Method and apparatus for improving energy efficiency in existing gas turbine combined cycle plants

Abstract

Method and apparatus for improving the energy efficiency of existing gas turbine combined cycle plants in which a compressor pressurises air which is combusted with fuel in a combustion chamber, followed by a turbine and a high temperature heat exchanger and a low temperature heat exchanger. In the secondary circulation after the steam turbine II, steam is condensed in the condenser into water, which is pressurised to the maximum pressure by means of a pump and preheated in low temperature heat exchanger and vaporised in a high temperature heat exchanger. After the high temperature heat exchanger, steam enters the steam turbine wherefrom a tap is taken, if necessary, which is injected after preheating into the combustion chamber of the gas turbine process or at the latest the beginning of the vanes of the turbine. Before steam turbine II, the enthalpy of steam (and additional water) at below 1 atm is increased by means of the condensation heat of the water contained in the combustion gases, after which intermediate superheating is applied to the saturated Rankine circulation steam using the excess heat of the low temperature heat exchanger.

Claims

1. A method for improving the energy efficiency of existing gas turbine combined cycle plants, wherein the compressor pressurises the combustion air which is combusted with fuel in a combustion chamber, followed by a turbine and a high temperature heat exchanger, wherein feed water pressurised by a pump is vaporized, the pump being arranged after a secondary process steam turbine II followed by a condenser-heat exchanger, wherein condensing heat of the steam contained by the combustion gases in atmospheric pressure and formed in combustion is used for increasing the enthalpy of the process circulation steam at a pressure of less than 1 bar by 100 to 350 kJ/kg in constant temperature subsequent to a steam turbine at a low temperature heat exchanger, subsequent to which the excess heat of the low temperature heat exchanger is used, subsequent to the high temperature heat exchanger, for intermediate superheating of steam between the steam turbine and the steam turbine II.

2. A method for improving the energy efficiency of existing gas turbine combined cycle plants, wherein the compressor pressurises the combustion air which is combusted with fuel in a combustion chamber, followed by a turbine and a high temperature heat exchanger, wherein feed water pressurised by a pump is vaporized, the pump being arranged after a secondary process steam turbine II followed by a condenser-heat exchanger, wherein condensing heat of the steam contained by the combustion gases in 1 atm pressure and formed in combustion is used for increasing the enthalpy of the circulation steam of Rankine process at a pressure of less than 1 bar by 100 to 350 kJ/kg in constant temperature subsequent to a steam turbine at a low temperature heat exchanger and/or for vaporising the liquid additional water fed to the Rankine process circulation in a pressure below 1 bar that is combined with the main Rankine circulation only at a pressure below 1 bar, subsequent to which steam is superheated in the low temperature heat exchanger by using, subsequent to the high temperature heat exchanger, the heat of the combustion gases between the steam turbine and the steam turbine.

3. The method according to claim 2, wherein the tapped steam from the secondary process and introduced into the combustion chamber or the beginning of the turbine vanes or a portion of the mixture formed by the air mass flow from the compressor and the above-mentioned steam is preheated in the high temperature heat exchanger.

4. The method according to claim 2, wherein the gas turbine process comprises a second turbine and/or a second combustion chamber.

5. The method according to claim 2, wherein the high temperature heat exchanger or the low temperature heat exchanger is provided with an additional combustion at a pressure of 1 atm at least partially above the vaporisation temperature of the Rankine process, the combustion being utilized in the vaporisation of steam of the Rankine cycle, superheating or intermediate superheating.

6. The method according to claim 2, wherein the intercooling of the compressor is carried out with water injected into the compressor or with partially vaporised water or the intercooling is carried out by heat transfer in a heat exchanger.

7. The method according to claim 2, wherein the injection water is taken partially from the tap outlet of the steam turbine or from the feed water after the low temperature heat exchanger.

8. The method according to claim 2, wherein a steam turbine II having a maximum pressure of at least 3 bar is supplied with an additional mass flow of water at a pressure of less than 1 atmosphere as an intermediate supply.

