Note: Descriptions are shown in the official language in which they were submitted.
~23~
PULVERIZED FUEL BURNER NOZZLE TIP
AND SPLITTER PLATE THEREFOR
Background of the Invention
The present invention relates to improving the low load
operation of fuel burners for use in pulverized coal-fired
furnaces and, more particularly, to improv~ng low load
operat~on of fuel-air admission assemblies for directing a
pulverized fuel-air mixture into the furnace by what is known
as the tangential method of firing.
In view of today's fluctuating electricity demand,
typified by peak demand occurring dur1ng weekday daytime hours
and minimum demand occurring at night and on the weekends,
electric utilities have chosen to cycle many of their
conventional coal-fired steam generator boilers by operating
them at full load dur~ng peak demand hours and reducing them to
low loads during per~ods of minimum demand.
As a consequence of th;s mode of operation, the
electric ut~lities have used large quantitites of natural gas
or oil to furnish add~tional ign~tion energy dur~ng low load
operation because the current generat~on from coal f~red steam
generator furnaces requ1re stabllizatlon of the coal flames
when operating at low loads. The required amount of auxiliary
fuel fired for stabilization purposes is s~gn~f~cant and, for
example, to maintain a 500 megawatt coal-f~red steam generator
at 10 to 15 percent load during min~mum demand per~ods could
require the use of 11,000 gallons of oil per day.
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One common method of firing a pulverized fuel such as
coal in a conven~ional steam generator furnace is known as
tangential firing. In this method, pulverized coal ~s
introduced to the furnace in a primary air stream through
burners, termed fuel-a;r admission assemblies, located in the
corners of the furnace. The fuel-air streams discharged from
these assemblies are aimed tan~entially to an ima~inary circle
in the middle of the furnace. This creates a fireball which
serves as a continuous source of ignition for the incoming
coal. Each fuel-air admission assembly is comprised of a fuel
delivery pipe through which pulverized fuel entrained in air
passes to the furnace, a secondary air conduit surrounding the
fuel delivery pipe through which additional air is introduced
into the furnace, and a nozzle tip which is pivotally mounted
to the outlet end of the fuel delivery pipe.
A typical nozzle tip comprises inner and outer shells
disposed coaxially in spaced relationship thereby defining a
first flow passageway within the inner shell through which the
pulverized fuel and air mixture discharging from the fuel
del~very pipe passes into the furnace and a second flow
passageway in the annular space between the inner and outer
shells through which the secondary air discharging from the
secondary air conduit passes into the furnace. Typically, one
or more splitter plates are disposed wlthin the inner shell
parallel to the axis of the nozzle tip to divide the flow
passageway within the inner shell into multiple subpassages.
The nozzle tip may be tilted upward or downward ln order to
direct the fuel-alr m~xture, discharging into the furnace from
the fuel del~very p~pe upwardly or downwardly as a means
controll1ng the tempera~ure of the superheated steam produced
~n heat exchange surface typ~cally d~sposed at the outlet of
the surface ~n the manner taught by U.S. Patent 2,363,875.
Dur~ng normal operation of a tangentially fired
furnace, a flame is establ~shed at one corner which in turn
suppl~es the requ~red lgnition energy to stabilize the flame
emanating from the corner downstream of and laterally ad~acent
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to it. When load is reduced, the flames emanating from each
corner become shorter and, as a consequence, a reduction in the
amount of ignition energy available to the downstream corner
occurs. As a result, auxiliary fuel such as oil or natural gas
must be introduced in each corner adjacent to the pulverized
coal-air stream to provide additional ignition energy thereby
insuring that a flameout and resultant unit trip will not occur.
Another problem associated with operating a coal-fired
burner at low loads results in the fact that the pulverizing
mills typically operate with a relatively constant air flow
over all load ranges. When furnace load is reduced, the amount
of coal pulverized in the mills decreases proportionally while
the amount sf primary air used to convey the pulverized coal
from the mills through the admission assemblies into the
furnace remains fairly constant, thereby causing the fuel-air
ratio to decrease. When the load on the furnace is reduced to
the low levels desired during minimum demand periods, the fuel-
air ratio has decreased to the point where the pulverized coal-
primary alr mixture has become too fuel lean for ignition to
stabillze w~thout significant supplemental ignition energy
being made available.
