Field of the invention

The present invention relates to a method for improving the efficiency in a power plant utilizing both steam and gas engine processes as well as oxygen combustion, particularly as presented in the preamble of claim 1.

Background of the invention

Recent development of gas turbine technology with inlet temperatures of 1600-1700° C (Ref. 1) offers a possibility of combined-cycle process efficiencies above the 60 % attained with a classical combined-cycle plant (Ref. 2). As the carbon dioxide emitted by these plants is diluted with large amounts of air, its capture requires large amounts of energy. In spite of interesting new developments (Ref. 3), such plants are not likely to provide a solution to the problem of carbon dioxide emissions.

In a newer development, Ref.4 (Fig. 1), the Brayton and Rankine processes are combined into a process where both mediums are brought to the high temperature of the Brayton process and the common medium formed is then expanded to produce mechanical energy. The oxygen combustion used eliminates problems in carbon capture and the production of NO _x 's. This "common medium" -solution of Ref. 4 has disadvantage due to fact that the temperature difference reaches large values, particularly at the lower temperature end. While the process of the known art shown in Fig. 1 represents an important step towards the realization of the common-medium principle, it does not achieve its full potential.

1. In the process of Ref. 4 the higher-temperature part of the exhaust heat of the first heat engine is transferred into the returning Brayton medium and passed into the combustion process while the lower-temperature part is transferred into the Rankine medium. 2. Losses incurred in the transfer the exhaust heat of the first heat engine into a Rankine cycle that involves a change of state, a situation studied in US 6,173,679.

OBJECTS AND SUMMARY OF THE INVENTION

The objective of the present invention is to improve the common medium process with an improved thermal effiency. The characteristic features of the invention are described in claim 1.

According to the invention said Rankine process uses supercritical pressure and a final enthalpy increase of the Rankine medium before said second means for utilization of heat energy takes place using the hottest common medium exiting from the first heat engine means.

A relative deviation from reversibility is approximately the relative difference of absolute temperatures of the hot and cold flows in the heat exchange, averaged over the whole heat exchange process. A deviation from ideal process is in a range of 1 - 10 %, preferably 1.5 - 5%. This depicts a temperature difference in the heat transfer in a chosen point, which is so called "Gradigkeit". As an example in 800 K a temperature difference of 2 % is 16 K.

The total increase is an integral through the exchanger.

The implementation of the present invention can be based on technically and commercially available technology. Components for supercritical conditions belong perhaps to "high tech", but the technology is anyway available.

In an embodiment said second means for utilization of heat energy comprises at least a high pressure steam turbine.

In another embodiment Rankine medium is mixed with Brayton medium at lower pressure and heated by at lower part of said heat exchange means.

In another embodiment said Brayton cycle uses argon and/or oxygen as means of increasing the polytrophic index of its medium. In another embodiment in the Brayton cycle a fraction of Rankine medium is mixed with Brayton medium.

In another embodiment the first heat engine means of the process contain a piston engine.

In the present invention both returning Brayton and Rankine mediums are preheated into the maximum temperature of the heat exchanger and passed into the combustion process,

Said losses incurred in the transfer the exhaust heat of the first heat engine into a Rankine cycle are minimized by transferring the exhaust heat to several counter flows of mixed Brayton and Rankine mediums at different pressures.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention is described in detail with reference to the accompanying drawings showing some applications of the invention, in which

Figure 1 shows schematically the process in Ref. 4.

Figure 2 shows the arrangement of the common-medium Brayton-Rankine process as presented in Finnish patent application 20155653 and realized in the present invention.

Figure 3 shows schematically an Embodiment where the common medium consists of a mixture of argon and/or oxygen, steam and carbon dioxide and the first heat engine is a gas turbine, where two steam turbines are used to utilize the excess heat present in the exhaust of the first heat engine.

Figure 4 shows schematically an arrangement of the C0 ₂ separation unit employed in this Embodiment.

Figure 5 shows the variation of the heat capacity of water under supercritical pressures as a function of temperature.

