Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TURBINE COMBUSTOR ~LAVING MORE UNIFORM
MIXING OF FUEL AND AIR FOR IMPROVED
DOWNSTREAM COMBUSTION
CROSS REFERENCE TO RELATED APPLICATIONS
None
BACKGROUND OF THE INVENTION
The present invention relates to combustors
employed in land based combustion turbines and more par-
ticularly to combustors in which substantially uniform
m;x;ng of ~uel and air across the combustor mix;ng zone is
needed prior to entry of the mix into the combustion zone,
i.e. the catalyst in catalytic combustors.
Prem;~;ng of fuel and air in premix combustors
is needed to provide long combustion life, high combustor
efficiency and low emissions through proper combustor
operating temperatures and chemistry. Catalytic combus-
tors provide a prospective commercial al~ernative for low
pollutant, and especially low NOx, combustion turblne
operation for electric power plants and other land based
applications, and proper catalytic combustion especially
requires substantial uniformity in the prem;~;ng of fuel
and air within the combustor m;x;ng zone.
With the operating compressor discharge pressure
in most engine designs, some preheating of fuel/air mixture
is needed for proper catalytic combustion. Thus, a catalytic
combustor may be provided with a generally tubwlar envelope
having a primary combustion ~one followed in sequence
first by a s~condary fuel injection and m;x;ng zone and
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finally by a catalyst. The primary combustion æone oper-
ates for example during startup when operating tempera-
tures do not adequately support catal~tic combustion.
During the catalytic combustion phase of operation, secon-
dary fuel is injected into the mixing zone where it mixeswith air for delivery to the flow channels through the
catalyst.
Typically, the secondary fuel injectors are
disposed circumferentially about the mixing zone and they
may inject fuel radially inwardly at a right or other
angle into the combustor mixing zone. Further, the fuel
must be essentially completely vapori~ed before entering
the catalyst which requires that the fuel nozzle produce
very small droplets which can evaporate rapidly. Small
droplets can be obtained by using a very high fuel nozzle
pressure drop (pressure atomization~, by using a small
amount of high energ~ atomizing air (air assist), or by
using a relatively large amount of low energy atomizing
air (air blast). In all cases, the momentum of the resul-
ting fuel spray is quite high. In fact the momentum ofthe fuel spray with respect to the momentum of the cross
flowing air inside the combustor is high enough that the
uel tends to penetrate to the center (axis) o the com-
bustor. This action produces a fuel rich core, i.e. the
fuel/air ratio profile has a sin~le center peak shape
across a reference diameter of a cross section of the
combustor mixing zone.
The fuel/air ratio is highest at the axis in the
fuel injection plane or region, and it decreases in the
radial outward direction. As the mix flows downstream
through the mixing zone, additional mixing action causes
the uel/air ratio profile to flatten somewhat. In gen-
eral, however, the scope of fuel penetration in the injec-
tion region has resulted in too much axial fuel concentra~
tion to permit available downstream mixing to produce a
substantially uniform fuel/air ratio distribution at the
catalyst entry plane.
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SUMMARY OF THE INVENTION
In accordance with the present invention, im-
proved operation is obtained in combustors and especially
catalytic combus-tors through structure which assists
deflection of injected fuel in the axial direction to
produce more uniform mixing of fuel and air in a mixing
zone located immediately upstream from the zone ~here
combustion occurs. Peferahly, the structure includes
circumferentially distributed holes in the combustor wall
upstream from the fuel injectors such that entering air
streams are angled downstream. The entering air streams
have high velocity due to the pressure drop across the
combustor wall and accordingly greatly assist the internal
gas flow in axially deflecting the injected fuel and
producing a substantially uniform fuel/air ratio profile
at the catalyst entry plane.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows an elevation view of a catalytic
combustor having portions thereof cut away and being
arranged in accordance with the principles of the inven-
tion.
Figure 2 shows a schematic diagram of a cata-
lytic combustor like that of Ei~ure 1 with operating
features of the invention illustrated in greater detail.
Fi~ure 3 shows a diagram like that of Figure 2
but representing an alternative embodiment in which exter-
nal air scoops are employed.
Figure 4 shows test results obtained with use of
the present invention as compared to results obtained with
a prior art reference.
Figure 5 shows a diagram of a prior art combus-
tor configuration used in obtaining comparative test
results.
DESCRIPTION OF THE PREFERRED EMBODIMENT
More particularly, a catalytic combustor 10 is
shown in Figure 1 for a land based combustion turbine
which is typically used in electric power and other indus-
trial plants.
