U.S. patent application number 12/203419 was filed with the patent office on 2009-03-12 for burner pilot with virtual spinner.
This patent application is currently assigned to Coen Company. Invention is credited to Pierre Begin, Vladimir Lifshits, Stephen B. Londerville.
Application Number | 20090068601 12/203419 |
Document ID | / |
Family ID | 40429311 |
Filed Date | 2009-03-12 |
United States Patent
Application |
20090068601 |
Kind Code |
A1 |
Londerville; Stephen B. ; et
al. |
March 12, 2009 |
Burner Pilot With Virtual Spinner
Abstract
A method and apparatus for preheating a furnace during a warm-up
phase of furnace operation. The furnace has main burners with a
tubular fuel supply surrounded by a main combustion air duct
defining an annular space between the supply and the duct that
extends in an axial direction of the main burner. A pilot nozzle in
the annular space extends in an axial direction of the burner
towards an interior of the furnace and discharges readily ignitable
fluid fuel jets through orifices in the nozzle toward the interior
of the furnace. Combustion air from the duct is directed past the
nozzle and is mixed with the fuel discharged from the orifices to
form an ignitable mixture that is ignited to form the furnace
heating pilot flame downstream of the nozzle. The flame is
stabilized and anchored to the pilot nozzle by recirculating
portions of the flame and its constituents from the furnace
interior back towards the nozzle by protecting the air passing
through the primary ignition zone from being directly affected by
air flowing through the main combustion air conduit, diverging the
fuel jets relative to the axial direction by an angle between about
20.degree. to 80.degree., and giving the fuel jets a tangential
directional component relative to the axial direction to spin the
flame about the axis of the pilot.
Inventors: |
Londerville; Stephen B.;
(Half Moon Bay, CA) ; Lifshits; Vladimir; (Redwood
City, CA) ; Begin; Pierre; (Sainte-Therese,
CA) |
Correspondence
Address: |
TOWNSEND AND TOWNSEND AND CREW, LLP
TWO EMBARCADERO CENTER, EIGHTH FLOOR
SAN FRANCISCO
CA
94111-3834
US
|
Assignee: |
Coen Company
Foster City
CA
|
Family ID: |
40429311 |
Appl. No.: |
12/203419 |
Filed: |
September 3, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60967915 |
Sep 6, 2007 |
|
|
|
Current U.S.
Class: |
431/6 ; 431/12;
431/187; 431/284 |
Current CPC
Class: |
F23D 2900/00014
20130101; F23D 1/00 20130101; F23D 2207/00 20130101 |
Class at
Publication: |
431/6 ; 431/187;
431/12; 431/284 |
International
Class: |
F23Q 9/00 20060101
F23Q009/00; F23D 14/48 20060101 F23D014/48; F23D 14/22 20060101
F23D014/22 |
Claims
1. A method of anchoring a high heat output pilot flame to a pilot
nozzle associated with a coal burner for industrial furnaces,
comprising flowing combustion air along an exterior of a coal
supply conduit having a downstream end for discharge of the air
into a furnace interior, placing a pilot nozzle proximate to the
downstream end of the coal supply conduit, delivering fuel gas to
the nozzle through a pipe placed in the combustion air flow,
shielding the pipe delivering fuel to the pilot nozzle in the part
adjacent to the nozzle with a tubular hood that is open in a
downstream direction, while permitting a limited amount of the
combustion air to flow into inside the hood, directing a plurality
of pilot fuel jets from the pilot nozzle into the furnace interior,
discharging at least one igniter fuel jet into an interior of the
hood, aspirating air into the hood with the fuel gas jets from at
least one of the pilot nozzle or the igniter jet in a quantity to
create a flammable mixture inside the hood, igniting the flammable
mixture inside the hood to generate an igniter flame that extends
past the open downstream end of the hood, and orienting the pilot
fuel jets so that they diverge relative to a longitudinal axis of
the pilot nozzle and so that the pilot fuel jets have a tangential
component relative to the pilot nozzle axis for spinning the fuel
about the pilot axis and recirculating a portion of the mixture of
fuel with air from the location downstream of the nozzle back to
the pilot nozzle.