Figure 6 shows schematically an Embodiment where the common medium consists of a mixture of argon, steam and carbon dioxide and the first heat engine is a gas turbine. A part of the returning Rankine medium water is injected into the counterflow of oxygen, argon and C02 entering the main heat exchanger to utilize the excess heat present in the exhaust of the first heat engine; the fuel is product gas generated in a pressurized-oxygen gasifier.

Figure 7 shows schematically an Embodiment where a large temperature ratio is obtained in the main heat engine, a gas turbine, without argon or excess oxygen in the medium by allowing its medium to expand to a pressure substantially below the atmospheric one. Figure 8 shows schematically an Embodiment where a large temperature ratio is obtained in the main heat engine using a cascade of two gas turbines.

Figure 9 shows schematically an Embodiment where a large temperature ratio is obtained in the main heat engine using a combination of a piston engine and a gas turbine. Figure 10 shows schematically a variation of the previous Embodiment where the combination of a piston engine and a gas turbine contains a high-temperature heat exchanger between the two heat engines and a large temperature ratio is obtained with argon. Figure 11 shows schematically a pinch analysis of the main heat exchanger process. DETAILED DESCRIPTION

Figure 1 uses reference numbers of the original document (Fig 1 in JP2003148112A)

The principle of the common-medium power cycle according to prior art presented in Fig. 1 of Ref. 5 is reproduced partly in Fig. 2. In it preheated streams of Brayton and Rankine mediums are combined and heated in an oxygen combustion process (10). The hot common medium formed is expanded in first heat engine (20, 30) and passed through main heat exchanger (40), where its heat is transferred to returning flows of Rankine (41) and Brayton (42, 43, 44) mediums. The common medium then passes through second means (50) of utilizing its heat energy and enters condenser (60), where most of its steam is condensed and the common medium is divided into separate flows of condensate water and

noncondensible gas. The amount formed in the combustion is separated from the condensate water and rejected (64), while the remainder is pressurized (63, 65) and returned as the Rankine counterflow to the heat exchanger (40). The amount of C0 ₂ formed in the combustion is separated (66) from the noncondensible gas and sent for further processing, while the remainder is pressurized (80) and returned as the Brayton counter- flow (42, 43, 44) into heat exchanger (40). However, when steam turbine 71 is used, the hottest enthalpy elevation of Rankine medium is not from the hottest common medium exiting from the gas turbine 30

Embodiment 1

In this Embodiment (Figs. 3 and 4) the common medium consists of a mixture of argon, steam and carbon dioxide and the first expansion engine is a gas turbine. Its combustor 10 receives fuel through connection 14, a mixture of argon, carbon dioxide and oxygen through connection 12 and steam through connection 13. The medium is expanded in gas turbine 20 and led to heat exchanger 40, where heat is transferred from it into a return flow of water at a pressure of 300 bar in manifold 41. The medium is then passed to turbine 50, from which it enters condenser 60, where most of its steam component is condensed by cold water in manifolds 6 land 62.

The condensate is pressurized in pump 63 and the part produced in the combustion is rejected at connection 64. Most of the remainder is preheated in manifold 61, brought to a pressure of 300 bar in pump 65, converted to supercritical steam in manifold 41 in heat exchanger 40 and expanded in high-pressure turbine 71. The exhaust of this turbine is reheated in manifold 43, expanded in medium-pressure turbine 72, reheated in manifold 44 and delivered to combustor 10.

If the common medium formed in the oxygen combustion of one kmol/s of methane consists of 9 kmol/s of an equimolar mixture of argon, steam and carbon dioxide, the noncondensible flow exiting condenser 60 consists of 6 kmol/s of an equimolar Ar-C0 ₂ mixture. This flow is pressurized in compressor 80 under water injection and led to separation unit 90, where one kmol/s of C0 ₂ produced in the combustion is liquified.

