METHOD AND PLANT FOR ENHANCING CO2 CAPTURE FROM A GAS POWER PLANT OR THERMAL POWER PLANT

Field of the invention The present invention relates to a method for enhancing CO2 capture from flue gases produced in a combustor in a gas power plant or a thermal power plant comprising a combustor with a flame tube, an oxygen separation device, a gas turbine comprising a turbine unit and a generator unit, one or more heat recovery steam generators, and a pipe system interconnecting the various parts, the method being based on the use of an open air- cycle and a semi-closed oxyfuel cycle, wherein the open- air cycle provides oxygen to the oxygen separation unit, in which part of the oxygen in the air is transferred to a flue gas flowing through a permeate side of the oxygen separation device, such oxygen enriched flue gas being supplied to the combustor

Background for the invention

One main category of power production systems with CO ₂ capture is called "oxyfuel". The commonality of this category is that nitrogen to a high degree is prevented from entering the combustion zone of the combustor. This may be achieved by producing oxygen in a conventional air separation unit (ASU) or by other methods that separate oxygen and nitrogen. The combustion products in such processes are mainly composed of water vapor and CO2. Separation and sequestration of CO ₂ are therefore simple compared to other categories of CO ₂ capture systems.

High and medium temperature oxygen transfer membranes (OTMs) are under development and several power cycles utilizing these have been proposed.

Ceramic Auto-thermal Recovery (CAR) reactors for oxygen production from air are also under development.

Process cycles utilizing these for power production with CO ₂ capture have been described.

Systems for power production with CO ₂ capture have significant efficiency penalties compared to systems without CO ₂ capture.

For oxyfuel systems this is often represented by the energy requirement for an air separation unit producing oxygen by distillation of air.

Systems utilising oxygen transfer membranes or ceramic auto-thermal recovery reactors have lower losses, but there still is a need for optimal integration in a power cycle.

EP 1172135 Al discloses a combustor, which indirectly heats preheated, compressed air. The air flows through an air separation device in order to produce the oxygen, the air separation device being in the form of a ceramic membrane separation system, producing a permeate stream that consist of oxygen supplied to the combustor in order to support combustion of a fuel. The combustor and the permeate side of the membrane operate more or less at the same pressure.

US 2004/0011048 Al discloses a method for operating a combustion plant where the combustion zone and the membrane permeate side are integrated in a membrane reactor device. Heat is transferred to the air supplied to a membrane reactor device. At least some oxygen is separated from the air by means of the membrane reactor device. The separated oxygen is combusted in the membrane reactor device. Since the combustor and the permeate side of the membrane form an integrated unit, there is no pressure difference between the said units.

WO 01/79754 Al discloses a combustor which indirectly heats the compressed air supplied to mixed conducting membrane which separates oxygen from the hot air stream

fed to a retentate side of the membrane, the oxygen being picked up by a hot sweep gas from the permeate side of the membrane and transporting the oxygen from the permeate side of the membrane to the combustor where a fuel is combusted with the oxygen enriched sweep gas. Both the oxygen lean air leaving the air side at the burner stage and the oxygen enriched exhaust are expanded subsequent to leaving the burner stage.

Summary of the invention

An object of the present invention is to provide a method and a plant for enhancing the CO ₂ capture from a gas power plant or thermal power plant.

Another object is to prevent nitrogen from mixing with the combustion products from the combustor.

A still further object of the invention is to provide a plant for power production with CO ₂ capture, avoiding the significant efficiency penalties inherent in prior art plants . Another object is to provide an integrated compete- tive system for power production with CO ₂ capture, using oxygen transfer membranes or ceramic auto-thermal recovery reactors .

Yet another object of the invention is to improve the efficiency of a power plant incorporating CO ₂ capturing.

According to the invention, the proposed power production plant operates on two connected cycles, the two cycles being:

- Open air cycle. - Semi-closed oxyfuel cycle.

The open air cycle generates power and provides hot, pressurised air to the oxygen transfer membrane. In the open air cycle air remains free of contamination of CO ₂ or other gases from the exhaust gases from the combustor (s) .

