The invention relates to a steam generation boiler in accordance with the preamble of claim 1 .

The reaction chamber of a circulating fluidized bed once-through steam generation boiler comprises typically an inner portion that has a rectangular horizontal cross-section and is defined by four sidewalls, a bottom and a roof, in which inner portion bed material containing solids and e.g. fuel is fluidized by means of fluidization gas, normally by means of oxygenous primary gas required by the exothermic reactions taking place in the reaction chamber, to be led through the bottom. The inner portion, i.e. the reactor chamber, is generally called a furnace and the reactor is called a fluidized bed boiler, when a combustion process is performed in a circulating fluidized bed once- through steam generation boiler. Typically, the sidewalls of the furnace are also provided with pipes for supplying at least fuel and secondary air.

The sidewalls of the furnace are normally manufactured so as to comprise panels consisting of pipes and fins between them, whereby the energy released in the chemical reactions of fuel is utilized for evaporating the water flowing in the pipes. Superheating surfaces are often adapted in a circulating fluidized bed once-through steam generation boiler in order to further increase the energy content in the steam. When the aim is to manufacture a high-power boiler, e.g . a boiler with a thermal capacity of several hundred megawatts, a large reaction volume and a lot of evaporation and superheating surface are required. It is known from prior art to arrange heat-exchange surfaces on the sidewalls of the boiler extending to the furnace in order to increase the evaporation and superheating area. For instance in US 4,442,796, such heat-exchange surfaces to be arranged in the furnace are disclozed. Also in EP 0 653588 B1 , heat-exchange walls arranged in conjunction with the sidewalls of the boiler and extending to the furnace are disclosed.

A heat exchange panel extending from the furnace wall into the furnace is known from US 2009/0084293 A1 , which panel comprises a pair of walls, where two walls comprise of evaporation tubes face each other. Here, only one side of each wall is directly exposed to the effect of the furnace.

The area of the boiler bottom is on the basis of the required volume and velocity of fluidization gas directly proportional to the boiler capacity. Typically, the cross-section of the reaction chamber is rectangular. Its lower part is arranged to taper towards the grid so that one set of sidewalls of the reaction chamber is inclined and another set of the sidewalls is straight and extends towards the grid. Here, the straight sidewalls extending towards the grid, also called as the end walls in this context, taper like a wedge towards the grid so that their edges meet the inclined sidewall sections. This applies to a reaction chamber with a rectangular cross-section. Reaction chambers in a boiler with cross-sectional shapes other than rectangles are also known from prior art, which reaction chambers do often, however, have such planar walls, the lower parts of which taper towards the grid.

To arrange steam generator pipes on the wall plane in a tapering wall section is likely to become a problem, if the tapering is large enough. It is important for reliable operation of a circulating fluidized bed once-through steam generation boiler that the heat exchange occurring on the steam generator surfaces in the pipes is uniform enough in the various parts of the furnace walls. This means, in practise, that it is disadvantageous for the operation of a once-through steam generation boiler if the heat delivery surfaces in the various parts of the furnace are exposed to a different impact of the fluidized bed and heat exchange, respectively, depending e.g. on the structures of the lower part of the grid and furnace and on the process control. Typically in known solutions, the lengths of the pipes in the tapering section, or at least the pipe sections remaining inside the furnace, may differ from one another in various parts of the wall. In US 7,516,719 B2 the structure of the lower section of the end walls in a once-through steam generation boiler is disclosed, the purpose of which structure is to reduce the varying heat exchange of the steam generator pipes in the tapering lower section and thus to enable as even and comparable heat exchange as possible in each of the parallel pipes. The document suggests reduction of the pipe diameter and the fin between the pipes in the tapering section instead of changing the pipe length . Then, according to the document, the various pipes are made equally long to a sufficient extent, which evens out the heat exchange they are exposed to. This kind of changing of the pipe size and fin width in the wall region requires a plurality of welding operations, which increases the number of working phases and the leak risk.

One object of the invention is thus to provide a steam generation boiler, the structure of the lower part of which makes it possible to provide a high-power and large-size boiler better than before.

A special object of the invention is to provide a circulating fluidized bed once- through steam generation boiler, the structure of the lower part of which makes it possible to provide a high-power and large-size boiler better than before.

