Patent Application
20120227372
The invention relates to power plants with integrated CO2 capture as well as CO2 capture ready power plants.
In recent years it has become obvious that generation of greenhouse gases lead to global warming and that further increase in greenhouse gas production will further accelerate global warming. Since CO2 (carbon dioxide) is identified as a main greenhouse gas, CCS (carbon capture and storage) is considered one potential major means to reduce the release of greenhouse gases into the atmosphere and to control global warming. In this context CCS is defined as the process of CO2 capture, compression, transport and storage. Capture is defined as a process in which CO2 is removed either from the flue gases after combustion of a carbon based fuel or the removal of and processing of carbon before combustion. Regeneration of any absorbents, adsorbents or other means to remove CO2 of carbon from a flue gas or fuel gas flow is considered to be part of the capture process.
The CO2 capture technology currently considered closest to large-scale industrial application is post combustion capture. In post-combustion capture the CO2 is removed from a flue gas. The remaining flue gas is released to the atmosphere and the CO2 is compressed for transportation and storage. There are several technologies known to remove CO2 from a flue gas such as absorption, adsorption, membrane separation, and cryogenic separation.
All known technologies for CO2 capture and compression require relatively large amounts of energy. There are many publications on the optimization of the different processes and the reduction of the power and efficiency penalty by integrating these processes into a power plant
EP1688173 gives an example for post combustion capture and a method for the reduction of power output penalties due to CO2 absorption, respectively the regeneration of the absorption liquid. Here it is proposed to extract steam for regeneration of the absorbent from different stages of the steam turbine of a power plant to minimize the reduction in the turbine output.
In the same context, WO2007/073201 suggests to use the compression heat, which results from compressing the CO2 flow for regeneration of the absorbent.
These methods aim to reduce the power requirements of specific CO2 capture equipments, however they reduce operational flexibility of the plant
The present disclosure is directed to a power plant including a steam power plant and/or a combined cycle power plant. A water steam cycle of the power plant includes two steam turbine arrangements, the first steam turbine arrangement includes steam turbines with at least two pressure levels and a second steam turbine arrangement having at least one back pressure turbine configured to expand steam to the supply pressure of a CO2 capture system.
According to another embodiment, the disclosure is also directed to a combined cycle power plant including flue gas ducting and a stack. A water steam cycle of the combined cycle power plant includes two steam turbine arrangements. The first steam turbine arrangement includes at least one intermediate pressure turbine configured to expand steam to a back pressure, which is suitable as supply pressure for a CO2 capture system. The second steam turbine arrangement includes a low-pressure turbine with an inlet pressure, which is matched to an outlet pressure of the at least one intermediate pressure turbine, and configured for a steam mass flow of the CO2 capture system to convert thermal energy of outlet steam of the at least one intermediate pressure steam turbine into mechanical energy when the CO2 capture system is not operating.
According to a further embodiment, the disclosure is directed to a method for operating a power plant including a steam power plant and/or a combined cycle power plant. A water steam cycle of the power plant includes two steam turbine arrangements, the first steam turbine arrangement includes steam turbines with at least two pressure levels and a second steam turbine arrangement including at least one back pressure turbine configured to expand steam to the supply pressure of a CO2 capture system. The power plant further includes a CO2 capture system, for removing CO2 from flue gas, flue gas ducting to the CO2 capture system and a stack downstream of the CO2 capture system. The method includes operating the first steam turbine arrangement to produce power during all steady state operating points of the steam cycle and at least a part of the second steam turbine arrangement is bypassed to the CO2 capture system and/or is operated to produce power and to release low-pressure steam to the CO2 capture system during periods when the CO2 capture system is in operation. The method also includes operating both steam turbine arrangements using all available steam to produce power when the CO2 capture system is not in operation.
According to a still further embodiment, the disclosure is directed to a method for retrofitting a power plant including a steam power plant and/or a combined cycle power plant. A water steam cycle of the power plant includes two steam turbine arrangements, the first steam turbine arrangement comprising steam turbines with at least two pressure levels and a second steam turbine arrangement including at least one back pressure turbine configured to expand steam to the supply pressure of a CO2 capture system. The power plant further includes a space required for a CO2 capture system configured to remove CO2 from flue gas of the power plant, the space is arranged to allow retrofitting of the CO2 capture system, and/or the plant includes flue gas ducting and a stack prepared for retrofitting a CO2 capture system. The method includes building the CO2 capture system in the space provided for the CO2 capture system while the power plant is operating. Operation of the power plant is only interrupted for connection of the CO2 capture system (12), and for subsequent recommissioning, where connecting the CO2 capture system includes connecting the low-pressure steam, condensate return, electric power supply, controls, and flue gas ducting and/or modifying the flue gas ducting.
