An ignition device, and turbomachine including the ignition device

Abstract

Disclosed herein is an ignition device for an internal combustion engine, such as specifically an expander. The ignition device includes an optical tube with a distal end, a proximal end, and an axial cavity extending from the proximal end to the distal end, the axial cavity forming an optical path for a laser radiation. The ignition device further includes; a first optical element located at the distal end of the optical tube. The first optical element forms a pressure barrier adapted to prevent penetration of gas from a combustion chamber into the axial cavity.

Description

AN IGNITION DEVICE, AND EXPANDER INCLUDING THE IGNITION DEVICEDESCRIPTIONTECHNICAL FIELD

[0001] The present disclosure pertains to igniters, i.e. ignition devices, for power generating turbomachines. Embodiments disclosed herein specifically refer to igniters adapted for use in oxyfuel combustion expanders or turbines, such as supercritical carbon dioxide expanders (sCO? expanders), and to expanders including such igniters.

[0002] As understood herein a SCO2 expander is an expander or turbine in which carbon dioxide in a supercritical state is present in at least a portion of the process gas flow path inside the expander.BACKGROUND ART

[0003] 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.

[0004] 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.

[0005] 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.

[0006] In recent years oxy-combustion cycles, also known as oxy-fuel cycles or oxyfuel 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.

[0007] 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.

[0008] 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.

[0009] 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.

[0010] Oxy-fuel combustion cycles, such as those described above, are particularly interesting in terms of efficiency, reduction of noxious emissions and ease of CO2 sequestration. However, they operate under CO2 supercritical conditions at the inlet of the expander and are characterized by high pressure and temperature values inside the expander and specifically inside the combustor. These operating conditions posedifficult constraints in the casing design.

[0011] Improvements in the design of the combustors adapted for supercritical carbon dioxide expanders or other expanders operating in similar conditions are highly desirable.

[0012] The present disclosure relates specifically to improvements to the igniters, i.e. ignition devices, for igniting a mixture of process gas and fuel in an expander, in particular an expander or turbine for a supercritical carbon dioxide expander.SUMMARY

[0013] According to one aspect, disclosed herein is an ignition device for an internal combustion engine, such as specifically an expander. The ignition device includes an optical tube with a distal end, a proximal end, and an axial cavity extending from the proximal end to the distal end, the axial cavity forming an optical path for a laser radiation. The ignition device further includes; a first optical element located at the distal end of the optical tube. The first optical element forms a pressure barrier adapted to prevent penetration of gas from a combustion chamber into the axial cavity. To detect possible failure of the first optical element, or of a seal therearound, a pressure detecting port can be provided, in fluid communication with the axial cavity of the optical tube. A pressure gauge can be fluidly coupled to the pressure detecting port and used to detect a pressure increase inside the axial cavity.

[0014] As understood herein the proximal end is the end intended to be positioned outside the casing of the internal combustion engine, and the distal end is the end intended to be positioned inside the casing of the internal combustion engine. Thus, in embodiments disclosed herein, the proximal end comprises a laser adapter for connecting a laser source to the optical tube.

[0015] The first optical element can be housed inside the axial cavity and can be flush with the terminal edge of the optical tube. However, in other embodiments the first optical element can be set back with respect to the terminal edge of the optical tube. “Located at the distal end of the optical tube” does not necessarily mean that the optical element is flush with the terminal end of the optical tube. Rather, the optical element can be positioned entirely inside the axial cavity of the optical tube, set back withrespect to the terminal edge of the optical tube. What matters, is that only a minor portion of the optical path of a laser beam exiting the first optical element is inside the axial cavity of the optical tube. For example, the external surface of the first optical element can be at a distance of 3D. 2D. D, V2D or less, from the terminal edge of the optical tube, where D is the inner diameter of the axial cavity.

[0016] By providing the optical element at the distal end of the optical tube, a more efficient operation of the ignition device is achieved, since laser energy, generated by a laser source coupled with the optical tube, travels mainly in a gas-free path inside the axial cavity, thus reducing energy losses due to absorption by the gas.

[0017] In some embodiments, the first optical element is a focusing lens. For example, the first optical element may have sufficient optical power to focus a collimated laser beam traveling in the axial cavity to a point at a required distance from the distal end of the optical tube and located in the combustion chamber of the internal combustion engine, for example within a liner of a combustor of an expander with which the ignition device is combined.

[0018] By placing the first optical element with an optical power at the distal end of the optical tube, a three-fold technical result is achieved. The energy density on the entrance surface of the optical element is the lowest possible, because the laser beam impinging on the optical element is a parallel (collimated) beam. This prevents or limits damage to the optical element.

[0019] The optical element prevents gas from the combustion chamber from entering into most of the axial cavity, thereby reducing the energy loss along the optical path. The length of the focusing path is reduced, resulting in more efficient focusing of the energy at the focal point. These factors reduce the power rate of the laser source, allowing a lower cost and simpler laser source to be used.

[0020] In some embodiments, the ignition device includes a second optical element positioned in the axial cavity of the optical tube, between the proximal end of the optical tube and the first optical element. The second optical device provides a further barrier against penetration of gas from the combustion chamber towards the proximal end of the optical tube. This can be particularly advantageous in internal combustionengines where the pressure in the combustion chamber reaches very high levels, for example 50 barA or above, or 100 barA or above, and even higher, around 200 barA or 300 barA or higher. For example, high pressures of several hundred bar can occur in supercritical carbon dioxide expanders used in oxy-combustion cycles, such as the so-called Allam cycles.

[0021] In advantageous embodiment, the second optical element is an optical window with zero optical power, such that a collimated laser beam from a laser source coupled to the proximal end of the ignition device will propagate through the second optical element without being focused and will thus reach as collimated, parallel beam the first optical element arranged at the distal end of the optical tube. Focusing is accomplished entirely by the first optical element, so that the energy density at the entrance surface of the first optical element remains as low as possible, reducing the risk of damage to the first optical element.

