Patent Application
20100170218
The subject matter disclosed herein relates to gas turbines and, more specifically, to an improved method for recovering energy from compressed bleed air discharged from a compressor of a gas turbine.
A gas turbine system may bleed air from a compressor to control operational conditions and health of various components (e.g., no stall conditions). Unfortunately, bleeding compressor discharge air from a gas turbine may decrease the net efficiency, because the energy expended to raise the pressure of air within the compressor is not being recovered by the combustor and turbine. The amount of air bled from the compressor (i.e., “bleed air”) may be a function of ambient conditions and gas turbine output. For example, the air bleeding may increase with lower gas turbine loads and lower ambient temperatures. By further example, air bleeding may increase when burning fuel with low energy values, e.g., measured in British thermal units (BTUs). In these low BTU fuel gas turbine applications, the fuel flow rate may be much higher than in natural gas fuel applications, and may cause additional backpressure on the compressor. Therefore, air bleeding the compressor flow may be desirable to reduce this backpressure and improve the stall margin of the compressor.
In one embodiment, a method for recovering energy from compressed air from a compressor of a gas turbine is provided. The method includes directing bleed air from a compressor of a gas turbine into a cooling unit. The bleed air is compressed air from the compressor, and the bleed air is not directed into a combustion chamber of the gas turbine. The method also includes cooling the bleed air within the cooling unit. The method further includes expanding the cooled bleed air within a turbo-expander to generate power.
In another embodiment, a gas turbine power generation system is provided. The system includes a gas turbine comprising a compressor, a combustion chamber, and a turbine component. The system also includes a cooling unit configured to receive and cool bleed air from the compressor. The bleed air is compressed air from the compressor, and the bleed air is not directed into the combustion chamber. The system further includes a turbo-expander configured to receive and expand the cooled bleed air to generate power.
In yet another embodiment, a bleed air and energy recovery system is provided. The system includes a cooling unit configured to receive and cool bleed air from a compressor. The system also includes a turbo-expander configured to receive and expand the cooled bleed air to generate power.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.
When introducing elements of various embodiments of the present invention, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.
In certain embodiments, the systems and methods described herein include recovering energy from bleed air discharged from a compressor of a gas turbine. Bleeding compressed air from a compressor of a gas turbine may help prevent the compressor from stalling. However, by simply venting the compressed air to the atmosphere, energy expended by the compressor to compress the air will not be recovered. Therefore, in certain embodiments, the bleed air may be directed into other processes where the energy within the bleed air may be recovered. For instance, the bleed air may be directed into a turbo-expander, within which the bleed air may be expanded, thereby generating power. This generated power may, in turn, be used to drive various loads, such as an electrical generator.
In addition, the bleed air from the compressor may be characterized by both high pressures and high temperatures. Therefore, prior to being expanded within the turbo-expander, the bleed air may be cooled within a pre-cooling unit. Specifically, within the pre-cooling unit, heat may be transferred from the bleed air into a cooling fluid, such as boiler feedwater. Since the temperature of the bleed air may be reduced before expansion within the turbo-expander, the turbo-expander may be sized to handle lower temperatures, thereby reducing the cost of the turbo-expander since, among other reasons, high temperature resistant materials may not be necessary. Furthermore, in addition to recapturing energy within the bleed air in the turbo-expander, the heat transferred from the bleed air into the cooling fluid may also be recaptured in other processes.
The system 10 may also include a steam turbine 22 for driving a second load 24. The second load 24 may also be an electrical generator for generating electrical power. However, both the first and second loads 14, 24 may be other types of loads capable of being driven by the gas turbine 12 and steam turbine 22. In addition, although the gas turbine 12 and steam turbine 22 may drive separate loads 14 and 24, as shown in the illustrated embodiment, the gas turbine 12 and steam turbine 22 may also be utilized in tandem to drive a single load via a single shaft. In the illustrated embodiment, the steam turbine 22 may include one low-pressure section 26 (LP ST), one intermediate-pressure section 28 (IP ST), and one high-pressure section 30 (HP ST). However, the specific configuration of the steam turbine 22, as well as the gas turbine 12, may be implementation-specific and may include any combination of sections.
The system 10 may also include a multi-stage HRSG 32. The components of the HRSG 32 in the illustrated embodiment are a simplified depiction of the HRSG 32 and are not intended to be limiting. Rather, the illustrated HRSG 32 is shown to convey the general operation of such HRSG systems. Heated exhaust gas 34 from the gas turbine 12 may be transported into the HRSG 32 and used to heat steam used to power the steam turbine 22. Exhaust from the low-pressure section 26 of the steam turbine 22 may be directed into a condenser 36. Condensate from the condenser 36 may, in turn, be directed into a low-pressure section of the HRSG 32 with the aid of a condensate pump 38.
