Description

Technical Field

[1] The invention relates to a gas premix burner with a fiber based cloth. An ionization pen measuring the flame current can be used to determine the air to gas ratio. The air to gas ratio of the gas premix burner according to the invention can be controlled over a broader burner load range by means of a control system using an ionization pen as sensor. Such gas premix burner can e.g. be used in boilers or in instantaneous water heaters.

Background Art

[2] Detection of the ionization current in the flame of a gas premix burner by means of an ionization pen is known as a way to detect whether or not ignition has occurred. However, in a growing number of gas premix burners (e.g. used in boilers or in water heaters), the ionization current is not only used to detect burner ignition, but its value is also used as a means for flame control, and more specifically for the control of the air to gas premix ratio. As an example, DE19632983 discloses an ionization pen to measure the flame current and an associated regulating device in a gas burner, wherein an air to gas ratio reference value for low emissions is set.

[3] Gas premix burners with a fiber based cloth as combustion surface are known. Such burners can have a metal fiber based knitted or woven fabric as combustion surface positioned on a perforated plate or woven screen which is acting as gas distribution plate and as support for the metal fiber based knitted or woven fabric. Such burners are e.g. known from US4657506 and WO2004/092647.

[4] For use in combustion control the ionization current of gas premix burners should be readily and reliably measurable over the load range of the burner. It is a problem, also with gas premix burners with a fiber based cloth as combustion surface, that in the low power range of the burner the ionization current drops drastically. This is rendering flame control by means of ionization current measurement unreliable in the low power range of gas premix burners. For a number of applications it is desirable that burners can operate in a broad load range, and that air to gas ratio control by means of measurement of ionization current with an ionization pen can be performed over the broad load range.

Disclosure of Invention

[5] A first aspect of the invention is a gas premix burner, preferably a fully premixed gas burner, comprising:

- a porous combustion surface, on which when the burner is in use combustion occurs after the premix gas has flown through the porous combustion surface,

- a fiber based cloth forming at least part of the porous combustion surface;

- a perforated plate, a woven wire mesh or expanded metal sheet; wherein the fiber based cloth is supported by the perforated plate, woven wire mesh or expanded metal sheet. Preferably the fiber based cloth is fixed over part of its surface onto the perforated plate, woven wire mesh or expanded metal sheet, e.g. by means of welding or in any other way.

The burner is characterized in that one or more zones of the porous combustion surface is not formed by the fiber based cloth, but by another porous substrate. With a porous substrate is meant a substrate has through perforations or open cell pores allowing the premix gas to flow through the porous substrate and to be combusted on its surface. This characterizing feature ensures that ionization current measurement by means of an ionization pen that is positioned covering at least part, and preferably the full length in the direction of the ionization pen, of the one or more zones of the porous combustion surface that are not formed by the fiber based cloth, can be used in a broader load range of the burner than prior art burners as a reliable indication of the air to gas ratio of the gas premix burner and therefore as input for the modulation of the air to gas ratio supplied to the gas premix burner. In such a burner, control of air to gas ratio over a broader load range by means of the use of the ionization current as measured by an ionization pen is allowed. A further and synergetic benefit is that a same broad range can be obtained for all burners of a same production batch and even between production batches.

The gas premix burner according to the invention can be provided in a wide range of different shapes. Examples are flat burners, cylindrical burners and burners that have a conical or frustoconical shape. As known by the person skilled in the art a flat burner can have and mostly has a curved shape or can even have an undulated shape. The class of flat burners is distinguished from the other main class of gas premix burners comprising burners that have a conical, cylindrical or frusto-conical shape.

In preferred burners of the invention the percentage of the surface of the combustion surface formed by fiber based cloth is at least 70%, more preferably at least 80%, for better overall performance of the burner.

A zone where the combustion surface is not formed by the fiber based cloth is defined as a surface area of the combustion surface where each two points on that surface area can be connected with a continuous line (straight or curved or multiple curved) that is not passing an area where fiber based cloth is acting as combustion surface.

A zone itself can comprise subzones that each have higher gas permeability (and preferably at least double gas permeability) than neighbouring or surrounding subzones within the zone. For instance it is beneficial when two or more subzones within the zone have higher gas permeability with in between a subzone with lower gas permeability. When ionization current is measured with an ionization pen spanning the subzones with higher gas permeability and the subzone in between with lower gas permeability, a better result is obtained in terms of use of the ionization signal for gas modulation over a broad load range.

Preferably, the surface area of a zone - and preferably of each of the one or more zones - of the one or more zones of the porous combustion surface that are not formed by the fiber based cloth, but by another porous substrate is between 150 and 500 square millimeter, more preferably between 300 and 450 square millimeter, as such ranges give best results. Preferred zones have a convex shape. Preferred zones are square or rectangular. Such zones are providing excellent results.

In a preferred embodiment, the burner comprises a gas premix chamber. In use the premix gas flows from the gas premix chamber partly through the combination of the fiber based cloth and the perforated plate, woven wire mesh or expanded metal sheet supporting the fiber based cloth; and partly through the one or more zones of the porous combustion surface that is or are not formed by the fiber based cloth but by another porous substrate. A specific gas permeability can be defined for the premix gas flow as the gas flow per unit of the surface area; or for part of surface area; of the porous combustion surface.

