EP4689353A1

EP4689353A1 - A rotor for a power-generating turbomachine, a turbomachine comprising said rotor, and a thermodynamic circuit using said turbomachine

Info

Publication number: EP4689353A1
Application number: EP24716634.1A
Authority: EP
Inventors: Gabriele GIREZZI; Giacomo RAGNI; Paolo Di Sisto; Damaso CHECCACCI; Lorenzo Cosi; Enrico Giusti; Fabio Valeri
Original assignee: Nuovo Pignone Technologie SRL
Current assignee: Nuovo Pignone Technologie SRL
Priority date: 2023-03-31
Filing date: 2024-03-26
Publication date: 2026-02-11
Also published as: AU2024247728A1; MX2025011566A; JP2026510413A; WO2024199729A1; CN120936787A; IT202300006354A1; KR20250164817A

Abstract

The rotor includes a plurality of rotor disks which are stacked and connected to one another by a plurality of tie rods. The tie rods comprise at least a first tie rod arrangement and a second tie rod arrangement. The first tie rod arrangement includes a plurality of first tie rods parallel to the rotation axis of the rotor at a first distance therefrom. The second tie rod arrangement can include a set of tie rods arranged parallel to the rotation axis of the rotor and at a second distance therefrom, the second distance being smaller than the first distance. Alternatively, or in combination, the second tie rod arrangement incudes a central tie rod coaxial with the rotation axis.

Description

A ROTOR FOR A POWER-GENERATING TURBOMACHINE, A TURBOMACHINE COMPRISING SAID ROTOR, AND A THERMODYNAMIC CIRCUIT USING SAID TURBOMACHINE

DESCRIPTION

TECHNICAL FIELD

[0001] The present disclosure concerns gas expanders particularly adapted for use in oxy-fuel power cycles operating with process gas at high pressures, for instance CO2 cycles, such as Allam cycles, aka NET Power cycles. More in general, the present disclosure concerns rotors for turbomachines and turbomachines including said rotors.

BACKGROUND ART

[0002] Fossil fuels are a major source of chemical energy used for the generation of mechanical power. Fossil fuels are mixed with air and combusted to generate a combustion gas at high pressure and temperature, which expands in a turbine or an expander. The expander converts combustion gas enthalpy into mechanical power available on the output shaft of the expander and used to drive a load, such as a compressor or compressor train, or to rotate an electric generator and convert mechanical power into electric power.

[0003] One of the major concerns regarding combustion of fossil fuels relates to the production of carbon dioxide, a greenhouse gas which is considered one of the main contributors of global warming and climate changes.

[0004] To reduce the environmental impact of power generation through combustion of fossil fuels, the option of post combustion capture of carbon dioxide has been investigated. Carbon dioxide capture facilities have been developed, to process flue gas exhausted from gas turbines and remove carbon dioxide therefrom, prior to discharging the flue gas in the environment. The cost of a carbon dioxide capturing facility is high, both in terms CAPEX, as well as in terms of energy required to run the facility, which reduces the overall thermodynamic efficiency of the system. The percentage of carbon dioxide in flue gas is low. This requires large volumes of flue gas to be processed through the carbon dioxide capturing facility and renders the capturing process particularly inefficient. [0005] In recent years oxy-combustion cycles, also known as oxy-fuel cycles, have been developed, wherein fuel, such as natural gas or another fossil fuel, is blended into a mixture of an oxidant consisting mainly of oxygen (O2) and carbon dioxide (CO2) at high pressure. The blend of fuel, oxygen and carbon dioxide burns in a combustor of an expander producing a pressurized flue gas consisting exclusively or almost exclusively of carbon dioxide and water.

[0006] The flue gas is expanded in the expander to generate mechanical power. The exhausted flue gas discharged at the discharge side of the expander is cooled in a regenerative heat exchanger and further chilled to condensate water, which can thus be removed from the chilled flue gas. The low-temperature flue gas, consisting mainly or exclusively of carbon dioxide, is pressurized and recycled through the regenerative heat exchanger towards the combustor of the expander.

[0007] Oxygen supplied to the combustor of the expander can be obtained by separation from ambient air, removing nitrogen therefrom, such that the working fluid supplied to the combustor mainly consists of oxygen and carbon dioxide and does not include nitrogen. The resulting flue gas mainly consists of water and carbon dioxide. Water is removed from the flue gas by condensation and the part of water-free flue gas, which is not recycled to the combustor, can be efficiently processed in a carbon dioxide capturing unit.

[0008] The oxy-fuel cycle summarized above is a semi-closed cycle, in that only a fraction of the flue gas exits the cycle after water has been removed therefrom.

