The invention relates to a burner according to the preamble of claim 1 capable of being installed in a furnace for burning a premixed air-fuel mixture.
The invention relates also to a furnace-burner assembly according to claim 16 for burning a premixed air-fuel mixture.
So-called premix burners are used for burning a mixture of premixed fuel and air. These burners are intended to attain low NOx emission levels. Particularly premix burners with a long combustion head, intended to attain low NOx emissions (less than 9 ppm NOx emissions in flue gases) without substantial oxygen excesses, are disclosed in the prior art, i.a. in the publication U.S. Pat. No. 6,238,206. This prior known burner model is provided with a combustion head associated with the frame and extending a long way into the interior of a furnace and the low NOx emissions require oxygen excesses of less than 12% or especially less than 6%. The biggest downside of this burner has nevertheless been found to be a continuously relatively high NOx emission level, not completely fulfilling the stringent emission standards of e.g. certain states in the United States provided that the burner is to be operated efficiently, in other words, with low oxygen excesses. The Applicant's own US application discloses this type of burner capable of reaching low emissions but being limited in its compatibility with commercially available furnaces.
The invention is intended to provide an improvement regarding the foregoing prior art or at least to alleviate the drawbacks existing in the above-described prior art. Therefore, a first objective of the invention is to provide a high-efficiency burner installable to a furnace, as well as a furnace-burner assembly in which a premixed air-fuel mixture can be combusted by the burner with oxygen excesses of less than 3% in such a way that the average NOx emissions in flue gases remain below 15 ppm, and with oxygen excesses of less than 7% in such a way that the average NOx emissions in flue gases remain below 5 ppm. It was a particular objective of the invention to attain a NOx level of less than 2.5 ppm with oxygen excesses of less than 8% by means of flame stabilization and phasing of mixture ratios.
A second objective of the invention was to provide a burner more readily installable to commercially available furnaces, as well as a furnace-burner assembly obtainable thereby.
The above objectives are attained with a burner of claim 1 capable of being installed to a furnace for burning a premixed air-fuel mixture, as well as with a furnace-burner assembly of claim 16 for burning an air-fuel mixture and for generating a flame in a combustion chamber inside the furnace.
More specifically, the invention relates to a burner of claim 1 installable to a furnace for burning an air-fuel mixture and for generating a flame in the furnace. The burner comprises a frame member provided with an elongated combustion head protruding from said frame member and being adaptable inside the furnace, what in a view from the burner's frame member is a distal end of said combustion head being adapted to generate both a main flame and a primary flame, said combustion head comprising an outer, larger diameter channel for a mixture of combustion air and fuel, as well as a smaller diameter, inner channel, surrounded by the outer channel, for primary air as well as for primary gas. Hence,
The space between the outer wall of the outer channel and the jacket of the outer tube of the inner channel is constructed as a discharge channel extending from the frame member to the distal end of the combustion head, whereby the discharge end of said discharge channel is turned away from the longitudinal center line of the discharge end so that the center line of said discharge end, or an extension thereof, forms an inclined angle of incidence with the combustion head's longitudinal center line, said angle of incidence being 90-140 degrees as said discharge end of the discharge channel is viewed from the direction of the burner's frame member.
A flow controller is provided in the flow space for primary air, at the distal end, for directing the flow of primary air in the flow space so that the primary air flows from the flow guide towards the mouth of the flow space, in the vicinity of the jacket of the outer tube of the inner channel,
In a furnace-burner assembly of the invention for burning an air-fuel mixture and for generating a flame in a combustion chamber present inside the furnace,
A main flame (B) is generable
A primary flame (E) is generable
The present invention is based on the combustion head being made up of two nested channels. In the outer channel, i.e. the discharge channel, a premixed air-gas mixture is passed, discharging into a combustion chamber from a discharge channel's discharge end, which becomes narrower and veers away as viewed from a center line of the combustion head. The discharge channel's cross-sectional area diminishes in the traveling direction of the air-gas mixture when proceeding towards the discharge end's mouth, which is located at the combustion head's mouth.