In a preferred embodiment of the invention, the ratio of the specific gas permeability where - in use of the burner - the premix gas flow from the gas premix chamber will occur through the one or more zones of the porous combustion surface that is or are not formed by the fiber based cloth but by another porous substrate; to the specific gas permeability where - in use of the burner -; the premix gals flow will occur through the combination of the fiber based cloth and the perforated plate, woven wire mesh or expanded metal sheet supporting the fiber based cloth is higher than 3; more preferably higher than 4; and preferably lower than 8; more preferably lower than 7. Experiments have shown that such values for the ratio provide best results in terms of stability of operation of ionization current measurement using an ionization pen.

In a preferred embodiment of the invention at least two zones of the porous combustion surface are not formed by the fiber based cloth. Burners according to this embodiment provide a better ionization signal over a broad load range when using an ionization pen spanning each of the at least two zones. The ionization signal is even significantly better compared to using a burner with one zone of the porous combustion surface that is not formed by the fiber based cloth and which one zone is having a same surface area as all the at least two zones of this embodiment. Similarly the ionization signal is even significantly better compared to using a burner with one zone of the porous combustion surface that is not formed by the fiber based cloth and which one zone has a same length under an ionization pen as the combined length of the ionization pens under all the at least two zones of this embodiment.

In a more preferred embodiment the closest distance between two zones of the porous combustion surface that are not formed by the fiber based cloth is at least 5 mm and preferably the fiber based cloth covers at least 5 mm of the closest distance between two zones; preferably the closest distance is smaller than 15 mm. Burners according to such more preferred embodiment even provide better results in terms of ionization signal over a broad load range, probably because of synergetic effects of recirculation flow in the combustion. In a burner equipped with an ionization pen, the projection line

(perpendicular projection of the ionization pen onto the combustion surface) of the ionization pen onto the combustion surface preferably has between two zones of the porous combustion surface that are not formed by the fiber based cloth a distance of at least 5 mm length (and preferably less than 15 mm) and preferably the fiber based cloth is covering at least 5 mm of that distance; preferably less than 15 mm.

In a preferred embodiment, the one or more zones are along their full circumference surrounded by the fiber based cloth. The technical benefit of the feature is that flame lift off is prevented from the one or more zones of the porous combustion surface that is or are not formed by the fiber based cloth but by another porous substrate. This technical feature synergistically adds to solving the problem in that it allows to have more stable and more reliable ionization current measurement.

In a first group of preferred embodiments, in the one or more zones of the porous combustion surface that are not formed by the fiber based cloth, the porous combustion surface is formed by the perforated plate, woven wire mesh or expanded metal sheet, preferably with higher porosity than outside the one or more zones. In such burners with a higher porosity of the perforated plate, woven wire mesh or expanded metal sheet in the one or more zones of the porous combustion surface that are not formed by the fiber based cloth a better functionality is obtained in terms of a higher load range over which the ionization current can be measured by means of an ionization probe.

Preferably, the porosity of the perforated plate, woven wire mesh or expanded metal sheet in the one or more zones of the porous combustion surface that are not formed by the fiber based cloth and where the perforated plate, woven wire mesh or expanded metal sheet acts as combustion surface is two to five times higher than where the perforated plate, woven wire mesh or expanded metal sheet is covered by the fiber based cloth.

Where a perforated plate is locally forming the combustion surface, the perforations in the perforated plate can be circular and preferably with a diameter between 0.5 and 1.5 mm, preferably between 0.6 and 0.9 mm. Alternatively, through slits (e.g. between 0.4 and 0.6 mm width and between 2 to 6 mm length); or a combination of such circular perforations and such through slits in the perforated plate can locally form the combustion surface. In a further preferred embodiment of the first group of preferred embodiment, the fiber based cloth is supported by a perforated plate. In the one or more zones of the porous combustion surface that are not formed by the fiber based cloth, the porous combustion surface is formed by the perforated plate, preferably with higher porosity in the one or more zones than outside the one or more zones (and more preferably the porosity is 2 to 5 times higher). The fiber based cloth is welded or otherwise fixed to the perforated plate around the edges of the of the one or more zones, and preferably around the edges of each of the one more zones. The fiber based cloth covers through slits that are present in the perforated plate under at least part of the edges of where the fiber based cloth is welded or otherwise fixed to the perforated plate around the one or more zones, and preferably around each of the one or more zones. Preferably the through slits have at least 90% of the length of the side of the zone (zone where not the fiber based cloth but the perforated plate forms the combustion surface - such zone can e.g. be square or rectangular) where the through slit is positioned. Preferably the distance between such through slit and the edge of the fiber based cloth around the zone where the perforated cloth forms the combustion surface is less than or equal to 5 mm, preferably less than or equal to 3 mm. Preferably, these slits have a width of between 0.35 and 1 mm, more preferably between 0.4 and 0.6 mm. The slits provide burners with higher lifetime, in that such burners have a higher resistance to differential thermal stresses in the combustion surface parts formed by and not formed by the fiber based cloth.

