Note: Descriptions are shown in the official language in which they were submitted.
Back~xound o~ -the Invention
(1) Field o the Invention
The present invention relates to burners designed for
the combustion of pulverized coal and particularly to such
burners as may be utilized in coal-Eired boilers of steam
generators. More speci~ically, t~is invention is directed to
a method for ~gniting pulverized coal in the absence of a
substantial energy input derived from the combustion of a
liquid or gaseous fuel. Accordingly, the general objects of the
present invention are to provide novel and improved apparatus
and method~ of such character.
(2~ Descr~ption of the Prior Art
Because of cost and availability, it is becoming
increasingly desirable to utilize coal rather than natural gas
or oil in electricity generating facilities. However, present
day coal-fired steam generator boilers of the types employed by
electric utilities nevertheless require substantial quantities
of natural gas or oil. Restated, in ordex to insure safe and
ef~icient operation, present coal-fired steam generators use
premium liquid and gaseous fuels to provide both ignition and
low-load ~lame-stabilizing energy. The required amount of these
auxiliary premium ~uels is signi~icant. For example, the use of
70,aoo. gallons of oil in a 500 megawatt unit for one start-up
is not uncommon. AccordinglY, a need exists for a means of
reducing the amount o~ auxiliary fuels needed in pulverized-
coal boilers.
_ mmary of the Invention
The present invention overcomes the above-discussed
and other deficiencies and disadvantages of the prior art by
providing for the direct ignition o~ a stream o~ pulverized coal
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and air suppli~d to a buxner.
In accordance with one broad aspect, the invention
resides in a method of caus~ng ignition and sustaining combustion
of pulverized coal in a comBustion zone comprising the steps of:
introducing into the com~ustion zone a fuel stream consisting
essentially o~ a mixture of coal particles and air, the fuel
stream h~ving an air-to-coal weight ratio below approximately
Q.5; causing a recixculation zone to occur in the combustion
zone; operating an ignitor in the recirculation zone in order
lQ to ignite the coal; and establish.ing a flow of secondary air
coaxial with the fuel stream subsequent to ignition of the coal,
the secondary aix supplying oxygen to support combustion.
In the interest of insuring ignition, in addition to
exercising control over the air~to-coal weight ratio upstream
of the combusti.on cham~er, the velocity of the coal/air mixture
will be varied in accordance wi,th the volatiles content and
grind of the coal; the. coal~air mixture in~ecti.on velocity being
less th,an 150 ft.~sec. and preferably in the range of 60-75 '~,
ft./sec. In some cases, the initiation of the secondary air
2~ flow which. causes the recirculation of the hot combustion
products- back to the coal~air ïnjecti.~n poïnt will'be delayed; :'
this.bei.ng particularly true in the case of hi.gh energy arc
~gnition. Furthe'x in accordance with the invention, the
secondar~ ai.r contributes as little as 15~ of the stoichiometric
combusti,on air. ~.:
In another broad aspect, the inYention resi.des in
apparatus for cauaing ignition and sustaining combustion of
pulveri2ed coal in a combustion zone comprising: means for
providing a fuel stream consi.sting essentially of a mixture
of coal and air having an air-to-coal weight ratio ~elow . :: .
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approximately ~.S; means fo~ delivering -the fuel stream into a
combustion zone, said delivering means produc.ing in khe
combustion zone a fuel stream having an axial re~ion comprised
of air and relatively ~ine coal particles; means for inter-
mittently providing ignit~on energy in the co~bustion zone, said
ignition energy provi~ding means compri~i.ng an electric ignition
energy source having a spark gap positioned in said combustion
zone in said fuel stream axial region; means for generating a
flo~ of secondar~ air in the combustion zone and generally
1~ coaxi,al with the fueI stream; and means for sensing the presence
of flame i.n the com~ust~on'zone ~hereby the ~lo~ of secondary
air may ~e delayed unti.l aubsequent to the. sensi.ng o~ flame.
