EP2932158A1

EP2932158A1 - Burner assembly, combustion chamber comprising said burner assembly and method for supplying fuel to said burner assembly

Info

Publication number: EP2932158A1
Application number: EP13831864.7A
Authority: EP
Inventors: Enrico GOTTARDO; Giulio Mori
Original assignee: Ansaldo Energia SpA
Current assignee: Ansaldo Energia SpA
Priority date: 2012-12-17
Filing date: 2013-12-17
Publication date: 2015-10-21
Anticipated expiration: 2033-12-17
Also published as: ITMI20122154A1; CN105102894B; WO2014097153A1; EP2932158B1; KR20150103039A; CN105102894A; WO2014097153A8

Abstract

A burner assembly (9) for a combustion chamber (4) of a gas turbine plant (1) is provided with: • a main burner (10) configured to produce a main combustion zone (12) having a main convective time (tp); • a secondary burner (11) configured to produce at least a secondary combustion zone (20) having a secondary convective time (ts); • the secondary burner (11) is configured so that the secondary convective time (is) is different from the main convective time (tp).

Description

BURNER ASSEMBLY, COMBUSTION CHAMBER COMPRISING SAID BURNER ASSEMBLY AND METHOD FOR SUPPLYING FUEL TO SAID BURNER ASSEMBLY TECHNICAL FIELD

The present invention relates to a burner assembly, to a combustion chamber comprising said burner assembly and to a method for supplying fuel to said burner assembly. In particular, the present invention relates to a burner assembly for a combustion chamber of a gas turbine of an energy production plant.

BACKGROUND ART

Known burner assemblies normally comprise a main low temperature combustion portion and a secondary high temperature combustion portion, which stabilizes the flame generated by the main portion.

There is a need for the stabilizing action of the secondary portion because thermo-acoustic instability is always present in combustion systems. Thermo-acoustic instability is a phenomenon characterized by the presence of thermo-acoustic flame oscillation, which generates?~a pressure wave. Such a pressure wave propagates in the combustion chamber, reflects on the walls of the combustion chamber and returns to act on the flame. These thermo- acoustic oscillations may increase unstably until they damage the walls of the combustion chamber, or they may- attenuate .

Hence, there is a critical condition of instability when the thermo-acoustic oscillations are excited by the combustion process and they increase in amplitude.

In accordance with that defined in the article by Tim Lieuwen et al . , "A Mechanism of Combustion Instability in Lean Premixed Gas Turbine Combustors", Transaction of ASME, Vol. 123, 2001, there is combustion instability when the thermal release fluctuations are in phase with the pressure fluctuations. Therefore, the instability is governed by the delay time T between the pressure oscillations and the thermo-acoustic oscillations.

The delay time T is greatly affected by the "convective times" τ , which are typical of each of the fuel supply lines in the burner assembly. In particular, the delay time T and the convective time τ are bound by the following relation:

T/Tperiod= f(t, flame length, kinetic chemical reactions)

Where

T is the delay time; and

Tperiod is the time corresponding to a complete oscillation of the fluctuations in the combustion chamber. Where fluctuations are intended both as the thermal fluctuations and the pressure fluctuations, which have the same period but different phase.

The accurate valuation of T may be performed with experimental investigations or- with Large Eddy Simulation (LES) calculation methods.

In combustion systems with "lean premix" technology, in which premixed flames are employed, the convective time τ is defined by the time employed by the mixture to travel the space between the fuel injection point and the flame base, in accordance with the following relation: τ = Linj/Uavg where :

Linj is the distance between the fuel injection point and the flame base;

Uavg is the average speed in the mixture in the stretch Lin .

Due to the high combustion temperature, the secondary portion is responsible for most of the polluting emissions. The kinetics of the chemical reactions which lead to the formation of pollutants indeed shows a dependency on the • temperature which is not highly linear, and an exponential increase of the pollutants according to the combustion temperature . Accordingly, the quantity of fuel supplied to the secondary portion is limited in order to prevent exceeding the legal limits related to polluting emissions. This also involves a subsequent limitation of the fuel flow rate to supply to the main portion, which requires the stabilizing action of the secondary portion to be able to burn without generating instability.

In conclusion, the performances of burner assemblies of known type are limited, because the methods for controlling the instabilities counter those for reducing the emissions and vice versa.

The stability limits in known burner assemblies may be exceeded only in given cases in which, generally, the complexity of the combustion . system increases significantly.