The condensate may then flow through a low-pressure economizer 40 (LPECON), e.g., which is a device configured to heat feedwater with gases, which may be used to heat the condensate. From the low-pressure economizer 40, the condensate may either be directed into a low-pressure evaporator 42 (LPEVAP) or toward an intermediate-pressure economizer 44 (IPECON). Steam from the low-pressure evaporator 42 may be returned to the low-pressure section 26 of the steam turbine 22. It should be noted that heated fluid from the air bleed and energy recovery system 13, as described in greater detail below, may be used to generate steam within the low-pressure evaporator 42.
Likewise, from the intermediate-pressure economizer 44, the condensate may either be directed into an intermediate-pressure evaporator 46 (IPEVAP) or toward a high-pressure economizer 48 (HPECON). In addition, water from the intermediate-pressure economizer 44 may be sent to a fuel gas heater (not shown) where the water may be used to heat fuel gas for use in the combustion chamber 18 of the gas turbine 12. Steam from the intermediate-pressure evaporator 46 may be sent to the intermediate-pressure section 28 of the steam turbine 22. Again, it should be noted that heated fluid from the air bleed and energy recovery system 13, as described in greater detail below, may be used to generate steam within the intermediate-pressure evaporator 46.
Finally, condensate from the high-pressure economizer 48 may be directed into a high-pressure evaporator 50 (HPEVAP). Steam exiting the high-pressure evaporator 50 may be directed into a primary high-pressure superheater 52 and a finishing high-pressure superheater 54, where the steam is superheated and eventually sent to the high-pressure section 30 of the steam turbine 22. Again, it should be noted that heated fluid from the air bleed and energy recovery system 13, as described in greater detail below, may be used to generate steam within the high-pressure evaporator 50. Exhaust from the high-pressure section 30 of the steam turbine 22 may, in turn, be directed into the intermediate-pressure section 28 of the steam turbine 22, and exhaust from the intermediate-pressure section 28 of the steam turbine 22 may be directed into the low-pressure section 26 of the steam turbine 22. It should also be noted that the connections between the economizers, evaporators, and the steam turbine 22 may vary across implementations as the illustrated embodiment is merely illustrative of the general operation of an HRSG system that may employ unique aspects of the present embodiments.
An inter-stage attemperator 56 may be located in between the primary high-pressure superheater 52 and the finishing high-pressure superheater 54. The inter-stage attemperator 56 may allow for more robust control of the exhaust temperature of steam from the finishing high-pressure superheater 54. Specifically, the inter-stage attemperator 56 may be configured to control the temperature of steam exiting the finishing high-pressure superheater 54 by injecting cooler feedwater spray into the superheated steam upstream of the finishing high-pressure superheater 54 whenever the exhaust temperature of the steam exiting the finishing high-pressure superheater 54 exceeds a predetermined value.
In addition, exhaust from the high-pressure section 30 of the steam turbine 22 may be directed into a primary re-heater 58 and a secondary re-heater 60 where it may be re-heated before being directed into the intermediate-pressure section 28 of the steam turbine 22. The primary re-heater 58 and secondary re-heater 60 may also be associated with an inter-stage attemperator 62 for controlling the exhaust steam temperature from the re-heaters. Specifically, the inter-stage attemperator 62 may be configured to control the temperature of steam exiting the secondary re-heater 60 by injecting cooler feedwater spray into the superheated steam upstream of the secondary re-heater 60 whenever the exhaust temperature of the steam exiting the secondary re-heater 60 exceeds a predetermined value.
In combined cycle systems such as system 10, hot exhaust may flow from the gas turbine 12 and pass through the HRSG 32 and may be used to generate high-pressure, high-temperature steam. The steam produced by the HRSG 32 may then be passed through the steam turbine 22 for power generation. In addition, the produced steam may also be supplied to any other processes where superheated steam may be used. The gas turbine 12 generation cycle is often referred to as the “topping cycle,” whereas the steam turbine 22 generation cycle is often referred to as the “bottoming cycle.” By combining these two cycles as illustrated in
Therefore, the main component of the topping cycle of the combined cycle power generation system 10 is the gas turbine 12.
