BACKGROUND OF THE INVENTION

[0001 ] Field of the Invention

[0002] The present invention relates to a boiler system comprising a furnace and a support construction in accordance with the preamble of claim 1 . More particularly, the invention relates to a boiler system, comprising a support construction and a furnace supported to the support construction at a middle section of the furnace, the furnace being enclosed by water tube walls comprising two side walls and two end walls, a roof and a bottom, the side walls having a total height H from the bottom to the roof, wherein each of the two side walls comprises a vertical upper portion that extends from the roof to a level of 30-70 % of the height H, a lower portion that extends from the bottom to a level of 30- 70 % of the height H and has a vertical upper portion, and an in downward direction outwards bent intermediate portion at a level between the upper portion of the side wall and the vertical upper portion of the lower portion of the side wall.

[0003] Description of related art

[0004] Relatively large boilers are conventionally arranged top-supported, i.e. they are supported so that the furnace of the boiler is arranged to hang from a support construction, usually a rigid support steel structure, extending around and above the furnace. Relatively small boilers are conventionally arranged bottom-supported, wherein vertical load of the furnace is balanced by a support construction arranged below the boiler. The main difference between top-sup- ported and bottom-supported constructions is that when the temperature of the furnace increases, thermal expansion of a top-supported boiler takes place mainly downwards whereas in a bottom-supported boiler thermal expansion takes place mainly upwards. Bottom-supported boilers are in case of relatively small boilers generally simpler and economically more advantageous than top- supported boilers, because they do not require a separate support construction extending around and above the furnace. A disadvantage of bottom-supported construction is that the walls of the furnace have to be strong enough to carry the vertical compression load of the furnace. [0005] A third alternative is to support the furnace at its middle-section to a rigid support construction. Thereby, the lower portion of the furnace, below the middle section, is top-supported, and the upper portion of the furnace, above the middle section, is bottom supported. Middle-supported construction is advantageous for some applications while it reduces the size of the support steel structure from that needed around the furnace of a top-supported boiler. Simultaneously such a middle-supported construction eliminates the need for very strong walls of the furnace as in large bottom-supported boilers. Different middle-supported boiler constructions are shown, for example, in U.S. Patent No. 2,583,599, U.S. Patent No. 2,856, 906, European patent publication EP

0073851 A1 and U.S. Patent Application publication No. 2015/0241054.

[0006] U.S. Patent No. 4,428,329 discloses a middle supported boiler construction with a support steel structure comprising multiple fixed cantilever arms at an intermediate height of the boiler. In order to absorb horizontal thermal ex- pansion, the tubewalls of the furnace are hanging from multiple levers flexibly connected to the cantilever arms by a number of vertical links attached to an in downward direction outwards bent section of the tubewall. [0007] A problem in designing middle-supported boilers is to find a simple and advantageous way to attach the middle section of the furnace to a rigid support construction around the furnace and simultaneously take into account the effects of horizontal thermal expansion.

[0008] An object of the present invention is to provide a boiler system having an advantageous support construction for a middle-supported furnace.

