The present invention relates to a furnace superheater having a number of tube elements mounted to each other in such a way that they form a planar surface.

The present invention also relates to a circulating fluidized bed reactor provided with a furnace superheater, comprising a reactor chamber, a solids separator connected thereto and a return duct, which connects the solids separator to the reaction chamber. Evaporator surfaces and a steam superheater mounted in the furnace of the circulating fluidized bed reactor are also provided.

Solid material, such as fuel, inert bed material and/or limestone is fluidized in the circulating fluidized bed reactor with gas supplied through gas feed means located in the lower part of the reactor. The fluidization velocity in circulating fluidized bed reactors is in normal conditions so high that a substantial portion of the bed material is entrained with the gas and discharged out of the reactor. It is especially a characteristic feature of the circulating fluidized bed that the amount of solid material being entrained with gas is so high that the operation of the bed could not be maintained without recirculation (or addition) of solid material.

In a circulating fluidized bed reactor significant amounts of solid material is discharged with the gases out of the reactor through the upper part thereof. In certain areas of the reactor both upward and downward flow take place. The absolute material flow varies both radially and axially in the reactor, and the downward flow is at its maximum close to the outer walls. When the particle density increases towards the lower part of the reactor chamber, also the particle accumulation along the outer walls increases downwards. Even a slight change in the flow direction of the particles causes erosion. Thus a circulating fluidized bed reactor provides very demanding conditions for different structures.

Due to the erosive conditions of the circulating fluidized bed reactor, it has been suggested that tube elements were used, which when connected to each other form a smooth outer surface. This kind of a structure has been suggested to be used, e.g. in a furnace superheater in power plant boilers based on circulating fluidized bed technique, whereby the superheater may be located directly in the furnace of the circulating fluidized bed reactor. Such tube elements are attached to each other by welding. Welding includes always a number of time-consuming operational steps, such as preheating, pre-welding, welding in tube direction on two sides of the tube, correction of deformations caused by stresses and inspection of the welding. A superheater panel formed by this welding method is therefore almost irrevocably assembled to form one single structure element. Welding is made on both sides, whereafter both surfaces are machined to give an as smooth as possible surface. This is recommended, as the conditions in a circulating fluidized bed reactor, even in the upper part thereof are so erosive that sufficient reliability cannot otherwise be guaranteed.

Forming superheater surfaces from parallel round tubes without welding them is appreciated from US patent 5,012,767. There adjacent tubes are attached with each other by a two sleeve-combination. The tubes run through the sleeves and the sleeves are welded to each other. This round, sleeve-like arrangement is very liable to erosion and the attaching sleeves also form a significant, easily eroding point of discontinuity. The same patent publication also discloses a possibility of welding separate attaching means between the tubes to connect them to each other. Even this arrangement provides a heat exchanger very liable to erosion and this kind of arrangement also increases the distance between the tubes, whereby even less parallel tubes can be fitted in the same space.

An object of the present invention is to eliminate or minimize the need of welding in connection with the manufacture of in furnace superheater tubes, whereby the manufacture time of a panel structure significantly diminishes. At the same time, operational welding steps, such as preheating, pre-welding, welding in tube

direction on both sides of the tube, levelling of deformations caused by stresses and inspection of the welding are eliminated or minimized.

Another object of the present invention is also to provide a circulating fluidized bed reactor, in which furnace superheaters are formed as planar surfaces to correspond to the prevailing conditions in a very simple manner.

Further, an object of the present invention is to provide a circulating fluidized bed reactor, in which furnace superheaters are formed as wear-resistant structures.

A purpose of the present invention is to eliminate or minimize disadvantages of known technique and to provide a completely novel construction. The invention is based on the concept that a furnace superheater is formed substantially without welding.

More accurately, a characterizing feature of a furnace superheater according to the present invention is mainly that the tube elements are connected to each other mechanically.

Also a characterizing feature of a circulating fluidized bed reactor according to the present invention, said reactor being provided with a furnace superheater, comprising a reaction chamber, a solids separator attached thereto and a return duct, connecting the solids separator to the reaction chamber, evaporator surfaces and a steam superheater arranged in the furnace of the circulating fluidized bed reactor, is mainly that said superheater comprises a planar tube panel formed of a number of superheater tube elements mechanically attached to each other.

According to a first embodiment of the invention a furnace superheater, which is especially suitable for a circulating fluidized bed reactor, is formed of superheater tubes of a rectangular cross-section, which are attached to each other by means of grooves or the like. In this connection, the term "rectangular" means a form having at least two substantially parallel sides, which are at least partially on top

of each other. A furnace superheater in accordance with the invention is thus formed by a number of separate superheater tubes attached to each other by means of connecting grooves or the like formed in the superheater tube itself in the contact surface thereof.