9. The method according to claim 2, wherein the plant operates as a district heating/back-pressure plant by heating the district heating water in a single-phase or two phases using condensation heat of steam from the steam turbine in a condensing heat exchanger and with condensation heat of steam contained by the combustion gases at a low temperature heat exchanger, or a portion of the water of the district heating circuit is heated in the heat exchanger and the remainder in the condensing heat exchanger, after which the water circulations are combined in the mixing heat exchanger.

10. The method according to claim 2, wherein the low-temperature heat exchangers are parallel or connected together to form a single integrated heat exchanger.

11. The method according to claim 2, wherein the high temperature heat exchanger and the low temperature heat exchangers are connected together, forming a single waste heat boiler.

12. The method according to claim 2, wherein the amount of water in the secondary process circulation is kept constant, for example, by introducing additional water into the process circulation and removing the required amount of water from the Rankine process circulation.

13. The method according to claim 2, wherein the maximum pressure of steam turbine II is from 3 to 8 bar.

14. The method according to claim 2, wherein the steam from the Rankine process circulation is superheated without additional combustion by a temperature increase equal to 0 to 400 Kelvin by means of combustion gases from the gas turbine process before the steam turbine II.

15. The method according to claim 9, wherein intermediate superheating of Rankine steam in a heat exchanger is provided between the steam turbines.

16. An apparatus for improving the energy efficiency of existing gas turbine combined cycle plants, wherein the compressor pressurises the combustion air which is combusted with fuel in a combustion chamber, followed by a turbine and a high temperature heat exchanger, wherein feed water pressurised by a pump is vaporized, the pump being arranged after a secondary process steam turbine II followed by a condenser-heat exchanger, wherein condensing heat of the steam contained by the combustion gases in atmospheric pressure and formed in combustion is used for increasing the enthalpy of the process circulation steam at a pressure of less than 1 bar by 100 to 350 kJ/kg in constant temperature subsequent to a steam turbine at a low temperature heat exchanger, subsequent to which the excess heat of the low temperature heat exchanger is used, subsequent to the high temperature heat exchanger, for intermediate superheating of steam between the steam turbine and the steam turbine II.

17. An apparatus for improving the energy efficiency of existing gas turbine combined cycle plants, wherein the compressor pressurises the combustion air which is combusted with fuel in a combustion chamber, followed by a turbine and a high temperature heat exchanger, wherein feed water pressurised by a pump is vaporized, the pump being arranged after a secondary process steam turbine II followed by a condenser-heat exchanger, wherein condensing heat of the steam contained by the combustion gases in 1 atm pressure and formed in combustion is used for increasing the enthalpy of the circulation steam of Rankine process at a pressure of less than 1 bar by 100 to 350 kJ/kg in constant temperature subsequent to a steam turbine at a low temperature heat exchanger and/or for vaporising the liquid additional water fed to the Rankine process circulation in a pressure below 1 bar that is combined with the main Rankine circulation only at a pressure below 1 bar, subsequent to which steam is superheated in the low temperature heat exchanger by using, subsequent to the high temperature heat exchanger, the heat of the combustion gases between the steam turbine and the steam turbine II.

18. The apparatus according to claim 17, wherein the tapped steam from the secondary process and introduced into the combustion chamber or the beginning of the turbine vanes or a portion of the mixture formed by the air mass flow from the compressor and the above-mentioned steam is preheated in the high temperature heat exchanger.

19. The apparatus according to claim 17, wherein the gas turbine process comprises a second turbine and/or a second combustion chamber.

20. The apparatus according to claim 17, wherein the high temperature heat exchanger or the low temperature heat exchanger is provided with an additional combustion at a pressure of 1 atm at least partially above the vaporisation temperature of the Rankine process, the combustion being utilized in the vaporisation of steam of the Rankine cycle, superheating or intermediate superheating.

21. The apparatus according to claim 17, wherein the intercooling of the compressor is carried out with water injected into the compressor or with partially vaporised water or the intercooling is carried out by heat transfer in a heat exchanger.

22. The apparatus according to claim 17, wherein the injection water is taken partially from the tap outlet of the steam turbine or from the feed water after the low temperature heat exchanger.

23. The apparatus according to claim 17, wherein a steam turbine II having a maximum pressure of at least 3 bar is supplied with an additional mass flow of water at a pressure of less than 1 atmosphere as an intermediate supply.

