Technical Field

The invention relates to a system for supplying energy to a C0 ₂ separation unit arranged to a power plant for desorption of CO2 of exhaust gases emitted from the power plant.

Also, the invention relates to a CO2 separation unit comprising the system for supplying energy thereto.

Further, the invention relates to a use of the system for supplying energy to a CO2 separation unit.

Background of the invention

Large-sized combined cycles will contribute to a great part of the future electricity market due to their high efficiency, relatively low investment cost and short construction time. Future legislation and political incentives may render that owners would be forced to either implement carbon capture and storage (CCS) or switch to bio-fuels in fossil fuel power plants.

Large-sized power plants would consume enormous amounts of bio-mass, which require large areas for growing and may cause logistical issues, considering this CCS might be more suitable. However, a separation unit requires energy for the process of capturing CO ₂ . The energy required is commonly supplied from the power plant itself, resulting in that the total efficient of the power plant will decrease. Or in other words, the amount of power from the power plant produced per unit fuel will be reduced when carbon capture is implemented. A major global task is to reduced global warming caused by CO2 emissions from combustion of fossil fuel, hence carbon capture is essential.

A combined cycle power plant (CCPP) is a combination of a Brayton cycle, for example a gas turbine, and a Rankine cycle, which is a steam cycle. A typical CCPP is based on a gas turbine, a heat recovery steam generator (HRSG) and a steam turbine. The hot exhaust from the gas turbine is used to generate steam in the HRSG for the steam cycle. In the steam cycle, water is pumped up to a high pressure before it enters the heat recovery steam generator where it is heated, evaporated and superheated. The steam expands in a turbine to a low pressure, at which it is condensed by a cooling medium, for example sea water is often used.

A state of the art CCPP includes three pressure levels in the steam cycle, which means that water is evaporated at three pressure levels. Such plants can achieve an electric efficiency close to 60 %. In general, CCPP:s are fired with natural gas, which is a fossil fuel, and therefore produce CO ₂ emissions. A CCPP based on three pressure levels is more complicated, for example it takes longer time to start up. Also, these type of CCPP:s are more expensive to build and more bulky in size compared to a CCPP having one or two pressure levels.

A CO ₂ separation unit, as shown in Fig. 1 , comprises an absorber, a stripper, a heat exchanger and a reboiler. Exhaust gas leaving a CCPP has to be cooled before entering the absorber in which a medium chemically absorbs or adsorbs the CO2 from the flue-gas. At a top of the absorber the treated flue-gas is let out into the atmosphere. At the bottom of the absorber the CO2-rich

absorbent/solvent-mixture is pumped via a heat exchanger to the stripper, where the CO2 is released from the absorbent/solvent as the chemical link between them is broken. This process requires heat that is normally added through the condensation of steam. The separated CO ₂ leaves the stripper. As the steam condenses, it is mixed with the absorbent/solvent and pores down to the bottom of the stripper, where it is collected into the reboiler. Heat is added from an external source to evaporate some of the water content and provide steam to the stripper. From the reboiler a lean absorbent/solvent is fed to the heat exchanger via the stripper and back to the absorber to close the cycle of the desorption process.

Regardless of the absorbent used, a large amount of heat is required for the CO2 desorption process. The most reasonable source for the heat supply is the power plant itself. However, the way of integration the desorption process is important for the total power output.

The performance penalty of the power plant can be reduced by integrating a CO ₂ separation unit into an existing natural gas-fuelled combined cycle (NGCC), however this integration will require major expensive modifications of the plant. One drawback raised, is that about sixty percent of the steam will be condensed at a higher pressure level in the reboiler resulting in that the last section of the heat recovery steam generator (HRSG) becomes inefficient due to a reduced flow on the cold side.

The heat is normally added through the condensation of steam. A known way is to extract steam before the low-pressure turbine and use the latent heat to supply the required energy. Then, a significant amount of steam must be extracted from the steam cycle to cover the heat duty of the CO2 separation unit. The condensate is returned at a much higher temperature than the feed-water leaving the main condenser. For a conventional steam power plant this can be accounted for in the feed-water preheating. For a combined cycle power plant this will result in an undesirably high feed-water temperature. The effect of this is that less energy can be recovered from the flue-gas. Fig. 2 shows a temperature to heat flux diagram i.e. the temperature profile of a HRSG of a CCPP, where an upper line A represents the exhaust gas, which enters at the hot side. The lower line has three sections, representing, when starting from the cold side, section B: water is heated until saturation in the economizer, section C: evaporation to steam at constant temperature in the evaporator, and D: superheating of the steam in the super-heater.

