Note: Descriptions are shown in the official language in which they were submitted.
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CATALYTIC COM~USTOR HAVING SECONDARY
FUEL INJECTION FOR LOW NOX STATIONARY
COMBUSTION TURBINES
BACKGROUND OF THE INVENTION
The present invention relates to stationary com-
bustion turbines and more particularly to the implementa-
tion of catalytic combustion in such turbines to charac-
terize the turbine operation with low NOX emissions.
Various schemes have been undergoing development
to provide combustion turbines which generate electric
power or run industrial processes without exceeding NOX
emission limits. The use of catalytic combustion is a
promising approach because catalytic combustion can occur
at about 2300F to 2500F to produce a high turbine inlet
temperature for turbine operating efficiency without any
significant side effect NOX generation from reactions
between nitrogen and oxygen. In contrast, conventional
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flame combustion at about 4500F results in NOX generation
which typically exceeds the limits set in more restrictive
areas such as California and Japan.
In the operatlon of the conventional turbine
combustion process, compressor discharge air is supplied
at an elevated temperature to support the combustion of
fuel supplied through one or more nozzles at the upstream
end of multiple combustor baskets. Combustion products
are directed through ducting to the turbine blades.
10For catalytic combustion to occur, fuel and air
must be mixed and supplied to the entry side of a catalyst
unit at an elevated temperature determined by chemical
characteristics of the catalyst employed in the catalyst
unit. In turn, the temperature of the compressor dis-
charge air used in the fuel-air mix depends on ,he com-
pression ratio of the compressor which is based on overall
turbine design considerations. For any particular com-
~; pressor design, the compressor discharge temperature also
depends on the operating point o:E the turbine during the
startup and load operating modes. Generally, as turbinespeed or load increases, the compressor discharge air
temperature increases.
Thus, in applying a catalytic combustion process
to combustion turbines a need exists to provide for tur-
bine system functioning where compressor discharge air issupplied at a temperature below the minimum temperature
needed for catalytic reaction. In the known prior art,
U.S. Patents 3,928,961 issued December 30, 1975 to W. C.
Pfeferle and 4,112,675 issued September 12, 1978 to Paul W.
:` 30 Pillsbury et al appear to address this need with various
limitations.
SUMMARY OF THE INVENTION
A catalytic combustor for a stationary gas tur-
bine comprises a combustor basket coupled to a catalytic
; 35 unit and l~aving a sidewall that defines an upstream pri-
mary combustion zone in which fuel is burned to produce
hot preheating gases in a downstream secondary zone.
Secondary fuel injection means is mounted relative to a
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casing of the turbine and the combustor basket to provide
for convenient secondary fuel assembly removal. The
secondary fuel is injected for mixing with air and the hot
in-ternal gases to provide a well mixed fuel-air mixture
for combustion in the catalytic unit when catalytic reac-
tion conditions are reached.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 schematically shows a catalytic com-
bustion system arranged to operate a stationary gas tur-
bine in accordance with the principles of the invention;
Figure 2 shows an elevational view of a cataly-
. tic combustion system disposed in a turbine and structurein accordance with principles of the invention;
Figures 3A and 3B show an enlarged view of the
combustion system of Figure 2;
Figure 4 shows an enlarged cross section of
secondary nozzle mounting structure taken along reference
line IV-IV of Fi~ure 3A; and
Figure 5 shows a portion of a vertical section
taken through a secondary fuel noz~le sho,wn in Figure 3A,
Figure 6 shows another embodiment of the inven-
tion in which the combustor basket is provided with a
necked down portion to promote secondary fuel-air mixing.
~ESCRIPTION OF THE PREFERRE,D EMBODIMENT
General Structural Concepts
More particularly, there is shown in Figure 1 a
~ generali~ed schematic representation of the preferred
,~ embodiment of the invention.
