Field of Invention The invention pertains to a combined cycle power plant with a carbon dioxide capture system and a system for cooling and heating power plant flue gases. It furthermore pertains to a method of operating such power plant.

Background Art

Fossil fuel fired power plants such as combined cycle power plants with a system for the capture of C02 from the exhaust gases have been described in the literature.

A C02 separation process requires additional energy for the separation process itself, for example for the regeneration of a C02 absorption solution, as well as for the cooling of flue gases or the treatment of wastewater resulting from the separation process. These energy requirements reduce the overall efficiency of the power plant.

A C02 capture system to separate the C02 from gas turbine flue gas typically operates at a temperature lower than the temperature of the flue gas when exhausted by a heat recovery steam generator. The operating temperature can be as low as ambient temperature. In order to ensure optimal operation of the C02 capture system, the flue gases must therefore be cooled prior to the C02 removal process. GB245266, for example, discloses a fossil fuel fired power plant with a C02 capture system, specifically a system using a chilled ammonia solution to remove the carbon dioxide from flue gases containing desulphurised water-vapor. The plant contains two apparatuses arranged in series to cool the flue gases prior to their C02 removal treatment. The condensate resulting from one of the cooling apparatuses is used for the C02 regenerator within the C02 capture system and optionally for the heating of clean flue gas. Summary of Invention

It is an object of the invention to propose a combined cycle power plant with a C02 capture system that is improved over those of the prior art in regard to the heat balance and overall power plant efficiency.

It is a further object of the invention to propose a method for the cooling of the flue gases containing C02 that is improved over those of the prior art.

A combined cycle power plant comprises one or more gas turbines and a heat recovery steam generator HRSG, which receives the exhaust gases from the gas turbine, one or more steam turbines driven by steam generated in the HRSG, and furthermore a C02 capture system to remove the C02 contained in the flue gas when exhausted by the HRSG. The exhaust gases are typically received from the gas turbine after expansion to ambient pressure, respectively after expansion to a pressure, which is equal to the ambient pressure plus the pressure drop of the exhaust system including heat exchangers, and the C02 capture system. According to the invention, the combined cycle power plant comprises a system for controlling the temperature of the C02 containing flue gases exhausted by the HRSG and of C02 lean flue gases resulting from the C02 capture system and being directed to a stack. The control system comprises in particular one single apparatus for cooling C02 containing flue gases exhausted by the HRSG arranged prior to the C02 capture system, where this cooling apparatus is configured to cool the flue gases having a temperature from 70 to ^~ \ 40 °C down to a temperature from 10 to 50 °C and where the control system is configured and arranged to cool the C02 containing flue gas by transfer of the heat from the C02 containing flue gas to C02 lean flue gas exiting from the C02 capture system and to heat the C02 lean flue gas having a temperature from 0 to 60 °C to a temperature from 40 to 80 °C.

A power plant with a temperature control system according to this invention allows the cooling of the C02 containing flue gases from the HRSG to a temperature suitable for the treatment of the flue gases in the C02 capture system including temperatures below ambient temperatures, where in particular the cooling process is realized by one single apparatus. By this manner, an additional auxiliary cooler can be avoided, which would otherwise require additional electric power to operate and thereby would reduce the overall performance of the power plant. The temperature control system ensures sufficient cooling capacity for the cooling of the C02 rich flue gas to temperatures below the ambient temperatures, which is lower than the lowest possible temperature reachable by using the power plant main cooling water. This effects a particular advantage in a power plant operating in hot climates, where the main cooling water of the power plant subsequently has a higher temperature. The temperature control system according to the invention allows a cooling of the flue gases also in such hot ambient conditions without the necessity of energy consuming

refrigeration. The power plant furthermore allows a full recovery of the heat gained from the cooling of the C02 rich flue gas and its full use in the heating process of the C02 lean gas. The full heat recovery and its use for operations within the power plant effects a minimization of the performance loss due to the C02 capture process. In particular, the power plant enables the use of the heat from the C02 rich flue gas for a reheating of the C02 lean flue gas.

