BURNER BODY WITH VARIABLE GAS OUTLET AREA

Description

A burner body for a gas burner, adapted to be used in a fully-premixed gas burner, is described below.

In particular, the burner body described below is suitable for being used in a fully-premixed gas burner with modulable power wherein the fuel gas comprises hydrogen or mainly consists of hydrogen, for example a gaseous fuel in which the volume percentage of hydrogen is greater than 90%.

In gas boilers it is known to use hydrocarbons as fuel gas, such as hydrocarbons containing methane (CH4), propane (C3H8) or butane (C4H10).

It is common practice to resort to the premixing of fuel gas with combustion air in order to reduce the emissions of pollutants, especially nitrogen oxides (NOx), as is also known to provide a quantity of air greater than the stoichiometric air, i.e. to work with excess air.

In such regard, the excess air factor λ is defined as the pure number that defines the relation between the actual air/fuel ratio of the mixture with respect to the stoichiometric air/fuel ratio.

However, an excess air leads to a reduction in the efficiency of the burner: typically a good compromise to minimise the emissions of pollutants, without excessively penalising the loss of efficiency of a burner, is to provide the burner with an excess air factor λ having a value of approximately 1.25-1.35.

However, the use of light hydrocarbons (e.g. methane) as a fuel still entails an important pollution problem, represented by carbon dioxide emissions.

The use of hydrogen as a fuel gas seems to be a promising solution for reducing the polluting emissions from gas boilers; however, hydrogen has a very different combustion from that of light hydrocarbons: in particular, the hydrogen molecule has a much higher combustion speed than the molecules of light hydrocarbons (indicatively the flame speed of hydrogen is approximately seven times higher than the speed of the methane flame).

The high laminar speed of gas combustion having a high content of hydrogen causes a much greater risk of flashback than in the combustion of fuel gases without, or with low hydrogen content.

In particular, it is observed that in the combustion of hydrogen the risk of a flashback is particularly significant at the time of the ignition of the air/gas mixture or when the burner works at reduced power, conditions in which the flame speed is reduced, causing it to anchor to the surface of the burner and the overheat of the same up to temperature values that favour the phenomenon of flashback.

A further drawback found in premixed burners fed by hydrogen is the low-modulation interval of the working power compared to burners working with traditional hydrocarbons (for example, in hydrogen-fed premixed burners, modulation ratio values higher than approximately 1:4-1:5 are not known).

In the premixed burners a burner having an elongated shape, for example substantially and/or essentially cylindrical, is provided inserted within a combustion chamber.

The air and fuel gas are first premixed, in an adequate ratio for complete combustion and with low pollutant emissions, and the mixture thus obtained comes out through a plurality of holes or openings positioned on the outer surface of the burner body, with consequent formation of a flame.

The shape, size and number of openings influence the speed of the fuel gas and air mixture that comes out of the openings of the burner body.

In order to avoid or reduce the risk of flashback, it is therefore important to keep such outflow speed of the mixture always higher than the laminar speed of the combustion.

All that being said, some solutions are proposed below in order to obviate, at least in part, the problems of the prior art.

In particular, solutions are proposed to reduce the risk of flashbacks in the combustion of premixed hydrogen when the burner is ignited and when the burner works with low powers.

Furthermore, solutions are described that allow a burner for the combustion of an air and hydrogen mixture to be managed more safely, also enabling a wide range of working power modulation to be obtained.

These and other objects are achieved by means of a burner body for a premixed gas burner according to claim 1.

Further objects may be achieved by means of the additional features of the dependent claims.

Some possible examples of a burner body for a premixed gas burner are hereinafter described with reference to the annexed drawing tables wherein:

FIG. 1 is a schematic drawing showing a burner body while in a first operating mode;

FIG. 2 shows the same burner body as in FIG. 1 while in a second operating mode;

FIG. 3 is a schematic drawing showing a second embodiment of a burner body while in a first operating mode;

FIG. 4 shows the same burner body as in FIG. 3 while in a second operating mode;

FIG. 5 is a schematic drawing showing a third embodiment of a burner body while in a first operating mode;

FIG. 6 shows the same burner body as in FIG. 5 while in a second operating mode;

FIG. 7 is a schematic drawing showing a burner device comprising one burner body according to the invention.