2. A method of anchoring a high heat output pilot flame to a pilot
nozzle associated with a coal burner for industrial furnaces, the
coal burner including a coal supply conduit extending in an axial
direction into an interior of the furnace, the method comprising
flowing combustion air along an exterior of the coal supply conduit
for discharge into the furnace interior, shielding the pilot burner
from the combustion air flowing along the exterior of the supply
conduit while permitting a limited, controlled amount of air from
the combustion air to flow past the pilot nozzle by placing a
tubular hood over the pilot nozzle which is open in a downstream
direction, discharging at least one high velocity igniter fuel jet
into an interior of the hood to thereby lower a pressure inside the
hood, using the lower pressure inside the hood to aspirate an
amount of air from the combustion air into the hood which depends
on the lowered pressure inside the hood, igniting fuel from the
igniter jets and air inside the hood to generate an igniter flame
that extends past the open downstream end of the hood, directing a
plurality of pilot fuel jets from a downstream end portion of the
pilot nozzle into the furnace interior, and orienting the pilot
fuel jets so that they diverge relative to a longitudinal axis of
the pilot nozzle and so that the pilot fuel jets are tangential
relative to the pilot nozzle axis for spinning the fuel about the
pilot axis and recirculating a portion of the pilot fuel from the
pilot fuel jets in an upstream direction toward the pilot
nozzle.
3. A method for preheating a furnace during a warm-up phase of
furnace operation, the furnace including at least one main burner
having a tubular fuel supply surrounded by a main combustion air
duct defining an annular space between the fuel supply and the duct
that extends in an axial direction of the main burner, the method
comprising positioning a pilot nozzle in the annular space so that
the nozzle generally extends in an axial direction of the burner
towards an interior of the furnace, discharging readily ignitable
fuel jets through orifices in the nozzle oriented toward the
interior of the furnace, directing air from the annular space past
the nozzle and mixing the air with the fuel discharged from the
orifices to form an ignitable mixture, igniting the mixture to form
a flame downstream of the nozzle, stabilizing the flame and
recirculating portions of the flame and/or the mixture from the
furnace interior back towards the nozzle by protecting the air
directed past the nozzle from being directly affected by air
flowing through the main combustion air conduit, and diverging the
fuel jets relative to the axial direction between about 20.degree.
to 80.degree. and giving the fuel jets a tangential directional
component relative to the axial direction.
4. A method according to claim 3 including positioning the nozzle
outside the furnace interior.
5. A method according to claim 3 wherein the angle is between about
20.degree. and 80.degree..
6. A method according to claim 3 wherein a heat output of the fuel
discharged through the orifices is between about 4 and 50 million
BTU per hour.
7. A method according to claim 6 wherein the nozzle comprises a
pilot burner, and including limiting a maximum width of the pilot
burner transverse to the axial direction to no more than about five
inches.
8. A method according to claim 3 wherein the fluid fuel comprises a
gas.
9. A method according to claim 3 wherein protecting the air
comprises placing a tubular hood having an open downstream end
about the nozzle, and inhibiting the flow of air from the
combustion air duct into the hood with an air flow restrictor
positioned proximate an upstream end of the hood.
10. A method of generating a high BTU output pilot flame during a
warm-up phase of operation of a furnace having a main production
burner that includes a first conduit for directing coal into an
interior of a furnace and a combustion air duct surrounding the
conduit defining an annular combustion air passage into the furnace
for mixing the coal with combustion air and ignition of the coal,
the method comprising placing a pilot nozzle in the combustion air
passage so that a downstream of the nozzle is proximate a
downstream end of the burner, surrounding the nozzle with a tubular
hood having an open downstream end proximate the downstream end of
the nozzle and an upstream end, preventing combustion air flowing
through the combustion air passage from directly entering the hood
through the upstream end thereof while maintaining flow
communication between the combustion air passage and an inside of
the hood via the upstream end thereof, flowing a pressurized fluid
fuel through igniter orifices in the nozzle located inside the
tubular hood at a sufficient rate to lower a pressure inside the
tubular hood to draw combustion air from the combustion air passage
via the upstream end of the hood into the hood, discharging a major
portion of the fluid fuel from a plurality of pilot orifices in a
downstream end portion of the nozzle, orienting fuel from the
plurality of pilot orifices so that fuel jets emitted therefrom
angularly diverge in a downstream direction toward the furnace
interior and have a tangential flow direction relative to a
longitudinal axis of the pilot nozzle, generating an igniter flame
that propagates past the downstream end of the hood by igniting the
fuel emitted by the igniter orifices inside the tubular hood, and
igniting a mixture of fuel from the pilot orifices and combustion
air from the combustion air conduit downstream of the main
production burner to generate a pilot flame that extends into the
furnace interior for heating the furnace interior while portions of
the pilot flame and its constituent gases recirculate from the
furnace interior rearwardly towards the downstream end of the
nozzle while simultaneously spinning relative to the nozzle axis
for maintaining a stable pilot flame.
11. A method according to claim 10 wherein the pilot flame
generates a heat output between 5 to 50 million BTU per hour.
