Note: Descriptions are shown in the official language in which they were submitted.
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Backqround of the Invention:
The present invention relates to the field of coal-fired
furnaces. It relates particularly to a furnace that can
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-se coal for ignition, warm-up, and low-load stabilization.
S It is sometimes desirable to use coal rather than gas
or oil in electrical generating facilities. In those
si~uations, the utility will naturally hava a coal-fired
unit built rather than an oil-fired unit. However, even in
coal-fired units, substantial quantities of gas or oil are
oten u9ed. In a typioal coal-fired unit, coal to be burned
in the furnace is dried and pulverized in a coal mill and
delivered directly from the coal mill to the load-carrying
coal no2zles in the furnace. Operation of the coal mills
requires that heated air be supplied to the mills for drying
and conveying the coal. This air is supplie~ by a forced-
draft fan that forces the air through an air preheater, a de-
vice that uses the hot products of combustion in the furnace
- to preheat the air. This preheated ~ y aiL, the air
used for drying and conveying coal, is delivered with the
coal to ~e coal nozzles and used to support combustion. The
primary air is typically not sufficient in quantity to support
combustion of all the coal, 50 secondary air is brought
directly from the air preheater to the furnace to supply the
rest of the air needed for combus~ion. The coal thus
supplied with air is caused to burn due to ignition enexgy
from the primary air, the secondary air, the heat in the coal
itself, radiation and conduction from flame in thP furnace,
and radiation ~rom furnare walls~
:
It is to he noted that almost all of these combustion
30 , energy sources presuppose that the furnace has already been
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operating, and, in the large furnaces used in power generat~
it presupposes that the furnace has been operating for a
fairly long time. Accordingly, in order to sause and sustain
comhustion of the coal, it is necessary to use an auxiliary
~uel for warming up the furnace walls, for providing ignition
flame, and for warming up the air preheater. This is usually
the function o oil- or gas-fired ignitors and warm-up guns.
In a typical installa-tion, a relatively high-capacity
oil bùrner is started by an ignitor, and this starts the
process or warming up the furnace walls and the heat-exchange
surfaces of the air preheater. This can take some time, and
the use of 70,000 gallons of oil in a SOO-megawatt unit for
one startup alone is not uncommon. In addition, there is
considerable capital expense involved in providing the hard-
ware that is used for supplying oil. Once the furnace has
been brought up to temperature, the coal noz21es are ignited
by oil- or gas-fired ignitors or by thewarm-up guns themselves.
The use of auiliary fuel is not necessaril~ over when
the coal nozzles have started to supply coal. At high boiler
loads--that is, when the amount of coal supplied by the
no2zles is great--the furnace can typically maintain stable
combustion of the pulverized coal. However, when the load
goes down and the coal supply is thereby decreased, the
stabLlity of the pulverized coal flame is also decreased, and
it is therefore common practice to use the ignitors or
w~rm-up guns to maintain flame in the furnace, thus avoiding
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the accumulation o unburned coal dust in the furnace and
the associated danger of explosion.
All of these functions of the oil- or gas-fired hurners
rely on the greater ease of ignition of these fuels; less
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heat LS re~uired, fro~ whatevex s.ouxce., to liberate the
volatiles and there~y in~.t~ate or sus:tain combustion.
Conversely, the ~reater d~.fficulty encountered in i.gniting coal
is the reason wh.y ~t has typically not been used for the
ignition, warm~up and low~load-stabilizati~on functions. An
incidental advantage of oil and gas that also contributes to
the greater des~ra~.ili`t~ of the~i`~ us-e fo~ these. ~unctions is
that it i5 possi~file to suppl~ th.em ~n relati~ely small pipes,
there~y~ keep~ng t~eir contri~ut~on to the congestion in the :
fuel-nozzle area to a minimum. The us~lal method of supplyiny
coal to nozzIes h~s ~e~u~æed rat~:e~ large p~ping~ and the
addition of more large~si`ze p~p~ng would not he we.lcome in the
area i:mmed~ately ~eh~nd th.e. ueI nozzles.