[0009] Oxy-fuel or oxy-combustion cycles, such as those described above, are particularly interesting in terms of efficiency and reduction of noxious emissions. However, they operate under CO2 supercritical conditions at the inlet of the expander and are characterized by a high pressure drop across the expander and a high torque applied to the rotor of the expander. These factors become critical and raise serious challenges in the expander rotor design, when the rated power of the expander increases, and may pose limitations to the maximum power rate of the expander.

[0010] A novel rotor and turbomachine design adapted to achieve higher power rates, for instance in an oxy-fuel cycle, would be welcomed in the art. SUMMARY

[0011] According to one aspect, disclosed herein is a rotor for a power-generating turbomachine, such as in particular for an expander or turboexpander. In some embodiments, the expander is an oxy-fuel combustion expander, adapted to process supercritical carbon dioxide at the inlet of the expander, for instance.

[0012] In embodiments disclosed herein, the rotor includes a plurality of rotor disks which are stacked and connected to one another by a plurality of tie rods. Each disk includes a plurality of rotor blades arranged according to an annular row around the rotation axis of the rotor. The rotor blades form part of an expansion flow path of the power-generating turbomachine, when the rotor is arranged in the turbomachine. The tie rods comprise at least a first tie rod arrangement and a second tie rod arrangement. The first tie rod arrangement includes a plurality of first tie rods parallel to the rotation axis of the rotor at a first distance therefrom. The second tie rod arrangement can include a set of tie rods arranged parallel to the rotation axis of the rotor and at a second distance therefrom, the second distance being smaller than the first distance. Alternatively, or in combination, the second tie rod arrangement incudes a central tie rod coaxial with the rotation axis.

[0013] The double tie rod arrangement provides a strong bound in the axial direction between components of the rotor. The axial bound is adapted to resist high torques and high pressures usually present in a supercritical carbon dioxide power generating turbomachine, such as an expander for an oxy-fuel or oxy-combustion cycle, for instance. A similar tie-rod arrangement can be used in other situations where the torque and/or pressure conditions are critical.

[0014] The rotor further includes a forward shaft portion and an aft shaft portion; wherein the set of rotor disks are arranged between the forward shaft portion and the aft shaft portion. Each tie rod of the first arrangement of tie rods extends from the forward shaft portion to the aft shaft portion and engages the forward shaft portion and aft shaft portion.

[0015] Further embodiments and features of the rotor are set forth in the appended claims and described below.

[0016] According to a further aspect, disclosed herein is a power-generating turbomachine including a rotor as outlined above. In embodiments disclosed herein, the turbomachine is an expander, and may be a supercritical CO2 expander, i.e., an expander wherein the carbon dioxide is in supercritical conditions at the inlet of the expansion flow path, and/or an expander for an oxy-fuel combustion cycle, such as an Allam cycle.

[0017] According to a further aspect, disclosed herein is a thermodynamic circuit using a turbomachine in the form of an expander as outlined above and described in more detail in the following description of exemplary embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Reference is now made briefly to the accompanying drawings, in which:

Fig. l is a schematic of an oxy-fuel power circuit;

Fig.2 is a sectional view along of an expander according to the present disclosure in a first embodiment;

Fig.3 is a sectional view of a rotor for an expander of Fig.2 in an embodiment;

Fig.4 is a sectional view of a rotor for an expander according to the present disclosure in a further embodiment;

Fig.5 is a sectional view according to line V-V of Fig.7 of a rotor for an expander according to the present disclosure in a yet further embodiment;

Fig.6 is a sectional view according to line VI- VI of Fig.7; and

Fig.7 is a sectional view according to line VII- VII of Figs.5 and 6.

DETAILED DESCRIPTION

[0019] The schematic of Fig. 1 illustrates a simplified supercritical carbon dioxide cycle (shortly SCO2 cycle), such as an Allam cycle or as similar oxy-fuel combustion cycle, in which the use of an expander including a rotor according to the present disclosure can be particularly beneficial. In general terms, a supercritical carbon dioxide cycle as understood herein is a cycle wherein carbon dioxide is in a supercritical condition at least at the inlet of the expansion flow path in the expander.

[0020] The power system 1 shown in Fig. l comprises an expander (aka turboexpander) 3 that includes an expansion section 5 and a combustor 7. The combustor 7 can be an annular combustor, a can-type combustor, a can annular combustor, or the like, for instance. In currently preferred embodiments, the combustor is a can-type combustor comprising a plurality of combustion chambers arranged around the rotation axis of the expander 3, as shown in more detail in Fig.2. In some embodiments, as schematically shown in Fig.2, the combustion chambers are housed in a high-pressure casing of the expander, as will described in more detail below. In some embodiments, each combustion chamber is housed in a respective seat formed in a high-pressure casing of the expander, as will be described in more detail below.

[0021] Reference number 7.1 in Fig.2 designates single combustor chambers of a can-type or can annular combustor. In other embodiments, not shown, the combustor can be an annular combustor, as mentioned.