A turbine or generally cylindrical catalytic
combustor 10 is combined with a plurality of like combus-
~' tors (not shown) to supply hot motive gas to the inlet of
a turbine (not shown in Figure l) as indicated by the
reference character 12. The combustor 12 includes a
catalytic unit 14 which preferably includes a conventional
monolithic catalytic structure 13 having substantial
distributed catalytic surface ar~a which effectively
supports catalytic combustion (oxidation) of a fuel-air
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mlxture flowing through the unit 14. Typically, the
catalytic structure 13 is a honeycomb structure having its
passages extending in ~he gas flow direction.
The combustor 10 includes a zone 11 into which
fuel, such as oil, is injected by nozzle means 16 from a
fuel valve 17 where fuel-air mixing occurs in preparation
for entry into the catalytic unit 14. Proper mixing
preferably entails vaporization of 80% to 90% of the
injected fuel for efficient and effective catalytic reac-
tion.
Typically, the fuel-air mix temperature (for
example 800F) required for catalytic reaction is higher
than the temperature (for example 700F) of the compxessor
discharge air supplied to the combustors from the enclosed
space outside the combustor shells. The deficiency in air
supply temperature in typical cases is highest during
` startup and lower load operation.
A primary combustion zone 18 is accordingly
provided upstream from the fuel preparation zone 11 within
the combustor 10. Nozzle means 20 are provided for in-
jecting fuel from a primary fuel valve 22 into the primary
combustion zone 18 where conventional flame combustion is
supported by primary air entering the zone 18 from the
space within the turbine casing through openings in the
combustor wall.
As a result, a hot gas flow is supplied to the
catalytic fuel preparation zone where it can be mixed with
the fuel and air mixture in the fuel preparation zone 11
to provide a heated fuel mixture at a sufficiently high
temperature to enable proper catalytic unit operation. In
this arrangement, the fuel injected by the nozzle means 1~
for combustion in the catalytic unit is a secondary fuel
flow which is mixed with secondary air and the primary
combustion products which supply the preheating needed to
raise the temperature of the mixture to the level needed
for entry to the catalytic unit~
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The catalytic combustion system is operated by a
generally conventional analog or digital computer or
digital/analog speed and load control 24 which operates
the primary and secondary fuel valves 22 and 17 through
conventional electropneumatic valve controls 26 and 28
respectively. The control 24 is preferably arranged to
operate the primary fuel system to energize the turbine
throu~h primary combustion only during startup and, after
synchronization, during loading up to a predetermined load
level. Thereafter, primary combustion is reduced by
primary fuel cutback as secondary fuel flow is initiated
by the control 24 to provide for turbine energization
primarily through catalytic combustion.
During the higher load catalytic combustion
phase of operation, primary combustion occurs at a reduced
level to provide secondary fuel-air mixture preheat as
previously described. Further, as catalytic activity
drops off with turbine operating time, compensatory in-
creases in primary combustion are instituted through
appropriate offset adjustments in the controls 26 and 28.
More description is presented subse~uently herein on the
coordinated operation of the prlmary and secondary fuel
valves.
During the startup/lower load phase of opera-
tion, primary combustion provides the turbine energizationneeded to drive the turbine operation to the point where
motive gas temperatures are sufficient for sustained
catalytic combustion operation.
During the higher load phase of operation, fuel
flow rates are increased but only a small part of the
- total fuel is supplied as fuel for primary combustion and
the rest of the fuel is supplied as secondary fuel for
catalytic combustion. Emission of NOX during the higher
load phase from the relatively small amount of primary
fuel combustion used to provide preheating of the secon-
dary fuel-air mixture thus is also well below the most
restrictive emission limits.
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Detailed ~tructural Arran~ement
In Figures 2 and 3A and 3B, there is shown a
structurally detailed catalytic combustion system 30
embodying the principles described for the combustor lO of
Figure l. Thus, the combustion system 30 generates hot
combustion products which pass through stator vanes 31 to
drive turbine blades (not shown). A plurality of the
combustion systems 30 are disposed about the rotor axis
within a turbine casing 32 to supply the total hot gas
flow needed to drive the turbine.