12. A method according to claim 10 wherein preventing combustion
air from flowing directly into the hood comprises placing a flow
restrictor proximate to and spaced apart from the upstream end of
the hood to define a gap between the flow restrictor and the
upstream end of the gap through which the combustion air enters the
hood.
13. A method according to claim 12 wherein the flow restrictor is a
plate, and varying a width of the gap by moving the plate relative
to the upstream end of the hood.
14. A method according to claim 10 wherein the nozzle includes a
fuel supply tube, and wherein the igniter orifices are formed in
the fuel supply tube.
15. Apparatus for preheating a furnace during a warm-up phase of
operation and prior to a production phase of operation of the
furnace comprising at least one main burner adapted to be extended
in an axial direction through a wall of the furnace including a
production fuel conduit for directing a production fuel into a
furnace interior during the production phase of the furnace, a
combustion air duct surrounding the production fuel conduit for
flowing combustion air along an annular passage past the main
burner into the furnace interior, an elongated pilot fuel nozzle
positioned in the air duct arranged substantially parallel to the
conduit and the duct and having a transverse extent slightly less
than a width of the annular passage, the nozzle including at least
one igniter orifice located upstream of a downstream end of the
nozzle and a plurality of pilot fuel orifices located proximate the
downstream end of the nozzle which angularly diverge in a
downstream direction relative to an axis of the nozzle and which
are tangentially positioned relative to the nozzle axis, a tubular
hood disposed in the annular passage and having an open downstream
end proximate the downstream end of the nozzle, a flow inhibitor
positioned proximate an upstream end of the tubular hood for
preventing combustion air from flowing from the annular passage
directly into the hood, and an igniter located inside the tubular
hood and proximate the igniter orifices for igniting the fuel
emitted by the igniter orifices and generating an igniter flame
inside the tubular hood which extends in a downstream direction
past the tubular hood for igniting a mixture of fuel emitted by the
pilot orifices and combustion air from the annular passage
downstream of the tubular hood, whereby the mixture generates a
pilot flame downstream of the hood and portions of the pilot flame
recirculate from the downstream part of the flame back to the
nozzle and spin relative to a longitudinal axis of the nozzle.
16. Apparatus according to claim 15 wherein the flow inhibitor
comprises a plate extending transversely across and axially spaced
from the upstream end of the hood to form a gap between the plate
and the upstream end of the hood through which air must flow in
order to enter the interior of the hood.
17. Apparatus according to claim 15 wherein the hood has an
interior cross-section, and wherein the nozzle has a lesser
cross-section than the hood and is positioned adjacent a wall of
the hood to define an enlarged space inside the hood where the
igniter flame is generated.
18. Apparatus according to claim 15 wherein the elongated fuel
nozzle includes a fuel supply tube, and wherein the igniter
orifices are formed in the fuel supply tube.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application claims the priority of Provisional Patent
Application No. 60/967,915 filed on Sep. 6, 2007, the disclosure of
which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to warming up large
utility-type furnaces, such as are used, for example, to generate
steam for major electrical power generating plants, particularly
but not limited to coal-fired plants, prior to the start-up of such
furnaces to commence their production phase of operation.
[0003] Such large furnaces can have the size of a large building,
and they often employ dozens of spaced-apart burners to provide the
needed heat for generating large amounts of electrical power. These
furnaces are fired with all types of fuels, including, but not
limited to, oil, gas, coal, bio-mass, etc. The furnaces require an
initial heat-up to bring their interior to the required operating
temperature at which all burners can be fired fully without causing
flameouts, generating large amounts of smoke and other pollutants,
potentially damaging portions of the furnace due to excessive heat
differentials, and the like.
[0004] Operators of such furnaces desire that the warm-up period is
as short as possible because during warm-up phase, expensive fuel
is consumed without generating any power. In the past, so-called
warm-up guns were preferred over smaller pilot burners, especially
for coal-fired furnaces, to generate sufficient amounts of heat
over a relatively short period of time so that the production phase
of the furnace can commence as soon as possible.
[0005] While natural gas and oil can be quickly ignited and do not
require long warm-up periods, coal-fired furnaces encounter the
problem of having to heat the furnace sufficiently so that the
large mass of coal consumed by the furnace during its production
phase can be ignited and will burn cleanly and completely without
emitting excessive pollutants. Conventional burner pilot lights
that are also used for the main burner ignition and flame
stabilization either have too low a heat output to accomplish the
required warm-up over a desired period of time, or require an
additional air supply besides the air passing through the main
burners, which makes their installation more complicated and
expensive.