[0022] According to another aspect, disclosed herein is an ignition device for a combustion engine, comprising an optical tube with a distal end, a proximal end, and an axial cavity extending from the proximal end to the distal end, the axial cavity forming an optical path for a laser radiation. The proximal end comprises a laser adapter for connecting a laser source to the optical tube. A first optical element is positioned at the distal end and forms a pressure barrier adapted to prevent penetration of gas from a combustion chamber into the axial cavity. The optical tube may comprise: a head forming the proximal end of the optical tube; and a tubular member forming the distal end of the optical tube. The tubular member extends within the head and from the head to the distal end of the optical tube.

[0023] In certain embodiments, the tubular member is connected to the head by a clamping arrangement comprising a thermal expansion compensation feature between the tubular member and the head.

[0024] In certain embodiments, the head comprises a thermal insulating ring positioned between the laser adapter and a main body of the head, whereto the tubular member is coaxially attached and wherefrom the tubular member projects opposite said thermal insulating ring.

[0025] Further features and embodiments are outlined in the attached claims and are described in the following description.

[0026] As mentioned, the ignition device is intended or an internal combustion engine, specifically an expander. In embodiments, the expander is a supercritical carbon dioxide expander. As understood herein, a supercritical carbon dioxide expander is an expander where carbon dioxide in a supercritical state is present in at least one portion of the flow path.

[0027] As used herein, an expander is a power generating turbomachine including a casing and a rotor housed in the casing for rotation therein, the rotor being provided with rotor blades defining, along with stator blades in the casing, an expansion path for the process gas. The expander further includes a combustor section, wherein pressurized and hot combustion gas is generated by igniting a mixture of oxidant and fuel. The term expander thus also includes a gas turbine engine.BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Reference is now made briefly to the accompanying drawings, in which:Fig.l illustrates a schematic of a power generation system including a supercritical carbon dioxide expander;Fig.2 is a sectional view of an expander, i.e. a turbine, in a simplified representation;Fig.3 is an enlarged and more detailed sectional view of a portion of the expander of Fig. 3, including an ignition device according to the present disclosure;Fig.4 is a sectional view of the ignition device in one embodiment; Fig.5 is an enlargement of Fig.4, showing the head of the ignition device of Fig.4;Fig.6 illustrates a detail of the distal end of the ignition device positioned in the combustion chamber; andFig.7 is an axonometric view of a clamp connection between the ignition device and the casing of an expander or gas turbine.DETAILED DESCRIPTION

[0029] The schematic of Fig. 1 illustrates a simplified power system including an oxy-fuel cycle operating with supercritical carbon dioxide at the expander inlet (shortly sCO2 cycle), such as an Allam cycle, or NET Power oxy-fuel cycle.

[0030] The power generating system 1 shown in Fig.l comprises a gas turbine, i.e. an expander 3, that includes an expansion section 5 and a combustor assembly 7. In some embodiments, the combustor assembly 7 comprises a plurality of combustors 8 (Fig.2), each provided with a combustion chamber 8.1, as will be described in more detail below.

[0031] In some embodiments, the combustors 8 are arranged circumferentially around a rotation axis A-A (Fig.2) of the expander 3, as shown in more detail in the subsequent figures and each combustor 8 is housed in a respective cylindrical seat as will be described in more detail below.

[0032] The combustor assembly 7 is supplied with an oxidant flow delivered by an oxidant source. The oxidant may be oxygen (O2). In some embodiments, the oxidant is a blend of oxygen and carbon dioxide (CO2). The oxidant flow, or the oxygen forming part of the oxidant blend can be produced by an air separation unit 9 which represents 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 supply line 11 to the combustor assembly 7 of the expander 3.

[0033] Reference number 13 indicates a fuel supply line, for instance adapted to supply natural gas, such as methane, to the combustor assembly 7, specifically to each combustor 8. The oxidant and the fuel are supplied at a forward side of the expander 3 to the combustor assembly 7 at high pressure, for instance at 50 barA or higher, preferably a pressure equal to or higher than 100 barA, more preferably equal to or higher than 150 barA, even more preferably equal to or higher than 200 barA. In some embodiments, the upper pressure of the cycle performed in the thermodynamic system depicted in Fig. 1 can be equal to or above 250 barA, or higher, for example equal to or lower than 1000 barA, or equal to or lower than 800 barA, or equal to or lower than600 barA. The oxidant-fuel blend is burned in the combustor assembly 7. Pressurized, hot combustion gas resulting from the combustion expands in the expansion section 5 of the expander 3.

[0034] In some embodiments, the temperature at the inlet of the gas expansion flow path, i.e., at the inlet of the rotor of the expander can be at or above 800°C, and preferably at or below 1500°C.

[0035] After expansion, the exhausted combustion gas is discharged at a discharge side of the expander 3 in a discharge line 15. The combustion 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.

[0036] 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 some 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 example, the rated power can be comprised between 200 MW and 650 MW.

[0037] Intermediate values of the upper limit and lower limit of each range mentioned above are also expressly disclosed herein.

[0038] The power system 1 further comprises a regenerative heat exchanger 17, wherein hot exhausted combustion gas flowing through a hot side 17.1 of the regenerative heat exchanger 17 is cooled in heat exchange with a flow of chilled exhausted combustion gas, which flows through a cold side 17.2 of the regenerative heat exchanger 17. The combustion 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 water vapor contained in the exhausted combustion gas. Condensed water is removed from the exhausted combustion gas in a water / gas separator 21.

[0039] The de-hydrated exhausted and chilled combustion gas, consisting mainly (e.g. up to 90% by weight) or exclusively of carbon dioxide, is compressed in a combustion gas compressor 23 to the pressure at the inlet side of the expander 3. While inthe schematic of Fig.1 the combustion gas compressor 23 is pictorially represented as a single compressor, in some embodiments a multiple compressor can be used. For instance, the combustion gas compressor 23 can be a multi-stage compressor, or a compressor train and can include one or more intercoolers.

[0040] The compressed combustion gas, consisting mainly of carbon dioxide and delivered by the combustion gas compressor 23, is partly removed from the cycle through a discharge line 24. The major part of the compressed combustion gas is divided into a first part of recycled combustion gas and a second part of recycled combustion gas. The first part of recycled combustion gas is delivered through the cold side 17.2 of the regenerative heat exchanger 17 and is heated by heat exchange with the hot combustion 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 combustion gas recycled through recycle line 25 is fed to the combustor assembly 7 and mixed with combustion gas generated therein as will be described in more detail later.