Summary of the In~ent~on:
It ~s accord~ngl~ an o~ject of the present invention
to accompl~sh th.e ~unctions of ignition t warm-up r and low-load
! stabilization ~th the use of a minimum of auxiliary fuel.
Acco~dingl~, in a furnace s~stem that i.ncludes a
l furnace, a ma~n coal nozzIe arranged to direct coal into the :
;l 20 furnace, an air preheater ha~ing a flue yas inlet, an air
j inlet, and an a~r outlet and being connected to receive flue .
gas:es from th.e furnace at its flue gas i`.nlet and transfer heat
from the 1ue gases to a~.r enteri.n~ the ai.x i.nlet and leaving
the air outlet, a main pul~erizer, condu~t.means connected to
conduct coal -from t~e pulYerizer outlet to the m~i.n coal nozzle,
means for forci:ng a ~irst air stream thxough the p~eheater . ;
from its aIr ~nlet to its ai.r outle.t, and from the ~ir outlet . -~
through the pulYer~zer and into t~e: main coal nQzzle, and ~ :
~eans for forcing ~ second air stream f~om the preheater outlet
3~ into th.e furnace, there. i~ provided ~ccording to one aspect of
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the present inye.ntion an i.gnito.r nozzle posi.ti.oned for ignition
of coal leaving the main coal nozzle, an ignitor pul~erizer
for pulverizing coal, a separator for separating coal from
aîr, me~ns for conveying coal ~ixed with air from the ignitor
pulverizer to t~e separator, means for conveying coal fxom
the separator to the ~gnitor nozzle, means. for causing a third
air stream hav~.ng a temperature higher th.an the te~pe.rature of
either the first or the: second air stream to flow to the
ignitor nozzle, and a li~h.ter, posi~tiona~le near the outlet
o~ the ignitor nozzle, ~or ~gniting coal issuing from the
ignitor nozzle.
According to anothex en~od~ment o~ the invention
t~ere is provi~ded an i~gnitor nozzIe po$it~oned or igniti.on
of coal leaving the:`coal nozzle, an ign~tor pulverizer for
pulverizing coal, a s.ep~ato~`fo~ sepa~atin~ coal from air,
means fox conYey~ng coal mi`xed ~th'a~r from the ~:~ni.tox ':
: pulveri.zer to the' s:epar~tor, means ~or co~veylng coal from the
separator to the i~gnitor nozzIe,`means ~or trans~erring heat
tQ a thi`.rd air stre~m from a sQurce other than the com~us.tion
products, means for causi~ng the' th~rd air s.tream to flo~ into
'~ the furnace`at the i`gni~tor nozzle, and a ligh.ter, positionable
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ne.a~ the outlet of the lgnitox`nQzzle~` fo~ ~gnit~ng coal issuing
from the ignitor nozzle.
Accordin~ to a furthe~ em~odiment~ the. inventi.on
resides in an ~mproved ~arm-up sys-tem for a ~urnace. system of
the type described, whi'ch ~arm-up ~s.tem comprise~ :
an i.~ni:tor nozzle.,
means ~or stox~n~ ~ suppl~ of pul~er~zed coal during normal
furnace opexat~on, ~ean~-for esta~ hi.n~ a second air ~tream,
3Q means for cont~oll~Iy re.le~in~ pulverized coal from
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said storing means into a stream of primar~ air to form a fuel
stream, means for trans:ferr~ng heat to the second air stream
from a source other th~n the furnace flue gas, means for
causing the heated second a~r stream and the fuel stream to
flow into the furnace at the i~nitor nozzle, said flow causing
means dividing the heated second air stream i~nto a pair of
streams coax~l wi~t~ the fuel s~tre~m, and a l~hter, position-
a~le near the outlet of the ~gnitor nozzle, for igniting coal
~ulng f~om t~e i`gni~tor nozzle w~erefi~ i~niti.on energy ~ill
be. s~ppl~.ed b~ said l~hter and ~e~t tr~ns~r mean~.
