Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
,_ WO 95/02789 ~ ~ PCT/US94/07745
APPARATUS AND METIiOD FOR REDUCING NOX, CO AND
HYDROCARBON EMISSIONS WHEN BURNING GASEOUS FUELS
FIELD OF THE INVENTION
The invention relates generally to combustion apparatus,
a-:d more specifically relates to a burner that combines
the advantageous operating characteristics of nozzle mix
and premixed type burners to achieve extremely low NOX,
CO and hydrocarbon emissions.
BACKGROUND OF THE INVENTION
NOx emissions from gas flames can be created either
through the Zeldevitch mechanism (often called thermal
NOx) or through the formation of HCN and/or NH3 which can
then be ultimately oxidized to NOx (prompt NOx).
Thermodynamic calculations typically show that NOx
emissions measured from natural gas flames are well
below, one to two orders of magnitude, the thermodynamic
equilibrium value. This indicates that in most
situations NOx formation is kinetically controlled.
Kinetic calculations indicate that thermal NOx emissions
are typically the most important source of NOx for natural
gas flames, with the NOx being created through the
following reactions:
N + OZ = NO + O (1)
N + OH = NO + H ( 2 )
NZ+O=NO+N (3)
Kinetic calculations were performed using a PC version of
the CHEMKIN computer program. Calculations using this
program have provided valuable insight into changes in
the burner fuel and air mixing characteristics which can
lower NOx emissions.
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2
As the name implies, thermal NOx can be controlled by
regulation of the peak flame temperature, and as shown in
Figure 1 using kinetic calculations, if the temperature
can be lowered enough the NOx emissions from a "true"
premixed natural gas flame operating at 15% excess air
can be reduced to extremely low values (less than 1
ppmv). In effect Figure 1 shows the relationship between
thermal NOx and temperature since for a premixed natural
gas flame with an excess of oxygen, thermal NOx is the
only route by which any significant NOx emissions are
created.
Under appropriate flame conditions the formation of
prompt NOx can also be important when burning natural gas.
The kinetic model used shows that under fuel rich
conditions, particularly when the stoichiometry is under
about 0.6, both HCN and NH3 can be formed through reaction
of CH with NZ to form HCN and N. These calculations were
conducted using gas and air mixtures with stoichiometries
ranging from 1.0 to 0.4. The model predicts that prompt
NOx becomes important at higher stoichiometries when the
temperature is lower; see Figure 2. Below a
stoichiometry of 0.5 almost all the NOx formed is prompt
NOx. The rate of prompt NOx formation (as the name
implies) is also very rapid, being nearly complete in
about 1 millisecond at a temperature of 2400° F.
Kinetic calculations also indicate that hydrocarbon
fragments, in addition to being important for prompt
NO~, are also important for thermal NOx formation since
they can act as a source of O atoms and OH radicals.
Kinetic calculations show the importance of the
hydrocarbon concentration in the formation of NOx, even
under oxidizing conditions. At a temperature of 3400° F
the predicted NOx emissions were about 4 ppmv after 5 ms
residence time for a mixture of N2, O2, H20, and COZ when
CA 02167320 2000-02-02
3
hydrocarbons were not present, as compared to 80 ppmv when
combustion of about 1% CH4 was present in the gas mixture.
If the concentration of methane initially present was
reduced to about 0.5%, the NOX concentration after 5 ms was
reduced to about 75 ppmv. The kinetic model used predicts
that the following mechanisms are important:
1. Reaction of CH4 with O2, OH and H to form CH3
2. Reaction of CH3 with 02 to form CH30 and O
3. Reaction of NZ with O to form NO and O
4. Various reactions to form OH
5. Reaction of NZ with OH to form NO and NH
Low NOX gas burners have been undergoing considerable
development in recent years as governmental regulations
have required burner manufacturers to comply with lower and
lower NOX limits. Most of the existing low NOX gas burner
designs are nozzle mix designs. In this approach the fuel
is mixed with the air immediately downstream of the burner
throat. These designs attempt to reduce NOX emissions by
delaying the fuel and air mixing through some form of
either air staging or fuel staging combined with flue gas
recirculation ("FGR"). Delayed mixing can be effective in
reducing both flame temperature and oxygen availability and
consequently in providing a degree of thermal NOX control.
However, delayed mixing burners are not effective in
reducing prompt NOX emissions and can actually exacerbate
prompt NOX emissions. Delayed mixing burners can also lead
to increased emissions of CO and total hydrocarbons.
Stability problems often exist with delayed mixing burners
which limit the amount of FGR which can be injected into
the flame zone. Typical FGR levels at which current
burners operate are at a ratio of around 20% recirculated
flue gas relative to the total stack gas flow.
