> are arranged, in an embodiment, in the primary part <NUM> of the furnace <NUM>.

In particular, in an embodiment, the first gap <NUM> is arranged in the primary part <NUM> of the furnace. In an embodiment, the first gap <NUM> is arranged above the division plane DP defined by the bend line BL or the bend lines BL of the side wall(s) of the furnace. In an embodiment, a second imaginary plane IP, of which normal is vertical, intersects the first gap <NUM> and is arranged at a higher vertical level than (i) the bend line BL of the side wall of the furnace <NUM>, or (ii) one of the bend lines BL of the side walls of the furnace <NUM>, or (iii) all of the bend lines BL of the side walls of the furnace <NUM> that define the secondary part <NUM> of the furnace <NUM>. The second imaginary plane IP is imaginary in the sense that it is not a material object, but a plane as defined in the field of geometry. This applies also to the second gaps <NUM> mutatis mutandis.

Referring to <FIG>, any embodiment of the wall <NUM> may be used as a side wall of the furnace <NUM> of the fluidized bed boiler <NUM>. In such an embodiment, a first part P1 of the furnace <NUM> is arranged on a first side of the wall <NUM>, and a second part P2 of the furnace <NUM> is arranged on a second, opposite side of the wall <NUM>, whereby the second side S2 of the plane P is exposed to the second part P2 of the furnace <NUM>. Thus, a first part P1 of the furnace <NUM> is arranged on a first side of the wall <NUM> and a second part P2 of the furnace <NUM> is arranged on a second, opposite side of the wall <NUM>.

However, when the wall <NUM> is a side wall, the parts P1 and P2 of the furnace <NUM> function differently. In this case, most of the combustion takes place in the first part P1, while the second part P2 is mainly used for preventing the formation of the vortexes as discussed above. Thus, referring to <FIG>, in an embodiment, the first part P1 of the furnace <NUM> is provided with air nozzles <NUM> for fluidizing bed material in the first part P1 of the furnace <NUM>. However, the second part P2 of the furnace <NUM> is free from air nozzles.

Moreover, to prevent bed material from escaping the furnace <NUM>, the fluidized bed boiler <NUM> comprises a plate <NUM>. At least part of the plate <NUM> is arranged on the second side S2 of the plane P. The plate <NUM> is fixed to the first fin <NUM> and configured to prevent bed material from escaping from the furnace <NUM>.

<FIG> show another embodiment of the wall <NUM>. As shown in <FIG>, when viewed from above (see <FIG> for the cross-section VIIb), the wall <NUM> has a shape of a cross. Thus, the wall <NUM> comprises a first part 200a and a second part 200b. The first part 200a and the second part 200b are planar and arranged perpendicular to each other. The first part 200a comprises the heat exchanger pipes <NUM>, <NUM>, <NUM> and their portions <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, as discussed above. The secondary portions <NUM>, <NUM> of the pipes define the imaginable plane P, as discussed above and shown in <FIG>. Such a wall <NUM> may be used as a division wall, as detailed above. The effects of this wall <NUM> are two-fold. First, having perpendicular parts may improve supportive capacity of the wall <NUM>. As detailed above, the wall <NUM> may support e.g. the grate <NUM>. Second, having perpendicular parts may increase the area of heat transfer surfaces, and in this way improve the heat recovery. However, heat recovery can be improved also by using a lot of heat exchanger pipes, e.g. a wide wall <NUM>.

Referring to <FIG>, <FIG>, heat recovery can be improved also by using auxiliary heat transfer surfaces. <FIG> show an embodiment of the wall <NUM>. In this embodiment, the wall <NUM> comprises, in its upper part, auxiliary heat transfer surfaces <NUM>, <NUM>. For example, the first primary portion <NUM>, the second primary portion <NUM>, and the first fin <NUM>, which form a part of the wall <NUM>, may constitute a planar part. A first auxiliary heat transfer surface <NUM> may protrude from this planar part, as shown in <FIG>. In a similar manner, a second auxiliary heat transfer surface <NUM> may protrude from the plane defined by the first primary portion <NUM>, the second primary portion <NUM>, and the first fin <NUM>, as shown in <FIG>. Such auxiliary heat transfer surfaces <NUM>, <NUM> may be referred to as wings. As shown in <FIG>, the second auxiliary heat transfer surface <NUM> may comprise e.g. a heat transfer tube for recovering heat. This applies to other auxiliary heat transfer surfaces (e.g. <NUM>), too. A purpose of the auxiliary heat transfer surfaces <NUM>, <NUM> is to improve heat recovery.

Referring to <FIG>, the walls with auxiliary heat transfer surfaces <NUM>, <NUM> may be used e.g. near side walls of the furnace <NUM> to improve heat recovery.

As shown in <FIG>, when the wall comprises auxiliary heat transfer surfaces <NUM>, <NUM>, such auxiliary heat transfer surfaces <NUM>, <NUM> may penetrate the fins <NUM>, <NUM>. However, as indicated in <FIG>, the auxiliary heat transfer surfaces <NUM>, <NUM> may form the connectors that connect the primary parts <NUM>, <NUM> of the pipes <NUM>, <NUM>. Thus, both a fin (<NUM>, <NUM>) and an auxiliary heat transfer surface (<NUM>, <NUM>) can be considered as a connector connecting the adjacent primary parts of the heat exchanger pipes.

Claim 1:
A wall (<NUM>) for a fluidized bed boiler (<NUM>), the wall (<NUM>) comprising
- a first heat exchanger pipe (<NUM>) comprising a first primary portion (<NUM>) and a first secondary portion (<NUM>),
- a second heat exchanger pipe (<NUM>) arranged in parallel with the first heat exchanger pipe (<NUM>), the second heat exchanger pipe (<NUM>) comprising a second primary portion (<NUM>) and a second secondary portion (<NUM>),
- a first connector (<NUM>, <NUM>) fixed to the first primary portion (<NUM>) and to the second primary portion (<NUM>), and
- a first gap (<NUM>) between the first secondary portion (<NUM>) and the second secondary portion (<NUM>), wherein
- at least a part of the first secondary portion (<NUM>) and at least a part of the second secondary portion (<NUM>) extend straight, in parallel, and within an imaginable plane (P) and
- at least a part of the first gap (<NUM>) penetrates through the plane (P) such that particulate material is able to propagate through the first gap (<NUM>) from a first side (S1) of the plane (P) to the opposite second side (S2) of the plane (P), the wall (<NUM>) comprising
- a refractory (<NUM>) arranged on at least one side of a part of the first secondary portion (<NUM>) and a part of the second secondary portion (<NUM>) such that
- a part of the first secondary portion (<NUM>) extends from the refractory (<NUM>) to a first direction (Dir1),
- a part of the second secondary portion (<NUM>) extends from the refractory (<NUM>) to the first direction (Dir1), and
- the first gap (<NUM>) extends from the refractory (<NUM>) to the first direction (Dir1); wherein
- an outer diameter (d12) of the first secondary portion (<NUM>) is less than or equal to an outer diameter (d11) of the first primary portion (<NUM>),
- an outer diameter (d22) of the second secondary portion (<NUM>) is less than or equal to an outer diameter (d21) of the secondary primary portion (<NUM>), and
- the first direction (Dir1) forms an angle of at most <NUM> degrees with an upward vertical direction (Sz).