ES2611304T3 - A steam boiler comprising a radiation element - Google Patents

Abstract

A steam boiler (1) comprising at least one water and/or steam transport element (30, 60) that is heated by the hot exhaust gases in the boiler, wherein the steam boiler (1) comprises at least one radiating element (70) therein; said radiation element (70) is an uncooled element; said radiating element (70) is disposed in the flow of hot exhaust gases (12), so that it is convectively heated by the exhaust gases; said radiating element (70) is located at a predetermined distance from said at least one water and/or steam transport element (30, 60), wherein said predetermined distance is adopted such that the flow of exhaust gases between the radiation element and the water and/or steam transport element occurs without difficulty, and in such a way that the water and/or steam transport element is heated by the radiation coming from the radiation element, characterized in that the radiation element (70) is formed by an alloy based on Fe or Ni and contains Al and which, when subjected to heat in an atmosphere containing oxygen, forms a protective alumina layer on its outer surface.

Description

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DESCRIPTION

A steam boiler comprising a radiation element Technical Field

The present invention relates to a steam boiler according to the preamble of claim 1. Background

Power plants with steam boilers are provided with elements that carry water and / or steam, such as steam pipes, on the boiler walls and also often superheaters to convert the saturated steam that is produced in the steam boiler Dry superheated that is efficient for power generation.

The global principle of operation of a boiler that produces hot water or steam is the generation of hot exhaust gases by burning a fossil fuel or a renewable fuel to generate hot water or steam that in the case of an energy plant is converted into electricity. The hot gas transfers the thermal energy through heat exchangers to hot water or pressurized value that can be used for domestic or industrial heating, in industrial processes, or in an energy boiler to generate electricity. The heat transfer in the final stage in the latter case is usually carried out in superheaters that typically consist of tube assemblies arranged in the energy boiler. During operation, the hot water or steam that is produced in the boiler wall pipes is redirected back to the boiler through superheater pipes. The hot exhaust gases coming from the boiler heat the superheater pipes mainly in a convective manner and the heat is conducted through the tube walls of the steam superheater flowing therein. The steam temperature is thereby increased so that superheated dry steam is produced.

Therefore, in conventional boilers, the transfer of heat from the exhaust gas to the boiler walls or superheaters is mainly restricted to convective heating from the hot exhaust gases in the boiler. Therefore, the boiler walls and superheaters must have a large surface area in order to achieve an effective heat transfer from hot exhaust gases to water or steam. As a consequence, conventional boilers and superheaters are associated with the problem of high material costs. An additional problem is that the boiler walls and bulky superheaters increase the total size of the power plant which results in high construction costs for the power plant.

An example of such a boiler is shown in document US4325328 which shows a steam boiler that is constructed from exchange tubes that define four enclosure walls and a partition, also manufactured from interconnected pipes, which is located within the walls. enclosure so that two combustion chambers are defined. The water is evaporated in steam on the walls and in the partition. The steam is then conducted through a stage heating surface for the first superheat and then through an additional superheater.

It is also known to provide exhaust gas baffles and baffles to control the flow of gas in the boilers.

Document US4226279 our fins that are welded to the tubes in a steam boiler to prevent the holflan particles from moving laterally over the steam pipes. Document US4226279 is aimed at overcoming the problem of warping and bulging of the baffle plates in the boilers. This problem is solved in document US4226279 by welding the fins directly to the tube section, so that the fins are cooled by the fluid flowing through the tube.

The document GB10233 aims to solve the problem of achieving a uniform gas flow in the boiler tubes. According to GB10233 this is achieved by introducing the baffle plates that have a resistance to gas flow in some positions in the boiler and therefore the gas is forced to flow uniformly into the boiler tubes. In GB10233 the baffle plates are in contact with the water tube or, alternatively, the baffle plates are arranged perpendicular to the gas flow. This restricts the flow of gas on the baffle plates and as a result, the baffle plates do not adopt a temperature much higher than the water tube.

JP49104001 shows baffle plates that are arranged at a considerable angle to the gas flow in order to collect the hollfn particles in a part of a boiler.

Additional attempts have been made to increase heat transfer with superheater tubes and to increase combustion efficiency. One such design marketed is the circulating fluidized bed boiler in which a heat transfer of 250-300 kW / m2 ° C is achieved by circulating hot solid particles (sand and ash) in the boiler volume and also around superheater pipes. Without

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However, this method is related to a high complexity and cost of the boiler and the increased wear of the superheater pipes and other parts of the interior of the boiler.

A similar method is described in document FR1154090 which describes a boiler in which the steam flows through a superheater-shaped plate and is heated by heat radiation of the hot hollow particles in the exhaust gases.

Therefore, it is an objective of the present invention to achieve an efficient energy boiler. A further objective of the present invention is to achieve an effective steam boiler from the point of view of the cost. Still another objective is to achieve a boiler in which emissions in the form of NOx gases and unburned hydrocarbons are reduced.