2167320
4
A further type of low NOx burner which has been developed
in recent years. is the premixed type burner. In this
approach, the fuel gas and oxidant gases are mixed well
upstream of the burner throat, e.g. at or prior to the
windbox. These burners can be effective in reducing both
thermal and prompt NOx emissions. However, problems with
premixed type burners include difficulty in applying high
air preheat, concerns about flashback and explosions, and
difficulties in applying the concept to duel fuel
burners. Premix burners also typically have stability
problems at high FGR rates.
SUMMARY OF INVENTION
Now in accordance with the present invention, extremely
low NOx, CO and hydrocarbon emissions are achieved, while
maintaining the desirable features of a nozzle mix
burner. Pursuant to the invention, this is accomplished
by injecting the fuel gas, such as natural gas, in a
position that would be typical for a nozzle mix burner,
while generating such rapid mixing that, effectively,
premixed conditions are created upstream of the ignition
point.
In burner apparatus in accordance with the invention an
outer shell is provided which includes a windbox and a
constricted tubular section in fluid communication
therewith. A generally cylindrical body is mounted in
the shell, coaxially with and spaced inwardly from the
tubular section so that an annular flow channel or throat
is defined between the body and the inner wall of the
tubular section. Oxidant gases are flowed under pressure
from the windbox to the throat, and exit from a
downstream outlet end. A divergent quarl is adjoined to
the outlet end of the throat and defines a combustion zone
for the burner. A plurality of curved axial swirl vanes
are mounted in the annular flow channel to impart swirl
A
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WO 95/02789 PCT/US94/0~745
to the oxidant gases flowing downstream in the throat.
Fuel gas injector means are provided in the annular flow
channel proximate or contiguous to the swirl vanes for
injecting the fuel gas into the flow of oxidant gases at
5 a point upstream of the outlet end. The fuel gas
injection means comprise a plurality of spaced gas
injectors, each being defined by a gas ejection hole and
means to feed the gas thereto. The ratio of the number
of gas ejection holes to the projected (i.e. transverse
cross-sectional) area of the annular flow channel which
is fed fuel ges by the injector means is at least 200/ft2.
One or more turbulence enhancing means may optionally be
mounted in the throat at at least one of the upstream or
downstream sides of the swirl vanes. These serve to
induce fine scale turbulence into the flow to promote
microscale mixing of the oxidant and fuel gases prior to
combustion at the quarl.
The gas injectors can be located at the leading or
trailing edges of the swirl vanes, and inject the fuel
gas in the direction of the tangential component of the
flow imparted by the swirl vanes. The gas injectors can
also be disposed on a plurality of hollow concentric
rings which are mounted in the throat downstream of the
swirl vanes. The injected can similarly comprise
openings disposed in opposed concentric bands on the
walls which define the inner and outer radii ~f the
annular flow channel. The gas injectors can also be
located at the surfaces of the swirl vanes, with the
vanes being hollow structures fed by a suitable manifold.
Preferably the geometry of the burner is such that the
product of the swirl number S and the quarl outlet to
inlet diameter ratio C/B is in the range of 1.0 to 3Ø
21 673 20
Pursuant to another aspect of the invention, a method is
provided for injection of gaseous fuel in a forced draft
burner of the type which includes an annular throat of
outer diameter B, having an inlet connected to receive a
forced flow of air and recirculated flue gases, and an
outlet adjoined to a divergent quarl. The gaseous fuel is
injected at an axial coordinate which is spaced less than B
in the upstream direction from the axial coordinate at
which the quarl divergence begins; and sufficient mixing of
l0 the gaseous fuel with the air and recirculated flue gases
is provided that these components are well-mixed down to a
molecular scale at the axial coordinate of ignition. This
procedure results in extremely low NOX, CO and hydrocarbon
emissions from the burner.
In a still further aspect of the invention, the swirl
vanes, which are mounted with their leading edges parallel
to the axial flow of fuel and oxidant gases, and then
slowly curve to the final desired angle, have a constant
radius of curvature along the curved portion of the vane,
whereby the curved portion is a section of a cylinder.
This shape simplifies manufacturing using conventional
metal fabricating techniques.
In another aspect of the invention there is provided an
improved forced draft burner of the type which includes an
annular throat of outer diameter B, having an inlet
connected to receive a forced flow of air and recirculated
flue gases, and an outlet adjoined to a divergent quarl;
and an ignition point defined at an axial location in said
quarl; the improvement enabling extremely low NOX, CO and
hydrocarbon emissions from the burner, and comprising:
means for injecting the gaseous fuel into said throat at an
axial coordinate which is spaced less than B in the
upstream direction from the axial coordinate at which quart
6a 21673Zp
divergence begins; and means for providing mixing at and
downstream of the point of injection such that the gaseous
fuel, air, and recirculated flue gases are well-mixed down
to a molecular scale at the point of ignition.