~rief Desc:r:iption of the Dra~ing
The present invention may ~e better understood and
its numerous objects and advantages will become apparent to
those skilled in the art by reference to the accompanying drawing
wherein like reference numerals refer to like elements in the
several figures and in whi.ch:
FIGURE 1 i~ a cross-sectional view of an arc-ignited
pulverized coal burner whi`.ch may be employed in th.e practice of
the present invention;
FIGURE 2 is a cross-sectional vi.ew of a pulverized
coal feed system whi:ch may be associated with the burner of
FIGURE l;
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FIGURE 3 is a front elevation view oF the feeder system of
FIGURE 2;
FIGURE 4 is a cross-sectional view of a main coal burner which
employs the burner of FIGURE 1 as an igniter; and
FIGURE 5 is a diagramatic representation of a coal supply system
for the igniter and burner of FIGURE 4.
Description of the Preferred Embodiment
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The apparatus shown in the accompanying drawing constitutes a
representative means for accomplishing the direct igni~ion of a stream of
pulverized coal and air in accordance with the invention and without any
significant consumption of petroleum or natural gas. The present invention
relies on having a dense phase coal/air mixture wherein the transport-air
stream-to-coal weight ratio, measured in a delivery conduit upstream of a
combustion zone, is 1.0 or less. The ignition energy source is positioned ~;~
so as to be either in or insertable in the flowing air/coal stream in the
combustion zone. The energy delivered to the air/coal mixture by the
ignition energy source ignites the coal particles. Considering the case
where the ignition energy source comprises a high energy electric arc, the ;~
ignitor thus being operated in a pulsed mode, a series of flame pockets are
created.
The technique of the present invention also contemplates the
establishment of secondary air flow to the combustion zone through burner ~ ~'
secondary-air registers. The burner secondary-air registers are designed,
in a manner known in the art, so as to establish a region of recirculatin~q~
air and combustion products (hot gases) whereby the pockets of burning coal
are recirculated back toward the point of in;tial coal injection and the
energy in the recirculation region will increase until th flame becomes
self-sustaining.
The technique of the present inven~ion has been successfully
practiced employing a high energy electric arc as the ignition energy source.
However, the ignition energy source may also be a resistance heater or
heaters or a hydrocarbon fueled pilot torch with minimal energy consump-
tion. In a preferred embodiment, the ignitor will be removed fro~ the
flame region once the existence of flame has been verified. When a high
energy arc supplies the ignition energy, secondary air flow will be delayed
until the existence of flame has been verified.
With reference to FIGURE 1, a burner in accordance with a first
embodiment of the present invention is shown. A coal pipe 16 is employed
to convey coal pneumatically to the ignition zone in the burner. Accord-
ingly, as the apparatus is shown in FIGURE 1, the left end o~ coal pipe16 is in communication with the coal feeder of FIGURES 2 and 3 while the
right end of coal pipe 16 terminates at a hollow-cone diffuser which is
mounted from coa1 pipe 16 by means of supports 21. An ignitor will be
positioned immediately downstream o~ the discharge end of coal pipe 16.
In the disclosed embodiment of -the invention the ignitor, which is indicated
at 23, enters through the side of the burner and comprises a high-energy
arc ignitor similar to the type presently used for igniting oil. It is
to be noted that any ignition source which imparts sufficient energy to
heat the reactants enough to ignite them may be used. Accordingly, a
resistance heater or small pilot torch fueled by natural gas could be
employed in place of the high energy arc ignitor. The high energy arc
ignitor is, however, preferred because of its rel;ability and controlla-
bility. Ignitor 23, as shown in FIGURE 1, w;ll typ;cally be retractably
mounted so that it can be removed from the combustion zone into a protective
area after the coal has been ignited.
The burner also includes a secondary air supply conduit 20
which is coaxial with coal pipe 16. Conduit 20 communicates with an air
chamber 14 which will typically be cylindrical chamber somewhat larger in
diameter than that of conduit 20. Air chamber 14 contains a plurality of ~-
vanes 12. Vanes 12 are arranged to impart a swirl to air entering conduit
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20 from chanlber 14~ An air inlet d~lct 10 leads to air chamber 14 from
a pressurized air supply, not shown. Air con~ui-t 20 termirlates in a
refractory lined quarrel 24 which de~ines a divergent nozzle. In one
reduction to practice of the invention coal pipe 16 had a one inch inner
diameter, conduit 20 had a six inch inner diameter and quarrel 24 had a
thirteen inch diameter at its open end and an angle of divergence of 35.