[0041] A side stream of chilled and dehydrated combustion gas, consisting of the second part of recycled combustion 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 combustion gas can be delivered through a line 28 to the air separator 9 and / or to the oxidant supply line 11 to add carbon dioxide to the oxygen from the air separation unit 9. The combustion gas from line 28 and the oxygen from the air separation unit are blended to form the oxidant flow which is delivered to the combustor assembly 7. The oxidant flow delivered to the combustor assembly 7 can contain for instance approximately 20% by volume of oxygen and 80% by volume of carbon dioxide. The addition of carbon dioxide to the oxidant stream prevents corrosive damages to the piping and expander components, which may be caused if pure oxygen were used as an oxidant. Moreover, carbon dioxide blended with oxygen in the oxidant stream mitigates safety issues related to the delivery of pure oxygen to the combustor assembly 7, and helps to tune the reactivity of the mixture within the combustor assembly 7. The percentages outlined above are by example only and shall not be understood as limiting the scope of the present disclosure.

[0042] To recover further heat from the regenerative heat exchanger 17, the oxidant supply line 11 can include a heating section 11.1 which extends through the regenerative heat exchanger 17, such that the oxidant is heated by heat exchange against the hot combustion gas flowing in the hot side 17.1 of the regenerative heat exchanger 17 prior to be fed to the combustor assembly 7.

[0043] 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 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 the illustrated embodiment, 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.

[0044] With continuing reference to Fig.1, Fig. 2 illustrates a simplified sectional view of the expander 3 in one embodiment. The expander 3 can comprise an outer casing 41, which houses an inner casing, a rotor supported for rotation in the inner casing, and the combustor assembly 7.

[0045] In some embodiments, the outer casing 41 includes a forward casing portion 41.1 and an aft casing portion 41.2. The forward casing portion 41.1 of the outer casing 41 can be in the form of a barrel, i.e. can be monolithic, and can include a monolithic annular body, for example manufactured by forging, casting, additive manufacturing, or combination thereof.

[0046] The monolithic body forming the forward casing portion 41.1 of the outer casing 41 extends around the longitudinal axis of the expander, i.e., around the rotation axis A-A thereof. As understood herein, a casing or casing portion having a monolithic body structure is made of a single piece of material, which is continuous in thetangential direction around the rotation axis of the expander.

[0047] Similarly, the aft casing portion 41.2 of the outer casing 41 can be in the form of a barrel. I.e. the outer casing 41 can be a vertically split casing.

[0048] A barrel -type structure the forward casing portion 41.1 and aft casing portion 41.2 of a vertically split outer of casing 41 is particularly adapted for a supercritical carbon dioxide expander, where the pressure of the process gas in the expansion flow path is substantially higher than in standard Bryton-cycle turbines.

[0049] In some embodiments, the aft casing portion 41.2 of the outer casing 41 forms a discharge plenum 41.3, through which exhausted combustion gas is discharged from the expander 3.

[0050] Reference numbers 45, 47 indicate bearing arrangements, which rotatingly support a rotor 43 of the expander 3 for rotation around the rotation axis A-A. The bearing arrangements 45, 47 can be arranged in bearing housings, not shown in detail The output shaft end 31 of the rotor 43 can be drivingly coupled to the driven machine (electric generator 33 in the exemplary embodiment of Fig.1) through a joint schematically shown at 49.

[0051] The rotor 43 is surrounded by an inner casing 51, which can be formed by a plurality of sections arranged in sequence in a forward-to-aft direction. The inner casing 51 can be horizontally split, i.e. can be comprised of 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 or more casing sections arranged in sequence in the axial direction (i.e. forward-to-aft direction), each section can in turn be horizontally split, i.e. comprised of two portions coupled along a plane containing the rotation axis of the rotor 43.

[0052] The inner casing 51 is fully or partly housed in the forward casing portion 41.1 of the outer casing 41. In some embodiments, as shown in Fig.2, the inner casing 51 extends in the aft casing portion 41.2 of the outer casing 41.

[0053] One or more annular fluid chambers 42 are formed between the inner casing 51 and the outer casing 41. In use, at steady state conditions, chilling or cooling fluid,e.g. chilled, dehydrated combustion gas from cooling line 27, can be supplied to the annular fluid chamber 42

[0054] 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 the annular fluid chamber 42 and the interior of the inner casing 51. Compressed recycled combustion gas, consisting mainly of carbon dioxide, can flow from the annular fluid chamber 42 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.

[0055] 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 assembly 7 a high number of expansion stages is preferred, for instance higher than three, preferably higher than five, in some examples equal to or higher than six. In some embodiments, the number of stages can be equal to or less than fifteen, in other embodiments, equal to or less than ten.

[0056] Each expansion stage includes an annular row of stationary blades 53, which are stationarily arranged in the inner casing 51. In some embodiments, intermediate supporting rings can be housed in the inner casing 51, between the inner surface of the inner casing 51 and the stationary blades 53. The stationary blades 53 and stationary shroud segments can be connected to the inner casing through said rings. The first expansion stage includes an annular row of stationary blades which form nozzles between the combustor assembly 7 and the inlet of the expansion flow path.

[0057] The stationary blades 53 extend radially from the inner casing 51 in the expansion flow path. Each expansion stage further includes an annular row of rotor blades 55, arranged downstream the respective annular row of stationary blades 53 along the expansion flow path. The rotor blades 55 extend radially from the rotor body in the expansion flow path.

[0058] In embodiments, the rotor 43 further comprises a forward shaft portion 65 andan aft shaft portion 67. In embodiments, the combustor assembly 7 extends around the forward shaft portion 65. In some embodiments, the discharge plenum 41.3 extends around the aft shaft portion 67.

[0059] 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.

[0060] With continuing reference to Figs 1 and 2, details of an embodiment of the combustor assembly 7 are described below with reference to Fig.3.

[0061] In the illustrated embodiment, the combustor assembly 7 includes a plurality of can-shaped combustors 8. The combustors 8 are arranged around the rotation axis A-A of the expander. Each combustor 8 is housed in a respective generally cylindrical seat 101 (Fig.3) formed in the forward casing portion 41.1 of the outer casing 41.