The features of some variants of the invention are now described, using the references contained in the figures. It should be noted that the above figures, although schematic, reproduce the elements of the invention according to proportions between spatial dimensions and orientations thereof that are compatible with a possible executive embodiment.

It should also be noted that any dimensional and spatial term (such as “lower”, “upper”, “inner”, “outer”, “front”, “rear” and the like) refers to the positions of the elements as shown in the annexed figures, without any limiting intent relative to the possible operating positions.

With particular reference to FIG. 7, reference number 1 indicates a burner body for a complete-premixing gas burner with modulable power.

The burner body 1 is intended to be part of a burner device 10 (hereinafter abbreviated to “burner 10”), comprising at least:

• • one fan 11 for the air inflow by a supply line 18,
  • one fuel gas supply line 12, essentially and/or mainly represented by hydrogen,
  • one valve 13 for regulating the fuel gas flow rate,
  • one air and gas mixing unit 14,
  • one combustion chamber 19 wherein the burner body 1 protrudes.

The fan 11 may be a variable speed (jerky or continuous within a given speed range) centrifugal fan, according to the required thermal power.

Therefore, the burner 10 allows for a discrete (step) or continuous regulation of the thermal power within a predetermined power range.

In the illustrated example, the air and fuel gas mixing unit 14 is positioned upstream of the fan 11; in alternative embodiments (not illustrated), the mixing unit 14 may be positioned downstream of the fan 11.

The burner 10 also provides for conventional ignition means 15, for example an ignition electrode and conventional sensor means 16 for detecting at least one combustion parameter, for example a temperature sensor or a sensor for detecting the presence of flames.

An electronic regulator 17 is also provided to regulate, in particular, the fuel gas flow rate and the air flow rate.

The burner body 1 (the outer shape thereof may be substantially and/or essentially cylindrical) comprises an inlet opening 2 for the inflow of an air and fuel gas mixture.

As more visible in the figures from FIG. 1 to FIGS. 6 and 7, such burner body 1 comprises a plurality n of outlet openings 31.a, 31.b; 32.a, 32.b; 33.a, 33.b, 33.c for the outflow of the air and fuel gas mixture that arrives through the inlet opening 2.

The shape of the outlet openings 31.a, 31.b; 32.a, 32.b; 33.a, 33.b, 33.c, the size and arrangement thereof on the surface of the burner body 1 may vary as needed. The burner body 1 comprises at least one movable element 41; 42; 43 for varying the number of outlet openings 31.a, 31.b; 32.a, 32.b; 33.a, 33.b, 33.c in communication with the inlet opening 2.

Thanks to at least one movable element 41; 42; 43 it is possible to vary the number of outlet openings 31.a, 31.b; 32.a, 32.b; 33.a, 33.b, 33.c and therefore the outlet area of the burner body 1 wherefrom the fuel gas and air mixture comes out.

As better described hereinafter, the partialisation of the number of outlet openings 31.a, 31.b; 32.a, 32.b; 33.a, 33.b, 33.c of the burner body 1 allows the length of the flames that come out of the burner body to be regulated and therefore the stability of the flames to be maintained even by significantly varying the working thermal power.

In other words, by modifying the flame outlet area (by changing the number of outlet openings 31.a, 31.b; 32.a, 32.b; 33.a, 33.b, 33.c), it is possible to improve the stability of the flame, maintaining the outflow rate of the air/gas mixture always higher than the propagation speed of the flame, reducing such outlet area when the burner 10 modulates at lower powers and increasing it when the burner 10 works at higher powers.

The solution described therefore allows operating both at reduced powers, avoiding risks of flashbacks, and at high powers, avoiding risks of flame detachment.