Such reheating of the C02 lean flue gas ensures sufficient buoyancy of the gas for its release through a stack. No additional heat is required to be extracted from the power plant for this reheating process, and the overall performance of the plant can be upheld. The temperature control system according to the invention is operable over a large range of temperatures, which allows its application in power plants operated with different types of fossil fuel and with several different types of C02 capture systems. The control system is able to handle an input of flue gases from the HRSG having a temperature in a range from 70-140°C. This can include flue gases resulting from fuels with different sulfur contents, depending on the fuel specification.

Furthermore, the system can be applied to power plants with different types of C02 capture systems in that it enables the cooling to a temperature range that would allow a treatment of the flue gases by different types of C02 capture. For example, a cooling to a temperature of 10°C allows a removal of C02 by means of chilled ammonia, whereas a cooling to a temperature of 50 °C allows a removal by means of an amine capture system.

In a first embodiment of the invention, the single apparatus for the cooling of C02 containing flue gas is a first heat exchanger configured and arranged for a heat transfer from the flue gas to a water flow. The temperature control system further comprises a second heat exchanger configured and arranged for a heat transfer from a water flow to the C02 lean flue gas produced by the C02 capture system. Lines for the water are arranged to form a closed loop from the first heat exchanger to the second heat exchanger and from the second to the first heat exchanger. In a first variant, the heat exchangers are direct contact heat exchangers, which by its nature allows a minimized pinch temperature and thereby allows a broad achievable temperature range with a fluid having a given temperature.

In this temperature control system, the hot water resulting from heat exchange with the flue gas in the direct contact cooler is pumped to a direct contact heater for the C02 lean flue gas, where the flue gas is heated prior to its release to the atmosphere through a stack. Part of the water content in the C02 rich flue gas may condense in the direct contact cooler. In the process of reheating the C02 lean flue gases in the direct contact heater, this water will again evaporate. The circulation of the water between the direct contact cooler and heater the amount of water condensed in the cooler can be balanced with the amount of water evaporated in the heater. By this measure, the amount of blow down flow from the direct contact cooler can be reduced. The flow of blow down to be handled by a system for treating the blow down as wastewater is thereby reduced.

Additionally, the need for makeup water is also reduced.

Alternatively, in a second variant, the heat exchanger for both cooling and heating the flue gas are realized as shell and tube heat exchangers.

In a further embodiment of the invention, the temperature control system comprises a gas-gas heat exchanger for the cooling of C02 rich flue gas configured and arranged for heat to be transferred from the C02 rich flue gas to the C02 lean flue gas from the C02 capture system. This embodiment has the advantage that no water circuit is necessary to realize the heat exchange. Additionally, as is the case in a direct-contact heat exchanger, a broad temperature range may be achieved with given temperatures of the gases flowing through the heat exchanger.

In an embodiment the second heat exchanger, i.e. the heat exchanger for reheating the C02 lean flue gas, comprises two heat exchangers arranged in series (in flow direction of the flue gas) is an arrangement of a direct contact heat exchanger (6) and a shell and tube heat exchanger (7) in series, wherein the direct contact heat exchanger (6) is arranged to receive C02 lean flue gas from the C02 capture system (5) and the shell and tube heat exchanger (7) is arranged to receive C02 lean flue gas from the direct contact heat exchanger (6), and wherein the water flow (4") from the first heat exchanger (4, 14) first passes through the a shell and tube heat exchanger (7), and then through the direct contact heat exchanger (6), before being returned to the single apparatus (4, 14) for the cooling of C02 rich flue gas. This arrangement allows reheating the C02 lean flue gases in a highly efficient direct contact heat exchanger. The further heating in the a shell and tube heat exchanger (7) allows to further heat the C02 lean flue gas without further evaporation of water and thereby reduces the relative humidity of the C02 lean flue gases to assure a release of the C02 lean flue gases to the atmosphere without condensation in the stack. In a further variant of this embodiment this arranged the shell and tube heat exchanger is arranged on top of the direct contact heat exchanger and a stack is arranged on top of the direct contact heat exchanger. This arrangement allows the use of a substantially shortened stack. Typically, such a stack has less than half the height of a freestanding stack. Further, the required plant area can be reduced.

