U.S. patent application number 10/389327 was filed with the patent office on 2003-09-18 for burner with high flow area tip.
Invention is credited to Bury, Mark E., Gauba, Gautam, Stephens, George.
Application Number | 20030175634 10/389327 |
Document ID | / |
Family ID | 28045495 |
Filed Date | 2003-09-18 |
United States Patent
Application |
20030175634 |
Kind Code |
A1 |
Stephens, George ; et
al. |
September 18, 2003 |
Burner with high flow area tip
Abstract
An improved burner and a method for combusting fuel in burners
used in furnaces, such as those used in steam cracking, are
disclosed. The burner includes a burner tube having an upstream end
and a downstream end and means for drawing flue gas from the
furnace or air from a source of air or mixtures thereof in response
to an inspirating effect of uncombusted fuel flowing through the
burner tube from its upstream end towards its downstream end. A
burner tip is mounted on the downstream end of the burner tube
adjacent a first opening in the furnace, the burner tip having a
plurality of main ports in an external surface thereof so that
combustion of the fuel takes place downstream of the burner tip,
the number and dimensions of said main ports in said external
surface being such that the ratio of the total area of the main
ports in said external surface is at least 1 square inch per
million (MM) Btu/hr burner capacity. The inspirating effect of the
fuel flowing though the burner tube can optionally be further
assisted by steam injection upstream of the burner tube.
Inventors: |
Stephens, George; (Humble,
TX) ; Bury, Mark E.; (Acton, MA) ; Gauba,
Gautam; (Marlborough, MA) |
Correspondence
Address: |
ExxonMobil Chemical Company
Law Technology
P.O. Box 2149
Baytown
TX
77522-2149
US
|
Family ID: |
28045495 |
Appl. No.: |
10/389327 |
Filed: |
March 14, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60365227 |
Mar 16, 2002 |
|
|
|
Current U.S.
Class: |
431/8 ; 431/115;
431/159; 431/187; 431/349; 431/5 |
Current CPC
Class: |
F23C 2202/10 20130101;
F23D 2900/00011 20130101; F23D 14/08 20130101; F23D 14/04 20130101;
F23C 7/008 20130101; F23L 7/005 20130101; F23M 5/025 20130101; F23C
6/045 20130101; F23C 2900/06041 20130101; F23D 2207/00 20130101;
F23M 11/042 20130101; F23C 9/00 20130101 |
Class at
Publication: |
431/8 ; 431/5;
431/115; 431/159; 431/187; 431/349 |
International
Class: |
F23D 014/16; F23D
014/48; F23C 009/06 |
Claims
What is claimed is:
1. A burner for the combustion of fuel in a furnace, said burner
comprising: (a) a burner tube having a downstream end and an
upstream end; (b) a fuel orifice located adjacent the upstream end
of said burner tube, for introducing fuel into said burner tube;
(c) a burner tip mounted on the downstream end of said burner tube
and adjacent a first opening in the furnace, said burner tip having
a plurality of main ports in an external surface thereof so that
combustion of the fuel takes place at said external surface of said
burner tip, the number and dimensions of said main ports in said
external surface being such that the total area of the main ports
in said external surface is at least 1 square inch per million (MM)
Btu/hr burner capacity; and (d) means for drawing flue gas from
said furnace or air from a source of air or mixtures thereof in
response to an inspirating effect of uncombusted fuel flowing
through said burner tube from its upstream end towards its
downstream end.
2. The burner according to claim 1, wherein the burner is a pre-mix
burner.
3. The burner according to claim 1, wherein the burner is a
flat-flame burner.
4. The burner according to claim 1, wherein said means for drawing
flue gas from said furnace is in fluid communication with an
external FGR duct.
5. The burner of claim 1, wherein the total area of the main ports
in said external surface is at least 1.2 square inch per million
(MM) Btu/hr burner capacity.
6. The burner of claim 1, further comprising a venturi intermediate
said upstream end and said downstream end of said burner tube.
7. The burner of claim 6, wherein said venturi includes a throat
portion having substantially constant internal cross-sectional
dimensions such that the ratio of the length to maximum internal
cross-sectional dimension of said throat portion is at least 3.
