Field of the invention

The invention relates to the use of a heat exchanger pipe in a fluidized bed. The invention also relates to the use of a heat exchanger in a fluidized bed, the heat exchanger comprising said heat exchanger pipe. The invention also relates to the use of a heat exchanger pipe in a fluidized bed boiler. The invention also relates to a fluidized bed reactor comprising said heat exchanger pipe. The invention also relates to a fluidized bed boiler comprising said fluidized bed reactor. The invention also relates to a method for heating a heat transfer fluid with granular solid substance. Background of the invention

A fluidized layer refers to a layer formed by a solid and granular substance, where the grains of the solid substance are in a fluidized state. The fluidized state can be achieved, for example, by fluidizing the grains by means of a fluidizing gas flow. The fluidized layer is formed in a fluidized bed reactor, which has been or is supplied with said granular solid substance. The fluidized bed reactor can be supplied with fluidizing gases from below, for fluidizing the solid substance. The fluidized layer can also be called a fluidized bed. Chemical reactions, such as burning or cracking of hydrocarbons, or physical reactions, such as cooling of solid substance, can take place in a fluidized bed reactor.

One application of a fluidized bed is the cooling of a solid substance. It is known to cool a hot solid granular substance in a fluidized bed in a fluidized bed reactor. The solid substance can comprise, for example, coke. The solid substance cools efficiently, when it is fluidized by a cooler fluidizing gas. Heat transfer to the fluidizing gas is very effective. Heat exchangers can be used to recover heat from the fluidizing gas. It would also be possible to recover heat by a heat exchanger from the fluidized bed. Another application of the fluidized bed is a fluidized bed boiler. The fluidized bed boiler comprises a fluidized bed reactor in which combustible material is burnt. In fluidized bed boilers, said solid substance comprises combustible material and non-combustible material, i.e. bed material, such as for example sand. In the fluidized bed boiler, the fluidized bed is formed of both combustible material and bed material by fluidizing with a fluidizing gas. The fluidiz- ing gas of the fluidized bed boiler comprises oxygen. Heat formed in the combustion is effectively transferred to the bed material. From the bed material, heat can be recovered by a heat exchanger which typically comprises heat exchanger pipes. The heat exchanger can be placed in the fluidized bed, or the heat exchanger can be used to recover heat from the fluidizing gas. The fluidized bed boiler can also comprise a second fluidized bed reactor, in whose fluidized bed the solid substance cools down or is cooled down. When using heat exchanger pipes placed in a fluidized bed, it has been found that the heat transfer coefficient from the fluidized bed to the heat exchanger pipe and further to the heat transfer fluid flowing in the heat exchanger pipe depends on the flow rate of the fluidizing gases. Furthermore, it has been found that said heat transfer coefficient is not constant around the pipe, but said heat transfer coefficient can be higher at the sides of the heat exchanger pipe than above or below the heat exchanger pipe. Heat transfer fluid refers to a liquid, steam or gas flowing inside the heat transfer pipe. The heat transfer coefficient from the fluidized bed to the heat exchanger pipe refers to the thermal power transferred from the fluidized bed to the heat exchanger pipe per temperature difference and surface area. If the heat transfer is not constant, the apparatus has not necessarily been optimized in the best possible way in view of the heat transfer.

Brief summary of the invention

It is an aim of the invention to enhance the heat transfer from the fluidized bed to the heat exchanger pipe. The heat transfer can be enhanced particularly on top of or below, or both on top of and below the heat exchanger pipe. Thus, the heat transfer coefficient around the pipe is, on one hand, more constant and, on the other hand, higher than in the solution of prior art. It has been found that low heat transfer on top of the pipe may be due to a heap of solid substance being formed on top of the heat exchanger pipe, reducing heat transfer from the fluidized bed to the heat exchanger pipe. It has been found that low heat transfer on top of the pipe may be due to a cushion of fluidizing gas being formed below the heat exchanger pipe, reducing heat transfer from the fluidized bed to the heat exchanger pipe. It has been found that these problems can be reduced by designing the heat exchanger pipe in such a way that it reduces the formation of said heap of solid substance and said cushion of fluidizing gas. Such a formation can be implemented, for example, fin structures of the heat exchanger pipe, extending in the direction of the flow of fluidizing gas and in the opposite direction, that is, in vertical directions. Said heat exchanger pipe can be placed at least partly in the fluidized bed, for example in a fluidized bed boiler. Description of the drawings

In the following, the invention will be described in more detail with reference to the appended drawings, in which: Fig. 1 a shows a fluidized bed reactor,

Fig. 1 b shows a circulating fluidized bed boiler ,

Fig. 1 c shows a circulating fluidized bed boiler ,

Fig. 1d shows a heat exchanger in a fluidized bed,

Fig. 1 e shows heat exchanger pipes of a heat exchanger according to

Fig. 1d in a fluidized bed,

Fig. 2 shows some heat exchanger pipes in a fluidized bed,

Fig. 3 shows heat exchanger pipes according to a first embodiment of the invention in a fluidized bed, and Figs. 4a to 4d show heat exchanger pipes according to some embodiments of the invention.

