U.S. patent application number 09/921254 was filed with the patent office on 2001-12-06 for low nox and low co burner and method for operating same.
Invention is credited to Gamburg, Michael, Guarco, John, Moore, Jon, Schindler, Edmund S., Tsirulnikov, Lev.
Application Number | 20010049076 09/921254 |
Document ID | / |
Family ID | 22218374 |
Filed Date | 2001-12-06 |
United States Patent
Application |
20010049076 |
Kind Code |
A1 |
Schindler, Edmund S. ; et
al. |
December 6, 2001 |
Low NOx and low CO burner and method for operating same
Abstract
A round burner capable of being operated with reduced CO and
NO.sub.x emissions includes a venturi tube positioned to direct a
flow of air through the burner and into a combustion zone in a
combustion chamber through an entrance in a wall of the combustion
chamber. The venturi tube has inlet and outlet ends and a throat.
The outlet end of the venturi tube has a larger internal diameter
than either the inlet end or the throat and the same is positioned
adjacent the entrance to the combustion chamber. The inlet end of
the venturi tube is also positioned further from the entrance than
the outlet end of the venturi tube. The burner may include a duct
system that includes an inlet disposed in fluid communication with
the combustion zone and an outlet disposed in fluid communication
with the venturi tube adjacent the throat thereof. The system is
arranged and adapted to recirculate a stream of flue gas from a
location within said combustion chamber adjacent the combustion
zone by induction into the venturi tube at a low pressure location
adjacent the throat of the venturi tube so that the recirculated
stream of internal flue gas is inducted into and intermixed with
the flow of air at the throat of the venturi tube. Alternatively or
cumulatively, the burner may include a fuel gas injector
arrangement having an injector nozzle extending through the wall at
a location adjacent the combustion zone. The injector nozzle is in
fluid communication with the combustion chamber and is positioned
to direct a flow of fuel gas into the combustion chamber at a
location in the wall radially beyond the inner edge of the
entrance. Also disclosed is a method for operating the burner to
reduce CO and NO.sub.x emissions.
Inventors: |
Schindler, Edmund S.;
(Fairfield, CT) ; Tsirulnikov, Lev; (Brooklyn,
NY) ; Guarco, John; (Wolcott, CT) ; Moore,
Jon; (Hamden, CT) ; Gamburg, Michael; (San
Francisco, CA) |
Correspondence
Address: |
James H. Marsh, Jr.
SHOOK, HARDY & BACON L.L.P.
1200 Main Street
Kansas City
MO
64105-2118
US
|
Family ID: |
22218374 |
Appl. No.: |
09/921254 |
Filed: |
August 2, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09921254 |
Aug 2, 2001 |
|
|
|
09335007 |
Jun 17, 1999 |
|
|
|
60089570 |
Jun 17, 1998 |
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Current U.S.
Class: |
431/5 |
Current CPC
Class: |
F23C 7/004 20130101;
F23C 2900/09002 20130101; F23D 17/002 20130101; F23M 5/025
20130101; F23C 2202/30 20130101; F23C 9/08 20130101; F23C 9/00
20130101; F23C 2900/06041 20130101; F23C 6/047 20130101 |
Class at
Publication: |
431/5 |
International
Class: |
F23C 009/06 |
Claims
We claim:
1. A round burner capable of reduced CO and NO.sub.x emissions
comprising: a venturi tube positioned to direct a flow of air
through the burner and into a combustion zone in a combustion
chamber through an entrance in a wall of the combustion chamber,
said venturi tube having an outer periphery and inlet and outlet
ends, said outlet end of the venturi tube having a larger internal
diameter than said inlet end, said outlet end of the venturi tube
being positioned adjacent said entrance to the combustion chamber,
said inlet end of the venturi tube being positioned further from
said entrance than said outlet end of the venturi tube, said
entrance having an inner edge; and a fuel gas injector arrangement
including at least one injector nozzle extending through said wall
at a location adjacent said combustion zone, said nozzle being in
fluid communication with said combustion chamber and positioned to
direct a flow of fuel gas into said combustion chamber at a
location in the wall beyond said inner edge of the entrance.
2. A round burner as set forth in claim 1, wherein said burner
includes at least one first fuel gas nozzle located in said venturi
and positioned to introduce a supply of fuel gas into said flow of
air.
