TECHNICAL FIELD

The present invention relates to a gas turbine unit with a multifluid fuel supply and a method of supplying a burner of a gas turbine unit .

BACKGROUND ART

As is known, in the energy production field, in particular power production via gas turbine plants, there is an increasingly felt need for reducing pollutant emissions, especially of nitrogen oxides (NOx) . It is also known that the methods of suppling fuel to the combustion chamber and the characteristics of the burners play an essential role in determining the level of pollutant emissions.

The need of minimizing pollutant emissions is accompanied by the need of maintaining adequate flame stability across a wide range of operating conditions, in order not to degrade plant performance .

In some cases, the above-mentioned needs may push toward conflicting directions, whereby it is not always possible to achieve satisfactory compromises.

For example, the use of a diffusion pilot burner ensures significant advantages in terms of flame stability, but at the expense of worsening pollutant emissions. In certain operating conditions, the pilot burner aids combustion stability with a flame that contributes to the overall thermal power for just a few percentage points, but is responsible for the majority of NOx emissions. On the other hand, partial premix pilot burners, better from the emission viewpoint, are less effective in stabilizing combustion. Among other things, even in partial premix pilot burners, a fraction of the supplied fuel flow still generates an uncontrolled diffusion flame, which can have negative repercussions on pollutant emissions.

DISCLOSURE OF INVENTION

The object of the present invention is therefore to provide a gas turbine unit and a method of supplying a burner of a gas turbine unit that enable overcoming the described limitations and, in particular, reducing the emission of polluting substances without compromising flame stability.

According to the present invention, a gas turbine unit and a method of supplying a burner of a gas turbine unit are provided as defined in claims 1 and 14, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described with reference to the accompanying drawings, which illustrate some non- limitative embodiments, in which:

- Figure 1 is a simplified block diagram of a gas turbine unit according to one embodiment of the present invention;

- Figure 2 is a more detailed block diagram of a portion of the gas turbine unit in Figure 1;

- Figure 3 is a side view, in cross-section along an axial longitudinal plane, of a burner assembly of the gas turbine unit in Figure 1;

- Figure 4 is a side view, in cross-section along an axial longitudinal plane, of an enlarged detail of the burner assembly in Figure 3;

- Figure 5 is a front view of the detail in Figure 4;

- Figure 6 is a side view, in cross-section along an axial longitudinal plane, of a detail of a burner assembly for a gas turbine unit according to a different embodiment of the present invention; and

- Figure 7 is a front view of the detail in Figure 6. BEST MODE FOR CARRYING OUT THE INVENTION

Figure 1 schematically shows a gas turbine unit 1 and a control device 2 for the gas turbine unit 1.

II gas turbine unit 1 comprises un compressor 3, an annular- type combustion chamber 5, a turbine 7 and a supply system 8 for supplying fuel to the combustion chamber 5.

Sensors 9.1, ..., 9.N measure respective plant quantities and provide corresponding measurement signals S _M i , ··· , S _MN to the control device 2.

The supply system 8 supplies the combustion chamber 5 with a regulated flow of one or more types of fuel, such as, and by way of non-limitative example, natural gas, HFO (Heavy Fuel Oil), Bunker C, syngas and diesel fuel. In one embodiment, depending to the operating conditions, the combustion chamber 5 uses natural gas as the primary fuel and diesel fuel as a secondary or back-up fuel. The primary fuel and the secondary fuel are supplied to the combustion chamber through a first fuel supply line 10 and a second fuel supply line 11, respectively, which are mutually independent.

In addition, the supply system 8 supplies the combustion chamber 5 with a flow of inert fluid through an inert supply line 12. An inert fluid means a fluid in a liquid or gaseous state that does not take part in the combustion reactions. In particular, an inert fluid is substantially without either combustible components, or oxidizing (combustion) agents. For example, the inert fluid supplied to the combustion chamber 5 could be nitrogen or demineralize water.

As schematically shown in Figure 2, the combustion chamber 5 is provided with a plurality of burner assemblies 15 (for example 24 burner assemblies 15), which use the fuels supplied by the supply system 8.

The supply system 8 comprises regulating members controlled by respective regulators to control the flow rates of primary fuel, secondary fuel and inert fluid to the burner assemblies 15. Each burner assembly 15 comprises a main burner 17, a partial premix pilot burner 18 and a lance injector 20, the structure of which shall be described in greater detail below. Here and in the following, a lance injector means an injector generally having an elongated shape, with a predominant axial longitudinal dimension and defined by one or more coaxial tubular bodies. In particular, it is meant to indicate a type of injector designed to be inserted along the main axis of a burner or burner assembly. Furthermore, a lance injector is meant to be supplied indifferently with a gas fuel (such as natural gas, syngas) or a liquid fuel (such as diesel fuel, HFO or Bunker C) . A lance injector can also receive a flow of a liquid or gaseous inert fluid, which does not take part in the combustion.

