The present invention relates to gas turbines in general and, in particular, to a gas- fuel supply system for a gas turbine.

INTRODUCTION

A gas turbine, also called combustion turbine, is an engine that creates a flow of combusted gas and extracts energy or power from the combusted gas. Figure 1 discloses a typical gas turbine having an upstream compressor 2 that is coupled to a downstream turbine 7 and a combustion system 100 fluidly coupled between the upstream compressor 2 and downstream turbine 7. During operation, the compressed air stream 3 is energised in a combustor (not shown, part of combustion system) through combusting fuel. The fuel 4 may be pre-heated in a heat-exchanger (not shown).

In this high-pressure environment of the combustor system 100, heating and combustion of the fuel produces high temperature gas 6 which is forced into the downstream turbine 7, where the hot gas flow is directed over the turbine's blades spinning the turbine 7, so as to power the compressor 2 via a shaft 13 and a generator 15 via a coupling 14.

An auxiliary medium 5 may be supplied to the heat exchanger at a suitable temperature and discharged 12 from the heat exchanger at a lower temperature so as to use the heat energy of the auxiliary medium 5 to heat the gas fuel supply 4 from its supply temperature to a predetermined higher temperature.

The gas fuel heating of the combustion system 100 therefore provides a means which is beneficial to the overall gas turbine plant efficiency by reducing the amount of gas fuel needed to produce the high-temperature gas 6 at a desired firing temperature, improving the overall rate of conversion of heat from fuel to power. In currently available gas turbines, one or more heat exchangers are generally used to heat the gas fuel stream 4 and the then heated gas fuel stream passes through respective regulator and block valves (not shown) which regulate the flow of fuel to each stage of the combustor (s).

Typical examples for a combustion system 100 comprising gas fuel heat- exchanger(s)is disclosed in Figures 2(a) and 2(b).

In the example combustion system disclosed in Figure 2(a), a heat-exchanger 20 is used to heat fuel stream 4 using an auxiliary medium 5 at a predetermined temperature that is higher than the supply temperature of the fuel 4, wherein the heat energy transferred to the fuel 4 is regulated by controlling the flow of the auxiliary medium through a valve 19. The then cooled auxiliary medium discharge 12 may be returned into the process cycle of the auxiliary medium 5 at a suitable location.

Downstream of the heat-exchanger 20, the pre-heated gas fuel streams 9A and 9B are mixed in the combustor 10 with an air stream 3, and ignited. In the high pressure environment of the combustor 10, combustion of the fuel produces a high temperature gas 6. It is imperative that the flow-rate of the fuel streams 9A and 9B is regulated so as to supply fuel to the combustor 10 at a suitable flow-rate according to the operating requirements of the gas turbine apparatus.

When comparing a gas turbine that uses gas fuel to a gas turbine that uses liquid fuel, gas fuel has a higher specific volume and a lower heat transfer coefficient, which means that a larger heat transfer surface area is required to provide the same heat transfer than liquid fuel, leading to a higher volume of the gas fuel heating system. The specific volume of a fluid is defined as the ratio of the substance's volume to its mass. Specific volume a fluid is therefore the reciprocal of the fluid's density and can be considered an intrinsic property of matter. Furthermore, the compressibility of a fluid is defined as the relative volume change in response to the applied pressure change.

When a gas turbine is shut down and the pressure in the combustion system 100 is relieved, the gas fuel still in the gas fuel heating system (i.e. heat-exchanger 20 and upstream fuel line 26) must be prevented from entering the combustor(s), which is effected using a shut-off valve 21.

For example, when closing the shut-off valve 21, the quantity of gas fuel downstream of the shut-off valve 21 may be too great, potentially causing dangerous combustion conditions in the combustor 10 and turbine 7. A potentially dangerous/unsafe combustion condition may be an explosion caused by a gas fuel to air ratio that decreases too slowly. Similarly, the relatively high volume of compressible gas in the gas fuel heating system means that gas fuel control valves 23A and 23B must be coupled downstream of the heat-exchanger 20 to the heated fuel line 26 in order to provide the required flow regulation for each fuel stream (i.e. A, 9B). This is because the compressible volume of the gas fuel causes a process lag, or a response with a negative phase angle, i.e. the response of the fuel flow lags behind the action of the regulator. Process lag is higher with higher compressible fluid volume, leading to the condition where the flow cannot reasonably be controlled as per the engine's dynamic performance requirements. Thus, as the regulation for the gas fuel streams are on the downstream side of the heat exchanger 20, the control valves 21, 23 A, 23B, flow measurement and other instrumentation associated with the flow regulation have to be suitable for a temperature operating range from relatively cold gas fuel conditions to relatively hot gas fuel conditions. That is, the equipment has to be suitable for a much larger temperature and flow volume range in order to regulate the gas fuel flow than equipment that is used upstream of the heat exchanger (i.e. cold side), where there is a considerably smaller temperature and flow volume range with the same mass flow.

