In industrial gas turbines recently used in thermal power plants or the like, there is a demand to ensure high power, high efficiency, and low NOx (nitrogen oxide). According to this demand, the following low NOx type combustor is employed as a combustor constituting the gas turbine.
For example, in Patent Document 1 below, a combustor having a combustor basket that is connected to an inlet of combustion gas of a turbine, burns fed fuel, and feeds generated combustion gas to the turbine, a pilot nozzle that is disposed in the center of the combustor basket, and injects the fuel into the combustor basket to form a diffusion flame, and a plurality of main premixing nozzles that are disposed around the pilot nozzle, inject a pre-mixture of compressed air and the fuel into the combustor basket, and ignite the pre-mixture to form a premixed flame by the diffusion flame is disclosed.
That is, in diffusion combustion caused by the pilot nozzle, since a local fuel-air ratio is relatively high in the combustion, combustion stability is good, but an amount of production of NOx is increased by high-temperature combustion. On the other hand, in premixing combustion caused by the main premixing nozzles, a uniform fuel-air ratio is accomplished, and thus the amount of production of NOx can be reduced by low-temperature combustion. In other words, in the combustor of Patent Document 1 below, a stable diffusion flame is formed, and the pre-mixture is ignited by this diffusion flame. Thus, the low NOx and relatively high combustion stability are compatible.
However, although the above-mentioned premixing combustion system is seemingly excellent in reduction of NOx, it easily causes combustion oscillation. The combustion oscillation is a phenomenon in which pressure fluctuation generated in the combustor by combustion and heat-generation fluctuation generated by temporal variation in fuel concentration caused by a nozzle part are synchronized at a combustion position, thereby oscillating. The combustion oscillation makes combustion unstable, and this makes it impossible to stably operate the combustor. As such, there is a need to prevent the combustion oscillation from being generated as much as possible.
As the combustor suppressing this combustion oscillation, there is a combustor as disclosed in Patent Document 2. In this combustor, main premixing nozzles injecting a pre-mixture are installed, for instance, in a plurality of sets, and then the plurality of sets of main premixing nozzles are alternately arranged in an axial direction. On the other hand, the pre-mixture injected from the main premixing nozzles is accompanied with the heat-generation fluctuation. The plurality of sets of main premixing nozzles are arranged in a positional relation in which the heat-generation fluctuations of fuel injected from the respective main premixing nozzles offset each other. Thereby, the heat-generation fluctuations are offset, and the combustion oscillation is reduced.