Thermal device, its use, and method for heating a heat transfer medium

A heat exchanger pipe in a flow duct for gases. The pipe first section has a second section with an inner pipe for transferring heat transfer medium; an outer pipe that radially encloses a part of the inner pipe; and a medium layer between the outer pipe and the part of the inner pipe. The second section of the heat exchanger pipe bends less than 90 degrees. Furthermore, the first section is insulated in its entirety, or non-insulated in the vicinity of other heat recovery surfaces only. In the device the temperature of the heat transfer medium flowing in the inner pipe is at least 500° C., the temperature of the outer surface of the outer pipe is higher than 600° C., or an auxiliary agent is fed to the thermal device.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is US national stage application of International App. No PCT/FI2014/050736 filed on Sep. 29, 2014, which claims priority on Finnish Application No. 20136013, filed on Oct. 11, 2014, the disclosures of which applications are incorporated by reference herein.

Not applicable.

BACKGROUND OF THE INVENTION

The invention relates to thermal devices, such as gasification reactors and boilers, particularly fluidized bed boilers, such as bubbling fluidized bed boilers. The invention relates to thermal devices for heating a heat transfer medium. In particular, the invention relates to thermal devices for heating a heat transfer medium, such as steam, to a very high temperature.

Boilers are used for burning combustible material and thereby for producing energy, such as heat. Heat is recovered from the heat transfer surfaces of the boiler by a heat transfer medium, such as water and/or steam. Hot steam can be used to generate electricity, for example by means of steam turbines.

The efficiency of generating energy is improved when the temperature of the heated heat transfer medium is raised. However, some challenges are involved in increasing the temperature. Increasing the temperature will inevitably increase the temperature of the outer surfaces of the heat transfer pipes. Because corrosive substances, such as salts, are condensed on the surfaces, and an increase in the temperature generally accelerates chemical reactions, corrosion is significantly accelerated due to the increase in the temperature.

Furthermore, for producing particularly hot heat transfer medium, the heat transfer pipe for recovering heat should be placed in a very hot environment. The pressure inside the heat transfer pipe is usually considerable (for example, dozens of bars, typically higher than 30 bar); for example, the pressure and the temperature may correspond to the pressure of saturated vapour, at least at low temperatures. At higher temperatures, the steam is normally superheated, wherein its temperature is higher than the temperature of saturated steam at a corresponding pressure, or the temperature of the critical point of the heat transfer medium, i.e. the critical temperature. (374° C. for water), is exceeded. The heat transfer pipe used in such a hot environment has to withstand the pressure prevailing inside the pipe and also the loads from the corrosive environment outside the pipe. Heat transfer pipes which are resistant to a hot environment and a high pressure under corrosive conditions are typically very expensive options.

Protected heat transfer pipes for a loopseal superheater are disclosed in US 2010/0000474. Therein, the superheating piping includes a steam pipe where the steam to be superheated is directed, and the steam pipe is separated by a protective shell. Such a superheater is preferably placed inside a loopseal. Same principles can be applied for a radiant superheater or a superheater arranged in a flue gas channel.

A device and method for altering the heat transfer characteristics of tubes exposed to the heat generated within a boiler is disclosed in U.S. Pat. No. 4,177,765. Therein a fluidized bed boiler is equipped with a plurality of slidable sleeves circumscribing the vapor generator tubes disposed therein. By selectively extending or retracting the sleeves over the tubes, the heat transfer characteristics can be altered.

SUMMARY OF THE INVENTION

It is an aim of the present invention to provide a thermal device, such as a gasification reactor or a boiler, for heating a heat transfer medium to a high temperature and simultaneously to apply conventional materials.

In an embodiment, the thermal device comprises

at least a first wall delimiting a flow duct for gases, and

a heat exchanger pipe comprising at least an inner pipe, at least a first section of said heat exchanger pipe being placed in said flow duct for gases and extending from said first wall to said first wall or to a second wall delimiting the flow duct for gases in said flow duct for gases, and

said first section of the heat exchanger pipe comprising a second section of the heat exchanger pipe, which extends in said flow duct for gases.

In the thermal device, the second section of the heat exchanger pipe comprises

at least a section of the inner pipe, for transferring heat transfer medium from the first end to the second end of the inner pipe and for recovering heat by the heat transfer medium,

an outer pipe which radially encloses said section of the inner pipe, and

a layer of medium left between said outer pipe and said section of the inner pipe in the radial direction.

the inner pipe of the first section of said heat exchanger pipe is noninsulated in one or more noninsulated areas in such a way that

the distance from all the points of the noninsulated areas in the first section of the heat exchanger pipe to the other heat transfer surfaces of the thermal device (except for the heat exchanger pipe itself) is not greater than 15 cm; or

the inner pipe of the first section of said heat exchanger pipe is, over its entire length, insulated from the flow duct for gases by means of said outer pipe and/or an insulator.

In an embodiment, the thermal device comprises several other heat transfer pipes inside the walls of the flow duct for gases, for recovering heat. Said heat exchanger pipe and said other heat transfer pipes constitute a continuous flow duct for the heat transfer medium, for heating the heat transfer medium.

Furthermore, in one such embodiment,

said flow duct for the heat transfer medium comprises the first section of said heat exchanger pipe as the last heat transfer element placed in the flow duct of gases, in the direction of the flow of the heat transfer medium, or

said flow duct for the heat transfer medium comprises the last first section of the heat exchanger pipe placed in the flow duct for gases, in the direction of flow of the heat transfer medium, and at least one heat transfer pipe in the flow duct for gases, placed downstream in the direction of flow of the heat transfer medium, and

said last first section of the heat exchanger pipe is arranged, in the flow direction of the gas flowing outside the outer pipe, upstream of said heat transfer pipes in the flow duct for gases, placed downstream in the flow direction of the heat transfer medium.

Preferably, said second section of the heat exchanger pipe extends in a straight line or bends less than 90 degrees.

In an embodiment, said second section of the heat exchanger pipe bends at least 90 degrees and thereby does not extend in a straight line.

The thermal device can be used, for example, for heating steam. In an embodiment of the use of the thermal device,

the heat transfer medium is allowed to flow in said inner pipe,

steam is used as the heat transfer medium, and

the temperature of the heat transfer medium flowing in the inner pipe is at least 500° C., preferably at least 530° C.

The thermal device can be used for heating the heat transfer medium in such a way that the surface temperature of a heat exchanger pipe in operation is considerably high. Thus, condensation of corrosive substances on the surface of the pipe is prevented or at least reduced. In an embodiment of the use, the temperature of the outer surface of the outer pipe exceeds 600° C.

Furthermore, in the presented boiler, the use of auxiliary agents for combustion is intensified when the means for supplying the auxiliary agent are placed in such a location where the operating temperature is typically favourable to the supply of the auxiliary agent.

The use of the thermal device will lead to performing a method. A corresponding method for heating a heat transfer medium comprises

producing gas heated by a thermal device,

conveying said gas into a flow duct for gases,

conveying heat transfer medium into a heat exchanger pipe comprising at least an inner pipe, at least the first section of the heat exchanger pipe being placed in the flow duct for gases and extending in said flow duct for gases from the wall of said flow duct to the same or another wall of said flow duct, and said first section of the heat exchanger pipe comprising a second section of the heat exchanger pipe, extending in said flow duct for gases, and

recovering heat by the heat transfer medium in the heat exchanger pipe.

In the method, the second section of the heat exchanger pipe comprises

at least a section of the inner pipe for transferring heat transfer medium from the first end to the second end of the inner pipe and for recovering heat by the heat transfer medium,

an outer pipe which radially encloses said section of the inner pipe, and

a layer of medium left, in the radial direction, between said outer pipe and said section of the inner pipe.

the inner pipe of the first section of said heat exchanger pipe is noninsulated from the flow duct for gases in one or more noninsulated areas in such a way that

the distance from all the points of the noninsulated areas in the first section to the other heat transfer surfaces of the thermal device is not greater than 15 cm; or

the inner pipe of the first section of said heat exchanger pipe is, over its entire length, insulated from the flow duct for gases by means of said outer pipe and/or an insulator.

In an embodiment of the method, too, the thermal device comprises several other heat transfer pipes inside the walls of the flow duct for gases, for recovering heat. Said heat exchanger pipe and said other heat transfer pipes constitute a continuous flow duct for the heat transfer medium, for heating the heat transfer medium.

In such an embodiment of the method,

said flow duct for the heat transfer medium comprises the first section of said heat exchanger pipe as the last heat transfer element placed in the flow duct of gases, in the direction of the flow of the heat transfer medium, or

said flow duct for the heat transfer medium comprises the last first section of the heat exchanger pipe placed in the flow duct for gases, in the direction of flow of the heat transfer medium, and at least one heat transfer pipe in the flow duct for gases, placed downstream in the direction of flow of the heat transfer medium, and said last first section of the heat exchanger pipe is arranged, in the flow direction of the gas flowing outside the outer pipe, upstream of said subsequent heat transfer pipes placed in the flow duct for gases, in the flow direction of the heat transfer medium.

Preferably, said second section of the heat exchanger pipe extends in a straight line or bends less than 90 degrees.

In an embodiment of the method, said second section of the heat exchanger pipe bends at least 90 degrees and thereby does not extend in a straight line.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Thermal devices are used for generating energy, such as electricity and/or heat, and/or for producing fuel from combustible material, such as municipal waste and/or raw material of biological origin, such as wood based raw material. For example, the thermal device may refer to a boiler in which combustible material is burnt for producing energy. Boilers can be classified according to the material to be burnt, wherein e.g. the following boilers are known: soda recovery boiler (fired with black liquor), oil fired boiler, coal fired boiler, pulverized fuel boiler, and waste fired boiler (in a waste-to-energy power plant). Boilers can be classified according to the structure of the boiler, wherein e.g. the following boilers are known: fluidized bed boiler, such as circulating fluidized bed boiler (CFB) and bubbling fluidized bed boiler (BFB); grate boiler; water-pipe boiler; and fire-pipe boiler. For example, the thermal device may refer to a gasification reactor, in which combustible material is oxidized to produce synthesis gas. Synthesis gas can be further refined to fuel, such as biofuel. For example, the thermal device may refer to a pyrolysis reactor, in which combustible material is pyrolyzed to produce pyrolysis oil. The pyrolysis oil can be further refined. Moreover, the thermal device may refer to a torrefaction reactor, in which combustible material is thermally treated to evaporate water and light hydrocarbons from the combustible material. The combustible material treated in this way can be later used as fuel in subsequent processes. In a corresponding way, the thermal process refers to a process in which energy and/or fuel is produced. In alignment with the above described reactors, the thermal process may be, for example, a combustion, gasification, pyrolysis, or torrefaction process. The above mentioned combustible material may be, for example, solid material of biological origin, such as wood based material.