Combustion oxygen is added at point 81 to the remaining flow of 3 kmol/ s of Ar and 2 kmol/s of C0 ₂ , the combined flow is heated in manifold 42 and fed into combustor 10. An arrangement of separator unit 90 is shown in Fig. 4. The Ar-C0 ₂ mixture passes through heat exchanger or regenerator 10' into inner jacket 11 ', where it is further cooled by the streams in outer jacket 12' and central column 13' . A part of its C0 ₂ condenses and passes through choke valve or liquid motor 20' into evaporator 30', where it boils under a pressure lower than that prevailing in inner jacket 11 ' . Separated gaseous C0 ₂ exits through outer jacket 12' and heat exchanger/regenerator 10', and the remaining C0 ₂ - Ar mixture exits through central column 13' and heat exchanger/regenerator 10' . The theoretical separation work W is the difference between that of a complete separation of 6 kmol/s of the equimolar mixture and that of the returned 5 kmol/s of the 3:2 mixtures: W = 6 RT In 0,5 - 5 RT (0,6 In 0,6 + 0,4 In 0,4). At 300 K W = - 2,0 MW or 0,26 % of the heat of combustion of one kmol/s of methane. This separation work, plus the energy required to cover losses, is delivered by compressor 80 by raising the partial pressure of C0 ₂ in the mixture to a value higher than the pressure of the separated gaseous C0 ₂ .

As mentioned above, combined-cycle technology of the present art suffers from the problem that while the heat capacity of the exhaust flow of the gas turbine changes only slowly with its temperature, the heat capacity of the water in heat exchanger 20 has a peak in the transition region where water changes from a liquid state to a gaseous one. The peak spreads out and shifts towards higher temperatures as the water pressure is increased to supercritical values (Fig. 5). For good efficiency the total heat capacity of the return flows should match the smooth trend of the heat capacity of the exhaust flow; this requires additional heat capacity both above and below this transition region.

In this Embodiment the returning flow of water is at a supercritical pressure with a phase transition region between 370° and 470° C, as shown in Fig. 5. By a suitable choice of process parameters this region can be placed in the central part of heat exchanger 40; above 470° C additional heat capacity is provided by steam being reheated in manifolds 43 and 44. Unless additional heat capacity is provided in the region below 370° C, the output flow of the common medium from heat exchanger 20 will be at a substantially higher temperature than that of the returning flow, reducing the efficiency of the heat exchange process. A solution to this problem is described in Embodiment 2 below.

Embodiment 2

In this Embodiment (Fig. 6) the above problem is solved by injecting a part of the returning Rankine medium water into the Brayton flow of oxygen, argon and C0 ₂ entering manifold 42. As the temperature of the mixed medium rises in the manifold, the water evaporates providing additional heat capacity. This heat capacity will increase with the rate of evaporation of the water and peak close to the point where all water has evaporated; the temperature of this peak can be adjusted by varying the proportion of water in the mixed medium. If the pressure of the Brayton medium is 24 bar, a molar ratio of 1 :2 between water and the Brayton medium will result in a partial steam pressure of 8 bar in it and a peak heat capacity close to 171° C. Above the peak the heat capacity of the mixed flow in manifold 42 will be ca. twice its value without water injection. The mixing process will be assisted by gravity in an arrangement of heat exchanger 40 where the common medium exiting turbine 20 travels upwards and the returning mediums travel downwards (not shown).

The fuel used in this Embodiment consists of product gas generated in pressurized gasifier 5 that receives fuel from connection 4 and a mixture of oxygen, argon and carbon dioxide from connection 6. When using fuels such as peat or black liquor, their water content can often be adjusted to provide an optimal amount of water for the gasification; in other cases and for dry fuels, steam can be added as shown dashed in the Figure. The gas produced is cleaned in cyclone 7 and fed into combustor 10 that also receives a mixture of argon, carbon dioxide and oxygen from connection 12 and steam from connection 13. The hot common medium produced passes through gas turbine 20 into heat exchanger 40, where it heats a return flow of Rankine water at a supercritical pressure in manifold 41 and a mixed Brayton return flow of argon, carbon dioxide, oxygen and water in manifold 42. The common medium also reheats exhaust steam from turbines 71 and 72 in manifolds 43 and 44.

