Premix Burner

Abstract

A premix burner (100) comprising a hollow body (1) with openings (22) in a boundary wall (2) defining a number of channels through which a mixture of air and hydrogen can escape and burn permanently without flashback on the outer surface (24) of the boundary wall (2). The particular porosity and load value of the burner (100) are selected to ensure stable operation of the burner (100).

Description

[Technical field]

[0001] The present invention relates to a premix burner for burning a gas mixture, preferably a gas mixture based on air and at least hydrogen. [Background technology]

[0002] This type of burner, frequently used in boilers for domestic heating, has a hollow body with an opening on the surface on which the flame is generated by the combustion of a gas mixture, traditionally consisting of air and one or more fossil gases, such as methane.

[0003] However, the burning of gas mixtures, including fossil gases, has several environmental drawbacks, such as the emission of substances such as carbon monoxide, carbon dioxide and nitrogen oxides, which pollute the environment, are harmful to health and contribute to the problem of global warming.

[0004] To overcome the drawbacks of burning fossil gas, major industrialized countries have increased investments in recent years to support research and development of burners capable of burning gas mixtures that include hydrogen.

[0005] Burning hydrogen in the presence of air makes it possible to eliminate the problem of monoxide and carbon dioxide emissions, because the reaction between hydrogen and oxygen in the air does not produce carbon compounds as unburned residues.

[0006] However, the use of hydrogen as a gas in a premix burner presents some practical difficulties due to hydrogen's flame temperature value, reactivity, and fast flame speed.

[0007] In particular, since hydrogen or a mixed gas containing hydrogen burns at a high speed, it is difficult to generate a stable flame, and there is a possibility that the so-called flashback phenomenon will occur.

[0008] A flame produced by the combustion of a gas mixture in a burner is stable when the flame speed, i.e., the speed at which the gas mixture burns at the burner surface, is equal to the speed at which the gas mixture itself propagates from the burner inlet to the combustion surface. When these two velocities are approximately equal, an equilibrium is reached between the amount of fresh gas mixture that can flow to the combustion surface and supply the flame per unit of time, and the amount of gas mixture that is burned per unit of time.

[0009] In the case of hydrogen or gas mixtures containing hydrogen, unless appropriate design measures are taken, the speed at which the gas mixture propagates (the so-called output speed) can be significantly lower than the flame speed, and a flame can form inside the burner rather than on the surface. This phenomenon is known as flashback and is particularly dangerous in that it can trigger an explosion of the burner or of other parts of the boiler in which the burner is usually installed, exposing users to significant risks.

[0010] Conversely, if the output velocity of the gas mixture is much greater than the output velocity of the flame, the flame will tend to peel off from the burner surface, a condition known as flame lift-off, and may cause the flame to go out.

[0011] It is clear that both flashback and flame lift-off conditions destabilize the flame.

[0012] From these brief introductory considerations, it can be seen how crucial the ratio of the mixture gas propagation speed to the flame speed is from the standpoint of flame stability.

[0013] In general, this ratio depends on various physical-chemical parameters, such as the type of combustible gas present in the combustion mixture, the mass ratio of air to the gas mixed with it, the pressure distribution inside the burner, and the heat exchange taking place between the burner and the flame.

[0014] The high flame temperature of hydrogen and the strong reactivity of this gas can contribute to the flashback phenomenon and also increase polluting emissions of nitrogen oxides.

[0015] Therefore, to use a burner for the combustion of a gas mixture containing hydrogen, it is essential to be able to control both the ratio of the propagation speed of the gas mixture to the flame speed and the temperature of the burner, the latter of which must be kept low to match the power required of the burner in order to reduce the possibility of flashback.

[0016] A prior art burner designed for the combustion of reactive gas mixtures such as hydrogen is described in the international application published under number WO 95 / 23315A1. This burner has a low porosity comprised between 0.5% and 4% and a diameter of 300 mm. 2 and an opening area value not exceeding

[0017] In general, it can be considered that for a given burner and a given gas mixture there exists a working region within which the burner can operate stably and the probability of flashback is substantially negligible, even if the above mentioned physical-chemical parameters are changed.

[0018] A design parameter that is often used as a practical indicator for the expansion of the working area is the specific heat load of the burner, defined as the ratio between the nominal power and the so-called open area of ​​the burner. Burners conventionally comprise a perforated deck, for example of cylindrical or parallelepiped shape, on whose surface the openings are formed (for example by a punching machine) in which the flame is formed when the air-gas mixture is burned. The open area is defined as the total area of ​​all openings on the deck, whereas the total area is defined as the total area of ​​the deck.

[0019] For a given nominal power that a burner is designed to produce, and for a given mixture of air and gas, the specific load, defined as the ratio of nominal power to open area, will vary within a limited and very precise range of values ​​when the burner is operating stably.

[0020] For example, for a burner designed to burn a mixture of air and methane and to provide a nominal power comprised between 10 and 150 kW, the specific load will be approximately 10 to 30 W / mm when the burner is operating at steady state. 2 The time varies between 0 and 100 ms. Summary of the Invention

[0021] SUMMARY OF THE PRESENT EMBODIMENT It is an object of the present invention to provide a premix burner which overcomes the aforementioned shortcomings of the prior art.

[0022] In particular, it is an object of the present invention to provide a premix burner for burning a mixed gas, preferably comprising air and at least hydrogen as the combustible gas, which is capable of generating a stable flame on the boundary wall and capable of providing heat output in a wide operating range without flashback.

[0023] More specifically, the object of the present invention is to provide a premix burner for burning a gas mixture, preferably comprising air and at least hydrogen as the combustible gas, in which the power to flame speed ratio and the boundary wall temperature can be maintained at values ​​at which the burner operates without flashback.