Boilers are given here as an example of thermal devices and their use. Boilers are used for burning combustible material and thereby for producing energy, such as heat. Heat is recovered from the heat transfer surfaces of the boiler by a heat transfer medium, such as water and/or steam. Hot steam can be used to generate electricity, for example by means of steam turbines.

A gasification reactor is given as a second example of thermal devices and their use. Gasification reactors are used to oxidise combustible material in oxygen deficient conditions, for producing synthesis gas. Heat can be recovered from the synthesis gas. Heat is recovered from the heat transfer surfaces of the gasification reactor by a heat transfer medium, such as water and/or steam. Hot steam can be used to generate electricity, for example by means of steam turbines.

Pyrolysis reactors are given as a third example of thermal devices and their use. They are used for forming pyrolysis steam which can be condensed. In the condensing, heat can be recovered in the above described way.

The efficiency of energy production is improved when the temperature of the heated heat transfer medium is raised. Water and/or steam is normally used as the heat transfer medium. In the present description, steam also refers to steam at a temperature exceeding the critical point of water (373° C.), which steam is sometimes call gas, because the steam at said temperature cannot be liquefied to water by increasing the pressure.

Thermal devices, such as boilers, comprise walls, which delimit, for example, a furnace, the gasification phase of the gasification reactor, and/or various gas ducts, such as flue gas ducts, synthesis gas ducts, or pyrolysis steam ducts. The term “wall” may refer to, for example, the walls or the ceiling of the reactor. Thermal reactors also comprise heat exchangers for recovering heat generated in the reactions. The surface temperature of the heat exchanger in operation has a significant effect on the corrosion of the surface of the heat exchanger. Basically, if said surface temperature is low, corrosive substances are condensed from the gases into solids. At the low temperature, the solids do not significantly corrode the surfaces. On the other hand, if said surface temperature is high, no significant amounts of corrosive substances are condensed from the gases, wherein the corrosion is relatively slow, too. In between, a range is left in which corrosive substances are condensed from the gases into liquid substances onto the heat recovery surfaces, wherein the corrosion is very rapid. The values of these temperatures will be given in more detail further below.

Raising the surface temperature of the heat exchanger pipe is very challenging, because the pipe has to withstand the pressure prevailing inside it at the operating temperature.

The present invention will be illustrated in the appended drawings. The figures, such asFIGS. 1aand 1g1, show a thermal device comprising

at least a first wall112delimiting a flow duct115for gases, and

a heat exchanger pipe200comprising at least an inner pipe210, at least the first section202of said heat exchanger pipe being placed in said flow duct115for gases and extending in said flow duct115for gases from said first wall112to said first wall112or to a second wall114(FIGS. 1ato 1e) delimiting the flow duct for gases, and

said first section202of the heat exchanger pipe comprising a second section240of the heat exchanger pipe, extending in said flow duct115for gases.

In this context, the “heat exchanger pipe” thus refers to a possibly long pipe whose (at least one) first section202is, over its entire length, placed in the flow duct115for gases. In a corresponding manner, the first section202refers to a continuous section of the pipe that is as long as possible and extends in the flow duct; that is, a section that extends from wall to wall (either the same or another wall). The second section240of the heat exchanger pipe, comprised in said first section202, is a shielded assembly in which an inner pipe210is shielded by an outer pipe220. The second section240may be shorter than the first section202, or equal in length.FIG. 1g1illustrates the structure of the second section240of such a heat exchanger pipe.

With reference toFIG. 1g1, in the presented embodiments, the second section240of the heat exchanger pipe comprises

at least a section of the inner pipe210for transferring heat transfer medium from the first end to the second end of the inner pipe and for recovering heat by the heat transfer medium,

an outer pipe220which radially encloses said section of the inner pipe, and

a layer230of medium left between said outer pipe220and said section of the inner pipe210in the radial direction.

Such a structure has the advantage that because of the medium layer230, the surface temperature of the outer pipe220is, when the thermal device is in operation, so high that no significant amounts of corrosive substances are condensed on its surface. Such a pipe with a layered structure is heavier than a single layered pipe. Furthermore, it has been found that if a pipe with a layered structure is bent, the outer pipe will come into contact with the inner pipe, wherein there will be no medium layer at the bending point. When there is no medium layer, heat will be conducted too well from the outer pipe to the inner pipe, which will reduce the surface temperature of the outer pipe to a range that is critical for corrosion, at least when the present configuration is applied in hot conditions and with a hot heat transfer medium. Furthermore, a relatively straight pipe is easier to make self-supporting than a pipe that bends to a great extent. For these reasons, in an advantageous embodiment,

said second section240of the heat exchanger pipe extends in a straight line or bends less than 90 degrees.

It has been discovered that with some technical solutions, it is possible to arrange the inner pipe210inside the outer pipe220, even when the outer and inner pipes are bent, in such a way that a medium layer sufficient for heat insulation is left between these pipes.

In an embodiment, said second section240of the heat exchanger pipe is bent at least 90 degrees, wherein said second section of the heat exchanger pipe does not extend in a straight line. Also in this case it is possible, by applying certain technical solutions, to provide a medium layer constituting a sufficient heat insulation between the outer pipe220and the inner pipe210.

The function of the outer pipe220is, among other things, to shield the inner pipe210. It is possible that in addition to the outer pipe220(FIGS. 1cand 1g4) or as an alternative to the outer pipe220(FIGS. 1band 1g2and1g3), the inner pipe210is shielded with an insulator260,255,257at least at some points of the flow duct for gases.

Moreover, it is possible that at a point where the temperature is already low in the flow duct115, the inner pipe is not shielded at all; not with an insulator nor with an outer pipe. Such points are typically found in the vicinity of the heat recovery surfaces, such as the walls112,114. Even in this case, the inner pipe210is shielded over almost its entire length in the flow duct115for gases. Consequently, in some embodiments

the inner pipe210of the first section202of said heat exchanger pipe is, in some parts, insulated from the flow duct115for gases by means of said outer pipe220and/or an insulator260, and

the inner pipe210of the first section202of said heat exchanger pipe is noninsulated from the flow duct115for gases in one or more noninsulated areas270(FIG. 1i) in such a way that

the length of even the largest noninsulated area270of the first section202does not exceed 15 cm; preferably, the length of even the largest noninsulated area270does not exceed 10 cm, the length being measured in the longitudinal direction of the inner pipe210; or

the distance from all the noninsulated areas270of the first section202to the other heat recovery surfaces of the thermal device (other than the heat exchanger pipe200itself) is not greater than 15 cm; preferably not greater than 10 cm; or

the first section202of said heat exchanger pipe, or the inner pipe210of said first section202, is, over its entire length, insulated from the flow duct115for gases by means of said outer pipe220and/or an insulator260(FIGS. 1ato 1f).

With reference to points (A, A1 and A2) andFIG. 1i, the first section202preferably comprises not more than two such noninsulated areas270(one at each end), and all the noninsulated areas270(the only one or both ones) extend from the wall (112,114) of the thermal device110to the flow duct115.

Point (A2) is also a possible solution, because the temperature of the gases in the flow duct115is typically lower in the vicinity of the heat recovery surfaces than far away from the other heat recovery surfaces. In the vicinity of the heat recovery surface, the heat exchanger pipe may also extend in the direction of the heat recovery surface or substantially in parallel with the heat recovery surface in the flow duct115. Typically, the heat exchanger pipe extends in a direction substantially perpendicular to the wall (FIG. 1i).

Yet more advantageously, the first section does not comprise any non-insulated areas270(FIGS. 1ato 1f), wherein the inner pipe210is shielded over its entire length in the flow duct115for gases (see point B above).

An embodiment of the present invention is illustrated inFIG. 1a. The thermal device100ofFIG. 1, such as a boiler, comprises

a first wall112(a wall in the figure) comprising the first area122of the wall of the boiler,

a second wall114(a wall in the figure) comprising the second area124of the wall of the boiler, and

a reaction area110for generating gases, such as (a) a furnace110for burning material and for forming flue gases, or (b) a gasification phase for oxidizing raw material and for forming synthesis gas, wherein

at least said first wall112of the thermal device delimits the flow duct115for gases in such a way that a section of the flow duct115for gases is left between the first area122of the wall of the device100and the second area124of the wall of the device100.

In the device according toFIG. 1a, said flow duct115for gases has a rectangular cross section, wherein the thermal device100comprises four walls. The invention can also be applied in such thermal devices in which the flow duct for gases has a circular cross section. Such a thermal device100comprises the first wall112only. Furthermore, if the heat exchanger pipe200extends through the duct115, the first wall112of the device also comprises the second area124of the wall, to which the heat exchanger pipe200(at least its inner pipe210) extends. In general, the thermal device thus comprises the second wall114only optionally. Advantageously, the thermal device comprises at least four walls delimiting the flow duct115for gases. In the embodiment ofFIG. 1a, the thermal device100comprises the second area124of the wall, comprised in said second wall114.

FIG. 1aalso shows a feeding duct104for feed gas. Combustion air can be supplied into boilers via the feeding duct104. Gasification plants, for example, can be supplied with oxygen and/or water vapour for gasifying the raw material. In a boiler, for example, combustion air is supplied via a duct104and a grate102into a furnace110. Advantageously, the type of the boiler100is a fluidized bed boiler, such as a bubbling fluidized bed boiler or a circulating fluidized bed boiler, preferably a bubbling fluidized bed boiler. In the fluidized bed boiler, such as a bubbling fluidized bed boiler, the combustion air is used to bring the solid material and the combustible material in the furnace110into a fluidized state; in other words, a fluidized bed is formed.