3. A round burner as set forth in claim 1, wherein said burner
includes a swirler positioned so that at least a primary portion of
said flow of air passes therethrough.
4. A round burner as set forth in claim 2, wherein said burner
includes a swirler positioned so that at least a primary portion of
said flow of air passes therethrough.
5. A round burner as set forth in claim 1, wherein said burner is
adapted for burning fuel oil introduced into said flow of air.
6. A round burner capable of reduced CO and NO.sub.x emissions
comprising: a venturi tube for directing a flow of air through the
burner and into a combustion zone in a combustion chamber through
an entrance in a wall of the combustion chamber, said venturi tube
having an inlet end, an outlet end and a throat having a lesser
internal diameter than either of said ends located between said
ends, said throat being operable when said flow of air passes
through the venturi tube to create a low pressure zone therein,
said venturi tube being positioned with its outlet end located
adjacent said entrance to the combustion chamber, said venturi tube
including a flue gas inlet connection located adjacent said throat
for introducing recirculated flue gas directly into said low
pressure zone, said tube further having an outer periphery and said
entrance having an inner edge; a fuel gas injector arrangement
including an injector nozzle extending through said wall at a
location adjacent said combustion zone, said nozzle being in fluid
communication with said combustion chamber and positioned to direct
a flow of fuel gas into said combustion chamber at a location in
the wall beyond said inner edge of the entrance; and a duct system
including at least one inlet disposed in fluid communication with
the combustion zone and at least one outlet connected to the flue
gas inlet connection of the venturi tube, said system being
arranged and adapted to recirculate a stream of flue gas from a
location in said combustion chamber adjacent said combustion zone
and into said flue gas inlet connection, whereby said stream of
flue gas is inducted into said low pressure zone through said flue
gas inlet connection and intermixed in the low pressure zone with
said flow of air.
7. A round burner as set forth in claim 6, wherein said burner
includes at least one first fuel gas nozzle located in said venturi
tube and positioned to introduce a supply of fuel gas into said
flow of air.
8. A round burner as set forth in claim 6, wherein said burner
includes a swirler positioned so that at least a primary portion of
said flow of air passes therethrough.
9. A round burner as set forth in claim 7, wherein said burner
includes a swirler positioned so that at least a primary portion of
said flow of air passes therethrough.
10. A round burner as set forth in claim 6, wherein said burner is
adapted for burning fuel oil introduced into said flow of air.
11. A round burner as set forth in claim 7, wherein said burner is
adapted for burning fuel oil introduced into said flow of air.
12. A method for operating a venturi tube equipped round burner
with reduced CO and NO.sub.x emissions, said method comprising:
directing a flow of air through the venturi tube and into a
combustion zone in a combustion chamber through an entrance in a
wall of the combustion chamber, said entrance having an inner edge;
and injecting a flow of fuel gas into said combustion chamber at a
location radially beyond said inner edge of the entrance and
adjacent said combustion zone.
13. A method for operating a venturi tube equipped round burner as
set forth in claim 12, wherein is included a step of introducing a
first supply of fuel gas into said flow of air.
14. A method for operating a venturi tube equipped round burner as
set forth in claim 12, wherein is included a step of passing at
least a primary portion of said flow of air through a swirler.
15. A method for operating a venturi tube equipped round burner as
set forth in claim 13, wherein is included a step of passing at
least a primary portion of said flow of air through a swirler.
16. A method for operating a venturi tube equipped round burner as
set forth in claim 12, wherein a supply of fuel oil is introduced
into said flow of air.
17. A method for operating a venturi tube equipped round burner so
as to achieve reduced CO and NO.sub.x emissions, said venturi tube
having an inlet end, an outlet end and a throat disposed between
said ends, said throat having a lesser internal diameter than
either of said ends and being operable when a flow of fluid passes
through the venturi tube to create a low pressure zone therein,
said venturi tube including a flue gas inlet connection located
adjacent said throat to permit induction of recirculated flue gas
directly into said low pressure zone, said venturi tube being
positioned with its outlet end located adjacent an entrance to a
combustion chamber, said entrance having an inner edge, said method
comprising: directing a flow of air through said venturi tube and
into a combustion zone in said combustion chamber through said
entrance to thereby create a low pressure zone in said venturi tube
adjacent said flue gas inlet connection; injecting a flow of fuel
gas into said combustion chamber at a location radially beyond said
inner edge of the entrance and adjacent said combustion zone; and
using the low pressure in said low pressure zone to induce a
recirculation of flue gas from a location in said combustion
chamber adjacent said combustion zone, through the flue gas inlet
connection and directly into said low pressure zone, whereby
recirculated flue gas is intermixed with said flow of air in said
low pressure zone.