BRIEF DESCRIPTION OF DRAWINGS
The invention is diagrammatically illustrated, by way of
example, in the drawings appended hereto in which:
FIGURE 1 is a graphical depiction showing calculated NOX
versus adiabatic flame temperature for a premixed flame
with 15a excess air;
FIGURE 2 is a further graph showing kinetic calculation of
prompt NOX (HCN and NH3) ;
WO 95/02789 216 7 3 2 0 pCT~S94107745
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FIGURE 3 is a schematic longitudinal cross-sectional
view, through a first embodiment of apparatus in
accordance with the present invention;
FIGURE 4 is a schematic view sic~:~iar to Figure 3, but
showing only sufficient of the apparatus to illustrate a
modification of same in which turbulence enhancing means
are provided;
FIGURE 5 is a schematic longitudinal cross-sectional view
similar to Figure 3, and showing a further embodiment of
apparatus in accordance with the invention;
FIGURE 6 is a perspective view of the apparatus of Figure
5;
FIGURE 7 is a cross-sectional schematic view similar to
Figure 4 and showing a further arrangement for the fuel
gas injection means;
FIGURE 7A is a simplified partially sectional perspective
view of a portion of a swirl vane, showing a further
arrangement for the fuel gas injection means;
FIGURE 8 is a graphical depiction showing the effect of
mixing rates on NOX emissions;
FIGURE 9 is a graph showing the relationship between
carbon monoxide and NOX for apparatus in accordance with
the invention, as compared with conventional nozzle mix
devices;
FIGURE 10 is a graph depicting the calculated effect of
stoichiometry on NOX for premixed flames;
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In FIGURE 11 a perspective view appears of a further
embodiment of burner apparatus in accordance with the
present invention;
FIGURE 12 is a longitudinal cross-sectional view through
the apparatus of Figure 11;
FIGURES 13 and 14 are respectively front and rear-end
views of the apparatus of Figures 12 and 13.
FIGURE 15 is a perspective view of a further embodiment
of burner apparatus in accordance with the present
invention;
FIGURE 16 is an elevational view simplified and partially
in cross-section of a further embodiment of burner
apparatus in accordance with the present invention;
FIGURE 17 is a top-plan view of the apparatus of Figure
16 which is partially in section;
FIGURE 18 is an elevational view showing details of one
of the swirl vanes which may utilize in the present
invention;
FIGURE 19 is a top-plan view of the swirl vane of Figure
18; and
FIGURE 20 is an in-view of the apparatus 51 of Figure 11
showing certain relationships between the swirl vane and
the remaining portions of the apparatus.
DESCRIPTION OF PREFERRED EMBODIMENTS
Figure 3 is a longitudinal highly schematic view of
forced burner apparatus 10 in accordance with the present
invention. Apparatus 10 includes an outer shell 8 having
..._._.... _..._...__....._ _.T..._._._........~...._..._.....
WO 95/02789 216 7 3 2 0 PCT~S94/07745
9
a plenum or windbox 12 and an adjoined tubular section
13. Air and recirculated flue gases 17 are provided
under positive pressure by conventional fan means (not
shown) via a conduit 11 to burner windbox 12, from which
they proceed into the burner throat 14. The latter is a
constricted annular space defined between outer
cylindrical wall 18 of section 13, and an inner coaxial
cylindrical body 16. The latter functions as a bluff
body, and may take the form of or include an oil gun 15
(which gives the burner both gas and oil firing
capabilities); or can simply be an open or closed end
tube in the case of a gas only burner.
A set of swirl vanes 20 are mounted in the throat 14.
Typically the swirl vanes can be approximately twenty in
number, although greater or lesser numbers of vanes can
be used depending upon the burner size and specific
conditions under which the burner may be operated. Swirl
vanes 20 are designed to impart a specific amount of
swirl (S) to the flow with a minimum pressure loss. Fuel
gas injection means 21 are provided proximate swirl vanes
20. Thus at the upstream, i.e. the leading edges of each
swirl vane, hollow tubes 22 are attached, each of which
is provided with a series of small holes 24 which serve
as ejection ports for fuel gas, e.g. natural gas. The
fuel gas is fed to the series of tubes 22 by a fuel gas
feed manifold 26 which extends in toroidal fashion about
the axis of apparatus 10 and is connected to provide an
input to each tube 22. The totality of gas injection
holes in effect define a grid of injection points. The
object of this arrangement is to provide extremely rapid
mixing with the air/FGR mixture. The mixing is effected
rapidly enough to minimize regions of fluid having a
stoichiometry of less than 0.6 downstream of the ignition
point. In the apparatus 10 shown, this occurs at an
approximate axial point 25.