The common medium exiting heat exchanger 40 enters turbine 50 where part of its steam component may condense. Nearly all of said steam component is then condensed in condenser 60 cooled by water flowing in manifolds 6 land 62. The condensate water is pressurized in pump 63 and the amount formed in the combustion is rejected at point 66. Most of the remainder is preheated in manifold 62, brought to a supercritical pressure in pump 65 and led to manifold 41 in heat exchanger 40, while a portion is pressurized to the inlet pressure of gas turbine 20 in pump 67 and inserted into manifold 42. The uncondensed gas exiting condenser 60, mainly argon and carbon dioxide, is

compressed to the inlet pressure of gas turbine 20 in compressor 80 under water injection and delivered to C0 ₂ separation unit 90. Further means of cooling can be used in this pressurization (not shown). The amount generated in the combustion is extracted for processing at point 66 and the remaining gas mixture is divided into streams that enter gasifier 5 and manifold 42 in heat exchanger 40; oxygen with an admixture of argon is added to these streams at points 94 and 95.

Embodiment 3

It is of course possible to attain a gas turbine exhaust temperature suitable for a steam cycle with a common medium consisting of carbon dioxide and steam with a minor amount of oxygen. As reported in the modelling study in Ref. 6, this requires a pressure ratio of the order of 87, At an inlet pressure of 20 bar the exhaust gas pressure would be 0.23 bar; this process is shown in Embodiment 3 (Fig. 7).

The exhaust of gas turbine 20 passes through heat exchanger 40 and enters condenser 60 where most of its water component is condensed. Most of the heat released is transferred to water flowing in manifold 61. The condensate is pressurized in pump 63 and the amount produced in the combustion is rejected at point 68.

Main heat exchanger 40 contains two manifolds extending through it. Manifold 41 carries condensate water from condenser 60 that is preheated in manifold 62 in the condenser and brought to a supercritical pressure in pump 65. After having been converted to steam, this water enters high-pressure turbine 71, whose exhaust is reheated in manifold 43 and passed to medium-pressure turbine 72. Its exhaust is then reheated in manifold 44 and injected into combustor 10 at connection 13. As in the previous Embodiment, a part of the preheated Rankine water is pressurized to the inlet pressure of gas turbine 20 in pump 67 and injected into the Brayton medium entering manifold 42.

The Brayton medium consists of carbon dioxide with minor impurities. It is extracted from condenser 60 and pressurized to the atmospheric pressure in the first section of compressor 80 under water injection. The amount produced in the combustion process is extracted for further processing at connection 66 and the rest is combined with combustion oxygen and compressed to the inlet pressure of the gas turbine in the second section of compressor 80 under water injection. Further means of cooling may be added to the compressor (not shown). This medium is then preheated together with a part of the Rankine medium in manifold 42 in heat exchanger 40 and fed at connection 12 into combustor 10, which also receives fuel from connection 14.

A similar process was modelled with Aspen Plus at Tampere University of Technology. The relevant data obtained in two cases, Case 1 with a medium containing argon, C02 and steam and Case 2 without argon, are presented in Tables 9-21 of Ref. 7. This modelling did not employ the improvements presented in the Embodiments described below; this causes the condenser cooling flow temperature to be as high as 800 °C . In spite of being actually a CHP process, the process using argon had a thermal efficiency of 53.9 % for the production of mechanical work, including the separation work for oxygen in the process described in Ref.11.

Embodiment 4

In this Embodiment (Fig. 8) a large effective pressure ratio is obtained with a heat engine consisting of two gas turbines 21 and 22 placed in series with respect to the heat flow of the process but in parallel with respect to the flow of the common medium. This produces effective pressure ratios close to the product of the pressure ratios of the individual turbines while maintaining the exhaust pressure of both turbines at or above the atmospheric one. Combustor 10 receives fuel, such as natural gas, through connection 14. Oxygen or oxygen/ C0 ₂ mixture pressurized in compressor 80 equipped with water injection is supplied through connection 12. The Brayton medium is formed from noncondensible gases extracted from condenser 60 and compressed in compressor 80 under water injection. The C0 ₂ formed in the combustion is removed for processing at connection 66 while the remainder is heated in manifold 43 of heat exchanger 40. Additional heat capacity in the lower part of the heat exchanger is created by mixing part of the preheated Rankine water at point 47 into this Brayton flow; the combined flow exiting heat exchanger 40 is divided between combustor 10 feeding turbine 21 and heat exchanger 15, where it is heated by the exhaust of turbine 21 and expanded in turbine 22. The remainder of the Rankine medium is brought to a supercritical pressure in pump 65 and led to manifold 41. There it is converted into steam that is expanded in high-pressure turbine 71, reheated in manifold 44 in the top part of exchanger 40, further expanded in medium- pressure turbine 72 and then mixed at point 46 with the returning Brayton medium in manifold 43.