18. A method for operating a venturi tube equipped round burner as
set forth in claim 17, wherein is included a step of introducing a
first supply of fuel gas into said flow of air.
19. A method for operating a venturi tube equipped round burner as
set forth in claim 17, wherein is included a step of passing at
least a primary portion of said flow of air through a swirler.
20. A method for operating a venturi tube equipped round burner as
set forth in claim 18, wherein is included a step of passing at
least a primary portion of said flow of air through a swirler.
21. A method for operating a venturi tube equipped round burner as
set forth in claim 17, wherein a supply of fuel oil is introduced
into said flow of air.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of prior co-pending
application Ser. No. 09/335,007 filed on Jun. 17, 1999, and
priority under 35 U.S.C. .sctn.120 is claimed herein from said
prior application. Application Ser. No. 09/335,007 in turn claims
priority under 35 U.S.C. .sctn.119(e) from provisional application
Ser. No. 60/089,570 filed on Jun. 17, 1998, and that claim for
priority under 35 U.S.C. .sctn.119(e) is repeated here for purposes
of this divisional application. The entireties of the disclosures
of said prior co-pending application and said provisional
application are hereby incorporated herein by this specific
reference thereto.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention of the present application relates to burners
for large scale industrial applications. Such burners may be
adapted for burning gaseous fuels including natural gas. Such
burners may also be adapted for burning fuel oil. And in many cases
the burners may be adapted for burning both gaseous fuels and fuel
oil either alternatively or at the same time. In particular the
invention relates to industrial burners which burn fuel gas and/or
oil and are specially constructed and engineered for emitting low
levels of nitrogen oxide (NO.sub.x) and carbon monoxide (CO) air
pollution. The invention also relates to the methodology for
operating such burners. More particularly the invention relates to
a burner and the methodology for operating the same whereby
substantial reductions of CO and NO.sub.x emissions are achieved
relative to existing burners.
[0004] 2. The Prior Art Background
[0005] Many designs exist for delivering fuel and air to a furnace
combustion chamber or firebox. Virtually all modem prior art
designs are intended to enhance combustion efficiency. In addition,
tube metal temperatures and other furnace component limitations
must be taken into consideration in designing furnace burners. More
recently governmental regulations and social pressures require
designers to take into consideration the reduction of CO and
NO.sub.x emissions.
[0006] One of the best of the more recently developed industrial
burners is the Todd Variflame No Internal FGR Injection and No
External Gas Injection Burner which uses an array of internal poker
tubes for delivering fuel and air to a furnace firebox. This system
is the subject matter of U.S. Pat. No. 5,860,803 to Schindler et
al. which issued on Jan. 19, 1999 (the "'803 patent"). The entirety
of the disclosure of the '803 patent is hereby incorporated herein
by reference.
[0007] In spite of the efforts of many prior art workers in the
field, a perfect solution to the CO and NO.sub.x emissions problem
remains elusive. Some have tried to reduce NO.sub.x emissions by
recirculating flue gas into the firebox. However, when flue gas is
recirculated from a downstream location, the costs associated with
providing and forcing such recirculation are substantial.
SUMMARY OF THE INVENTION
[0008] The present invention provides a device and methodology for
efficiently and economically reducing the amount CO and NO.sub.x
emission from a combustion chamber without substantially effecting
thermal efficiency and/or reaction parameters of the same. In
particular the invention provides a novel burner design and novel
operating methodology which utilizes internal flue gas
recirculation and/or external fuel injection in a venturi tube
burner system. More particularly, the invention provides a venturi
tube burner system which provides swirled primary and straight line
secondary combustion air in the venturi tube and straight line
tertiary air outside the venturi tube to provide novel effects in
the burner flame formed under the above conditions. Preferably the
burner includes internal flue gas recirculation and/or external
fuel injection.
