WO2011147654A1

WO2011147654A1 - Burner element having local differences in physical properties

Info

Publication number: WO2011147654A1
Application number: PCT/EP2011/056573
Authority: WO
Inventors: Patrick Lenoir; Nicolas Even
Original assignee: Solaronics S.A.
Priority date: 2010-05-25
Filing date: 2011-04-26
Publication date: 2011-12-01

Abstract

A radiant burner (1) comprises a body (2) defining a premixing space (9) and a combustion space (10), the premixing space being separated from the combustion space by a radiant burner element (5) having its burner part (11) at the side of the combustion space, wherein the burner part's emissivity and/or conductivity and/or temperature resistance is not equal at each location of it while the gas permeability is substantially equal at each location of it. At least one location has a different value of emissivity and/or conductivity and/or temperature resistance than the radiant burner element base material.

Description

Title: Burner element having local differences in physical properties

Description

Technical Field

[0001 ] The present invention relates to radiant burners comprising a radiant burner element with a burner part at the side of a combustion space.

Background Art

[0002] Radiant burners are known in wide variety of embodiments. For example US-4,799,879 shows a radiant burner comprising a body defining a premixing space and a combustion space. The two spaces are separated from each other by a radiant burner element comprising a base with a burner surface at the side of the combustion space. The radiant burner element has a number of functions. First of all it serves to transport and distribute a fuel mixture from the premixing space towards the combustion space. Further its burner surface which delimits the combustion zone serves to produce infrared radiation or radiant heat flux when being heated during combustion of the fuel mixture. Finally the radiant burner element serves as a thermal insulator which needs to prevent backfire of ignited fuel mixture back into the premixing space.

Besides being delimited by the radiant burner element, the combustion space is also delimited by a screen. The screen is also heated during combustion and thus also produces infrared radiation or radiant heat flux. The screen together with the radiant burner element provides a total radiation output of the burner, which averages at levels around 50% efficiency. The radiant burner element is made of refractory material, for example with a ceramic base, and here comprises a honeycomb pattern of through holes or perforations. This honeycomb pattern has the positive effect of increasing the temperature level because radiation from the burner surface at the location of the through holes is reflected several times along the walls of these holes before emanating there from.

Consequently the honeycomb pattern also helps to increase the radiative output of the burner.

[0003] A disadvantage with this is that the honeycomb pattern may create local overheating of the radiant burner element. Also the temperature uniformity of the burner surface may be poor. Temperature differences up to a maximum of 100 degrees Celsius between respective zones of the burner surface may even occur. Local overheating would result in early failure of the ceramic material of the radiant burner element. This may make it necessary to lower the input of fuel mixture such that a relative low average burner surface temperature is kept in order to avoid that local hot spots reach too high temperature levels. Thus the possibility of the temperature becoming locally too high, forms a limitation in the use of this known radiant burner, in particular a limitation on the maximum amount of radiation energy which can be obtained with it.

[0004] Another way of trying to achieve a higher radiation output was

proposed in US-3,847,536 which uses two radiation screens above a ceramic radiant burner element. The provision of a double set of screens helped to improve the radiation efficiency.

[0005] A disadvantage with this however, was that the combination of the two screens appeared to cause local overheating of the radiant burner element in the middle part thereof. This made it necessary to keep the input of fuel mixture such that acceptably low (local) temperatures of the radiant burner element were obtained in order to prolong the lifetime thereof.

[0006] In order to prolong the lifetime of radiant burner elements it has also been tried to manufacture them out of a material having a high conductivity, in particular metal.

[0007] This however had the disadvantage that it took away the function of the radiant burner element forming a thermal insulator towards the premixing space and severely increased the risk of back fire of the fuel mixture back into the premixing space. Moreover a metal radiant burner element had the disadvantage of having an unacceptably high thermal expansion.

[0008] It has been tried to further improve the efficiency of radiant burners by manufacturing the radiant burner element as a multilayered porous plate of which the distinctive layers have distinctive conductivity and/or emissivity properties.

[0009] For example US-4,643,667 shows an embodiment having a first layer made out of a material which has a low inherent thermal conductivity, whereas a second layer is made out of a material having a high inherent thermal conductivity. In that way the first low conductivity layer is able to operate as a preheating zone for a fuel mixture, without having the risk of backfire. Ignition of the fuel mixture then only takes place in pores of the second high conductivity layer, because there the temperature is able to reach to a value above the ignition level of the fuel mixture.