[0024] More specifically, the object of the present invention is to provide a premix burner for burning a mixed gas, preferably comprising air and at least hydrogen as combustible gas, the flame being controlled by an opening in the wall, which is capable of generating a stable flame on the boundary wall at a given distance.

[0025] Said object is fully achieved by a burner as claimed in claim 1.

[0026] In particular, the burner object of this claim comprises a hollow body having a boundary wall of a non-null thickness and provided with openings formed between an inner surface facing the interior of the hollow body and an outer surface opposite the inner surface, the openings being configured to define a plurality of channels between said surfaces allowing escape of a gas mixture to be combusted. The burner is sized to provide a predetermined nominal power output.

[0027] The burner has a specific load of 25W / mm 2 from 70W / mm 2 and characterized in that the porosity of the boundary wall, i.e. the deck through which the flame is formed, is comprised between 10% and 45%. The porosity is defined as the percentage ratio of the open area of ​​the deck to the total area of ​​the deck, the open area of ​​the deck being given by the sum of the areas of all openings formed in the deck.

[0028] Generally, 25W / mm 2 from 70W / mm 2 The specific load values ​​included between 10 kW and 150 kW, in particular for nominal powers included between 10 kW and 150 kW, make it possible to burn a gas mixture containing a highly reactive gas such as hydrogen without substantial risk of flashback. These load values ​​make it possible to adjust the output speed of the gas mixture so that it approximates the flame speed of the gas mixture itself. As will be explained in more detail below, the flame front in the burner reaches a stable state when the two speeds are comparable. In this state, the possibility of flashback phenomena is significantly reduced. Recalling that for a given nominal power, the specific load value of the burner is given by the ratio of said power to the aforementioned opening area, the specific load of the burner according to the invention can in fact be adjusted by choosing an appropriate value of the opening area of ​​the deck.

[0029] The porosity values ​​adopted for the burner deck according to the invention make it possible to control the heat exchange and thermo-hydrodynamic behavior of the burner so as to increase its resistance to flashback. In particular, said porosity values ​​make it possible to reduce the degree of recirculation of the combustion gases outside the opening, i.e. the amount of flame heat that returns to the burner deck due to the vortexes that occur in the vicinity of the opening. An increase in porosity reduces the volume of the swirl area, lowering the recirculation speed and generally weakening the degree of swirl. The resulting reduction in recirculation leads to a decrease in the deck temperature, which significantly reduces the risk of flashback. An increase in deck temperature increases the likelihood of flashback.

[0030] As mentioned above, the probability of flashback also depends on the ratio of the output velocity of the combusted gas mixture to the flame speed. In general, a flame generated on the burner deck surface is stable when, for each point on this surface, the output velocity of the gas mixture is equal to the flame speed, since under these conditions the flame front does not change with time.

[0031] The output velocity of the mixed gas depends on various factors, such as the type of gas present, the ratio of the amount of carburant (usually air) to the amount of another combustible gas, and the heat exchange established within the burner. In general, the output velocity has a spatial distribution. Highly reactive gases, such as hydrogen, are characterized by high flame speeds. In the absence of proper design arrangements, the flame speed of the highly reactive gases will generally be higher than the output velocity of the mixed gas from the openings formed in the surface of the deck.

[0032] In the burner according to the invention, by acting on the value of the deck open area, it is possible to adjust the output velocity of the mixed gas to approximate the output velocity of the flame in its intended operating range. Due to the relationship between the specific load, open area and nominal power of a burner as described above, the application of a specific load equivalently means that for a burner sized to deliver a certain nominal power, the open area and therefore the output velocity are fixed.

[0033] In conclusion, for a burner according to the invention, it is possible to significantly reduce the risk of flashback by controlling the output rate of the mixed gas by selecting the open area value of the deck (i.e. for a particular load) and by controlling the degree of recirculation by selecting the porosity value.

[0034] It is important to emphasize that, for the same open area, a burner with high porosity is less exposed to the risk of flashback than a burner with low porosity. A more porous burner obviously has a smaller "closed" surface, i.e. a surface without openings, and therefore a smaller surface with which the very hot combustion gases come into contact, causing an increase in the temperature of the burner. As a result, a more porous burner has a lower deck temperature than a less porous burner with the same open area. The burner of the present invention therefore has a combined technical effect, produced by the deck porosity and open area, in terms of reducing the risk of flashback.

[0035] The upper limit of the maximum porosity achievable in the deck is determined by the technical characteristics of the machine used to perforate the deck, such as the punch dimensions, if the openings are formed by punching, and by the limits of resistance to stress of the materials used to manufacture the boundary walls. Conversely, the lower limit is determined by the occurrence of the flashback phenomenon: for a given nominal power and a given gas mixture, there is a porosity value below which the degree of recirculation of the combustion gases near the openings, and therefore the flame heat returning to the burner, exceeds a level at which the burner is exposed to a substantial risk of flashback.

[0036] Porosity is a relative quantity, since it represents the ratio of the total area of ​​the openings in the deck (or in the boundary walls) to the total area of ​​the latter. It is therefore possible in principle to make burners with the same porosity but with different opening area values.

[0037] The burner according to the invention has an opening area of ​​25 W / mm 2 from 70W / mm 2The thickness is selected in combination with the porosity within a predetermined range of values ​​maintained between 0.1 and 0.5.

[0038] Selecting an aperture area value for a given nominal power output allows for the adjustment of the output rate of the mixed gas. As the aperture area decreases, the output rate increases, but the total flow rate of the mixed gas required to deliver the required power remains the same. For gases characterized by high reactivity and therefore high flame speeds, such as hydrogen, controlling the aperture area allows the ratio of power to flame speed to be appropriately modified and adjusted in order to reduce the risk of flashback.