The cross-section of the discharge channel is continuously reduced, reaching its minimum at the mouth of the discharge channel. This gives the advantage that the premixed air-gas mixture (main flow) is continuously accelerated in the discharge channel. The maximum flow rate of the main flow is thus reached at the mouth of the discharge channel.
Primary air and primary gas flow in the inner channel. The primary air is directed towards the combustion head's mouth by way of a flow controller, such as vanes, co-directionally with the main flame. The flow controllers direct the primary air to flow from the flow controller towards the mouth of the flow space, in the vicinity of the jacket of the outer tube of the inner channel. In this way, the path of the primary air from the flow controller towards the mouth of the flow space can be directed to follow the curvature of the inner wall of the discharge channel, away from the center line of the combustion head.
The primary gas travels in the inner channel in its own tube and is directed to the combustion head's mouth by means of nozzles.
In the burner, the intensity of flame and the amount of air are adjusted, and the adjustment is precise and has a strong effect on the main flame. It is by virtue of good adjustability that the burner is better-than-before compatible with diverse applications and combustion chambers. In particular, the burner is more compatible than before with water-tube boilers and other special applications.
It is by virtue of the more reliable adjustment that the novel burner enables attainment of the emission limit of 5 ppm with acceptable stability in combustion chambers more diverse than before. In addition to this, the novel concept enables attainment of a NOx level of less than 2.5 ppm with oxygen excesses of less than 8% by virtue of the further optimized flame stabilization and phasing of mixture ratios (see also Table 1). It is by virtue of the innovative combustion head design that the risk of occurring backfire, typical for premix burners, is minimized, thus improving both safety and convenience of use.
The invention and benefits attainable thereby will next be illustrated in even more detail with reference to the accompanying drawings.
Next follows a brief review of those aspects of a burner, as well as a furnace-burner assembly, of the invention which are detailed in each
The burner 1 in
The burner's 1 elongated combustion head 2 protrudes from the frame member 6, and in the interior 90 of the furnace 9 have been generated a main flame B and a primary flame E, which are located downstream of a mouth 23 of what in a view from said frame member 6 is a distal end 2a of the combustion head 2. The buildup of other flame zones (A, C, D) will be described later.
The combustion head 2 comprises an outer, larger diameter channel 3 for a mixture 80 of combustion air and fuel, as well as a smaller diameter, inner channel 4, surrounded by the outer channel 3, for primary air 60 as well as for primary gas 70.
The combustion head's 2 inner channel 4, which is surrounded by the combustion head's 2 outer channel 3, has been provided with supplies of primary gas 70 and primary air 60. The combustion head's inner channel 4 extends from the burner's frame member 6 all the way to the mouth 23 of the combustion head's 2 distal end 2a.
The combustion head's 2 inner channel 4 comprises an inner tube 4; 41 for a gaseous fuel (primary gas) 70 and an outer tube 4; 42. The outer tube 4; 42 surrounds said inner tube 4; 41 in a ring-shaped way. A flow space 4; 43 for primary air 60 is left between an external surface 41a of the jacket 40 of the inner tube 41 (i.e. the side of the jacket 41 of the inner tube 41 of the inner channel 4 facing the outer tube 4; 42) and an internal surface of the jacket 42a of the outer tube 4; 42 (i.e. the side of the jacket 40 of the outer tube facing the inner tube) is left. The flow space 4; 43 is thus limited in the direction of the outer channel 3 by the jacket of the outer tube 4; 42 of the inner channel 4 which is thus the inner wall of the outer channel 3.
This annular flow space 43 between the inner channel 4, the outer tube 42 and the inner tube 41 is now provided with a supply of primary air 60 from the frame member 6 of the burner 1, or from that section 2b of the combustion head 2 which is in communication with the frame member 6 and is located upstream of the combustion chamber 90 in the flowing direction of primary air 60.
Thus, primary air 60 flows in the flow space 43, from the end 43b of said flow space 43 on the side of the frame member 6 of the burner 1 all the way to the mouth 43a of the flow space. The mouth 43a of the flow space 43 is located at the distal end 2a of the combustion head 2. The mouth 43a of the flow space 43 is part of the mouth 23 of the combustion head 2, into which primary air 60 is passed through said flow space and primary gas 70 is arranged to flow via the inner tube 41 of the inner channel 4.