The catalytic combustor 30 includes a combustor
basket 40, a catalytic unit 36 and a transition duct 3R
which directs the hot gas to the annular space througb
which it passes to be directed against the turbine blades.
15Th~ combustor basket 40 is mounted on the casing
32 by bolt means 42 and preerably is provided with a
primary and plural (six) secondary sidewall fuel nozzles
44 and 46. Fuel supplied through the primary noæzle 44
(readily removable for maintenance) is mixed with primary
combustion support air, which enters the basket 40 through
sidewall scoops 48 (or openings), and burned in a primary
combustion zone 50 to provide hot gas for driving the
- turbine or preheating a downstream fuel-air mixture to the
level required for catalytic reaction. Primary combustion
support air also enters the basket 40 in this case through
swirlers 52 which are disposed coaxially about the primary
nozzle 44. Dilution air enters the zone 50 primarily
through scoops 49. The length of the primary zone 50
accordingly is sufficient to provide the space needed for
primary combustion to occur followed by the space needed
for mixing of the primary combustion products with dilu-
tion air. The primary zone sidewall is conventionally
structured from a plurality of sidewall rings which are
securely held together in a telescopic arrangement by
corrugated spacer bands. The spacer bands thus provide an
annular slot between adjacent sidewall ring members
through which air is admitted to cool the internal side-
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wall ring surfaces. As a result, the cross-section of the
primary zone increases slightly in the downstream direc-
tion.
~rimary ignition is provided by a conventional
spark igniter in a tube 35 in one or more of the combus-
tors 40. Cross flame tube connectors indicated by refer-
ence character 37 are employed to ignite the other com-
bustors 40.
The supplemental use of a conventional burner to
produce part of the total fuel combustion in the system 30
enables compensation to be made for dropoff in catalytic
activity with turbine operation time. As previously
noted, the ratio of conventional combustion to catalytic
combustion is sufficient under all higher output operating
~5 conditions to achieve the needed combustion assistance
without the production of an unacceptable N0x penalty.
Gases flow downstream within the combustor
basket ~0 from the primary combustion zone 50 to the entry
to a secondary zone 54 where the secondary fuel nozzles 46
inject fuel along an injection plane preferably with re-
spective surrounding jets of air through sidewall scoops
55 for mixing with the primary gas flow. The resultant
mix expands as it passes through an outwardly flared
diffuser 56 which forms an end portion of the basket 40.
It then enters a catalytic reaction element 27 in the
catalytic unit 36.
Proper penetration of secondary air jets into
the combustor is important from the standpoint of fuel/air
mixing because the jets carry the secondary fuel with
them. If penetration is excessive, the center of the
catalyst element receives too much fuel; if too little
penetration is obtained, the edges of the catalyst receive
too much fuel. For optimum mixing, the maximum penetra-
tion should be 33% of the tubular combust-or diameter.
With proper jet penetration, ~ood atomization of
secondary fuel (such as 30 micron droplets) is the key to
achieving rapid fuel vaporization. With preheat to 800F,
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30 micron fuel droplets are normally completely vaporized
within a few inches of the injection plane, but even drops
as large as 90 microns, of which there would normally be
very few, should be more than 99% vaporized at the cata-
lyst inlet.
The diffuser 56 is employed because a smaller
path diameter is needed for satisfactory fuel mixing in
the combustor basket 40 as compared to the path diameter
needed for catalytic combustion. Thus, injection of
secondary fuel into a smaller diameter basket provides
improved fuel/air mixing and better fuel/air uniformity
across the face of the catalyst 27. On the other hand,
the use of a larger basket diameter enables use of a
larger catalyst diameter which results in a lower catalyst
inlet velocity and produces a lower pressure drop and
improved combustion efficiency.
The flared shape of the diffuser 56 is pref-
erably formed to prevent hot gas flow separation (i.e. to
prevent turbulent layer formation near the diffuser wall).
Back pressure from the catalyst structure provides forces
needed to expand gas streamlines out to the diffuser wall
and prevent turbulent layer buildup.
