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Title:
DUAL FUEL BURNER WITH SWIRL ARRANGEMENT
Document Type and Number:
WIPO Patent Application WO/2019/134748
Kind Code:
A1
Abstract:
Invention relates to an arrangement in a burner, comprising a duct part (14) having a circular inner cross section, an inlet (16) in a first end of the duct part (14), an outlet (18) at the second end of the duct part (14) and means (20) for introducing gaseous fuel into the burner, wherein the arrangement is provided with a mixing section (22) in the duct part (14), which mixing section (22) comprises plurality of stator vanes (24) arranged into the duct part (14) so as to induce tangential component to a gas flow in the duct part (14) when in use, the stator vanes (24) having an inlet side surface (24') and an outlet side surface (24''). The means (20) for introducing the gaseous fuel into the burner comprises at least one fuel feeding opening (26) arranged to the outlet side surface (24'') of more than one of the stator vanes (24).

Inventors:
SVEBERG, Stian (Værftsgata 13, 1511 Moss, 1511, NO)
STEINGRIMSEN, Terje (Værftsgata 13, 1511 Moss, 1511, NO)
SPYDEVOLD, Geir (Værftsgata 13, 1511 Moss, 1511, NO)
LINDLØV, Odd Ivar (Værftsgata 13, 1511 Moss, 1511, NO)
RUUD, Alexander (Værftsgata 13, 1511 Moss, 1511, NO)
ALFREDSSON, Henrk (Værftsgata 13, 1511 Moss, NO)
HÄGGLUND, Joakim (Værftsgata 13, 1511 Moss, NO)
Application Number:
EP2018/050199
Publication Date:
July 11, 2019
Filing Date:
January 04, 2018
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
WÄRTSILÄ MOSS AS (Værftsgata 13, 1511 Moss, 1511, NO)
International Classes:
F23D14/02; F23D17/00
Domestic Patent References:
WO2000061992A12000-10-19
Foreign References:
EP1909030A22008-04-09
US20100293956A12010-11-25
US20150285503A12015-10-08
US5240410A1993-08-31
Attorney, Agent or Firm:
GENIP OY (Heikinkatu 7, Kotka, 48100, FI)
Download PDF:
Claims:
Claims

1 . An arrangement in a burner, comprising a duct part (14) having a circu- lar inner cross section, an inlet (16) in a first end of the duct part (14), an outlet (18) at the second end of the duct part (14) and means (20) for introducing gas- eous fuel into the burner, wherein the arrangement is provided with a mixing section (22) in the duct part (14), which mixing section (22) comprises plurality of stator vanes (24) arranged into the duct part (14) so as to induce tangential component to a gas flow in the duct part (14) when in use, the stator vanes (24) having an inlet side surface (24’) and an outlet side surface (24”), characterized in that the means (20) for introducing the gaseous fuel into the burner comprises at least one fuel feeding opening (26) arranged to the outlet side sur- face (24”) of more than one of the stator vanes (24).

2. An arrangement according to claim 1 , characterized in that that the ar- rangement is provided with a center tube (36) having a circular outer cross sec- tion and arranged into the duct part (14), and that the vanes (24) are arranged into the duct part (14) extending radially from an inner wall of the duct part (14) towards the center of the duct part (14), and there is a gap (38) arranged between each one of the stator vanes (24) and the center tube (36).

3. An arrangement according to claim 2, characterized in that the arrange- ment comprises a sleeve member (40) to which the vanes (24) are attached at their radially inner ends, and that the gap (38) is arranged between the sleeve member (40) and the center tube (36).

4. An arrangement according to claim 3, characterized in that sleeve mem- ber (40) is supported to the center tube (36).

5. An arrangement according to claim 3, characterized in that the sleeve member (40) is coaxial with the center tube (36) and the gap (38) has a constant width.

6. An arrangement according to claim 1 , characterized in that the at least one fuel feeding opening (26) is a rectangular fuel feeding opening (26).

7. An arrangement according to claim 1 , characterized in that the means for introducing gaseous fuel comprises a gaseous fuel feeding conduit (32) which is a manifold circumscribing the duct part (14) arranged to feed the gaseous fuel into the stator vanes (24).

