Field of the invention

The invention relates to an arrangement for recirculating bed material in connection with a boiler. The invention relates particularly to an arrangement for recirculating bed material in connection with a fluidized bed boiler. By means of the arrangement, bed sand can be recirculated and fresh sand can be supplied into the fluidized bed boiler.

Background of the invention

In boilers utilizing a bubbling fluidized bed (BFB) and a circulating fluidized bed (CFB), the furnace is supplied with air, fuel and bed material, such as sand. An arrangement of prior art is illustrated in Fig. 1 a. A furnace 1 15 is supplied with fresh sand 130 which has not been removed from the furnace 1 15 but stored in a silo 132. In addition, the furnace 1 15 is supplied with recyclable sand 170 which has been previously removed from said furnace 1 5, for example with ash 150. Recyclable sand 70 can be separated from the ash removed from the furnace, for example, by screening.

In arrangements of prior art, the silo 132 in which the fresh sand 130 is stored, is typically placed inside a boiler house 100. The boiler house 100 also accommodates the boiler 1 10 which comprises the furnace 1 15. The boiler 1 10 may be, for example, a fluidized bed boiler. Furthermore, in arrangements of prior art, the silo 132 is located so high in relation to the furnace 1 15 that fresh sand 130 can be transferred from the silo 132 to the furnace 115 by a screw conveyor 35. In arrangements of prior art, a second conveyor 175 is used for transporting recyclable sand.

Some problems are involved in the state of the art. The storage for fresh sand, i.e. the silo, is typically arranged inside the boiler house, which increases the investment costs related to the building. Moreover, the silo is always placed above the grate of the boiler to secure the supply of sand, wherein the equipment for filling the silo has to be capable of lifting the sand relatively high. This is reflected in the investment costs of the transportation equipment.

Brief summary of the invention

It is an aim of the present invention to provide an arrangement for supplying bed material, such as sand, into a boiler, such as a fluidized bed boiler. By the arrangement, it is possible to feed fresh bed material, such as sand, and recyclable bed material, such as recyclable sand, into a boiler, such as a flu- idized bed boiler. By means of the arrangement, the sand silo can be placed lower than in the arrangements of prior art, wherein the filling of the silo is easier than in arrangements of prior art. This reduces the costs involved in the sand transportation equipment. In the arrangement of prior art, recyclable sand and fresh sand are supplied by separate conveyors into the furnace of the boiler. In the new arrangement, the same conveyor is arranged to transfer both recyclable sand and fresh sand into the fluidized bed boiler. The arrangement reduces investment costs because works with a single conveyor instead of two conveyors. Further- more, by means of the arrangement, the sand silo can be placed outside the boiler house, which reduces the investment costs on the boiler house.

The arrangement according to the invention is characterized in what is presented in the characterizing part of the independent claim . The method according to the invention is characterized in what is presented in the characterizing part of the independent claim 11.

Description of the drawings In the following, the invention will be described in more detail with reference to the appended drawings, in which:

Fig. 1a shows an arrangement of prior art for supplying fresh sand into a fluidized bed boiler, Fig. 1 b shows an arrangement of prior art for recirculating sand in connection with a fluidized bed boiler,

Fig. 2 shows a fluidized bed boiler in a boiler house,

Fig. 3 shows an arrangement according to an embodiment of the invention for transferring fresh sand into a fluidized bed boiler and for recirculating recyclable sand from the fluidized bed boiler into the fluidized bed boiler,

Fig. 4 shows an arrangement according to an embodiment of the invention for transferring fresh sand into a fluidized bed boiler and for recirculating recyclable sand into the fluidized bed boiler, Fig. 5a shows an arrangement according to an embodiment of the invention for transferring fresh sand into a fluidized bed boiler and for recirculating recyclable sand into the fluidized bed boiler, in a side view, Fig. 5b shows an arrangement according to an embodiment of the invention for transferring fresh sand into a fluidized bed boiler and for recirculating recyclable sand into the fluidized bed boiler, in a side view transverse to Fig. 5a, Fig. 5c shows a side view of a power plant comprising an arrangement of Figs. 5a and 5b for transferring sand into a fluidized bed boiler.