[0009] As a result of extensive research and development conducted
by the present inventors, an improved burner design has been
developed whereby it is possible to achieve substantial reductions
in CO and NO.sub.x emissions without substantial loss of burner
efficiency. Thus, in accordance with one aspect of the present
invention, a novel round burner is provided which comprises a
venturi tube positioned to direct a flow of air through the burner
and into a combustion zone in a combustion chamber through an
entrance in a wall of the combustion chamber. The venturi tube has
inlet and outlet ends and a throat located between the inlet and
outlet ends. The outlet end has a larger internal diameter than
either the inlet end or the throat. The outlet end of the venturi
tube is positioned adjacent the entrance to the combustion chamber
and the inlet end of the venturi tube is positioned further from
the entrance than the outlet end.
[0010] The novel burner of the invention also provides a duct
system that includes at least one inlet disposed in fluid
communication with the combustion zone, and at least one outlet
disposed in fluid communication with the throat of the venturi
tube. The duct system is arranged and adapted to recirculate flue
gas from a location within said combustion chamber adjacent said
combustion zone and into said venturi tube at a location adjacent
said throat, whereby the recirculated flue gas is inducted into and
intermixed with said flow of air at said throat of the venturi
tube. Thus, NO.sub.x emission reduction may be achieved without the
expense of an external flue gas recirculation system.
[0011] In another aspect of the invention, the invention provides a
round burner which comprises a venturi tube positioned to direct a
flow of air through the burner and into a combustion zone in a
combustion chamber through an entrance in a wall of the combustion
chamber. The novel burner of this aspect of the invention includes
a fuel gas injector arrangement including at least one injector
nozzle extending through the wall of the combustion chamber at a
location adjacent said combustion zone. Such injector nozzle is in
fluid communication with the combustion chamber. The injector
nozzle is positioned to direct a flow of fuel gas into said
combustion chamber at a location in the wall radially outward of
and beyond the inner edge of the entrance.
[0012] In yet another aspect of the invention, the novel burner may
include both the duct system for recirculated flue gas and the fuel
gas injector arrangement described above.
[0013] In its more specific aspects, the burner of the present
invention may include a first fuel gas nozzle that is located in
the venturi tube and which is positioned to introduce a supply of
fuel gas into the air flowing through the venturi tube. The burner
may also include a swirler positioned so that at least a primary
portion of the air flow passes therethrough. Ideally the
arrangement of the outlet end of the venturi tube and the swirler
may be such that a secondary portion of the air flow does not pass
through the swirler. Even more ideally, an annular gap may be
provided between the outer periphery at the outlet end of the
venturi tube and an inner edge of said entrance. Such gap may be
positioned to direct a tertiary air flow around the periphery of
the venturi tube and through the entrance into said combustion
chamber.
[0014] Preferably, at least one first fuel gas nozzle may be
positioned centrally of the venturi tube adjacent a longitudinal
axis thereof and at a location to introduce fuel gas into said
primary portion of the flow of air. At least one fuel gas poker
nozzle may also be included at a position to introduce fuel gas
into said secondary portion of the flow of air.
[0015] The burner of the invention may be equipped to burn either
fuel gas or oil.
[0016] The invention also provides a method for operating a venturi
tube equipped round burner of the sort described above. In
accordance with this aspect of the invention, the method comprises
directing a flow of air through said venturi tube and into a
combustion zone in said combustion chamber through said entrance
and recirculating flue gas from a location in said combustion
chamber adjacent said combustion zone and into the venturi tube at
a location adjacent the throat of the venturi tube, whereby said
recirculated flue gas is inducted into and intermixed with the
combustion air flow at the low pressure throat of the venturi
tube.
[0017] In another aspect of the invention, the method may comprise
directing a flow of air through the venturi tube and into a
combustion zone in a combustion chamber through an entrance in a
wall of the combustion chamber and injecting a flow of fuel gas
into said combustion chamber at a location radially outward and
beyond the inner edge of the entrance and adjacent to said
combustion zone. Furthermore, the novel method may include both the
recirculation of flue gas and external fuel gas injection as
described above.
[0018] In a more specific sense, the method may include a step of
introducing a first supply of fuel gas into said flow of air. The
method also may include a step of passing at least a primary
portion of said flow of air through a swirler. Even more
specifically, the method may be such that a secondary portion of
said flow of air does not pass through the swirler.
[0019] In another important preferred aspect of the invention, the
method may include a step of causing a tertiary air stream to flow
around the periphery of the venturi tube, through a gap provided
between the large end of the venturi tube and an inner edge of the
entrance to the combustion chamber, and on into the combustion
zone.
