EP4689362A1

EP4689362A1 - An expander for oxy-fuel combustion cycles and the like

Info

Publication number: EP4689362A1
Application number: EP24716636.6A
Authority: EP
Inventors: Frederico BUCCIARELLI; Enrico Giusti; Massimiliano Mariotti; Damaso CHECCACCI; Lorenzo Cosi; Giancarlo GAUDIUSO; Lorenzo Succi
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: AU2024246603A1; MX2025011225A; KR20250159273A; WO2024199731A1; IT202300006339A1; JP2026510466A; CN120936791A

Abstract

The supercritical expander comprises an outer casing and a rotor housed in the outer casing for rotation around a rotation axis. At least one combustor is housed in the casing or associated to the casing. The outer casing comprises a high-pressure casing and a low-pressure exhaust casing coupled to one another with a flange connection along a plane orthogonal to the rotation axis. The low-pressure exhaust casing comprises a discharge plenum, adapted to collect combustion gas expanded through a gas expansion flow path; the low-pressure exhaust casing comprising at least one exhaust aperture.

Description

AN EXPANDER FOR OXY-FUEL COMBUSTION CYCLES AND THE LIKE

DESCRIPTION

TECHNICAL FIELD

[0001] The present disclosure concerns gas expanders particularly adapted for use in oxy-fuel combustion cycles operating with process gas at high pressures, for instance CO2 cycles (SCO2 cycles), such as Allam cycles., aka NET Power Cycle.

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 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 are high, both in term 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 or oxy- fuel combustion 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, oxidant and carbon dioxide bums 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 exhaust 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 combustion cycles, such as those described above, are particularly interesting in terms of efficiency, reduction of noxious emissions and CO2 sequestration. However, they operate under CO2 supercritical conditions at the inlet of the expander and are characterized by high pressure values inside the expander casing. These operating conditions pose difficult constraints in the casing design.

[0010] A novel expander design adapted to achieve higher power rates in an oxy-fuel combustion cycle would be welcomed in the art.

SUMMARY

[0011] To address the above-mentioned needs, disclosed herein is a high-pressure expander comprising an outer casing, a rotor housed in the outer casing for rotation around a rotation axis, and at least one combustor. The outer casing comprises a high- pressure casing and a low-pressure exhaust casing coupled to one another with a flange connection along a plane orthogonal to the rotation axis of the rotor. The low-pressure exhaust casing comprises a discharge plenum, adapted to collect combustion gas expanded through a gas expansion flow path and at least one exhaust aperture.

[0012] Specifically, the expander can be a supercritical expander. As understood herein, a supercritical expander is an expander adapted to receive a working fluid in a supercritical condition at the first expansion stage. Embodiments described herein are particularly adapted as CO2 supercritical expanders, i.e. expanders adapted to expand carbon dioxide starting from a supercritical condition.

[0013] The combustor can be mounted on or around the casing of the expander. In some embodiments the combustor is at least partly housed in the casing.

[0014] Further features and embodiments of the expander are described below with reference to the enclosed drawings and outlined in the appended claims.

[0015] According to a further aspect, disclosed herein is an oxy-fuel combustion system, comprising an expander as outlined above, and a compression system adapted to provide a flow of compressed process gas to the expander.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] 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 of an expander in one embodiment;

Figs. 3, 4, 5, and 6 illustrate axonometric views of a low-pressure exhaust casing with multiple discharge apertures, in several embodiments;

Figs. 7, 8, and 9 illustrate sectional views of discharge apertures of the low-pressure exhaust casing with measures adapted to prevent thermal damages and reduce thermal load on the low-pressure exhaust casing, in several embodiments;

Figs.10 to 21 illustrates details of the coupling between the high-pressure casing and the low-pressure exhaust casing in several embodiments;

Fig.22 illustrates a schematic sectional view of an expander designed for forward insertion of the rotor;

Fig.23 is a sectional view of two adjacent combustion chambers; and

Figs. 24, 25, 26 and 27 are schematic representation of alternative arrangements of the exhaust apertures provided in the low-pressure exhaust casing.

DETAILED DESCRIPTION

[0017] The schematic of Fig. 1 illustrates a simplified oxy -fuel cycle operating with supercritical carbon dioxide at the expander inlet (shortly SCO2 cycle), such as an Al- lam cycle or NET Power oxy -fuel cycle.

[0018] The power system 1 shown in Fig.l comprises an expander 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 combustor chambers arranged around the rotation axis of the expander 3, as shown in more detail in Fig.2. The combustor chambers are housed in an outer casing of the expander, as will described in more detail below.