[0010] A disadvantage with this multilayered construction however was that despite the high inherent thermal conductivity of the second layer, an uneven heat distribution at the burner surface still occurred.

[001 1 ] Another example of a multilayered radiant plate is WO 2007/1 14852, which shows an embodiment in which the entire radiant plate is coated with a material having high emissivity. The thus obtained higher emissivity value for the burner surface provides more efficient radiation and/or cooler operating temperatures for a given rate of infrared emission.

[0012] A disadvantage with this coated construction is that local hot spots still appear. As above those local hot spots cause a decrease in output because they make it necessary to drop the overall temperature. This temperature drop is only partly compensated for by the increase in emissivity.

Disclosure of Invention

[0013] The present invention aims to at least partly overcome the

abovementioned disadvantages or to provide a usable alternative. In particular the invention aims to provide a radiant burner which has a radiant burner element which makes it possible to further improve the efficiency of the radiant burner.

[0014] This aim is achieved by a radiant burner that comprises a body

defining a premixing space and a combustion space. The premixing space is separated from the combustion space by a radiant burner element comprising a burner part, for example a plate shaped base having one or multiple burner surfaces, at the side of the combustion space. According to the inventive thought, the burner part's emissivity and/or conductivity and/or temperature resistance is not equal at each location of it, while the gas permeability is substantially equal at each location of the burner part. Furthermore at at least one location of the burner part, the burner part has a different value of emissivity and/or conductivity and/or temperature resistance than a base material of the radiant burner element. Preferably, this locally different value of emissivity and/or conductivity and/or temperature resistance than a base material of the radiant burner element; is a higher value for the emissivity and a higher value for the conductivity and a higher value for the temperature resistance than a base material of the burner element. Even more preferably is that the locally different value of emissivity and conductivity and temperature resistance are each at least 15% higher than the values of the base material of the radiant burner element. Most preferred is a radiant burner, wherein the burner part has a zone with a value of the emissivity and of the conductivity and of the temperature resistance of at least 15% higher than the values of the emissivity and of the conductivity and of the temperature resistance in another zone of the burner part, while the gas permeability is substantially equal at each location of the burner part.

[0015] With conductivity is meant the thermal conductivity. With the gas

permeability is substantially equal at each location of the burner part is meant that the differences in emissivity and/or conductivity and/or temperature resistance are not creating a difference in the gas flow through the radiant burner element. With the gas permeability is "substantially" equal is meant that the gas permeability does not differ more than 5%, more preferably not more than 2% between zones with different values of emissivity and/or conductivity and/or temperature resistance. The benefit of having substantially the same gas permeability is that the same amount of combustion happens, meaning a same amount of heat generation. The temperature resistance is meant as the resistance to withstand a temperature expressed in degrees Celsius.

[0016] The invention advantageously makes it possible to arrange for locally different thermal distribution characteristics and locally different thermal behaviour of the burner part, and/or to focus local

temperature decreases only there on the burner part where it is necessary and/or to only locally increase the resistance of the burner part against high temperatures, etc. It allows for a longer lifetime of the radiant burner element even if operated at higher power densities or with higher view factor, for example because of the use of a multilayered or honeycomb pattern of through holes or combination thereof in the radiant burner element or because of the use of one or more radiant screens. Local hot spots caused by such a multilayered or honeycomb pattern of through holes or combination thereof or screen can no longer cause problems. The overall (or average) temperature of the radiant burner element can thus be kept higher which helps to improve the efficiency of the radiant burner and without having a negative effect on the life span of the element. Temperature differences at the burner part can now be kept within an acceptable range. Thus a greater temperature uniformity of the burner part can be attained. This greater temperature uniformity may also result in a significant higher energy efficiency of the entire radiant burner.

[0017] The invention even enables the use of alternative burner element geometries, e.g. with more efficiency through higher view factor from combustion support to itself. For example the burner element may be provided with concavely shaped hollows with sharp edges at the side of the combustion space. According to the invention those more critical zones of the sharp edges can now be properly protected by giving them differing physical properties. Such geometries would be nearer to the theoretical black body.

[0018] The burner element may comprise a fibrous texture or locally hollow volumes, having one or multiple burner parts at the side of the combustion space. Other shapes are also possible.

