Power plants – Combustion products used as motive fluid – Process
Reexamination Certificate
2002-01-17
2003-12-09
Freay, Charles G. (Department: 3746)
Power plants
Combustion products used as motive fluid
Process
C060S039370, C060S723000, C060S772000, C060S794000
Reexamination Certificate
active
06658856
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to a method and system for combusting hydrocarbon fuels with resulting ultra-low emissions over a wide range of power levels, fuel properties and ambient operating conditions.
The conventional gas turbine combustor, as used in a gas turbine power generating system, requires a mixture of fuel and air which is ignited and combusted uniformly. Generally, the fuel injected from a fuel nozzle into the inner tube of the combustor is mixed with air for combustion, fed under pressure from the air duct, ignited by a spark plug and combusted. The gas that results is lowered to a predetermined turbine inlet temperature by the addition of cooling air and dilutent air, then injected through a turbine nozzle into a gas turbine.
It is well known within the art that exhaust gases produced by combusting hydrocarbon fuels can contribute to atmospheric pollution. This occurrence is attributed to the development of localized high temperature zone, which can exceed 2,000° C. Exhaust gases typically contain many undesirable pollutants such as nitric oxide (NO) and nitrogen dioxide (NO
2
), which are frequently grouped together as Nitrogen Oxides (NO
x
), unburned hydrocarbons (UHC), carbon monoxide (CO), and particulates, primarily carbon soot.
Several methods are known in the art to decrease NO
x
emissions. For example, the formation of fuel-bound NO
x
can be minimized or avoided entirely by burning a low nitrogen or nitrogen-free fuel. However, burning a low nitrogen fuel does nothing to reduce the formation of thermal or prompt NO
x
. The formation of thermal NO
x
can be reduced by operating under uniformly fuel-lean conditions, such as by using a lean diffusion flame or a lean premixed/prevaporized (LPP) system. The excess air used to achieve fuel-lean combustion acts as a diluent to lower flame temperatures, thereby reducing the amount of thermal NO
x
formed. Prompt NO
x
can also be reduced by operating under fuel-lean conditions. However, the extent to which thermal and prompt NO
x
formation can be reduced by fuel-lean combustion may be limited by flame instability that occurs at very lean conditions.
By way of example, Honeywell Air Staged Combustion Systems as used in the ASE120 and ASE50DLE industrial engines are air-staged lean, premixing (LP) combustion systems. Air from the compressor flows over the combustor wall to provide convective cooling and then to at least one three-way air staging valve. Depending on their position, these valves direct air either to the premixers, where the fuel is added and mixed prior to burning in the combustor, or to a bypass manifold which returns the air to the main gas stream downstream of the flame but upstream of the turbine. By modulating the air staging valves the flame temperature can be held substantially constant from no-load to peak conditions. At no-load conditions, a large amount of air is bypassed, while at high power a relatively small amount of air is bypassed allowing the flame temperature to be held close to the ideal for low emissions throughout the power range. An advantage of this system is that all of the compressed air is routed through the turbine, and there is no loss of efficiency as in bleed-type air staging systems. A further advantage is that the combustion system pressure drop remains substantially constant as the air staging valves are modulated. Thus there is little or no impact of the air staging system operation on overall engine efficiency. This provides a system that is accurate and controllable over a wide range of power levels, fuel properties and ambient operating conditions. However, it is not capable of achieving ultra-low emissions.
Catalytic combustion systems, though, are capable of achieving ultra-low emissions. Catalytic combustion systems using a solid phase catalyst are known within the art. However, Catalytic combustion systems are not able to offer the accuracy and controllability of the air staging system over a wide range of power levels, fuel properties and ambient operating conditions.
U.S. Pat. No. 4,040,252, issued to Mosier, a Catalytic Premixing Combustor, discloses a combustor arrangement for a power plant having a tandem, self-regulating arrangement wherein a combustion chamber for burning a fuel-air mixture is placed in line with a catalytic reaction device. This is a representative example of a combination fuel-air combustion chamber and catalytic reaction devices. While such devices are known within the art they are difficult to use, cumbersome, require a great deal of hardware, expensive, and generally require a pre-heater. Pre-heaters are cumbersome and expensive to supply and operate. However, if eliminated in such a systems the catalyst will not activate and this would result in extremely high HC or CO emissions, or the flame will be too lean to sustain. Therefore, a preheater is needed in prior art systems.
Accordingly, what is needed in the art is an easy to use, inexpensive method and system for combusting hydrocarbon fuels that is accurate, controllable, easily adapted to a wide range of power levels, fuel properties and ambient operating conditions, and offers ultra-low emissions without the need for a preheater.
SUMMARY OF THE INVENTION
The present invention is directed to an easy to use and inexpensive method and system for combusting hydrocarbon fuels over a wide range of power levels, fuel properties and ambient operating conditions that results in ultra-low emissions.
One aspect of the invention is a system for combusting hydrocarbon fuel, comprising an air supply for supplying air from a compressor to an air inlet, at least one air staging valve, at least one fuel preparation and mixing section for receiving fuel and air directed from the air staging valves, at least one catalyst section for receiving said fuel and air mixture, a combustor, a secondary air stream and an exit for delivering the exiting effluent gas stream generated by the system to a turbine.
The system may be operated in different manners to allow for low and high power operation, as well as according to a controlled schedule that may be programmed. Under low power operation oxidation does not occur in the catalyst section. However, the mixing of the fuel and air in the fuel preparation and mixing section is enhanced by the presence of the catalyst. As the engine power level increases the compressor outlet air temperature will become high enough to activate the catalyst, and partial oxidation reactions will occur.
In one aspect of the present invention, a method of combusting a hydrocarbon fuel is disclosed. According to this method, air is compressed, then divided into at least one air staging valve air stream and at least one secondary air stream. Each air staging valve air stream is divided into at least one bypass flow stream, and at least one primary air stream. The bypass flow stream flows through a bypass manifold, combines with the secondary air stream and the output is an output bypass flow stream. It should be noted that the secondary air stream may consist solely of the control air stream. For instance, at high power, where the temperature profile is important, the output bypass flow stream is low and therefore has little effect. The primary air stream is introduced into a fuel preparation and mixing section, wherein fuel is injected and mixed to form a fuel/air mixture stream, which is introduced into a catalyst section. During certain conditions, which vary, depending on the specific catalyst, no oxidation will occur, but premixing is enhanced by the presence of the catalysts. The product stream that exits the catalyst section is then fed into the combustor. The temperature and composition of the product stream are selected to control simultaneously the amounts of NO
x
formed in the combustor and the stability of the flame in the combustor, thereby controlling the total amount of NO
x
in the exit effluent gas stream. Where conditions are desired such that no oxidation occurs in the catalyst section, the air staging valve sche
Belena John F.
Freay Charles G.
Thomas Kayden Horstemeyer & Risley, L.L.P.
Vericor Power Systems LLC
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