In Figs. 1a to 5c, corresponding numbers or symbols are used for corre- sponding parts.

Detailed description of the invention

Energy, such as heat and/or electricity, is produced in power plants. In this description, a power plant refers to a power plant in which energy is produced by burning a material. Such a power plant comprises a boiler for burning the material, particularly solid material of biological origin. The boiler comprises at least a furnace for burning the combustible material. Typically, the boiler comprises heat transfer surfaces for recovering heat into a heat transfer medium, such as water. One boiler type is a fluidized bed boiler. In the fluidized bed boiler, material mixed in bed material, such as sand, is burnt. The bed material is granular and incombustible. The aim of the bed material is to improve the heat transfer from the combustible material. In several examples in this description, sand is used as the bed material, but it is obvious than another inert and granular material can also be used instead of sand as the bed material. The bed material and the combustible material constitute the solid material in the furnace. In the fluidized bed boiler, solid material is fluidized by means of fluidizing air. Boilers utilizing a bubbling fluidized bed (BFB) and a circulating fluidized bed (CFB) are known. In the fluidized bed boilers, the furnace is supplied with air, fuel and bed material, such as sand. The sand comprises fresh sand which has not been removed from the furnace but stored in a silo. The sand also comprises recyclable sand removed from the furnace. The recyclable sand can be separated, for example, from ash. By means of processing, such as screening, the recyclable sand can be separated from the ash, and this sand can be recirculated into the fluidized bed boiler.

Figure 1 a shows some parts of a fluidized bed boiler 110, a boiler house 100 and a silo 132 according to the prior art. The silo 132 and the fluidized bed boiler 110 are arranged inside the boiler house 100. The walls 120 of the boiler house enclose the interior of the boiler house 100. The silo 132 is used for storing fresh sand 130 which will be used in the fluidized bed boiler 110. The term "fresh sand" refers to bed material, such as sand, which has been introduced into the silo 132 from elsewhere than the fluidized bed boiler 110. Correspondingly, the term "fresh bed material" refers to bed material which has been introduced into the silo 132 from elsewhere than the fluidized bed boiler 110. Typically, the fresh sand 130 has not been used previously as bed material in another boiler either. The figure also shows a screw conveyor 135 for conveying fresh sand 130 from the silo 132 into the fluidized bed boiler 110, particularly into the furnace 112 of the fluidized bed boiler 110, for facilitating the transportation of the fresh sand. The silo 132 is not placed directly above the grate 112, but the distance of the grate from the ground level is smaller than the distance of the silo 32 from the ground level. The figure does not show means for supplying fluidizing air, or means for supplying fuel into the furnace; these are obvious for a person skilled in the art.

Further referring to Fig. 1 b, bed material such as sand is also recirculated in connection with a fluidized bed boiler in solutions of prior art. Ash 150 is removed from the furnace of the fluidized bed boiler 110, and screened. The ash is conveyed by a conveyor 155 to a screen 160. The screen 160 is used for separating recyclable sand 170 from the ash 150. The terms "recyclable sand" and "recyclable bed material" refer to a fraction of material, such as ash 150, that is removed from the fluidized bed boiler 110 and recirculated as bed material to the fluidized bed boiler 110. The recyclable material comprises bed material, and sand is a bed material. The recyclable bed material (sand) 170 may thus be fresh bed material (sand) 130 which has been passed through the fluidized bed boiler 110.

When recyclable sand 170 is separated from the ash 150, coarse bottom ash 180 is also separated which can be collected in a container 185. The recyclable ash 170 separated in this way is transferred by a conveyor 175 to the furnace 115 of the fluidized bed boiler 110. In Fig. b, the sand recirculation apparatus is placed outside the boiler house 132.