[0019] The burner element may comprise perforated tiles of a ceramic

material. The ceramic material is having high temperature resistance. The ceramic material is preferably having excellent mechanical and thermodynamic properties. Examples of ceramic materials that can be used are e.g. cordierite or zirconia; partially stabilized zirconia (PSZ), alumina, silicon carbides or other high level technical ceramics.

[0020] In an embodiment the variation in emissivity and/or conductivity

and/or temperature resistance may totally or partially result from one or several uneven chemical and/or physical treatments of the burner part.

[0021 ] In another embodiment the variation in emissivity and/or conductivity and/or temperature resistance may result from one or several outer covering layers of which at least one is only partly covering the burner part. Thus, for example, the surface of the burner part can only partly be covered with an outer covering layer which is made from a material that has a configuration with a different (and preferably higher) emissivity and/or conductivity and/or temperature resistance than the base material of the radiant burner element. For example, zones of the burner part that were already exhibiting uncritical temperatures may advantageously remain uncovered and thus remain able to act as efficiently as possible. Those zones that are exhibiting or would otherwise exhibit too high temperatures can now be covered with the high emissivity and/or high conductivity and/or temperature resistant layer. The outer covering layers are applied in such a way as not to modify substantially the gas permeability of the burner part. It means that in case the burner part is made of perforated plates, the perforations are not substantially affected by the covering layers. In a preferred embodiment, the outer covering layers are made out of a material that is having a higher emissivity and a higher conductivity and a higher temperature resistance.

[0022] It has appeared most optimal if the outer covering layer covers less than 85 % of the burner surface, preferably less than 60 %, and preferably more than 20%. Thus the hot spots can adequately be dealt with in such a way that no unnecessary covering of the burner surface takes place.

[0023] For the same reason the outer covering layer preferably only covers zones of the burner surface which obtain or otherwise would obtain higher temperatures during combustion and radiation than other zones of the burner surface, in particular at least 30 degrees higher.

[0024] In an advantageous embodiment a zone having higher emissivity and/or conductivity and/or temperature resistance, for example by means of an outer covering layer, is provided in a centre part of the burner surface, whereas the part surrounding, which is susceptible to be colder, is kept uncovered. This makes it possible to provide the radiant burner also with one or more screens to delimit its

compression space at the side opposite to the burner surface without having to lower the input. Back radiation of the screen(s) towards the burner part no longer has to lead to local overheating in the for this embodiment most critical centre zone of the radiant burner part, and thus also not in an early failure thereof.

[0025] The radiant burner element may be made with a substantially flat burner surface. The element may also be made with multiple levels at which combustion takes place in its burner surface, for example because of the provision of a honeycomb or otherwise stepped pattern of bumps and troughs and/or through holes therein. The zone having higher emissivity and/or conductivity and/or temperature resistance, for example the mentioned local outer covering layer, can then be provided on only a limited set of those multiple levels, for example the highest ones of those levels, or the highest ones lying in a centre part of the burner surface. In the alternative or in addition it is also possible to provide the zone having higher emissivity and/or conductivity and/or temperature resistance, for example the mentioned local outer covering layer, on the lowest ones of those levels, or on the lowest ones lying in a centre part of the burner surface.

[0026] The radiant burner element can be positioned in various positions relative to the horizontal, for example substantially horizontal or substantially vertical, or any slanted position in between. If positioned substantially vertical then the zone having higher emissivity and/or conductivity and/or temperature resistance, for example the mentioned local outer covering layer, may advantageously be provided on a top part of the burner surface, whereas the part beneath that may be kept uncovered.

[0027] The radiant burner element can be made out of one unitary element, preferably plate shaped. It is also possible to assemble it out of a plurality of segments. It is than possible to only provide the zone having higher emissivity and/or conductivity and/or temperature resistance locally on a limited number of those segments, for example only locally cover them with the outer covering layer. For example in the vertical orientation it is possible to only provide the a zone having higher emissivity and/or conductivity and/or temperature resistance on one or more upper segment(s), whereas in the horizontal or any other orientation in between the horizontal and vertical, it is possible to only provide the a zone having higher emissivity and/or conductivity and/or temperature resistance on one or more centre segments.