In a further aspect, the i.n~enti.on broadly resides in
a method of operating a coal-fired furnace that includes
heating a first air stream ~ transferring heat from combustion
products of the furnace to the air in the firs.t air ~tream,
:~ usiny the first air stream to entra~.n coal ground i.n a main
pulverizer and convey ~t to the furnace, heating a second air
stream ~y transfe.rri`ng heat from combust~on products of the
furnace to the air ~n the s~cond ~ix stream, and conducting it
to the furnace, the improve~ent compri.s~n~ steps of
.; 2Q separ~ting pul~erized coal f~om a ~xture of pul~erized :~
coal and air, s:upplying the sepa~ated coal to an ignitor
nozzle poslti.oned to ;~.gnite the coal entrained in the fir~t
air s.tream as it enters the furn~ce, he.atlng a th~rd air
s-tream to a temperature high.ex than t~.e temperatu~e of both
the fir~t and second air stxeam~, cau~ng the thi.rd air stream
to flow to the furnace at the: ignitor nozzle, and
igniting the coal lea~lng the l~nitox nozzle.
In an alternati`~e em~odi.~ent, a third a;r stream `
m~y be heated f~o~ a source oth.er than th.e co~bustion products,
3~ and us:ed fox th.e purpose$ descr~.bed In the p~eceding paragraph. -- -
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~rief Description of the Dr'awin~:
These and further features and advantages of the
invention become evi~dent ~n the descripti.on of the embodiment
shown in the dra~ngs attac~ed, where~n:
FIGURE 1 i.s a diagrammatic ~iew o~ fuel system for the
load-carrying nozzIes;
FI~URE 2 is a diagrammat~c vi.ew o~ the ~uel system for
the ignitor nozzle of the presen-t invent~on; and
FIGURE.3 i.s a side'elevatlon, partly a cross section,
lQ of a typical ignitor nozzIe for use with the present invention.
'~ De:tai:led:De:s:cript:ion:'o:f::t:h'e: pre::~e:rr'e'd Embod~ment:
FIGURE 1 sho~s the system for s.uppl~.ng air and fuel
to the load-carr~ing nozzles of a pulverized~coal boi.ler. The
furnace is generally sho~n at lQ. A conduit 36 connects the
outlet o~ th.e furnace to the ~lue gas ~nlet of ai.X preheater . `
38. Condui.t 4Q conne.ct~ the'flue-gas outlet to a stack, not
f sh.own, th~t reIeases th.e products of combusti.on to the
atmosphere. A fan 42 draws ~rom the ~tmpsphere and blows
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air through the air inle~ of air preheater 38, Conduit 34
connects the air outlet of air preheater 38 to windboxes 12
and 30 located on either side cf the furnace. The typical
furnace would actually have four windboxes, one at each
corner, but, for the sake of simpLicity, only two are shown.
Another conduit 32 conducts air from conduit 34 to the air
inlet of pulverizer 22. The outlet of pulverizer 22 is con-
nected by conduit 21 to exhauster 20, whose outlet communi-
cates with several conduits. Conduits 18 and 24 lead from
the ~xhauster outlet to coal nozzles 19 and 25, which are
arranged so as to direct coal fed to them into the interior
of furnace 10. ~ozzles 16 and 26 are fed by a second
pulverizer-exhauster combination that is not shown in the
drawing, while a third pulverizer-exhauster combination,
! 15 also not shown, feeds nozzles 14 and 28. Again, for each
pair of nozzles shown there is typically another pair of
nozzles not shown that is fed by the same pulverizer.
Windboxes 12 and 30 communicate with the interior of the
furnace through openings in the vicinity of the nozæles.
Dampers, not shown in the drawing, control the allocation
o~ air from the windbox among the openings.