The arrangement according to Figs. 1a and 1b comprises a silo 132 which is arranged inside the boiler house 100. The arrangement further comprises two conveyors arranged for the transport of sand.

Figure 2 shows a new boiler house 200 for a power plant. The boiler house 200 comprises walls 120 which enclose the interior of the boiler house. A boiler 110 is placed inside the boiler house. The power plant also comprises a conveyor 155 for transporting ash 150 away from the boiler 110. The power plant further comprises a conveyor 215. The conveyor 215 is part of a novel arrangement for recirculating bed material in connection with the boiler. Only a part of the conveyor is shown 215 in Fig. 2. The boiler 110 is advantageously a fluidized bed boiler, such as a circulating fluidized bed boiler (CFB boiler) or a bubbling fluidized bed boiler (BFB boiler). The conveyor 215 is configured to convey both fresh bed material 130 and recyclable bed material 170 (Fig. 3) into the boiler 110, for example into the furnace 115 of the boiler. The conveyor 215 is configured to convey bed material 210, such as sand 210, the bed material 210 comprising fresh bed material 130, recyclable bed material 170, or a mixture of these. Depending on the situation, the conveyor 215 may convey fresh bed material 130, recyclable bed material 170, or a mixture of these. In an embodiment of the invention, however, the conveyor is configured to transport both fresh bed material 130 and recyclable bed material 170 in the long term, possibly only one of these at a time, depending on the use situation. Typically, for example during a week, both fresh bed material 130 and recyclable bed material 170 are transported by the conveyor 215, while at times it is possible to transport only one of the bed materials, or possibly a mixture of these, by the conveyor 215. There is no silo 132 inside the boiler house 200 shown in Fig. 2, but the silo 132 is arranged outside the boiler house 200, as shown in Fig. 3, wherein the wall 120 of the boiler house 200 is left between the boiler 0 and the silo 132.

Figure 3 shows an arrangement according to an embodiment of the invention for conveying bed material (sand) 210 into a fluidized bed boiler. The sand 210 comprises at least one of the following: fresh sand 130 and recyclable sand 170. The arrangement comprises means 235 for transferring fresh sand 130 from the silo 132 to the conveyor 215. The silo 132 is arranged outside the boiler house (not shown in the figure). The conveyor 215 is arranged to transport sand 210, i.e. fresh sand 130 and recyclable sand 170, to the fluid- ized bed boiler 110 (Fig. 2). The conveyor 215 may be arranged to transport only one of the following at a time to the fluidized bed boiler: fresh sand 130 and recyclable sand 170. The conveyor 215 is arranged, together with means 235, to transport fresh sand 130 from the silo 132 to the fluidized bed boiler 110. The arrangement further comprises means 240 for transferring recyclable sand to said conveyor 215. The means 240 may comprise, for example, a hood or a funnel, by means of which recyclable sand 170 separated from the ash 150 is collected by a screen 160 to said conveyor 215. In this arrange- ment, the conveyor 215 is arranged to transport fresh sand 130 and recyclable sand 170 to the fluidized bed boiler 110.

As the conveyor 215, it is possible to use, for example, a scraper conveyor and/or a screw conveyor, or several such conveyors. Advantageously, the conveyor 215 comprises a compressed air transmitter arranged to transfer sand 210 by compressed air along a pipe to the fluidized bed boiler. Thus, the conveyor 215 also comprises said pipe for transferring the sand 210 to the fluidized bed boiler. Advantageously, the conveyer is of a compressed air conveyor type that comprises a compressed air transmitter and a pipe for transferring the sand 210 to the fluidized bed boiler. By means of such a compressed air conveyor, the sand 210 can be transferred not only in the horizontal direction but also in another angle. As shown in Figs. 3 and 2, both fresh sand 130 and recyclable sand 170 are conveyed not only in the horizontal direction but also in the vertical direction when sand is transported by the conveyor 215 to the fluidized bed boiler 110.

