Apparatus and method for start-up of a power plant

A power plant is provided which includes a gas turbine having a compressor for producing compressed air and a combustor for combusting the compressed air with a combustible fuel to produce a heated combustion gas. The power plant also includes a heat recovery steam generator for generating a flow of steam from an exhaust of the gas turbine and a steam turbine for expanding the flow of steam from the heat recovery steam generator. The steam turbine has a rotor having a rotor bore disposed axially therein. The power plant also includes a conduit for directing at least a portion of the compressed air or at least a portion of the heated combustion gas from the gas turbine to the rotor bore of the steam turbine, wherein the compressed air or the heated combustion gas may warm the rotor bore of the steam turbine.

TECHNICAL FIELD

The present application relates to a start-up process for a combined cycle power plant.

BACKGROUND OF THE INVENTION

Combined cycle power plants generally include a gas turbine, which utilizes the Brayton cycle, and a steam turbine, which utilizes the Rankine cycle. Greater efficiency may be achieved by utilizing a gas turbine and a steam turbine in combination than may be achieved by utilizing a gas turbine or a steam turbine independently. A combined cycle power plant typically includes a gas turbine, a heat recovery steam generator, and a steam turbine. A gas turbine is coupled with a generator to generate electricity. An exhaust gas from the gas turbine is introduced into the heat recovery steam generator to generate a flow of steam. The steam drives the steam turbine, which is coupled with a generator to generate additional electricity.

Minimizing start-up times improves the availability of combined cycle power plants and reduces maintenance cost and start-up emissions. The overall start-up time for a combined cycle power plant is limited by the start-up time of the steam turbine. Gas turbine start-up is fast relative to steam turbine start-up. During start up there is a relatively rapid increase in the exhaust temperature from the gas turbine. As the load of the gas turbine increases, a limit is reached on the exhaust temperature. A gas turbine controller then increases the airflow of the unit while maintaining the exhaust temperature limit. The exhaust flow and exhaust temperature is directly related to the amount of energy discharged in the heat recovery steam generator and the steam temperature generated by the heat recovery steam generator.

Steam turbine start-up is slow relative to gas turbine start-up. The start-up time of the steam turbine is limited by thermal stresses caused by temperature gradients between the rotor core and blades. These thermal stresses are monitored by measuring the temperature difference between the rotor and the steam at the inlet of the steam turbine. The allowable steam inlet temperature is limited by the rotor temperature. As the rotor temperature increases, higher inlet steam temperatures are allowed. Because the steam turbine rotor temperature sets a limit on the allowable inlet steam temperature and the gas turbine exhaust temperature controls the steam temperature, the gas turbine may not increase in load until the steam turbine rotor is heated to a sufficient temperature. This may reduce revenue by causing the power plant to operate for an extended period at a lower efficiency condition. Start-up emissions also may be increased because the load of the gas turbine may be too low for the combustor to operate in an efficient manner, thus causing concentrations of emissions such as NOx and CO to be greater than they would at higher load conditions.

A method and apparatus for warming a steam turbine during start-up of the gas turbine is desirable in order to reduce thermal stresses and decrease start-up times of a combined cycle power plant.

BRIEF DESCRIPTION OF THE INVENTION

In one embodiment, the present application provides a power plant. The power plant may include a gas turbine having a compressor for producing compressed air and a combustor for combusting the compressed air with a combustible fuel to produce a heated combustion gas. The power plant also may include a heat recovery steam generator for generating a flow of steam from an exhaust of the gas turbine and a steam turbine for expanding the flow of steam from the heat recovery steam generator. The steam turbine may have a rotor having a rotor bore disposed axially therein. The power plant also may include a conduit for directing at least a portion of the compressed air or at least a portion of the heated combustion gas from the gas turbine to the rotor bore of the steam turbine, wherein the compressed air or the heated combustion gas may warm the rotor bore of the steam turbine.

Another embodiment of the present application provides for a method of heating a steam turbine during start-up of a power plant having (i) a gas turbine having a compressor, a combustor, and a turbine for expanding a heated combustion gas, (ii) a heat recovery steam generator, and (iii) a steam turbine having a rotor having a rotor bore disposed axially therein. The method of heating the steam turbine during start-up includes compressing ambient air in the compressor to produce compressed air, removing at least a portion of the compressed air from the compressor, and providing the compressed air to the rotor bore of the steam turbine, wherein the compressed air heats the rotor.

A further embodiment of the present application provides for a method of heating a steam turbine during start-up of a power plant having (i) a gas turbine having a compressor, a combustor, and a turbine for expanding a heated combustion gas, (ii) a heat recovery steam generator, and (iii) a steam turbine having a rotor having a rotor bore disposed axially therein. The method of heating the steam turbine during start-up includes compressing ambient air in the compressor to produce compressed air, combusting the compressed air with a combustible fuel to produce a heated combustion gas, removing at least a portion of the heated combustion gas from the combustor, and providing the heated combustion gas to the rotor bore of the steam turbine, wherein the heated combustion gas heats the rotor.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, in which like numerals indicate like elements throughout the separate views,FIG. 1shows a schematic view of a power plant10of a particular embodiment of the present application. The power plant10may include a gas turbine11, a heat recovery steam generator (“HRSG”)12, and a steam turbine13.