In a preferred embodiment of this first group of embodiments, the perforated plate, woven wire mesh or expanded metal sheet is dome shaped in the zone or zones where the combustion surface is formed by the perforated plate, woven wire mesh or expanded metal sheet, thereby extending from the surface shape formed by the perforated plate, woven wire mesh or expanded metal sheet outside the one or more zones of the porous combustion surface that are not covered by the fiber based cloth. It is a benefit of this embodiment that a burner with longer lifetime can be obtained, as the dome shape allows the perforated plate, woven wire mesh or expanded metal sheet to better absorb and withstand the different thermal stresses when using the burner.

In a second group of preferred embodiments an additional woven wire mesh (e.g. made out of a Fe Cr and Al containing alloy, such as e.g. Kanthal AP) forms the porous combustion surface in the one or more zones of the porous combustion surface that are not formed by the fiber based cloth. This way an appropriate combustion surface is formed to allow the use of an ionization pen for burner modulation over a broad load range. The additional woven wire mesh can be connected or fixed to the supporting plate, woven wire mesh or expanded metal sheet (e.g. by welding or by mechanical means). Alternatively the fiber based cloth can be connected or fixed at the edges of the zones to the additional woven wire mesh, preferably without connection to the supporting plate, woven wire mesh or expanded metal sheet. Such a solution allows better absorption of thermal stresses in the different parts (supporting plate, woven wire mesh and fiber based cloth) resulting in a longer lifetime of the gas premix burner. A woven wire mesh provides more constant porosity between products compared to the use of fiber based cloth. Connection of the fiber based cloth (e.g. by means of welding) at the edges of the zones to the additional woven wire mesh can be with the fiber based cloth located between the additional woven wire mesh on the one hand and on the other hand the perforated plate, the woven wire mesh or the expanded metal sheet that is fully or partly covered by the fiber based cloth.

Alternatively, connection of the fiber based cloth (e.g. by means of welding) at the edges of the zones to the additional woven wire mesh can be with the additional woven wire mesh between the fiber based cloth on the one hand and on the other hand the perforated plate, the woven wire mesh or the expanded metal sheet that is fully or partly covered by the fiber based cloth.

In a third group of preferred embodiments an additional porous object is positioned to form the porous combustion surface in the one or more zones of the porous combustion surface that are not formed by the fiber based cloth. It is meant that in such zone the combustion surface is not formed by the fiber based cloth nor by the perforated plate, woven wire mesh or expanded metal sheet that supports the fiber based cloth.

Preferably such additional porous object is an object made from perforated plate, but it can also be made from or with woven wire mesh or from a fiber based cloth with different permeability or from expanded metal sheet. The additional porous object can have through perforations and/or can have an open cell porous structure.

Preferably, the additional porous object has a higher air permeability than the perforated plate, woven wire mesh or expanded metal sheet; and a higher air permeability than the fiber based cloth.

The additional porous object can be placed in the burner in different ways. It can e.g. be placed over the perforated plate, woven wire mesh or expanded metal sheet such that the premix gas is first flowing through the perforated plate, woven wire mesh or expanded metal sheet and then through the porous object after which the premix gas is combusted on the surface of the additional porous object. Alternatively, one or more openings can be present in the perforated plate, woven wire mesh or expanded metal sheet and the additional porous object covers such opening such that at least part of the premix gas does not flow through the pores or perforations of the perforated plate, woven wire mesh or expanded metal sheet but through the one or more openings and then through the additional porous object before being combusted on the surface of the additional porous object.

Preferably the additional porous object is connected or held in the burner in such a way to allow thermal expansion of the additional porous object independently of the thermal expansion of the perforated plate, woven wire mesh or expanded metal sheet. Besides the broad load range over which an ionization pen can be used to measure the flame current in order to modulate the burner, a burner with long lifetime is obtained. Different thermal expansions that exist due to different temperatures in the different zones of the combustion surface can be absorbed more easily.

A first example of connection to allow thermal expansion of the additional porous object independently of the thermal expansion of the perforated plate, woven wire mesh or expanded metal sheet is where the porous object is connected to the perforated plate, woven wire mesh or expanded metal sheet by clamping the porous object to the perforated plate, woven wire mesh or expanded metal sheet, e.g. according to leaf spring action, e.g. in openings in the perforated plate, woven wire mesh or expanded metal sheet at the one or more zones of the porous combustion surface that are not formed by the fiber based cloth. A second example of connection to allow thermal expansion of the additional porous object independently of the thermal expansion of the perforated plate, woven wire mesh or expanded metal sheet is where the additional porous object is held in the perforated plate, woven wire mesh or expanded metal sheet via geometrical constraint, e.g. as a rivet, e.g. in openings in the perforated plate, woven wire mesh or expanded metal sheet at the one or more zones of the porous combustion surface that are not formed by the fiber based cloth.

