Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE INVENTION
LOW NOX EMISSIONS BURNER ASSEMBLY AND METHOD FOR REDUCING THE NOX CONTENT OF
FU
RFACE FLUE GAS
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to burners for large scale industrial applications.
Such burners may be adapted for burning either gaseous fuels including natural
gas or
liquid fuels including fuel oil. In particular the invention relates to a low
NOX, low noise
burner assembly which provides structure for recirculating flue gas directly
from the
inside of the fire box.
The State of the Prior Art
Environmental concerns today fuel a continuing search for burners which
operate efficiently, economically, with a minimum amount of noise, and with a
minimum
amount of contamination, such as NOX, in the flue gases. It has been
determined
previously that NOx contamination may be reduced by recirculating flue gases
back into
the combustion zone. Various methods have been envisioned and/or developed for
accomplishing this recirculation, including motor driven fans and eductor
devices.
Generally speaking, conventional burners utilize only the motive force of
combustion fuel
to entrain furnace flue gas. This prior methodology is limiting in that the
gas has high
velocity but low mass resulting in low entrainment rates.
In addition, prior methodology has involved the use of burners including
internal structural conduits for transporting the recirculated flue gases back
to a place
where the same may be reintroduced into the combustion zone. The structural
requirements are extensive adding greatly to the cost to the overall
installation.
Accordingly there has been a need for structural features which will simplify
the
recirculation of flue gases.
SUMMARY OF THE INVENTION
The present invention provides a novel burner assembly which addresses
the problems and shortcomings of the prior art. In particular, the invention
provides a
burner assembly that is of simple construction and facilitates the use of a
simple and
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effective motive force for recirculating flue gases. In accordance with the
concepts and
principles of the present invention, it has been determined that the use of
combustion air
for entrainment of flue gases is much more efficient than the use of fuel
gases for this
purpose due to the tremendous difference in mass. The mass of combustion air
is
typically 10 to 12 times that of the combustion fuel gases. The invention also
contemplates the simultaneous use of both combustion air and fuel gases to
provide even
greater motive forces for recirculating flue gases.
The burner assembly of the invention is adapted for being mounted on a
wall or floor or roof of a furnace or fire box or the like which defines a
combustion zone.
In accordance with the concepts and principles of the invention, the burner
assembly
comprises a tile formation that has a portion which protrudes beyond the
furnace wall and
into the combustion zone. The tile formation presents an air passageway
configured for
conducting combustion air into the combustion zone and a recirculated flue gas
(RFG)
conduit that intercommunicates the air passageway and an area within the
furnace
adjacent the combustion zone when the burner assembly is mounted on the wall.
In
accordance with the invention, the RFG conduit is preferably located entirely
within the
protruding portion of the tile formation, whereby it is unnecessary to provide
RFG
conduits built into the furnace structure.
In a preferred form of the invention, for ease of assembly, the tile
formation may include first and second tiles with the RFG conduit defined
between the
tiles. In this preferred form of the invention, the second tile may be mounted
on a seating
structure provided on the first tile. The first and second tiles may each have
an annular
body segment and the tiles may be arranged such that the annular body segment
of the
first tile is disposed in surrounding relationship to the combustion air
passageway and the
annular body segment of the second tile is disposed in surrounding
relationship to at least
a portion of the annular body segment of said first tile. Preferably, in
accordance with
the principles and concepts of the invention, the annular body segment of the
first tile
may have an outer surface and the annular body segment of the second tile may
have an
inner surface, and these surfaces may be arranged and positioned such that the
RFG
conduit is defined between them. Ideally, the annular body segments may be
arranged
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concentrically with the outer surface of the first tile spaced radially from
the inner surface
of the second tile such that the RFG conduit is essentially annular in shape.
In a particularly preferred mode of the invention, the second tile may have
an annular edge which sits on a seating structure of the first tile. In this
preferred mode,
the seating structure may include at least one, preferably at least two and
ideally four
evenly circumferentially spaced shoulder members which extend radially
outwardly from
the outer surface of annular body segment of the first tile. In a most
preferred
embodiment of the invention, the shoulder members each include a notch
configured for
receiving and securing the annular edge of the second tile. Such structure
facilitates the
positioning of the tiles relative to one another and insures concentricity.
In a further preferred form of the invention, the burner assembly may
include a fluid fuel burner nozzle disposed in the combustion air passageway.