WO 95/02789 216 7 3 2 0 PCT/US94/07745
As is known in the art, an igniter is only used for
start-up. Once,the main flame is established, the
igniter is removed and the flame is self-stabilizing.
The ignition point is determined by the temperature and
5 mass flow rate of the internal recirculation gases, which
in turn is determined by the burner geometry and the
amount of external FGR that is used.
The high degree of mixing should be achieved down to a
molecular scale. The grid of gas injection points is
10 designed to provide a minimum of 200 injection points per
square foot of the projected area of a transverse cross-
section taken through the throat 14 at the plane of the
grid, with,the mutual spacing of the injection points
being such as to provide uniform mixing with the oxidant
gases. In Figure 3 the injection points are located to
inject the gas in the direction of the tangential
component of the flow imparted by the swirl vanes 20. In
this approach the gas injection also acts to enhance the
swirl number of the flow. The diameter (A) of the
cylindrical body 16 defines the inner diameter of the
swirling flow.
A modification of the apparatus of Figure 3 is shown in
the partial view of Figure 4. In this instance a
turbulence enhancer 27 has been mounted in the throat 14
downstream of the swirl vanes 20. The turbulence
enhancer may take the form of a fine mesh screen, its
function being to generate fine scale mixing as the
fuel/air/FGR mixture passes through the screen. In a
typical instance the screen openings will be no greater
than 1 mm.
A further embodiment of the invention appears in Figure
5. In this instance, it will be seen that the gas
injection means 21, i.e. consisting of the same basic
arrangement as aforementioned, is such that the hole
7,. _ _. ...._ _____._..~~~_____..._....T~_~.~~ i
T T
WO 95/02789 216 7 3 2 0 PCT/US94107745
11
carrying tubes 22 are now mounted at the trailing edge of
the swirl vanes 20. In this instance again the
turbulence enhancer 27, i.e. a fine screen is provided.
The screen is located a minimum of 30 gas injection hole
diameters downstream of the gas injection grid 21 in
order to provide adequate distance for macromixing to
occur. Depending upon the specific shape of the swirl
vanes 20, tubes 22 can also extend along radials, i.e.
the tubes would be vertically oriented as shown in Figure
3.
The mixture of air, recirculated flue gas and fuel gas
are flowed into a divergent quarl 28, which may be formed
of a suitable refractory in view of the high temperature
combustion taking place within such flame zone. The
quarl has a sufficient diameter expansion, i.e. the ratio
C/B (see Figure 3), to provide the desired flame
stability.
An isometric perspective depiction of the Figure 5
embodiment of apparatus 10 appears in Figure 6, which
shows the relative location of the furnace windbox 12,
the location of the swirl vanes 20 in the air/FGR throat
14 and the location of the turbulence enhancer 27. It
should be appreciated that the turbulence enhancer need
not necessarily be used with the present invention,
although in many instances it will assist in providing
the desired enhanced mixing.
A further embodiment of the present invention is depicted
in the partial longitudinal cross-sectional schematic
view of Figure 7. This apparatus is generally similar to
that of Figures 5 and 6 except that in this instance the
gas injection means instead of or in addition to
comprising tubes located just downstream of the trailing
edges or just upstream of the leading edges of the swirl
vanes, comprises a pair of hollow rings or torroids,
WO 95/02789 216 7 3 2 0 PCT~S94I07745
12
which are mounted to reside within throat 14. The outer
ring 30 carries a plurality of gas ejection openings,
oriented to eject gas in an axial direction. The smaller
hollow ring 32 similarly carries a series of ejecting
holes disposed to eject gas away from the axis of
apparatus 10. Thus the ejection holes on each of rings
30 and 32 direct the fuel gas toward the opposed ring, to
assist in mixing. Additional rings of this type may be
used in pairs or otherwise. For small burner sizes this
arrangement can simplify to gas injection from holes
provided at the inner and outer walls of the annular flow
channel which defines the throat 14.
Figure 7A shows a further fuel gas injector arrangement.
In this instance a small section 35 is shown toward a
lateral edge 37 of a swirl vane. The vane is hollow as
seen at 39 and is fed fuel gas under pressure from an
open end 41 connected to a gas feed manifold (not shown).
The fuel gas is ejected from a plurality of holes 43
provided at the surface of the swirl vane.