[0028] In a further embodiment the outer covering layer is formed by a

relative thin layer, preferably in the range of 10-100 micrometer. Preferably the outer covering layer is formed by a coating. The outer covering layer can easily be provided on only part of the burner surface by means of a (programmed) spray robot. It is also possible to provide the outer covering layer on only part of the burner surface by means of suitably masking the zones not to be covered with the outer covering layer. An example of a coating that can be applied is a silicon carbide coating, e.g. with a thickness in the range of 10 - 100 micrometer. The outer covering layer may have a uniform thickness. It is however also possible to have its thickness and/or composition differ depending on its location on the burner surface. Thus an even more uniform temperature distribution and/or heat resistance distribution over the burner surface can be achieved.

[0029] In a more preferred embodiment, a radiant burner according to the invention comprises two radiant screens. The two radiant screens are both arranged parallel to the radiant burner element.

[0030] The invention further relates to radiant burner elements for use in a radiant burner.

[0031] Another aspect of the invention is a method for manufacturing a

radiant burner element for a radiant burner according to the invention. The method is comprising the steps of manufacturing the burner element; and providing the burner part thereof with an emissivity and/or conductivity and/or temperature resistance that is not equal at each location of it while the gas permeability is substantially equal at each location of it and where at least one location has a different value of emissivity and/or conductivity and/or temperature resistance than the radiant burner element base material.

[0032] In a specific embodiment one or several outer covering layers are provided on top of the burner part for obtaining the variation in emissivity, conductivity or thermal resistance, and where at least one of the said outer covering layer(s) is only partly covering the burner part.

[0033] In a more specific embodiment, the outer covering layer(s) is

provided on only part of the burner part by means of a spray robot, wherein the outer covering is applied such not to substantially modify the gas permeability of the burner part. Alternatively, after application of the outer by means of a spray robot a process is performed to ensure that gas permeability is not substantially different between zones that are and zones that are not covered by the outer covering layer(s). [0034] In an even more specific embodiment, the outer covering layer(s) is provided on only part of the burner part by means of dipping, wherein the outer covering layer is applied such not to substantially modify the gas permeability of the burner part. Alternatively, after application of the outer covering layer by means of a spray robot a process is performed to ensure that gas permeability is not substantially different between zones that are and zones that are not covered by the outer covering layer(s).

[0035] In a further specific method, the outer cove ng layer(s) is provided on only part of the burner part by means of plasma spraying, wherein the outer covering is applied such not to substantially modify the gas permeability of the burner part. Alternatively, after application of the outer by means of a spray robot a process is performed to ensure that gas permeability is not substantially different between zones that are and zones that are not covered by the outer covering layer(s).

[0036] In another further specific method, the outer covehng layer is

provided on only part of the burner part by means of masking the zones not to be covered with this outer covering layer.

Brief Description of Figures in the Drawings

[0037] Example embodiments of the invention are described hereinafter with reference to the accompanying drawings, in which:

[0038] Fig. 1 shows a side view, partially in cross section, of a radiant burner according to the invention with a segmented vertically orientated radiant burner element with only the top segment coated;

[0039] Fig. 2a, b, c show a side view, a top view and an enlarged partial view of an embodiment of a radiant burner element according to the invention in a radiant burner and which has been locally coated at its highest zones;

[0040] Fig. 3 shows an alternative of fig. 2 with a non-uniform coated circular centre zone; and

[0041 ] Fig. 4 shows a top view of a further alternative of fig. 2 with a top- coated rectangular centre zone. Mode(s) for Carrying Out the Invention

[0042] In fig. 1 the entire radiant burner has been given the reference

numeral 1 . The burner 1 comprises a housing with a body 2 which together with a peripheral band 3 encloses a plate shaped radiant burner element 5 and a screen 6 (figure 1 shows a radiant burner with one screen, in a similar way radiant burners with two superimposed parallel screens can also be provided according to the invention). The body 2 has an inlet 8 which connects to a premixing space 9. The premixing space is delimited between the body 2 and the element 5. Between the element 5 and the screen 6 a combustion space 10 is delimited. The outer side of the plate shaped base of the element 5 which is directed towards the combustion space 10 is referred to as the burner surface 1 1 , the rest of the element 5 is referred to as the base.