[0006] It is not normally feasible to increase the size of the
pilot burners to enhance their heat output because pilots are
arranged in the relatively small annular passage between the main
burner and the surrounding combustion air duct. This limits the
size of pilots, for most industrial installations, to no more than
about four to five inches in diameter. With such size limitations,
the maximum heat output of pilots not using additional air supply
is typically limited to around 3 to 7 million BTUs per hour.
[0007] Increasing the fuel flow rate through the pilots beyond that
range results first in unstable operation sensitive to the regime
of air flow through the main burner. The operation becomes
unsatisfactory because the resulting high velocity fuel jets can
snuff out the pilot flame.
[0008] Further, the high capacity pilots need to be located at the
main burner discharge end to generate the flame in the furnace
interior and prevent the flame from burning the main burner. At the
same time the pilots need to be protected from the main burner
flame. In order to meet both these requirements, such installations
often require complicated mechanisms to subsequently retract the
pilot rearwardly out of the heat and away from fuel particles into
the combustion air duct, which are costly, require much
maintenance, and are subject to early failures.
[0009] Separate warm-up guns not tasked with the main burner
ignition were therefore widely employed for warming up furnaces.
Although such guns are capable of generating large amounts of heat
and, therefore, can significantly shorten the warm-up period for
even coal-fired utility-type furnaces, they require their own
combustion air supplies as well as relatively complicated
installations including their own piping, fans, motors, controls,
gun retracting mechanism and the like, all of which make separate
warm-up guns expensive to install and maintain.
BRIEF SUMMARY OF THE INVENTION
[0010] The present invention improves the manner in which large
furnaces, such as are used for commercial power generation, and
particularly coal-fired furnaces, are preheated during the initial
start-up phase of furnace operation when the interior furnace
temperature must be raised sufficiently to commence the production
firing of the furnace. This is accomplished in accordance with the
present invention by placing a pilot burner (hereafter typically
"pilot") with a much higher heat output in the same limited annular
passage between the fuel, e.g. coal supply conduit, and the
surrounding combustion air supply duct, where prior art burners
have been commonly placed in the past. The pilot of the present
invention is operated with combustion air for the main burner and
eliminates the need for a separate air supply for the pilot. The
pilot flame is ignited and stabilized by injecting a portion of
gaseous fuel delivered to the pilot in a spinning pattern that
creates intense recirculation and mixing of the discharged pilot
fuel with appropriate amounts of air passing through the main
burner around the pilot.
[0011] Such pilots can provide a heat output in the range between
about 4 to 50 million BTU per hour, which is much higher than the
heat output that could be achieved with prior art pilots operating
without the additional air, and assures a rapid heat-up of the
furnace and a relative quick start-up of its production phase.
Substantial amounts of fuel otherwise used by the pilot without
producing useable steam or electricity are thereby saved.
[0012] The present invention provides both a method and an
apparatus for preheating furnaces, particularly large utility-type
furnaces that have many burners which often operate with
difficult-to-ignite coal during the warm-up phase of furnace
operation. Generally speaking, this involves a main production
burner that includes a first conduit for directing a fuel, for
example coal, into an interior of a furnace. An air duct surrounds
the coal conduit to define an annular combustion air passage into
the furnace where the coal and combustion air are mixed and ignited
during the production phase of furnace operation.
[0013] A pilot nozzle is positioned in the air passage of the main
burner so that a downstream end of the nozzle is proximate the
downstream end of the burner. The nozzle is surrounded by a tubular
hood which has an open downstream end proximate the downstream end
of the nozzle and an upstream end. Air flowing through the main
burner passage for air is prevented from directly entering the hood
by placing a flow inhibitor, such as a plate, over the upstream end
of the hood while permitting air to enter the hood via a gap formed
between the hood and the plate.
[0014] A relatively lesser portion of a pressurized fluid fuel,
e.g. a gas, is flowed through igniter orifices in the nozzle
located inside the tubular hood at a rate commensurate with the
amount that can be burned within the limited space inside the
tubular hood. The fuel gas jets inside the hood are directed at
angles that facilitate entrainment of air through the hood. A major
portion of the pilot gas is discharged from main pilot orifices--a
plurality of spaced-apart pilot orifices in a downstream end
portion of the nozzle immediately adjacent to the hood. The fuel
jets from the main pilot orifices are oriented so that the emitted
fuel jets angularly diverge in the downstream direction and have a
tangential flow component relative to the longitudinal axis of the
pilot nozzle. As fuel jets discharging from the main pilot orifices
pass in the vicinity of the hood downstream end, they also
facilitate the flow of air through the hood.