One way in which the need for auxiliary fuel firing
during low load operation on coal-fired boilers can be reduced
is presented in U.S. Patent 4,252,069. Thls patent discloses
an improved fuel-alr admission assembly incorporating a split
coal bucket which permits a pulverized coal-~ired furnace to be
operated at low loads without use of auxiliary fuel to provide
stabllizatlon. As dlsclosed thereln, the spllt coal bucket
comprlses lndependent upper and lower coal nozzles plvotally
mounted to the coal dellvery plpe, the upper and lower coal
nozzles be~ng independently tiltable. When the furnace is
operating at low loads such as durlng the minlmum demand
per~ods, the primary alr and pulverlzed coal stream dlschar~ing
from the coal dellvery pipe ~s split into an upper and a lower
coal-air stream and independently directed into the furnace by
tllt~ng the upper coal nozzle upward and the lower coal nozzle
downward. In doing so, an ignition stabilizing pocket is
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established in the locally low pressure zone created in the
void between the spread apart coal-air streams. Hot combustion
products are drawn, i.e., recirculated into this low pressure
zone, thus providing enough additional ignition energy to the
incoming fuel to stabilize the ,lame.
An additional nozzle tip designed to improve ignition
stability, albeit directed to the ignition of low volatile
coal rather than ignition at low load operation, is presented
in U.S. Patent 2,608,168. Disclosed therein is a coal bucket
p~votally mounted to the coal delivery pipe with the flow
passageway defined within the inner shell bifurcated into two
parallel but spaced apart flow subpassages. Secondary air is
discharged into the furnace from the secondary air conduit
surrounding the coal delivery pipe through the flow passageway
between the inner and outer shells and through the central
channel formed between the parallel but spaced apart
subpassages formed within the inner shell. Ignition is said to
be improved by increasing the contact area between the coal-air
mixture discharged from the spaced flow passages of the inner
shell and the bounding secondary air streams.
Despite the aforementioned nozzle tip designs, there
stfll ex~sts a need for a nozzle tip of a relatively simple
design which inherently provides improved ignition stability at
low load operation. There also ex~sts a need for such a nozzle
tip which is readlly manufactured by fabrication and/or casting.
Summary of the Invention
The present ~nvention provides a novel tip for a burner
on a pulverized fuel fire~ furnace whlch ~s partlcularly
adapted to provlde Improved Ignttlon stab111ty durlng low load
operat.lon of the furnace. The nozzle tip of the present
Invention comprises an open-ended inner shell definlng a flow
passageway through which a mixture of pulverized fuel and
transport air passes from the burner into the furnace, an open-
ended outer shell spaced from and surround~ng the inner shell
thereby definin~ an annular flow passage therebetween through
which additional air for combustion passes from the burner into
the furnace, and plate means disposed within the inner shell
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for dividing the flow passageway therethrough into first and
second flow passages which extend from the inlet of the inner
shell to the outlet of the inner shell in a diverging manner
with a void region established therebetween through wh~ch flow
is precluded. The coal-air mixture discharging from the burner
is split by the plate means into a first stream which is
directed into the furnace through the first flow passageway
through the inner shell and a second stream which is directed
into the furnace through the second flow passageway of the
inner shell. Thus, the coal-air mixture is directed into the
furnace in two diverging streams. In doing so, an ignition
stabilizing pocket is established in the locally low pressure
zone created between the spread-apart and diverging coal-air
streams in the furnace just downstream of the void region
established between the diverging first and second flow
passageways through the inner shell of the noz le tip. Coal is
concentrated in this pocket and hot combustion products are
drawn back into the pocket from the flame to provide additional
ignition energy to the incoming fuel to stabllize the flame.
Preferably, the plate means comprises first and second
splitter plates disposed within the inner shell with their
leading edge portion disposed transversely across the flow
passageway of the inner shell at the inlet thereof and their
trailing edge portion extending transversely across the flow
passageway of the inner shell at the outlet end thereof. The
first and second splitter plates converge along a line at the
inlet end of the inner shell and extend outwardly therefrom in
a diverging manner toward the outlet end of the lnner shell.
In this manner, the first and second spl~tter plates divlde the
3n flow passageway through the ~nner shell into a first flow
passage bounded by the first splitter plate and the inner shell
and a second flow passage bounded by the second splitter plate
and the inner shell. The first and second flow passages
diverge ~n the direction of flow through the nozzle tip and are
separated by a void region established between the first and
second divergent splitter plates through which flow is
precluded. Accordingly, a low pressure recirculation zone will
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be established in the furnace just downstream of the void
region of the nozzle tip between the diverging fuel-air streams
as they discharge into the furnace from the divergent first and
second flow passages through the inner shell.