In this
form of the invention, the fluid fuel burner nozzle may be adapted for
delivering a
gaseous fuel, preferably natural gas or a specially blended combustible
mixture of gases,
to the combustion zone. In accordance with the invention, the fluid fuel
burner nozzle
may preferably be adapted for accommodating pressurized gaseous fuels at
various
pressures within a given range. Alternatively, the fluid fuel burner nozzle
may be
adapted for delivering a liquid fuel, preferably a fuel oil, to the combustion
zone.
The burner assembly of the invention may also comprise at least one fluid
fuel burner nozzle disposed adjacent an entrance to the RFG conduit. In
addition, or
alternatively, the burner assembly of the invention may include a fluid fuel
burner nozzle
disposed on an outer peripheral surface of the annular body segment of the
second tile.
In further accordance with the concepts and principles of the invention,
the burner assembly may be adapted for use in connection with a furnace having
a
firebox defining a combustion zone. This burner assembly may include a first
annular
tile defining a path for flow of combustion air and a second annular tile
concentric with
the first annular tile. The second annular tile may have an internal diameter
which is
larger than an external diameter of the first annular tile, and the second
annular tile may
be positioned in surrounding relationship relative to at least a portion of
the first annular
tile so that a ring-shaped RFG conduit is defined therebetween. In this form
of the
invention, the tiles may be adapted for placement in the combustion zone with
the RFG
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conduit in direct fluid communication with flue gases in an area surrounding
the
combustion zone. The assembly may preferably be such that combustion air
flowing
along the flow path induces a flow of flue gas through the RFG conduit for
entrainment
by the flow of combustion air. Ideally the assembly may include a gas jet
positioned
adjacent an inlet to the RFG conduit providing a flow of gas for admixture
with the flow
of flue gas.
In still further accord with its concepts and principles, the invention
provides an RFG inducing burner assembly. The assembly comprises a fuel nozzle
arrangement that includes a nozzle positioned to direct a flow of fluid fuel
along a flow
path and into a combustion zone inside a furnace firebox. The assembly also
includes
a first tile structure having a central opening and which is located so that
the central
opening surrounds the nozzle and directs an annular flow of combustion air
past the
nozzle in surrounding relationship to the fluid fuel flow path. The first tile
structure
desirably has an outer peripheral face that extends there around in
surrounding
relationship relative to the central opening. Also included is a second tile
structure that
has a central passageway. The second tile structure desirably is located so
that its central
passageway surrounds at least a portion of the first tile structure. The
second tile
structure may have an internal face which extends around its central
passageway and such
internal face may be disposed in spaced, facing relationship relati ve to the
peripheral face
of the first tile structure. An annular space is defined between the outer
peripheral face
of the first tile structure and the internal face of the second tile
structure. Preferably, the
tile structures are arranged such that the annular space therebetween is in
direct
intercommunication with an interior area within the firebox adjacent the
combustion zone
when the burner is operationally installed relative to the furnace. In
accordance with the
principles and concepts of the invention, the assembly of the tile structures
may desirably
be such that a flow of RFG from the interior area and through the annular
space is
induced by combustion air flowing through the opening of the first tile
structure. In this
regard it is to be noted that the assembly of the present invention may be
used to augment
flue gas recirculation in existing applications which employ a forced air
design.
In another form the invention provides a low NOX furnace that includes
an RFG inducing burner assembly and a firebox providing a combustion zone. In
this
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form of the invention, the burner assembly is operationally installed on a
wall or roof or
floor of the furnace to provide combustion air and a fluid fuel to the
combustion zone.
In accordance with the invention, the burner assembly preferably comprises a
fuel nozzle
arrangement that includes a nozzle positioned to direct a flow of fluid fuel
along a flow
path and into the combustion zone within the firebox, a first tile structure
that has a
central opening, and a second tile structure that has a central passageway.
The first tile
structure may be located so that its central opening surrounds the nozzle and
directs an
annular flow of combustion air past the nozzle in surrounding relationship to
the fluid
fuel flow path. The first tile structure may also have an outer peripheral
face which
extends there around in surrounding relationship relative to the central
opening thereof.