The mouth 23 of the combustion head 2 is divided into a discharge end 10a of the discharge channel 10, and a combined mouth 43a for the flow space 43, in which primary gas 70 flows via the inner tube 41 and primary air 60 flows via the flow space 43.
The free end of the discharge channel 10, facing the mouth 23 of the combustion head 2, comprises a discharge end 10a, whose mouth opens into the mouth 23 of the distal end 2a of the combustion head 2. The discharge channel 10 will be described in more detail below.
The mouth 43a of the flow space 43 of the inner channel 4 is limited, seen from the center line of the combustion head, i.e. in the radial direction of the combustion head 2, by the discharge end 10a of the discharge channel 10, particularly the distal end of the jacket 42a of the outer tube 42.
A flow controller 7 is installed at the mouth 43a of the flow space 43 of the inner channel 4, located at the free end of the flow space, seen from the frame member 6 of the burner 1, and constituting a part of the mouth 23 of the distal end 2a of the combustion head. The flow controller 7 is thus located at the distal end 2a of the combustion head 2, seen from the frame member 6 of the burner, close to the mouth 23 of the combustion head 2.
The flow space 4; 43 for primary air 60 extends from the burner's 1 frame member 6 to the mouth 23 of what (in a view from the frame member 6) is the combustion head's 2 distal end 2a. The outer tube's 4; 42 jacket 42a of the inner channel 4 constitutes at the same time a boundary surface between the inner channel 4 and the outer channel 3 and separates flow space 43 and discharge channel 10 from each other.
As stated above, the jacket 42a of the outer tube 4; 42 of the inner channel 4 also separates the discharge end 10a of the discharge channel 10 and the mouth 43a of the flow space 43 from each other at the mouth 23 of the combustion head 2.
Viewed from the frame member 6 of the burner 1, the outer end 42A of the outer tube 42 of the inner channel 4, that is, the free end 42A, is arranged to veer outward, i.e. in the direction of a free end 31A of the outer channel's 3 external wall 31, as viewed from the longitudinal center line of the combustion head 2. The centre of the radius of curvature is located outside the combustion head 2.
As seen in
To put it more precisely, said free end 42A of the external wall of the jacket 42a of the outer tube 42 of the inner channel 4 is located on such a circular arc whose radius is R1, the center of said circular arc being located outside the combustion head 2.
In this context, the distal end 42A or free end 42A of the outer tube 42 of the inner channel 4 refers to that part of the outer tube 42 which is placed at the outer end 2a of the combustion head 2, approximately downstream of the flow controller 7, seen from the frame member 6 of the burner.
The flow controller 7 directs the flow of primary air 60 in the flow space 43 to pass co-directionally with the main flame B when said primary air 60 exits the flow space 43. The structure and the function of the flow controller 7 are shown in more detail below in
The outer channel 3 of the combustion head, in turn, extends from the frame member of the burner 1 to the mouth 23 of the distal end 2a of the combustion head 2. The outer wall 31 of the outer channel 3 simultaneously constitutes the outer wall of the combustion head 2. The free end 3A of the outer channel 3, that is, the free end 31A of the outer wall of the outer channel, veers outwards seen from the center line 10L of the discharge channel 10, that is, away seen from the center line P of the combustion head (cf.
The space remaining between an internal side 30 of the outer channel's 3 external wall 31 as well as an external side of the jacket 42 of the inner channel 42 constitutes a discharge channel 10, in which travels a premixed air-fuel mixture 80 for generating a main flame B. The jacket 42A of the outer tube defines the inner channel 4 and thereby serves simultaneously as the outer wall of the inner channel 4. The discharge channel 10 has its free end, which is closer to the combustion head's 2 mouth 23, comprising a discharge end 10a whose mouth is a part of the mouth 23 of the combustion head's 2 distal end 2a.
