CLAIM TO PRIORITY

This application is a U.S. national stage application of PCT International Patent Application No. PCT/FI2016/050447, filed Jun. 20, 2016.

FIELD OF THE INVENTION

The present invention relates to a bottom-supported boiler. More particularly, the invention relates to a bottom-supported boiler comprising a boiler pressure body, such as a furnace, a convection cage, an empty pass, a solid separator, or a horizontal pass, and a construction for supporting the boiler pressure body.

DESCRIPTION OF THE RELATED ART

Relatively large boiler pressure bodies, such as tube walled furnaces, are generally arranged top-supported, i.e., so that the boiler pressure body is arranged to hang from a rigid supporting structure. Relatively small boiler pressure bodies are conventionally arranged as bottom-supported, wherein a vertical load of the boiler pressure body is balanced solely by a support construction arranged below the boiler pressure body. The main difference between top-supported and bottom-supported constructions is that when the temperature of the boiler pressure body increases, thermal expansion of a top-supported boiler pressure body takes place mainly downwards whereas in a bottom-supported boiler pressure body thermal expansion takes place mainly upwards. Bottom-supported boiler pressure bodies are generally simpler and economically more advantageous than top-supported boiler pressure bodies, because they do not require a separate supporting structure. A disadvantage of bottom-supported construction is that the walls have to be strong enough to carry the vertical compression load of the boiler pressure body. Therefore, bottom-supported construction is conventionally applied only for relatively small boiler pressure bodies, such as furnaces of power boilers with a capacity of less than 40 MWe.

U.S. Pat. No. 3,280,800 discloses a conventional bottom-supported boiler, comprising a self-standing tube-walled furnace and a boiler bank downstream of the furnace, supported by a downcomer. Bottom support for the furnace is provided by a plurality of support members mounted atop pedestals and adapted to engage, either fixedly or slidably, headers of the wall tubes.

U.S. Pat. No. 3,927,714 discloses a bottom-supported boiler comprising a frame of vertical down pipes and return pipes connected by manifolds and headers, and provides from below vertical support to vertical water tubes connected between the manifolds and headers.

German Patent No. 27 48 650 discloses a bottom-supported boiler comprising in U-form bent evaporation tubes within the boiler, in which the outermost portions of the evaporation tubes are connected to an opposite water tube wall so as to provide vertical support for the evaporation tubes.

An object of the present invention is to provide a bottom-supported boiler with an improved support construction for vertical water tube walls of a boiler pressure body.

SUMMARY OF THE INVENTION

According to one aspect, the present invention provides a bottom-supported boiler comprising a boiler pressure body having a rectangular horizontal cross section formed by joining four planar water tube walls pairwise together so as to form four corner sections, and a support construction, wherein the support construction comprises four vertical columns vertically supported to the ground, the vertical columns being arranged outside the boiler pressure body so that adjacent to each of the corner sections is arranged one of the four vertical columns, wherein each of the vertical columns is attached to the respective corner section.

According to the invention, vertical loads, such as gravitational forces and forces caused by seismic loads and direct wind loads, of the boiler pressure body are balanced, i.e., transferred to the ground or foundations of the boiler, by the four vertical columns, advantageously, solely by the four vertical columns. This differs from a conventional solution in which the weight of the boiler pressure body is transferred to be through the wall tubes, whereby the bottom sections of the water tubes are heavily loaded and relatively densely disposed buckstays are required to avoid excess compression and flexular buckling of the water tube panels. The present invention also minimizes the need to use especially large diameter thick-walled tubes, as required if conventional solution is applied to relatively large boilers, to obtain sufficient stiffness and strength of the tube walls. When using the present solution, the use of multiple support members, most of them sliding support members, below each bottom header of the wall tubes, to support the bottom headers from supporting steel structure, is also avoided, as well as tedious shimming of the support members to balance force distribution between the support members.

The four vertical columns are preferably substantially as high as the boiler pressure body, but, in some applications, they may alternatively have a smaller height than the boiler pressure body. Thereby, the four vertical columns are preferably attached to the respective corner sections in a region having a height of at least 30% of the height of the boiler pressure body, even more preferably, a height of at least 60% of the height of the boiler pressure body. In some cases, when the vertical columns also have other functions than just supporting the pressure body, for example, supporting a steam drum, the vertical columns, or at least some of them, may extend even higher than the boiler pressure body. When vertical columns as high as, or higher than, the boiler pressure body are used, the four vertical columns are advantageously attached to the respective corner sections in a region having a height that substantially equals the height of the boiler pressure body.

