CROSS-REFERENCE TO RELATED APPLICATIONS This patent application claims priority from Italian patent applications no. 102020000028394 and no. 102020000028400 filed on November 25, 2020, the entire disclosure of which is incorporated herein by reference. TECHNICAL FIELD

The invention relates to a reduced-emission industrial burner and apparatus to heat a fluid. In particular, the invention finds advantageous, though not exclusive application in monoblock burners with a diffusion flame, to which explicit reference will be made in the description below without because of this loosing in generality. CONTEXT OF THE INVENTION

The heating of fluids in industrial applications, for example air or water for boilers, foundries, roasting systems, kilns, etc., usually takes place through the use of industrial burners placed in combustion chambers of boilers and kilns delimited at least by two opposite walls and by a roof or by a tubular or box-shaped body. These chambers are usually heated by one or more burners arranged in series, depending on the use application. In particular, the burners installed in boilers usually are monoblock burners, namely having a respective fan and a control panel on board the burner.

The operating cycle of burners using methane (or mixtures or LPG) is typically designed with utmost precision both in order to obtain a quick and uniform heating and to optimize efficiency and emissions. To this regard, the emissions that have most frequently been the centre of attention in recent years definitely are nitrogen oxides (NOx).

During thermal processes, NOx are formed starting from the nitrogen usually present in the oxidizer (atmosphere) in the presence of high temperatures and of a large quantity of oxygen. However, in case of ideal combustion, nitrogen oxides are not part of the combustion products, since nitrogen is known to be inert at relatively small temperatures. Therefore, it is because of the temperature peaks reached during the intermediate (transition) phases of the combustion that nitrogen molecules (N2) dissociate into atomic nitrogen, which, on the other hand, is extremely reactive in contact with oxygen, which is also atomically dissociated, thus leading to the formation of NOx.

Furthermore, the extreme temperature decrease, which takes place in the final phase of the combustion or far from the flame in the burners, freezes the aforesaid reaction, thus forbidding the re-association of nitrogen and oxygen, hence discharging the sub-product NOx downstream. Nitrogen oxides are commonly deemed to be polluting substances as well as possible causes of pulmonary and/or atmospheric problems and, therefore, their reduction is a common goal in the field of industrial combustion. To this aim, different types of industrial burners were developed in order to obtain a desired temperature inside the combustion chamber, though reducing the emissions of nitrogen oxides as much as possible both for environmental purposes and for energy-efficiency purposes.

However, the attempt to reduce emissions determined a decrease in the gas flow used and a consequent extension of the times needed to reach a desired flame temperature. Indeed, in order to avoid temperature peaks, which are responsible for the greatest part of the emissions (upon turning on and under steady conditions), power is usually decreased with an increase in the flow of air cooling down the flame, thus reducing, as a consequence, the production of Nox emissions. In particular, the emission reduction is in contrast to the requirements to be fulfilled in order to obtain a steady flame quickly reaching the desired temperature, since the minimization of the fuel used jeopardizes the depth and the stability of the so-called flame root. The object of the invention is to provide an apparatus and a burner, which are designed to at least partially overcome the drawbacks of the prior art and, at the same time, are cheap and easy to be manufactured.

SUMMARY According to the invention, there are provided an industrial burner and apparatus according to the independent claims attached hereto and, preferably, according to any one of the dependent claims directly or indirectly depending on the independent claims.

The appended claims describe preferred embodiments of the invention and form an integral part of the description. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the accompanying drawings, which show some non-limiting embodiments thereof, wherein:

- figure 1 is a schematic sectional side view of a first embodiment of an industrial burner according to the invention;

- figure 2 is a schematic perspective view of a second embodiment of an industrial burner according to the invention; - figure 3 is a sectional side view of the part of figure 2;

- figure 4 is a schematic perspective view of the cross section of figure 3; and

- figure 5 is a schematic front view of the burner of figures 2 to 4.

DETAILED DESCRIPTION

In figure 1, number 1 indicates, as a whole, a reduced- emission industrial burner according to a first aspect of the invention. The burner 1 can be installed (i.e. is installable) in a combustion chamber 3, for example of a boiler or of a kiln, in particular in the area of a wall 4 of the combustion chamber 3. More in particular, the burner 1 can be installed (i.e. is installable) in a plant for firing the enamels of ceramic articles, for booth painting, for the drying of sand and/or gravel, for pre-cooking food products (e.g. deep freeze goods), in heat generators using hot water, overheated water, steam, overheated steam with diathermic oil (thermal oil boiler).