To protect the catalytic element 27 and the
combustor basket 40, the system operates so that the
residence time for the gaseous mixture (in this case,
preheated to 800F) in the seconclary fuel preparation zone
54 is less than the ignition delay time from the primary
zone 50. In this way, flame is contained in the primary
combustion zone 50 away from the catalytic element 26.
Thus, the secondary fuel injection plane 5~ is spaced from
the catalyst face by a distance which is sufficient to
permit proper fuel mixing (substantial uniformity across
the catalyst face) and preparation for the catalyst 27 but
which is less than the critical distance which allows the
fuel-air mixture to auto-ignite before it crosses the
secondary zone 54 into the catalytic element 27. Normal-
ly, the fuel-air mixture is driven across the zone 54
within several milliseconds to avoid auto-ignition.
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The secondary fuel nozzles 46 are supported
preferably with a predetermined spacing outwardly from the
combustor sidewall. In this case, the nozzles are angled
for transversely directed fuel injection with a predeter-
mined angle of spread. Each nozzle 46 is connected (seeFigure 5) to a tubular fuel supply line 60 which is sup-
ported coaxially within an outer tubular air line 62. The
air tube 62 in turn is supported by a sliding rail ar-
;; rangement 64 (see Figure 4) which includes a bracket 65
attached to the sidewall of the combustor basket 40. A
flexible joint 69 (Figure 3A) provides for longitudinal
expansion of the fuel nozzle assembly.
The air tube 62 is supported at its casing entryend by a mounting plate 66 which is bolted to a flange on
a sleeve 70 as indicated at 68. The sleeve 70 is secured
suitably to the turbine casing 32 and it thus provides an
opening through which the fuel nozzle assembly extends
into the space within the casing 32. All secondary fuel
nozzle assemblies are thus readily removable for mainten-
ance simply by removing the bolts 68 and first sliding the
tubular assembly so that mount 63 slides free of the rail
bracket 65 and then continuing to slide the assembly until
it is removed from the turbine casing.
With the provision of the air supply line 62
about the fuel line 60, air Gooling is provided for the
fuel as it is delivered to the downstream secondary fuel
injection nozzles. By supplying secondary fuel at the
secondary nozzles at a temperature lower than what it
would otherwise be, added protection is provided against
auto-ignition in the fuel preparation zone 50 as a result
of added time required to raise the injected fuel to the
auto-ignition temperature.
The cooling air also atomizes the fuel to a fuel
fog as it is injected through the scoops 55 into the
combustor fuel preparation zone 50. An additional air jet
joins the nozzle flow in the scoop 55 and provides any
additional air needed to achieve the desired uel-air
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ratio (preferably lean) in the fuel preparation zone 50.
The scoop size and nozzle placement both can be varied to
modify the amount of such air jet flow.
The diameter of the catalytic element 26 is
5 determined mainly by the maximum allowable reference gas
velocity for complete emissions burnout at an acceptable
pressure loss. Higher gas velocities require longer cata
lyst beds and result in higher emissions. The mass trans
fer units required for complete emissions burnout are
10 inversely proportional to the square root of reference
velocity in laminar flow, but the effect of reference
velocity on the mass trans~er rate decreases with an
increase in channel Reynolds number. Thus, the maximum
allowable reference velocity is lirr.ited in turbulent flow
15 by the restriction of pressure losses. However, the low
limit boundary of reference velocity for the region of
operability may be determined by flashback considerations
in the fuel preparation zone.
The catalytic element 26 includes a can 30 with-
20 in which a catalytic honeycomb structure is conventionally
supported through a compliant layer 39. The catalyst
characteristics can be as follows:
DATA FOR DXE-442 CATALYST
I. Substrate
Size (2" + 2") long-
(~5" gap between two
sections)
Material Zircon Composite
Bulk Density 40-42 lb/ft3
Cell Shape Corrugated Sinusoid
Number 256 Channels/in
Hydraulic Diameter 0.0384"
Web Thickness 10 _ 2 mils.