FIGURES 2 and 3, which wi11 discussed simultaneously, show a
pulverized-coal feed system for supplying a coal-air mixture to the coal
pipe or fuel conduit 16. The feed system consists of a pulverized coal
hopper 40 that can be supplied by any of a number oF means known in the
art. Preferably, hopper 40 should be sized to store sufficient pulverized
coal to supply the burner throughout the warm-up period of the furnace in
which the burner is to be used. Hopper 40 communicates with gravimeteric
feeder 43. Feeder 43 consists of a variable-speed feed device 42, a
weight-sensitive conveyor 44 and appropriate control circuitry, not shown.
The speed of rotation of variable-speed feeder 42 determines the amount of
coal allowed to drop onto weight-sensitive conveyor 44, and the weight
sensed by weight-sensitive conveyor 44 controls the speed of this rotation.
Gravimetric feeder 43 introduces coal into a rotary air-lock feeder 46 at
a constant rate.
Rotary air-lock feeder 46 is a cylindrical chamber with blades
47 that approach an air-tight fit with the chamber. At the bottom of the
chamber are entrance opening 48 and exit opening 49. The fit of blades 47;
is such that there is almost no free air path between openings 48 or 49
and feeder 43. Accordingly, it is possible for an air stream entering
opening 48 to continue out through opening 49 without being deflected into
gravimetric feeder 43. The rotation of blades 47 carries pulverized coal
dropped onto blades 47 by gravimetric feeder 43 into the air path between ~ ~ ;openings 48 and 49. Compressed air is supplied to feeder 46 by an appro-
priate source 50 at a controlled rate whereby a coal-air mixture measured
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in conduit 16 h~s an air-to-coal weiyht ratio of 1~0 or less and preferably
0.5 or less.
In one reduction to practice of the invention, ;n wh;ch subb;-
tum;nous C rank coal that had been pulverized to 70 percent minus 200
mesh was employed, the upper limit on air-to coal weight ratio was 1Ø
The optimum air-to-coal weight ra-tio will vary with coal type.
In order to operate the burner of Figure 1, ignitor 23 is
moved to its inserted position and turned on. Employing an arc ignitor,
sparks produced by the ignitor having an energy contact of approximately
25 joules, lasting about 10 microseconds each, and having a repetition
rate of 10 Hertz have been successfully employed. Compressnr 50 is turned
on once the ignitor has begun operation, and gravimetr;c feeder 43 is also
started. The compressed air flowing through rotary-air-lock feeder 46
entrains measured amounts of pulverized coal and carries it through conduit
16 and hollow-cone diffuser 22. While it is possible to operate the burner
of Figure 1 without hollow-cone diffuser 22, it is considered desirable to
include diffuser 22 in order to introduce a minor amount of recirculation
during the ignition stage of operation. The coal-air mixture brought into
the vicinity of ignitor 23 is ignited by the energy imparted by the ignitor
and the resultant flame propagates through the coal. As a result, ignition
occurs, and a relatively unsteady flame exists at the outlet of the burner.
At this point, an observer 26 or an automatic flame-detection system
determines that ignition has occurred and causes a secondary air (ambient
or heated) flow through air inlet duct 10. Vanes 12 introduce a rotation
into the air flow, and this results in a spiraling, or swirling, stream of
air that flows down conduit 20 and through quarrel 24. Quarrel 24 is a
divergent nozzle that enhances the recirculating effect that naturally -
occurs due to the vortical flow of air. The swirling stream of air envelops
the combustion zone, and as a result the hot combustion products are drawn -
back into the region of fresh coal injection. The observable effect of
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this recirculation is that the flame becomes steady, and the stability
of the flame is such that ignitor 23 can be turned off and withdrawn.
Thus retracted, ignitor 23 remains in a protective area, thereby prevent-
ing damage to the ignitor due to the in-tense heat of combustion.
It is thought that the direct ignition of pulverized coal
provided by the present invention is su~cessful because it provides
appropriate conditions for propagation of flame within the flowing coal
stream after the ignitor has caused ignition of some of the coal partic1es;
above approximately a 1.0 air-to-coal weight ratio, propagation of the
flame is difficult with unheated air, and direct ignition of pulverized
coal by an arc ignitor accordingly does not work effectively. On furnace
startup~ preheated air is not available and can be produced only through ~;
the expenditure of a significant quant1ty of energy and/or liquid or
gaseous fuel. Combined with the provision of proper conditions For propa-
gation of the flame is the provision of a recirculation zone that contributes
to stability of the resultant coal flame. The recirculation causes hot
products to be drawn back into the combustion zone, thereby causing the
flame to provide its own ignition energy. Whatever the reasons for its
success, however, the present invention does provide a means for satisfactory
direct ignition of flowing pulverized coal.