According to figures 1, 3 and 4, the burner 1 comprises an tubular discharge element 5, inside which at least one duct 6 feeding a primary flow PF of a fuel (for example, natural gas or liquefied petroleum gas) extends, said primary flow PF being configured to develop a flame root FR. In particular, the flame root FR is developed in a radially central region of the burner 1, in the area of a longitudinal (symmetry) axis LA of the burner.

In particular, the burner 1 comprises at least one duct 7 feeding a secondary fuel flow SF, which is configured to develop a main flame MF (radially on the outside, relative to the longitudinal axis LA, of the central flame root FR).

The burner 1 further comprises at least one duct 8 feeding an oxidizer OX (generally ambient air) . In particular, the fuel introduced through the fuel feeding ducts 6 and 7 substantially is methane, whereas the oxidizer OX introduced through the oxidizer OX feeding duct 8 substantially is ambient air (with approximately 21% of oxygen).

In the non-limiting embodiments of the accompanying figures, the tubular discharge element 5 is configured to be (completely) crossed by the oxidizer OX, hence by the duct 8, and, in particular, also by the ducts 6 and 7 feeding the primary fuel flow PF and the secondary fuel flow SF, respectively.

According to the non-limiting embodiment of figures 1 and 3, the tubular discharge element 5 is provided with an end 9, which is configured to be installed outside the combustion chamber 3, and with an end 10, which is opposite the end 9 and is configured to be installed inside the combustion chamber 3.

Advantageously, though not necessarily, the burner 1 also comprises a tubular discharge element 11, which extends from the end 10, on the opposite side relative to the end 9, namely towards the (more, precisely, the inside of the) combustion chamber 3. In particular, the tubular discharge element 11 is configured to be at least partially crossed by the duct 6 and by the duct 7. More in particular, the discharge element 11 is configured to be flown through by the fluids flowing out of the tubular discharge element 5.

According to some non-limiting embodiments, the tubular discharge element 5 is coupled to a support element comprising a flange, which is configured to fix the burner 1 to the wall 4 of the combustion chamber 3. Advantageously, though not necessarily, the burner further comprises a suction element 12, which is configured to lead at least part of the gases G present on the outside of the burner 1 (namely, inside the combustion chamber 3) into the tubular discharge element 11 and is provided with at least one opening 13, which is arranged between the tubular discharge element 5 and the tubular discharge element 11 and sucks the gases G present on the outside of the burner

1 due to a depression generated in the area of the suction element 12. In particular, the opening 13 has an annular shape. In this way, distributed gas G suction can be obtained. By so doing, it is further possible to use the oxygen left inside the combustion chamber 3 and complete the combustion of those gases G, G' that were not completely burnt with a first passage inside the burner 1, namely with a primary combustion. Furthermore, the gases G, G' (presumably, also in consideration of the fact that they have a relatively high temperature) help improve the efficiency of the combustion. In addition, some gases G' are sucked back by the main flame MF because of the out-flowing speed of the mixture consisting of the secondary flow SF and of the secondary portion OX (accelerated due to the narrowing 20). The term "primary combustion" indicated the combustion generated by the burner without the gas G, G' recirculation.

In the non-limiting embodiments of the accompanying figures, the burner 1 is configured to generate an acceleration of the (sole) oxidizer OX in the area of the suction element 12.

According to some non-limiting embodiments, the suction element 12 comprises, in particular is, a Venturi tube.

According to the non-limiting embodiments of the accompanying figures, the tubular discharge element 11 is connected to the tubular discharge element 5 in an integral manner and is substantially coaxial to the tubular discharge element 5. In other words, the longitudinal symmetry axis LA of the tubular discharge element 11 coincides with the longitudinal symmetry axis LA of the tubular discharge element 5. Advantageously, though not necessarily, the tubular discharge element 11 is (completely) located on the inside of the combustion chamber 3.

According to the non-limiting embodiments of the accompanying figures, the burner 1 comprises a partition element 14 for the oxidizer OX, which is configured to divide the oxidizer OX (upstream of the opening 13 of the suction element 12) into a primary portion OX' to be mixed with said primary flow PF and a secondary portion OX' to be mixed with said secondary flow SF. In particular, the oxidizer partition element 14 comprises (is) a mixing head 15, which houses, on the inside, the fuel feeding duct 6. In detail, the head 15 is in a radially central position relative to the longitudinal axis LA of the burner. More precisely, the head 15 is partially on the inside of the tubular discharge element 5 and partially on the inside of the tubular discharge element 11.