[0015] Fuel emitted by the igniter orifices inside the tubular hood
is mixed with air passing through the hood and is ignited to
generate an igniter flame that propagates past the downstream end
of the hood. The igniter flame in turn ignites the mixture of fuel
from the main pilot orifices and air passing through the main
burner to generate a pilot flame that extends into and heats the
furnace interior. Portions of the pilot flame and its constituent
gases recirculate from the furnace interior rearwardly towards the
nozzle while the flame as a whole spins relative to the nozzle axis
to maintain a stable pilot flame.
[0016] One reason why placing high heat output pilot burners inside
the combustion air duct of the main burner has heretofore been
unsuccessful was that the volume of air flowing through the duct
may vary substantially so that the pilot fuel flowing with a fixed
rate often fails to ignite, or to maintain the flame, due to
unfavorable fuel-to-air ratios, unless the pilot has its own air
supply and controls. This is overcome by the present invention
because the amount of air entering the hood is substantially
proportional to the fuel delivered inside the hood through the
igniter orifices and only to a small degree affected by the amount
of flow through the duct as its upstream flow inhibitor effectively
shields the pilot fuel from the effect of the high velocity air
flowing through the duct. The hood forms a small combustion chamber
where a relatively minor amount of the pilot fuel is initially
ignited to form the igniter flame which propagates in a downstream
direction past the downstream end of the hood, where the major
portion of the pilot fuel is discharged via the appropriately
positioned and oriented pilot orifices.
[0017] The hood, including the earlier mentioned flow diverter,
also effectively shields sensitive components like the spark
electrode inside the hood from the heat of the main pilot flame and
the furnace, which allows operating the burner without having to
retract the pilot into the burner.
[0018] In addition, to maintain a pilot flame, it must be stable
and remain anchored to the nozzle. High heat output pilots require
high fuel velocities through the burner orifices of as much as
500-1500 ft./sec. Such high fuel jet velocities lead to undesirable
flame instabilities which are significantly reduced or entirely
eliminated in accordance with the present invention by imparting a
spin to the pilot flame downstream of the nozzle that facilitates
establishing a recirculating flow downstream of the nozzle. To
attain such a spin, the axes of the pilot orifices are tangentially
offset relative to the pilot axis as described in more detail
below. The tangential flow component of the jets provides the
spinning results obtained with common prior art burners by placing
relatively large spinners around the nozzles that cannot be applied
here due to the earlier mentioned space limitations.
[0019] Another important advantage of the present invention is that
the amount of air entering the interior of the hood automatically
adjusts itself to the amount of fuel emitted by the igniter
orifices inside the hood because as the volume of emitted fuel
varies, its speed varies correspondingly, which in turn lowers or
raises the fuel pressure inside the hood inversely to the velocity
of the fuel emitted from the igniter orifices. With the lowered
pressure, more air from the air duct is aspirated into the hood
interior so that an approximate stoichiometric balance between the
fuel and the air in the hood is maintained. This assures an
uninterrupted igniter flame to maintain the main pilot flame even
in the event of a temporary flameout. The amount of air drawn into
the hood is correspondingly lowered as less fuel is emitted from
the igniter orifices of the nozzle and the pressure inside the hood
rises correspondingly.
[0020] Thus, the pilot burner of the present invention is
relatively inexpensive because it has no moving parts and needs no
internal or external controls.
[0021] A further advantage attained with the present invention is
that the pilot burner is shielded from the high temperature and
abrasive/corrosive/contaminating influences of the gases, dust and
particles on the furnace interior because the pilot is located
inside the air duct, which reduces maintenance costs and prolongs
the life of the burner. Still further, since the pilot burner of
the present invention requires no external controls, separate air
supply lines and the like, it can be made relatively larger in the
limited space available in the air ducts of industrial burners.
This in turn makes it possible to increase the heat output of the
burner and thereby shorten the warm-up period for the furnace, all
of which reduces operating costs for the furnace warm-up and pilot
burner maintenance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 schematically illustrates a large, e.g. utility-type,
furnace arrangement for driving a steam turbine as used in large
electric power generating plants;
[0023] FIG. 2 is a schematic, cross-sectional view through a
burner, including a high heat-output pilot constructed in
accordance with the present invention for installation in the
furnace shown in FIG. 1;
[0024] FIGS. 3A and 3B are sectional views of the pilot of the
present invention;
[0025] FIG. 4 is a schematic front elevational view of the main
burner and the pilot shown in FIG. 1;
[0026] FIG. 5 schematically illustrates the formation of a pilot
flame recirculation zone in accordance with the present
invention;
[0027] FIG. 6 is an end view of an air flow restrictor plate of the
pilot shown in FIGS. 3A and 3B;
[0028] FIG. 7 is an end view of an air flow straightener that
prevents combustion air from flowing directly into a hood
surrounding the pilot; and
[0029] FIGS. 8A and 8B are end and side elevational views,
respectively, of the nozzle of the pilot shown in FIGS. 3A and
3B.