It is meant that the perforated plate, woven wire mesh or expanded metal sheet is held between parts of the additional porous object without connection, but merely by geometrical obstruction for the additional porous object to be removed out of the perforated plate, woven wire mesh or expanded metal sheet. A way to accomplish such an embodiment is as a rivet, by placing an additional porous object with rivet shape inside the appropriate opening in the perforated plate, woven wire mesh or expanded metal sheet and folding over or buckling the tail of the rivet in order to create the geometrical obstruction for the additional porous object so that it cannot be removed out of the burner. A third example of connection to allow thermal expansion of the additional porous object independently of the thermal expansion of the perforated plate, woven wire mesh or expanded metal sheet is where the additional porous object is connected (e.g. via welding, via riveting...) to the fiber based cloth at the edges of the fiber based cloth around the one or more zones of the porous combustion surface that are not formed by the fiber based cloth.

Such connection can be made with the additional porous object between the perforated plate, woven wire mesh or expanded metal sheet on the one hand and the fiber based cloth on the other hand. Alternatively such connection can be made with the fiber based cloth between the of the perforated plate, woven wire mesh or expanded metal sheet on the one hand and the additional porous object on the other hand.

In a preferred embodiment the gas premix burner comprises an ionization pen to measure the ionization current of the gas premix burner. The ionization pen is positioned covering the one or more zones of the porous combustion surface that are not formed by the fiber based cloth. This way the ionization pen is where the flames are generated in the porous combustion surface and effectively affected by the part of the combustion surface that is not formed by fiber based cloth.

In an even more preferred embodiment the gas premix burner further comprises a control system using the ionization current measured by the ionization pen as input value and wherein the control system is adapted to modulate the air to gas ratio in the premix supply to the burner. The ionization current depends on the burner load (as determined by the amount of gas supply) and the air to gas ratio in the gas premix supply.

When the ionization current can be reliably measured, and knowing the burner load (amount of gas supply) the air to gas ratio of the premix can be derived from the ionization current as measured by the ionization pen. A correct air to gas ratio of the burner is required to obtain clean combustion. The modulation of the premix supply can e.g. be performed by means of volume control of the supply of combustible gas (e.g. natural gas) or air to the premix in order to obtain for each burner load (determined by the amount of gas supply) the correct air to gas ratio leading to clean and optimum combustion.

The fiber based cloth can comprise metal fibers. Examples of preferred ranges of metal fibers are stainless steel fibers. A specifically preferred range of stainless steel fibers are chromium and aluminium comprising stainless steel fibers as in DIN 1 .4767, e.g. as are known under the trademark FeCrAlloy. Preferred are fibers with equivalent diameter less than 40 μιτι. With equivalent diameter of a fiber is meant the diameter of a circle with the same surface area as the cross sectional area of that fiber.

Metal fibers for the fiber based cloth, e.g. stainless steel fibers, with an equivalent diameter less than 40 micrometers, e.g. less than 25 micrometers, can be obtained by a bundle drawing technique. This technique is disclosed e.g. in US-A-2050298, US-A- 3277564 and in US-A-3394213. Metal wires are forming the starting material and are covered with a coating such as iron or copper. A bundle of these covered wires is subsequently enveloped in a metal pipe. Thereafter the thus enveloped pipe is reduced in diameter via subsequent wire drawing steps to come to a composite bundle with a smaller diameter. The subsequent wire drawing steps may or may not be alternated with an appropriate heat treatment to allow further drawing. Inside the composite bundle the initial wires have been transformed into thin fibers which are embedded separately in the matrix of the covering material. Such a bundle preferably comprises no more than 2000 fibers, e.g. between 500 and 1500 fibers. Once the desired final diameter has been obtained the covering material can be removed e.g. by solution in an adequate pickling agent or solvent. The final result is the naked fiber bundle.

Alternatively metal fibers for the fiber based cloth, such as stainless steel fibers can be manufactured in a cost effective way by machining a thin plate material. Such a process is disclosed e.g. in US-A-4930199. A strip of a thin metal plate is the starting material. This strip is wound around the cylindrical outer surface of a rotatably supported main shaft a number of times and is fixed thereto. The main shaft is rotated at constant speed in a direction opposite to that in which the plate material is wound. A cutter having an edge line expending perpendicularly to the axis of the main shaft is fed at constant speed. The cutter has a specific face angle parallel to the axis of the main shaft. The end surface of the plate material is cut by means of the cutter.

Yet an alternative way of producing metal fibers for the fiber based cloth is via extracting or extrusion from a melt.

Another alternative way of producing metal fibers is machining fibers from a solid block of metal.

As an alternative for or in combination with metal fibers, ceramic fibers can be used in the fiber based cloth. Metal fibers are preferred however, because of the good electrical conductivity beneficial when measuring the ionization current of the flame on the combustion surface of the burner.

[33] The fiber based cloth can e.g. comprise or be a woven fabric, or a knitted fabric, or a braided fabric comprising yarns with e.g. metal fibers, preferably stainless steel fibers. The yarns can be spun from stretch broken fibers (such as bundle drawn stretch broken fibers) or yarns made from shaved or machined fibers. The yarns can be plied yarns, e.g. two ply, three ply... Preferred fabrics made from metal fibers have a weight of between 0.6 and 3 kg/m ² ; preferably between 0.7 and 3 kg/m ² , even more preferred between 1.2 and 2.5 kg/m ² .