24. The apparatus according to claim 17, wherein the plant operates as a district heating/back-pressure plant by heating the district heating water in a single-phase or two phases using condensation heat of steam from the steam turbine in a condensing heat exchanger and with condensation heat of steam contained by the combustion gases at a low temperature heat exchanger, or a portion of the water of the district heating circuit is heated in the heat exchanger and the remainder in the condensing heat exchanger after which the water circulations are combined in the mixing heat exchanger.

25. The apparatus according to claim 17, wherein the low-temperature heat exchangers are parallel or connected together to form a single integrated heat exchanger.

26. The apparatus according to claim 17, wherein the high temperature heat exchanger and the low temperature heat exchangers are connected together, forming a single waste heat boiler.

27. The apparatus according to claim 17, wherein the amount of water in the secondary process circulation is kept constant, for example, by introducing additional water into the process circulation and removing the required amount of water from the Rankine process circulation.

28. The apparatus according to claim 17, wherein the maximum pressure of steam turbine II is from 3 to 8 bar.

29. The apparatus according to claim 17, wherein the steam from the Rankine process circulation is superheated without additional combustion by a temperature increase equal to 0 to 400 Kelvin by means of combustion gases from the gas turbine process before the steam turbine II.

30. The apparatus according to claim 24, wherein intermediate superheating of Rankine steam in a heat exchanger is provided between the steam turbine.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0015] FIG. 1 illustrates a basic process of the invention. The gas turbine process will in the following be referred to as the primary process and the steam turbine process referred to also as the secondary process.

[0016] FIG. 2 illustrates the process according to FIG. 1, wherein a part of the steam turbine mass flow is introduced into the combustion chamber. Table 2 illustrates the values of the status points of FIG. 2.

[0017] FIG. 3 illustrates the district heating version of the process according to FIG. 2. In addition, it has an additional combustion in the waste heat boiler and the heat exchangers 15a and 15b form a single integrated heat exchanger.

[0018] FIG. 4 illustrates a version of FIG. 2, wherein the heat exchangers 15a and 15b form a single integrated heat exchanger assembly.

[0019] FIG. 5 is a design figure of a waste heat boiler according to FIG. 3.

[0020] FIG. 6 illustrates a practical version of the process of FIG. 3, i.e. it also includes steam turbine II 16 for electricity production in summer. Table 1 shows the values of the status points of FIG. 6.

[0021] FIG. 7 illustrates a process with the same maximum gas turbine pressure level as in aviation

and additionally intercooling of the compressor with preheated (and partially vaporised) water. The process also comprises two combustion chambers and two turbines in the gas turbine process.

[0022] FIG. 8 illustrates a construction in which the utilization of the condensation heat of the steam contained by the combustion gases and the intermediate superheating of the steam are not connected to each other but take place in a different phase/pressure level with respect to the Rankine process.

[0023] FIG. 9 depicts a dual-fuel version of FIG. 8. The difference to FIG. 8 is the heat exchanger 15a, b according to FIG. 1.

[0024] FIG. 10 illustrates an innovative CHP application.

EMBODIMENTS

[0025] In the tables 1 and 2, the status points are shown with number plus the letter c (e.g. 1c) and the components without the letter c.

TABLE-US-00001 TABLE 1 h(N2) h)O2) h(CO2) h(H2O) p(bar) T(K) kJ/kg kJ/kg kJ/kg kJ/kg  1c 1.0 288.0 15.4 13.5  2c 29.0 792.5 554.8 509.8  3c 28.4 1767 1732.2 1598.4 1779.1 5935  4c 1.04 917.5 696.8 643 679 3804  5c 6 1649 1623.3 5615  7c 173.5 626  8c 0.7 1700 11c 177.0 313 130 13c 170.0 873 3560 14c 0.036 2609 15c 0.035 300 111 17c 1.0 330 59.1 52.1 48.2 238 20c 0.71 2438 26c 1.02 671 420.5 383.7 391.9 3279 30c 1.02 31c 1.0 330 34c 0.71