[0019] Reference number 7.1 in Fig.2 designates the combustion chamber of the combustor of an annular combustor or each combustion chamber of a can-type or can annular combustor. In some embodiments, each combustion chamber 7.1 is housed in a respective seat 41.4 formed in the high-pressure casing 41.1. The combustion chambers 7.1 are arranged circumferentially around the rotation axis of the expander 3.

[0020] The combustor 7 is supplied with an oxidant flow delivered by an oxidant source. The oxidant may be oxygen (O2) or a blend 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.

[0021] 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 combustor 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, for instance at 50 barA or higher, preferably equal to or higher than 100 barA, more preferably equal to or higher than 150 barA, more preferably equal to or higher than 200 barA. In some embodiments, the upper pressure of the cycle can be equal to or above 250 barA, or higher, for instance at or above 300 barA. 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.

[0022] In some embodiments, the temperature at the inlet of the gas expansion flow path, i.e., at the inlet of the rotor can be at or above 800°C, and preferably at or below 1500°C.

[0023] The exhausted flue gas is discharged at a discharge side of the expander 3 in a discharge line 15. The flue gas in the discharge line 15 can be at around 600°C, for instance, and at a pressure which may range between 10 barA and 100 barA, for instance between 20 barA and 60 barA.

[0024] The power rate of the expander 3 can be higher than 50MW, for instance equal to or higher than 100 MW, for instance 150 MW or higher, e.g. 200 MW or higher. In embodiments the rated power is equal to or higher than 300 MW. In some embodiments the rated power is equal to or lower than 2000 MW, for instance equal to or lower than 1500MW, or equal to or lower than 1000 MW, For instance the rated power can be comprised between 200 MW and 650 MW. Intermediate values of the upper and lower limit of each range mentioned above are also expressly disclosed herein.

[0025] 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 in 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.

[0026] 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 and can include one or more intercoolers.

[0027] The compressed flue gas delivered by the flue gas compressor 23 is partly removed from the cycle through a discharge line 25. The major part of the 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.

[0028] 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. A further side stream of chilled, dehydrated flue gas can be delivered through a line 28 to the air separator 9 and/or to the oxidant line 11 to add carbon dioxide to the oxygen delivered to the combustor 7.

[0029] 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. 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.

[0030] 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.

[0031] Fig. 2 illustrates a sectional view of the expander 3 in one embodiment. The expander 3 can comprise an outer casing 41, which houses the combustor 7. In embodiments, the outer casing 41 includes a high-pressure casing 41.1 and a low-pressure exhaust casing 41.2. The high-pressure casing 41.1 can be in the form of a barrel including a monolithic body, for example manufactured by forging, casting, or combination thereof. The monolithic body extends around the longitudinal axis of the expander, i.e., around the rotation axis.

[0032] While in some embodiments the monolithic body is manufactured starting from a single block, as understood herein a monolithic body may also comprise a body assembled by several portions manufactured separately from one another and subsequently assembled to one another by welded to obtain the final monolithic body. The several portions are irreversibly coupled to one another. To achieve a higher mechanical strength, each part of the monolithic body preferably extends around the rotation axis of the expander, i.e., the parts of the monolithic body are coupled to one another along surfaces extending transversely to the rotation axis. Each part has an annular structure, i.e. a continuous structure surrounding the rotation axis. As understood herein, a continuous structure surrounding the rotation axis of the expander is a structure, wherein closed lines surrounding the rotation axis of the expander do not cross a welding or mechanical interface between parts forming the monolithic body.

[0033] A continuous structure surrounding the rotation axis has a higher mechanical resistance towards radial and tangential loads generated by internal pressure.

[0034] The single piece formed by the monolithic body (either manufactured from a single piece or by individual components welded together and forming the final barrel with no reversible mechanical connections) provides suitable resistance against high pressure of the process fluid inside the expander 3 and specifically the most forward portion thereof.

[0035] The low-pressure exhaust casing 41.2 can be positioned on the discharge side, i.e. the aft side, of the expander 3, i.e., on the side opposite the combustor 7. Also, the low-pressure exhaust casing 41.2 can be monolithic, i.e., can consist of a single piece as mentioned above having a continuous structure developing around the rotation axis.

[0036] In other embodiments, the low-pressure exhaust casing 41.2 can be manufactured in two or more pieces, connected to one another. For instance, the low-pressure exhaust casing 41.2 can be manufactured as a two-pieces body separated along a plane containing the rotation axis of the rotor. The two pieces can be welded to one another, i.e. they can be connected irreversibly to one another. In other embodiments, the pieces whereof the low-pressure exhaust casing 41.2 is made can be coupled to one another by tie rods, screw bolts, or other reversible coupling means. By reversible coupling means, coupling devices are understood which allow the two or more pieces forming the casing to be separated again without irreversible damages thereto.

