Patent Application
20090053036
The present application relates generally to gas turbines and more particularly relates to methods and systems for extending gas turbine emissions compliance at lower loads.
Due to rising fuel costs, natural gas fired power plants that were designed to operate at mostly full power output are now being operated on a intermittent basis. Coal and nuclear energy now generally make up the majority of stable power output. Gas turbines are being increasingly used to make up the difference during peak demand periods. For example, a gas turbine may be used only during the daytime and then taken off line during the nighttime when the power demand is lower.
During load reductions or “turndowns”, gas turbines typically can remain in emissions compliance down to about forty-five percent (45%) of full rated load output. Below this load, carbon monoxide (CO) emissions can increase exponentially and cause the system as a whole to go out of emissions compliance. Generally described, emissions compliance requires that the turbine as a whole to produce less than the guaranteed or predetermined minimum emissions levels. Such levels may vary with the ambient temperature, system size, and other variables.
If a gas turbine has to be shutdown because it cannot remain in emissions compliance due to a low power demand, the other equipment in a combined cycle application also may need to be taken offline. This equipment may include a heat recovery steam generator, a steam turbine, and other devices. Bringing these other systems online again after a gas turbine shutdown may be expensive and time consuming.
Such startup requirements may prevent a power plant from being available to produce power when the demand is high. There may be a strategic operational advantage in being able to keep a gas turbine online and in emissions compliance during periods of low power demand so as to avoid the start up time and expense.
There is a desire therefore for methods for extending gas turbine emissions compliance during periods of reduced loads. Reducing the load on the gas turbine while remaining in emissions compliance may enable the operator to take advantage of these peak demand opportunities.
The present application thus provides a gas turbine system for operation at low loads. The gas turbine system may include a number of inlet guide vanes, a compressor, a turbine, and an air movement system for maintaining an emission from the gas turbine system below a predetermined level.
The present application further describes a gas turbine for operation at low loads. The gas turbine may include a number of inlet guide vanes, a compressor, and an air recirculation system to raise a temperature of an outlet air stream leaving the compressor.
The present application further describes a gas turbine system for operation at low loads. The gas turbine system may include a number of inlet guide vanes, a compressor, a turbine, and an air extraction system to extract air from the compressor.
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,
A first technique involves the use of inlet bleed heat and reducing the angles for the inlet guide vanes 150. Reducing the minimum angles for the inlet guide vane 150 reduces the core airflow through the gas turbine system 100 so as to raise the reaction zone temperature in the combustor 130. During a turndown, the angles of the inlet guide vanes 150 may be reduced until the minimum angle or an exhaust temperature isotherm is reached. Operation above this temperature level may cause damage to downstream components. After reaching either of these limits, a decrease in the load requires a reduction in the fuel flow. This reduction, however, may decrease the reaction zone temperature in the combustor 130 and may promote CO formation. A further reduction in the minimum angle for the inlet guide vanes 150 therefore may allow operation along the exhaust temperature isotherm at a lower load before a reduction in fuel flow may be needed. These minimum angles may result in an improved turndown over a portion of the ambient temperature range. Depending on the nature of the gas turbine 100, angles of about 30 to about 50 degrees may be used herein, with a typical full operating range extending from about 40 to about 90 degrees. Other angles may be used herein.
The angles of the inlet guide vanes 150 generally are opened to maintain exhaust temperatures at or below the isotherm. Increasing the exhaust temperature isotherm also may permit operation at lower angles of the inlet guide vanes 150. Increasing the isotherm may be accomplished by adjusting the operating parameters of the gas turbine 100 as a whole. Further, variations in the isotherm may be caused by adding duct insulation, different material selection, and varying other components.
In the embodiment of
For example, a first compressor cooling line 190 may extend from a thirteenth stage of the compressor 110 to a stage two nozzle in the turbine 140 with a second compressor cooling line 190 extending from a ninth stage of the compressor 110 to a stage three nozzle in the turbine 140. Introduction into the exhaust path may be upstream or downstream of any type of exhaust temperature measurement location. The extractions may be taken from any stage of the compressor 110. There may be a common extraction location for cooling or there may be separate locations specifically for the purpose of bypassing air. The choice of location may depend on factors such as recycle efficiency, compressor operability, durability, and acoustics. Existing extraction locations may be used.
A number of these techniques may be employed depending on the overall configuration of the gas turbine 100. In fact, each method may be applicable for improving turndown performance. The selection of the methods and their operation and interaction will depend on the overall design of the gas turbine system 100 and related combustion technology. Specifically, the level of turndown improvement may depend upon the frame size of the gas turbine 100 and the particular combustion technology used.
For example, in a 7FA+e gas turbine with a dry-low-Nox 2.6 combustion system, the preferred configuration may include reducing the minimum angle of the inlet guide vanes 150, doubling the extraction flows, and adding an extraction from the compressor discharge casing 120 to bypass additional air to the exhaust. The 7FA+e gas turbine is available from the General Electric Company of Schenectady, N.Y. For a 9FB gas turbine with a similar combustion system, only reducing the minimum angle of the inlet guide vanes 150 with an increase in the isotherm may be required. The 9FB gas turbine also is available from the General Electric Company of Schenectady, N.Y. Other types of gas turbines may be used herein. By employing these various methods, emissions compliance may be maintained down to about thirty percent (30%) of the load. Other improvements may be possible.
It should be apparent that the foregoing relates only to the preferred embodiments of the present application and that numerous changes and modifications maybe 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.