In certain applications, the gas turbine 12 pressure ratio may reach a limit for the compressor 20. For instance, in applications where low-BTU fuels are used as fuel sources in the combustion chamber 18, or in locations characterized by lower ambient temperatures, the compressor 20 pressure ratio (i.e., the ratio of the air pressure exiting the compressor 20 to the air pressure entering the compressor 20) may become lower than the turbine 16 pressure ratio (i.e., the ratio of the hot gas pressure exiting the turbine 16 to the hot gas pressure entering the turbine 16). In order to provide compressor 20 pressure ratio protection (i.e., prevent the compressor 20 from stalling), air discharged from the compressor 20 may be bled off via the air bleed and energy recovery system 13.
The amount of air bled from the compressor 20 may be a function of ambient conditions and the gas turbine 12 output. More specifically, the amount of air bled may increase with lower ambient temperatures and lower gas turbine 12 loads. In addition, as described above, in gas turbine 12 applications utilizing low-BTU fuel sources 66, the flow rate of the fuel source 66 will generally be much higher than in comparable natural gas fuel applications. This is primarily due to the fact that more low-BTU fuel must be used in order to attain comparable heating. As such, additional backpressure may be exerted on the compressor 20. In these applications, the air discharged from the compressor 20 may also be bled to reduce the backpressure and improve the stall margin (i.e., margin of design error for preventing stalling) of the compressor 20.
However, bleeding compressed air discharged from the compressor 20 may decrease the net efficiency of the combined cycle power generation system 10, because the energy expended to raise the pressure of the air 64 within the compressor 20 is not recovered by the combustion chamber 18 and turbine 16 of the gas turbine 12. However, the energy introduced into the bleed air 68 discharged from the compressor 20 need not be wasted. Rather, using the various embodiments of the air bleed and energy recovery system 13, the energy in the bleed air 68 may be recaptured.
One technique for recapturing the energy in the bleed air 68 may be to introduce the bleed air 68 into the flow of heated exhaust gas 34 from the turbine 16 of the gas turbine 12. The flow of the bleed air 68 may, for instance, be controlled by a bleed control valve 70. In addition, one or more controllers 71 of the combined cycle power generation system 10 may be used to control the operation of the bleed control valve 70, as well as other valves and equipment described herein, to ensure that an adequate amount of compressed air is bled from the compressor 20. In particular, the controller(s) 71 may take into consideration operating parameters of the compressor 20, as well as operating parameters of the other components of the gas turbine 12, to determine how much compressed air to bleed off from the compressor 20. For instance, the controller(s) 71 may take into consideration the pressure ratio of the compressor 20, the pressure ratio of the turbine 16, and so forth.
By combining the bleed air 68 with the heated exhaust gas 34 from the gas turbine 12, the energy in the bleed air 68 may be recaptured within the HRSG 32. Specifically, the energy (e.g., heat) within the bleed air 68 may be used to help generate steam for use in the steam turbine 22 via the high-pressure economizer 48, high-pressure evaporator 50, primary high-pressure superheater 52 (PHPSH), finishing high-pressure superheater 54 (FHPSH), and the other heat transfer components of the HRSG 32, described above in greater detail with respect to
However, even more efficient techniques for recovering energy in the bleed air 68 may be implemented. For instance,
However, since the bleed air 68 from the compressor 20 may be characterized by relatively high temperatures and pressures, the actual turbo-expander used would be sized accordingly to tolerate higher temperatures. As such, the turbo-expander 72 could, in certain applications, be somewhat bulky and expensive. Therefore, it may be advantageous to reduce the temperature of the bleed air 68 prior to directing it into the turbo-expander 72. By doing so, the turbo-expander 72 may be sized to handle lower temperatures and, therefore, may be less bulky and expensive than if the bleed air 68 were not cooled, as with the embodiment illustrated in
The heated fluid 80 discharged from the pre-cooling unit 76 may, in turn, be used by other processes within or external to the combined cycle power generation system 10. For instance, the heated fluid 80 may be introduced into components of the HRSG 32, where the heated fluid 80 may be used to help recover additional energy, e.g., via steam generation in the HRSG 32. In addition, it should be noted that heat may also be recovered from the bleed air 68 after the bleed air 68 has been discharged from the turbo-expander 72. For instance, the discharged bleed air 68 may be combined with the heated exhaust gas 34 from the turbine 16 of the gas turbine 12, for use in the HRSG 32.
The concept of recapturing energy from the heated fluid 80 discharged from the pre-cooling unit 76, as shown in
However, in the embodiment illustrated in
The flow of the heated fluid 80 into the high-pressure evaporator 50 may be controlled by a pressure control valve 88. In addition, since the heated fluid 80 (e.g., heated boiler feedwater 82 from the boiler feedwater pump 84) will generally be at a higher pressure than the pressure in the high-pressure evaporator 50 drum, the pressure control valve 88 downstream of the pre-cooling unit 76 may also be used to control the pressure of the heated fluid 80 before sending it into the high-pressure evaporator 50 drum.