[0022] The combustor 7 is supplied with an oxidant flow delivered by an oxidant source. The oxidant may be oxygen (O2) or a blend comprising, or mainly consisting of oxygen and carbon dioxide (CO2). The oxidant flow can be produced by an air separation unit 9, which features an oxidant source. The air separation unit 9 may remove nitrogen or nitrogen and carbon dioxide from ambient air to produce the required oxidant stream, which is supplied through an oxidant line 11 to the combustor 7 of the expander 3. In some embodiments, for the sake of easier handling of the oxidant flow, the latter may include around 20% in volume of oxygen and 80% in volume of carbon dioxide. The CO2 and O2 percentages mentioned above are by way of example. The carbon dioxide can be added to the oxygen through a recycling line 12, as explained in more detail below.

[0023] Reference number 13 indicates a fuel supply line, for instance adapted to supply natural gas, such as methane to the combustor 7, specifically to each combustion chamber 7.1. The oxidant and the fuel are supplied at the inlet side of the expander 3 to the combustor 7 at high pressure. The upper pressure of the thermodynamic cycle can be for instance 50 barA or above, preferably 100 barA or above, for instance around or above 200 barA, preferably around or above 250 barA, or higher, for instance equal to or higher than 300 barA. In general, the pressure can be lower than 800 barA, or lower than 650 barA. In some embodiments, the upper pressure of the thermodynamic cycle, i.e. the pressure in the combustor and at the first expansion stage can be 100, 150, 200, 250, 300, or 350 barA.

[0024] The oxidant-fuel blend is burned in the combustor 7. Pressurized, hot combustion gas resulting from the combustion expands in the expansion section 5 of the expander 3. The temperature at the exit side of the stationary nozzles downstream the combustor can be comprised between 800°C and 1500°C, for instance.

[0025] The exhausted flue gas is discharged after expansion at a discharge side of the expander 3 in a discharge line 15. The flue gas in the discharge line 15 can be at a temperature between about 400°C and about 700°C, for example around 600°C, and at a pressure which may range between about 10 barA and about 100 barA, preferably between about 20 barA and about 60 barA.

[0026] The circuit further comprises a regenerative heat exchanger 17, wherein hot flue gas flowing through a hot side 17.1 of the regenerative heat exchanger 17 is cooled in heat exchange with a flow of chilled flue gas which flows through a cold side 17.2 of the regenerative heat exchanger 17. The flue gas discharged from the hot side 17.1 of the regenerative heat exchanger 17 is further chilled in a chilling heat exchanger 19 to a temperature which causes condensation of steam contained in the exhausted flue gas. Condensing water is removed from the exhausted flue gas in a water/gas separator 21.

[0027] The de-hydrated exhausted and chilled flue gas, consisting mainly or exclusively of carbon dioxide, is compressed in a flue gas compressor 23 to the pressure at the inlet side of the expander 3. While in the schematic of Fig.1 the flue gas compressor 23 is pictorially represented as a single compressor, in some embodiments a multiple compressor can be used. For instance, the flue gas compressor 23 can be a multi-stage compressor or a compressor train. In some embodiments, the compressor can be an intercooled compressor. In some embodiments, a main compressor can be set in series with two sequentially arranged pumps.

[0028] The compressed flue gas delivered by the flue gas compressor 23 is partly removed from the cycle through a discharge line 25. If the compressor 23 is featured by a plurality of compressing turbomachines arranged in series, the discharge line 25 can be connected between two sequentially arranged turbomachines and/or at the discharge side of the most downstream compressing turbomachine.

[0029] The remaining compressed flue gas is delivered through the cold side 17.2 of the regenerative heat exchanger 17 and is heated by heat exchange with the hot flue gas flowing through the hot side 17.1 of the regenerative heat exchanger 17, and recycled to the expander 3 through a recycle line 25. The flue gas recycled through recycle line 25 is mixed with the combustion gas generated in the combustor 7, or with the oxidant stream from oxidant line 11.

[0030] A side stream of chilled flue gas is delivered through a cooling line 27, which bypasses the regenerative heat exchanger 17, towards components of the expander 3 which require cooling.

[0031] The expander 3 may include an output shaft end 31 which can be integral with the central portion of the rotor, or can be assembled with the central portion of the rotor by bolting, welding, Hirth or spline connections, or the like, or a combination thereof. The mechanical power generated by the expansion of the combustion gas in the expansion section 5 of the expander 3 is available on the output shaft end 31 for mechanical drive or power generation purposes. In the exemplary embodiment of Fig. 1 the output shaft end 31 is drivingly coupled to an electric generator 33 directly or through a gearbox, a joint or combinations thereof. A flanged connection between the outputs shaft end 31 and the electric generator 33 is shown at 49 in Fig.2. The electric generator 33 is in turn electrically coupled to an electric power distribution grid 35. In Figs 1 and 2 the output shaft end 31 is shown at the aft side of the expander 3. In other embodiments, not shown, the output shaft end 31 can be arranged at the forward side of the expander. In yet further embodiments, not shown, two output shafts ends can be provided, one at the forward side and one at the aft side of the expander.

