WO2023057937A1

WO2023057937A1 - Pre-mixing burner

Info

Publication number: WO2023057937A1
Application number: PCT/IB2022/059530
Authority: WO
Inventors: Fabio SCHIRO; Francesco Maria Ferrari; Domenico Peserico; Laura DALLA VECCHIA
Original assignee: Polidoro S.P.A.
Priority date: 2021-10-07
Filing date: 2022-10-06
Publication date: 2023-04-13

Abstract

Premix burner (100) provided with a hollow body (1) provided with openings (22) on a boundary wall (2). The openings (22) define a plurality of channels through which a mixture of air and hydrogen can outflow and be burned permanently, without flashbacks, on the outer face (24) of the boundary wall (2). The specific porosity and load values of the burner (100) are selected so as to ensure stable operation of the burner (100).

Description

DESCRIPTION

PRE-MIXING BURNER

Technical field

The present invention relates to a pre-mixing burner for the combustion of a gas mixture, preferably based on air and at least hydrogen.

Prior art

Burners of this type are frequently used in boilers for domestic heating and have a hollow body provided with openings and on the surface of which the combustion of the mixture produces flames. Conventionally, the mixture comprises air and one or more fossil gases such as methane.

However, the combustion of a mixture containing fossil gases has a number of environmental disadvantages, like the emission of substances such as carbon monoxide and dioxide and nitrogen oxides, which pollute, damage health and contribute to the problem of climate warming.

To overcome the disadvantages of the combustion of fossil gases, major industrial countries have been increasing investments for several years in support of the study and development of burners capable of burning gas mixtures containing hydrogen.

The combustion of hydrogen in the presence of air makes it possible to eliminate the problem of the emission of monoxide and carbon dioxide, since a possible reaction of hydrogen with the oxygen present in the air does not produce carbon compounds as unburned residues.

However, the use of hydrogen as a gas in a pre-mixing burner presents a number of practical difficulties due to the high flame speed, reactivity and flame temperature values of hydrogen.

In particular, the high speed at which hydrogen or a mixture containing hydrogen burns makes it difficult to produce stable flames and can cause the so-called flashback phenomenon.

The flame produced by the combustion of a mixture in a burner is stable when the flame speed, i.e., the speed at which the mixture is burned at the surface of the burner, equals the speed of propagation of the mixture itself from the burner inlet towards the combustion surface: when these two speeds are approximately equal, an equilibrium condition is achieved between the amount of fresh mixture that flows to the combustion surface and is available in the unit of time to feed the flame and the amount of mixture that is burned in the unit of time.

In the case of hydrogen or of mixtures containing hydrogen, in the absence of appropriate design measures, the speed at which the mixture propagates (the so-called output speed) may be significantly lower than the flame speed and cause the flames to form inside the burner instead of at the surface. The phenomenon is known as flashback and is particularly dangerous, in that it can trigger an explosion of the burner or of other components of the boiler in which the burner is typically located, exposing users to serious risks.

If the output speed of the mixture is, on the contrary, much greater than that of the flame, the latter will tend to detach from the surface of the burner: in these conditions, called flame lift-off, the latter can extinguish.

It is evident that both the flashback condition and the flame lift-off condition make the flame unstable.

It can be understood from these brief introductory considerations how the ratio between the speed of propagation of the mixture and the flame speed is decisive from the point of view of flame stability.

In general terms, this ratio varies according to different physical-chemical parameters, such as the type of combustible gases present in the burned mixture, the ratio between the air mass and that of the gases mixed with the latter, the distribution of pressure inside the burner and the heat exchange that takes place between the burner and the flames.

The high flame temperature of hydrogen and the strong reactivity of this gas contribute to the flashback phenomenon and can also cause an increase in polluting emissions of nitrogen oxides. In order for a burner to be used to burn a mixture containing hydrogen, it is therefore essential to be able to control both the ratio between the mixture propagation speed and the flame speed, and the temperature of the burner; the latter, in particular, must be kept low, compatible with the power values required of the burner, to reduce the likelihood of a flashback.

A burner of the prior art designed for the combustion of reactive gas mixtures such as hydrogen is described in the international application published with number WO 95/23315 A1 ; this burner is characterized by a low porosity, comprised between 0.5% and 4%, and open area values not exceeding 300 mm².

In general, for a given burner and for a given mixture there is a working region within which, as the physical-chemical parameters listed above vary, the burner is able to operate in a stable manner and in which the likelihood of a flashback can be considered substantially negligible.

A design parameter that is often used to give a practical indication of the extension of the working region is the specific thermal load of the burner, defined as the ratio between the nominal power and the so-called open area of the burner. The burner conventionally comprises a perforated deck, for example cylindrical or parallelepiped in shape, on the surface of which openings are formed (for example by means of a punching machine) at which flames are formed, when the mixture of air and gas is burned. The open area is defined as the overall area of all the openings on the deck; instead, the gross area is defined as the total area of the deck.

For a given nominal power that the burner is supposed to produce as per design and for a given mixture of air and gas, the specific load, defined as the ratio between the nominal power and the open area, assumes values within a limited and very precise range when the burner operates in a stable manner.

For example, in the case of burners designed to burn mixtures of air and methane and deliver a nominal power comprised between 10 and 150 kW, the specific load varies approximately between 10 and 30 W/mm², when the burner operates under stable conditions.

Object of the invention

It is an object of the present invention to provide a pre-mixing burner which overcomes the aforementioned drawbacks of the prior art.

