Patent Application
20060016198
The present invention relates to a gas turbine combustion system and more specifically to an apparatus and method of providing an off-gas fuel to a combustion system.
A gas turbine engine is a common source of mechanical output either for propulsion requirements or to provide mechanical energy to drive a generator for generating electricity. A typical gas turbine engine comprises at least a compressor, combustion system, and turbine. If the gas turbine is designed for use in generating electricity, a generator is coupled to the shaft of the gas turbine to convert the mechanical energy into electrical generation. Combustion systems provide a region for mixing compressed air and fuel and containing the reaction that occurs when this mixture is introduced to an ignition source. The hot combustion gases are then directed into the turbine, which is coupled to at least the compressor, and optionally a generator.
A fuel supply system feeding a typical combustion system is shown in
A common issue for both propulsion gas turbine operators and electrical power plant operators is exhaust emissions exiting the engine. Specifically, emission levels of concern are oxides of nitrogen (NOx) and carbon monoxide (CO). Recently, government regulatory agencies have introduced more stringent rules regarding the amount of emissions that can be produced by electrical power plants, thereby requiring power plant operators to upgrade their gas turbine engines to comply with the new regulations, face fines for exceeding pollutant levels, or restrict the amount of time an engine can operate in order to stay within the emissions regulations. As a result of these emissions regulations, gas turbine manufacturers have spent significant time and resources developing new combustion systems to lower polluting emissions. However, these combustion system designs have primarily focused on changes within the existing gas turbine structure in order to improve fuel/air concentrations and mixing or reducing combustion flame temperature in order to lower emissions.
Further reductions to gas turbine emissions are possible when modifications are made with specific regard to the type of fuel used and how it is mixed prior to injection in the combustion chamber. Such changes to fuel type can allow a present combustion system to emit lower levels of emissions without requiring major hardware changes to the combustion system.
A fuel supply system and method thereof that utilizes an off-gas in addition to the primary fuel to lower the emissions of a gas turbine combustion system is disclosed. The combustion system of the present invention preferably operates on a gas fuel such as natural gas. The fuel supply system apparatus comprises a fuel gas supply conduit and an off-gas supply conduit in fluid communication with the fuel gas supply conduit such that the flow of an off-gas to the fuel gas can be regulated as required by the operator. The fuel gas supply conduit further comprises a main gas pressure regulator, at least one fuel gas control valve, at least one fuel gas splitter valve, and contains a fuel gas such as natural gas. The off-gas supply conduit further comprises a means for compressing an off-gas, an off-gas main stop valve, an off-gas control valve, a first mixed fuel stop valve, and contains an off-gas selected from a group of off-gas fuels preferably comprising the common constituents hydrogen and methane.
The fuel supply system of the present invention is preferably configured to direct fuel to a gas turbine combustion system comprising a plurality of fuel nozzles, including a pilot fuel nozzle. The pilot fuel nozzle may also include a plurality of fuel circuits if necessary. The fuel supply system of the present invention includes multiple embodiments depending on where the off-gas is connected to the main fuel gas supply to form a fuel gas/off-gas blended mixture. In the preferred embodiment, the off-gas supply conduit is connected to fuel gas supply conduit such that only the pilot fuel nozzle receives the blended mixture of fuel gas and off-gas, while in an alternate embodiment, the off-gas supply conduit is connected to the main fuel gas supply conduit such that all fuel nozzles in the combustion system receive the blended mixture of fuel gas and off-gas.
It is an object of the present invention to provide a fuel supply system to a gas turbine engine that improves stability and flameout margin thereby allowing a combustion system to operate at reduced emissions levels.
It is another object of the present invention to provide a method of providing an off-gas fuel to a combustion system.
In accordance with these and other objects, which will become apparent hereinafter, the instant invention will now be described with particular reference to the accompanying drawings.
The selection of the combustor region in which to use a blended mixture of off-gas with a fuel gas for the reduction of emissions depends on the required emissions levels and individual combustor operation. Historically, pilot fuel nozzles are a source of high emissions due to their flame stability function for the combustor. Therefore, it is desirable for a combustion system that operates on a blended fuel mixture to utilize this blended mixture on at least the pilot fuel nozzle.
The preferred embodiment of the present invention, a fuel supply system for introducing an off-gas to a combustion system pilot fuel nozzle, is shown in
In the preferred embodiment of the present invention, fuel supply system 20 is providing fuel to a gas turbine combustor that utilizes a plurality of fuel nozzles, including a pilot nozzle, that are arranged in multiple circuits. In order to accomplish this fuel flow arrangement, at least one fuel gas control valve 24 comprises three gas control valves, 24a located in gas supply conduit 21 and additional control valves 24b and 24c to control the amount of fuel flowing to the main fuel nozzles and pilot fuel nozzle, respectively, for a combustor. Furthermore, at least one fuel gas splitter valve 25 comprises a first fuel gas splitter valve 25a and a second fuel gas splitter valve 25b. First fuel gas splitter valve 25a divides the fuel flow from gas supply conduit 21 into a primary circuit 32 and a secondary circuit 33. Second fuel gas splitter valve 25b is located within secondary circuit 33 such that it further divides the fuel flow within secondary circuit 33 into a secondary manifold 34 and a transfer manifold 35.