[0039] Minimum specific load (25W / mm 2 ) and maximum value (70W / mm 2 ), the minimum and maximum values ​​between which the open area value of the burner according to the invention is selected, define operational limits within which a premix burner sized to produce a nominal power output, preferably comprised between 10 kW and 150 kW, can stably support on its outer surface a flame obtained by combustion of a gas mixture containing hydrogen, i.e. without substantial risk of flashback or detachment (lift-off) of the flame itself.

[0040] The dimensions and spatial density of the openings present in the boundary wall, i.e. deck, of the burner according to the invention therefore have the particular technical effect of making it possible to generate stable flames over a wide operating range starting from gas mixtures including highly reactive gases such as hydrogen.

[0041] Since a reduction in the opening area leads to an increase in the pressure loss, for a given power output there is a limit to the minimum value for which the opening area can be compatible with the maximum allowable pressure loss, which is assigned as a design specification together with the range of nominal power values ​​within which the burner must be able to burn. Conversely, there is an upper limit to the maximum value that the opening area can take, which is essentially determined by the occurrence of the flashback phenomenon: for a given gas mixture and a given nominal power output there is a maximum value for the opening area, beyond which the output velocity of the gas mixture becomes too small relative to the flame speed and the burner is exposed to a significant risk of flashback.

[0042] The burner according to the invention is designed to provide a nominal power output preferably comprised between 10 kW and 150 kW. The maximum nominal power output required determines the physical dimensions of the burner, which are larger the higher the required power output.

[0043] As an example, a cylindrical burner with a diameter of 70 mm, typically used for commercial equipment and distributed in the central area of ​​the deck, located between a first blind zone close to the fixing flange and a second blind zone close to the cover that closes the burner at the top, usually has a height of about 42 mm if designed to deliver a nominal power of 25 kW, and with the same diameter the height reaches about 74 mm if the burner is sized to deliver a power of 150 kW. In the case of a flat burner, for example a flat burner of parallelepiped shape, a device sized to generate a power of 25 kW has for example a width of about 50 mm and a length of about 90 mm. In the case of a device designed to deliver 150 kW, the width and length reach 100 mm and 176 mm, respectively.

[0044] The open area of ​​the deck of the burner according to the invention is preferably 400 mm for a nominal power comprised between 10 kW and 150 kW. 2 From 3000mm 2 Between 450mm and 500mm, more preferably 450mm 2 From 1800mm 2For these open area values, the burner of the invention has a lower deck temperature in the indicated operating range (10-150 kW) than the prior art burners described above, for example with reference to International Application WO 95 / 23315 A1, in particular when used to burn gas mixtures containing hydrogen.

[0045] The reduction in temperature leads to an increase in the operating range in which the burner can burn hydrogen-based gas mixtures and deliver the required nominal power without experiencing flashback, compared to known burners. This effect is particularly evident in combination with porosity values ​​comprised between 21% and 35%. These values ​​are within the range of 400 mm 2 From 3000mm 2 Between 450mm and 500mm, preferably 2 From 1800mm 2 In combination with the aforementioned open area values ​​included between 0.1 and 0.5, it can be used to obtain a reduction in deck temperature compared to the prior art.

[0046] The technical effects mentioned above in terms of the opening area and porosity values ​​are achieved by the burner according to the invention, regardless of the shape of the opening. It is therefore possible to use openings of any known shape, for example round or with elongated slits (also called slots), in order to control the local flame distribution or pressure loss in combination with the above-mentioned porosity and opening area values. By combining openings of different shapes, it is also possible to create a distribution of openings characterized by the porosity and opening area values ​​already described. By combining openings of different shapes, it is also possible to control the distribution of the flame on the surface of the deck.

[0047] The technical effects mentioned above with regard to the open area and porosity values ​​can also be achieved by the burner according to the invention using, for example, a uniform distribution of periodic openings or utilizing a non-uniform distribution.

[0048] The boundary wall, i.e. the deck, of the burner according to the invention preferably has a ratio between the thickness of the wall and the characteristic dimension of each opening, which is at least equal to 1. By characteristic dimension is meant the diameter in the case of circular openings or the width in the case of openings of elongated shape, e.g. slits. Thus, the thickness of the deck is preferably at least equal to or greater than the characteristic dimension of each opening, more preferably the ratio is at least equal to 3.

[0049] The use of boundary walls with a considerable thickness and at least equal to the characteristic dimension of each opening has the technical effect of significantly reducing the temperature of the deck in the working area of ​​the burner. The thickening of the boundary walls results in the formation of channels at each opening between the inner and outer surfaces, the length of which is equal to the thickness of the wall. Thanks to the length of the channels thus formed, the mixed gas takes heat from the burner and cools it while it flows through the channels to the outside. This effect is particularly important in the case of premix burners burning highly reactive gases, as it contributes to further reducing the risk of flashback phenomena. The cooling effect is particularly noticeable when the ratio between the wall thickness and the characteristic dimension of each opening is at least equal to 3.

[0050] By superimposing two or more perforated decks of conventional thickness, for example 0.6 mm, and sealing the decks together, it is possible to achieve boundary walls characterized by a very high characteristic thickness / dimension ratio, for example a characteristic thickness / dimension ratio at least equal to 3. In this way, it is possible to overcome the current technical limitations of the machines used for perforation: for example, punching machines are usually not able to form holes of precise shape and diameter in decks with thickness / diameter ratio values ​​greater than 1.

[0051] The burner according to the invention may optionally have, on the outer surface, at each opening, a protruding wall that partially or completely defines the boundary of the opening and extends from the opening to a predetermined height in a direction substantially perpendicular to the wall. Each protruding wall thus forms an extension of an existing channel at each opening between the inner and outer surfaces of the boundary wall, this extension extending from the face of the boundary wall in which the opening is formed. In this way, the combustion of the mixed gas generates a flame at a distance from the outer surface at least equal to the distance of the distal end of each extension from said outer surface.