The invention, its nature as well as its advantages, shall be described in more detail below with the aid of the accompanying drawings. Referring to the drawings:
A main objective of the present invention is to provide a fossil fired power plant with a CO2 capture system with increased operational flexibility, which allows operation with high efficiency when a steam extraction for a CO2 capture system is in operation as well as when the CO2 capture system is not operating and no steam is extracted. A method to operate this type of plant is a further objective of the invention. An additional objective is to provide a power plant, which is prepared for future retrofitting of a CO2 capture system, and which is already provided with a steam cycle capable of operation with high efficiency with or without steam extraction.
One main aspect of the invention is to provide a water steam cycle with two steam turbine arrangements. One steam turbine arrangement is operating independently of a CO2 capture operation, and one steam turbine arrangement can at least partially be shut down during a CO2 capture operation.
To this end, a power plant, which comprises a water steam cycle with two steam turbine arrangements, is proposed. The first steam turbine arrangement comprises steam turbines with at least two pressure levels, and the second steam turbine arrangement comprises at least one back pressure turbine designed to expand steam to a back pressure, which is suitable as supply pressure for a CO2 capture system or an intermediate pressure turbine designed to expand steam to a pressure, which is suitable as supply pressure for a CO2 capture system.
To optimize the steam cycle it is advantageous to extract heat in a boiler at different pressure levels. In one embodiment of the invention, the inlet pressure of the second turbine arrangement's back pressure steam turbine is therefore higher or lower than the inlet pressure of the first turbine arrangements intermediate pressure steam turbine. For example the inlet pressure of the second turbine arrangement's back pressure steam turbine is higher than the inlet pressure of the first turbine arrangement's intermediate pressure steam turbine. As a result, the total water steam cycle will therefore effectively have an additional pressure level dedicated to the second steam turbine in order to maximize the plant's performance and or profitability.
In order to efficiently use the second turbine arrangement's back pressure steam turbine's outlet steam at all times, a low-pressure steam turbine designed for the back pressure turbine's outlet pressure and outlet mass flow is part of the second turbine arrangement. It converts the back pressure turbine's outlet steam flow into mechanical energy if the steam is not directed to a CO2 capture system.
Further, the second steam turbine arrangement typically has a smaller design mass flow than the first steam turbine arrangement. In consequence it can be advantageous if the second steam turbine arrangement or the back pressure turbine of the second steam turbine arrangement is operating at a higher frequency than the first steam turbine arrangement. To supply power to an electric grid it is either connected to a generator, which is operating at grid frequency, with a gearbox or is driving a generator, which is running at an elevated frequency. In the latter case the second generator is connected to the grid with the help of a frequency converter, for example a matrix converter.
In another embodiment the power plant is a combined cycle power plant comprising a CO2 capture system for removing CO2 from the flue gas or prepared for retrofitting a CO2 capture system. It comprises flue gas ducting and a stack as well as two steam turbine arrangements. The first one comprising at least one back pressure turbine designed to expand steam to a back pressure, which is suitable as supply pressure for the installed or planed CO2 capture system. The second steam turbine arrangement comprises a low-pressure turbine with an inlet pressure, which is matched to the outlet pressure of the back pressure turbine, and designed for the steam mass flow of the CO2 capture system to convert the thermal energy of the outlet steam of the back pressure steam turbine into mechanical energy when the CO2 capture system is not operating.
This kind of steam turbine arrangement is for example suitable for CO2 capture systems, which are based on Ammines. They typically need relatively large amounts of steam at a low-pressure level. For these the back pressure steam turbine expands to a low back pressure. The low-grade steam has only a relatively low specific enthalpy so that the total mass flow of the back pressure turbine is required for CO2 capture. This allows omission of a low-pressure steam turbine. This arrangement is particularly suitable for combined cycle power plants, where the specific steam mass flow relative to the flue gas mass flow is low. Another embodiment, in which the low-pressure turbine can be omitted, is a CO2 capture system based on Ammonia as absorbent. In this case additional process steam is typically available for extraction.