[0037] The high-pressure casing 41.1 and 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. The outer casing 41 is therefore a so-called “vertically split” casing.

[0038] In some embodiments, the low-pressure exhaust casing 41.2 forms a 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 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 may include a radial bearing. A reversed arrangement is also possible, with a bearing having axial load capability arranged on the combustor side.

[0040] The output end 31 of the rotor 43 can be drivingly coupled to the driven machine (electric generator 33) through flanges 49. The bearing arrangements 45, 47 can be arranged in bearing housings, not shown in detail.

[0041] The rotor 43 is surrounded by an inner casing 51, which can be formed by plurality of sections arranged in sequence in a forward-to-aft direction. In Fig.2 the inner casing 1 comprises two casing sections sequentially arranged in the forward-to- aft direction, i.e., parallel to the rotation axis. The inner casing 51 can be horizontally split, i.e. can include two portions which are coupled to one another along a plane containing the rotation axis of the rotor 43. If the inner casing comprises two casing sections arranged in sequence in the axial direction, each section can in turn be split into two portions along a plan containing the rotation axis of the rotor 43.

[0042] The inner casing 51 is fully or partly housed in the high-pressure casing 41.1. In some embodiments, as shown in Fig.2, the inner casing 51 projects in the low-pressure exhaust casing 41.2.

[0043] In some embodiments, the inner casing 51 is provided with cooling ducts, one of which is schematically shown at 51.1 in Fig.2. The cooling ducts provide a fluid coupling between one or each of the annular fluid chambers 42.1, 42.2 with the interior of the inner casing 51. A cooling duct, such as compressed recycled flue gas consisting mainly of carbon dioxide, can flow from the annular fluid chambers into the interior of the inner casing 51 to cool or purge annular cavities inside the inner casing 51. External cooling ducts can be provided in combination or as an alternative to cooling ducts extending through the inner casing.

[0044] One or more annular fluid chambers 42 are formed between the inner casing 51 and the outer casing 41. Specifically, in the exemplar embodiment shown in Fig.2 the annular chamber 42 includes two sequentially arranged annular fluid chambers 42.1 and 42.2 separated by a septum 44. The fluid pressure inside the two annular fluid chambers 42.1 and 42.2 can be different. For instance, the forward annular fluid chamber 42.1 can be at a higher pressure than the aft annular fluid chamber 42.2. In use, at steady state conditions, the aft fluid chamber 42.2 and the forward fluid chamber 42.1 can be fed with chilling or cooling fluid, e.g. with chilled, dehydrated flue gas from cooling line 27.

[0045] The septum 44 or other connection member adapted to connect the inner casing 51 to the outer casing 41 is configured such as to allow differential thermal expansions of the inner casing 51 and outer casing 41 to take into account the different temperature conditions of the casings at steady state operating conditions and during startup and shut-down transients. [0046] The expander can be adapted to expand the combustion gas through the gas expansion flow path with a pressure drop of at least 150 bar, preferably of at least 250 bar, more preferably 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 Fig.2, the expander 3 includes eight stages, each configured as an axial expansion stage. 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.

[0047] Each expansion stage includes an annular row of stationary blades or stationary vanes 53 that are stationarily arranged in the inner casing 51. Each expansion stage further includes a respective annular row of rotor blades 55, arranged downstream the respective annular row of stationary blades 53 along an expansion flow path which extends from the combustor 7 through the expansion section 5 to the discharge plenum 41.3 in a forward-to-aft direction.

[0048] The rotor blades 55 form part of the rotor 43, i.e., they are connected thereto for co-rotation with the rotor shaft. In some embodiments, each annular row of rotor blades 55 is connected to a respective rotor disk, not shown in detail. The structure of the rotor and of the rotor disks is not relevant and is not shown in detail.

[0049] In embodiments, the rotor 43 further comprises a forward shaft portion 65 and an aft shaft portion 67. In embodiments, 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.

[0050] A balance drum 69 can be constrained to the rotor 43 for co-rotation therewith. In the embodiment of Fig.2 the balance drum 69 includes a first balance drum portion 69A and a second balance drum portion 69B connected to one another by tie rods 70.

[0051] In the embodiment of Fig.2 the expander 3 is configured such that the rotor 43 is mounted in the outer casing 41 from the aft side, i.e. from the low-pressure side of the expander. In some embodiments, a bundle comprising the inner casing 51 and the rotor 43 is assembled and introduced axially from the aft side into the high-pressure casing 41.1. Finally, the low-pressure exhaust casing 41.2 is mounted on the high- pressure casing 41.1.

