Note: Descriptions are shown in the official language in which they were submitted.
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CONTROLLED FLOW, SPLIT STREAM
BURNER ASSEMBLY WITH SORBENT INJECTION
BACKGROUND OF THE INVENTION
This invention relates generally to a burner assembly
and more particularly to an improved burner assembly which
operates in a manner to reduce the formation of nitrogen
oxides and sulfur dioxides as a result of fuel combustion.
In a typical arrangement for burning coal in a vapor
generator, several burners are disposed in communication
with the interior of the furnace and operate to burn a mix-
ture of air and pulverized coal. The burners used in these
arrangements are general~y of the type in which a fuel-air
mixture is continuously injected through a nozzle so as to
form a single, relatively large, flame.
In the burning of coal in this manner, unacceptable
levels of sulfur dioxide are produced which must be reduced
in order to meet government standards of air quality. Also,
when the flame temperature at the burner exceed~ 2800F, the
amount of fixed nitrogen removed from the combustion sup-
porting air rises exponentially with increases in the tem-
perature. This cond1tion leads to the production of high
levels of nitrogen oxides in the final combustion products,
which also causes se~vere air pollution problems.
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Control of sulfur dioxide emissions is usually
achieved by external means such as wet or dry flue gas
desulfurization. In-situ control (i.e., within the furnace)
has been under investigation for man~ years and utilizes
either a pre-mixing of limestone (or other sorbent) with
coal, or an injection of pulverized sorbent external to the
burner throat through separate ports or small injection
nozzles. However, both of these techniques have distinct
drawbacks. The injection of the sorbent with the coal
usually yields low sulfur dioxide capture ratios due to
deadburning of the sorbent and can lead to increased
slagging. The external injection of the sorbent requires
numerous wall penetrations, tube bends and expensive piping
and burner staging controls for the ports.
Also, sorbent injection between or above the burners can
limit suIfur capture due to several effects:
~ Inadequate mixing between the products of combustion
and the sorbent particles;
- Insufficient residence time in the boiler's radiant
2a zone; and
- Increased slagging and sorbent deposition to the
boiler's sidewalls when sorbent is injected to the
lower burner levels of a multiple level boiler. This
injection location also reduces sulfur capture since
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sorbent particles can be re-entrained in the high
temperature portion of the flame.
These deficiencies can be corrected by in jecting sorbent in
conjunction with an internally staged lo~ N0X burner. This
type of burner reduces N0X by at least 50%, as compared to
turbulent burners, without simultaneous use of external com-
bustion air staging systems such as overfire or tertiary
air ports. ~owever, when overfire air ports are used, N0X
reductions as great as 75% can be obtained. An internally-
staged low N0X burner can be defined as one which yieldsfuel-rich and fuel-lean zones within a flame envelope simi-
lar to that of a turbulent burner. This is in contrast to
delayed mixing burners which produce very long narrow flames
which gradually combust the fuel over a substantially
greater distance than is characteristic of either turbulent
or internally staged burners.
Other attempts, including two-stage combustion, flue gas
reclrculation and the introduction of an oxygen-deficient
fuel-air mixture suppress the flame temperature and reduce
the quantity of available oxygen during the combustion pro-
cess and thus reduce the formation of nitrogen oxides.
However, although these attempts singularly may produce some
beneficial results they have not resulted in a reduction of
-nitrogen oxides to minimum levels. Also, these attempts
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have often resulted in added expense in terms of increased
construction cos~s and have led to other related problems
such as the production of soot and the like, nor do they
lend themselves to sulfur control via sorbent injection.
SUMMARY OF THE INVENTION
Accordingly, the present invention seeks to provide
a burner assembly which operates in a manner to considerably
reduce the production of sulfur dioxides and nitrogen oxides
in the combustion of fuel without any significant increase
in cost or other related problems.
Further, the present invention seeks to provide a
controlled flow/split-flame low NOX burner of an internally
staged design which, when combined with sorbent injection,
effectively reduces sulfur emissions.
More specifically, the present invention seeks to
provide a burner assembly in which the surface area of the
flame per unit volume is increased which results in a
greater flame radiation, a lower flame temperature, and a
shorter residence time of the gas component within the
flame at maximum temperature, thereby reducing the formation
of thermal nitrogen oxides by fixation of atmospheric
nitrogen.
