BACKGROUND OF THE INVENTION
Field of the Invention
[0001] The present invention relates to the field of radiant wall burners. In particular
the invention relates to radiant wall burners wherein a number of technologies are
combined in a single burner arrangement so as to achieve low NO
x and low noise.
The State of the Prior Art
[0002] Reduction and/or abatement of NO
x in radiant burners has always been a desirable aim. Some NO
x abatement has been achieved in the past by staging a portion of the gaseous fuel.
Low pressure staged gas may be introduced into the combustion zone either from low
pressure gas tips arranged around the periphery of the burner or from a center gas
tip which protrudes through the center of the end cap of the radiant burner nozzle.
These arrangements have not always been successful because, for NO
x abatement purposes, the staged fuel should not be introduced into areas of the combustion
zone where the oxygen concentration is greater than about 4% by volume.
SUMMARY OF THE INVENTION
[0003] Various problems encountered in prior art burners are addressed by the concepts and
principles of the present invention. In particular, the invention addresses the ever
present need for NO
x abatement. In accordance with one aspect of the invention, it has been found that
when gas is burned in a staged manner it may sometimes be responsible only for about
6 ppm (vol) of the total NO
x emissions of an individual burner. Accordingly it has been thought to be desirable
to adapt the concept of fuel staging to radiant wall burners. Several different configurations
have been tried, some more successful than others, but none with complete satisfaction.
In some configurations, staged fuel has been delivered through a plurality of tubes
at very low pressure around the circumference of the burner. In such a case the staged
fuel is introduced in proximity to a combusting mixture which is still quite rich
in oxygen. This excess oxygen leads to higher flame temperatures and higher NO
x content in flue gases.
[0004] In other configurations, staged gas has been introduced into the combustion zone
from the axially distal end of the premix discharge nozzle. This configuration, where
the staged fuel is injected coaxially at the center line of the premix burner assembly,
has been somewhat more successful in achieving lower NO
x emissions than the first configuration discussed above, at least in part due to the
fact that the introduction point is located in spaced relationship to the face of
the tile as well as away from the oxygen rich stream leaving the premix discharge
nozzle. The down side of this particular methodology is that the momentum of the staged
gas jet can and often does pull the primary oxygen rich premixed stream into the jet
as an entrained flow thereby increasing the availability of excess oxygen as well
as the production of NO
x. This problem is exacerbated in applications requiring a multiplicity of individual
burners in an array because of the interactions between burners.
[0005] In accordance with an important aspect of the invention, a low NO
x burner nozzle assembly is provided for a radiant wall burner. The assembly includes
an elongated hollow burner tube and a discharge nozzle. The burner tube has a central,
longitudinally extending axis and defines a conduit extending along the axis for supplying
a mixture of fuel and air to a radiant combustion area of a combustion zone that extends
radially and surrounds the nozzle assembly. This mixture may desirably be fuel lean.
The discharge nozzle is mounted on the tube at a downstream end of the conduit adjacent
the combustion zone, and the same is adapted for receiving the mixture of fuel and
air from the conduit and directing the same into the radiant combustion area in an
essentially radial direction relative to the axis of the tube. The discharge nozzle
may include a plurality of flow directing members arranged in an array which extends
circumferentially around the discharge nozzle and the members may desirably be arranged
to define therebetween a plurality of passageways which extend in a generally radial
direction relative to the axis. The discharge nozzle may also include an end cap that
is mounted on the members in a position to close the conduit and prevent flow of the
mixture in a direction along the axis. Thus, the mixture is caused to flow through
the passageways in a generally radial direction.
[0006] Preferably, the flow directing members may be arranged so that some of the passageways
therebetween have a larger flow area than others. Desirably, the members may be in
the form of plates which are essentially rectangular in shape. Ideally, the passageways
may also extend in an axial direction. In a much preferred form of the invention,
the end cap may have a lateral edge which is located at a first radial distance from
the axis, and the members may each have an outer edge located at a second radial distance
from the axis. The second radial distance ideally may be greater than the first radial
distance such that passageways defined by the members extend radially outward beyond
the lateral edge of the end cap.
[0007] In accordance with another preferred form of the invention, the nozzle may include
an internal baffle positioned and arranged to redirect at least a portion of the mixture
flowing through the conduit and cause the same to flow through the passageways in
a generally radial direction.
[0008] In yet another preferred form of the invention, the end cap may have an axially extending
hole therein, and the nozzle assembly may include a centrally located staged fuel
burner nozzle made up, for example, of a length of tubing which extends along the
axis of the conduit. The assembly may also include a staged burner nozzle tip at a
downstream end of the length of tubing. In accordance with this aspect of the invention,
the staged fuel burner nozzle may desirably be arranged so as to protrude axially
through the hole. Importantly, the tip ideally may have a fuel delivery orifice therein
for delivering fuel to the combustion zone in spaced relationship to the radiant combustion
area.
[0009] In one desirable form of the invention, the delivery orifice may be disposed so as
to introduce fuel gas into zone 20 at an upward and outward angle relative to a plane
that is perpendicular to the axis. Preferably, the angle may be at least about 30°,
and for some purposes in accordance with the invention, the delivery orifice may be
disposed to introduce fuel gas in a direction along the axis.