Further with reference toFIG. 1a, heat can be recovered in the boiler100by primary superheaters152placed in a smoke passage160downstream of the furnace. Heat can be recovered by superheaters154at the top150of the furnace. Heat can be recovered, for example, by tertiary superheaters156at the top150of the furnace. Conveying the flue gases to the next heat recovery surfaces, to removal, to purification, or to after-treatment is illustrated with an arrow175. The boiler100may also comprise a nose180for guiding the flue gases and for shielding the tertiary superheaters156from direct radiation heat, for example. InFIG. 1a, the nose180is drawn with broken lines to illustrate that the boiler100does not necessarily comprise the nose180. InFIG. 1a, the superheaters are arranged in the following order in the flow direction of the flue gases: secondary superheater154, tertiary superheater156, and primary superheater152. The heat transfer medium (such as water and/or steam) is arranged to flow (and flows during the operation) from the primary superheater152to the secondary superheater154and further to the tertiary superheater156.

InFIG. 1a, the boiler also comprises a heat exchanger pipe200that is particularly suitable for this purpose, as described above. The first section202of the heat exchanger pipe is provided in the flow duct115for gases. In the case ofFIG. 1a, the first section202of the heat exchanger pipe consists of the above described second section240of the heat exchanger pipe, whose structure is illustrated inFIG. 1g1. In other words, the second structure240with the layered structure also extends over the entire length of the flue gas duct115.

In an embodiment, the second section240of the heat exchanger pipe extends in a straight line or bends less than 90 degrees, as described above. Advantageously, the second section240bends less than 45 degrees, less than 30 degrees, or less than 15 degrees.

In a corresponding manner, in some other embodiments, the second section240of the heat exchanger pipe bends at least 90 degrees, at least 45 degrees, at least 30 degrees, or at least 15 degrees.

With reference toFIGS. 1h1to1h3, the phrase “bends less than α degrees” means that

said heat exchanger pipe200extends in such a way that the second section240extends in the flow duct115, and

said second section240of the heat exchanger pipe has a first longitudinal direction S1at its first point p1(FIGS. 1h1to1h3), and

said second section240of the heat exchanger pipe has, at all its points p2, a longitudinal direction S2which is parallel to or forms an angle with a magnitude smaller than said a degrees to the first direction S1of the second section of said heat exchanger pipe.

In this context, the longitudinal direction of the heat exchanger pipe refers to the longitudinal direction in the flow direction of the heat transfer medium. For example inFIG. 1h1, the direction S2of the heat exchanger pipe is parallel with the direction S1irrespective of the selection of the points p1and p2. Consequently, inFIG. 1h1, the second section240of the heat exchanger pipe extends in a straight line.

For example inFIG. 1h2, the direction S2of the heat exchanger pipe deviates from the direction S1, for a certain selection of points p1and p2, but the directions are parallel for some other selections. However, irrespective of the selection of the points p1and p2, the angle α left between the directions S2and S1is smaller than 90 degrees. Consequently, inFIG. 1h2, the second section240of the heat exchanger pipe bends less than 90 degrees.

For example inFIG. 1h3, the direction S2of the heat exchanger pipe deviates from the direction S1, for a certain selection of points p1and p2. For the selection shown in the figure, the directions S2and S2are opposite, so that the angle α is 180 degrees. Consequently, inFIG. 1h3, the second section240of the pipe bends more than 90 degrees.

In the embodiment ofFIG. 1a,

said heat exchanger pipe200extends so that the second section240of the heat exchanger pipe extends from said first area122of the wall of the device to said second area124of the wall of the device in such a way that

said second section240of the heat exchanger pipe has a central axis having a radius of curvature that indicates, at each point, the direction, or the change in the direction, of the central axis and is at least 25 cm, at least 50 cm, at least 1 m, at least 5 m, and most advantageously at least 10 m.

Thanks to the large radius of curvature, a medium layer230is also provided at each point between the outer pipe220and the inner pipe210when the pipe with a layered structure is bent. Furthermore, such a relatively straight pipe is easier to make self-supporting.

As presented above, with some technical solutions it is possible to arrange the inner pipe210inside the outer pipe220, also when the outer and inner pipes are bent, in such a way that a medium layer sufficient for the heat insulation is left between these pipes.

Consequently, in an embodiment

said heat exchanger pipe200extends so that the second section240of the heat exchanger pipe extends from said first area122of the wall of the device to said second area124of the wall of the device in such a way that

said second section240of the heat exchanger pipe has a central axis having a radius of curvature that indicates, at each point, the direction, or the change in the direction, of the central axis, and being shorter than 10 m, shorter than 5 m, shorter than 1 m, shorter than 50 cm, or shorter than 25 cm.

In an embodiment, the first area122of the wall (such as a wall) of the device is placed on the opposite side of the flow duct115, with respect to the second area124of the wall of the device. In an embodiment, the first wall112of the device is opposite to the second wall114of the boiler.

In an embodiment, the first area122of the wall of the boiler and the second area124of the wall of the boiler are parallel to each other, or the angle between the planes parallel to the areas is smaller than 45 degrees, such as smaller than 30 degrees or smaller than 15 degrees. The areas of the walls can also be perpendicular, for example if the first section of the heat exchanger pipe extends between two walls at an angle to each other.

The extension of the second section240of the pipe in the flow duct115can be represented by one or more of the following:

by the curvature of the second section240, particularly the angle of curvature (angle α), and

by the radius of curvature of the central axis of the second section240.

For example, the second section240can curve not more than 45 degrees so that the radius of curvature is at least 1 m. In a corresponding manner, the second section240can curve more than 45 degrees so that the radius of curvature is shorter than 1 m.

In an advantageous embodiment, as illustrated inFIGS. 1aand2,

the section240of the heat exchanger pipe extends straight from said first area122of the wall of the boiler to said second area124of the wall of the boiler.

In this embodiment, the section240of the heat exchanger pipe has, at all points thereof, a longitudinal direction that is parallel with the first longitudinal direction of said heat exchanger pipe. As presented above, the heat exchanger pipe200can bend in the flow duct215, for example, less than 90 degrees, or according to the radius of curvature, but bending is not technically advantageous in view of the manufacture. In view of the manufacture, it is technically advantageous that the inner pipe210can be inserted through the outer pipe220in its longitudinal direction. This is possible, for example, when the outer pipe220is straight.

As presented above and inFIG. 1a, the first section202can consist of the second section240. With reference toFIG. 1b, the first section202of the heat exchanger pipe does not necessarily consist of the second section of the heat exchanger pipe. In the embodiment ofFIG. 1b,

the thermal device100comprises insulator255adjacent to the first wall112and extending from said first area122of the wall of the device to the flow duct115for gases,

said insulator255adjacent to the first wall112is arranged to insulate at least the inner pipe210of the heat exchanger pipe200from the flow duct115for gases,

the thermal device100comprises insulator257adjacent to the second wall114and extending from said second area124of the wall of the device to the flow duct115for gases,

said insulator257adjacent to the second wall is arranged to insulate at least the inner pipe210of the heat exchanger pipe from the flow duct115for gases, and

said second section240of the heat exchanger pipe extends from said insulator255adjacent to the first wall of the device to said insulator257adjacent to the second wall of the device.

Such an insulated structure is illustrated inFIG. 1g2, in which the inner pipe210is only insulated by the insulator255,257adjacent to the (first or second) wall.

It is obvious that the insulator can be alternatively used in connection with only one wall, for example the first wall (not shown in the figure). Thus,

the thermal device100comprises insulator255adjacent to the wall and extending from said first area122of the wall of the device to the flow duct115for gases,

said insulator255adjacent to the wall is arranged to insulate at least the inner pipe210of the heat exchanger pipe from the flow duct115for gases, and

said second section240of the heat exchanger pipe extends from said insulator255adjacent to the wall to said second area124of the wall of the device.

Alternatively, it is possible, for example, that

the thermal device100comprises insulator255adjacent to the wall and extending from said first area122of the wall of the device to the flow duct115for gases,

said insulator255adjacent to the wall is arranged to insulate at least the inner pipe210of the heat exchanger pipe from the flow duct115for gases,

the inner pipe of the first section of said heat exchanger pipe is non-insulated from the flow duct for gases in one noninsulated area270in such a way that

said noninsulated area270extends from the second area124of the wall of the device to the flow duct115for gases, and

said second section240of the heat exchanger pipe extends from said insulator255adjacent to the wall of the device to said noninsulated area270.

The length of the noninsulated area270is advantageously short, as presented above.

With reference toFIG. 1c, it is possible that the heat exchanger pipe comprises a bend, or a fold, possibly even an abrupt bend. As presented above, in such a bend it is, however, very difficult to secure the local heat conductivity of the pipe with a layered structure, because the thickness of the medium layer230(FIGS. 1g1and1g4) is difficult to control. Thus, as shown inFIG. 1c, the heat exchanger pipe comprises a first second section240and a second section240b. These second sections240and240bare represented by the above-presented features relating to the second section, such as straightness and layered structure.

Thus, the heat exchanger pipe comprises a thermally insulated section250, in which section250the first section202of the pipe can bend even abruptly. In the thermally insulated section250, the insulator260(FIGS. 1g3and1g4) can insulate merely the inner pipe210from the flow duct115for gases, as shown inFIG. 1g3, or the thermal insulator260can insulate the outer pipe220from the flow duct115for gases, as shown inFIG. 1g4. In these embodiments,

said first section202of the heat exchanger pipe comprises a thermally insulated section250in said flow duct115for gases, in which thermally insulated section250

the inner pipe210is not enclosed by the outer pipe, and the inner pipe210in said thermally insulated section250is thermally insulated by means of a thermal insulator260from the gases of the flow duct115, as shown inFIG. 1g3, or

the inner pipe210is enclosed by the outer pipe220, and the outer pipe220in said thermally insulated section250is thermally insulated by means of the thermal insulator260from the gases of the flow duct115, as shown inFIG. 1g4.