[0062] Each combustor 8 comprises a tubular liner 103 partially housed in the respective generally cylindrical seat 101. Each liner 103 has a longitudinal axis B-B which can be coincident with, or parallel to the longitudinal axis of the of the cylindrical seat 101. The longitudinal axes B-B of the cylindrical seats 101 and of the respective liners 103 are inclined toward the rotation axis A-A of the expander and converge towards said rotation axis. In some embodiments, the axes B-B can be positioned on a conical surface, the axis whereof is coincident with the rotation axis A-A of the expander 3.

[0063] In some embodiments, the angle between the axes A-A and B-B can be between 0° and 80°, or between 0° and 60°, in some embodiments between 15° and 40°. The angle between the axes A-A and B-B is selected as a compromise between the need to reduce radial dimensions of the expander and improve the combustor design (which would be improved using smaller angles), and the overall design constraints of the expander, such as the dimension and position of the rotor shaft and bearings (which require larger angles).

[0064] Each liner 103 has a forward end 103F and an aft end 103 A. Each liner 103 further includes a generally cylindrical or tubular sidewall 105 which extends from theforward end 103F to the aft end 103 A of the liner 103. The sidewall 105 has an outer surface 105A and an inner surface 105B and surrounds a combustion chamber 8.1 of the combustor 8.

[0065] Each combustor 8 comprises at least one burner 107 at the forward end 103F of the respective liner 103. In some embodiments, each combustor 8 may comprise a plurality of burners 107, i.e. a burner cluster 107.

[0066] A forward-end closure lid 106 is provided at the forward end of each generally cylindrical seat of each combustor 8, on the forward side of the burner or burner cluster 107.

[0067] The burner or burner cluster 107 is fluidly coupled with a fuel inlet 109 and with an oxidant inlet 111. The fuel inlet 109 is in turn fluidly coupled with the fuel supply line 13 (Fig.1) and receives a fuel, for instance a gaseous fuel, such as natural gas, therefrom. The oxidant inlet Ill is fluidly coupled with the oxidant supply line 11 and receives oxidant therefrom, the oxidant mainly consisting of oxygen and carbon dioxide, as mentioned above.

[0068] Each combustor 8 further comprises a transition piece 113 positioned at the aft end 103 A of the liner 103 and extending therefrom. Each transition piece 113 forms an extension of the respective liner 103 towards the first annular row of stationary blades 53 and guides the combustion gas generated in the combustion chamber 8.1 toward the expansion flow path formed by the stationary and rotary blades of the expander.

[0069] In some embodiments, the liner 103 of each combustor 8 can be surrounded by a generally tubular or cylindrical sleeve 115, which can be coaxial with the liner 103.

[0070] Each sleeve 115 comprises a forward end 115F and an aft end 115A. In some embodiments, the forward end 115F of the sleeve 115 is coupled to an inner surface of the corresponding cylindrical seat 101 of the combustor 8. Each sleeve 115 divides a space between the inner surface of the cylindrical seat 101 and the liner 103 into an inner annular space 117 and an outer annular space 119. The outer annular space 119 surrounds the inner annular space 117.

[0071] In some embodiments, the sleeve 115 comprises a flange 115.1 which connects the sleeve 115 to the cylindrical seat 101 of the combustor 8. In some embodiments, the flange 115.1 can be bolted or otherwise connected to an annular abutment formed inside the cylindrical seat 101 and the sleeve 115 extends in the aft direction from the flange 115.1 towards the rotation axis A-A of the expander 3 from the flange 115.1 projecting outside of the cylindrical seat 101.

[0072] In the embodiment of Fig.3 the forward end 115F of the sleeve 115 is positioned between the forward end 103F of the liner 103 and the flange 115.1. In other embodiments, the sleeve 115 can terminate at the flange 115.1, and the forward end 115F thereof will therefore be coincident with the flange 115.1. In yet further embodiments, the sleeve 115 can extend forward till the forward end 103F of the liner or even beyond the forward end 103F of the liner 103.

[0073] Each inner annular space 117 may extend parallel to the axis B-B in the forward direction towards the forward end 103F of the corresponding liner 103. In the aft direction, the inner annular space 117 can extend beyond and outside the cylindrical seat 101 of the combustor 8 to the transition piece 113.

[0074] Similarly, the outer annular space 119 can extend in the aft direction outside the cylindrical seat 101 of the combustor 8 towards the transition piece 113.

[0075] As described above, recycled combustion gas is returned towards the combustor assembly 7 of the expander 3. More specifically, a flow of recycled combustion gas is delivered to each combustor 8 of the combustor assembly 7.

[0076] As described in connection with Fig.l, de-hydrated combustion gas is added to oxygen separated from air by the air separation unit 9. The oxidant consisting of the blend of oxygen and recycled combustion gas is delivered to the burner or burner cluster 107 of each combustor 8 through the respective oxidant inlet 111 of the combustor 8.

[0077] A further flow of de-hydrated recycled combustion gas is delivered to the expander 3 through the recycle line 25 and is fed to each combustor 8 through a first process gas inlet 120 of each combustor 8. Each first process gas inlet 120 is fluidly coupled with the inner annular space 117 through a corresponding forward plenum127 formed in the cylindrical seat 101 of the combustor 8 and surrounding the forward portion of the liner 103 and of the sleeve 115.

[0078] A yet further flow of recycled combustion gas is delivered to the expander 3 through the cooling line 27, which is fluidly coupled to one or more second process gas inlets 121. The second process gas inlet(s) 121 is(are) fluidly coupled with the outer annular space 119, preferably in a position downstream of the forward end 115F of each sleeve 115, i.e. in a position between the expansion flow path of the expander 3 and the forward end 115F of the sleeves 115.

[0079] Each first process gas inlet 120 is therefore connected to the respective combustor 8 in a position upstream of the position of the second process gas inlet(s) 121. For each combustor 8, a thermal insulation chamber is formed between the second process gas inlet(s) 121 and the forward end 115F of each sleeve 115. Each insulation chamber is formed by the outer annular space 119, or part thereof, which extends around the corresponding sleeve 115, and the corresponding liner 103. The thermal insulation chamber can be filled with stagnant process gas representing an inert, thermal insulating gas, consisting mainly of carbon dioxide at high pressure, which represents an efficient insulation material.

