GB2614037A

GB2614037A - Gas turbine arrangement

Info

Publication number: GB2614037A
Application number: GB2115800.1A
Authority: GB
Inventors: Corbett Nicolas; Batt Simon
Original assignee: Siemens Energy Global GmbH and Co KG
Current assignee: Siemens Energy Global GmbH and Co KG
Priority date: 2021-11-03
Filing date: 2021-11-03
Publication date: 2023-06-28
Also published as: GB202115800D0

Abstract

A gas turbine arrangement 100 comprises a gas turbine 10, a water injection system 94 and a fuel 98. The gas turbine comprises a compressor 14, a combustor 16, a turbine 18, an exhaust 19, and a controller 74. The water injection system comprises a condenser 62 and a conduit system 100 connected between the condenser and the combustor. Exhaust gases 60 are arranged to flow into the condenser. In use, water flows from the condenser to the combustor via the conduit system. The fuel comprises at least 10% by volume of hydrogen gas. The water injection system may be closed-loop. Water may additionally flow from the condenser to the compressor. The conduit system may comprise valves 84, 80 controlled by the controller to regulate the waters flow based on certain parameters including combustor entry temperature, turbine temperature, fuel composition, exhaust temperature, engine shaft speed, power output, and ambient temperature. Also disclosed is a method of operating a gas turbine arrangement as described above.

Description

GAS TURBINE ARRANGMENT

FIELD OF INVENTION

The present invention relates to a gas turbine arrangement having a closed loop water injection system and a method of operating the gas turbine arrangement for minimising emissions.

BACKGROUND

Gas turbine engines are used in both electricity generation and oil and gas industries for many years. These industries continue to be heavily reliant on these gas turbine engines and many thousands of gas turbine engines remain in use. Some industrial gas turbine engines can be configured to be dry low emission (DLE) or wet low emission (WLE). WLE gas turbines comprise a water or steam injection and management system for delivering water or steam to a combustor of the gas turbine engine. WLE gas turbine engines must have clean water, without particulates or harmful chemicals, from an external source otherwise the injection system will clog with the particulates and the chemicals may corrode components. A WLE gas turbine produces relatively low emission because the temperature of combustion is reduced by the presence of water or steam. A DLE gas turbine achieves low emissions by virtue of a lean burn combustion flame which is controlled by sophisticated software and hardware.

However, these existing and future gas turbine engines will be expected to operate on a different range of fuel compositions as the energy transition requires "clean" energy.

One alternative fuel which may become commonly used is hydrogen because the combustion products from burning hydrogen gas alone contain no carbon dioxide or less carbon dioxide if hydrogen gas is combined with existing hydrocarbon fuels such as methane. One of the drawbacks of using hydrogen gas as a fuel is that it has a high combustion temperature which leads to higher NOx emissions. Another problem is the higher temperatures cause damage to combustor and turbine components. As NOx has a global warming potential (GWP) of nearly 300, there is a strong desire to find ways of reducing the NOx emissions when using hydrogen gas as a fuel, as well as methods to manage the temperature of the combustor hardware.

SUMMARY OF INVENTION

Thus, the following objects alone or in any combination are addressed for a gas turbine that operates using a fuel having a hydrogen content: reduction of NOx emissions, reduction of temperature of the combustor hardware, provision of power augmentation, operation without an external source of water.

The above objects are achieved by a gas turbine arrangement comprises a gas turbine, a water injection system and a fuel. The gas turbine comprises a compressor (14), a combustor, a turbine, an exhaust, a controller. The water injection system comprises a condenser and a conduit system. The exhaust is arranged to flow exhaust gases into the condenser. The conduit system is connected between the condenser and the combustor and in use condensed water flows from the condenser to the combustor via the conduit system and condensed water is injected into the combustor. The fuel comprises at least 10% by volume of hydrogen gas.

The fuel (98) comprises at least 15% by volume of hydrogen gas. The fuel may comprise at least 15% by volume of hydrogen gas. The fuel may comprise at least 30% by volume of hydrogen gas. The fuel may comprise between and including 10% and 40% by volume of hydrogen gas. The fuel may comprise between and including 15% and 30% by volume of hydrogen gas.

The water injection system may be closed-loop. Preferably, the water injection system does not use any water that has not been condensed from the exhaust gases.