According to the invention, the connecting groove or the like may be formed by machining or the contact surfaces may be formed also by milling. The connecting groove or the like may be realized in many ways. It is, however, preferable that the freedom of movement of the tube elements relative to each other is eliminated to a reasonable extent.

The joint is formed according to a preferred embodiment of a recess, a connecting groove or the like, in one tube element and a protrusion in the other tube element, which when fitted together form a mechanical joint.

The superheater elements are preferably rectangular in cross-section and are attached to each other by means of a groove or the like and a protrusion or the like, said groove and protrusion having a substantially similar profile. A protrusion or the like refers to an element either stationarily or movably attached to a tube element arranged next to a tube element provided with a groove, the protrusion having the form of the groove.

A tube element comprises according to the invention a preferably round duct forming a flow channel, limited by tube material in such a way that the outer surface of the tube element is formed within a distance from the surface of the flow channel of two substantially parallel planar surfaces and two contact surfaces substantially perpendicular to said parallel planar surfaces. A joining groove or the like and/or a protrusion or the like substantially similar of their cross-sectional profile are formed on said contact surfaces.

The cross-section of the joining groove or the like in the tube element and the protrusion or the like is preferably a parallelogram, a serriiparallelogram, or it is

circular, hemi-spherical, or triangular in form or a combination thereof. Even some other forms realizing the purpose of the invention may be applied, if so desired. The joining groove or the like of a tube element is preferably larger of cross-section or as large as the protrusion or the like.

By means of the present invention it is possible to achieve significant advantages over the prior art technique. The invention eliminates or minimizes the need of welding in the tube direction of the furnace superheaters. The time needed for manufacturing of panel structures significantly diminishes, whereby also the time needed for the panel structures to pass through the manufacturing process is diminished. The operational steps of welding, such as preheating, pre-welding, welding of both sides in tube direction, levelling of deformations caused by stresses and inspection of welding are replaced by an arrangement in accordance with the present invention.

The invention is discussed more in detail, by way of example, with reference to the accompanying drawings, in which

Fig. 1 schematically illustrates an example of a circulating fluidized bed reactor provided with a furnace superheater in accordance with the present invention;

Fig. 2 schematically illustrates a preferred embodiment of a furnace superheater of

Fig. 1;

Fig. 3 schematically illustrates a second embodiment of a furnace superheater of

Fig. 1; Figs. 4-6 schematically illustrate different cross-sectional profiles of a furnace superheater in accordance with the present invention; and

Fig. 7 schematically illustrates yet another embodiment in accordance with the invention to form a furnace superheater of Fig. 1.

Fig. 1 illustrates a circulating fluidized bed reactor 2, comprising a combination of a reaction chamber 4, a hot gas separator 6 and a return duct 8, into which reactor fluidizing gas is introduced through conduit 10 to the lower part of the reactor

chamber 4 or elsewhere (not shown). Solid material, such as fuel, is supplied through a conduit 12 or through a number of conduits (not shown). Product gases are discharged from the hot gas separator through a conduit 14. A heat-generating process is maintained in the reaction chamber. This heat is recovered to the evaporating boiler water, for example, through reaction chamber walls 16 formed of evaporator tubes. Water to be evaporated is introduced into the evaporator tubes from a cylinder 18 through conduits 20 and the steam/water mixture flows back to the cylinder, for example, via a conduit 22. The described circulation cycle contains a lot of generalizations and simplifications, in reality such a cycle includes a variety of other components. Also the hot gas separator is preferably formed of a evaporator or other heat exchange surfaces. The circulation cycle may according to the invention be of natural circulation or forced circulation type; the invention may be applied to both arrangements. A furnace superheater 24 is provided in the upper part of the reaction chamber, and steam is supplied to the furnace superheater 24 via means 26. Steam is led from the superheater via a channel 28 to a turbine 30 and from there further back to the cylinder via a channel 32. This circulation cycle is also heavily simplified.

The furnace superheater 24 in the upper part of the reaction chamber 4 is arranged to superheat steam by utilizing heat released by the solids-gas-suspension of the reactor. The superheater 24 is arranged in the circulating fluidized bed reactor in such a way that it preferably extends at both ends outside the opposed walls of the reaction chamber. Thus it may, on one hand, be steadily and reliably supported on the ends to the reaction chamber. On the other hand, a relatively long distance between the reaction walls sets certain requirements on the panel. In practice, a structure, which is poorly supported easily begins to vibrate or it may bend due to possible thermal expansion. In other words, the superheater panel must be sturdy enough. This is arranged according to the present invention by supporting the tube elements longitudinally to each other by means of a mechanical joint. This kind of a mechanical joint is according to the invention achieved by providing all surfaces between adjacent tube elements with contoured interconnecting surfaces so that contact surfaces facing each other fit into each other and said contact surfaces also

to a sufficient extent prevent movement of elements relative to each other. The ends of the superheaters are provided with conduits 34 to control and/or divide the steam flow in a desired manner between the separate tube elements of the superheater. The furnace superheater is according to the invention formed by joining tube elements mechanically to each other, whereby no welding in the tube direction is necessary. A tube panel formed in accordance with the present invention is, however, tightly connected to conduits 34, preferably by welding the tube panel to these conduits, so that the demands set by steam production are fulfilled.