Compendium of the Invention

According to the invention, at least one of these objectives is achieved by means of a steam boiler comprising at least one water and / or steam transport element that is heated by the exhaust gases in the boiler, characterized in that the steam boiler comprises at least one radiation element therein; said radiation element is an uncooled element; said radiation element is arranged in the flow of hot exhaust gases that is conveniently heated by the exhaust gases; said radiation element is located at a predetermined distance from said at least one water and / or steam transport element, such that the flow of hot exhaust gases between the radiation element and water transport element and / or Steam is heated by heat radiation from the radiation element.

The general principle of the invention can be explained as follows; During the operation of the steam boiler, both the water and / or steam transport tubes and the radiation element are conductively heated by the exhaust gases and subsequently both the water and / or steam transport element and the water element. radiation, when geometric assumptions are applied, will emit heat energy in accordance with the Stefan-Boltzman Law:

where

P is the radiated energy per surface area of the radiation element in W / m2

9 is a geometry factor that depends on the amount of radiation element self-emission, between 0 and 1 a is 5.67 10-8 W / m2

£ is the surface emissivity (typically between approximately 0.7 and 0.9 for oxidized surfaces) and Te and Ts are the temperature of the radiation element and surrounding in ° K

image 1

However, the difference between the radiation element and the pipes is that the water and / or steam transport tubes are cooled by the fluid flowing through them, while the radiation element is an uncooled element. Therefore, no steam or water flows circulate in the radiation element.

Since the radiation element is located in the hot exhaust gas and is not cooled by steam, it will be heated to a temperature much higher than that of the steam transport tubes.

In the operation of the steam boiler, even the temperature of the hottest components in contact with the steam or water, namely the temperature of the outer surfaces of the steam transport tubes, is typically no more than 50 ° C higher than the maximum temperature of the steam flowing through the tube, which typically is not greater than 400-650 ° C. The radiation element, on the other hand, adopts a temperature closer to that of the exhaust gases that is typically between 800-1250 ° C. The radiation element, therefore, when the temperatures have reached a stable value, will emit more heat than the steam transport tubes, typically of the order of 3 to 10 times more heat per surface area. This heat will be largely absorbed by the steam transport tubes.

Due to the radiation element, the transfer of total heat to the superheater pipes and / or the boiler walls increases in comparison with conventional superheaters that are mainly heated by convection. Due to the heat transfer per unit of increased area, the size of the boiler wall and superheaters can be reduced while maintaining the efficiency of the boiler.

The invention also provides additional advantages.

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Emissions in the form of NOx gases and unburned hydrocarbons are reduced in the exhaust gases because the surfaces of the radiation elements and the water and / or steam transport elements of the boiler are heated to a temperature at which gas / surface reactions are kinetically stimulated. Therefore, due to the high temperature of the radiation element, chemically more active surfaces are arranged where the exhaust gases can react to more stable components.

In conventional boilers, the exhaust gases condense on the relatively cold surfaces of the boiler pipes and form large amounts of deposits. In the boiler of the invention, these problems are minimized since the preceding heat radiation of the radiation elements provides a possibility to control the location of the condensation of deposits in the regions of the boiler where the consequences are minor. . It is also believed that the introduction of heat radiation on the surface of condensed deposits can alter or even dissolve condensed coatings. The increased radiation in a part of for example the steam pipes in the boiler wall can raise the surface temperature to a level where the unwanted components will not condense.

In the present invention it is important that the radiation elements are arranged in such a way that the exhaust gases flow without difficulties in the radiation element, in the steam transport tubes and also flow without difficulties in the space between the radiation elements. and steam transport tubes. The smooth flow of the exhaust gas is important to ensure that as much heat as possible is conveyed by convection from the hot exhaust gases to the steam transport tubes. The flow without difficulty in the radiation elements is also important to ensure that the convective heating of the radiation element is maximized, because the higher the temperature that the radiation element adopts, the more heat energy will be transferred by heat radiation from the radiation element to the steam transport tubes.

By "without difficulty" flow is meant that the exhaust gas flows freely over the surfaces of the radiation element and the surfaces of the steam transport tubes and that the flow of the exhaust gas is not restricted to any path between the exhaust element. radiation and steam transport element. Therefore, the radiation element should be arranged at a predetermined distance from the steam transport tubes, that is, in such a way that there is an open space between the radiation element and the steam transport tubes.

The predetermined distance between said at least one radiation element and said at least one vapor and / or water transport element is therefore preferably adopted so as to allow a smooth flow of the exhaust gases between said radiation element and said steam and / or water transport element. Preferably, the predetermined distance between the radiation element and the steam and / or water transport element is large enough to ensure that the flow of the exhaust gases between the latter is sufficient to generate a positive heating effect of the gases. Exhaust and water and / or steam transport element. The predetermined distance should be optimized for each specific case, depending on the boiler pressure conditions, the exhaust gas temperature, etc. It is preferred that the radiation element is not too distant from the water and / or steam transport element, in order to provide a positive heating effect of said radiation element. A maximum distance of approximately 500 cm, preferably 250 cm, more preferably 100 cm, even more preferable 60 cm, and most preferably, 30 cm is conceived for most applications.