DISCLOSURE OF INVENTION

It is one object of the present invention to make a high-performance burner assembly, capable of giving rise to a stable combustion and, at the same time, capable of complying with the .legal limits related to polluting emissions.

In particular, it is one object of the present invention to make a particularly flexible burner assembly capable of increasing the capacity range at which the plant may operate while at the same time keeping the emissions below the legal limits.

In accordance with such objects, the present invention relates to a burner assembly for a combustion chamber of a gas turbine plant in accordance with claim 1.

It is a further object of the present invention to make a high-performance combustion chamber, capable of generating a stable combustion and capable of complying with the legal limits related to polluting emissions.

In accordance with such objects, the present invention relates to a combustion chamber as claimed in claim 12.

Finally, it is a further object of the present invention to provide a method for supplying fuel to a burner assembly capable of contributing to generating a stable combustion and capable of complying with the legal limits related to polluting emissions.

In accordance with such objects, the present invention relates to a method for supplying fuel to a combustion assembly as claimed in claim 13.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will become more apparent from the following description of a non- limiting embodiment thereof, with reference to the figures of the accompanying drawings, in which:

- figure 1 is a diagrammatic view of a gas turbine plant 1 comprising the combustion chamber according to the present invention;

- figure 2 is a cross-section view, with parts removed for clarity, of a burner in accordance with the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Numeral 1 in figure 1 indicates a power plant comprising a gas turbine 2 extending along an axis A, a compressor 3, a combustion chamber 4, an assembly 6 for supplying fuel to the combustion chamber 4 and a generator 7, which transforms the mechanical power supplied by the gas turbine 2 into electric power.

The combustion chamber 4 comprises a plurality of seats 8, each of which is adapted to be employed by a burner assembly 9 (better shown in figure 2) . The seats 8 are arranged along a circular path close to a peripheral edge of the combustion chamber 4. In the non- limiting example described and shown. herein, the combustion chamber 4 is of the annular type and there are twenty- four seats 8 and burner assemblies 9.

With reference to figure 2, each burner assembly 9 extends along an axis B and comprises a main burner 10 and a secondary burner 11.

The secondary burner 11 extends substantially along axis B, while the main burner 10 extends about the secondary burner 11 parallel to axis B.

The main burner 10 is configured to supply an air-fuel mixture and to define a main combustion zone 12 (schematically shown in figure with a dash-dot line) .

The air and the fuel are supplied along a supply direction Dl directed towards the inside of the combustion chamber 4.

In particular, the main burner 10 comprises an air supplying annular conduit 13 and a fuel supplying annular conduit 14.

The air supplying annular conduit 13 receives air from compressor 3 and has a decreasing radial height in the supply direction Dl so as to generate a substantially truncated-conical shaped conduit.

The fuel supplying annular conduit 14 extends parallel to axis B and ends with a plurality of nozzles 15, which face directly onto the air supplying annular conduit 13.

The air supplying annular conduit 13 is provided at an end portion .16 with a cylindrical outer element 18, which extends parallel to axis B.

Arranged along the air supplying annular conduit 13, preferably downstream of the nozzles 15, is a swirler 17, configured to promote the generation of vortexes so as to facilitate the mixing between air and fuel. In particular, swirler 17 is adapted to cause a rotation about axis B to the mixture which flows through it in order to stabilize the flame generated and to allow better control of the flame position inside the combustion chamber 4.

Swirler 17 comprises a plurality of blades (not shown in the accompanying figures) , which are fastened to the walls which define the air supplying annular conduit 13.

Extending close to the end portion 16 of the air supplying annular conduit 13 and the cylindrical outer element 18 is the main combustion zone 12, in which the combustion occurs of the air- fuel mixture originating from the air supplying annular conduit 13.

The main combustion zone 12 has a main convective time ΐρ. In accordance with the above definition, the main convective time τ_ρ is the time employed by the fuel to travel the distance between the nozzles 15 and the main combustion zone 12.

The main convective time T_p depends on the geometry of the main burner 10, on the position of the fuel emitting nozzles 15, and on the fluid dynamic and kinetic conditions that determine the position of the main combustion zone 12.

Although it is responsible for a low production of polluting emissions (due to a low combustion temperature) , the main combustion zone 12 is characterized by a low combustion stability. The poor combustion stability involves the entire main combustion zone 12 in which the thermal release is localized.

The fuel supplying annular conduit 14 is supplied with a main fuel flow rate QP (see figure 1) , which, as seen in detail below, is conveniently regulated by a control device 19 of the assembly 6 for supplying fuel (figure 1) . The secondary burner 11 is configured to supply a partially premixed air-fuel mixture and to define one or more secondary combustion zones 20 (schematically shown in figure with a broken line) .