The hatched area in Fig. 2 shows the reduction of recovered heat (ΔΟ.) due to the increased feed-water temperature when a C0 ₂ separation unit is integrated. Ti is the feed-water temperature without CO ₂ capture and T ₂ is the feed-water temperature when C0 ₂ capture is implemented.

The straightforward and widely used method is to extract steam from before the low-pressure turbine and use the latent heat to supply the required energy. In WO 2007/012143 A1 it is suggested that the heat that normally is rejected in the flue-gas cooling device prior to the CO ₂ separation unit should be recovered and used in the absorbent regeneration process. However this heat is limited and at a low temperature and will therefore require a heat pump in order to fulfil a part of the reboiler heat duty.

It is desirable to reach a high efficient of a CCPP and simultaneously keep the emission of CO ₂ controlled, while the construction of the power plant should be simple and comparatively inexpensive.

Summary of the invention

An object of the present invention is to mitigate or eliminate one or more deficiencies and disadvantages of the prior art, singly or in any combination.

In a first aspect, the invention relates to a system provided for recovering energy from a power plant for supplying energy to a CO ₂ separation unit having a reboiler. The CO ₂ separation unit is arranged to the power plant for capturing CO ₂ of exhaust gases emitted from the power plant. The system comprises tubings containing a heat carrier and is arranged between a heat recovering steam generator (HRSG) of the power plant and the reboiler. The tubings are connected to a first additional heat exchanger, which is arranged within the reboiler, and the HRSG and are forming a circuit.

In a first embodiment, the tubings of the system are arranged for bringing the heat carrier of the tubings in fluid communication with feed-water of an economizer of the HRSG, in such way that one of the tubings is connected to the inlet of the feed-water and the other one is connected to the outlet of the feed- water. The heat carrier according to the first embodiment is then water, or a mixture of water and steam. In a second embodiment, a second additional heat exchanger is arranged within the HRSG and is connected to the tubings forming a closed circuit comprising the first additional heat exchanger of the reboiler. In the second embodiment the heat carrier is water, or a mixture of water and steam, or is any suitable medium, for example a heat resistant oil. The system having the closed circuit makes it suitable for retrofitting of existing power plants.

The system will supply energy to the reboiler, which partly is supplied by water extracted from the economizer and partly by steam extracted from a steam turbine of the power plant. The system comprises control means for controlling the temperature of the heat carrier of the tubings.

In a second aspect, the invention relates to a CO2 separation unit for capturing CO ₂ of exhaust gases emitted from a power plant. The CO2 separation unit comprises a system as described above for recovering energy from the power plant for supplying energy to said CO ₂ separation unit.

In a third aspect, the invention relates to use of a system as described above for supplying energy to a C0 ₂ separation unit.

In a fourth aspect, the invention relates to a power plant comprising a CO ₂ separation unit for capturing CO ₂ of exhaust gases emitted from said power plant.

Further objects, features and advantages of the present invention will appear from the following detailed description, from the attached drawings and from the dependent claims.

Brief description of drawings

In order to explain the invention, embodiments of the invention will be described below with reference to the drawings, in which:

Fig 1 is a schematic view of an absorption CO ₂ separation unit,

Fig 2 is a diagram showing the heat flux as a function of the feed-water temperature,

Fig 3 shows a system according to a first embodiment of the invention, Fig 4 shows a system according to a second embodiment of the invention, and

Fig 5 is a diagram showing excess energy recovered in the economizer. Same reference numerals have been used to indicate the same parts in the figures to increase the readability of the description and for the sake of clarity.

Description of embodiments of the invention

Below, embodiments of the invention will be described. These embodiments are described in illustrating purpose in order to enable a person skilled in the art to carry out the invention and to disclose the best mode. However, the embodiments do not limit the invention. Moreover, other combinations of the different features are possible within the scope of the invention.

A typical post-combustion absorbent/solvent-based C0 ₂ separation unit, herein called a C0 ₂ separation unit, is firstly described for a better understanding of the invention.

