Combustion system with low polluting emissions

Combustion system (10) for a gas turbine equipped with a premixing chamber (12) for air which is mixed with the fuel injected from a series of holes (11) creating a main central flame which is formed in a flame tube (14), the premixing chamber (12) is convergent towards a connection end with a combustion chamber comprising the flame tube (14), the combustion system (10) comprises a series of pilot devices (20) with premixing of the fuel gas, which create a series of corresponding pilot flames suitable for stabilizing the main central flame itself, at the same time reducing the polluting emissions.

TECHNICAL FIELD

The present invention relates to a combustion system with low polluting emissions, in particular a combustion system for a gas turbine.

BACKGROUND

It is known that gas turbines are machines consisting of a compressor and a turbine with one or more phases, which are connected to each other by at least one combustion chamber.

Air from the outside environment is fed to the compressor to bring it under pressure.

The pressurized air passes through a premixing chamber, terminating with a nozzle or convergent portion. In said chamber, at least one injector feeds fuel which is mixed with the air to form an air-fuel mixture to be burnt.

The fuel necessary for producing combustion is therefore introduced into the combustion chamber by means of a pressurized network. The purpose of said combustion is to cause an increase in the temperature and enthalpy of the gas.

In order to improve the flame stability characteristics, a parallel fuel feeding system, suitable for generating a pilot flame, is also generally envisaged.

U.S. Pat. No. 5,381,652 indicates a combustion system for gaseous fuels in which a main flame is stabilized by means of a pilot device.

In this case, the pilot device injects pilot fuel gas into the combustion chamber, with which the combustion system is equipped.

A duct is present in the combustion system, for premixed air which flows to a first central duct in which there is a first series of radial holes for the fuel gas which feeds the main flame.

In the main duct, the air is premixed with the gaseous fuel fed by injection through a second series of holes, which are situated on cylinders positioned on a section of the main air duct, at the same distance along the perimeter of the section itself.

In this case, the injection of the pilot fuel is effected by means of a first series of radial holes situated at the outlet of the premixing duct, surrounding the main flame.

As this pilot fuel has not been previously mixed with comburent air, it burns in flames mainly controlled by the diffusion process of the fuel in the comburent air, which are consequently called diffusion flames.

Diffusion flames are characterized by the presence of areas in which the temperature is extremely high, therefore causing high polluting emissions.

The emission of nitrogen oxides is, in fact, proportional to the maximum flame temperature and flames of the diffusion type having high temperatures are consequently most responsible for polluting emissions, in particular emission of nitrogen oxides.

One of the disadvantages of the present system is that it has moderate polluting emissions due to the presence of diffusion-type flames.

U.S. Pat. No. 5,666,044 describes the use of pilot devices for stabilizing the main flame, which inject fuel gas through nozzles into the combustion chamber.

Air, necessary for the combustion of the fuel gas injected by means of the pilot device is contemporaneously injected into the combustion chamber, in an area adjacent to each pilot device. Also in this case, the mixing takes place in the combustion chamber, and the flames are consequently of the diffusive type, even if the presence of the high quantity of air in the area adjacent to the injection, reduces the extension of high temperature areas.

A combustion system is also known from the patent application EP 1321715A2, which can be used with liquid or gaseous fuel, in which there are devices which inject the pilot fuel directly into the combustion chamber.

One of the disadvantages of this solution is that it generates significant emissions of polluting agents, also in this case due to the presence of flames of the diffusion type used for stabilizing the main premixed flame.

SUMMARY

An objective of the present invention is to provide a combustion system with low polluting emissions which allows a reduction in the polluting emissions of gas turbines.

Another objective is to provide a combustion system with low polluting emissions which allows reduced oscillations of the pressure inside the combustion chamber and at the same time allows the flame to be stabilized.

A further objective is to avail of a combustion system with low polluting emissions which allows a high duration of the components subject to high temperatures.

Yet another objective is to avail of a combustion system with low polluting emissions which allows a high combustion efficiency and at the same time has low polluting emissions.

These objectives according to the present invention are achieved by providing a combustion system with low polluting emissions as specified in claim1.

Further characteristics of the invention are described in the subsequent claims.