The second tile structure may be located so that its central passageway
surrounds at least
a portion of the first tile structure. The second tile structure may also have
an internal
face which extends around the central passageway thereof. Desirably, in
accordance with
the concepts and principles of the invention, the internal face of the second
tile structure
is disposed in spaced, facing relationship relative to the peripheral face of
the first tile
structure, and the faces are preferably arranged so as to define an annular
space
therebetween. The tile structures further may be arranged such that the
annular space
therebetween is in direct intercommunication with an interior area of the
firebox adjacent
the combustion zone when the burner assembly is operationally mounted.
Desirably, the
arrangement of the tile structures is such that a flow of RFG from an area in
the firebox
adjacent the combustion zone and through the annular space is induced by
combustion
air flowing through the opening of the first tile structure. Again, the
assembly of the
present invention may be used to augment flue gas recirculation in existing
applications
which employ a forced air design.
In further accord with the concepts and principles of the invention, a
method is provided for efficiently and effectively reducing the NOx content of
furnace
flue gas produced by combusting air and a fluid fuel in a combustion zone of a
furnace.
In this form of the invention, the method involves the use of a burner
assembly as
described above for introducing RFG into the combustion zone.
In yet another aspect of the invention, a method is provided for efficiently
and effectively reducing the NOX content of furnace flue gas. This method may
comprise
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providing a flow of a fluid fuel to a combustion zone in a firebox of the
furnace,
providing a flow of combustion air to the combustion zone, combusting the
fluid fuel and
the air in the combustion zone to thereby produce a flame, and using the
motive force of
the flow of combustion air to induce a flow of RFG directly from an area in
the firebox
adjacent the combustion zone and cause it to interact with the fluid fuel and
the
combustion air in the combustion zone. The formation and emission of NOx in
the
furnace is thus reduced. Preferably, in accord with the concepts and
principles of the
invention, the combustion air flows along a longitudinal axis of an elongated
burner
assembly.
The invention also provides a method for operating a burner for a gas fired
furnace having a fire box. In accordance with this aspect of the invention,
the method
preferably includes providing a flow of combustion air, providing a flow of
combustion
fuel, admixing the fuel and air, combusting the admixture of air and fuel in a
combustion
zone in the firebox, and inducing a flow of flue gas directly from an area in
the fire box
adjacent the combustion zone and into the admixture of combustion air and
fluid fuel,
and thus the flame, using the motive force of the flow of combustion air.
Desirably, the
foregoing method may further include a step of using the motive force of the
flow of
combustion gas fuel to further induce a flow of flue gas from the area
adjacent the
combustion zone. Desirably, in this form of the invention also, the combustion
air flows
along a longitudinal axis of an elongated burner assembly.
One of the important features of the invention involves the use of primary
air flowing axially through the burner assembly for entrainment of RFG
directly from an
area at the periphery of the combustion zone. The axial entrainment system of
the
invention enables the provision of a much more uniform and homogenous
combustible
gas mixture leaving the burner whereby typical stratification problems and
resultant
flame instability are alleviated. The advantages of the invention over pr7or
art burner
assemblies include, but are not limited to, (1) shorter flame length (0.5 to
1.1 foot per
MMBtuh vs. 1.5 to 2 feet per MMBtuh); (2) larger turn down ratios (10 to 1 vs.
3 to 1);
(3) much lower noise around burner; (4) tiles are not subjected to hot spots
caused by
burning jets piercing through the tile; (5) coking tendencies are reduced if
not eliminated
completely; (6) stability is greatly improved facilitating operation under
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substoichiometric conditions; (7) flame anchoring in an oxidizing zone is
facilitated; (8)
homogenous mixtures are provided leading to uniformity of flame patterns; (9)
both
prompt and thermal NOX are lowered; (10) potentially both air and fuel gas may
be used
for flue gas entrainment; (11) flue gas entrainment is much more efficient;
and (12) axial
flame patterns reduce burner to burner interaction; (12) the assembly of the
invention
may be coupled with RFG in forced air applications; (13) the assembly of the
invention
is even more efficient when used in connection with a forced air design.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a side elevational view of a burner assembly which
embodies the concepts and principles of the present invention;
FIGURE 2 is a plan view looking upwardly at the lower end of the burner
as it is positioned in FIG. 1;
FIGURE 3 is an enlarged plan view looking downwardly at the upper end
of the burner as it is positioned in FIG. 1;
FIGURE 4 is an enlarged detail view illustrating the portion of the burner
assembly within the Circle 4. of FIG. 2;
FIGURE 5 is an enlarged detail view illustrating the portion of the burner
assembly within the Circle 5. of FIG. 3;
FIGURE 6 is an enlarged detail view illustrating the portion of the burner
assembly within the Circle 6. of FIG. 3;
FIGURE 7 is an enlarged view, partly in cross-section illustrating the tile
formation of the burner assembly of FIG. 1;
FIGURE 8 is a top plan view of the lower (upstream) tile of the tile
formation of FIG. 7;
FIGURE 9 is a side elevational view of the lower tile of FIG. 7;
FIGURE 10 is a side elevational view, partly in cross-section, illustrating
the upper (downstream) tile of the tile formation of FIG. 7; and
FIGURES 11 through 16 are side elevational schematic views illustrating
several alternative embodiments of the burner assembly of the invention.