Still further, the present invention seeks to pro~ide
a burner assembly of the above type in which the stoichio-
metric combustion of the fuel is regulated to reduce thequantity of available oxygen during the combustion process
and achieve an attendant reduction in the formation of
nitrcgen oxides from the fuel-bo~ld nitrogen.
In its broader aspects, the present invention seeks to provide a
burner assembly of the ab~ve type in which pre-pulverized sorbent is
injected through the outer parallel path of the above-mentioned secondary
air stream to reduce the formation of sulfur dioxide without the problems
set forth above.
More particularly, the invention pertains to a burner assembly for
introducing fuel and air into an inlet passage formed through a furnace
wall, the assembly comprising nozzle means having an outlet for
discharging a mixture of air and fuel into the inlet passage, m~ans for
discharging additional air into the inlet passage in an inner and outer
radially spaced path, each of which being radially spaced from the
nozzle, and mea~s for discharging sorbent into the inlet passage
downstream of the nozzle outlet and downstream of the discharge area of
the additional air from the radially outer path. The sorbent mixes with
the additional air during their passage through the inlet passage before
they enter the interior of the furnace.
The invention also pertains to a method of burning fuel comprising
the steps of discharging a mixture of air and fuel into an area of an
inlet passage formed through a furnace wall, discharging air into the
inlet passage in an inner and outer radially spaced path, each of which
being radially spaced from the discharge area of the nozzle, and
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discharging sorbent into the inlet passage downstream of the discharge
area of the mixture and downstream of the discharge of the air from the
radially outer path, with the sorbent mixing with the air during their
passage through the inlet passage before they enter the interior of the
furnace.
ERIEF DESCRIPTION OF THE DR~WINGS
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The above brie-f description as well as further objects, features and
advantages of the present invention will be more fully appreciated by
reference to the following detailed description of presently preferred
but nonetheless illustrative embodiments in accordance with ~he present
invention when taken in conjunction with the accompanying drawings
wherein:
Fig. 1 is a sectional view depicting the burner assembly of the
present invention;
Fig. 2 is a partial perspective view of a component of the burner
assemb~y of Fig. l;
Fig. 3 is an enlarged elevational view, partially cutaway, of the
burner portion of the assembly of the present invention;
Fig. 4 is an end view of the burner portion of Fig. 3; and
Fig. 5 is a cross-sectional view taken along the line 5-5 of Fig. 3.
DESCRIPTION OF THE PREFERRED EMBODIMENrS
Referring specifically to Figure 1 of the drawings the
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reference numeral 10 refers in general to a burner assembly
which is disposed in axial alignment with a through opening
12 formed in a front wall 14 of a conventional furnace. It
is understood that the furnace includes a back wall and side
walls of an appropriate configuration to define a combustion
chamber 16 immediately adjacent the opening 12. Also simi-
lar openings are provided in the furnace front wall 14 for
accommodating additional burner assemblies identical to the
burner assembly 10~ The inner surface of the wall 14 as
well as the other walls of the furnace are lined within an
appropriate thermal insulation material 18 and, while not
specifically shown, it is understood that the combustion
chamber 16 can also be lined with boiler tubes through which
a heat exchange fluid, such as water, is circulated in a
conventional manner for the purposes of producing steam.
It is also understood that a vertical wall is disposed
in a spaced parallel relationship with the furnace wall 14
in a direction opposite that of the furnace opening 12 along
with correspondingly spaced top, bottom and side walls to
form a plenum chamber, or wind box, for receiving combustion
supporting air, commonly referred to as "secondary air", in
a conventional manner.
The burner assembly lG includes a nozzle 20 having an
inner tubular member 22 and an outer tubular member 24.
The outer tubular member 24 extends over the inner tubular
member 22 in a coaxial, spaced relationship thereto to
define an annular passage 26 which extends towards the fur-
nace opening 12.
A tangentially disposed inlet 28 co~nunicates with the
outer tub~llar member 24 for introducing a stream of fuel
into the annular passage 26 as will be explained in further
detail later.