It should be noted that while certain embodiments described herein have been presented in the context of utilizing high-pressure components of the HRSG 32 and associated equipment (e.g., high-pressure boiler feedwater pump 84, high-pressure economizer 48, high-pressure evaporator 50, and so forth), any components of the HRSG 32 and associated equipment may, in fact, be utilized. For example, the boiler feedwater 82 used within the pre-cooling unit 76 may be supplied from an intermediate-pressure boiler feedwater pump. Regardless of the particular equipment used in any particular application, the underlying methods for recovering energy from bleed air 68 from the compressor 20 of the gas turbine 12 may prove beneficial.
In step 94, the bleed air 68 may be cooled within the pre-cooling unit 76. Cooling the bleed air 68 within the pre-cooling unit 76 may include transferring heat from the bleed air 68 into the cooling fluid 78 received by the pre-cooling unit 76. As described above, the particular components of the pre-cooling unit 76 and the cooling fluid 78 used may vary from application to application. However, in certain embodiments, the cooling fluid 78 used may be boiler feedwater 82 from a high-pressure boiler feedwater pump 84. The boiler feedwater 82 may be received by the pre-cooling unit 76, wherein heat from the bleed air 68 may be transferred into the boiler feedwater 82. As described above, the heat transferred into the resultant heated fluid 80 may also be recovered. For instance, in the embodiment where the boiler feedwater 82 is used as the cooling fluid 78, the heated fluid 80 (i.e., heated boiler feedwater 82) may be directed into the drum of the high-pressure evaporator 50 of the HRSG 32. Within the high-pressure evaporator 50, the heated fluid 80 may be converted into steam, which may ultimately be used in the bottoming cycle of the combined cycle power generation system 10.
In step 96, the cooled bleed air 68 may be expanded within the turbo-expander 72, thereby generating power. In particular, the cooled bleed air 68 may be used to drive blades of the turbo-expander 72, causing rotation suitable for driving any suitable load 74. For example, the generated power may be used to drive an electrical generator for producing supplementary electrical power.
Technical effects of the invention include bleeding compressed air from the compressor 20 of the gas turbine 12 of the combined cycle power generation system 10. The bleed air 68 may be expanded within the turbo-expander 72, thereby generating power which may drive various loads 74, such as an electrical generator. By expanding the bleed air 68 within the turbo-expander 72 to generate power, energy expended within the compressor 20 to compress the bleed air 68 may be recaptured.
In addition, prior to expansion within the turbo-expander 72, the bleed air 68 may be cooled within the pre-cooling unit 76. By reducing the temperature of the bleed air 68, the turbo-expander 72 may be sized to handle lower temperatures, thereby reducing the cost of the turbo-expander 72. More specifically, since there may be no need for expensive, high temperature resistant materials for the turbo-expander 72, a less expensive, standard, off-the-shelf turbo-expander 72 may be used. In addition to the reduced costs, these lower temperature turbo-expanders may also be more readily available with more vendors selling them, quicker turnaround times, and so forth.
Furthermore, heat transferred into the cooling fluid 78 (e.g., the boiler feedwater 82) within the pre-cooling unit 76 may also be recaptured. For example, as described above, the heated fluid 80 discharged from the pre-cooling unit 76 may be directed into the HRSG 32, where heat from the heated fluid 80 may be used to generate steam used in the bottoming cycle of the combined cycle power generation system 10. An advantage of using the boiler feedwater 82 as the cooling fluid 78 within the pre-cooling unit 76 may be that either the same boiler surface area may generate more steam or the boiler surface area may be reduced while maintaining the same steam cycle power.
It should be noted that while embodiments described herein have generally been presented in the context of the combined cycle power generation system 10, the techniques described herein are not limited to combined cycle power generation systems. Indeed, energy may be recaptured from bleed air from compressors of simple gas turbines as well. For instance, the pre-cooling unit 76 and turbo-expander 72 presented herein may cool and expand, respectively, compressed air bled from a compressor of a simple gas turbine in the same manner as compressed air bled from the compressor 20 of the combined cycle gas turbine 12 presented herein. Furthermore, heat within the heated fluid 80 discharged from the pre-cooling unit 76 need not necessarily be recovered within the HRSG 32 of the combined cycle power generation system 10. Indeed, this heat may be recaptured in various other processes, such as standalone boiler systems.
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.