[0052] The technical effect of the protruding walls is to make it possible to form a flame at a given distance from the outer surface of the deck, thus making it possible to keep the deck temperature low. Furthermore, the extension of the channel that each protruding wall defines makes it possible to further enhance the cooling effect of the walls mentioned above in relation to the thickness of the boundary walls.

[0053] The protruding walls are an integral part of the outer surface of the deck and can be obtained, for example, by removing and / or deforming parts of the deck by punching, deep drawing, laser cutting or by additive manufacturing techniques such as three-dimensional printing. Realizing the protruding walls as an integral part of the outer surface of the deck is advantageous from a cost point of view, since it does not require the expensive step of manufacturing the protruding walls separately and the equally expensive and laborious phase of mounting individual walls at each opening. Moreover, forming the protruding walls as an integral part of the outer surface has the advantage that the final structure, which in principle consists of a single element, is mechanically more robust and also more resistant to thermal stresses.

[0054] Each protruding wall may bound a respective opening along the entire perimeter of that opening, defining a protruding frame around that opening. The cooling effect of the deck from the use of protruding walls is particularly noticeable when protruding frames are used.

[0055] Alternatively, it is possible to use for each opening a protruding wall in the form of a rectangular blade, connected along at least one side of the boundary of the opening. The protruding blade-like wall can be manufactured when forming the relevant opening. The deck is cut to lift its rectangular part, which remains attached to the deck at least along one side of the opening. The part of the deck thus lifted forms an inclined blade that gradually rises and extends above the opening. The use of an inclined protruding blade-like wall for each opening allows to direct the flow of the mixed gas leaving the burner in a preferential direction, resulting in cooling and a reduction in the amount of heat carried by the solid part of the deck.

[0056] Optionally, a further protruding wall can be formed on the inner surface of the deck opposite the protruding wall formed on the outer surface so as to additionally bound the respective openings partially or entirely on the inner surface of the burner, the further protruding wall also being integrally formed with the inner surface. The use of a protruding wall inside the burner makes it possible to increase the cooling effect of the deck, thus further contributing to the reduction of the flashback phenomenon.

[0057] Preferably, the protruding wall has a height comprised between 0.10 mm and 2.50 mm, more preferably between 0.20 mm and 1.50 mm, even more preferably between 0.25 mm and 0.50 mm. With protruding walls having a height comprised between these values, in particular between 0.25 mm and 0.50 mm, it is possible to maintain the temperature of the deck below 500° C. in most operating ranges of the burner according to the invention (i.e. between 10 kW and 150 kW).

[0058] Particularly advantageous is the use of protruding walls with openings arranged according to a periodic pattern on the outer surface of the deck. The pitch of the pattern can be advantageously utilized to adjust the rate of heat release by the deck, the temperature of the deck and the local distribution of gas around the openings. A reduction in pitch in particular makes it possible to reduce the amount of heat transferred to the outside by conduction through the solid parts of the deck, resulting in a lower deck temperature and a reduced incidence of flashbacks.

[0059] The protruding walls may be formed such that the channel extension of each opening has a straight or tapered profile in cross section in a plane perpendicular to the exterior surface of the deck. In the first case, the width of each extension of the channel remains constant proceeding from the base of the extension, i.e., from the opening, in the direction of the distal end of the extension, i.e., away from the opening. In the second case, the width of each extension of the channel gradually narrows proceeding away from the opening.

[0060] Particularly advantageous are curved tapered profiles, since in this case the cooling of the deck due to the formation of flames at a distance from the base of the deck itself, i.e. at a distance from the individual openings, becomes very pronounced.

[0061] The burner according to the invention can advantageously be used in combination with a perforated distributor arranged inside the burner itself at a distance comprised between 0.20 mm and 4 mm, preferably between 0.60 mm and 2.50 mm, from the inner surface of the deck. For distance values ​​comprised between 0.60 mm and 0.80 mm, the behavior of the burner in terms of flashback is particularly good.

[0062] The distributor's internal opening may be the same shape and size as the burner opening, and may be aligned with the burner opening or offset from the burner opening. [Brief description of the drawings]

[0063] The above features will be better understood from the following description of a preferred embodiment, illustrated purely as a non-limiting example in the accompanying drawings, in which: [Figure 1] 1 shows a perspective view of a burner according to a first embodiment of the invention; [Diagram 2] 2 shows an internal view of the burner of FIG. 1. [Diagram 3] FIG. 2 shows a perspective view of a burner according to a second embodiment. [Figure 4A] 1 shows a perspective view of a burner according to a third embodiment. [Figure 4B] 4B shows a front view of the burner of FIG. 4A. [Figure 4C] FIG. 4B shows a detail of the exterior of the burner of FIG. 4A. [Figure 4D] 4B shows a bottom view of the burner of FIG. 4A with the associated flange and inlet for the introduction of the mixed gas. [Diagram 5] 4 shows a detail of a burner opening according to a variant embodiment of the burner according to the invention; [Figure 6] The limit curve λ(P) that separates the flashback-free operating region from the region where flashback occurs as the opening area of ​​a burner with a flat slot-shaped opening without a protruding frame is changed is shown. [Figure 7] The limit curve λ(P) is shown when the opening area of ​​a burner with a circular hole-shaped opening changes. [Figure 8] The limit curves λ(P) for varying porosity for a burner with circular hole-like openings are shown. [Figure 9] The limit curves λ(P) for varying porosity for a burner with a flat slot-shaped opening are shown. [Figure 10] The limit curve λ(P) is shown for a burner with a circular hole-shaped opening as the thickness changes. [Figure 11] The limit curve λ(P) is shown for a burner with a circular hole-shaped opening as the thickness changes. [Figure 12]The limit curves λ(P) are shown for two burners of equal porosity and comparable opening area, with slot-like and circular hole-like openings, respectively. [Figure 13A-13B] Simulation results of temperature and heat release rate distributions in a burner with a straight profile orifice and in a burner with a curved tapered profile orifice are presented for two different nominal power outputs. [Figures 14A-14C] 13 shows simulation results of temperature and heat release rate distributions in a burner with slot-like openings with a protruding frame according to the fourth embodiment. [Figure 15] FIG. 13 shows a front view of a burner according to a fourth embodiment of the invention. [Figure 16] FIG. 16 shows a top view of the burner of FIG. [Figure 16a] 16 shows an enlarged view of area A in FIG. [Figure 17] 16 shows a cross section of the burner of FIG. 15 taken along line B-B. [Figure 18] FIG. 16 shows a perspective view of the burner of FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0064] Figures 1 and 2 show, by way of example only, a premix burner (100) according to a first embodiment of the invention. The burner comprises a cylindrical hollow body (1) with a boundary wall or deck (2) of non-null thickness, in which an opening (22) is formed extending between the inner surface (23) and the outer surface (24) of the deck. The opening has the shape of a circular hole with a diameter equal to 0.60 mm in the example shown, and the boundary wall (2) has a diameter of 494 mm. 2 The deck has an open area equal to 28.1%, the porosity of the deck is equal to 28.1%, and the burner has a rated power of 60.7 W / mm for a nominal power of 30 kW. 2 It is designed to secure a specific load.