The conduit system may be connected between the condenser and the compressor and in use condensed water flows from the condenser to the compressor via the conduit system and condensed water is injected into the compressor.

The conduit system may comprise at least one valve, the at least one valve may be controllable by the controller to regulate the flow rate of condensed water passing therethrough.

The water injection system may comprise a reservoir for storage of condensed water.

The water injection system may comprise a pump and preferably a filter. The filter may be arranged to filter particulates from the condensed water.

The controller may be programmed to operate the water injection system based on any one or combination of the following parameters: combustor entry temperature, turbine temperature, exhaust temperature, engine shaft speed, power output, ambient temperature, fuel composition such as hydrogen gas content or an emission in the exhaust gas.

The controller may be programmed with a schedule of mass flow rate of the condensed water to be injected into the combustor.

The controller may be programmed with a threshold value or values for any one or more of the parameters and once the threshold is reached the controller operates the water injection system to inject condensed water into the combustor in a predetermined quantity.

In another aspect of the present invention there is provided a method of operating a gas turbine arrangement. The gas turbine arrangement comprises a gas turbine and a water injection system. The gas turbine comprises a compressor, a combustor, a turbine, an exhaust and a controller. The water injection system comprises a condenser and a conduit system. The method comprising the steps supplying a fuel to the combustor, the fuel comprises at least 10% by volume of hydrogen gas, exhausting a flow of exhaust gases from the gas turbine into the condenser, condensing water from the exhaust gases and injecting the condensed water into the combustor.

The method may comprise the step injecting the condensed water into the compressor.

The method may comprise adjusting the flow rate of condensed water being injected into the combustor dependent on the power setting of the gas turbine, preferably increasing the flow rate of condensed water when the power setting is increased and/or decreasing the flow rate of condensed water when the power setting is decreased thereby maintaining NOx emissions below the required level.

The method may comprise adjusting the flow rate of condensed water being injected into the combustor dependent on the composition of the fuel, preferably the flow rate of condensed water being injected into the combustor is greater the greater the volume of hydrogen gas in the fuel.

The controller may be programmed to operate the water injection system based on any one or combination of the following parameters: combustor entry temperature, turbine temperature, exhaust temperature, engine shaft speed, power output, ambient temperature, fuel composition such as hydrogen gas content or an emission in the exhaust gas. The method may comprise the step controlling the mass flow rate of the condensed water injected into the combustor via a schedule of parameter values and mass flow rates of the condensed water or controlling the mass flow rate of the condensed water injected into the combustor where a threshold value or values for any one or more of the parameters and once the threshold is reached and operating the water injection system to inject condensed water into the combustor in a predetermined mass flow rate.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned attributes and other features and advantages of this invention and the manner of attaining them will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawing, wherein FIG. 1 shows part of a schematic sectional view of a gas turbine arrangement having a gas turbine engine and a water injection system in accordance with the present invention.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows a gas turbine arrangement 11 in accordance with the present invention and having a dry low emission gas turbine engine 10 and a water injection system 94. The gas turbine engine 10 comprises, in flow series, an inlet 12, a compressor section 14, a combustor section 16, a turbine section 18 and an exhaust 19 which are generally arranged in flow series and generally about and in the direction of a longitudinal or rotational axis 20.

The gas turbine engine 10 further comprises a shaft 22 which is rotatable about the rotational axis 20 and which extends longitudinally through the gas turbine engine 10. The shaft 22 drivingly connects the turbine section 18 to the compressor section 14.

In operation of the gas turbine engine 10, air 24, which is taken in through the air inlet 12 is compressed by the compressor section 14 and delivered to the combustion section or burner section 16. The burner section 16 comprises a burner plenum 26, one or more combustion chambers 28 and at least one burner 30 fixed to each combustion chamber 28. The combustion chambers 28 and the burners 30 are located inside the burner plenum 26. The compressed air passing through the compressor 14 enters a diffuser 32 and is discharged from the diffuser 32 into the burner plenum 26 from where a portion of the air enters the burner 30 and is mixed with a gaseous or liquid fuel. The air/fuel mixture is then burned and the combustion gas 34 or working gas from the combustion is channelled through the combustion chamber 28 to the turbine section 18 via a transition duct 17.