In particular, it is an object of the present invention to provide a pre-mixing burner for the combustion a mixture preferably containing air and at least hydrogen as a combustible gas, capable of producing stable flames on a boundary wall and of delivering thermal power in a wide operating range without flashbacks.

More particularly, the object of the present invention is to provide a premixing burner for the combustion of a mixture preferably containing air and at least hydrogen as a combustible gas, capable of maintaining the ratio between the output and flame speeds and the temperature of the boundary wall at values for which the burner operates without flashbacks.

Even more particularly, the object of the present invention is to provide a pre-mixing burner for the combustion of a mixture preferably containing air and at least hydrogen as a combustible gas, capable of producing stable flames on the boundary wall at a predetermined distance and controlled by the openings on the wall.

Said objects are fully achieved by the burner according to claim 1 .

In particular, the burner object of this claim comprises a hollow body having a non-null thickness boundary wall and provided with openings formed between an inner face, facing the interior of the hollow body, and an outer face opposite the inner face, the openings being configured so as to define between said faces a plurality of channels that allow the mixture to be burned to escape. The burner is sized so that a predetermined nominal power can be delivered.

The burner is characterized in that the specific load of the burner is comprised between 25 W/mm² and 70 W/mm² and the porosity of the boundary wall, i.e.,, of the deck on which the flames form, is comprised between 10% and 45%. Porosity is defined as the percentage ratio between the open area of the deck and the overall area of the deck, where the open area of the deck is given by the sum of the areas of all the openings formed in the deck.

Specific load values comprised between 25 W/mm² and 70 W/mm² in general allow, especially for nominal powers comprised between 10 kW and 150 kW, to burn gas mixtures comprising highly reactive gases such as hydrogen without substantial risks of flashback. These load values allow the output speed of the mixture to be adjusted so that it approximates the flame speed of the mixture itself. As explained in more detail below, the flame front in a burner becomes stationary in time when the two speeds are comparable: in this condition, the likelihood of flashback phenomena is significantly reduced. Recalling that, for a given nominal power, the specific load value of a burner is given by the ratio between said power and the aforementioned open area, the specific load of a burner according to the present invention can actually be adjusted by selecting an appropriate value of the open area of the deck.

The porosity values adopted for the deck of the burner according to the present invention make it possible to control the thermal-fluid dynamic behaviour and the thermal exchange of the burner so as to increase the resistance to flashback: in particular, the aforementioned porosity values make it possible to reduce the degree of recirculation of the burnt gases outside the openings, that is, the amount of flame heat that returns to the deck of the burner due to vortices that are created near the openings. The increase in porosity reduces the volume of the swirling regions, lowers the recirculation speed and generally weakens the swirling degree. The consequent reduction in recirculation results in a lowering of the temperature of the deck and significantly reduces the danger of flashback: the likelihood of flashback increases as the temperature of the deck increases. As mentioned above, the likelihood of flashback also depends on the ratio between the value of the output speed of the mixture to be burned and the flame speed: in general terms, the flames produced on the surface of the deck of a burner are stable when, for each point of this surface, the output speed of the mixture equals the flame speed because, under such conditions, the flame front is stationary over time.

The output speed of a gas mixture depends on various factors, such as the types of gases present, the ratio between the amount of carburant (generally air) and the amount of the different combustible gases, as well as the heat exchange that is established in the burner; in general, the output speed has a spatial distribution. Highly reactive gases such as hydrogen are characterized by high flame speeds; in the absence of appropriate design arrangements, the flame speed of a highly reactive gas is generally higher than the output speed of the mixture from the openings made on the surface of the deck.

In the burner according to the present invention, it is possible to adjust the output speed of the mixture so that it approximates that of the flame in the intended operating range, by acting on the value of the open area of the deck. As mentioned above, by virtue of the link between the specific load of the burner, open area and nominal power, the imposition of a specific load means, in an equivalent way, that the open area and, consequently, the output speed are fixed, for a sized burner so as to deliver a certain nominal power.

In conclusion, for a burner according to the present invention, the control over the output speed of the mixture, by choosing the open area value of the deck (or, equivalently, of the specific load), and the control over the degree of recirculation, by choosing the porosity value, allow to significantly reduce the risk of flashback.

It is important to underline that, for the same open area, a burner with a higher porosity is less exposed to the risk of flashbacks than a burner with a lower porosity. The more porous burner obviously has a smaller "closed" surface, i.e., without openings, and therefore a smaller surface with which the very hot burnt gases can come into contact and cause a rise in the temperature of the burner: consequently, a more porous burner will have a lower temperature of the deck than a less porous burner, with the same open area. There is therefore in the burners of the present invention a combined technical effect, in terms of reducing the risk of flashback, produced by the porosity of the deck and by the open area.

The upper limit to the maximum porosity achievable on a deck is dictated by the technical characteristics of the machinery used for perforating the deck, such as the dimensions of the punch, in the case where the openings are made by punching, and by the limits of resistance to the stresses of the materials used to manufacture the boundary wall. At the opposite extreme, the lower limit is determined by the occurrence of flashback phenomena: for a given nominal power and for a given gas mixture, there is a porosity value below which the degree of recirculation of the burnt gases near the openings and, therefore, the flame heat returning to the burner exceeds a level beyond which the burner is exposed to a substantial risk of flashbacks.