For example ceramics, mortar, or putty can be used as the insulator255,257adjacent to the wall and/or as the insulator260in the thermally insulating area250. The thermal conductivity κ of the insulator (255,257,260) is advantageously lower than 75 W/mK (Watts per meter and Kelvin), more advantageously lower than 50 W/mK, or even more advantageously lower than 10 W/mK, the thermal conductivities being given at room temperature 20° C. For example mortar can be used as the insulator. For example in this case the thermal conductivity of the insulator (255,257,260) can be lower than 2.5 W/mK. The thermal conductivity of e.g. some ceramics is some dozens of W/mK, for example 60 W/mK for silicon carbide (SiC), 32 W/mK for aluminium oxide (Al2O3), and 20 W/mK for silicon nitride (Si3N4). The thickness t of the insulator (255,257,260) is advantageously at least 0.5 mm, more advantageously at least 1 mm, and even more advantageously at least 2 mm. If necessary, the ceramic coating can be thin. Preferably, the coating is thicker when mortar or putty is used. Thus, the outer surface of the heat exchanger pipe can be equipped with protrusions, such as pins, to keep the insulator in place. This can be done particularly when fastening the insulator255,257adjacent to the wall. In this case, the thickness of the insulator can be, for example, 10 to 30 mm. In an embodiment, the insulator255,257adjacent to the wall is fastened to the heat exchanger pipe (outer pipe or inner pipe) by means of protrusions.

The insulator255,257,260, for example gunning or ceramics, can be selected so that the insulator260is heat resistant and it provides the desired heat transfer level from the flow duct115to the heat exchanger pipe200. The desired heat transfer level may depend on e.g. the location of the heat exchanger pipe. For example, the thickness and the thermal conductivity can be selected so that the ability to conduct heat (i.e. conductance) of the insulation layer, as determined by the thermal conductivity κ and the thickness t by the formula κ/t, is not higher than 80,000 W/m2K, more advantageously not higher than 30,000 W/m2K, where the thermal conductivity κ is given at room temperature 20° C. Furthermore, the insulator (255,257,260) should withstand temperatures corresponding to the operating temperature. Advantageously, the insulator (255,257,260) withstands temperatures higher than 900° C., such as higher than 1000° C., without melting or burning; optionally, the insulator does not have to withstand temperatures higher than 1500° C.

With reference toFIG. 1d, in an embodiment comprising a thermally insulating area250, the heat exchanger pipe is insulated in said area by both insulation material and a shield252. The insulation material may be mortar or putty, as described above. Furthermore, the shield252may be, for example, a heat resistant piece that is at least partly open at the top, such as a trough or a box. The piece that is at least partly open at the top may be, for example, a metal trough or box. The bending section of the heat exchanger pipe200can be provided inside said piece252, and the mortar or putty can be cast in the box, wherein the mortar or putty will act as the insulator260. Such a configuration is easy to implement, and furthermore, the piece252that is open at the top will shield the insulation material260left between the heat exchanger pipe200and the piece252.

Advantageously, in such a bending insulating area250, the heat exchanger pipe200does not comprise an outer pipe220. This is due to the fact that the heat exchanger pipe is normally made of a straight pipe by bending. During the bending, damage, such as microfractures, takes place particularly at the bending point. If no outer pipe220is used at the point to be bent, the condition of the bent point of the inner pipe210after the bending can be secured more easily than the condition of a structure in which the inner pipe210would enclosed by the outer pipe220.

As can be seen fromFIGS. 1ato 1c, in these embodiments,

at least the inner pipe210extends from said first area122of the wall to the outside of the flow duct115for gases, and

at least the inner pipe210extends from said second area124of the wall to the outside of the flow duct115for gases.

According toFIGS. 1ato 1f, at least a section of the heat exchanger pipe200, particularly the second section240, is arranged in the flow duct155for gases delimited by the walls112,114, and thereby at least a section of said heat exchanger pipe, particularly its second section240, is arranged at a distance from the walls112,114. Such a distance can be, for example, greater than 15 cm, such as greater than 50 cm or greater than 1 m. Consequently, the “heat exchanger pipe200” does not refer to a heat exchanger pipe possibly extending on the wall112,114. A burner typically comprises several heat exchanger pipes of the above described kind, and/or their sections, which constitute a heat exchanger, such as a superheater. However, the heat exchanger is not necessarily a separate unit placed in the flow duct115, because the inner pipe210may also extend outside the flow duct115, thanks to through holes placed in the areas122and124of the wall (or walls). If the areas122and124are opposite or angular to each other, the distance between the areas122and124can be, for example, at least 0.5 m, such as at least 1 m, typically at least 2 m or at least 3 m. In the embodiments according toFIGS. 1ato 1c, the distance between the areas122and124can be, for example, 1 m to 10 m, advantageously 3 m to 6 m. A short distance will secure sufficient stability of the structure; on the other hand, a long distance will secure a sufficient heat recovery capacity. In these embodiments, the length of the second section240can also be, for example 1 to 10 m, advantageously 3 to 6 m, as described above. In these embodiments, the first section202of the heat exchanger pipe is subjected to significant shear forces, because the pipes extend substantially perpendicular to the force of gravity.

Yet some embodiments are shown inFIGS. 1dand 1e. In these embodiments, the first section202of the heat exchanger pipe bends 180 degrees, but the bend is, as shown inFIG. 1c, shielded with an insulator260; in other words, the first section202of the heat exchanger pipe comprises a thermally insulated section250in said flow duct115for gases. Said thermally insulated section250divides said first section202into two second sections: the first second section240and the second section240b. InFIGS. 1dand 1e, the first wall of the device is the top of the device.

In the embodiment ofFIG. 1d,

said first wall112comprises the first area122of the wall of the device, and

the thermal device100comprises insulator255adjacent to the wall and extending from said first area122of the wall of the device to the flow duct115for gases,

said second section240of the heat exchanger pipe extends from said insulator255adjacent to the wall of the device to said thermally insulated section250, and

said insulator255adjacent to the wall is arranged to insulate at least the inner pipe210of the heat exchanger pipe from the flow duct115,

In the embodiment ofFIG. 1e, in turn,

said first wall112comprises a first area122of the wall of the device, and

said second section240of the heat exchanger pipe extends from said first area122of the wall of the device to said thermally insulated section250.

InFIG. 1f, the heat exchanger pipe200comprises two first sections: a first first section202aand a second first section202b. Each first section202a,202bcomprises a second section; for example, the first first section202acomprises a first second section240, and the second first section202bcomprises a second second section240b. InFIG. 1f, the top of the structure acts as the first wall112. The thermal device comprises a nose, and each first section202a,202bextends from the wall112to the nose180. Each first section202a,202bcomprises, at each end, an insulator255,257adjacent to the wall. The second sections240and240bextend between the insulators. The insulator257extends from the nose180to the flow duct for gases. The nose180constitutes a second wall114.

In the embodiments according toFIGS. 1dto 1f, the length of the second section240can also be clearly longer than that described above. For example, the length of the second section can be 1 to 25 m, advantageously 3 to 15 m. In these embodiments, the first section202,202a,202bof the heat exchanger pipe is not subjected to significant shear forces, because the ducts extend at a small angle to the force of gravity.

Preferably, and as shown inFIGS. 1a, 1eand2, said second section240,240bof the heat exchanger pipe extends from said first area of the wall of the device to said flow duct for gases. This gives the advantage that at least the section of the heat exchanger pipe adjacent to the wall is insulated by at least the outer pipe from the flow duct for gases. The outer pipe220has been found to be a solution that is more durable in view of corrosion protection and more serviceable (for example replaceable) than using the insulator255. In addition, the structure can thus be made mechanically even more stable by connecting the outer pipe to the wall, for example by welding.

Such a structure has some technical advantages.

Firstly, the medium layer230insulates the inner pipe210thermally from the outer pipe220. Thus, there is little transfer of heat from the outside to the inner pipe210and further to the heat transfer medium. As a result, heat losses in such a duct take place mostly in the medium layer230and not in the inner pipe210.

Consequently, even if the heat exchanger pipe200is placed in an environment (duct115) in which a very high temperature prevails, wherein the surface temperature of the heat exchanger pipe200rises high, the temperature of the inner pipe210does not become too high in view of the regulations for designing the material of the inner pipe. In a corresponding manner, if the temperature of the inner surface of the inner pipe210is to be raised in order to form a hotter heat transfer medium, the layered structure according toFIG. 1g1can be used, particularly by adjusting the thickness of the medium layer230, to secure that the temperature of the outer surface of the inner pipe210does not become too high in view of the durability of the material. Because the inner pipe210contains heat transfer medium under pressure during the use, the inner pipe210should withstand the respective pressure. It is known that materials are less capable of withstanding pressure at a high temperature than at a low temperature. Said “too high” temperature refers to the temperature at which the inner pipe210is no longer capable of withstanding the pressure prevailing in it. In a corresponding manner, the medium layer230does not need to withstand pressure, because the pressure is taken by the inner pipe210. Moreover, the outer pipe220does not need to withstand pressure. In the flow duct for gases, the first section202of said heat exchanger pipe, or the inner pipe210of the first section202of said heat exchanger pipe is, over its entire length or almost its entire length, insulated from the flow duct for gases by means of said outer pipe and/or an insulator, as presented above. In this way, it is prevented that the temperature of the inner pipe would become too high in view of the prevailing pressure level locally, for example at a noninsulated point. Furthermore, condensing of a corrosive substance on the outer surface of the inner pipe is avoided. The solution may comprise noninsulated areas270as presented above (FIG. 1i). Preferably, however, such areas are only present in the vicinity of other heat recovery surfaces, such as the wall112,114. This has been described in more detail above. Advantageously, the distance from all the points of the noninsulated areas270to the heat recovery surfaces of the thermal device (excluding the heat exchanger pipe200itself) is not greater than 15 cm, more advantageously not greater than 10 cm. At such a point, the temperature of the gases in the flow duct115is typically clearly lower than in the centre of the flow duct.