This exemplary gas turbine engine 10 has a cannular combustor section arrangement 16, which is constituted by an annular array of combustor cans 19 each having the burner 30 and the combustion chamber 28, the transition duct 17 has a generally circular inlet that interfaces with the combustor chamber 28 and an outlet in the form of an annular segment.

An annular array of transition duct outlets form an annulus for channelling the combustion gases to the turbine 18.

The turbine section 18 comprises a number of blade carrying discs 36 attached to the shaft 22. In the present example, two discs 36 each carry an annular array of turbine blades 38. However, the number of blade carrying discs could be different, i.e. only one disc or more than two discs. In addition, guiding vanes 40, which are fixed to a stator 42 of the gas turbine engine 10, are disposed between the stages of annular arrays of turbine blades 38.

Between the exit of the combustion chamber 28 and the leading turbine blades 38 inlet guiding vanes 44 are provided and turn the flow of working gas onto the turbine blades 38.

The combustion gas from the combustion chamber 28 enters the turbine section 18 and drives the turbine blades 38 which in turn rotate the shaft 22. The guiding vanes 40, 44 serve to optimise the angle of the combustion or working gas on the turbine blades 38.

The turbine section 18 drives the compressor section 14. The compressor section 14 comprises an axial series of vane stages 46 and rotor blade stages 48. The rotor blade stages 48 comprise a rotor disc supporting an annular array of blades. The compressor section 14 also comprises a casing 50 that surrounds the rotor stages and supports the vane stages 48. The guide vane stages include an annular array of radially extending vanes that are mounted to the casing 50. The vanes are provided to present gas flow at an optimal angle for the blades at a given engine operational point. Some of the guide vane stages have variable vanes, where the angle of the vanes, about their own longitudinal axis, can be adjusted for angle according to air flow characteristics that can occur at different engine operations conditions.

The casing 50 defines a radially outer surface 52 of the passage 56 of the compressor 14. A radially inner surface 54 of the passage 56 is at least partly defined by a rotor drum 53 of the rotor which is partly defined by the annular array of blades 48.

For the present invention, the gas turbine 10 is adapted to burn a fuel 98 comprising hydrogen gas in the combustor 16. It should be noted that the hydrogen gas described herein is H2 gas, i.e., hydrogen molecules, as opposed to molecules of hydrocarbons (or ammonia NH3) having hydrogen atoms, e.g. CH4(methane). Hydrocarbons or ammonia for example are not within the present definition of 'hydrogen gas'. The composition of the fuel 98 contains hydrogen gas between and including 10% and 100% by volume (1.5% and 100% by mass) of the fuel being burnt in the gas turbine 10. The present invention is particularly useful where the composition of the fuel 98 contains hydrogen gas greater than and including 15% by volume or 2% by mass of the fuel being burnt and even more advantageous where the composition of the fuel 98 contains hydrogen gas greater than and including 30% by volume or 5% by mass. For existing combustor and gas turbine engine designs the present invention is advantageous using fuel 98 containing hydrogen gas between and including 10% and 40% by volume (1.5% and 8% by mass) and more preferably between and including 15% and 30% by volume (2% and 5% by mass) of the fuel being burnt in the gas turbine 10. For combustors specifically designed to burn fuel with significant hydrogen gas content, the fuel 98 may contain hydrogen gas between and including 10% and 100% by volume (1.5% and 100% by mass) and more preferably greater than and including 30% by volume (5% by mass) of the fuel being burnt.

The present invention is applicable to both DLE and WLE gas turbine engines 10 as well as other gas turbine engines, particularly those with aeroengine-style or aeroenginederivative combustion systems. In both cases the hydrogen gas in the fuel causes the combustion flame to increase in temperature and in so doing increases emissions and particularly NOx emissions above the emissions emitted for the same power output using a fuel having no or insignificant hydrogen gas content. The present invention is realised by injecting condensed water into the combustor 16 to reduce the temperature of the flame. The present gas turbine arrangement 11 comprises the gas turbine 10 and the water injection system 94 operating in a closed loop manner such that water is condensed from the exhaust gases and injected into the combustor. There is no external source of water in the present gas turbine arrangement or in its method of operation. For any gas turbine engine, the condensed water may be additionally injected into the compressor 14 for power augmentation on demand. For application to WLE gas turbine engines, the condensed water is injected into the combustor, usually continuously and during normal operation and the volume of condensed water injected into the combustor is according to a schedule of power settings.