In a further embodiment the power plant is a combined cycle power plant, which comprises a gas turbine with flue gas recirculation. Part of the flue gas is recirculated into the inlet air or the gas turbine after its useful heat is extracted in a heat recovery steam generator. This increases the CO2 concentration in the flue gas and facilitates CO2 capture from the flue gas.
According to a further embodiment the second turbine arrangement's low-pressure turbine is engageable to the second turbine arrangement's shafting with an overrunning clutch. If steam is provided to the low-pressure turbine, and the low-pressure turbine is operating, the overrunning clutch is engaged, and the low-pressure turbine drives the second generator. If no steam is available during CO2 capture operation the gear can disengage and the steam turbine can remain at standstill.
In one embodiment the first and second steam turbine arrangements are aligned on one common shafting with only one generator between the two steam turbine arrangements.
All embodiments either comprise a CO2 capture system for removing CO2 from the flue gas, flue gas ducting to the CO2 capture system, and a stack downstream of the CO2 capture system or comprise the space required to retrofit such a capture system. CO2 capture system can comprise auxiliaries, such as CO2 treatment, and compression facilities. If the plant is built for later retrofit of a CO2 capture system, the plant and reserved space has to be arranged to allow later connection of the CO2 capture system. Additionally, space for flue gas cooling before CO2 capture, and for construction shall be also reserved for later retrofit if required.
In one embodiment of a plant for future retrofit, the stack is arranged at its final location considering the future CO2 capture system. The flue gas ducting is designed for continuous operation before retrofitting, and can be used as bypass duct once the CO2 capture system is retrofitted. To minimize the impact of retrofitting the CO2 capture system, the flue gas ducting may already comprise a flap, damper or diverter to direct the flue gas to the CO2 capture system, once it is retrofitted. Further, it may comprise a branch connection to return CO2 depleted flue gas from the future CO2 capture system to the flue gas ducting. Once the CO2 capture system is in full operation the part of the original flue gas duct of the power plant, which is downstream of the damper or diverter, becomes a bypass duct.
To take the additional pressure losses of the CO2 capture system into account, space for a flue gas blower shall be provided if applicable. This allows the optimization of the power plant for the initial back pressure of the ducting, which is directly leading the flue gas to the stack. In one embodiment the flue gas blower is installed downstream of the diverter or damper and is only needed for CO2 capture operation.
Besides the flue gas ducting's mechanical interfaces, low-pressure steam piping to the CO2 capture system including condensate return lines, control interfaces between the power plant, and the CO2 capture system are required.
One major advantage of the proposed plant arrangement is the possibility to retrofit or upgrade an existing conventional power plant without CO2 capture to a power plant with CO2 capture without any significant modifications to the existing power plant.
One element of the current invention is a method of retrofitting an existing conventional power plant without CO2 capture to a power plant with CO2 capture. According to this method a CO2 capture system is built next to the existing power plant.
According to an embodiment of this method the CO2 capture system, the additional flue gas ducting, and the necessary steam, power and controls connections are built while the conventional power plant is in normal operation. Operation of the existing conventional power plant is only interrupted for connecting the existing conventional power plant to CO2 capture system. For this only the additional or changed flue gas ducting is connected. Further, the low-pressure steam, condensate, electric power supply, and controls are connected to the CO2 capture system. Further subsequent recommissioning has to be carried out. No or very limited changes or retrofits have to be carried out on the major components of original power plant itself, e.g. on the steam or gas turbine or the boiler because the proposed steam cycle has the provisions for retrofit of the CO2 capture system.
The CO2 capture system can be cold commissioned independently of the operation of the existing power plant. For change over to CO2 capture from the conventional power plant the direction into which the flap, damper or diverter releases flue gas simply has to be changed. Depending on the arrangement the return branch from the CO2 capture system has to be connected to an existing stack.
Interruption of the commercial operation of the existing power plant can thus be minimized. The time needed for reconnecting or changing the flue gas ducts can be reduced below the time needed for a normal scheduled maintenance outage of the existing power plant.