Secondly, the outer pipe220shields the structures inside it, that is, the medium layer230and the inner pipe210, from corrosion and mechanical wear. The outer pipe220is advantageously a single piece, wherein the outer pipe effectively shields the medium layer230and the inner pipe210from mechanical wear. Such a single-piece outer pipe220is, for example, weldless. In addition or alternatively, such a single-piece outer pipe220is, for example, without holes. Moreover, the outer pipe220can shield the insulation layer230and the inner pipe210over at least the whole length of the flow duct115for gases. Consequently, the second section240of the duct advantageously comprises a single-piece outer pipe220extending over its entire length. Yet more advantageously, such a second section extends over the entire length of the first section202.

Thirdly, because the surface temperature of the outer pipe220is high, as described above, no corrosive substances, such as salts, will condense on its surface. The same also applies to the insulated area250. Salts condense from flue gases onto heat recovery services when the partial pressures of steam in the flue gas exceeds the pressure of saturated steam. The pressure of saturated steam, in turn, is significantly dependent on the temperature. In a combustion process, salts in steam phase are formed in flue gases in such amounts that condensing takes place, typically for example when the temperature of the heat recovery surface is lower than 500° C., lower than 550° C., or lower than 600° C. In a corresponding manner, condensing does not take place if the surface temperature of the heat recovery surface is higher. Advantageously, during the operation, the temperature of the outer surface of the outer pipe220of the heat exchanger pipe200is at least 550° C., at least 600° C., or at least 650° C., such as about 670° C. or higher. In a use of the thermal device,

the heat transfer medium is allowed to flow in said inner pipe in such a way that

the temperature of the outer surface of the outer pipe is higher than 600° C. Furthermore, steam is advantageously used as the heat transfer medium.

As for other noninsulated areas270in the vicinity of the heat recovery surfaces, it is noted that at lower temperatures, the corrosion problem is reduced for the above described reasons.

Fourthly, the structure makes it possible to use fuels having a higher content of heavy metals or chlorine than usual. As presented above, the temperature of the outer surface of the outer pipe220rises high because of the insulation layer230. Thus, the condensing of heavy metals and/or chlorides (e.g. NaCl, KCl) on the outer surface of the outer pipe220is prevented or at least reduced to a very significant extent. Consequently, the boiler100can be used even for long times without maintenance even if the contents of heavy metals and/or chlorides in the flue gases were higher than in the flue gases of boilers of prior art. Further, this enables the application of said fuels in the boiler.

Fifthly, even though the presented layered structure of the heat exchanger pipe200increases the mass of the heat exchanger pipe200, the presented structure will carry the mass of the heat exchanger pipe200, because the second section240of the heat exchanger pipe extends in the flow duct115for flue gases approximately in the same direction over its whole length, or it does not have abrupt bends, as described above in more detail. If the second section240of the pipe twisted in the flow duct115for flue gases, the second section240of the heat exchanger pipe would subject its supporting structures to a relatively high torque, or the flow duct115should be fitted with separate supporting structures. Due to this supporting, the length of the second section240is advantageously relatively short, at least when the second section is horizontal, as will be presented further below.

Advantageously, the ducts210,220have a circular cross section. Pipes of this kind are technically easy to manufacture, and furthermore, they are more resistant to pressure than pipes of other shapes.

The inner diameter of the inner pipe210can be, for example, 30 to 60 mm, such as 40 to 50 mm, advantageously about 45 mm, such as 42 to 46 mm. The thickness of the shell of the inner pipe can be, for example, 4.5 to 7.1 mm. The thickness of the shell refers to the thickness of the wall of the duct, that is, the half of the difference between the outer diameter and the inner diameter. The inner pipe210can comprise for example steel. The inner pipe210can comprise for example ferritic or austenitic steel. Advantageously, the inner pipe210comprises austenitic steel.

The thickness of the medium layer230is advantageously 0.5 to 4 mm, such as 1 to 2 mm. The medium layer may comprise solid, liquid or gaseous medium. The medium layer may comprise at least one of the following: gas (such as flue gas, air, synthesis gas, pyrolysis steam), putty, and ceramics. Advantageously, the medium layer comprises putty, and the thickness of the putty layer is 1 to 2 mm. The putty can be selected, for example, so that the putty is resistant (without burning and/or melting) to temperatures higher than at least 700° C. but possibly not higher than 1000° C.

The inner diameter of the outer pipe220is dimensioned according to the outer diameter of the inner pipe210and the thickness of the medium layer230. Because the medium layer230can comprise gas, increasing the inner diameter of the outer pipe220will increase the thickness of the insulation layer230if the outer diameter of the inner pipe210is not increased in a corresponding way. The inner diameter of the outer pipe220can be, for example, 35 to 70 mm. The thickness of the shell of the outer pipe220can be, for example, 4.5 to 7.1 mm. The outer pipe220can comprise for example steel. The outer pipe220can comprise for example ferritic or austenitic steel. Advantageously, the outer pipe220comprises austenitic steel.

Typically, in a thermal device, such as a boiler, the temperature depends on the location, and particularly the height in view from the furnace110. InFIGS. 1ato 1cand inFIG. 2,

said first section202of the heat exchanger pipe is horizontal, or the longitudinal direction of said first section forms an angle smaller than 30 degrees at its every point with the horizontal plane. The angle can also be, for example, smaller than 20 degrees, smaller than 10 degrees, or smaller than 5 degrees. The term “horizontal” refers to a line in the horizontal plane, such as a pipe curved in the horizontal plane, or a horizontal pipe. The term “every point” specifies that the longitudinal direction of the pipe depends on the point of viewing, if the pipe is not straight.

This gives the advantage that the whole outer pipe220of the second section240of the heat exchanger pipe will be substantially at the same temperature. By the placement of the second section240in the height direction it is possible to make sure that the whole outer pipe is at the same, sufficiently high temperature in view of condensing of corrosive substances. When the whole second section240of the heat exchanger pipe200is placed at substantially the same temperature, it is considerably easier, on one hand, to dimension the structure to enable the production of hot heat transfer medium and, on the other hand, not to exceed or go below the operating temperatures of the materials even locally, than in a situation in which the heat exchanger pipes extended for example vertically (FIGS. 1dand 1e) or in another direction (FIG. 1f). It should be mentioned that even if the second section240(or the second sections240,240b) were horizontal, that section of the pipe200which is outside the flow duct115can extend in another direction, such as the vertical direction, as shown inFIG. 2.

In an advantageous embodiment, the length of the first section202of the heat exchanger pipe200is, for example, shorter than 6 m, wherein the first section202of the heat exchanger pipe200is self-supporting in the horizontal direction as well. Self-supporting refers to a structure which is supported at its ends only. Thus, no separate supporting structures will be needed for the first section202of the pipe in the flow duct115for flue gases. The heat exchanger pipe200, particularly the inner pipe210, is supported to the first and second areas (122,124), from which the inner pipe is conveyed through the wall or walls. The length of the first section202is advantageously not greater than 5 m and more advantageously not greater than 4.5 m. For achieving a sufficient heat transfer capacity, the length of the first section240is advantageously at least 1 m, such as at least 2 m, and more advantageously at least 3 m. The length of the first section240can be, for example, about 4 m. What has been said here about the length of the first section202also applies to the length of the second section240.

Moreover, in the self-supporting structure, there is no need to support the heat exchanger pipe200or its sections in the flow duct115for flue gases. In an embodiment, the first section202of the heat exchanger pipe extends freely in the flow duct115. Thus, the first section202of the heat exchanger pipe is not supported to the rest of the structure, such as the wall (112,114) of the thermal device100, the top of the thermal device100, another heat exchanger pipe200, another first section202bof the same heat exchanger pipe200, or another second section240bof the same heat exchanger pipe200. Such a freely extending structure is technically easier to manufacture than a supported structure. Furthermore, the freely extending structure does not involve supporting structures which would conduct heat to the heat exchanger pipe. Moreover, the presence of supporting structures would make it more difficult to design the suitable operating temperature and to maintain the thermal device.

With the presented solution, it is possible to raise the outer temperature of the outer pipe220of the heat exchanger pipe200so high that no corrosive substances condense on its surface, such as heavy metals and/or alkali salts, particularly sodium chloride (NaCl) or potassium chloride (KCl). During the operation, the temperature of the outer surface of the pipe200is advantageously high, as described above. In a corresponding manner, during the operation, the temperature of the heat transfer medium, such as steam, flowing inside the inner pipe210, is, for example, at least 500° C., such as at least 530° C., and advantageously at least 540° C. In a use of the thermal device,

the heat transfer medium is allowed to flow in said inner pipe210,

steam is used as the heat transfer medium, and

the temperature of the heat transfer medium flowing in the inner pipe210is at least 500° C., preferably at least 530° C.

In such use, the temperature of the inner pipe210is, for example, between 500° C. and 700° C. and advantageously between 500° C. and 600° C.

To achieve these values, some measurements have been presented above. Furthermore, in an embodiment of the thermal device100, the heat exchanger pipe according to the invention is placed in such a way with respect to the other heat exchanger pipes and flow directions that said temperature values are fulfilled. In some embodiments, said first section of the heat exchanger pipe is placed in a desired temperature zone in the thermal device100, by selecting a desired height position for said first section202of the pipe in the thermal device100, such as a boiler.

FIG. 2shows an advantageous way of selecting said desired height position and placing the first section202of the heat exchanger pipe. In this embodiment,

the thermal device100comprises several other heat transfer pipes, such as superheaters154and156, inside the walls of the flow duct115for gases for recovering heat,

said heat exchanger pipe200and said other heat transfer pipes (154,156) constitute a continuous flow duct for the heat transfer medium, for heating the heat transfer medium, and

said flow duct for the heat transfer medium comprises, as its last heat transfer element placed in the flow duct115for gases in the flow direction of the heat transfer medium, a first section202bof said heat transfer pipe200. Because the different first sections can be named as desired, such a first section can be said first section202(not shown in the Figure).