[0065] As will be explained below with reference to experimental data, the burner shown in Figures 1 and 2 and characterized by the porosity and specific load values ​​described above is capable of burning a mixture of air and hydrogen and generating a stable flame on the surface of the deck (2) within the range of the designed nominal power output without substantial risk of flashback.

[0066] Figure 3 shows a premix burner (100) according to a second embodiment of the invention. The burner of Figure 3 differs from that of Figures 1 and 2 in that the openings (22) are in the form of slots or slits with a length equal to 2.50 mm and a width equal to 0.50 mm, the opening surface of each slot being approximately 1.20 mm. 2 The boundary wall (2) of the burner of FIG. 3 has 416 openings and therefore is approximately 498 mm 2 The deck has an open area equal to 17%, the deck porosity equal to 17%, and the burner has a rated power of 50.2 W / mm for a nominal power of 25 kW. 2 The load is designed to ensure a specific load.

[0067] In the embodiment of Figure 3, each opening (22) is bounded at the deck's exterior surface (24) by a protruding wall (33) that extends along the entire boundary of the opening (22) to form a protruding frame. The addition of a protruding wall to bound each opening (22) on the exterior surface (24) allows the flame to form away from the exterior surface (24) at a distance substantially at least equal to the height of the protruding wall, thereby making it possible to reduce the temperature of the deck during combustion. Reducing the deck temperature contributes to further reducing the risk of flashback.

[0068] Figures 15, 16, 16a, 17, 18 show a premix burner (100) according to a further embodiment of the invention, also based on the use of slot-like openings (22). This embodiment differs from the embodiment of Figure 3 in that each opening (22) is also bounded on its inner surface (23) by a protruding wall (34) that extends along the entire boundary of the opening (22) and forms a protruding frame (35). Figure 16a is an enlarged view of area A of Figure 15, showing a part of the protruding frame. The addition of a protruding wall (34) also on the inner surface (23) of the boundary wall (2) further reduces the temperature of the deck.

[0069] The slit-like openings (22) have, in the embodiment of Figures 15, 16, 16a, 17, 18, a length equal to 6 mm and a width equal to 0.50 mm, the opening surface of each slot being 2.95 mm. 2 In the embodiment shown in the previous figures, the boundary wall (2) comprises 216 openings and is therefore 637 mm 2 The deck has an open area equal to 14% and the burner has a porosity of 47.1 W / mm for a nominal power of 30 kW. 2 The load is designed to ensure a specific load.

[0070] Figure 17 is a view along the section B-B of figure 15 and makes it possible to see how the protruding walls protrude from both the outer surface (23) and the inner surface (24) of the deck (2) and define at each opening (22) a channel allowing the passage of the mixed gas from the inside to the outside of the hollow body (1). The width of the channel varies in the example shown from a value of 0.47 mm on the inside of the hollow body (1) to a maximum value of 0.53 mm on the outside. The profile of the protruding walls in the direction perpendicular to the outer surface (24) is substantially linear. The change in the width of the channel from the inside to the outside is due to the fact that in the example shown, the protruding walls extend perpendicular to the cylindrical surface.

[0071] Figures 4A and 4B show a burner according to a third embodiment of the invention, in which the openings have the shape of slots or slits, but unlike the embodiment shown in Figures 15-18, the openings (22) do not have a protruding frame. Figure 4C shows an enlarged detail of the deck of a prototype of the burner according to Figures 4A and 4B. Each slot is about 2.20 mm long and about 0.45 mm wide. The open area of ​​the deck of Figures 4A and 4B is 610 mm 2 The porosity is equal to 26.5%. The burner has a nominal power of 25 kW and a power output of 41 W / mm 2 The load is designed to ensure a specific load.

[0072] FIG. 5 shows a particularly advantageous variant of the invention, for reasons that will be explained in more detail below, in which the opening has a protruding wall with an outwardly tapered curved profile.

[0073] All burners according to the invention are provided with an inlet for introducing the gas mixture into the hollow body of the burner. Figure 4D shows, for example, the inlet of a burner according to the embodiment of Figures 4A and 4B. The inlet is surrounded by a flange (50).