Typical hole size of each injector is~approximately 1/16"
or smaller, and the number of injection holes will
typically range from 625 to 1043 for ratios of the
gas/air velocity ranging from 3 to 5. However, different
gas/air velocity ratios may be used generating different
numbers of injectors according to the method described
below. The injection grid should be spaced uniformly in
the azimuthal direction, but varied in the radial
direction to give equal number of injectors per annulus
cross-sectional area (i.e. the area increases with radius
squared.
The injector hole size is based on the entrainment rate
of the air/FGR mixture into the gas jets. For a typical
case the volume of the air and FGR mixture is 15 times
the volume of the air and the desired mixing distance is
T ~. .__.__..__.~..-...r..T_._ i
WO 95/02789 216 7 3 2 0 PCT/US94/07745
13
4". The diameter of the gas jets can be calculated
according to the entrainment rate:
Mc/I"Ia = CO .32 (p,/Po) uzx/d~ _ 1
M~ - mass entrained
Mo - mass of gas j et
p, - density of air/FGR mixture
po - density of the gas
x - axial distance
d - diameter of the gas jet
For M~/Mo = 15 and x = 4" and pe/po = 1, the diameter of
the gas jet is 0.087". The gas jet diameter should be
smaller than 0.087".
The number.of gas jets can be defined by the ratio of the
gas/air velocity used. Typically the velocity ratio will
be in the range 3/1 to 5/1 and will depend on the
available gas pressure and the direction of gas injection
relative to the air velocity. The number of gas
injectors can be defined by the relative total area of
air/FGR area to the gas area and the ratio of the gas/air
injection velocity. This number is given by:
Number of gas injectors/square foot = 1/(volume ratio
oxidant/gas~gas/air velocity ratio~area single gas
injector ft2) .
For an oxidant/gas ratio = 15, an gas/air velocity ratio
of 4 and a hole size of 1/16" the number of gas injectors
per square foot of air/FGR cross-sectional area is 782.
The number of injectors should consequently be at least
782.
Pursuant to a further aspect of the present invention,
the dimensions of the annular region defined by the ratio
of the inner diameter of the swirl vanes divided by the
outer diameter of the swirl vanes, i.e. the ratio A:B in
WO 95/02789 216 7 3 ~ 0 PCT/US94/07745
14
Figure 3, is preferably in the range of 0.6 to 0.8. In
addition, the product of the swirl number (S) with the
quarl outlet-to-inlet ratio, i.e. the factor S~(C/B) is
preferably in the range of 1.0 to 3.0 in order to assure
the adequate mixing of interest to the invention. The
outlet of the quarl can be shaped to provide control of
the flame shape. For example, the outlet of the quarl
can be parallel to the burner throat to minimize the rate
of expansion of the flame in a narrow furnace. The
quarl, as mentioned, can be constructed from refractory
material or can form part of a water wall where water
cooling is utilized.
The combination of the parameters S~(C/B) between 1.0 and
3.0 and the annular ratio of the swirling flow between
0.6 and 0.8 generates a strong internal recirculation
flow far back into the quarl. This insures a sufficient
supply of hot combustion gases to the mixture of gas/air
and recirculated flue gas to insure a stable flame at
high recirculation rates.
Experiments were conducted with burners having geometries
similar to those of Figures 3 through 7, in an 80 hp
boiler where 3 MMBtu/hr represents full load. During the
burner experiments various gas injection methods were
investigated. These methods included (1) axial injection
through the central recirculation zone ("slow mixing");
(2) injection of the gas into the combustion air just
downstream of the burner throat using an annular injector
("nozzle mix"); (3) gas injectors attached to the leading
edge of the swirl vanes as per Figure 3; and (4) annular
ring injectors as per Figure 7. Procedures (3) and (4)
are in accordance with the invention. Results from these
tests are shown in Figure 8. Without flue gas
recirculation, the NOX emissions were reduced by going to
slower fuel and air mixing rates. But for FGR rates
greater than about 10%, the reverse became true, the
WO 95/02789 3 ~ p PCT/US94I07745
faster the mixing rate the lower the NOX emissions. When
the fuel/air mixing rate is slow, flue gas recirculation
has only a small effect on the NOX emissions indicating
that the majority of the NOx is prompt NOX. The rapid mix
5 cases (3) and (4) show a much greater effect on the NOX
emissions than the more traditional nozzle mix case (2).
The data in Figure 8 show that if unmixed pockets of gas
can be eliminated downstream of the ignition point, NOX
emissions can be reduced to less than 10 ppm with about
10 25% FGR (as compared to 50% for the nozzle mix injector).
If desired, the NOX emissions can be reduced to less than
4 ppm using about 50% FGR.