Fig 1 shows a schematic view of a C0 ₂ separation unit 10, known in the art. The C0 ₂ separation unit 10 comprises a first column 11 , called an absorber, a second column 12, called a stripper, a heat exchanger 13 and a reboiler 14. The reboiler has a heat exchanger 15 for recovering energy from steam 16 from a steam turbine of a power plant, e.g. a CCPP (not shown).

A flue-gas cooler 17 is arranged between the CCPP and the C0 ₂

separation unit 10 and is connected to the absorber 1 . Exhaust gas or flue-gas

18 leaving a CCPP are still warm and needs to be cooled by the condenser 17 to a temperature appropriate for an absorbent used in the C0 ₂ separation unit 10. The cooled flue gas 18 enters the absorber 11 at a bottom thereof. The liquid absorbent/solvent mixture contained in the absorber 11 chemically absorbs or adsorbs the C0 ₂ from the flue-gas. At a top of the absorber 11 the treated flue-gas

19 is let out into the atmosphere. At the bottom of the absorber 11 the now C0 ₂ - rich absorbent/solvent-mixture 21 is pumped via the heat exchanger 13, in which the mixture is heated, by a lean absorbent/solvent to the top of the stripper 12. In the stripper, the C0 ₂ is released from the absorbent/solvent as the chemical link between them is broken.

This process requires heat that is normally added through the

condensation of steam. The separated C0 ₂ 20 leaves the stripper 12 for further treatment at the top of the stripper 12. As the steam condenses, it is mixed with the absorbent/solvent and pores down to the bottom of the stripper 12 where it is collected into the reboiler 14. Heat is added from the steam 16 extracted from the steam turbine to evaporate some of the water content and provide steam to the stripper 12. From the reboiler 14 a lean absorbent/solvent 22 is fed via the stripper 12 and the heat exchanger 13 back to the absorber 11 for closing the cycle of the desorption process.

Fig. 3 shows a first embodiment of the invention, illustrating a system for recovering energy of a power plant (not fully shown) for supplying energy to a C0 ₂ separation unit 10 arranged to said power plant, which comprises a heat recovering steam generator (HRSG) 30 having an economizer 31. The system comprises tubings 32, 33 forming a first circuit with the HRSG 30 and the reboiler 14. The tubings 32, 33 are connected to the economizer 31 and are arranged to be in communication with the feed-water to the HRSG 30 through a connection 37 to the inlet feed-water 34 and a connection 38 to the outlet feed-water 35, respectively. The connection 37 represents the cold or less hot side of the feed- water and the connection 38 represents the hot side thereof. The reboiler 14 has a first heat exchanger 15 and a second heat exchanger 36. The first heat exchanger 15 is in communication with the steam turbine (not shown) of the power plant receiving heat from steam 16 thereof. The second heat exchanger 36 is a part of the first circuit and is connected to the tubing 32 and 33, respectively. The tubings 32 and 33 are containing a heat carrier of water or a mix of water and steam.

Saturated water is extracted after the economizer 31 and is fed to the reboiler 14 by the tubing 32. Then, heat is rejected to the absorbent of the CO2 separation unit 10, thereafter the water is returned via the tubing 33 to the economizer 31. The economizer is a heat exchanger adapted for transferring heat from the flue-gas to the feed-water. The feed-water is flowing within pipes of the economizer 31 and is absorbing heat from the flue-gas flowing outside of the tubes.

Fig. 4 shows a second embodiment of the invention, illustrating a system for recovering energy of a power plant (not fully shown) for supplying energy to a CO2 separation unit 10 arranged to said power plant, which comprises a heat recovering steam generator (HRSG) 30 having an economizer 31. The system comprises tubings 40, 41 forming a second circuit with the HRSG 30 and the reboiler 14. The tubings 40, 41 are connected to an additional heat exchanger 42 of the HRSG 30 and the second heat exchanger 36 of the reboiler 14. This circuit forms a closed circuit between the HRSG 30 and the reboiler 14. The first heat exchanger 15 of the reboiler 14 is in communication with the steam turbine (not shown) of the power plant receiving heat from steam 16 thereof. The heat carrier of the second circuit can be water, e.g. extracted from the economizer 31 , or a mixture of water and steam. Also, the heat carrier of the second circuit can be a medium different from water, such as a heat resistant oil. This latter medium of the heat carrier will allow different pressures. A closed circuit is more suitable for retrofitting existing power plants with CO2 capture, since less encroachment to or rebuilding of the plant is required.