According to an advantageous embodiment of the present invention, each of the four vertical columns is attached to the respective corner section by a continuous metal strip that provides, in the vertical direction, a rigid joint. The metal strip is advantageously dimensioned so that it provides, in addition to the desired rigidity, also so good thermal contact between the corner section and the vertical column that they stay in substantially equal temperature. The attaching is advantageously made with a continuous weld to a corner fin between outermost water tubes of the water tube walls forming the corner section.

Thermal expansion of the planar water tube walls of the boiler pressure body takes place mainly upwards. However, thermal expansion generally takes place, usually, to a smaller amount, also in the horizontal direction. According to an advantageous embodiment of the present invention, horizontal thermal expansion is taken into account by supporting at least some of the four vertical columns to the ground by using conventional sliding elements. In such a case, most preferably, all four vertical columns are supported by a sliding element, and horizontal movements of the boiler pressure body are prevented by suitable guiding elements. It is, however, also possible that one of the vertical columns is fixed to the ground or foundation of the boiler, and the other three vertical columns are supported by a sliding element.

Advantageously, the four vertical columns are supported to the ground by a support steel construction below the vertical columns. The support steel construction advantageously comprises a second vertical steel column below each of the four vertical columns. When sliding supporting elements are used, the support steel construction is advantageously substantially rigid. According to another preferred embodiment, the support steel construction is designed to be flexible to render sufficient bending of the support steel construction possible, to allow horizontal thermal expansion of the planar water tube walls without sliding elements between the vertical columns and the support steel construction. According to a preferred embodiment of the present invention, the support steel construction comprises below the boiler pressure body an upper portion that is flexible to allow horizontal thermal expansion of the planar water tube walls without sliding elements, and a rigid lower portion, such as foundations of the boiler.

According to a preferred embodiment of the present invention, at least one, preferably each, of the four vertical columns is a boiler pipe. Advantageously, the boiler pipes are downcomer pipes of the boiler, but, in some applications, they could also be, for example, steam pipes. By using the downcomer pipes as the vertical columns, the need for special supporting of the downcomer pipes is minimized. Because the water in the downcomer pipes is nearly at the same temperature as the water in the water wall tubes, there is not any significant thermal stress between the water tube walls and the downcomer pipes.

According to another preferred embodiment of the present invention, which is especially applicable when downcomer pipes or other suitable boiler pipes are not available, at least one of the multiple vertical columns is a not a boiler pipe. Such a vertical column can be, for example, a separate hollow vertical beam with a square cross section. Such separate hollow vertical beams, which are dedicated to the use as the vertical columns, have the advantage that their sizes are more freely selected, and their sizes can even change with the elevation. When using such separate hollow beams as the vertical columns, minimizing temperature difference between the water tube walls and the vertical columns has to be ensured by using especially good thermal conductivity providing metal strips between the water tube walls and the vertical columns. In order to minimize the temperature difference, each of the four vertical columns, no matter of being, for example, a boiler pipe or a hollow vertical beam, is preferably arranged inside a common thermal insulation with the boiler pressure body.

The present invention is especially applicable when the boiler is a bottom-supported fluidized bed boiler, such as bubbling bed boiler, or a pulverized coal (PC) boiler. By using the present invention, it is possible to make bottom-supported boilers with a capacity of, for example, as high as 70 MWe, or even higher. According to other advantageous embodiments of the present invention, the boiler pressure body is one of a furnace, a convection cage in connection with a furnace, an empty pass in connection with a furnace, a solid separator in connection with a furnace, or a horizontal pass in connection with a furnace.

The present invention also renders possible an especially straight forward design of the boiler, clearly faster erection of the boiler than by using conventional methods, and a remarkable reduction in the quantities of the required steel structures.

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 currently preferred, but nonetheless illustrative, embodiments in accordance with the present invention, when taken in conjunction with the accompanying drawings.