The gas turbine11may include a compressor14, a combustor15, and a turbine16. In normal operation, ambient air17may be compressed by the compressor14to produce compressed air18. The compressed air18may be provided to the combustor15along with a combustible fuel19. In the combustor15the combustible fuel19may be combusted with the compressed air18to produce a heated combustion gas20. In particular embodiments the combustible fuel may include natural gas, hydrogen, propane, butane, isopropane, gasoline, diesel fuel, jet fuel, kerosene, ethanol, isopropyl alcohol, or synthetic gases derived from coal. The heated combustion gas20then may be provided to the turbine16, wherein the heated combustion gas20may be expanded, thereby generating rotary work. The turbine16may be coupled with a first generator21to generate electricity.

The exhaust22from the turbine16of the gas turbine11may be directed to the heat recovery steam generator12. In the heat recovery steam generator12, heat may be transferred from the gas turbine exhaust22to a feedwater flow23thereby generating a flow of steam24. After flowing through the heat recovery steam generator12, the cooled exhaust gas then may be discharged to the atmosphere via a stack25. In a particular embodiment the heat recovery steam generator12may include ductwork including finned tubes for the feedwater flow23. The hot exhaust gas may flow over the finned tubes, transferring a considerable portion of its heat to the feedwater flow, and thereby produce steam.

The flow of steam24from the heat recovery steam generator12then may be directed to the steam turbine13. The steam turbine13may include a rotor26having a rotor bore27disposed axially therein. In normal operation the steam24may be introduced to the steam flow path30of the steam turbine13where it may be expanded, thereby generating rotary work. The rotor26of the steam turbine13may be coupled with a second generator28to generate electricity. In a particular embodiment, both the gas turbine and the steam turbine may be coupled to the same generator to generate electricity. After exiting the steam turbine13, the steam may be dumped to a condenser29.

During start-up, the gas turbine11may be brought to steady-state operation as quickly as possible. Exhaust22from the gas turbine11may be directed to the heat recovery steam generator12which yields a flow of steam24. A period of time may elapse before the heat recovery steam generator12is capable of generating steam at sufficient temperature and pressure to be introduced to the steam turbine13. Introducing low temperature, low pressure steam to the steam turbine13could result in undesirable condensation within the steam turbine flow path30. Steam may be dumped to the condenser29via conduit31until it reaches a sufficient temperature and pressure for introduction into the steam turbine13.

An outlet32may be provided for removing at least a portion of the compressed air18from the compressor14of the gas turbine11. In another embodiment, a compressor separate from the compressor14of the gas turbine11may be provided to supply compressed air. The compressor separate from the compressor14of the gas turbine11may be powered by an electric motor, a piston engine, a gas turbine, a steam turbine, or another power source. During start-up the portion of compressed air18may be provided to the rotor bore27of the steam turbine13via a conduit33. The conduit33may include any means by which the compressed air18may be transmitted from the compressor14to the rotor bore27. A valve34may be provided for controlling flow through the conduit33. During start-up the valve34may be opened to allow flow from the compressor14to the rotor bore27, thereby heating the rotor26before steam is introduced to the steam flow path30. Once start-up is complete, the valve34may be closed, thereby directing the compressed air through the turbine16to produce useful work.

The steam turbine13may include a system35for introducing the compressed air flow to the rotor bore27of the steam turbine13. Once the compressed air flow passes through the rotor bore27of the steam turbine13, a second system36may be provided for removing the compressed air from the rotor bore27. Systems suitable for introducing a flow to the rotor bore of a turbine are known to those of skill in the art. For example, a suitable system for introducing a gas to the rotor bore of a steam turbine is described in U.S. Pat. No. 5,498,131 to Minto. Once it exits the rotor bore27, the compressed air may be vented to the atmosphere.

FIG. 2shows a schematic view of a power plant in accordance with an embodiment of the present application. By contrast withFIG. 1, the power plant ofFIG. 2includes an outlet37for removing at least a portion of the heated combustion gas20from the combustor15of the gas turbine11. During start-up the portion of heated combustion gas20may be provided to the rotor bore27of the steam turbine13via a conduit33. A valve34may be provided for controlling flow through the conduit33. In a particular embodiment, a cooling system may be provided for cooling the conduit. During start-up the valve34may be opened to allow flow from the combustor15to the rotor bore27, thereby heating the rotor26before steam is introduced to the steam flow path30. Once start-up is complete, the valve34may be closed, thereby directing the heated combustion gas20through the turbine16to produce useful work.

The steam turbine13may include a system35for introducing the heated combustion gas to the rotor bore27of the steam turbine13. Once the heated combustion gas passes through the rotor bore27of the steam turbine13, a second system36may be provided for removing the heated combustion gas from the rotor bore27. Once it exits the rotor bore27, the heated combustion gas may be vented to the atmosphere. In another embodiment, the heated combustion gas may be provided to the heat recovery steam generator12after passing through the rotor bore27.

By heating the steam turbine during start-up, the apparatus and methods of the present application may reduce thermal stresses and may decrease overall start-up times of combined cycle power plants.

It should be understood that the foregoing relates only to the preferred embodiments of the present application and that numerous changes and modifications may be made herein without departing from the general spirit and scope of the invention as defined by the following claims and the equivalents thereof.