The burner of Figure 1 can be used as a warm-up burner for
utility boilers. In utility-boiler operation, it is necessary for the
boiler to be brought to an elevated temperature in order for its conven-
tional coal burners to work properly. The burners of the present invention
can be used to bring the furnace up to a temperature high enough for stable
combustion in conventional burners. The present invention can also be used
for both ignition and low-load stabilization.
Figure 4 is a side sectional view of a main coal nozzle. The
center of the main coal nozzle is occupied by a version of the burner of
Figure 1 adapted to use as a center ignitor in a tilting tangential nozzle.
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Corresponding numbers refer to correspon~ing parts, most of which have
already been explained in connection w;th Figure 1. One difference is
that quarrel 24 is shown to have a shape in Figure 4 that is somewhat
different from its shape in Figure 1. Another difference is that vanes
12 are located within conduit ZO rather than in an air inlet duct.
A main coal pipe 28 is shown coaxial with -the ignitor burner.
Coal and primary air, the air used to dry and convey the pulverized coal,
are supplied through main coal pipe 28. Secondary air, the remainder of
the air required for combustion, is supplied through a conduit 30 that is
concentric with main coal pipe 28. Vanes 38 are positioned at the outlet
of conduit 30 and are arranged for tilting so that the air flowing out
of conduit 30 may be directed as required. The tilting of vanes 38 would
typically be accompanied by the tilting of the ignitor burner, so flexible
sections 32, 34, 36 are included in the ignitor burner. In the typical
arrangement, vanes 38 and the exit of the ignitor burner would be appro-
priately connected so as to tilt together.
In operation, the ignitor burner would be operated as pre-
viously explained, and after the ignitor had achieved stable combustion,
coal and primary air would be sent to conduit 28, and secondary air would
be sent to conduit 30. Ignition would occur due to the presence of the
flame from the ignitor burner, and under low-load conditions the stabiliza-
tion of the main flame would also be accomplished by the ignitor burner.
Several advantages are inherent in the apparatus of Figure 4
and the method described in using it. In the typical coal nozzle used
in a tangentially fired unit, the ignitor is mounted beside the main
burner. The apparatus of Figure 4 is an improvement over the typical
arrangement because the center position affords greater proximity to the
main coal stream, thereby allowing the use of a smaller amount of ignitor
energy to produce the sarne effectiveness in igniting the main coal stream.
Another advantage of the Figure 4 arrangement is that the ignitor flame
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reaches the coal stream while it is still fuel rich. This is not
typically the case when side ignitors are used, and present theory
susgests that oxides of nitrogen are less likely -to be produced when a
significant amount of combustion occurs in a fuel-rich zone. Test results
with the present invention have verified that the center position of the
ignitor is environmentally beneficial. A final advantage of the apparatus
of Figure 4 is that, since the ignitor uses coal exclusively~ the fuel
cost penalty for continuous use no longer exists. Thus, the additional
complication of turning the ignitor off after ignition and turning it on
again at low loads may be avoided. Furthermore, a single flame-proving
device would be adequate, since it is only necessary, if the ignitor is
intended to be used continuously, to insure that the ignitor flame is
present. No additional flame-proving device need be provided to insure `
that the main flame is present.
Figure 5 schematically depicts a fuel supply system for the
ignitor burner and main coal nozzle of Figure 4. Pulverizer 56 is one
of the types of coal mills known to the art. Its function is to grind
coal for use in pulverized-coal units. It is typical for pulverizers to
require hot air for the drying function that is usually carried out in
conjunction with the grinding process. The hot air used for this function
comes from an air preheater, not shown, that transFers heat from flue gases
to air. The resulting hot air is drawn by mill fan 52 through line 51,
and it is sent through line 54 to pulverizer 56. The hot air thus blown
through the pulverizer dries the coal and entrains pulverized coal in
order to convey it out of the pulverizer by way of line 58. ~oal to be
pulverized is supplied by means of coal llne 55.