For example inFIG. 2, the flow duct for heat transfer medium comprises superheaters154and156as well as a heat transfer pipe200, e.g. its second sections240and240b. InFIG. 2, the second sections240are also the first sections202; the insulator (255,257) adjacent to the wall is not shown. Thus, a first section (section202bin the figure) of the heat exchanger pipe is exactly the last heat transfer element, such as a heat exchanger pipe or a heat transfer pipe, in said circulation, placed in the flow duct115for gases. From such a first section202b, which inFIG. 2comprises the second second section240b, the heated heat transfer medium is conveyed via the return circulation420to, for example, energy production. After said first section202, the heated heat transfer medium is not conveyed to a heat transfer element (such as a heat transfer pipe or the heat exchanger pipe) in the flow duct for gases.

Another advantageous height position is also realized in the embodiment ofFIG. 1d. In this embodiment,

the thermal device100comprises several other heat transfer pipes152,156inside the walls of the flow duct115for gases, for recovering heat,

said heat exchanger pipe200and said other heat transfer pipes152,156constitute a continuous flow duct for the heat transfer medium, for heating the heat transfer medium, and

said flow duct for the heat transfer medium comprises the last first section202of the heat exchanger pipe placed in the flow duct for gases, in the direction of flow of the heat transfer medium, and at least one heat transfer pipe (such as pipe152inFIG. 1d) placed downstream in the flow duct for gases, in the direction of flow of the heat transfer medium, and

said last first section202of the heat exchanger pipe is arranged, in the flow direction of the gas flowing outside the outer pipe, upstream of said heat transfer pipes (pipes152inFIG. 1d) placed downstream in the flow duct for gases in the flow direction of the heat transfer medium.

For example, the flow duct for heat transfer medium shown inFIG. 1dcomprises superheaters152and156as well as a heat exchanger pipe200, e.g. its second sections240and240b. InFIG. 1d, the first section202comprises the second sections240and240b. Thus, the first section202shown inFIG. 1dis, in the flow direction of the heat transfer medium, the last first section202of the heat exchanger pipe placed in the flow duct for gases. Furthermore, the flow duct for the heat transfer medium comprises a heat transfer pipe152placed downstream of said section202in the flow direction of the heat transfer medium in the flow duct for gases. InFIG. 1d, the last first section202of the heat exchanger pipe, i.e. the first section202, is arranged, in the flow direction of the gas flowing outside the outer pipe220, upstream of said heat transfer pipes152in the flow direction of the heat transfer medium. The flow direction of the gases is illustrated with arrows175. Obviously, the pipe152is placed downstream of the pipe200in the flow direction of the gases.

In such use, the noninsulated heat transfer pipe downstream of the last first section202of the heat exchanger pipe in said medium circulation may be placed, in the flow duct for flue gases, in an area whose temperature is, for example, below 500° C. In addition, when the temperature of the heated medium in said last first section202of the heat exchanger pipe is advantageously at least 500° C., no condensing takes place on the surface of the noninsulated pipe. In a use

heat transfer medium is heated to a first temperature in said first section202of the heat exchanger pipe placed last in the flow duct for gases, in the direction of the heat transfer medium,

at least one said heat transfer pipe152downstream in the flow direction of the heat transfer medium is arranged in an area where a second temperature is prevailing in the flow duct for gases, and

the second temperature is not higher than the first temperature.

Thus, the heat exchanger pipe200with a layered structure, particularly the first section202,202bof the heat exchanger pipe placed last in the flow duct for gases, is arranged in a hotter place than the other heat transfer pipes. In the first section202,202bof the heat exchanger pipe placed last in the flow duct for gases, the heated heat transfer medium is, in such a solution, typically so hot that no significant condensing of corrosive substances will take place on the surface of the heat transfer pipes downstream. If the heat transfer element placed last in the flow duct115for gases, in the flow direction of the heat transfer medium, is a structure of the above described kind, the structure comprises no heat transfer pipes on which corrosive substances would condense downstream.

Advantageously, the heat exchanger pipe200is arranged close to the point of forming heat. For example in a boiler, the distance between the first section202of the heat exchanger pipe200with a layered structure, closest to the grate102(in the flow direction of flue gases), and the grate102can be, on one hand, at least 5 m or at least 10 m, to secure a sufficiently large furnace110. On the other hand, the distance between a first section202of the heat exchanger pipe200with a layered structure and the grate102can be, for example, not greater than 50 m, not greater than 40 m, or not greater than 35 m, to secure the hotness of the environment of the heat exchanger pipe200during the operation. In a corresponding manner, the height of the first section202of the heat exchanger pipe200in the thermal device100above the earth's surface can be, for example, not greater than 50 m, not greater than 40 m, or not greater than 35 m. In a corresponding manner, the height of the first section202of the heat exchanger pipe200in the thermal device100above the earth's surface can be, for example, at least 5 m or at least 10 m.

With reference toFIG. 2, the thermal device according to an embodiment comprises

means300for feeding an auxiliary agent, for feeding an auxiliary agent for the process, such as an auxiliary agent for the combustion process, for example to the furnace or the process area,

a part of which means300for feeding an auxiliary agent is placed in the flow duct115for gases, and

said part of the means300for feeding an auxiliary agent is placed downstream of the first section202or another first section202of said heat exchanger pipe200in the flow direction of gases.

This gives the advantage that the auxiliary agent is only supplied to the flue gases cooled by the heat exchanger pipe200, whereby the effect of the auxiliary agents is improved.

The auxiliary agent is preferably liquid, for example an aqueous solution of a reacting agent. The means300comprise a pipe or the like for feeding the liquid auxiliary agent to the flow duct115for gases, and one or more nozzles310. Advantageously, the feed means300extend through the flow duct115over its entire length in one direction, wherein auxiliary agent can be supplied over substantially the entire area of the flow duct in the direction of its cross section.

The auxiliary agent comprises at least one of the following: ammonia (NH3), ammonium ion (NH4+), ferric sulphate (Fe2(SO4)3), ferrous sulphate (FeSO4), aluminium sulphate (Al2(SO4)3) ammonium sulphate ((NH4)2SO4), ammonium hydrogen sulphate ((NH4)HSO4), sulphuric acid (H2SO4), and sulphur (S), as well as aqueous solutions of these. Advantageously, the auxiliary agent comprises ammonia (NH3) or ammonium ions (NH4+). One way of operating the boiler100is to use said means for feeding auxiliary agent to supply the boiler with an auxiliary agent that comprises ammonia (NH3) or ammonium ions (NH4+). In a use of the thermal device,

said means for feeding an auxiliary agent are used for supplying the thermal device with an auxiliary agent,

the auxiliary agent comprising at least one of the following: ammonia (NH3), ammonium ion (NH4+), (Fe2(SO4)3), (FeSO4), (Al2(SO4)3), ((NH4)2SO4), ((NH4)HSO4), (H2SO4), and sulphur (S), as well as aqueous solutions of these. In an advantageous embodiment, the auxiliary agent comprises ammonia (NH3) or ammonium ions (NH4+).

Further with reference toFIG. 2, an embodiment comprises

a first heat exchanger comprising said heat exchanger pipe200and further several heat exchanger pipes200which comprise some inner pipe210, at least one outer pipe220and a medium layer230remaining between the outer pipe and a section of an inner pipe,

a second heat exchanger comprising several heat transfer pipes,

the first heat exchanger being arranged upstream of said second heat exchanger in the flow direction of gases,

the second heat exchanger being spaced from the first heat exchanger, wherein a space350is left between the second heat exchanger and the first heat exchanger,

part of the means300for feeding an auxiliary agent being placed in the flow duct115for gases, and

said part of the means300for feeding an auxiliary agent being arranged in said space350.

For example, the second heat exchanger can be arranged in the top of the process area110of the thermal device100, as shown inFIG. 2. The second heat exchanger can be, for example, a conventional pipe assembly comprising several heat transfer pipes. In an embodiment shown inFIG. 2, the second heat exchanger is a secondary superheater154.

Obviously, a part of the pipes of the means for feeding an auxiliary agent is placed outside the boiler. Furthermore, it is obvious that other means for feeding an auxiliary agent can be placed in other parts of the boiler.

With reference toFIG. 2, one embodiment of the boiler100comprises

a first section202of said heat exchanger pipe, that is, the first first section202of the heat exchanger pipe,

said heat exchanger pipe comprises a second first section202bextending from one wall (the second wall114,FIG. 2) to the same or another wall (the first wall112,FIG. 2) in the flow duct for gases,

the second first section202bor the inner pipe of said second first section202bbeing insulated over its entire length from the flow duct for gases by means of a second outer pipe and/or an insulator, and

said inner pipe210connecting said first first section of the heat exchanger pipe to said second first section of the heat exchanger pipe outside said flow duct for gases.

In this way it is easy to guide the inner pipe210back to the duct115, and a separate insulated area150is not necessarily needed although the first sections extend straight in the flow duct115.

It is also possible that the second first section202bis only insulated over almost its entire length from the flow duct115, as presented earlier (see alternatives A, A1, A2, and B above). The second first section comprises at least an inner pipe which is, in the above described way, insulated, for at least the most part, from the flow duct115for gases. Furthermore, the second first section may, and advantageously does, comprise a second second section where an outer pipe encloses the inner pipe of the second first section.

InFIG. 2, the first first section202extends from the first area122of the wall of the device to said second area124of the wall of the device in the flow direction of the heat transfer medium, and the second first section202bextends from said second area124of the wall of the device to said first area122of the wall of the device in the flow direction of the heat transfer medium.

As described above, the first first section202comprises the first second section240. Advantageously, the second first section202balso comprises a second second section240b. Furthermore, it would be possible for either of the first sections202,202bto comprise several second sections, as shown inFIG. 1c.

Advantageously, the sections240,240bextend straight in the flue gas duct115. In an embodiment,

said first second section240of the heat exchanger pipe extends straight in the flow duct for gases, wherein said first second section240extends in a longitudinal direction Sx parallel with the flow direction of the medium flowing in the first pipe,

the heat exchanger pipe comprises a second second section240bextending straight in the flow duct for gases, wherein said second second section240bextends in a longitudinal direction −Sx parallel with the flow direction of the medium flowing in the second pipe,

the second longitudinal direction −Sx is opposite to the first longitudinal direction Sx, and

said inner pipe210connects said first first section202of the heat exchanger pipe to said second first section202bof the heat exchanger pipe outside said flow duct115for gases.