SUMMARY OF THE INVENTION

[0009] According to one aspect, the present invention provides a boiler system, comprising a support construction and a furnace supported to the support construction at a vertically middle section of the furnace, the furnace being en- closed by water tube walls comprising two side walls and two end walls, a roof and a bottom, the side walls having a total height H from the bottom to the roof, wherein each of the two side walls comprises a vertical upper portion that extends from the roof to a level of 30-70 % of the height H, a lower portion that extends from the bottom to a level of 30-70 % of the height H and has a vertical upper portion, and an in downward direction outwards bent intermediate portion at a level between the upper portion of the side wall and the vertical upper portion of the lower portion of the side wall, wherein the support construction comprises horizontal wall supporting beams that are arranged parallel to the side walls at a level below the roof of the furnace and directly above the vertical up- per portions of the lower portions of the two side walls, and the furnace is supported to the support construction by having the intermediate portions of the side walls connected to adjacent horizontal wall supporting beams so as to balance vertical loads of the furnace. [0010] To enable efficient and reliable supporting of the furnace, for example, the furnace of a fluidized bed boiler, at an intermediate height of the furnace, at a vertically middle section of the side walls of the furnace is arranged an in downward direction outwards bent intermediate portion. Due to the out- wards bent intermediate portion of the side walls, it is possible to arrange horizontal supporting beams, hereinafter called horizontal wall supporting beams, directly above the vertical upper portions of the lower portions of the side walls, and support the side walls vertically to the horizontal wall supporting beams. [001 1 ] The support construction comprises advantageously two horizontal wall supporting beams, one adjacent to each of the sidewalls. Thereby, the length of the horizontal wall supporting beams is generally at least as long as the width of the sidewalls. In some applications it may also be possible to use piecewise horizontal wall supporting beams, whereby, for example, adjacent to each of the sidewalls are arranged two horizontal wall supporting beams, one after the other. The supporting of the sidewalls is advantageously made by a plurality of short vertical hanger rods connected between the outwards bent intermediate portions of the sidewalls and the respective horizontal wall supporting beams.

[0012] A main feature of the present invention is that the furnace is middle- supported, i.e. that vertical loads, such as gravitational forces and seismic forces, affecting to the furnace are balanced to a rigid support construction at an intermediate height, between the bottom and roof, of the furnace. Because of the middle-supporting, the lower portion of the furnace, below the middle section, is top-supported, and the upper portion of the furnace, above the middle section, is bottom supported. Thus, when the temperature of the furnace increases from ambient temperature to the normal operating temperature, such as 850 DegC, the upper portion of the furnace expands, typically by more than ten centimeters, upwards and the lower portion of the furnace expands similarly downwards. However, thermal expansion of the furnace naturally also takes place in the horizontal direction. Therefore, the supporting of the middle-supported furnace has to be performed so as to be able to also absorb the horizon- tal thermal expansion.

[0013] A similar in downward direction outward bent intermediate portion at a level between the upper portion and lower portion of the side wall of a middle- supported furnace is shown also in U.S. Patent No. 4,428,329. However, in the solution shown in U.S. Patent No. 4,428,329, groups of the water tubes of the side walls are supported by vertical links via multiple horizontal thermal expansion absorbing levers to corresponding fixed cantilever arms. The U.S. Patent No. 4,428,329 does not teach supporting the tube walls to horizontal wall supporting beams arranged parallel to the side walls directly above the vertical lower portions of the side walls.

[0014] To render possible simple and reliable horizontal thermal expansion absorbing supporting of the furnace at an intermediate height of the furnace, the support construction comprises advantageously a first portion having multi- pie vertical columns supported to the foundations of the boiler system and multiple fixed horizontal beams firmly supported to the vertical columns and a second portion movably connected to the first portion and comprising the horizontal wall supporting beams. According to a preferred embodiment of the present invention, each of the horizontal wall supporting beams is movably supported to at least two of the fixed horizontal beams, which at least two fixed horizontal beams are arranged parallel to the end walls of the furnace.

[0015] The horizontal wall supporting beams are preferably arranged at a level below the roof of the furnace, more preferably at a level of 30-70 %, even more preferably at a level of 40-60 %, of the height H from the bottom of the furnace. The horizontal wall supporting beams are advantageously connected to fixed horizontal beams of the support construction located nearly at the same level as the horizontal wall supporting beams, either below the horizontal wall supporting beams or slightly above the horizontal wall supporting beams.

Thereby, the supporting arrangement of the present invention renders possible to use a simple and economically advantageous fixed support construction having a clearly smaller height than that of a conventional support construction of a top-supported furnace, which extends to a level clearly higher than the roof of the furnace.