Therefore, fuel shut-off valve 21 and fuel control valves 23A and 23 B utilized in currently available gas turbine systems must be suitable for operation in higher temperature and for higher specific volume of the heated gas fuel. However, current design requirements for such fuel valves limit the maximum temperature of the heated fuel, subsequently minimizing the benefits potentially achieved from gas fuel heating to the overall rate of conversion of heat from fuel to power.

In addition, because the gas fuel in the heat-exchanger 20 does not have a flow path when the shut-off valve 21 is closed, a pressure relief valve 22 is required on the heat-exchanger fuel outlet 26, so as to relieve any build-up of pressure from heating the fluid resident (i.e. gas fuel) in the heating system, which may otherwise reach a pressure that exceeds the pressure capacity of the system.

Furthermore, the discharge 25 of the relief valve(s) 22 must be to a safe area and may require cooling and/or containment according to specific engineering and regulatory requirements.

Figure 2b discloses another example combustion system similar to the one disclosed in Figure 2a, where a by-pass valve 24 is added upstream of the heat-exchanger 20 so as to allow the regulation of the temperature of the heated gas fuel by mixing the heated gas fuel stream 26 with gas fuel from the fuel supply stream 4 at the supply temperature.

Alternatively, additional fuel supply stream(s) may also bypass heat-exchanger 20 to the combustor 10, wherein fuel stream(s) 11 is regulated via control valve(s) 27 to other stages of the combustor 10. A separate shut-off valve 28 may be required for these non-heated fuel streams. Accordingly, it is an object of the present invention to provide an improved fuel supply system for a gas turbine allowing pre-heating of the fuel to any suitable temperature without compromising the functionality and material integrity of the fuel flow control means (e.g. shut-off valves, control valves), improving the durability of control components and the overall energy efficiency of the gas turbine.

SUMMARY OF THE INVENTION Preferred embodiments of the invention seek to overcome one or more of the above disadvantages of the prior art.

According to a first aspect of a first embodiment of the present invention, there is provided a fuel supply system for a gas turbine having at least one combustor, comprising:

at least one first heat exchanger, adapted to receive a fuel from a fuel source at a first temperature through a first fuel line and provide said fuel to the at least one combustor at a second temperature;

at least one shut-off valve, adapted to selectively stop the fuel-flow through said first fuel line to the at least one combustor;

at least one first control valve, adapted to regulate the flow-rate of said fuel through said at least one first heat exchanger to the at least one combustor, wherein said at least one shut-off valve and said at least one first control valve are operably coupled to said first fuel line upstream of and in fluid communication to said at least one first heat exchanger, and wherein said at least one shut-off valve is arranged upstream of and in fluid communication to said at least one first control valve.

This provides the advantage that the heated fuel temperature is not limited by the valve component temperature limits, the flow regulation of each individual fuel stream can be simplified to the extent that the temperature and flow volume regulation system can be rated for the conditions at the supply temperature (i.e. the temperature of the fuel before it is heated in the heat-exchanger), while maintaining the function of temperature regulation and flow regulation of each fuel stream. Furthermore, a pressure relief valve and discharge line are no longer required on each heat-exchanger system when shutting-down the gas turbine and relieving the pressure in the combustor, because the fuel supply system allows discharge to the combustion system so as to relive the pressure through the turbine section.

Preferably, the at least one first heat exchanger may comprise an auxiliary fluid circuit adapted to transfer heat energy from an auxiliary fluid to said fuel.

Advantageously, the at least one first heat exchanger may be configured to provide a predetermined 'heat-transfer-surface-area to fuel-volume' ratio that is adapted to limit the quantity of fuel, so as to minimize the fuel flow response time of said at least one shut-off valve and/or said at least one first control valve.

This provides the advantage of minimising potential process lags between actuating actuating control- and or shut-off valve (s) and respective fuel flow response.

