DESCRIPTION

The present invention relates to a combustion head for a liquid fuel burner capable of reducing the emission of nitrogen oxides (NOx) in an economical way compared to known combustion heads.

In the burners field it is known that NOx is one of the most important pollutants arising from combustion processes, caused in part by the presence of nitrogenous compounds in fuel (the so-called " chemical NOx ") and partly by the atmospheric air present in the air (the so-called "thermal NOx").

NOx formation is favored by high combustion temperatures, and by the time it takes to stay at these temperatures.

Even insufficient mixing of fuel and combustion air can favor NOx formation, because it increases chances that oxygen meets nitrogen and forms NOx before combining with carbon or hydrogen of the fuel.

A lack of air, even a local one, due to inadequate mixing, can cause the formation of unburnt and carbon monoxide CO.

According to European EN 267/2009 regulations, burner types are divided into the following efficiency classes, depending on the amount of nitrogen oxides NOx and CO carbon monoxide emitted per kWh of thermal energy generated: - Class 1 for those old-generation burners whose combustion generates NOx in quantities greater than 180 mg kWh;

- Class 2 for NOx burners with quantities between 120 and 180 mg/kWh; - Class 3 for the most advanced burners, capable of generating NOx less than 120 mg/kWh.

NOx limitation can be achieved chemically by employing fuels free of nitrogenous compounds (obviously more expensive), thus reducing chemical NOx while the thermic NOx due to nitrogen present in combustion air is reduced only by treating the combustion quality which, in turn, can also prevent the formation of chemical NOx.

Ultimately, low-grade burners must necessarily use purified fuels while the goal is to use non-purified, less expensive fuels, just increasing the quality of combustion.

To achieve such quality, attention must be paid both to the dosage and to the fluid dynamics of the combustion fluid. In particular, it is important to ensure:

- the nebulization of the fuel so that it provides the maximum contact surface with combustion air A;

- the proportions between fuel C and combustion air A;

- the distribution of combustion air A along the flame.

Such combustion air A is divided into primary air Ap and possible secondary air As, where for primary air Ap is meant that part of combustion air A mixed with fuel upstream of the point where combustion begins to produce flame F, while for secondary air As is meant the possible remaining air necessary for the combustion provided along the flame F.

Here are provided the following definitions:

- "Rich" mixture: a mixture rich of fuel C where combustion air A is insufficient for a complete combustion;

- "poor" mixture: a mixture poor of fuel C where combustion air A is excessive for complete combustion;

- "stoichiometric" mixture: a mixture where fuel C and combustion air A are in the exact proportions for complete combustion and free of residual oxygen.

In practice, since full combustion is only obtained with a mixture at least slightly poor, the term "stoichiometric" may mean a poor mixture with air in excess just as much as necessary for complete combustion. To a stoichiometric mixture corresponds the minimum emission of combustion fumes FC and hence the minimal dispersion of heat and pollutants.

When combustion uses both primary Ap and secondary As, the mixture obtained with the primary air Ap upstream of the flame root F is a rich mixture; the total air supplied by adding the secondary air As along the flame F forms a poor mixture, this will be closer to a stoichiometric one, the finer is the nebulising of the fuel C, the more accurately the combustion air A is divided between primary

Ap and secondary As and the more carefully the secondary air As is driven along the flanks of the flame.

These aspects, are known well and with depth of details to industry experts.

However it is also well known, how difficult, with solutions of reasonable cost, it is to supply primary Ap and secondary As air in the desired quantities and proportions while varying the thermal power.

It is useful here to list the essential components of the combustion head T of a typical burner which are present in the figures attached to this description.

The combustion head T comprises a casing generally referred to as a mouthpiece

1, connected with a flange 8 to the ventilation unit 9.