8. An arrangement according to claim 1 , characterized in that the stator vane (24) provided with at least one fuel feeding opening (26) is provided with at least one winglet (40) arranged to the outer side surface (24”) of the stator vane (24).

9. An arrangement according to claim 8, characterized in that the winglet (40) is arranged to extend angularly from a leading edge of the stator vane to a trailing edge of the stator vane.

10. An arrangement according to claim 8, characterized in that the winglet (40) is arranged inside the at least one fuel feeding opening (26) in radial direc- tion.

1 1. An arrangement according to claim 1 , characterized in that the stator vanes (24) provided with at least one fuel feeding opening (26) are of hollow structure.

12. An arrangement according to claim 1 , characterized in that the vanes (24) are arranged into the duct part (14) at a same longitudinal location evenly around the duct part (14).

13. An arrangement according to anyone of the preceding claims, characterized in that the inlet side surface (24’) and an outlet side surface (24”) of the stator vane is curved.

14. An arrangement according to anyone of the preceding claims 1 - 13, char- acterized in that the inlet side surface (24’) and an outlet side surface (24”) of the stator vane is flat and the surfaces are arranged parallel with each other.

15. An arrangement according to claim 1 or 2, characterized in that the ar- rangement is provided with a center tube (36) having a circular outer cross sec- tion and arranged coaxially with and into the duct part (14), which center tube (36) comprises a liquid fuel lance (44), and which liquid fuel lance (44) is enclosed by an outer wall (35) of the center tube (36), which outer wall (35) is arranged coaxially with the liquid fuel lance (44) such that there is an annular space formed between the liquid fuel lance (44) and the outer wall (35), and that the outer wall (35) is provided with one or more openings (48) configured to allow gas flow into the annular space at a location upstream the vanes (24).

16. An arrangement according to claim 1 or 2, characterized in that the ar- rangement is provided with a center tube (36) having a circular outer cross sec- tion and arranged coaxially with and into the duct part (14), and the duct part (14) at the outlet (18) has an inner diameter (D) which is substantially constant over the length (L), and that the center tube (36) extends from the mixing section (22) towards the outlet (18) over the length (L).

Description:
DUAL FUEL BURNER WITH SWIRL ARRANGEMENT

Technical field

[001] The present invention relates to an arrangement in a burner comprising a duct part having a circular inner cross section, an inlet in a first end of the duct part, an outlet at the second end of the duct part and means for introducing gas- eous fuel into the burner, wherein the arrangement is provided with a mixing section in the duct part, which mixing section comprises plurality of stator vanes arranged into the duct part so as to induce tangential component to a gas flow in the duct part according to the preamble of claim 1.

Background art

[002] Combustion is a major source of pollutants. Oxides of nitrogen, or NOx, (NO or nitric oxide, N02 or nitrogen dioxide, and N2 O or nitrous oxide) contribute to acid rain, smog, global warming, and ozone depletion. CO As NOx emissions from combustion sources primarily consist of nitric oxide, an understanding of how NO is generated is important. There are three principal mechanisms which produce NO during combustion: the fuel NO mechanism; prompt NO mecha- nisms; and the "thermal NO”. Carbon monoxide is colorless, odorless, and taste- less, but highly toxic gas. Partial oxidation of carbon containing fuel, such as incomplete combustion, results in undesired carbon monoxide emissions.

[003] The fuel to be combusted can be premixed with the combustion air or it can be introduced into the air immediately prior to combustion. Swirling flows have been used to stabilize combustion in a variety of burners, both premixed and non-premixed. Swirl in these burners is generally created either by generat- ing tangential flow motion in a cylindrical chamber, as in cyclone combustion chambers, or swirling a co-axial air flow. In both cases, the function of the swirl is to create a toroidal recirculation zone. When combusting gaseous fuel pre- mixed with the combustion air it is important that gaseous fuel is thoroughly mixed with the air. [004] US5240410 discloses a structure of a burner which can be fuelled with gas fuel or oil fuel. The main features includes: a specially designed swirl gener- ator; an annular hollow gas gun; an oil gun received in the gas gun where the gas jets of the gas gun and the oil jets of the oil gun have a predetermined angle with respect to the centerline. Under designed operating conditions, a swirling air flow can be generated with a low pressure drop and low turbulences, which is beneficial to flame stability, reducing flame temperature, and delaying the mixing of air and fuel, thus inhibiting the formation of NOx. Staging air and flue gas re- circulation are available for further reduction of nitrogen oxides. The gas gun and the oil gun are detachable and their positions are adjustable, whereby an oper- ating person can adjust the fuel supply.