[0010] Even more desirably, the staged fuel burner nozzle may be positioned such that a
downstream portion of the length of tubing protrudes beyond the end cap so that the
tip is positioned in axially spaced relationship relative to the end cap. Ideally,
in this particularly desirable form of the invention, the low NO
x burner nozzle may include an elongated protective sheath disposed in surrounding
relationship to the protruding portion of the length of tubing and the tip. Such sheath
may desirably include an opening disposed in alignment with the orifice. The sheath
may also be provided with one or more vent openings configured to permit gases between
the sheath and the length of tubing to escape into the combustion zone. In accordance
with the foregoing aspects of the invention, the staged burner nozzle may be of significant
value, regardless of the form of the discharge nozzle. Thus, the staged burner tip
of the invention may be used with any sort of radial discharge nozzle that operates
to spread a combustible mixture of fuel and air radially across the face of a radiant
tile.
[0011] In accordance with yet another aspect of the invention, the burner tube may comprise
a venturi tube having a throat that is in communication with an air supply and a source
of fuel gas under pressure. The venturi tube may desirably be arranged such that the
flow of fuel gas through the throat induces a flow of air from the air source whereby
the mixture of fuel and air is created in the throat and caused to flow toward the
discharge nozzle.
[0012] The invention also provides a low NO
x radiant wall burner comprising a burner tile having a central opening surrounded
by a radiant tile face and an elongated low NO
x burner nozzle assembly as described above that extends through such opening. The
face of the burner tile may be either dished or flat.
[0013] In addition, the invention provides a method for operating a burner comprising providing
a mixture of fuel and air at a centrally located point adjacent a face of a burner
tile, separating the mixture into a plurality of separate streams and causing such
streams to flow radially outwardly from the centrally located point across the face
of the tile, and causing the velocity of some of the streams to be greater than the
velocity of others of the streams.
[0014] The invention further provides a method for operating a burner which includes the
steps of providing a mixture of fuel and air at a centrally located point adjacent
a face of a burner tile, separating the mixture into a plurality of separate streams
and causing the streams to flow radially outwardly from the point across the face
of the tile, causing the streams to combust to form flames, each having an outer peripheral
terminus spaced radially from the point, and providing secondary air to the flame
at a location adjacent the termini.
[0015] In yet another form, the invention provides a method for operating a burner that
comprises providing a mixture of fuel and air, causing the mixture to flow along a
path to a centrally located point adjacent a face of a burner tile, separating the
mixture into a plurality of separate streams and causing the streams to flow radially
outwardly from the path across the face of the tile, causing the streams to combust
to form flames in an area of a combustion zone adjacent the face, and providing staged
fuel to the zone at a location spaced from the area. In accordance with this form
of the invention, the oxygen content of the gases at the location where the staged
fuel is introduced is desirably not more than about 4% by volume.
[0016] The invention also provides a low NO
x burner assembly which includes an elongated hollow burner tube providing a longitudinally
extending conduit for supplying a mixture of fuel and air to a combustion zone. The
burner tube has an outer wall surrounding the conduit, a longitudinally extending
central axis and a pair of spaced ends. The assembly also includes a discharge nozzle
at one of the ends of the burner tube, an inlet for a mixture of fuel and air at the
other end of the burner tube, and at least one port extending through the wall at
a location between the discharge nozzle and the inlet to communicate the conduit with
an external area located outside the burner tube. Desirably the port may have a center
axis which is essentially perpendicular to the central axis of the tube. Alternatively,
the port may have a center axis which is at an angle relative to the central axis
of the tube. Ideally the assembly may include a plurality of ports extending through
the wall of the tube at respective locations between the discharge nozzle and the
inlet. In one preferred form of the invention, the ports may be arranged in one or
more rows which extend around the outer wall of the tube.
[0017] In another form of the invention, the ports described above may be utilized in combination
with a discharge nozzle that includes a plurality of flow directing members as described,
which are arranged to define therebetween a plurality of passageways which extend
in generally radial directions relative to said axis, and an end cap mounted on said
members in a location to redirect at least a portion of the mixture flowing from the
end of the conduit and cause the same to flow through said passageways in a generally
radial direction. In accordance with the invention, the members may be arranged so
that some of the passageways have a larger flow area than others of the passageways.
[0018] The nozzle assembly having at least one port extending through the wall of the burner
tube may be used as a component of a low NO
x radiant wall burner that includes a burner tile having a central opening. In such
a case, the nozzle assembly may extend through the central opening of the tile. Desirably,
the discharge nozzle may include a plurality of flow directing members which are arranged
to define therebetween a plurality of passageways which extend in generally radial
directions relative to the axis of the burner tube, and an end cap mounted on said
members in a location to redirect at least a portion of the mixture flowing from the
end of the conduit and cause the same to flow through said passageways in a generally
radial direction so that when ignited, the redirected mixture of fuel and air provides
a generally laterally extending flame having an outer peripheral extremity at a location
in said zone spaced radially from said axis.
