The invention is directed to an apparatus for expanding a compressed combustion gas comprising a combustor, an axial turbine rotor and a diffusor.

Numerous apparatuses, such as gas turbines, are known for expanding a compressed combustion gas comprising a combustor, a turbine rotor and a diffusor. Typically the turbine rotor will be fixed to a shaft and the rotational energy as generated by the rotating turbine rotor and shaft is usually converted to electricity by making use of a generator also fixed to the shaft. Such apparatuses are typically optimised to expand a compressed combustion gas having a high pressure ratio. In certain applications it may be desirable to apply a lower pressure ratio of about 2 to 5, wherein the pressure ratio is the pressure upstream the turbine rotor and the ambient pressure. A problem with the state of the art apparatuses is that the efficiency of recovering rotation energy from the expanding combustion gas by means of the turbine rotor is low.

For small scale gas turbines used in industry in non-aviation applications combinations of a centrifugal compressor, annular burners and axial turbine rotors are used. The compressor is used to compress combustion air. A fuel is typically combusted with the compressed air in one or more annular positioned burners. The compressed combustion gas thus obtained is expanded using an axial turbine rotor or rotors and discharged via a diffusor. A disadvantage of such apparatuses is that NOx emissions are typically high. If electricity is required to be produced using the rotational energy produced by the turbine rotor by means of a generator the rotational speed is often too high to directly fix a generator to a common shaft. Gear boxes will be required which makes the device complex and less energy efficient.

US2014/0260325 describes such a gas turbine. In this apparatus the diffusor through which the expanded combustion gasses pass consists of two separate compartments having a relatively sharp upward directed bend and fluidly connected to an outlet at the upper side of the apparatus which outlet is directed radially outward relative to the axis of the turbine rotor. A disadvantage of such a design is that it is complex due to the multiple compartments. A further disadvantage is that the boundary layer of the gas may separate from the wall at the sharp bends to form eddies, also referred to as diffusor stall. These eddies will block the flow in the diffusor and will make an efficient conversion of gas velocity to gas pressure more difficult. A less efficient conversion to gas pressure will result in a less energy efficient apparatus.

GB2005355 describes a gas turbine wherein the expanded combustion gasses make two sharp turns wherein the axial direction of the gas reverses twice. Such a design will have the same issues with gas being separated from the wall surface as described above.

US2012167591 describes a gas turbine wherein the expanded combustion gasses are axially transported to a ring shaped outlet connected to a collector. A disadvantage of such a design is that a less efficient conversion of gas velocity to pressure is expected in such a design.

US3831374 describes a gas turbine wherein the expanded combustion gasses are radially transported wherein at a certain point the axial direction of the expanded gasses reverses to be provided to a heat exchanger. Apart from the fact that such a design is complex it is also not expected that such reversal will result in an efficient apparatus.

The object of the present invention is to provide an energy efficient apparatus when operated at these lower pressure ratio's and which does not have one or more of the above described disadvantages. The apparatus comprises a combustor, an axial turbine rotor and a diffusor.

This object is achieved with the following apparatus. Apparatus for expanding a compressed combustion gas comprising a combustor, an axial turbine rotor and a diffusor, wherein

the combustor is comprised of an axially extending space around an axially extending combustor axis provided with one or more burners at one end and an circular slit as discharge opening at its opposite downstream end,

the turbine rotor being an axial turbine rotor, optionally having a radial component, which turbine rotor being rotatable along the combustor axis and provided with an inlet to receive combustion gas fluidly connected to the discharge opening of the combustor and a discharge outlet to discharge combustion gas, wherein the turbine rotor is fixed to a rotatable shaft, which shaft is positioned axial with the combustor axis and wherein the diffusor is comprised of a circular slit-shaped inlet opening fluidly connected to the discharge outlet of the turbine rotor, a flow path for combustion gas which extends axially away from the combustor and extends radially outward towards a circular slit-shaped outlet, which circular slit -shaped outlet is fluidly connected along its circumference to a receiving space.