The objects of the invention are achieved by a steam generation boiler comprising a bottom portion and a roof portion as well as walls to extend vertically between the bottom portion and the roof portion, thus forming the reaction chamber of the steam generation boiler, the walls of which reaction chamber embody a structure comprising of steam generator pipes, and which steam generation boiler comprises in its lower part at least one wall section tapering towards the bottom portion. The invention is mainly characterized in that a first group of steam pipes in said tapering wall section is arranged to pass from the wall plane into the reaction chamber and extend from the wall plane to the bottom portion of the steam generation boiler on the side of the reaction chamber and a second group of steam pipes is arranged to pass to the bottom portion along the wall plane. By this kind of a solution a steam generation boiler, the structure of the end wall of which comprising steam pipes tapers towards the bottom portion, is provided, which structure is advantageous from the viewpoint of steam production. In particular, by this kind of a solution a once-through steam generation boiler, the structure of the end wall of which comprising steam pipes tapers towards the bottom portion thus enabling a sufficiently uniform heat exchange to each steam pipe in the structure, is provided , wh ich structure is advantageous from the viewpoint of the operation of the once- through steam generation boiler. Said wall section comprises, according to one embodiment of the invention, a wall section that tapers symmetrically towards the bottom portion with respect to the middle axis of the wall section, in which wall section the first group of steam pipes comprises steam pipes on both sides of the middle axis. According to one preferable embodiment of the invention, the steam pipes of said first group pass in two different subgroups at a distance from one another so that they essentially face one another on one side. Accordingly, one side of said first group of steam pipes included in the wall is essentially free from the heat flow of the reaction chamber, whereby their conditions correspond essentially to those of the second group of steam pipes. This is particularly advantageous in conjunction with a once-through steam generation boiler.

According to one embodiment, said different subgroups of the first group of steam pipes pass in the wall on different planes, which are located at a distance from one another, to the bottom portion of the steam generation boiler. Then, it is further advantageous that the distance between the first subgroup and the second subgroup is such that there is a space arranged between them, which space is also gas-tightly separated from the reaction chamber.

According to one embodiment, feed members for medium are arranged in said space for feeding medum into the reaction chamber through the space and/or said space is provided with one or several measuring transducers for determining the conditions prevailing in the reaction chamber. The feed members are preferably arranged so as to deliver oxygenous gas.

Preferably, the steam pipes of the first group and the second group are arranged so as to receive an essentially equal heat flow, respectively, from the reaction chamber. Then, the steam generation boiler is preferably a once- through boiler.

According to one embodiment, the steam pipes of the first group and second group are equally long, respectively, whereby the size of the wall away from the plane of the end wall is preferably determined by the number of pipes in the first group.

According to one preferable embodiment, the first group of steam pipes extends from the plane of the end wall to the bottom portion of the steam generation boiler on the side of the reaction chamber passing at least a part of the way in an angle deviating from the right angle with respect to the plane, and forms a wall, the upper surface of which is inclined, in the reaction chamber.

According to one embodiment, the first and second group of steam pipes are connected to a common distributor of the substance to be evaporated.

The steam generation boiler according to the invention is preferably a circulating fluidized bed once-through steam generation boiler arranged to carry out an exothermic reaction in the circulating fluidized bed maintained in its reaction chamber. The walls of the reactor of the circulating fluidized bed once-through steam generation boiler comprise steam pipes.

Then, at least the walls of the lower part of the reaction chamber and especially said at least one wall section, the lower part of which tapers towards the bottom portion, and the wall formed therein, are preferably coated with refractory material on their side facing the reaction chamber.

Other additional characteristic features of the invention are disclosed in the appended claims and in the following description of the embodiments shown in the figures.

