Method for operating a combustion device including injecting a fluid together with diluent fuel to address combustion pulsations

A method for operating a combustion device includes supplying a fuel and an oxidizer into the combustion device and burning them. According to the method, during at least a part of a transient operation, an additional fluid is supplied together with the fuel, and its amount is regulated to counteract combustion pulsations.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit under 35 USC §119 to European Patent Application No. 11179344.4 filed Aug. 30, 2011, the entire contents of which are incorporated by reference herein as if fully set forth.

FIELD OF THE INVENTION

The present invention relates to a method for operating a combustion device. In particular, the method according to the invention allows operation of a combustion device with reduced pulsations. Preferably the combustion device is a part of a gas turbine.

BACKGROUND

In the following particular reference to combustion devices that are part of a gas turbine is made; it is anyhow clear that the method can also be implemented in combustion devices for different applications. Thus, before the combustion device a compressor and after the combustion device a turbine are typically provided.

Combustion devices are known to include a body with a fuel supply for either a liquid fuel (for example oil) or a gaseous fuel (for example natural gas) and an oxidizer supply (usually air).

During operation, the fuel and the oxidizer react within the combustion device and generate high pressure and temperature flue gases that are expanded in a turbine.

During transient operation, such as for example when the gas turbine is started up, switched off, during fuel switch over or also during other transient operations, problems can occur.

In fact, during transient operations pressure waves can generate within the combustion device.

FIG. 1shows an example of a possible circumferential pressure wave (it can be a static or a rotating pressure wave).FIG. 1shows the pressure P as a function of the angular position φ over the combustion device at a period in time t=t0(solid line) and t=t1(dashed line). From this figure it is apparent that an injector located at a position φ1:

at the period in time t=t0faces an environment at a low pressure P1; this promotes fuel supply through the injector; and

at the period in time t=t1faces an environment at a high pressure P2; this hinders fuel supply through the injector.

Likewise,FIG. 2shows an example of a possible axial pressure wave.FIG. 2shows the pressure P as a function of the axial position x (L indicates the combustion device length) at a period in time t=t0(solid line) and t=t1(dashed line).

Also in this case, an injector will face a combustion device having a pressure that fluctuates with time; as explained above, this fluctuating pressure adversely influences fuel injection.

FIG. 3shows the effect of the fluctuating pressure within the combustion device on the fuel injection. In particularFIG. 3shows an example in which the fuel mass flow is reduced; this could be an example of a switch off, nevertheless the same conditions are also present at the beginning of a start up or at the beginning and end of a switch over and in general each time the fuel mass flow supplied decreases and falls below a given mass flow.

FIG. 3shows the fuel mass flow M injected through an injector as a function of time t. FromFIG. 3at least the following phases can be recognized:

before t=t3: steady operation with substantially constant fuel mass flow through the injector (curve1),

between t=t3and t=t4(the fuel mass flow stays above a critical fuel mass flow Mc): the amount of fuel injected decreases, but the fluctuating pressure within the combustion device does not noticeably affect fuel injection (curve2),

after t=t4(i.e. when the fuel mass flow falls below the critical fuel mass flow Mc): in these conditions, since the amount of fuel is low, the fluctuating pressure within the combustion device alternatively promotes and hinders fuel injection, causing a fluctuating fuel injection. In particular inFIG. 3, curve2shows a theoretical run of the reducing fuel mass flow and curve3an example of a possible real run of the reducing fuel mass flow.

Fluctuating fuel supply into the combustion device generates large combustion pulsations.

Combustion pulsations, largely mechanically and thermally, stress the combustion device and the turbine downstream of it, therefore they must be counteracted.

SUMMARY

An aspect of the present invention thus includes providing a method by which combustion pulsations generated during transient operation are counteracted.

This and further aspects are attained by providing a method in accordance with the accompanying claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method can be implemented with any kind of combustion device, for example adapted to generate a premixed flame, a diffusion flame, a mixed flame, etc.

For example the combustion device can be a premixed combustion device5(FIG. 4), with conical swirl chamber6and combustion chamber7extending downstream of the swirl chamber6; a front plate8is provided between them. This combustion device further includes fuel supply (for example a lance9that typically injects a liquid fuel) and tangential slits10at the swirl chamber6for oxidizer supply (typically air). Additional fuel supply includes injectors11(FIG. 5) provided on lines12that are connected to the wall of the swirl chamber6, at positions close to the slits10, for fuel injection (typically gaseous fuel). This kind of combustion device5is well known and is schematically shown inFIGS. 4, 5 and 9.

A different kind of premixed combustion devices15is for example schematically shown inFIG. 6. This combustion device15includes a body16(for example a tubular body with square or trapezoidal cross section) with an inlet17and outlet. Within the body16, vortex generators19(for example tetrahedral vortex generators but also different shapes and concepts are possible) and fuel supply including a lance20with fuel injectors21are housed. Downstream of the body16, a combustion chamber22is provided.

FIGS. 7 and 8show further examples of combustion devices that are arranged to generate a diffusion flame.

These combustion devices25have a body26with fuel supply including fuel injectors27(liquid or gaseous fuel) and oxidizer supply including oxidizer injectors28.

In all the figures, reference30indicates the flame and reference G indicates the hot gases generated in the combustion device and directed toward the turbine.

In the following, particular reference to the embodiment ofFIG. 3is made; it is anyhow clear that the same method can be implemented in all kind of combustion devices (i.e. those described or others).