A further subject of this invention is a method to operate a power plant for the combustion of carbon-based fuels with a CO2 capture system as described above. According to this operation method, the first steam turbine arrangement is operated to produce power during steady state operation of the steam cycle. The second steam turbine arrangement is either at least partially bypassed to provide steam for the CO2 capture system or is operated to produce power and to release low-pressure steam to the CO2 capture system (12) during periods when the CO2 capture system (12) is in operation. When the CO2 capture system is not in operation both steam turbine arrangements are operated using all available steam to produce power during steady state operation. All available steam is the steam produced by the boiler or HRSG reduced by steam flows required for operation of the power plant itself, e.g. for fuel preheating, or reduced by steam diverted as process steam, e.g. in cogeneration.
In a further embodiment, the CO2 capture unit is switched off and a low-pressure turbine of the second steam turbine arrangement is brought into operation to produce peak power during periods of high electricity demand or during which CO2 capture is not required or cannot be operated.
To allow a quick start up and/or loading of a low-pressure turbine of the second steam turbine arrangement if additional power is required during periods of high electricity demand, the low-pressure turbine of the second steam turbine arrangement is charged with a minimum steam flow to keep it warm.
This split of the water steam cycle into two parts allows high efficiency operation when steam is used for a CO2 capture system as well as when no steam is used for a CO2 capture system. It allows a flexible operation with CO2 capture and different operating methods depending on the optimization target. Possible optimization targets are for example maximum power, maximum efficiency, and maximum CO2 capture rate.
The steam power plants as described here are typically coal fired steam power plants. However, the invention is also applicable to any other kind of fossil fired steam power plants such as oil or gas fired steam power plants.
The main components of the power plant with CO2 capture according to this invention are a power plant 1, 2 comprising a first steam turbine arrangement 14 and a second steam turbine arrangement 15, and a CO2 capture system 12.
A first example of a plant arrangement according to one embodiment of the invention is shown in
The first steam turbine arrangement 14 comprises a HPT (high pressure steam turbine) 24, an IPT (intermediate pressure steam turbine) 25 and a LPT (low-pressure steam turbine) 26. The HPT 24 and IPT 25 are driven by high-pressure live steam 9 and intermediate pressure live steam 10 from the boiler 3. The LPT 26 is driven by low-pressure steam 11a, which is obtained from the IPT 25, and/or low-pressure live steam 37 from the boiler 3. The arrangement further comprises a generator 5, which produces electric power, and a condenser 18, from which feed water 19 is returned to the boiler 3. The steam cycle is simplified and shown schematically without feed water pumps, and details of the boilers etc., as these elements are not subject of the invention.
The second steam turbine arrangement 15 comprises a back pressure steam turbine 27, and a second LPT (low-pressure steam turbine) 28. The back pressure steam turbine 27 is driven by intermediate pressure live steam 10 from the boiler 3. Alternatively the back pressure steam turbine 27 can be driven by extracted intermediate pressure steam 10′ from the first IPT 25. The steam extraction can be controlled by the steam extraction valve 29. The intermediate pressure steam 10 can be controlled by the steam control valve 38. The second LPT 28 is driven by low-pressure steam 11b, which is obtained from the exhaust steam of the back pressure steam turbine 27 if the CO2 capture system 12 is not in operation. The steam flow to the second LPT 28 is controlled by the second LP control valve 41. The arrangement further comprises a generator 45, which produces electric power, and a condenser 18 from which feed water 19 is returned to the boiler 3. The steam cycle is simplified and shown schematically without feed water pumps, and details of the boilers etc., as these elements are not subject of the invention.
The CO2 capture system 12 is schematically shown as a box, which removes CO2 from the flue gas 4. The CO2 depleted flue gas 16 is released from the CO2 capture unit 12 to a stack 16. In case the CO2 capture unit 12 is not operating, it can be bypassed via the flue gas bypasses 40. To control the bypass flow a bypass flap 21 is provided in the flue gas ducting.
A CO2 capture system 12 comprises for example a CO2 absorption unit, in which CO2 is removed from the flue gas 4 by an absorbent, and a regeneration unit, in which the CO2 is released from the absorbent. Depending on the temperature of the flue gas and the operating temperature range of the CO2 absorption unit a flue gas cooler 6 might also be required. The captured CO2 is sent for compression and storage 17. Typically a large amount of thermal energy is required by the CO2 capture system 12 for its regeneration unit. This is supplied to the CO2 capture system as low-pressure steam 11c from the back pressure steam turbine 27. The low-pressure steam 11c flow to the CO2 capture system 12 is controlled by the capture system control valve 42. Condensate or low-grade steam is returned from the CO2 capture system 12 to the water steam cycle in the condensate return 13.