[0032] As used herein “forward” and “aft” are referred to the direction of flow of the process gas through the expander 3. Therefore, “forward” indicates a position on the side of the combustor 7 and “aft” indicates a position on the side opposite the combustor 7, i.e., the discharge side of the expander 3.

[0033] The high pressure drop across the expander 3, the high absolute pressure in the combustor 7 and in the cooling ducts of the expanders, as well as the high torque applied to the expander shaft pose serious challenges in the design of the expander 3, in particular for high rated powers, for instance at or around 150 MW or above, such as between 100 MW and 2000 MW. The expander, and specifically the rotor design disclosed herein, can be used to achieve high power rates, preferably higher than 50 MW, for instance in the order of 100MW o higher, for instance of 150MW or higher, e.g. 200MW or higher, or 300MW or higher. In embodiments, the rated power can be lower than 2000MW, preferably lower than 1500 MW, for instance lower than 1000 MW, or lower than 800 MW. For instance, the rated power can be comprised between 200 MW and 650 M. Intermediate values of the upper and lower limit of each range mentioned above are also expressly disclosed herein.

[0034] With continuing reference to Fig.1, Fig. 2 illustrates a sectional view of the expander 3 in one embodiment. By way of example, the expander of Fig.2 includes a rotor according to Fig.4. In other embodiments, the expander of Fig.2 can include a rotor according to Fig.3 or to Figs. 5, 6 and 7, as disclosed in more detail below.

[0035] In some embodiments, the expander 3 comprises an outer casing 41, which houses the combustor 7. In some embodiments the outer casing 41 includes a main body 41.1, referred to herein as a high-pressure casing, and a closure 41.2, referred to herein as a low-pressure exhaust casing. The high-pressure casing 41.1 can include a main body which can be monolithic, i.e., can consist of a single piece, for instance manufactured by forging, machining, casting, or combinations thereof. In some embodiments, the high-pressure casing 41.1 can be manufactured by welding to one another a plurality of components, preferably interfaced along a plane orthogonal to the rotation axis.

[0036] The low-pressure exhaust casing 41.2 can be positioned at the discharge side, i.e. the aft side, of the expander 3, i.e., on the side opposite the combustor 7. The low- pressure exhaust casing 41.2 can be monolithic, i.e. can consist of a single piece, for instance manufactured by forging, machining, casting, or combinations thereof. In some embodiments, the low-pressure exhaust casing 41.2 can be manufactured in two or more components which can be connected irreversibly to one another, e.g., by welding, or which can be coupled to one another in a reversible manner, e.g., by means of bolts or the like. For instance, the low-pressure exhaust casing 41.2 can be split into two portions along a plane containing the rotation axis of the expander.

[0037] The high-pressure casing 41.1 and the low-pressure exhaust casing 41.2 can be connected to one another along a plane P which is orthogonal to a rotation axis A- A of a rotor 43 supported for rotation in the outer casing 41.

[0038] In some embodiments, the low-pressure exhaust casing 41.2 forms a discharge volute or discharge plenum 41.3, through which exhausted flue gas is discharged from the expander 3.

[0039] Reference numbers 45, 47 indicate bearing arrangements, which rotatingly support the rotor 43. For instance, the bearing arrangement 45 on the side opposite the combustor 7, i.e. on the aft side, may include an axial or thrust bearing in combination with a radial bearing, or a bearing having an axial-radial bearing capability. The bearing arrangement 47 on the combustor side, i.e. on the forward side, may include a radial bearing. A reversed arrangement is also possible, with a bearing having axial load capability arranged on the combustor side. The bearing arrangements 45, 47 can be housed in bearing casings, not shown in detail.

[0040] In some embodiments, the rotor 43 is surrounded by one or more inner casings 51, stationarily housed in the outer casing 41. Each inner casing 51 can be split into two portions along a plane parallel to the rotation axis A-A of the rotor 43, for instance a plane containing the rotation axis A-A. The arrangement of inner casings 51 and outer casing 41 is particularly beneficial when the combustion gas reaches high pressures, around 200-300 barA or higher. The monolithic high-pressure casing 41.1 can withstand the loads generated by the high pressure inside the outer casing, while the inner casings 51 facilitate mounting of the stationary vanes or stationary blades, described below.