[0080] The forward plenum 127 is fluidly coupled with the inner annular space 117. The first process gas inlet 120 is fluidly coupled to the forward plenum 127, such that process gas, i.e. combustion gas recycled from the exhaust of the expander 3 through the recycle line 25 and the first process gas inlet 120, flows through the forward plenum 127 and therefrom into the inner annular space 117 in a forward-to-aft direction. The inner annular space 117 and / or the forward plenum 127 can be fluidly coupled with the interior of the liner 103 through holes, apertures, or ports extending through the side wall 105 of the liner 103.

[0081] Each combustor 8 can further include an ignition device 129, which penetrates from the exterior of the outer casing 41 through the forward plenum and faces the combustion chamber 8.1. The ignition device 129 will be described in detail below with reference to Figs 4 to 6.

[0082] In some embodiments, the combustor assembly 7 further comprises anannular aft plenum 131 positioned at the aft end 103 A of the liners 103 of the combustors 8.

[0083] In the embodiment of Fig.3, the second process gas inlet 121, or each one of a plurality of second process gas inlets 121 is positioned at the aft plenum 131, e.g. directly fluidly coupled therewith. In other embodiments, not shown, the second process gas inlet(s) 121 can be positioned in an intermediate position along the development of the outer annular space 119, between the aft end thereof and the aft plenum 131.

[0084] The aft plenum 131 can be fluidly coupled through cooling ducts (not shown) to components which face the expansion flow path, such as the stationary vanes and / or the rotary blades of the rotor.

[0085] In the embodiment of Fig.3, the inner annular space 117 of each combustor 8 is fluidly coupled at the aft end thereof with a cooling annulus 113 A of the corresponding transition piece 113. The cooling annulus can be formed between an inner duct and an outer duct of the transition piece 113, wherein the inner duct forms a hot gas path adapted to fluidly connect the combustion chamber 8.1 with the expansion flow path of an expander 3. The aft plenum 131 extends around the outer duct of the transition piece 113.

[0086] The process gas delivered through the first process gas inlet 120 flows through the inner annular space 117 and partly enters the combustion chamber 8.1 through the apertures extending through the wall of the liner 103. The remaining process gas flows from the first inner annular space 117 into the cooling annulus 113 A of the transition piece 113 in a direction of flow concordant with the direction of flow of the combustion gas generated in the combustion chamber 8.1 and which flows towards the expansion flow path. The process gas flows from the cooling annulus 113 A through cooling apertures formed in the transition piece, in the hot gas path formed by the inner duct of the transition piece 113.

[0087] An embodiment of the above-mentioned ignition device 129 is shown in Figs 4, 5 and 6 and is described in detail below.

[0088] The ignition device 129 is based on the use of a laser source. A laser beamgenerated by the laser source is conveyed from the laser source into the combustion chamber 8.1 of the combustor 8, or of each of several combustors 8 of the combustor assembly 7 when the expander 3 is to be started, in order to start the combustion of an oxidant and fuel mixture supplied into the combustion chamber 8.1.

[0089] The ignition device comprises an optical tube 151, with a distal end 151.1 and a proximal end 151.2. An axial cavity 153 extends through the optical tube 151, from the proximal end 151.2 to the distal end 151.1 and forms an optical path for a laser beam generated by a laser source, schematically shown at 155.

[0090] In some embodiments the laser source 155 can be a solid-state laser source. For example, the laser source 155 can include a laser diode. The laser source can be selected based on the material, whereof the optical element 157 and possibly other optical elements positioned in the axial cavity 153 are made of. Specifically, the material of the optical elements can be sapphire and the wavelength of the laser source can be selected such as to minimize absorption thereof by sapphire. In some embodiments the wavelength can be around 1000 and 1000 nm, for example between 1050 and 1070 nm, for example around 1064 nm.

[0091] The axial cavity 155 can be an empty cavity, which ensures propagation of the laser radiation generated by the laser source 155, from the proximal end 151.2 to the distal end 151.1 of the optical tube 151. The distal end 151.1 of the optical tube 151 can be positioned inside the combustion chamber 8.1 of the combustor 8, or flush with the liner 103 of the combustor 3, as shown by way of example in Fig.6. The arrangement of Fig. 6 is beneficial in terms of preservations of the ignition device from damages provoked by high temperatures inside the combustion chamber 8.1 during operation of the expander 3.

[0092] To prevent of high temperature combustion gas from entering the axial cavity 153 from the chamber 8.1 toward the source 155, a first optical element 157 is disposed at the distal end, the first optical element forming a pressure barrier.

[0093] The material of the optical element 157 must also withstand the high temperature generated by the combustion inside the combustion chamber 8.1, which can be above 1000°C. A temperature around 500°C or above can be present in the area wherethe optical element 157 is positioned. Sapphire can be a suitable material for the production of the optical element 157 thanks to the high thermal resistance thereof. The optical element 157 can be housed in a seat formed inside the axial cavity 153, at or adj acent the distal end 151.1.

[0094] In advantageous embodiments, the first optical element 157 is an optical element with optical power, i.e. a focusing lens. This allows a collimated laser beam to be received at the side of the first optical element 157 facing the proximal end 151.2. The focal length of the first optical element 157 can be such that, when the ignition device 129 is mounted on the expander, the focus of the laser beam generated by the laser source 155 is positioned in a suitable position within the combustion chamber 8.1. The focal length may be selected based on design considerations and may also depend, for example, also on the shape and configuration of the burner or burner cluster 107.

[0095] The proximal end 151.2 of the optical tube 151 is provided with a laser adapter 159, for connecting the laser source 155 to the optical tube 151. The laser adapter 159 can be configured such that an optical output window of the laser source 155 is optically aligned with the axial cavity 153 and with the first optical element 157. Specifically, the laser adapter 159 can be configured such that an output window 155.1 of the laser source 155 is coaxial with the axial cavity 153 and the first optical element 157 disposed at the distal end 151.1 of the optical tube 151.