In a particular variant of the embodiment above, the power plant further comprises a flue gas recirculation line, which directs flue gas from the HRSG back to the gas turbine compressor inlet. This generates an additional efficiency gain for the power plant as a whole, because the percentage C02 present in the flue gas effectively treated in the C02 capture system is greater than in a power plant without flue gas recirculation. Thereby the C02 capture operation is more energy efficient per mass of C02 captured.

In a method of operating a combined cycle power plant with a C02 capture system according to the invention, flue gas rich in C02 exhausted by a heat recovery steam generator of the CCPP is directed through and cooled in one single heat exchanger, wherein it is cooled from a temperature from 140-70 °C down to a temperature in the range from 10-50°C, and the flue gas is then treated within a C02 capture system to remove the C02 and produce a C02 lean flue gas flow and pure C02 gas flow. In particular, all the heat removed from the C02 rich flue gas is used to heat the C02 lean flue gas. The heat transfer is performed either by direct contact heat transfer between water and the flue gases or by tubes in a shell and tube heat exchanger.

According to another embodiment of the method, the C02 rich flue gas is cooled in the direct contact by the water flow and thereby the C02 rich gas flow is washed in order to condition the C02 rich gas flow for C02 removal within the C02 capture system. In a further embodiment the C02 is removed by chilled ammonia process within the C02 capture system and an ammonia slip from the chilled ammonia process is washed from the C02 lean flue gas flow by the direct contact heater with the water flow when reheating C02 lean flue gas for release through the stack. Brief Description of the Figures

Figure 1 shows a schematic of a first embodiment of the power plant according to the invention comprising direct contact heat exchangers for the cooling and heating of flue gas.

Figure 2 shows a schematic of a second embodiment of the power plant according to the invention comprising shell and tube heat exchangers for the cooling and heating of flue gas.

Figure 3 shows a schematic of a third embodiment of the power plant according to the invention comprising a gas-gas heat exchanger for the cooling and heating of flue gas.

Figure 4 shows a schematic of a forth embodiment of the power plant according to the invention comprising direct contact heat exchangers for the cooling and heating of flue gas and comprising shell and tube heat exchangers for additional heating of the flue gas.

Same numerals in different figures indicate same elements.

Detailed Description of the Invention

Figure 1 shows in simplified schematic a combined cycle power plant 1 with a power generating unit 2 containing at least one gas turbine, at least one steam turbine, and a heat recovery steam generator HRSG 3, which receives the exhaust gases from the gas turbine and uses for generating or heating steam. The exhaust gases released from the HRSG, having a temperature in the range from 140 to 70 °C, are directed via line 3' to a first direct contact cooler 4, where it is brought into contact with water, which is provided via line 4" from a direct contact heater 6 and cools the flue gas down to Ι Ο-δΟ 'Ό. In order to facilitate a cooling within the given temperature ranges, the direct contact cooler is configured with standard features.

The thus cooled flue gas is then directed via line 4' to a C02 capture system 5, which separates the C02 from the flue gas by means of a chilled ammonia process or by means of an amine capture process using a C02 absorption solution. The C02 capture system 5 generates a C02 lean flue gas flow, which is directed via line 5' to a second direct contact heat exchanger and heater 6. A line 5" directs the pure C02 gas to a system C for C02 compression and further treatment. In the second heat exchanger 6, the C02 lean flue gas is heated back to a temperature of between 40 and 80 °C and directed via line 6' to a stack S. Having a temperature within that range it will have sufficient buoyancy in order enter the atmosphere via the stack S at the given ambient conditions.

In some climate situations a higher temperature is necessary to ensure the buoyancy. For such situations, a further heat exchanger 7 arranged in series with the heater 6 can be operated using heat from another source such as steam condensate from the C02 absorption solution reboiler, HRSG feedwater extraction, hot cooling water from a C02 compressor, or a steam extraction a steam turbine the HRSG.