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FIGURE 17 is a side elevational view, partly in cross-section, illustrating
an alternative form of a burner assembly which embodies the concepts and
principles of
the present invention;
FIGURE 18 is a top plan view of the burner of FIG. 17;
FIGURE 19 is a cross sectional view taken substantially along the line
19-19 of FIG. 17;
FIGURE 20 is a cross sectional view taken substantially along the line
20-20 of FIG. 18;
FIGURE 21 is a cross sectional view taken substantially along the line
21-21 of FIG. 18; and
FIGURE 22 is a side elevational view, partly in cross-section, illustrating
yet another alternative form of a burner assembly which embodies the concepts
and
principles of the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE
INVENTION
A burner assembly which embodies the concepts and principles of the
invention is illustrated in Fig. 1 where it is identified broadly by the
reference numeral
20. The assembly 20 is particularly adapted for being mounted as shown on a
wall 22 of
a fire box of a furnace or the like. Wall 22 defines a combustion zone 24
disposed
generally at and around the upper end 26 of assembly 20 as the latter is
depicted in Fig.
1. It will be appreciated by those skilled in the burner art, however, that
the orientation
of the assembly 20 is not necessarily critical and the same may be used in any
one of a
variety of positions such that the flame generated thereby is projected
upwardly,
downwardly, horizontally or at an angle relative to horizontal. Accordingly,
defining end
26 as an upper end is done only for convenience of reference. Moreover, it
will be
appreciated that the principles and concepts of the invention are not limited
to use with
round burner tiles. In this regard, the principles and concepts of the
invention may also
be applied to burners having rectangular shaped burner tiles, flat flame
burners and
burners with an axisymetric design.
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Burner assembly 20 includes the conventional accessories needed for
operation, including an air inlet 28 adapted for connection to a source of air
for
combustion, an air box 30, and a manifold system 32 adapted for connection to
a source
of fuel and distributing the same to respective individual burner nozzles of
the burner
assembly. These devices are conventional and well known to those skilled in
the burner
art. Assembly 20 may include a flange 33 for attaching the same to wall 22 as
can be
seen in Figs. 1 and 7. The flange 33 may be secured to the wall 22 using a
conventional
nut and bolt (or stud) arrangement (see Fig. 4).
Burner assembly 20 is particularly adapted for inducing a flow of flue
gases from an area 34 adjacent combustion zone 24 and recirculating the same
for
diluting the air and fuel mixture combusting in the combustion zone 24. These
recirculated flue gases are often referred to as RFG (or FGR), and it is a
well known
phenomena that when RFG is mixed in with the combusting air and fuel in the
combustion zone, NOX emissions are reduced.
In accordance with the invention, assembly 20 includes a tile formation
36 having a portion 38 which protrudes beyond wall 22 and into the combustion
zone 24
as shown in Fig. 1. That is to say, portion 38 of tile formation 36 protrudes
into and is
positioned essentially within combustion zone 24. Formation 36 includes a
first, lower,
upstream, inner tile structure 40 and a second, upper, downstream outer tile
structure 42.
As can particularly be seen viewing Fig. 7, tile 42 is mounted on a seating
structure 44
of tile 40.
As can best be seen viewing Figs. 7, 8 and 9, in the preferred embodiment
of the invention depicted in the drawings, tile 40 includes a generally
annular body
segment 46 which surrounds a central opening 48, a base 50, and a plurality of
shoulder
members 52 which preferably are essentially identical and evenly spaced about
the
periphery of body segment 46. As shown in the drawings, the depicted assembly
includes
four of the shoulder members 52; however, those skilled in the art would
recognize that
the assembly could just as well be designed to employ only three shoulder
members 52.
