Patent Application
20070151257
This invention relates generally to rotary machines and, more particularly, to methods for improving the ability to operate at low loads. Many known combustion turbine engines bum a hydrocarbon-air mixture in a combustor assembly and generate a combustion gas stream that is channeled to a turbine assembly. The turbine assembly converts the energy of the combustion gas stream to torque that may be used to power a machine, for example, an electric generator or a pump. In many cases the engine is coupled to a generator who's rotational speed is a fixed rate that is defined by the electrical frequency of the electric grid. The temperature of the combustion gas stream is referred to as the combustor exit temperature. A common range of combustion gas stream temperatures is approximately 2400° F. to 2600° F. In some of these engines, a lower temperature limit may exist due to the ability of the combustor to completely bum the hydro-carbon fuel at low temperatures. When the combustion process is not completed, high levels of carbon-monoxide (CO) will exist in the turbine exhaust system. High CO emission levels are prohibited by regulatory agencies. Typically, when a turbine is operated at a high load, the combustor exit temperature is high and CO emissions are held to a minimum. As turbine load is decreased, it is necessary, in many gas turbines, to reduce the combustor exit temp, which may result in increased CO emissions. To prevent this increase in CO emissions it is desired to employ a method that can maintain high combustor temperatures while the engine is at low loads.
In order to maintain the emissions below a desired limit, the combustor exhaust temperature must be maintained within a specific range. Since the structural integrity of turbine hot gas path components such as nozzles and buckets is related to working fluid flow velocity and temperature, and coolant temperature and flow rate, managing the gas turbine generator load reduction can have significant life benefits while meeting the stringent regulatory emissions requirements.
Disclosed herein is a method for enabling turn down of a turbine engine, comprising: extracting compressor discharge air from a working fluid path before it enters a combustion zone of the turbine engine; and reintroducing the extracted air to the working fluid path downstream of a combustor exit.
Further disclosed herein is an apparatus related to a gas turbine, comprising: a compressor section, one or more combustors downstream from the compressor section, a turbine section downstream from the compressor section; and at least one conduit for extracting compressor discharge air from a working fluid path prior to a combustion zone and reintroducing the extracted air to the working fluid path downstream of a combustor exit in response to the turbine being in a turned down condition.
The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike:
Gas turbines generally include a compressor section, a combustion section and a turbine section. The compressor section is driven by the turbine section typically through a common shaft connection. The combustion section typically includes an array of spaced combustors. A fuel/air mixture is burned in each combustor to produce a hot energetic gas, which flows through a transition piece to the turbine section. For purposes of the present description, only one combustor is discussed and illustrated, it being intended that any number of the other combustors arranged about the turbine can be substantially identical to the first including all combustors being substantially identical to one another.
It should be appreciated by those skilled in the art that alternate embodiments of the invention may be applied to machines with multiple shaft turbines and to those with single chamber combustor sections, which may be annular or may be positioned non-symmetrically around the machine.
Referring to
The combustion process takes place within the combustor 30, and the parameters necessary to meet desired emissions limits are substantially controlled within the combustor 30. It has been determined that the temperature of the combustion process plays a key role in whether or not an engine meets the desired emissions limits. The temperature at the combustor exit 46, in particular, has a strong correlation to emissions output, in that, if the combustor exit 46 temperature falls below a certain level, the emissions quickly increase. The combustor exit 46 temperature depends on factors such as, air flow and fuel flow, for example. By reducing both the air flow and the fuel flow, the total amount of air and fuel that combust in the combustor 30 is decreased resulting in a decreased level of enthalpy entering the turbine. This reduction in enthalpy causes a reduction in engine output at a constant speed. In this case, since the air fuel ratio is maintained at acceptable levels, the temperature of the combustor exit 46 is also maintained thereby preserving an acceptable level of emissions.
It should be appreciated by those skilled in the art that embodiments of the invention may be applied to machines that reduce their load with turbine variable vanes configurations, compressor variable guide stator configuration and gas turbine variable rotor speed configuration.
An embodiment of the invention maintains the air fuel ratio in the combustion zone 23 by varying the amount of extracted air 25 for a given level of fuel delivered to the nozzle 34. More specifically, the extracted air 25 is removed from somewhere upstream of the combustion zone 23, by porting it into extraction sleeve 50. It is then ported through an extraction conduit 54, which may be insulated, and an optional valve 27 and is combined with extracted air from the other combustor heads 26; if more than one combustor head 26 is having air extracted, before being fed to a booster pump 58. Although this embodiment illustrates the use of a booster pump 58, it should be understood that embodiments without a booster pump 58 may also be utilized as will be described in more detail below. Additionally, alternate embodiments may use the valve 27 to vary the amount of extracted air 25 without the pump 58, and still other embodiments may use the valve 27 and the pump 58, however when both the valve 27 and the pump 58 are used the pump 58 should be of the non-positive displacement type thereby allowing the flow variation to be controlled by the valve 27. It should be appreciated, by one skilled in the art, that it is not necessary to extract air from all combustor heads 26, of a turbine engine 10, however, if balancing of air flow through all combustors 30 is desired, then it is an option. The booster pump 58 is located outside of the engine casing 18 and is driven by a pump driver 62. The pump driver 62 may be any motive system for example a variable speed electric motor or a steam turbine. If a steam turbine is used then expanding steam from a heat recovery steam generator (HRSG) of a combined cycle power plant, for example, as is shown in
The pressurized outlet flow 60 is then directed back through a return conduit 72 and enters a working fluid path 94 through the first stage nozzle 42. By reintroducing the outlet flow 60, downstream of the combustor exit 46, to a first stage nozzle airfoil 96 and platform 102, the air enters the working fluid path 94 without having a significant impact on the temperature profile at the axial plane of the nozzle trailing edge. Establishing a proper ratio of airfoil 96 and platform 102 flow will allow the system to minimize the impact to the critical core flow temperature profile. A change to a temperature profile for a hot gas path piece of hardware (
An embodiment of the invention introduces the outlet flow 60 into the working fluid path 94 in a way that will reduce the average temperature of the turbine working fluid path 94 while minimizing the impact on the temperature profile. This reduction in the average temperature results from the outlet flow 60 mixing with the combustion gases 98 resulting in a lower average temperature and extending the life of the turbine hardware.
Referring now to
In the exemplary embodiment illustrated, in addition to increasing the cooling of the hot gas path components, recombining all of the extracted air 25 (cooling fluid) with the combustion gases 98, prior to the first stage nozzle 42, assures that maximum power production will be achieved since all compressor discharge air 20 (working fluid) will pass through all of the turbine sections 44 of the gas turbine engine 10.
A doubling of the pump outlet flow 60 through the first stage nozzle 42 will allow a significant extension of engine turn down. To minimize an increase in pressure inside the first stage nozzle 42 at double the pump outlet flow 60, diameters of the impingement holes 100, in the inserts 80, 82, and the cooling holes 104 in the nozzle airfoil 96 and/or platform 102 should be sized to meet the back pressure requirements of the booster pump 58
Some advantages of some embodiments of the invention include: increase in the range of engine turn down while meeting desire emission limits, improved and uniform cooling of hot gas path components, increased life of hot gas path components, and reduced fuel consumption at low loads.
While the embodiments of the disclosed method and apparatus have been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the embodiments of the disclosed method and apparatus. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the embodiments of the disclosed method and apparatus without departing from the essential scope thereof. Therefore, it is intended that the embodiments of the disclosed method and apparatus not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out the embodiments of the disclosed method and apparatus, but that the embodiments of the disclosed method and apparatus will include all embodiments falling within the scope of the appended claims.