[0052] In some embodiments the low-pressure exhaust casing 41.2 can include a plurality of exhaust apertures, rather than a single exhaust aperture. The provision of multiple exhaust apertures results in a structure of the low-pressure exhaust casing 41.2 which is more suited to resist to the high pressure and temperature values involved. The total cross-sectional area of the multiple exhaust apertures can be the same as if one single aperture where provided, but the overall structure is mechanically more resistant.

[0053] Figs. 3, 4, 5 and 6 show axonometric views of the low-pressure exhaust casing 41.2 in various embodiments, each including multiple exhaust apertures.

[0054] Fig.3 illustrates an axonometric view from the aft side of the low-pressure exhaust casing 41.2 in an embodiment comprising two exhaust apertures 46.1, 46.2 oriented radially with respect to the axis of the expander 3, i.e., the rotation axis of the rotor 43. In the embodiment of Fig.3, the two exhaust apertures 46.1, 46.2 are arranged symmetrically around the rotation axis and with respect to a plane containing the rotation axis.

[0055] Fig.4 illustrates an axonometric view from the aft side of the low-pressure exhaust casing 41.2 in an embodiment comprising two exhaust apertures 46.3, 46.4 oriented tangentially with respect to the axis of the expander 3, i.e. the rotation axis of the rotor 43. In the embodiment of Fig.4 the two exhaust apertures 46.3, 46.4 are symmetrical with respect to the rotation axis.

[0056] Fig.5 illustrates an axonometric view from the aft side of the low-pressure exhaust casing 41.2 in an embodiment comprising four exhaust apertures 46.5, 46.6, 46.7. 46.8 oriented radially with respect to the axis of the expander 3, i.e. the rotation axis of the rotor 43. In the embodiment of Fig.5 the four exhaust apertures 46.1, 46.2 are arranged symmetrically around the rotation axis and with respect to a plane containing the rotation axis.

[0057] Fig.6 illustrates an axonometric view from the aft side of the low-pressure exhaust casing 41.2 in an embodiment comprising four exhaust apertures 46.9, 46.10, 46.11, 46.12, oriented tangentially with respect to the axis of the expander 3, i.e. the rotation axis of the rotor 43. In the embodiment of Fig.6 the two exhaust apertures 46.9, 46.10 and the two exhaust apertures 46.11, 46.12 are symmetrical with respect to a plane containing the rotation axis.

[0058] In other embodiments, the low-pressure exhaust casing 41.2 can comprise a different number or a different arrangement of the exhaust apertures. For instance, in some embodiments the low-pressure exhaust casing 41.2 can include three exhaust apertures.

[0059] For instance, in some embodiments the exhaust apertures can be arranged at a constant pitch according to a tangential direction. Fig.24 shows such an arrangement with four exhaust apertures 46 at a constant pitch of 90° from one another. Fig.26 illustrates a similar arrangement with only three exhaust apertures 46 at 120° from one another in a tangential direction.

[0060] In Fig.27 the three exhaust apertures 46 are at a constant pitch of 120° from one another but are oriented radially rather than tangentially as in Fig.26.

[0061] In Fig.25 two exhaust apertures 46 (in the lower part of the low-pressure exhaust casing in the embodiment) are arranged parallel to one another and two exhaust apertures 46 (in the upper part of the low-pressure exhaust casing) are oriented with an inclination over the horizontal but neither parallel to one another nor coaxial with the opposite exhaust apertures at the bottom side of the low-pressure exhaust casing. In other embodiments, not shown, the upper apertures can be coaxial.

[0062] In some embodiments the exhaust apertures can have the same cross sectional area. In other embodiments, the exhaust apertures may have different cross sectional areas.

[0063] In some embodiments the low-pressure exhaust casing 41.2 can be made of a metal alloy adapted to withstand the high temperatures of the exhaust flue gas after expansion. In such case the low-pressure exhaust casing 41.2 can be devoid of any thermal shield, as shown schematically in Fig.7. In some embodiments the low-pressure exhaust casing 41.2 of Fig. 7 is made of a cast nickel-based alloy, adapted to withstand temperatures in the range of 600°C in a continuous operating mode of the expander 3. In other embodiments, the low-pressure exhaust casing can be made of austenitic steels or Fe-based alloys, or any other metal alloy adapted to withstand the operating conditions in terms of chemical and thermal resistance.