[0096] In some embodiments, the laser adapter 159 comprises a flange 159.1 having a first surface facing the distal end 151.1 and a second surface, opposite the first surface, wherefrom a connector 159.2 projects. The connector 159.2 can be a threaded connector adapted to mechanically couple the laser source 155 thereto, for instance by means of a threaded nut 155.2 of the laser source 155. In other embodiments the connector 159.2 may have other mechanical features for coupling the laser source 155 thereto. For instance, the connector 159.2 can feature a bayonet mount, or a magnetic coupling, or other mechanical connection means.

[0097] The configuration of the connector 159.2 can be such that, once attached to the optical tube 151, the laser source 155 is oriented with an output window 155.1 thereof, wherefrom the laser beam exits, optically aligned with the axial cavity 153 ofthe optical tube 151. The coupling between the laser source 155 and the optical tube 151 can therefore be configured and manufactured to provide an accurate optical alignment between the laser source 155 and the optical tube 151, enabling the laser beam emitted by the laser source to propagate coaxially into the axial cavity 153 to the distal end 151.1 of the optical tube 151.

[0098] By placing the laser source (and thus the output window thereof) coaxially with the axial cavity 153 and with the first optical element 157 the need for reflective mirrors or other waveguide components is avoided, which would make the ignition device cumbersome and prone to malfunction, for example due to optical misalignment of the optical components, and which would also reduce the optical efficiency of the ignition device. With a coaxial arrangement of the laser source 155 and the optical tube 151, the laser beam exiting the laser source 155 directly reaches the first optical element 157.

[0099] The first optical element 157 can be sufficient to maintain the axial cavity 153 free of combustion gas and to prevent hot combustion gas from approaching the laser source 155. The laser source 155 is thus protected from excessive thermal stress and mechanical stress caused by the pressure of the combustion gas, which can reach 300 bar or more. The absence of combustion gas in the axial cavity 153 contributes to the optical efficiency of the ignition device 129.

[0100] In some embodiments, the optical tube 151 may include a second optical element 161 positioned within the axial cavity 153 of the optical tube 151 between the proximal end 151.2 of the optical 151 tube and the first optical element 157, to increase the operational reliability of the ignition device 129. A threaded nut 162 can be screwed in the axial cavity 153, to lock the second optical element 161 therein.

[0101] If the first optical element 157 has an optical power, i.e. is a convergent lens, the second optical element 161 can be an optical element with zero optical power, i.e. with parallel input and output surfaces, which forms a simple optical window, which neither focuses nor defocuses the incoming laser beam generated by the laser source 155. The first optical element 157 will in this case be the only focusing element along the optical path from the laser source 155 to the focus of the ignition device 129, pictorially represented by dot F in Fig. 6.

[0102] Thus, the laser beam impinging on the incident surface of the first optical element 157 is collimated and has the lowest possible energy density. This is advantageous with respect to maintaining the integrity of the first optical element 157.

[0103] However, the possibility of using a first optical element 157 and a second optical element 161 both having an optical power such that the laser beam from the laser source 155 is focused partially by the second optical element 161 and partially by the first optical element 157 is not excluded.

[0104] In still further embodiments, the first optical element 157 may have zero optical power and the laser beam may be focused by the second optical element 161 only. This embodiment is currently less preferred because a higher energy density is achieved at the incident surface of the first optical element 157.

[0105] The second optical element 161 forms a second pressure barrier, which protects the laser source 155 from high pressure and high temperature combustion gas generated in the combustion chamber 8.1. Should the first optical element 157 fail, the second optical element 161 will still prevent combustion gas from reaching the laser source 155.

[0106] In some embodiments, the optical tube can comprise a pressure detecting port 163, fluidly coupled with the axial cavity 153 of the optical tube 151. In the exemplary embodiment of Figs. 4 to 6 the pressure detecting port 163 is fluidly coupled with a pipe 165, which may connect the axial cavity 153 with a pressure sensor or pressure gauge schematically shown at 167. When the first optical element 157 and the second optical element 161 are provided in combination, the pressure detecting port 163 can be positioned between the first optical element 157 and the second optical element 161, so that an increase in pressure following failure of the first optical element 157, or of the seal therearound, may be immediately detected.

[0107] A different position of the pressure detecting port can be between the proximal end 151.2 and the second optical element 161. However, in this case the pressure sensor 167 would detect a failure only if both the first optical element 157 and the second optical element 161 fail, which may be less preferred. Two pressure detecting ports may also be provided, once between the first optical element 157 and the secondoptical element 161, and another between the second optical element 161 and the laser source 155, i.e. the proximal end 151.2 of the optical tube 151 for redundant safety check.

[0108] As best shown in Fig.5, in this embodiment the optical tube 151 comprises a head 171 at the proximal end thereof, i.e. forming the proximal end 151.2 of the optical tube. The head 171 is mechanically coupled with a tubular member 173 extending within the head 171 and from the head 171 to the distal end of the optical tube 151. The tubular member 173 effectively forms the distal end 151.1 of the optical tube 151.

[0109] In some embodiments, the tubular member 173 is connected to the head 171 by a clamping arrangement 175 that may include a thermal expansion compensation feature 179 positioned between the tubular member 173 and the head 171. The thermal expansion compensation feature compensates for differential thermal expansion of the components forming the optical tube 151, specifically the head 171 and the tubular member 173.

[0110] In some embodiments, the clamping arrangement 175 comprises a clamping nut 181 coaxial with the tubular member 173 and adapted to axially lock the tubular member 173 to the head 171. The thermal expansion compensation feature may comprise a resilient ring 183 coaxial with the tubular member 173 and positioned between the clamping nut 181 and a flange 173.1 of the tubular member 173. A sealing ring 185 can be positioned between the flange 173.1 and an inner collar 171.1 formed by the head 171.

[0111] To thermally protect the laser source 155, in some embodiments the head 171 can comprise a thermal insulating ring 187 positioned between the laser adapter 159 and a main body 171.2 of the head 171, whereto the tubular member 173 is coaxially attached and wherefrom the tubular member projects opposite said thermal insulating ring 187.