[0019] The invention further provides a method for operating a burner which includes the
steps of causing a mixture of fuel and air to flow toward a centrally located point
adjacent a face of a burner tile, causing additional air to flow toward a location
adjacent said face which is spaced laterally from said point, and separating a portion
of said mixture and intermixing the same with said additional air to create an ultra
lean admixture capable of flameless oxidation before the additional air reaches said
location. More particularly, the method may include the steps of causing a mixture
of fuel and air to flow toward a centrally located point adjacent a face of a burner
tile, separating a first portion of said mixture into a plurality of separate streams
and causing said streams to flow radially outwardly from said point across the face
of said tile, causing said streams to combust to form flames, each having an outerperipheral
terminus spaced radially from said point, providing secondary air to said flame at
a location adjacent said termini, adding a second portion of said mixture to said
secondary air at a location upstream from said location to create an admixture capable
of flameless oxidation at the face of said tile, and flamelessly oxidizing said admixture
at said face to create relatively cool oxidation products. In accordance with the
concepts and principles of the invention, oxidation products may be admixed with the
combusting gases to thereby dilute and cool the same. In further accordance with the
principles and concepts of the invention, a flow of recirculated flue gas may be provided
to said flame at a location adjacent said termini.
[0020] Prior art burners of the premix type of design have not been able to utilize as many
NO
x abatement technologies in a single burner as are provided in the burner arrangements
of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021]
FIG. 1 is a side elevational view, partly in cross-section, of a low NOx radiant wall burner which embodies the concepts and principles of the invention;
FIG. 2 is a side elevational view of the nozzle arrangement of the burner of FIG. 1;
FIG. 3 is a schematic plan view of a preferred embodiment of the discharge nozzle of the
nozzle arrangement of FIG. 2;
FIG. 4 is an enlarged elevational, cross-sectional view of the discharge nozzle of FIG. 3;
FIG. 5 is an enlarged view, partly in cross-section, of the discharge nozzle of FIG. 3;
FIG. 6 is an enlarged view, similar to FIG. 5, which is partly in cross section to illustrate an embodiment of an internal baffle;
FIG. 7 is a schematic view of the nozzle arrangement of FIG. 1;
FIGS. 8A and 8B respectively are side elevational and plan views an embodiment of a central staged
nozzle tip for the nozzle arrangement of FIG. 2;
FIG. 9 is a side elevational view of an embodiment of a tile for use with the burner of
FIG. 1;
FIG. 10 is a plan view of the tile of FIG. 9;
FIG. 11 is a schematic side elevational view of one embodiment of a nozzle arrangement that
is useful in connection with the invention;
FIG. 12 is a schematic side elevational view of an alternative embodiment of a nozzle arrangement
that is useful in connection with the invention;
FIG. 13 is a schematic side elevational view of another alternative embodiment of a nozzle
arrangement that is useful in connection with the invention;
FIG. 14 is a schematic side elevational view of yet another alternative embodiment of a nozzle
arrangement that is useful in connection with the invention;
FIG. 15 is a side elevational view of yet another burner arrangement which embodies the concepts
and principles of the invention;
FIG. 16 is an enlarged cross sectional view of the discharge nozzle of the burner of FIG. 15; and
FIG. 17 is a schematic view illustrating the operation of the burner of FIG. 15, including a schematic showing of the flow paths of the several combustion and flameless
oxidation streams.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
[0022] A burner 10 which embodies the concepts and principles of the invention is illustrated
in Fig. 1 where it can be seen that the same includes a burner tile 12 having a centrally
disposed opening 14 and a burner nozzle assembly 16 including a burner discharge nozzle
18 which protrudes through opening 14 and extends into a combustion zone 20. The burner
10 may also include conventional components such as a muffler 22, an air door 24 and
an inlet tube 26 facilitating connection to a source of fuel gas.
[0023] With reference to Fig. 2, it can be seen that the nozzle assembly 16 may include
spud 28 whereby a supply of fuel gas is supplied to the nozzle assembly 16. Spud 28
is connected to a coupling 42 (See Fig. 7) whereby fuel gas is supplied to the discharge
nozzle 18. The assembly 16 also includes an elongated, hollow burner tube 30 and a
base 32. Tube 30 extends between base 32 and discharge nozzle 18 and provides a passageway
for primary combustion air and accommodates the gas supply system (not shown) which
interconnects spud 28 and coupling 42.
[0024] Discharge nozzle 18 preferably includes a fuel distribution section 36 and an end
cap 38. With reference to Figs. 5, 6 and 7, it can be seen that the downstream portion
34 of tube 30 may preferably include a venturi tube 40 whereby fuel gas being ejected
through an opening 44 in coupling 42 induces a flow of combustion air from the interior
of tube 30. Preferably coupling 42 may be provided with a plurality of openings 44
as shown. The fuel gas ejected through the openings 44 mixes with the induced air
to preferably form a combustible fuel lean fuel gas/air mixture which travels through
portion 34 of tube 30 toward and into fuel distribution section 36.