A pair of spaced annular plates 30 and 32 extend around
the burner 20, with the inner edge of the plate 30 ter-
minating on the outer tubular member 24. A liner member 34
extends from the inner edge of the plate 32 and in a general
longitudinal direction relative to the burner 20 and ter-
minates adjacent the insulation material 18 just inside the
wall 14. An additional annular plate 38 e~tends around the
burner 20 in a spaced, parallel relation with the plate 30.
An air divider sleeve 40 extends from the inner surface of
the plate 38 and between the liner 34 and the nozzle 20 in a
substantially parailel relation to the burner 20 and the
liner 34 to define two air flow passages 42 and 44.
A plurality of outer register vanes 46 are pivotally
mounted between the plates 30 and 32 to control the swirl of
secondary air from the wind box to the air flow passages 42
and 44. In a similar manner a plurality of inner register
vanes 48 are pivotally mounted between the plates 30 and 38
to further regulate the swirl of the secondary air passing
through the annular passage 44. It is understood that
although only two register vanes 46 and 48 are shown in
FIG. l, several more vanes extend in a circumferentially
spaced relation to the vanes shown. Also~ the pivotal
mounting of the register vanes 46 and 48 may be done in any
conventional manner, such as by mounting the vanes on shafts
(shown schematically in YIG. l) and journaling the shafts
in proper bearings formed in the plates 30, 32 and 38.
Also, the position of the vanes 46 and 48 may be adjustable
by means of cranks or the like. Since these types of com-
ponents are conventional they are not shown in the drawings
nor will be described in any further detail.
A plurality of sorbent injectors 49 are pro~ided, each
of which extends through the plates 30 and 38, between two
vanes 48 and into the air flow passage 420 The inlet end
portion (not shown) of each injector 49 is connected to a
source of sorbent such as limestone, Ca(OH)2, or the like,
2a and the discharge end is located at the opening 12 of the
front wall 14. Although not clear from the drawing, it is
understood that more than two injectors 49 can be provided
-~ in a equilangularly spaced relation around the nozzle 20,
and that the velocity of injection and injection angle can
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be controlled at each injector in a conventional manner.
The quantity of air flow from the wind box into the
register vanes 46 is controlled by movement of a sleeve 50
which is slidably disposed on the outer periphery of ~he
plate 32 and is movable parallel to the longitudinal axis of
the burner nozzle 20. An elongated worm gear 52 is providea
for moving the sleeve 50 and is better shown in Figure 2.
The worm gear 52 has one end portion suitably connected to
an appropriate drive means (not shown) for rotating the worm
1~ gear and the other end provided with threads 52a. The worm
gear 52 extends through a bushing 54 (FIG. 1) which is
attached to the plate 30 to provide rotatable support. The
threads 52a of the worm gear 52 mesh with appropriate aper-
tures 55 formed in the sleeve 50 so that, upon rotation of
the worm gear, the sleeve moves longitudinally with respect
to the longitudial axis of the burner 20 and across the air
inlet defined by the plates 30 and 32. In this mannerr the
quantity of combuskion supporting air from the wind box
passing through the air flow passages 42 and 44 can be
controlled by axial displacement of the sleeve 50. A per-
forated air hood 56 extends between the plates 30 and 32
immediately downstream of the sleeve 50 to permit indepen-
dent measurement of the air flow to the burner 20.
As shown in FIGS. 3-5, which depict the details of the
nozzle 20, the end portion of the outer tubular member 24
and the correspvnding end portion of the inner tubular
member 22 are tapered slightly radially inwardly toward the
furnace opening 12. A divider cone 58 extends between the
inner tubular member 22 and the outer tubular member 24.
The divider cone 58 had a straiqht portion 58a (FIG. 5)
which extends between the straight portions of inner tubular
member 22 and the outer tubular member 24, and a tapered
portion 58b which extends between the tapered portions of
the tubular members for the entire lengths thereof. The
function of the divider cone 58 will be described in greater
detail later.