Within the combustion head T are housed:

- a pre-chamber 4 in which the combustion air A is introduced by the ventilation unit 9, before being sent and distributed to the downstream components;

- a nozzle 3 mounted on a nozzle holder 301 ; within the nozzle 3, the fuel passes through helical channels (not shown in the figures) that impart to the liquid fuel C a roto-traslatory motion with an angular velocity ω (whether it is clockwise or counterclockwise) and swirling in a vortex in order to exit the output orifice 302 of the nozzle 3 nebulized and in a cone-open jet;

- spark ignition electrodes 5, located near orifice 302, trigger the electric spark to start the combustion of the fuel-air mixture;

- a so called, flame disk 7, also known as a turbolator or stabilizer disk, placed immediately downstream of orifice 302; the flame disk 7 has a central hole 704 for the passage of fuel and of primary air Ap and has turbolator slots 705 having radial development and generally with helical trend of the surface that when crossed by a first part of secondary air As, here said secondary central air Asl, produce a vortex motion coaxial with the axis of the mouthpiece 1 with the same direction of the angular velocity co transmitted to the fuel C favoring the nebulizing and mixing with combustion air A;

- not shown in the figures, a flame detector is provided with sensors sensitive to the light emitted by the combustion.

The pre-chamber 4 has an outer diameter D which is characteristic of the nominal thermal output of the combustion heads T of the type described herein and its magnitude can be taken as a reference for all the other values that will be quoted below.

Generally, the flame disk 7 is not really a disc but has a first cone-shaped portion 702 diverging in the direction of fluid progression followed by a second cylindrical portion 703 provided with an output edge 706. The flame disk 7 can be slid axially forward until the output edge 706 runs to the rest 101 of the mouthpiece 1, punched as a rim bent inside the mouthpiece 1. By adjusting the distance between the edge of output 706 and the rest 101 it is possible to regulate the inflow of a remaining portion of secondary air As here said peripheral secondary air As2. The output edge 706 may be notched (not shown in the figures) so that, even when driven to rest to the rest 101, a portion of peripheral secondary air As2 is always output towards the flame. The flame disk 7 is driven by air guides 103 whose main function is to drive the peripheral secondary air As2 parallel to the axis of the combustion head T towards the output edge 706.

The creation of vortex motion in the combustion air A requires a fan designed to generate sufficient head pressure. At the same thermal efficiency of the combustion head T, the head pressure and therefore the initial cost of the fan and its consumption may vary greatly with the architecture of said combustion head T.

Advantageously, combustion is often distributed/extended downstream of the mouthpiece 1 within an optional over mouthpiece 2, fixed to the mouthpiece 1 by appendices 201 consisting of fins obtained by trimming the end of the tube from which the over mouthpiece 2 is obtained. Between the mouthpiece 1 and the over-mouthpiece 2 there is a circumferential slit 202 through which the recirculation of FC combustion fumes occurs for Venturi effect. Over mouthpiece 2, by heating up, acts as a post burner for the residual combustion- fuel; such combustion begins shortly before the output of mouthpiece 1 while the recirculation of FC combustion fumes, which are colder than the flame, reduces the combustion temperature thus limiting the formation of NOx.

Although the red-hot over-mouthpiece 2 supports combustion, this is not complete highly probably because the means envisaged are not sufficient for proper distribution of combustion air A. Notwithstanding that, the reduction in combustion temperature does reduce NOx thus allowing to reach efficiency class 3, however just the limit is reached with no confidence margin and only with the use of said purified fuels.

A drawback of the burners of the type described is that the air coming from the fan is subject to turbulent motions in disordered directions that can counter the angular velocity co that is to be transmitted.

Further disadvantage of such burners is that the peripheral secondary air As2 does not blend satisfactorily with the flame while also partly reducing the vortex motion impressed by the turbolator slots 705.

A further drawback is that between the output edge 706 and the rest 101 unburned carbon deposits build up because the secondary air As is unable to sweep them away.

A further drawback is that the said appendices 201, consisting of a sort of scalloping, are an obstacle to the recirculation of FC combustion fumes.

Another drawback is that over-mouthpiece 2, to be effective, has a length L2.a bulky, while it would be advantageous to contain it for cost and space reasons. Another drawback is that at the outlet end of the over mouthpiece 2, where this is colder, carbon deposits are formed.