Alternative fiber based cloths that can be used in the invention can comprise or can be nonwovens, e.g. comprising metal fibers (preferably stainless steel fibers). The nonwovens can be consolidated by different techniques (e.g. needle punching) and can be sintered or unsintered; unsintered fabrics are preferred however.

[34] A second aspect of the invention is a method to control the air to gas ratio of a gas premix burner, wherein a gas premix burner is used as in the first aspect of the invention and wherein the ionization current measured by means of the ionization pen is used in a control system to modulate the air to gas ratio. The ionization pen is preferably installed in such a way that it covers, e.g. at least partly, the one or more zones where the combustion surface is not formed by the fiber based cloth. Specific examples allow e.g. a modulation range from 1 :3 to 1 :15 of the burner.

[35] In a preferred method to control the air to gas ratio of a gas premix burner installed in a boiler, the ratio of the ionization current at maximum load of the gas premix burner installed in the boiler is less than 50% higher than the ionization current at minimum load of the gas premix burner installed in the boiler. It is a further benefit of this embodiment that even better control possibility exist, as the ionization signal is less dependent from the burner load and more constant at a high level over a broad range of the burner load.

[36] A third aspect of the invention relates to the use of the burner of the first aspect of the invention. Examples of use are boilers (e.g. central heating boilers) or water heaters, e.g. instantaneous water heaters or direct fired water heaters, that comprise a gas premix burner as in the first aspect of the invention and/or that are using a method as in the second aspect of the invention to control the air to gas ratio supplied to the gas premix burner. The hot flue gas generated by the gas premix is transferring its heat to a fluid (mostly water) in a heat exchanger. Specific examples of use are such boilers or water heaters with a capacity between 8 and 1000 kW, preferably between 8 and 60 kW.

Specific examples allow e.g. a modulation range from 1 :3 to 1 : 15.

Brief Description of Figures in the Drawings

[37] Figure 1 shows an example of a burner according to the invention.

Figures 2, 3, 4, 5, 6 and 7 show cross sections along line ll-ll of figure 1 of burners according to the invention.

Figure 8 shows an alternative embodiment of the invention. Figures 9-13 show embodiments in which an additional woven wire mesh is used to form part of the combustion surface.

Figures 14-16 show embodiments of the invention wherein the perforated plate acts as combustion surface in the one or more zones and wherein these zones are on one or more of their sides surrounded by through slits.

Figure 17 shows ionization current values for a burner according to the invention compared to a prior art burner.

Mode(s) for Carrying Out the Invention

[38] Figure 1 shows an example of a burner 100 according to the invention. The burner 100 (here a flat burner, but the invention can be performed on a cylindrical or frusto-conical burner as well in similar way as on a flat burner) has a perforated plate 1 10 (in the same way a woven wire mesh or expanded metal sheet can be used) and supports a fiber based cloth 120 over the largest part of its surface, wherein the fiber based cloth 120 forms part of the combustion surface. The combustion surface has two zones (140, 142) that are not formed by the fiber based cloth 120, but by another porous substrate. Such porous substrate that acts as combustion surface in these zones can be the perforated plate 1 10 (preferably with in such zone or zones higher porosity than outside those zones) or an additional porous object, e.g. a woven wire mesh. Figure 1 shows two of such zones, but it is possible to use one zone, or three zones, or four zones as well. Figure 1 further shows the position of an ionization pen 130 which is preferably positioned in the burner such that it is positioned over the zones 140, 142 where the porous combustion surface is not formed by the fiber based cloth 120. Preferred is when the zones 140, 142 are lengthwise oriented so that a long length of the zone is covered by the ionization pen 130. Preferred is also that the ionization pen 130 is positioned substantially centrally compared to the zones 140, 142, as is indicated in figure 1.

The length L1 of a flat burner of the invention is preferably between 80 and 700 mm; and the width L2 preferably between 80 and 700 mm. The sides L3 and L4 of rectangular zones where the fiber based cloth 120 does not form the combustion surface are preferably between 12 and 30 mm.

In the example, the zones have a square shape, but they can be rectangular or circular or of another shape, preferably a convex shape. Square and rectangular shapes are preferred however.

[39] Figure 2 shows a cross section 200 along lines ll-ll of figure 1 of an embodiment of the invention. The cross section 200 shows the perforated plate 210 and the fiber based cloth 220. In the zone where the combustion surface is not formed by the fiber based cloth 220, an opening is present in the perforated plate 210 and another porous object 260 is present (alternatively another porous object can be placed on top of the perforated plate 210). The additional porous object 260 can be a perforated plate object, possibly shaped to fit the location. The fiber based cloth 220 can be welded onto the additional porous object 260, preferably at the edges of the additional porous object 260 via welds indicated with the arrows 281 and 282, this way the additional porous object 260 is held in place. It is also possible to additionally weld the fiber based cloth 220 to the perforated plate 210 around where the additional porous object 260 is located, by means of welds indicated at arrows 285 and 286. Figure 2 also indicates the ideal position of an ionization pen 230 when such burner is in use with ionization current measurement for air to gas ratio modulation.