Figure 3 also shows a screen 160 for separating recyclable sand 170 from the ash 150. As the screen, it is possible to use, for example, a vibrating screen or a trommel screen. Advantageously, a vibrating screen is used, for which the investment costs are lower than for a trommel screen. Figure 3 also shows an ash conveyor 155 for transporting ash from the fluidized bed boiler 110 to said screen 160. Furthermore, means 240 for transporting recyclable sand 170 from said screen 160 to said first conveyor 215 are shown. The conveyor 215 shown in Fig. 3 is arranged to transport sand 210 to the fluidized bed boiler 110, above the grate 112 of the fluidized bed boiler (Fig. 2). The conveyor 215 is not necessarily arranged to transport sand directly above the grate. The conveyor 215 may be arranged to transport ash to a higher level than the height of the grate. Furthermore, part of the con- veyor 215 is placed substantially flush with the ground level 300. Part of the conveyor 215 is placed substantially flush with the ground level 300, when the distance of said part from the ground level 300 is shorter than 0.5 metres. This gives the advantage that the silo 132 can be placed at a low level in the height direction. The silo 132 can be placed, for example, so that at least part of the silo 132 is placed below the grate 112 of the boiler 110 but not necessarily directly underneath the grate. Thus, the distance between the silo 132 and the ground level 300 is shorter than the distance between the grate 112 and the ground level 300. The distance between the silo 132 and the ground level 300 refers to the distance between the lowest point of the silo 132 and the ground level 300. The distance between the grate 112 and the ground level refers to the distance between the top of the grate and the ground level 300. At least part of the furnace of the boiler is placed above the top of the grate 112. When the silo 132 is placed in this way at a low level in the height direc- tion, the silo is easier to fill, compared with a situation in which the silo is placed at a higher level. Typically, the silo 132 is filled from a truck which has transported and thus contains fresh sand 130. The silo 132 can be filled from said truck by, for example, a pneumatic conveyor. When the silo 132 is placed at a low level, a smaller pressure will be sufficient for transferring the fresh sand 130 from the truck into the silo 132. It should be noted that the silo 132 is normally filled at the top, wherein particularly the height of the top of the silo 132 is decisive on what kind of equipment can be used for filling the silo 132. Figure 3 also shows a first control means 231 for controlling the passage of fresh sand 130 from the silo 132 to the conveyor 215. Furthermore, a second control means 271 for controlling the passage of recyclable sand 170 to the conveyor 215 is shown. By means of the first and second control means 231 and 271 , it is possible, for example, to select, which sand, fresh sand 130 or recyclable sand 170, is transported by the conveyor 215 onto or into the flu- idized bed boiler 110. The control means 231 , 271 can be used, for example, to control the ratio of fresh sand 130 and recyclable sand 170 in the sand 210 to be transported by the conveyor 215. In the long-term operation of the flu- idized bed boiler, both of said sand types are transported by the same con- veyor. The first control means 231 can be, for example, a shutter, which can be used to prevent or limit the passage of fresh sand 130 to the conveyor 215. The second control means 271 can also be, for example, a shutter, which can be used to prevent or limit the passage of recyclable sand 170 to the conveyor 215. Figure 4 shows some parts of an embodiment of the invention. For the sake of clarity, only part of the ash conveyor 155 is shown. The conveyor 215 comprises a scraper conveyor. The conveyor 215 is arranged to transport fresh sand 130 and recyclable sand 170 to the fluidized bed boiler. The control means 231 , 271 can be used to select, which sand type is transported by the conveyor 215 to or into the fluidized bed boiler 110. A difference to the embodiment of Fig. 3 is, among other things, the fact that the conveyor 215 is arranged to receive fresh sand 130 and recyclable sand 170 at different points. The conveyor 215 also comprises a lifting device 410, i.e. an elevator for elevating the sand 210, as well as a means 420, such as a trough or the like, for transferring the sand from the lifting device 410 into the fluidized bed boiler 110. The scraper conveyor is arranged to transfer sand 210 to the lifting device 410, and the other parts of the conveyor 215 (the lifting device 410 and the trough 420) are arranged to transfer sand 210 further into the fluidized bed boiler. Instead of or in addition to the scraper conveyor, the conveyor 215 may comprise a screw conveyor. In particular, the conveyor 215 is arranged to transfer sand 210 above the grate (112, Fig. 2) of the fluidized bed boiler. It is feasible that the conveyor 215 comprises a scraper conveyor and an elevator. Alternatively, it is feasible that the conveyor 215 consists of a scraper conveyor for transferring sand 2 0 to the fluidized bed boiler. Thus, another conveyor, such as the lifting device 410, can be used for transferring sand into the fluidized bed boiler. Consequently, the conveyor 2 5 is arranged to transfer bed material 2 0 to or into the boiler 0.