Further in accordance with the present invention,
ignition stabi1ity may be further enhanced by providing
splitter plates having their trailing edge portion scalloped.
The trailing edge portion of a splitter plate is preferably
scalloped by forming the trailing edge portion of the
I0 splitter plate of a plurality of longitudinally elongated
strips which extend longitudinally outward from the leading
edge portion of the splitter plate in side-by-side relationship
transversely across the flow passageway through the inner
shell. A first portion of the trailing edge strips, ~isposed
lS alternately between a second portion of the trailing edge
strips, is bent radially away from the leading edge of the
splitter plate in one direction while the second portion of the
tra~llng edge strips is bent radlally away from the leading
edge portion of the splitter plate in a direction opposite to
that in wh~ch the first portion of the trailing edge strips are
bent away from the leading edge portion of the splitter plate.
In this manner, a scalloped edge is provided along the trailing
edge portion of the splitter plates which serves to generate
turbulence along the boundries between the fuel-air streams
discharging from the divergent flow passages and the void
region establlshed therebetween whereby the mixing of
pulverized fuel and hot combust~on products drawn into the low
pressure recirculatlon zone formed in the furnace ,iust
downstream of the void region of the nozzle ~ip thereby further
stabiliz~ng ignition.
Brief Descript~on of the Drawings
Figure 1 is a diagrammatic plan view of a furnace
employing the tangential firing method;
Figure 2 is a elevational cross-sectional view, taken
along line 2-2 of Figure 1, showing a set of three coal-air
admission assemblies, the upper coal-air admission assemblies
having a nozzle tip designed in accordance with the present
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invention and the lower two coal-air admission assemblies
equipped with a nozzle tip typical of the prior art;
Figure 3 is a elevational cross-sectional view of a
single coal-air admission asse~bly equipped with a nozzle tip
designed in accordance with the present invention;
Figure 4 shows an elevational cross-sectional view of a
nozzle tip of the present invention,
Figure 5 shows an elevational end view taken along line
5-5 of Figure 4 of the nozzle tip of the present invention; and
Figure 6 is an eleva~ional end view of an alternate
embodiment of the nozzle tip of the present invention.
Description of the Preferred Embodiment
While the present invention may be applied, in spirit
and in scope, to a number of different burner designs employed
in the various firing methods commonly used in conventional
pulverized fuel-fired steam yenerator boiler furnaces, it may
be best described when em~odied on a pulverized coal-air
admission assembly of the type employed in pulver~zed coal
fired furnaces utllizing the tangential firing method
illustrated in Figure 1. In the tangential firing method,
pulverized coal and air are introduced into the furnace through
coal-air admission assemblies 10 mounted in the four corners of
the furnace 1. The coal-air admission assemblies 10 are
oriented so as to dellver the pulverized coal and alr streams
tangentially to an imaginary circle 3 in the center of the
furnace 1 so as to form therein a rotating vortex-like flame
termed a fire ball.
As shown in Figure 2, a plurallty of coal-air admlsslon
assemblles 10 are arran~ed ln the corners of the furnace ln a
vertlcal column separated by auxlllary alr compartments 20 and
20'. One or more of thes auxiliary air compartments, such as
compartment 20', ~s adapted to accommodate an oll or gas burner
22, which ls used when starting and warmlng up the bo~ler and
which, ln the prior art, is used when necessary to prov~de
addit~onal ignition ener~y to stabllize the coal flame when
operating the furnace at low loads.
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Each coal-air admission assembly 10 comprises a coal
delivery pipe 12 extending therethrough and opening into the
furnace, and a secondary air conduit 14 which surrounds coal
delivery pipe 12 and opens into an air supply plenum 18, termed
S a windbox. Pulverized co~l entrained in transport air is
discharged into the furnace through the coal delivery pipes 12
from a supply source such as a pulverizer wherein the coal is
dried and comminuted. Secondary air is passed into the furnace
through the secondary air conduits 14 as a stream surrounding
the pulverized coal and transport air stream discharged from
each coal-delivery pipe 12. Additional combustion air is
passed into the furnace from windbox 18 through the auxiliary
air compartments 20.
Each coal delivery pipe 12 is provided with a nozzle
tip, often referred to as a coal bucket, which is pivotally
mounted to the coal delivery pipe 12 so that the nozzle tip may
be tllted about an axis 16 transverse to the longitudinal axis
of the coal dellvery pipe 12 in order to direct the pulverized
coal and alr mixture into the furnace at either an upward angle
or a downward angle as a means of controlling the position of
the fire ball within the furnace whereby the temperature of the
superheat steam leaving the steam generator, not shown, Is
controlled in the manner taught by U.S. Patent 2,363,875 issued
November 2~, 1944 to Krelsinger et al for "Combustlon Zone
Control". Nozzle tips 28, shown in Figure 2, are typical of
the standard prior art nozzle tip commonly mounted to the coal
delivery pipe 12.