The energy to the reboiler 14 can be partly supplied by water extracted from the economizer 31 and partly by steam 16 extracted from the steam turbine of the power plant. If the heat duty of the CO2 separation unit 10 is small, it is possible to cover the entire demand thereof by energy supply from the HRSG 30 via the first or second circuit.

The temperature of the system has to be controlled. For the purpose of controlling the temperatures of the heat carrier of the tubings 32, 33, 40, 41 , the system comprises control means (not shown), such as a pump and a control valve, or a frequency controlled pump. A cross coupling having a valve 39 could be provided between the hot and cold side of the inventive circuit 32, 33 or 40, 41 for controlling the temperature thereof. Also, a cross coupling including a valve 43 arranged between an inlet and an outlet of the economizer 31 could be provided for controlling the temperature of the feed-water in the economizer 31. The embodiment in Fig. 4 will require a circuit to dump the heat from the heat exchanger 42 if the CO2 capture plant is not operated, the heat could be realised to the deaerator or the condenser.

By the system, excess heat in the economizer 31 of the HRSG 30 is utilized to regenerate the absorbent of the CO2 separation unit 10. This heat is carried by the heat carrier in the inventive circuit. The hat carrier is water mixed with the main flow according to the first embodiment, or could be a different medium as offered by the second embodiment. In order to contribute to the regeneration process, the heat exchanger of the economizer 31 has to be enlarged both in terms of cold fluid capacity and heat transferring area, compared to a CCPP without a CO2

separation unit, to recover a larger amount of heat from the flue-gas when the first embodiment of the system is chosen This enlarging of the heat exchanger of the economizer 31 is not necessary when the excess heat is recovered by the additional heat exchanger 42 according to the second embodiment of the system.

The reboiler 14 is configured to utilize two heat sources, the first one is the steam turbine, of which steam is extracted, and the second one is the economizer 31 , of which heat is recovered from flue gases. The temperature difference in the reboiler 14 is higher than for a conventional steam extraction reboiler, which will reduce the required heat transferring area.

Equation (1) shows the relation between temperature and recovered heat, which indicates that if the feed-water mass flow through the economizer 31 is increased, the slope of the water side in the corresponding temperature to heat flux diagram will be reduced. Q denotes the heat, m the mass flow, and c _p the specific heat, respectively, wherein the mean value of the specific heat has been used to simplify the equation.

Q = c _p (T ₂ - T _l ) (1 )

The invention is based on this equation. Thus, when a circuit, containing water as a heat carrier, is arranged between the economizer 31 and the reboiler 14 more heat can be recovered from the flue-gas from the gas turbine of the power plant and be supplied to the reboiler 14, leading to lower extraction of steam from the steam turbine. The result is that the mass flow of water through the economizer 31 is increased, while the feed-water temperature remains constant compared to when steam only is used for supporting the heat duty of the reboiler 14.

Fig. 5 shows a temperature to heat flux diagram i.e. the temperature profile of a HRSG of a CCPP, where an upper line A represents the exhaust gas, which enters at the hot side. The lower line has three sections, representing, when starting from the cold side, section B: water is heated until saturation in the economizer, section C: evaporation to steam at constant temperature in the evaporator, and D: superheating of the steam in the super-heater.

Fig. 5 shows that the amount of recovered heat from the flue-gas is increased by the inventive circuit. The solid lines A, B, C, D represent the case when the circuit is not used, and the dashed lines show that more heat can be recovered when the mass flow through the economizer is increased. An increased mass flow is resulting in a reduced slope of the diagram B ^' , i.e. more heat can be recovered for a given feed-water temperature.

The process in a heat exchanger is driven by the temperature difference between the hot and cold side, which means that in order to reduce the

temperature difference it is necessary to increase the heat transfer area.

In a conventional CCPP, the economizer heat transfer area is large enough to give a sufficient approach point on the hot side. The approach point is defined as the difference between the evaporation temperature and the economizer outlet temperature. The heat exchanger surface temperature on the hot side is also an important parameter for the economizer, if it drops below a critical value there is a risk of flue-gas condensation, which can be reduced by recirculating a part flow of water through the economizer for controlling the surface temperature of the tubes.

By arranging a circuit between the economizer and the reboiler, the approach point can be controlled by the mass flow through the circuit. The mass flow to the reboiler can be increased if the heat transferring area of the economizer is increased, which will reduce the temperature difference at the cold side of the HRSG resulting in increased total efficiency of the power plant.