Advantageously, though not necessarily, the burner 1 is configured to generate an acceleration of the secondary portion OX'' and/or of the primary portion OX'. In particular, the conformation of the oxidizer partition element 14 combined with the suction element 12 allows the burner 1 to generate an acceleration of the secondary portion OX'' (narrowing the passage of the secondary portion OX'') and a slowing down of the primary portion OX' (widening the passage of the primary portion OX').

Advantageously, though not necessarily, the burner 1 comprises a diffuser terminal 16, which is fluidically connected to fuel feeding duct 6 (for the primary flow PF) and is configured to diffuse the primary fuel flow PF and cause it to swirl. In particular, the diffuser terminal 16 divides and deflects the primary flow PF so as to generate a steady and turbulent flame root FR. In detail, "steady flame" means a flame whose (oscillation) frequency) belongs to the range 5-200Hz, preferably to the range 10-100Hz.

In particular, the diffuser terminal 16 is integral to the tubular discharge element 5.

Is some non-limiting example, the tubular discharge element 11 can be adjusted longitudinally (along the longitudinal axis LA) so as to widen or narrow the opening 13 of the suction element 12.

Alternatively or in addition, the diffuser terminal 16 can be adjusted longitudinally (along the longitudinal axis LA) so as to vary the separation of the oxidizer OX (namely, how and in which point of the burner it is divided) into the primary and secondary portions OX', OX''.

Advantageously, though not necessarily, the diffuser terminal 16 comprises a turbulator disc 17 (shown in all the embodiments of the accompanying figures, in particular frontally in figure 5), which is arranged perpendicularly to the longitudinal axis LA of the burner 1. In particular, the turbulator disc 17 comprises a plurality of through slits 18 and/or holes 19, which are configured to generate a turbulent motion of the primary flow PF. More precisely, the slits 18 are oblong openings, which extend from the main surface of the turbulator disc 17 in an inclined manner, whereas the holes 19 are circular openings, which extend perpendicularly to the main surface of the turbulator disc 17, hence parallel to the longitudinal axis LA of the burner 1.

Advantageously, though not necessarily, and according to the non-limiting embodiments shown in figures 1 to 4, the suction element 12 is arranged upstream of the flame root FR along the longitudinal axis LA of the burner 1. In other words, the root FR is generated downstream of the suction element 12.

In particular, the suction element 12 is arranged, along the longitudinal axis LA, between the flame root FR and the tubular discharge element 5. In other words, the primary flow PF meets the oxidizer OX (in particular OX') after having passed the tubular discharge element 5, namely inside the tubular discharge element 11. In the non-limiting embodiments of figures 1, 3 and 4, the suction element has at least one narrowing 20 arranged in the area of the end 10.

In particular, the narrowing is determined by a narrowing of the tubular discharge element 5 or by a widening of the oxidizer partition element 14 without interruptions.

Advantageously, though not necessarily, the narrowing 20 has (at least) a segment TT', TT' with the shape of a truncated cone, which is delimited by a larger base and a smaller base.

According to a limiting embodiment, the the tubular discharge element 11 has an open end 21 facing the suction element 12 and an open end 22 facing the inside of the combustion chamber. In the non-limiting embodiments of figures 1, 3 and 4, the narrowing 20 comprises (consists of) a segment TT' with the shape of a truncated cone, which radially extends, moving along a flow direction FD, towards the central longitudinal axis LA of the burner 1, in particular whose larger base coincides with the end 10 of the tubular discharge element 5. Furthermore, the narrowing 20 comprises a segment TT' with the shape of a truncated cone, which radially extends, moving along the flow direction FD, from the central longitudinal axis LA of the burner 1, in particular whose larger and smaller bases are determined by the geometry of the partition element 14 for the oxidizer OX (namely, of the head 15). Advantageously, though not necessarily, the segment TT' with the shape of a truncated cone is arranged so as to be externally radial (along a radial direction RD, which is shown in figures 1 and 3) relative to the segment TT'' with the shape of a truncated cone.

Advantageously, though not necessarily, the segment TT' with the shape of a truncated cone and the segment TT'' with the shape of a truncated cone are at least partially staggered along the flow direction FD. In this way, the acceleration of the oxidizer OX is spaced out. In particular, according to the non-limiting embodiments of figures 1, 3 and 4, the segment TT' with the shape of a truncated cone is at least partially arranged downstream relative to the segment TT'' with the shape of a truncated cone along the flow direction FD.