[0074] In the burners described above with reference to Figures 1, 3, 4A, 15, the openings are evenly distributed according to a periodic pattern. For example, in Figure 4C, the slots are distributed in the boundary wall in the tangential direction of the cylindrical surface of the hollow body (1) with a regular pitch equal to about 1.57 mm. In the burner shown, the openings extend in the tangential direction and form a series of rows of openings in the axial direction of the burner. The openings of adjacent rows are offset along the tangential direction.

[0075] In the examples given above, the burner has a cylindrical shape. The object of the invention can also be achieved with burners of different shapes, such as, for example, a parallelepiped with flat parallel faces. The openings can have various shapes, such as, for example, circular holes with a diameter comprised between 0.35 mm and 0.80 mm, or more preferably between 0.45 mm and 0.65 mm, or slits (also called slots) with a length comprised between 2 mm and 6 mm, or more preferably between 2.24 mm and 2.90 mm, and a width comprised between 0.10 mm and 0.60 mm, or more preferably between 0.20 mm and 0.53 mm. The object of the invention can also be achieved by combining slot-like openings and hole-like openings with dimensions comprised in any of the respective ranges just mentioned.

[0076] The hollow body (1) shown in the above figures is manufactured from a flat deck, which is perforated by a punching machine so as to achieve the spatial distribution of the openings (22) shown. The perforations can also be performed by a multi-punch press or a laser press. In the case of a cylindrical outer body, the flat deck is perforated and then folded to form the cylindrical body (1). The openings and the protruding walls of the deck in FIG. 5 were instead made by deep drawing with a special tool that perforates and lifts the deck locally at each opening. The flat deck used to manufacture the hollow body (1) is preferably a metal sheet, for example a steel sheet. The protruding walls shown in FIGS. 3, 15 can be manufactured by additive manufacturing techniques such as three-dimensional printing. In this case, under the guidance of a computer, a layer of powdered metal material is deposited around the openings preformed in the flat deck and is melted in situ to create walls of the desired height that form an integral part with the deck.

[0077] The variation of the embodiment of FIG. 5 having a protruding wall with an outwardly tapered curved profile is particularly advantageous for purposes of flashback reduction, as it allows the flame to form away from the outer surface of the hollow body of the burner in order to reduce the temperature of the solid portions of the hollow body itself.

[0078] 13A and 13B show the simulation results of the temperature distribution and the heat release rate distribution in a burner with a straight profile opening (such as the burner in FIG. 4A) and a burner with a curved tapered profile opening of the type shown in FIG. 5, respectively. The simulations concern two different nominal powers (6.8 kW and 15.5 kW). It can be seen from the figures that the burner with the curved tapered profile opening (570° C. at 6.8 kW and 466° C. at 15.5 kW) has a significantly lower hollow body temperature than the burner with a straight profile opening (748° C. at 6.8 kW and 601° C. at 15.5 kW). The heat release distribution in the case of the burner with the curved tapered profile opening, seen in FIG. 13B, clearly suggests that the flames form at the distal end of each opening and tend to be distributed far from the solid parts of the burner deck, which has a clear advantage in terms of reducing the burner temperature during operation.

[0079] Figures 14A-14C show the simulation results of the temperature distribution and the heat release rate distribution, respectively, for a burner with a slot-like opening with a protruding frame of the type shown in Figure 15. Even in this case, the advantages seen in Figures 13A-13B of the burner with a curved tapered profile opening are observed: the temperature of the burner deck is reduced compared to a burner with a straight profile opening without protruding walls, and the heat release distribution shows that the flame is formed at a position away from the burner body.

[0080] 6-9 show the effect of selection of open area and porosity values ​​on resistance to flashback in a burner according to the present invention.

[0081] The diagram shows the trend of the ratio λ of the amount of air to the amount of combustible gas (including hydrogen) versus the power P delivered by the burner. Each of the curves λ(P) represents the limit between the flashback-free operating region of the burner (the zone above the curve) and the region where flashback occurs (the zone below the curve).

[0082] FIG. 6 shows the behavior of three burners of the type shown in FIGS. 4A and 4B, designated by the abbreviations A, B, and C, respectively. The three burners have dimensions of 0.50 mm by 2.50 mm, and each has a diameter of 1152 mm. 2 (A), 896 mm 2 (B), 768mm 2 (C), the porosity of the three burners is identical and equal to 17.9%. As the aperture area decreases, the power rate of the mixed gas increases, so that the ratio of local power to flame speed approaches 1, making it possible to prevent flashbacks. Burner C, characterized by a smaller aperture area, behaves better in terms of resistance to flashbacks for the reasons mentioned above, as can be seen from the significant decrease in the associated curve λ(P) towards the value λ=1 (combustion reaction in the presence of equal amounts of air and gas) compared to the curves of the other two burners. Thus, the working area in which burner C can operate without flashbacks (between about 5 kW and 20 kW) is significantly wider than that of the other two burners A, B.

[0083] FIG. 7 shows the resistance to flashback of three burners of the type shown in FIG. 1. The three burners, designated A, B and C respectively, have openings in the form of circular holes with a diameter of 0.6 mm, and are each 1146 mm 2 , 896mm 2 , 746mm 2 The porosity is 17.9%. As with the slotted burner whose performance is illustrated in FIG. 6, the resistance to flashback of the burner in FIG. 7 improves as the opening area decreases. The minimum opening area (746 mm 2 Burner C, which features a 2.0 mm diameter burner, has a larger flashback-free working area than the other two burners.

[0084] The specific load values ​​for burners A, B, and C are 21.8 W / mm2 for a nominal power of 25 kW. 2 , 27.9W / mm 2 , 33.5W / mm2 is equal to.

[0085] Figures 8 and 9 show the effect of porosity on flashback.