Another important advantage of the present invention is
15 the effect on CO and other unburned combustible
emissions. Figure 9 shows the relationship between NOX
and CO emissions for the tests described above. As the
mixing improved and lower NOX emissions could be
obtained, the CO emissions were also reduced. For the
rapid mix cases (3) and (4), less than 4 ppm NOX
emissions could be obtained with CO emissions less than
the detection limits of the analyzer (1 ppm).
The present invention provides methods of obtaining a
high degree of mixing upstream of the ignition point
while maintaining a gas injection point downstream of the
axial swirl vanes (i.e. the burner would effectively
remain a nozzle mix burner avoiding the drawbacks of a
premix burner). The method of rapid mixing is combined
with a burner and quarl geometry which provides strong
internal recirculation of hot combustion products to the
root of the flame, and an extremely stable fla~:,e. The
combination of the parameters S~(C/B) being between 1.0
and 3.0 and the annular ratio of the swirling flow
WO 95/02789 216 ~ 3 2 Q PCT~S94/07745
16
between 0.6 and 0.8 provides a suitable internal
recirculation pattern and the required flame stability.
If desired the rapid mixing of fuel and air may be
combined with air staging to reduce NOX emissions while
minimizing the amount of FGR which may be required. As
shown in Figure 10, kinetic calculations show that at an
FGR rate of 20%, a reduction in NOX emissions from 20
ppmv to 8 ppmv can be achieved if rapid mix conditions
can be created. Figure 10 also shows that operations at
stoichiometries of 0.5 must be avoided if prompt NOX is
to be eliminated.
In Figure 11 herein, an isometric perspective view
appears of a further embodiment of burner apparatus 51 in
accordance with the present invention. This Figure may
be considered simultaneously with Figures 12, 13 and 14,
which are respectively longitudinal cross-sectional; and
front and rear end views of apparatus 51. Apparatus 51
may be compared with the apparatus 10 in Figure 6, from
which it will be seen that certain similarities are
present, but also a number of differing features.
In burner apparatus 51 combustion air (which can be mixed
with recirculated flue gas) is provided to the windbox 53
through a cylindrical conduit 55. Windbox 53 adjoins a
tubular section 57 which terminates at a flange 59, which
as in prior embodiments is secured to a divergent quarl
58 (Figure 12). In the apparatus 10 of Figure 3, fuel
gas is provided by an external manifold 26. In the
arrangement shown in Figure 11, the inner co-axial
cylindrical body 61 is comprised of a central hollow
cylindrical tube 63 intended for receipt of an oil gun or
a sight glass and a surrounding tubular member or
cylinder 65 which is spaced from the outside wall of tube
63 and closed at each end, by closures 67. A hollow
annular space 68 is thereby formed between tubular member
_ ,
I _ ..._.._. _..__~~..~_.,T._~.__. , _ i
WO 95/02789 216 7 3 2 0 pCT~S94/07745
17
63 and cylinder 65, which serves as a manifold 68 for the
fuel gas which is provided to such space via connector
69. The cylindrical body 61 is positioned and spaced
within wind box 53 and tubular section 61 by passing
through flanges, one of which is seen at 71. The latter
is secured to a plate 73 at the end of the wind box by
bolts 75 and suitable fasteners (not shown). This
arrangement enables easy disassembly, as for servicing
and the like.
In the arrangement of burner 51, a series of swirl vanes
77 are again provided in the annular space or throat 79
which is defined between tubular body 61 (specifically,
between the outer wall of cylinder 65) and the inner wall
of tubular member 57. At the immediately upstream end of
each of the swirl vanes 77, gas injector means are
provided which take the form of a plurality of tubes 81,
each of which is provided with multiple holes 83, this
arrangement being in such respect similar to the device
shown in Figure 3. It will be evident that the tubes 81,
being hollow members, are in communication at their open
one end with the interior of the gas manifold 68 defined
within member 65, which therefore serves as a feed source
for the fuel gas. The fuel gas is discharged in the
direction of the openings 83, so that in each instance
fuel is injected into the throat directly at the leading
edges of the swirl vanes and in the direction of the
tangential component of the flow imparted by the swirl
vanes 77. Accordingly, the gas injection also acts to
enhance the swirl number of the flow.
In Figure 15 a further perspective view appears of burner
apparatus in accordance with the invention. The
apparatus 85 in Figure 15 is in most respects similar to
apparatus 51 in Figures 11 through 14, and identical
components are identified by corresponding reference
numerals. In the instance of apparatus 85, the method of
WO 95/02789
216 7 3 2 0 pCT~S94/07745
18
fuel gas introduction is different from that shown in
Figures 11 through 14, and in fact uses principles
similar to those shown in the apparatus of Figure 7.
