Turbomachinery component for so 2 doped co 2 power cycles

Abstract

The subject-matter disclosed herein relates to the use of a composition comprising or consisting of Carbon Dioxide and Sulphur Dioxide as working fluid for a supercritical and / or transcritical power cycle wherein said supercritical / transcritical power cycle is performed in an energy generation system whose at least one component configured to be in contact with the working fluid during the supercritical / transcritical power cycle consist of or comprises an alloy comprising Cobalt. The subject-matter disclosed herein also relates to a turbomachinery component configured to be in contact with said composition during a supercritical and / or transcritical power cycle and to an energy generation system configured to perform a supercritical and / or transcritical power cycle which comprises said turbomachinery component.

Description

TITLETurbomachinery component for SO2 doped CO2 power cyclesDESCRIPTIONTECHNICAL FIELD

[0001] The subject-matter disclosed herein relates to the use of a composition comprising or consisting of Carbon Dioxide and Sulphur Dioxide as working fluid for a supercritical and / or transcritical power cycle wherein said supercritical / transcritical power cycle is performed in an energy generation system whose at least one component configured to be in contact with the working fluid during the supercritical / transcritical power cycle consists of or comprises an alloy comprising Cobalt. The subject-matter disclosed herein also relates to a turbomachinery component configured to be in contact with said composition during a supercritical and / or transcritical power cycle and to an energy generation system configured to perform a supercritical and / or transcritical power cycle which comprises said turbomachinery component.BACKGROUND ART

[0002] The increase in global warming potential and CO2 emissions due to consumption of fossil fuels in conventional power plants is demanding new power production technologies which are efficient and economically sustainable. This especially considering that global energy demand continues to increase.

[0003] In this context, Carbon Dioxide based supercritical and / or transcritical power cycles are promising candidates for the production of renewable energy from thermal energy sources, such as nuclear, solar, geothermal energy and waste heat recovery. In particular, concentrated Solar Power (CSP) stands outas a very promising solution, utilizing the concentration of solar energy to generate electricity. Supercritical Carbon Dioxide (sCCh) refers to Carbon Dioxide operating above its critical point (304.13K, 7.38MPa). Supercritical CO2 power cycles operate between two pressure limits where both the heat addition and heat rejection pressures are higher than the critical point of the working fluid. Consequently, the compression process takes place in the supercritical phase using a compressor. CO2 is a colourless and odourless gas, relatively non-flammable, non-toxic and falls under the category of substances "generally regarded as safe" (GRAS). Thanks to its favourable thermodynamic properties, the interest in utilising CO2 has experienced a significant surge, particularly in recent years. Two significant factors influence the choice of SCO2 for a power cycle. The first factor is the enhanced cycle efficiency in power cycles using supercritical fluids when operating near the critical range of the fluid. The second factor is the critical temperature of CO2 which is closed to ambient temperature (31°C), facilitating the use of SCO2 power cycles with a wide range of heat sources and the operation of cycle variations that discharge heat to sinks at nearly ambient conditions. In summary, the main attractive features of supercritical Carbon Dioxide power cycles are higher cycle efficiency, compact size of turbines and heat exchangers compared to conventional steam Rankine cycle and their integration capability with different heat sources ranging from low to high temperature.

[0004] However, there are some challenges of SCO2 power cycles which includes lower heat recovery effectiveness and higher size footprint of cycle layouts which lead to higher specific costs and limited efficiency. This especially in geographic areas wherein the average ambient temperature is higher than the critical temperature of CO2, thus making necessary the use of a compressor instead of a pump (as known, the use of compressor is more expensive than the use of the pump in terms of power consumption).To address the abovementioned drawbacks, several sCCh-based mixtures have been proposed to enhance thermodynamic performance of sCCh power cycles. Said mixtures comprise halide base dopants such as Titanium Tetrachloride (TiCh), Hexafluoro-benzene (CeFe) and Perfluorobutane (C4F10). Said mixtures raise the critical temperature of the working fluid, thus allowing it to condense at ambient temperatures also in the locations described above. In this way, the resulting supercritical / transcritical power cycles have lower compression work and higher thermal efficiency.