[0041] The pressure drop across the expander 3 can be around 150 bar or higher, preferably around 200 bar or higher, for instance between 250 and 400 bar. To expand the combustion gas generated in the combustor 7, a high number of expansion stages is preferred. In the exemplary embodiment of Figs.2 to 7, the expander 3 includes eight stages. In other embodiments, a different number of expansion stages can be foreseen, preferably equal to or higher than four, more preferably equal to or higher than five. In some embodiments, the number of expansion stages can be higher than eight, for instance nine, ten, eleven or more, and preferably lower than fifteen. The expansion stages form an axial expansion flow path for the process gas being expanded in the expander 3.

[0042] Each expansion stage includes an annular row of stationary vanes or stationary blades 53 that are stationarily arranged in the outer casing 41. In the exemplary embodiment of Fig.2, the annular rows of stationary blades are housed in the inner casings 51. Each expansion stage further includes a respective annular row of rotor blades 55, arranged downstream the respective annular row of stationary blades along the expansion flow path, which extends from the combustor 7 to the discharge plenum 41.3 in a forward-to-aft direction, through the expansion section 5.

[0043] In some embodiments, a first annular row of stationary blades 53.1 can be arranged at the discharge end of the combustor 7 and form an array of nozzles directing hot, high-pressure gas from the combustion chambers of the combustor to the first row of rotor blades. A first annular row of rotor blades labeled 55.1 can be arranged directly downstream of the first annular row of stationary blades 53.1 adjacent the combustor 7. A last annular row of stationary blades 53.8 can be positioned near the discharge plenum 41.3, upstream of the last annular row of rotor blades shown at 55.8. Reference number 55 generically refers to any one of the annular rows of rotor blades or to a rotor blade as such.

[0044] The rotor blades 55 form part of the rotor 43, i.e., are connected thereto for co-rotation with the rotor shaft. In embodiments, each annular row of rotor blades 55 (i.e. each row 55. i, with i=l to 8) is connected to a respective rotor disk. The rotor disks are labeled 57. i, with i =1 to 8. Reference number 57 indicates a generic rotor disk. The structure of the rotor 43 in different embodiments is illustrated in Figs. 2 to 7.

[0045] More specifically, Fig.4 illustrates an enlargement of the rotor shown in Fig.2, while Fig.3 illustrates an alternative embodiment of a rotor with a similar structure. A modified rotor, to be described in more detail later, is shown in Figs. 5 to 7. In Figs 5- 7 the rotor blades 55 are omitted for clarity.

[0046] The rotor blades of each stage can be manufactured separately from the respective rotor disk 57 and mechanically mounted thereon. In other embodiments, the rotor blades and the rotor disk of each stage can be manufactured as a monolithic body, for instance by additive manufacturing. In yet further embodiments, the two design options can be combined. One or some stages may include respective monolithically manufactured components including rotor blades and disks, and one or some stages may include a rotor disk and separately manufactured rotor blades mechanically coupled to the rotor disk.

[0047] The rotor disks 57 are and connected to one another by tie rods. In the exemplary embodiment of Figs.2 to 4, the tie rods comprise a first tie rod arrangement including a plurality of tie rods 61 and a second tie rod arrangement including a single tie rod 63.

[0048] More specifically, the tie rods 61 of the first tie rod arrangement are arranged around the rotation axis A-A of the rotor 43, at a distance from the rotation axis A-A. Preferably, all tie rods 61 are positioned at the same distance dl from the rotation axis A-A of the rotor 41. Preferably, the tie rods 61 are distributed with a constant angular pitch around the rotation axis A-A of the rotor 43.

[0049] In these embodiments, the single central tie rod 63 of the second tie rod arrangement is co-axial to the rotor 43.

[0050] In some embodiments, the rotor 43 further comprises a forward shaft portion 65 and an aft shaft portion 67. Each one of said forward shaft portion 65 and aft shaft portion 67 can include one or more sections, connected to one another, e.g. by means of tie rods.

[0051] In some embodiments, see Fig.2, the combustor 7 extends around the forward shaft portion 65. In some embodiments, the discharge plenum 41.3 extends around the aft shaft portion 67.

[0052] In some embodiments, the forward shaft portion 65 and the aft shaft portion 67 are connected to the rotor disks 57 in a stacked configuration by the tie rod arrangements mentioned above.

[0053] More specifically, in the embodiment of Figs 2 and 4, the first tie rods 61 of the first tie rod arrangement extend through respective through holes of the rotor disks 57 and include a first end 61.1 connected to the forward shaft portion 65 and a second end 61.2 connected to the aft shaft portion 67. The first end portion 61.1 and the second end 61.2 of each tie rod 61 is threaded for engagement with a respective nut 62.1 and 62.2.

[0054] In some embodiments, each rotor disk 57 comprises front teeth on each side thereof, for torsional engagement with respective front teeth of the adj acent rotor disks, the forward shaft portion 65, and/or the aft shaft portion 67. The front teeth form Hirth joints between rotor disks 57 and between the first and last rotor disk 57.1, 57.8 and the forward shaft portion 65 and aft shaft portion 67, respectively.