A p~urality of ~-shaped splitters 60 are circumferen-
tially spaced in the annular space between the outer tubular
- 15 member 24 and the divider cone 58 in the outlet end portion
of the nozzle 20. As shown in FIGS. 3 and 4, four such
splitters 60 are spaced at 90 intervals and extend from the
outlet to a point approximately midway between the tapered
portions of the tubular members 22 and 24. Each splitter 60
is formed by two plate members welded together at their ends
to form a V-shape. The plate memhers are also welded along
their respective longitudinal edges to the outer tubular
member 24 and the divider cone 58 to support the splitters
and the divider cone in the nozzle 20. The apex of each
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splitter 60 is disposed upstream of the nozzle outlet so
that the fuel-air stream flowing in the annular space bet-
ween the divider cone 58 and the outer tubular member 24
will. be directed into the adjacent spaces defined between
the splitters to facilitate the splitting of the fuel stream
into four separate streams.
Four pie-shaped openings 62 are formed through the outer
tubular member 24 and respectively extend immediately over
the splitters 60. These openings are for the purpose of
lQ admitting secondary air from the inner air flow passage 44
(FIG. 1) into the annular space defined between the divider
cone 58 and the outer tubular member 24 for reasons that
will be explained in detail later.
As shown in FIG. 5, a tip 54 is formed on the end of the
tapered portion of the inner tubular member 22 and is
movable relative to the latter member by means of a plura-
lity of rods 66 extending within the tubular member and
affixed to the inner wall of the tip. The other ends of the
rods 66 can be connected to any type of act,uator device (not
shown) such as a hydraulic cyllnder of the like to effect
longitudinal movement of the rods and therefore the tip 64
in a conventional manner.
It can be appreciated from a view of FIG. 5 that the
longitudinal movement of the tip 64 varies the effective
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outlet opening defined between the tip and the divider cone
58 so that the amount of fuel~air flowing through this
opening can be regulated. Since the divider cone 58 divides
the fuel-air mixture flowing through the annular passage 26
into two radially spaced parallel streams extending to
either side of the divider cone 58, it can be appreciated
that movement of the tip 64 regulates the relative flow of
the two streams while varying their velocity.
It is understood that appropriate ignitors can be pro-
lQ vided adjacent the outlet of the nozzle 20 for igniting thecoal as it discharges from the nozzle. Since these ignitors
are of a conventional design they have not been shown in the
drawings in the interest of clarity.
In operation of the burner assembly of the present
invention, the movable slee~e 50 associated with each burner
i5 adjusted during initial start up to accurately balance
the alr to each burner. After the initial balancing, no
further movement of the sleeves 50 are needed since normal
control of the secondary air flow to the burners is
accomplished by operation oE the outer burner vanes 46.
However, if desired, flow control can be accomplished by the
sleeve.
Fuel, preferably in the form of pulverized coal
suspended or entrained within a source of primary air, is
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introduced into the tangential inlet 28 where it swirls
through the annular chamber 26. Since the pulverized coal
introduced into the inlet 28 is heavier than the air, the
pulverized coal will tend to move radially outwardly towards
the inner wall of the outer tubular member 24 under the
centrifugal forces thus produced. As a result, a great
majority of the coal along with a relatively small portion
of air enters the outer annular passage defined between the
outer tubular member 24 and the divider cone 5B tFIG. 5)
lQ where it encounters the apexes of the splitters 60. The
stream is thus split into four equally spaced streams which
discharge from the nozzle outlet and, upon ignition, form
four separate flame patterns. Secondary air from the inner
air passage 44 (FIG. 1) passes through the inlets 62 formed
in the outer tubular member 24 and enters the annular
passage between the latter member and the divider cone 58 to
supply secondary air to the streams of coal and air
disch~rging from the outlet.
The remaining portion of the air-coal mixture passing
through the annular passage 26 enters the annular passage
defined between the divider cone 58 and the inner tubular
mem~er 22. The mixture entering this annular passage is
mostly air due to the movement of the coal radially out-
wardly, as described above. The position of the movable tip
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64 can be adjusted to precisely control the relative amount,
and therefore velocity, of the air and coal discharging from
the nozzle 20 from the annular passages between the outer
tubular member 24 and the divider cone 5B and between the
divider cone and the inner tubular member 220
Secondary air from the wind box is admitted through the
perforated hood 56 and into the inlet between the plates 30
and 32. The axial and radial velocities of the air are
controlled by the register vanes ~6 and 48 as it passes
1~ through the air flow passages 42 and 44 and into the furnace
opening 12 for mixing with the coal from the nozzle 20. The
igniters are then shut off after steady state combustion has
been achieved.