Applicants found that the efficiency of such an apparatus is high when applying a lower pressure ratio. The design of the diffusor allows for that more kinetic energy of the combustion gas as it passes the diffusor is converted into pressure. The apparatus is further advantageous because its design allows for other elements, such as a generator and compressor to be fixed to the shaft and close to the turbine rotor such to result in a practical length of the shaft.

Furthermore the apparatus may be operated at rotational speeds allowing for direct coupling of a generator to the shaft. Further advantages will be described in relation with the preferred embodiments described below.

The diffusor is comprised of a circular slit-shaped inlet opening fluidly connected to the discharge outlet of the turbine rotor. By circular slit-shaped opening is meant an annular opening or an opening shaped like or forming a ring as also shown in Figure 1. The opening is suitably a single opening and the flow path for combustion gas is suitably present in a single space. The diffusor is comprised of a flow path for combustion gas which extends axially away from the combustor and extends radially outward towards a circular outlet. By circular slit- shaped outlet is meant an annular opening or an opening shaped like or forming a ring as also shown in Figure 1. At its upstream part the flow path may be provided with a bend to direct the expanded combustion gas from a substantially axial direction as it is discharged from the turbine rotor to the above described path way which extends both radially and axially. The radius of such a bend should not be too small in order to avoid unnecessary additional pressure drop and not too large in order to avoid a very large apparatus. The inlet area of the bend and thus also of the circular slit-shaped inlet opening of the diffusor is suitably equal or greater than the outlet area of the bend.

Part of the flow path for the combustion gas of the diffusor is suitably defined by two frusto conical walls. This part will thus extend both in a radial and axial direction when viewed in the direction of the gas as its passes the diffusor. These walls may run parallel wherein the distance between the two walls remains constant along the length of the flow path. The distance may also vary, for example increase or decrease, along the length of the flow path.

The angle between the general direction of the flow path between the two frusto conical walls and the combustor axis is between 20 and 60°. The angle of the general direction is the numerical average of the angle of the two frusto conical walls with the combustor axis. In case additional elements, such as a generator and compressor are to be fixed to the shaft it is preferred that this angle is greater than 30°, more preferably greater than 40°. The angle is preferably smaller than 50° to achieve higher efficiencies. As is clear from the above description is the fact that the flow of gas as it follows the flow path from the circular inlet of the diffusor to the circular outlet of the diffusor does not reverse direction as viewed along the combustor axis.

Preferably the area ratio between the end and the start of the flow path between the two frusto conical walls is above 1.5, more preferably above 1.8 and preferably below 6 and more preferably below 3. The area is here defined as the cross sectional area of the flow path as present the two frusto conical wall parts and not including the optional bends. Preferably the increase of this area in the direction of the flow path is low enough to avoid that the boundary layer of the gas separates from the wall to form eddies, also referred to as diffusor stall. These eddies will block the flow in the diffusor and are preferably avoided. This increase in area will be dependent of the above described general direction of the flow path, the length of the flow path and the positioning of the two walls relative with respect to each other. The two frusto conical walls may run parallel, divert or convert. The distance between the frusto conical walls may increase in the flow direction, wherein a diffusion angle γ is suitably below 20°, preferably below 10° and even more preferably below 5°. Applicants found good results with an angle close to zero. The angle may also be slightly negative, wherein the two walls converge slightly in the flow direction. The diffusion angle is herein twice the angle of one frusto conical wall with the average direction of the flow path. The length of the part of the flow path between the two frusto conical walls may be limited by the available space. Further the length is determined by the above mentioned area ratio, general direction of the flow path, and the positioning of the two walls relative with respect to each other. The diffusor is also provided with an circular outlet fluidly connected along its circumference to a receiving space. Preferably the receiving space is designed to direct the axially and radially flowing gas in a single radially moving gas flow. Preferably the receiving space has the design of a so-called volute. In such a volute combustion gas as discharged from the outlet opening of the diffusor will be collected. The volute will have an increasing cross- sectional area along its circumference such that the velocity of the combustion gas in the volute remains substantially the same while combustion gas is collected along the circular slit-shaped outlet opening. The volute may be in a plane perpendicular to the combustor axis as illustrated in the figures. The receiving space may have one or more outlets, suitably tangentially directed, openings for discharge of the combustion gas to other process apparatuses, such as a boiler or a further burner. The diffusor and collecting space as described above thus results in that the axial outflow or axial outflow component of combustion gas as it is discharged from the turbine rotor is deflected in a radial direction.