Figure 3 shows a cross section 300 along lines ll-ll of figure 1 of an embodiment of the invention. The cross section 300 shows the perforated plate 310 and the fiber based cloth 320. In the zone where the combustion surface is not formed by the fiber based cloth 320, an opening is present in the perforated plate 310 and another porous object 360 is placed. The additional porous object 360 can be a perforated plate object, possibly shaped to fit the location. The fiber based cloth 320 can be welded onto the additional porous object 360, preferably at the edges of the additional porous object 360 via welds indicated with the arrows 381 and 382. It is also possible to weld the fiber based cloth 320 to the perforated plate 310 around where the additional porous object 360 is located, by means of welds indicated at arrows 385 and 386. In this example the additional porous object 360 is shaped such that it is geometrically stuck in the burner as it is held by the perforated plate 310 as the perforated plate 310 is geometrically stuck between the welds 381 and 382 that connect the additional porous object 360 to the fiber based cloth 320 and the base of the additional porous object 360. Figure 3 also indicates the ideal position of an ionization pen 330 when such burner is in use with ionization current measurement for air to gas ratio modulation.

Figure 4 shows a cross section 400 along lines ll-ll of figure 1 of an embodiment of the invention according to figure 1 . The cross section 400 shows the perforated plate 410 and the fiber based cloth 420. In the zone where the combustion surface is not formed by the fiber based cloth 420, an opening is present in the perforated plate 410 and another porous object 460 is placed (alternatively another porous object can be placed on top of the perforated plate 410). The additional porous object 460 can be a perforated plate object. In figure 4 the additional porous object 460 is supported by the perforated plate 410. The fiber based cloth 420 can be welded onto the additional porous object 460, preferably at the edges of the additional porous object 460 via welds indicated with the arrows 481 and 482, at the other side of the additional porous object 460 than where the perforated plate 460 is located. It is also possible to weld the fiber based cloth 420 to the perforated plate 410 around where the additional porous object 460 is located, by means of welds indicated at arrows 485 and 486. Figure 4 also indicates the ideal position of an ionization pen 430 when such burner is in use with ionization current measurement for air to gas ratio modulation.

Figure 5 shows an alternative possible cross section 500 along lines ll-ll of figure 1 of an embodiment of the invention according to figure 1. The cross section 500 shows the perforated plate 510 and the fiber based cloth 520. In the zone where the combustion surface is not formed by the fiber based cloth 520, an opening is present in the perforated plate 510 and another porous object 560 forms in that zone the combustion surface. The additional porous object 560 can be a perforated plate object, but can also be another object with through perforations or open cell porosity, e.g. a piece of woven wire mesh. In figure 5 the additional porous object 560 is supported by fiber based cloth 520 onto which it is welded e.g. at positions 581 , 582 or otherwise fixed onto the additional porous object 560. Preferably there is no direct fixation (and hence no weld) between the additional porous object 560 and the perforated plate 510. It is also possible to weld the fiber based cloth 520 to the perforated plate 510 around where the additional porous object 560 is located, by means of welds indicated at arrows 585 and 586. Figure 5 also indicates the ideal position of an ionization pen 530 when such burner is in use with ionization current measurement for air to gas ratio modulation.

Figure 6 shows a cross section 600 of another embodiment of the invention. A porous object 662 is via geometrical constraint held in an opening in the perforated plate 610 (or in the woven wire mesh or expanded metal sheet). The porous object 662 forms locally the combustion surface of the burner. Around the zone where the porous object 662 forms the combustion surface the fiber based cloth 620 is positioned and fixed e.g. via welding to the perforated plate 610 around the opening in the perforated plate where the porous object 662 is positioned. It is also possible that the fiber based cloth 620 is additionally welded onto the additional porous object 662 (not shown in figure 6). Figure 6 also indicates the ideal position of an ionization pen 630 when such burner is in use with ionization current measurement for air to gas ratio modulation.

A way of accomplishing such an embodiment is by placing a porous object with rivet shape inside the appropriate opening in the perforated plate, woven wire mesh or expanded metal sheet and folding over or buckling the tail of the rivet shape in order to create the geometrical obstruction for the additional porous object so that it cannot be removed out of the burner.

Figure 7 shows an alternative cross section 700 of a burner according to the invention. A porous object 764 is clamped into an opening in the perforated plate 710 and forms there the combustion surface. The clamping can be achieved by having a porous object 764 with legs which have a certain elastic flexibility. By compressing the legs the porous object 764 can be positioned into the opening in the perforated plate 710 and when releasing the legs, they return by elasticity to their original shape thereby clamping the porous object 764 in the opening in the perforated plate 710. The fiber based cloth 720 is present around the opening in the perforated plate 710. The fiber based cloth 720 can be fixed onto the perforated plate 710 around the opening, e.g. by welding (e.g. welds at locations 785 and 786). It is also possible to fix the fiber based cloth 720, e.g. via welding, also to the porous object 764 (not shown in the figure). Figure 7 also indicates the ideal position of an ionization pen 730 when such burner is in use with ionization current measurement for air to gas ratio modulation. Figure 8 shows an alternative cross section 800 of a burner according to the invention. A zone is present where the fiber based cloth 820 is not the combustion surface. In that zone the combustion surface is formed by a dome 895 in the perforated plate 810.