[0020] In another preferred aspect of the invention, the method for
operating a venturi equipped round burner may include a step of
introducing a first supply of fuel gas into the primary portion of
the flow of air, and introducing a second separate supply of fuel
gas into said secondary portion of the flow of air.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a sectional elevational view of an embodiment of a
combustion chamber burner of the invention and its associated
elements taken essentially along the vertical centerline of the
combustion chamber windbox;
[0022] FIG. 2 is a front end elevational view of the burner of FIG.
1;
[0023] FIG. 3 is a graph illustrating the number of scanner signals
and the air pressure drop data obtained in a combustion chamber
burner of the invention as the ratio of swirled air flow to
straight air flow is changed;
[0024] FIG. 4 is a graph illustrating the amount of the relative
available internal flue gas recirculation flow measured when the
ratio of primary and tertiary air flows is changed;
[0025] FIG. 5 is a graph illustrating the improved performance of
the burner of the invention in terms of achieving reduction of CO
and NO.sub.x emissions as the ratio between excess air factors in
the secondary and tertiary air flows is varied;
[0026] FIG. 6 is a graph illustrating the improved performance of
the burner of the invention in terms of achieving reduction of CO
and NO.sub.x emissions as the injector fuel gas flow rate is varied
with respect to the total fuel gas flow rate; and
[0027] FIG. 7 is a graph illustrating the improved performance of
the burner of the invention in terms of achieving reduction of
NO.sub.x emissions when internal flue gas is recirculated.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] A burner assembly which embodies the features, concepts and
principles of the invention is illustrated in FIG. 1 where it is
identified by the reference numeral 10. As is conventional and well
known to those of ordinary skill in the relevant art, the burner 10
may be surrounded by a windbox 12 which provides combustion air to
the burner at a pressure sufficient to cause it to flow into the
combustion zone 14 in a combustion chamber or firebox 16 through an
entrance 18 in a wall 20 of the combustion chamber 16. As is also
well known to those of ordinary skill in the art, an entrance, such
as the entrance 18, may preferably be in the form of a generally
circular opening which extends through the wall 20 of combustion
chamber 16.
[0029] The burner 10 is equipped with an elongated venturi tube 22
having an inlet end 25 that is spaced from entrance 18 and a outlet
end 26 that is positioned adjacent to and in alignment with
entrance 18. The venturi tube 22 also has a throat 24 disposed
between inlet end 25 and outlet end 26. As would be well known to
the routineer in the burner art, the venturi tube 22 may generally
be circular in cross-sectional configuration, and the outlet end 26
thereof should preferably and generally be larger in diameter than
either the inlet end 25 or the throat 24.
[0030] As illustrated in FIG. 1, outlet end 26 of venturi tube 22
is preferably positioned within and surrounded by entrance 18.
Additionally, the outer periphery 28 of outlet end 26 is smaller in
diameter than the annular inner edge surface 30 of entrance 18.
Thus, an annular gap 32 is presented between the outer periphery 28
of the outlet end 26 of the venturi tube 22 and the inner edge
surface 30. An annular shroud 33 is positioned within entrance 18
and is mounted on edge surface 30 so as to provide a mouth 35 for
the gap 32.
[0031] The burner assembly 10 is also provided with a swirler 34
which is positioned centrally within the outlet end 28 of the
venturi tube 22. As can be clearly seen in FIG. 1, the outer
diameter of the swirler 34 is smaller than the internal diameter of
the venturi tube 22 at the outlet end 28 of the latter. This
provides an annular space 36 which surrounds the swirler 34 within
the venturi tube 22.
[0032] The burner assembly 10 of the invention also may preferably
be provided with a conventional ignitor 38 and one or more central
fuel gas nozzles 40. Only a single nozzle is shown in FIG. 1;
however, one of ordinary skill in the burner art would understand
that the burner 10 may include a plurality of central fuel gas
nozzles spaced evenly around the longitudinal axis of the venturi
tube 22. The determinative factor in choosing the number of central
fuel gas nozzles to use is simply to make sure that the central or
primary gas flow is evenly distributed in the combustion air. The
nozzle or nozzles 40, as the case may be, provide fuel gas to the
air flowing through the center of the venturi tube 22. The burner
assembly 10 may also preferably be equipped with a conventional
steam operated fuel oil atomizer unit 42 so that the burner 10 is
adapted to burn fuel oil as well as gaseous fuels including natural
gas.