[0025] Fuel distribution section 36 is illustrated in Figs. 3 and 4. Section 36 includes
a plurality of fin-like flow directing members 46 and 48 which define therebetween
a plurality of passageways 50 and 52 which extend in a generally radial direction
relative to the central axis 54 of the nozzle assembly 16. The members 46 and 48 may
be sized and arranged such that the passageways 50 defined between adjacent members
46 may be larger in cross-sectional flow area than the passageways 52 defined between
adjacent members 48. In operation, the fuel/air mixture flows through portion 34 of
tube 30 in a direction which is generally parallel to axis 54. As the air/fuel mixture
approaches end cap 38, the air/gas mixture is redirected so that it flows radially
outward through the passageways 50 and 52. It is to be noted in this regard that the
respective outer extremities 46a and 48a of members 46 and 48 are preferably spaced
further from axis 54 than the outer edge 38a of cap 38. This provides respective openings
50a and 52a (see Fig. 5) at the axially outer ends of passageways 50 and 52 which
permit a portion of the air/fuel combustible mixture adjacent thereto to bend slightly
and flow toward zone 20 rather than in a direction at right angles to axis 54.
[0026] With particular reference to Fig. 3, it can be seen that the members 46 and 48 may
be arranged so as to provide respective groups 56 and 58 of passageways 50 and 52.
As shown, each group 56 includes five passageways 50 and each group 58 includes five
passageways 52. As can clearly be seen from Fig. 3, the passageways 50 are wider than
the passageways 52 so that the cross sectional flow areas provided by passageways
50 are greater than the cross-sectional flow areas provided by passageways 52. As
shown, the groups 56 and 58 are arranged around section 36 in alternating positions.
It can also be seen that in the presently preferred embodiment, the section 36 includes
four groups 56 and four groups 58. However, it is to be noted that the passageways
may be arranged in a variety of equally acceptable arrangements, depending upon the
design and operational characteristics desired. The sizes of the passageways 52 and
50 may be varied to facilitate increased velocity, particularly through passageways
52. Increased velocity through passageways 52 relative to the velocity through passageways
50 provides increased diffusion of recirculated flue gas
[0027] In one preferred embodiment of the invention, shown particularly in Figs. 11 and
12, a central secondary staged fuel nozzle 60 protrudes through a hole 64 provided
in end cap 38. Nozzle 60 includes a length of gas supply tubing 86 that extends along
axis 54 and through portion 34. A staged burner tip 62 is mounted at the downstream
end 88 of the tubing 86. Tip 62 may be as is illustrated in Figs. 8A and 8B, where
it can be seen that the same may be provided with delivery openings 66 for directing
the flow of preferably raw fuel into zone 20 in spaced relationship relative to a
radiant combustion area 75 in zone 20 adjacent face 74 (see Fig. 1). As shown in Figs.
8A and 8B, openings 66 may be disposed at an approximate angle of 45° from the plane
of the tile face; however, the angle required for any given installation may vary
depending upon the desired operational and performance characteristics of the burner.
In this latter regard, the angle of openings 66 should desirably not be less than
about 30°, as shown schematically in Fig. 11, to insure that premature mixing of the
staged fuel with an oxygen rich mixture is avoided. Likewise, the number and spacing
of the openings 66 is a function of the desired performance characteristics.
[0028] In another embodiment of the invention, nozzle 60 may be as shown in Figs. 12, 13
and 14, where the downstream portion 90 of tubing 86 protrudes beyond end cap 38 such
that the tip 62 is positioned in spaced relationship relative to end cap 38. In this
case, the assembly 16 may preferably include a cylindrical sheath 92 which is mounted
on end cap 38 and extends along the entire length of protruding portion 90 in surrounding
relationship to the latter. Appropriately positioned openings 94 may be provided in
sheath 92 to permit the entirety of tip 62 to be protected from the heat of the combustion
zone and yet allow egress of staged fuel from tip 62.
[0029] As shown in Fig. 12, the sheath 92 may also have an open end 96 which is adapted
to vent the sheath 92 by permitting gases between the sheath 92 and the tubing 86
to escape into the combustion zone. Alternatively, the arrangement may be as shown
in Figs. 13 and 14, where the end of the sheath 92 is closed by a flat (Fig. 13) or
domed (Fig. 14) cap 98. In this case, appropriate vent holes 99 may be provided in
the wall of the sheath 92. These vent holes 99 serve essentially the same purpose
as the open end 96, but as shown, the same may preferably be disposed at a downwardly
inclined angle of about 10° relative to a plane which is perpendicular to the longitudinal
axis of the sheath 92. Desirably, nozzle 60 may also be provided with an orifice 68
as shown in Fig. 5 to control the amount of fuel which flows into the combustion zone
via nozzle 60.
[0030] In accordance with the concepts and principles of the invention, the tip 62 desirably
may be positioned far enough away from the premixed discharge nozzle 18 such that
the flow patterns of the oxygen rich and radially moving combusting gases in the radiant
combustion area 75 and the staged fuel injected via nozzle 60 are mechanically decoupled
so as to avoid burning of the staged fuel in an oxygen rich environment. Thus, the
staged gas jet leaving tip 62 is far enough from the premixed flow envelope such that
the momentum of the jet is insufficient to cause the staged gas and the premixed gas/air
mixture to intermingle, at least until the fuel from nozzle 60 has had an opportunity
to become mixed with flue gas. This is extremely important, particularly when considered
in conjunction with the ultralean concept of the primary air/fuel mixture where the
large amount of excess air left over from the combustion in the radiant heating area
75 is significant enough to cause localized combustion to start at the tip of the
staged riser, thus increasing NOx emissions. Desirably, for best results in NO
x abatement, the staged fuel should be combusted in an atmosphere which contains no
more than about 4% oxygen by volume.