The exhausts of gas turbines 21 and 22 pass through heat exchanger 40 and are then expanded in turbine 50; in some applications a part of their steam component condenses in this turbine. The medium is then passed into condenser 60 cooled by water flowing in manifolds 61 and 62. The condensate water is pressurized in pump 63 and the amount produced in the combustion is rejected at point 68. Most of the remainder is preheated manifold 62 in the condenser; a part of it is brought to a supercritical pressure in pump 65 and led to manifold 41 in heat exchanger 40, while the remainder is pressurized to the inlet pressure of turbines 2 land 22 in pump 67 and mixed at point 47 with the Brayton medium entering manifold 43.

In this Embodiment the combustion oxygen, mixed with C0 ₂ if desired, is pressurized separately in compressor 81, preheated in manifold 42 and injected into combustor 10 at connection 12. Embodiment 5

If the supercritical Rankine medium being heated in heat exchanger 40 contains a gaseous phase, this phase will also contain steam at the vapor pressure of water at each temperature. An equimolar mixture of argon and water at 300 bar will at 342° C contain a partial pressure of 150 bar of steam in its gas phase, i.e., all water has already entered the gas phase at this temperature, well below the critical temperature of 374° C. Adding a gas phase into the

Rankine medium will thus cause water to evaporate into it, increasing the heat capacity of the medium well below the supercritical transition region. As reported in Ref. 8, this effect is enhanced in the case of carbon dioxide because of the interactions between water and C0 ₂ molecules; water-C0 ₂ mixtures at a high pressure attain complete miscibility already at 366° C, below the critical point at 374° C. This phenomenon is used in this Embodiment (Fig. 9) to provide an additional degree of freedom in optimizing the efficiency of heat exchanger 40 in a CHP plant where the combustion takes place in piston engine 20 followed by gas turbine 30 and the common medium is a mixture of carbon dioxide and steam. The cooling of the piston engine provides additional heating capacity. Piston engine 20 receives fuel at connection 14, supercritical steam with an admixture of C0 ₂ at connection 12 and a low-pressure mixture of oxygen, steam and C0 ₂ at connection 13. Its exhaust is expanded in turbine 30 and led into heat exchanger 40, where it heats said supercritical mixture in manifold 41 and said low-pressure mixture in manifold 42.

The common medium exiting heat exchanger 40 is led into condenser 60; most of the heat liberated is transferred to manifold 62 to provide heat for industrial or district heating purposes. If the medium consists of an equimolar mixture of C0 ₂ and steam at atmospheric pressure, its steam component will start to condense at 81° C with one half of the water condensing by 65° C. This gradual condensation improves the thermal efficiency of the heat transfer.

The condensate water is pressurized in pump 63 and the amount produced in the combustion is rejected at connection 68. The remaining water is preheated in manifold 61 and some of it is pressurized in pump 67 and mixed at point 48 with the low-pressure Brayton medium that enters manifold 42 in heat exchanger 40. The remainder is pressurized to ca. 300 bar in pump 65 and inserted into manifold 41 in the heat exchanger.

Of the noncondensible carbon dioxide exiting condenser 60, the amount produced in the combustion is delivered for processing at point 66. The remainder is pressurized in compressor 80 under water injection to the intake pressure of piston engine 20. Part of this pressurized mixture is mixed with oxygen and water at point 48 and delivered into manifold 42 in the heat exchanger; the remainder is pressurized to ca. 300 bar in compressor 8 lunder water injection and mixed with the supercritical Rankine medium at point 49. The resulting mixed medium is preheated in manifold 41 and injected into the piston engine during the first part of its power stroke. Embodiment 6

In this variation of the above Embodiment a heat exchanger 15 is inserted between the piston engine and the gas turbine, which is equipped with a combustor to increase its inlet temperature (Fig. 10). This way a part of the exhaust heat of the piston engine is recycled into it, improving its Carnot efficiency and reducing the temperature of its exhaust to a value suitable for the gas turbine combustor. The common medium contains argon, leading to high temperature ratios in the expansion processes in both heat engines.