[0033] In accordance with the concepts and principles of the
invention, the burner assembly includes at least one fuel gas poker
44 for delivering fuel gas to the air traveling through the venturi
tube 22 on its way to the combustion zone 14. Although only a
single poker 44 is shown in FIG. 1, the burner assembly 10 may
preferably include three or more fuel gas pokers 44 spaced evenly
around the inside of the venturi tube 22. Conventionally the burner
may include six to eight pokers 44 as illustrated in FIG. 2;
however, if the invention of the '803 patent is employed, the
burner 10 may need only three pokers 44. The pokers 44 may each
include an elongated tube 45 and a nozzle 47, and the same may
conventionally be linked together by a fuel gas manifold 46 as
shown in FIG.2. The principal design consideration in selecting the
correct number of pokers for any given installation is that the
fuel gas be distributed evenly around the entire circumference of
the venturi tube 22.
[0034] Desirably burner assembly 10 of the invention may include
one or more ducts 48 for internal recirculating flue gas 49 from a
point within the combustion chamber 16 adjacent combustion zone 14
to the air flowing through venturi tube 22 at the low pressure zone
72 in throat 24 thereof. A single duct 48 is shown in FIG. 1 for
illustrative purposes. However, burner assembly 10 preferably may
include four ducts 48 spaced 90 degrees apart around the periphery
of the venturi tube 22 as best shown in FIG. 2. Again, the
principal design consideration in selecting the correct number of
ducts 48 for a given application is simply that the recirculated
flue gas be distributed evenly around the entire circumference of
the venturi tube. Ducts 48 may each be provided with an outlet 50
which is connected to the venturi tube at a point adjacent to the
low pressure zone 72 at the throat 24 of the venturi tube 22 so
that recirculated flue gas 49 is inducted into the venturi tube 22.
Each duct 48 also preferably has an inlet 52 which is in fluid
communication with the interior of the combustion chamber via an
opening 54 in wall 20. Thus, flue gas 49 from adjacent the
combustion zone 14 in chamber 16 may be inducted into the air
flowing through the venturi tube 22 and intermixed therewith at
throat 24.
[0035] As is illustrated in FIG. 1, the burner 10 of the invention
may also be provided with at least one external fuel gas injector
56. The injector 56 may preferably include an elongated tube 58 and
a nozzle 60. The nozzle 60 protrudes through an opening 62 which
extends through wall 20 such that the nozzle 60 is positioned in
outwardly spaced relationship relative to entrance 18. That is to
say, opening 62 is positioned outwardly beyond the inner edge
surface 30 of entrance 18 and therefore the nozzle 60 is positioned
to direct a flow of fuel gas into said combustion chamber 16 at a
location adjacent to and externally of the combustion air flowing
into combustion zone 14.
[0036] A single fuel gas injector 56 is shown in FIG. 1 for
illustrative purposes. However, as shown in FIG. 2, the burner
assembly 10 may preferably include four to eight fuel gas injectors
56 spaced 45 degrees apart around the periphery of the venturi tube
22. Again, the principal design consideration in selecting the
correct number of fuel gas injectors 56 for a given application is
that the fuel gas be distributed evenly around the entire periphery
of the combustion zone 14. The injectors 56 are provided with a
manifold 64 which distributes fuel gas thereto.
[0037] In operation, combustion air enters the burner 10 from
windbox 12 and is divided into three separate and distinct
portions. The flow path of primary air is designated by the arrow
66, the flow path of secondary air is designated by the arrow 68
and the flow path of tertiary air is designated by the arrow 70. As
dictated by the shape and size of the venturi tube 22, the shape
and configuration of the swirler 34 and the shape and size of the
entrance 18, primary air 66 moves to the center of the venturi tube
22 where it is mixed with fuel gas from the centrally located fuel
nozzle 40 and caused to flow through the swirler 34 which rotates
the primary air/central fuel gas mixture in a manner well known to
the routineer in the burner art. Thus, primary air 66 and central
fuel gas from nozzle 40 are thoroughly mixed and agitated as the
same are directed into the center core of the combustion zone
14.