[0005] Even if said mixtures allow to raise the critical temperature of the working fluid, they present some drawbacks. In particular, they are too aggressive towards the materials comprised in the system equipment (typically said materials are represented by an alloy). This drawback is relevant especially considering that the system equipment is expected to remain operative for 20 years or more. Furthermore, TiCh has limitations due to its high reactivity with moisture in the air and the formation of HC1 and TiCh, with obvious hazardous risks for humans and the environment.

[0006] In view of the observations made above, there is still the need in the sector to overcome the drawbacks which characterize the prior-art.SUMMARY

[0007] The object of the present invention relates to the use of a composition comprising or consisting of Carbon Dioxide and Sulphur Dioxide as working fluid for a supercritical and / or transcritical power cycle wherein said supercritical / transcritical power cycle is performed in an energy generation system whose at least one component configured to be in contact with the working fluid during the supercritical / transcritical power cycle consists of or comprises an alloy comprising Cobalt.

[0008] The invention also refers to a turbomachinery component configured tobe in contact during a supercritical and / or transcritical power cycle with a working fluid comprising or consisting of Carbon Dioxide and Sulphur Dioxide, wherein said turbomachinery component consists of or comprise an alloy comprising Cobalt.

[0009] In addition, the invention refers to an energy generation system configured to perform a supercritical and / or transcritical power cycle which comprises the turbomachinery component described above.DRAWINGS

[0010] Figure l is a simplified process flow diagram of an illustrative energy generation process to generate electricity using supercritical / transcritical CO2 in a simple closed-loop Brayton cycle.

[0011] Figure 2 shows the experimental data (expressed in terms of mass gain in mg / cm2) of the test described in Example 1.

[0012] Figures 3-6 show the results of the SEM analysis of the samples tested in Example 1.DETAILED DESCRIPTION OF EMBODIMENTS

[0013] Reference now will be made in detail to embodiments of the disclosure. The examples are provided by way of explanation of the disclosure and should not be construed as a limitation of the disclosure. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope or spirit of the disclosure.

[0014] According to an aspect, the subject-matter disclosed herein relates to the use of a composition (which is present in form of a gas mixture) comprising or consisting of Carbon Dioxide and Sulphur Dioxide as workingfluid for a supercritical and / or transcritical power cycle wherein said supercritical and / or transcritical power cycle is performed in an energy generation system whose at least one component configured to be in contact with the working fluid during the supercritical / transcritical power cycle consists of or comprises an alloy comprising Cobalt.

[0015] In a preferred embodiment, the molar fraction of Sulphur Dioxide in said composition varies from 5 and 35%, more preferably from 10 to 25%, even more preferably equal to 20%. In particular, said composition may consists of CO2 and SO2, wherein the molar ratio of CO2 is equal to 80% and the molar ratio of SO2 is 20%.

[0016] As said before, the subject matter disclosed herein also relates to a turbomachinery component configured to be in contact during a supercritical and / or transcritical power cycle (in particular during one or more steps of said cycle) with the composition described above wherein said turbomachinery component consists of or comprise an alloy which comprises Cobalt.

[0017] As known, heat-resistant Nickel-based alloys, cobalt-based alloys and / or Co-bearing alloys are commonly employed to build the components of devices for heat treatments (such as for example gas turbines). The durability of these components is strongly menaced by the presence of Sulphur gas compounds (which are present as contaminants in the gas mixture at very low amounts, usually between 50 and 1500 ppm). In particular, it is known that Sulphur gas compounds are dangerous for the equipment at high temperatures (i.e., equal to or higher than 680°C) because they are featured by high diffusivity and they manage to form low-melting eutectics, especially with Nickel, thus damaging the alloy. Cobalt-based alloys and Cobalt-bearing alloys have superior hot corrosion and sulfidation resistance compared to Nickelbased superalloys due to low diffusion of Sulphur in the matrix. However, it is known that said difference is only observed at high temperatures (i.e., equal toor higher than 680°C) and / or with Sulphur amounts lower than 1 wt%, while a similar behaviour (i.e., degradation rate in same order of magnitude) is described for both alloys in low oxidant or reducing environments, and in strongly oxidant environments having limited content (i.e., equal or lower to 4 wt%) of Sulphur species , see for example the textbook “ High-Temperature Corrosion and Materials Applications'" (Author: George Y. Lai, ISBN: 978-0- 87170-853-3, in particular Fig. 7.36 and Fig. 7.49).