Porosity is a relative quantity, since it represents the ratio between the total area of the openings on the deck (or boundary wall) and the overall area of the latter: it is therefore in principle possible to create a burner with the same porosity for different open area values.

The burner according to the present invention is characterized in that the open area is selected, in combination with the porosity, within a predetermined range of values for which the specific load is maintained between 25 W/mm² and 70 W/mm².

Choosing the open area value for a given nominal power allows the adjustment of the output speed of the mixture: as the open area decreases, the output speed grows, with the same total mixture flow required to deliver the required power. In the case of gases such as hydrogen, characterized by high reactivity and, therefore, by a high flame speed, the control over the open area allows to modify and adjust appropriately the ratio between the output and the flame speed, so as to reduce the risk of flashback.

The minimum (25 W/mm²) and maximum (70 W/mm²) values of specific load among which the open area values of the burners according to the present invention are selected define the operating limits within which a pre-mixing burner, sized to produce a nominal power preferably comprised between 10 kW and 150 kW, is able to support on its outer face flames obtained by combustion of a mixture containing hydrogen in a stable manner, that is without a substantial risk of flashback or detachment (liftoff) of the flame itself.

The dimensions and the spatial density of the openings present on the boundary wall, i.e., on the deck of the burner according to the present invention, therefore have the specific technical effect of allowing the generation of stable flames over a wide operating range starting from a mixture comprising a highly reactive gas such as hydrogen.

Since the reduction of the open area leads to an increase in the pressure drops, there is - for a given power - a limit to the minimum value that the open area can assume compatibly with the maximum permissible value of pressure drops, assigned - together with the range of nominal power values that the burner must be able to burn - as a design specification. At the opposite extreme, there is an upper limit to the maximum value that the open area can assume, essentially determined by the occurrence of the flashback phenomenon: for a given mixture and for a predetermined nominal power, there is a maximum value of open area beyond which the output speed of the mixture becomes too small, in relation to the flame speed, and the burner is exposed to a significant risk of flashbacks.

The burners according to the present invention are designed to supply nominal powers preferably comprised between 10 kW and 150 kW; the maximum nominal power required determines the physical dimensions of the burner, which are greater, the greater the required power is. By way of example, a cylindrical burner with a diameter of 70 mm, commonly used in commercial devices, and in which the openings are distributed in a central region of the deck arranged 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, typically has a height of about 42 mm, if designed to deliver a nominal power of 25 kW; the height rises to about 74 mm, with the same diameter, in the event that the burner is sized to deliver a power of 150 kW. In the case of flat burners, for example parallelepiped-shaped, a device sized to produce a power of 25 kW may for example have a width of about 50 mm and a length of about 90 mm; width and length rise to 100 mm and 176 mm respectively for a device designed to deliver 150 kW.

The open area of the deck of the burners according to the present invention is preferably comprised between 400 mm² and 3000 mm² and more preferably between 450 mm² and 1800 mm² for nominal powers comprised between 10 kW and 150 kW. For these open area values, the burners of the invention have in the indicated operating range (10-150 kW), in particular when used to burn mixtures comprising hydrogen, a lower temperature of the deck than the burners of the prior art discussed above, for example with reference to the international application WO 95/23315 A1.

The lowering of the temperature leads to an extension, compared to the known burners, of the working range in which the burner is able to burn a hydrogen-based mixture and to deliver the required nominal power without being subject to flashback. This effect is particularly marked in combination with porosity values comprised between 21% and 35%; these values can be used in combination with the aforementioned open area values comprised between 400 mm² and 3000 mm² and, preferably between 450 mm² and 1800 mm², in order to obtain a lowering of the temperature of the deck compared to the prior art.

The technical effects described above with reference to the open area and porosity values are achieved by the burner according to the present invention regardless of the shape of the openings: it is therefore possible to use openings of any known shape, for example circular or with an elongated slit (also called slot), to control the local flame distribution or pressure drops, in combination with the porosity and open area values discussed above. Openings of different shapes can also be combined to make distributions of openings characterized by the porosity and open area values already discussed; the combination of openings of different shapes also allows to control the distribution of the flames on the surface of the deck.

The technical effects described above with reference to the open area and porosity values can also be achieved by the burner according to the present invention either by using uniform distributions of openings, for example periodic, or by resorting to non-uniform distributions.

The boundary wall, that is, the deck of the burner according to the present invention preferably has a ratio between the thickness of the wall and a characteristic dimension of each opening equal to at least one; by characteristic dimension it is meant the diameter, in the case of circular openings, or the width in the case of openings of elongated shape, for example slits. The thickness of the deck is thus preferably at least equal to or larger than the characteristic dimension of each opening; more preferably, the ratio is at least equal to three.

The use of boundary walls with a significant 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 operating field of the burner. The thickening of the boundary wall creates between the inner and outer face, at each opening, a channel of length equal to the thickness of the wall; thanks to the length of the channel thus formed, the mixture removes heat from the burner and cools it during the outflow through the channel to the outside. This effect is particularly important in the case of pre-mixing burners for the combustion of highly reactive gases, as it contributes to further reducing the risk of flashback phenomena. The cooling effect is particularly pronounced for values of the ratio between the thickness of the wall and the characteristic dimension of each opening at least equal to 3.