■The combustion temperature in the secondary combustion zone 20 is greater than the combustion temperature in the main combustion zone 12 and is characterized by a low temperature variability.

The secondary burner 11 is preferably configured so that the secondary combustion zone 20 is at least partially overlapping the main combustion zone 12. Preferably, the secondary combustion zone. 20 is substantially entirely contained in the main combustion zone 12.

The secondary combustion zone 20 has a secondary convective time τ₃. In accordance with the above definition, the secondary convective time τ₃ is the_. time employed by the fuel to travel the - distance between the nozzles for supplying the fuel of the secondary burner 11 and the secondary combustion zone 20.

The secondary convective time T_s depends on the geometry of the secondary burner 11, on the position of the nozzles which emit fuel into the secondary burner 11, and on the fluid dynamic and kinetic conditions which determine the position of the secondary combustion zone 20.

The secondary burner 11 is configured to supply the secondary combustion zone 20 with a secondary convective time T_s that is different from the main convective time τ_Ρ.

The thermo-acoustic waves generated by the main burner 10 and the thermo-acoustic waves generated by the secondary burner 11 propagate independently from one another.

A suitable configuration of the secondary burner 11 allows the convective time τ₃ to be varied so that the thermo-acoustic waves generated by the secondary burner 11 do not give rise to a constructive interference which could lead to increased instability and accordingly, to damage to the combustion chamber 4.

Preferably, the secondary burner 11 is configured so that there is disruptive interference between the thermo- acoustic waves generated by the main burner 10 and the thermo-acoustic waves generated by the secondary burner 11. Essentially, it is important to prevent the thermo-acoustic waves generated by the secondary burner 11 from being in phase with the thermo-acoustic waves generated by the main burner 10 and giving rise to constructive interference. Such a situation is satisfied if the secondary convective time T_S is different from the main convective time τ_Ρ. Thereby, the overall delay time T is different from the one there would be with the main combustion zone 12 alone, and can be modulated by means of varying the secondary convective time x_s .

In the non-limiting example described and shown herein, the secondary burner 11 comprises a first auxiliary burner 24 and a second auxiliary burner 25, which are both capable of generating a partially premixed diffusive combustion.

In particular, the first auxiliary burner 24 generates a partially premixed diffusive combustion in a respective first auxiliary combustion zone 27 (shown with broken line) , while the secondary auxiliary burner 25 generates a partially premixed diffusive combustion in a respective second auxiliary combustion zone 28 (shown with broken line) .

The first auxiliary combustion zone 27 and the second auxiliary combustion zone 28 contribute to defining a secondary combustion zone 20. As mentioned above, it is preferable that the first auxiliary combustion zone 27 and the second auxiliary combustion zone 28 be completely contained in the main combustion zone 12.

The first auxiliary combustion zone 27 has a first auxiliary convective time τ₅ι and extends up to a distance dl from the cylindrical outer element 18, while the second auxiliary combustion zone 28 has a second auxiliary convective time τ₃2 and extends u to a distance d2 from the cylindrical outer element 18.

Preferably, distance d2 is greater than distance dl .

In accordance with that defined above, the first auxiliary convective time t_si is the time required by the fuel supplied to the first auxiliary burner 24 to reach the first auxiliary combustion zone 27, while the second auxiliary convective time τ_Ξ2 is the time required by the fuel supplied to the secondary auxiliary burner 25 to reach the second auxiliary combustion zone 28.

Preferably, the second auxiliary convective time T_S2 differs from the main convective time τ_Ρ by about 1-2 ms, while the first auxiliary convective time T_SI differs from the main convective time τ_Ρ by about 2-3 ms .

It has been experimentally discovered that for the purposes of stability, the effects caused by the first auxiliary burner 24 and by the second auxiliary burner 25 are complementary and are characterized by a different efficacy on the basis of the load conditions. The first auxiliary burner 24 is preferably designed to stabilize the main combustion zone 12 under full load operating conditions, while the second auxiliary burner 25 is designed to stabilize the main combustion zone 12 at low loads .

The studies on the distribution of the temperature of the flame 16 show that the second auxiliary burner 25 is also capable of facilitating the stabilizing action caused by the first auxiliary burner 24 in the main combustion zone 12. This determines increased temperature uniformity in the main combustion zone 12.

A first embodiment of the first auxiliary burner 24 and a first embodiment of the second auxiliary burner 25 are described below.