8. The burner of claim 6, wherein the ratio of the length to
maximum internal cross-sectional dimension of said throat portion
is from about 4 to about 10.
9. The burner of claim 6, wherein the ratio of the length to
maximum internal cross-sectional dimension of said throat portion
is from about 4.5 to about 8.
10. The burner of claim 6, wherein the ratio of the length to
maximum internal cross-sectional dimension of said throat portion
is from about 6.5 to about 7.5.
11. The burner according to claim 7, including one or more steam
tubes terminating adjacent the upstream end of said burner tube for
introducing steam into said burner tube.
12. The burner according to claim 1, including one or more steam
tubes terminating adjacent the upstream end of said burner tube for
introducing steam into said burner tube.
13. The burner according to claim 11, further comprising at least
one air port for introducing staged secondary air.
14. The burner according to claim 1, further comprising at least
one air port for introducing staged secondary air.
15. The burner according to claim 14 wherein said fuel orifice is
located within a gas spud.
16. The burner according to claim 1 wherein said fuel orifice is
located within a gas spud.
17. A method for combusting fuel in a burner of a furnace,
comprising the steps of: (a) combining fuel with air, flue gas or a
mixture thereof at a predetermined location adjacent a fuel
orifice; (b) passing the fuel and air, flue gas or mixture thereof
through a burner tube; (c) discharging the fuel and air, flue gas
or mixture thereof at a burner tip downstream of the predetermined
location, the burner tip having a plurality of main ports in an
external surface thereof and a plurality of further side ports in a
peripheral surface thereof; and (d) combusting the fuel downstream
of said burner tip, wherein the total area of the main ports in
said external surface of the burner tip is at least 1 square inch
per million (MM) Btu/hr burner capacity.
18. The method according to claim 17, wherein the burner is a
pre-mix burner.
19. The method according to claim 17, wherein the burner is a
flat-flame burner.
20. The method according to claim 17, wherein the burner further
comprises an external FGR duct.
21. The method according to claim 17, further comprising the step
of injecting steam into the burner tube to mix with the fuel and
air, flue gas or mixtures thereof upstream of said zone of
combustion.
22. The method according to claim 21 wherein the furnace is a
steam-cracking furnace.
23. The method according to claim 17 wherein the furnace is a
steam-cracking furnace.
24. The method of claim 17 wherein said fuel orifice is located
within a gas spud.
25. The method of claim 17 wherein the total area of the main ports
in said external surface is at least 1.2 square inch per million
(MM) Btu/hr burner capacity.
Description
RELATED APPLICATIONS
[0001] This patent application claims priority from Provisional
Application Serial No. 60/365,227, filed on Mar. 16, 2002, the
contents of which are hereby incorporated by reference.
FIELD OF THE INVENTION
[0002] This invention relates to an improved burner of the type
employed in high temperature furnaces. More particularly, the
invention relates to a burner having a tip with increased flow area
so as to allow increased flue gas recirculation and thereby reduce
NO.sub.x emissions.
BACKGROUND OF THE INVENTION
[0003] As a result of the interest in recent years to reduce the
emission of pollutants from burners of the type used in large
industrial furnaces, significant improvements have been made in
burner design. In the past, burner design improvements were aimed
primarily at improving heat distribution to provide more effective
heat transfer. However, increasingly stringent environmental
regulations have shifted the focus of burner design to the
minimization of regulated pollutants.
[0004] Oxides of nitrogen (NO.sub.x) are formed in air at high
temperatures. These compounds include, but are not limited to,
nitrogen oxide and nitrogen dioxide. Reduction of NO.sub.x
emissions is a desired goal to decrease air pollution and meet
government regulations.
[0005] The rate at which NO.sub.x is formed is dependent upon the
following variables: (1) flame temperature, (2) residence time of
the combustion gases in the high temperature zone and (3) excess
oxygen supply. The rate of formation of NO.sub.x increases as flame
temperature increases. However, the reaction takes time and a
mixture of nitrogen and oxygen at a given temperature for a very
short time may produce less NO.sub.x than the same mixture at a
lower temperature, over a longer period of time.