DETAILED DESCRIPTION OF THE EMBODIMENTS

With reference to the figures, these illustrate a combustion system with low polluting emissions, indicated as a whole with10, which comprises a combustion chamber containing a “liner” or flame tube14suitable for containing a main flame.

The flame tube14has a tapered connection end15to a premixing chamber12of the air, in correspondence with a terminal section13of the premixing chamber12.

The air premixing chamber12is substantially a duct in the form of a circular crown which has a tapered section, or narrower section, towards a connecting end to the combustion chamber, to accelerate the air after this has been premixed using fuel gas fed by a series of holes11preferably situated on cylindrical elements positioned in the perimeter of the premixing chamber12.

Said series of holes11is situated along a section which precedes the tapered end of the air premixing chamber12.

The tapered end of the air premixing chamber12has a circular section in which there is a central body17which extends into said tapered end for the injection of air suitable for preventing the flame from touching the surface of the central duct17.

The combustion system10preferably comprises a series of thermocouples19, outside the central duct17.

The flame is normally formed in the combustion chamber downstream of the premixing chamber12.

If, as a result of some abnormal functioning, the flame rises into the premixing chamber12, the thermocouples19detect the increase in temperature and the flow of gas is interrupted to protect the premixing chamber12from excessive temperatures.

The tapered end15of the flame tube14has a surface in the form of a circular crown on which there are a series of pass-through holes18for housing a respective series of premixed fuel gas injection pilot devices20in order to stabilize the main central flame and at the same time maintain low levels of polluting emissions.

This is achieved by using a series of fuel gas premixing pilot devices20, in which the fuel gas is mixed with a turbulent stream of air, obtaining a homogeneous mixture of fuel gas/air, before the injection of the mixture itself into the combustion chamber.

This allows a reduction in the polluting emissions of NOx as diffusion-type flames are avoided.

The combustion system10has a series of pilot devices20with premixing of the fuel gas which allow the main central flame to be stabilized, at the same time reducing the polluting emissions through a corresponding series of external flames.

Each pilot device of the series of pilot devices20comprises a premixing duct29, a series of holes28, inside the premixing duct29, for the feeding of the fuel and a unit24acting as a swirler; said unit24has the function of creating a turbulent and whirling flow of air capable of uniformly mixing with the fuel gas fed through the series of holes28inside the device itself.

The turbulent flow of air is mixed with the fuel gas fed through the series of ducts28, forming a homogeneous mixture of air and fuel gas inside the premixing duct29, before the introduction of the combustion system10into the combustion chamber.

In this way, for each pilot device20a corresponding flame is obtained, having a maximum temperature lower than that of diffusion flames.

Consequently, as the maximum flame temperature is lower, the polluting emissions, among which NOx emissions, are correspondingly reduced.

The combustion system10comprises a series of pilot devices20each arranged in a respective hole of the series of ducts18which are distributed at an equal distance on a circumference lying on the surface of the end15of the flame tube14and coaxial with its axis.

The unit24of each pilot device20comprises at least one shaped element capable of increasing the turbulence of the flow of air in the premixing duct29.

The air flow for each premixing pilot device20is preferably fed directly from the air flow coming from a compressor.

Furthermore, the flame of each pilot device20is stabilized.

The stabilization of the flame of premixed pilot burners20is effected by the injection of pilot fuel gas (therefore a sub-pilot) which stabilizes the premixed pilot flame which in turn stabilizes the main premixed flame.

The sub-pilot fuel gas produces diffusion flames but as only the premixed pilot fuel gas has to be stabilized, only a small quantity of fuel is used, thus producing lower polluting emissions of NOx.

According to an aspect of the present invention, a first preferred embodiment is provided, shown inFIG. 4, of a pilot device20equipped with a unit24in which said at least one shaped element comprises a series of shaped blades27suitably inclined to create a swirling motion of the air inside the premixing duct29, ensuring that the turbulent air flow is uniformly mixed with the fuel gas fed through the series of ducts28inside the premixing duct29itself.

The pilot device20comprises a central element42inside the premixing duct29which internally comprises a central duct43in turn inside and coaxial to an annular duct34.

The annular duct34has the function of feeding the fuel gas which, through the series of holes28, is premixed with the air inside the premixing duct29.