According to the invention, a minimum distance between the radiation element and the water and / or steam transport element is 20 cm, preferably 10 cm, preferably 5 cm, more preferably 1 cm, more preferably 2 cm, even more preferably 5 mm, most preferably 3 mm. This achieves a sufficient flow of exhaust gases for most conceivable boiler applications. The separation is open and will allow the flow of exhaust gases between said elements.

To ensure maximum convective heating of the radiation element, it is also important that the radiation element is arranged in the flow of the exhaust gas, so that all sides of the radiation element are exposed to the flow of exhaust gas, i.e. there should be a flow of exhaust gas on all sides of the radiation element. The radiation element, therefore, should be arranged at a predetermined distance from other parts of the boiler, for example the circumferential walls, so that there is an open space between all the lateral surfaces of the radiation element and the other parts of the boiler. Preferably, the radiation element is arranged at a distance of at least 5 mm, preferably at least 1 cm, more preferably at least 5 cm, even more preferably at least 10 cm from the boiler parts that are not part of the radiation element. .

To maximize the transfer of convective heat to the steam transport tubes it is also important that the radiation element does not deflect the flow of the exhaust gas away from the steam transport tubes. Therefore, the radiation element should be arranged so that the exhaust gas can flow past the radiation element in a direction towards the vapor transport element without essentially changing the main direction of flow. Thus, the radiation element should be arranged so that the exhaust gases can flow in a direction of constant flow over the radiation element.

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The "main flow direction" is the flow direction from the burn section to the gas outlet of the boiler, or from a gas inlet to the gas outlet.

To prevent deflection of the gas flow, the radiation element is preferably arranged so that it extends in a direction that is essentially parallel to the main flow direction of the exhaust gases.

According to a first embodiment, the radiation element is a sheet, such as a flat steel sheet. An advantage with a flat sheet is that it has a larger surface area in relation to weight. This is advantageous for radiation efficiency. The steel sheet should be arranged so that one normal to its large lateral surfaces is perpendicular to the direction of the gas flow, that is, that the lateral surfaces of the steel sheet are parallel to the flow direction. The steel sheet should also be arranged so that its edge portion, which is relatively narrow, and therefore has low resistance to gas flow, faces the gas flow.

It is also possible that the radiation element is a corrugated steel sheet, that is, with a meander shape.

The radiation element may also be a flat or wavy strip (typically 1-20 cm wide). The radiation element should be as thin as possible to minimize weight. However, to ensure thermal stability and to avoid rapid failure due to corrosion, the thickness of the radiation element should be at least 0.5 mm. Typically, the thickness of the radiation element is 0.5-20 mm, preferably 1.5-10 mm. The length and weight of the radiation element are selected depending on the application in question.

According to a second embodiment, the radiation element is an elongated bar element. The bar element could have a circular cross-section, such as a round bar or a cable or a thick-walled tube. It could also have a rectangular cross section. The radiation element in the form of a bar should be arranged so that the longitudinal axis of the bar element is parallel to the gas flow and such that the normal to the longitudinal axis of the bar element is perpendicular to the gas flow.

It should be appreciated that the radiation element is a separate, individual element, which is deliberately arranged in the boiler in order to increase the efficiency of the boiler by emitting heat to the steam and / or water transport elements.

In order to maximize the convective heating of the radiation element, it is preferred that the radiation element be extended in the flow of the exhaust gases from an upstream side of the steam transport tubes to the downstream side thereof. A superheater arrangement is preferred if the radiation element is disposed in an upstream portion of the superheater arrangement. The reason for this is that the exhaust gases are cooled when they flow over the steam transport tubes. In order to expose the radiation element to the hottest gas, preferably it should be disposed as far as possible upstream water relative to the steam transport tubes. If the radiation element is disposed in a downstream part of the superheater arrangement, it will be exposed to lower temperature exhaust gases and the convective heating of the radiation element will not be effective.

The radiation element should preferably be attached to a surface in the boiler having a temperature as close as possible to the radiation element, preferably the same temperature. This avoids temperature gradients in the radiation element that would lead to mechanical stresses and the bulging and cracking of the radiation element. Therefore, it is suitable to attach the radiation element to a part of the boiler roof that is not covered with steam pipes.

Preferably, the radiation element is flexible, that is, it is mobile, is attached to the boiler so that the radiation element can be moved and is subjected to thermal extension during heating. The advantage thereof is that the accumulation of mechanical stresses is avoided in the radiation element.

Preferably, the radiation element comprises fasteners in the form of hooks or rings, so that the radiation element can be hung on, for example, a boiler bar. Hooks or rings allow the radiation element to move during thermal expansion.