[0064] In other embodiments, the exhaust aperture(s) of the low-pressure exhaust casing 41.2 can be provided with an inner thermal protection, adapted to reduce heat exchange between the exhaust flue gas and the inner surface of the low-pressure exhaust casing, specifically the inner surface of the ducts forming the exhaust aperture(s). Figs. 8 and 9 show a sectional view of a single exhaust aperture of the low-pressure exhaust casing 41.2 in two embodiments.

[0065] In Fig.8 a thermal shield or a thermal cladding 48 is arranged inside a generic exhaust aperture 46. The thermal shield 48 can be manufactured with a metal alloy able to withstand high temperatures for a long period of time, thus allowing continuous operation of the expander 3. In some embodiments, the thermal shield 48 can be made of a nickel-based alloy while the low-pressure exhaust casing 41.2 can be made of ferritic-martensitic steel provided with a suitable protective weld overlay, or austenitic steel.

[0066] In some embodiments, an additional thermal insulation 48.2 can be provided between the thermal shield 48 and the inner surface of the low-pressure exhaust casing 41.2.

[0067] In Fig.9 a thermal shield 48 is provided inside the exhaust aperture 46 and a cooling chamber or jacket 48.1 is formed between the thermal shield 48 and the low- pressure exhaust casing 41.1. A cooling gas at suitable temperature, for instance recycled flue gas mainly consisting of carbon dioxide can be circulated in the cooling chamber or jacket 48.1. The thermal shield 48 of Fig.9 can be made of a nickel-based alloy and the low-pressure exhaust casing 41.2 can be made of ferritic-martensitic steel or austenitic steel, similarly to the embodiment of Fig.8.

[0068] Efficient connection between the high-pressure casing 41.1 and low-pressure exhaust casing 41.2 can be obtained through suitably shaped flanges and relevant connection means. In Fig.2 the high-pressure casing 41.1 comprises a high-pressure closure flange 41.5 and the low-pressure exhaust casing comprises a low-pressure closure flange 41.6. The high-pressure closure flange 41.5 and the low-pressure closure flange 41.6 are coupled to one another by a set of studs or bolts shown in more detail and various embodiments in Figs. 10 to 21

[0069] With continuing reference to Figs 2 to 9, Fig.10 shows a sectional view of a portion of the outer casing 41 in one embodiment. The high-pressure closure flange 41.5 and the low-pressure closure flange 41.6 are coupled to one another by studs 81 each having a first threaded end screwed in a respective threaded blind hole 82 in the low-pressure closure flange 41.6. The studs 81 extend through respective holes in the high-pressure closure flange 41.5 and a second threaded end of each stud 81 is engaged by a respective nut 83.

[0070] In some embodiments, one of the low-pressure closure flange 41.6 and high- pressure closure flange 41.5 may have at least one rabbet co-acting with the other flange. Rabbets are particularly useful in embodiments where the high-pressure casing is made of a heat resistant alloy usually characterized by a thermal expansion coefficient that is higher than the thermal expansion coefficient of the high-pressure casing that can be made, e.g., of a martensitic alloy.

[0071] In general terms, the rabbet can be internal, i.e., arranged at the inner side of the outer casing, external, i.e., arranged at the outer side of the outer casing, or intermediate. In some embodiments, more than one rabbet can be provided at the interface between the flanges 41.6, 41.5, and can be arranged either on the high-pressure flange, on the low-pressure flange, or on both.

[0072] In Fig.10 the low-pressure closure flange has an internal rabbet 85. During operation the rabbet is engaged driven by the higher radial displacement of the exhaust casing with respect to the high-pressure casing

[0073] With continuing reference to Figs. 2 to 10, Fig.11 illustrates a further embodiment of the low-pressure closure flange 41.6 and high-pressure closure flange 41.5. In this embodiment, both the low-pressure closure flange 41.6 and high-pressure closure flange 41.5 comprise through hole; a set of bolts 87 and respective nuts 88 connect the low-pressure exhaust casing 41.2 and the high-pressure casing 41.1 to one another. [0074] With continuing reference to Figs. 2 to 11, Fig.12 illustrates a further embodiment of the low-pressure closure flange 41.6 and high-pressure closure flange 41.5. The low-pressure closure flange 41.6 and high-pressure closure flange 41.5 are coupled to one another by a set of bolts 87 extending through holes in the low-pressure closure flange 41.6 and in high-pressure closure flange 41.5. External nuts 91 are screwed on respective first ends of the bolts 87, which are placed outside the outer casing 41. Inner nuts 93 are screwed on the opposite ends of each bolt 87. Each inner nut 93 is housed in a seat 95 facing the interior of the discharge plenum 41.3. This embodiment results in a reduced external diameter of the flanges, while still allowing the use of standard screws and nuts.