It is possible to realize boundary walls characterized by a very high characteristic thickness/dimension ratio, for example at least equal to three, by overlapping two or more perforated decks with conventional thickness, for example 0.6 mm, and making the decks adhere to each other. In this way, it is possible to overcome the current technological limits of the machinery used for perforating: a punching machine, for example, is not normally able to form accurate shape and diameter holes on decks where the value of the thickness/diameter ratio is greater than the unit.

The burner according to the present invention can optionally have on the outer face, at each opening, a projecting wall delimiting - partially or completely - the boundary of the opening and extending in a direction substantially perpendicular to the wall up to a predetermined height from the opening. Each projecting wall thus forms an extension of the existing channel at each opening between the inner and outer face of the boundary wall. This extension extends from the face of the boundary wall in which the openings are formed: in this way, the combustion of the mixture produces flames at a distance from the outer face that is at least equal to the distance of the distal end of each extension from said outer face.

The technical effect of the projecting walls is to allow the formation of the flames at a predetermined distance from the outer face of the deck, which in turn allows the temperature of the latter to be kept low. In addition, the extension of the channel that each projecting wall defines makes it possible to further strengthen the cooling effect of the wall discussed above in relation to the thickening of the boundary wall.

The projecting walls are an integral part of the outer face of the deck and can be obtained either by removal and/or deformation of portions of the deck, for example by punching, deep-drawing or laser cutting, or by means of additive manufacturing techniques such as three-dimensional printing. The realization of the projecting walls as integral parts of the outer face of the deck is advantageous from the point of view of costs, because it does not require an expensive step of individual manufacture of the projecting walls and an equally expensive and laborious phase of application of the individual walls at each opening. Furthermore, the formation of the projecting walls as integral parts of the outer face has the advantage that the final structure, being in principle constituted by a single element, is mechanically more robust and better withstands thermal stresses.

Each projecting wall can bound the respective opening along the entire boundary of the latter, so as to define a projecting frame around the opening. The cooling effect of the deck due to the use of projecting walls is particularly pronounced in case projecting frames are used.

Alternatively, it is possible to use projecting walls in the form of a rectangular blade and connected to the respective opening along at least one of the sides of the boundary of the opening. The projecting bladeshaped walls can be manufactured at the time of forming the relative opening: the deck is cut in such a way as to lift a rectangular-shaped portion of it, which however remains attached to the deck at least along one side of the opening; the portion of the deck thus lifted forms an inclined blade that rises and extends progressively above the opening. The use of projecting blade-shaped walls inclined on the respective openings allows the flow of the mixture exiting the burner to be conveyed in a preferential direction and produces a cooling and a reduction in the amount of heat conveyed by the solid parts of the deck.

Optionally, further projecting walls can also be formed on the inner face of the deck, on the opposite side with respect to the projecting walls formed on the outer face, so as to partially or entirely bound the respective openings, additionally, on the inner face of the burner; also the further projecting walls are formed integrally with the inner face. The use of projecting walls inside the burner makes it possible to accentuate the cooling effect of the deck, thus further contributing to the reduction of flashback phenomena.

Preferably, the projecting walls have a height comprised between 0.10 mm and 2.50 mm, more preferably between 0.20 mm and 1.50 mm and even more preferably between 0.25 mm and 0.50 mm. For these values and, in particular, for projecting walls with a height comprised between 0.25 mm and 0.50 mm, it is possible to keep the temperature of the deck below 500 °C in most of the operating range (i.e., between 10 kW and 150 kW) of the burners according to the present invention.

Particularly advantageous is the use of projecting walls with openings arranged according to a periodic pattern on the outer face of the deck: the pitch of the pattern can be advantageously exploited to adjust the heat release rate by the deck, the temperature of the latter and the local distribution of the gases around the openings. The reduction of the pitch allows, in particular, to reduce the amount of heat transferred to the outside by conduction through the solid parts of the deck, with a consequent lowering of the temperature of the latter and a reduction in the incidence of flashbacks.

The projecting walls can be made in such a way that the extension of the channel at each opening has in section, in a plane orthogonal to the outer face of the deck, a straight or tapered profile. 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., moving away from the opening. In the second case, the width of each extension of the channel decreases progressively moving away from the opening.

Particularly advantageous is a curved tapered profile: in this case, the cooling of the deck due to the formation of the flames at a distance from the base of the deck itself, that is, at a distance from the respective opening, becomes very marked.

The burner according to the present invention can advantageously be used in combination with a perforated distributor located inside the burner itself, at a distance from the inner face of the deck comprised between 0.20 mm and 4 mm, preferably between 0.60 and 2.50 mm. For values of the distance comprised between 0.60 mm and 0.80 mm the behaviour of the burner from the point of view of the flashback is particularly good.

The internal openings of the distributor may be of the same shape and dimensions as the openings of the burner and may be aligned with or offset from the burner openings.