[0031] With reference to Figs. 9 and 10, for some important applications utilizing the concepts
and principles of the invention, the opening 14 may desirably be larger in internal
diameter than the outer diameter of tube portion 34 so that secondary combustion air
may flow into zone 20 through the annular space between opening 14 and cylindrical
section 34. In accordance with the invention this aspect of the invention, and as
illustrated in Figs. 9 and 10, secondary air ducts 70 may be provided to facilitate
and improve the flow of secondary air. One end 72 of duct 70 is in communication with
zone 20 at the face 74 of tile 12. The other end 76 of duct 70 is in communication
with opening 14. As can be seen from Fig. 10, end 72 is arcuate in shape so that the
same projects a fan-shaped flow of air into zone 20. End 76 is also arcuate in shape
and in general is in the shape of a slot which extends around the internal surface
78 of opening 14. In accordance with the invention, the face 74 of tile 12 may be
dished or flat. Dishing may facilitate recirculation of flue gas inside the dish.
[0032] In the operation of a burner which incorporates the tile illustrated in Figs. 9 and
10, the fuel lean fuel/air mixture leaving passageways 50 and 52 travels radially,
outwardly of axis 54 and generally across face 74 of tile 12 where it is burned in
a radiant combustion area 75 adjacent face 74. The combustion products of the fuel/air
mixture eventually intermix with raw fuel from nozzle 60. In many embodiments of the
invention, the intermixture may be fuel rich, and after combustion, the same may provide
a generally laterally extending flame having an outer peripheral extremity at the
radial periphery of area 75, which periphery is spaced radially from the axis. Preferably,
end 72 of duct 70 may be positioned so as to provide a fan a air to the flame at the
outer peripheral extremity of the laterally extending flame.
[0033] An embodiment of the nozzle of the invention which includes an internal baffle 84
is shown in FIG. 6. Baffle 84 is generally in the shape of an inverted cone and the
same is positioned for redirecting the flow of the air/fuel mixture traversing tube
40. The combustible mixture travels along tube 40 in a generally axial direction until
it encounters baffle 84 which redirects the flow so that it moves in a generally radial
direction. In FIG. 6 the baffle is shown in combination with a nozzle structure which
includes a centrally located raw fuel nozzle 60. However, it will be recognized by
one of ordinary skill that the internal baffle will be highly useful regardless of
the presence or absence of the central nozzle.
EXAMPLE
[0034] A burner embodying the concepts and principles of the invention was operated as follows:
the burner is fired at 0.63 MMBtuh; excess air is 10%; furnace temperature is 1800
°F; burner differential pressure is 0.25 inches of water; secondary and primary burner
damper is fully opened; combustible gas is 50% natural gas and 50% hydrogen; burner
is aligned with outer cupped tile edge and then pushed in 0.25 inch.
Measured results using a single burner: 2.5% O2; 0 ppm CO; and 8 to 10 ppm NOx. Measured results using an array of 13 burners: 2.5 % O2; 0 ppm CO; and 15 to 19 ppm NOx.
[0035] As a result of the experiment it was noted that with deeper staging of air through
the tile, NO
x emissions can be brought down by a significant percentage of the overall emissions.
[0036] The advantages provided by the invention described above include very low NO
x, low noise, partial premix with a rich gas stream axially staged for low NO
x, prompt NO
x alleviation with fuel induced furnace gas recirculation, simplicity, short flame
profile, high pressure utilization at turndown for jet stability, high stability,
operation with either flat or cupped tile face, facilitation of the manipulation of
L/D for defined combustion of premixed fuel and air, staged air tile further decreases
NO
x formations with staged air technology, staged gas is directed away from furnace wall
for slowed combustion, and secondary air staging is integral part of tile such that
no excess air is needed at the base of the premix tip.
[0037] The burner of the invention is of a premix design. The burner may also include a
venturi that is preferably optimized sufficiently to deliver an extremely fuel lean
premix of air and fuel to the main discharge nozzle of the burner. The discharge nozzle
may be designed so that its slots have a significant L/D (width to depth ratio) to
keep each individual premixed jet as a defined individual flame envelope. This allows
for the natural recirculation patterns of the tile and furnace to inject furnace flue
gas into each stream. This is one factor in the reduction of NO
x.
[0038] The discharge nozzle may be arranged in eight sections, four (4) that are high flow
and four (4) that are of lower flow. Since the webbing between each section is proportional
the recirculation of flue gases in the tighter restricted area is more pronounced.
The variation of area assures stability in the larger flow areas while the smaller
areas are subjected to a higher percentage of flue gas by diffusion due to the smaller
mass.
[0039] As described above, a center riser 60 may be inserted through the burner tube 34,
which preferably may be a venturi, so that the riser protrudes through the end plate
38 of the discharge nozzle 36. The secondary or staged nozzle 60 is fed pure gas fuel
(unpremixed) at a pressure of about 10 psig. The gas is then expelled via a staged
tip 62 designed to handle the high temperatures of the furnace and subsequently burned.