Another disadvantage is that when the ignition is started, the fan sends a pressure stroke that disturbs the ignition, making the flame unstable and sometimes extinguishing it.

Known art lists many means and methods to solve at least part of these drawbacks but all are rather expensive and therefore difficult to implement particularly for small sizes burners for building/residential uses, i.e. power up to about 70 kW.

The general aim of the present invention is to at least partly overcome the drawbacks outlined above by means of a series of new design solutions which are low cost, mutually compatible but also independently usable, each of which is capable of reducing NOx emissions and/or improving the completeness of combustion and/or favoring flame stability in the critical phases of ignition and/or to reduce the dimensions and/or to reduce the carbon deposits in specific areas.

The features of the present invention will be better disclosed by the following description of preferred embodiments according to the patent claims and illustrated, by way of non-limiting examples, in the accompanying drawings, in which:

- picture 1 shows in perspective and longitudinal section a combustion head according to the invention where the elements of interest for the invention are marked with identification numbers;

- picture 2 is identical to picture 1 except that, instead of indicating said identification numbers, it highlights the linear magnitudes of interest for the invention;

- picture 3 shows, with and without the fuel nozzle, the same combustion head of previous pictures, shown by longitudinal sections and some cross sections; the linear magnitudes of interest for the invention are rated in preferred ratios with respect to a baseline value; - picture 4 shows in cross section the same combustion head of previous figures with indicated combustion air flows and combustion fumes;

- picture 5 shows in cross section, the detail E of picture 3 doubled in size. The features of the invention are now described with particular reference to the embodiments shown in the figures.

It should be noted that any possible absolute spatial term used (such as "lower", "top", "interior", "exterior" and the like) refers to the position whereby the above mentioned elements are illustrated in the attached figures without limitative intent to the possible operating conditions, while any relative spatial terms (such as "upstream", "downstream", "input", "output") are given with reference to the flow of fluids under operating conditions.

All the aforementioned sizes are identified by alphanumeric symbols and in an appendix rated with reference to a baseline value which in turn is characteristic of the Pnom nominal power of the combustion head.

It is well known that in fluid dynamics the relations between dimensions of the ducts in which fluids flow are critical to the correct functioning of the devices. Even some construction details have an effect that often is not understood and ignored. Fuel burners do not escape this rule.

The attached drawings are provided by way of example and show a possible preferred combustion head T on scale according to possible but not compulsory proportions.

Often the innovative designs described herein, have a synergistic effect, yet retaining their effectiveness even if they are used independently from each other; to understand both their synergic and autonomous effect, it is appropriate therefore to describe them all.

According to many combustion heads T of the prior art, the mouthpiece 1 according to the invention, is of a conical type ending at the outlet with an outer diameter Die.

According to a first aspect of the invention, at the entrance of the combustion head T i.e. at the entrance of the pre-chamber 4, it is possible to place a diffuser disc 401, provided with a series of pass through holes 402, for combustion air A supplied from the fan. The purpose of the diffuser disc 401 is to dampen the , vortexes at the entrance of the combustion head T so that this, immediately downstream, can more easily assume the direction desired and transmitted by the shape of the mouthpiece 1 and the flame disk 7.

The diffuser disc 401 has an outer diameter substantially equal to the diameter T) of the pre-chamber 4.

According to a second aspect of the invention, in the pre-chamber 4, downstream from the inlet of the combustion head T and preferably upstream of the opening of the air guides 103, one or more air vent small holes 102 may be provided outward of the combustion head T having total passage section Sit. The purpose of these air vent holes 102 is to dampen the abrupt change in pressure generated at the start-up of the fan or at any sudden change in speed and which is responsible for instability and flame lift and ignition difficulties. Preferably, said total passage section Sit is distributed in at least Nl air vent holes 102, even more preferably these will have radial symmetry.