[0018] In this context, it has been surprisingly found that the composition according to the present invention is particularly suitable as working fluid when the turbomachinery components (or component parts) of the energy generation system which come in contact with the working fluid during the supercritical and / or transcritical power cycle consist of or comprise an alloy comprising Cobalt. In particular, it has been found that this kind of alloy has an improved sulfidation resistance compared to Nickel-based superalloys when the composition according to the present invention is used as working fluid (especially at temperatures below 650°C). In this view, turbomachinery components (or component parts) made by or comprising Ni-Cr-Co alloys, Cobased alloys and / or Co-bearing alloys are particularly preferred. The alloy may also be present in form of weld overlay (i.e., the alloy is applied as a layer atop the surface of the component by welding). Indeed, the weld overlays of said types of alloys on the surface of the turbomachinery components (or component parts) were found very efficient to increase the corrosion resistance (and, consequently, to increase the expected service life of the component). In addition, the presence of said types of alloys in form of weld overlay may allow the use of components (or component parts) made of or comprising standard stainless steel, with advantages in terms of costs of the energy generation system. In addition, the presence of said types of alloys in form of weld overlay allows to avoid the problems related to the production of components of great dimensions (which would be otherwise difficult to produce). These aspects areparticularly relevant for components such as for example the casing, the cover flanges, the rotor, and valve bodies.

[0019] In a preferred embodiment, the weight concentration of Cobalt with respect to the total weight of the alloy is equal to or higher than 8%. More preferably, the weight concentration of Cobalt with respect to the total weight of the alloy is equal to or higher than 10%. Even more preferably the weight concentration of Cobalt with respect to the total weight of the alloy is equal to or higher than 12%. The weight concentration of Cobalt with respect to the total weight of the alloy may be comprised between 8 and 75%, more preferably between 12 and 60%. For example, the weight concentration of Cobalt with respect to the total weight of the alloy is comprised between 10 and 30%, more preferably between 12 and 25%, even more preferably between 15 and 20%. The alloy may further comprise Nickel and / or Chromium, more preferably said alloy is a Nickel-chromium-cobalt based alloy. In particular, the weight concentration of Chromium with respect to the total weight of the alloy may be equal to or higher than 10%, while Nickel is used as remainder. For example, the weight concentration of Chromium with respect to the total weight of the alloy may be comprised between 10 and 35%, more preferably between 15 and 30%, while the weight concentration of Nickel with respect to the total weight of the alloy may be comprised between 20 and 70%, more preferably between 40 and 60%. The amount of Nickel may be lower than 60 wt%, more preferably lower than 50 wt%, even more preferably lower than 40 wt% with respect to the total weight of the alloy.

[0020] In a preferred embodiment:The weight concentration of Cobalt with respect to the total weight of the alloy is equal to or higher than 10%;The weight concentration of Chromium with respect to the total weight of the alloy is equal to or higher than 10%.The alloy may further comprise Aluminium, Titanium and / or Niobium. In particular, said components allows to ensure precipitation hardening using gamma prime (y1) or double prime (y") phase. More preferably, the total weight concentration of said components with respect to the total weight of the alloy of said components may be for example higher than 1%. Optionally, said alloys may comprise also Molybdenum and / or Tungsten. Said components (which confer high temperature straight) may be added for example by solid solution hardening.In preferred embodiment, the alloy comprises Cobalt (more preferably, the weight concentration of Cobalt with respect to the total weight of the alloy is equal to or higher than 8%), Nickel, Chromium, Aluminium and Titanium. Optionally, said alloy further comprises Molybdenum and / or Tungsten.

[0021] In a preferred embodiment, the alloy comprises:• At least 8.0 wt% of Cobalt;• 10.0-30.0 wt% of Chromium and• At least 35.0 wt% of Nickel.

[0022] In a further preferred embodiment, the alloy comprises or consists of:• At least 8.0 wt% of Cobalt;• 10.0-30.0 wt% of Chromium;• At least 35.0 wt% of Nickel;• 0.5-3.0 wt% of Aluminium and• 0.5-5.0 wt% of Titanium.

