Gas turbine equipped with a gas burner and axial swirler for the burner

A gas turbine equipped with a burner, which has an axial swirler for generating turbulence in a flow of comburent air, a secondary supply line of fuel gas, a main supply line of fuel gas arranged concentrically around the secondary line, and a pilot line of fuel gas. Accordingly, the burner is ignited by firing a spark and feeding, towards the spark, comburent air and fuel gas which is synthesis gas.

TECHNICAL FIELD

The present invention relates to a method for starting a gas turbine equipped with a gas burner.

BACKGROUND ART

So as to produce electrical power, it is known to use electric machines powered by gas turbines comprising a burner assembly which is ignited and started using a natural gas as fuel and then, when the electric machine has exceeded a predetermined value of load, is fed by a so called synthesis gas or syngas, that is a fuel gas rich in hydrogen and having a relatively low calorific value (low-BTU), for example a fifth of the calorific value of natural gas.

However, the use of the two different types of fuel gas for starting the burner assembly of the turbine is unsatisfactory, since it is necessary to envisage double connecting systems to deliver fuel gas from different sources, and it is necessary to bear a relatively high fixed cost for the natural gas supply contract, while the number of times the burner assembly is ignited is in effect relatively low.

DISCLOSURE OF INVENTION

The object of the present invention is to provide a method for starting a gas turbine equipped with a gas burner, which allows to resolve in a simple and economic manner the above-mentioned problems and which, preferably, can be carried out without flashback and localised overheating of the burner, and which can be also implemented in existing burners with only slight structural modifications.

According to the present invention a method is provided for starting a gas turbine equipped with a gas burner comprising:an axial swirler to generate turbulence in a flow of comburent air,a secondary fuel gas supply line,a main fuel gas supply line arranged concentrically around said secondary line, anda fuel gas supply pilot line;
the method comprising the step of igniting said burner causing a spark and feeding comburent air and fuel gas towards said spark; characterised in that the fuel gas fed towards said spark is synthesis gas.

The present invention also relates to an axial swirler for a burner on a gas turbine.

BEST MODE FOR CARRYING OUT THE INVENTION

InFIG. 1, it is indicated by1a burner assembly, which is feeded with fuel gas and forms part of a gas turbine (not illustrated) which, in use, drags in rotation a shaft of an electric machine (not illustrated) to produce electrical power.

The assembly1extends along an axis2, generates, in use, combustion in a chamber3(partially illustrated) and comprises a central burner4and a peripheral burner5coaxial to and arranged around the burner4.

The burner5comprises a main supply line6for fuel gas, known as “the main diffusion line”, which comes out into the chamber3, through an outlet7concentric and external with respect to the burner4and supplies synthesis fuel gas, or syngas, to generate a diffusion flame with a flow of comburent air.

The comburent air comprises primary air for combustion coming from burner4and secondary air for combustion coming from a bladed device8, generally known as a swirler, which is arranged around the outlet7and generates turbulence in the flow of air which has been flown into the chamber3with a so-called “diagonal” mean path.

Similarly, the burner4comprises its own bladed device known as axial swirler, indicated by reference numeral16, to generate turbulence in the flow of primary air.

The device16comprises an internal cone17and an external cone18, which are partially opposite each other and are reciprocally coupled by a plurality of blades21having respective pressure sides and respective depression sides that define in between a series of spaces22(FIG. 2).

With reference toFIGS. 3 and 4A, the device16, at the rear where the primary air comes from, comprises two coaxial walls23,24, which are substantially cylindrical, and extend as an axial extension of the cones17and18and are respectively fixed to the cones17,18themselves in a manner not described in detail, and are partially opposed to each other in a radial direction. The wall24comprises a slit25arranged at one of the spaces of the device16, indicated by the reference numeral22a. An intermediate portion26of the space22ais radially outwardly closed by a wall27(FIG. 4A), which is fixed watertight to the edges of the slit25and to the cone18and extends axially for a lesser cut of the wall24, with respect to the cone18, that is, it is not flush with the rear edge of the wall24directed towards the primary air inlet of device16.