The typ~cal pr~or art nozzle t~p 28 ls comprlsed of a
open-ended ~nner shell def~nlng therethrough a flow passageway
3n through wh~ch the mixture of pulverlzed coal and transport air
passes from the coal delivery pipe 12 into the furnace
surrounded by an open-ended outer shell spaced therefrom so as
to deflne an annular flow passage therebetween through which
secondary air passes from the secondary air conduit 14 into the
furnace. The inner and outer shell are adapted to be mounted
to the outlet end of the coal delivery pipe 12 by means of a
pivot pin so as to be tiltable about axis 16. Typically, one
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or more baffle plates 26 are disposed within the inner shell of
the prior art nozzle tip 28 along an axis parallel to the
nozzle tip and the coal delivery pipe 12 so as to form two or
more parallel flow passages within the inner shell through
which the pulverized coal and air passes from the coal delivery
pipe 12 into the furnace as a single strea~ subdivided into one
or more parallel and contiguous substreams. As indicated
earlier, when a furnace equipped with the prior art nozzle tips
28 was operated at low load, ignition became unstable and
supplemental fuel, such a natural gas or oil, had to be fired
in order to provide sufficient additional ignition energy to
stabilize the ignition of the single coal-air streams
discharging from nozzle tips 28.
In accordance with the present invention, stable
ignition at low loads is insured by providing a nozzle tip 30
which inherently provides improved ignition stability during
low load operation. Nozzle tip 30 comprises an open-ended
inner she11 32, an open-ended outer shell 34 spaced from and
surrounding the inner shell 32, and plate means 40 disposed
within the inner shell for dividing the interior of the inner
shell into first and second flow passageway. The inner shell
32 has an outlet end 36 opening into the furnace and an inlet
end 38 adapted to be mounted about the outlet end of the coal
delivery pipe 12 so as to receive the pulverized coai and a1r
d~scharging therefrom. An annular flow passageway 50 is
defined between the 1nner shell 32 and the outer shell 34
through which additional combustion air passes from the
secondary air condu~t 14 into the furnace. In accordance wlth
the present invention, plate means 4n 1s d1sposed wlthin the
inner shell 32 for dlvld~ng the flow passage therethrough Into
f1rst and second flow passages 52 and 54, respectively,
extend~ng from the inlet end 3~ of the inner shell 32 to the
outlet end 36 thereof In a diverging manner with a void region
56 established therebe~ween through which flow is precluded.
The nozzle tip accomplishes the deslred objective of improving
ignition stabllity at low load operation by providing two
separate and distinct diverging flow passages 52 and 54 through
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the inner shell 32 which are spaced to lie above and below a
central void 56 through which flow is precluded. As is evident
from the drawingg the stream of pulverized fuel and transport
air discharging from the coal delivery pipe 12 into the nozzle
tip 30 will be split into two portions. One portion would pass
into the furnace through the first f~ow passa~e 52 of the
nozzle tip 30 to be dlscharged upwardly into the furnace while
the second portion of the pulverized coal and transport air
stream would pass into the furnace through the second flow
passage 54 of the nozzle tip 30 to discharge downwardly into
the furnace as best seen in Figure 3. A low pressure zone 80,
which serves as an lgnition stabilizing region, will he created
in the furnace at the outle~ of the nozzle tip 30 downstream of
the void region 56 between the diverging coal-air streams 60
and 70. Coal particles from the streams 60 and 70 will be
drawn into the low pressure zone 80 from the diverging coal-air
streams 60 and 70. Ignition will be stabilized because a
portion of the hot combustion products formed during the
ignltion process are recirculated within the low pressure
lgnltion stabillzing zone 80, thereby providing sufficient
ignition energy for igniting coal partlcles which are
subsequently drawn into the zone 80 from the diverging coal-air
streams 60 and 70.