The conditions in a circulating fluidized bed reactor are very erosive: a relatively high temperature (e.g. 600-1200 °C) combined with a solid particles suspended in the gas. In these conditions, surfaces erode very easily, especially at points of discontinuity. Therefore, a furnace superheater in accordance with the invention is preferably formed of a number of tube panels arranged one on top of the other, which again are formed of a number of tube elements 36 joined to each other and the surfaces of which are arranged substantially planar by means of the method of forming a furnace superheater in accordance with the invention.

Fig. 1 illustrates a circulating fluidized bed reactor arranged for fuel combustion.

A furnace superheater in accordance with the invention may, if so required, be used also in other kind of furnaces, even for example in so called pulverized fuel boilers. The best characteristics of a furnace superheater in accordance with the present invention become apparent especially in conditions, in which the superheater is surrounded by a gas mixture contaiiiing solid material. Thus the circulating fluidized bed reactor may also act as a hot gas cooler, for example, in a grate incinerator, whereby mainly the grate arrangement deviates from the above described. A furnace superheater in accordance with the present invention may be located in the furnace at different levels, although it is in Fig. 1 located in the upper part of the furnace.

Fig. 2 illustrates an arrangement of a furnace superheater tube panel according to the invention. It shows two tube elements 362, 366 connected to each other by means of a joint groove 364, which is formed by a recess 368 in one tube element 362 and a protrusion 370 in another tube element. The protrusion and recess form together a joint, which to a sufficient extent bind the elements to each other and makes the superheater panel sturdy. The protrusion or recess may be arranged in the tube elements so that each element comprises a recess at one end and a protrusion at the other end. Alternatively, there are two kinds of tubes: tubes with alternatively protrusions or recesses, whereby equal amounts of both tube types are required to form a tube panel. The first mentioned alternative is more advantageous, because in that way only one kind of tube elements are needed. This alternative is illustrated in Fig. 3. Flow channel 372 for steam to be superheated is arranged inside a tube element. The tube element is formed according to the invention so that it comprises two substantially parallel and planar surfaces 374, 376 spaced apart at a distance which is determined so that a duct or flow channel

372 and also enough material between the surfaces and the channel 372 is present in the area 378 between the surfaces. The element has a mainly rectangular cross- sectional profile with ends 380, 382 in the contact surfaces being connected to joining grooves or the like in adjacent elements. Surfaces 374 and 376 extend far enough, so as to obtain contact surfaces with recesses and protrusions and a sufficient material strength.

Figs. 4-6 illustrate different ways of forming contact surface required for the joints of the tube elements. In Fig. 4 the protrusion and the recess have one perpendicular end surface 41 and one inclined end surface 43. In Fig. 5, protrusions and recesses are approximately formed as circular surfaces, whereas in Fig. 6 they are triangular.

The method shown in Fig. 7 of joining tube elements of a furnace superheater to each other is very advantageous. There the opposed contact sides of two adjacent tube elements 70 are both provided with recesses 72, and the tube elements 70 when arranged adjacently form a space 74 between the tubes. The tubes are joined

to each other by fitting a bar or the like 76 imitating the form of the space 74 into it. This bar 76 corresponds to the protrusion or the like, but is not stationarily connected with the tube elements. This bar or the like does not have to extend continuously from one end of the tubes to the other, but the joint may be formed of several shorter pieces, which are located to the space 74 according to a particular positioning. The form of the bar or the like is preferably elected so that it substantially prevents the movement of the tube elements, at least the movement spacing the elements apart from each other. By means of the arrangement in Fig. 7 it is possible to release, for example, one tube element from the tube panel by removing the bar from both sides thereof or element 76 respectively by means of a suitable tool. This is a very advantageous feature especially in connection with a circulating fluidized bed reactor, as a possible repairing of a furnace superheater by replacing a tube with another may be realized very simply and quickly.

The accompanying Fig. 1 illustrates a circulating fluidized bed reactor operating at atmospheric conditions, but a furnace superheater in accordance with the invention is especially suitable in combustors, which operate at pressures above atmosphere, for example, at 2-50 bar.

Although the accompanying drawings illustrate a number of embodiments, it is clear that all modifications evident for a person skilled in the art are within the scope of invention determined by the enclosed patent claims.