Preferably, the auxiliary fluid circuit may comprise an auxiliary control valve adapted to regulate the flow-rate of said auxiliary fluid through said auxiliary fluid circuit of said at least one first heat exchanger. Even more preferably, the at least one auxiliary control valve may be operably coupled to said auxiliary fluid circuit downstream of and in fluid communication to said at least one first heat exchanger, and may be adapted to regulate the flow-rate of said auxiliary fluid through said at least one first heat exchanger.

This provides the advantage that heat energy transferred to the fuel (i.e. the temperature of the heated fuel) can be controlled via flow-rate of the auxiliary fluid. According to a second aspect of the present invention, the fuel supply system may comprise at least one second heat exchanger adapted to receive said fuel from said fuel source at said first temperature through said first fuel line, and to provide said fuel to the at least one combustor at said second temperature or a third temperature.

Advantageously, at least one second heat exchanger may comprise at least one second control valve adapted to regulate the flow-rate of said fuel through said at least one second heat exchanger to the at least one combustor, wherein said at least one second control valve is operably coupled to said first fuel line upstream of and in fluid communication to said at least one second heat exchanger and downstream of and in fluid communication to said at least one shut-off valve.

This provides the advantage that more than one fuel line provides fuel to the combustor, wherein the fuel in each fuel line may be pre-heated to a different temperature. According to a third aspect of the resent invention, the at least one first and second heat exchanger may be operably combined in a dual-compound heat exchanger, comprising a single auxiliary fluid circuit adapted to transfer heat from said auxiliary fluid to said fuel. According to a fourth aspect of the present invention, the fuel supply system may further comprise at least one temperature control valve adapted to selectively provide fuel from said first fuel line at said first temperature to a fuel output of said at least one first heat exchanger, so as to bypass said at least one first heat exchanger.

Advantageously, the temperature control valve may be operably coupled to said first fuel line upstream of and in fluid communication to said at least one first heat exchanger and downstream of and in fluid communication to said at least one first control valve.

This provides the advantage of an improved temperature control of the pre-heated fuel of at least one fuel line to the combustor. According to a fifth aspect of the present invention, the fuel supply system may further comprise at least one fuel source control valve, adapted to selectively provide fuel from said fuel source at said first temperature directly to said at least one combustor.

Advantageously, the at least one fuel source valve may be operably coupled to said first fuel line upstream of and in fluid communication to said at least one first control valve and downstream of and in fluid communication to said at least one shut-off valve.

This provides the advantage of an improved fuel mixture control (flow, temperature) in the combustor.

Preferably said fuel is a gas fuel.

According to a second embodiment of the present invention, there is provided a gas turbine comprising a compressor, a combustor, a fuel supply system operably coupled to said combustor, and a turbine operably coupled to said compressor and said combustor, wherein said fuel supply system is in accordance with any one of the various aspects of the first embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention will now be described, by way of example only and not in any limitative sense, with reference to the accompanying drawings, in which:

Figure 1 (Prior Art) shows a schematic diagram of a typical combustion gas turbine comprising, inter alia, a compressor, a turbine, a generator, and a gas fuel performance heat exchanger and combustor system comprising a fuel supply stream and heating medium; Figure 2 (Prior Art) shows (a) a detailed schematic diagram of a fuel heater arrangement for the gas turbine disclosed in Figure 1, comprising, inter alia, a single input fuel stream, pre-heated utilising an auxiliary medium at a predetermined temperature, wherein the temperature of the pre-heated fuel is regulated by controlling the flow of the heating medium utilising a temperature control valve, and wherein the flow downstream of the heat-exchanger is regulated utilising two control valves that supply the heated output stream(s) to the combustor, and (b) a detailed schematic diagrams of a fuel heater arrangement disclosed in 2(a), further comprising a first bypass valve in order to control the temperature of the output stream by mixing the pre-heated fuel with the input fuel, a second bypass valve regulating unheated fuel provided to the combustor, and a shut-off valve supplying unheated fuel in addition to one or more pre-heated fuel stream(s) to the combustor; Figure 3 shows a schematic diagram of a fuel supply system (a) according to the second aspect of the present invention, comprising, inter alia, two separate gas fuel performance heat-exchangers; and (b) according to a third aspect of the present invention, comprising two heat-exchanger that are combined in a dual- compound heat-exchanger, wherein a single auxiliary fluid circuit provides heat energy from both fuel lines;

Figure 4 shows a schematic diagram of a fuel supply system according to the fourth aspect of the present invention, further comprising a by-pass fuel line to the output of the first heat-exchanger allowing independent temperature/flow control of one of the two fuel streams, and