[005] W00061992 A1 discloses a swirler with a hollow guide pipe supporting multiple blades adjacent a furnace end is provided. The swirler blades each in- clude tunnels in fluidic communication with a transport passage located inside the hollow guide pipe and openings in the swirler blade edges facing the furnace combustion chamber. The tunnels may be used to transport oxygen, natural gas, or other combustible gaseous substances. The tunnels may comprise a single passage or multiple passages interconnecting the transport passage to one or more openings in the trailing edge of the swirl blade. [006] An object of the invention is to provide a burner in which the performance is considerably improved compared to the prior art solutions.

Disclosure of the Invention

[007] Objects of the invention can be met substantially as is disclosed in the independent claim and in the other claims describing more details of different embodiments of the invention.

[008] According to an embodiment of the invention a burner comprises a duct part having a circular inner cross section, an inlet in a first end of the duct part, an outlet at the second end of the duct part and means for introducing gaseous fuel into the burner, wherein the burner is provided with a mixing section in the duct part, which mixing section comprises plurality of stator vanes arranged into the duct part so as to induce tangential component to a gas flow in the duct part when in use, the stator vanes having an inlet side surface and an outlet side surface. The means for introducing the gaseous fuel into the burner comprises at least one fuel feeding opening arranged to open into the outlet side surface of more than one of the stator vanes. This provides an efficient manner of admitting gaseous fuel into the stream of air in the duct part.

[009] Due to the construction of the arrangement in burner the pressure of the gaseous fuel needed for introducing the gas is smaller, and the mixing of the gaseous fuel in to the air is excellent. These together provide more advantageous effects to the operation of the burner. According to an embodiment of the inven- tion the vanes are arranged into the duct part extending radially from an inner wall of the duct part towards the center of the duct part.

[0010] According to an embodiment of the invention the vanes are arranged into the duct part to a predetermined longitudinal location evenly around the duct part.

[0011] According to an embodiment of the invention the inlet side surface and an outlet side surface of the stator vane is curved.

[0012] According to an embodiment of the invention the inlet side surface and an outlet side surface of the stator vane is flat and the surfaces are arranged parallel with each other.

[0013] According to an embodiment of the invention the burner is provided with a center tube having a circular outer cross section, arranged into the duct part, and there is a gap arranged between each one of the stator vanes and the center tube.

[0014] According to an embodiment of the invention the burner is provided with a center tube arranged coaxially with the duct part. [0015] According to an embodiment of the invention the stator vanes are sup- ported only to the duct part and detached from the center tube.

[0016] According to an embodiment of the invention the stator vane provided with one fuel feeding opening is provided with at least one winglet arranged to extend angularly on the outer side surface of the stator vane. Advantageously the vane is of hollow structure for providing adequate gaseous fuel feeding oper- ation.

[0017] According to an embodiment of the invention the winglet is arranged to extend angularly from a leading edge of the stator vane to a trailing edge of the stator vane.

[0018] According to an embodiment of the invention the winglet is arranged ra- dially inside the at least one fuel feeding opening.

[0019] According to an embodiment of the invention the stator vane provided with at least one fuel feeding opening comprises more than one fuel feeding openings.

[0020] According to an embodiment of the invention the arrangement in the burner is provided with a center tube having a circular outer cross section and arranged coaxially with and into the duct part, and the duct part at the outlet sec- tion has an inner diameter which is substantially constant over the length, which length substantially equals to or is greater than the diameter and that the center tube extends from the mixing section towards the outlet section over the length.

[0021] This this way the mixing of the gaseous fuel in the air is efficient and the combustion is efficient as well minimizing the NOx and CO emissions.

[0022] According to an embodiment of the invention the arrangement comprises a sleeve member to which the vanes are attached at their radially inner ends, and that the gap is arranged between the sleeve member and the center tube.

[0023] According to an embodiment of the invention that sleeve member is sup- ported to the center tube of the burner.

[0024] According to an embodiment of the invention the sleeve member is coax- ial with the center tube and the gap has a constant width.