[0016] It may in some embodiments of the present invention be possible to supplement the above described middle-supporting of the furnace by flexible auxiliary top-supporting or bottom-supporting, but in any case, according to the present invention, most of the vertical loads of the furnace are balanced by the middle-support. According to a preferred embodiment of the present invention, vertical loads of the furnace are balanced solely by the by the horizontal wall supporting beams. The expression that a furnace is supported solely by the horizontal wall supporting beams does not mean that there are no connections to the surrounding structures, but that such other connections, such as means for conveying flue gas from the furnace or water to the water tubes, or means for feeding air and fuel to the furnace, do not provide any essential balancing of vertical loads of the furnace.

[0017] As mentioned above, the support construction comprises advantageously multiple vertical columns supported to the foundations of the boiler system, multiple fixed horizontal beams firmly supported to the vertical columns and horizontal wall supporting beams which are movably supported to at least two of the multiple fixed horizontal beams, which are arranged parallel to the end walls of the furnace. The at least two fixed horizontal beams comprise advantageously at least two of one or more horizontal beams arranged outside the end walls of the furnace and one or more cantilever beams protruding towards a central portion of a side wall of the furnace.

[0018] Horizontal wall supporting beams having a length greater than the width of the side walls of the furnace are usually supported to fixed horizontal beams arranged outside the end walls of the furnace. In case the width of the side walls is a relative large, for example, as in a fluidized bed boiler with two or more particle separators side by side, the horizontal wall supporting beams are advantageously also supported to at least one fixed cantilever beam protruding toward a central portion of the respective side wall. Usually the number of cantilever on each side wall is one less than the number of adjacent particle separators, whereby there is a cantilever beam between each pair of separators.

When using one after the other arranged piecewise wall supporting beams, it is naturally necessary to support at least the inward ends of the piecewise wall supporting beams to cantilever beams protruding towards a central portion of the respective side wall. [0019] The temperature of the horizontal wall supporting beams advantageously follows the temperature of the furnace. Therefore, according to a preferred embodiment of the present invention, the horizontal wall supporting beams are arranged inside a common thermal insulation with the furnace. This arrangement provides the advantage that the horizontal wall supporting beams stay in all conditions nearly at the same temperature as the furnace, and the thermal expansion of the horizontal wall supporting beams is nearly the same as the thermal expansion of the furnace. [0020] Due to the nearly same temperature of the horizontal wall supporting beams and the furnace, there is no need for a flexible connection of the water- tube walls to the horizontal wall supporting beams. Instead, it is possible to hang the furnace from the two horizontal wall supporting beams simply by multi- pie relatively short hanger rods which are connected to the side walls of the furnace. Preferably the length of the hanger rods is at most two meters, even more preferably at most one meter.

[0021 ] The hanger rods are advantageously attached to the two side walls by a support lug. The support lug is advantageously welded to the edge between the lower end of the intermediate portion of the side wall and the upper end of the vertical upper portion of the lower portion of the side wall. The lug is advantageously designed so as to have the hanger rod aligned with the vertical upper portion of the lower portion of the respective side wall. Because the hori- zontal wall supporting beams stay in all conditions closely at the same temperature as the furnace, the hanger rods stay in practice aligned with the vertical upper portions of the lower portions of the side walls.

[0022] The support lugs of the hanger rods are welded close to each other to the tubes or fins of the tubewall of the side wall. In order to provide a nearly uniform support to the furnace, the distance between adjacent hanger rods is advantageously a small multiple of the distance between adjacent water tubes of the water tube wall. The distance of the hanger rods is thus N times the distance between adjacent water tubes of the water tube wall, where N is a small integer. Preferably N is at most three, more preferably at most two, and even more preferably N is one. [0023] On the other hand, the horizontal wall supporting beams do not stay at the same temperature as the fixed support construction. Therefore, it is necessary to connect the horizontal wall supporting beams to the fixed support construction in a differential horizontal thermal expansion allowing way. Corre- spondingly, according to an advantageous embodiment of the present invention, each of the horizontal wall supporting beams is supported on sliding surfaces arranged on the at least two fixed horizontal beams.