Advantageously, only the inner pipe210connects said first first section202of the heat exchanger pipe to said second first section202bof the heat exchanger pipe outside said flow duct115for gases, because the structure will thus become simpler. It is naturally possible that also the outer pipe220extends outside the flow duct115. This solution has the advantage that in this way, the heat exchanger pipe200or a corresponding heat exchanger can be connected to the water circulation of the device100in such a way that the feed and return circulations are on the same side of the boiler, inFIGS. 2 and 5bon the left side. The same effect can also be achieved by using an insulated and bent pipe as shown inFIGS. 1dand 1e. In these embodiments, the thermal device comprises

a feed circulation410of heat transfer medium, for feeding heat transfer medium to the heat exchanger pipe200, and

a return circulation420of heat transfer medium, for returning heat transfer medium from the heat exchanger pipe200, and

the heat exchanger pipe200is connected to the feed circulation410and the return circulation420on the same side of the first wall112of the boiler.

Advantageously, the heat exchanger pipe200is used as the last superheater of the boiler100. Thus, the boiler comprises

means for conveying heat transfer medium from a tertiary superheater156to said heat exchanger pipe200.

At this stage, superheated steam typically acts as the heat transfer medium.

If the thermal device100comprises two or more insulated first sections202of the above described kind in such a way that at least two sections (202,202b) of the heat exchanger pipe are spaced in the flow direction of gases, the sections (202,202b) are advantageously placed downstream in the flow duct for gases; downstream with respect to both the medium and the gases. To put it more precisely, in such a thermal device,

said second first section202bof the heat exchanger pipe is placed downstream of said first first section202of the heat exchanger pipe in the flow direction of the medium flowing in the inner pipe210, and

said second first section202bof the heat exchanger pipe is placed downstream of said first first section202of the heat exchanger pipe in the flow direction of the gas flowing outside the heat exchanger pipe.

For example, inFIG. 2, the second first section202bis placed above the first first section202. When superheated steam passes from the inside of the first first section202to the inside of the second first section202b, at the same time gases flow upwards, that is, from the outer surface of the first first section202towards the outer surface of the second first section202b.

In such an arrangement, both sections202and202bare heated more evenly with respect to each other than in an arrangement in which the sections202,202bwere placed upstream relative to said flows. Said more even heating will reduce thermal stresses caused and will improve durability.

Preferably, the tertiary superheater156is also directed downstream, as shown inFIG. 2. The flow direction of heat transfer medium flowing from the tertiary superheater156is illustrated with an arrow405. Superheated steam from the return circulation of the tertiary superheater156is conveyed further to the feed circulation410of the heat exchanger pipe200with a layered structure.

During the operation of the thermal device, the heat transfer medium and the flue gas flow in the above described way. At other times, the heat transfer medium and the flue gas in the boiler100are arranged to flow in the above described way. The flow direction from the thermal device is obvious for a person skilled in the art. The heat transfer medium flows from the input to the use, such as to power production or to the use of heat. Gases flow from the process area to the use, such as to heat recovery or discharge.

In the embodiment shown inFIG. 2,

the wall of the thermal device, such as a boiler, comprises a nose180, and

said first section202of the heat exchanger pipe extends from said nose180.

InFIG. 2, the nose180comprises the second area124of the wall of said device. Areas and walls can be named freely, whereby the nose could alternatively comprise said first area122of the wall of the boiler. Furthermore, the first wall112of the boiler can comprise the nose180, or another wall of the boiler can comprise the nose180.

When the nose180comprises said first122or second124area of the wall of the device, the span of the first section202(or202b) of the heat exchanger pipe200becomes shorter, because the nose180extends from the wall of the boiler towards the flow duct115for gases. In this way, the nose forms a protrusion in the wall, extending into the flow duct for gases. The nose makes the flow duct for gases narrower. The shorter span stabilizes the structure of the heat exchanger pipes200. Above, advantageous lengths were presented for the first section202and the second section240of the heat exchanger pipe200, the length corresponding to said span.

FIG. 3ashows a way of connecting the heat exchanger pipe200to the first wall112of the thermal device100in the first area122of the wall. A corresponding connection can be provided in the second area124of the wall.FIG. 3ashows the first area122of the wall, and its vicinity, in a side view.

The wall112of the boiler shown inFIG. 3acomprises heat transfer pipes510for recovering heat. In the first area122, inner pipes210ato210fare introduced through the wall and arranged, on the side of the flow duct for flue gases, inside the outer pipes220a,ato220a,fand220b,ato220b,fin the above described way. Thus, the outer pipes belong to the first second sections240a,xand the second second sections240b,x, where x is a, b, c, d, e, or f. In a corresponding manner, the inner pipe210xis divided into a first first section202a,xand a second first section202b,x. At least part of the first sections202a,xand202b,xare enclosed by an outer pipe220a,xor220b,x, respectively, in the above described way. Because the outer pipes are connected to the areas122,124and the temperature of said areas is lower than the temperature in the flow duct115, the temperature of the outer pipes220will increase when moving from the vicinity of the area122,124to the central parts of the flow duct. This will result in a temperature gradient in the outer pipe, and said temperature gradient may impair the service life of the outer pipe220.

In the embodiment shown inFIGS. 3aand3b,

the first122or second124area of the wall of the thermal device100comprises a housing450,

which housing450protrudes from the wall of the thermal device, for example from the first112or second114wall, outwards from said flow duct115for gases, the housing450comprising a through hole for conveying said inner pipe210,210ato210eout from the reaction area110of the thermal device110, such as from a furnace110of a boiler or from the flow duct115for gases, andthe inner surface of the housing450being provided with said outer pipe220,220ato220efor shielding the inner pipe210of the heat exchanger pipe200and optionally the medium layer230,insulator255,257adjacent to the wall, extending from the inner surface of the housing450to the reaction area110of the thermal device or to the flow duct115for gases; orsaid noninsulated area470of the first section202(seeFIG. 1i) extending from the inner surface of the housing450to the reaction area110of the thermal device or to the flow duct115for gases.

Preferably, the outer pipe220is tightly fastened to the inner surface of the housing450so that the flue gases of the flue gas duct115cannot contact the insulation layer230or the inner pipe210. The outer pipe can be, for example, welded to the housing450.

The housing450can also be applied in the embodiments shown in FIGS.1band1c. Thus,

the insulator255adjacent to the wall extends from the inner surface of the housing450to the flow duct115for gases, for shielding the inner pipe of the heat exchanger pipe.

Furthermore, as shown inFIGS. 1iand 3b, it is possible that the noninsulated area270of the inner pipe210is placed in the housing450.

When the housing450protrudes from the wall of the boiler in the above described way, the flow of gases in the housing450is very slow compared with the flow in the flow duct115for gases. Thus, very little corrosive condensation takes place in the housing. Firstly, because the flow is very slow, the amount of gas from which condensation can take place, is reduced. Thus, the condensing is reduced as well. Secondly, because heat is recovered from the gases in the housing, too, the gas in the housing will cool down to a lower temperature than the gas flowing in the flow duct115. In such colder ranges, corrosion is slow, as described above.

Furthermore, the temperature in the housing450increases from the edge area towards the flow duct115. In the embodiment with the housing, the temperature of the outer pipe220increases over a clearly greater length of travel than in a situation in which there is no such protruding housing. The greater length of travel, in turn, means a lower temperature gradient, which increases the service life compared with an embodiment without said housing. To reduce corrosion and to sufficiently reduce the temperature gradient, the depth L of the housing (FIG. 3b) can be, for example, at least 10 cm, more advantageously at least 15 cm or at least 20 cm.

FIG. 3bshows a principle view of the situation ofFIG. 3aseen from above. InFIG. 3b, a distance d is left between the inner surface of said housing450and the outer surface of said outer pipe220, wherein said outer pipe220(and thereby also the inner pipe210) is thermally insulated from the boiler wall. The distance d can be, for example, at least 1 mm, at least 5 mm, or at least 10 mm. As presented above, the inner pipe210in the housing can, in some embodiments, be insulated by means of an insulator255,257adjacent to the wall (FIGS. 1b, 1c). In this embodiment, a distance d is advantageously left between the inner surface of the housing450and the outer surface of said insulator255,257, wherein said insulator is also thermally insulated from the housing. Also in this case, the distance d can be, for example, at least 1 mm, at least 5 mm, or at least 10 mm. Furthermore, in an embodiment in which part of the inner pipe is noninsulated, a distance d is left between the inner surface of said housing450and the noninsulated area470. Thus, the inner pipe210is thermally insulated from the wall of the thermal device. Such a distance will further thermally insulate the heat exchanger pipe200from the wall (112,114) of the boiler and increase the expected service life, i.e. the probable service life, of the heat exchanger pipe200. Such a distance will thermally insulate the heat exchanger pipe200from the wall (112,114) of the boiler, because a thermally insulating medium is thus left between the heat exchanger pipe200and the boiler wall (112,114). As will be presented further below, the distance d is not necessarily constant, if, for example, the inner surface of the housing450is curved. The distance d refers to the shortest distance from the outer surface of the outer pipe220or the insulator260to the line segment formed as the housing450coincides with that wall of the boiler, from which the housing450protrudes (e.g. the first wall112, seeFIGS. 4aand 4b). Put more broadly, the distance d is the distance between the outer surface and the wall112of the device100at the end of the housing450on the side of the flow duct115.

Advantageously, at least one of the walls of the housing450does not comprise the heat exchanger pipe510, to maintain a high temperature of the housing. This will further reduce said temperature difference. For technical reasons relating to the construction, one heat transfer pipe510′ which in the normal design would extend in the wall112, can be moved aside, out of the way for the housing450and the heat exchanger pipes200(210,220). Advantageously, as shown inFIG. 3b, a distance is left between such a heat transfer pipe510′ moved aside and the housing450, for thermally insulating the housing from said heat transfer pipe as well. This distance d2(FIG. 3b) can be, for example, at least 1 mm or at least 2 mm, such as at least 5 mm.