The open cycle comprises the following main components: a compressor, which may be a standard component, compressing ambient air. An integrated combustor/heat exchanger where the compressed air supplied by the compressor is circulated through or around the combustion zone in a separate circuit without coming into contact with the flue gas, the compressed air being indirectly heated by the combustor.

An air separation device, which either may be an oxygen transfer membrane or a ceramic auto-thermal recovery reactor, in which oxygen is transferred from the open cycle to the semi-closed oxyfuel cycle.

If desired, an additional combustor of a modified standard type. Fuel may be combusted in this optional combustor in order to increase turbine inlet temperature and thus power output or to start up the system. The fuel for such additional combustor may preferably, but not necessary be of a type non-carbon containing fuel. Such fuel may for example be hydrogen. If hydrocarbons are used as a fuel, the CO ₂ resulting front this combustion is not captured.

An expander, which expands hot exhaust gas from the combustor. - Heat recovery steam generator (s) , producing steam, either for use in a conventional steam power cycle or for injection into the integrated combustor. The semi-closed oxyfuel cycle generates power, heats

the compressed air in the open air cycle and has a working fluid mainly composed of water vapour and CO ₂ - greatly simplifying capture of CO ₂ . The semi-closed oxyfuel cycle comprises the following main components: - An integrated combustor/heat exchanger

(oxyfuel side) where the compressed air supplied by the compressor is circulated through or around the combustion zone in a separate circuit without coming in contact with the flue gas, the compressed air being indirectly heated by the combustor, and which combusts fuel using oxygen enriched sweep gas as oxidant. Heat is transferred from the oxyfuel side to the pressurized air side. Hence, the exhaust gas will be cooler than if heat had not been transferred. Simultaneously, the air fed to the air separation device will be preheated by the transferred heat from the combustor. - An oxygen enriched sweep gas/cycle fluid containing oxygen which has passed through the air separation device, where oxygen has been added to the fluid. An air separation device, which either may be oxygen transfer membranes or ceramic auto-thermal recovery reactors. In this device, oxygen is transferred from the air cycle into the oxyfuel cycle. An expander - One or more heat recovery steam generator, producing steam, either for use in a conventional steam power cycle (Oxycap Figure 3 and 4) or for injection into the integrated combustor (Oxycap Figure 2).

One or more compressors, compressing the oxygen enriched cycle fluid from the air separation device and allowing recirculation to the integrated combustor/heat exchanger.

A cycle fluid scrubber for example of a standard scrubber type, separating liquid water from the cycle fluid.

- One or more steam turbines of a standard type, producing power by expanding steam.

The objects of the present invention are achieved by a method and a plant as further described in the characterizing part of the independent claims, read in conjunction with the preamble. According to the invention, the flue gas from the combustor is expanded prior to being supplied to the permeate side of the air separation device while the mixed permeate and the sweep gas from the air separation device are compressed prior to being supplied to the combustor as oxidant in the combustion process.

According to a further embodiment, the combustor operates at a considerably higher pressure than the permeate side of the air separation device.

The air separation device may either be an oxygen transfer membrane or a ceramic auto-thermal recovery reactor .

The supplied air is indirectly heated by the combustor, prior to being supplied to the air separation device . Further, the flue gas may be compressed prior to being supplied to the permeate side of the air separation device. The reason for compressing the exhaust gas prior to supply to the permeate side of the air separation device is that certain systems may prefer increased

pressure of the gas supplied to the permeate side in order to function optimally.

In order to enhance the process, steam from one or more heat recovery steam generators may be injected into the combustor together with the fuel and the oxygen enriched flue gas.

According to the invention, a turbine is arranged between the flue gas exit of the combustor and the inlet to the permeate side of the air separation device, expanding the flue gas prior to entering the permeate side. Further, a compressor is arranged between the exit of the permeate side of the air separation device and the inlet to the combustor, compressing the oxygen enriched flue gas prior to injecting the flue gas into the combustor, the oxygen enriched flue gas functioning as an oxidant in the combustion process.

An expander is arranged upstream of the inlet of the permeate side of the air separation device, expanding the flue gas prior to injecting it into the permeate side of the air separation device.