DETAILED DESCRIPTION OF THE INVENTION
[0030] FIG. 1 schematically illustrates a large power generation
installation, as is commonly used, for example, by public utility
companies for generating electricity for the public. The
installation has at least one large, utility-type furnace 2 and
many, typically dozens, of main production burners 4 which extend
through at least one wall 6 of the furnace into its interior 8.
Such furnaces can be and are fired with all kinds of fuels, with
oil, coal and natural gas being the most common. The present
invention has particular (but not sole) applicability to firing the
furnaces with coal which is typically ground to fine powder or
dust. As is well known, the heat generated by the fuel on the
interior of the furnace generates steam 10 that can be used to
drive a turbine 12 which may be connected, for example, to an
electric generator (not shown). Exhaust gas from the furnace is
released to the atmosphere through a stack 14, typically (but in
many of the areas of the world not necessarily) after having been
appropriately cleaned and/or scrubbed to limit atmospheric
pollution.
[0031] FIG. 2 schematically shows the use of the present invention
with a main production burner 4 mounted on and operatively
extending through one of the furnace walls 6 and constructed to
burn coal, typically finely ground or pulverized coal. It has a
coal supply source 18 and a coal supply conduit 20 in which
powdered, pulverized or the like coal flows in a downstream
direction to a discharge end 22, which may include a spinner or
diverter 24 for discharging the coal via an outwardly flared burner
throat 26 in furnace wall 6 into the interior 8 of the furnace.
Main burner 4 further has a combustion air supply duct 32 which
concentrically surrounds coal supply conduit 20 to form an annular
combustion air passage 34 between the coal supply conduit and
combustion air duct. During operation, combustion air needed to
burn the coal (or other fuel) is discharged from the downstream end
22 of the burner into the furnace interior. The main burner may
include a supplemental fuel supply tube 28 which runs coaxially
through (the horizontal portion of) the main burner and has a fuel
discharge end cap 30 that can be used to provide additional heat
from firing oil or gas, for example during peak demand periods for
electricity when more heat output is needed.
[0032] The construction and operation of such main burners is well
known to those of ordinary skill in the art and, therefore, is not
further described herein.
[0033] Burner installation 4 includes a pilot burner 36 constructed
in accordance with the present invention to initiate combustion in
the furnace interior and, during a start-up phase of operation of
the furnace, to warm up the furnace interior until main burners 16
can be fired after the furnace interior has reached the required
temperature for maintaining a stable and complete combustion of the
coal (or other fuel). The pilot has a feed tube 38 through which a
fluid fuel, such as natural gas for example, is supplied from an
appropriate source (not shown) to a pilot nozzle 40. The nozzle is
surrounded by a tubular shield or hood 42, the ends of which are
open, and an igniter, e.g. an electrical spark igniter 44, is
provided for igniting the fuel, as is further described below.
[0034] FIGS. 3A, B and 4 show the pilot burner of the present
invention in greater detail. Nozzle 40 includes and is attached to
a downstream end of feed tube 38, has a discharge (or downstream)
end 50, and has a plurality of pilot fuel discharge orifices 52
from which pilot fuel jets flow. The pilot fuel jets are discharged
at an oblique angle relative to the longitudinal axis of the pilot
burner, and they are additionally tangential to the axis of the
pilot as is further described below.
[0035] Tubular hood 42 has open upstream and downstream ends 66,
68, respectively. A flow straightener and conditioner 70 (shown
also in FIG. 7) is positioned inside the upstream end of the hood
and extends some distance into the hood. A fuel feed tube 38 and an
igniter support pipe 76, respectively, extend into the hood 42. The
flow straightener includes a plurality of ribs 80 placed between
the hood 42 and the fuel feed pipe 38 parallel to the igniter
burner axis 96. The ribs define multiple flow straightening
passages 82 that extend in an axial direction of the pilot. Air
flowing between the ribs 80 becomes better oriented in the axial
direction of the pilot, a feature which is particularly useful in
instances when air flowing through the passage 34 is at an oblique
angle relative to the pilot axis.
[0036] Pilot 36 is further fitted with a damper plate 84 (also
shown in FIG. 6) which is spaced apart from the upstream end 66 of
hood 42. The damper includes a tubular hub 86 that surrounds pilot
fuel feed tube 38 and is slidably movable therealong. Opposite hub
86 is a U-shaped cutout 88 through which igniter support pipe 76
extends.
