Patent Application
20090056342
The present application relates generally to gas turbines and more particularly relates to methods and systems to extend gas turbine turndown values during part load operations.
Gas turbines generally have high efficiency at peak and base load operations. This efficiency, however, generally decreases during part-load operations. Turbine operation and exhaust emissions compliance may become an issue at such lower loads. Specifically, reducing the load on the turbine or “turndown” generally may be accomplished by reducing the fuel flow to the combustor. This reduction in fuel flow, however, makes the air-fuel mixture leaner such that sustaining combustion becomes more problematic as combustion temperatures are reduced. Unstable combustion may lead to excessive gas emission levels as well as to mechanical instability. Such instability potentially may damage elements of the gas turbine system as a whole. A typical turndown value of about forty percent (40%) to about thirty percent (30%) of full load may be expected while maintaining emissions compliance.
There is a desire, therefore, for improved methods and systems for gas turbine part-load operating conditions. Preferably, the improved methods and systems can extend the turndown value of a gas turbine within emissions compliance while maintaining or improving overall system efficiency.
The present application thus provides a method of operating at partial load a gas turbine system having a compressor, a combustor, and a turbine. The method may include the steps of lowering a flow of fuel to the combustor, extracting air from the compressor so as to lower a flow of air to the combustor, and returning the extracted air to the turbine or a component of the gas turbine system other than the combustor. Extracting air from the compressor raises a combustion temperature within the combustor. Raising the combustion temperature maintains a combustion exhaust below a predetermined level such as a predetermined emissions compliance level.
The present application further describes a gas turbine system. The gas turbine system may include a compressor with a compressor discharge, a combustor in communication with the compressor, and a turbine in communication with the combustor. A compressor discharge extraction may extend from the compressor discharge to the turbine such that air from the compressor discharge may be extracted and returned to the turbine during partial load operations.
The present application further describes a gas turbine system. The gas turbine system may include a compressor and a combustor in communication with the compressor. The compressor may include a compressor discharge valve such that air from the compressor may be extracted during partial load operations.
These and other features of the present application will become apparent to one of ordinary skill in the art upon review of the following detailed description when taken in conjunction with the several drawings and the appended claims.
Referring now to the drawings, in which like numerals refer to like elements throughout the several views,
Emissions compliance levels may vary according to location, type of generating equipment, operating conditions, and other variables. For the purposes herein, emissions compliance means a predetermined limit on gas turbine emissions that should not be exceeded. Emissions compliance generally focuses on NOx and COx emissions and other types of byproducts.
One known method of staying within emissions compliance during part-load operations is to reduce the angle of the inlet guide vanes about the compressor 110 and to activate an inlet bleed heat flow while considering a Fuel Stroke Reference. Such a control system is shown in commonly owned U.S. Pat. No. 7,219,040 entitled “Method and System for Model Based Control of Heavy Duty Gas Turbine.”
In addition to the existing turbine designs, another emissions compliance method is to bleed off some of the compressed discharge air from the compressor 110 before it reaches the combustor 120. Specifically, the fuel flow to the combustor 120 may be reduced during turndown. The reduction in fuel flow makes the air/fuel mixture leaner and reduces the temperature within the combustor 120. Bleeding some of the compressor air also forces the temperature within the combustor 120 to increase so as to allow the gas turbine system 100 as a whole to operate at its intended fuel mixture.
In addition to raising the temperature in the combustor 120, this bleed air may be used to cool the parts of the turbine 130 within the hot gas path in a manner similar to existing compressor extractions. Specifically, in addition to existing extractions, the gas turbine system 100 also may have a number of cooling compressor stage extractions 160. For example, a stage 9 compressor extraction 160 may be used to cool turbine stages 2 and 3 while compressor extractions 160 from stages 13, 17, and 18, may be used to cool stages 1, 2 and 3 of the turbine 130. Other extraction locations and combinations may be used herein.