In the following, the invention and its operation will be explained with reference to the appended schematic drawings, of which

Figure 1 shows schematically one embodiment of a circulating fluidized bed once-through steam generation boiler accord ing to the invention, and

Figure 2 shows the pipe structure of the lower section of the end wall of the circulating fluidized bed once-through steam generation boiler according to Figure 1 . Figure 1 shows schematically one embodiment of the steam generation boiler 10 according to the invention, the type of which boiler is a circulating fluidized bed once-through steam generation boiler. The steam generation boiler 10 comprises a bottom portion 12 and a roof portion 16 and walls 14 extending between them. Further, it is obvious that a circulating fluidized bed once-through steam generation boiler comprises a number of such parts and elements that are not shown herein for the sake of clarity. The bottom portion, the roof portion and the walls 14 form a reaction chamber 20, which in the case of a boiler is a furnace. The bottom portion 12 also includes a grid 25, through which e.g. fluidization gas is led into the reactor. In addition, the fluidized bed reactor comprises a solids separator 18, which is typically a cyclone separator. The solids separator 18 is connected to the reaction chamber at its upper part, in the vicinity of the roof section, by means of a connecting channel 22, through which a mixture of reaction gas and solids may flow into the solids separator 18. In the solids separator, solids are separated from the gas and returned into the reaction chamber 20, i.e. to the furnace, after an optional treatment, such as cooling. For this purpose, the solids separator is connected to the lower part of the reaction chamber 20 by means of a return channel 24. The gas, from which solids have been separated, is led in the system to further treatment through a gas outlet 26.

The two opposite sidewalls 14.1 , 14.2 of the reaction chamber 20 are arranged so as to be inclined in the lower part of the circulating fluidized bed once-through steam generation boiler so that the sidewalls approach each other when coming closer to the bottom portion 1 2. Here, the reaction chamber 20 has a quadrangular cross-section, whereby it is, in addition to the sidewalls, defined by end walls, of which only one 14.3 is shown herein. The lower sections 14.31 of the end walls taper when approach ing the bottom portion 12. The end walls comprise steam generator pipes 30, which are preferably arranged so that the heat load from the reactor they are all exposed to, is essentially the same, respectively. Figure 2 shows schematically the lower section 14.31 of the end wall as for the structure of the steam generator pipes. It is to be noted that the pipes in the figure are, for the sake of simplicity, depicted by lines and the fins that in practise connect the pipes are indicated by the distances between the lines.

The lower sections 14.31 of the end walls comprise a tapering section 14.33, to which the inclined section of the sidewalls is connected. The steam pipes of a first group 30.1 (Figure 2) in the tapering wall section 14.31 are arranged so as to pass from the tapering wall section to the reaction chamber 20 and extend from the wall plane Y-Z (Figure 2) to the bottom portion 12 of the steam generation boiler on the side of the reaction chamber 20 forming a wall 1 1 in the reaction chamber 20, and the steam pipes of a second group 30.2 are arranged so as to pass to the bottom portion along the wall plane Y-Z (Figure 2). In this manner, essentially all the steam generator pipes of the tapering section 14.33 are exposed to the reaction taking place in the reaction chamber 20. Thus for instance, the forming of the tapering section requires neither any reduction of the pipe size nor any essential reduction of the distance between the pipes. Above the lower section, the end wall 14.3 is of uniform width essentially all the way to the roof portion 16, i.e. its width does not essentially change, whereby the number of steam generator pipes 30 and their distance from one another is more or less constant, except for any special points, such as openings. The pipes pass in the wall essentially parallel with the longitudinal axis Y of the wall. The pipes in the tapering section passing on the wall plane Y-Z are arranged so as to pass at least partially in an angle with respect to the longitudinal axis Y towards the wall 1 1 arranged in the tapering section 14.33 of the end wall. The steam pipes 30.1 of the first group are bent outwards from the wall plane Y-Z towards the reaction chamber and further towards the bottom portion 12. The steam pipes of the second group 30.2 in the tapering section of the end wall pass on the wall plane all the way to the bottom portion 12 either the whole distance in the above-mentioned ankle with respect to the longitudinal axis Y, or so that the pipes are rebent to be parallel with the longitudinal axis Y at the end facing the bottom portion. In Figure 1 , the tapering wall section 14.41 is with respect to its middle axis Y symmetrically tapering towards the bottom portion 12. Then, the wall 1 1 is formed essentially in the middle of the end wall.