Accordingly, the permeate (oxyfuel) side of the air separation device operates at a significantly lower pressure than the oxyfuel side of the combustion device. The driving force in the air separation device is the difference in oxygen partial pressure between the open air side and the oxyfuel side. In order to make the separation process possible, the oxygen partial pressure must be higher on the open side. Further, the size of the air separation device will to some degree be dependent upon the difference in oxygen partial pressure between the two sides, both in case of oxygen transfer membranes and ceramic auto-thermal recovery reactors. Generally, a smaller difference will require a larger air separation device, and vice versa. The present invention maximizes

this difference by having a large difference in total pressure between the two sides as well as by using combustion products as sweep gas on the permeate side. The use of combustion products as a sweep gas results in a requirement of oxygen concentration in the oxygen enriched flue gas that is far lower than 100%, typically 5-40%.

The invention relates to the following type of systems :

- A system for production of power based on combustion of a carbon containing fossil fuel.

- Capture of the majority of the CO2 coming from the combustion process.

- An air separation device being fed by air that has been compressed, e.g. by at least one compressor and indirectly heated by a combustion device, e.g. the integrated combustor/heat exchanger.

The combustion device uses the oxygen separated in the air separation device as oxidant.

Brief Description of the Drawings

The invention may take physical form in certain parts and arrangement of parts, preferred embodiment of which will be described in detail and illustrated in the accompanying drawings which form a part hereof and wherein:

Figure 1 shows in principle the new and inventive part of the invention, showing the semi-closed cycle;

Figure 2 shows a specific embodiment of a oxyfuel cycle where steam is added to the fuel injected into the combustor, resulting in a cycle fluid with a high steam content;

Figure 3 shows an embodiment of a oxyfuel cycle where the total efficiency is maximized; and

Figure 4 shows a plant providing moderate inlet

temperature and pressure to the air separation device.

Detailed Description of preferred embodiments

Figure 1 shows in principle the new and inventive semi-closed cycle according to the invention. As shown, the cycle comprises a combustor 16, comprising a combustion chamber 15 and a surrounding, separated jacket 14. Further the cycle comprises air separation device 18; a fluid scrubber 27 and a turbine 22 and a compressor 24, with a pipe line system associated therewith.

The semi-closed cycle according to the invention functions in the following way:

Fuel, such as for example natural gas, is injected into the combustion chamber 15 together with re-circulated exhaust and possibly steam. The stream of re-circulated exhaust is -mainly composed of CO2, steam and oxygen. The hot exhaust gas from the combustor chamber 15 is expanded in the turbine 22 to a temperature in the range of 300-900 ⁰ C, preferably in the range of 500-700 ⁰ C, and more preferably in the range of 500-600 ⁰ C, for example about 550 °C. The absolute pressure is in the range of 0.1-4 bar, preferably in the range of 0.2-3 bar, and more preferably in the range of 0.2-2 bar, for example about 0.3 bar. The expanded exhaust gas is then separated into two streams, one of which is transported to a CO ₂ compression plant (not shown) where water vapor is removed from the exhaust gas, producing CO ₂ that may be compressed and sequestered or transported to an oil field and used for enhanced oil recovery. The second stream of hot, expanded exhaust gas, the semi-closed stream, is transferred to a permeate side of the air separation device 18 where the exhaust gas is enriched by oxygen, transferred from the air side of the air separation device 18. The oxygen enriched exhaust gas

is then cooled to a temperature in the range of 0-100 °C and transported to the fluid scrubber 27 where water may, if required, be removed. From the fluid scrubber, the oxygen enriched exhaust gas is transported to a compressor which compresses it to an absolute pressure in the range of 1-40 bar. The oxygen enriched exhaust gas is then added to the combustion chamber 15, together with the fuel, and possibly steam, as defined above.

Further, it should be appreciated that the combustor 16 is equipped with a jacket 14, surrounding the combustion chamber, allowing a separate stream of compressed air at an absolute pressure in the range of 4- 40 bar to pass through the jacket 14. In this manner the compressed air is heated to a temperature in the range of 500-1000 ⁰ C, prior to being transported to the air side of the air separation device, where oxygen is transferred from the air side of the air separation device 18 to the permeate side of the unit 18, thereby oxygen enriching the exhaust gas flowing through the permeate side of the unit 18. The heated, compressed air with reduced oxygen content is then vented to the atmosphere.