As can be seen, the shoulder members 52 extend radially outwardly from the
outer
peripheral surface 54 of annular body segment 46, and together, they provide
the seating
structure 44 for tile 42 mentioned above. To this end, the shoulder members 52
are each
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provided with a notch 56 which is configured to receive and secure an annular
edge
portion 58 of tile 42. That is to say, when the tiles 40 and 42 are assembled
so as to
present the tile formation 36, the edge portion 58 of tile 42 sits on a
generally horizontal
ledge 60 which is part of the corresponding notch 56. The manner in which the
edge
portion 58 is received and secured by the notches 56 is best illustrated in
Fig. 7. As can
be seen, surface 54 essentially extends around tile 40 and surrounds opening
48. The tiles
40, 42 of the tile formation 36 may be connected together using well known
conventional
attachment devices. If the tile formation 36 is mounted on a bottom wall of a
furnace,
tile 42 may simply rest on the seating structure 44. On the other hand, if the
tile
formation 36 is mounted in some other position, tile 42 may be attached to
tile 40 using
elongated studs that are cast into tile 42 at positions that correspond to the
locations of
the notches 56. Stud receiving holes may then be provided to extend downwardly
through shoulder members 52 from the notches 56 to the flange 33 where nuts
may be
used to secure the tiles 40, 42 to flange 33 via the elongated studs.
With reference to Figs. 3, 7 and 10, in the preferred embodiment of the
invention depicted in the drawings, tile 42 has an annular body segment 62
that surrounds
a central opening 64 thereof. The internal diameter of tile 42 is greater than
the external
diameter of tile 40. Thus, as can particularly be seen in Fig. 7, when tile 42
is mounted
on tile 40, annular body segment 62 of tile 42 surrounds at least a portion of
annular body
segment 46 of tile 40. In particular, the lower edge portion 58 of annular
body segment
62 surrounds the upper frusto-conical portion 68 of annular body segment 46.
Thus, it
can be seen that the annular body segments 46 and 62 are arranged
concentrically with
the outer annular surface 70 of the upper frusto-conical portion 68 of tile 40
spaced
radially inwardly from the inner annular surface 72 of lower edge portion 58
of tile 42.
Manifestly, therefore, inner surface or face 72 of tile 42 is disposed in a
surrounding
relationship relative to frusto-conical portion 68 of tile 40.
When tile 42 is seated on tile 40 in an operational position as shown in
Figs. 3 and 7, it can be seen that central opening 48 of tile 40 together with
central
opening 64 of tile 42 present a passageway 74 defining a path 76 along which
combustion
air from air box 30 is conducted into combustion zone 24. As can be seen
viewing Figs.
3 and 7, passageway 74 is surrounded by the respective annular body segments
46 and
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62 of tiles 40 and 42 as well as by annular face 72 (not seen in Fig. 3) which
extends
there around.
As mentioned above, outer annular surface 70 of the upper frusto-conical
portion 68 of tile 40 is spaced radially inwardly from the inner annular
surface 72 of
lower edge portion 58 of tile 42. Thus, surfaces 70 and 72 are disposed in
spaced, facing
relationship relative to one another. The annular space 78 between surfaces 70
and 72
(see Fig. 3) provides an annular or ring-shaped RFG conduit 80 which
intercommunicates
air passageway 74 with the area 34 which is adjacent combustion zone 24. Thus,
RFG
conduit 80, which is defined between upper and lower tiles 40, 42, is in
direct
communication with flue gases in the area 34 surrounding combustion zone 24 so
that
combustion air flowing along path 76 is able to induce a flow of flue gas from
area 34
and through conduit 80 for entrainment by the combustion air and dilution and
cooling
of the gases combusting in combustion zone 24 to thereby reduce the production
of NOX.
