EP1764553A1

EP1764553A1 - High-stability premix burner for gas turbines

Info

Publication number: EP1764553A1
Application number: EP05425639A
Authority: EP
Inventors: Luciano Carrai; Giordano Tanzini; Giancarlo Benelli
Original assignee: Enel Produzione SpA
Current assignee: Enel Produzione SpA
Priority date: 2005-09-14
Filing date: 2005-09-14
Publication date: 2007-03-21

Abstract

A premix burner for a gas turbine for use in power generation plants, comprises an axial swirler (12) to which pilot fuel gas and pilot air flow are delivered, and a duct for the premix flow (9) extending annularly and diagonally around said axial swirler (12), which contains a diagonal swirler (8) to which premix fuel gas and premix air are delivered. The axial swirler and diagonal swirler converge substantially at the outlet section (16) of the burner. A discharge throat (15) is attached to the outlet of the axial swirler (12) and extends axially from the outlet section (16) of the burner within the combustion chamber (14). The presence of the discharge throat contributes significantly towards improving the working stability at high and medium loads of the burners in Siemens gas turbine machines of the VX4 type.

Description

The present invention relates generally to gas turbines for power generation and more particularly to a high-stability premix burner for installation in such machines.
The gas turbine machines in current use are frequently equipped with Dry Low NOx (DLN) burners, as in the case of the SW V94.3A2 gas turbine machine, for instance, which is installed in a number of combined cycle plants of the Applicant.
These burners are used in annular combustion chambers. Twenty-four such burners are installed in the SW V94.3A2 gas turbine machine, 20 of which are fitted with CBO (Cylindrical Burner Outlet). As is known a CBO is a cylindrical extension situated at the burner outlet, with the same diameter as the outlet, the purpose of which is to modify the flame "time lag" and thus change the acoustic coupling between the flame and the combustion chamber.
To obtain low pollutant emissions, the main part of the flame in this type of combustor is obtained by premixing air with the fuel before it is delivered to the combustion chamber, using "lean" combustion mixtures. The premixed flame is supported by a diffusion pilot flame.
The burner includes two coaxial swirlers, one axial and the other diagonal, a fuel oil lance and a metal grid placed over the diagonal swirler inlet. The gas is delivered to the combustion chamber along three lines (premix, pilot and diffusion).
The diffusion gas is only used in the ignition stage; in normal operation the fuel is distributed between the pilot and premix lines in a ratio that varies according to the load.
The diagonal swirler has a truncated cone-shaped section and is complete with blades (e.g. 20 in the case of the above-mentioned burner) slightly inclined with respect to the outflow direction. Over 90%, in particular about 92%, of the air delivered to the burner as a whole passes through the diagonal swirler. The premix gas is delivered through an annular manifold that serves small ducts formed inside the blades on the diagonal swirler; in a particular embodiment, each of these ducts leads to 9 gas nozzles, 4 on the extrados and 5 on the intrados of each diagonal swirler blade.
The axial swirler is a relatively narrow duct, complete with a number of blades (e.g. 8) lying at a steep angle to the direction of the flow of air, and ending at the oil lance placed inside the swirler.
This component feeds a diffusion flame that can be produced by injecting the gas from so-called "diffusion nozzles" and/or "pilot nozzles".
The gas diffusion nozzles are situated upstream of the axial swirler. They consist of a number of small holes (e.g. 24 holes 3.5 mm in diameter) arranged symmetrically around the circumference in groups of three. The igniter is installed in the section where the diffusion nozzles inject the fuel.
The "pilot gas" nozzles are also located upstream of the axial swirler, but slightly downstream of the igniter; they consist, for instance, of four holes 4.8 mm in diameter lying at right angles to one another.
The oil lance is positioned inside the axial swirler, ends at about the swirler blades and has a nozzle at the end that extends into the combustion chamber to deliver fuel oil. On the Applicant's gas turbine machines, which run exclusively on natural gas, the oil lance is replaced by a so-called "dummy" lance, which has no effect whatsoever on the combustion process.
The perforated metal grid is situated at the diagonal swirler inlet and serves the purpose of making the flow delivered to the burner more uniform, eliminating any turbulence.
The CBO, which is installed in the majority of combustors installed in such machines, was introduced by Siemens with a view to modifying the fluid-dynamic field at the burner outlet. This component changes the time lag between the fuel inlet and the air-fuel mixture ignition, thereby improving the stability conditions of the burner.
Modern gas turbine combustors designed to guarantee lower and lower NOx emissions adopt premix flame technologies with lean fuel/air mixtures that reduce the temperature of the flame and consequently also the production of nitrogen oxides. This solution has the drawback, however, of giving rise to combustion instability phenomena, which become evident in the form of pressure oscillations in the combustion chamber.
These oscillations occur as a result of interaction between the heat released by the flame and the acoustics of the combustion chamber: fluctuations in the amount of heat released by the flame excite the acoustic field of the pressures in the combustor which, in turn, triggers further fluctuations in the release of heat. If the fluctuations in the pressure and the heat released are in a favorable phase relationship, the oscillations that are generated are self-supporting and become amplified up to a limit cycle determined by the non-linearity of the system. Under certain circumstances, the forces generated by the pressure oscillations can reach amplitudes that reduce the average life and performance of the combustor. Preventing or reducing combustion instability is one of the priorities in the conduction of gas turbines equipped with high energy density combustors.
The onset of oscillation phenomena in the combustion process, commonly called "humming", frequently makes it necessary to reduce the load and sometimes demand an immediate shutdown of the machine.
When the machine is in operation, the combustion process control system monitors the values recorded by sensors capable of measuring the machine mechanical oscillations and thus manages the different critical conditions that can occur with respect to preset system thresholds. In the specific case of the SW V94.3A2 gas turbine machines, for instance, the safety devices are enabled 55 seconds after reaching a velocity of 47 Hz or 2820 rpm and they are activated when the signals coming from the acceleration sensors (accelerometers) positioned just outside the combustion chamber reach the following acceleration values:

1^st limit: 2.5 g, triggers load reductions in steps of 6 MW every 4 s and a shutdown after 19 s;
2^nd limit: 3.0 g, triggers load reductions in steps of 15 MW per s and a shutdown after 13 s;
3^rd limit: 8.0 g, triggers an immediate shutdown;
4^th limit: 1.5 g - 1.9 g and position of the IGV (Inlet Guide Vane) on the compressor inlet opened to an extent greater than 95%, trigger a load reduction in steps of 6 MW every 0.5 s.

As regards the Applicant's machines, instabilities have occured mainly in two different operating conditions, i.e. high load (240-260 Mwe) and medium load (180-200 Mwe). The latter type of critical condition happens mainly with load reduction transients and coincides with increments in the pressure oscillations up to values of a few dozen mbar, generally recovered by the machine safety systems by means of a rapid drop in load.
The phenomenon is symptomatic of certain working conditions; it is thought that a correlation would exist with the various process parameters, such as air and fuel flow rates, working pressure and temperature, fuel gas composition, ambient conditions, P/T ratio (i.e. the ratio of the amount of fuel gas sent to the pilot to the total amount of fuel gas burnt).
No anomalous events in the combustion process severe enough to demand a machine shutdown were encountered in operation under a partial load, whereas the same cannot be said for the full load conditions, when a machine shutdown was "tripped".
Tripping involves the immediate shutdown of the gas turbine due to the enabling of a safety device by the control system. Analyses conducted on the Applicant's SW V94.3A2 gas turbine machines demonstrated that these trips occur as a result the third limit being exceeded and always under full load in steady state operating conditions.
During the interval between September 2004 and January 2005, countless cases of combustion instability were observed in the Applicant's SW V94.3A2 gas turbine systems. These events occurred at different levels of intensity: in 13 cases a machine trip was caused, while a controlled load reduction (or "load rejection") was needed in 10, and in many other cases a situation of incipient instability developed.
These phenomena have a heavy fallout in terms of the deterioration of the machine and, when a machine shutdown is tripped, they also immediately cause considerable economic damages due to the loss of production and all the costs relating to the tests and inspections needed before the machine can be started again.
The object of the present invention is to provide a premix burner for gas turbines that is highly stable in operation so as to enable a wider use of gas turbine machines which are characterized by instability problems in operation at high and medium loads.
A particular object of the present invention is to provide a burner of the above-mentioned type suitable for installation in Siemens gas turbine machines of the VX4 series, such as the SW V94.3A2.
Another object of the present invention is to provide a burner of the above-mentioned type that can be installed in gas turbine machines in the place of the original burner without needing any structural adaptation, and that demands no changes to the system and machine modulation logic for its operation.
The characteristic feature of the burner according to the invention consists in that it is provided with an discharge throat installed at the outlet of the axial swirler conceived and designed so as:

to promote a smooth interaction between the premixed flow produced by the diagonal swirler and the flow produced by the axial swirler, facilitating the damping of the Precessing Vortex generated at the burner outlet;
to improve the recirculation of the primary flow by incrementing both the swirl number and the mass quantity recirculated, thereby increasing the pilot burner flame stability, so that it can operate at lower P/T values.

The features and advantages of the burner according to the present invention will become more apparent from the following description of an embodiment, given as a non-limiting example with reference to the attached drawings, wherein:

figure 1 shows the burner according to the invention in a schematic longitudinal section;
figure 2 shows the isolines of the current function of the field of motion at the burner outlet;
figure 3 shows three front-view images of the original burner to illustrate the PVC (Precessing Vortex Core) phenomenon;
figure 4 is a graphic representation of the response of the turbulence at the outlet of the original burner and of the burner according to the invention vs. the relative radial position r/D;
figure 5 graphically represents the response of the pressure fluctuations in the original burner and in the burner according to the invention vs. frequency.