[0037] The axial position of damper plate 84 relative to the
upstream end of the hood can be adjusted by moving the plate along
fuel supply tube 38 of the pilot burner to vary the width of a gap
90 between the upstream end of the hood and the damper plate to
accommodate specific characteristics of the fuel and provide a
range of air flows through the burner 32.
[0038] The downstream end of igniter support pipe 76 ends at a
bluff body 92 (FIGS. 3A, B) attached to the inside of the tubular
hood 42. An electronic igniter 94 is placed inside the support tube
76 end about flush with the bluff body 92. On the side facing the
flow, the bluff body 92 is shaped with a slope 93 that eliminates
stagnation areas to the flow upstream of the igniter 94. Suitable
hardware and wiring (not shown) for the electronic igniter extends
through the igniter pipe 76 to an igniter control (not shown).
[0039] In a presently preferred embodiment of the invention, pilot
nozzle 40 is configured as a cap attached to the downstream end of
fuel feed tube 38 and has a multiplicity of fuel discharge orifices
52 arranged in a plurality of, e.g. two, rows 52A, 52B that are
spaced apart in the axial direction of the nozzle, as illustrated
in FIG. 3B. Each orifice diverges in a downstream direction
relative to the pilot burner axis 96 by an angle .alpha. (shown in
FIG. 8B) in a range between about 20.degree. to 80.degree.,
preferably in a range between about 35.degree. to 75.degree. and in
the presently preferred embodiment at an angle of about
60.degree..
[0040] In addition, each orifice 52 is arranged so that its center
line 98 is offset relative to a radius line 100 with its origin at
the center 96 of the nozzle so that each orifice is also tangential
relative to this center, as is illustrated in FIG. 8A. This causes
the fuel flow and flame in the wake of the nozzle 50 to spin in a
manner analogous to a conventional spinner and anchors the flame to
the pilot in spite of the high velocity fuel jets emitted from the
orifices.
[0041] In a presently preferred embodiment, the pilot nozzle 40
additionally includes relatively small-diameter center holes 102.
In use, gas flows through the center holes which cools the nozzle
center.
[0042] Referring to FIG. 4, pilot nozzle 40 and igniter 44 are
offset relative to the axis of tubular hood 42 so that the pilot
nozzle is adjacent one side of hood 42, to thereby define an
enlarged space 104 between the periphery of the pilot nozzle and
the opposite wall of the hood where an initial igniter flame is
generated, as is further described below. Arrows 106 in FIG. 4
illustrate the tangential positioning and orientation of fuel jets
53 (shown in FIG. 5).
[0043] Turning to the operation of pilot 36 for starting up a cool
furnace, combustion air flows through annular passage 34 of burner
32 in a downstream direction past tubular hood 42 and then into the
furnace interior 8. The gas for the pilot is flowed through feed
tube 38 to orifices 46 and pilot nozzle 40. Sizing of the orifices
46 is such that a relatively minor portion of the fuel exits
through igniter orifices 46 in the feed tube 38 which are oriented
to direct resulting fuel jets into the enlarged space 104 inside
the hood and in the vicinity of igniter 44. At the same time, air
from annular passage 34 of the main burner enters the interior of
hood 42 via gap 90 between the upstream end of the hood and damper
plate 84. Flow straightener 70 straightens out the incoming air so
that it flows generally in the direction of the pilot axis and
becomes mixed with fuel from igniter orifices 46. The resulting
mixture is ignited by spark igniter 94 to form an igniter flame 47
in the enlarged space 104 which propagates in a downstream
direction past downstream end 68 of the hood, as is illustrated in
FIG. 5.
[0044] The bulk of the fuel for preheating the furnace is ejected
through orifices 52 in nozzle 40 as gas jets 53 which diverge
outwardly in the downstream direction so that the ejected fuel
becomes mixed with combustion air that flows through the annular
passage 34 of the main burner. This mixture is ignited by the
igniter flame 47 exiting from the downstream end of the hood which
maintains the main pilot flame 54.
[0045] The amount of combustion air typically flowing through the
annular passage 34 depends on the operational needs of the regime
and is substantially independent of the pilot burner operation. The
rate at which fuel is needed for the pilot also may be changed for
operational reasons. To maintain the igniter flame 47, the amount
of air fed to the burner must reflect the amount of fuel ejected by
the igniter orifices to maintain an overall flammable mixture
inside the hood 42 on the downstream part of bluff body 92.