[0112] In some embodiments, the thermal insulating ring 187 is positioned between the main body 171.2 of the head 171 and the flange 159.1 of the laser adapter 159. A plurality of screws 188 can extend into through holes of the flange 159.1 and connect the laser adapter 159 to the main body 171.2 of the head 171. A resilient feature, forinstance a resilient ring 190 can be positioned between the face of the flange 159.1 facing the main body 171.2 of the head 171 and the main body 171.2, to elastically load the screws 188 and prevent unscrewing thereof.

[0113] The use of a flange and screw connection of the laser adapter to the main body 171.2 of the head 171 allows to correct possible optical misalignments due to machining tolerances. If the laser beam exiting the laser source 155 is not correctly aligned with the axial cavity 153 and co-axial with the optical element 157 (and with the optical element 161, if present), the misalignment can be corrected arranging shims between the flange 159.1 and the thermal insulating ring 187, or between the thermal insulating ring 187 and the main body 171.2 of the head 171. In some embodiments, the head 171 can comprise a flange 171.3 for connection to the casing of the expander 3. The flange 171.3 can have an outer conical surface, the purpose whereof will become apparent from the description below.

[0114] To provide an efficient alignment of the laser beam generated by the laser source 155 with the first optical element 157, precise machining tolerances should be suitably used. For instance, the machining precision is such as to achieve a tolerance between 0.01 and 0.07 mm, preferably between 0.01 and 0.03 mm on the parallelism of the mechanical components with respect to the axis of the axial cavity 153.

[0115] As can be seen in Figs 3, 6 and 7, the optical tube 151 passes through an access port 191 formed in the casing 41 of the expander 3, which in this embodiment forms the seat 101 of the combustor 8. In particular, if the combustor assembly 7 is a can combustor assembly comprising a plurality of can combustors 8 arranged annu-larly around the rotation axis A-A of the expander 3, each seat 101 may have a respective ignition device 129, whose optical tube 153 passes through a respective combustion chamber access port 191 reaching the combustion chamber. As best shown in Fig.6 the tip, i.e. the distal end 151.1 of the optical tube 151 can reach an aperture 103.1 in the liner 103. In some embodiments, the optical tube 151 can terminate with the distal end 151.1 thereof just slightly behind the aperture 103.1 or within said aperture formed in tubular sidewall 105, i.e. without projecting in the interior volume of the liner 103, thus avoiding direct exposure of the first optical element 157 to the flame in the combustion chamber 8.1. In any case, even if the distal end 151.1 of the opticaltube is set back with respect to the opening 103.1 of the liner, the focus point F, i.e. the point at which the laser beam from the laser source 155 is focused by the optical element(s) 157 (and 161), is within the volume surrounded by the liner 103, i.e. within the combustion chamber 8.1.

[0116] In any event, even if the distal end 151.1 of the optical tube is set back with respect to the aperture 103.1 of the liner, the focus point F, i.e. the point where the laser beam from the laser source is focused by the optical element(s), is within the volume surrounded by the liner 103, i.e. inside the combustion chamber 8.1.

[0117] Fig.7 illustrates a clamping arrangement adapted to connect the ignition device 129 to the combustion chamber access port 191. In this embodiment, the combustion chamber access port 191 includes a terminal flange 191.1, which can have a conical shape and which can be coupled with the flange 171.3 of the head 171. A pair of clamps 193 surround the flanges 171.3 and 191.1 and are tighten around the flanges by means of bolts or tie rods 195.

[0118] While in the above-described embodiments the combustor assembly 7 features a can-shaped combustor assembly, i.e. comprises a plurality of combustors 8 each having a generally tubular liner 103 housed in a generally cylindrical seat 101 and fluidly coupled with a respective transition piece 113, the ignition device described above and illustrated in Figs 4 to 6 can be used also in a turbine or expander having a cannular (aka can-annular) combustor assembly or an annular combustor assembly.

[0119] 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 attached claims. Further aspects are provided by the subject matter of the following clauses:Clause 1. An ignition device for a combustion engine, the ignition device comprising:an optical tube with a distal end, a proximal end, and an axial cavity extending from the proximal end to the distal end, the axial cavity forming an optical path for a laser radiation; wherein the proximal end comprises a laser adapter forconnecting a laser source to the optical tube; anda first optical element located along the optical tube, between the distal end and the proximal end thereof;wherein the first optical element is a focusing lens having a focus preferably outside the axial cavity.Clause 2. The ignition device of clause 1, wherein the laser adapter is configured such that an optical output window of the laser source is optically aligned with the first optical element.Clause 3. The ignition device of clause 1 or 2, wherein the optical tube further comprises a second optical element positioned in the axial cavity of the optical tube, between the proximal end of the optical tube and the first optical element.Clause 4. The ignition device of clause 3, wherein the second optical element is an optical window with zero optical power.Clause 5. The ignition device of clause 4, wherein the second optical element is a focusing lens.Clause 6. The ignition device of any preceding clause, comprising a pressure detecting port, fluidly coupled with the axial cavity of the optical tube.Clause 7. The ignition device of clause 3, 4 or 5, comprising a pressure detecting port, fluidly coupled with the axial cavity of the optical tube; and wherein the pressure detecting port is positioned between the first optical element and the second optical element.Clause 8. The ignition device of any preceding clause, wherein the optical tube comprises: a head forming the proximal end of the optical tube; and a tubular member forming the distal end of the optical tube; wherein the tubular member extends within the head and from the head to the distal end of the optical tube.Clause 9. The ignition device of clause 8, wherein the tubular member is connected to the head by a clamping arrangement comprising a thermal expansion compensation feature between the tubular member and the head.Clause 10. The ignition device of clause 9, wherein the clamping arrangement comprises a clamping nut coaxial to the tubular member and adapted to axially lock the tubular member to the head; and wherein the thermal expansion compensation feature comprises a resilient ring coaxial to the tubular member and positioned between the clamping nut and a flange of the tubular member.Clause 11. The ignition device of claim 8, 9 or 10, wherein the head comprises a thermal insulating ring positioned between the laser adapter and a main body of the head, whereto the tubular member is coaxially attached and wherefrom the tubular member projects opposite said thermal insulating ring.Clause 12. The ignition device of clause 11, wherein the laser adapter comprises a flange, coupled to the main body of the head and wherefrom a connector projects opposite the main body of the head and coaxial to the axial cavity, the connector comprising a coupling feature for connection of a laser source to the ignition device; and wherein the thermal insulating ring is positioned between the main bod of the head and the flange.Clause 13. The ignition device of any one of clauses 8 to 12, wherein the head comprises an annular flange for connection to a casing of an internal combustion engine.Clause 14. The ignition device of any preceding clause, further comprising a laser source connected to the laser adapter.Clause 15. The ignition device of claim 14, wherein the laser source is connected to the laser adapter such that an output window of the laser source is optically aligned with the first optical element.Clause 16. A power generating turbomachine, comprising: a casing;a combustion chamber located within the casing;a laser source adapted to generate a laser radiation for igniting a fuel and oxidant mixture in the combustion chamber; andan ignition device according to any one of clauses 1 to 14; wherein the optical tube of the ignition device extends through a combustion chamber access port of thecasing.Clause 17. The turbomachine of clause 16, wherein the distal end of the optical tube is positioned proximate to an aperture of a liner of the combustion chamber.Clause 18. The turbomachine of clause 16 or 17, wherein the turbomachine is a gas turbine or an expander, preferably a supercritical carbon dioxide turbine or expander.Clause 19. The turbomachine of any one of clauses 16 to 17, wherein the casing comprises a combustion chamber access port; and wherein the optical tube of the ignition device extends through the combustion chamber access port.Clause 20. The turbomachine of clause 19, wherein the combustion chamber access port comprises an annular flange; and wherein removable clamps or bolts connect the annular flange of the combustion chamber access port and a flange of the ignition device to one another.Clause 21. The turbomachine of any one of clauses 16 to 20, wherein the distal end of the optical tube is positioned at a side wall of a liner arranged in the combustion chamber, in an aperture of the liner or set back with respect to the aperture and outside the liner, at a distance from the aperture, such that a focus point of the first optical element of the ignition device is inside the inner volume surrounded by the liner.