In this example the back pressure steam turbine 27 is not fed by intermediate steam 10 with the same pressure level as the IPT 25 of the first steam turbine arrangement 14 but with elevated intermediate pressure steam 20 at an elevated pressure level compared to the intermediate pressure of the first steam turbine arrangement 14. The flow of elevated intermediate pressure steam 43 to the back pressure steam turbine 27 can be controlled by the steam control valve 38. By using two different pressure levels for the intermediate pressure steam turbines the water steam cycle effectively becomes a four pressure level cycle. This allows better utilization of the extracted heat and leads to a higher overall efficiency.
The gas turbine 30 comprises a compressor 31, in which inlet air 7 is compressed, a combustor 32, and a turbine 33 and drives a generator 5. The compressed gas is used for combustion of the fuel 8 in the combustor 32, and the pressurized hot gas expands in the turbine 33. Its main outputs are electric power from the generator 5, and hot flue gas 34. The hot flue gas 34 passes through the heat recovery steam generator (HRSG) 39, which generates live steam 9, 10, 37. The flue gas leaves the HRSG 39 at a lower temperature level and is directed to the CO2 capture system 12.
In this example, gas turbine is shown as a gas turbine with flue gas recirculation. A controllable fraction of the flue gas is diverted by a damper, or a control flap and/or a blower (not shown) for flue gas recirculation 22, and recirculated to the inlet air 7 via the flue gas recirculation line 35. The recirculated flue gas is cooled in the flue gas cooler 36 to limit or control the inlet temperature of the gas turbine compressor 31. The flue gas cooler 36 typically comprises a condensate separator (not shown), which removes condensate from the cooled flue gas.
Further, the combined cycle power plant 2 comprises a first and second steam turbine arrangement 14, 15 analogues to those shown in
The second steam turbine arrangement 15 is also simplified and only consists of a LPT 28. It is arranged along the same shaft as the first steam turbine arrangement 14. If the LPT 28 is operating it is engaged to the generator 5 of first steam turbine arrangement 14 by an overrunning clutch 23. If the low-pressure steam 11c is used for the operation of the CO2 capture system 12 the second steam turbine arrangement 15 is off-line and the overrunning clutch 23 is disengaged.
The low-pressure steam 11b flow to the second LPT 28 is controlled by the second LP control valve 41. The low-pressure steam 11c flow to the CO2 capture system 12 is controlled by the capture system control valve 42.
A further embodiment of the invention is shown in
Exemplary embodiments described above and in the drawings disclose to a person skilled in the art embodiments, which differ from the exemplary embodiments and which are contained in the scope of the invention.
For example all Figures show power plants 1, 2 including CO2 capture systems. To provide a capture ready power plant 1, 2 according to the invention the same arrangement and two steam turbine arrangements 14, 15 can be used. Simply, the CO2 capture system 12 including the flue gas cooler 6 is omitted and replaced by space for future retrofit. The flue gas ducting, which is needed to connect the CO2 capture system and the steam line, which provides the CO2 capture system 12 with low-pressure steam 11c, can be omitted or provided with blind flanges for later connection.
In the examples given above, single combustion gas turbines are described. It is to be understood that sequential combustion gas turbines, also called gas turbine with reheat combustor, as described for example in U.S. Pat. No. 5,577,378, can equally be used. A combination of sequential combustion gas turbine and singe combustion gas turbine based power plants can also be used. The application of sequential combustion gas turbines can be advantageous, as the CO2 concentration in their flue gas is typically higher that in single combustion gas turbines.
In the examples no single shaft power trains are shown for the combined cycle power plants. However, arrangements with single shaft power trains in which the first steam turbine arrangement 14 and or the second steam turbine arrangement 15 can be coupled by an overrunning clutch to the gas turbine's generator 5 are equally conceivable.
Further, the term boiler should not be used in a restrictive manner. For example supercritical steam generators are also to be considered as boilers in the context of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 09171636.5 | Sep 2009 | EP | regional |
This application is a continuation of International Application No. PCT/EP2010/064469 filed Sep. 29, 2010, which claims priority to European Patent Application No. 09171636.5, filed Sep. 29, 2009 the entire contents of all of which are incorporated by reference as if fully set forth.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2010/064469 | Sep 2010 | US |
| Child | 13431400 | US |