Brief description of the drawings

The above-mentioned characteristics will be better understood from the following description of a preferred embodiment, illustrated purely by way of non-limiting example in the accompanying drawings, in which:

- figure 1 illustrates a perspective view of a burner in accordance with a first embodiment of the present invention;

- figure 2 illustrates a view of the interior of the burner of figure 1 ;

- figure 3 illustrates a perspective view of a burner in accordance with a second embodiment;

- figure 4A illustrates a perspective view of the burner in accordance with a third embodiment;

- figure 4B illustrates a front view of the burner of figure 4A;

- figure 4C shows a detail of the outer surface of the burner of figure 4A;

- figure 4D illustrates a bottom view of the burner of figure 4A with the relative flange and the inlet for the introduction of the mixture;

- figure 5 illustrates a detail of the openings of a burner according to an embodiment variant of the burner according to the present invention;

- figure 6 shows the limit curve A(P) separating the operating region without flashback from the region where the flashback occurs as the open area for the burners with openings in the form of flat slots without projecting frames varies; - figure 7 shows the limit curves A(P) as the open area for the burners with circular hole-shaped openings varies;

- figure 8 shows the limit curves A(P) as the porosity for the burners with circular hole-shaped openings varies;

- figure 9 shows the limit curves A(P) as the porosity for the burners with openings in the form of flat slots varies;

- figure 10 shows the limit curves A(P) as the thickness for the burners with circular hole-shaped openings varies;

- figure 11 shows the limit curves A(P) as the thickness for the burners with circular hole-shaped openings varies;

- figure 12 shows the limit curves A(P) for two burners of equal porosity and comparable open area, provided respectively with slot-shaped and circular hole-shaped openings;

- figures 13A and 13B show the results of simulations of the distributions of temperature and heat release rate in a burner with straight profile openings and in one with curved tapered profile openings, for two different nominal powers;

- figures 14A-14C show the results of simulations of the distributions of temperature and heat release rate in a burner with slot-shaped openings provided with projecting frames according to a fourth embodiment;

- figure 15 illustrates a front view of a burner in accordance with the fourth embodiment of the present invention;

- figure 16 illustrates a top view of the burner of figure 15;

- figure 16a illustrates an enlargement of region A of figure 15;

- figure 17 illustrates section B-B of the burner of figure 15;

- figure 18 illustrates a perspective view of the burner of figure 15.

Detailed description of the preferred embodiments of the invention

Figures 1 and 2 illustrate, by way of example only, a pre-mixing burner (100) according to a first embodiment of the present invention. The burner comprises a hollow body (1 ) of cylindrical shape with a boundary wall or deck (2) having non-null thickness, in which openings (22) are formed extending between the inner face (23) and the outer face (24) of the deck. The openings have, in the illustrated example, the shape of circular holes with a diameter equal to 0.60 mm; the boundary wall (2) has an open area equal to 494 mm²; the porosity of the deck is equal to 28.1%; the burner is designed to ensure a specific load of 60.7 W/mm² for a nominal power of 30 kW.

As illustrated below with reference to experimental data, the burner illustrated in figures 1 and 2 and characterized by the above-mentioned porosity and specific load values is capable of burning air and hydrogen mixtures and of producing stable flames on the surface of the deck (2) in the range of nominal powers for which it is designed, without substantial risks of flashback.

Figure 3 illustrates a pre-mixing burner (100) according to a second embodiment of the present 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 open surface of each slot is approximately 1 .20 mm². The boundary wall (2) of the burner in figure 3 is provided with 416 openings and therefore has an open area equal to approximately 498 mm²; the porosity of the deck is equal to 17%; the burner is designed to ensure a specific load of 50.2 W/mm² for a nominal power of 25 kW.

In the embodiment of figure 3 each opening (22) is bounded, on the outer face (24) of the deck, by a projecting wall (33) that extends along the entire boundary of the opening (22) to form a projecting frame. The addition of projecting walls to bound the boundary of each opening (22) on the outer face (24) causes the flames to form at a distance from the outer face (24), substantially at a distance at least equal to the height of the projecting walls, and thus makes it possible to reduce the temperature of the deck during combustion. The lowering of the temperature of the deck in turn contributes to further lowering the risk of flashbacks. Figures 15, 16, 16a, 17 and 18 illustrate a pre-mixing burner (100) according to a further embodiment of the present invention, also based on the use of slot-shaped openings (22). This embodiment differs from that of figure 3 in that each opening (22) is also bounded on the inner face (23) by a projecting wall (34) that extends along the entire boundary of the opening (22) to form a projecting frame (35). Figure 16a is an enlargement of the region A of figure 15 and shows some of the projecting frames. The addition of projecting walls (34) also on the inner face (23) of the boundary wall (2) further reduces the temperature of the deck.

The slit-shaped openings (22) have, in the embodiment of figures 15, 16, 16a, 17 and 18, a length equal to 6 mm and a width equal to 0.50 mm; the open surface of each slot is 2.95 mm². In the embodiment illustrated in the aforesaid figures, the boundary wall (2) is provided with 216 openings and therefore has an open area equal to 637 mm²; the porosity of the deck is equal to 14%; the burner is designed to ensure a specific load of 47.1 W/mm² for a nominal power of 30 kW.

Figure 17 is a view along section B-B of figure 15 and allows to appreciate how the projecting walls protrude from both the outer face (23) and the inner face (24) of the deck (2) and define, at each opening (22), a channel that allows the passage of the mixture from the inside of the hollow body (1 ) towards the outside. The width of the channel varies, in the example illustrated, from a value of 0.47 mm, inside the hollow body (1 ), to a maximum of 0.53 mm outside. The profile of the projecting walls in the direction perpendicular to the outer face (24) is substantially straight; the change in width of the channel from the inside to the outside is due to the fact that, in the illustrated example, the projecting walls extend perpendicularly to a cylindrical surface.