By way of a non-limiting example, by using pure hydrogen as fuel gas it is possible to achieve power modulation ratios of approximately 1:10.

It should also be noted that by being able to regulate the height of the flames coming out of the burner, it is possible to work with A constant even at low powers.

In other words, the solution described guarantees high efficiency both at low and high working powers.

Furthermore, the solution described allows the burner 1 to be used with fuels of different nature while maintaining a stable flame.

In some embodiments, such as those exemplified in the figures from FIG. 1 to FIG. 6, the burner body 1 may comprise a plurality n of chambers 51.a, 51.b; 62.a, 62.b; 73.a, 73.b, 73.c, for example two chambers (as in the embodiments referred to in the Figures from FIG. 1 to FIG. 4) or three chambers (as in the embodiments referred to in the FIGS. 5 and 6).

The chambers 51.a, 51.b; 62.a, 62.b; 73.a, 73.b, 73.c are delimited by fixed partition walls inside the burner body 1 and by portions of the outer wall of the burner body 1.

In the embodiments in which the burner body 1 comprises a plurality n of chambers 51.a, 51.b; 62.a, 62.b; 73.a, 73.b, 73.c, each of them is in communication with the inlet opening 2 and each chamber 51.a, 51.b; 62.a, 62.b; 73.a, 73.b, 73.c is in communication with a part of the n outlet openings 31.a, 31.b; 32.a, 32.b; 33.a, 33.b, 33.c.

For example, in the versions of burner bodies 1 with two chambers 51.a, 51.b; 62.a, 62.b (see the figures from FIG. 1 to FIG. 4), it may be provided that each of the two chambers 51.a, 51.b; 62.a, 62.b is substantially connected to the half of the total outlet area of the flames, outlined by the openings 31.a, 31.b; 32.a, 32.b provided on the surface of the burner body 1.

Therefore, in this embodiment, each of the two chambers 51.a, 51.b; 62.a, 62.b is capable of developing approximately the half of the maximum thermal power that may be developed by the burner 10.

In the versions with burner bodies 1 having three chambers 73.a, 73.b, 73.c (see FIGS. 5 and 6), each chamber is in communication, substantially, with approximately one third of the total outlet area of the flames, outlined by the outlet openings 33.a, 33.b, 33.c.

Therefore, in this embodiment, each of the three chambers 73.a, 73.b; 73.c is capable of developing approximately one third of the maximum thermal power that may be output by the burner 10.

In particular, the burner body 1 of FIGS. 5 and 6 may work with one chamber 73.a, with two chambers 73.a, 73.b or with all the three chambers 73.a, 73.b, 73.c.

In the embodiments of the burner body 1 of the attached figures, chambers 51.a, 51.b; 62.a, 62.b; 73.a, 73.b, 73.c are shown in communication with outlet openings 31.a, 31.b; 32.a, 32.b; 33.a, 33.b, 33.c which define total flame outlet areas having a substantially similar dimensional development in each of the chambers.

Embodiments of the burner body 1 may however be provided in which said total flame outlet areas have a non-similar dimensional development in each chamber 51.a, 51.b; 62.a, 62.b; 73.a, 73.b, 73.c, which are therefore capable of producing an unequal fraction of the thermal power that may be developed by the burner 10.

According to the invention, n−1 movable elements 41; 42; 43, are then provided positioned upstream of n−1 chambers 51.b; 62.b; 73.b, 73.c and downstream of the inlet opening 2 to open and close the communication between such inlet opening 2 and the n−1 chambers 51.b; 62.b; 73.b, 73.c.

In some of the illustrated embodiments, the n−1 movable elements 41; 42; 43 are shutters adapted to open and close a corresponding passage port which puts the inlet opening 2 in communication with the n−1 chambers 51.b; 62.b; 73.b, 73.c. Therefore, in the embodiments of the burner body 1 that provide for a plurality of chambers, one of them 51.a; 62.a; 73.a is always crossed by the flow of the air and fuel gas mixture, ensuring the burner 10 the minimum power necessary for the correct operation thereof, while the other chambers 51.b; 62.b; 73.b, 73.c are crossed by the air and fuel gas mixture only if the corresponding shutters 41; 42; 43 are open.