This tip 62 desirably provides a L/D sufficient to ensure that the gas can be directed
at an angle as required to oxidize the gas in a stable manner away from the heat of
the furnace wall. This ensures that the combustion process is impeded, but not enough
to induce appreciable amounts of CO.
[0040] The tip pressure is maintained by an integral orifice 68 located in the line from
the main gas spud to the staged tip. The discharge nozzle 36 and the staged tip 62
interact together in flow patterns created by the open slots in the face of the discharge
nozzle 36 to insure appropriate staging of the raw fuel and the subsequent recirculation
of the CO and CO
2 formed to lower the NO
x further in the primary premixed section of the flame.
[0041] Another aspect of the burner of the invention is its capability to utilize a truly
staged air tile formation, whereby secondary air is mixed into the premixed portion
of the flame at its peripheral tip. The NO
x can be further impeded by the mixing mechanics of this secondary air tile as it stages
the air out instead of allowing the secondary air to come into contact with the base
of the premixed flame envelope.
[0042] In another preferred embodiment of the present invention, and as illustrated in Figs.
15, 16 and 17, the burner may be provided with one or more, preferably several, and
ideally eight or more radially extending ports 100 in the wall of the centrally disposed
tube 34 which provides a conduit for delivering the central air/fuel mixture to the
burner tip. These ports 100 communicate with the space 102 surrounding the tube 34
whereby a portion of the air/fuel mixture flows through the ports 100 and becomes
admixed with secondary air flowing along the outside of the tube 34 toward the combustion
zone 20. The admixture thus formed may generally be too lean to support a conventional
flame; however, low temperature oxidation thereof occurs at face 174 of the burner
tile 104, whereby NO
x emissions are minimized.
[0043] In accordance with a particularly preferred form of the invention described above,
where the ports 100 are used in connection with a radiant burner having a cupped tile
104, the ports 100 provide for a prestaging of some of the premixed air and fuel resulting
in decreased tip velocity through discharge nozzle 36, enhanced stability and minimization
of NO
x emissions. The cupped tile 104 enables the placement of the ports 100 at a location
about 3 full inches upstream from the discharge nozzle 36 whereby, as shown in Fig.
17, the already lean air/fuel mixture 152 escaping from the central tube 34 through
the ports 100 is able to become thoroughly admixed with secondary air flowing in the
direction of the arrows 154 along the outside of tube 34 to present an ultra lean
admixture well before the latter reaches the face 174 of the tile. This ultra lean
admixture undergoes low temperature oxidation without conventional flame on the face
of the tile. The products of this low temperature oxidation are then entrained into
the main flame 150 created at the discharge nozzle 36 and provide a quenching, cooling
effect to thereby reduce NO
x in the main flame. The overall effect provides in a reduction of NO
x emissions to a level well below 10 parts per million on a volumetric basis (ppmv).
In accordance with the principles and concepts of the present invention, NO
x emissions below 5 ppmv can be achieved consistently.
[0044] The attributes of this form of the invention include: 1) low NO
x emissions with staged fuel; 2) flameless combustion coupled with rapid oxidation
in the proximity of the tile; 3) low noise as a function of tip pressure and heat
release; 4) staged gas jets entraining flue gas external to the burner; 5) prompt
NO
x alleviation; 6) secondary air has less effect on NO
x emissions; 7) short flame profile; 8) high turndown ratios with added premix tip
velocities; 9) high stability; 10) minimization of CO emissions; 11) very lean premixed
zone; 12) oxidation against radiant tile with stoichiometry below LEL's (cold combustion);
and 13) three separate fluid flow zones containing different stoichiometries of gas
and air.
[0045] As shown in Fig. 16, the holes 100 for directing a portion of the primary air/fuel
mixture into the flow of secondary air on the outside of the burner tube 34 to thus
create an ultra lean mixture of air and fuel, desirably may be used in conjunction
with a burner nozzle which includes flow directors such as the directors 46, 48, a
central nozzle such as the nozzle 60, and an internal baffle such as the baffle 84.
[0046] Broadly, in accordance with the concepts and principles of the configuration illustrated
in Figs. 15, 16 and 17, by prestaging a volume of an ultra lean premixed air and fuel
gas in conjunction with a premixed burner and a fuel rich staged tip, ultra low NO
x emissions may be achieved in conjunction with a tile designed to facilitate flameless
combustion of the prestaged ultra lean admixture while maintaining separation of the
latter from the main flame until a appropriate product mix is achieved to dilute and
cool the main flame so as to lower emissions therein.
[0047] In the burner of Figs. 15, 16 and 17, once some fraction of the fuel, ranging from
about 15% of the fuel to all of the fuel, is mixed with air, a small portion of the
mixture is removed prior to the main discharge nozzle and redirected into a secondary
air stream. In the case of a radiant wall burner, the premix is removed from the central
tube 34, which may be in the form of a venturi, by means of ports (radially drilled
holes) 100 positioned around the body of the burner prior to the tip. In another configuration
the premix may be mixed with recirculated flue gas that is ported back through the
tile using special ports. This creates a mixture that is below the flammability limits
and incapable of sustaining combustion. This stream must pass then pass through the
highly radiant tile section that is capable of accelerating the kinetics of the gas
and causing a rapid oxidation of the fuel even though it is below its flammability
limits. When substantial oxidation, if not complete oxidation, has taken place this
stream is then remixed with the main premixed air and gas stream that is exiting the
main burner tip and is just within its flammability limits. The main premix stream
sustains and stabilizes the combustion. The oxidized stream has a quenching effect
on the main flame, lowering its theoretical temperature by putting a heat load on
the flame by means of extra mass.