In Figs. 1 to 4, corresponding numerals or symbols refer to corresponding elements.

Detailed description of the invention

Figure 1 a shows a fluidized bed reactor 110 provided with a fluidized bed 100. The fluidized bed 100 is delimited by walls 115 and the bottom 108 of the fluidized bed reactor 1 10. The fluidized bed 100 is supplied with fluidizing gas 105 through the bottom 108, wherein the solid and granular material in the fluidized bed is in a fluidized state. The fluidized bed reactor 110 can be, for example, a bubbling fluidized bed boiler (BFB boiler), wherein the fluid- ized bed 100 comprises combustible material and non-combustible bed material, and the fluidizing gas 105 comprises oxygen. Heat can be recovered by a heat exchanger from the fluidized bed 100 of the bubbling fluidized bed boiler. Figure 1 b shows a circulating fluidized bed boiler (CFB boiler) 120. The furnace 130 is limited on the sides by the walls 125 of the fluidized bed boiler. The furnace 130 is a fluidized bed reactor of the fluidized bed boiler. From below, the furnace 130 is limited by a grate 128. The furnace of the fluidized bed boiler contains non-combustible solid bed material, such as sand, and combustible material, such as wood-based biomass. The solid substance in the boiler comprises bed material and combustible material.

Air is supplied through the grate 128 to the furnace, which is shown by an arrow 126. By means of the air supply 126, the solid substance is fluidized and circulated, and the combustible material mixed in the bed material is burnt. In the circulating fluidized bed boiler, the quantity of air to be supplied is so high that the bed material and the material to be burnt rise upwards in the furnace 130. The circulation of the bed material and the fluidizing gas is indicated with an arrow 132. Fluidizing gas can be removed from the boiler, which is indicated with an arrow 138. In a cyclone 135, solid substance is separated from flue gases. The solid substance may comprise burnt material, that is ash and impurities, such as metal and rocks. Solid substance is returned via a first chamber 136, a gas trap 137 and a second chamber 139 to the furnace 130. The circulation of the solid substance is indicated with arrows 134.

In the circulating fluidized bed boiler, the solid substance is in a fluidized state, among other things, in the furnace 130, in the first chamber 136, and in the second chamber 139. In the circulating fluidized bed boiler, heat can be recovered from the fluidized bed by a heat exchanger which can be placed, for example, in the furnace 130, in the chamber 136, or in the chamber 139. In this description, the fluidized bed reactor refers to the space where the fluidized bed forms or is formed. Chemical reactions, such as combustion, take place in the furnace 130, and cooling of the solid substance takes place in the chambers 136, 139. Said first chamber 136 or second chamber 139 can thus be the second fluidized bed reactor of the circulating fluidized bed boiler 120.

Fluidizing gas is entrained in the solid substance entering the chamber 136 from the cyclone 135. Thus, the solid substance is in a fluidized state in the chamber 136, and the direction of the flow of the solid substance 136 is substantially downwards. In the chambers 136, 139, the solid substance is fluidized by means of fluidizing gas (not shown in the figure) introduced to the bottom of said chamber. Thus, the solid substance in the gas trap 137 is also in a fluidized state. From the chamber 136, the solid substance enters the gas trap 137. The function of the gas trap 137 is to provide a pressure difference between its inlet and outlet sides, that is, between the first chamber 136 and the second chamber 139. The pressure difference is arranged by means of solid substance columns of different heights on the inlet and outlet sides of the gas trap 137. Thereby the function of the gas trap 137 is also to prevent the circulation of fluidizing gases via the gas trap 137 and the chambers 139, 136 to the cyclone 135, Thus, no flue gases circulate in the chamber 136 or in the gas trap 137, wherein corrosion caused by flue gases in the heat exchanger is reduced. The chamber 136 is not necessarily an element separate from the gas trap 137 but the chamber 136 may also be, for example, the upper part of the inlet side of the gas trap 137. In one embodiment of the invention, the heat exchanger is arranged on the inlet side of the gas trap 137. The heat exchanger may be arranged in the first chamber 136. In one embodiment of the invention, the heat exchanger is arranged on the outlet side of the gas trap 137. The heat exchanger may be arranged in the second chamber 139.

