Power plants – Combustion products used as motive fluid – Process
Reexamination Certificate
2001-09-15
2004-06-15
Kim, Ted (Department: 3746)
Power plants
Combustion products used as motive fluid
Process
C060S723000, C431S007000, C431S326000
Reexamination Certificate
active
06748745
ABSTRACT:
FIELD OF THE INVENTION
The present invention is generally directed to combustion, and more specifically to a method of operating a main burner wherein the main combustion occurring therein is supported by a catalytic pilot that oxidizes a fuel rich mixture and a main burner for use therewith.
BACKGROUND
Power is generated in a gas turbine engine by the expansion of heated gases against a rotating turbine. To accomplish this heating and expansion a gas turbine has at least one combustor having at least one main burner positioned therein. The main burner combines a fuel and air into a fuel/air mixture and combusts the mixture thereby creating the expanding hot gases. Combustion of the mixture generally occurs by a flame mechanism.
A problem commonly associated with the operation of gas turbines employing a flame mechanism is that at high flame temperatures, particularly above 2800 degrees F., oxygen and nitrogen present in the air combine by a thermal formation mechanism to form pollutants such as NO and NO
2
, collectively referred to as NO
x
. In a gas turbine, temperatures of most common fuels combusting in air can easily exceed this value. Accordingly, it has been an objective of gas turbine combustion system designers to develop methods and associated apparatuses for combustion that produce reduced temperatures at or below 2800 degrees F., so that such thermal formation of NO
x
is limited.
Modern combustion methods employed in gas turbine combustors reduce flame temperatures, and thereby NO
x
, by using excess air to create lean fuel/air mixtures, e.g. mixtures that contain more air than needed to fully combust all the fuel present. Quantitatively, the mixture has a fuel/air equivalence ratio less than one. The equivalence ratio is the ratio of the actual fuel/air ratio to the stoichiometric fuel/air ratio, where the stoichiometric coefficients are calculated for the reaction giving full oxidation products CO
2
and H
2
O. An equivalence ratio greater than one defines a fuel-rich fuel/air mixture, and an equivalence ratio less than one defines a fuel-lean fuel/air mixture. For any given substantially premixed fuel/air mixture, the combustion temperature will be at its highest temperature when the fuel/air mixture being combusted has a fuel/air equivalence ratio of about one.
The more excess air added to and well mixed in a fuel/air mixture, the leaner the resulting fuel/air mixture becomes and the lower the flame temperature of that mixture. However, if too much excess air is added the resulting fuel/air mixture will become so lean that it will not homogeneously combust. In this situation, the mixture is said to have reached its lower flammability limit. Therefore, excess air to limit flame temperature can only be added to a well mixed fuel/air mixture until this limit is reached.
In order to obtain the benefits of lower flame temperatures in fuel/air mixtures, the fuel/air mixture being combusted must be substantially mixed. Typically, the lower the unmixedness the lower the NO
x
that will be produced. While unmixedness defines a continuum such that mixtures can only be categorized as being mixed to some degree, a “substantially premixed mixture” can be defined based on the fuel/air mixture's potential to produce a certain level of NO
x
when combusted within the context of acceptable NO
x
production based on existing environmental regulation. In other words, the mixture is mixed sufficiently to produce a level of NO
x
that will meet current environmental regulations.
Thus based on current environmental regulation, substantially premixed fuel/air mixtures are mixtures wherein the average variation of fuel/air ratio from the mean is less than about 20 percent of the mean value and more preferably in the range from about 10 percent to about 2 percent, with less than 2 percent being a practical minimum. Mean fuel/air ratio refers to the average fuel/air ratio as measured at various points in the region of interest. Variation from the mean refers to the magnitude of the difference between the mean and the measured fuel/air ratio at some single measured point, and the average variation from the mean is the average of all measured variations from the mean. For a combustible fuel/air mixture the region of interest is generally immediately prior to combustion.
In a combustor, the air stream and the fuel stream must form a fuel/air mixture prior to combustion. To mix two flowing fluid streams to form a single flowing stream, the individual streams must be brought into contact and travel some distance together. If mixing is done within a duct, the length of the duct will determine the degree of unmixedness. Generally speaking, the longer the duct the lesser the degree of unmixedness.
As a lean fuel/air mixture is made ever leaner but above the mixture's lower flammability limit, the rate of combustion associated with the mixture decreases, i.e. the flame is becoming less robust. In order to maintain the flame, the environment within the flame must be made ever more conducive to combustion, e.g. the flow velocity must be reduced, otherwise the flame could be blown out, much like one blows out a candle. In a gas turbine when the fuel/air mixture has been leaned to the point that the rate of combustion of the mixture is too low to sustain combustion under the existing conditions, the extinguishing of the flame by its environment is termed blowout. Flame anchoring, i.e. the ability to provide proper environmental conditions to support a flame, and flame stability thus become problematic for fuel-lean combustion.
The management of combustion within a gas turbine operating on lean fuel/air mixtures to avoid blowout and assure flame anchoring and stability is complex. Gas turbines are generally designed to operate at a given or peak condition, i.e. an optimum condition which is highly efficient. However, during startup or at other times, it may be desirable to operate at other, or off-peak, conditions. Therefore, a gas turbine must have the ability to transition from the peak condition to off-peak conditions. This ability to go from a peak to off-peak condition is generally referred to by those skilled in the art as the ability to turndown the gas turbine.
Turndown is accomplished by reducing the fuel supply to the combustor, thereby making the fuel/air mixture being combusted therein leaner. As the gas turbine at its peak condition is already operating with a fuel/air mixture that is quite lean to meet current environmental standards, when the fuel/air mixture is made ever leaner to achieve the desired off-peak operating condition, sustaining combustion within the combustor becomes ever more problematic. In some cases, turndown is simply insufficient to permit acceptable off-peak operation conditions.
To increase the ability of a gas turbine to turndown, pilots can be used to support combustion within the combustor. Specifically, the pilots are supporting what is termed main combustion. Pilots that use flames operate at very favorable fuel/air mixtures, which may even be at fuel/air ratios at or near 1.0, providing highly stable and high temperature flames. Initially, pilot emissions were a small percentage of the overall emissions from the gas turbine. Currently, however, gas turbines have main combustion occurring at such lean fuel/air mixtures that NO
x
discharge is acceptable, and it is the emissions from these flame based pilots that must be further reduced to reduce overall gas turbine NO
x
emissions.
Conventional catalytic pilots on the other hand are highly stable but operate at lower temperatures, because of catalyst material considerations, thereby producing less NO
x
than flame pilots. However, these lower temperatures hamper the ability of the catalytic pilot to support combustion of lean fuel/air mixtures.
Based on the foregoing, it is the general object of the present invention to provide a method and apparatus for use therewith to support main combustion that overcomes the problems and drawbacks of the prior art.
SUMMARY OF THE INVENTION
The method of comb
Etemad Shahrokh
Lyle Kent
Pfefferle William C
Smith Lance L.
Ul Karim Md. Hasan
Kim Ted
McCormick Paulding & Huber LLP
Precision Combustion Inc.
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