[0048] In addition, a secondary staging of pure fuel gas is also being introduced from a
secondary tip 60 downstream of the main burner premix discharge nozzle. The secondary
fuel is introduced further into the furnace and uses the kinetic energy of its sonic
jets to entrain and mix in substantial amounts of furnace flue gas before it is pulled
back into the main flame by the momentum of the main flame and the force of recirculating
furnace gases. This also has a quenching effect to the main flame and also serves
to bring the flammability limits of the overall mixture into a range that is once
again flammable. The stabilizing affect of the refractory helps to maintain a stable
flame envelope during turndown and low oxygen regimes seen during operational excursions
within the furnace.
[0049] It is important to note that the premix prestaging technique described in connection
with Figs. 15, 16 and 17 provides NO
x reductions to approximately half of what was already an ultra-low NO
x burner. It should be noted in this regard that the premix prestage concept facilitated
by the holes 100 can be extended for use with essentially any burner shape and/or
mounting pattern. In other words, the premix prestaging technique can be extended
to essentially any burner application. Thus, this concept may be used to make very
low NO
x round flame, upfired or sidefired burners, as well as rectangular flat flame burners
and downfired burners. The concept may also be utilized in both, what are fundamentally
diffusion flame burners as well as full fledged premixed type burners.
[0050] The use of a lean primary air/fuel mixture augmented by a flameless combustion zone
within the tile located in proximity to the main flame, plus a substantial staged
portion of the gas in fuel rich form that is subsequently returned to the main flame
by entrainment and momentum via a nozzle such as the nozzle 60 to provide reduction
of theoretical temperature by additional mass, is a very important feature of the
invention.
[0051] Overall, the invention is adaptable so as to provide several families of burners
ranging from radiant wall burners to horizontal, upfired, and even downfired burner
designs with the capability of delivering NO
x emissions much below current burner technologies.
[0052] In another configuration, in accordance with the concepts and principles of the invention,
the ported nozzle arrangement may be used in conjunction with a specially ported version
of a tile that is adapted to recirculate flue gas which may then be used instead of
secondary air to dilute the ported primary air/fuel mixture. Such an arrangement also
may be used to provide and maintain a lean premix behind the tile assuring that combustion
which would be detrimental to the burner tip does not take place. The spin off to
this is the loading of the flame that helps to lower the theoretical temperature of
the flame much more than is typically seen in burner designs. The flameless combustion
zone may be controlled and kept separate from the main flame until most of the initial
oxidation is complete.
[0053] The concepts and principles of the present invention add a new twist to an already
evolving technology. The creation of a flameless combustion zone (lean premixed) coupled
with specific tile designs to control and stabilize the combustion process operate
together to provide low NO
x without the use of flue gas recirculation and/or other dilution methods for reducing
flame temperature.
[0054] The burner of Figs. 15, 16 and 17 provides single digit NO
x numbers in what may be considered "within the parts that are usually included in
a conventional burner". By adding the flameless combustion zone behind the main flame,
new ground has been broken in addressing what is considered the "prompt NO
x regime" of NO
x production.
[0055] The joining of all of these various aspects of the invention allows the burner of
the invention to deliver NO
x emissions in the range of single digits to the mid teens (ppm) depending on the number
of burners in the array, and the species and concentrations of the species in the
fuel mix. Thus, in accordance with the invention, it has been discovered that it is
possible to combine many known theories of NO
x abatement into a single burner that provides stable operation and appropriate turndown
while performing in a range that has not previously been thought possible. In accordance
with the invention, shorter flame patterns are possible especially when the fuel comprises
heavy hydrocarbons; larger turn down ratios are possible on high hydrogen fuels, particularly
when an internal baffle is utilized; much lower noise is experienced around a burner
with multiple ports and small jets; either cupped or flat tiles may be utilized interchangeably;
staged air tile design allows for NO
x adjustment while running; burner adjustment capabilities in the tile allow for NO
x adjustment; tips are easily removed and serviced by design; and the direction of
the staged jets at turndown help to stabilize the primary flame.
1. A method for burning fuel in a combustion zone comprising:
providing a fuel lean mixture of fuel and air;
delivering said mixture to a centrally located point adjacent a face of a burner tile,
said point being disposed adjacent said combustion zone;
causing said mixture to flow radially outwardly from said point in a plurality of
streams across the face of said tile;
combusting the fuel in said mixture in an area of said combustion zone adjacent said
face;
providing staged fuel to said zone at a location spaced from said area; and
combusting said staged fuel in an environment containing flue gases and not more than
about 4% oxygen by volume.
2. A method for burning fuel as set forth in claim 1, wherein said mixture includes all
of the oxygen necessary for burning all of the fuel in the mixture plus all of the
staged fuel.
3. A method for burning fuel as set forth in claim 1 or 2, wherein said staged fuel is
provided to said zone as pure fuel.