Advantageously, though not necessarily, the duct 6 feeding the primary fuel flow PF is arranged in a central position of the burner along a longitudinal axis LA of the burner 1. Furthermore, the duct 7 (or the ducts 7) feeding the secondary fuel flow SF is arranged on the outside of the duct 6 in a concentric manner. In particular, the burner 1 comprises a plurality of ducts 7 (for example six in the embodiment of figures 2 to 5), which are arranged radially relative to the duct 6. According to the non-limiting embodiments of the accompanying figures, said at least one duct 7 is arranged so as to cross a space S' extending between the tubular discharge element 5 and the partition element 14 for the oxidizer OX. In particular, said at least one duct 7 is arranged so as to also at least partially extend through a space S'' comprised between the tubular discharge element 11 and the oxidizer partition element 14.

Advantageously, though not necessarily, the discharge element 5 has a circular cross section, in particular with a constant diameter.

Advantageously, though not necessarily, the discharge element 11 has a circular cross section, in particular with a constant diameter.

Advantageously, though not necessarily, the ducts 6 and 7 have a circular cross section.

Advantageously, though not necessarily, the head 15 has a circular cross section with an at least partially variable diameter.

In particular, in the area of the narrowing 20, the cross section for the passage of the secondary portion OX'' has a passage that is smaller than two thirds of the space S' and/or of the space S''. More in particular, in the area of the narrowing 20, the cross section for the passage of the secondary portion OX'' has a passage that is smaller than half the space S' and/or the space S''. The more the passage in the area of the narrowing 20 decreases relative to the space S', the more the speed variation of the oxidizer portion OX'', which, in use, is mixed with the secondary flow SF for the formation of the main flame MF, increases. In some non-limiting cases, the space S' is substantially equal to the space S''. In other non-limiting cases, the space S' is substantially greater or smaller than the space S''. Advantageously, though not necessarily, the diameter of the tubular discharge element 5 is smaller than the diameter of the tubular discharge element 11.

Advantageously, though not necessarily, and according to the non-limiting embodiments of figures 1, 2 and 5, the burner 1 further comprises a fuel feeding system 23, which is configured to adjust the inflowing volume of the primary flow PF inside the duct 6, and a second fuel feeding system 24, which is configured to adjust the inflowing volume of the secondary flow SF inside the duct (or the ducts) 7. In particular, the feeding system 23 and the feeding system 24 are adjustable independently of one another. In this way, for example, the primary flow PF can be varied keeping the secondary flow SF constant and vice versa, based on the load of the burner 1. According to some preferred, though non-limiting embodiments, the burner 1 further comprises a feeding system 25 for the oxidizer OX. In particular, the oxidizer feeding system 25 comprises at least one fan 26 (schematically shown in figure 1) with variable revolutions, which is controlled, in its rotation, by an actuator system of its own.

Advantageously, though not necessarily, the burner 1 comprises an electronic control unit 27, which is configured to control (in a coordinated manner) the fuel feeding system 23, the fuel feeding system 24 and the feeding system 25 for the oxidizer OX. In particular, the control unit 27 is configured to vary the proportion between the primary flow PF and the secondary flow SF depending on a load requested to the burner 1.

Advantageously, though not necessarily, the control unit 27 is configured to vary the primary flow PF between 5% and 50% of the total fuel resulting from the sum of the primary flow PF and of the secondary flow SF.

Advantageously, though not necessarily, the control unit 27 is configured to vary the secondary flow SF between 95% and 50% of the total fuel (resulting from the sum of the primary flow PF and of the secondary flow SF). According to some advantageous non-limiting embodiments, the control unit 27 is configured to minimize the inflowing volume of the primary flow PF as long as a further decrease does not cause the extinguishing of the main flame MF. In the non-limiting embodiments of figures 1 and 3, the burner 1 is configured in such a way that the main flame MF develops in a volume of the combustion chamber 3 that is separate from the flame root FR.

In this way, it is possible to have lower values of the mean flame temperature and to reduce, as a consequence, the contribution of thermal NOx.

In some non-limiting cases, the feeding system 23 and the feeding system 24 each comprise a respective electric actuator device M, in particular to adjust a respective valve V, preferably a throttle valve. In detail, the electric actuator systems M are stepper motors or brushless motors.

According to a second aspect of the invention, there is provided an industrial apparatus to heat a fluid.

In particular, the apparatus comprises the combustion chamber 3 and a burner 1 according to the description above. Advantageously, the burner 1 is arranged so that the suction element 12 is placed inside the combustion chamber 3 and causes at least part of the gases G present inside the combustion chamber 3 to flow through the tubular discharge element 11.