Piston engine 20 receives a mixture of oxygen, argon, carbon dioxide and steam that has been preheated to ca. 900° C in the heat exchanger 15 through connection 13 and fuel through connection 14. In the exhaust stroke the combustion gases that still contain oxygen enter heat exchanger 15 at 15 bar and ca. 900° C. The gases then enter at ca. 600° combustor 10 of gas turbine 30, which also receives fuel through connection 11. The combustion products enter the gas turbine at 1300° C and its exhaust enters main heat exchanger 40 at 1 bar, 400° C. There its heat is nearly isentropically transferred to a mixed flow of the Rankine and Brayton mediums at a supercritical pressure in manifold 41 and a mixed flow of the Brayton and Rankine mediums at 16 bar in manifold 42. The exhaust flow of the main heat exchanger enters at ca. 85° C turbine 50, where a part of its steam component may condense, and thence into condenser 60. The condensate water collecting in it is brought to the atmospheric pressure in pump 63, the amount produced in the combustion is rejected at point 64 and the remainder is returned to the common-medium process.

The noncondensible gas, a mixture of argon, carbon dioxide and oxygen plus water vapor at the saturation pressure, is compressed to 16 bar in compressor 80 under water injection (not shown). Part of this flow enters carbon dioxide separation unit 90 where the amount formed in the combustion processes is separated and sent for further processing at connection 66, while the rest of this flow is mixed at point 91 with oxygen-argon mixture and condensate water that has been preheated in manifold 61 in condenser 60, heated in manifold 42 in heat exchanger 40 and passed into combustor 10 of gas turbine 30. The latter also receives fuel through connection 1 l.The remaining exhaust flow of compressor 80 is pressurized to a supercritical pressure in compressor 81 under water injection (not shown), mixed with preheated water pressurized in pump 65, heated in manifold 41 in heat exchanger 40 and injected into heat exchanger 15.

In a CHP variation of this Embodiment (not shown), turbine 50 is omitted and the exhaust flow of heat exchanger 40 is led directly into condenser 60 at 85° C, 1 bar in analogy with Embodiment 5.

The outstanding feature of this invention is that it provides a nearly isentropic heat transfer in the main heat exchanger after the first heat engine of the common-medium process. The situation is illustrated in the pinch analysis shown schematically in Fig. 11. Curve A depicts the heating of a "cold" flow of the Rankine water component of the common medium at a pressure of 300 bar in manifold 41 of heat exchanger 40 as a function of its temperature, while curve B shows the cooling of a "hot" exhaust flow of the common medium from the first heat engine matched to curve A in the region between 350° and 470° C, where the transition from a liquid state to a gaseous one takes place in the cold flow. As explained above, a satisfactory match is obtained in this region using supercritical pressure. For a nearly isentropic process more heat must be transferred to the cold flow from the hot flow above 470° C; this is accomplished in the steam reheat processes described in

Embodiment 1 and Claim 9 below. The crucial problem of the mismatch below 350° C is solved by two novel principles: first, by adding Rankine water to the returning Brayton flow as described in Embodiment 2, and second, by adding Brayton gas phase to the returning Rankine water flow or flows, as described in Embodiments 5 and 6. These inventions provide new degrees of freedom to the optimization of the heat exchange process, opening the way to the nearly isentropic heat transfer depicted as dashed lines in Fig. 11.

In Fig. 11, the hot and cold curves can be matched in the area above the transition region, above 500 °C, within ca. 10 K using the reheat processes described above.

In the transition region between 500 °C and 350 °C the delta T is ca. 30 K, while a value of 10 K can be realized below 350 °C using the degrees of freedom described above.

In summary, it appears possible to achieve a heat transfer process in the central heat exchanger that is within a few per cent of reversibility. This is a major improvement over the known art. References

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