With reference to Fig. 1 c, in one embodiment of the invention, solid substance is cooled in the fluidized bed which is arranged in a third chamber 133. Said third chamber 133 may thus be a fluidized bed reactor of the cir- culating fluidized bed boiler 120. In Fig. 1 c, the flow of solid substance 134 is divided into two fractions 134a and 134b in the gas trap. The fraction 134a is directed, as shown in Fig. 1 c, via the gas trap, particularly its outlet side, to the furnace 130. The fraction 134b is directed from the gas trap to a separate heat exchanger for the solid substance. Said heat exchanger for the solid substance comprises said third chamber 133. From the heat exchanger for the solid substance, the fraction 134b is returned to the furnace 130, as illustrated in Fig. 1 c. In the heat exchanger for the solid substance, the solid substance can be kept in a fluidized state by supplying fluidizing gas (not shown in the figure) into the heat exchanger for the solid substance. In the third chamber 133 in the embodiment of the figure, the solid substance flows substantially from above downwards and is in a fluidized state.

Figure 1d shows a heat exchanger 140 in a fluidized bed 145. The fluidized bed 145 is in a fluidized bed reactor. The fluidized bed 145 can be, for exam- pie, in a fluidized bed boiler. The fluidized bed can be, for example, in a circulating fluidized bed boiler, in the furnace 130 of the circulating fluidized bed boiler, wherein the direction of flow of the fluidized bed is substantially from below upwards. The fluidized bed can be, for example, in a circulating fluidized bed boiler, in a chamber 136 in connection with the inlet side of a gas trap in the circulating fluidized bed boiler, wherein the direction of flow of the solid substance is substantially from above downwards. The fluidized bed can be, for example, in a chamber 139 in connection with the outlet side of a gas trap in the circulating fluidized bed boiler, wherein the direction of flow of the solid substance is substantially from below upwards. Furthermore, the fluidized bed can be in a chamber 133 separate from the gas trap of the cir- culating fluidized bed boiler. In the separate chamber 133, the direction of flow of the solid substance can be substantially from above downwards.

The fluidized bed can also be in a bubbling fluidized bed boiler. In a bubbling fluidized bed boiler, the direction of flow of the fluidizing gases is substantially from below upwards. The fluidized bed 145 can also be in a fluidized bed reactor 1 10 for cooling the bed material, wherein the direction of flow of the fluidizing gases is substantially from below upwards. In some cases, for example in a flow from the cyclone 135 of a circulating fluidized bed boiler to the gas trap 137, the gas flow can be directed from above downwards.

Cooled heat transfer fluid 142 is supplied to the heat exchanger 140 of Fig. 1d for heating the heat transfer fluid to heated heat transfer fluid 144. The heat transfer fluid 142, 144 may comprise at least one of the following: liquid and gas. Advantageously, the heat transfer fluid comprises at least one of the following: water, steam and superheated steam. Steam refers to gaseous water. Superheated steam refers to steam at a temperature higher than the condensation point. At the condensation point, the heat transfer fluid may comprise both liquid and gas. Particularly at the condensation point of water, the heat transfer fluid may comprise both water and steam. It is possible that the cooled heat transfer fluid 142 is liquid, for example water, and the heated heat transfer fluid 144 is gas, for example steam. The heated heat transfer fluid 144 can be used, for example, in the production of energy, for example in a steam turbine. The heated heat transfer fluid 144 can be used in heating and particularly in heating a moist material for drying it.

Figure 1 e shows the internal structure of a heat exchanger. The heat exchanger comprises heat exchanger pipes 150 with a circular cross-section. The heat exchanger is arranged in a fluidized bed 145. In the fluidized bed 145, the direction 160 of flow of the fluidizing gas can be from below upwards. The heat exchanger can be arranged in a fluidized bed in which the direction 162 of flow of solid substance can be from above downwards. In a fluidized bed, the direction of flow of both the fluidizing gas and the solid substance can be from below upwards. In general, in a fluidized bed, the direc- tion of flowing of the fluidizing gas or the solid substance is substantially vertical. The heat exchanger pipe 150 comprises a surface that encloses the inner part of the heat exchanger pipe. The heat exchanger pipe has a profile extending in its longitudinal direction. The inner part of the heat exchanger pipe is configured to transfer a heat transfer fluid through the heat exchanger, wherein the surface of the heat exchanger pipe is not permeable to the heat transfer fluid. Consequently, the surface of the heat exchanger pipe is leak- proof. In Fig. 1 e, the heat exchanger pipe 150 of the heat exchanger is placed in a fluidized bed in such a way that said profile is substantially horizontal. Thus, the profile of the heat exchanger pipe is substantially transverse to a direction of flow of the fluidized bed. The heat exchanger pipes may also extend outside the fluidized bed 145, wherein at least part of the heat exchanger pipe 150 is placed in the fluidized bed 145. Figure 1 e shows a cross-section of the heat exchanger pipe 150 in a substantially vertical plane, wherein the cross-section of the heat exchanger pipe is substantially circular, as illustrated in the figure. In a direc- tion transverse to the plane shown in the figure, the heat exchanger pipes are substantially straight and have a continuous cross-sectional shape.