Figure 5a shows an advantageous embodiment of an arrangement for transferring sand into a fluidized bed boiler. Figure 5a show the arrangement in a side view. The fluidized bed boiler (not shown in the figure) is arranged in a boiler house whose wall 120 is on the background in the figure. A silo 132 for storing fresh sand 130 is left outside the boiler house. At the bottom of the silo, a control means 231 is provided for controlling the passage of fresh sand 130 to the conveyor 215. The arrangement comprises means 235 for transporting fresh sand 130 from the silo 132 to the conveyor 215. The arrangement of Fig. 5a also comprises a screen 160 for separating recyclable sand 170 from ash 150. The arrangement further comprises a conveyor 155 for transporting the ash 150 from the fluidized bed boiler to the screen 160. Under the screen, a control means 271 is provided for controlling the passage of recyclable sand 170 to the conveyor 215. The arrangement comprises means 240 for transporting the recyclable sand 170 from the screen 160 to the conveyor 215. Figure 5a also comprises supporting structures 500 for supporting the silo 132 and the recirculation apparatus, for example, to the ground surface 300.

Figure 5b shows the arrangement of Fig. 5a in another side view transverse to Fig. 5a. In addition to the arrangement, the figure shows the wall 120 of the boiler house. The wall 120 may be placed farther away from the arrangement than shown in the figure. In addition to the features shown in Fig. 5a, Fig. 5b shows a container 185 for storing separated bottom ash. Furthermore, a channel 510 for by-passing the screen 160 is shown. The screen 160 can be by-passed, if it is not desired to use the recyclable sand 170. The screen 160 is used for separating recyclable sand 170 and bottom ash 180 from the ash 150.

Figure 5b also shows means 520 for transferring bottom ash from the screen to a bottom ash container 185. The means 520 may be, for example, a pipe, a trough, or the like. The arrangement also comprises means 530 for col- lecting fly ash from the ash 150. The means 530 may comprise, for example, a suction device or the like for collecting the loose fly ash. The fly ash refers to light ash which is easily entrained in an air flow. The ash 150 may comprise fly ash in addition to the bottom ash 180 and the recyclable sand 170. Figure 5c further shows the arrangement of Figs. 5a and 5b in a side view according to Fig. 5b. In addition to the arrangement, the figure shows the boiler house 200. The fluidized bed boiler 110 is placed inside the boiler house. A conveyor 155 is arranged for transporting ash from the fluidized bed boiler 110 to the screen 160, for recirculating the sand. Means 550 for cool- ing the ash are provided in connection with the fluidized bed boiler 110. The silo 132 is placed outside the boiler house 200, wherein the wall 120 of the boiler house is left between the silo 132 and the boiler 110. The distance between the silo 132 and the ground level 300 is smaller than the distance between the grate 1 2 of said fluidized bed boiler 110 and the ground level 300. The conveyor 2 5 is arranged to transport fresh sand from the silo 32 into the fluidized bed boiler 110. The conveyor 215 is also arranged to transport recyclable sand from the screen 160 into the fluidized bed boiler 110. The distance between the silo 132 and the wall 120 of the boiler house 200 facing the silo may be several metres, for example 5 to 25 m.