[0055] In some embodiments, a respective sealing disk 68, also known as distancing disk, is positioned between each pair of consecutive (i.e., adjacent) rotor disks 57. If sealing disks 68 are interspersed between pairs of rotor disks 57, front teeth are provided on the opposite sides of each sealing disk 68, and consecutively arranged rotor disks 57 are torsionally connected to one another through the interposed sealing disk 68, Hirth joints being formed between opposing sides of each rotor disk 57 and respective sealing disk 68. As used herein, “torsionally connected” means a connection adapted to transmit a torque between the torsionally connected components, such that the torsionally connected components rotate as a single body around the rotation axis A-A of the rotor.

[0056] The provision of front teeth can be beneficial to increase the torque which can be transmitted from the forward shaft portion 65 to the first rotor disk 57.1, from the last rotor disk 57.8 to the aft shaft portion 67, as well as between each rotor disk 57 and the adjacent rotor disk or sealing disk 68. In some embodiments, front teeth can be provided only in some sections of the rotor, where the torque to be transmitted is higher, i.e., in the most downstream section of the rotor 43, for instance. The region where the highest torque shall be transmitted depends on where the load is applied, i.e. whether on the aft side (opposite the combustor 7) or on the forward side (the side of the combustor 7) of the expander 3. Output shafts on both sides (aft and forward) can also be foreseen.

[0057] While in Figs. 2, 3 and 4, the sealing disks or spacers 68 extend radially inwardly between adjacent rotor discs 57, in other embodiments the rotor disks 57 can be stacked in direct mutual contact with one another, and may be provided with front teeth of respective Hirth joints, which transmit torque from one rotor disk 57 to the adjacent rotor disk 57 directly. In this case, annular seal runners can be provided between adjacent rotor disks.

[0058] In both cases, the seal runners or the sealing disks 68 provide a seal against the stationary blades to prevent gas from leaking outside the expansion flow path defined by interspersed rows of stationary blades and rotor blades.

[0059] In the exemplary embodiment of Figs. 2 and 4, the central tie rod 63 has a first threaded end 63.1 , which extends through a central hole 65.1 of the forward shaft portion 65. The central tie rod 63 extends through the disks 57 and has a second threaded end 63.2 threadedly engaged in a threaded blind hole 67.2 of the aft shaft portion 67. A nut 71.1 is threadedly engaged on the first threaded end 63.1 of the central tie rod 63, to tighten the forward shaft portion 65, the aft shaft portion 67, the eight rotor disks 57.1-57.8, and the sealing disks 68 (if present) to one another in axial direction. In other embodiments, not shown, the aft shaft portion 67 can be provided with a through hole and the second threaded end 63.2 of the central tie rod 63 can be engaged by a nut similar to nut 71.1.

[0060] With the tie rod arrangements described above, an enhanced axial connection between components of the rotor 43 is achieved, which allows the rotor to withstand high axial thrust generated in use by the pressure drop across the expander 3.

[0061] Enhanced coupling achieved by the double tie rod arrangement is also beneficial in that it increases the torque which can be transmitted through the rotor to the driven shaft.

[0062] The double tie rod arrangement has further beneficial effects regarding cooling of the rotor. Cooling is performed by feeding a pressurized cooling fluid to the rotor 43. Specifically, in some embodiments, compressed and cooled carbon dioxide can be delivered to a cooling chamber 66 in the interior of the rotor 43. For instance, pressurized carbon dioxide delivered by the flue gas compressor 23 through cooling line 27 (Fig.1) can be fed in the cooling chamber 66 and can flow through radial holes (not shown) in the rotor 43 to buffer and cool spaces 58 between adjacent disks 57. The double tie rod arrangement can withstand the load generated by the high-pressure cooling carbon dioxide delivered to the interior of the rotor 43.

[0063] In some embodiments, the expander 3 comprises a balance drum drivingly coupled to the rotor 43. In the embodiment of Figs. 2 and 4 a balance drum 75 is provided at the forward end of the rotor 43, between the first expander stage and the forward bearing arrangement 47.

[0064] In some embodiments the balance drum 75 is integrally formed with the forward shaft portion 65.

[0065] In some embodiments, as shown in Figs. 2 and 4, the forward shaft portion 65 includes two sections 65 A and 65B. The two sections 65 A, 65B of the forward shaft portion 65 can be connected to one another by one tie rod. Preferably, as shown in Figs. 2, 2A, the two sections 65A, 65B are connected to one another by a plurality of tie rods 77, arranged around the rotation axis A-A of the rotor 43. The tie rods 77 extend through holes provided in two flanges 75.1, 75.2 of the two sections 65A, 65B of the forward shaft portion 65, wherein the flanges 75.1, 75.2 cumulatively form the balance drum 75.