Figure 2 shows some heat exchanger pipes 150 in a fluidized bed 145. In Fig. 2, the heat exchanger pipes are intermeshed with each other in the verti- cal direction. The heat exchanger pipes could be vertically aligned to each other, as shown in Fig. 1d. The fluidized bed 100 is fluidized by fluidizing gases 160, whose direction of flow is substantially from below upwards. It is obvious that when flowing past the heat exchanger pipe 150, the fluidizing gas circumvents the heat exchanger pipe 150. Thus, the direction of the flu- idizing gas flowing past the pipes varies locally, which is illustrated by an arrow 161 drawn with a broken line.

When the heat exchanger pipes 150 guide the flow according to the arrow 161 , an area 210 is formed underneath the heat exchanger pipe, where the flow of the fluidizing gas is substantially reduced. In a corresponding manner, turbulence 220 is formed in the flow above the heat exchanger pipe. It has been found that a cushion 215 of fluidizing gas is formed underneath the heat exchanger pipe in the fluidized bed. In the cushion 215 of fluidizing gas, the flow of fluidizing gas is low, wherein the cushion 215 of fluidizing gas can act as a thermal insulation from the fluidized bed 45 to the heat exchanger pipe 150. It has also been found that a heap 225 of solid substance is formed above the heat exchanger pipe in the fluidized bed. In the heap 225 of solid substance, the flow (turnover) of fluidizing gas is low, wherein the heap 225 of solid substance can act as a thermal insulation from the fluidized bed 145 to the heat exchanger pipe 150. The formation of the cushion 215 of fluidizing gas or the heap 225 of solid substance may be due to the guidance of the flow by the heat exchanger pipe 150. In the fluidized bed, the direction of flowing, either the direction of flowing of the fluidizing gas or that of the solid substance, is substantially vertical. The heat exchanger pipe can be substantially horizontal or at another angle to the horizontal plane.

If the direction of flowing of the solid substance is from below upwards, for example in the furnace 130 or the second chamber 139 of a circulating fluidized bed boiler (Fig. 1b), the heap 225 of solid substance can also be formed underneath the heat exchanger pipes. In a corresponding manner, the cush- ion 215 of fluidizing gas can be formed above the heat exchanger pipe.

It has been found that by the design of the heat exchanger pipe, it is possible to affect the size of the cushion 215 of fluidizing gas and the heap 225 of solid substance formed. By diminishing the cushion 215 of fluidizing gas or the heap 225 of solid substance, or both, it is possible to enhance the heat transfer from the fluidized bed 145 to the heat exchanger pipe. Furthermore, the size of the cushion 215 of fluidizing gas and the heap 225 of solid substance formed can be affected by the flow rate. When the flow rate increases, said heat transfer coefficient increases. A high flow rate, however, increases the wear of the heat exchanger pipe and thereby reduces its service life.

With reference to Fig. 3, in a first embodiment of the invention, a heat exchanger pipe 300 comprises a pipe 150 with a substantially circular cross- section. In the first embodiment of the invention, the heat exchanger pipe 300 also comprises an upper fin 314 and a lower fin 312. The upper fin 314 is planar and fastened outside and above said pipe 150 in such a way that the orientation of said fin at said fastening point is substantially transverse to said pipe, that is, to the surface of said pipe. The lower fin 312 is planar and fastened outside and below said pipe 150 in such a way that the orientation of said fin at said fastening point is substantially transverse to said pipe, that is, to the surface of said pipe. The length, that is, the substantially greatest dimension of the planar fin extends in the longitudinal direction of the pipe 150. The height and the thickness of the planar fin can be selected as will be described below. The shape of the fins 312, 314 can be continuous in the direction transverse to the plane of Fig. 3. The fins 312, 314 can consist of several parts. The length of the parts can be, for example, shorter than the length of the pipe 150. Thus, gaps can be left between the parts of the fin, wherein the fin 312, 314 is not necessarily uniform, and the shape of the fin is not necessarily continuous in the direction transverse to the plane of Fig. 3.