In one embodiment, the regulating members of the supply system 8 comprise: a first main regulating valve 21, arranged between the first fuel supply line 10 and the main burners 17 and controlled by a first main regulator 22; a second main regulating valve 32, arranged between the second fuel supply line 10 and the main burners 17 and controlled by a second main regulator 34; a partial premix regulating valve 23, arranged between the first fuel supply line 10 and the pilot burners 18 and controlled by un partial premix regulator 24; a first lance regulating valve 26, arranged between the first fuel supply line 10 and a lance supply line 25, which is fluidically coupled to the lance injectors 20; a second lance regulating valve 27, arranged between the inert supply line 12 and the lance supply line 25; and a secondary regulating valve 28, arranged between the second fuel supply line 11 and the lance injectors 20. The first lance regulating valve 26 and the second lance regulating valve 27 are jointly controlled by a first lance regulator 30. The definition "first lance regulator" generically means the assembly of physical components (hardware) and possible program code portions (software) that enable performing the functions of regulating the flow-rate of primary fuel and the flow-rate of inert fluid to the lance injectors 20 via the first lance regulating valve 25 and the second lance regulating valve 27, based on the set- points determined by the control device 2 and the measurement signals. For example, it is possible to include processing modules that are configured to determine fluid flow rates based on the assigned load conditions and the degrees of opening of the first lance regulating valve 25 and the second lance regulating valve 27 based on the determined flow rates; and actuator members that can be operated to achieve the desired degrees of opening of the first lance regulating valve 25 and the second lance regulating valve 27. Furthermore, it is understood that the definition also includes an embodiment where separate regulator devices operating in a coordinated manner are provided for the first lance regulating valve 25 and the second lance regulating valve 27.

The secondary regulating valve 28 is controlled by a second lance regulator 31.

The supply system 8 may also comprise further elements not shown, in particular manifolds for uniformly distributing the fluid flows leaving the regulating members to the burner assemblies 15.

One of the burner assemblies 15 is shown in detail in Figure 3. It is understood that the other burner assemblies 15 have the same structure.

The burner assembly 15 extends along an axis A, around which the main burner 17 is peripherally arranged and with the pilot burner 18 arranged centrally. The lance injector 20 is inserted inside the pilot burner 18, always along axis A.

The main burner 17 is of the premix type, is arranged around the pilot burner 18 and is equipped with a swirler, indicated with reference numeral 33.

The swirler 33 is radially defined between a body 35 and a substantially truncated cone shaped wall 36. The body 35 has a cylindrical axial cavity and, outwardly, also has a truncated cone shape. The swirler 33 also comprises a plurality of blades 37, defining between them respective flow channels 38 to convey a flow of combustion air, with a diagonal flow path with respect to axis A, towards the combustion chamber 3.

The main burner 17 receives the flow of primary fuel supplied by the first main regulating valve 21 through a first premix conduit 39, which extends around axis A and terminates in an annular manifold, made inside the body 35 and communicating with the flow channels 38 through passageways made in the blades 37.

A second premix conduit 40 receives the flow of fuel supplied by the second main regulating valve 32 and injects it downstream of the blades 37.

The swirler 33 provides vigorous mixing of air that forms in the flow channels 38 and the flow of fuel, with an amount of air exceeding the theoretical stoichiometric ratio to generate a lean premixed flame.

The pilot burner 18 comprises a first truncated cone shaped body 41 and a second truncated cone shaped body 42, between which an axial type of swirler 43 is defined (Figures 1 and 3) . The first truncated cone shaped body 41 extends around axis A and has an axial through cavity, in which the lance injector 20 is housed. The second truncated cone shaped body 42, which is hollow, is arranged around the first truncated cone shaped body 41, is coaxial to the latter and is connected to outer truncated cone shaped surface of the body 35.

The swirler 43 comprises a plurality of blades 45, which are arranged in the space between the first truncated cone shaped body 41 and the second truncated cone shaped body 42. Flow channels 44 are defined between adjacent pairs of blades 45. In one embodiment, the blades 45 extend beyond the front edge of the first truncated cone shaped body 41, up to the front edge of the second truncated cone shaped body 42. Thus, a radially internal edge of the blades 45 is free for a distance, while the radially outer edge ends close to the front edge of the second truncated cone shaped body 42.

The pilot burner 18 is provided with its separate nozzle systems and receives the flow of primary fuel coming from the partial premix regulating valve 23 through the partial premix conduits 46, 47 concentric about axis A. Alternatively, the pilot burner 18 is supplied only by partial premix conduit 46.

Referring to Figures 4 and 5, the lance injector 20 comprises an outer tubular body 50 that extends along axis A. An inner tubular body 51 is housed in an axial through cavity of the tubular body 50 and defines a secondary delivery line 52 and a secondary return line 53. More precisely, the secondary delivery line 52 is defined by the inner tubular body 51 and is coupled to a respective branch of the second fuel supply line 11 through a connector 54. The secondary return line 53 is defined by an annular air space between the outer tubular body 50 and the inner tubular body 51. A terminal element 55, placed at an injection tip of the lance injector 20, forms a chamber that connects the secondary delivery line 52 and the secondary return line 53. The terminal element 55 also has one or more central nozzles 56 shaped to produce a diffusion flame in the combustion chamber 2.