[0025] According to an embodiment of the invention more than one of the stator vanes is provided with at least one fuel feeding opening. [0026] According to an embodiment of the invention each one of the stator vanes is provided with at least one fuel feeding opening.

[0027] According to an embodiment of the invention at least one fuel feeding opening is a rectangular fuel feeding opening. [0028] According to an embodiment of the invention the means for introducing gaseous fuel comprises a gaseous fuel feeding conduit which is a manifold cir- cumscribing the duct part arranged to feed the gaseous fuel into the stator vanes.

[0029] The exemplary embodiments of the invention presented in this patent ap- plication are not to be interpreted to pose limitations to the applicability of the appended claims. The verb "to comprise" is used in this patent application as an open limitation that does not exclude the existence of also unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. The novel features which are considered as charac- teristic of the invention are set forth in particular in the appended claims.

Brief Description of Drawings

[0030] In the following, the invention will be described with reference to the ac- companying exemplary, schematic drawings, in which

Figure 1 illustrates a burner according to an embodiment of the invention, Figure 2 illustrates a mixing section of a burner according to an embodiment of the invention,

Figure 3 illustrates a mixing section of a burner according to another embodiment of the invention,

Figure 4 illustrates a mixing section of a burner according to another embodiment of the invention,

Figure 5 illustrates a mixing section of a burner according to another embodiment of the invention, Figure 6 illustrates a mixing section of a burner according to another embodiment of the invention,

Figure 7 illustrates a burner according to another embodiment of the invention, and Figure 8 illustrates a burner according to still another embodiment of the inven- tion.

Detailed Description of Drawings

[0031] Figure 1 depicts schematically a burner 10 according to an embodiment of the invention. Figure 2 depicts a mixing section and means 20 for introducing gaseous fuel into the burner of the figure 1 in more detailed manner.

[0032] Now turning to the figures 1 and 2 the burner 10 comprises a body 12 into which a duct part 14 of the burner is provided. The body may be simply the wall of the duct part but the duct part 14 can be made also into a bulkier body part. The duct part is a part of the burner via which the air is conveyed from an inlet 16 of the duct part to an outlet 18 of the duct part for combustion of fuel by the burner. The duct part has a generally circular inner cross section. The burner 10 shown in the figure 1 comprises means 20 for introducing a gaseous fuel into the burner 10. The burner is also provided with a mixing section 22 for inducing tan- gential component to a gas flowing in the duct part 14, as is illustrated by the arrows in the duct part 14, and for mixing the gaseous fuel into the air.

[0033] The mixing section 22 comprises plurality of stator vanes 24, advanta- geously at least four vanes. The stator vanes may be called here also simply as vanes. The vanes 24 are arranged into the duct part so as to bring about a swirl into the duct part 14, when gas is flown through the vanes 24. The vanes are arranged generally to an angle D in respect to a longitudinal axis A of the duct part 14 so that a leading edge is nearer to the inlet 16 and a trailing edge is nearer to the outlet 18. In this manner the stator vanes have an inlet side surface 24’ and an outlet side surface 24”. The inlet side and the outlet side refers to general flow direction of the gas in the duct part when in use. The inlet side surface 24’ has its face projection towards the inlet 16 of the duct part and the outlet side surface 24” has its face projection towards the outlet 18 of the duct part. The burner is configured for burning at least gaseous fuel and the means 20 for intro- ducing gaseous fuel into the burner is provided to be used for that purpose. More particularly the means 20 for introducing the gaseous fuel into the burner corn- prises at least one fuel feeding opening 26 arranged to open into the outlet side surface 24” of more than one of the stator vanes 24 in the duct part 14. In the vane shown on the left side in the figure 2 has the openings on a visible side of the vane and on the right side vane the openings at behind the vane. The vane 24 provided with the opening or openings 26 comprises an internal conduit 28 or a cavity into which the opening 26 opens. Even if the gaseous fuel is fed in to the duct part 14 from the blades at the mixing section 22, the mixing of gas and air continues until the flame front is reached. The vanes 24 are arranged into the duct part 14 extending radially from an inner wall 31 of the duct part towards the center of the duct part. The vanes 24 with the openings are provided with an opening 30 at the radial distal end thereof, which serves for introducing gaseous fuel into the cavity 28 and further through the fuel feeding openings 26 into the swirl generated by the vanes 24. Even if not shown in the figure, the inlet of opening 30 can be provided with a perforated plate in order to further adjust the flow area into the opening. The vanes 24 are arranged into the duct part 14 to a same longitudinal location evenly around the duct part. In practise there may be some variation in the locations of the vanes and adequate operation may be ob- tained if the vanes 24 are arranged to essentially same location and essentially evenly around the duct part. In the embodiment shown in the figure 1 and 2 the inlet side surface 24’ and an outlet side surface 24” of the stator vane is curved preferably such that the pressure on the outlet side surface 24” is lower than on the inlet side surface 24’.