The presented housing450can also be applied in connection with such a heat exchanger pipe which does not comprise the outer pipe at all but only the first, at least partly insulated part. The presented housing450can also be applied in connection with a heat exchanger pipe that does not comprise a substantially straight outer pipe. Such a thermal device comprises

at least a first wall delimiting a flow duct for gases, and

a heat exchanger pipe comprising at least an inner pipe, at least the first section of said heat exchanger pipe being placed in said flow duct for gases and extending, in said flow duct from gases, from said first wall to said first wall or another wall delimiting the flow duct for gases, in such a way that

the inner pipe210of the first section202of said heat exchanger pipe is, in some parts, insulated from the flow duct115for gases by means of said outer pipe220and/or an insulator260, and

the inner pipe210of the first section202of said heat exchanger pipe is noninsulated from the flow duct115for gases in one or more noninsulated areas270(FIG. 1i) in such a way that

the length of even the largest noninsulated area270of the first section202does not exceed 15 cm; advantageously, the length of even the largest noninsulated area270of the first section202does not exceed 10 cm, the length being measured in the longitudinal direction of the inner pipe; or

the distance from all the noninsulated areas270of the first section202to all the other heat recovery surfaces of the thermal device (other than the heat exchanger pipe200itself) is not greater than 15 cm, advantageously not greater than 10 cm; or

the first section of said heat exchanger pipe or the inner pipe of the first section of said heat exchanger pipe is insulated over its entire length from the flow duct for gases by means of an outer pipe and/or an insulator.

the wall of the thermal device comprises a housing,

the housing protruding outwards from the wall of the thermal device, seen from the flow duct for gases,

the housing comprising a through hole for conveying said inner pipe out of the process area of the thermal device or from the flow duct for gases.

Said outer pipe can be connected to the inner surface of the housing. Insulator adjacent to the wall may extend from the inner surface of the housing to the flow duct for gases, for shielding the inner pipe of the heat exchanger pipe.

FIGS. 4aand 4bshow some embodiments of the housing450seen from above. In the figures, the wall452of the housing constitutes a flexible structure in the housing450, arranged to receive the thermal expansion of the thermal device100and the heat exchanger pipe200.

For exampleFIG. 4ashows a housing450in a principle view from above. In the embodiment ofFIG. 4a,

at least one wall452of said housing450forms at least two bends455, wherein

said wall452of the housing450constitutes a flexible structure in the housing450, arranged to receive the thermal expansion of the thermal device100, such as the boiler100and the heat exchanger pipe200.

Further,FIG. 4bshows an embodiment which receives the thermal expansion in a more efficient way. In the embodiment ofFIG. 4b,

at least one wall452of said housing450forms at least one fold460which deviates from the line of the wall of the housing450, wherein

said fold460constitutes a flexible structure in the housing450, arranged to receive the thermal expansion of the thermal device, such as the boiler100and the heat exchanger pipe200. The fold460converts the housing450into bellows, i.e. a tubular structure that becomes shorter and longer when pressed and pulled, respectively. The length of such a bellows-like housing450is arranged to change by the effect of thermal stresses.

The line of the wall of the housing450refers to a plane that is best fitted to the shape of the wall of the housing (with a fold). When the wall of the housing comprises a fold460, it comprises at least three bends455(not shown with reference numerals inFIG. 4b).

InFIG. 4b, the housing450protrudes (deviates outwards) from the first wall112of the thermal device100. Furthermore, the fold460protrudes from the line of the wall452of the housing450in such a way that the fold460extends in parallel with said first wall112. Instead of protruding, the fold could deviate inwards into the housing450from the line of the wall452. Furthermore, in the case of at least two folds, the first fold460can deviate outwards (protrude) and the second one inwards. InFIG. 4b, both walls of the housing450presented comprise two folds460.

Above, receiving the thermal expansion of the thermal device100and the heat exchanger pipe200refers to the fact that even if the heat exchanger pipe200and the thermal device100(such as a boiler, for example a boiler wall) expand to a different extent due to the different operating temperatures and/or different heat expansion coefficients of the thermal device100and the heat exchanger pipe200, no significant thermal stresses are formed in the structure because the structure is flexible, i.e. receives the thermal expansion. In such a structure, at least part of the wall452of the housing450is arranged to bend as a result of thermal stresses. When the wall452of the housing comprises a bend, as a result of thermal expansion the bend is straightened out or curved more, which requires considerably smaller stresses than, for example, expanding or compressing the straight wall of the housing450in the direction of the wall of the housing.

FIG. 5shows yet another advantageous embodiment in a boiler.FIG. 5shows a side view of a heat exchanger comprising heat exchanger pipes of the above described kind, and parts thereof. Part IIIa ofFIG. 5has been presented above in connection withFIG. 3a. The embodiment comprises several inner pipes210ato210f. Each inner pipe comprises a first first section and a second first section; for example, the inner pipe210fcomprises a first first section202a,fand a second first section202b,fThe first sections202a,fand202b,fconsist of the described second sections240a,fand240b,f(respectively); in other words, the second sections extend straight and comprise the outer pipes220a,fand220b,frespectively.

The heat exchanger pipe (such as the pipe200) extends from the first wall112to the opposite wall114of the boiler. InFIG. 5, the heat exchanger pipe extends from the first wall112of the boiler to the nose180of the opposite wall114, as shown inFIG. 2. The heat exchanger shown inFIG. 5comprises several heat exchanger pipes200with a layered structure, shown inFIG. 1b, extending straight in the flow duct115for gases and bending outside the flow duct115, in this case inside the nose180(cf.FIGS. 2 and 3a).

A housing450ais provided in the first area122for conveying inner pipes220, such as the inner pipe210f, from the outside of the flow duct115for flue gases to the flow duct115. Furthermore, on the side of the flow duct115, the inner pipes are provided inside the outer pipes220, such as the outer pipes220a,fand220b,f, as presented above. In a corresponding manner, a second housing450bis provided in the second area124, for conveying the inner pipe210out from the side of the flow duct115into the nose180. The second housing450bcomprises two folds460bfor receiving thermal expansion.

InFIG. 5, several inner pipes220are conveyed through via the same housing. It is also possible to provide a single housing for each through hole for one pipe. Such a single housing can comprise, in the above described way, at least two bends455, such as a fold460. This arrangement provides the advantage that at an uneven operating temperature, each heat exchanger pipe200can expand in a different way because each single housing will receive the thermal expansion of each single pipe section240,240b.

The embodiment ofFIG. 5can also be implemented in a more general thermal device. In general, the thermal device shown inFIGS. 1 to 5can be, for example, one of the following types:

a pyrolysis reactor,

a gasification reactor, or

a boiler, such as a fluidized bed boiler, for example a bubbling fluidized bed boiler or a circulating fluidized bed boiler; preferably a bubbling fluidized bed boiler.

In addition to the thermal device, a method has been presented above for heating a heat transfer medium. The method comprises:

producing gas heated by the thermal device100,

conveying said gas to a flow duct115for gases,

introducing heat transfer medium to a heat exchanger pipe200, at least a first section202of said heat exchanger pipe being placed in the flow duct115for gases and extending, in said flow duct115for gases, from the wall (112,114) of said flow duct to the same (112,114) or another (114,112) wall of said flow duct115, said first section202of the heat exchanger pipe comprising a second section240of the heat exchanger pipe, extending in said flow duct115for gases, and

recovering heat into the heat transfer medium by means of said heat exchanger pipe200.

In the method, the heat exchanger pipe200used for recovering heat is such that said second section240of the heat exchanger pipe200comprises

at least part of an inner pipe210for transferring heat transfer medium from the first end to the second end of the part of the inner pipe, and for recovering heat by the heat transfer medium,

an outer pipe220radially enclosing said part of the inner pipe210, and

a medium layer230placed between said outer pipe and said part of the inner pipe in the radial direction, and

the inner pipe210of the first section202of said heat exchanger pipe is, in some parts, insulated from the flow duct115for gases by means of said outer pipe220and/or an insulator260, and

the inner pipe210of the first section202of said heat exchanger pipe is noninsulated from the flow duct115for gases in one or more noninsulated areas270(FIG. 1i) in such a way that

the length of even the largest noninsulated area270does not exceed 15 cm; advantageously, the length of even the largest noninsulated area270does not exceed 10 cm; the length being measured in the longitudinal direction of the inner pipe; or

the distance from all the points of the noninsulated areas270to the other heat recovery surfaces of the device (other than the heat exchanger pipe200itself) is not greater than 15 cm, advantageously not greater than 10 cm; or

the first section202of said heat exchanger pipe200, or the inner pipe210of the first section202of said heat exchanger pipe200is, over its entire length, insulated from the flow duct115for gases by means of said outer pipe240and/or an insulator260.

In an advantageous embodiment of the method, the thermal device comprises several other heat transfer pipes inside the walls of the flow duct for gases, for recovering heat. Said heat exchanger pipe and said other heat transfer pipes constitute a continuous flow duct for the heat transfer medium, for heating the heat transfer medium.

In such an embodiment,

said flow duct for heat transfer medium comprises a first section of said heat exchanger pipe as the heat transfer element placed last in the flow duct for gases in the flow direction of the heat transfer medium, or

said flow duct for the heat transfer medium comprises the first section of the heat exchanger pipe placed last in the flow duct for gases, in the flow direction of the heat transfer medium, and at least one heat transfer pipe placed downstream in the subsequent flow duct for gases, in the direction of flow of the heat transfer medium, and

said first section of the heat exchanger pipe placed last is arranged, in the flow direction of the gas flowing outside the outer pipe, upstream of said heat transfer pipes placed downstream in the flow duct for gases in the flow direction of the heat transfer medium.

In an advantageous embodiment of the method, said second section240of the heat exchanger pipe extends in a straight line or bends less than 90 degrees.

In an embodiment of the method, said second section240of the heat exchanger pipe bends at least 90 degrees.

Features of the method relating to temperatures have been presented above in connection with the use of the device. Features of the method relating to the supply of auxiliary agent have been presented above in connection with the use of the device. Technical features of structures used in the method have been presented above as features of the thermal device.