Around the dome 895 the fiber based cloth 820 is present. The fiber based cloth 820 can be fixed onto the perforated plate 810 around the dome 895 or at the start of the dome 895, e.g. by welding (e.g. welds at locations 885 and 886). Figure 8 also indicates the ideal position of an ionization pen 830 when such burner is in use with ionization current measurement for air to gas ratio modulation.

Figure 9 shows an example of a burner 900 according to the invention. The burner 900 (here a flat burner, but the invention can be performed on a cylindrical or frusto-conical burner as well in similar way as on a flat burner) has a perforated plate 910 and supports a fiber based cloth 920 over the largest part of its surface, wherein the fiber based cloth 920 forms part of the combustion surface. The combustion surface has two zones that are not formed by the fiber based cloth 920, but formed by woven wire meshes (992, 994). Such woven wire meshes preferably have a higher gas permeability than the fiber based cloth 920. Figure 9 shows two zones with woven wire meshes forming the combustion surface, but it is possible to use one zone, or three zones, or four zones as well. Preferably between the two zones with wire meshes 992, 994 fiber based cloth is present (see 914) and acting there as combustion surface. Figure 9 further shows the position of an ionization pen 930 which is preferably positioned in the burner such that it is positioned over the zones where the porous combustion surface is not formed by the fiber based cloth 920. The woven wire meshes 992, 994 can be placed onto the perforated plate 910 or in or on openings in the perforated plate 910.

The length L1 of a flat burner of the invention is preferably between 80 and 700 mm; and the width L2 preferably between 80 and 700 mm. the sides L3 and L4 of rectangular zones where the fiber based cloth 120 is not forming the combustion surface are preferably between 12 and 30 mm.

In the example, the zones have a square shape, but they can be rectangular or circular or of another shape. Square and rectangular shapes are preferred however.

Figure 10 shows a cross section 1000 along lines X-X of figure 9 of an embodiment of the invention. The cross section 1000 shows the perforated plate 1010 and the fiber based cloth 1020. In the zone where the combustion surface is not formed by the fiber based cloth 1020, a woven wire mesh 1092 is positioned onto the perforated plate 1010 (and preferably the porosity of the perforated plate is higher in that zone). The fiber based cloth 1020 can be welded or otherwise fixed onto the woven wire mesh 1092, preferably at the edges of woven wire mesh 1092 via welds indicated with the arrows 1081 and 1082. It is also possible to weld the fiber based cloth 1020 to the perforated plate around where the woven wire mesh is positioned (1085, 1086). Figure 10 also indicates the ideal position of an ionization pen 1030 when such burner is in use with ionization current measurement for air to gas ratio modulation. Figure 1 1 shows a cross section 1 100 along lines X-X of figure 9 of an embodiment of the invention. The cross section 1 100 shows the perforated plate 1 1 10 and the fiber based cloth 1 120. In the zone where the combustion surface is not formed by the fiber based cloth 1 120, a woven wire mesh 1 192 is positioned onto the perforated plate 1 1 10 (and preferably the porosity of the perforated plate is higher in that zone), with the woven wire mesh 1 192 resting onto the edges of the fiber based cloth 1 120 around where the woven wire mesh 1 192 forms the combustion surface. The fiber based cloth 1 120 can be welded or otherwise fixed onto the woven wire mesh 1 192, preferably at the edges of woven wire mesh 1 192 via welds indicated with the arrows 1 181 and 1 182. It is also possible to weld the fiber based cloth 1 120 to the perforated plate around where the woven wire mesh is positioned (1 185, 1 186). Figure 1 1 also indicates the ideal position of an ionization pen 1 130 when such burner is in use with ionization current measurement for air to gas ratio modulation.

Figure 12 shows a cross section 1200 along lines X-X of figure 9 of an alternative embodiment of the invention. The cross section 1200 shows the perforated plate 1210 and the fiber based cloth 1220. In the zone where the combustion surface is not formed by the fiber based cloth 1220, a woven wire mesh 1 192 is positioned over an opening in the perforated plate 1210. The fiber based cloth 1 120 can be welded or otherwise fixed onto the woven wire mesh 1292, preferably at the edges of woven wire mesh 1292 via welds indicated with the arrows 1281 and 1282. It is also possible to weld the fiber based cloth 1220 to the perforated plate around where the woven wire mesh is positioned (1285, 1286). Figure 12 also indicates the ideal position of an ionization pen 1230 when such burner is in use with ionization current measurement for air to gas ratio modulation. Figure 13 shows an example of a burner 1300 according to the invention. The burner 1300 (here a flat burner, but the invention can be performed on a cylindrical or frusto- conical burner as well in similar way as on a flat burner) has a perforated plate 1310 and supports a fiber based cloth 1320 over the largest part of its surface, wherein the fiber based cloth 1320 forms part of the combustion surface. The combustion surface has one zone that is not formed by the fiber based cloth 1320, but formed by a woven wire mesh 1392. The woven wire mesh 1392 preferably has a higher gas permeability than the fiber based cloth 1320. Figure 13 further shows the position of an ionization pen 1330 which is preferably positioned in the burner such that it is positioned over the zones where the porous combustion surface is not formed by the fiber based cloth 1320. The woven wire mesh 1392 can be placed onto the perforated plate 1310 or in or on openings in the perforated plate 1310.