When the radiation element comprises hooks or rings it can be hung directly on a steam and / or water transport element, such as a superheater tube. The hooks or rings allow the radiation element to move and with it the mechanical stresses due to the temperature differences between the radiation element and the steam and / or water transport element are avoided. Preferably, the fastening examples are manufactured from cables (for example 1-5 mm thick) to minimize heat transfer between the radiation element and the steam and / or water transport elements.

The function of the hooks of the fasteners is purely mechanical and does not contribute to the radiation function.

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Preferably, the radiation element is attached to the boiler so that at least one end of the radiation element is free. This allows the expansion of the radiation element, usually the heat expansion and the slow sliding elongation.

According to an alternative, the steam and / or water transport element is at least one steam transport superheater tube.

According to an alternative, said steam and / or water transport element are the water and / or steam transport tubes in at least a part of the boiler wall.

According to an alternative, said water and / or steam transport element is a double wall lining.

Preferably, the surface area of the part of the radiation element facing said at least one water and / or steam transport element is at least 3 percent of the outer surface area of said at least one transport element of water and / or steam.

According to an alternative, said radiation element comprises a concave surface that is turned towards a water and / or steam transport element. Therefore, the radiation from the radiation element is focused on the water and / or steam transport element, in particular if the latter has a tubular shape and is therefore partially enclosed by the radiation element. Preferably, the concave surface is a surface of a folded sheet forming said radiation element. Preferably, the radius of said concave surface is within the range of 0.8-2 times, preferably 1.0-1.5 times the radius of the water and / or steam transport element in the event that the latter is tubular .

The radiation element is formed by an alloy based on Fe or Ni and contains Al and which, when subjected to heat in an oxygen-containing atmosphere, forms a protective alumina layer on its surface. Such steel has the advantage of having superior heat resistance and presenting a long life as elements in the harsh environment generated in a boiler.

According to a particularly preferred embodiment, the radiation element is formed by a steel containing between 10-30% by mass, preferably 15-25% by mass of Cr, 2-7% by mass of Al, Compensation Fe e inevitable impurities. Such steel has excellent heat resistance, corrosion resistance and ability to generate a protective alumina layer when subjected to heat in an oxygen-containing atmosphere. Preferably, the steel should be subjected to temperatures of 700 ° C, preferably 1050 ° C or higher to obtain such a protective alumina layer.

According to yet another embodiment, the radiation element is formed by steel containing between 10-30% by mass, preferably between 15-25% by mass of Cr, 2-7% by mass of Al, 1-4% by mass. Mass of Mo, Compensation Faith and unavoidable impurities. The presence of Mo in this steel contributes to an improved heat resistance.

According to yet another embodiment, the radiation element is formed of steel containing between 10-30% by mass, preferably 15-25% by mass of Cr, 2-7% by mass of Al, 1-4% by mass Mo, 0.01 -1.0% by mass of rare earth metals (REM), compensation Fees and unavoidable impurities. REM contributes to improved corrosion and oxidation resistance.

According to yet another embodiment, the radiation element is formed by steel containing between 10-30% by mass, preferably 15-25% by mass of Cr, 2-7% by mass of Al, 1-4% by mass. of Mo, 0.01 -1.0% by mass of rare earth metals (REM), 0.05-2.0 by mass of Ti Zr, Y, and Hf, Fe compensation and unavoidable impurities. REM contributes to improved corrosion and oxidation resistance.

The steam boiler may comprise a plurality of radiation elements. In a boiler design in which the water and / or steam transport element is arranged in one or more rows, the radiation elements may then be located between such rows or on each side of each row. With this, each of such rows can be heated from two opposite sides thereof by adjacent radiation elements arranged on opposite sides of said row.

The radiation elements may be distributed in a predetermined manner, so as to cover certain parts of the water and / or steam transport element. With this, the radiation element would have the technical effect of making it possible to control the condensation of the deposits.

Description of the drawings

Figure 1 is a schematic illustration of a steam boiler plant according to a first embodiment of the invention.

Figure 2 is a schematic illustration of a section of a configuration of superheater pipes in a steam boiler of the invention.

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Figure 4 is a schematic illustration in side view of a configuration of radiation and superheater elements in a steam boiler according to a second preferred embodiment of the invention.

Figure 5 is a schematic illustration in top view of a configuration of the radiator and superheater element in a steam boiler according to a second preferred embodiment of the invention.

Figure 6 is a schematic top plan illustration of a conventional superheater configuration that forms the basis for heat transfer calculation.

Figure 7 is a schematic illustration in top view of a superheater configuration of the invention that forms the basis for heat transfer calculation.

Description of the realizations

Figure 1 schematically shows a steam boiler according to a first preferred embodiment of the invention. For reasons of clarity, only the components that are relevant to the invention are shown.

Boiler 1 is a coal steam boiler. This type of boiler comprises a combustion zone 11 in which burners 11 produce hot exhaust gases at a temperature of up to 1250 ° C. The temperature of the steam produced in the boiler is between 400 - 700 ° C. The boiler could also be a bubbling fluidized bed steam boiler, in which combustion takes place in a combustion zone of a layer of sand one meter deep at the bottom of the boiler.

