Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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IMPROVED LOW-NOX, RICH-LEAN COMBUSTOR
BACKGROUND_ OF THE INVENTION
The present invention relates to combustion
turbines as may be employed in a variety of uses, such as
industrial processes, electric power generation, or air-
craft engines. More particularly, the present inventionis directed to combustors employed in combustion turbines
for heating motive gases which drive the turbine.
In general terms, a typical prior art combustion
turbine comprises three sections: a compressor section, a
combustor section, and a turbine section. Air drawn into
the compressor section is compressed, increasing its
temperature and density. The compressed air from the
compressor section flows through the combustor section
where the temperature of the air mass is further increased.
From the combustor section the hot pressurized gases flow
into the turbine section where the energy o the expanding
gases is transformed into rotational motion of a turbine
rotor.
A typical combustor section comprises a plurality
of combustors arranged in an annular array about the cir
cumference of the combustion turbine. In conventi.onal
combustor techIlology, pressurized gases flowing from the
compressor section are heated by a diffusion flame in the
combustor before passing to the turbine section. In the
difusion flame technique, fuel is sprayed into the up-
stream end o a combustor by means of a nozzle. The flame
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is maintained immediately downstream of the nozzle by
strong aerodynamic recirculation. The lack of thorough
mixing of the fuel results in pockets of high fuel concen-
tration and correspondingly high combustion reaction
temperatures. Because the reaction temperature is high,
hot gases flowing from the combustion reaction must be
diluted downstream by cool air so as to prevent damage to
turbine components positioned downstream. In addition,
the flame diffusion technique produces emissions with
significant levels of undesirable chemical compounds,
including NOx.
NOx results from two basic mechanisms. Thermal
NOx is produced from the combination of nitrogen and
oxygen in the fuel oxidizer (air) during and after the
combustion process when the temperature level is suffi
ciently high to permit the overall reaction of
N2 2 2NO
to occur at a measurable rate. The thermal NOx reaction
occurs for all co~bustion processes using air and is
essentially independent of the fuel.
NOx is also formed from fuel-bound nitrogen,
which forms NO-type compounds in the combustion process in
a manner somewhat analogous to the formation of CO and C02
from fuel carbon and H20 from fuel hydrogen. The difer-
ences between the two mechanisms for forming NOx lie inthe time and temperature of the combustion process.
Fuel-bound nitroyen compound~ appear virtually simultan-
eously with the CO, C02, and H20, while the formation of
NOx from the oxidi7er appears later and is governed by a
kinetic rate mechanism.
Increasing environmental awareness has resulted
in more stringent emission standards for NOx. The more
stringent standards are leading to development of improved
combustor technologies. One such improvement is a pre-
mixing, pre-vapori7ing combustor. In this type of com-
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bustor, fuel is sprayed into a fuel preparation zone whereit is thoroughly mixed to achieve a homogeneous concentra-
tion which is everywhere within definite limits of the
mean concentration. Additionally, a certain amount of
fuel is vaporized in the fuel preparation zone. Fuel
combustion occurs at a point downstream from the fuel
preparation zone. The substantially uniform fuel concen-
tration achieved in the fuel preparation zone results in a
uniform reaction temperature which may be limited to
approximately 2000 to 3000F. Due to the uniformity of
the combustion, the pre-mixing, pre~vaporizing combustor
produces lower levels of thermal NOx than does a conven-
tional combustor using the same amoun-t of fuel. NOx
formed from fuel-bound nitrogen is tolerable because of
the comparatively low nitrogen content of the typical
petroleum fuel utilized.
The increased environ~ental awareness of recent
years regarding emissions standards has been accompanied
by a recognition of the limited availabilit~ of petroleum
fuels. Consequently, a trend has developed focusing on
the use of nonpetroleum fuels for combustion turbines.
Nonpetroleum fuels typically have a higher nitrogen content
than do petroleum fuels. For example, a typical petroleum
fuel might have a nitrogen content of 0.1% by weight,
while coal-derived liquids contain fuel-bound nitrogen up
to 1% by weight and oil shale-derived liquid fuels contain
fuel-bound nitrogen up to 2% by weight. Because they do
not inhibit NOx formed from fuel-bound nitrogen, pre-
mixing, pre-vaporizing combustors would likely fail the
stringent Nx standards when operated with nonpetroleum
fuels.
Hence, it appears that known prior art combustors
do not adequately provide for low-NOx emissions when
operated with nonpetroleum fuels.