According to a further aspect of the invention, there is provided a method to control the burner 1 comprising the step of separately controlling the electric actuator systems M so as to minimize the primary flow PD used to generate the flame root FR.

In use, in the area of the turbulator disc, the burner 1 generates a first combusted mixture, in particular the flame root FR, out of the primary flow PF and the primary oxidizer portion OX', whose gases at least partially flow through the discharge element 11, which introduces the flame root FR into the combustion chamber 3. At the same time, the burner generates a second combusted mixture, in particular the main flame MF, out of the secondary flow SF and the accelerated secondary oxidizer portion OX'', whose gases at least partially flow through the discharge element 11, which introduces the main flame root MF into the combustion chamber 3 deeper than the flame root.

The products of the combustion emitted by the burner 1 are not totally burnt when they go through the discharge element 11 for the first time, but the combustion is increased (completed) thanks to the continuous recirculation of the gases G (present inside the combustion chamber 3) through the suction element 12 in the discharge element 11. In addition, the combustion is further improved thanks to the continuous return of the gases G' into the volume of the main flame MF due to the high outflow speed of the secondary flow SF and of the mixture formed by the secondary oxidizer portion OX'' and by the gases G. In other words, the burner 1 generates a primary combustion of the gases introduced by the ducts 6, 7 and 8 (fuel and oxidizer) and a secondary combustion thereof, using the gases G, G' recirculated from the inside of the combustion chamber 3, as they were not completely burnt (and, therefore, have residual oxygen), so that they are sucked by the suction element 12 or by the main flame MF (separated from the flame root FR).

In particular, the narrowing 20 of the suction element 12 determines an increase in the speed of the secondary oxidizer portion OX'' flowing out of the discharge element 5. The change in the speed of the oxidizer, making use of the Venturi effect, determines a depression in the area of the opening 13. This depression determines, in turn, the suction of the gases G present inside the chamber 3, thus allowing for a secondary combustion using these gases F (where there still is a moderate percentage of oxygen - variable up to 10%).

In the non-limiting embodiments shown in the accompanying figures, the suction element 12 determines an increase in the turbulent motions inside the combustion chamber 3. Furthermore, the secondary combustion generates a further heat exchange increase. As a consequence, there are an increase in the total heat exchange coefficient and a greater temperature homogeneity inside the combustion chamber 3.

Furthermore, the independent adjustment of the flows PF and SF limits the useless use of fuel, thus further improving, together with the suction element 12, the efficiency of the combustion and the reduction of emissions (in particular of NO _x ).

Therefore, it is evident that an apparatus 1 according to the invention allows user to obtain a greater uniformity of the temperature inside the combustion chamber 3, whose mean is particularly lower compared to prior art solutions.

It should be pointed out that, when using a burner according to the description above, the temperature peaks occurring close to the discharge of the burner 1 are (at least partially) flattened, thus determining a smaller amount of emissions. Even though the invention described above makes particular reference to a precise example of application, it should not be deemed as limited to said example of application, for its scope of protection also encompasses all those variants, changes or simplifications that are covered by the appended claims, such as for example a different geometry of the head 15 and of the suction element 12, a different arrangement of the ducts 6 and 7 inside the burner of the discharge element 5 and 11 (in terms both of position and of alignment), different feeding systems, a different type of diffuser terminal, etc..

The apparatus and the burner described above have many advantages.

First of all, the invention, thanks to a plurality of factors combined together, significantly reduces - compared to a standard burner - the NO _x and CO emitted by the burner 1. In particular, the invention allows for a high fuel staging due to the arrangement of the ducts 6 and 7 and to independence of the feeding systems 23 and 24. Hence, this staging determines a volume separation between the flame root FR and the main flame MF.

In addition, the burner 1, given its geometry, allows for an effective oxidizer staging and easily be installed as a replacement (improvement) of a standard architecture.

Furthermore, the invention also leads to a high burner power modulation ratio (thanks to the electronic control of the oxidizer feeding system 25 capable of changing the revolutions of the fan 26).

Finally, the dispersion decrease, the increase in combustion and uniformity of the temperature inside the combustion chamber 3, the independent control of the primary flow PF and of the secondary flow SF determine, for the apparatus and the burner 1 according to the invention, the need for a smaller quantity of gas (usually methane) to be introduced into the burner 1 in order to maintain a given temperature, compared to prior art solution, thus determining, given the same power and beside an emission reduction, a saving in terms of energy and raw materials.