TABLE-US-00002 TABLE 2 h(N2) h)O2) h(CO2) h(H2O) p(bar) T(K) kJ/kg kJ/kg kJ/kg kJ/kg  1c 1.0 288.0 15.4 13.5  2c 24.0 751.8 509.4 467.1  3c 23.52 1767 1732.2 1598.4 1779.1 5935  4c 1.04 979.1 768.1 709.6 754.1 3943  7c 173.5 626 1700  8c 0.7 3170 11c 177.0 130 13c 170.0 873 3560 14c 170.0 2609 15c 0.035 300 111 17c 1.0 330 59.1 52.1 48.2 238 20c 0.71 2438 21c 1.02 26c 1.02 671 420.5 383.7 391.9 3279 27c 24.0 29c 1.02 30c 1.02 31c 1.0 330 32c 24.5 3026 33c 24.0 873 3686 34c 0.71

[0026] FIG. 1 illustrates a gas turbine (combined cycle) process in which a compressor 1a,b pressurises the air which is burned with fuel (e.g. natural gas) in a combustion chamber 2 followed by a turbine 3 and a high temperature heat exchanger 12. It is advantageous to preheat natural gas prior to combustion. For example, it is advantageous to do this by separating a part of the combustion gases subsequent to the high temperature heat exchanger 12 (alternatively the combustion gases exiting the heat exchanger 15b are suitable for this either alone or mixed with the above). Thus these combustion gases heat the natural gas in the heat exchanger 42. The rest of the combustion gas mass flow goes to the low-temperature heat exchangers 15a, 15b. In practice, these could often be an integrated heat exchanger as in FIG. 7, but here they are presented as separate units to clarify the inventive idea. In the secondary process, the pump 11 pressurises the water to the maximum pressure and the water is preheated in a low-temperature heat exchanger 15a by means of combustion gases. The feed water is then vaporized and superheated in a high temperature heat exchanger 12 and subsequently expanded in a steam turbine 14a,b to below atmospheric pressure (such as 0.7 bar). The vapour concentration of steam is then finally increased to 100% (X=1.0) by means of the condensation heat of steam produced by combustion, at the latest at the low temperature heat exchanger 15b. Since excess energy is can be recovered from steam generated in combustion, additional water is mixed with the steam from the steam turbine 14a,b. This is the preheated in the low-temperature heat exchanger 15a and partly vaporised partly with the condensation heat of steam formed by combustion. It should be noted that, of course, in all figures in which additional water is introduced into the process, water must be removed if it is necessary to maintain a constant amount of water in the process circulation. In practice, this could be done, for example, by having a two-part pump 11 and by removing excess water between them. The rest of the Rankine cycle steam is thus vaporised in the low temperature heat exchanger 15b and also the intermediate superheating of saturated steam is effected there by means of the heat of the combustion gases. Subsequent to this, steam is then expanded in the steam turbine II 16 and condensed into liquid in the condensing heat exchanger 10.

[0027] In FIG. 1, a higher gas turbine pressure ratio can be used, as a lower exit temperature is sufficient. This means a lower exergy loss in the waste heat boiler and a higher single efficiency of the gas turbine process.

[0028] FIG. 2 illustrates the process according to FIG. 1, wherein a part of the steam turbine mass flow is introduced into the combustion chamber 2. Naturally, part of the mass flow of steam can be introduced to the beginning of the turbine 3 vanes to cool the first vanes of the turbine 3. Since the water introduced into the combustion chamber is removed from the process circulation, it must be replaced by additional water. The intermediate outlet from the steam turbine 14a,b is heated to 600° C. in the first part of the high temperature heat exchanger 12, so the temperature of the combustion gases after the turbine 3 can be slightly higher than in the case of FIG. 1. The additional water is vaporised by means of condensation heat of steam of the combustion gases of the heat exchanger 15a and mixed with the mass flow after the steam turbine 14a,b and vaporised/superheated in the heat exchanger 15b.

[0029] It should be noted that in FIG. 2 (likewise FIG. 6) the status point 3c corresponds to the turbine mixing temperature and the post-combustion chamber temperature is typically about 100° C. higher due to the cooling of the vanes.