The gas turbine 10 comprises a controller 74 that is programmed to operate the water injection system 94. The controller 74 is also known as an engine control unit (ECU) or engine management system (EMS).

The water injection system 94 comprises a conduit system 100, a pump 88, optionally a filter 90 and optionally a reservoir 92. The pump 88 is controllable by the controller 74 to deliver the condensed water at the required pressure to the water injectors via the conduit system 100. The condensed water may be filtered by the filter 90 to remove particles that might otherwise block the water injectors. An amount of condensed water may be stored in the reservoir 92 so that there is sufficient condensed water immediately available. The reservoir 92 may be situated on or part of the conduit system 100.

The working gas 34 that passes through the turbine 18 forms an exhaust gas 60 which passes through the exhaust 19. The exhaust gas 60 is channelled into a condenser 62 which condenses water vapour from the exhaust gas 60 and channels the condensed water along the conduit system 100 for use in the gas turbine 10. The remaining exhaust gas 66 is vented to ambient or other devices as known. The conduit system 100 comprises a main conduit 64 and connected thereto is a first conduit 68 and a second conduit 70. Although not shown, the first conduit 68 and the second conduit 70 may each comprise a manifold, extending around part or all the gas turbine 10, and a plurality of pipes extending from the manifold to a plurality of injectors that inject the condensed water into the combustor 16 or array of combustors 16 and if additionally required compressor 14 respectively.

The condensed water is injected into the combustor 16 via the first conduit 68 which branches from the main conduit 64. The first conduit 68 has a first valve 80 that can be operated to control the flow rate of condensed water passing through the first conduit 68 and is operable between and including fully open and closed. Alternatively, or as well as, condensed water is injected into the compressor 14 via a second conduit 70 which branches from the main conduit 64. The second conduit 70 has a second valve 84 that controls the flow rate of condensed water passing through the second conduit 70 and is operable between and including fully open and closed.

The controller 74 is programmed to determine when to inject condensed water into the combustor 16 by commanding the first valve 80 to open or to close when condensed water is not required. The controller 74 is programmed to operate the water injection system 94 based on any one or combination of the following parameters: combustor entry temperature, turbine temperature, exhaust temperature, engine shaft speed, power output, ambient temperature, fuel composition such as hydrogen gas content or an emission in the exhaust gas 60. The controller 74 may be programmed with a schedule of mass flow rate of the condensed water injected into the combustor 16. Alternatively, the controller 74 may be programmed with a threshold value or values for any one or more of the parameters and once the threshold is reached the controller 74 operates the water injection system 94 to inject water into the combustor 16 is a predetermined quantity.

The amount of condensed water or its mass flow rate injected into the combustor 16 may be either a fixed flow rate or a variable flow rate depending on the magnitude of the parameter. Alternatively, the variable flow rate of condensed water injected into the combustor 16 may be dependent on the power setting or demanded output of the gas turbine 10; the greater the demanded power the greater the amount of condensed water being injected into the combustor 16.

Where condensed water is injected into the combustor 16 and where required the compressor 14 simultaneously the mass flow rate of the condensed water being injected into each may be independently controlled via the appropriate first valve 80 or second valve 84.

In general, the greater the amount of hydrogen gas in the fuel 98 the greater the amount of condensed water injected into the combustor 16 to limit nitrous oxides; thus, the controller 74 can be scheduled to control the pump 88 and valve 80 to deliver the required flow rate of condensed water to maintain emissions of nitrous oxides below a required emissions threshold. Further, the quantity of condensed water being injected is also dependent on the subsequent emission of the component gas e.g. 25ppm, and of sufficient quantity to maintain the emission of the component gas or particulate in the gas below the predetermined threshold. The predetermined threshold of emissions may be adjusted according to location or other statutory emission requirement.

