GB2292793A

GB2292793A - Turbine combustion chamber

Info

Publication number: GB2292793A
Application number: GB9417643A
Authority: GB
Inventors: Simon Mario Depietro; Michael Francis Cannon
Original assignee: European Gas Turbines Ltd
Current assignee: Alstom Power UK Holdings Ltd
Priority date: 1994-09-02
Filing date: 1994-09-02
Publication date: 1996-03-06
Anticipated expiration: 2014-09-02
Also published as: GB9417643D0; GB2292793B

Abstract

A combustion chamber for a gas or liquid fuelled turbine, has a central fuel injector module (13) surrounded by other fuel injector modules (5) and projecting downstream of them and their associated flame zone (2). The central module (13) comprises a radial jet fuel nozzle (18) the radial jets passing into an axial flow air chamber (16) where it is diverted to a secondary flame zone (14) downstream of the central module. The central nozzle (18) and chamber (16) are contained by tubes (27 and 37) which between them form an annular duct (10) which is fed with air and controllable fuel to feed in turn the secondary zone (14). This dispersed feeding of the secondary zone (14) is desirable during the transition from primary zone operation to secondary zone operation with increase in load. The fuel can be distributed to the injector modules so that (a) 100% passes to the outer modules, (b) a major portion passes to the outer modules with all modules individually ignited and (c) a major portion passes to the outer modules with a single flame zone fed by all the modules but centered on and downstream of the central module. The transition between conditions (b) and (c) is achieved by feeding 100% fuel to the central module to extinguish the outer modules and then reducing the proportion to the central module while increasing the proportions of the outer modules without igniting them individually. <IMAGE>

Description

COMBUSTION CHAMBER This invention relates to a combustion chamber particularly for use with a gas turbine.

The emission pollutant requirements for industrial combustion turbines are becoming ever more stringent. One of the main groups of pollutants hitherto produced by such engines are the nitrogen oxides (NOx). It is an object of the present invention to provide a combustion chamber for a gas turbine which can operate with low NQt emissions and good flame stability over a wide range of load conditions.

It has previously been proposed to use an array of fuel injectors mounted on a back plate through which combustion air is fed. Such arrangements may be used in staged fuel systems by feeding fuel (gas or liquid) to the fuel injector modules selectively. The stability of the flame zones in this system is not, however, ideal.

According to one aspect of the invention, a combustion chamber for a gas or liquid fuelled turbine comprises a central fuel injector module surrounded by, and projecting downstream of a plurality of other fuel injector modules and their associated flame zone, the central module comprising a fuel injector nozzle within air-fed tube means, the tube means comprising concentric inner and outer tubes providing an annular duct, means for feeding the annular duct with fuel and air, and means for controlling the supply of fuel to the annular duct to provide a dispersed fuel-air supply to a secondary flame zone downstream of the central module when only the central module is operative.

The fuel injector nozzle preferably comprises a two-stage fuel-air mixing arrangement.

The fuel injector nozzle may comprise an axial fuel duct closed at its downstream end and having lateral holes in the downstream end, an air-fed chamber concentrically surrounding the downstream end of the fuel duct, the air-fed chamber being closed at its downstream end and having lateral holes aligned with the lateral holes in the fuel duct.

The concentric air-fed chamber may be located within the inner tube by a back plate which extends laterally between the air-fed chamber and the inner tube at an axial position upstream of the lateral holes, the back plate having a plurality of apertures for projecting air axially past the lateral holes and providing the second stage of fuel-air mixing.

The upstream end of the inner tube may be provided with a plurality of apertures which confront the ends of corresponding fuel supply tubes and the upstream end of the annular duct then being open to a source of pressurised air to provide, with fuel supplied through the apertures, a fuel-air mixture for feeding the secondary flame zone.

The fuel supply tubes may extend radially from a fuel supply pipe concentrically surrounding the axial fuel duct and controlled independently of the axial fuel duct.

The downstream end of the annular duct may be fitted with swirlers to disperse fuel-air mixture fed to the secondary flame zone.

There is preferably an annular restriction within the combustion chamber wall at an axial position in the region of the central module downstream end, for increasing the velocity of fuelair mixture from the other fuel injector modules on entering the secondary flame zone.

There are preferably included means for controlling the relative proportions of fuel supplied to the central fuel injector module on one hand and to the surrounding fuel injector modules on the other.

According to another aspect of the invention, in a method of operating a combustion chamber as aforesaid, with load conditions increasing from zero to 100% rated full load, fuel is distributed between the central and the outer fuel injector modules in the proportions (a) 100% to the outer modules with central module inactive, (b) a major proportion to the outer modules with all modules individually ignited and (c) a major proportion to the outer modules with a single flame zone fed by all of the modules but centred on and downstream of the central module.

The transition between the conditions (b) and (c) is preferably achieved by feeding 100% fuel to the central module to extinguish the outer modules and then reducing the proportion to the central module while increasing the proportion to the outer modules without igniting them individually. In condition (b) the major proportion is preferably approximately 70% and in condition (c) the major proportion is approximately 83%.