[0030] FIG. 3 illustrates the district heating version of the process according to FIG. 2. It also has additional combustion in the high temperature heat exchanger 12 (waste heat boiler) and the low temperature heat exchangers 15a and 15b form a single integrated heat exchanger. Also the high temperature heat exchanger 12 and the low temperature heat exchanger 15a,b could be integrated to form a waste heat boiler, but for easier understanding of the invention they are presented separately. Additional combustion takes place in the process shown in the figure by means of natural gas, but in practice wood, for example, can be used as well. Similarly, additional combustion could equally take place in the low temperature heat exchanger 15a,b. It is advantageous to heat the water for district heating network in two stages, because about half of the mass flow of the steam turbine is directed into the combustion chamber. Thereby the first stage of preheating can be done with steam at about 0.5 bar, whereby more work can be recovered from the steam turbine than in the case of single-phase heating. If there is no intermediate supply from the steam turbine 14a,b to the combustion chamber, preheating of the district heating water can be carried out in two stages using so-called tap preheating. Naturally, the mass flow of the steam circulation must be kept constant by feeding additional water thereto. It should be emphasised that in practice, a CHP construction would normally be in accordance with FIG. 6, so that during summer, with low demand for district heat, the plant would be a normal CC plant, i.e. it would also have a steam turbine II 16 and the flows would be controlled by valves according to the current situation. FIG. 3 is included mainly to simplify the matter.

[0031] FIG. 4 illustrates a version of FIG. 2, wherein the heat exchangers 15a and 15b form a single integrated heat exchanger assembly. The additional water is vaporised with the condensation heat of steam of the combustion gases of the heat exchanger 15a, b and superheated with the excess heat of the heat exchanger 15a,b at a pressure of, for example, about 0.7 bar. After this the steam is expanded in steam turbine II 16. This solution is simpler and the arrangement of mass flow is clearer than in the case of FIG. 2.

[0032] FIG. 5 shows a structural diagram of steam circulation of a waste heat boiler (heat exchangers 12 and 15a, b of FIG. 3. Naturally, that is just one possible structural solution among many different options. The steam entering the combustion chamber 2 is first preheated at the combustion gas inlet of the heat exchanger 12. At the end of the heat exchanger 15a,b, in the direction of flow of the combustion gases, additional water is vaporised and in the other part of the heat exchanger, feed water is preheated and intermediate superheating of steam is carried out in parallel.

[0033] FIG. 6 therefore shows a practical CHP version of FIG. 3. The adjustment can be done, for example, by means of sliding pressure and additional combustion. In the figure, additional combustion is effected with natural gas in the high temperature heat exchanger 12, but it can also occur in the heat exchanger 15a,b. Additional combustion can also be effected, for example, with coal or wood, whereby a very high partial efficiency of additional combustion in district heating use is achieved. The valves 36, 37, 38, 39, 50, etc. in the pattern control the flows when the district heat is on/off. The valves are not optimally positioned and skilled in the art will find a better practical solution without any particular inventiveness. In the calculations related to the figure, waste heat boiler additional combustion is not in use, i.e. its mass flow rate is zero.

[0034] FIG. 7 illustrates a process using maximum pressure level of gas turbines used in aviation (approx. 50 to 60 bar) plus intercooling of the compressor with preheated (and partially vaporised) water. The process also comprises two combustion chambers and two turbines in the gas turbine process. The compressor 1a pressurizes the air to, for example, a pressure of 20 to 30. The partially vaporised branch of the secondary process feed water is then connected to the air mass flow and the compressor 1b pressurises the air to the maximum pressure. In the process this is followed by combustion in the combustion chamber 2 and expansion in the turbine 3. After that there is a second combustion chamber 21. Gas turbines with two combustion chambers are commercially available, the first one was manufactured by Alstom in the 1990s. However, many manufacturers have not manufactured gas turbines with two combustion chambers. This may be due to the fact that cooling of the vanes of the second turbine may be challenging in existing gas turbines, because the temperature of the combustion gases, used to cool the vanes of the second turbine, is much higher than that of single combustion chamber design for cooling the turbine with compressed air. However, in this design, the pressure ratio of the first turbine is much higher than that in the above-mentioned Gas turbine by Alstom, and thus the temperature of the combustion gases after the first turbine is much lower. In addition, as far as construction is concerned, it be reduced even further, for example by mixing additional air or steam into the primary process, i.e. the gas turbine process.

