BURNER Field of the invention The present invention relates to a burner of the type comprising: -a first tubular duct having at least an end portion with rectilinear axis including an outflow opening ; -a second tubular duct, external and coaxial to the first duct with which it defines a free space with substantially annular section comprising, at one end, a respective outflow opening, surrounding the outflow opening of the first duct ; -means for feeding in the first duct and in the free space independent fluids including a fuel and a comburent, respectively, or viceversa ; Burners of the aforesaid type are widely, but not exclusively, used for the partial or total oxidation of hydrocarbons to obtain, for example: synthesis gas, fuel gas or reduction gas.

In this case, the first duct (inner duct) of the burner, is for example crossed by a gaseous flow containing oxygen (comburent), while the second duct (outer duct) is for example crossed by a hydrocarbon flow (fuel), both said ducts having the corresponding outflow openings facing a combustion chamber, external to the burner.

Only to simplify in the following description it will be made reference to the aforesaid field of application.

Regarding to the use of a burner of the aforesaid type, the technical disadvantage mainly complained is the thermal wear of the outflow openings that causes an interruption of the burner operation. This implies negative consequences such as the need of maintenance operations and even the replacement of the end part of the burner

Such operations involve moreover the shutdown of the equipment connected to the burner with consequent elevated losses both in productive and in economic point of view.

Prior Art In order to overcome such disadvantage, the prior art has proposed to control and to reduce the temperature in the burner. Therefore in patent EP 0 640 679 different means are described, such as cooling coils winded externally to the burner or cooling chambers defined in proximity of the outflow openings and finally the use of a gaseous flow comprising a temperature moderator fluid, that crosses an appropriate duct of the burner coaxial to the reagents feeding ducts,.

In the field of application moreover, burners with cavities have been proposed.

The cavities are made within the thickness of the feeding ducts Despite the general control of the temperature, such devices have turned out to have low effect in the control and the reduction of the temperature in the outflow openings.

In fact it has been found that, independently by the cooling means used according to the prior art, the end portion of the outflow openings endures a fast thermal wear, causing the interruption of the burner activity.

The aforesaid disadvantage is due-among the others-to the elevated temperature found in the outflow openings of the burner (in the magnitude of 1000 °C and more), to the presence of fast and high temperature vortices of gas in the ends of such openings and to the corrosive effects caused by the fluids.

Such disadvantage is particularly present in of the terminal part of the outflow opening of the inner duct (first duct) and in presence of fluids comprising hydrogen and oxygen.

In order to overcome the aforesaid disadvantage the teaching of the prior art, the present invention takes part during the motion of the fluid outflowing from the first duct (inner

duct) so as to protect such duct from the corrosive action of high temperature gases, preventing in particular the formation of corrosive effects.

Summarv of the invention Consequently, the technical problem underlying the present invention is to make available a burner, particularly but not exclusively indicated for the partial or total oxidation of hydrocarbons, having structural and functional features such to avoid the building up of fast and high temperature fixed gas vortices in the outflow opening of the first duct of the burner.

This problem is solved, according to the present invention, by an outflow opening of the first duct comprising a surface with axial symmetry, countersunk towards the outside of the same duct. In other words, the outflow opening of the first duct is advantageously countersunk towards the outside of such duct.

Because of the particular shape of the aforesaid outflow opening, the present invention, allows to solve in a simple and extremely effective way the problem of the thermal wear of the inner duct (first duct) that, as it has been mentioned with reference to the prior art, is the duct mainly exposed to such corrosion.

It has, in fact, surprisingly been found that with an outflow opening of the first duct countersunk towards the outside, it is possible to maintain the outflowing fluid thereof such duct constantly in contact with the whole surface of the outflow opening, therefore completely protecting such surface by high temperature gases.

Advantageously, is obtained a mixing of the fluid outflowing from the first duct with the fluid outflowing from the free space, defined between the first and second duct, such second duct is placed at the beginning of the outflow opening of the first duct.

In other words, making the outflow opening of the first duct with an axial symmetry surface countersunk towards the outside of the same duct, advantageously enables to avoid the formation of a shadow area (area not interested by the primary fluid passage) placed between the terminal part of the first duct and the mixing point of fluid reagents.

According to the test made by the inventor, in such shadow area-of the burners according to the prior art-highly turbulent fluid flows at high temperature take place, in the shape of high speed fixed vortices, responsible of the fast thermal wear of the outflow opening of the first duct.

Such advantage is mainly enhanced in case the fluid flowing in the inside of the first duct, outflows from the latter with helical motion.