A combustion chamber in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an axial section of part of the combustion chamber; Figure 2 is an end view of a detail of Figure 1; and Figures 3(a), 3(b), 3(c) and 3(d) are diagrammatic illustrations of the combustion chamber in various operating modes.

The combustion chamber has an outer cylindrical wall 1 and an axis 3. Fitted to the back wall of the chamber in a hexagonal array are six fuel injectors of which one, 5, is illustrated.

This taxs an axial fuel path 7 and a concentric air path 9 leading into a short mixing tube 11 which in turn opens into the primary combustion zone 2. A secondary combustion zone 14 is situated downstream of combustor wall restriction 43.

The fuel injectors 5 surround a central fuel injector module 13. This comprises an axial fuel duct 15 which is fed with fuel by means not shown at its upstream end. At its downstream end it is closed off by a cap 17 but the cylindrical wall has six lateral holes 19 whereby fuel is sprayed out radially. Surrounding the fuel duct 15 is a concentric air chamber 21 which is open at its upstream end and closed by cap 17 at its downstream end. Holes 25 are provided in the cylindrical wall of the air chamber 21 in radial alignment with the holes 19 in the fuel duct.

The nozzle 18 provided by the fuel duct 15, cap 17 and air chamber 21 is located within a concentric air-fed module inner tube 27 by means of an annular back plate 29 which is welded to the air chamber 21 at an axial location upstream of the holes 25. The back plate 29 has an array of holes which are rotationally aligned with the holes 25 so that air applied under pressure to the upstream side of the back plate deflects and moves with the radial jets of fuel/air to provide a combustion mixture downstream of the nozzle.

The nozzle further comprises a second axial fuel duct 33 closed at its downstream end surrounding the first fuel duct 15 and having connecting radial tubes 35 aligned with holes 41 in the module inner tube 27 positioned upstream of the two stage arrangement, the radial tubes to discharge fuel into the concentric air passage 10 formed between the module inner tube 27 and the outer tube 37 and thus to secondary combustion zone via swirler 12.

A significant feature of the design is the extension of the module tube 27 downstream of the two stage nozzle to provide a localised flame zone at least partially isolated laterally within the combustion chamber. The flame zone of the central module is thus isolated from the flame zones of the surrounding modules.

At the upstream end of the central module the fuel duct 15 extends to a controlled fuel supply (not shown) outside the back wall 31 of the combustion chamber. Pressurised air is supplied through wall 31, some to concentric chamber 10 and some to central chamber 16. Air passing through concentric chamber 10 has single stage mixing with fuel from holes 41. Air passing through chamber 16 divides, some into chamber 21 and some through holes in back plate 29 thus giving two stage mixing, first in the air chamber 21 as fuel jets from the holes 19 in the fuel duct pass through and entrain air from the air chamber, and secondly, as the fuel/air jets from the holes 25 mix with and are deflected by air from the holes in the back plate 29.

Surrounding the module inner tube 27 is an outer tube 37 which is spaced from the inner tube by a small amount to form an annular duct 10. The module inner tube is incidentally, protected to some extent when the surrounding modules are in operation and the module tube is surrounded by flame zones. The main function of the double module wall 27/37 is to provide an annular duct 10 for the passage of fuel-air mixture in a transitional mode to be explained subsequently. An annular fuel pipe 33, controlled independently of the fuel duct 15, feeds a number of radial tubes 35 the ends of which confront apertures 41 in the upstream end of the inner tube 27. A sealed connection is not necessary at these positions since air fed to the duct 10 will suck fuel from the tubes 35.

While the construction illustrated employs a single central fuel injector module it will be appreciated that several 'central' injectors of the kind described and illustrated could be arranged surrounded by 'upstream injectors' in the manner described. The central injectors would then be arranged in a group centred on the axis 3 of the chamber.

The surrounding injectors 5 may be of conventional or other design but are basically upstream of the central injector nozzle (13).

In certain operational conditions (to be described) the fuel/air mixture from the surrounding injector modules is not ignited until it is downstream of the central module. As mentioned previously, an annular restriction 43 fitted to the chamber wall assists in preventing ignition back flash to the surrounding modules.

The reduction of nitrogen oxide pollutants has conventionally been achieved by the use of steam or water injection. Such a facility obviously increases costs in equipment and water provision and is avoided by means of the design described. Noxious emissions can be reduced by reducing the fuel to air ratio but problems arise with maintaining flammability at very low ratios. In the present combustion chamber there is provision, not shown, for controlling the relative proportion of fuel fed to the central injector nozzle to fuel fed to the surrounding modules. This proportion can be varied from zero to 100%.

Referring now to Figures 3(a) to 3(d), at very low loads, from zero to 20% (Figure 3(a)) only the surrounding modules are fed with fuel. In the load range 20 to 40% (Figure 3(b)) operation is achieved by a fuel distribution of 70%/30% outer to inner modules. In this mode all modules are ignited individually and, incidentally, with a lean, ie low fuel/air ratio, mixture.