The combustor is comprised of an axially extending space around an axially extending combustor axis provided with one or more burners at one end and an circular slit as discharge opening at its opposite downstream end. The axially extending space may have any design, such as regular or irregular shapes. Its cross-sectional area may for example be circular, elliptical or rectangular. The cross-sectional area may increase or decrease along the length of the combustor axis. Suitably the axially extending space of the combustor is a tubular space.

The burner may be centrally positioned when a single burner is present. When multiple burners are present it is preferred to arrange the burners in a somewhat symmetrical manner to assist in the creation of an uniform combustion flow in the combustion space. The direction of the burner, i.e. the direction of the combustion gasses as exiting from the burner, may be along the combustor axis in case a single burner is used. In case multiple burners are present the direction of the burner may be parallel to the combustor axis or under an angle with this axis. The burner or burners are suitably of the so-called can type.

The position of the burner or burners at the end of the combustor allows fast dismounting for service purposes. Further it is found that when a single burner is axially aligned with the turbine rotor and shaft an optimal inflow conditions are created for the turbine rotor. Suitably at the downstream end of the axially extending space of the combustor a gas diverting body is present positioned along the combustor axis defining a combustor gas pathway between said body and the wall of the axially extending space and terminating at the circular slit at the downstream end of the combustor. Such a gas diverting body, sometimes referred to as a dome, may have a cross-sectional design being or resembling the half of an ellipse, wherein the upper end of the ellipse is positioned closest to the burner on the combustor axis.

The axially extending space of the combustor has a closed end provided with one or more burners, an opposite end provided with a circular slit as discharge opening and optionally a dome and a wall running in the direction of the combustor axis and connecting both said ends. Around the wall of the axially extending space of the combustor a second shell is suitably placed defining a cooling zone between said wall and second shell. The cooling zone is provided with an inlet for compressed air and one or more outlets for compressed air, wherein at least one of these outlets is to provide compressed air to the burner. Suitably more than one outlet for compressed air is provided as openings in the wall of the axially extending space. In case of a tubular space these openings may be arranged in a circle along the circumference of the tubular wall. The inlet for compressed air is suitably positioned such that substantially the entire wall of the axially extending space can be cooled at its exterior by the compressed air. The cooling space may be provided with guiding vanes to create a pathway in the cooling zone which enhances the cooling and converts any radial movement of the compressed air as it is supplied to the cooling space to an axial movement.

Applicants found that the design of combustor and diffusor and collecting space result in a better mixing of fuel and compressed air, a lower NOx emissions, a better stability of the flame and a lower heat load on the wall of the axially extending space. Furthermore because the turbine rotor, the axially extending space and burner are aligned very low loss of energy is obtained of the pressurised combustion gas when a tubular axially extending space of the combustor is used

The apparatus is provided with a turbine rotor. This may be a single rotor or a multiple rotor. Preferably a single rotor is used to obtain a more simple design. The rotor itself is an axial turbine rotor. Optionally the rotor may have a radial component. Such a radial component suitably directs the combustion gas towards the direction of the radially and axially extending flow path of the diffusor. The bend as described above could then be omitted.