The outer tubular body 50 houses an internal supply line 58 that is arranged concentrically around the secondary delivery line 52 and the secondary return line 53. The internal supply line 58 is fluidically coupled with the lance supply line 12 through a connector 59 to receive primary fuel, inert fluid or a mixture of primary fuel and inert fluid, according to the settings of the first lance regulator 30 determined according to the operating conditions of the gas turbine unit 1.

The internal supply line 58 can be defined by an annular air space in the outer tubular body 50 or by a plurality of separate conduits. In both cases, a plurality of peripheral nozzles 60 arranged in a ring around axis A and the central nozzles 56 enables injection into the combustion chamber 2 of the fluid flow received through the internal supply line 58. In particular, the peripheral nozzles 60 are shaped so as to generate a diffusion flame in the combustion chamber 2 when the fluid flow determined by the first lance regulator 30 contains a fuel fraction (in practice, when the first lance regulating valve 26 is not completely closed) .

As mentioned, the composition of the fluid supplied to the combustion chamber 2 through the internal supply line 58 is controlled by the first lance regulator 30 according to the operating conditions. In particular, the injection of a controlled flow rate of inert fluid enables controlling combustion conditions, in particular the temperature, in the central region of the burner, where the flame is of the diffused type. This allows achieving benefits in pollutant emission levels of the pilot flame and preserving the combustion stabilization effect.

In one operating mode, the diffusion flame is generated via the injection of diesel fuel through the central nozzles 56. In this case, the first lance regulator 30 can supply a controlled flow rate of just inert fluid, which locally cools the diffusion flame region, especially at the base of the flame. The peripheral nozzles 60 are thus used exclusively to control flame temperature and the reduction of pollutant emission levels, while the stabilization function is performed by the central nozzles 56.

In a different operating mode, the injection of diesel fuel could be suspended and the peripheral nozzles 60 could be used to generate a diffusion flame. Moreover, the first lance regulator 30 determines the proportioning between the flow rate of primary fuel and the flow rate of inert fluid according to required load conditions of the gas turbine unit 1. The first lance regulator 30 balances the stabilization effect according to the power requested, by acting on the first lance regulating valve 26 to regulate the flow rate of primary fuel; and the flame temperature control effect, by acting on the second lance regulating valve 27 to regulate the flow rate of inert fluid. In some operating conditions, the lance regulator 30 can interrupt the supply of inert fluid and provide just the primary fuel. In this case, the flame generated by the lance injector 20 with the primary fuel is purely diffusive, but of extremely small size, given the position of the peripheral nozzles 60 in the immediate vicinity of axis A of the burner assembly 15.

Independently of the supply of secondary fuel, primary fuel and inert fluid, when both flows are present, they mix together in the portion of the fluidic circuit between the lance regulating valves 26, 27 and the peripheral nozzles 60 (lance supply line 25 and internal supply line 58, respectively upstream of and inside the lance injector 20) .

In general, the mixture of fuel and inert fluid injected close to axis A of the burner assembly 15 in controlled proportions enables reducing pollutant emissions from the diffusion flame to the minimum allowed by the stability limit of the burner assembly 15, which has, at the same time, been improved.

Furthermore, the possibility of using an internal line inside the lance injector 20 in different modes enables making the entire burner assembly 15 more efficient and flexible without penalizing bulk.

According to a different embodiment of the invention, to which Figures 6 and 7 refer, the burner assemblies 15 comprise lance injectors 120 set up to inject primary fuel and inert fluid (or a mixture of the two) and does not have lines and nozzles for the injection of a secondary fuel. In this case, in particular, each lance injector 120 comprises a tubular body 150, which extends along an axis A' and has an empty axial through cavity 151. The tubular body 151 houses an internal supply line 158 that is fluidically coupled with the lance supply line 12 through a connector 159 to receive primary fuel, inert fluid or a mixture of primary fuel and inert fluid, according to the settings of the first lance regulator 30 determined according to the operating conditions of the gas turbine unit 1. If necessary, the axial cavity 151 may be used to allow the passage of cooling air, in particular for the purpose of avoiding overheating of the tubular body 151 close to the flame region.

Peripheral nozzles 160 arranged in a ring around axis A' enable injection into the combustion chamber 2 of the fluid flow received through the internal supply line 158. In particular, the peripheral nozzles 160 are shaped so as to generate a diffusion flame in the combustion chamber 2 when the fluid flow determined by the first lance regulator 30 contains a fuel fraction. With respect to the scheme shown in Figure 2, with the lance injectors 120, the second fuel supply line 11, the secondary regulating valve 28 and the second lance regulator 31 are superfluous and can be omitted.

Finally, it is clear that modifications and variants can be made with respect to the described gas turbine unit and method without departing from the scope of the present invention, as defined in the appended claims.