Figure 5 shows a schematic diagram of a fuel supply system according to the fifth aspect of the present invention, further comprising a second by-pass fuel line directly to the combustor allowing independent fuel mixture control in the combustor. DETAILED DESCRIPTION OF EMBODIMENTS

Referring now to Figure 3 (a), an example of a second aspect of the first embodiment of the present invention is disclosed showing only the fuel supply system 100 suitable for a gas turbine as disclosed in Figure 1. The fuel supply system 100 includes an input fuel stream 4, which is pre-heated utilising an auxiliary medium 5 at a predetermined temperature, and two gas fuel performance heat exchangers 200A and 200B. A gas fuel shut-off valve 28 is located upstream (i.e. cold side) of the heat-exchangers 200A and 200B. There is no requirement of a pressure relief valve (as shown in the examples of the available prior art).

Preferably, the auxiliary medium 5 may either be water heated to a predetermined temperature and at a pressure lower than the critical pressure of water, or a water/steam mixture heated to a predetermined temperature at a pressure higher than the critical pressure of water. However any other suitable auxiliary medium 5 may be used in the fuel supply system for transferring heat energy to the fuel. After heat is transferred from the auxiliary medium 5 to the fuel, the then cooled auxiliary medium 12 is discharged and may be returned to the process cycle of the auxiliary medium 5 at a suitable location.

The temperature of each of the pre-heated gas fuel streams A and B is regulated through controlling the auxiliary medium 5 flow utilising respective temperature control valves 19A and 19B, wherein the flow of each of the gas fuel streams 9A and 9B is regulated utilising respective valves 27A and 27B, both located upstream (i.e. cold side) of respective heat-exchangers 200A and 200B.

The two pre-heated gas fuel streams 9A and 9B supply fuel to the combustor 10, where it is combusted with a compressed air stream 3, producing high temperature gas 6 that is utilised to run the turbine.

The heat-exchangers 200A and 200B may be of any suitable type of heat-exchanger adapted to provide a sufficiently high ratio of heat transfer surface area to volume on the fuel side, so as to allow providing a fuel volume downstream of the control valves 27A and 27B that is sufficiently small to maintain the required regulator response, and so as to further allow providing gas fuel downstream of the shut-off valve 28 at a quantity sufficiently small so as to minimise the quantity of fuel entering the combustor 10 after the flow of fuel is stopped to safe limits.

Figure 3 (b) discloses an alternative third aspect of the present invention, wherein the two heat-exchangers 200A and 200B shown in Figure 3 (a) are replaced by a compound fuel heat-exchanger 200, allowing two gas fuel streams to be heated utilising one stream of an auxiliary heating medium 5. A control valve 19 serves to control the heat transferred to both fuel lines in the compound heat-exchanger 200.

Referring now to Figure 4, an alternative fourth aspect of the present invention is disclosed, where the fuel supply system 100 disclosed in Figure 3 (a) further comprises an additional first bypass valve 24 that is coupled to the fuel line 4 upstream of the first heat exchanger 200A allowing temperature control of preheated fuel stream 9A by simply mixing the pre-heated fuel stream output 26 from the first heat-exchanger 200A with the input fuel stream 4 at the supply temperature. The temperature in each of the pre-heated fuel streams A and 9B may therefore be regulated to different temperatures, so as to allow compliance with specific requirements of the combustion system.

Referring now to Figure 5, an alternative fifth aspect of the present invention is disclosed, where the fuel supply system 100 disclosed in Figure 4 further comprises a second bypass valve 27 adapted to regulate supply of an unheated fuel stream 4 at the supply temperature directly to the combustor 10 and in addition to the preheated fuel output streams 9 A, 9B. The fuel supplied by the pre-heated fuel streams 9 A, 9B and the unheated fuel stream 11 is then combusted with compressed air stream 3, producing a high temperature gas 6 that is utilised to drive a turbine 7.

Figures 3(a) & (b), 4 and 5 illustrate typical examples of the fuel supply system 100 of the present invention, comprising suitable combinations of one or more pre- heated fuel streams A, 9B and unheated fuel streams 4 (bypassing heat-exchanger 200A) and 11 in the same combustor 10.

It will be appreciated by persons skilled in the art that the above embodiment have been described by way of example only and not in any limitative sense, and that various alterations and modifications are possible without departing from the scope of the invention as defined by the appended claims.