[0038] Secondary air 68 moves in a generally straight line through
the venturi tube 22 and passes into the combustion zone. As the
secondary air 68 passes around the swirler 34, it is in the shape
of an annular envelope that surrounds the swirler 34 and the
swirled primary air 66. As can be seen viewing FIG. 1, the fuel gas
pokers 44 are positioned radially outwardly relative to the swirler
34 and such that the fuel gas from the poker nozzles 47 is
intermixed with the secondary air 68. Thus, straight line secondary
air 68 and the fuel gas from poker nozzles 47 are directed in a
straight line into the combustion zone 14 at a position which is
radially outward of the center of the latter.
[0039] Tertiary air 70 moves in a straight line around the
periphery of the venturi tube 22 and is guided by the mouth 35 so
that it passes through the gap 32 between the outlet end 26 of the
venturi tube 22 and the inner edge surface 30 of the entrance 18.
The tertiary air 70 is in the shape of an annulus which surrounds
the venturi tube 22 and the secondary air 68 as it is introduced
into the combustion zone 14.
[0040] Fuel gas from the injectors 56 is introduced into the
combustion chamber 16 at a position which is radially outward
relative to the center of the combustion zone 14 and to the
primary, secondary and tertiary air flows 66, 68 and 70.
[0041] Generally speaking, the outlet end of the venturi tube 22
may preferably be about 6 to about 40 inches in diameter. The shape
of the venturi tube 22 is not necessarily critical to the operation
of the burner 10. That is to say, the shape of the venturi tube is
in some measure dictated by the desired air flow rate
characteristics. However, it has been determined experimentally
that the venturi tube 22 may preferably be shaped such that the
ratio of the diameter of the throat 24 to the diameter of the
outlet end 26 may preferably be in the range of from about 1:1.2 to
about 1:1.6. It has also been determined experimentally that the
ratio of the total cross-sectional area of the annular gap 32 to
the total cross-sectional area of the outlet end 26 of the venturi
tube 22 may preferably, but not necessarily, be in the range of
from about 1:6 to about 1:8. It is also preferred, but not
necessarily required, that the swirler 34 be positioned at a
distance from the outlet end 26 which is within the range of from
about 0.4 to about 0.6 times the internal diameter of outlet end
26.
[0042] The difference between the forward velocity of the swirled
primary air stream 66 and the forward velocity of the straight line
secondary air stream 68 is associated with the physical design of
the burner. Conceptually, all of the primary air stream 66 passes
through the swirler 34. On the other hand, the secondary stream 68
passes around the swirler 34 and theoretically none of it passes
through the swirler 34. Clearly none of the tertiary air flow 70
passes through the swirler 34. The swirler 34 imposes a degree of
aerodynamic resistance on the primary stream 66 passing
therethrough. Thus, the velocities of the straight line streams 68
and 70 are greater than the velocity of the primary stream 66. As
can be seen from FIG. 3, when the ratio of swirled primary air flow
to straight line air flow (secondary+tertiary) is greater than
about 0.2, air resistance increase rapidly. On the other hand, when
the ratio of swirled primary air flow to straight line air flow is
less than about 0.08, flame stability problems occur. From these
parameters, the preferred relative air flow velocities may be
determined. Thus, in actual operation, it is preferred that the
ratio of the forward velocity of the primary swirled air stream 66
to the forward velocities of the straight line air streams 68 and
70 should be in the range of from about 1:1.1 to about 1:1.5.
[0043] As set forth above, the preferred lower limit of the
tertiary air flow velocity is about 1.1 times the primary air
velocity. In accordance with FIG. 4, an increase in the velocity of
the tertiary air velocity is accompanied by a decrease in the
amount of recirculated flue gas 49 which can be induced into the
combustion air by the venturi effect at low pressure zone 72 in
venturi tube 22. There is a comparatively small influence on the
amount of flue gas recirculated by induction when the ratio of the
velocities of the tertiary and primary air streams is 1.5 or less.
However, when this ratio exceeds 1.5, the recirculated flue gas
rate drops off quickly. This phenomena also supports the preference
for a primary air velocity to tertiary air velocity ratio of 1.5 or
less. In accordance with the invention, the recirculated internal
flue gas rate should preferably be within the range of from about
4% to about 8%, inclusive, based on the total amount of combustion
air supplied to the burner. The effectiveness of such recirculation
is apparent from FIG. 7.