[0034] The burner 10 is provided with a gaseous fuel feeding conduit 32, which is configured to feed gaseous fuel through the openings 30 into the cavity 28 of the vane 24. The conduit 32 is provided with gaseous fuel inlet 34. In the em- bodiment of the figures 1 and 2 the conduit 32 is a manifold circumscribing the duct part 14. Preferably there are several vanes 24, optionally all of the vanes, which are provided with the fuel feeding openings 26 and therefore the duct part 14 is provided with equal number of openings 30 along its inner surface 30. [0035] The positioning of the fuel feeding openings 26 on the outlet side surface 24” of the vane 24 results in very even mixing of the gaseous fuel in a simulta- neously formed low swirl of gas flow. This makes it possible to combust the gas- eous fuel in very efficient manner and with absolutely minimized NOx and CO emissions using a so called low swirl combustion.

[0036] Additionally, positioning the fuel feeding openings on the outlet side sur- face 24’ facilitates the fuel feeding into the duct part 14 because the disclosed design creates circumstance which decreases the pressure on the outlet side surface 24’. Thus, this way the gas feed pressure in the conduit 32 can also be decreased. This in turn improves the energy efficiency of the burner.

[0037] As can be seen in the figure 1 the burner 10 is provided with a center tube 36 in the duct part 14. The center tube has a circular outer cross section and it is arranged coaxially with the duct part 14. The center tube is according to an em- bodiment of the invention arranged to serve as means 21 for introducing liquid fuel into the burner.

[0038] Now turning to figure 3, which illustrates another embodiment of the in- vention, one can see that there is a gap 38 arranged between each one of the stator vanes and the center tube. In this embodiment the vanes 24 are supported to the center tube 36 by a slender studs, but still, there is a partial gap 38 between the vane 24 and the tube 36.

[0039] In the figure 4 there is shown another embodiment of the invention, ac- cording to which the stator vanes 24 are supported only to the duct part 14 and detached from the center tube 36. In this embodiment the vanes 24 are totally detached from the center tube 36. This way the gap 38 between the vanes 24 and the center tube 36 is a complete gap.

[0040] The gap 38 between the vanes 24 and the center tube 36 provides a following advantageous effect. Namely, the gap 38 forms a bypass flow channel parallel with the vanes 24 in the mixing section. A partial flow of air is generated which flows directly through the gap 38 with a less effect caused by the vanes. The partial flow minimizes a gas recirculation phenomenon which would other- wise take place radially near the center tube 36. Since the gas which would be otherwise recirculated contains gaseous fuel, the gaps 38 also minimizes a risk of too early combustion of the fuel.

[0041] In the figure 5 there is shown an embodiment of the invention which is otherwise corresponding to the embodiment of the figure 4, but in addition to the features of figure 4 such stator vanes 24, which are provided with at least one fuel feeding opening 26, is provided with a winglet 40 arranged on the outlet side surface 24” of the stator vane. It is not absolutely essential to provide each one of the vanes 24 with the fuel feeding openings 26, but the most even distribution and mixing of the gaseous fuel into the air is accomplished if each one of the vanes 24 participates in fuel feeding. The winglet 40 is attached to the outlet side surface 24” of the vane 24 and it extends away from the surface. The winglet 40 is curved in respect to the center axis A so that its radial distance from the center tube 36 is preferably constant.