[0086] Figure 8 shows a slot (0.50 mm x 2.50 mm), 716 mm 2 The behavior of two burners (A and B) with an opening area of ​​17.9% and porosity equal to 12.9%, respectively, is shown. Burner A, with its high porosity, has a good resistance to flashback for nominal powers comprised between 5 kW and 20 kW. The specific load of burners A and B is 27.9 W / mm for a nominal power of 20 kW. 2 Figure 9 shows the behavior of three burners, designated A, B and C, with openings in the form of circular holes with a diameter of 0.55 mm. The opening area of ​​the three burners is 884 mm 2 , and the porosity is equal to 19.5% (A), 15.8% (B), and 10.3% (C), respectively. All burners in Figure 9 have a power output of 28.9 W / mm 2 With increasing porosity, the resistance to flashback increases, as can be easily seen from the fact that the curve λ(P) of the burner with the highest porosity (19.5%) is lower than the corresponding curves of the other burners and bounds a larger working area (above the curve).

[0087] Figures 6-9 show that resistance to flashback increases as the burner opening area decreases and porosity increases, regardless of the specific shape of the opening. Further experimental confirmation that the technical effect of the present invention is achieved regardless of the shape of the opening, when the opening area and porosity values ​​are selected in the ranges defined in the main claims, is provided by Figure 12, which shows that the resistance to flashback increases as the burner opening area decreases and porosity increases, regardless of the specific shape of the opening. 2) and the behavior of two burners with the same porosity (17.9%) but with different shapes of orifices. Burner A has a slot with dimensions of 0.50 mm x 2.50 mm, and burner B has a hole with a diameter of 0.60 mm. The behavior of the two burners is practically equivalent, as can be seen from the closeness of the curves, despite the fact that they employ different shapes of orifices.

[0088] The behavior of a burner according to the invention when the thickness of the boundary wall, or deck, is varied is shown in FIGS.

[0089] FIG. 10 shows a 0.60 mm diameter, 840 mm diameter, and 0.60 mm diameter. 2 for three burners characterized by holes with an opening area of ​​0.4 mm, 0.60 mm and 0.80 mm, respectively, and therefore the ratios of hole diameter to thickness (characteristic dimension of the opening) are equal to 0.67, 1 and 1.33, respectively. As can be deduced from the fact that the curve λ(p) of burner C is located below the curves of burners B and A (in the intermediate position), the resistance of the burners to flashback increases with increasing deck thickness (minimum for burner A and maximum for burner C).

[0090] Figure 11 shows a diameter of 0.80 mm, 889 mm 2 10, porosity of 18% and thicknesses equal to 0.40 mm, 0.60 mm and 0.80 mm, respectively. Also in this case, characterized by holes with a larger diameter than in the case of FIG. 10 and thickness / characteristic dimension ratios equal to 0.5, 0.75 and 1, the best results in terms of resistance to flashbacks are achieved by burner C, which has a higher value of the thickness / diameter ratio, as can be deduced from the fact that the curve λ(p) of burner C is close to the value λ=1 of the other two curves.

[0091] Figures 10 and 11 clearly show the influence of the thickness of the deck, in relation to the characteristic dimensions of the openings, on the flashback resistance of the burner according to the invention. The burner according to the invention can be used in combination with an internal distributor. Figure 2 shows an example of a burner according to the invention, which is equipped on the inside of the deck (2) with an internal distributor, which is marked without reference number in the figures but is clearly visible in cross section.

[0092] The main function of the distributor is to allow the mixed fluids supplied to the burner (100) to pass and diffuse towards the combustion surface of the burner. The mixed gas, usually consisting of air and gas, is introduced through one or more openings located on the surface of the head located at the base of the burner (100) and fixed to the base by a flange. The internal distributor and the hollow body of the burner (100) are fixed to the head (1).

[0093] The inner distributor is separated from the hollow body (1) of the burner (100) by a gap of non-null thickness, the thickness being comprised between 0.20 mm and 4 mm. The gap (300) is empty and contains air.

[0094] The experiments carried out have shown that the presence of a gap having a non-null thickness, preferably a thickness comprised between 0.60 mm and 2.50 mm, improves the resistance to flashback of the burner according to the invention, regardless of the shape of the opening. [Prior art documents] [Patent documents]

[0095] [Patent Document 1] WO 95 / 23315 A1

Claims

1. A premix burner (100) for burning a gas mixture preferably comprising air and at least hydrogen, comprising: - a hollow body (1) comprising a boundary wall (2) having a non-null thickness and having an inner surface (23) facing the interior of the hollow body (1) and an outer surface (24) opposite said inner surface (23); an inlet for introducing said gas mixture inside said hollow body (1); - openings (22) formed in said boundary wall between said inner surface (23) and said outer surface (23) so as to define individual channels adapted to allow the outflow of said mixed gas from inside said hollow body (1); the burner is sized to produce a given nominal power (P); The specific load of the burner is defined as the ratio of the nominal power (P) to the open area of ​​the boundary wall (2) and is 25 W / mm ２ to 70 W / mm ２ Included between The open area of ​​the boundary wall (2) is defined as the sum of the areas of all the openings (22), The porosity of the boundary wall is defined as the percentage ratio of the open area of ​​the boundary wall (2) to the total area of ​​the outer surface (24), and is comprised between 10% and 45%, preferably between 21% and 35%.

2. The burner (100) of claim 1, wherein said predetermined nominal power (P) is comprised between 10 kW and 150 kW.

3. The opening area of ​​the boundary wall (2) is 400 mm ２ From 3000mm ２ Between 450 mm and 500 mm, preferably ２ From 1800mm ２ The burner (100) of claim 2, which is included between

4. 4. The burner (100) according to any one of claims 1 to 3, wherein the opening (22) is a circular hole of a predetermined diameter, preferably comprised between 0.35 mm and 0.80 mm, more preferably between 0.45 mm and 0.65 mm.