In the preferred embodiment of the present invention,
the plate means 40 comprises first and second splitter plates
41 and 42 disposed within the inner shell 32 so as to divide
the ~nterlor of the inner shell 32 into a first flow passage 52
bounded by the f~rst spl1tter plate 41 and the lnner shell 32
and a second flow passage 54 bo~nded by the second splltter
plate 42 and the lnner shell 32. Each of the splltter plates
41 and 42 has a lead~ng edge portlon 43 dlsposed transversely
across the flow passage of the lnner shell 32 at the lnlet end
38 thereof and a trailing edge portlon 44 extending
transversely across the flow passage o~ the inner shell 32 a~
the outlet end 36 thereof. The first and second split.ter
plates 41 and 42 converge along the line at the lnlet end 38 of
the inner shell 32 and extend outwardly therefrom in a
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diverging manner, preferably at an included angle of
approximately 20, toward the outlet end 36 of the inner shell
32 and defined therebetween a void region 56 throu~h which flow
is precluded.
Achievement of the obJective of improving ignition
stability may be further enhanced by providing that the
trailing edge portion 44 of the first and second splitter
plates 41 and 42 is scalloped as best, seen in Figures 4, 5 and
6. To scallop the trailing edge portion 44 of each of the
first and second splitter plates 41 and 42, the t~railing edge
portion 44 thereof comprises a plurality of longitudinally
elongated strips ext,ending longitudinally outward from the
leading edge portion 43 of the splitter plates in side-by-side
relationship transversely across the flow passageway of the
inner shell 32. A first portion 45 of the trailing edge strips
extend~ng longitudinally outward from the leadin~ edge portion
of the first and second splitter plates is disposed alternately
across the inner shell 32 between a second portion 47 of the
trailing ed~e strips and are bent radially away from the second
portion 47 of the trailing edge strips thereby form7ng the
desired scalloped trailing edge on the splitter plates 41 and
42. Preferably, the first portion 45 of the trailing edge
strips are bent radially away from the leading edge portion 43
of each splitter plate in one direct~on while the second
portion 47 of the trailing edge strips is bent, radially away
from the leading edge portion 43 of each of the splitter plates
~n the direction opposite ~,o that in which the f~rst portion 45
are bent.
By providing a scalloped tra~ling edge port~on on 0ach
of the splitter plates 41 and ~2, a turbulent zone ls
established along the ~nterface between each of the coal-air
streams 60 and 70 in the low pressure rec~rculation zone 80
formed therebetween. Such a turbulent interface insures that
coal and alr will be drawn out of the coal-air streams and
mlxed thoroughly w~th hot ignition products in the low pressure
recirculation zone 80 t~hereby further enhancing ignition
stability.
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It is also preferable to provide filler plates 4~ which
extend transversely between adjacent first and second portions
4~ and 47 of the trailing ed~e strips along the interface
between the trailing edge strips, as best seen in Figures 5 and
6, to preclude the flow of pulverized fuel and transport air
across the interface formed between adjacent diverging leading
edge strips 45 ann 47. If a significant amount of pulverized
fuel and transport air were allowed to pass into the void
region 56 through the divergent trailing ed~e strips 45 and 47,
the establishment of a low pressure recirculation zone between
the diverging coal-air streams 60 and 70 could be adversely
affected. Additionally, the splitter plates 41 and 42 may be
arranged within the inner shell 32 of the nozzle tip 30 so that
the scalloped trailing edge portions thereof are disposed in an
in-line arrangement as shown in Fi~ure 5 or a sta~gered
arrangement as shown in Figure 6.
Although the splitter plates 41 and 42 are shown in the
drawing as being fabricated of various p~eces of plate metal
welded together, it is to be understood that the splitter
plates 41 and 42 may also be readily manufactured by well-known
casting processes. Additionally, it is to be a'ppreciated that
the lifetime of the splitter plates with~n the coal flow
passage through the inner shell 32 may be enhanced in
accordance with the teachings of U.S. Patent 4,356,975 issued
November 2, 1982 to Chadshay for "Nozzle Tip for Pulverized
Coal Burner" by manufacturing the splitter plates 41 and 42
with their leading edge portion 43 formed of a relatlvely
abrasion reslstant material such as sllicon carblde or Ni-hard,
and their tralllng edge portion ~ formed of a materlal
relatively resistant to high temperatures such as certain well-
known stainless steels.
While the preferred embodiment of the present invention
has been ~llustrated and described when Incorporated into a
coal-air admission assembly of the type typically employed on a
tangent~ally-fired furnace, it is to be understood that the
invention should not be limited thereto. The nozzle tip of the
present invention could be readily modified by those skilled in
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the art to be applied within the spirit and scope of the
present invention to any number of burner configurations
wherein pulverized coal or other abrasive pulverized solids are
to be combusted.
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