[0006] A strategy for achieving lower NO.sub.x emission levels is
to install a NO.sub.x reduction catalyst to treat the furnace
exhaust stream. This strategy, known as Selective Catalytic
Reduction (SCR), is very costly and, although it can be effective
in meeting more stringent regulations, represents a less desirable
alternative to improvements in burner design.
[0007] Burners used in large industrial furnaces may use either
liquid fuel or gas. Liquid fuel burners mix the fuel with steam
prior to combustion to atomize the fuel to enable more complete
combustion, and combustion air is mixed with the fuel at the zone
of combustion.
[0008] Gas fired burners can be classified as either premix or raw
gas burners, depending on the method used to combine the air and
fuel. They also differ in configuration and the type of burner tip
used.
[0009] Raw gas burners inject fuel directly into the air stream,
and the mixing of fuel and air occurs simultaneously with
combustion. Since airflow does not change appreciably with fuel
flow, the air register settings of natural draft burners must be
changed after firing rate changes. Therefore, frequent adjustment
may be necessary, as explained in detail in U.S. Pat. No.
4,257,763. In addition, many raw gas burners produce luminous
flames.
[0010] Premix burners mix some or all of the fuel with some or all
of the combustion air prior to combustion. Since premixing is
accomplished by using the energy present in the fuel stream,
airflow is largely proportional to fuel flow. As a result,
therefore, less frequent adjustment is required. Premixing the fuel
and air also facilitates the achievement of the desired flame
characteristics. Due to these properties, premix burners are often
compatible with various steam cracking furnace configurations.
[0011] Floor-fired premix burners are used in many steam crackers
and steam reformers primarily because of their ability to produce a
relatively uniform heat distribution profile in the tall radiant
sections of these furnaces. Flames are non-luminous, permitting
tube metal temperatures to be readily monitored. Therefore, a
premix burner is the burner of choice for such furnaces. Premix
burners can also be designed for special heat distribution profiles
or flame shapes required in other types of furnaces.
[0012] One technique for reducing NO.sub.x that has become widely
accepted in industry is known as combustion staging. With
combustion staging, the primary flame zone is deficient in either
air (fuel rich) or fuel (fuel lean). The balance of the air or fuel
is injected into the burner in a secondary flame zone or elsewhere
in the combustion chamber. As is well known, a fuel-rich or
fuel-lean combustion zone is less conducive to NO.sub.x formation
than an air-fuel ratio closer to stoichiometry. Combustion staging
results in reducing peak temperatures in the primary flame zone and
has been found to alter combustion speed in a way that reduces
NO.sub.x. Since NO.sub.x formation is exponentially dependent on
gas temperature, even small reductions in peak flame temperature
can dramatically reduce NO.sub.x emissions. However this must be
balanced with the fact that radiant heat transfer decreases with
reduced flame temperature, while CO emissions, an indication of
incomplete combustion, may actually increase.
[0013] In the context of premix burners, the term primary air
refers to the air premixed with the fuel; secondary, and in some
cases tertiary, air refers to the balance of the air required for
proper combustion. In raw gas burners, primary air is the air that
is more closely associated with the fuel; secondary and tertiary
air are more remotely associated with the fuel. The upper limit of
flammability refers to the mixture containing the maximum fuel
concentration (fuel-rich) through which a flame can propagate.
[0014] U.S. Pat. No. 4,629,413 discloses a premix burner that
employs combustion staging to reduce NO.sub.x emissions. The premix
burner of U.S. Pat. No. 4,629,413 lowers NO.sub.x emissions by
delaying the mixing of secondary air with the flame and allowing
some cooled flue gas to recirculate with the secondary air. The
entire contents of U.S. Pat. No. 4,629,413 are incorporated herein
by reference.
[0015] U.S. Pat. No. 5,092,761 discloses a method and apparatus for
reducing NO.sub.x emissions from premix burners by recirculating
flue gas. Flue gas is drawn from the furnace through recycle ducts
by the inspirating effect of fuel gas and combustion air passing
through a venturi portion of a burner tube. Air flow into the
primary air chamber is controlled by dampers and, if the dampers
are partially closed, the reduction in pressure in the chamber
allows flue gas to be drawn from the furnace through the recycle
ducts and into the primary air chamber. The flue gas then mixes
with combustion air in the primary air chamber prior to combustion
to dilute the concentration of oxygen in the combustion air, which
lowers flame temperature and thereby reduces NO.sub.x emissions.