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The combustor 10 includes a generally tubular
sidewall 12 having successive circumferential row of
holes 14, 16 for entry of air used in the combustion
process. A-t a head end 18 of the combustor 10, a primary
fuel nozzle 20 admits fuel for burning in a primary zone
22 to yenerate the energy needed for startup until opera-
ting conditions support catalytic combustion. In addi-
tion, the primary nozzle 20 supplies some fuel for primary
combustion during catalytic operation to provide any
preheating needed to keep the gas temperature at a cata-
lyst entry pla~e~4 at the value needed (i.e. approxi-
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mately 1~00~-- 195~Q~) for efficient catalytic combustion.
The overall combustor operation involves amounts of pri-
mary fuel combustion such that NOx production is well
below prescribed environmental limits.
An outlet end 26 of the combustor wall 12 is
outwardly flared and coupled to a conventional catalyst
element 28 ha~ing a honeycomb structure. In turn, the
catalyst outlet 30 is coupled to a transition duct (not
shown) which directs the ho-t gases to the turbine (not
shown3.
Secondary fuel is injected into the combustor 10
during the catalytic combustion phase of operation by a
set of circumferentially spaced nozzles 32 at the down-
stream end o the primary combustor zone 22. Air may ormay not enter the combustor 10 at the nozzle locations. A
combustor region 34 between the primary zone and the
catalyst element 28 provides for mixing of the secondary
fuel and air prior to its entry into the catalyst 28. The
region 34 is referred to as a mixing zone, and combustion
does not occur in this zone since 1ashback can damage the
combustor and/or catalyst 28. As more fully described in
conn~ction with Figures 2 and 4, a circumferential row of
air holes 36 immediately upstream of the secondary fuel
~JCCtlon3 in the combustor sidewall are angled to admit
air in the downstream direction to produce uniform mixing
of the secondary fuel and air in the mixing zone 34.
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As shown in the enlarged view of Figure 2,
internal angular scoops 37 are provided for produciny an
angled air stream flow 39 through the holes 36 so as to
assist in deflecting the secondary fuel to produce a
substantially uniform fuel/air mixture~_for the catalyst
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38. As shown in Figur~e 2, the angled~air stream 39 signif-
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icantly assists~internal crossflow air 41 in deflecting
the fuel spray produced by the secondary fuel nozzles 32.
In Fiyure 3, an alternate embodiment is illustrated in
which external scoops 33 produce similar fuel-air mixing
action.
Generally, the fuel/air distribution is con-
trolled and the center peaked fuel/air mix situation is
avoided by taking advantage of the pressure drop across
the combustor wall or liner 12. This pressure drop is
typically high enough that the velocity of the air enter-
ing the combustor 10 through holes is much higher than
that of the air already flowing inside the combustor 10.
Therefore, the momentum flux (momentum per unit area per
unit time) of the entering air is much higher. With
plunged holes or scoops located just upstream of the fuel
spray and angled downstream, the high velocity of the air
admitted through the holes provides a basis for avoiding
the nonuniform center peaked fual/air mix situation. In
fact, the angle of the holes can be varied to control the
fuel/air mix profile entering the catalyst 28.
With the provision of angled air admission as
described, sidewall injection of fuel for catalytic com-
bustors is capable of giving the needed even fuel/air
mixture approaching the catalyst.
As shown by test results in Figure 4, the cata-
lyst outlet temperature, which reflects the catalyst entry
fuel/air ratio profile, shows a relatively even distribu
tion 44 (i.e. a generally flattened shape) for an embodi-
ment of the invention as compared to the cent.er peaked
distribution for the prior art. Figure 5 shows the config~
uration used for the prior art in the test while Eigure
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shows the invention configuration used in the test. The
provision of angled air streams in the invention confiyura-
tion is the principal reason for the improvem~nt. I'he
improved mixing is believed to occur as a result of deflec-
tion of the fuel spray by the angled air s'cream to a moreadvantageous mix location and/or possibly as a result of
air boosted turbulent kinetic energy in the region where
the secondary fuel spray enters the combustor.
The following are the conditions applicable to
the test of Figure 4:
COMPARISON OF CATALYST OUTLET TEMPERATURE
PROFILES FROM FULL SCALE TESTS IN CONCORDVILLE LAB
Prior Art Invention
AIR INLET TEMP.: 757F 701F
AIR INLET PRESS.: 100.1 psig 151.6 psig
CATALYST APPROACH
VELOCITY: 73.6 fps 79.4 fps
PRESSURE DROP: 3.47% 3.85%
(CATALYST3
FUEL/AIR RATIO: 0.0139 0.0136
ATOMIZING AIR
PRESS. DROP,
~SECONDARY NOZZ.:3 73.5 psid 185.3 psid