The water condensing at the bottom of the direct contact heater 6 is collected and directed via a line 6" and by means of a pump back to the direct contact cooler 4. The lines 4" and 6" form the circuit for the water serving as heat transfer medium for both heating and cooling in the control system 1 . They can be additionally connected by recirculation lines 8 or 9 or both lines 8 and 9, which connect the return water lines 4" and 6". They can be activated by means of valves 8' and 9' in order to regulate the rate of cooling and heating in the system. For example, valve 8' is opened in order to generate a short circuit, which recirculates some of the warmer fluid returning from the first heat exchanger 4 to line 6" and back into the same exchanger 4 and thereby cools the flue gas in heat exchanger 4 to a lesser degree. This may be necessary in the case of load changes in the power plant. On the other hand, for example in the case of changing ambient conditions in the atmosphere, valve 9' can be opened to bleed some of the cooler fluid returning from the second exchanger 6 to line 4" and back into the same exchanger 6 and thereby decreasing the degree of heating the C02-lean flue gas in that second heat exchanger. Optionally, additional valves 15 and/or 17 can be provided to better control the flow spit between the short circuits in the lines 8, respectively 9. The valve 15 may be slightly closed when the valves 9' is opened to generate a sufficient pressure drop for good control. The valve 17 may be slightly closed when the valves 8' is opened to generate a sufficient pressure drop for good control.

Blow down from the direct contact cooler is led via line 1 1 to a wastewater treatment system 12. Due to the circulation of the cooling and heating water between the two direct contact heat exchangers 4 and 6, the amount of blow down is reduced and the system 12 can be designed for smaller volumes or be operated at a lower level.

A makeup water tank and makeup waterline 13 are provided to replenish the flow of heat transfer medium due to the losses in the blow down.

In an exemplary operation of the power plant, C02 rich flue gas from the HRSG 3 with a temperature of 140 °C is cooled in heat exchanger 4 to a temperature of 50 °C. The water resulting from such cooling in line 4" will have a temperature of 95 °C and is used in direct contact heater 6. C02 lean flue gas resulting from the C02 capture system using an amine capture process has a temperature of 40 °C. In order to ensure its successful release to the atmosphere via the stack of a given design under all ambient conditions, a temperature of 40 ° to 80 °C is required, which can be provided by heating with the hot 95 °C water in line 4" from the cooler 4. However, if the temperature of the flue gases from the HRSG 3 drop, and less heat is transferred to the water in heat exchanger 4, an auxiliary heat exchanger 10 may be activated to support the heating in heater 6. Instead of heat exchanger 10, a further auxiliary heat exchanger 7 may be activated to complete the heating to 80 °C of the flue gas for the stack. The recirculation valve 8' can be used to control the temperature of the water passing through the direct contact cooler, and the recirculation valve 9' may be used to control the temperature of the water passing through the direct contact heater. Figure 2 shows a variant of the power plant of figure 1 . It comprises power generating unit 2 and HRSG 3, which exhausts its flue gases to line 3 and to a heat exchanger for cooling. The heat exchanger is a shell and tube cooler 14 arranged to receive cooling water via line 16" from a heater 16. After being cooled in heat exchanger 14 to a temperature from 10 to 50 °C the C02 rich flue gas is directed via line 14' to the C02 capture system 5, and C02 lean flue gas is led through line 5' to a second heat exchanger, a shell and tube heater 16, which receives its heating water via line 14" from cooler 14. As in figure 1 , the heated C02 lean flue gases are released to the atmosphere via line 16' and stack S. Again, depending on the particular climate conditions, an auxiliary heat exchanger 7 may be operated to augment the heating of the flue gases using heating sources as mentioned in connection with figure 1 .

The heat balance is similar as for the temperature control system in figure 1 . A finer temperature control can be activated by means of recirculation lines 18 and /or 19 and recirculation valves 18' and/or 19'. Recirculation line 18 leads from the water line 14" to water line 16" and can be activated by means of valve 18' in case of necessary temperature adjustments. Recirculation line 19 leads from the water line 16" to water line 14", which recirculates cooler water back to the heater 16, and thereby lowers the heating effect in case, for example, of a decrease in ambient temperatures.