[0046] To properly control the flow of air into hood 42, damper
plate 84 blocks combustion air flowing through annular passage 34
directly into the hood. Instead, combustion air must first flow
from the annular passage in a radial direction (relative to hood
42) through gap 90 and is then redirected past flow straightener 70
into the interior of the hood, thus minimizing the effects of air
flow velocity through the passage 34 onto the amount of air flow
entering the hood 42. The axial position of damper plate 84
relative to the upstream hood end can be adjusted by moving the
plate, including its flange 86, along feed tube 38 to set the
proper width for gap 90 to permit a sufficient air flow into the
hood while preventing variations in the combustion air flow in the
annular passage from materially affecting the air flow rate through
the hood.
[0047] In use, the position of the damper plate is not normally
changed. The air intake via gap 90 into the hood is nevertheless
automatically varied as a function of the gas flow rate through
igniter orifices 46 because as the gas velocity through the igniter
orifices increases or decreases, the pressure inside the hood
changes inversely to the pressure changes. An increase in the gas
velocity through the igniter orifices lowers the pressure in the
hood, which causes an increase in the air flow rate through gap 90
into the hood and vice versa. This air flow variation occurs
automatically and requires no controls of any type.
[0048] Accordingly, the pilot burner of the present invention is
self-regulating and maintains the igniter and pilot flames 47, 54
regardless of changes in the combustion air flow rate while
stabilizing the pilot flame 54 and anchoring it to the end of the
pilot burner. This assures a continuing, uninterrupted,
self-regulating operation of the pilot burner to fully heat up the
furnace as quickly as possible.
[0049] It is typically preferred to maintain the igniter flame 47
inside hood 42 for the duration of the pilot burner operation so
that in the event the main flame generated by the pilot becomes
extinguished, it is immediately reignited by the pilot flame.
[0050] FIG. 5 schematically illustrates the main pilot flame 54
generated downstream of the pilot burner 36 and its interaction
with pilot flame 47 extending from downstream end 68 of the hood.
As was earlier described, fuel jets 53 emanating from orifices 52
of pilot nozzle 40 are directed outwardly and away from pilot axis
96 into the furnace interior. To achieve the required heat input,
the gaseous fuel jets 53 have velocities which typically range
between 500 to 1500 ft./sec. These high velocities also help mix
fuel jets with sufficient air to efficiently burn large quantities
of fuel gas delivered through the pilot.
[0051] In order to assure reliable flame propagation from the flame
47 through the high velocity fuel jets 53, flammable mixtures in
substantial parts of the flow immediately adjacent to the nozzle 40
have to be achieved and maintained over the duration necessary to
ignite the fuel. This is accomplished by placing orifices 52 about
the circumference of the nozzle 40 in two or more staggered rows
axially spaced from each other and by the tangential positioning of
the orifices spinning off fuel emitted from pilot orifices 52. In
each row, the orifices are typically spaced by about one to three
times the diameter of the orifices. In a presently preferred
embodiment, the spacing between the orifices is approximately twice
the nozzle diameter.
[0052] Propagation of the flame through gas jets 53 is not
sufficient for the flame 54 stabilization. Flow recirculation 58
enhanced by the spinning of fuel emitted from pilot orifices 52
caused by the tangential positioning of the orifices makes the
pilot operation efficient and reliable.
[0053] As is well known to those skilled in the art, a tangential
component imparted to fuel jets to form a forward-directed spiral
motion facilitates the formation of gaseous recirculation patterns.
The greater the spiral effect, the better the recirculation. The
recirculation component of the gas is a function of the so-called
"swirl number" S according to the following formula:
S = axial flux of angular momentum axial thrust .times. R
##EQU00001## [0054] wherein the axial thrust is the axial force
exerted by the combustion air and gas flows entering the
recirculation zone, [0055] R is the radial distance (from the
center of the pilot nozzle) of pilot orifices 52, and the angular
momentum is the rotational force at R generated by the gas jets
53.
[0056] For certain fuels, such as oil, for example, pilot nozzle 40
can extend past the downstream end of main burner 4 into burner
throat 26. However, for coal-fired burners, the pilot is recessed
into the annular space 34 between coal supply conduit 20 and
combustion air conduit 32 to keep the pilot away from the heat,
smoke, dust, particulates and the like that are typically present
on the interior of coal-fired furnaces, but which are kept out of
annular passage 34 and therefore also away from the pilot nozzle by
the flow of combustion air.
[0057] The combustion of fuel from pilot 36 is continued until the
furnace interior has reached the desired temperature, at which time
the production fuel, e.g. coal, can be ignited and stably combusted
without generating large amounts of pollutants as would occur if
combustion were commenced before the required furnace temperature
has been reached.
* * * * *