The space22ain a circumferential direction is defined by two blades21ahaving axial length greater than the other blades21of the device16. In particular, the blades21aextend between the wall23and the edges of the slit25and comprise respective end portions29which protrude axially with respect to the other blades21, towards the primary air inlet of the device16, converge on each other in the direction of the flow of primary air, and form an intake30which is adjacent to the portions26and is not covered or defined by the wall27.

Still with reference toFIG. 1, the burner4comprises a tubular axial body34, which fits into a diesel or fuel oil burner35, which extends axially from the centre of the cone17towards the space3.

The body34and the burner35together radially define a duct36, which defines what is known as a “secondary diffusion line” to feed a flow of fuel gas which generates a diffusion flame in the chamber3.

With reference toFIG. 2, the duct36comes out into the spaces22through a plurality of holes37made in the cone17and houses, in a manner not illustrated, four pipes38, which are isolated from the gas flowing into the duct36, are parallel to the axis2, and define what is known as the “pilot line” which comes out into the spaces22through respective outlets39, to feed in fuel gas in particular operating conditions of the turbine, such as a drop in the electrical load applied to the electric machine.

With reference toFIG. 1, the primary comburent air flowing into the spaces22comes from an annular duct40, which is defined, upstream by the device16, by the body34and by a tubular coaxial body41, and houses an electric line42, of known type and not described in detail (FIGS. 3 and 4A), provided with a pair of electrodes43fixed on the portion23and having respective ends44arranged in the portions26of the space22a, facing one of the outlets of the pipes38, indicated by reference numeral39a, to fire an ignition spark.

According to the present invention, the ignition of the assembly1is effected by firing the spark and directing towards said spark a fuel gas known as synthesis gas, or “syngas” while the turbine operates loadlessly, that is without being connected to the electric machine.

In particular, during the ignition step, the synthesis gas is fed to the pipes38, that is through the pilot line. Preferably, the space22ain which the ignition spark is fired has only one outlet39a, while being free from the holes37(FIG. 2), so as to optimise the fluid mechanical conditions in the space22aitself.

Once the turbine has reached a predetermined number of revolutions, the supply of syngas is commutated from the pilot line to the secondary diffusion line (duct36). The speed of rotation of the turbine is then increased to reach a full speed and loadless condition. At this point, synchronisation occurs, that is the coupling between the turbine and the electric machine, in particular at a speed of about 3000 rpm but with zero electric charge applied to the electric machine, so as to only allow it to coast.

The electrical load is then increased to a reference value, in particular to up to 50% of the basic load of the electric machine, while continuing to feed syngas from the secondary diffusion line.

Finally, the supply of synthesis gas is gradually commutated from the secondary diffusion line to the primary diffusion line, so as to reach the basic load of the electric machine.

From the above description it is clear how it is possible to operate the gas turbine from the ignition of the assembly1to the basic load of the electrical machine, feeding to the assembly1fuel gas which is exclusively synthesis and not natural gas, after gauging the passage sections of syngas in the secondary diffusion line and the pilot line so as to reach sufficient flow rates of fuel for correct functioning according to the calorific value of the syngas.

Consequently, the electrical power production plant is simplified and costs are reduced, thanks to the elimination of the supply lines and supply contracts for natural gas.

The constructional and fluid mechanical characteristics of the device16allow to avoid flashback and overheating in all operational conditions.

In particular, during ignition, the blades21aof the space22a, thanks to the absence of the holes37and the length of said blades, guide the comburent air in an optimal manner and, moreover, accelerate the flow of air towards the spark fired by the electrodes, providing an invitation in the intake portion30. Moreover, the cut of the wall26forms a suitable compromise between the need for a high flow rate of incoming air to the space22aand the need to avoid dispersing the gas flowing from the outlet39a.

The method of the present invention may be easily carried out in existing gas turbines, preferably by replacing the existing axial swirler with the device16described above and correctly gauging the sections for the passage of the syngas in the secondary diffusion line and the pilot line.

From the above description, finally, it is clear that modifications and variations to the method described with reference to the accompanying drawings can be effected without leaving the scope of protection of the present invention.

In particular, the ignition could occur by directing syngas towards the spark from an outlet other than the one indicated by way of example.