Condensate resulting from the cooling in heat exchanger 14 is collected and led through line 21 to a cooling tower 22 or any suitable operation of the power plant.

Water flow volume in lines 14" and 16" are replenished as needed via a tank and line 23. The tank serves as an expansion vessel and ensures a constant pressure in the water circuit system. Figure 3 shows a further power plant 1 with a flue gas temperature control system according to the invention. A gas turbine GT receives ambient air A via a gas turbine compressor inlet and exhausts its flue gases to a heat recovery steam generator 3, which generates steam to drive a steam turbine ST. The C02 rich flue gases from the HRSG 3 are directed through line 3' to an optional air quality control system 30, which can limit corrosion in a gas-gas heat exchanger to follow. A gas-gas heat exchanger 31 is arranged to receive the flue gases through line 30' and cools them to a temperature down to 10 to 50 °C depending on the type of C02 capture system 5. The C02 lean flue gas flow generated in the system 5 is led through line 5' back through the gas-gas heat exchanger 31 for reheating. This heats the gases to a temperature from 40-80°C such that they can be released through the stack S.

The gas-gas heat exchanger 31 can be any one of several types. In a regenerative type gas-gas heater having a rotating membrane, the pressure difference between the two gas sides is controlled to be as low as possible.

Further possible heat exchangers for the heating of the C02-lean flue gas are a standard shell and tube type heat exchanger or a heat pipe type exchanger. Figure 4 shows another exemplary embodiment based on Figure 1 . In order to ensure that the humidity of the flue gasses is sufficiently low for its release to the atmosphere via the stack of a given design under all ambient conditions, an auxiliary heat exchanger 7 is arranged downstream (in the flue gas glow) of the direct contact heater 6. While the C02 lean flue gas 5' are heated and typically humidified close to 100% relative humidity in the direct contact heater 6 the auxiliary heat exchanger 7 allows a further heating without evaporation of water. Thereby the humidity of the C02 lean flue gas leaving the direct contact heater 6 is reduced before it is sent to the stack S. The hot cooling water 4" from the direct contact cooler 4 is first feed to the auxiliary heat exchanger 7 and then used in the direct contact heater 6 to heat the C02 lean flue gas 5'.

Optionally a further auxiliary heater 7 with an additional heat source such as for example low-grade steam or cooling water may be provided. Depending on load and ambient conditions the further auxiliary heat exchanger 7 may be activated to complete the heating of the flue gas for the stack.

The power plant 1 can furthermore comprise a flue gas recirculation line 40, which directs a partial flue gas flow exhausted from the HRSG 3 back to the gas turbine compressor inlet together with ambient air A. A flue gas cooler 41 may be activated depending on the operation mode of the gas turbine.

The flue gas recirculation effects a higher C02 concentration in the flue gas directed to the C02 capture system.

Terms used in the Figures

1 power plant

2 power generating unit (Gas turbine, steam turbine, generator)

3 heat recovery steam generator HRSG

3' line for hot C02 rich flue gas

4 direct contact cooler

4' line for cooled C02 rich flue gas

4" line for hot cooling water resulting from cooler 4

5 C02 capture system

5' line for C02 lean flue gas

5" line for C02 gas

6 direct contact heater

6' line for heated C02 lean flue gas

6" line for cool water resulting from heater 6

7 further heat exchanger

8, 9 recirculation line

8\9' recirculation valve

10 heat exchanger to support heater

1 1 line for blow down

12 wastewater treatment system

13 line for makeup water

14 shell and tube heat exchanger

14' line for cooled C02 rich flue gas

14" line for hot cooling water resulting from cooler 14

15 valve

16 shell and tube heat exchanger

17 valve

18,19 recirculation line

18', 19' recirculation valve

20 heat exchanger to support heater

21 line for condensate

22 cooling tower or wastewater system

23 line from head tank

30 air quality control system

30' line for hot flue gas

31 gas-gas- heater 31 ' line for cooled flue gas

32 line for heated flue gas

40 flue gas recirculation line

GT gas turbine

ST steam turbine

A ambient air

C C02 compressor

S stack