Such embodiments are described by way of non- limiting example. Indeed, further embodiments exist of the first auxiliary burner and of the second auxiliary burner which are adapted to generate a partially premixed diffusive combustion in a first auxiliary combustion zone and in a second auxiliary combustion zone, respectively. The first auxiliary combustion zone and the second auxiliary combustion zone are at least partially contained in the main combustion zone. The first auxiliary convective time Tsi and the second auxiliary convective time τ₅₂ are different from the main convective time τ_Ρ.

In particular, the first auxiliary burner 24 is provided with an air supplying annular channel 30, which flows directly into the combustion chamber 4, and with a fuel supplying annular channel 31, which extends parallel to axis B and flows into the end portion of the annular channel 30 close to an outer edge 33 of the annular channel 30.

Arranged at the outer edge 33 of the annular channel 30 and at the edge of the annular channel 31 is a swirler 34, which is configured to generate vortexes and to promote the mixing between the air and the fuel originating from the annular channel 30 and from the annular channel 31, respectively. In particular, swirler 34 is adapted to cause a rotation about axis B to the mixture which flows through it in order to stabilize the flame generated and to allow better control of the flame position inside the combustion chamber 4.

As will be seen in detail below, the annular conduit 31 is supplied with a first auxiliary fuel flow rate QA1 under the control of the control device 19 of the assembly 6 for supplying fuel (figure 1) .

The second auxiliary burner 25 is provided with a fuel supplying annular channel 35, which flows directly into the air supplying annular conduit 13 of the main burner 10.

In particular, the annular channel 35 is provided with a plurality of nozzles 36, arranged along the inner wall of the air supplying annular conduit 13 in a position comprised between an injection point of the nozzles 15 and the cylindrical outer element 18. Preferably, the- nozzles 36 are arranged downstream of swirler 17.

The position of the nozzles 36 affects the second auxiliary convective time^' T_S2- In particular, the position of the nozzles 36 is established following a design step and an experimentation step.

Calculation tools are employed in the design step which are capable of considering the gasdynamic, acoustic and chemical kinetic conditions. The fluid dynamic effects are generally valued with CFD (Computational Fluid Dynamics) simulation programs.

The experimentation step involves executing a series of tests and on- field adjustments designed to optimize and select the solutions which emerged from the preceding design steps.

In the non-limiting example described and shown herein, the nozzles 36 are arranged substantially equidistant from swirler 17 and from the cylindrical outer element 18.

In the non-limiting example described and shown herein, the nozzles 36 have a circular section and are uniformly distributed along the air supplying annular conduit 13 of the main burner 10. Variants (not illustrated) provide the employment of nozzles 36 with an increased diameter and provided with a suitable narrowing configured to prevent the transmission of the acoustic disturbances. Preferably, the narrowing is inserted between nozzle 36 and the annular channel 35 in order to regulate the second auxiliary convective time τ_Ξ2.

The annular channel 35 is supplied with a second auxiliary fuel flow rate QA2 under the control of the control device 19 of the assembly 6 for supplying fuel (figure 1) . _ .

The control device 19 is configured to regulate the main fuel flow rate QP, the first auxiliary fuel flow rate QA1 and the second auxiliary fuel flow rate QA2. In particular, the control device 19 regulates the fuel flow rates so as to respect a given ratio between the second auxiliary fuel flow rate QA2 and the total fuel flow rate QTOT.

The total fuel flow rate QTOT is defined as the sum of the main fuel flow rate QP and the secondary fuel flow rate QS . While the secondary fuel flow rate QS is defined as the sum of the first auxiliary fuel flow rate QA1 and the second auxiliary fuel flow rate QA2.

In particular, the control device 19 regulates the fuel flow rates so as to respect a given ratio between the second auxiliary fuel flow rate QA2 and the total fuel flow rate QTOT on the basis of the capacity emitted by plant 1.

Indeed, the control device 19 is configured to supply a second auxiliary flow rate QA2 equal to approximately 40% of the total flow rate QTOT when the plant operates at low loads, while it is configured to supply a second auxiliary flow rate QA2 which is reduced with respect to the flow rate supplied to the low loads, which is generally equal to 10% of the total flow rate QTOT, when plant 1 operates at intermediate loads .

Indeed, at low loads the main problem is the polluting emissions, in particular CO. The second auxiliary burner 25 produces a second auxiliary combustion zone 28 capable of causing a uniform temperature distribution in the main combustion zone 12 and a complete combustion of the fuel. Indeed, incomplete combustion is the main cause of the presence of CO emissions.