When a CO2 separation unit is used, the temperature of the metal on the gas side of the economizer is much higher than in a conventional CCPP, then there is a great temperature margin to flue-gas condensation. If the plant is operated without the C0 ₂ separation unit, the circuit between the economizer and the reboiler can be bypassed, then part of the flow of heat carrier (water) is recirculated from the economizer outlet to control the temperature of the tube surface.

Fig. 5 shows the amount of excess energy recovered in the economizer to provide part of the reboiler heat duty, illustrated in a temperature to heat flux diagram. The invention utilizes excess energy in the economizer for regenerating the absorbent by using a heat carrier to bring the heat from the economizer to the reboiler. Thus two heating circuits are arranged to the reboiler, the original condensing steam circuit 16, known per se, and the inventive circuit between the economizer and the reboiler containing the heat carrier.

Example

Combined cycle power plants have been modelled to accommodate absorbent-based CO ₂ capture using hot water to partly cover the reboiler heat duty. When the economizer-reboiler circuit was included, the mean temperature difference in the economizer of the HRSG was reduced. The result of this is that the HRSG efficiency is no longer dependent on the number of pressure levels. On the other hand, if the CO ₂ separation unit employs an absorbent that cannot withstand high temperatures, a second pressure level can improve the total efficiency. This is a consequence of the increasing amount of entropy generated in the reboiler with increasing temperature difference. The low pressure of such a plant should be much higher than in a conventional dual-pressure CCPP. If the absorbent can be regenerated at a high temperature, and if the energy saved in the CO2 compression process exceeds the energy consumption related to the high pressure desorption, a single-pressure CCPP is advantageous. In this case it is advantageous to divide the regenerator into two parts operated at different temperature levels. The high temperature part should be heated by the water from the inventive circuit and the low temperature should be heated be the remaining heat of the said water and steam extracted from the turbine. A chilled ammonia separation unit offers the possibility of using a high regenerator temperature. The possibility of better utilizing the potential of the hot water in a chilled ammonia CO ₂ separation unit appears very promising.

Table 1 shows the efficiencies of different CCPP with CO ₂ capture. The plant connected to the most left pile and to the most right pile does not use the economizer-reboiler circuit. All plants except for the sixth from the left use ΜΕΞΑ as absorbent. The example shows that great economical savings can be obtained by implementing the inventive circuit.

An advantage of the invention is higher efficiency of the power plant, e.g. an increased power output is obtained. Another advantage is that the need for cooling of exhaust gas prior to the CO ₂ separation unit is significantly reduced.

Another advantage is that the invention eliminates the need for multiple pressure levels. A single pressure plant using the invention can achieve an equal high efficiency as a conventionally multiple pressure plant. A single pressure plant is much cheaper to construct and has a faster start up, since less material has to be brought up in temperature, compared to multiple pressure plants.

Still another advantage is that the invention is suitable for retrofitting existing plants, since the constructional changes for arranging the inventive circuit only have to be made at the low temperature part of the HRSG, i.e. of the economizer.

The invention presents a solution for redesigning combined cycle power plants to better fit the heat demands of the CO ₂ separation units.

Also, the invention relates to a C0 ₂ separation unit comprising the system for recovering energy from a power plant comprising the C0 ₂ capture plant.

Further, the invention relates to use of a system for supplying energy to a C0 ₂ separation unit for separating C0 ₂ from exhaust gases from a power plant.

Also, the invention relates to a power plant comprising the CO ₂ separation unit comprising the system for recovering energy from said power plant.

In summary, the present invention increases the total efficiency of combined cycle power plants with absorption based CO ₂ capture. The invention is based on that the heat required for regenerating the absorbent and hence separating the CO ₂ from the solvent should be partly extracted from the HRSG or a waste heat recovery unit (WHR). When carbon capture is implemented on combined cycle power plants excess heat will be available in a part of the HRSG where the economizer is arranged, which can be used for the desorption process of CO ₂ . The invention uses a heat carrier to transfer this heat from the flue-gas to the reboiler where the absorbent is heated.

In the claims, the term "comprise/comprising" does not exclude the presence of other elements or steps. Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented. Reference signs in the claims are provided as a clarifying example and shall not be construed as limiting the scope in any way.