CA1169257A - Catalytic combustor having secondary fuel injection for low no.sub.x stationary combustion turbines - Google Patents

Abstract

16 49,416 ABSTRACT OF THE DISCLOSURE
A combustion turbine is provided with a plural-ity of catalytic combustors each of which includes a combustor basket coupled to a transition duct through a catalytic unit such that the parts are free to grow axial-ly with operating temperature changes. The combustor basket is provided with a primary nozzle at its upstream end to provide fuel for conventional combustion and dilu-tion in a primary zone. A plurality of secondary nozzles are provided for fuel injection through the basket side-wall at the downstream end of the primary zone. A fuel preparation zone is provided within the basket from the secondary fuel injection location to the catalytic unit to provide uniform mixing of the fuel in the gas flow.
During startup and lower loads, primary fuel is supplied to energize the turbine without secondary fuel. At a predetermined load, secondary fuel flow is initiated and primary fuel is cut back to a level sufficient to provide any preheat needed to raise the secondary fuel mixture to a level required for catalytic activity.

Description

~ ~ ~ 9 ~ 5 '7 1 49,416 CATALYTIC COMBUSTOR HAVING SECONDARY
FUEL INJECTION FOR LOW NOX STATIONARY
COMBUSTION TURBINES
CROS'S~REFERENCES TO RELATED APPLICATIONS
Canadian Application Serial No. 396,,712, entitled IMPROVED CATALYTIC SYSTEM FOR A STATIONARY COMBUSTION
TURBINE HAVING A TRANSITION DUCT MOUNTED CATALYTIC ELEMENT
and filed by P. E. Scheihing et al. concurrently herewith.
Canadian Application Serial No. 396,743, entitled CATALYTIC COMBUSTOR HAVING SEGONDARY FUEL INJECTION FOR LOW
NOX STATIONARY COMBUSTION TURBINES and filed by P. W.
Pillsbury et al. concurrently herewith.
'BACKGROUND OF I~E'INVENTION
The present invention relates to stationary com-bustion turbines and more particularly to the implementa tion of catalytic combustion in such turbines to charae-terize the turbi'ne 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 ~ ~ 69~5'~

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 operation of the conventional turhine 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.
For 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 the 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 of the turbine during the startup and load operating modes. Generally, as turbine speed 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 is supplied at a temperature below the minimum temperature needed for catalytic reaction. In the known prior art, U.S. Patents 3,928,961 and 4,112,675 appears 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 unit and having 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 ,, ~ 1163~5'~

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 internal gases to provide a well mixed fuel-air mixture for combustion in the catalytic unit when catalytic reac-tion conditions are reached.
B EF DESCRIPTION OF THE DRAWINGS
Figure 1 schematically shows a catalytic combus-tion system arranged to operate a stationary gas turbine in accordance with the principles of the invention;
Figure 2 shows an elevational view of a cataly-tic combustion system disposed in accordance with princi-ples of the invention;
Figures 3A and 3B show an enlarged view of the lS combustion system of Figure 1;
Figure 4 shows an enlarged cross section of secondary nozzle mounting structure taken along reference line IV-IV of Figure 3A; and Figure 5 shows a portion of a vertical section taken through a secondary fuel nozzle shown in Figure 3A.
DESCRIPTION OF THE PREFERRED EMBODIMENT
General Structural Concepts More particularly, there is shown in Eigure 1 a generalized 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 inclu~es a conventional monolithic catalytic structure 13 having substantial distributed catalytic surface area which effectively supports catalytic combustion (oxidation) of a fuel-air mixture flowing through the unit 14. Typically, the catalytic structure 13 is a honeycomb structure having its p~ssages extending in the gas flow direction.

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4 ~
The combustor 10 includes a zone ll 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 tha 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 700~F) of the compressor 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 16 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.
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 ~ ~ 6~32~ ~

conventional electropneumatic valve controls 26 and 28 respectively. The control 24 is preerably arranged to operate the primary fuel system to energize the turbine through 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 or turbine energixation 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 subsequently herein on the coordinated operation of the primary and secondary fuel valves.
During the startup/lower load phase of opera-tion, primary combustion provides the turbine energization needed 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 N0x duri~g the higher load phase from the relatively small amount of primary ~uel combustion used to provide preheating of the secon-dary fuel-air mixture thus is also well below the most restrictive emission limits.
Detailed Structural Arrangement - In Figures 2 and 3A and 3B, there is shown a structurally detailed catalytic combustion system 30 embodying the principles described for the combustor 10 of 5, Figure 1. 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 5 Withill 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 38 which directs the hot gas to the annular space through which it passes to be directed against the turbine blades.
The combustor basket 40 is mounted on the casing 32 by bolt means 42 and preferably is provided with a primary and plural (six~ secondary sidewall fuel nozzles 44 and 46. Fuel supplied through the primary nozPle 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 re~uired 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 5COOpS 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 combu~tion product~ 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-wall ring surXaces. As a result, the cross-section of the primary zone increases slightly in the downstream direc-tion.