[0035] Thus, the combustion gases flow from the second combustion chamber 21 to the second turbine 22. After this, the combustion gases (vaporise and) superheat feed water in the high temperature heat exchanger 12. The superheated steam exiting from it expands in the steam turbine 14a, b to a pressure less than 1 atm. The above-mentioned combustion gases pass from the high temperature heat exchanger 12 to the low temperature heat exchanger 15a,b and to a small branch to a heat exchanger 42 for preheating natural gas. Feed water from the post-condenser 10 pump 11 is heated in the low temperature heat exchanger 15a,b. From the pump 11, considered to be a two-stage one in the diagram, supplies additional water, if necessary, to the steam from the steam turbine 14a,b. If the pump 11 is a single-phase one, the water pressure must naturally be reduced by means of a valve. The above-mentioned mass flow of steam also goes to the low-temperature heat exchanger 15a,b. A branch is taken from the water preheated between the low-temperature heat exchanger 15a,b and the high-temperature heat exchanger 12 to the pressure reducing valve 57, wherein the water is partially vaporised and then introduced to between the above-mentioned compressor 1a and the compressor 1b. For the above-mentioned reason, the intermediate superheating in the low temperature heat exchanger 15a,b after the constant-temperature heating of steam can be reduced and in some versions the intermediate superheating can be (almost) non-existent. Because of this, the additional claims include limits for enthalpy and temperature with regard to the heat transfer in the heat exchanger mentioned above. Increasing enthalpy at a constant temperature before the intermediate superheating is naturally performed by means of the condensation heat of the steam contained by the combustion gas. From the low temperature heat exchanger 15a,b the steam is introduced into the steam turbine II 16 and thereafter to the condenser 10 at a pressure of less than 1 atm. If necessary, additional water is also fed into the process circulation.

[0036] It should be emphasised that the pre-cooling of the steam used for the intercooling of the compressor of the gas turbine process can also be taken, for example, by tapping the steam turbine and mixing it with additional water, if necessary, and by countless other means.

[0037] Intercooling of the compressor 1a,b shown in the figures could, of course, also be carried out in plants which do not have the inventive idea disclosed in this patent application, which differs from those in existing plants. In this case, excess heat, such as the waste heat from the oxygen compressor of an IGCC plant, is used to preheat the water injected into the compressor 1a, b. Thus that is an additional invention.

[0038] In FIG. 7, the combustion gases can expand in the gas turbine process to even below the maximum temperature of the Rankine process if the waste heat boiler is provided with additional combustion.

[0039] FIG. 8 illustrates a process with an intermediate supply of steam to steam turbine II 16 at a pressure of less than 1 bar. Compared to the previous figures, the minimum pressure of the steam turbine 14a, b is, for example, about 3 to 8 bar, and this steam turbine is followed by the intermediate superheating of the circulation medium, i.e. steam. Since the recovery of the condensation heat from steam formed in the combustion is a separate step, it is carried out in another heat exchanger or at the final part of the same waste heat boiler. In this case, additional water at a pressure of about 0.5 to 0.8 bar is introduced to the process circulation and vaporized as the steam produced during combustion at the low temperature heat exchanger 15a, b condenses into liquid. The evaporated additional water is then introduced as a tap feed to steam turbine II 16.

[0040] Additional water can be taken between the two-stage pump or also from the main steam circulation before the vaporisation phase. In this case, the valve is used to reduce the pressure to a suitable level (such as 0.7 bar). If the heat exchanger 15b were separate, it would be advantageous to direct the combustion gases from it, for example, to the heat exchanger 15a. This case is disclosed in the claims of the CHP plant for the sake of clarity, but it also includes an integrated heat exchanger.

[0041] FIG. 9 illustrates a plant using natural gas as fuel and having an additional combustion with natural gas in the heat exchanger 15b. In a practical application wood or black liquor would be more likely to be used as additional fuel than natural gas, as the invention allows a very high partial load efficiency for combustion of wood. Since the final temperature of the intermediate superheating is relatively low (about 600K in the example calculations), the final temperature of the intermediate superheating can be increased from 200 to 300 K to a value of 800 to 900 K with, for example, intermediate superheating pressure of 4 bar. The plant according to the figure is also very suitable for combustion of black liquor. Safety requirements are important in recovery boilers, so a low steam pressure level is an advantage. A CHP design is also possible, and a very competitive design would be, for example, one in which steam turbine II does not have a tap feed, with the rest of the heat of the heat exchanger 15a being utilised as heat energy instead. A part of that heat energy can also be used for heating, for example, natural gas or injected water. Naturally, the above-mentioned temperature of 800 to 900 K is also achievable if the steam enters the high temperature heat exchanger from intermediate superheating before steam turbine II 12.