In the direction transverse to the plane of Fig. 3, the fins 312, 314 can extend longer than the fluidized bed 145. In said direction, the fins can be shorter than the fluidized bed 145. In said direction, the fins extend preferably sub- stantially or at least in the length of the fluidized bed 145.

The heat exchanger pipe 300 comprises a surface that encloses the inner part of the heat exchanger pipe. The heat exchanger pipe 300 has a substantially continuous profile shape in its longitudinal direction transverse to its cross-sectional plane. The inner part of the heat exchanger pipe is configured to transfer a heat transfer fluid through the heat exchanger pipe, wherein the surface of the heat exchanger pipe is not permeable to the heat transfer fluid. Consequently, the surface of the heat exchanger pipe is leak-proof. The lower fin 312 of the heat exchanger pipe 300 significantly reduces the size of the cushion 215 of fluidizing gas formed. The height of the lower fin 312 of the heat exchanger pipe 300 can be greater than the thickness of a typical cushion 2 5 of fluidizing gas, as shown in Fig. 3. Thus, at least part of the fin 312 penetrates through the cushion 215 of fluidizing gas. Thus, the heat exchanger pipe 300, which comprises the fin 312, is in direct contact with the fluidized bed 145 via the fin 312, wherein the heat transfer from the fluidized bed 145 to the heat exchanger pipe 300 is good. Advantageously, the fin 312 of the heat exchanger pipe comprises a material of high thermal conductivity, wherein the heat transfer from the fluidized bed 145 to the pipe 140 and to the heat transfer fluid flowing inside the pipe 150 is also enhanced. For example, the fin 312 can consist of a metal. Advantageously, the fin 312 is connected to the pipe 150 by a thermally conductive joint. The fin 312 can be connected to the pipe 150 by, for example, welding.

The upper fin 314 of the heat exchanger pipe 300 significantly reduces the size of the heap 225 of solid substance formed. The height of the upper fin 314 of the heat exchanger pipe 300 can be greater than the height of a typical heap 225 of solid substance, as shown in Fig. 3. Thus, at least part of the fin 314 penetrates through the heap 225 of solid substance. Thus, the heat exchanger pipe 300, which comprises the fin 314, is in direct contact with the fluidized bed 145, wherein the heat transfer from the fluidized bed 145 via the fin 314 to the heat exchanger pipe 300 is good. Advantageously, the fin 314 of the heat exchanger pipe comprises a material of high thermal conductivity, wherein the heat transfer from the fluidized bed 145 to the pipe 150 and to the heat transfer fluid flowing inside the pipe 150 is also enhanced. For example, the fin 314 can consist of a metal. Advantageously, the fin 314 is connected to the pipe 150 by a thermally conductive joint. The fin 314 can be connected to the pipe 150 by, for example, welding.

In the embodiment of the invention, wherein the direction of flow of solid sub- stance is from below upwards, the lower fin 312 reduces the size of the heap 225 of solid substance and the upper fin 314 reduces the size of the cushion 225 of fluidizing gas.

Figures 4a to 4d show some embodiments of the heat exchanger pipe 300. In its use, the heat exchanger pipe is advantageously in a substantially horizontal direction. Thus, the longitudinal direction determined by the length of the heat exchanger pipe is substantially horizontal. The heat exchanger pipe can also be at another angle. Thus, the longitudinal direction determined by the length of the heat exchanger pipe forms an angle a to the horizontal plane. The angle a can be, for example, smaller than 60 degrees. If the angle a is greater than this, the solid substance will automatically slide in the longitudinal direction of the pipe, wherein the above-described problem is alleviated. The angle a can also be smaller. The angle a can be, for example, smaller than 45 degrees. The angle a can be, for example, smaller than 30 degrees or smaller than 15 degrees.