Figures 4A and 4B show the burner according to a third embodiment of the present invention, in which the openings have the shape of slots or slits but, unlike the embodiment illustrated in figures 15-18, the openings (22) are without projecting frames; figure 4C shows an enlarged detail of the deck of a prototype of the burner according to figures 4A and 4B. Each slot is approximately 2.20 mm long and approximately 0.45 mm wide. The open area of the deck of figures 4A and 4B is equal to 610 mm²; the porosity is equal to 26.5%; the burner is designed to ensure a specific load of 41 W/mm² for a nominal power of 25 kW.

Figure 5 illustrates a particularly advantageous variant of the invention, for the reasons discussed in more detail below, in which the openings have projecting walls with a curved profile that is tapered outwards.

All burners according to the present invention are provided with an inlet for the introduction of the mixture into the hollow body of the burner: figure 4D shows for example the inlet of the burner according to the embodiment of figures 4A and 4B; the inlet is surrounded by a flange (50).

In the burners discussed above with reference to figures 1 , 3, 4A and 15 the openings are evenly distributed according to a periodic pattern: for example, in figure 4C the slots are distributed on the boundary wall with a regular pitch equal to about 1.57 mm in the direction tangent to the cylindrical surface of the hollow body (1 ). In the burners illustrated, 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.

In the examples presented above, the burners have a cylindrical shape; the objects of the invention can also be achieved with burners of different shapes, for example parallelepiped with flat and parallel faces. The openings can assume different shapes, for example a circular hole with a diameter comprised between 0.35 mm and 0.80 mm or, more preferably, between 0.45 mm and 0.65 mm, or a slit (also called a slot) 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 objects of the invention can also be achieved by combining slot-shaped openings and hole-shaped openings with dimensions comprised in any of the respective ranges just described.

The hollow body (1 ) illustrated in the figures discussed above is manufactured from a flat deck that is perforated by means of a punching machine, so as to achieve the spatial distribution of openings of (22) illustrated; the perforation can also be carried out by means of multi-punch or laser presses. In the case of a cylindrical-shaped outer body, the flat deck, once perforated, is folded over to form the cylindrical body (1 ). The openings and the projecting walls of the deck of figure 5 were instead made by deep-drawing, by means of a special tool that perforates and raises the deck locally at each opening. The flat deck used to manufacture the hollow body (1 ) is preferably a metal sheet, for example of steel. The projecting walls illustrated in figures 3 and 15 can be made by additive manufacturing techniques such as three-dimensional printing: in this case, under the guidance of a calculator layers of powdered metal material are deposited around the openings, previously formed in the flat deck, and melted in situ, until walls of the desired height are made, which thus form a single piece with the deck.

The embodiment variant of figure 5 with projecting walls with a curved profile tapered outwards is particularly advantageous, for the purposes of flashback reduction, because it allows flames to be formed at a distance from the outer face of the hollow body of the burner, so as to lower the temperature of the solid parts of the hollow body itself.

Figures 13A and 13B show respectively the results of simulations of the distributions of temperature and heat release rate in a burner with straight profiled openings (e.g., as in the burner of figure 4A) and in one with curved tapered profile openings of the type illustrated in figure 5; the simulations relate to two different nominal powers (6.8 kW and 15.5 kW). From the figures it can be seen that the burner with curved tapered profile openings has significantly lower temperatures of the hollow body (570 °C at 6.8 kW, 466 °C at 15.5 kW) than a burner with straight profiled openings (748 °C at 6.8 kW, 601 °C at 15.5 kW). The heat release distribution in the case of the burner with curved tapered profile openings, visible in figure 13B, clearly suggests that the flames form at the distal end of each opening and tend to distribute far from the solid parts of the deck of the burner, with clear advantages in terms of lowering the burner temperature during operation.

Figures 14A-14C show the results of simulations of the distributions of temperature and heat release rate respectively in a burner with slotshaped openings provided with projecting frames, of the type illustrated in figure 15. Also in this case, the benefits seen in figures 13A-13B for the burner with curved tapered profile openings are observed: the temperature of the deck of the burner is lowered, compared to a burner with straight profile openings without projecting walls, and the heat release distribution shows that the flames form at a distance from the body of the burner.

Figures 6-9 illustrate the effect of the choice of open area and porosity values on the resistance to flashback in the burners according to the present invention.

The figures show the trend of the ratio A between the amount of air and that of combustible gas (comprising hydrogen) with respect to the power P delivered by the burner. Each of the curves A(P) represents the limit between the operating region of the burner without flashbacks (zone above the curve) and the region where flashbacks occur (zone below the curve).

Figure 6 shows the behaviour of three burners of the type illustrated in figures 4A and 4B, respectively identified by the abbreviations A, B and C. The three burners have slot-shaped openings with dimensions 0.50 mm x 2.50 mm and an open area equal to 1152 mm² (A), 896 mm² (B) and 768 mm² (C) respectively; the porosity of the three burners is the same and is equal to 17.9%. As the open area decreases, the output speed of the mixture increases and, consequently, the ratio between the local output and flame speeds approaches the unit, which makes it possible to prevent flashbacks. The burner C, characterized by the smaller open area, consequently exhibits a better behaviour in terms of resistance to the flashback, as can be seen from the fact that the relative curve A(P) decreases significantly, compared to the curve of the other two burners, towards the value A = 1 (combustion reaction in the presence of equal amounts of air and gas). The working area in which the burner C can operate (between about 5 and 20 kW) without flashbacks is therefore significantly larger than that of the other two burners A and B.