In this case, the flame relating to the pilot device20is centrally stabilized by the injection of combustible fluid through the duct43.

Each pilot device of the series of fuel gas premixing pilot devices20, also comprises two ducts32inside the premixing duct29to detect the flow-rate of the fluid inside the pilot device itself.

According to another aspect of the present invention, a second embodiment is provided, shown inFIG. 5, of a pilot device20equipped with a unit24in which said at least one shaped element, as in the first embodiment, also comprises a series of shaped blades27having the same function.

This embodiment is analogous to the first, except for the stabilization of the flame.

In this case, in fact, the flame relating to the pilot device20is centrally stabilized by the injection of combustible fluid by means of a central mini-burner45inside the premixing chamber29.

Said mini-burner45is connected downstream of a central element inside the premixing duct29, also present in the second embodiment, which internally comprises a central duct43, in turn inside and coaxial to an annular duct34.

The annular duct34has the function of feeding the fuel gas which, through the series of holes28, is premixed with the air inside the premixing duct29.

The duct43feeds the sub-pilot fuel gas for the mini-burner45, whereas the air flow for the mini-burner45is fed by a series of holes35situated therein and is made turbulent by means of a further swirler present inside the mini-burner45itself.

According to another aspect of the present invention, a third preferred embodiment of a pilot device20is provided, shown inFIG. 3, in which a unit24is present, in which said at least one shaped element comprises two series of shaped blades25to create a turbulent flow of air in the premixing duct29.

Each premixing pilot device20comprises a duct36for the fuel gas connected to a series of holes28inside the premixing duct29.

The fuel gas of the duct36is injected into the premixing duct29, through the series of holes28, for premixing with the turbulent air flow.

The flame of each pilot device20in this embodiment is also stabilized by the injection, into the combustion chamber, of fuel gas of a duct37fed through a series of holes30outside the premixing duct29, situated on a base surface31of an end of the pilot device20.

The duct37is therefore connected to the series of outer holes30to fed fuel gas in order to stabilize the flame of the pilot device20itself.

Each pilot device of the series of fuel gas premixing pilot devices20, also comprises at least one thermocouple33and two ducts32inside the premixing duct29to detect the flow-rate of the fluid inside the pilot device20.

It can be observed that a further fuel feeding duct70, which surrounds the central duct17, is also present in the combustion system10.

Furthermore, said duct70is close to the tapered end15of the flame tube14where this is connected to the premixing chamber12.

The duct70serves to feed the diffusion pilot fuel which surrounds the main flame, which can also be used for ignition or for stabilizing the flame under transitory conditions, when the emissions are not significant.

With reference toFIG. 2, this also shows the air flows, indicated with80and82, inside the combustion system10, respectively regarding the premixed air flow of the premixing chamber12and the premixed air flow relating to one of the pilot devices20indicated in the figure.

The air coming from the compressor is mixed inside the premixing chamber12with the fuel fed from the series of holes11.

The premixed air is subsequently accelerated and optionally mixed by diffusion with fuel gas fed from the duct70.

The main flame is stabilized by means of a series of pilot flames functioning with premixed fuel gas.

With reference toFIG. 6, it is possible to observe, in particular, the fuel flow, indicated with83, fed from the holes of the series of holes28, adjacent to the unit24.

Furthermore, the pilot devices of the series of fuel gas premixing pilot devices20are preferably situated in the tapered end15of the flame tube14, but a different arrangement can also be adopted.

The combustion system10comprises preferably a duct60for feeding air to the series of premixing pilot devices20.

In conclusion, it can be summarized that a combustion system10for a gas turbine comprises a series of pilot devices20with premixing of the fuel gas, situated outside the main flame, or in a tapered connection end15to the premixing chamber12, which create a series of corresponding pilot flames suitable for stabilizing the main central flame itself, at the same time reducing polluting emissions.

It can thus be seen that the combustion system with low polluting emissions according to the present invention achieves the objectives specified above.

Numerous modifications and variants can be applied to the combustion system with low polluting emissions of the present invention thus conceived, all included within the same inventive concept.

Furthermore, in practice, the materials used, as also the dimensions and components, can vary according to technical demands.