[0024] According to another advantageous embodiment of the present in- vention, each of the horizontal wall supporting beams is supported hanging from the at least two fixed horizontal beams arranged parallel to the end walls of the furnace. Advantageously each of the two horizontal wall supporting beams is supported hanging by at least two main hanger rods from the at least two fixed horizontal beams. The main hanger rods are generally relatively long so as to enable sufficient tilting of the main hanger rods to absorb the by differential horizontal thermal expansion caused relative movement between the fixed horizontal beams and the horizontal wall supporting beams. The length of the main hanger rods is thus preferably at least three meters, even more preferably at least five meters.

[0025] The present invention renders possible an especially straightforward design of the boiler, clearly faster erection of the boiler than by using conventional methods, and in many cases a remarkable reduction in the quantities of the required steel structures.

[0026] The above brief description, as well as further objects, features, and advantages of the present invention will be more fully appreciated by reference to the following detailed description of the presently preferred, but nonetheless illustrative, embodiments in accordance with the present invention, when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 schematically illustrates a side view of a boiler system according to a preferred embodiment of the present invention. [0028] Figure 2 schematically illustrates a horizontal top view of the boiler system of Fig. 1 .

[0029] Figure 3 schematically illustrates a detail of the suspension of a furnace according to an embodiment of the present invention.

[0030] Figure 4 schematically illustrates a side view of a boiler system according to another preferred embodiment of the present invention.

[0031 ] Figure 5 schematically illustrates a side view of a boiler system ac- cording to a third preferred embodiment of the present invention.

[0032] Figure 6 schematically another side view of the boiler system of Fig. 5

DETAILED DESCRIPTION OF THE INVENTION [0033] The diagram of Fig. 1 schematically illustrates a side view of a fluid- ized bed boiler system 10, in accordance with an embodiment of the present invention. The fluidized bed boiler system 10 comprises a furnace 12 having a bottom 14 and a roof 16 at a height H from the bottom, two side walls 18 and two end walls 20, only one of which is seen in Fig. 1 . The side walls 18 and end walls 20 are of conventional type, consisting of vertical water tubes 22 connected together by fins. The boiler can be either a drum boiler or a once- through boiler. The furnace comprises also other conventional equipment, such as a flue gas duct 24 and means for feeding fuel 26 and primary air 28 to the furnace. Because such equipment is not relevant for understanding the present invention, they are, however, not described here in details.

[0034] The side walls 18 comprise a vertical upper portion 30 and a lower portion 32 with an inwards slanted bottom portion 34 and a vertical upper por- tion 36. Between the vertical upper portion 30 and the vertical upper portion 36 of the lower portion 32 of the side wall 18, there is an in downward direction outwards bent intermediate portion 38.

[0035] In case of a fluidized bed boiler having a rectangular furnace en- closed by two longer wall and two shorter walls, the shorter walls, which are generally vertical, are usually considered as the end walls. Thus, the supporting of the furnace according to the present invention is made on the longer walls, which are to be considered as the side walls. A cross section of such a furnace is seen in Fig. 2. On the other hand, the furnace of a circulating fluidized bed boiler with only one particle separator has usually an at least nearly square cross section. In such a case, any two of the enclosing walls can be considered as the side walls as in the present description, i.e. the supporting of the furnace according to the present invention can be made on any two mutually opposing walls of the enclosing walls [0036] The furnace 12 is supported to the ground 40 by a support construction 42 that comprises multiple vertical columns 44 supported to the ground 40 and multiple fixed horizontal beams 46 firmly attached to the vertical columns 44. The support construction 42 comprises also horizontal wall supporting beams 48 arranged parallel to the side walls 18. In addition to the inner vertical columns 44 close to the furnace 12, there are usually also, for example, outer vertical columns 44', which are needed to provide support to other equipment of the boiler system 10, such as a steam drum, fuel bins or particle separators, not shown in Fig. 1