The boiler 1 comprises a first section 10 and a second section 20 which is defined by circumferential walls 9. It is possible that the boiler only comprises one section, ie the first section 10. It is also possible that the boiler comprises more than two sections. The burners 11 are arranged in the combustion zone in the lower part 8 of the first section 10 of the boiler, in this case the burners burn carbon, however the burners could be driven by fuels of other types of combustible material such as gas natural. The burners11 produce hot exhaust gases 12 which, under high turbulence, flow through the section 10 of the boiler, over the second section 20 and out, through a gas outlet 40. In the event that the boiler Only comprise one section, the gas outlet 40 is located in this section. Expelled exhaust gas 12 is then subjected to catalytic purification and released or used for other purposes. These stages are shown in Figure 1. The boiler 1 also comprises a roof 13.

The internal surfaces of the circumferential walls, ie the surfaces facing the combustion gases, of the first and second sections 10, 20 are lined with a water and / or steam transport element in the form of steam pipes 30. In Figure 1, only the parts of the tubes 30 in the lower part of the boiler section are shown in order not to obscure other relevant parts of the boiler. However, the tubes 30 run from the bottom of each boiler section 10, 20 to the top of each section 10, 20, so that essentially the entire interior of the boiler is covered by tubes. Water enters the steam pipes at the water inlet 21 in the first boiler section 10 and is then pumped through the boiler by a circulation pump, not shown. As water is pumped through the steam pipes 30 from the first boiler section 10, the second boiler section 20 is steam heated by the hot fluid gases of the boiler.

The boiler also also includes water and / or steam transport elements in the form of two superheater tube arrangements to increase the temperature of the steam that arrives from the steam tubes 30. In Figure 1 a primary superheater arrangement 50 is arranged in the second section 20 of the boiler, a secondary superheater arrangement 60 is arranged in the first section 10 of the boiler. However, it is obvious that any number of superheater arrangements could be arranged in the boiler.

Saturated steam is conducted from the steam pipes 30 in the second boiler section to the primary superheater 50. The steam is circulated through the first superheater arrangement 50 and conducted to the second secondary superheater arrangement 60 in the first section 10 of the boiler comprising a steam outlet 52 where the dry superheated steam is extracted from the boiler.

Superheater arrangements 50 and 60 typically comprise several U-shaped tube sections 61 that are arranged side by side, so that a large tube volume is achieved. Figure 2 shows a detailed view of a section 61 of a superheater arrangement comprising several U-shaped tubes 61a, 61b, 61c.

According to a first embodiment of the invention, the superheater arrangements 50, 60 comprise radiation elements 70, which, unlike the superheater pipes or steam tubes of the boiler, are not cooled by steam or water.

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The radiation elements are arranged between the U-shaped tube sections 61 in the superheater, so that as far as possible the surface of the radiation elements is facing the superheater tubes. In Figure 1 the radiation elements are arranged parallel to the main flow direction of the exhaust gases. The radiation elements are partially obstructed by superheater arrangements 60 and 50.

Figure 3 schematically shows in detail a radiation element in the three sections of the superheater arrangements 60.

For illustrative purposes, Figure 3 shows three superheater sections 61 and a radiation element 70. However, it is obvious that the superheater arrangement could comprise any number of tube sections 61 and also any number of radiation elements 70. For example, two additional radiation elements could be arranged in the empty spaces between the tube sections 61. The superheater sections 61 in Figure 3 are identical to the superheater section 61 of Figure 2, however, in order not to obscure the element of radiation, only the outermost U-shaped tube is shown in each section.

The radiation element 70 is a flat sheet of heat-resistant steel. A flat steel sheet is advantageous as a radiation element since it is available at a relatively low cost and covers a large area. The flat steel sheet has two large flat side surfaces 71, 72 and a circumferential edge portion 73. Typically, the steel sheet has a thickness of 3 - 50 mm. As can be seen in Figure 3, the radiation element 70 is arranged so that the normal N to its flat side surfaces is perpendicular to the main flow direction of the exhaust gas 12 and in such a way that its edge portion 73 It is facing the flow of gas.

Preferably, the steel is a FeCrAl aluminum-forming steel that has high resistance to oxidation and corrosion of the exhaust gas. Preferably, the steel comprises in% by weight, 15 to 25% Cr, 2 to 7% Al, 1-4% Mo, 0.01-1.0% rare earth metals, and Compensation Fe e inevitable impurities. Such an alloy is the commercially available Kanthal APMT alloy, marketed by Sandvik AB. This alloy, which is reinforced by dispersion by powder metallurgy, has good corrosion properties, good mechanical resistance and high resistance to deformation by slow sliding at high temperatures.

Another group of suitable alumina-forming alloys are NiFeCrAl alloys containing 15-30% Cr and 2-7% Al, plus minor additions. Ni is compensation but it could also be partially substituted by Fe.