The flue gas recirculating system may be retrofitted into existing
burners or may be incorporated in new low NO.sub.x burners. The
entire contents of U.S. Pat. No. 5,092,761 are incorporated herein
by reference.
[0016] Analysis of burners of the type disclosed in U.S. Pat. No.
5,092,761 has shown that the flue gas recirculation (FGR) ratio is
generally in the range of 5 to 10%, where the FGR ratio is defined
as: 1 FGR ratio ( % ) = 100 .times. [ ( l b . of flue gas drawn
into venturi ) ( l b . fuel combusted in burner + lb . air drawn
into burner ) ]
[0017] The ability of existing burners of this type to generate
higher FGR ratios is limited by the inspirating capacity of the
fuel orifice/gas spud/venturi combination. Although further closing
of the primary air dampers can further reduce the pressure in the
primary air chamber and thereby enable increased FGR ratios; the
resultant reduction of primary air flow is such that insufficient
oxygen is present in the venturi for acceptable burner
stability.
[0018] There is therefore a need to provide a burner that allows
improved flue gas recirculation while minimizing any accompanying
reduction in burner stability. In accordance with the invention,
this result is achieved by increasing the flow area of the burner
tip. It is to be appreciated that, in designing a burner tip, it is
normal to target a specific velocity for the fuel/air mixture
flowing through the tip. The velocity should be high enough to
prevent flashback, but low enough to prevent the flame lifting off
the tip.
SUMMARY OF THE INVENTION
[0019] In one aspect, the present invention is directed to a burner
for the combustion of fuel in a furnace, said burner comprising:
(a) a burner tube having a downstream end and an upstream end; (b)
a fuel orifice located adjacent the upstream end of said burner
tube, for introducing fuel into said burner tube; (c) a burner tip
mounted on the downstream end of said burner tube and adjacent a
first opening in the furnace, said burner tip having a plurality of
main ports in an external surface thereof so that combustion of the
fuel takes place at said external surface of said burner tip, the
number and dimensions of said main ports in said external surface
being such that the total area of the main ports in said external
surface is at least 1 square inch per million (MM) Btu/hr burner
capacity; and (d) means for drawing flue gas from said furnace and
primary air from a source of air in response to an inspirating
effect of uncombusted fuel flowing through said burner tube from
its upstream end towards its downstream end. In one embodiment, a
venturi includes a throat portion having substantially constant
internal cross-sectional dimensions such that the ratio of the
length to maximum internal cross-sectional dimension of said throat
portion is at least 3.
[0020] In another embodiment, ratio of the length to maximum
internal cross-sectional dimension of said throat portion is from
about 4 to about 10, more preferably from about 4.5 to about 8 and
most preferably from about 6.5 to 7.5.
[0021] In a further aspect, the invention resides in a method for
combusting fuel in a burner of a furnace, comprising the steps of:
(a) combining fuel with air, flue gas or a mixture thereof at a
predetermined location adjacent a fuel orifice; (b) passing the
fuel and air, flue gas or mixture thereof through a burner tube;
(c) discharging the fuel and air, flue gas or mixture thereof at a
burner tip downstream of the predetermined location, the burner tip
having a plurality of main ports in an external surface thereof and
a plurality of further side burner openings in a peripheral surface
thereof; and (d) combusting the fuel at the external surface of
said burner tip, wherein the total area of the main ports in said
external surface of the burner tip is at least 1 square inch per
million (MM) Btu/hr burner capacity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The invention is further explained in the description that
follows with reference to the drawings wherein:
[0023] FIG. 1 is an elevation partly in section of a burner
according to the present invention;
[0024] FIG. 2 is an elevation partly in section taken along line
2-2 in FIG. 1;
[0025] FIG. 3 is a perspective view of the burner tip of the FIG. 1
embodiment;
[0026] FIGS. 4A and 4B are plan views of the tip of the burner of
the FIG. 1 embodiment and the tip of a conventional burner,
respectively;
[0027] FIG. 5 is an elevation view of a burner according to another
embodiment of the present invention employing an external flue gas
recirculation duct;
[0028] FIG. 6 is an elevation partly in section of a flat-flame
burner according to a further example of the present invention;
and
[0029] FIG. 7 is an elevation partly in section taken along line
7-7 of FIG. 6.
