Patent Application
20120015311
The present invention relates to a gas combustor burner particularly arrangement of inlets in the burner which supplies combustible gas to the combustor and its operation for fuel staging.
Gas turbines are used to convert heat energy to mechanical energy, for example in power plants. Gas turbines have combustion chambers in which a fuel is burned with air. The combustion chambers of gas turbine plants are supplied with liquid and/or gaseous fuel using burner consisting of one or more nozzle or inlets. The burner can also be used to carry air required for the combustion. For the optimal operation of the gas compressors, stable flames should be formed in the combustion chamber during combustion. Common techniques for flame stabilization include the formation of small eddies or recirculation zones in the combustion chamber. The temperature in the recirculation zone needs to be above a threshold especially at lower load conditions to sustain combustion which will result in stable operation.
A flame will be inherently unstable if the energy release from the combustion is insufficient to raise the temperature to a level at which combustion is self sustaining under conditions of heat loss including radiation to and from the flame and under hot gas recirculation. The ability to keep a sustainable temperature to maintain combustion at different loads especially at lower loads and simultaneously achieve emission targets is a great challenge in gas turbine operations.
In view of the foregoing, an embodiment herein includes a burner for a gas combustor. The burner comprises a front surface area divided into a plurality of subareas. Inlets are arranged on the front surface area such that each subarea is encircled by at least four inlets such that during operation of the burner, a gas recirculation in the combustor is facilitated corresponding to each subarea.
In view of the foregoing, another embodiment herein includes a method of operating a burner with a plurality of inlets on the front surface area of the burner to provide combustible gas to a combustor. The method comprises dividing the front surface area into a plurality of subareas, and for at least one subarea selecting at least four inlets which are encircling the respective subarea. The method further comprise providing combustible gas only through the selected inlets such that a gas recirculation in the combustor is facilitated corresponding to each subarea.
The underlying idea here is to provide gas recirculation inside a combustor by providing and operating inlets in at least one subarea which is smaller than that of front surface area of the burner. By operating the inlets encircling the subarea a gas recirculation or gas recirculations, if more than one subarea are operated, is formed which can maintain the sustainable temperature for the combustion. The number of subareas operated is based on the load of the gas turbine which can also be directly mapped to the combustor load. Operating the inlets in a subarea or subareas also enables staging the supply of combustible gas to the combustor for combustion. The combustion resulting in the creating of gas recirculation which is hot thereby resulting in maintaining the required temperature throughout the entire load range. By operating a single smaller subarea at lower loads, the combustion associated with that subarea can be sustained. This combustion creates hot recirculation which also provides stability to flames in the combustor provided for the combustion. As the load increases other subareas can be made operational to supply the combustible gas to the combustor. Practically, at least four inlets are required to realize a gas recirculation in a subarea. The staging of the combustible gas referred in the invention should also be interpreted as the staging of the fuel since the inlets are generally supplied continuous by air in industrial operations.
According to a preferred embodiment, the burner further comprises a pilot inlet in at least one of the subareas. This pilot inlet helps in supplying flames to provide adequate temperature to start the combustion process.
In an alternative embodiment, the inlets encircling the subarea are spaced equally. This enables to create a stable recirculation using the combustible gases injected into the combustor by the inlets in the subarea.
In another alternative embodiment, the front surface area comprises of two or three or more subareas. Having plurality of subareas enables more flexibility or control of the supply of the fuels to the combustor for combustion. That means the supply of the fuels to the combustor can be staged. The more the number of subareas, more the number of staging that can be realized. During staging, the amount of combustible gas supplied through the inlets in one or more subareas are regulated or controlled based on the load of gas turbine. Also simultaneous operation of multiple subareas will result in plurality of gas recirculation, which further provides more tuning flexibility regarding thermo acoustic oscillations in the gas combustor. Practically two or three gas recirculations are optimal even though more gas recirculations can be realized for the operation.
In another alternative embodiment, the shape of said burner is circular or elliptical. The shape of the burner enables to arrange the inlets in multitude of possibilities to get a gas recirculation.
In another alternative embodiment, the subareas formed by the arrangement of the inlets on the front surface area have symmetrical configuration. Symmetrical configuration of the subareas and corresponding inlets arranged will enable to provide more stability to the combustor, if operated simultaneously.
In another alternative embodiment, the subareas formed by the arrangement of the inlets on the front surface area has asymmetrical configuration. This enables the burner to have subareas having different area configurations and also possibly with different number of inlets encircling them which further helps in staging the combustible gas by selecting the required subarea based on the combustor load. For example if the load is very low, the smallest subarea can be selected for the operation.
In another alternative embodiment adjacent subareas on the front surface area of the burner are adapted to use at least one inlet in common. This helps in effective utilization of the front surface area of the burner to generate effective gas recirculation.
In another alternative embodiment, the inlets arranged on the front surface area, to supply the combustible gas to the combustor, are of at least two different diameters. This will help in the operation of the required subarea which further regulates the flow of combustible gas based on the load requirement. For example, at lower loads the inlets with smaller diameter can be operated and at higher loads, when more combustible gas is required the inlets with larger diameter could be used.
In another alternative embodiment, said burner operates on pre-mixed jet flames. Combustion systems based on pre-mixed jet flames offer special advantages over for example, swirl stabilized systems from the thermo-acoustic point of view, owing to the distributed heat release zones and the absence of swirl induced vortices. By appropriately selecting the jet impulse, small scale eddy structures can be created which dissipate the acoustically induced fluctuations of heat release, thereby suppressing the pressure pulsations which are typical for swirl stabilized flames.
The present invention is further described hereinafter with reference to illustrated embodiments shown in the accompanying drawings, in which:
It is known that undesired thermoacoustic oscillations frequently occur in combustors of gas turbines. The term “thermoacoustic oscillations” designates mutually self reinforcing thermal and acoustic disruptions. In the process, high oscillation amplitudes can occur, which can lead to undesired effects, such as to high mechanical loading of the combustor and increased NOx emissions as a result of inhomogeneous combustion. In order to ensure a high output in relation to pulsations and emissions over a wide operating range, further tuning of the fuel distribution and active or passive control of the combustion oscillations may be necessary.
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Also it is possible to merge the subareas during operation. At high load operation all the inlets of the burner are used to supply combustible gas to the combustor. The operation of all inlets in the burner results in two hot recirculations; one formed by the inlets of the subarea 102 and another formed by the inlets of the subarea 104.
Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternate embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that such modifications can be made without departing from the embodiments of the present invention as defined.