In normal operation, coal and air enters furnace 10
through one or more elevations of nozzles. Combustion takes
plaoe in the interior of furnace 1~, producing hot flue
gases that flow out conduit 36, through air preheater 38, and
i through conduit 40 to a stack. Air preheater 38 has moving
heat-exchange surfaces that alternately contact the hot flu2
gases and the air enteriny the preheater from fan ~2. The
surfaces thus absorb heat from the flue gases and release
i 30 it to the air from fan 42. Part of the heated air leaving
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57~7
air preheater 38 passes throuyh conduit 32 ~nd into pulverizer
22. Pulverlzer 22 is an apparatus for drying and crushing
coal, and the hot air brought by conduit 32 is used to dry
the coal~ The air stream ~lowing in conduit 32 and pulverizer
22 also flows through con~uit 21, exhauster 20, and conduits
18 and 24 to the associated nozzles. In flowing through
pulverizer 22, the air stream entrains thP coal that has
been sufficiently pulverized and carries it to nozzles 19
and 25. Since fan 42 and exhauster 20 both provide motivating
force for this motion, it can be seen that together they
constitute means for forcing a first air stream from the air
- outlet of preheater 38, through pulverizer 22, and into either
nozzle 19 or nozzle 25.
The air forced by fan 42 and exhauster 20 through
pulverizer 22 is referred to as primary air and is delivered
~ with the coal to main coal nozzles 19 and 25. However,
- there is not usually enough primary air to support co~bustion
of all of the coalt so some of the air leaving air preheater
38 goes through conduit 34 to windboxes 12 and 30. Windboxes
12 and 30 supply the secondary air, the remainder of the air
reguired to support combustion of all the coal~ ~ :
It is evident `that the above discussion presupposes ~.
that hot flue gases are flowing through conduit 36. Of course,
~5 at the beginning of furnace operation, the gases flowing
through the conduit 36 are relatively cooI. A typical coal-
~ired unit includes supplementary burners that burn oil or
natural gas, and it is the func~ion of these hurners to
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; operate when the gases coming through conduit 36 are relati~ely
30 cool. This is because pulverized coal is relatively difficult . . .
to ignite, and ~table combustion cannot be guaranteed unless
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~ignificant amounts of heat energy are present in the
combustion area~ Th_s heat energy that is used to start
or maintain combustion comes from many sources. It could
come directly by radiation from flame that is already in
the furnace, by radiation from the walls of the furnace,
by conduction from the generally hot gases in the furnace,
or by conduction from the primary and secondary air flowing
into ~e the furnace. In actuality, all of these sources
contribute to the ignition energy, and at high-load conditions
10 they all add up to a sufficient amount of .ignition energy
~ for stable combustion of the coal. However, in many situa-
; tions the com~ination of these energy sources is not
sufficient to guarantee stable combustion. One of these
situations is that of a cold furnace, in which there is
15 little radiation from the furnace walls and little energy
transferred to the primary and secondary air by the air
preheater. In such cases the supplementary burners are used.
Another situation in which supplementary burners are used
; is the case in which the furnace is operated at a relatively
- 20 low load, when the amount of reactants burniny is sufficiently
low to cause a redu~tion~in the energy derived ~rom the
various sources. In this case again, supplementary burners
arc used to maintain stable combustion. In the past, these
supplementary burners have all burned oil or natural gas.
,- 25 Thi~ is a natural choice, since oil and natural gas are much
easierJto light than pulverized coal is~
FIGURE 2 shows a system that enables the supplementary
burner~ tQ be fired by pulverized coal. An iynitor pulverizer
110 receives air at inl~t 112 from air preheater 38 of
E'IGURE 1. Conduit 100 conducts the coal=air mixture leaving
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pulverizer 110 to exhauster 102, and conduit 98 connects
the outlet of exhauster 102 to further conduits 96. Conduits
96 lead to cyclone separators such as separator 65. The
number of such separators deponds on the designer; only one
is necessary, but more could be used The outlet ~f separator
65 is connected by an air line 62 to a point in the interior
of the furnace remote from the fuel nozzles~ Bin 66 is
positioned to receive the coal leaving separator 65, and the
outlet of the bin is controlled by valve 67. Coal from bin .