[0075] The low-pressure closure flange 41.6 further includes an internal rabbet 97 co-acting with the high-pressure closure flange 41.5.

[0076] Fig.13 illustrates an embodiment of the high-pressure and low-pressure closure flanges 41.5 and 41.6 in a further embodiment, similar to the embodiment of Fig.10. The same reference numbers of Fig.10 are used in Fig.13 to designate the same or equivalent components. The main difference between Figs. 13 and 10 concerns the position of the centering rabbet. In the embodiment of Fig.13 an external rabbet 86 is integrally formed with the high-pressure closure flange 41.5 and replaces the internal rabbet 85 which in Fig.10 is integrally formed with the low-pressure closure flange 41.6.

[0077] For an improved sealing between the low-pressure closure flange 41.6 and high-pressure closure flange 41.5, in some embodiments energized seals can be provided. Figs. 14 and 15 illustrate a sectional view of the outer casing 41 including a radially-working energized seal 101 and an axially-working energized seal 103, respectively.

[0078] In some embodiments, to provide a more rigid structure of the low-pressure exhaust casing 41.2, ribs can be provided in the discharge plenum 41.3. An embodiment with reinforcing ribs 105 in the discharge plenum 41.3 is shown in Figs. 16, 17, 18, wherein Fig.16 illustrates a sectional view of the outer casing and Figs. 17 and 18 illustrate detailed sectional axonometric views of the low-pressure exhaust casing. The reinforcing ribs 105 reduce flexural deformations of the low-pressure exhaust casing 41.2 caused by the inner pressure of the exhaust flue gas. Additionally, the reinforcing ribs 105 can improve the distribution of the exhaust flue gas towards the exhaust apertures 46.

[0079] In some embodiments, the interface between the low-pressure closure flange 41.6 and high-pressure closure flange 41.5 can be provided with locking teeth engaging into recesses provided in the opposite. An embodiment of locking teeth and recesses is shown in Figs. 19, 20 and 21. In this embodiment on a surface of the high- pressure closure flange 41.5, which is in pressure contact with a corresponding surface of the low-pressure closure flange 41.6, locking teeth 107 are provided, which engage in recesses 109 in the opposite surface of the low-pressure closure flange 41.6. An opposite arrangement, with recesses in the high-pressure closure flange 41.5 and locking teeth in the low-pressure closure flange 41.6 is also possible.

[0080] While in the embodiments disclosed above the outer casing 41, the inner casing 5, and the rotor 43 of the expander 3 are adapted to introduce the inner casing 51 and the rotor 43 in an aft-to forward direction, in other embodiments the bundle including the inner casing 51 and the rotor 43 can be introduced in the outer casing 41 in a forward-to aft direction. Fig.22 illustrates a schematic sectional view of an expander 3 adapted for insertion of the expander bundle from forward, i.e., in a forward- to-aft direction. The same reference numbers used in the previous figures designate in Fig.22 the same or equivalent components, which are not described in detail again.

[0081] To allow introduction of the bundle in a forward-to-aft direction (i.e., in the direction of arrow f in Fig.22), the forward end of the outer casing 41 comprises an aperture, the diameter whereof is sufficiently large to accommodate the inner casing 51. The combustion chambers 7.1 of the combustor 7 are introduced in the respective seats 41.4 once the bundle 51, 43 has been accommodated in the outer casing 41. The forward end of the outer casing 41 is then closed by a closure member 111 arranged opposite the low-pressure exhaust casing 41.2.

[0082] In some embodiments, if the combustor comprises a plurality of combustion chambers 7.1 at least partly housed in the outer casing, two or more combustion chambers 7.1 can be connected to one another by cross fire ducts housed in the body of the outer casing, specifically in the high-pressure casing 41.1. An embodiment of a cross fire duct connecting two adjacent combustion chambers 7.1 is shown in the sectional view of Fig. 23. This sectional view shows the combustion chambers 7.1 and a cross fire duct 7.2. The cross-fire duct fluidly couples the interior of the combustion chambers 7.1 and propagates the flame from one chamber to the other if required, for instance at start-up. More uniform operating conditions are maintained in the various combustion chambers 7.1 coupled by respective cross fire ducts 7.2 and the risk of the flame extinguishing in one of the combustion chambers is prevented.

[0083] 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.

[0084] Some features disclosed in combination with embodiments of the expander described herein can be used in a power-generating turbomachine having a different structure. According to one aspect, for instance, the low-pressure exhaust casing can be used in an expander or in a turbine having a different casing structure, such as a horizontally split structure, rather than a barrel structure as outlined above.