Each of said steam generator pipes 30.1 of the first group forms preferably an essentially equally long flow path as the steam generator pipes 30.2 of the second group. In this connection, it is to be kept in mind that some minor variation may be allowed also in a once-through steam generation boiler. This has an impact on the temperature of each parallel pipe/each pipe being on the same vertical plane, and thereby on the stresses appearing in the pipe wall. In practise, the possible length difference is determined at the design stage according to the calculated temperature difference (for instance the temperature of a certain pipe differing from the mean temperature) between the pipes, which temperature difference is given a specific maximum value. The maximum value is dependent, for instance, on the allowed stresses in the wall structure.

The wall 1 1 comprises preferably steam pipes 30.1 that are bent on both sides of the longitudinal axis Y of the wall. Further, the steam pipes 30.1 bent on both sides, i.e. the first group of steam pipes 30.1 , pass in two different subgroups 30.1 ', 30.1 " (Figure 2) at a distance X' - X" from one another. Here, the pipes of both subgroups, and the walls formed by them, are in connection with the reaction chamber 20 on one side and lack the connection on the other side. Preferably, the first group and second group of steam pipes face each other on one side. In practice, the first group and second group of steam pipes form gas-tight walls or panels. Consequently, also the first group of steam pipes 30.1 passing via the wall 1 1 is exposed to a similar heat flow as the second group of steam pipes 30.2, which pass on the plane Y-Z of the end wall of the reactor. Preferably, the steam generation boiler according to the invention is a circulating fluidized bed once-through steam generation boiler, whereby the operation of the once-through boiler with a circulating fluidized bed is, due to the above-described feature, better than before.

The distance X-X" between the pipes of the first group 30.1 ' and those of the second group 30.1 " is preferably such that there is a space 32 separated from the reaction chamber 20 arranged between them. The space makes it possible to arrange feed members 36 for medium in conjunction with the wall 1 1 , whereby the delivery of medium via the space into the reaction chamber can end up closer to the centre of the reaction chamber 20. The distance X'- X" may vary within certain limits. If, in one embodiment particularly, the distance X'-X" is longer than the diameter of two steam pipes and the width of the fin between them, the roof of the space 32 is formed of at least one of the steam pipes in the first group. When the distance is selected to be still longer, the roof may be formed of more than one parallel steam pipe. Further, one or several measuring transducers 38 can be arranged in the space 32 for measuring the conditions prevailing in the reaction chamber. In this manner, measured values are received closer to the centre of the reaction chamber 20, which gives often a more real picture of the process. Preferably, the steam pipes 30.1 of the first group form in the wall two parallel planar structures on different planes Y-X' ; Y-X"(Figure 2). The wall is preferably vertical on the plane Y-X, whereby the abrasive effect of the solids flow in the reactor with a circulating fluidized bed is minimized. The pipes in the wall are joined together preferably by means of a fin structure. In addition, the wall 1 1 is preferably coated with refractory material on the surface facing the reaction chamber 20 in a manner known per se. The wall 1 1 is preferably perpendicular with respect to the plane Y-Z of the end wall 14.3 and parallel with the longitudinal axis Y of the end wall.

Figure 2 shows further that the pipes on the upper surface of the wall are inclined. Preferably, also the actual upper surface 1 1 .1 of the coated wall is inclined. The inclined upper surface reduces, e.g., the abrasive effect of the solids moving in the reaction chamber 20 during its operation (a circulating fluidized bed once-through steam generation boiler). The inclined upper surface is also provided with coating material. In the wall 1 1 , the steam pipes of the first group 30.1 extend from the wall plane Y-Z into the reaction chamber 20 and further to the bottom portion 12 of the steam generation boiler passing at least a part of the way in an angle deviating from the right angle with respect to the plane Y-Z forming a wall 1 1 , the upper surface 1 1 .1 of which is inclined, in the reaction chamber 20. The steam connection may be realized for instance so that the first 30.1 and second group 30.2 of the steam pipes are connected to a common distributor 34 for the substance to be evaporated.

It is to be noted that only a few most advantageous embodiments of the invention are described in the above. For instance, the cross-sectional shape of the boiler may also be another than a quadrangle. Thus, it is clear that the invention is not limited to the above-described embodiments, but may be applied in many ways. The features described in conjunction with the different embodiments may be used in conjunction with other embodiments as well and/or various combinations of the described features may be made within the frame of the basic idea of the invention, if so desired, and if technical feasibility for this exists.