Thereby, the technical minimum environmental load may be reduced with respect to the values which can be obtained in known plants, without running into unwanted increases of CO emissions.

At intermediate loads, the temperature in the combustion chamber 4 is high and allows the complete oxidation of the CO. Moreover, the thermo-acoustic instabilities at intermediate loads are not high. In this configuration, the second auxiliary burner 25 does not contribute to optimizing the combustion and therefore, the second auxiliary fuel flow rate QA2 is significantly reduced.

At rated load, the second auxiliary fuel flow rate QA2 is again increased until it reaches 20% of the total flow rate QTOT. Indeed, under normal load conditions, the thermo-acoustic oscillations increase in the combustion chamber. The increase of the second auxiliary fuel flow rate QA2 and the modulation of the second auxiliary convective time x_S2 allow such oscillations to be controlled.

One variant (not illustrated) provides for the coupling between main burner and secondary burner of the burner assembly to be configured so as to generate a flameless combustion process in the main combustion zone 12. Flameless combustion process is intended as a combustion process in which the oxidation process occurs when the air-fuel mixture is beyond the inflammability limits and exceeds the autoignition temperature. These conditions may be obtained by means of preheating the comburent air and recirculating the combusted gases. The conditions for preheating the air are obtained by recirculating the fumes. Indeed, thereby the heat is transferred from the products to the reactants and the comburent is diluted with the fuel and with the combustion products. The recirculation of the fumes may be established by a recirculation outside the chamber or inside the chamber, governed by a specific fluid dynamic attitude of the burner. The "flameless" definition derives from the absence of a defined flame front and having a reduced thickness with respect to the extension of the combustion volume during the occurrence of the combustion reaction.

Advantageously, the burner assembly 9 according to the present invention is capable of ensuring increased combustion stability with respect to burner assemblies from the known art, polluting emissions being equal.

This allows thermal release variations to be obtained more quickly with respect to those obtained with burner assemblies from the known art. Thereby, the burner assembly 9 according to the present invention is capable of quickly meeting the capacity variations required by plant 1.

In particular, the increased stability of the combustion is mainly due to the fact that the secondary burner 11 is configured so that the secondary convective time t_s is different from the main convective time τ_Ρ. Indeed, thereby the thermo-acoustic oscillations of the main burner 10 are reduced by the counter action of the secondary burner 11.

Furthermore, the fact that the secondary burner 11 comprises a first auxiliary burner 25 and a second auxiliary burner 28, which are supplied independently by respective fuel supply lines, generates a homogeneous distribution of different convective times related to several fuel sources. A homogeneous distribution of convective times generates more stable combustion with respect to the combustion that would be obtained in a burner assembly in which there is only one convective time associated with the main burner. Indeed, the homogeneous distribution of the convective times allows the delay time to be regulated so as to counter the effects of the thermo- acoustic instabilities.

Moreover, the fact that the secondary burner 11 is configured so that the secondary combustion zone 20 is completely contained within the main combustion zone 12 causes a reduction of the combustion temperature variability, with apparent advantages in terms of polluting emissions, and an effective stabilization of the entire combustion zone.

In accordance with the present invention, the stabilizing effect is, at least partly, delegated to modulating the convective times, while the reduction of the polluting emissions is delegated to improving the temperature distribution in the combustion zone.

Known burner assemblies generally assign combustion stabilization to the control burner. However, the control burner involves the presence of combustion zones with high temperatures, and accordingly polluting. Finally, the regulation of the fuel flow rates carried out by the control device 19 ensures a given ratio is respected between the second auxiliary flow rate QA2 and the total combustion flow rate QTOT on the basis of the capacity emitted by plant 1. Thereby, the stability of the combustion is ensured under all the operating conditions of plant 1.

Finally, it is apparent that modifications and variants may be made to the burner assembly, to the combustion chamber and to the method for supplying said burner assembly herein described without departing from the scope of the appended claims .

Claims

1. Burner assembly (9) for a combustion chamber (4) of a gas turbine plant (1) comprising:

a. main burner (10) configured to produce a main combustion zone (12) having a main convective time

(T_P) ;

a secondary burner (11) configured to produce at least a secondary combustion zone (20) having a secondary convective time (τ₃) ;

the secondary burner (11) being configured so that the secondary convective time (T_s) is different from the main convective time (τ_Ρ) .

2. Burner assembly according to claim 1, wherein the secondary convective time (τ_Ξ) is lower than the main convective time (τ_Ρ) .