The distal end 42A of the jacket 42a of the outer tube 42 of the inner channel 4, serving as the inner surface of the discharge end 10a of the discharge channel 10, and the outer wall 31 of the outer channel 3, serving as the outer surface of the discharge end 10a of the discharge channel 10, thus veer towards each other and simultaneously away from the center line P of the combustion head, when the combustion head is seen from the direction of the frame member 6. Thus, the combustion end 10a has a trumpet-like appearance at the mouth 23 of the combustion head 2, and the cross-section of the discharge end 10a of the discharge channel 10 decreases continuously, reaching its minimum at the mouth of the discharge channel 10.
This provides the advantage that the premixed air-gas mixture (main flow) is continuously accelerated at the discharge end 10a of the discharge channel. The maximum flow rate of the main flow is thus achieved at the mouth of the discharge end 10a.
As mentioned above, the free ends 31A and 42A of the discharge channel's 10 external wall 31 and the discharge channel's internal wall, i.e. those of the outer tube's 42 jacket 42a, have both a “trumpet-like” general appearance at the combustion head's 2 distal end 2a. Hence, the respective free ends 31A and 42A of the discharge channel's 10 external wall 31 and the discharge channel's internal wall 42a curve respectively outward from the discharge channel's center line 10L as well as from the combustion head's center line P, the radii of curvature thereof being respectively R and R1, wherein R and R1 are equal or unequal. The centers of these radii R and R1 of curvature are located outside the combustion head.
In a preferred embodiment of the invention, the centers of the radii R and R1 of curvature are located, seen from the frame member 6 of the burner, on the side of the such a cross-sectional plane of the center line P of the combustion head, facing the mouth 23 of the combustion head 2, which extends approximately via the flow controller 7 located at the distal end of the combustion head 2.
Since the free end 31A of the discharge channel's 10 external wall 31 and the free end 42A of the discharge channel's internal wall 42a curve outward as seen from the discharge channel's 10a center line 10L and as seen from the combustion head's 2 center line P, respectively, the entire discharge end 10a is directed away as seen from the combustion head's center line P.
Since the discharge channel 10a in itself is ring-shaped, the discharge channel's center line 10L refers here to what is a ring-shaped center line 10L of the ring-shaped discharge channel 10a as seen in a longitudinal section of the discharge channel (cf.
Generally, the free end 42A of the discharge channel's 10 internal wall 42 (the free end 42A of the outer tube 42), which has a radius of curvature R1, arches more vigorously than the free end 31A of the discharge channel's 10 external wall 31, which has a radius of curvature R, whereby R>R1. Hence, the discharge channel's 10 entire discharge end 10a becomes narrower and turns outward when proceeding in the traveling direction of a premixed air-gas mixture 80 towards the mouth of the discharge channel's 10 discharge end 10a, which is a part of the combustion head's mouth 23 (cf.
This provides that the flow rate of the premixed air-gas mixture 80 flowing in the discharge channel is continuously accelerated when passing towards the mouth of the discharge end 10a. The flow rate and the magnitude of its acceleration will depend on the angle of incidence 5 between the center line 10A of the discharge channel and the longitudinal center line P of the combustion head, as well as on the mutual relationship between the radii R and R1 of curvature.
It is by virtue of the novel design of the discharge channel's 10 discharge end 10a that the risk of causing backfire, typical for premix burners, has been minimized, thereby improving both safety and convenience of use.
The outer channel's 3 discharge channel 10 is provided with a supply of premixed air-fuel mixture 80 for generating a main flame B visible in
What in the flowing direction of a primary gas 70 is a distal end 41a of the inner tube 4; 41 is in turn provided with a plurality of nozzles 8 for conducting the primary gas 70 into a flow space 4; 43 and then to the mouth 43a of the flow space 43. The primary gas is conveyed into the flow space in the flowing direction of a primary air 60 upstream or downstream of a flow deflector 7 located at a free end 43 of the flow space 43. The inner channel's 4 inner tube 4; 41 is provided with a supply of primary gas 70 from the burner's 1 frame member 6, or from that section 2b of the combustion head 2 which is associated with the frame member 6 and located, in the flowing direction of primary gas 70, upstream of the combustion chamber 90.