[0042] The winglet 40 is arranged radially inside the at least one fuel feeding opening 26 in the vane 24. The winglet provides an effect of minimizing the gas- eous fuel concentrating and channelling towards the center of the swirl i.e. near to the center tube 36. The channelling tendency is caused by the swirl and differ ences between the densities of the air and the gaseous fuel. This way the winglet 40 improves the even distribution of the gaseous fuel into the air. [0043] In some practical application it may also be feasible to arrange the winglet

40 such that its radial distance from the center tube 36 not constant, but e.g. increasing from its leading edge to its trailing edge, in the direction of the swirl flow.

[0044] In the embodiment of the figure 5 the winglet 40 is arranged to extend angularly from a leading edge of the stator vane 24 to a trailing edge of the stator vane. It also conceivable that the winglet is arranged to extend angularly from a leading edge of the stator vane over the trailing edge of the stator vane i.e. in this case the length of the winglet 40 is longer than the span of the vane 40. The winglet may also be shorter than the span of the vane 40. The winglet may also be comprised of more than one several winglets arranged angularly and/or radi- ally one after the other. [0045] In the embodiment shown in the figure 6 the inlet side surface and an outlet side surface of the stator vane is flat and arranged parallel with each other. When the stator vane is flat, it is easier to manufacture and the installation of the winglet 40 is easier. In this embodiment of the invention the stator vanes 24 are supported only to the duct part 14 and detached from the center tube 36. This way the gap 38 between the vanes 24 and the center tube 36 is a complete gap.

[0046] Now referring to the figure 7 which shown a still another embodiment of the invention. This is similar to that shown in the figure 1 so that that the outlet 18 comprises in the longitudinal direction, which is also the general flow direction of the gas in the burner 10, a conically convergent section 18.1 and conically divergent section 18.2 arranged one after the other. The duct part 14 has an inner diameter D which is substantially constant over the length L between the mixing section 22 and the conically convergent section 18.1. The conically con- vergent section 18.1 and the conically divergent section 18.2.form a throat part between them and the conically divergent section 18.2 functions as a diffusor part. The diffusor part begins from the throat part and has its diameter gradually extending from its first end to its second end. The length L between the mixing section 22 and the conically convergent section 18.1 can be slightly less, but is advantageously longer than the diameter D of the duct part 14. The center tube 36 extends from the mixing section 22 towards the outlet section 18) over the length L. This improves the maintaining of the swirl of the gas until the gas exits the conically divergent section 18.2. In the figure 7 the center tube 36 is sup- ported at its end by support beams 37 and one of the support beams comprises means 21 for introducing liquid fuel into the burner. [0047] Figure 8 depicts schematically a burner 10 according to an embodiment of the invention. The burner 10 comprises a body 12 into which a duct part 14 of the burner is provided. The body may be simply the wall of the duct part 14 but the duct part 14 can be made also into a bulkier body part. The duct part is a part of the burner via which the air is conveyed from an inlet 16 of the duct part 14 to an outlet 18 of the duct part for combustion of fuel by the burner. The burner 10 is a dual fuel burner configured to combust gaseous and liquid fuel. The duct part 14 has a generally circular inner cross section. The burner 10 shown in the figure 8 comprises means 20 for introducing a gaseous fuel into the burner 10 as well as means 21 for introducing liquid fuel into the burner. The burner is also provided with a mixing section 22 for inducing tangential component to a gas flowing in the duct part 14, as is illustrated by the arrows in the duct part 14, and for mixing the gaseous fuel into the air. [0048] The mixing section 22 comprises plurality of vanes 24. The vanes 24 are arranged into the duct part so as to bring about a swirl into the duct part 14, when gas is flown through the vanes 24. Similarly to the burner shown in the figure 1 the vanes are arranged generally to an angle in respect to a longitudinal axis A. In this manner the stator vanes have an inlet side surface 24’ and an outlet side surface 24”. The burner is configured for burning at least gaseous fuel and the means 20 for introducing gaseous fuel into the burner is provided to be used for that purpose. More particularly the means 20 for introducing the gaseous fuel into the burner comprises one fuel feeding opening 26 arranged to the outlet side surface 24” of more than one of the stator vanes 24 in the duct part 14, advanta- geously to each one of the vanes 24. The opening 26 in the outlet side surface