Although not shown, it should be appreciated that one or more heat recovering steam generators (not shown) may preferably be included in both the semi-closed cycle and in the air cycle passing through the jacket 14 of the combustor 16. A conventional air compression may be used to supply compressed air and a conventional air turbine may be used to extract energy from the compressed air with reduced oxygen content before it is vented to the atmosphere.

Figure 2 shows a specific example of an embodiment of an oxyfuel cycle where steam is injected into the combustor, also indicating possible operation temperatures, pressures and O ₂ and CO ₂ contents, etc. It

should be appreciated, however, that these parameters are disclosed as example only, in order to identify one possible working condition of the plant, and may vary without thereby deviating for the inventive idea. The plant according to Figure 2 comprises two cycles, one open air cycle and a semi-closed exhaust or flue gas cycle. The open air cycle functions as follows: Air is compressed by means of a compressor 11 in a turbine unit 10, also comprising a turbine 12 and a generator 13, the latter producing electricity. The compressed air from the compressor 11 is fed to a jacket 14, surrounding the combustion zone 15 in a combustor 16, the compressed air being heated during its passage through the jacket. The temperature of the compressed air is about 400 ⁰ C, while the pressure being about 17 bar. When leaving the jacket 14 the temperature of the compressed air is about 700 ⁰ C. The indirectly heated, compressed air is fed into the compressed air side 17 of an air separation device 18, comprising a means 19 allowing only oxygen to be transferred from the compressed air side 17 to a permeate side 20 of the air separation device, oxygen enriching the fluid passing through the permeate side 20.

From the air separation device 18, the oxygen lean air is fed to a second combustor 21, where a fuel is injected, the exhaust or flue gas from the second combustor 21 being expanded in a turbine 12 and then fed to a heat recovery steam generator 22, producing steam. The fuel may be a carbon containing fuel or a non-carbon containing fuel such as hydrogen. The expanded gas has a temperature of about 440 ⁰ C when leaving the turbine 12. Thereupon the air/flue gas is discharged to atmosphere. The oxygen content of the fluid from the air separation device 18 is 6, 1% prior to entering the secondary combustor 21 and 3% upon leaving the secondary combustor

21. The temperature at the outlet of the exhaust gas from the combustor is about 1040 ⁰ C. It should be appreciated that use of such combustor 21 is optional and that the amount of fuel combusted here may be adjusted. The semi-closed cycle operates in the following manner :

The combustor 16 is driven by injecting fuel, steam and oxygen enriched flue gases. The temperature of the flue gas leaving the combustor 21 is 1246 ⁰ C, while the pressure is 16,5 bar. The flue gas leaving the exit of the combustor 16 is expanded by the turbine 22 to a temperature in the range of 300-900 ⁰ C, preferably in the range of 500-700 ⁰ C, and more preferably in the range of 500-600 ⁰ C, for example 550 ⁰ C, the turbine 22 forming a part of a turbine unit 23, a compressor 24 and a generator, the latter producing electricity. The absolute pressure is in the range of 0.1-4 bar, preferably in the range of 0.2-3 bar, and more preferably in the range of 0.2-2 bar, for example 0,3 bar. The flue gas contains 3,2% O ₂ , 28,8% CO ₂ and 67,0% H ₂ O. When leaving the turbine 22, a part of the expanded flue gas is transported to the permeate side 20 of the air separation device 18, and thereupon to a heat recovery steam generator 26. The remaining part of the flue gas is transported directly to the heat recovery steam generator 26. It should be appreciated that the heat recovery steam generator is of a type which separate the two different flows. The flow transported directly to the heat recovery steam generator is transported to a CO ₂ capture plant, while the first flow, i.e. the flow from the permeate side of the air separation device 18 is transported to cycle fluid scrubber, separating liquid water from the cycle fluid. When leaving the heat recovery steam generator and entering the cycle fluid scrubber, cycle fluid comprises

24,3% CO ₂ , 57,5% H ₂ O and 18,2% O ₂ . The cycle fluid is cooled to a temperature in the range of 0-100 ⁰ C, preferably 20 ⁰ C and liquid water is extracted in the fluid scrubber 27. The oxygen enriched cycle fluid containing 39,1% O ₂ is transported to the compressor 24, the pressure being about 0,27 bar. The cycle fluid is compressed to an absolute pressure of in the range of 1-40 bar, for example 17 bar. From the compressor 24 the cycle fluid is injected into the combustor together with fuel and steam delivered from the heat recovery steam generator(s) 22,26.