[0042] The claim related to additional combustion relates to FIG. 9 and is also applicable to existing plants. In the existing gas turbine combined cycle plants, additional combustion is not used for increasing the final temperature of intermediate superheating, but it rather has an effect on the mass flows of the dual-pressure level process. Using the energy of the additional combustion in intermediate superheating and for increasing the final temperature of the intermediate superheating achieves a very high partial efficiency of solid fuel.

[0043] FIG. 10 differs from FIG. 1 in that it does not have a low-pressure turbine 16, i.e. it is a pure CHP plant. The inventive idea is based on the fact that a portion of the previously mentioned ⅓ of the mass flow is used for heating a portion of the district heating water (pressure sufficient to prevent vaporization) in the heat exchanger 15b and the remaining water is heated in the condensation heat exchanger 10. The heat exchanger 15b provides water at a temperature of up to 200 to 300 C, so the desired 100 C for district heating water can be obtained by expanding in a steam turbine 14b to below 1 atm, possibly from 0.4 to 0.6 bar. This is due to the fact that the mass flows of water are then mixed in a mixing heat exchanger 88 wherefrom they are directed to transfer of district heat. This will produce more work from the steam turbine. The figure also shows the intermediate superheating between the steam turbine 14a and 14b. This is not required, but is suitable for the installation. In the heat exchanger 15b, the intermediate superheating is thus effected at a higher temperature (e.g. pressure of 4 to 15 bar) and water is heated at a lower temperature. This invention can also be utilised in existing plants. The remaining heat of combustion gases can also be utilised, for example, with a flue gas scrubber. Naturally, intermediate superheating can be effected differently from this disclosure, in the high temperature heat exchanger 12.

[0044] If pure oxygen is introduced to applications according to the invention, the power demand for its pressurisation is naturally lower if it is introduced to the second combustion chamber. It is sensible to take any additional air introduced to this combustion chamber from the compressor 1a,b by tapping. The air used for production of oxygen can also be taken from the tap of the compressor 1a,b. The compressor 1a,b naturally forms a single integrated compressor or is formed by two compressors. The additional air or oxygen can also be preheated in a waste heat boiler.

[0045] In the low-temperature part of the waste heat boiler (heat exchangers 15a,b), water is preheated and in the high-temperature part, water is vaporised and superheated. Intermediate superheating of steam also takes place in the low-temperature part. However, the additional combustion mentioned earlier also allows the use of the high temperature part for intermediate superheating, at least partly.

[0046] The invention has many advantages over existing installations. Naturally, the very high efficiency is the most significant advantage. Similarly, a smaller amount of expensive superheater materials is needed, because the intermediate superheating temperature and pressure are lower. The share of the gas turbine process of the total power is proportionally higher in CHP applications, which is important for competitiveness. Modern technology can also be applied to start-up and shut-down as well as to the shaft arrangement of compressors and turbines. The mass flow of water to the combustion chamber can naturally also be 100% of the mass flow of the steam turbine 14a,b.

[0047] Naturally, the expansion energy of natural gas and/or the cold energy generated by pressure decrease can be utilized, for example, for the cold cleaning/separation of the combustion gas, for production of oxygen or for the separation of carbon dioxide formed in combustion.

[0048] A thermal battery can also be connected to the process, if necessary. In some constructions, the additional water can also be introduced directly to point 35c (the place corresponding to point in FIG. 2).

[0049] It should be noted that the pressure range is disclosed in the main claim as wide as possible, but in practice the range is around 0.5 to 0.8 bar, even 0.3 to 0.95 bar may be possible.

[0050] The embodiment examples of the invention have been described only to illustrate the invention, and they do not limit the scope of invention, because details such as water treatment and generator(s), which are not necessary for understanding the invention, have been omitted for the sake of clarity. Thus, the invention is not limited to the described embodiments, but it includes everything covered by the scope of the appended claims.