24” is a rectangular opening which is arranged to extend from near a trailing edge of the vane 24 towards the leading edge. In the vane shown on the left side in the figure 8 has the opening on a visible side of the vane and on the right side vane the opening at behind the vane 24. The positioning of the fuel feeding open- ings 26 on the outlet side surface 24” of the vane 24 results in very even mixing of the gaseous fuel in a simultaneously formed low swirl of gas flow. This makes it possible to combust the gaseous fuel in very efficient manner and with abso- lutely minimized NOx and CO emissions using a so called low swirl combustion. Additionally, positioning the fuel feeding openings on the outlet side surface 24’ facilitates the fuel feeding into the duct part 14 because the disclosed design creates circumstance which decreases the pressure on the outlet side surface 24’ of the vane 24. Thus, this way the gas feed pressure inside the space in the vane 24 and in the conduit 32 can also be decreased. This in turn improves the energy efficiency of the burner. [0049] The vane 24 is formed of sheet material and has a hollow space 28 as an internal conduit 28 or a cavity into which the opening 26 opens. The vanes 24 are arranged into the duct part 14 extending radially from an inner wall 31 of the duct part towards the center of the duct part. The vanes 24 with the openings are provided with an opening 30 at the radial distal end thereof, which serves for introducing gaseous fuel into the cavity 28 and further through the fuel feeding openings 26 into the swirl generated by the vanes 24.

[0050] The arrangement in the burner 10 is provided with a gaseous fuel feeding conduit 32, which is configured to feed gaseous fuel through the openings 30 into the cavity 28 of the vane 24. The conduit 32 is provided with gaseous fuel inlet 34. The conduit 32 is a manifold circumscribing the duct part 14 and the vanes 24 are arranged to extend radially out of the duct part 14, through the wall of the duct part 24, into the manifold 32. The hollow space in the vanes 24 is in flow communication with the manifold for feeding the gaseous fuel

[0051] The arrangement comprises a sleeve member 40 to which the vanes 24 are attached at their radially inner ends. The sleeve member 40 is supported to the center tube 36 by supports 42 between the center tube 36 and the sleeve member 40. There is a gap 38 arranged between the sleeve member 40 and the center tube 36. The gap 38 forms a bypass flow channel parallel with the vanes 24 in the mixing section as is depicted by an arrow in the figure 8. Even if not shown in the figure, the inlet of the gap 38 can be provided with a perforated plate in order to further adjust the flow area into the gap. A partial flow of air is gener- ated which flows directly through the gap 38 with a less effect caused by the vanes. The partial flow minimizes a gas recirculation or backflow phenomenon which would otherwise take place radially near the center tube 36. Since the gas which would be otherwise recirculated contains gaseous fuel, the gaps 38 also minimizes a risk of too early combustion of the fuel. The sleeve member 40 is coaxial with the center tube 36 and the gap 38 has a constant width. [0052] The center tube 36 in the duct part 14 has generally a circular outer cross section and it is arranged coaxially with the duct part 14. However, the center tube 36 is provided with successive divergent 36’ and convergent cone sections 36” downstream the vanes 24 and the gap 38. The cone sections are formed into an outer wall 35 of the center tube 36. Longitudinally at the area of the convergent section 36” also the duct part is converging towards the outlet 18. The center tube 36 is arranged to serve as means 21 for introducing liquid fuel into the burner and it comprises a liquid fuel lance 44 which is provided with a liquid fuel nozzle 46 at its end. The liquid fuel lance 44 is enclosed by the outer wall 35 of the center tube 36 which outer wall is arranged coaxially with the liquid fuel lance such that there is an annular space formed between the liquid fuel lance 44 and the outer wall 35. The annular space opens into the outlet 18 of the duct part 14. The outer tube 35 is provided with one or more openings 48 at a location upstream the vanes 24. Advantageously the opening 48 is a slit extend- ing around the outer wall 35 of the center tube 36, which is configured to allow gas flow into the annular space. The opening(s) 48 allow gas flow into the annular space for cooling the center tube 36, fuel lance 44 and the outer wall 35 of the center tube 36. Even if not shown in the figure, the inlet of the opening 48 can be provided with a perforated plate in order to further adjust the flow area into the opening.

[0053] While the invention has been described herein by way of examples in connection with what are, at present, considered to be the most preferred em- bodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations or modifications of its features, and several other applications included within the scope of the in- vention, as defined in the appended claims. The details mentioned in connection with any embodiment above may be used in connection with another embodi- ment when such combination is technically feasible.