Below, embodiments of the invention will be described with reference to a cross-section of the pipe. Said cross-section is the section transverse to the longitudinal direction of the heat exchanger pipe. When applying the heat exchanger pipe in the above-described way, the cross-sectional plane of the heat exchanger pipe intersects the horizontal plane. Thus, at the section of said planes, one direction of the cross-sectional plane is parallel to the horizontal plane. The other directions of the cross-sectional plane deviate from the horizontal plane. Figure 4a shows a cross-section of the heat exchanger pipe according to Fig. 3 in more detail. The heat exchanger pipe 300 comprises a pipe 150 with a substantially circular cross-section. Typically, the diameter d of a heat exchanger pipe used in a fluidized bed can be 30 to 100 mm, for example about 45 to 55 mm in superheaters and about 70 to 80 mm in preheaters. At present, commonly used diameters are, for example, 50 mm and 75 mm. It is possible that with enhanced heat transfer, the size of heat exchangers can be reduced and the diameter of heat exchanger pipes can be changed. The diameter refers to the outer diameter of the pipe. It has been found that if no fins are used, the height of the heap 225 of solid substance formed on top of the heat exchanger pipe with a diameter of 50 mm (Fig. 2) can be approximately in the order of the radius of the pipe, for example about 25 mm, and the thickness t _g of the cushion 215 of fluidizing gas formed below the heat exchanger pipe (Fig. 2) can be in the same order. The height ¾ of the forming heap 225 of solid substance and the thickness t _g of the cushion 215 of fluidizing gas (Fig. 2) are affected by the flow rate in the fluidized bed. Moreover, the profile of the flow can be scaled as the size of the heat exchanger pipe increases, wherein the height ¾ of the heap 225 of solid substance in relation to the diameter d of the pipe 150 can be about 1/2. Correspondingly, the thickness t _g of the cushion 215 of fluidizing gas in relation to the diameter c of the pipe 150 can be about 1/2. The ratio of the height hi of the upper fin 314 of the pipe (Fig. 4a) to the diameter d of the pipe can thus be at least 1/2. The ratio of the height h ₂ of the lower fin 312 of the pipe (Fig. 4a) to the diameter d o\ the pipe can be at least 1/2, correspondingly. In view of the manufacturing technique, it may be advantageous to make the upper fins 314 and the lower fins 312 equally high, wherein h ₁ = h ₂ . Thus, it is possible to select hi = h ₂ ≥d/2, correspondingly. Because the fins diminish both the heap of solid substance and the cushion of fluidizing gas, slightly lower fins have also been found to enhance the heat transfer. In particular, the height of the fins 312, 314 can be selected such that the ratio of the height i, h ₂ of the fin to the diameter d of the pipe 150 is at least 1 :4. Fur- thermore, the fins can be selected to be substantially equal in height.

The thickness of the fin is significant for thermal conductivity, because a thick fin conducts heat better than a thin one. On the other hand, the manufacturing cost of a thick fin is higher than that of a thin fin. The thickness of the fin can be, for example, 3 to 15 mm.

It is obvious that a first dimension of the heat exchanger pipe 300 of Fig. 4a in its cross-sectional plane is d + hi + h ₂ . Correspondingly, a second dimension of the heat exchanger pipe in the direction of the cross-sectional plane transverse to the first dimension is d. The first dimension is greater than the second dimension. When the heat exchanger pipe 300 is used in a fluidized bed, the second dimension is horizontal.

Furthermore, the heat exchanger pipe 300 tapers towards its upper and lower edges, which edges are determined by said first dimension of the heat exchanger pipe. In other words, said first dimension is placed between the upper edge and the lower edge of the cross-sectional plane of the heat exchanger pipe. Furthermore, the horizontal width of the cross-section of the heat exchanger pipe 300 becomes smaller from the centre of the heat exchanger pipe 300 towards the upper or lower edge of the heat exchanger pipe.

Figure 4b shows another embodiment of the heat exchanger pipe 300. The heat exchanger pipe 300 comprises a pipe 150 with a substantially circular cross-section. The upper and lower fins 314, 312 of the pipe 150 consist of a solid fin structure 316. The fin structure 316 consists of the upper fin 314 and the lower fin 312 and a curved part 317 connecting these. The shape of the curved part 317 is fitted with the pipe 150. The fin structure 316 can be welded to the pipe 315. The fins can be dimensioned as explained in connection with the first embodiment (Fig. 4a).

Figure 4c shows a third embodiment of the heat exchanger pipe 300. The heat exchanger pipe 300 comprises a pipe 150 with a substantially circular cross-section. The pipe 150 comprises a surface that encloses an inner part. The inner part of the pipe 150 is configured to transfer heat transfer fluid through the pipe 150, wherein the surface of the pipe 150 is not permeable to the heat transfer fluid. Consequently, the surface of the pipe is leak-proof. The diameter of the pipe 150 is d.

The heat exchanger pipe 300 of Fig. 4c also comprises an upper fin 314 and a lower fin 312. The fins 312, 314 are welded to the pipe 150 by welded joints 400. The welded joints 400 increase the contact area between the fins and the pipe 150, thereby increasing the heat transfer between the fins and the pipe. In the third embodiment, said fins are substantially circular in cross- section. The fins 312, 314 and the pipe 150 have a substantially parallel lon- gitudinal direction. The fin 312, 314 comprises a surface that encloses an inner part. Said fins can be hollow or solid. If the fin is hollow, the fin is also configured to transfer heat transfer fluid through the heat transfer pipe, wherein the surface of the fin is not permeable to the heat transfer fluid. Consequently, the surface of the fin is leak-proof. The diameter of the upper fin is di and the diameter of the lower fin is afe. The diameters can be equal.