[0037] The horizontal wall supporting beams 48 are arranged slidingly, by using suitable sliding surfaces 50, on two fixed horizontal beams 46 arranged parallel to the end walls 20. As can been seen from Fig. 2, there can also be cantilever beams 58 protruding towards central sections of the sidewalls 18 for providing additional support to the horizontal wall supporting beams 48. The same reference numbers are generally used for the same or corresponding elements in the all the Figures 1 -6. [0038] The horizontal wall supporting beams 48 are arranged close to the outwards bent intermediate wall portions 38, directly above the vertical upper portions 36 of the lower portions 32 of the side walls 18. The furnace 12 is then supported by hanging the side walls 18 of the furnace from the horizontal wall supporting beams 48 by multiple short hanger rods 52.

[0039] The horizontal wall supporting beams 48 are resting on the fixed horizontal beams 46 at a level C which is vertically at a middle section of the furnace. When the furnace 12 heats up from ambient temperature to the operating temperature, such as 850 DegC, thermal expansion lengthens the height and width of the furnace. When the furnace is middle-supported, as in Fig. 1 , the middle portion of the furnace remains at its original level, and the upper portion of the furnace 12, upwards from the level C, expands upwards, and the lower portion of the furnace, downwards from the level C, expands downwards. In ad- dition to the vertical expansion, the furnace experiences thermal expansion also in horizontal direction. The effect and absorption of horizontal thermal expansion will be considered below.

[0040] The level C is clearly below the roof of the furnace, preferably at a level of 30-70 %, even more preferably at a level of 40-60 %, of the height H from the bottom of the furnace. The total height of the support construction 42 can thereby be clearly smaller than that of a conventional top-supported furnace, in which the support construction extends clearly above the roof of the furnace.

[0041 ] Fig. 2 is a horizontal top view of the boiler system of Fig. 1 . As is seen in Fig. 2, the end walls 20 furnace 12 are shorter than the side walls 18. In addition to the fixed horizontal beams 46 outside and parallel to the end walls 20, there are also inner and outer fixed horizontal beams 56, 56' outside and parallel to the side walls 18. There are also additional inner and outer vertical columns 60, 60' to support the inner and outer fixed horizontal beams 56, 56' and cantilever beams 58 protruding towards the side walls 18. In practice the furnace of a fluidized bed with more than one particle separator arranged on a side wall of the furnace has a cantilever beam 58 between each pair of adjacent particle separators.

[0042] Fig. 3 shows more in details the suspension of the furnace 12 from the horizontal wall supporting beam 48. More particularly, Fig. 3 shows the sliding supporting of the horizontal wall supporting beam 48 by a sliding surface 50 in the location of a cantilever beam 58 that ends in the vicinity of the vertical upper portion 30 of the side wall 18. Supporting of the horizontal wall supporting beams 48 to the fixed horizontal means 46 outside and parallel to the end walls 20 is generally similar than that shown in Fig. 3. The lower ends of the hanger rods 52 are attached by support lugs 60 to the edge 62 between the intermediate outwards bent wall portion 38 and vertical upper portion 36 of the lower portion 32 of the side wall 18. In order to maintain the horizontal wall supporting beams 48 in the same temperature with the furnace 12, they are covered by a common insulating layer 64.

[0043] As seen in Fig. 3, the top ends of the hanger rods 52 are fixed to the horizontal wall supporting beam 48 by suitable fixing nuts 54, or other suitable means. Locations of the hanger rods 52 along the side walls 18, on the horizontal wall supporting beams 48 were also seen in Fig. 2 on the basis of the fixing nuts 54 above the vertical upper portion 36 of the lower portion 32 of the side- walls 18. The cantilever beams 58 may have vertical bores to run the hanger rods 52 through the cantilever beams at the location of the beams, as was shown in Fig. 2. Alternatively, the hanger rods may be omitted from the locations of the cantilever beams.