3. Burner assembly according to claim 1 or 2, wherein the main convective time (τ_Ρ) and the secondary convective time (τ_Ξ)- cause disruptive interference between the thermo-acoustic waves of the main burner (10) and the thermo-acoustic waves of the secondary burner (11) .

4. Burner assembly according anyone of the foregoing claims, wherein the secondary combustion zone (20) is at least partially contained in the main combustion zone (12) .

5. Burner assembly according anyone of the foregoing claims, wherein the secondary combustion zone (20) is completely contained in the main combustion zone (12) .

6. Burner assembly according anyone of the foregoing claims, wherein the main burner (10) extends about the secondary burner (11) and comprises an air supplying annular conduit .(13) and a fuel supplying annular conduit (14).

7. Burner assembly according to claim 6, wherein the secondary burner comprises a first auxiliary burner (24) configured to produce a first auxiliary combustion zone (27) having a first auxiliary convective time (T_SI) an a second auxiliary burner (25) configured to produce a second auxiliary combustion zone (28) having a second auxiliary convective time (τ₃₂) ; the first auxiliary convective time (T_S1) and the second auxiliary convective time being (T_S2) different from the main convective time (τ_Ρ) .

8. Burner assembly according to claim 7, wherein the first auxiliary convective time (τ₃1) and the second auxiliary convective time (τ₃₂) are lower than the main convective time (τ_Ρ) .

9. Burner assembly according to claim 8, wherein the first auxiliary cohvective time (τ_Ξι) is lower than the second auxiliary convective time (τ₃₂) .

10. Burner assembly according to anyone of the claims from 7 to 9, wherein the second auxiliary burner (25) comprises a fuel supplying annular channel (35) , which flows directly into the air supplying annular conduit (13) of the main burner (10) .

11. Burner assembly according to claim 10, wherein the fuel supplying annular channel (35) is provided with a plurality of nozzles (36) , arranged along the inner wall of the air supplying annular conduit (13) in a position comprised between a fuel injection point of the main burner (10) and an outer edge (18) of main burner (10) .

12. Combustion chamber comprising at least one burner assembly (9) as claimed in anyone of the foregoing claims.

13. Method for supplying fuel to a burner assembly

(9) of a combustion chamber (4) of a gas turbine plant (1) as claimed in anyone of the foregoing claims; the method comprising the steps of :

supplying a main fuel flow rate (QP) to the main burner

(10) ;

supplying a secondary fuel flow rate (QS) to the secondary burner (11) ;

regulating the secondary fuel flow rate (QS) on the basis of the load of the plant (1) .

14. Method according to claim 13, wherein the secondary burner (11) comprises a first auxiliary burner (24) configured to produce a first auxiliary combustion zone (27) and a second auxiliary burner (25) configured to produce a second auxiliary combustion zone (28) ; the method comprising the steps of:

supplying a first auxiliary fuel flow rate (QA1) to the first auxiliary burner (24) ;

supplying a second auxiliary fuel flow rate (QA2) to the 5. second auxiliary burner (25) ;

regulating the second auxiliary fuel flow rate (QA2) on the basis of the load of the plant (1) .

15. Method according to claim.14, wherein the step of regulating the second auxiliary fuel flow rate (QA2) on the0 basis of the load of the plant (1) comprises reducing the second auxiliary fuel flow rate (QA2) when the load of the plant (1) increases.

16. Method according to claim 14 or 15, wherein the step of regulating the second auxiliary fuel flow rate5 (QA2) comprises supplying a second auxiliary fuel flow rate (QA2) which is equal to the 40% of the total fuel flow rate (QTOT) , intended as the sum of the main fuel flow rate (QP) and the secondary fuel flow rate (QS) , when the load is lower than a first threshold value.

0 17. Method according to anyone of claims from 14 to

16, wherein step of regulating the second auxiliary fuel flow rate (QA2) comprises supplying a second auxiliary flue flow rate (QA2) which is equal to the 20% the total fuel flow rate (QTOT) , intended as the sum of the main fuel flow5 rate (QP) and the secondary fuel flow rate (QS) , when the load is comprised between a first threshold value and a second threshold value.

18. Method according to anyone of the claims from.14 to 17, wherein the step of regulating the second auxiliary fuel flow rate (QA2) comprises supplying a second auxiliary fuel flow rate (QA2) equal to the 10% of the total fuel flow rate (QTOT) , intended as the sum of the main fuel flow rate (QP) and the secondary fuel flow rate (QS) , when the load is greater than a second threshold value.