66 is fed through coal pipe 70 to appropriately valved coal
pipes 74, 78, and 82, each of which terminates in coal
nozzles not shown in. FIGURE 2. Similar coal pipes 86, 90,
and 94 also receive coal either from coal bin 66 or another
coal bin not shown and feed it to nozzles positioned at their
exits.
Those skilled in the art will recognize that it is not
: essential that pulverizer 110 ~e a separate pulverizer.
The functions o~ pulverizer 110 and pulverizer 22 could be
combined in the same pulverizer, the output being divided
between a direct connection to the furnace and a connection
to a separator 65. Accordingly, the main pulverizer and the
ignitor pulverizer in the claims can be embodied in the same
hardware.
Fan 118 draws air from the air preheater shown in
FIGURE 1, and this air stream is divided among conduits 119,
120 and 122. Conduit 119 feeds an in-duct air heater, possibly
an electric heater, and the output of air heater 116 is sent
by means of conduit 114 to the ignit~r no~zles at the ends
of coal pipes 82 and 94. The temperature of the a-ir leaving
30 air heater 116 is preferably between 300E and 1000F. A
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~imilar heater and similar connections exist between conduit
120 and th~ noz~les at the end of coal pipes 78 and 90 and
between conduit 122 and the nozzles at the ends of coal
pipes 74 and 86.
FIGURE 3 shows an ignitor nozzle of the type that would
be fed by coal pipe 82. The ignitor nozzle is actually made
of three concentric nozzles 128, 130 and 134. ~ozzles 128
and 134 are both fed by conduit 80, which is attached to
noæzle 128 by flexible connector 126. Coal pipe 82 is
connected through ball joint 138 to coal-pipe extension 1440
Interior to and concentric with coal pipe 82 and coal-pipe
extension 144 is lighter 142. Lighter 142 may be a small
version of an ordinary coal-/or gas-fired ignitor, or it may
be a high-energy arc ignitor. In either case, the ignitor
is flexible at least thrvugh the area of the ball joint in
order to allow it to move with coal-pipe extension 144. Air
conduit 124 communicates with windbox 12 of FIGURE 1 and has
nozzl~ 130 fitted on its exit~ Accordingly, no~zle 130 is
' in communication with windbox 12. A typical unit would have
1 20 a discriminating flame detector 132 of any desired type in
order to determine whether or not there is flame at the end
of the ignitor nozzle.
To start up the furnace when it is cold, pulverizer 110
i9 ~tarted, receiving coal at i~s inlet and crushing it~
The air inlet of pulverizer 110 receives air that has been
blown through air preheater 38 by fan 42. In a cold start up,
this air is still relatively cool. The cool air-is blown
through pulverizer 110, conduit 100, exhauster 102, and
conduits 98 and 96 to separator 65. Separator 65 removes the
~oal that has been entrained by the air blown through pulver-
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izer 110, and it drops it into bin 66. Simulatan~ously,
the air separated from the coal is exhausted into the furnace
through line 62. Alternately, bin 66 could be a storage bin
larye enough to hold the amount of coal needed for a startup.
I~ such a case, the pulverized coal left in bin 66 from
previous oper~tion of the furnace would fuel the operation
until the furnace has heated up. Inerting line 64 is used
to maintain an atmosphere in bin 66 during storage that dis-
courages spontaneous combustion. Af~er the furnace has
heated up, ignitor pulverizer 110 starts to work, replenishing
the supply of stored coal in bin 66.
Whichever method is used, coal is supplied by bin 66.