[0066] By splitting the balance drum into balance drum portions 75.1, 75.2, manufacturing of the balance drum, for instance by forging, is facilitated and provides the possibility to assemble and tighten the central tie rod 63.

[0067] With continuing reference to Figs 2 and 4, a further embodiment of a rotor 43 for the expander 3 is shown in Fig. 3. The same reference numbers in Figs 2, 4 and in Fig. 3 are used to designate the same or equivalent components, parts or elements of the rotor 43, which will not be described again.

[0068] The rotor 43 of Fig.3 differs from the rotor of Figs 2 and 4 mainly in that the second threaded end 63.2 of the central tie rod 63 engages in a through hole 64.1 of an inner flange 64 integrally formed with the disk 57.6. A nut 71.2 is screwed on the threaded end 63.2 of the central tie rod 63.

[0069] In this embodiment the central tie rod 63 contributes to the axial force connecting to one another only some rotor disks. Specifically, in the embodiment illustrated in Fig. 3 the central tie rod 63 connects rotor disks 57.1-57.6, the respective sealing disks 68 and the forward shaft portion 65 of the rotor 43. This block is in turn mechanically connected to the remaining disks 57.7, 57.8, and to the aft shaft portion 67, by the tie rods 61 of the first tie rod arrangement.

[0070] While in the embodiments described above the second tie rod arrangement includes a single central tie rod 63 coaxial with the rotor 43, in other embodiments the second tie rod arrangement can include a plurality of second tie rods as an alternative to, or in combination with, the central tie rod. The second tie rods are arranged around the rotation axis A-A of the rotor 43 and extend parallel thereto in quite the same manner as the tie rods 61 of the first tie rod arrangement.

[0071] An embodiment with a second tie rod arrangement including a plurality of tie rods is shown in Figs. 5, 6 and 7, wherein Fig.7 is a section of the rotor according to a plane orthogonal to the rotation axis A-A of the rotor 43. The same reference numbers are used in Figs 5, 6 and 7 to designate similar or corresponding parts, elements or components, which have been described in connection with Figs 2, 3 and 4. These parts will not be described again.

[0072] The tie rods of the second tie rod arrangement shown in Figs 5, 6 and 7 are arranged at a distance d2 from the rotation axis A-A of the rotor 43. The distance d2 is smaller than the distance dl. In the embodiment of Figs.5, 6 and 7 the tie rods 63 extend axially from the forward shaft portion 65 to the aft shaft portion 67 and engage both said forward shaft portion and aft shaft portion. In other embodiments, not shown, the tie rods 63 may have a shorter axial length and engage with an inner flange of one of the rotor disks 57, for instance the sixth rotor disk 57.6, quite in the same way as shown in Fig.3.

[0073] Exemplary embodiments have been disclosed above and illustrated in the accompanying drawings. It will be understood by those skilled in the art that various changes, omissions and additions may be made to that which is specifically disclosed herein without departing from the scope of the invention as defined in the following claims.

[0074] For instance, while in the embodiment disclosed herein the combustor is housed in the expander, in other embodiments, the combustor can be arranged outside the expander.

[0075] In yet further embodiments, the expander disclosed herein can be used in a close-loop thermodynamic cycle, such as a supercritical carbon dioxide cycle, where heat is introduced in the thermodynamic cycle through a heat exchanger, rather than using a combustor.

[0076] Moreover, while in the illustrated embodiments the aft shaft portion 67 is manufactured as a monolithic single body, in other embodiments the aft shaft portion 67 can be split in two or more sections, quite in the same way as the forward shaft portion 65. The two sections can be connected to one another by the same tie rod 63. Alternatively, additional rods, similar to rods 77, can connect the two sections of the aft shaft portion 67 to one another.

Claims

1. A rotor for a power-generating turbomachine, the rotor comprising: a set of rotor disks; wherein each rotor disk comprises a respective annular row of rotor blades of an expansion flow path of the power-generating turbomachine; wherein each annular row of rotor blades surrounds a rotation axis of the rotor; and a forward shaft portion and an aft shaft portion; wherein the set of rotor disks are arranged between the forward shaft portion and the aft shaft portion; wherein the forward shaft portion, the aft shaft portion and the rotor disks are stacked and connected to one another by a plurality of tie rods comprising at least: a first tie rod arrangement comprising a plurality of first tie rods parallel to the rotation axis of the rotor at a first distance therefrom; and a second tie rod arrangement; wherein the second tie rod arrangement comprises:

(i) a set of tie rods arranged parallel to the rotation axis of the rotor and at a second distance therefrom, the second distance being smaller than the first distance; or

(ii) a central tie rod coaxial with the rotation axis of the rotor; or

(iii) a set of tie rods arranged parallel to the rotation axis of the rotor and at a second distance therefrom, the second distance being smaller than the first distance, and a central tie rod coaxial with the rotation axis of the rotor; and wherein each of said first tie rods extends from the forward shaft portion to the aft shaft portion and engages the forward shaft portion and the aft shaft portion.