For such a reason the burner according to the present invention comprises advantageously further means for causing a helical motion to the fluid fed in the first duct in proximity of at least an end portion.

Therefore making, the fluid outflowing from the first duct expands towards the outside easing a constant adhesion of such fluid against the countersunk surface of the outflow opening, avoiding in a still safer way a premature crawling of the fluid from the outflow opening and therefore the formation of a shadow area downstream the outflow opening where the high temperature gas is flowing.

With the aim to obtain an optimal adhesion of the fluid outflowing from the first duct against the outflow opening, three preferred embodiments of realisation of the present invention are disclosed.

According to a first embodiment of the present invention, the axial symmetry surface of the outflow opening of the first duct, advantageously is a cone log (conical countersink).

Particularly satisfactory results have been obtained by the surface with axial symmetry that has a taper comprised between 20° and 80°, and preferably between 30° and 60°.

According to a second embodiment of the present invention, the surface of the outflow opening, with axial symmetry of the first duct has a profile-with respect to an axial section-curvilinear.

In this case, particularly satisfactory results have been obtained by the tangent of the curvilinear profile with respect to a rectilinear axis of the end section of the first duct that varies with continuity from 0° to a maximum of 90°.

In agreement with a further embodiment of realisation of the present invention, the axial symmetry surface of the outflow opening of the first duct comprises-with respect to an axial section-a portion with curvilinear profile and a portion with rectilinear profile.

This embodiment of the invention is particularly favourable in the fact that it allows a simple and extremely precise countersunk shape of the outflow opening of the first duct.

In this respect, the portion with rectilinear profile is arranged preferably in the most peripheral part of the surface with axial symmetry.

According to this embodiment, particularly satisfactory results have been obtained with an angle formed by the portion of rectilinear profile with respect to a rectilinear axis-of the end portion of the first duct-comprised between 30° and 90°, and preferably between 40° and 80°.

The features and the advantages of the invention will turn out mainly from the description of the following example of the process according to the said invention for suggesting and not restricting, with reference with the attached figures reported

Brief description of the drawing -Figurel shows an axial section schematic view of a burner according to a preferred embodiment of the present invention.

Detailed description of a preferred embodiment With reference to figure 1, with 1 is schematically represented a detail of a burner-and more in particular the terminal part thereof-according to a preferred embodiment of the present invention.

The burner 1 comprises a first tubular duct 2 having at least an end portion 3 with rectilinear axis 12 including an outflow opening 4.

In figure 1, the end portion 3, is represented in the shape of a straight cylindrical portion.

With 5 is instead indicated a second tubular duct, external and coaxial to first duct 2 with which it defines a free space 6 of substantially annular section, comprising, at one end 7, a corresponding outflow opening 8, surrounding the outflow opening 4 of first duct 2.

The corresponding length of ducts 2 and 5 is not critical according to the present invention. In the example of figure 1, the outside duct 5 has a slightly greater length with respect to the one of the inside duct 2.

The burner 1 comprises moreover specific means in order to feed, in first duct 2 and in the free space 6, independent fluids including correspondingly, a fuel and a comburent or viceversa.

Such means themselves not conventional and thereof not represented in the detail of burner of figure 1, comprise for example fluid reagent inlet nozzles both in ducts 2 and 5.

The burner 1 according to the present invention differs from the burners of the prior art in the fact that the outflow opening 4 of first duct 2 advantageously comprises one surface 9 with countersunk axial symmetry towards the outside of the same duct.

In particular, according to the preferred embodiment of figure 1, the outflow opening 4 comprises, in axial section, a portion 10 with curvilinear profile and a portion 11 with rectilinear profile.

Portion 11 with rectilinear profile is moreover advantageously placed in the more peripheral part of the outflow opening 4. In this way, the mechanical construction of the outflow opening 4 of inner duct 2 becomes considerably simpler.

Particularly satisfactory results regarding the adhesion of the fluid outflowing from the inner duct 2 against the outflow opening 4, have been obtained making the portion 11 with rectilinear profile so as to form an angle with respect to the axis 12 of end portion 3 rectilinear comprised, for example, between 50° and 70°.

According to the other embodiments of the present invention, also advantegeous but not represented, surface 9 with axial symmetry of the outflow opening 4 of first duct 2 is a cone log or, alternatively, a profile-in axial section-curvilinear.

In the first case, surface 9 with axial symmetry has a taper, for example, comprised between 40° and 50°.

In the second case, the tangent of the curvilinear profile, with respect to the rectilinear axis 12 of end portion 3 of first duct 2, varies preferably with continuity from 0° to a maximum of 90°.