The heat exchanger pipe 300 of Fig. 4c comprises a surface that encloses the inner part of the heat exchanger pipe 300. The inner part of the heat exchanger pipe 300 is divided into three compartments: the inner part of the pipe 150, and the inner parts of the fins 3 2, 314, of which each inner part can be configured to transfer heat transfer fluid through the heat exchanger pipe 300. The inner part of the heat exchanger pipe 300 can be divided into two or four or more compartments as well. In an embodiment of the invention, each compartment is configured to transfer heat transfer fluid through the heat exchanger pipe. In an embodiment of the invention, at least one compartment is configured to transfer heat transfer fluid through the heat exchanger pipe.

It is obvious that a first dimension of the heat exchanger pipe 300 of Fig. 4c in its cross-sectional plane is d + di + d _∑ . Correspondingly, a second dimension of the heat exchanger pipe in the direction of the cross-sectional plane transverse to the first dimension is d. When applying the heat exchanger pipe 300 in a fluidized bed, its second dimension is the substantially horizontal dimension of the cross-sectional plane of the heat exchanger pipe 300. Fur- thermore, the first dimension is greater than the second dimension, and the heat exchanger pipe is tapered towards its upper and lower edges, which edges are determined by said first dimension of the heat exchanger pipe. Thus, the horizontal width of the cross-section of the heat exchanger pipe 300 reduces towards the upper or lower edge of the heat exchanger pipe. Depending on the shape of the welded joints, the width of the heat exchanger pipe 300 does not necessarily decrease continuously from the central area of the heat exchanger pipe 300 towards the upper or lower edge of the heat exchanger pipe, but at some points, the width may increase. Figure 4d shows a fourth embodiment of the heat exchanger pipe 300. The cross-section of the heat exchanger pipe 300 of Fig. 4d is a parallelogram with rounded corners. The radii of curvature of the upper and lower corners are substantially equal. The radii of curvature of both side corners are substantially equal. The radius of curvature of the upper corner is smaller than the radius of curvature of the side corner. A first dimension of such a heat exchanger pipe in its cross-sectional plane is d _mx . Correspondingly, a second dimension of the heat exchanger pipe in the direction of the cross-sectional plane transverse to the first dimension is d _mn . Because of the rounding of the corners and the different radii of curvature, d _mx > d _mn - When applying the heat exchanger pipe in a fluidized bed, the smaller one of the dimensions, d _mn , is placed substantially horizontally. Furthermore, the heat exchanger pipe is tapered towards its upper and lower edges. Thus, the dimension of the cross- section of the heat exchanger pipe 300 transverse to the flowing direction decreases towards the upper or lower edge of the heat exchanger pipe. The heat exchanger pipe 300 can also have an elliptical {i.e. oval) cross-section. The ellipse has first and second dimensions corresponding to the axes of the ellipse. The first dimension can be selected to be greater than the second dimension (unless the ellipse is circular). In a fluidized bed, said second dimension can be substantially horizontal. Thus, the dimension of the cross-section of the heat exchanger pipe 300 transverse to the flowing direction decreases towards the upper or lower edge of the heat exchanger pipe.

The heat exchanger pipe is used for recovering heat. Thus, the heat exchanger pipe 300 is advantageously made of a material with good thermal conductivity. In use, heat can be recovered from a very hot material. For example, the temperature of solid material in a fluidized bed boiler can be about 800Ό. Thus, the heat exchanger pipe 300 is a dvantageously made of a material with good heat stability. The heat exchanger pipe 300 can be made of a material comprising a metal. The heat exchanger pipe 300 can consist of a metal. If the heat exchanger pipe 300 comprises fins 312, 314 and a pipe 150, the fins 312, 314 can comprise the same metal as the pipe 150, or a different metal. In the presented embodiments, the heat exchanger pipe 300 has a first dimension d _max in its cross-sectional plane. In the embodiments shown in Figs. 4a to 4d, the first dimension is one of the following: d + h ₁ + h ₂ , d + di + d∑, and d _mx . In the presented embodiments, the heat exchanger pipe 300 has a second dimension d _mm in the horizontal direction transverse to the first direction. In the embodiments shown in Figs. 4a to 4d, said dimension dmin is one of the following: d and d _mn . In the presented embodiments, the first dimension d _max is greater than said second dimension dmn transverse to this. When the heat exchanger pipe is applied in a fluidized bed, the smaller of said dimensions is placed horizontally. Thus, the horizontal dimen- sion dmin of the heat exchanger pipe 300 is smaller than the first dimension dmax- Furthermore, the horizontal width of the cross-section of the heat exchanger pipe 300 decreases towards the upper or lower edge of the heat exchanger pipe. The upper and lower edges of the heat exchanger pipe are determined by said first dimension. In the presented embodiments, the radius of curvature of the lower part of the heat exchanger pipe 300 is also smaller than the radius of curvature of the side part of the heat exchanger pipe 300. The lower part can comprise a fin 312 with a small radius of curvature. In the presented embodiments, the radius of curvature of the upper part of the heat exchanger pipe 300 is smaller than the radius of curvature of the side part of the heat exchanger pipe 300. The upper part can comprise a fin 314 with a small radius of curvature.