5. 4. The burner (100) according to any one of claims 1 to 3, wherein said opening (22) is an elongated slit having in a first direction a predetermined length, preferably comprised between 2 mm and 6 mm, more preferably between 2.21 mm and 4 mm, even more preferably between 2.24 mm and 2.90 mm, and in a second direction, said opening (22) has a predetermined width which is smaller than said predetermined length, preferably comprised in a first range between 0.10 mm and 0.60 mm, more preferably between 0.20 mm and 0.53 mm, even more preferably between 0.40 mm and 0.44 mm, or alternatively comprised in a second range between 0.47 mm and 0.53 mm.

6. 6. The burner (100) according to any one of the preceding claims, wherein the ratio between the thickness of the boundary wall (2) and the diameter or width of the opening (22) is at least equal to 1.

7. 7. The burner (100) according to claim 6, wherein the boundary wall (2) is formed by two or more perforated decks of the same thickness, preferably at least equal to 0.6 mm, superimposed in tight contact with one another, each of the two or more decks having perforations adapted to allow the passage of the mixed gas through it, the perforations of the decks being aligned to define the openings (22) and individual channels for the outflow of the mixed gas from the hollow body (1) of the burner.

8. 8. The burner (100) of claim 7, wherein the ratio between the thickness of the boundary wall (2) and the diameter or width of the opening is at least equal to 3.

9. 9. The burner (100) according to any one of claims 1 to 8, wherein each opening (22) is at least partially delimited, at the outer surface (24) of the boundary wall (1), by a protruding wall (33) which protrudes from the boundary wall (2) in a direction substantially perpendicular to the boundary wall (2) to a predetermined height and which forms an integral part of the outer surface (24), the protruding wall (33) defining an extension of the respective channel of each opening (22) adapted to guide the mixed gas, so that a flame generated by combustion of the mixed gas is formed at a position away from each opening (22) at a distance at least equal to the height of the respective protruding wall (33).

10. Burner (100) according to claim 9, wherein said protruding wall (33) is obtained by removing a part of said boundary wall (2), preferably by punching or deep drawing, or by adding a layer of material to said boundary wall, preferably by additive manufacturing techniques, even more preferably by three-dimensional printing.

11. Burner (100) according to claim 10, wherein the height of each protruding wall (33) is comprised between 0.10 mm and 2.50 mm, preferably between 0.20 mm and 1.50 mm, even more preferably between 0.25 mm and 0.50 mm.

12. 12. The burner (100) of claim 11, wherein the protruding walls (33) defining each opening (22) are shaped such that the extension of the channel defined by the protruding walls (33) has a straight profile in cross section and the width of the extension in cross section remains constant.

13. 12. The burner (100) of claim 11, wherein the protruding walls (33) defining each opening (22) are shaped such that the extension of the channel defined by the protruding walls (33) has a tapered profile in cross section and the width of the extension in cross section decreases from the end of the channel close to the opening (22) in the direction to the opposite end of the channel.

14. The burner (100) of claim 13, wherein the channel extension has a curved, tapered profile in cross section.

15. 15. The burner (100) according to any one of claims 9 to 14, wherein each opening (22) comprises a further protruding wall (34) arranged on and forming an integral part of the inner surface (23) of the boundary wall (2), each of the further protruding walls (34) at least partially delimiting the respective opening (22) and defining a second extension of the channel directed towards the interior of the hollow body (1).

16. 16. The burner (100) of claim 15, wherein each of the protruding walls (33) of the outer surface (24) and each of the further protruding walls (34) of the inner surface (23) define a protruding frame that completely bounds the respective opening (22).

17. 12. The burner (100) according to any one of claims 9 to 11, wherein each protruding wall has the shape of a substantially rectangular blade connected along at least one of its sides to the outer surface of the boundary wall and protruding from the boundary wall to extend downward in front of the respective opening.

18. 18. The burner (100) according to any one of the preceding claims, wherein the burner has an axis of symmetry and has the shape of a cylinder or a parallelepiped with flat and parallel faces.

19. 20. The burner (100) of claim 18, wherein the openings are arranged in the boundary wall according to a periodic pattern formed by a series of rows parallel to one another in the direction of the axis of symmetry, each row of the pattern comprising openings arranged at a predetermined mutual distance along the row.

20. 20. The burner (100) of claim 19, wherein the openings of adjacent rows along the axis of symmetry are offset from one another in a direction perpendicular to the axis of symmetry.

21. 21. The burner (100) according to any one of the preceding claims, comprising a distributor provided with an opening and arranged inside the hollow body at a distance from the hollow body comprised between 0.20 mm and 4 mm, preferably between 0.60 mm and 2.50 mm.

22. 22. Burner (100) according to claim 21, wherein the distributor opening (22) has the shape of a circular hole with a diameter comprised between 0.50 mm and 5 mm, preferably between 1.50 mm and 3.00 mm.

23. 22. The burner (100) according to claim 21, wherein the distributor opening (22) has the shape of an elongated slit, the shape of the elongated slit having a predetermined length in a first direction, preferably between 2 mm and 6 mm, more preferably between 2.21 mm and 4 mm, even more preferably between 2.24 mm and 2.90 mm, and a predetermined width in a second direction smaller than said predetermined length, preferably comprised between 0.10 mm and 0.60 mm, more preferably comprised between 0.20 mm and 0.53 mm, even more preferably comprised in a first range comprised between 0.40 mm and 0.44 mm, or alternatively in a second range comprised between 0.47 mm and 0.53 mm.

24. 24. The burner (100) according to any one of claims 21 to 23, wherein the opening of the distributor is aligned with the opening (22) formed in the boundary wall (2) of the burner.

25. 24. The burner (100) according to any one of claims 21 to 23, wherein the opening of the distributor is offset with respect to the opening (22) formed in the boundary wall (2) of the burner (100).

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