The radiation element is arranged so that at least one of its two large lateral surfaces faces the tube section 61 of the superheater. It is also sized so that the total surface area of the part of the radiation elements facing the superheater tubes is equal to at least 3 percent of the total outer surface area of the superheater tubes.

It has been shown that the total surface area of the radiation elements is at least 3 percent of the outer surface area of the superheater tubes, a significant contribution by the radiation is provided to the steam in the superheater tubes. However, it is advantageous if the surface area of the radiation elements is large compared to the total outer surface area of the superheater tubes, since the transfer of heat by radiation to the superheater tubes increases with it. Preferably, the surface area of the part of the radiation elements facing the superheater tube has a surface area that is at least 5 percent of the total outer surface area of the superheater tubes, more preferably 7 percent , more preferred at least 10 percent thereof, more preferred at least 15 percent thereof, more proffered at least 25 percent thereof.

The maximum dimensions of the radiation elements are limited by the boiler flow conditions as well as the boiler operating and design conditions and are determined in each case separately.

In the described embodiment, each of the radiation elements has a rectangular shape with a height of 6 meters and a width of 2 meters. The radiation element may also be mounted from several smaller parts.

In order not to impede the flow of the exhaust gas around the superheaters, an opening could be arranged in the radiation element. Figure 3 schematically indicates with dashed lines the position of a rectangular opening 71 in the radiation element 70. The rest of the steel sheet, that is, the edge 71 around the opening covers the superheater tubes. In addition, the radiation element could be provided with turbulence stimulation elements (not shown) to stimulate turbulent flow around the superheater and the radiation element.

The intensity per unit area of heat radiation from a point or line shaped heat source decreases with distance. To maximize the radiation exchange between the radiation element and the superheater it is therefore important, for a given geometric size of the radiation element, that the

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However, it is also important that the distance between the radiation element and the large superheater tube be large enough to allow the exhaust gases to flow without difficulty over the superheater tube. Preferably, the distance should be large enough to allow turbulent flow of the exhaust gases between the superheater tube and the radiation element.

The radiation elements and the superheater tubes can have various shapes and dimensions and therefore the exact distance between the superheater tubes and the radiation elements must be determined for each application in question. In the preferred embodiment shown schematically in Figure 3, the distance between the radiation elements and the superheater tube is 20 to 60 cm.

The radiation elements 70 are preferably attached to the ceiling of the boiler. According to an alternative, one or several steel bars 90 are attached to the roof of the boiler on the superheater pipes. The radiation elements 70 comprise fasteners 80, for example pins, or hooks or rings that are attached, for example by welding or riveting to the upper edge of the sheet. The radiation element could comprise any number of fasteners, for example two or three or five. The fasteners are attached to the steel bar 90 so that the radiation elements hang down between the superheater tubes. This can be achieved in many different ways, the fastening elements can for example be welded to the bar so that the radiation element hangs fixedly. It is also possible to join the upper edge of the radiation element directly to the ceiling of the boiler. It is also possible to attach the radiation elements to other parts of the boiler, for example to the walls. However, to avoid bulging and warping, it is preferred that the radiation element is attached to uncooled surfaces, that is to say surfaces that are not cooled with water or steam, for example to a part of the roof or to the bar 90.

Thermal expansion in combination with temperature gradients during boiler operation can introduce mechanical stresses in the radiation elements and cause deformation, such as bending or bulging. In order to avoid or reduce the mechanical stresses in the radiation elements, it is therefore preferred to arrange the radiation element so that at least one of them can be freely expanded, for example by hanging the radiating elements of the boiler roof as described above

To achieve this, the radiation element comprises fasteners in the form of rings and hooks and is hung from the bar 90. This allows the radiation elements to expand in all directions and the amount of mechanical stress is further reduced.

When the radiation element comprises fasteners in the form of rings or hooks, it is also possible to hang the radiation element directly from the water and / or steam transport elements of the boiler, for example in a superheater tube.

It is also possible to arrange the radiation element so that it moves from the outside of the boiler approaching or away from the superheater tube, or so that the angle between the radiation element and the superheater tube can be changed from the outside of the boiler . Accordingly, a radiation element displacement element may be arranged, which is in engagement with the radiation element and extends to the outside of the boiler, so that it can be operated from the outside of the boiler in order to move the radiation element 70. This can be achieved by joining the steel bar 90, on which the radiation element 70 is attached, to a pivot key in the boiler roof or by arranging the sliding steel bar in a slot of the roof. The steel bar 90 can be maneuvered from the outside by a lever.

The function of the superheater arrangement should have been clarified based on the foregoing. Thus, in operation, the exhaust gases 12 from the burners 11 heat the radiation elements 70 which in equilibrium reach a temperature given by the temperature of the exhaust gas and the loss of heat of radiation. The net effect is that the heat of radiation is absorbed by the superheater tubes, which are cooler than the radiation elements, and driven further to the steam flowing in the tubes.