[0043] The element 5 has a plurality of through holes which make it possible for a fuel mixture, for example a gas/air mixture, to enter the combustion space 0 after being thoroughly mixed in the premixing space 9. The fuel mixture can then ignite or be ignited in this combustion space 10 after which it heats up at least the burner surface 1 1 of the element 5 as well as the screen 6 to a temperature of approximately 900-1300 Degrees Celsius, in particular about 1 150 degrees Celsius. This high temperature has the effect of making both the screen 6 and the outer burner surface 1 1 red hot such that they start to produce significant infrared radiation or radiant heat flux. Because of this the radiant burner is also known as infrared emitter. It is mostly used in industrial processes such as the drying of coated paper, board and steel, and the like.

[0044] The burner surface 1 1 comprises a plurality of slits 12 positioned

adjacent one another. The bottoms of the slits 12 are referred to as the inner/lowest level and the top sides of the wall parts delimiting the slits 12 are referred to as the outer/highest level of the burner surface 1 1 .

[0045] The plate shaped element 5 here extends in a vertical direction and is built up out of two segments 5a, 5b positioned above one another. The burner surface of the upper segment 5b is provided with a coating 15, whereas the burner surface of the lower segment 5a has remained uncovered. The coating 15 is applied in such a way or the coating application is followed by a manufacturing process, such that the gas permeability of the plate shaped radiant burner elements is not substantially different in the region where the coating is present from the region where no coating is present. The coating 15 has material characteristics which make it better resistant to high temperatures than the rest of the radiant burner element 5, the so- called base material thereof. During use the upper part of the radiant burner tends to get hotter than the lower part, because of hot air being lighter than colder air. This causes the upper segment 5b to get hotter than the lower segment 5a. Since the upper segment 5b is coated with the more temperature resistant coating, this is no problem. Because of the coating, the upper segment 5b is well able to withstand higher temperatures. Those higher temperatures thus do not have a negative effect on the life span of the upper segment, and owing to the partial coating according to the invention, it is

advantageously possible to operate the entire burner at a higher level without running the risk of early failure. The upper zone of the element 5, which otherwise would be facing too high temperatures has now been adequately protected there against. Moreover, because of highest emissivity, the coated zone has still high radiation contribution (cf. Boltzmann law).

[0046] A successfully tested configuration is to manufacture the plate

shaped base out of cordierite and to apply a partial local coating of silicon carbide upon local hot spot(s) of its burner surface.

[0047] The coating 15 can for example be applied by means of dipping the upper segment 5b only with its burner surface 1 1 in a bath with coating material, or to apply the coating by means of spraying onto only the burner surface 1 1 of the upper segment 5b. In order to increase the adherence between the coating and the segment 5b, it is possible to use one or more compatibility layers. The compatibility layer can for example be a mix between the material out of which the base is made of and the material of the coating.

[0048] The coating process can be followed by a plate machining operation to further improve the adjustment of the coating against the base.

[0049] In fig. 2 an alternative embodiment is shown of the burner element 5.

Similar parts have been given the same reference numerals. It is now formed by two plates forming a base, which can be used in any orientation in a radiant burner, in which the burner surface 1 1 is formed by the top side of the base. The burner surface 1 1 has multiple levels formed by the slits 12 which here run diagonal. In this embodiment the coating 15 is provided on only the highest ones of the multiple levels, that is to say on the tops 20 of the wall parts extending between the slits 12. The bottoms 21 of the slits 12 remain uncovered with the coating material. The coating 15 is applied in such a way or a the coating application is followed by a

manufacturing process, such that the gas permeability of the plate shaped radiant burner elements is not substantially different in the region where the coating is present. Between the coating 15 and the base a compatibility layer 24 is provided. During use the tops 20 tend to get hotter than the bottoms 21 , in particular when the element 5 is used in combination with a screen positioned at a distance there above. Since the tops 20 are coated with the more temperature resistant coating, this is no problem, and they are well able to withstand the higher temperature without the base material deteriorating too soon. Moreover, the highest conductivity of the coated zone allows a more even heat repartition, increasing bottom part temperature, allowing more radiation. Highest emissivity allows same emission from the top zone as before coating, even at lower temperature.

[0050] The coating 15 in this embodiment can for example be applied by means of dipping the element 5 only with the tops 20 of its burner surface 1 1 in a bath with fluid coating material. It is also possible to spray the coating onto only the tops of the burner surface 1 1 by using a mask that suitably covers the bottoms 21 of the slits 12 during the coating process. And followed by a manufacturing process, such that the gas permeability of the plate shaped radiant burner element is not substantially different in the region where the coating is present.