In a cold start-up, the air provided by the air preheater is
not at the temperature normally desired for p-lverizer operation. Accord-
ingly, a fuel-storage silo is provided. This silo contains enough
pulverized coal to supply all the ignitors in the Furnace and one elevation
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of warm-up coal nozzles for aslong as it takes to warm-up the furnace
sufficiently to provide hot enough air for the air preheater for proper
operation of the pulverizer. Silo 70 accordingly might have a capacity
of around 800 tons of pulverized coal. A separator 60 is provided between
pulverizer 56 and bin 70. The function of separator 60 is to separate
out the air that has entrained the coal in pulverizer 56. The air leaves
separator 60 through line 62, and the coal leaves separator 60 through
line 6~. Since the air leaving separator 60 is ordinarily not completely
free of coal dust, it may not always be desirable to exhaust the separator
directly into the boiler. Therefore, appropriate valving may be provided
in order to filter the air output of the separator or to add the ou~put of
the separator to the main coal feed o~ the boiler. This is illustrated by
lines 64 and 66 and valves 63 and 65. Line 64 might be a line to a filter,
valve 63 being opened during initial start-up when the main coal feed to -
the boiler is not operating, and valve 65 might be open, allowing the air
to flow through line 66 to the main coal feed when the main coal feed is
operating. ~
Line 72 represents the communication of bin 70 with ignitors ~ -
and warnn-up nozzles. Line 72 may, for instance9 be a screw conveyor that
supplies coal to a feed system of the type that is illustrated in Figures
2 and 3. Alternately, bin 40 of Figures 2 and 3 could be embodied in bin
70 itself, with feeder 42 being one of several feeders supplied by the
same bin. Feed system 76 supplies lines 82, 84, 86, and 88, the feed lines
to the ignitors, hy means of line 78 and a riffle distributor 80. Each
of the lines 82, 84, 86, and 88 supplies a different level of ignitors.
It may be determined that it is desirable to operate all ignitors whenever
the furnace is operating, regardless of whether all elevations of coal
nozzles are operating. In such a case, there would be no need to valve any
of the lines 82, 84, 86, or 88. Alternately, it could be determined that
only those ignitors would be operated that are associated with a coal
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no~zle that is to be operated. I~ this is the case7 appropriate
valving, not shown, would have to be supplied.
In addition to supplying the ignitors, bin 70 would also
supply an elevation of warm-up nozzles. Each nozzle would be ignited and ~ -
stabili7ed by one of the ignitors fed by line 88. A gravimetric feeder
90 supplied from bin 70 by means of line 72 would supply an exhauster ~,
which would force coal and air from the air preheater through line 96 to
riffle distributor 98. Riffle distributor 98 supplies each of the warm-up
nozzle lines 100, 102, 104, and 106. It would be typical for lines 100,
102, 104, and 106 to be operated together, because they are on khe same
elevation. In a typical cold start-up, bin 70 would have been filled with
pulverized coal during a previous operation of the boiler. Of course,
during the first start up after the unit is built, it would be necessary
to ship in enough pulverized coal to fill bin 70 or provide a temporary
means of coal drying to allow pulverization. After the first operation
of the boiler, however, the silo could always be kept filled by means of
the prior operation of the boiler. Feed system 76 would start to operate,
sending coal and air mixed accordiny to the present invention to at least
the ignitors supplied by line 88, which supplies the ignitors for the
warm-up elevation. The ignitors would operate as previously indicated,
and an igniting flame would be present at the center of each of the warm-up
nozzles. As soon as flame has been proven, feeder 90 and exhauster 94
begin operation, conveying coal and air from bin 70 and the air preheater,
respectively, to the warm-up nozzles, where the coal is ignited by the
ignitors supplied by line 88. The boiler continues to be operated in this
manner, pulverizer 56 and mill fan 52 remaining off, until the boiler has
warmed up sufficiently to provide a high enough air temperature at the
air-preheater exit. At this point, further elevations in the furnace would
typically gegin operation, with the ignitors at each elevation providing
ignition and stabilization. Mill fan 52 and pulverizer 56 would also
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begin operation at this time, contro11ed by appropriate sensing devices
that monitor the amount of coal stored by silo 70. Thus, the silo is
replenished, and enough pu1verized coal is stored to supply the next
boiler start-up.
While preferred embodiments have been shown and described,
various modifications and substitutions may be made thereto without
departing from the spirit and scope of the invention. Accordingly, it will
be understood that the present invention has been described by way of
illustration and not limitation.
What is claimed is:
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