Patent Application
20090183492
The subject invention relates to gas turbines. More particularly, the subject invention relates to control of combustors of gas turbines.
A typical gas turbine has a plurality of combustors, and each combustor may include a quantity of cans, which in turn include a number of individual nozzles. Fuel/air mix may be routed to individual nozzles in unequal amounts, depending on the operating conditions of the combustor. The ratios of these amounts are vernacularly referred to as fuel splits. Fuel flow to the individual burner tubes is regulated in order to control combustion dynamics to achieve a desired load and/or combustion temperature, and to control emissions of, for example, NOx and CO2. To minimize emissions of NOx, it is often desired to operate the turbine with a lean fuel mixture (one where the fuel to air ratio is low), but as the fuel mixture in the combustor gets leaner and leaner to minimize NOx emissions, the risk of lean blow out (LBO) increases, especially at certain operating conditions of the gas turbine. LBO is a phenomenon where there is not enough fuel in the combustion chamber relative to the amount of air in the chamber, and the combustor fails to ignite the mixture. To prevent LBO, a combustor-level fuel to air ratio, which is adjusted for the fuel splits between burner tubes, is scheduled versus combustor severity parameter, which is a function of combustor load, pressure, temperature, and relative humidity. For a particular severity parameter value, a combustor-level fuel to air ratio is prescribed to prevent LBO. This method of preventing LBO produces conservative results when the combustor is at extremities of the operating envelope, particularly cold day and/or low load. Additionally, the current method presumes that all nozzles are in operation, which is not the case in some circumstances, for example startup of the combustor.
The present invention solves the aforementioned problems by providing a method and system for controlling a combustor of a gas turbine utilizing fuel nozzle equivalence ratio. The equivalence ratio of at least one fuel nozzle of the combustor, the combustor having at least one fuel nozzle disposed in at least one combustor can, is measured. The measured equivalence ratio is compared to a threshold value for lean blowout. The fuel flow from the at least one nozzle is modified thereby adjusting the equivalence ratio to prevent lean blowout.
These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings.
The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:
The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings.
Shown in
A manifold, schematically shown at 20, mixes fuel and air and regulates the flow of the fuel air mixture through the nozzles 12. The manifold 20 divides fuel/air mix flow into separate circuits such that differing volumes of fuel/air mix, and different fuel/air mixture ratios can be provided to each group of nozzles, PM1 nozzle 14, PM2 nozzles 16, and PM3 nozzles 18.
Equivalence ratio or phi (Φ) for the combustor is defined as a ratio of an actual fuel-to-air ratio (Wfuel/Wair) to a stoichiometric fuel-to-air ratio (Wsfuel/Wsair). In general, for given combustion conditions, for example, load, pressure, temperature, and relative humidity, the lower the value of Φ, the leaner the fuel-to-air ratio, and the greater likelihood of lean blowout (LBO). Since severity parameter is a function of load, pressure, temperature, and relative humidity, Φ can be plotted versus severity parameter as illustrated in
To protect against LBO in the operating conditions, such as startup, when not all of the groups of nozzles, PM1 nozzle 14, PM2 nozzles 16, and PM3 nozzles 18, are operating, LBO lines 22 are determined for specific groups of nozzles. In one embodiment, LBO prevention is provided by scheduling Φ of PM1 nozzle 14 (ΦPM1) and Φ of PM3 (ΦPM3) versus severity parameter. For the PM1 nozzle 14, ΦPM1 is the ratio of an actual PM1 fuel-to-air ratio (Wfuel/Wair)PM1 to a stoichiometric PM1 fuel-to-air ratio (Wsfuel/Wsair)PM1. A schematic PM1 LBO line 24 of a minimum ΦPM1 versus severity parameter is shown in
In operation, at a particular severity parameter corresponding to machine operating conditions, an equivalence ratio of a desired quantity of nozzles 12 is measured and compared to a threshold value. The threshold value corresponds to the value of Φ on, for example, line 24 for PM1, for the given severity parameter. Adjustments to Φ if it falls below, or near, the threshold may be accomplished by adjusting the fuel flow and/or the fuel/air mix from the manifold 20 to one or more of the nozzles 12.
In some embodiments, it may be desirable to modify the PM1 LBO line 24, to incorporate a minimum ΦPM1 at which there are other detrimental effects on combustor performance, for example, an undesirable dynamic signature. This is shown as ΦPM1SIG in
While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.