From 40% to 100% load (Figure 3(d)) premixed operation is achieved by feeding the outer modules with 83% of the fuel and the inner/central module with 17%. However, in this case the fuel/air mixture from the outer module is not ignited until it has passed the annular restriction 43 in the chamber wall where it is ignited by the secondary flame.

The transition from the 20% - 40% load condition (Figure 3(b)) to the 40 - 100% load condition (Figure 3(d)) is achieved by reducing the outer module supply to zero, i.e. 100% fuel to the central module, as shown in Figure 3(c), and then, with the outer modules not ignited, adjusting the fuel supply to give an 83% - 17% distribution between outer and inner modules and a single (secondary) flame zone as shown in Figure 3(d).

However, transitional operation according to Figure 3(c) produces an increased risk of mal-operation, since 100% of the fuel is, or would be but for the invention, injected from the single central nozzle 18. Such concentration of fuel, particularly in a situation where there is in the region of 60% load, can cause flame-out or increased combustion pollution output. In this transitional condition therefore, part of the fuel supply is diverted from the fuel duct 15 to the annular fuel supply pipe 33 to feed the radial tubes 35 and the annular duct 10.

In Figures 3(b) and 3(d) it will be appreciated that the given fuel distribution percentages are only approximate and variations in the region of these figures will be acceptable in general.

Variations within plus and minus 5% of the major proportion given are contemplated.

Claims

CLALMS

1. A combustion chamber for a gas or liquid fuelled turbine, the combustion chamber comprising a central fuel injector module surrounded by, and projecting downstream of a plurality of other fuel injector modules and their associated flame zone, said central module comprising a fuel injector nozzle within air-fed tube means, said tube means comprising concentric inner and outer tubes providing an annular duct, means for feeding the annular duct with fuel and air, and means for controlling the supply of fuel to said annular duct to provide a dispersed fuel-air supply to a secondary flame zone downstream of the central module when only the central module is operative.

2. A combustion chamber according to Claim 1, wherein said fuel injector nozzle comprises a two-stage fuel-air mixing arrangement.

3. A combustion chamber according to Claim 2, wherein said fuel injector nozzle comprises an axial fuel duct closed at its downstream end and having lateral holes in said downstream end, an air-fed chamber concentrically surrounding said downstream end of said fuel duct, said airfed chamber being closed at its downstream end and havinglateral holes aligned with the lateral holes in said fuel duct.

4. A combustion chamber according to Claim 3, wherein the concentric air-fed chamber being located within said inner tube by a back plate which extends laterally between the air-fed chamber and the inner tube at an axial position upstream of said lateral holes, said back plate having a plurality of apertures for projecting air axially past said lateral holes and providing the second stage of fuel-air mixing.

5. A combustion chamber according to Claim 3 or Claim 4, wherein the upstream end of said inner tube is provided with a plurality of apertures which confront the ends of corresponding fuel supply tubes and the upstream end of said annular duct is open to a source of pressurised air to provide, with fuel supplied through said apertures, a fuel-air mixture for feeding said secondary flame zone.

6. A combustion chamber according to Claim 5 wherein said fuel supply tubes extend radially from a fuel supply pipe concentrically surrounding said axial fuel duct and controlled independently of said axial fuel duct.

7. A combustion chamber according to any preceding claim, wherein the downstream end of said annular duct is fitted with swirlers to disperse fuel-air mixture fed to said secondary flame zone.

8. A combustion chamber according to any preceding claim and comprising an annular restriction within the combustion chamber wall at an axial position in the region of the central module downstream end, for increasing the velocity of fuel-air mixture from said other fuel injector modules on entering said secondary flame zone.

9. A combustion chamber according to any preceding claim including means for controlling the relative proportions of fuel supplied to said central fuel injector module on one hand and to the surrounding fuel injector modules on the other.

10. A combustion chamber substantially as hereinbefore described, with reference to Figures 1 and 2 of the accompanying drawings.

11. A method of operating a combustion chamber according to any preceding claim, wherein, with load conditions increasing from zero to 100% rated full load, fuel is distributed between the central and the outer fuel injector modules in the proportions (a) 100% to the outer modules with central module inactive, (b) a major proportion to the outer modules with all modules individually ignited and (c) a major proportion to the outer modules with a single flame zone fed by all of the modules but centred on and downstream of the central module.

12. A method according to Claim 11, wherein the transition between the conditions (b) and (c) is achieved by feeding 100% fuel to the central module to extinguish the outer modules and then reducing the proportion to the central module while increasing the proportion to the outer modules without igniting them individually.

13. A method according to Claim 12, wherein said 100% of fuel supplied to the central module is distributed between the fuel injector nozzle and said annular duct.

14. A method according to Claim 11 or Claim 12, wherein in condition (b) the said major proportion is approximately 70%.

15. A method according to any of Claims 11, 12 or 14, wherein in condition (c) the said major proportion is approximately 83%.

16. A method of operating a combustion chamber for a gas turbine, substantially as hereinbefore described with reference to the accompanying drawings.