The turbine rotor is fixed to a rotatable shaft. Suitably a generator and a compressor is further directly fixed to the rotatable shaft. The sequence along the shaft in this preferred embodiment is turbine rotor, generator and compressor. This sequence is advantageous because it results in a very compact apparatus. Furthermore because no gear box is present between shaft and generator a more efficient apparatus is obtained. The compressor is provided with an outlet for compressed air which is fluidly connected to the combustor. The outlet for compressed air may be fluidly connected to the combustor via a compressed air flow path and wherein no heat exchanger is present in the compressed air flow path. This is advantageous when the expanded combustion gas as obtained in the collecting space is further used to for example heating purposes. In a situation wherein the generation of electricity is more desired a heat exchanger may be present in said flow path to heat the compressed air against expanded combustion gas as obtained in the collecting space by indirect heat exchange in said heat exchanger.

The generator is suitably configured to also be useable as an electrical motor. The apparatus can then be started-up using the generator as electrical motor. In such a preferred embodiment the generator is equipped to either generate electric power or deliver a shaft torque to the shaft such as for example described in US2002/0070716. Suitably use is made of a converter in combination with such a combined generator-electric motor combination. Such a converter may comprise of a grid-side converter and a generator/motor-side converter. The grid-side converter has two operating modes. In one mode it can transform DC current as generated by the generator into grid alternating current. In another mode it can convert grid alternating current into DC current which is used when the combination is operated as an electrical motor. The grid side converter can perform both conversions. The generator/motor side converter can suitably additionally control the rotational speed of the generator by changing the frequency of the current at the generator/motor side. The generator/motor side converter and grid side converter may also be combined into one apparatus. The generator- motor combination is suitably connected to the electrical grid or any other external source of electricity via the above converter. The generator may comprise of a rotor as fixed to the shaft and equipped with permanent magnetic materials. Around the rotating rotor a stator is positioned with electric spools in which electricity is generated or supplied.

The compressor may be an axial compressor having one or more stages. The compressor is preferably a centrifugal compressor and even more preferably a single centrifugal compressor. Centrifugal compressors are preferred because of their simplicity because they require less stages to obtain the same pressure rise as compared to an axial compressor.

The apparatus is especially suited to prepare a heated gas and generate electricity at a low pressure ratio. The invention is therefore also directed to a process to prepare a heated gas and generate electricity using the apparatus according to the invention at a pressure ratio of between 2 and 5 and preferably between 3 and 4. The temperature of the heated gas is suitably between 650 and 750 °C. The heated gas may find use in many applications. Suitably the heated gas is used to generate steam by indirect heat exchange of water and the heated gas.

Figure 1 shows an apparatus according to the present invention. Figure 1 shows a can- type burner 1 as part of a combustor 2. The combustor 2 is comprised of a tubular combustor space 3 extending axially from the axis 4 of the shaft 5 and provided with the burner 1 at one end 6 and a circular slit 7 as discharge opening at its opposite downstream end 8. At circular slit 7 stator vanes may be present. Through slit 7 the compressed combustion gas flows to the turbine rotor 9. The apparatus is further provided with a single stage centrifugal compressor 10 connected to rotatable shaft 5. In compressor 10 air is compressed to obtain compressed air which compressed air is routed via a compressed air flow path to the burner 1 via conduit 11 to an annular space 12 where the compressed air is used to cool the exterior wall 13 of the tubular combustor space 3 before being fed to the burner 1 via conduit 14. Part of the compressed air is supplied directly to the combustor space 3 via openings 17. Burner 1 is further provided with a fuel supply conduit 16. Combustor space 3 is provided with a dome 18 at end 8.

The compressed combustion gas as present in combustor space 3 is expanded via turbine rotor 9 connected to the rotatable shaft 5 to obtain the heated gas. The heated gas after passing the turbine rotor 9 will flow substantially in an axial direction via a diffusor 19 and volute type collecting space 20. The diffusor 19 will divert the direction of the heated gas flow from a substantial axial flow to a radial flow. The diffusor 19 has a flow path positioned between an inner 22 and outer 23 frusto conical wall. At the end of the flow path a circular outlet 24 is present which fluidly connects along its circumference the flow path to the receiving space 20. In the receiving space 20 the heated gas is collected and redirected to a circular flow and tangentially discharged from said collecting space. As shown the receiving space 20 has the design of a volute having an increasing cross-sectional area along its circumference wherein the tangentially arranged outlet is positioned at the larger cross- sectional part of space 20.