[0030] FIGS. 8A and 8B are plan views of the tip of the burner of
the FIG. 6 embodiment and the tip of a conventional burner,
respectively;
DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS
[0031] Although the present invention is described in terms of a
burner for use in connection with a furnace or an industrial
furnace, it will be apparent to one of skill in the art that the
teachings of the present invention also have applicability to other
process components such as, for example, boilers. Thus, the term
furnace herein shall be understood to mean furnaces, boilers and
other applicable process components.
[0032] Referring to FIGS. 1 to 3, the burner of this embodiment of
the invention includes a freestanding burner tube 12 located in a
well in a furnace floor 14. Burner tube 12 includes an upstream end
16, a downstream end 18 and a venturi 19. Burner tip 20 is located
at downstream end 18 of tube 12 and is surrounded by an annular
tile 22. A fuel orifice 11, which may be located within gas spud 24
is located at upstream end 16 of tube 12 and introduces fuel into
burner tube 12. Fresh or ambient air is introduced into primary air
chamber 26 through adjustable damper 28 to mix with the fuel at
upstream end 16 of burner tube 12. Combustion of the fuel/air
mixture occurs downstream of the burner tip 20.
[0033] Burner tip 20 has an upper end 66 which, when installed,
faces the burner box and a lower end 68 adapted for mating with the
downstream end 18 of burner tube 12. Mating of the lower end 68 of
burner tip 20 to the burner tube 12 can be achieved by swaging or,
more preferably, by welding or threaded engagement.
[0034] Referring specifically to FIG. 3, the upper end 66 of the
burner tip 20 includes a plurality of main ports 64 in a centrally
disposed end surface 69 and a plurality of side ports 68 in an
annular side surface 60. In operation, the side ports 68 direct a
portion of the fuel/air mixture across the face of the tile 22,
whereas the main ports 64 direct the major portion of the mixture
into the furnace.
[0035] Referring now to FIG. 2, a plurality of air ports 30
originate in secondary air chamber 32 and pass through furnace
floor 14 into the furnace. Fresh air enters secondary air chamber
32 through adjustable dampers 34 (see FIG. 1) and passes through
staged air ports 30 into the furnace to provide secondary or staged
combustion.
[0036] In order to recirculate flue gas from the furnace to the
primary air chamber, FGR duct 76 extends from opening 82, in the
floor of the furnace into the primary air chamber 26.
Alternatively, multiple FGR ducts may be used instead of a single
FGR duct 76. Flue gas is drawn through duct 76 by the inspirating
effect of fuel passing through venturi 19 of burner tube 12. In
this manner, the primary air and flue gas are mixed in primary air
chamber 26, which is prior to the zone of combustion. Therefore,
the amount of inert material mixed with the fuel is raised, thereby
reducing the flame temperature, and as a result, reducing NO.sub.x
emissions. Closing or partially closing damper 28 restricts the
amount of fresh air that can be drawn into the primary air chamber
26 and thereby provides the vacuum necessary to draw flue gas from
the furnace floor.
[0037] Unmixed low temperature ambient air, having entered
secondary air chamber 32 through dampers 34 and having passed
through air ports 30 into the furnace, is also drawn through duct
76 into the primary air chamber 26 by the inspirating effect of the
fuel passing through venturi 19. The ambient air may be fresh air
as discussed above. The mixing of the ambient air with the flue gas
lowers the temperature of the hot flue gas flowing through duct 76
and thereby substantially increases the life of the duct and
permits use of this type of burner to reduce NO.sub.x emission in
high temperature cracking furnaces having flue gas temperature
above 1900.degree. F. in the radiant section of the furnace.