[0044] Fig. 2 shows horizontal wall supporting beams 48 arranged on sliding surfaces 50 even on the cantilever beams 58. Alternatively, the horizontal wall supporting beams 48 could be fixed to the cantilever beams 58 at a central location of the side wall 18. It is also possible that the horizontal wall supporting beams 48 are piecewise extending, for example, from a cantilever beam 58 to a fixed horizontal beam 46 outside and parallel to an end wall 20. The parts of such a piecewise horizontal wall supporting beam are usually connected together to ensure desired longitudinal thermal movement of the piecewise wall supporting beam. [0045] Fig. 4 schematically illustrates a side view of another boiler system 10' as seen towards a side wall 18. The boiler system 10' corresponds otherwise the boiler system 10 shown in Figs. 1 and 2, but it has shorter side walls 18, whereby there is no need for additional supporting of the horizontal wall supporting beams 48 by cantilever beams at a central portion of the side walls 18. Fig. 4 particularly shows that, in order to provide a nearly uniform support to the side walls 18 of the furnace 12, the multiple hanger rods 52 are at a short distance from each other. Advantageously the distance between adjacent hanger rods 52 is a small multiple of the distance between adjacent water tubes 22 of the water tube wall. The distance of the hanger rods is thus advantageously N times the distance between adjacent water tubes 22 of the water tube wall, where N is a small integer. Preferably N is at most three, more preferably at most two, and even more preferably N is one.

[0046] According to the present invention, the horizontal wall supporting beams stay, particularly due to the thermal isolating layer 64 shown in Fig. 3, advantageously in all operating conditions at the same, or at least nearly the same, temperature with the furnace 12. Therefore, the thermal expansion of the horizontal wall supporting beams 48 is in practice identical with that of the width of the side walls 18. Due to the sliding surfaces 50, the horizontal wall supporting beams are able to slide with respect the fixed horizontal beams 46, whereby the hanger rods 52 stay during thermal expansion parallel with each other, and vertical in the direction of the plane parallel to the vertical portions of the adja- cent side wall 18. In the direction of the vertical plane perpendicular to the adjacent end wall 18 the hanger rods may be uniformly tilted to a small angle which is to be taken into account in the fixing of the hanger rods to the horizontal wall supporting beams 48 and to the lugs 60. [0047] Fig. 5 schematically illustrates a side view of another embodiment of the present invention. The boiler system 10" shown in Fig. 5 differs from that of Fig. 1 mainly in that the horizontal wall supporting beams 48' are not supported slidingly on fixed horizontal beams but the horizontal wall supporting beams 48' are hanging from fixed horizontal beams 46' by main hanger rods 66. As is seen in Fig. 6, there is advantageously a single main hanger rod 66 connected to the fixed horizontal beams 46' outside and parallel to the end walls 20. It is also possible that there are cantilever beams, similarly as shown in Fig. 2, to arrange additional main hanger rods also at central portions of the horizontal wall supporting beams 48'.

[0048] The horizontal wall supporting beams 48' are advantageously inside a common insulating layer with the furnace 12, whereby the horizontal wall supporting beams 48' stay at the same temperature with the furnace 12. Differential horizontal thermal expansion between the horizontal wall supporting beams 48' and the fixed horizontal beams 46' is absorbed by tilting of the main hanger rods 66. In order to avoid too large tilting angles, the hanger rods have to have a sufficient length, such as at least about three meters. Longer main hanger rods absorb horizontal thermal expansion by less tilting but they have the disad- vantage of possibly increasing the height of the support construction needed for supporting the furnace at a certain height.

[0049] It is to be understood that Figs. 1 -6 show only exemplary embodiments of the present invention, and features shown in the different embodi- ments can be changed to corresponding features shown in other embodiments, or to those based on general teaching of the present description, whenever it is technically possible. [0050] As becomes clear from above, different embodiments of a furnace of a boiler system with a simple and reliable supporting construction are provided. It should be understood that the elements described in connection with an embodiment can be used also in other embodiments, when possible.

[0051 ] While the invention has been described herein by way of examples in connection with what are at present considered to be the most preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations or modifica- tions of its features and several other applications included within the scope of the invention as defined in the appended claims.