[0044] The center core of the burner flame is located in the
central part of the combustion zone 14. This part of the flame,
which is fed primarily by the primary air flow and the fuel from
the central fuel nozzles 40, is responsible for stability and
vibration of the entire flame. In addition, the core of the flame
plays a role as a flame pilot whenever the heat load is reduced to
a minimum. It is well known to the routineer in the burner art that
the most stable flame occurs when the conditions in the burner are
stoichiometric. From a practical viewpoint, however, flames are
sufficiently stable whenever the amount of air is at least 70% of
the amount that is theoretically sufficient to burn all of the fuel
and no greater than 110% of such amount. Thus, the fuel/air ratio
in the primary air stream should be maintained such that the
available oxygen ranges from about 70% to about 110% of theoretical
at the time the primary air stream enters the combustion zone.
[0045] As can be seen from FIG. 5, however, there is an effective
reduction in emitted NO.sub.x without a corresponding increase in
emitted CO when the ratio of the excess air factor in the secondary
stream 68 to the excess air factor in the tertiary air stream 70 is
in the range of from about 1.3:1 to about 2.7:1. When this ratio is
less than about 1.3:1, NO.sub.x reduction is negligible. When this
ratio is above about 2.7, CO emission becomes unacceptable. Coupled
with the foregoing information, one must take into consideration
the fact that the state of the art knowledge is that the local
excess air factor should preferably never be more than 2.0 to
prevent local cooling of the flame, and should preferably never be
less than about 0.7 to avoid the unacceptable concentrations of
incompletely combusted products in the flue gas. Based on these
considerations, and in accordance with the concepts and principles
of the present invention, it has been determined that the excess
air factor provided by the primary stream 66 should preferably be
in the range of from about 0.7 to about 1.1, that the excess air
factor provided by the secondary stream 68 should preferably be in
the range of from about 0.7 to about 2, and that the excess air
factor provided by the tertiary stream 70 should preferably be in
the range of from about 0.5 to about 0.7.
[0046] With reference to the foregoing considerations the preferred
relative primary fuel gas flow can be determined. Thus, the primary
fuel gas flow is a multiplication product of the relative primary
air flow and the primary excess air factor, which is
(0.08-0.20).times.(0.7-1.1)=(0.05- 6-0.22). It is known that in
order to avoid stability and vibration problems when the heat load
is reduced, such reduction should be accompanied by an increase in
the proportion of the fuel gas fed to the core of the flame.
Usually, under full load conditions, the amount of fuel fed to the
core of the flame should be about 6% of the total fuel flow rate.
Tests have shown that the amount of fuel gas fed to the center of
the flame should be increased at a rate which is about the fourth
degree root of the burner turndown. Thus, to accommodate a standard
turndown of 12.5:1, the fuel fed to the core of the flame should
amount to 6.sup.-4.times.12.5=19.6% of the total fuel rate. So the
amount of the total fuel in the primary air stream 66 should
preferably range from about 6% to about 19%. These numbers are
comparatively close to the numbers calculated above.
[0047] With reference to FIG. 6, it can be seen that a desirable
degree of NO.sub.x reduction is achieved without an unacceptable
increase in CO emissions when the ratio of the fuel gas rate from
the injector nozzles 60 ranges from about 65% to about 85% of the
total fuel rate. Thus, under full load, the secondary fuel gas flow
from the poker nozzles 47 should preferably range from about 9% to
about 29% of the total fuel gas flow. Under partial loads, the
secondary fuel gas flow from the poker nozzles 47 should preferably
be a little less than about 5% of the total fuel gas flow. So the
overall secondary fuel gas flow rate from the poker nozzles 47
should preferably range from about 5% to about 29% of the total
fuel gas flow.
[0048] In sum, and in accordance with the concepts and principles
of the present invention, it has been determined that the flow rate
of the primary fuel gas from nozzles 40 should preferably be in the
range of from about 6% to about 19% of the total fuel supplied to
the burner, that the flow rate of the secondary fuel fed from poker
nozzles 47 should preferably be in the range of from about 5% to
about 29% of the total fuel supplied to the burner, and that the
flow rate of the tertiary fuel supplied from nozzles 60 should
preferably be in the range of from about 52% to about 89% of the
total fuel supplied to the burner.
[0049] It has also been determined in accordance with the
principles and concepts of the invention, that the ratio of
recirculated internal flue gas 49 to total combustion air flow (66,
68 and 70) should preferably be in the range of from about 0.04:1
to about 0.08:1. This factor is determined by a balance between
flame stability and emission reduction and is controlled by the
various flow rates of the combustion air as discussed above.
* * * * *