Specifically, it will be seen that the fuel gas
introduced by connector 69 to the interior gas manifold
68 (see Figure 12) defined between tubular member 65 and
the inner tube 63, is injected into throat 79 by a series
of holes or openings 87 which are~disposed in a band
extending circumferentially about the tubular member 65.
These holes are seen to be directly adjacent and
virtually contiguous to the downstream end, i.e. to the
trailing edges of swirl vanes 77. Gas injection from
openings 87 is seen to be in an outward radial direction
with respect to the axis 101 of apparatus 85. Further,
it is seen that a second gas manifold 95 is formed as an
annular space surrounding tube 57, by a cylinder 89 which
is closed at both ends 91 and 93. The annular gas
manifold 95 is thus seen to be present between cylinder
89 and tubular member 57. An inlet for fuel gas is again
provided by a connector 97. A second series of holes or
openings 99 are disposed in a band about the wall of
cylinder 57, so that fuel gas may be injected from
manifold 95 through such openings 99. In this instance,
the gas is injected radially but toward the axis 101 of
the apparatus. Thus, as was discussed in connection with
Figure 7, the holes on the one hand at 99 and at the
other at 87, provide opposed gas injection between the
bands of holes, to produce a high degree of turbulence
and mixing directly at the trailing edges of swirl vanes
77. The arrangement shown is particularly suitable where
the apparatus 85 is of relatively compact dimensions.
In Figures 16 and 17, elevational and plan views appear
of further apparatus 110 in accordance with the
invention. These views are somewhat simplified and
schematic in nature and may be considered simultaneously
in connection with this description. Windbox 112, as
t __. _ . _. _.._~~_ __.._~._._... _ _.
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19
best seen in Figure 16, is fed combustion air and flue
gas in the direction 114 (by pressurizing means not
shown). The entirety of the windbox is not shown, but is
rather broken away at its upper end. The arrangement of
apparatus 110 enables a more compact device than certain
of the prior apparatus discussed. Specifically, it will
be seen that a constricted tubular section 115 is
provided, which is in direct communication with the
interior of wind box 112 through the open end defined by
diverging flange 116. The flange 116, while shown to
diverge linearly, can also be dish-shaped to assist in
air flow. Combustion air proceeds through the annular
space 118 defined between tubular member 115 and the
generally cylindrical body 120 mounted coaxially within
said member. Cylindrical body 120 consists of a central
tube 122 within which is received an oil gun 124
terminating in a nozzle 126. Oil is provided to gun 124
by port 125. Tube 122 in turn is surrounded by a spaced
tubular member 130. The spacing between tube 130 and
tube 122 defines an annular space 132 the function of
which will be indicated below. Tube 130 is, in turn,
surrounded toward its forward end by a further cylinder
134 which is closed at each end and defines within same
an annular fuel receiving manifold 136. Fuel from
manifold 136 is fed via a connector 138. The gas
injector means in the present device 110 comprises a
series of prism-like hollow members 139 which are mounted
transversely to cylinder 134 and intersect and are open
to the interior manifold space 136 within same. The
members 139 are provided on their lateral faces with
openings 140 which substantially correspond in function
to the openings 83 in Figure 11. The members 139 are
directly in contact with and contiguous with the leading
edges of swirl vanes 142, so that the gas is injected
directly at such leading edge. As in prior embodiments,
a diverging quarl 144 is provided at the outlet end of
burner throat 118.
WO 95/02789 216 7 3 2 0 pCT~S94/07745
It is seen further that an elbow-shaped conduit 148
connects the interior of one side of windbox 112 to the
interior annular space 132. A manually or non-manually
operated gate valve 150 may be actuated to open or close
5 a flow path between the windbox 112 and space 132. When
the valve is in an open position, an air flow is provided
into space 132 which then proceeds forwardly in the
device and passes about the periphery of the oil nozzle
126. Steam or other actuating gas (such as compressed
10 air) may be fed to the rear of the tube 131 to assist ~(as
is known in the art) in spraying or atomizing the oil
into the combustion volume. The air stream flow moving
past nozzle 126 prevents or limits coke and ash particles
from depositing on the oil gun during oil firing.
15 The details of the swirl vane constructions which are
preferably used in accordance with the present invention
are set forth in the views of Figures 18, 19 and 20.
Referring first to Figure 18, a vane 160 is shown which
may be considered to be one of the vanes 77 in apparatus
20 51 of Figure 11. This is representative of the swirl
vanes which may be used in any of the. apparatus depicted
in the drawings herein. The shadow line version of the
vane as seen at 162 indicates the form of the vane in
plan view before it is bent to achieve a desired
curvature in accordance with the invention. The vane 160
is seen to be secured, as previously discussed in
connection with apparatus 51, to the injector tube 83 as,
for example, by being welded to same. The corresponding
plan view of the vane of Figure 18 is seen in Figure 19,
which again shows the vane before and after the bending
to achieve the desired shape for use in the invention.