According to a third aspect of the invention, the air guides 103 may follow a plane inclined with respect to the burner axis in order to contribute to the vortex like motion of the flame with angular velocity ω by transmitting a rotary motion to combustion air A; this is particularly effective on peripheral secondary air As2, which, on the contrary, according to the known art, enters the flame in the direction of exclusively radial and centripetal direction. The angle of inclination a of said plane is that sufficient (and of course in the needed direction) to favor the peripheral secondary air rotation As2 at said angular velocity ω. Such angle depends on the geometry of the mouthpiece 1 and other elements that guide the combustion air A but preferably is within 15° and 30° with respect to the burner axis. Even more preferably it is essentially equal to 23°.

A fourth aspect of the invention relates to the resting surface between the output edge 706 of the flame disk 7 and the rest 101 of the mouthpiece 1 of internal diameter Dli. As has already been said, the rest 101 of the mouthpiece 1 is currently punched as a bent rim inside the mouthpiece 1, not realizing that instead a careful execution of a flat surface, which cannot be obtained by sheet punching, is very important:

- for the correct dosing and uniform circumferential distribution of secondary peripheral air As2;

- for its correct transmission of the centripetal motion which activates also the recirculation of combustion fumes FC and their convergence towards the flame;

- as well as for the effective cleaning of the rest 101 from carbon deposits. It has been also found that for one or more of said scopes, it is important that the edge 104 between the converging portion and the rest 101 of the mouthpiece 1 be sharp, result that is impossible to obtain by sheet metal punching.

Thus, according to the fourth aspect of the invention, it is preferred that the rest 101 of the mouthpiece is obtained by a circular flange 101 of internal diameter Dli, obtained separately by shearing or preferably by lathing and then bound to the end of the mouthpiece 1 preferably by welding. The length of the inner diameter Dli is also important to obtain the desired convergence angle of the secondary air As2 towards the flame.

The fact that the rest 101 of the mouthpiece 1 is obtained by a circular flange 101 enables a further improvement, very difficult to achieve if said surface 101 were obtained by punching, in particular as a simple bent rim.

As shown in fig. 5, on the outer part of said circular flange a countersink of angle 5° and 15° and preferably about 10° is performed. Strong experimental evidence shows that such countersink greatly contributes to the quality of the of combustion; probably because it favors the mixing with the part of combustion fumes which is re-circulated by Venturi effect. Such countersink, could be obtained also by minting and/or flattening of the rest 101 as this gets made by punching or trimming, however this would cause great stress of the tools, so achieving it by a lathing process is much more suitable.

It should be noted that the secondary air As2 is canceled, at least in principle, when exit edge 706 and rest 101 are in contact, since it is preferred according to the invention that the exit edge 706 is free of notches foreseen in known art. Concerning the flame disk 7, nothing prevents that in its second cylindrical part 703, in axis with the turbolator slots 705 foreseen on the first part 702 of conical shape, are made pass through anti carbon deposits holes as already known from the IT 102015000017565 document and equally placed and dimensioned for the same purpose.

Further aspects of the invention concern the possible over-mouthpiece 2.

According to a fifth aspect of the invention, the over-mouthpiece 2 has at its inlet edge 203 an inlet ring 204 whose inner diameter D2i measures less than the

D2 the diameter of the over-mouthpiece 2 itself. This narrowing contributes to converging the combustion fumes FC recirculated towards the flame axis.

Preferably, anyway, the measure D2i of such inner diameter is larger than the inner diameter Dli of the exit edge 706 of the flame disk 7. Even more preferably, said measure D2i is larger than the outer diameter Die of the mouthpiece 1 at its outlet.

According to a sixth innovative solution, it has been found very advantageous to provide recirculation holes 206 of quantity N2 and diameter D2r placed just before the output edge 205.