It is also possible to arrange radiation elements adjacent to the water and / or steam transport elements in the steam boiler.

According to a second embodiment (not shown), the radiation elements are arranged adjacent to the steam or water pipes 30 that form the boiler walls. Also in this case, the surface area of the part of the radiation elements facing the steam pipes should be at least 3 percent of the total area of the steam pipes on the boiler walls in order to achieve a significant transfer of heat to steam or water from the tubes. However, depending on the design and size of the boiler, a significant heat transfer could be achieved when the surface area of the radiation elements is at least 3 percent of the total surface area of the tubes 30 in a part of the walls of the boiler. For example, at least 3 percent of the total surface area of the tubes in one of the boiler sections 10, 20.

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Preferably, the total surface area of the radiation elements should be at least 5 percent of the total outer surface area of the steam pipes, more preferred at 7 percent, more preferred at least 10 percent thereof. , more preferred at least 15 percent thereof, more preferred at least 25 percent thereof.

It is also possible that at least a part of the boiler wall comprises a vapor and / or water transport element in the form of a double wall liner (not shown in the figures). This is typically an elongated rectangular enclosed space that is manufactured from steel sheets that are welded together. Water is introduced at one end of the double wall liner and at the other end the water is distributed over a distributor to the steam pipes that line the boiler wall.

According to a third embodiment (not shown in the figures), the radiation elements are arranged adjacent to said double wall lining. Also in the case of a double wall lining, the surface area of the part of the radiation element facing the double wall lining should be at least 3 percent of the total area of the double wall lining to achieve a transfer of significant heat. Preferably, the total surface area of the radiation elements should be at least 5 percent of the total outer surface area of the double wall liner, more preferred at least 10 percent thereof, more preferred at least 15 percent of it, more preferred at least 25 percent thereof.

Of course, it is possible to arrange radiation elements both in the vicinity of the steam pipes in the boiler wall and in the vicinity of the superheaters and the double wall lining. It is also possible to have only radiation elements in the vicinity of some of these water and / or steam transport elements. A selective application of the radiation elements provides the possibility of controlling the amount of heat flow in the different parts of the boiler. Therefore, it is possible to compensate for a variable heat distribution that can be produced by fuels of variable composition or ash content.

The radiation elements could be separated in any way on the water and / or steam transport elements. For example, several radiation elements could be arranged close to each other in one part of the steam pipes in the boiler wall, while other elements could be arranged further apart in other parts of the boiler wall. As mentioned, it is therefore possible to control the amount and location of the deposits that condense in the boiler.

According to a fourth embodiment, see Figure 4, the radiation element is a rod-shaped element, in the form of an elongated bar of circular cross-section, such as a round bar. However, the radiation element could also have a rectangular cross section. The radiation element could also be hollow, for example a thick-walled tube. The radiation element could have any suitable diameter, for example 2-20 mm and be of any length, depending on the size of the steam transport element, for example 6 meters.

An advantage with the round cross section radiation elements is that an equal amount of heat is emitted 360 ° around the radiation element. It is therefore possible to heat several steam transport elements with relatively few radiation elements. The compact round radiation elements take up little space therefore they have little impact on the gas flow.

The rod-shaped radiation element is arranged in the flow of the exhaust gases so that the exhaust gas is at the end surface of the radiation element. The radiation element is arranged so that its longitudinal axis L is parallel to the gas flow and its normal N is perpendicular to the gas flow.

Examples

The heating effect of the radiation element of the invention in a steam boiler, will be shown in the following by a calculated example. In the example, calculations of temperatures and heat transfer have been made based on empirical data from conventional boiler designs. The coefficient of absorption and emission of exhaust gas is assumed to be equal and all surfaces are assumed to have a coefficient of emission and absorption of 0.8 and also have the same convective heat transfer properties. For the calculation, the primary radiation, the first and second reflections and the absorption in the volume of gas have been considered.

The calculation shows that the heat that is absorbed in a superheater arrangement in a compact boiler that burns oil. A calculation is made for an overheating arrangement of the invention with a radiation element and a calculation is made for a conventional overheating arrangement without a radiation element.

Figure 6 shows a conventional superheater tube arrangement in a side view. The gas flow is transverse to the tubes in the vertical direction. The superheater arrangement consists of several superheater tubes 60. The distance between the tubes is approximately 80 mm in this example. Figure 7 shows a superheater arrangement of the invention in which flat radiation elements 70 are disposed between the superheater tubes 60. Note that the radiation elements are arranged at a

distance of superheater pipes.

The input data and the results for the calculation are shown below in Table 1.

The results show that the total heat absorbed by the superheater arrangement of the invention is increased by 19% compared to the conventional superheater arrangement, that is from 57 to 68 5 kW / m2.