2. The rotor of claim 1, wherein the central tie rod has a first end engaging the forward shaft portion and a second end engaging the aft shaft portion.

3. The rotor of claim 1, wherein the central tie rod has a first end engaging one of the forward shaft portion and aft shaft portion; and wherein the central tie rod has a second end engaging one of said rotor disks intermediate the forward shaft portion and the aft shaft portion.

4. The rotor of claim 3, wherein the first end of the central tie rod engages the forward shaft portion.

5. The rotor of claim 1, wherein each tie rod of the second tie rod arrangement extends from the forward shaft portion to the aft shaft portion and engages the forward shaft portion and aft shaft portion.

6. The rotor of one or more of the preceding claims, wherein the rotor further comprises sealing disks interposed between adjacent rotor disks and torsionally coupled therewith.

7. The rotor of one or more of the preceding claims, wherein at least one of said rotor disks includes front teeth adapted to rotationally couple said rotor disk to at least one of: an adjacent rotor disk, a forward shaft portion, an aft shaft portion, and an adjacent sealing disk, or any combination thereof.

8. The rotor of one or more of the preceding claims, further comprising a balance drum.

9. The rotor of claim 8, wherein the balance drum is formed on one of the forward shaft portion and aft shaft portion, preferably on the forward shaft portion.

10. The rotor of claim 9, wherein the balance drum comprises a first balance drum portion and a second balance drum portion connected to one another through at least one tie rod and preferably through a plurality of tie rods arranged peripherally around the rotation axis of the rotor.

11. The rotor of one or more of the preceding claims, comprising at least four expander stages, preferably at least five expander stages.

12. The rotor of one or more of the preceding claims, wherein the rotor comprises an axial cooling chamber adapted to receive a cooling fluid and radially fluidly connected to annular spacers between adjacent disks of the rotor.

13. A power-generating turbomachine, comprising: an outer casing; a plurality of annular rows of stationary blades arranged in the casing; and a rotor according to any one of the preceding claims, housed for rotation in the casing; wherein each annular row of rotor blades and a respective annular row of stationary blades upstream thereof form an expansion stage of the expansion flow path.

14. The turbomachine of claim 13, wherein the turbomachine is a powergenerating turbomachine, in particular a gas turbine or an expander.

15. The turbomachine of claim 13 or 14, wherein the outer casing comprises a high-pressure casing and a low-pressure exhaust casing; wherein the high- pressure casing and the low-pressure exhaust casing are coupled along a plane orthogonal to the rotation axis of the rotor.

16. The turbomachine of claim 15, wherein the high-pressure casing comprises a monolithic barrel body.

17. The turbomachine of claim 15 or 16, wherein the low-pressure exhaust casing is configured as a monolithic body.

18. The turbomachine of any one of claims 13 to 17, further comprising at least one combustor housed in the outer casing.

19. The turbomachine of claim 18, when dependent upon claim 16, wherein the at least one combustor is housed in a seat formed in the high-pressure casing.

20. The turbomachine of claim 19, wherein the low-pressure exhaust casing is arranged at a low-pressure side of the expander, opposite the combustor; and wherein the low-pressure exhaust casing forms a discharge plenum.

21. The turbomachine of any one of claims 13 to 20, further comprising at least one inner casing, and preferably a plurality of inner casings, stationarily housed in the outer casing, and surrounding the rotor; wherein each inner casing is split into a first casing portion and a second casing portion along a plane parallel to the rotation axis of the rotor; and wherein the inner casing(s) contains annular rows of stationary blades.

22. The turbomachine of any one of claims 13 to 21, wherein the rotor is adapted to receive process gas at a temperature T comprised between 800°C and 1500°C.

23. The turbomachine of any one of claims 13 to 22, wherein the rotor is adapted to receive process gas at a pressure higher than 50 bar A, preferably higher than 100 bar A, more preferably equal to or higher than 200 bar A, and preferably lower than 800 barA, more preferably lower than 650 barA.

24. The turbomachine of any one of claims 13 to 23, adapted to generate a power higher than 50 MW, preferably equal to or higher than 100 MW, preferably lower than 2000 MW, more preferably lower than 1500 MW.

25. A supercritical carbon dioxide thermodynamic circuit comprising: an oxidant source; an expander with an inlet side and a discharge side, wherein the inlet side is fluidly coupled to the oxidant source; a flue gas recycling line, adapted to recycle flue gas from the discharge side of the expander to a combustor of the expander; in the flue gas recycling line, a cooler adapted to cool the flue gas from the discharge side of the expander and condense water contained in the flue gas; a regenerative heat exchanger, wherein flue gas from the expander flows in heat exchange with chilled flue gas from the cooler; wherein the expander is an expander according to any one of claims 13 to 24.