Combustors are commonly used in industrial and power generation operations to ignite fuel to produce combustion gases having a high temperature and pressure. For example, gas turbines typically include one or more combustors to generate power or thrust. A typical gas turbine used to generate electrical power includes an axial compressor at the front, one or more combustors around the middle, and a turbine at the rear. Ambient air may be supplied to the compressor, and rotating blades and stationary vanes in the compressor progressively impart kinetic energy to the working fluid (air) to produce a compressed working fluid at a highly energized state. The compressed working fluid exits the compressor and flows through one or more fuel nozzles into a combustion chamber in each combustor where the compressed working fluid mixes with fuel and ignites to generate combustion gases having a high temperature and pressure. The combustion gases expand in the turbine to produce work. For example, expansion of the combustion gases in the turbine may rotate a shaft connected to a generator to produce electricity.
Various parameters influence the design and operation of combustors. For example, higher combustion gas temperatures generally improve the thermodynamic efficiency of the combustor. However, higher combustion gas temperatures also promote flame holding conditions in which the combustion flame migrates towards the fuel being supplied by the fuel nozzles, possibly causing damage to the fuel nozzles in a relatively short amount of time. In addition, higher combustion gas temperatures generally increase the disassociation rate of diatomic nitrogen, increasing the production of nitrogen oxides (NOX). Conversely, a lower combustion gas temperature associated with reduced fuel flow and/or part load operation (turndown) generally reduces the chemical reaction rates of the combustion gases, increasing the production of carbon monoxide and unburned hydrocarbons.
In a particular combustor design, one or more injectors, also known as late lean injectors, may be circumferentially arranged around the combustion chamber downstream from the fuel nozzles. A portion of the compressed working fluid exiting the compressor may be diverted through the injectors to mix with fuel to produce a lean fuel-air mixture. The lean fuel-air mixture may then be injected into the combustion chamber for additional combustion to raise the combustion gas temperature and increase the thermodynamic efficiency of the combustor.
The late lean injectors are effective at increasing combustion gas temperatures without producing a corresponding increase in the production of NOX. However, the diverted compressed working fluid that flows through the injectors necessarily reduces the amount and velocity of compressed working fluid available to flow through the fuel nozzles. Reduced flow and/or velocity of compressed working fluid through the fuel nozzles create conditions more conducive to flame holding conditions in the fuel nozzles. In addition, the reduced amount and velocity of compressed working fluid flowing through the fuel nozzles may impact the ability to operate the combustor using liquid fuel without implementing additional NOX abatement measures, such as richer fuel-air ratios and/or emulsifying the liquid fuel. Therefore, an improved system and method that can vary the amount of working fluid diverted through the injectors would be useful.