The method for operating a combustion device5comprises supplying a fuel35and an oxidizer36into the combustion device5and burning them.

In addition, during at least a part of a transient operation such as for example a start up, a switch off or a switch over, an additional fluid37is supplied into the combustion device5together with the fuel35.

The additional fluid37is advantageously supplied through the same injectors as the fuel35and it is typically at least partly mixed with the fuel35.

The amount of the additional fluid37is thus regulated to counteract combustion pulsations.

With reference toFIG. 14, a first parameter FP indicative of the fuel feed is chosen and the additional fluid supply starts only when the first parameter reaches a critical value FPc. The critical value FPc can be chosen such that when the first parameter reaches or passes it, pulsations start to generate or to substantially generate. In this respectFIG. 14shows the first parameter FP and its critical value FPc; supply of the additional fuel starts only at t5, when the first parameter reaches its critical value FPc.

In different examples, the first parameter can be the fuel mass flow M or the differential pressure ΔP between a fuel supply and the inside of the combustion device5; in these cases additional fluid supply starts when the fuel amount supplied into the combustion device or the differential pressure falls below the critical value Mc or ΔPc.

In addition, a second parameter SP indicative of the fuel and additional fluid feed is also chosen; the regulation includes maintaining the second parameter above or below a given value (FIG. 15) or preferably maintaining the second parameter SP within a prefixed range R (FIG. 16).

The given value can be a critical value SPc of the second parameter SP. Also in this case, the critical value can be chosen such that when the second parameter reaches or passes it, pulsations start to generate or to substantially generate.

Preferably, the bottom or the top of the range corresponds to the critical value SPc of the second parameter.

The second parameter SP can be the fuel and additional fluid mass flow M or the differential pressure ΔP between a fuel and additional fluid supply and the inside of the combustion device5. In these cases the regulation includes maintaining the total mass flow of fuel35and additional fluid37or differential pressure AP above the critical value or maintaining them within the prefixed range R.

FIG. 17shows an example in which the first and the second parameter are the same physical entity (for example mass flow M or differential pressure AP as indicated above). In this case the first parameter and the second parameter can be measured through the same sensors. In particularFIG. 17shows that before t=t6(i.e. when the fuel mass flow M or differential pressure ΔP between the fuel supply and the inside of the combustion device) are above the critical value Mc or ΔPc the sensors measure the first parameter and only fuel is injected and when the first parameter (i.e. M or ΔP) reaches the critical value Mc or ΔPc also the additional fluid37starts to be fed and the sensors measure the second parameter SP; in this example the second parameter is kept at the critical value Mc or ΔPc but as already described it can be kept above or below it or within a range R.

To measure the differential pressure ΔP the control device shown inFIG. 9can be used.

FIG. 9shows a control device45connected to sensors46for measuring the pressure in a line supplying the fuel (or fuel and additional fluid) to the combustion device5and sensors47for measuring the pressure within the combustion device; the control device45elaborates the signals from the sensors46,47and provides a control signal (to a valve48or different component) to regulate the amount of the additional fluid37.

The fuel35is supplied into the combustion device5via a fuel supply (for example the lance9or the lines11but, in the other examples of combustion devices15,25, also lance20); the additional fluid37is preferably also supplied into the same fuel supply (i.e. into the lance9or the lines11or lance20).

Advantageously, the additional fluid37is at least partly mixed with the fuel35and in this respect a mixer49can be provided.

The additional fluid37is preferably an inert fluid; inert fluid is a fluid that does not react during burning, i.e. it is neither a fuel nor an oxidizer.

In addition, when the fuel is a liquid fuel, the inert fluid is preferably a liquid fluid (for example the fuel can be oil and the additional fluid water) and when the fuel is a gaseous fuel the additional fluid is preferably a gaseous fluid (for example the fuel can be natural gas or methane and the additional fluid nitrogen).

Advantageously, since when the amount of fuel becomes low the additional flow is injected with it, no fluctuating amounts of fuel are injected into the combustion device; this prevents or hinders thermal and mechanical pulsations.

In the following some embodiments of the invention are described in detail.

Switch Over From a Fuel Being Premix Gas to Premix Oil

InFIG. 10curve50shows the reducing amount of premix gas injected into the combustion device and curve51indicates the increasing amount of premix oil. In addition, curve52indicates the water that is supplied together with the premix oil51and curve53indicates the differential pressure as defined in the present disclosure. The amount of water is at its maximum at the beginning of its supply and then decreases. When the first parameter for the premix oil exceeds the critical amount (for example mass flow Mc or differential pressure ΔPc), the supply of water is stopped (curve52goes to zero). In this example, the additional fluid is only fed together with the premix oil (but not with the premix gas).

Switch Over From a Fuel Being Premix Gas to Premix Oil

This example is similar to the first example. In particular, in this second example two speeds for the fuel regulation are provided: a slow speed during water supply and a faster speed when no water supply is provided.

Switch Over From a fuel Being Premix Gas to Premix Oil

Also this example is similar to the first example and, in particular, water52and nitrogen54are supplied when a first parameter of both the gas premix and the oil premix50,51are below their critical value.

Switch Over From a Fuel Being Premix Gas to Premix Oil

Also this example is similar to the first example and, in particular, supply of water starts before premix oil supply.

Naturally, the features described may be independently provided from one another.

In practice the materials used and the dimensions can be chosen at will according to requirements and to the state of the art.

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