In one example, the amount of condensed water injected into the combustor 16 is determined by virtue of a power setting of the gas turbine 10. In general, the higher the power setting the greater the amount of condensed water being injected. This determination of the amount of condensed water being injected can be termed the base amount of condensed water. In addition, the amount of condensed water being injected into the combustor 16 may be adjusted from the base amount of condensed water by a factor dependent on the fuel composition in terms of its calorific value and/or hydrogen gas content. For example, where the fuel 98 nominally contains approximately 30% hydrogen gas and the engine is operating at a certain power setting a predetermined mass flow rate of condensed water is injected into the combustor 16, now if the fuel composition changes to say containing approximately 20% hydrogen gas then the predetermined mass flow rate of condensed water for that power setting is reduced by an increment or factor and which maintains the nitrous oxide emissions below a desired amount.

It has been estimated that a typical range of mass flow rate of condensed water being injected into the combustor 16 to fuel 98 being injected into the combustor 16 is between 1:1 and 5:1.

In another aspect of the present invention, for any gas turbine 10 its maximum power output may be increased by injecting condensed water into the compressor 14. The condensed water may be injected into the inlet 12 or directly into the compressor 14 via a spray and by known means. Injecting condensed water reduces the work done by the compressor and subsequently increase the amount of work done by the turbine, thereby increasing power output. Thus, when the controller 74 is commended to produce an output above the 'dry' maximum power output i.e., above a threshold value, the controller 74 commands the second valve 84 to open. Condensed water is then injected into the compressor 14. The amount of condensed water being injected into the compressor 14 may be either a fixed flow rate or a variable flow rate depending on the demanded power output. Where condensed water is injected into the combustor 16 and the compressor 14 simultaneously the mass flow rate of the condensed water being injected into each may be independently controlled via the appropriate first valve 80 or second valve 84.

As described above there are three conditions where a threshold value might be exceeded and the controller 74 commands one or more of the valves 80, 84 to open thereby injecting condensed water into the combustor 16 and/or compressor 14. The controller 74 is programmed to command the valves 80, 84 to open when the first of any of the threshold values are exceeded. The percentage content of hydrogen gas in the fuel will dictate the quantity of water vapour in the exhaust gas 60 and therefore the amount of condensed water that can be condensed and used for condensed water injection. Where the amount of condensed water is limited the controller 74 is programmed to control condensed water injection for the primary purpose of reducing emissions i.e., NOx abatement.

The invention provides a sustainable or closed loop condensed water supply in the required quantities from the gas turbine exhaust gas 60 alone. Nonetheless, an option is for the gas turbine arrangement to have a reservoir 92. The reservoir 92 may be located on the main conduit 92. The reservoir 92 may initially contain an amount of condensed water such that on engine start-up condensed water may be immediately injected into the compressor 14 or combustor 16 for reduction of NOx. The reservoir 92 may also assist in providing an immediate larger quantity of condensed water in conditions where power augmentation of the gas turbine is required and to prevent lag in the condensation and condensed water injection process.

It is estimated that even in steady state operation, the quantity of condensed water required for injection into the combustor 16 will be less than that being condensed. The condensed water that is not used for injection into the combustor may be used to fill the reservoir 92. Further condensed water may be used to other purposes such as agriculture. Furthermore, condensing more water vapour from the exhaust gas than needed for injecting into the combustor 16 has an additional benefit because it reduces water vapour being discharged into the atmosphere which would otherwise act as an insulator or 'green gas'.

The present invention is described with reference to the above exemplary turbine engine having a single shaft or spool connecting a single, multi-stage compressor and a single, one or more stage turbine. It should be appreciated that the present invention is equally applicable to two or three shaft engines and which can be used for industrial, aero, aero-derivative or marine gas turbine applications.

The terms upstream and downstream refer to the flow direction of the airflow and/or working gas flow through the engine unless otherwise stated. The terms forward and rearward refer to the general flow of gas through the engine. The terms axial, radial and circumferential are made with reference to the rotational axis 20 of the engine.