The diffusor 19 as shown in Figure 1 is comprised of one space, meaning that no separation walls are present between the walls 22 and 23 of the diffusor which would create multiple flow paths.

The radially and axially extending diffusor 19 allows the positioning of a generator 25. The generator 25 has a permanent magnet 26 connected to the shaft 5 and windings 27 in which electricity is generated.

Figure 2 shows a detail of the diffusor 19 of Figure 1. A cross-sectional view of one half of the diffusor 19 is shown. The diffusor 19 is rotatable symmetric with respect to axis 4.

Diffusor 19 is provided with a circular slit-shaped inlet opening 28 fluidly connected to the discharge outlet of the turbine rotor. The dimensions of the inlet 28 is defined by radius rla, rl and rib. The first part of the diffusor is comprised of a bend 29 such that the axial direction of the gas as it enters inlet 19 is diverted to a more radial direction. The bend terminates at an outlet 30, wherein the direction of the plane of outlet 30 and the plane of inlet 28 make an angle a. The bend has a radius rlr. The dimensions of the outlet 30 is defined by radius r2a, r2 and r2b. The bend 29 is fluidly connected to a flow path 31 as present between the inner 22 and outer 23 frusto conical wall. The length of the flow path 31 is Id. The width of the flow path is h2 at its upstream end and h3 at its downstream end. In Figure 2 h2 is equal to h3. The width of the flow path 31 may vary along its length as illustrated in Figure 2a, wherein the width h3 at its downstream end is greater, equal or smaller than the width h2 at its upstream end and the distance between the frusto conical walls 22 and 23 increases, stays equal or decreases in the flow direction. Figure 2 also shows the diffusion angle γ being twice the angle of one frusto conical wall with the average direction of the flow path. The area ratio of the flow path 31 between the two frusto conical walls 22 and 23 is suitably between 2 and 3. This is the area of the frusto conical shape defined by radius r2a and r3a for outer wall 23 and the area of the frusto conical shape defined by r2b and r3b for inner wall 22. Also circular outlet 24 is shown which fluidly connects the diffusor 19 with the receiving space 20 of Figure 1. The dimensions of the circular outlet 24 is defined by radius r3a, r3 and r3b. Figure 3 shows the diffusor 19 of Figure 1 and the volute shaped receiving space 20. In this Figure a part of the diffusor 19 and receiving space 20 has been cut out for illustrative reasons. The diffusor 19 has a flow path positioned between an inner 22 and outer 23 frusto conical wall, a circular inlet 30 and a circular outlet 31. As shown the axial direction of the gas as it flows along the flow path in the diffusor does not reverse or make sharp bends. The outlet 31 of the diffusor 19 is fluidly connected to a circular inlet 32 of the volute shaped receiving space 20. The volute shaped receiving space 20 has an increasing cross-sectional area along its circumference wherein the tangentially arranged outlet 33 is positioned at the larger cross-sectional part 34 of receiving space 20.

Figure 3 and 4 shows the diffusor 19 of Figure 1 and the and the volute shaped receiving space 20 from different angles. In both Figures a part of the diffusor 19 and receiving space 20 has been cut out for illustrative reasons. The diffusor 19 has a flow path positioned between an inner 22 and outer 23 frusto conical wall, a circular inlet 30 and a circular outlet 31. As shown the axial direction of the gas as it flows along the flow path in the diffusor does not reverse or make sharp bends. The outlet 31 of the diffusor 19 is fluidly connected to a circular inlet 32 of the volute shaped receiving space 20. The volute shaped receiving space 20 has an increasing cross-sectional area along its circumference wherein the tangentially arranged outlet 33 is positioned at the larger cross-sectional part 34 of receiving space 20.