It should be noted that in all these embodiments of realisation, the end portion with rectilinear axis of the external wall 2a of first duct 2 (horizontal in figure 1), ends in the tip 13 of the countersunk outflow opening 4. In fact it has surprisingly been found that, in this way, an optimal adhesion is obtained with the fluid outflowing from duct 2 against the outflow opening 4 and therefore it is able to protect completely the portion of duct 2 from the corrosive action of high temperature gases.

In order to ensure a better adhesion of a fluid outflowing from duct 2 against the outflow opening 4, it is preferred to make flowing the fluid with helical motion in the end section 3. In this way when the fluid reaches the outflow opening 4, it expands licking the countersunk surface 9.

In this way, the burner 1 according to the preferred embodiment of the present invention represented in figure 1, comprises advantageously means for causing a helical motion to the fluid fed in first duct 2 in proximity of an end portion 3.

Such means comprise, for example, one or more fins 14, opportunely shaped and eventually angled with respect to longitudinal axis 12, placed in proximity of a support rod end 15, that extends coaxially for one predetermined length in the passage 16 defined by first duct 2.

Fins 14 are provided so as to confer to the fluid the wished spin or swirl. Preferably, a plurality of fins 14, are provided that extends helically on the rod 15.

According to an alternative embodiment not represented, such means can be for example made by same first duct 2 or of rod 15, opportunely shaped.

In figure 1, with 17 and 18 are indicated the cavities obtained in the thickness of the wall of first duct 2 and of the second duct 5, respectively, for the passage of a cooling fluid, for example water, to control the temperature of ducts 2 and 5.

Such cavities 17 and 18 are however entirely optional and can be omitted depending from the operating temperature conditions of the burner.

Although in the example of figure 1, burner 1 only comprises two tubular ducts 2 and 5, inner and outer, respectively, the present invention can advantageously have also application in burners of the type comprising three or more ducts for reagent fluids, being coaxial one with respect to another.

When in use, the burner 1 according to the present invention, operates in the following way.

A first reagent fluid, for example a comburent, is let to flow in the inside of first duct 2 (passage 16), while a second reagent fluid, for example a fuel, is let to flow in the free space 6, defined between the inner duct 2 and outer duct 5.

Such fluids are fed in duct 2 and respectively in free space 6 by feeding means previously described but not represented in figure 1.

The burner 1 according to the invention can also operate with the comburent in the outer duct 5 and the fuel in the inner duct 2.

As comburent fluid can, for example, be used a gaseous flow comprising oxygen, air, air enriched in oxygen or, preferably, pure oxygen.

As fuel fluid can instead be used a flow comprising hydrocarbons, preferably gaseous.

With :"hydrocarbons", is generically indicated a hydrocarbon or hydrocarbon mixtures saturate and/or not saturated, light and/or heavy (for example C1-C6).

With"gaseous flow comprising hydrocarbons", instead is indicated a fluid comprising gaseous hydrocarbons, like for example methane or natural gas, or a gaseous flow comprising solid fuels in suspension (for example powder of coal or lamp-black) or still a gaseous flow comprising liquid hydrocarbons in dispersion (for example light hydrocarbons or heavy which the naphtha and the fuel oil).

Once crossed the passage 16 of inner duct 2 as well as the free space 6 between ducts 2 and 5, the fluids flow-through corresponding outflow openings 4 and 8-in a combustion chamber external to burner 1 (not represented) where the combustion takes place.

The burner 1, according to the invention, is particularly but not exclusively indicated for the partial or total oxidation of gaseous hydrocarbons.

Advantageously, thanks to the particular countersunk shape of the outflow opening 4 of first duct 2, the fluid outflowing from the latter is opportunely diverted and expanded

against all surface 9 of such outflow opening 4, that is therefore protected from the corrosive action of high temperature gases, hydrogen and oxygen, circulating in the outflow openings of the burner.

Such expansion is then further promoted by the presence in the inner duct 2 of means 14 for causing a helical motion to the fluid outflowing from the outflow opening 4.

The presence of the reagent fluid flow that licks completely the outflow opening 4 of the inner duct 2, advantageously prevents the formation of fixed vortices of high temperature gases in the outflow opening 4, protecting such outflow opening from a premature thermal wear.

It should be noted that, the outflow openings, and the end part of most of the burners according to the prior art, have a duration of only few months, hereinafter the opening have to be replaced with a consequent shutdown of production.

Thanks to the present invention, the duration of the end part of the burners is instead considerably increased being able to last various years without any need of replacement and therefore allowing the burner to operate with long operational cycles.

In this way the losses in production and the maintenance costs are remarkably reduced.