4. A low NO
x burner nozzle assembly comprising:
an elongated hollow burner tube (30) providing a longitudinally extending conduit
for supplying a mixture of fuel and air to a combustion zone (20), said burner tube
having an outer wall surrounding said conduit, a longitudinally extending central
axis (54) and a pair of spaced ends;
a discharge nozzle (18) at one of the ends of the burner tube:
an inlet for a mixture of fuel and air at the other end of the burner tube;
an air passageway (120) located outside the outer wall of the burner tube; and
at least one port (100) extending through said outer wall at a location between the
discharge nozzle (18) and said inlet intercommunicating the conduit and the air passageway.
5. A nozzle assembly as set forth in claim 4, wherein said air passageway (102) is annular
and surrounds said outer wall.
6. A nozzle assembly as set forth in claim 4 or 5, wherein said port (100) has a center
axis which is essentially perpendicular to said central axis.
7. A nozzle assembly as set forth in claim 14 or 15, wherein said port has a center axis
which is at an angle relative to said central axis.
8. A nozzle assembly as set forth in claim 14 or 15, comprising a plurality of ports
(100) extending through said wall at respective locations between the discharge nozzle
and said inlet.
9. A nozzle assembly as set forth in claim 8, wherein said ports (100) are arranged in
one or more rows which extends said outer wall.
10. A nozzle assembly as set forth in claim 8 or 9, wherein each of said ports (100) has
a center axis which is essentially perpendicular to said central axis.
11. A nozzle assembly as set forth in claim 10, wherein said center axes are arranged
in a common plane which is essentially perpendicular to said central axis.
12. A nozzle assembly as set forth in any one of claims 4 to 7, wherein said location
is closer to said discharge nozzle than it is to said inlet end.
13. A nozzle assembly as set forth in claim 11, wherein said common plane is positioned
closer to said discharge nozzle than to said inlet end.
14. A nozzle assembly as set forth in any one of claims 4 to 13, wherein said discharge
nozzle includes a plurality of flow directing members (46, 48) which are arranged
to define therebetween a plurality of passageways (50, 52) which extend in generally
radial directions relative to said axis, and an end cap (38) mounted on said members
in a location to redirect at least a portion of the mixture flowing from the end of
the conduit and cause the same to flow through said passageways in a generally radial
direction.
15. A nozzle assembly as set forth in claim 14, wherein said members are arranged so that
some of said passageways (50) have a larger flow area that others of said passageways
(52).
16. A nozzle assembly as set forth in claim 14 or 15, wherein said air passageway (102)
is annular and surrounds said outer wall.
17. A nozzle assembly as set forth in claim 16, comprising a plurality of ports (100)
extending through said outer wall, and wherein said ports are arranged in one or more
rows which extend around said outer wall.
18. A low NOx radiant wall burner assembly comprising a burner tile (12) having a central opening
(14) and a nozzle assembly as set forth in claim 4, the burner tube (30) of said nozzle
assembly being adapted and arranged so as to extend through said central opening (14).
19. A burner assembly as set forth in claim 18, wherein the discharge nozzle of said nozzle
assembly includes a plurality of flow directing members (46, 48) which are arranged
to define therebetween a plurality of passageways (50, 52) which extend in generally
radial directions relative to said axis, and an end cap (38) mounted on said members
in a location to redirect at least a portion of the mixture flowing from the end of
the conduit and cause the same to flow through said passageways in a generally radial
direction.
20. A burner assembly as set forth in claim 19, wherein said members (46, 48) are arranged
so that some of said passageways (50) have a larger flow area than others of said
passageways (52).
21. A burner assembly as set forth in claim 18, 19 or 20, wherein said air passageway
(102) is annular and surrounds said outer wall.
22. A burner assembly as set forth in claim 21, comprising a plurality of ports (100)
extending through said wall, and wherein said ports are arranged in one or more rows
which extend around said wall.
23. A burner assembly as set forth in claim 19 or 20, wherein the passageways (50, 52)
are arranged such that the redirected mixture of fuel and air, when ignited, provides
a generally laterally extending flame having an outer peripheral extremity at a location
in said zone spaced radially from said axis.
24. A method for operating a burner comprising:
causing a mixture of fuel and air to flow toward a centrally located point adjacent
a face of a burner tile;
causing a stream of at least one of additional air and recirculated flue gas to flow
toward a location adjacent said face which is spaced laterally from said point; and
separating a portion of said mixture and intermixing the same with said stream to
thereby create an fuel lean admixture capable of flameless oxidation before the same
reaches said location.
25. A method for operating a burner as set forth in claim 24, said method further comprising
separating a second portion of said mixture into a plurality of separate streams,
causing said streams to flow radially outwardly from said point across the face of
said tile and causing said streams to combust to form a flame which surrounds said
point, and flamelessly oxidizing said admixture at said face to create relatively
cool oxidation products.
26. A method as set forth in claim 25, comprising admixing said oxidation products with
said flame to thereby dilute and cool the same.
27. A method as set forth in claim 24, wherein said stream comprises additional air.
28. A method as set forth in claim 24, wherein said stream comprises recirculated flue
gas.
29. A method as set forth in claim 24, wherein said stream comprises recirculated flue
gas and additional air.