In this example, a compressor discharge extraction 170 from a compressor discharge 175 of the compressor 110 also may be used to cool an early stage of the turbine 130 in a manner similar to the compressor stage extractions 160 described above. The compressor stage extraction 170 may extend from the compressor discharge 175 to the first or second stage of the turbine 130. Other positions may be used herein.
Alternatively, the energy of the compressor discharge extraction 170 may be used for any desired operation with respect to the gas turbine system 100 or the power plant as a whole via a heat exchanger 180 or other type of heat transfer device. The heat exchanger 180 may be of conventional design. For example, the heat exchanger 180 may be in communication with the compressor discharge 175 and other elements of the combined cycle power plant as described above.
Operation of the extractions 160, 170 may be performed with the use of an exhaust temperature sensor 190. The exhaust temperature sensor 190 may be in communication with the exhaust flow from the turbine 130 so as to sense the output temperature therein. The exhaust temperature sensor 190 may be of conventional design. The exhaust temperature sensor 190 may be in communication with an extraction flow control valve 200. The extraction flow control valve 200 may be a conventional three-way valve that forwards the air of the compressor discharge extraction 170 either towards the turbine 130 for cooling therein or towards the heat exchanger 180 for use with the combined cycle power plant or otherwise. A further turbine temperature sensor 195 may be used with respect to the parts within the hot gas path of the turbine 130. Other sensors may be used herein.
A similar flow control valve 165 may be positioned about the compressor stage extractions 160 such that the compressor stage extractions also may be used to control the temperature of the combustor 120 or for other purposes. For example, the compressor extraction 160 may be used to cool the various stages of the turbine 130 as described above as well as for the stability of the combustor 120 during part-load operations. Specifically, the compressor stage extractions 160 may be used during part load operations to limit the air sent to the combustor 120 while cooling the turbine 130 or otherwise. The extraction flow control valve 165 may be a three-way valve as described above and may be in communication with the heat exchanger 180 or a similar type of device such that the heat and energy of the compressor stage extractions 160 also may be in communication with other elements of the combined cycle power plant as described above.
The amount, location, and temperature of the extractions 160, 170 may be determined by the temperature sensors 190, 195 in association with a controller 210. The controller 210 may be any type of programmable microprocessor. More than one controller 210 may be used. The controller 210 may store performance parameters, curves, equations, look up tables, other data structures as well as immediate feedback from the temperature sensors 190, 195, from the load sensor 155, and from other types of input. Specifically, the controller 210 may adjust selectively the location and volume of the source and the destination of the extractions 160, 170 based upon the exhaust temperature, the temperature of the parts in the hot gas path of the turbine 130, and/or the load on the gas turbine system 100 as a whole. The controller 210 also may completely shutdown certain cans 125 within the combustor 120. Shutting the combustor cans 125 down may further extend turndown values. The controller 210 may provide for shutdown of one or more of the cans 125 and vary the extractions 160, 170 so as to maintain a predetermined exhaust temperature and maintain the gas turbine system 100 within emissions compliance.
As is shown in
In use, the combination of these various techniques may reduce the turndown value of the gas turbine 100 as a whole to about 14.3% or less of full-load with a fuel consumption decrease of about nine percent (9%) or more. These turndown values may be achieved by maintaining the temperature of the combustor 120 above the minimum operating limits by controlling the amount of intake air. Air for part-load operations may be controlled by the selected extractions 160, 170 from the compressor discharge 175 and the compressor stages, by decreasing the number of compressor cans 125 in operation, and/or by returning exhaust gases selectively to the combustor 120, the compressor 110, and/or the turbine 130. Various combinations of these techniques also may be used. Likewise, the use of the compressor extractions 160, 170 reduces the temperature of the parts in the hot gas path of the turbine 130 so as to extend part life. The heat and energy of the extractions 160, 170 further may be delivered to the heat exchanger 180 so as to increase overall plant thermal efficiency or for other purposes.
It should be apparent that the forgoing relates only to the preferred embodiments of the present application and that numerous changes and modifications may be made herein by one of ordinary skill in the art without departing from the general spirit and scope of the invention as defined by the following claims and the equivalents thereof.