[0085] A further subject of the present disclosure is therefore a casing for a powergenerating turbomachine including: a discharge plenum; and a plurality of discharge apertures.

[0086] While in the above described embodiments the outer casing of the expander comprises a high-pressure casing and a low-pressure exhaust casing, the novel feature of a plurality of exhaust apertures can be embodied in a turbomachine wherein the outer casing comprises two or more sections coupled along a plane containing the rotation axis, i.e. a horizontally split casing, wherein the aft part of the casing features the discharge plenum and the plurality of exhaust apertures.

[0087] The exhaust apertures can be arranged in a radial or in a tangential direction. In some embodiments, the exhaust apertures can be arranged symmetrically around the rotation axis or with respect to a plane containing the rotation axis.

[0088] In some embodiments, the discharge plenum may include reinforcing ribs, such as ribs 105.

[0089] The casing containing the discharge plenum and featuring the plurality of exhaust apertures can be monolithic, i.e. formed as a single body. In other embodiments the casing can be split into two or more components, parts or sections which can be coupled to one another by screw-bolts, tie-rods or other reversible coupling means.

When the casing is split in two or more portions, the portions can be coupled one another along a plane containing the rotation axis of the turbomachine or parallel to said axis.

[0090] In some embodiments, casing containing the discharge plenum can be a por- tion of an outer casing of the turbomachine, e.g. can form a low-pressure exhaust casing or an aft-portion casing of the power-generating turbomachine. The low-pressure exhaust casing or aft-portion casing can include a flange for connection to a high- pressure, or forward-portion casing of the turbomachine.

Claims

1. A supercritical expander comprising: an outer casing; a rotor housed in the outer casing for rotation around a rotation axis; and at least one combustor; wherein the outer casing is formed by less than four annular components flanged to one another along respective planes orthogonal to the rotation axis; the outer casing comprises a high-pressure casing and a low-pressure exhaust casing coupled to one another with a flange connection along a plane orthogonal to the rotation axis; each annular component of the high-pressure casing is monolithic; the flow path is entirely contained in the outer casing; and wherein the low-pressure exhaust casing comprises a discharge plenum, adapted to collect combustion gas expanded through a gas expansion flow path; the low-pressure exhaust casing comprising at least one exhaust aperture.

2. The expander of claim 1, wherein the at least one combustor is at least partially housed in a seat in the high-pressure casing.

3. The expander of claim 2, wherein the at least one combustor comprises a plurality of combustion chambers; wherein the expander further comprises a plurality of seats in the high-pressure casing, said seats being positioned around the rotation axis; wherein each seat houses a respective combustion chamber; and wherein preferably adjacent combustion chambers are fluidly connected to one another by respective cross fire ducts.

4. The expander of any one of the preceding claims, wherein the outer casing is formed by two annular components flanged to one another along a plane orthogonal to the rotation axis; wherein one of said annular components is a monolithic annular component forming the high-pressure casing; and wherein the other of said annular components forms the low-pressure discharge casing.

5. The expander of any one of the preceding claims, wherein the low- pressure exhaust casing comprises a main body configured as a monolithic block, preferably extending annularly around the rotation axis.

6. The expander of any one of the preceding claims, further comprising a closure member connected at the outer casing, opposite the low-pressure exhaust casing.

7. The expander of any one of the preceding claims, further comprising an inner casing, stationarily housed in the outer casing; wherein the rotor is housed in the inner casing for rotation therein.

8. The expander of claim 7, wherein a plurality of annular rows of stationary blades is housed in the inner casing; wherein a respective annular row of rotor blades is positioned downstream of each annular row of stationary blades, forming therewith an axial expander stage; and wherein the sequentially arranged annular rows of stationary blades and rotor blades form the gas expansion flow path; wherein the gas expansion flow path is preferably fully contained in the inner casing.

9. The expander of claim 7 or 8, wherein the inner casing is mechanically coupled to the outer casing by at least one connection member arranged between a forward end and an aft end of the inner casing.

10. The expander of claim 9, wherein the at least one connection member is adapted to allow differential thermal expansions of the inner casing and outer casing.

11. The expander of any one of claims 7 to 10, wherein the inner casing is entirely housed in the high-pressure casing and surrounded thereby, or partly housed in the high-pressure casing and projects in the low-pressure exhaust casing.

12. The expander of any one claims 7 to 11, wherein the outer casing and a bundle comprising the inner casing and the rotor housed therein are configured such that the bundle is adapted to be introduced into the high-pressure casing in an aft- to-forward direction; and wherein the low-pressure exhaust casing comprises an abutment, against which the bundle is pushed during operation of the expander by a thrust generated by gas expanding through the expander.

