Power plants – Combustion products used as motive fluid
Reexamination Certificate
1998-11-13
2002-02-05
Kim, Ted (Department: 3746)
Power plants
Combustion products used as motive fluid
C060S039530, C060S039550, C060S736000
Reexamination Certificate
active
06343462
ABSTRACT:
BACKGROUND OF THE INVENTION
Industrial electric power generation gas turbines are designed to operate over a pre-specified range of ambient temperature, barometric pressure, fuel calorific value, and steam injection rate. This operational flexibility is realized by designing the gas turbine air compressor and hot gas expander to operate effectively over a range of flows and pressure ratios. At most operating points, the air compressor limits the capacity of the gas turbine, while, at some other points, the hot gas expander is limiting. The net result of this operational flexibility is unused gas turbine capacity, and/or higher heat rate.
The prior art has attempted to devise thermodynamic cycles and process equipment arrangements that increase power output and/or minimize heat rate (efficiency) penalties. In many cases the primary focus has been on increasing the mass flow of air by cooling (lower suction temperature, higher density) or by humidification. The practice of intercooling to reduce compression energy consumption also has been considered.
Gas turbine manufacturers primarily have focused on improving the gas turbine mechanical design and process control technology to deliver lower heat rates, reduced NOx and CO emissions, higher reliability and reduced maintenance. Historically, the practice of water or steam injection in gas turbines has been utilized for NOx emissions control that also results in power augmentation as a secondary benefit. Steam injection solely for power augmentation is also commonly practiced. However, with the development of Dry Low NOx (DLN) units, the usage of water or steam injection for NOx emissions control has decreased.
A gas turbine operating system has been developed including an integrated air separation and gas turbine process that returns high purity or waste nitrogen product from the air separation plant for injection into the gas turbine. Another prior art system incorporates an integrated air separation gasification/partial oxidation gas turbine process that separates a portion of gas turbine air in the air separation plant to make oxygen and nitrogen products; uses the oxygen to gasify or partially oxidize a carbonaceous fuel to make syngas; burns the syngas in the gas turbine combustor; and returns nitrogen product for injection into the gas turbine compressor discharge, and/or combustor.
While such prior art approaches have proven useful in increasing power output and/or enhancing efficiency, there remains a need for a more efficient and effective technique and apparatus for increasing the power output and lowering NOx emissions, while minimizing heat rate penalties.
SUMMARY OF THE INVENTION
Now, an improved apparatus and process has been developed for enhancing the power output of gas turbines while lowering NOx emissions and minimizing heat rate penalties.
The invention enables power augmentation with potentially lower NOx emissions. In practice, a fuel gas mixture containing pre-set concentrations of fuel and diluent such as nitrogen and water vapor is prepared and used. Thermal properties of the fuel mixture are tuned by adjusting the concentrations of water vapor (relatively higher heat capacity) and nitrogen. The fuel gas mixture is delivered superheated to a fuel gas manifold of a gas turbine combustor unit. Depending on the gas turbine design, this fuel gas mixture needs to be delivered at a pressure sufficiently greater than the gas turbine combustor operating pressure. For some gas turbines this means the fuel should be at a pressure in the range of 150 psia to 300 psia, where as for some others greater than 300 psia. This invention enables the use of low pressure steam (steam whose pressure is less than that of the required fuel gas mixture delivery pressure) to prepare a fuel gas mixture with desired water vapor content and superheat. The fuel gas mixture is prepared by moisturizing the nitrogen gas and then mixing moist nitrogen with fuel. The moisture for this purpose is derived from steam at a pressure of 30 psia or greater but not exceeding the gas turbine fuel delivery pressure. For most commercially available gas turbines the preferred steam pressure is at least 50 psi below the gas turbine fuel delivery pressure. Moisturization is accomplished by contacting nitrogen and hot water in a countercurrent contactor with appropriate design and engineering features to obtain high mass transfer rates with minimum pressure losses. The water vapor content in the moist nitrogen can be as high as 60 mole %, preferably in the 30-50 mole % range. The moist nitrogen gas is superheated prior to mixing with fuel such as natural gas to prevent condensation. The mixing is accomplished such that fuel gas mixture of consistent calorific value and composition is obtained. The fuel content in the fuel gas mixture can be 25 to 75 mole %, and preferably 35-50 mole %. The resulting fuel gas mixture is superheated to obtain 50 F superheat to avoid condensation in the gas turbine fuel gas manifold system. This invention enables the use of low pressure steam as both the source of moisture, as well as the source of thermal energy. This invention also enables deriving moisture primarily from a low pressure steam, and using a relatively higher pressure steam as the source of additional moisture and/or thermal energy. This invention, however does not preclude the use of relatively higher pressure steam as the primary source of moisture and/or thermal energy. Low grade heat such as low pressure steam is utilized to the maximum possible extent to minimize heat rate penalties.
Compared to air cooling or air humidification, this invention offers more rangeability in the amount of additional mass that can be injected into the gas turbine. The invention does not depend on the integration of the air separation plant and the gas turbine. The invention can use nitrogen supplied by pipeline from a remotely located air separation plant or from an on-site plant. The invention does not depend on the technology used for air separation such as cryogenic distillation, or pressure swing adsorption, or vacuum pressure swing adsorption, or membrane technology. The invention can use both high purity nitrogen product (less than 10 ppm oxygen), as well as lower purity nitrogen product such as the waste nitrogen stream in an air separation plan (<5% oxygen).
The disclosed invention enables enhancing the power output of the gas turbines over a range of operating conditions. It is particularly applicable to situations were the gas turbine compressor is the bottleneck due to site conditions. It is also particularly applicable where significant low grade heat such as low pressure saturated steam at 50 to 200 psig is available.
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Ahmed M. Mushtaq
Drnevich Raymond Francis
Biederman Blake T.
Kim Ted
Praxair Technology Inc.
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