[0051 ] In fig. 3 an alternative embodiment is shown of the burner element 5.

It is again formed by two plates forming a base with slits 2 on its top side. The coating 15 is now provided on only a centre part 30 of the burner surface 1 1 , both on the tops 20 as well as on the bottoms. The coated centre part 30 here is circular, but also can be given other shapes. The centre zone 30 is coated such that it has different thickness and composition between the highest and lowest zones, that is to say between the tops 20 and the bottoms 21 lying in this centre zone 30. For example the tops 20 can then be coated with a more temperature resistant coating material, whereas the bottoms 21 can be coated with a more conductive coating material. In the alternative it is also possible to use one similar coating material but to apply it such that it becomes thicker at the tops 20 and thinner at the bottoms 2 . During use this centre part 30 has appeared to get hotter than the part of the burner surface 1 1 surrounding it, and inside this centre part 30 the tops 20 appeared to get hotter than the bottoms 21 , in particular when the element 5 was used in combination with one or more screens. Thus temperature differences which also still occur in the more vulnerable centre zone 30 between the tops 20 and the bottoms 21 can even more adequately be dealt with.

[0052] The coating 5 in this embodiment can most easily be applied by means of spraying it onto only the centre part 30 of the burner surface 11 by using a complementary mask. The coating 15 is applied in such a way or a the coating application is followed by a manufacturing process, such that the gas permeability of the plate shaped radiant burner elements is not substantially different in the region where the coating is present from the region where no coating is present.

[0053] In fig. 4 a further alternative embodiment is shown of the burner

element 5. The coating 15 is now provided on only the tops 20 of a rectangular centre part 40 of the burner surface 1 1 , whereas the bottoms 21 in this centre part 40 remain free from the coating material. As coating material a more temperature resistant and/or higher emissivity and/or higher conductivity coating can then be used.

[0054] Besides the embodiments shown various variants are possible. For example the shown embodiments may be combined with each other. The various parts may be given different shapes and dimensions or other material combinations may be used. Instead of a coating it is also possible to use a somewhat thicker covering layer, in particular with a thickness in a range of 5-500 μηη (dip coating), 20-60 μιτι (spray coating). Depending its position on the element, the covering layer can have any constant or variable thickness, be made of one or of multiple, or of a variable number of layers exhibiting constant or variable compositions and thus variable material characteristics. The radiant burner element is preferably made of a ceramic material with high temperature resistance, and excellent mechanical and

thermodynamic properties, such as e.g. cordierite or zirconia, partially stabilized zirconia (PSZ), alumina, silicon carbides or other high level technical ceramics. Also possible are aluminum titanates, silicon oxides, corundum or mullite, silicon nitrides or metal-infiltrated ceramics, such as silicon-infiltrated silicon carbide, as well as other heat resistant materials (non ceramics) e.g. materials with >50% weight metal silicide such as molybdenum disilicide, tungsten disilicide, highly heat-resistant steel grades such as Kantal APM or APMT or FeCrAI alloys, Cr/Ni steel grades like Avesta 253 MA, 153 MA, Inconel 60 , Incoloy 800HT, Incoloy MA936. The element may also be fibrous, comprise empty spheres or other empty hollow shapes for combustion support.

[0055] The height difference in between two levels of a multiple levelled

burner surface is preferably between 1 -20 mm. The covering material is preferably made from platinum, silver, magnesium, or the like if higher conductivity is required, and from the abovementioned silicon carbide, silicon, silica, aluminium, silicon nitride, carbon, iron oxide, platinum, silver, magnesium, or the like if higher emissivity and/or temperature resistance are required. The covering layer can also be connected to the base in other ways, for example by means of chemical bonding and/or mechanical bonding.

[0056] In particular the burner part at its at least one location has an

emissivity and/or conductivity and/or temperature resistance which is at least 15% higher than the emissivity and/or conductivity and/or temperature resistance of the radiant burner element base material. For example, the theoretical values, for silicon carbide as bulk coating material compared to those of cordierite, are the following:

[0057]

[0058] The burner element may be provided with a honeycombed pattern.