With reference to the embodiment of FIGS. 5 and 6, at the time of the ignition of the burner 10 or when the burner 10 is working at a power lower than approximately one-third of the maximum power, only the chamber 73.a is open, while the other two chambers 73.b and 73.c remain closed.

When the burner 10 is working at a power approximately comprised between one/third and two/thirds of the maximum power, one shutter 43 is open and the other one is closed, and therefore only two chambers 73.a, 73.b out of three are operational.

When the burner 10 is working at a power comprised between approximately two/thirds and three thirds of the maximum power, both shutters 43 are open and therefore all the three chambers 73.a, 73.b, 73.c are operational.

The movable elements 41; 42; 43 (hereinafter “shutters 41; 42; 43”) may be axially movable or able to swing (for example by) 90°.

Such shutters 41; 42; 43 placed upstream of the chambers 51.b; 62.b; 73.b, 73.c may be, for example, eccentrically hinged tilting plate-shaped shutters.

In some particular embodiments the shutters 41; 42; 43 may be motorised shutters (see the variants shown in FIGS. 5, 6 and 7): for such purpose, motor means 430 may be provided to operate the shutters 41; 42; 43, for example electric motors with a 90° rotation.

The motorised shutters 41; 42; 43 may go from a configuration in which they completely close the passage port to a configuration in which the passage port is totally open.

The transition between the two operating position configurations of the motorised shutters 41; 42; 43 may be carried out, for example, according to the number of revolutions of the fan 11.

Naturally, it is also possible to use other operating parameters to regulate the motorised shutters 41; 42; 43.

For example, in alternative embodiments of the burner 10, the regulation of the operating positions of the motorised shutters 41; 42; 43 may be carried out according to the mass flow rate of the air/fuel gas mixture, the mass flow rate of the air or the mass flow rate of the fuel gas.

The speed of the air and fuel gas mixture flowing out through the outlet openings 31.a, 31.b; 32.a, 32.b; 33.a, 33.b, 33.c may therefore be regulated by the operating position of the motorised shutters 41; 42; 43 in order to maintain the height of the flames within a range in which the same is stable, avoiding risks of flashbacks or flame detachment.

Furthermore, in other forms of use of the burner body 1, at the time of the ignition of the burner 10, it may be provided that the motorised shutters 41; 42; 43 remain closed, so as to work with a reduced number of outlet openings 31.a; 32.a; 33.a and to reduce the risks of flashbacks.

Preferably, a hysteresis function is provided for the operation of the shutters 41; 42; 43, so as to avoid excessive swings of the burner regulation system 10.

In other embodiments, the shutters 41; 42; 43 are devoid of motor means 430 and are adapted to open mechanically in a self-adapting manner when the pressure upstream of the shutter is sufficiently greater than the pressure downstream of the same.

Conversely, said self-adapting shutters 41; 42; 43 close mechanically when the pressure difference is less than the pre-set value.

According to such embodiments, the opening of the shutters 41; 42; 43 occurs in contrast with a recall force towards the closed position (for example the weight force of the shutter and/or the force of elastic means, for example a spring that works under compression).

Therefore, the shutters 41; 42; 43 take an angular position that corresponds to a point of balance between the torque created by the thrust of the air and gas mixture and the resisting-torque created, for example, by the weight of the shutter and/or by the elastic means.

The embodiments of the burner body 1, shown in the figures from FIG. 1 to FIG. 6, have proved to be suitable for solving the technical problem of regulating the outflow speed of the air/gas mixture from the outlet openings 31.a, 31.b; 32.a, 32.b; 33.a, 33.b, 33.c, in relation to the working power of the burner 1, in such a way as to avoid flashbacks when the burner 10 operates at lower powers.