That is to say, the arrangement of the tiles 40, 42 is such that a flow of RFG
from area
34 and through annular space 78 is induced by the action of the combustion air
flowing
along passageway 74 and through the openings 48 and 64. The combustion air
flowing
past the downstream end 81 of conduit 80 creates a motive force for inducing a
flow of
RFG through conduit 80. Thus, the arrangement of the tiles provides an action
much like
an eductor or an ejector. That is to say, the mass of combustion air flowing
in the
direction of arrow 76 entrains RFG directly from the area 34 in the interior
of the firebox
which surrounds the tile assembly 20 of the invention. The RFG flows in the
direction
of the arrows 85 from the area 34, through the ring-shaped conduit 80 and past
end 81 of
the latter where it joins and is entrained by the combustion air flowing along
path 76.
The mass of combustion air flowing in the direction of the arrow 76 thus
provides the
motive force for entraining the flue gases directly from the area 34 along the
path of the
arrows 85 and delivering it to the outlet end 83 of gas nozzle 82 where it is
diffused and
admixed with the combustion mixture, thus slowing the rate of reaction of the
fuel with
oxygen from the combustion air.
In accordance with the invention, the downstream tile 42 is located around
the upstream tile 40 so as to form the ring-shaped conduit 80 which is open to
the furnace
gases in the interior of the furnace. These furnace gases are able to interact
with the air
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flowing along path 76 through the center of tile 42 to create a combined flow
leaving the
tile system of approximately 118% of inlet air quantity (volume). This added
furnace gas
is swept around the flame in combustion zone 24 slowing the diffusion of air
into the
main flame in a very uniform and homogenous manner.
At this point it is to be noted that although tile 40 is shown as having four
shoulder members 52, the number actually desired and/or required for a given
application
may vary depending upon, for example, such things as the overall dimensions of
the tiles,
the orientation of the assembly, the weight of tile 42 and/or the flow areas
required for
passageway 74 and conduit 80.
In the preferred embodiment of the invention illustrated in Figs. 1 through
10 of the drawings, burner assembly 20 may include primary, secondary and
tertiary fluid
fuel burner nozzles. Primary nozzle 82, which is illustrated particularly in
Figs. 3 and 7,
is surrounded by tiles 40 and 42 and is generally centrally located in
passageway 74,
whereby an annular flow of combustion air is directed past nozzle 82 in
central opening
64. Accordingly, the combustion air initially flows in a surrounding
relationship relative
to the flow of fuel emanating from nozzle 82. Although the nozzle 82 as shown
is
particularly adapted for delivering a gaseous fuel to the combustion zone, it
is to be
understood that in accordance with the concepts and principles of the
invention, nozzle
82 may be adapted for delivering either a liquid fuel, for example a fuel oil,
or a gaseous
fuel, for example natural gas, along a path which is generally parallel to
path 76 to the
combustion zone.
In the preferred embodiment of the invention, burner assembly 20 may
also include a plurality, preferably four (4), secondary burner nozzles or gas
jets 84.
These nozzles 84, which are particularly illustrated in Figs. l, 3, 5 and 7,
are disposed
adjacent the inlet 86 of RFG conduit 80 whereby fuel gas is admixed with RFG.
In
addition, the gas emanating from nozzles 84 provides a motive force for
further inducing
a flow of RFG from area 34. As will be appreciated by those skilled in the
burner art,
there is nothing critical about the number of nozzles 84, and for any given
application,
the number thereof may vary from zero (0) to eight (8) or more. However, when
the
assembly includes a multiplicity of such secondary nozzles, the same should be
spaced
evenly about the outer periphery of annular tile segment 46 as shown.
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Burner assembly 20 may also include a plurality, preferably four (4),
tertiary burner nozzles or gas jets 88. These nozzles 88, which are
particularly illustrated
in Figs. l, 3, 6 and 7, may preferably be mounted in respective channels 90
provided in
the outer peripheral surface 92 of annular tile segment 62. Desirably, the
shoulder
members 52 may be provided with end faces 94 which provide support for the
tubes 96
supplying fuel to nozzles 88. As will once again be appreciated by those
skilled in the
burner art, there is nothing critical about the number of nozzles 88, and for
any given
application, the number thereof may vary from zero (0) to eight (8) or more.
However,
when the assembly includes a multiplicity of such tertiary nozzles, the same
should be
spaced evenly about the outer periphery 92 of annular tile segment 62 as
shown.
SPECIFIC EXAMPLE
A specific example of the operation of a furnace using the embodiment
of the present invention described above and shown in the FIGS. 1 through 10
of the
drawings is as follows:
Burner is fired at 8.0 MMBtuh. Excess air is 15°70. Furnace
temperature
is 1500 °F. Burner differential pressure is 0.3 inches of HBO. Burner
damper is fully
opened. Combustible gas is 100% natural gas. Single central fuel nozzle
configuration.