Preferably however, the burner element is a multilevel plate with humps and troughs, having holes in both humps and troughs. With such a multilevel plate preferably the highest zones are covered. This is profitable in any position, even vertical. The influence on

temperature of being in the highest zone of the plate is more important than that of being the top plate in a vertical configuration. The same goes for the variant in which the center is coated. Then also the use position is not necessarily horizontal, because the influence on temperature of being in the center of the plate can be more important than that of being the top plate in a vertical configuration.

[0059] Furthermore, the coating may comprise multiple layers of various compositions, including compatibility layer(s). The composition of such compatibility layer(s) can be intermediate between that of the base material and that of the burner part an/or top layer. Preferably the adherence has been increased prior to coating by specific treatment. For example the manufacturing may include one/multiple machining step(s) and some of/all those step(s) may be carried out after a coating process. Also the change in emissivity and/or conductivity and/or temperature resistance may result from multiple infiltration(s) or gradient(s) in composition, in particular infiltration of metal, alloy, oxide or ceramics.

Thus according to the invention an economic radiant burner is provided with a burner element having a burner part with differing physical properties which makes it possible to operate the burner at higher temperatures and thus obtain a higher degree of efficiency. The invention can thus be used for drying web materials at even higher speeds than according to the state of the art.

Claims

1 . A radiant burner comprising a body defining a premixing space and a combustion space, said premixing space being separated from the combustion space by a radiant burner element having its burner part at the side of the combustion space, wherein said burner part's emissivity and/or conductivity and/or temperature resistance is not equal at each location of it while the gas permeability is substantially equal at each location of it; and wherein at least one location has a different value of the emissivity and/or conductivity and/or temperature resistance than the radiant burner element base material.

2. A radiant burner according to claim 1 , wherein the burner element comprises perforated tiles of a ceramic material.

3. A radiant burner according to claim 1 or 2, wherein the burner part has a zone with a value of the emissivity and of the conductivity and of the temperature resistance of at least 5% higher than the values of the emissivity and of the conductivity and of the temperature resistance in another zone of the burner part.

4. A radiant burner according to any of the preceding claims, wherein the variation in emissivity and/or conductivity and/or temperature resistance totally or partially results from one or several uneven chemical and/or physical treatment of the burner part.

5. A radiant burner according to any of the preceding claims, wherein the variation in emissivity and/or conductivity and/or temperature resistance results from one or several outer covering layers or coating layers and where at least one of the said outer covering layer(s) or coating layer(s) is only partly covering the burner part.

6. A radiant burner according to claim 5, wherein the coating layer is a silicon carbide coating.

7. A radiant burner according to any of the preceding claims 4-6, wherein the at least one of the said outer covering layer(s) only partly covering the burner part covers less than 85 % of the burner part.

8. A radiant burner according to any of the preceding claims, wherein a zone having higher emissivity and/or conductivity and/or temperature resistance is provided in a centre part of the burner part.

9. A radiant burner according to any of the preceding claims 1 -8, wherein the burner part has multiple levels at which combustion takes place, and wherein a zone having higher emissivity and/or conductivity and/or temperature resistance is provided on highest ones of those levels.

10. A radiant burner according to any of the preceding claims 1 -8, wherein the burner part has multiple levels at which combustion takes place, and wherein a zone having higher emissivity and/or conductivity and/or temperature resistance is provided on lowest ones of those levels.

1 1 . A radiant burner according to any of the preceding claims, wherein the radiant burner element extends in a vertical direction, and wherein a zone having higher emissivity and/or conductivity and/or temperature resistance is provided on a top part of the burner part. 2. A radiant burner according to any of the preceding claims, wherein the radiant burner element comprises a plurality of segments together forming a plate shaped base with a burner surface, and wherein a zone having higher emissivity and/or conductivity and/or temperature resistance is provided on only some of those segments.

13. A radiant burner according to any of the preceding claims, wherein the radiant burner comprises two radiant screens.

14. A radiant burner element for use in a radiant burner according to any of the preceding claims.

15. Method for manufacturing a radiant burner element for a radiant burner according to any of the preceding claims, comprising the steps of:

- manufacturing the burner element; and

- providing the burner part thereof with an emissivity and/or conductivity and/or temperature resistance that is not equal at each location of it while the gas permeability is substantially equal at each location of it and where at least one location has a different value of emissivity and/or conductivity and/or temperature from the radiant burner element base material.