[0038] Advantageously, a mixture of from about 20% to about 80%
flue gas and from about 20% to about 80% ambient air should be
drawn through recirculation duct 76. It is particularly preferred
that a mixture of about 50% flue gas and about 50% ambient air be
employed. The desired proportions of flue gas and ambient air may
be achieved by proper sizing, placement and/or design of duct 76,
and air ports 30, as those skilled in the art will readily
recognize. That is, the geometry and location of the air ports may
be varied to obtain the desired percentages of flue gas and ambient
air.
[0039] A lighting port 50 is provided in the primary chamber 26,
both to allow inspection of the interior of the burner assembly,
and to provide access for lighting of the burner with lighting
element (not shown). As shown, a tube 84 provides access to the
interior of secondary air chamber 32 for an optional pilot 86.
[0040] Referring now to FIGS. 4A and 4B, the upper end 66 of the
burner tip 20 of FIG. 1 is shown in FIG. 4A, whereas FIG. 4B shows
the upper end 66 of a conventional burner tip 20. It will be seen
that the number and size of the main ports 64 in the centrally
disposed end surface 69 of the burner tip of the invention are
significantly larger than those of the conventional tip. In
particular, the number and dimensions of the main ports 64 in the
tip of the invention are such that the total area of the main ports
64 in the end surface 69 is at least 1 square inch, preferably at
least 1.2 square inch, per million (MM) Btu/hr burner capacity. In
contrast, in the conventional burner tip shown in FIG. 4B, the
total area of the main ports 64 in the end surface 69 is less than
1 square inch per million (MM) Btu/hr burner capacity. In one
practical embodiment of a burner tip according to the invention,
wherein the design firing rate of the burner is 6.0 MMBtu/hr, the
total area of the main ports 64 in the end surface 69 is 8.4
in.sup.2 whereas, in the conventional burner tip for use at the
same design firing rate, the total area of these openings is only
5.8 in.sup.2.
[0041] Intuitively, it would be expected that raising the tip flow
area would proportionally reduce tip velocity, but instead, it is
found that the drop in velocity can be mitigated by the fact that
raising tip flow area raises FGR.
[0042] Increasing the total area of the main ports 64 in the burner
tip 20 increases the flow area of the burner tip 20, which in turn
enables higher FGR, rates to be induced without increasing the
velocity for the fuel/air mixture flowing through the tip. In this
way, stable operation of the burner can be retained with higher FGR
rates.
[0043] In a preferred embodiment of the invention, the venturi 19
of the burner shown in FIG. 1 includes a throat portion 19a that is
of substantially constant internal cross-sectional dimensions along
its length and a divergent cone portion 19b, wherein the ratio of
the length to maximum internal cross-sectional dimension of the
throat portion 19a is least 3, preferably from about 4 to about 10,
more preferably from about 4.5 to about 8 and most preferably from
about 6.5 to about 7.5. Increasing the ratio of the length to
internal cross-sectional dimensions in the throat portion of the
venturi allows the venturi to induce more flue gas recirculation
thereby reducing flame temperature and NO.sub.x production. In
addition, the increased flue gas recirculation mitigates the
reduction of the tip velocity that results with the increased
opening area in the tip. A longer venturi throat also promotes
better flow development and hence improved mixing of the fuel/air
stream prior to the mixture exiting the burner tip 20. Better
mixing of the fuel/air stream also contributes to NO.sub.x
reduction by producing a more evenly developed flame and hence
reducing peak temperature regions.
[0044] In addition to the use of flue gas as a diluent, another
technique to achieve lower flame temperature through dilution is
through the use of steam injection. Steam can be injected in the
primary air chamber through stream injection tube 15 or the
secondary air chamber (not shown). Preferably, steam is injected
upstream of the venturi 19.
[0045] Referring to FIG. 5, another embodiment of the present
invention is illustrated in which the high flow area burner tip
described above is employed with an external FGR duct 376
communicating with an exhaust 300 of the furnace. It will be
understood by one of skill in the art that several burners may be
located within the furnace, all of which may be connected to
furnace exhaust 300 through the external FGR duct 376.
[0046] Benefits similar to those described above through the use of
the burner tip of the present invention can also be achieved in
flat-flame burners, as will now be described by reference to FIGS.
6 and 7.
[0047] A burner 110 includes a freestanding burner tube 112 located
in a well in a furnace floor 114. Burner tube 112 includes an
upstream end 116, a downstream end 118 and a venturi portion 119.