Open Area 65.5%
Heat Capacity 0.17 BTU/lb, F
Thermal Expansion
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Coefficient 2.5 x 10 6 in/in, F
Thermal Conductivity 10 BTU, in/hr, ft , F
~elting Temperature 3050F
Crush Strength
Axial 800 PSI
90 25 PSI
II. Catalyst
Active Component Palladium
Washcoat Stabilized Alumina
10The catalytic unit can 30 is supported within a
clam shell housing 43 by lugs 45 and spring means 47. The
clam shell housing 43 is supported in turn by spring means
51 on the combustor diffuser 56 and by a ring 53 which is
supported by spring means 55 on the transition duct 38.
15The spring means 47, 51 and 53 allow for axial growth o~
the hot combustion and duct parts as operating tempera-
tures change. More detail is provided on the structure
and operation of the catalytic combustor support arrange-
ment in the patent application previously cross-referenced
herein.
With operation of the catalytic combustors 30 in
;~ the manner described, hot motive gases are supplied to the
turbine inlet essentially free of oxides of nitrogen and
at efficient operating temperatures above 2200F. As
indicated by the following table, primary combustion
occurs throughout the startup mode and during initial
loading until 47% load is reached. At that point, the
control sequences the secondary fuel valve into operation
and cuts back on the primary fuel supply. Further load
increases are then met by increases in secondary fuel.
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More particularly~ as the com~ustion turbine is
cranked and ignited ~see eolumn 1 of the tablej and ~rought
to ~dle speed within a time period of about ten minutes,
; primary ~uel flo~ (column 51 is increased from zero to ,286
lbs. per second with secondar~ ,uel shut o~f~ The inlet
guide vanes (column 151 are partially closed for about the
~irst seven minutes o~ the startup sequence in accordance
witK conventional practice to protect against surge. There-
a~ter the inlet guide vanes are opened to the 0 position.
10During the startup period, catalyst inlet temper~
ature ~column 81 rises from 555R to 1553R. I~thout seeond-
ary ~uel, no catalytic burning occurs and the catalyst exit
temperature (column ~1 is the same as the catalyst inlet
temperature~ The inlet temperature required ~or catalyt~c
reaction may be 80QF as previously noted or it could be a
lower value such as 6aOF depending on the catalytic material
and depending on the catalytic burning e~ficiency desired as
well understood in the prior art.
In the case o~ a 600F catalytic reac-tion threshold,
2Q the combustor inlet gas temperature (column 1~ would satisfy
the requirement at idle speed (1002R as shown in column 2)
without added heat from the burning of primary fuel. However,
at that -time the catalyst exit tempera-ture is 1553R or
llQQF which is significantly belo~ the catalyst exit temper-
ature wh~ch results ~hen secondary ~uel is turned on ~i.e.over 2300 R as shown in the table :row 47~ After ~rans.~
As the tur~ine is loaded to 47~ load, pr~mary ~uel
continues to be increased w~th seeondary ~uel shutoff. The
catal~st inlet and exit temperatures, and the turbine parts
temperatures, continue to rise gradually to 2300R temper~
ature which occurs with the switchover to secondary ~uel,
~ ith the switch~ver to secondary fuel, primar~ ~uel
is cut back to the nominal preheat level needed (about 0.1
lbs~sec as shown in column 5) and the inlet guide vanes are
again partially closed Ccolumn 15 I w~th a reduction in air
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flow from a~out 44 l~s~sec to about 31 lhs~sec ~column 41~ :
Catalyst inlet temperature drops to 126QR as the combined
result of the cut~ack in primary fuel and the reduction in
air flo~. Simultaneously, catal~st exit temperature increases
to 2334R ~hen the hot turhine parts are at or near the pre-
transfer exit temperature of 2060R. Continued increase of
secondary fuel meets increasing load and the inlet guide
~anes are eventuall~ fully opened ~71% load~ when catalyst
exit temperature is held a~ove 230QR, i.e. 2319R, with full
air flow.
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