Through said holes a local vortex of combustion fumes FC originates with a double advantage:

- first is that from the output edge 205 unburned deposits are more easily removed thus blocking the formation of carbon deposits. Carbon deposits in the over-mouthpieces, usually build up especially at the output edge where the flame is cooler and further cooled at the start by the fact that the over- mouthpiece 2 has not yet reached a sufficient temperature;

- second is that such local vortex allows to reduce the over-mouthpiece from its usual length L2.a to a shorter L2.b while delivering the same results that known art obtains with usual length L2.a in terms, for example, of flame stability, combustion completeness, NOx reduction. According to a seventh possible innovation, it is preferred that said appendices 201 which secure the over-mouthpiece 2 to the mouthpiece 1 are radial orientation blades 201, i.e. laying on planes passing through the mouthpiece 1 axis so as to oppose the minimum resistance to the combustion fumes FC recirculating through the circumferential slit 202, whose width is determined by the distance L2d between the rest 101 of the mouthpiece 1 and the inlet edge 203 the over-mouthpiece 2 in turn determined by the length of said appendices 201. Finally, while according to known art, over-mouthpiece 2 has a considerable thickness of 2 or more millimeters to give it sufficient thermal capacity to stabilize combustion temperature, according to the invention, for the over- mouthpiece 2 to best contribute to combustion stability since the starting of the ignition, the thickness s2 must is at least half of what is present current practice, so that its thermal mass is lower and hence it is more quickly brought to high temperatures. This also reduces the possibility of unburnt deposits in the phases following the ignition.

Concerning preferred dimensions of the elements afore mentioned, the outer diameter D of the diffuser disc 401 practically coincides with the diameter of the flange connecting the combustion head T to of the fan unit and can be considered a dimensional reference value for the majority of the geometrical sizes listed above. In fact, the input section of the diffuser disc, proportional to D2, generally varies linearly with the nominal power of the burner and almost all the other linear magnitudes should vary proportionally to D.

As an indicative non-limiting example, D may be in the order of 100 mm for Pnom nominal burner power between 50 and 60 kW while it is in the order of 125 mm for Pnom in the order of 100 kW.

Basically, for purely indicative purposes:

D2 = k. Pnom [mm] where k is about 150 mm2/kW.

As for the other sizes, without limitative intent here below are provided, as example, possible field of values mostly expressed in relation to D:

- Outlet diameter Die of the mouthpiece 1 : - preferably between 0.48 x D and 0.85 x D;

- still more preferably 0.64 x D.

Inner diameter Dli of the rest 101 < Die

- and preferably between 0.42 x D and 0.77 x D;

- even more preferably between 0.56 x D and 0.58 x D.

Total passage section Sit of air vent holes 102

- preferably between 0,0004 x D2 and 0,007 x D2;

- still more preferably equal to 0.0005 x D2.

Nl number of air vent holes 102

- preferably Nl = 6 and preferably distributed by radial symmetry.

L2d distance between over-mouthpiece 2 and mouthpiece 1

- preferably between 0.05 x D and 0.08 x D;

- even more preferably equal to 0.06 x D.

D2 diameter of the over-mouthpiece 2

- preferably between 0.7 x D and 1.2 x D;

- still more preferably equal to 0.9 x D.

Inside diameter D2i of inlet 203 in the over-mouthpiece 2

- preferably between 0,4 x D and 0,95 x D;

- even more preferably between 0.56 x D and 0.73 x D.

D2r diameter of recirculation holes 206

- preferably between 0.05 x D and 0.09 x D;

- even more preferably 0.07 x D.

Recirculation holes 206

- distributed with radial symmetry;

- preferably in number N2 such that they are spaced apart by about twice the diameter D2r;

- even more preferably in the number N2 = 18;

- preferably of a diameter of 0.07 x D.

L2b length of over-mouthpiece 2 (at least when recirculation holes 206 are provided)

- preferably between 0.35 x D and 0.6 x D;

- even more preferably at 0.46 x D.

- L2d length of the distance between the rest 101 of the mouthpiece 1 and the inlet edge 203 of the over-mouthpiece 2)

- preferably between 0.05 x D and 0.07 x D;

- even more preferably 0.06 x D.

- s2 thickness of the over-mouthpiece 2

- less than 2 mm;

- preferably about 1 mm.

Advantageously, the mouthpiece 1 is obtained by die-casting of a suitable metal alloy. Even more advantageously, then, and differently than what shown in the accompanying figures, the air guides 103 are then formed with the mouthpiece 1 during the same die-casting process.