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Primary 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 conditions to achieve the needed combustion assistance without the production of an unacceptable N0x penalty.
Gases flow downstream within the combustor basket 40 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 combustor diameter.
With proper jet penetration, good atomization of secondary fuel (such as 30 micron droplets) is the key to achieving rapid fuel vaporization. With preheat to 800F, 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 IJ6~

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 bas~et diameter enables use of a larger cata].yst 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 wall3.
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 secondary 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 58 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 butwhich 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.
The secondary fuel nozzles 46 are supported preferably with a predetermined spacing outwardly from the ~ 3 ~;~25'i' combustor sidewall. In this case, the nozzles are angled for transversely directed fuel injection with a predeter-mined angle of spread. Each nozzle 46 i5 connected (see Figure 5) to a tubular fuel supply line 60 which is sup-ported coaxially within an outer tubular air line 62. Theair tuba 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 entry end by a mounting plate 66 which is bolted to a flange on a sleeve 70 as indicated at 58. 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 cooling 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 fuel-air 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.

5~~ The diameter of the catalytic element 2~ is determined mainly by the maximum allowable reference gas velocity for complete emissions burnout at an acceptable pressure loss. ~igher gas velocities require longer cata-lyst beds and result in higher emissions. The mass trans-fer uniks required for complete emissions burnout are inversely proportional to the sq~lare root of reference v210ci ty in laminar 10w, but the effect of reference velocity on the mass transfer rate decreases with an increase in channel Reynolds number. Thus, the maximum allowable reference velocity is limited in turbulent flow 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 ~ includes a can 30 with-in which a catalytic honeycomb structure is conventionally supported through a compliant layer 39. The catalyst characteristics can be as follows:

I. Substrate Size (2" + 2") long-(~" gap between two sections) Material Zircon Composite Bulk Density 40-42 lb/ft3 Cell Shape Corrugated Sinusoid Number 256 Channels/in2 Hydraulic Diameter 0.0384"
Web Thickness 10 + 2 mils.
Open Area 65.5%
Heat Capacity 0.17 BTU/lb, F
Thermal Expansion Coefficient 2.5 x 10 6 in/in, F
Thermal Conductivity 10 BTU, in/hr, ft2, F

2 ~ 7 Melting Temperature 3050F
Crush Strength Axial 800 PSI
go 25 PSI
II. Catalyst Active Component Palladium Washcoat Stabilized Alumina The 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.
The spring means 47, 51 and 53 allow for axial growth of the hot combustion and duct parts as operating tempera-tures change. More detail is provided on the structureand 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 tempexatures 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 p~int, 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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Claims (4)

What is claimed is:

1. A catalytic combustion system for a stationary gas turbine comprising a combustor basket having a tubular sidewall defining a primary combustion zone therein, prim-ary nozzle means for supplying fuel for combustion in the primary zone, said combustor basket sidewall defining a secondary zone downstream from the primary zone, secondary means for injecting fuel and air into the secondary zone for mixing with the primary combustion product flow to pro-vide a fuel-air mixture at a combustor basket outlet suffic-iently mixed and heated to undergo catalytic reaction, and said basket sidewall being structured along the primary com-bustion zone such that the primary zone outlet cross-section is at least as great as the primary zone cross-section over the primary zone upstream from the primary zone outlet, said combustor basket including a downstream diffuser end portion having a sidewall which defines an expanding path for the fuel-air mix over the entire secondary zone from the fuel injection plane to the basket outlet, a catalytic unit, means for supporting said catalytic unit relative to said diffuser end portion to receive the outlet flow from said combustor basket, said diffuser end portion having an end sidewall section which is outwardly flared, said catalytic unit co operating with said outwardly flared end sidewall section to apply back pressure on hot gas steams and move them under resultant flow forces toward the flared end section sidewall to prevent turbulent layer buildup, and means for supplying fuel to said primary nozzle means and said secondary inject-ing means in a predetermined coordinating manner,

2. A catalytic combustion system as set forth in claim 1 wherein said secondary injecting means includes a plurality of secondary nozzle means spaced circumferen-tially about said combustor sidewall in the secondary fuel injection plane, fuel line means connected to each of said secondary nozzle means to form a secondary fuel assembly, means for mounting each of said secondary fuel assemblies relative to a casing of the turbine, and means for releas-ably supporting each of said secondary fuel assemblies relative to said combustor basket so that each secondary fuel assembly can be removed from the casing when the associated mounting means is disconnected from the casing.

3. A catalytic combustion system as set forth in claim 2 wherein each of said fuel supply means comprises an inner fuel supply tube and a coaxial outer tube through which air flows to cool said inner tube and the fuel supp-lied therethrough, said inner and outer tubes being connect-ed to the associated nozzle means to supply air and fuel thereto to provide an air assisted fuel injection spray.

4. A catalytic combustion system as set forth in claim 2 wherein each of said mounting means is flexible to permit thermal growth of the associated secondary fuel assembly.