With reference to figure 1, the burner comprises a main tubular body 1 from which a flange 2, in an intermediate position, extends radially to connect the body 1 to a wall 3 of the outer casing of the gas turbine (not shown). An oil lance 4, shown only partially in figure 1 and inactive in the event of gas firing, is installed coaxially in the tubular body 1 and defines an annular duct 5 for the diffusion gas together with the inner wall of said tubular body 1. A second tubular body 6, supported by the flange 2 and lying parallel to the tubular body 1, serves as a duct for delivering the premix gas to an annular manifold 7 coaxial to the tubular body 1. From the annular manifold 7 the premix gas is delivered through nozzles 7a to a diagonal swirler 8 with a truncated cone-shaped cross-section and equipped with blades lying at a slight angle to the outflow direction. The diagonal swirler 8 is situated inside an annular duct 9 for the premix flow, through which more than 90% of the combustion air flows. The premix gas is delivered through small ducts formed inside the blades of the diagonal swirler 8, which in turn serve groups of nozzles positioned on the extrados and intrados of each blade of the diagonal swirler 8. These structural details, which are characteristic of the Siemens VX4 burners, are well known to a person skilled in the art and are consequently not further illustrated or described herein.
At the inlet to the diagonal swirler 8, in the annular duct 9, there is a perforated metal grid 10, the purpose of which is to smoothen the air flow arriving at the burner inlet from the plenum (not shown), eliminating any turbulence.
On the outside of the end portion of the tubular body 1 there is an annular chamber 11, through which the pilot air is fed to an axial swirler 12, which is a duct with a relatively narrow cross section and is fitted with blades lying at a steep angle to the direction of the outgoing air flow. The pilot gas is fed to the axial swirler 12 through four nozzles, only one of which is shown in figure 1, indicated by the numeral 17, and located upstream of said swirler. The ducts of the axial swirler 12 and diagonal swirler 8 substantially converge at the outlet section 16 of the burner.
Figure 1 also shows a cylindrical extension (or CBO) 13 attached to the outlet of the duct 9 and extending from the outlet section 16 of the burner into the combustion chamber, generically indicated at 14.
According to the invention, a discharge throat 15 is attached to the outlet from the axial swirler 12, coaxially to the longitudinal axis X of the burner and extending from the outlet section 12a of the axial swirler duct through which the premix gas is delivered to the combustion chamber 14, roughly at the outlet section 16 of the burner.
The discharge throat 15 consists of a tubular sleeve with a cross-section that opens outwards towards the outlet, the length of which comes typically between 25 mm and 55 mm, with an half-opening angle amplitude comprised between 10° and 30°. The shape of the outermost part of said discharge throat 15 has been designed so as to assure continuity with the profile of the inner wall of the channel 9, thus avoiding any discontinuity in the premix flow.
The divergent shape of the discharge throat 15 serves the purpose of "expanding" the flow coming from the burner, i.e. it increases the swirl number (as defined below). The discharge throat has a straight profile on the inside, but a curved profile may also be used. Though it gives rise to a modest discontinuity, the straight profile is acceptable so long as the half-opening angle amplitude is not excessive. Any sharp corners and steep half-opening angles can induce flow separation, which would give rise to aerodynamic noise.
Tests conducted on the prototype enabled the Applicant to show that the presence of the discharge throat 15 contributes significantly towards improving the operating stability of the burners of the Siemens V94.3A2 gas turbine machine at high and medium loads.
This result can be explained by the fact that experimental analyses conducted by the Applicant on the original burner showed that the flow produced by the burner is not axially symmetric.
The fluid-dynamic field promoted by this burner is characterized by a swirl number S (defined as the axial flux of the tangential component of the momentum divided by the axial flux of the axial component of the momentum times the equivalent nozzle radius) equal to 0.56; this generates a narrow central recirculation zone characterized by a recirculated mass of just 2% of the total mass of processed fluid. Experimental analyses have also shown that the central region of the forced swirl is unstable, generating a precessing motion around the symmetry axis of the system, known as "PVC" (Precessing Vortex Core), a motion that occurs in swirled flows at S values beyond a given limit.
The PVC is located at the limit of the mean recirculation zone between the isoline zero of the stream function, which identifies the recirculation zone, and the zero velocity line defined by the points with a velocity value equal to zero within the recirculation zone (see figure 2).
Comparing the precessing frequency of the PVC with the profile of the mean tangential velocity shows that, considering the radius Rp of the precessing motion, the frequency of the PVC, f_PVC, gives a tangential velocity equal to the mean tangential velocity corresponding to the radius Rp: $w (R_{p}) = 2 π \begin{matrix} \end{matrix} f_{PVC} \cdot R_{p}$
The presence of this phenomenon has been emphasized in flow visualization experiments conducted on the SW V94.3A2 burner.
Figure 3 shows the images recorded using a pulsed laser stripe placed at the original burner outlet during aerodynamic tests, after injecting a tracer only in the air flow produced by the diagonal swirler. There is clearly a central swirling core that continuously changes in position, rotating around the burner axis.
There is a strong conviction that one of the causes of thermoacoustic instability in combustion chambers is the separation of coherent and large-scale, swirling structures associated with the turbulence of the main flow. These structures are capable of disrupting the flame and thus contribute to the generation of noise in the combustion process. The "PVC" phenomenon is consequently an important element involved in the phenomenon of pressure fluctuation in the SW V94.3A2 burner.
Experimental measurements were taken in isothermal conditions characteristic of the fluid-dynamic field produced at the outlet of the prototype burner, according to the invention. These experiments were conducted with respect to a kinematic similitude corresponding to 65% of the real working conditions of the burner installed in a machine.
The similitude of the velocity field - or kinematic similitude - stems from the assumption of a non-viscous fluid, suitably scaling down the volumetric flow rate in the tests. The limits of the air flow rate that the aerodynamic test bench at the Applicant's laboratories can deliver enable tests to be conducted with a 65% kinematic similitude with the working conditions of the burner in a machine.
The results of the aerodynamic tests were judged to be representative, since the test conditions maintained Reynolds numbers high enough to make the viscous effects negligible.
Experiments conducted on the same burner using LDV (Laser Doppler Velocity) methods also confirmed that the velocity field at the outlet of the SW V94.3A2 burner on the test bench maintains the same characteristics as the velocity field of the burner installed in a machine.
Measurements were taken with a hot-wire anemometer to determine the turbulence spectrum and the characteristic frequencies of the steady-state field of motion in the discharge region, both of the original burner and of the prototype according to the invention. The instrument adopted was the Dantec "Stream-Line" hot-wire anemometer, consisting of:

4 CTA modules,
1 automatic calibration module,
1 National Instrument PCI_MIO_16E_1 A/D card,
Sample and Hold National Instrument SC- 2040.

The graph in figure 4 clearly shows that the velocity fluctuations measured in the prototype configuration, i.e. using the burner fitted with the discharge throat according to the invention, are considerably lower than those measured with the original burner, highlighting a greater stability of the field of motion at the burner outlet. Full-scale combustion tests confirmed the greater stability of the prototype burner.
Figure 5 shows the comparison between the pressure fluctuation spectra measured at rated load using a 6% P/T ratio between the original burner and the burner adapted according to the invention: there is evidence of the suppression of the greatest pressure peaks typical of unstable combustion that occur with the original configuration.
The burner according to the invention consequently assures a greater flame stability than the original burner over the entire working range, thus keeping combustion noise at a low level.
The modification to the original burner according to the invention also enables higher specific powers to be achieved in stable operating conditions, thus extending the operating range of the gas turbine machine.
Moreover, the burner according to the invention keeps the production of the various pollutant chemical species at the same levels as the original burner.
Variations and/or modifications to the premix burner for gas turbines according to the present invention may be made without departing from the scope of the invention, as set forth in the following claims.

Claims

Premix burner for a gas turbine for use in power generation plants, with an outlet section (16) placed in the turbine combustion chamber (14), comprising an axial swirler (12) to which pilot fuel gas and pilot air flow are delivered, and a premixed flow duct (9) extending annularly and diagonally around the axial swirler (12), in which there is a diagonal swirler (8) to which premix fuel gas and premix air are delivered, said swirlers converging substantially at the outlet section (16) of the burner, characterized in that a discharge throat (15) is mounted at the outlet from said axial swirler (12), extending axially from said outlet section (16) within the combustion chamber (14).
Burner according to claim 1, wherein said discharge throat has a diverging cross-section, starting from the outlet section (12a) of said axial swirler (12).
Burner according to claim 1 or 2, wherein said discharge throat (15) is a tubular sleeve with a progressively increasing cross-section, with half-opening angle amplitude comprised between 10° and 30°.
Burner as set forth in any of the previous claims, wherein said discharge throat (15) is between 25 mm and 55 mm long.
Burner as set forth in any of the previous claims, wherein a CBO (13) extends from said premix flow channel (9) coaxially to said discharge throat (15).
Burner as set forth in any of the previous claims, wherein said sleeve has a straight inner profile.
Burner as set forth in claims from 1 to 5, wherein said sleeve has a curved inner profile.
Burner as set forth in any of the previous claims, wherein said turbine is a Siemens gas turbine of the VX4 type.