The inner part of the heat exchanger pipe 300 can be divided to compartments. In an embodiment, each compartment is configured to transfer heat transfer fluid through the heat exchanger pipe. In an embodiment, at least one compartment is configured to transfer heat transfer fluid through the heat exchanger pipe. If the inner part of the heat exchanger pipe 300 is not divided, the inner part constitutes an enclosure configured to transfer heat transfer fluid through the heat exchanger pipe. Particularly when used in a fluidized bed, the above-presented heat exchanger pipes 300 have the above-described technical advantage: the heat exchanger pipe 300 eliminates or diminishes at least one of the following: the cushion 215 of fluidizing gas and the heap 225 of solid substance. Thus, the heat transfer coefficient increases from the fluidized bed to the heat exchanger pipe 300. Furthermore, the fins 312, 314 enhance the heat transfer from the fluidized bed to the heat transfer fluid flowing in the heat exchanger pipe, because the fins 312, 314 increase the surface area of the heat exchanger pipe. The heat transfer coefficient between the heat exchanger pipe 300 and the heat transfer fluid flowing inside the same is not necessarily changed.

The fluidized bed can be placed in a fluidized bed reactor for forming the fluidized bed. Said fluidized bed reactor can comprise a heat exchanger comprising a heat exchanger pipe 300. Said fluidized bed reactor can be a part of a fluidized bed boiler. For example, the fluidized bed can be placed in one of the following:

- the furnace 130 of a circulating fluidized bed boiler,

- the gas trap 137 of a circulating fluidized bed boiler,

- a chamber 136, 139 in connection with the gas trap of a circulating fluidized bed boiler, - a chamber 133 separate from the gas trap of a circulating fluidized bed boiler, and

- a bubbling fluidized bed boiler.

The fluidized bed can also be placed in a fluidized bed reactor used for cool- ing solid substance.

The above-presented heat exchanger pipe 300 and its use in a fluidized bed increases the heat transfer from the fluidized bed to the heat exchanger pipe 300 and further to the heat transfer fluid flowing in the pipe. In many cases, a fluidized bed reactor, such as a fluidized bed boiler, is used to achieve a given heat output transferred to the heat transfer fluid. When comparing a conventional heat exchanger pipe 150 having a circular cross section and the heat exchanger pipe 300 according to the presented embodiments, it can be found that the same heat output can be transferred by a smaller quantity of the new heat exchanger pipe 300 than of the old heat exchanger pipe 150. Thus, the heat exchangers can be constructed smaller, which, in turn, reduces the investment costs.

In a test comparing a conventional heat exchanger pipe 150 having a circular cross-section and a heat exchanger pipe 300 according to the first embodiment of the invention (Figs. 3 and 4a), it was found that the heat transfer coefficient for the conventional heat exchanger pipe, from the fluidized bed to the pipe 50, varied between 250 and 500 W/m ² K. The highest values of the heat transfer coefficient were detected at the sides of the heat exchanger pipe and the lowest values above and below the heat exchanger pipe. The supply pressure of fluidizing gas was selected so that the fluidizing rate in said test was 0.23 m/s. The fluidizing rate refers to the flow rate of fluidizing gas at the selected supply pressure of fluidizing gas, after the solid substance has been removed from the fluidized bed boiler. When this heat transfer was compared with the heat exchanger pipe 300 according to the first embodiment (Figs. 3 and 4a), it was found that the thermal power transferred to the heat transfer fluid by the heat exchanger pipe according to the first embodiment of the invention was about 10% higher. The flow rate was also varied in said tests. The fluidizing rate used in the tests was between 0.2 and 0.5 m/s. In the test, the heat exchanger pipes were in a substantially horizontal position.