Burner tip 120 is located at downstream end 118 and is surrounded
by a peripheral tile 122. A fuel orifice 111, which may be located
within gas spud 124 is located at upstream end 116 and introduces
fuel into burner tube 112. Fresh or ambient air may be introduced
into primary air chamber 126 to mix with the fuel at upstream end
116 of burner tube 112. Combustion of the fuel and fresh air occurs
downstream of burner tip 120. Fresh secondary air enters secondary
chamber 132 through dampers 134.
[0048] In order to recirculate flue gas from the furnace to the
primary air chamber, a flue gas recirculation passageway 176 is
formed in furnace floor 114 and extends to primary air chamber 126,
so that flue gas is mixed with fresh air drawn into the primary air
chamber from opening 180 through dampers 128. Flue gas containing,
for example, 0 to about 15% O.sub.2 is drawn through passageway 176
by the inspirating effect of fuel passing through venturi portion
119 of burner tube 112. Primary air and flue gas are mixed in
primary air chamber 126, which is prior to the zone of
combustion.
[0049] A small gap exists between the burner tip 120 and the burner
tile 122. By keeping this gap small, the bulk of the secondary
staged air is forced to enter the furnace through staged air ports
(not shown) located some distance from the primary combustion zone,
which is located immediately on the furnace side of the burner tip
120.
[0050] In operation, fuel orifice 111, which may be located within
a gas spud 124 discharges fuel into burner tube 112, where it mixes
with primary air, recirculated flue-gas or a mixture thereof,
before being discharged from burner tip 120. The mixture in the
venturi portion 119 of burner tube 112 is maintained below the
fuel-rich flammability limit; i.e. there is insufficient air in the
venturi to support combustion. Staged, secondary air is added to
provide the remainder of the air required for combustion. The
majority of the staged air is added a finite distance away from the
burner tip 120 through staged air ports (not shown). However, a
portion of the staged, secondary air passes between the burner tip
120 and the peripheral tile 122 and is immediately available for
combustion.
[0051] Referring now to FIGS. 8A and 8B, the upper end 166 of the
burner tip 120 of FIG. 6 is shown in FIG. 8A, whereas FIG. 8B shows
the upper end 166 of a conventional burner tip 120. It will be seen
that the number and size of the main ports 164 in the centrally
disposed end surface 169 of the burner tip of the invention are
significantly larger than those of the conventional tip. In
particular, the number and dimensions of the main ports 164 in the
tip of the invention are such that the total area of the main ports
164 in the end surface 169 is at least 1 square inch, preferably at
least 1.2 square inch, per million (MM) Btu/hr burner capacity. In
contrast, in the conventional burner tip shown in FIG. 8B, the
total area of the main ports 164 in the end surface 169 is less
than 1 square inch per million (MM) Btu/hr burner capacity. In
addition to the use of flue gas as a diluent, another technique to
achieve lower flame temperature through dilution is through the use
of steam injection. Steam can be injected in the primary air
chamber through stream injection tube 15 or the secondary air
chamber (not shown). Preferably, steam is injected upstream of the
venturi 19.
[0052] In addition to the use of flue gas as a diluent, another
technique to achieve lower flame temperature through dilution is
through the use of steam injection. Steam can be injected into the
primary air chamber through stream injection tube 184 or into the
secondary air chamber (not shown). Preferably, steam is injected
upstream of the venturi 119.
[0053] It will also be understood that the burner tip described
herein also has utility in traditional raw gas burners and raw gas
burners having a pre-mix burner configuration wherein flue gas
alone is mixed with fuel gas at the entrance to the burner tube. In
fact, it has been found that the pre-mix, staged-air burners of the
type described in detail herein can be operated with the primary
air damper doors closed, with very satisfactory results.
[0054] Although illustrative embodiments have been shown and
described, a wide range of modification change and substitution is
contemplated in the foregoing disclosure and in some instances,
some features of the embodiment may be employed without a
corresponding use of other features. Accordingly, it is appropriate
that the appended claims be construed broadly and in a manner
consistent with the scope of the embodiments disclosed herein.
* * * * *