The length L1 of a flat burner of the invention is preferably between 80 and 700 mm; and the width L2 preferably between 80 and 700 mm. The sides L3 and L4 of rectangular zones where the fiber based cloth 120 does not form the combustion surface are preferably between 12 and 30 mm. In the example, the zone has a square shape, but it can be rectangular or circular or of another shape. A square or a rectangular shape is preferred however.

Figure 14 shows part 1410 of a perforated plate that can be used in a burner according to the invention. The perforated plate 1410 has a perforation pattern with perforations 1413 where it will support the fiber based cloth (the fiber based cloth is not shown on the figure). It has one or more zones (two zones shown in figure 14, but a perforated plate according to the invention can e.g. have one, two, three or four such zones) with perforations 1417 that create a higher porosity of the perforated plates in such zones. Such zones will fully or partly not be covered by fiber based cloth and such zones will form part of the combustion surface of the burner. The perforated plate 1410 has through slits 1415 along sides of the one or more zones, and preferably around each of the one or more zones. Preferably the through slits have a width of between 0.35 and 1 mm, more preferably between 0.4 and 0.6 mm. Preferably, such through slits have a length of at least 90% of the length of the side of the zone where the perforated plate is forming the combustion surface along which the through slits are positioned.

Figure 15 shows an alternative perforated plate 1510 that can be used in a burner according to the invention. The perforated plate 1510 has a perforation pattern with perforations 1513 where it will support the fiber based cloth (the fiber based cloth is not shown on the figure). It has one or more zones (two zones shown in figure 15, but a perforated plate according to the invention can e.g. have one, two, three or four such zones) with perforations 1517 that create a higher porosity of the perforated plates in such zones. Such zones will fully or partly not be covered by fiber based cloth and such zones will form part of the combustion surface of the burner. The perforated plate 1510 has through slits 1515 along each of the sides of the one or more zones, and preferably around each of the one or more zones. Preferably the through slits have a width of between 0.35 and 1 mm, more preferably between 0.4 and 0.6 mm.

Figure 16 shows a burner 1600 according to the invention. A perforated plate 1610 is used as is shown in figures 14 or 15. A fiber based cloth 1620 forms the major part of the combustion surface of the burner 1600. The fiber based cloth 1620 is supported by the perforated plate 1610. The fiber based cloth 1620 is fixed onto the perforated plate 1610, preferably by means of welding, around the zones where the perforated plate 1610 forms the combustion surface, the fiber based cloth 1620 thereby covers the through slits 1415 of the perforated plate of figure 14, or of the perforated plate of figure 15. Preferably the through slits have a least 90% of the length of the side of the zone where the through slit is positioned. Preferably the distance between such through slit and the edge of the fiber based cloth around the zone where the perforated cloth forms the combustion surface is less than or equal to 5 mm, preferably less than or equal to 3 mm.

The zones where the perforated plate 1610 forms the combustion surface has perforations 1617. The porosity of the perforated plate 1610 where it forms the combustion surface is preferably two to five times higher than the porosity outside that zone.

Figure 16 further shows the possible position of an ionization pen 1630 which is preferably positioned in the burner such that it is positioned over the zones where the porous combustion surface is not formed by the fiber based cloth 1620.

Fabrics knitted from yarns made out of stainless steel fibers according to DIN 1.4767 can be used as fiber based cloth in each of the examples that have been shown. The knitted fabric has e.g. a surface weight of 1.4 kg/m ² . In a similar way a woven fabric made out of the same or similar yarns can be used.

Figure 17 shows comparative ionization current measurements (Y, in relative units) as a function of the relative burner load (X, in per cent of the full burner load) for:

- curve A: a burner according to the invention as shown in figure 8. The fiber based cloth is a 1.4 kg/m ² knitted fabric made out of yarns from stainless steel fibers.

- curve B: a burner of otherwise the same dimensions and features, but without the dome and in which the combustion surface is completely formed by a 1 .4 kg/m ² knitted fabric made out of yarns from stainless steel fibers (same fiber based cloth as used in the burner of the invention for which the ionization current is shown in curve A).

Figure 17 shows that the ionization current of the burner according to the invention can be used into a lower range of burner load for modulation of the burner. Even at 10% of the burner load the ionization current can still be used for the modulation of the burner. Furthermore, when testing different same burners, the ionization curves of the burners of the invention do not vary much in the lower load range compared to prior art burners, also making the burner of the invention more suited for use with modulation of the burner in the low load ranges.

Elements of different embodiments and/or elements of different examples of the invention can be combined within the scope of the invention.