Power plants – Combustion products used as motive fluid – Process
Reexamination Certificate
2001-03-20
2002-08-27
Kim, Ted (Department: 3746)
Power plants
Combustion products used as motive fluid
Process
C060S737000, C060S742000, C060S748000
Reexamination Certificate
active
06438961
ABSTRACT:
TECHNICAL FIELD
The present invention relates to heavy duty industrial gas turbines and, in particular, to a burner for an industrial gas turbine including a fuel/air premixer enabling high-efficiency operation without producing undesirable air polluting emissions.
BACKGROUND
Gas turbine manufacturers are currently involved in research and engineering programs to produce new gas turbines that will operate at high efficiency without producing undesirable air polluting emissions. The primary air polluting emissions usually produced by gas turbines burning conventional hydrocarbon fuels are oxides of nitrogen, carbon monoxide, and unburned hydrocarbons. It is well known in the art that oxidation of molecular nitrogen in air breathing engines is highly dependent upon the maximum hot gas temperature in the combustion system reaction zone. The rate of chemical reactions forming oxides of nitrogen (NOx) is an exponential function of temperature. If the temperature of the combustion chamber hot gas is controlled to a sufficiently low level, thermal NOx will not be produced.
One preferred method of controlling the temperature of the reaction zone of a heat engine combustor below the level at which thermal NOx is formed is to premix fuel and air to a lean mixture prior to combustion. The thermal mass of the excess air present in the reaction zone of a lean premixed combustor absorbs heat and reduces the temperature rise of the products of combustion to a level where thermal NOx is not formed.
There are several problems associated with dry low emissions combustors operating with lean premixing of fuel and air. That is, flammable mixtures of fuel and air exist within the premixing section of the combustor, which is external to the reaction zone of the combustor. There is a tendency for combustion to occur within the premixing section due to flashback, which occurs when flame propagates from the combustor reaction zone into the premixing section, or autoignition, which occurs when the dwell time and temperature for the fuel/air mixture in the premixing section are sufficient for combustion to be initiated without an igniter. The consequences of combustion in the premixing section are degradation of emissions performance and/or overheating and damage to the premixing section, which is typically not designed to withstand the heat of combustion. Therefore, a problem to be solved is to prevent flashback or autoignition resulting in combustion within the premixer.
In addition, the mixture of fuel and air exiting the premixer and entering the reaction zone of the combustor must be very uniform to achieve the desired emissions performance. If regions in the flow field exist where fuel/air mixture strength is significantly richer than average, the products of combustion in these regions will reach a higher temperature than average, and thermal NOx will be formed. This can result in failure to meet NOx emissions objectives depending upon the combination of temperature and residence time. If regions in the flow field exist where the fuel/air mixture strength is significantly leaner than average, then quenching may occur with failure to oxidize hydrocarbons and/or carbon monoxide to equilibrium levels. This can result in failure to meet carbon monoxide (CO) and/or unburned hydrocarbon (UHC) emissions objectives. Thus, another problem to be solved is to produce a fuel/air mixture strength distribution, exiting the premixer, which is sufficiently uniform to meet emissions performance objectives.
Still further, in order to meet the emissions performance objectives imposed upon the gas turbine in many applications, it is necessary to reduce the fuel/air mixture strength to a level that is close to the lean flammability limit for most hydrocarbon fuels. This results in a reduction in flame propagation speed as well as emissions. As a consequence, lean premixing combustors tend to be less stable than more conventional diffusion flame combustors, and high level combustion driven dynamic pressure activity often results. This high level dynamic pressure activity can have adverse consequences such as combustor and turbine hardware damage due to wear or fatigue, flashback or blow out. Thus, yet another problem to be solved is to control the combustion driven dynamic pressure activity to an acceptably low level.
Lean, premixing fuel injectors for emissions abatement are in common use throughout the industry, having been reduced to practice in heavy duty industrial gas turbines for more than two decades. A representative example of such a device is described in U.S. Pat. No. 5,259,184, dated Nov. 9, 1993, invented by Richard Borkowicz, David Foss, Daniel Popa, Warren Mick and Jeffery Lovett; and assigned to the General Electric Company. Such devices have achieved great progress in the area of gas turbine exhaust emissions abatement. Reduction of oxides of nitrogen, NOx, emissions by an order of magnitude or more relative to the diffusion flame burners of prior art have been achieved without the use of diluent injection such as steam or water.
These gains in emissions performance, however, have been made at the expense of incurring several problems. In particular, flashback and flame holding within the premixing section of the device result in degradation of emissions performance and/or hardware damage due to overheating. In addition, increased levels of combustion driven dynamic pressure activity results in a reduction in the useful life of combustion system parts and/or other parts of the gas turbine due to wear or high cycle fatigue failures. Still further, gas turbine operational complexity is increased and/or operating restrictions on the gas turbine are necessary in order to avoid conditions leading to high-level dynamic pressure activity, flashback, or blow out.
In addition to these problems, conventional lean premixed combustors have not achieved maximum emission reductions possible with perfectly uniform premixing of fuel and air.
An example of a method for reducing the amplitude of combustion driven dynamic pressure activity in lean premixed dry low emissions combustors can be found in U.S. Pat. No. 5,211,004 dated May 18, 1997, invented by Steven H. Black, and assigned to General Electric Company. The current invention builds upon the principles disclosed in this prior patent by controlling both fuel/air radial profile and fuel injection pressure drop to minimize or eliminate the amplification resulting from the weak limit oscillation cycle.
DISCLOSURE OF THE INVENTION
The current invention is an improvement relative to the prior art in that the unique features of the premixer cause it to achieve performance improvements relative to the prior art in all of the problem areas noted above.
It is an object of the invention to achieve gas turbine exhaust emissions performance that is superior to current technology lean premixed dry low emissions combustor performance at elevated firing temperatures of the most advanced heavy-duty industrial gas turbines. In particular, the emissions of oxides of nitrogen (NOx) are to be minimized without compromising carbon monoxide (CO) or unburned hydrocarbon (UHC) emissions performance. It is another object of the invention to improve upon the resistance to flashback and flame holding within the premixer relative to current technology lean premixed dry low emissions combustors for heavy-duty industrial gas turbine application. It is yet another object of the invention to reduce the level of combustion driven dynamic pressure activity and increase the margin to lean blow out over the entire operating range of the gas turbine relative to current technology lean premixed dry low emissions combustors for heavy duty industrial gas turbines.
These and other objects of the invention are realized through the use of an inlet flow conditioner (IFC) located upstream of the premixer inlet. The IFC improves the air flow velocity distribution through the premixer, which improves the uniformity of the fuel/air mixture exiting the premixer. The premixer is made less sensitive to air flo
Battaglioli John Luigi
Bechtel, II William Theodore
Benoit Jeffrey Arthur
Black Stephen Hugh
Bland Robert James
General Electric Company
Kim Ted
Nixon & Vanderhye P.C.
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