Sorbent is injected, by the injectors 49, into the
secondary air stream flowing through the flow passage 42 at
the opening 12 to capture the sulfur dioxide produced as a
result of combustion of the coal.
: As a result of the foregoing, several advantages result
from the burner assembly of the present invention. For
example, since the pressure drop across the perforated air
hoods 56 associated with the burner assemblies can be
equalized by balancing the secondary air flow to each burner
by initially adjusting the sleeves 50, a substantially uni-
form flue gas distribution can be obtained across the Eur-
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nace. This also permits a common wind box to be used and
enables the unit to operate at lower excess air with signi-
ficant reductions in both nitrogen oxides and carbon monoxi-
des. Also, the provision of separate register vanes 46 and
48 for the outer and inner air flow passages 42 and 44
enables secondary air distribution and f:Lame shape to be
independently controlled resulting in a significant reduc-
tion of nitrogen oxides, and a more gradual mixing of the
primary air coal stream with the secondary air since both
streams enter the furnace on parallel paths with controlling
mixing.
Further, the provision of multiple flame patterns
results in a greater flame radiation, a lower average flame
temperature and a shorter residence time of the gas com-
ponents within the flame at a maximum temperature, all ofwhich, as stated above, contribute to reduce the formation
of nitric oxides.
Still further, the provision of the tangential inlet 26
provides excellent distribution of the fuel around the annu-
2Q lar space 26 in the nozzle 20, resultin~ in move completecombustion and reduction of carbon loss and making it
possible to use individual burners witb capacities signifi-
cantly higher than otherwise could be used. Provision of
the inlet openings 62 in the outer tubular member permits
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the introduction of a portion of the secondary air to be
entrained with the fuel-air stream passing through the annu-
lar passage between the outer tubular member 24 and the
divider cone, since the majority of this stream will be pri-
marily pulverized coal. As a result, a substantially uni-
form air-coal ratio across the entire cross-section of the
air-coal stream is achieved. Also, the provision of the
movable tip 64 to regulate the flow of the coal-air mixture
passing through the inner annular passage defined between
lQ the divider cone 58 and the inner tubular member 22 enable
the air flow on both sides of the divider cone to be regu-
lated thereby optimizing the primary air velocity with
respect to the secondary air velocity.
Also, by injecting the sorbent into the outer secondary
air annulus the particles will by-pass the hottest part of
the flame so that a minimum of deadburning of the sorbent
will occur. Also, since the sorbent particles will be
rapidly entrained in the swirling secondary air ~rom this
outer secondary annulus they will be intimately mixed with
2Q the products of combustion as soon after passing the peak
flame temperature zone as is feasible. This increases the
efficiency of the sulfur capture and results in capture
that is e~ual to or better than capture methods external to
the burner throat.
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It is understood that several variations and additions
may be made to the foregoing within the scope of the inven-
tion. For example, since the arrangement of the present
invention permits the admission of air at less than
S stoichiometric for further reductions in NOX emmissions,
overfire air ports, or the like can be provided as needed to
supply air to complete the combustion. Also, the distripu-
tion of the sorbent injectors 49 around the periphery of the
burner can be varied to obtain optimum sulfur capture.
Additionally, the ~urner levels which receive sorbent injec-
tors are dependent on the number of burner levels, slagging
characteristics of the coal ash and the gas temperature at
the exit to the furnaces radiant zone~ Boilers with three
or more burner levels need only have the top two levels con-
tain sorbent injectors. This is sufficient to provide aneffective calcination zone for calcium-based sorbents along
with a long residence time for sulfation reactions to occur
prior to the furnace e~it.
As will be apparent to those skilled in the art,
2Q various changes and modifications may be made to the embodi-
ments of the present invention without departure from the
spirit and scope of the present invention as defined in the
appended claims and the legal equivalent.