In order to lower the combustor emissions, an off-gas is used where the off-gas is preferably formed from at least hydrogen and methane at different concentration levels. The exact concentrations of hydrogen and methane, as well as any other gases making up the off-gas, are dependent on the desired emissions reduction, off-gas availability, and individual operating requirements. Although it is preferred to use an off-gas comprising at least hydrogen and methane, one skilled in the art of gas turbine combustion will understand that the off-gas composition is not limited to the constituents listed, but could also comprise other gases including, but not limited to, propane, butane, and ethane. Utilizing an off-gas such as a hydrogen-based gas is preferred because hydrogen gas is well known to have a higher flame speed and greater flammability limits, such that it provides greater flame stability at leaner conditions. Combining this fuel type with natural gas results in the blended fuel having these similar traits, such that it can be stably burned at a lower flame temperature to produce lower amounts of carbon monoxide, thereby leading to lower NOx levels for a given CO level.
In the preferred embodiment of the present invention, the off-gas is directed exclusively to the pilot fuel nozzle of the combustor. When the use of an off-gas is desired, first mixed fuel stop valve 30 is opened and the off-gas, which preferably comprises the constituents hydrogen and methane, is flowed through off-gas supply conduit 22, including means for compressing the off-gas 26, off-gas pressure regulator 27, and off-gas stop valve 28 such that off-gas control valve 29 is adjusted to a specified position to regulate the amount of off-gas directed towards secondary manifold 34. The fuel gas and off-gas are then blended in secondary manifold 34 to form a blended mixture, which then flows into a pilot fuel nozzle within a combustion system.
A series of tests were conducted on a pilot fuel nozzle in accordance with the alternate embodiment of the present invention. The pilot fuel nozzle is commonly used in a combustion system that contains a plurality of main fuel injectors arranged in an annular array about the pilot fuel nozzle. In the series of combustion tests, a baseline case was run exclusively with natural gas flowing through the pilot fuel nozzle, with NOx and CO emissions being recorded. Other tests were conducted in which the natural gas flowing to the pilot fuel nozzle was blended with various levels of an off-gas comprising at least the constituents hydrogen and methane. The general trends, with respect to NOx and CO emissions, as a result of this testing are shown in
While these specific tests focused only on blending an off-gas with natural gas for a pilot fuel nozzle in accordance with the preferred embodiment of the present invention, it is expected that similar reductions in CO and flame temperature will be achieved when an off-gas is blended with natural gas for all fuel nozzles within a combustion system. Depending on the operation mode of the combustor and the desired level of emissions reduction, the off-gas from off-gas supply conduit may be supplied to fuel gas supply conduit 21 such that a blend of fuel gas and off-gas passes through first fuel splitter valve 25a and into primary circuit 32 and secondary circuit 33, and as a result, into all fuel nozzles of the combustor.
This alternate embodiment of the present invention is shown in
The method under which this off-gas to fuel gas blending occurs and is provided to a plurality of fuel nozzles within a combustor is best described in the following steps. A combustion system is provided having a plurality of main fuel nozzles and a pilot fuel nozzle. A fuel gas, such as natural gas, is flowed through main gas pressure regulator where its supply pressure is lowered as necessary to comply with the combustion requirements of the engine. If the use of the off-gas is desired, and if not already in the closed position, first mixed fuel stop valve 30 is closed such that the flow of off-gas is directed into passage 37. Next, an off-gas, which preferably comprises the constituents hydrogen and methane, is flowed through off-gas supply conduit 22, including means for compressing the off-gas 26, which is typically an auxiliary compressor, off-gas pressure regulator 27, and off-gas stop valve 28 such that off-gas control valve 29 is adjusted to a specified position to regulate the amount of off-gas flowing into passage 37. The amount of off-gas to blend with the fuel gas depends on the chemical composition of the off-gas, but is usually up to approximately 20% of the total fuel flow to the combustion system. Second mixed fuel stop valve 31 is then opened in passage 37 to allow the off-gas to flow through passage 37 and towards fuel gas supply conduit 21. It is desired for this embodiment, that the off-gas enter the fuel gas supply conduit upstream of the first fuel gas control valve 24a, so that the overall flow of fuel to the combustor can be regulated by control valve 24a and blending may occur before being split to the various circuits of the combustor. Specifically, when a blended mixture is flowing from first fuel gas control valve 24a, first fuel gas splitter valve 25a splits the blended mixture between primary circuit 32 and secondary circuit 33, depending on the flow requirements of each circuit. However, for the combustion system in fluid communication with the fuel supply system of
While the invention has been described in what is known as presently the preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment but, on the contrary, is intended to cover various modifications and equivalent arrangements within the scope of the following claims.