Claims

CLAIMS1. A gas turbine arrangement (100) comprises a gas turbine (10), a water injection system (94) and a fuel, the gas turbine (10) comprises a compressor (14), a combustor (16), a turbine (18), an exhaust (19), a controller (74), the water injection system (94) comprises a condenser (62) and a conduit system (100), the exhaust (19) is arranged to flow exhaust gases (60) into the condenser (62), the conduit system (100) is connected between the condenser (62) and the combustor (16) and in use condensed water flows from the condenser (62) to the combustor (16) via the conduit system (100) and condensed water is injected into the combustor (16), the fuel (98) comprises at least 10% by volume of hydrogen gas.
2. A gas turbine arrangement (100) as claimed in claim 1 wherein the fuel (98) comprises at least 15% by volume of hydrogen gas.
3. A gas turbine arrangement (100) as claimed in any one of claims 1-2 wherein the water injection system (94) is closed-loop.
4. A gas turbine arrangement (100) as claimed in any one of claims 1-3 wherein the conduit system (100) is connected between the condenser (62) and the compressor (14) and in use condensed water flows from the condenser (62) to the compressor (14) via the conduit system (100) and condensed water is injected into the compressor (14).
5. A gas turbine arrangement (100) as claimed in any one of claims 1-4 wherein the conduit system (100) comprises at least one valve (84, 80), the at least one valve (84, 80) is controllable by the controller (74) to regulate the flow rate of condensed water passing therethrough.
6. A gas turbine arrangement (100) as claimed in any one of claims 1-5 wherein the water injection system (94) comprises a reservoir (92) for storage of condensed water.
7. A gas turbine arrangement (100) as claimed in any one of claims 1-6 wherein the water injection system (94) comprises a pump (88) and preferably a filter (90), the filter (90) is arranged to filter particulates from the condensed water.
8. A gas turbine arrangement (100) as claimed in any one of claims 1-7 wherein the controller (74) is programmed to operate the water injection system (94) based on any one or combination of the following parameters: combustor entry temperature, turbine temperature, exhaust temperature, engine shaft speed, power output, ambient temperature, fuel composition such as hydrogen gas content or an emission in the exhaust gas (60).
9. A gas turbine arrangement (100) as claimed in claim 8 wherein the controller (74) is programmed with a schedule of mass flow rate of the condensed water to be injected into the combustor (16).
10. A gas turbine arrangement (100) as claimed in claim 8 wherein the controller (74) is programmed with a threshold value or values for any one or more of the parameters and once the threshold is reached the controller (74) operates the water injection system (94) to inject condensed water into the combustor (16) in a predetermined quantity.
11. A method of operating a gas turbine arrangement (100), the gas turbine arrangement (100) comprises a gas turbine (10) and a water injection system (94), the gas turbine (10) comrpises a compressor (14), a combustor (16), a turbine (18), an exhaust (19), a controller (74), the water injection system (94) comprises a condenser (62) and a conduit system (100), the method comprising the steps supplying a fuel (98) to the combustor (16), the fuel (98) comprises at least 10% by volume of hydrogen gas, exhausting a flow of exhaust gases (60) from the gas turbine (10) into the condenser (62), condensing water from the exhaust gases (60) and injecting the condensed water into the combustor (16).
12. A method of operating a gas turbine arrangement (100) as claimed in claim 11, wherein the method comprises the step injecting the condensed water into the compressor (14).
13. A method of operating a gas turbine arrangement (100) as claimed in any one of claims 11-12, wherein the method comprises adjusting the flow rate of condensed water being injected into the combustor (16) dependent on the power setting of the gas turbine (10), preferably increasing the flow rate of condensed water when the power setting is increased and/or decreasing the flow rate of condensed water when the power setting is decreased thereby maintaining NOx emissions below the required level.
14. A method of operating a gas turbine arrangement (100) as claimed in any one of claims 11-13, wherein the method comprises adjusting the flow rate of condensed water being injected into the combustor (16) dependent on the composition of the the fuel (98), preferably the flow rate of condensed water being injected into the combustor (16) is greater the greater the volume of hydrogen gas in the fuel.
15. A method of operating a gas turbine arrangement (100) as claimed in any one of claims 1144, wherein the controller (74) is programmed to operate the water injection system (94) based on any one or combination of the following parameters: combustor entry temperature, turbine temperature, exhaust temperature, engine shaft speed, power output, ambient temperature, fuel composition such as hydrogen gas content or an emission in the exhaust gas (60), wherein the method comprises the step controlling the mass flow rate of the condensed water injected into the combustor (16) via a schedule of parameter values and mass flow rates of the condensed water or controlling the mass flow rate of the condensed water injected into the combustor (16) where a threshold value or values for any one or more of the parameters and once the threshold is reached operating the water injection system (94) to inject condensed water into the combustor (16) in a predetermined mass flow rate.