Figure 7 illustrates the resistance to flashback of three burners of the type shown in figure 1 . The three burners, identified by the abbreviations A, B and C respectively, have openings in the shape of a circular hole with a diameter of 0.6 mm and have an open area equal to 1 146 mm², 896 mm² and 746 mm² respectively; the porosity is 17.9%. As in the case of the burners with slots whose performance has been discussed in figure 6, the resistance to flashback of the burners in figure 7 improves as the open area decreases: the burner C, characterized by the lowest value of open area (746 mm²), has a working region without flashbacks that is wider than the other two burners.

The specific load values of the burners A, B and C are equal to 21.8 W/mm², 27.9 W/mm² and 33.5 W/mm² respectively for a nominal power of 25 kW.

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

Figure 8 shows the behaviour of two burners (A and B) with slots (0.50 mm x 2.50 mm), open area of 716 mm² and porosity equal to 17.9% and 12.9%, respectively. The burner A with higher porosity has a better resistance to flashback for nominal powers comprised between 5 kW and 20 kW. The specific load for the burners A and B is equal to 27.9 W/mm² for a nominal power of 20 kW.

Figure 9 illustrates the behaviour of three burners, indicated by the abbreviations A, B and C, provided with openings in the form of circular holes with a diameter of 0.55 mm. The open area of the three burners is 884 mm²; the porosity is equal to 19.5% (A), 15.8% (B) and 10.3% (C) respectively. All burners in figure 9 have a specific load of 28.9 W/mm². As the porosity increases, the resistance to flashback improves, as can be easily seen from the fact that the curve A(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 region (above the curve).

Figures 6-9 show how, regardless of the specific shape of the openings, the resistance to flashback increases with decreasing open area and increasing porosity of the burner. A further experimental confirmation that the technical effects of the present invention are achieved regardless of the shape of the openings, provided that the open area and porosity values are selected in the ranges defined in the main claim, is given by figure 12, which illustrates the behaviour of two burners with a substantially similar open area (about 750 mm²) and identical porosity (17.9%) but with differently shaped openings. The burner A has slots with dimensions of 0.50 mm x 2.50 mm, while the burner B has holes with a diameter of 0.60 mm. The behaviour of the two burners is substantially comparable, as can be seen from the closeness of the curves, although they employ openings of different shapes.

The behaviour of the burners according to the present invention as the thickness of the boundary wall, i.e., of the deck, varies, is illustrated in figures 10 and 11 .

Figure 10 relates to three burners indicated with A, B and C and characterized by holes with a diameter of 0.60 mm, an open area of 840 mm2 and a porosity equal to 18.5%; the thickness of the deck of the three burners is equal to 0.40 mm, 0.60 mm and 0.80 mm, respectively: the ratio between the thickness and the diameter of the holes (characteristic dimension of the openings) is therefore respectively equal to 0.67, 1 and 1.33. As the thickness of the deck increases (minimum for the burner A, maximum for the burner C), the resistance to flashback of the burner increases, as can be deduced from the fact that the curve A(P) for the burner C is arranged below the curves for the burners B (in the intermediate position) and A.

Figure 11 shows the results relative to three burners, indicated by the letters A, B and C, respectively, with holes with a diameter of 0.80 mm, an open area of 889 mm², a porosity of 18% and a thickness equal to 0.40 mm, 0.60 mm and 0.80 mm, respectively. Also in this case, characterized by holes with a greater diameter than in the case of figure 10 and thickness/characteristic dimension ratios equal to 0.5, 0.75 and 1 , the best result in terms of resistance to flashback is achieved by the burner C with a higher value of the thickness/diameter ratio, as can be deduced from the fact that the curve A(P) for the burner C is closer to the value A = 1 of the other two curves.

Figures 10 and 11 clearly illustrate the effect of the thickness of the deck, in relation to the characteristic dimension of the openings, on the resistance to flashback in the burners according to the present invention.

The burner according to the present invention can be used in combination with an internal distributor. Figure 2 illustrates an example of a burner according to the present invention, provided with an inner distributor shown in the figure without reference number but clearly recognizable in the section, inside the deck (2).

The main function of the distributor is to allow the passage and the spreading of a fluid mixture fed into the burner (100) towards the combustion surface of the burner. The mixture, typically consisting of air and gas, is introduced through one or more openings disposed on the surface of a head located at the base of the burner (100), and fixed to the latter by means of a flange. Both the internal distributor and the hollow body of the burner (100) are fixed to the head (1 ).

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

The experiments carried out have indicated that the presence of a gap with non-null thickness and preferably comprised between 0.60 mm and 2.50 mm improves the resistance to flashback of the burners according to the present invention, regardless of the shape of the openings.

Claims

25 CLAIMS

1. A pre-mixing burner (100) for the combustion of a gas mixture which preferably includes air and at least hydrogen, comprising: a hollow body (1 ) with a boundary wall (2) having a non-null thickness and provided with an inner face (23), facing the interior of the hollow body (1 ), and with an outer face (24) opposite the inner face (23); an inlet for introducing said mixture inside the hollow body (1 ); openings (22) formed in the boundary wall between the inner face (23) and the outer face (23), so as to define respective channels adapted to allow the mixture to outflow from inside the hollow body (1 ); the burner being sized to be able to produce a predetermined nominal power (P) and characterised in that the specific load of the burner, defined as the ratio of the nominal power (P) to the open area of the boundary wall (2), is comprised between 25 W/mm² and 70 W/mm², the open area of the boundary wall (2) being defined as the sum of the areas of all the openings (22), and the porosity of the boundary wall, defined as the percentage ratio between the open area of the boundary wall (2) and the overall area of the outer face (24), is comprised between 10% and 45% and preferably between 21 and 35%.