The results obtained in the flue gas include 3°lo measured oxygen, 0
ppm measured CO
and approximately 20 to 35 ppm measured NOx.
In the foregoing description and specific example, the fuel has been
described principally as being fuel gas or natural gas. It is to be noted in
this regard, that
the burner of the invention may also be used effectively for reducing NOx
emissions when
burning other fluid fuels, such as, for example, normally liquid fuels
including fuel oil
or other gas blended fuels. It is also to be noted that existing installations
may be
retrofitted so as to use the principles and concepts of the present
application.
As illustrated and described above, the burner assembly 20 of the
invention includes primary, secondary and tertiary burners. Alternative
embodiments are
illustrated schematically in Figs. 11 through 16. In each case, the
schematically shown
tile formation including concentric tiles may be essentially the same as the
tiles 40 and
42 described above. As shown in Fig. 11, the illustrated burner assembly may
include
only a single centrally located primary burner nozzle. As shown in Fig. 12,
the illustrated
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burner assembly may include two or more burner nozzles located adjacent the
outer
peripheral surface of the upstream tile near the inlet to the RFG induction
conduit. As
shown in Fig. 13, the illustrated burner assembly may include two or more
burner nozzles
located on the internal surface of the central opening in the upstream tile
adjacent the
RFG induction conduit. As shown in Fig. 14, the illustrated burner assembly
may include
a centrally located primary burner nozzle and two or more burner nozzles
located on the
outer peripheral surface of the downstream tile. As shown in Fig. 15, the
burner
assembly may include a single centrally located primary burner nozzle that is
equipped
with a premixer and conventional swirler mechanism. As shown in Fig. 16, the
burner
assembly may be arranged essentially as described above in connection with
Figs. 1
through 10, except in this case the central burner nozzle is particularly
adapted for
delivering a liquid fuel. In each case, the flow of RFG from area 34 and
through conduit
80 is induced by combustion air flowing along path 76 past end 81 of the
conduit.
An alternative form of a burner arrangement which embodies the concepts
and principles of the invention is illustrated in FIGS. 17 through 21 where
the
arrangement is identified broadly by the reference numeral 120. The burner
arrangement
120 is similar in configuration to the burner arrangement 20 except for the
manner in
which the upper tile 142 sits on the lower tile 140. This in turn changes the
configuration
of the annular space 178 defined between the tiles 140 and 142.
With reference to FIGS. 17 through 21, it can be seen that the lower tile
140 includes a plurality of shoulder members 152 which project radially
outwardly from
the outer peripheral surface 154 of annular body segment 146. Together the
upper
surfaces 153 of the members 152 provide a seating structure 144 for tile 142.
To this
end, upper tile 142 is provided with a plurality of downwardly projecting feet
159. In
accordance with the invention, the number of feet 159 on upper tile 142
corresponds with
the number of members 152 on lower tile 140, and as can be seen, the feet 159
are
positioned to sit on the upper surfaces 153 when the tiles are assembled. With
this
arrangement, the lower edge portion 158 of the upper tile 142 is spaced
vertically from
the upper edge portion 161 of the lower tile 140. Such spacing provides a
configuration
for the annular space 178 which is different than the configuration of the
annular space
78 of burner assembly 20. This spacing also changes the configuration of the
path 185
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provided for the flow of flue gas from the interior of the furnace and into
the central
passageway 174 within the tiles 140, 142. Such flow, of course, is induced by
the flow
of combustion air along the path 176 as well as by the flow of fuel gas from
nozzles 184.
Tile 142 may be attached to tile 140 using conventional means, for example,
studs
extending downwardly from feet 159 through stud receiving holes extending
through the
shoulder members 152.
With reference to FIG. 19, it can be seen that the channels 190 are offset
angularly relative to the shoulder members 152, whereas, in the burner
arrangement 20,
the channels 90 are not offset angularly relative to the shoulder members 52.
In
accordance with the concepts and principles of the invention, the channels 190
are
preferably offset 45 ° relative to the shoulder members 152. This
angular offset places
the nozzles 184 in positions where there is a minimum of structure to hinder
the flow of
fuel from the nozzles 184 and into the annular space 178.