The end view of Figure 201 shows the vane in its
installed position in apparatus 51. Corresponding parts
as discussed in Figure 11, are identified by
corresponding reference numerals.
T_.~__ _~.__~.__.~ .._ _ i
T
WO 95/02789 216 7 3 2 0 pCT~S94/07745
21
The burner apparatus of the invention uses the fixed
curved axial swirl vanes in the burner throat 79 to
impart a given swirl level to the flow. The vanes are
called axial swirl vanes because of the manner in which
they convert an axial flow to a swirling flow. The swirl
vanes used in the burner are designed to provide the
desired flow pattern with a minimum pressure drop.
Additionally, the vane is shaped to simplify
manufacturing using conventional metal fabricating
techniques.
Swirling flows are commonly used in burners to improve
flame stability and to improve fuel and air mixing. When
a swirling flow is expanded, an internal recirculation
zone is created which recirculates hot combustion
products to incoming air and fuel, thus providing an
ignition source. The objective of a swirl generator is
to provide a certain level of rotation to the flow in
order to provide the required amount of internal
recirculation with a minimum energy requirement (pressure
drop). In the burner apparatus of the invention, the
swirl vanes also act as a mixing device for the gas and
air/FGR mixture. When the gas is injected at the leading
edge of the vane, fine scale turbulence is created as the
flow acquires rotation generating the desired rapid
mixing between the gas and oxidant.
Pursuant to the present invention, the leading edge of
the vane is parallel to the axial flow. The fluid is
accordingly slowly curved to the final angle. A constant
radius of curvature is used along the surface of the
curved portion of the vane (the curved portion of the
vane is a section of a cylinder). This is significant in
allowing the vanes to be easily manufactured using
conventional rolling equipment. The swirl vane assembly
is annular with an inner to outer radius R1/R3 (as shown
in Figure 20) in the range of 0.6 to 0.7. The exit angle
PCT/US94/07745
WO 95/02789 2 1 b 7 3 2 0
22
of the vane is constant along the height of the vane
producing a constant flow angle of the azimuthal to axial
velocity (W/U). A straight vane section oriented at the
final flow angle is attached to the trailing edge of the
curved section. This is shown at ABC in Figure 19. The
straight vane section is designed to produce a constant
overlap between the vanes as a function of the vane
height.
Referring to Figure 19, the vane is curved with a
constant radius of curvature until the exit angle is
achieved (point A). The vane then continues at a fixed
angle (straight portion). The length of the straight
portion varies with the vane height. In Figure 18 the
length of the straight portion of the vane along the
inner tube is given by the distance from points A to B,
while the length along the outer tube is given by the
distance from points A to C.
The vane is curved in a circular arc with the radius of
curvature given by:
(1) R~ _ (Overlap Factor) (6.28 ~ RI) /N/ (1-cos (a) )
where
Overlap factor = (100 + % vane overlap)/100
R, = innet tube radius
N = total number of vanes, and
a = vane exit angle
A typical % overlap to be used in equation (1) is 50%,
but may be any number greater than about 20%. A typical
exit angle is 45 degrees but may vary in the range 30 to
60 degrees.
Because the use of a constant radius of curvature results
in a reduction of the vane overlap for the curved portion
of the vane with increasing height of the vane between RZ
T ___ _ . _~_ .w. i
PCT/US94/07745
WO 95!02789 216 7 3 ~ fl
23
and R,, a straight section is added to the vane. The
shape of the straight section of the vane which continues
at the vane exit angle, a, is determined according to the
following criteria.
Along the inner surface of the vane, the surface attached
to R1, the straight vane length is a small fraction of the
curved vane length (5 to 10% of the curved vane length
would be a typical fraction). The length of the straight
vane along the outer surface is determined such that the
distance Y2, Figure 20, is the same as the distance
Y, ~R2/R, along the inner surface . The length of the
straight vane section at any intermediate radi-.as R~ is
determined such that Y" is equal to Y~ ~R~/R~ .
The vane 160 may be fabricated by several techniques.
These include cutting out the flat vane shape and
following the curved portion of the vane; cutting out the
flat vane shape and stamping the final shape; and casting
the blade directly into the final shape.
While the present invention has been particular set forth
in terms of specific embodiments thereof, it will be
understood in view of the present disclosure, that
numerous variations on the invention are now enabled to
those skilled in the art, which variations yet reside
within the scope of the present teaching. Accordingly,
the invention is to be broadly construed and limited only
by the scope and spirit of the claims now appended
hereto.