2. Burner (100) according to claim 1 , wherein the predetermined nominal power (P) is comprised between 10 kW and 150 kW.

3. Burner (100) according to claim 2, wherein the open area of the boundary wall (2) is comprised between 400 mm² and 3000 mm², preferably between 450 mm² and 1800 mm².

4. Burner (100) according to any one of the preceding claims, wherein the openings (22) are circular holes of predetermined diameter, preferably comprised between 0.35 mm and 0.80 mm and even more preferably between 0.45 mm and 0.65 mm.

5. Burner (100) according to any one of claims 1 to 3, wherein the openings (22) are elongated slits 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 and even more preferably between 2.24 mm and 2.90 mm, and in a second direction they have a predetermined width, less than said predetermined length and preferably comprised between 0.10 mm and 0.60 mm, more preferably between 0.20 mm and 0.53 mm, even more preferably comprised in a first range between 0.40 mm and 0.44 mm or, alternatively, in a second range between 0.47 mm and 0.53 mm.

6. 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 openings (22) is at least equal to 1 .

7. Burner (100) according to claim 6, wherein the boundary wall (2) is formed by two or more perforated decks of identical thickness, preferably at least equal to 0.6 mm, superimposed so as to adhere to each other, each of the two or more decks having perforations adapted to allow the passage of the mixture through the deck, the perforations of said decks being aligned so as to define the openings (22) and the respective channels for the outflow of the mixture from the hollow body (1) of the burner.

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

9. Burner (100) according to any one of the preceding claims, wherein each opening (22) is at least partially delimited on the outer face (24) of the boundary wall (1 ) by a projecting wall (33) which protrudes from said boundary wall (2) in a direction substantially perpendicular to the boundary wall (2) up to a predefined height and which forms an integral part of said outer face (24), the projecting wall (33) defining an extension of the respective channel of each opening (22) adapted to guide the mixture, so that the flames produced by the combustion of said mixture form at a distance from each opening (22) at least equal to the height of the respective projecting wall (33).

10. Burner (100) according to claim 9, wherein the projecting walls (33) are obtained by removing a portion of the boundary wall (2), preferably by punching or deep-drawing, or by adding layers of material on the boundary wall, preferably by means of additive manufacturing techniques, even more preferably by means of three-dimensional printing.

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

12. Burner (100) according to claim 11 , wherein the projecting wall (33) delimiting each opening (22) is shaped so that the extension of the channel defined by said projecting wall (33) has a straight profile in crosssection and the width in cross-section of said extension remains constant.

13. Burner (100) according to claim 11 , wherein the projecting wall (33) delimiting each opening (22) is shaped so that the extension of the channel defined by said projecting wall (33) has a tapered profile in cross- 28 section, the width in cross-section of said extension decreasing from the end of said channel near the opening (22) in the direction of the opposite end of said channel.

14. Burner (100) according to claim 13, wherein the channel extension has a curved tapered profile in cross-section.

15. Burner (100) according to any one of claims 9 to 14, wherein each opening (22) comprises a further projecting wall (34) arranged on the inner face (23) of the boundary wall (2) and forming an integral part of said inner face (23), each of the further projecting 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. Burner (100) according to claim 15, wherein each of the projecting walls (33) on the outer face (24) and each of the further projecting walls (34) on the inner face (23) define projecting frames which completely bound the respective openings (22).

17. Burner (100) according to any one of claims 9 to 11 , wherein each projecting wall has the shape of a substantially rectangular blade connected along at least one of its sides to the outer face of the boundary wall and protruding from said boundary wall so as to extend in front of the respective underlying opening.

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

19. Burner (100) according to claim 18, wherein the openings are arranged on the boundary wall according to a periodic pattern formed by a series of 29 rows parallel to each other in the direction of the axis of symmetry, each row of said pattern comprising openings arranged at a predetermined mutual distance along said row.

20. Burner (100) according to claim 19, wherein the openings of adjacent rows in the direction of the axis of symmetry are mutually offset in the direction perpendicular to said axis of symmetry.

21. Burner (100) according to any one of the preceding claims, comprising a distributor provided with openings and placed inside the hollow body at a distance from said hollow body comprised between 0.20 mm and 4 mm, preferably between 0.60 mm and 2.50 mm.

22. Burner (100) according to claim 21 , wherein the openings (22) of the distributor have the shape of circular holes with a diameter comprised between 0.50 mm and 5 mm, preferably between 1 .50 mm and 3.00 mm.

23. Burner (100) according to claim 21 , wherein the openings (22) of the distributor have the shape of elongated slits 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 and even more preferably between 2.24 mm and 2.90 mm, and in a second direction a predetermined width, less than said predetermined length and preferably comprised between 0.10 mm and 0.60 mm, more preferably between 0.20 mm and 0.53 mm, even more preferably comprised in a first range between 0.40 mm and 0.44 mm or, alternatively, in a second range between 0.47 mm and 0.53 mm.

24. Burner (100) according to any one of claims 21 to 23, wherein the openings of the distributor are aligned with the openings (22) formed in the boundary wall (2) of the burner. 30

25. Burner (100) according to any one of claims 21 to 23, wherein the openings of the distributor are offset with respect to the openings (22) formed in the boundary wall (2) of the burner (100).