Valve 67 regulates the amount of coal that is allowed to
~all from bin 66, and this coal is forced by appropriate
means through conduits 70 and 82 and out the ignitor nozæleO
Similarly, coal is also forced through coal pipe 94 and through
the nozzle fitted at its exit. Due to the fact that the coal
is sent to conduits 82 and 94 with almost no air, coal pipes
82 and 94 can be made relatively small, so they do not
contribute to the congestion in the furnace corners. At the
same time that the coal is being delivered to the ignitor
nozzles, air from preheater 38 is drawn by fan 118 through
~onduit 119 to heater 116. Heater 116 heats the air to a
temperature high enough to provide stable combustion. Without
heater 116, the only heat in the air would be that imparted
to it by air preheater 38, and on a cold start this is not
very much heat. The hot air leaving heater 116 is fed by
conduit 114 to conduits 80 and 92. Part of the air flowing
through conduit 80 passes through nozzle ~34 of FIGURE 3~
According to the present state of the art, nozzle 134 may have
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vanes 136 to properly direct the air flow, and this air flow
imparts an appropriate flow pattern to the coal that l~aves
the openings of coal-pipe extension 144.
It is to be noted that th~ present system allows ~he
a~nount of heat introduced by air heater 116 to be kept to
a minimum. Since the air that is heated is used only to add
to the ignition energy at the ignitor nozzle, the n~cessity
of adding heat to the entire volume of air flowing through
preheater 38 i5 avoided. Furthermore, since the inert water
vapor that result~ from the drying of the coal has been
separated from the coal before the coal reaches the ignitor
nozzles, none of the energy supplied by air heater 116 is
used up in he-ating inerts. The rest of the air that ~lows
through conduit 80 is conducted through nozzle 128 and past
vanes 140, which also impart a flow pattern appropriate for
stable combustion. Though the amount of air heated by
heater 116 will normally be kept as low as possible, system
designs may provide suf~icient capacity to heat 100 per cent
stoichiometric air if required. Thus, the amount of air
supplied through nozzles 128 and 134 may be stoichiometrically
sufficient for combustion of the coal. If it is not, windbox
air will be introduced through nozzle 130 Even if the
amount of heated air introduced through no2zles 128 and 134
is suf~icient for combustion o~ all the coal, however, it
may be desirable, depending on the characteristics of nozzles
128 and 134 and vanes 136 and 140, to introduce windbox air
in order to cause a flow pattern adapted to feeding hot
combustion products back into the combustion zone, thereby
~ontributing t~ ignition energy and the stability of the
ignitor flame.
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Typically, the coal leaving coal-pipe extension 144
would have its volatiles liberated by liyhter 142, and
combustion of some of the coal would also be started in the
presence of the air flowing through ignitor 134. The
remainder of the air needed for combustion would be supplied
by nozzle 128, so combustion is completed after the coal
and air leaving nozzle 134 meets the air in nozzle 128. As
was noted before, the air coming throuyh conduit 80 is
hot enough so that its ~ontribution to ignition energy pro-
vides for a stable flame~
It is to be understood that the nozzle of FIGURE 3 is
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merely illustrative; it merely shows the functions that would
- typically be performed by a nozzle used with the present
invention.
The stable flame at the outlet of the ignitor nozzle
begins to warm the furnace walls and steam pipes, and as they
warm up, the flue-gas temperature increases. Eventually,
the air preheater becomes hot enough for operation o~ the
` main coal no~zles, and their pulverizers are started. The
coal issuing from the main coal nozzles is ignited by flame
from the ignitor nozzles, and normal operation begins. If
the urnace is operating at low loads, the ignitor nozzles
remain on, providing low-]oad stabilization. It may ~e
determined that the cost penalty in leaving the ignitors
in operation is minor, so they may be left operating even at
high loads.
While the invention has been described in terms of a
~pecific embodiment, the use o~ a specific embodiment is by
no means meant as a limitation. Accordingly~ any modification
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within the scope of the appended claims that is apparent to
those skilled in the art in light of the foregoing descrip-
tion is meant to be included in the invention.
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