Steam from the heat recovery steam generator 26 may also be used for powering a steam turbine 28, producing electricity. The energy produced by the turbine 28 is about 23 MW. The turbine 22 produces an effect of 266 MW, while the energy consumption of the compressor 24 is 63,4 MW. The energy consumption of the CO ₂ capture unit is 14,1 MW.

The energy consumption of the compressor 11 is 92,4 MW while the energy produced by the turbine is 120 MW, the net effect of the turbine unit 10 being 27', 6 MW.

The energy added through the injected fuel in the combustor (s) is 483,7 MW, while the net effect energy effects produced by the turbines 10,23 and 28 is 248,3 MW. Hence the efficiency of the plant according to this example is 51,5%.

According to this embodiment, steam is injected into the combustor, resulting in a steam-rich cycle fluid that may be preferable, compared to the CO ₂ rich cycle fluid, ref. Oxycap 2. Consequently, an indirectly heated steam cycle is avoided, resulting in a capital cost reduction at the cost of lowered efficiency.

The expander in the semi-closed oxyfuel cycle runs according to this embodiment on ~ 67% steam.

Figure 3 shows a second embodiment of the invention, the only structural difference being that steam is not added to the cycle flow injected into the combustor 16. Instead, all the steam produced by the heat recovery steam generator is used to power a steam generator 28. It should be appreciated, however, that according to this embodiment, the temperatures, pressures, effects and percentage of the various fluids differ from the embodiment described in conjunction with Figure 2. The effect requirements for powering the compressor 11 is 82,4 MW while the turbine 12 produces 120,1 MW. The temperature of the compressed air leaving the compressor 11 is 400 ⁰ C, while the pressure is 17 bar. The temperature of the compressed air leaving the heating jacket 14 is approximately 700 ⁰ C. The compressed air has a O ₂ content of 6, 1% when leaving the air separation device .

In the optional combustor 21 additional fuel is added, increasing the temperature of the cycle fluid to approximately 1040 ⁰ C. Subsequent to expansion in the turbine 12, the temperature is 440 ⁰ C, while the O ₂ content is approximately 3%. The effect output from the turbine is 120,1 MW, while the effect emanating from the heat recovery steam generator is 23,6 MW. At the inlet to the compressor 11, the percentage O ₂ is 20,7%. The temperature of the flue gas discharged from the combustor 16 is 1255 ⁰ C. Similar to the embodiment disclosed in Figure 2, the flue gas leaving the exit of the combustor 16 is expanded by the turbine 22 to a temperature in the range of 300-900 ⁰ C, preferably in the range of 500-700 ⁰ C, and more preferably in the range of 500-650 ⁰ C, for example 600 ⁰ C. The absolute pressure is in the range of 0.1-4 bar, preferably in the range of 0.2- 3 bar, and more preferably in the range of 0.2-2 bar, for

example 0.3 bar. The flue gas contains 2,8% O ₂ , 77% CO ₂ and 20% H ₂ O. When leaving the turbine 22, part of the flue gas is transported to the permeate side 20 of the air separation device 18, and thereupon to a heat recovery steam generator 26. It should be appreciated that the heat recovery steam generator 26 is of a type which separates the two different flows. The flow transported directly to the heat recovery steam generator 26 is transported to CO ₂ capture plant, while the first flow, i.e. the flow from the permeate side of the air separation device 18 is transported to a cycle fluid scrubber 27, separating liquid water from the cycle fluid. The cycle fluid is cooled to a temperature in the range of 0-100 ⁰ C, preferably 20 °C. In the scrubber 27, water is extracted from the cycle fluid. The oxygen enriched cycle fluid leaving the air separation device 18 and the scrubber 27 contains 16,2% O ₂ and the pressure is 0,27 bar. The cycle fluid is compressed to an absolute pressure in the range of 1-40 bar, for example 17 bar, in the compressor 24. From the compressor 24, the compressed cycle fluid is injected into the combustor together with fuel.