SU~MAR~ OF THE INVENTION
.,
Accordingly, a combustion turbine combustor
arranged to achieve low-NOx emissions comprises a basket,
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means for injecting fuel into the basket, means for pro-
viding fuel-rich combustion in a primary combustion zone,
and means for providing fuel-lean combustion in a secondary
combustion zone. The fuel-rich combustion disassociates
fuel-bound nitrogen and inhibits the formation of NOx due
to the oxygen-deficient atmosphere. The fuel-lean combus-
tion, while completing the combustion process, is carried
out at temperatures too low to enable the formation of
thermal NOx. Hence, stringent NOx emission standards may
be adhered to when nonpetroleum as well as petroleum fuels
are used to uel the present combustor.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows a longitudinal section of a
land-based combustion turbine arranged for the production
of electric power; in particular, a combustor is depicted
within the combustion turbine;
Figure 2 shows a sectional view of the combustor
shown in Figure l;
Figure 3 shows an alternative embodiment of the
wall of the combustor shown in Figure 2;
Figure 4 shows a third embodiment of the wall of
the combustor shown in Figure 2; and
Eigure 5 shows an alternative embodiment of the
downstream portion of the combustor shown in Figure 2.
2S DESCRIPTION OF THE PREFERRED EMBODIMENT
More particularly, there is shown in Figure 1 a
combustion turbine lO having a plurality of generally
cylindrical combustors 12. Fuel is supplied to the com-
bustors 12 through a nozzLe structure 14 and air is sup-
plied to the combustors 12 by a compressor 16 through air
flow space 18 within a combustion casing 20.
Hot gaseous products of combustion are directed
from each combustor 12 through a transition duct 22 where
they are discharged into the annular space through which
turbine blades 24, 26 rotate under the driving force of
the expanding gases.
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In accordance with the principles of the inven-
tion, combustor 12 is arranged to provide improved, low-
N0x combustion emissions when operated with nonpetroleum
fuels as well as with petroleum fuels. The combustor 12,
shown in greater detail in Figure 2, comprises a generally
cylindrical outer metal jacket 30 having a conical-shaped
upstream end 32 and being open-ended at the downstream end
34. The conical end 32 of the metal jacket defines a
centrally positioned opening 36 having a pressure atomiziilg
fuel injector 38, of a type well known in the art, pro-
truding therethrough.
A ceramic cylinder 40, within the metal jacket
30, surrounds a rich burn zone 42 within the combustor 12.
The ceramic cylinder 40 may comprise a monolithic cylinder
or a cylinder formed from a plurality of sections. An
expansion layer 44, comprising, for example, a network of
wire mesh, separates the ceramic cylinder 40 from the
metal jacket 30. The expansion layer 44 compensates for
the different rates of thermal expansion of the ceramic
cylinder 40 and the metal jacket 30. A plurality of bleed
ports 45 in the metal jacket 30 provide a source of cooling
air to the expansion layer 44. ~n insulating layer 46,
comprised of suitable insulating material, separates the
ceramic cylinder 40 from the expansion layer 44.
2S A flame tube 48 protrudes through the combustor
wall (comprising at this point metal jacket 30, the expan-
sion layer 44, the insulating layer 46, and the ceramic
cylinder 40) at a polnt immediately downstream of the fuel
injector 38. The flame tube 48 connects a torch igniter
50 to the rich burn zone 42, provlding a hot flame jet for
positive ignition of the combustor. Downstream of the
flame tube 48, the combustor wall defines an annular ring
of radially extending primary air ports 52 for delivery of
an air supply for combus~ion in the rich burn zone 42.
A ~uench zone 54, downstream of the rich burn
zone 42, comprises a Venturi-shaped section of the interior
combustor wall. The combustor wall surrounding the quench
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zone 54 comprises the metal jacket 30 encasing cast ceramic
56. The cast ceramic, which is shaped to achieve the
Venturi effect, is affixed to the metal jacket 30 by metal
retainers 58 which are attached, such as by welding, to
the metal jacket 30 and cast within the ceramic 56. The
metal retainers 58 may be arranged in any fashion, such as
the helical arrangement depicted in Figure 2, which ensures
the rigid attachment of the cast ceramic to the metal
jacket 30.
The throat of the Venturi-shaped combustor wall
surrounding the quench zone 54 defines a plurality of
annularly disposed cooling air ports 60 extending radially
through the combustor wall (comprising at this point the
metal jacket 30 and the cast ceramic 56) for the delivery
of cooling air to hot gaseous products produced in the
primary burn zone 42.