Another alternative form of a burner arrangement which embodies the
concepts and principles of the invention is illustrated in FIG. 22 where the
arrangement
is identified broadly by the reference numeral 220. The burner arrangement 220
is
essentially the same as the burner arrangement 120 except for the placement of
the burner
nozzles 284 and the radial dimension of the shoulder members 252. The
arrangement of
FIG. 22 facilitates the employment of a burner which may be smaller in
diameter than the
burner arrangement 120.
The burner arrangements of the present invention achieve low NOx
emissions, low noise, air entrainment of RFG, prompt NOX alleviation,
simplicity of
construction, short flame profile, high turndown ratios, high stability and
low CO
emissions. Flow of RFG is induced without mechanical devices such as blowers
by using
the combustion air as a motive force for entrainment of RFG instead of fuel
gas. In
addition, the arrangement is such that the secondary gas tips are located
behind a burner
tile so that jet noise is shielded from the outside of the burner. Moreover,
diffusion of
RFG directly from the interior of the furnace directly into the gas jet prior
to combustion
results in reduction of prompt NOx. The arrangement is simple resulting in low
manufacturing costs while providing ease of operation.
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An important feature of the invention is the provision of a non-powered
means of supplying flue gas to a burner to reduce NOX emissions and enhance
mixing of
combustion air, fuel gas and RFG to facilitate homogeneity of the mixture. The
burner
arrangement is based at least in part upon the important concept of using the
mass of the
axially flowing combustion air to entrain RFG from the furnace and mix it with
the
entering fuel gas in sufficient amounts while ensuring that the mixture
remains
combustible. As mentioned previously, the invention also contemplates the use
of both
the mass of the axially flowing combustion air and the mass of the fuel
provided through
gas jets to entrain RFG from the furnace and mix it with the entering fuel.
The impedance of combustion caused by the flue gas entering the nozzle
arrangement directly in accordance with the invention lowers the flame
temperature, thus
inhibiting the formation and emission of NOX. Prior to the design of the
invention, only
small amounts of flue gas were entrained by individual gas jets inspirating
flue gases into
the tile block. The old design proved to be fairly inefficient with the
stratification of flue
gases causing long flames and burner instability. The present invention
utilizes the
energy of the large mass of flowing combustion air leading to a greater mass
of flue gas
entrainment and consequent more efficient mixing with the gas jet and flame.
An
important note here is that the air induced flue gas entrainment is performed
in the
furnace space and the flue gases are not required to go back into the burner
body as with
some previous burner arrangements.
In the simplest configuration of the burner the flame is ignited and
stabilized in an oxidizing zone within the primary tile. The flame envelope is
quickly
extended into the flue gas rich secondary zone where NOX formation and
emission is
inhibited. The flame envelope never loses stability as it enters the secondary
flue gas
diluted zone as it is firmly anchored and defined in the primary (oxidizing)
zone
contained in the primary tile section. This rapid diffusion and rapid
quenching without
instability conserves short flame lengths and large turndown ratios. Flame
lengths are
typically 1. l feet or less per million BTUs of firing rate (fuel dependent)
and turndown
rates typical of natural draft low NOX burners on the order of 10 to 1 may
easily be
achieved. CO emissions are typically zero, but are somewhat dependent upon
furnace
temperature.
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Configurations of the burner of the invention other than as shown in the
drawings include staged fuel with either a gas gun being utilized or with
primary risers
inserted into the ring-shaped conduit for additional NOX abatement. When an
annular
primary tile is utilized as shown in the drawings, the riser is shielded from
the outside of
the burner by the primary tile greatly reducing the jet noise emitted by the
burner to the
exterior. The primary risers are set low enough in the ring-shaped conduit so
that the gas
jet has sufficient time to entrain pure flue gas prior to entering the
oxidizing zone. This
premixing of flue gas with the combustion gas lowers the speed at which the
combustion
gas can react thus additionally reducing the formation of NOX. Once the gas
jet starts to
burn it does so in an atmosphere rich with flue gas which is constantly being
entrained
by the combustion air. The overall effect is ultra low NOX as both prompt and
thermal
NOX are greatly reduced. The overall simplicity of the burner combined with
the stability
achieved in the oxidizing zone make the burner arrangements of the invention
an
important advance relative to existing technologies.
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