13. The expander of any one of claims 7 to 11, wherein the outer casing and a bundle comprising the inner casing and the rotor housed therein are configured such that the bundle is adapted to be introduced into the high-pressure casing in a for- ward-to-aft direction; and wherein the high-pressure casing forms an abutment against which the bundle is pushed during operation of the expander by a thrust generated by gas expanding through the expander.

14. The expander of any one of claims 7 to 13, wherein the inner casing is split into a first casing portion and a second casing portion along a plane containing the rotation axis.

15. The expander of any one of claims 7 to 14, comprising at least one annular fluid chamber between the inner casing and the high-pressure casing; wherein the annular fluid chamber is adapted to receive a pressurized cooling fluid.

16. The expander of any one of claims 7 to 14, comprising a plurality of annular fluid chambers between the inner casing and the high-pressure casing; wherein the plurality of annular fluid chambers are placed sequentially in a forward-to-aft direction and are adapted to receive a pressurized cooling fluid at a gradually decreasing pressure from a most upstream annular fluid chamber to a most downstream annular fluid chamber.

17. The expander of claim 16, wherein at least two sequentially arranged annular fluid chambers are separated from one another by a pressure-reduction device or a sealing device.

18. The expander of claim 17, wherein sequentially arranged annular fluid chambers are separated from one another by a connection member which couples the inner casing to the outer casing.

19. The expander of any one of claims 15 to 18, comprising cooling ducts extending through the inner casing and fluidly coupling at least one annular fluid chamber with an interior of the inner casing.

20. The expander of any one of claims 7 to 19, wherein the inner casing comprises in turn a plurality of inner casing sections arranged sequentially in a forward-to-aft direction parallel to the rotation axis.

21. The expander of any one of the preceding claims, wherein the low- pressure exhaust casing comprises a plurality of exhaust apertures.

22. The expander of claim 21, wherein the exhaust apertures are arranged in a radial or in a tangential direction.

23. The expander of claim 21 or 22, wherein the exhaust apertures are arranged symmetrically around the rotation axis or with respect to a plane containing the rotation axis.

24. The expander of any of the preceding claim wherein the low-pressure exhaust casing is made of a heat resistant alloy, such as a nickel-based alloy.

25. The expander of any one of the preceding claims, wherein the low- pressure exhaust casing comprises an inner thermal protection, adapted to reduce heat exchange between an exhaust flow and an inner surface of the low-pressure exhaust casing.

26. The expander of claim 25, wherein the inner thermal protection comprises a thermal shield at least partly covering an inner surface of the low-pressure exhaust casing.

27. The expander of claim 26, wherein the inner thermal protection comprises a cooling chamber defining a flow passage for a cooling fluid between the thermal shield and an inner surface of the low-pressure exhaust casing.

28. The expander of claim 26 or 27, wherein the inner thermal protection comprises a layer of a solid thermal insulation material between the thermal shield and the inner surface of the low-pressure exhaust casing.

29. The expander of any one of the preceding claims, wherein the at least one combustor is selected from the group consisting of: a can combustor; a cannular combustor; an annular combustor; a double annular combustor.

30. The expander of any one of the preceding claims, wherein the high- pressure casing comprises a high-pressure closure flange and the low-pressure exhaust casing comprises a low-pressure closure flange, wherein the high-pressure closure flange and the low-pressure closure flange are coupled to one another by a set of studs or bolts; comprising at least one of the following features: bolts engaging nuts arranged in the low-pressure exhaust casing or high pressure casing; at least one rabbet on at least one of said low-pressure closure flange and high- pressure closure flange; a radially working energized seal; an axially working energized seal; reinforcing ribs extending from the low-pressure closure flange in a discharge plenum formed in the low-pressure exhaust casing; locking teeth projecting from a sealing surface of one of said low-pressure closure flange and high-pressure closure flange and engaging in recesses formed in a sealing surface of the other of said low-pressure closure flange and high-pressure closure flange.

31. The expander of any one of the preceding claims, wherein the expander is adapted to receive a process fluid at a pressure equal to or higher than 50 bar A, preferably equal to or higher than 100 barA, more preferably equal to or higher than 150 barA, more preferably equal to or higher than 200 barA.

32. The expander of any one of the preceding claims, wherein the expander is adapted to expand a combustion gas through the gas expansion flow path with a pressure drop of at least 150 bar, preferably of at least 250 bar, more preferably between 250 and 400 bar

33. An oxy-fuel combustion system comprising an expander according to any one of the preceding claims and a compression system adapted to provide a flow of compressed process gas to the expander.