A lean burn zone 62, positioned downstream of
the quench zone 54, comprises a catalytic section 64 for
secondary combustion of the gaseous products from the rich
burn zone 42. The catalytic section 64 is surrounded by
an expansion layer 66 of the same structure as the expan-
sion layer 44 surrounding the rich burn zone 42. The
expansion layer 66 is surrounded and contained by the
metal acket 30
In operation, the atomi~ing fuel injector 38
sustains a diffusion flame in the fuel-rich atmosphere of
the rich burn zone 42. Utilization of a diffusion flame
for combustion of nonpetroleum liquid fuels has heretofore
not been acceptable (according to known prior art) due to
the problems associated with this technique. The ceramic
cylinder 40 encasing the rich burn zone 42 eliminates the
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;~ ~ for~film-cooling~the interior wall of the com-
bustor~ fea4~nF-ryF~ ~r~r~a~. The lacX of
film cooling within the rich burn zone enables the success
of fuel-rich combustion and actually enhances the combus-
tion process by maintaining the walls at an elevated
temperature.
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The fuel equivalence ratio of a combustion zone
is defined as the ratio of the actual fuel-to-air ratio to
the stoichiometric fuel-to-air ratio. A lean combustion
zone may have a fuel equivalence ratio as low as 0.4,
while a rich combustion zone may operate at a value as
high as 2.5. It is suggested that the rich burn zone of
the present invention may operate favorably at a fueL
equivalence ratio of 1.7.
Fuel-rich combustion provides an oxygen deficient
atmosphere in which the relatively inactive fuel-bound
nitrogen molecules, disassociated from the fuel by the
combustion process, cannot compete with carbon and hydrogen
for the limited oxygen molecules. Consequently, most of
the nitrogen leaving the rich burn zone 42 is in the form
of free nitrogen (N2), rather than in the form of N0x.
The hot gaseous products leaving the rich burn
zone 42 are quickly diluted to a cooler temperature within
the quench zone 54. The Venturi shape of the quench zone
54 promotes thorough and homogeneous mixing of the cooling
air supplied to the poxts 60 with the gaseous products
from the rich burn zone.
The combustion process is completed in the lean
burn zone 62, where the gaseous products from the rich
burn zone 42, such as C0, smoke, and other unburned fuel
components, are passed through the catalytic section 64.
Combustion within the catalytic section 64 occurs at a
temperature significantly reduced from the reaction tem-
perature in the rich burn zone. The formation of thermal
NOX is minimized by the lower lean combustion reaction
temperature, which in essence limits the reaction rate of
the formation of N0x. Hence, the combustor 12 produces
low-N0x emissions by disassociating the fuel-bound nitrogen
in a rich combustion reaction in the rich burn zone 42 and
completing the combustion procass at temperatures too low
for the formation of thermal N0x. The formation of thermal
N0x within the rich burn zone is inhibited by the defi-
ciency of the oxygen molecules necessary for the reaction.
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Figure 3 shows an alternative embodiment for the
combustor wall surrounding the rich burn zone 42. This
embodiment comprises a structure substantially similar to
that of the combustor wall surrounding the quench æone 54.
In the alternative embodiment, the rich burn zone is
surrounded by a ceramic layer 70 cast to the metal jacket
30 and affixed to the metal jacket by metal retainers 72.
Figure 4 depicts an alternative embodiment for
the wall of the combustor 12. This embodiment comprises
the outsr metal jacket 30 surrounding an inner metal
jacket 74, the jackets 30, 74 extending from the dome 32
to the downstream end 34 of the combustor 12. Cooling
air, depicted at 76, enters the space between the metal
jackets 30, 74 at the upstream end of the rich burn zone
42. The cooling air circulates around the primary air
supply ports 52 to reach the cooling air ports 60. In
thi~ embodiment, the cooling air which entered at 76 cools
the inner metal jacket 74 along the rich burn zone and
provides the sole source of cooling air used within the
quench zone to dilute the temperature of the hot gaseous
products leaving the rich burn zone. Some of the cooling
air which entered at 76 is diverted to cool the inner
metal jacket downstream of the cooling air ports 60.
Figure 5 depicts an alternative embodiment for
the lean burn zone 62. In this embodiment, the lean burn
zone comprises a straight cylindrical section, structured
substantially similar to the rich burn zone 42 of Figure
2, or the rich burn zone of Figure 3. In this embodiment,
lean combustion is accomplished at the lower temperatures
of the gases within the lean burn zone, which temperatures
are still high enough to ensure combustion. Further, the
ceramic wall 80 surrounding the lean burn zone 6~ enhances
the secondary combustion process.
Hence, the present invention provides an effi~
cient combustor for achieving low-N0x emissions from the
combustion of nonpetroleum as well as petroleum fuels.
Combustion in a fuel-rich burn zone disassociates fuel-
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inhibits the formation of thermal NOx and combustion is
completed in a fuel lean combustion zone at temperatures
too low to allow the formation of thermal NOx.
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