Claims

CLAIMS1. An ignition device for a combustion engine, the ignition device comprising:an optical tube with a distal end, a proximal end, and an axial cavity extending from the proximal end to the distal end, the axial cavity forming an optical path for a laser radiation; wherein the proximal end comprises a laser adapter for connecting a laser source to the optical tube;a first optical element located at the distal end, the first optical element forming a pressure barrier adapted to prevent penetration of gas from a combustion chamber into the axial cavity; anda pressure detecting port, fluidly coupled with the axial cavity of the optical tube.

2. The ignition device of claim 1, wherein the laser adapter is a mechanical adapter configured such that an optical output window of a laser source, coupled to the optical tube through the laser adapter, is optically aligned with the first optical element.

3. The ignition device of claim 1, wherein the first optical element is housed inside the axial cavity.

4. The ignition device of claim 1 or 2, wherein the first optical element is a focusing lens.

5. The ignition device of claim 4, wherein the focus of the first optical element is positioned outside the axial cavity of the optical tube.

6. The ignition device of any preceding claim, wherein the optical tube further comprises a second optical element positioned in the axial cavity of the optical tube, between the proximal end of the optical tube and the first optical element.

7. The ignition device of claim 6, wherein the second optical element is an optical window with zero optical power.

8. The ignition device of claim 6, wherein the second optical element is a focusing lens.-28-9. The ignition device of any one of claims 6 to 8, wherein the pressure detecting port is positioned between the first optical element and the second optical element.

10. The ignition device of any preceding claim, wherein the optical tube comprises: a head forming the proximal end of the optical tube; and a tubular member forming the distal end of the optical tube; wherein the tubular member extends within the head and from the head to the distal end of the optical tube.

11. The ignition device of claim 10, wherein the tubular member is connected to the head by a clamping arrangement comprising a thermal expansion compensation feature between the tubular member and the head.

12. The ignition device of claim 11, wherein the clamping arrangement comprises a clamping nut coaxial to the tubular member and adapted to axially lock the tubular member to the head; and wherein the thermal expansion compensation feature comprises a resilient ring coaxial to the tubular member and positioned between the clamping nut and a flange of the tubular member.

13. The ignition device of any one of claims 10 to 12, wherein the head comprises a thermal insulating ring positioned between the laser adapter and a main body of the head, whereto the tubular member is coaxially attached and wherefrom the tubular member projects opposite said thermal insulating ring.

14. The ignition device of claim 13, wherein the laser adapter comprises a flange, coupled to the main body of the head and wherefrom a connector projects opposite the main body of the head and coaxial to the axial cavity, the connector comprising a coupling feature for connection of a laser source to the ignition device; and wherein the thermal insulating ring is positioned between the main bod of the head and the flange.

15. The ignition device of any one of claims 10 to 14, wherein the head comprises an annular flange for connection to a casing of an internal combustion engine.

16. The ignition device of any preceding claim, further comprising alaser source connected to the laser adapter.

17. The ignition device of claim 16, wherein the laser source is connected to the laser adapter with an output window of the laser source optically aligned with the first optical element and the axial cavity of the optical tube.

18. A power generating turbomachine, comprising:a casing;a combustion chamber located within the casing;a laser source adapted to generate a laser radiation for igniting a fuel and oxidant mixture in the combustion chamber; andan ignition device according to any one of claims 1 to 14; wherein the optical tube of the ignition device extends through a combustion chamber access port of the casing.

19. The turbomachine of claim 18, wherein the distal end of the optical tube is positioned proximate to an aperture of a liner of the combustion chamber.

20. The turbomachine of claim 18 or 19, wherein the turbomachine is a gas turbine or expander, preferably a supercritical carbon dioxide turbine or expander.

21. The turbomachine of any one of claims 18 to 20, wherein the casing comprises a combustion chamber access port; and wherein the optical tube of the ignition device extends through the combustion chamber access port.

22. The turbomachine of claim 21, wherein the combustion chamber access port comprises a flange; and wherein removable clamps or bolts connect the flange of the combustion chamber access port and a flange of the ignition device to one another.

23. The turbomachine of any one of claims 18 to 22, wherein the distal end of the optical tube is positioned at a side wall of a liner arranged in the combustion chamber.

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