Steam from the heat recovery steam generators 22,26 may also be used for powering a steam turbine 28. The turbine 22 produces an effect of 295 MW, while the energy consumption of the compressor 24 is about 15 MW. The energy consumption of the CO ₂ capture unit is about 13 MW. The energy consumption of the compressor 11 is around 82 MW, while the energy produced by the turbine 10 is about 120 MW. According to this embodiment, the flow rate on the permeate side of the air separation device is maximized at a low pressure resulting in maximum driving forces in the air separation device, giving a positive impact on unit size. Further, the overall efficiency is very good. With

reference to Figure 2, it should be appreciated that expander 22 arranged between the exit of the combustor 15 and the air separation device 17 in the semi-enclosed oxyfuel cycle runs on about 77% CO ₂ . Figure 4 shows a further embodiment of the invention. This embodiment provides a moderate inlet temperature and pressure to the air separation device, which may be preferable for a specific air separation device.

According to this embodiment compressed air is supplied by a compressor 11, the compressed air being delivered to the compressed air side of the air separation device 18 at a pressure in the range of 2 to 20 bar and a temperature in the range of 50 to 300 ⁰ C. Similar to the other two embodiments oxygen is transferred from the compressed oxygen side to the permeate side of the air separation device 18. The oxygen lean air is then compressed by a second compressor 29. The compressed oxygen lean air is then transported to the jacket 14, surrounding the combustor chamber 15. In the jacket 14, the compressed oxygen lean air is indirectly heated and thereupon transported to a second combustor 21. The exhaust from the second combustor 21 is then expanded in a turbine 12, and then transported to a heat recovery steam generator 22, generating steam for a steam generator 28. From the heat recovery steam generator, the exhaust is fed unclean to the atmosphere.

Hence, according to this embodiment, the turbine unit 10 comprises a first compressor 11, compressing the air delivered to the compressed air side of the air separation device 18, a second compressor 29; compressing the oxygen lean air from the air separation device 18; a turbine 12, expanding the lean air and the exhaust from the second compressor 21; and a generator 13, generating energy.

The second, semi-enclosed cycle operates as follows:

Fuel is injected into the combustor chamber 15 of the combustor. The exhaust or the flue gas from the combustion is then expanded by means of a turbine 22 and the transported to a heat recovery steam generator 26. From the heat recovery steam generator 26, the exhaust is transported to a cycle fluid scrubber 27, separating water from the cycle fluid. Part of the cycle fluid is then extracted from the cycle as described below, while the remaining part of the cycle fluid is compressed by a compressor 30 and then delivered to the permeate side of the air separation device 18 at a pressure in the range of 2 to 20 bar and a temperature in the range of 50 to 300 ⁰ C. A heat source and heat exchange internal to the air separation device may be used to obtain the desired operating temperature. Oxygen enriched cycle fluid is then compressed in a second compressor 22 and injected together with fuel into the combustor.

Hence, the second turbine unit 23 comprises a first compressor 30, compressing the cycle fluid from the cycle fluid scrubber 27 and delivering compressed cycle fluid to the permeate side of the air separation device; a second compressor compressing the oxygen enriched cycle fluid from the air separation device 18, delivering the compressed oxygen enriched cycle fluid to the combustor 16; a turbine 22 expanding the flue gas from the combustor 16; and a generator 25, producing energy.

According to the invention, a constant stream of fuel is added to the oxyfuel cycle through the combustor 15. Additionally, oxygen is added through the air separation device. A similar mass flow of combustion products needs to be removed from the cycle in order to avoid build-up of inventory in the cycle. Thus, combustion products are removed downstream of the expander 22. Heat is recovered from the combustion products, water is condensed and a

stream of relatively pure CO ₂ will remain. It should be appreciated that the means for CO ₂ compression, drying and potential oxygen removal, liquefaction, etc., could follow and may be of any conventional type and is not shown from ) a clarity point of view.