 Conventional superheating arrangement Superheating arrangement of the invention with radiation element

 Radiation surface

   No Yes

 Gas speed

 Wg T34 T34 m / s

 Gas temperature

 tg 1000 1000 ° C

 Gas emission coefficient

 £ g 0.07 0.07 -

 Surface temperature cooled

 tk 480 480 ° C

 Coefficient of emission of cooled surface

 £ k 0.8 0.8

 Radiation surface emission coefficient

 £ s 0.8

 Radiation surface temperature

 ts 760 ° C

 Heat absorbed by the surface cooled by projected surface

 Convection

 qk 49 49 kW / m2

 Radiation

 qs 8 19 kW / m2

 Total

 q 57 68 kW / m2

Table 1: Calculation showing the effect of a radiation element in combination with superheater pipes.

10 Table 2 shows more simulation results of the present invention. In Table 2 the temperature of the water and / or steam transport and the temperature of the flat sheet radiation elements have been calculated for various exhaust gas temperatures and various boiler components.

 Chilled surface temperature ° C Radiation element temperature ° C Exhaust gas temperature ° C

 Tertiary boiler superheater

 555 670 790

 Hollow hollow

 450 675 800

 Panel superheater

 490 710 900

 Chilled surface temperature ° C Radiation element temperature ° C Exhaust gas temperature ° C

 Panel superheater

 490 745 1000

 Boiler surface

 400 710 1000

Table 2. Relationship between the temperature of the water and / or steam transport surface, the temperature of the radiation element, and the gas temperature.

Although particular embodiments have been described in detail, it has been done for illustrative purposes only, and they are not intended to be limiting. In particular it is contemplated that various substitutions, alterations and modifications can be made within the scope of the appended claims. For example, the radiation element can have any type of geometric shape, such as the shape of a plane wing or reel shape. The boiler could also be of a type that only comprised steam and / or water transport elements in the form of steam / water pipes in the boiler wall.

10

Claims (14)

5 10 fifteen twenty 25 30 35 40 Four. Five fifty 55 60 65 REIVINDICACI ONES 1. A steam boiler (1) comprising at least one water and / or steam transport element (30, 60) which is heated by hot exhaust gases in the boiler, where the steam boiler (1) it comprises at least one radiation element (70) therein; said radiation element (70) is an uncooled element; said radiation element (70) is arranged in the flow of hot exhaust gases (12), so that it is convectively heated by the exhaust gases; said radiation element (70) is located at a predetermined distance from said at least one water and / or steam transport element (30, 60), wherein said predetermined distance is adopted so that the flow of the exhaust gases hot between the radiation element and the water and / or steam transport element is produced without difficulty, and so that the water and / or steam transport element is heated by the radiation from the radiation element, characterized in that The radiation element (70) is formed by an alloy based on Fe or Ni and contains Al and which, when subjected to heat in an oxygen-containing atmosphere, forms a protective alumina layer on its outer surface. 2. The steam boiler according to claim 1, wherein said radiation element (70) is arranged so that the flow of hot exhaust gases (12) can pass said radiation element (70) essentially without Change the direction of the flow. 3. The steam boiler according to claim 1 or 2, wherein the radiation element is arranged in the flow of the exhaust gases so that the entire radiation element is exposed to the flow of the exhaust gases. 4. The steam boiler according to any one of claims 1-3, wherein the water and / or steam transport element (30, 60) is at least one superheater tube (60) that carries steam. 5. The steam boiler according to any one of claims 1-4, wherein the surface area of the radiation element (70) is at least 3 percent, preferably at least 10 percent, of the area of said at least one water and / or steam transport element (30, 60) that is directly exposed to the exhaust gases. 6. The steam boiler according to any one of claims 1-5, wherein the radiation element extends essentially parallel to the flow direction of the exhaust gases. 7. The steam boiler according to any one of claims 1-6, wherein said radiation element (70) is a sheet (70) comprising two lateral surfaces (71, 72) and a circumferential edge (73 ), wherein the radiation element is arranged so that one normal to one of its lateral surfaces (71, 72) is perpendicular to the flow direction of the hot exhaust gases (12) and so that an edge part of the sheet (70) is facing the flow of hot exhaust gases. 8. The steam boiler according to any one of claims 1-7, wherein said radiation element (70) is a corrugated sheet. 9. The steam boiler according to claims 1-6, wherein the radiation element is an elongated rod element, wherein the radiation element is arranged such that its radial axis R is perpendicular to the direction of flow of exhaust gases. 10. The steam boiler according to claim 9, wherein the radiation element has a circular cross section or a rectangular cross section. 11. The steam boiler according to any one of claims 1-10, wherein the radiation element is flexibly bonded to an uncooled surface of the boiler (1). 12. The steam boiler according to any one of claims 1-11, wherein the radiation element (70) is flexibly attached to the boiler, such that at least one end of the radiation element is free to expand or contract. 13. The steam boiler according to any one of claims 1-12, wherein the radiation element (70) is arranged so that it hangs flexibly from the water and / or steam transport element. 14. The steam boiler according to any of claims 11-13, wherein the radiation element comprises hooks or rings for hanging the radiation element on the water and / or steam transport element.