The primary flame E is generated as primary air 60 introduced into the flow space 43 and the flow of primary gas 70 merge with each other after the mouth of the flow end of the flow space 43. The flow space mouth is a part of the combustion head's 2 mouth 23.
As depicted in
The flow controller 7, used for directing the primary air 60, may come in diverse designs and shapes, the most important aspect being, however, that the flow of primary air 6, arriving at the flow controller 7 from the flow space 43, will be directed by the flow controller 7 towards the main flame B and the outer wall of the flow space, which is at the same time the jacket 42a of the outer tube. The main flame is generated by means of an air-fuel mixture 80 flowing in the discharge channel 10.
The flow controller 7; 7a may comprise, as depicted for example in
The vanes 71, 72, 73 . . . 7n are at least in a partially crosswise orientation with respect to the flowing direction 60 of primary air 60 in the flow space 43. Preferably, the vanes 71, 72, 73 . . . 7n are at an angle of 20-90 degrees relative to the primary air's flowing direction 60.
In
Next follows a still further review of a few important details of the invention with reference to the preceding description of the
The flow space 4; 43 enlarges when proceeding towards the combustion head's 2 mouth 23, because the free end 42A of the outer tube's jacket 42 curves away as viewed from a center line P of the combustion head 2.
It is by virtue of the reliable main flame B adjustment that enables the burner to attain a 5 ppm emission limit with acceptable stability in several combustion chambers that were earlier difficult to control. In addition to this, the novel concept enables attainment of a NOx level of <2.5 ppm (O2 ref 3%) in the combustion chamber by virtue of further optimized flame stabilization and phasing of mixture ratios.
In order to generate a primary flame E, the amount of supplied primary air 60 is 5-30% of the total amount of air delivered into the burner's combustion head 2, and it is the adjustment of the relative amount and flow rate of primary air 60 and primary gas 70 that enables precise control of the intensity of primary flame E. This provides a major contribution to the intensity and stability of the main flame B. Preferably, the amount of primary air 60 supplied for generating the primary flame E is about 20% of the total amount of air used for generating a main flame B by means of a premixed air-gas mixture 80 as well as for generating a primary flame E by means of primary air 60.
All the premixed air and fuel is delivered into the site of a main flame B or into a B-zone in
In an A-flame zone, visible in
In front of the combustion head's 2 mouth 23 there is a strong backflow. The flue gas discharges from the main flame's B-zone along the furnace's 9 walls, while cooling down at the same time, and a portion thereof returns back by way of a middle section of the combustion chamber 9 in the form of a backflow D-C. The backflow D-C both cools down and dilutes the B-zone established by the main flame. In a D-zone of the flame, at an end of the combustion chamber 90, there are no significant backflows, yet there still occurs some complete combustion of carbon monoxide.
In the burner-furnace combination according to the invention, thanks to the radial direction of the fluids, the flame becomes very different when compared with competing technologies. The flame becomes compact (wide and short) but still surprisingly large in volume. Increasing the diameter of the flame will affect the volume of the flame more than increasing the length. (Cylinder volume=PI*(D/2){circumflex over ( )}2*)
Because of the shape of the flame, the return flows (A and D) dilute and cool the flame more efficiently than in competing technologies. These differences provide the following advantages: the flame is cool, and therefore the NOx emissions are low.
If the burner is used with high excess air, a smaller excessive air quantity will be sufficient to achieve the require NOx emission. If the burner is used in combination with external flue gas recirculation (FGR), a smaller amount of recirculated flue gas will be sufficient. Moreover, a shorter flame is advantageous in many applications, because a shorter furnace is sufficient.
With the exemplary burner-furnace combination according to the invention, low Nox values were achieved, thanks to the design of the combustion head and the shape of the resulting flame. Table 1 gives the quantity of Nox emissions resulting from flue gases of the furnace as a function of excess oxygen [dry, mol-%].
Number | Date | Country | |
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63329969 | Apr 2022 | US |