[0023] In a further preferred embodiment, the alloy comprises or consists of:• At least 8.0 wt% of Cobalt;• 10.0-30.0 wt% of Chromium;• At least 35.0 wt% of Nickel;• 0.01-0.5 wt% of Carbon;0.5-2.0 wt% of Silicon and0.2-2.5 wt% of Manganese.

[0024] In a further preferred embodiment, the alloy comprises or consists of:• At least 8.0 wt% of Cobalt; • 10.0-30.0 wt% of Chromium;• At least 35.0 wt% of Nickel;• 0.01-0.5 wt% of Carbon;• 0.5-2.0 wt% of Silicon;• 0.2-2.5 wt% of Manganese; • 0.01-1.0 wt% of Phosphorus;• 0.01-1.5 wt% Sulphur;• 0.5-2.0 wt% of Aluminium;• 0.5-5.0 wt% of Iron;• 0.5-10.0 wt% of Molibdenum; • 0.5-5.0 wt% of Titanium;• Optionally, up to 1 wt% of Boron;• Optionally, up to 2 wt% of Copper and• Optionally, up to 1 wt% of Zirconium.

[0025] For example, the alloy may have the following compositions:

[0026] The surface of the turbomachinery components (or component parts) which is configured to be in contact with the working fluid may further comprise a coating. For example, the turbomachinery components (or component parts) can be further coated with a:Cr-N based coating;Cr-N-0 based coating;Al-Cr-0 based coating and / orMCrAlY coating, wherein M is preferably Nickel, Cobalt or a combination thereof.This coating allows to further increase the corrosion resistance of the component. Indeed, said coatings allow strong reduction of reaction kinetics on critical components strongly increasing the expected service life. This aspect is particularly relevant for components such as for example rotor and stator blades. Alternatively, said coating may also confer further properties to the surface of the component.

[0027] Said coating can be added to the turbomachinery components (or component parts) through known methods such as physical vapor deposition (PVD coating) and chemical vapor deposition (CVD coating).

[0028] As said before, the subject matter disclosed herein also relates to an energy generation system configured to perform a supercritical and / or transcritical power cycle which comprises the turbomachinery component described above. In a preferred embodiment, said turbomachinery component is an expander and / or an expansion turbine (since these components are theones which are more exposed to the corrosive power of the working fluid). In particular, said expander may include an inner chamber wall made of or comprising an alloy comprising Cobalt. Further components or components parts may be for example: external and internal casings, cover flanges, rotors, stator and rotor blades, closing flanges and valve bodies. However, all components (or component parts) of said energy generation system which are configured to be in contact with the working fluid during the supercritical and / or transcritical power cycle may advantageously consist of or comprise the alloy according to the present invention. Typically, an energy generation system (100) based on a simple closed-loop Brayton cycle comprises: a heat exchanger (110) which heats the working fluid; an expander (120) which includes an inner chamber with an expansion turbine; a heat exchanger (130) to cool the exhausted gas from the expander; a compressor (140) which compress the working fluid to the desired pressure.

[0029] In particular, in a simple closed-loop Brayton cycle, the working fluid (i.e., CO2) is heated indirectly from a heat source through a heat exchanger. Then, energy is extracted from the CO2 as it is expanded in the turbine. The CO2 exiting the turbine is then cooled in a heat exchanger to the desired compressor inlet temperature, and after compression to the required pressure, the CO2 is sent back to the heater to complete the cycle.

[0030] A known variant of said closed-loop Brayton cycle incorporates thermal recuperation through a heat exchanger that is introduced between the expander and the compressor.

[0031] As can be seen from the experimental results obtained in the following examples, the supercritical and / or transcritical power cycle may be advantageously performed with a turbine inlet temperature comprised between400 and 750°C, more preferably between 500 and 680°C, even more preferably equal to 550°C.EXAMPLE 1 - High-temperature corrosion tests

[0032] High-temperature corrosion tests were carried out by setting the samples inside a flanged (removable) pressure vessel maintained at 550°C and filled with an 80% CO2+20% SO2 mixture for an overall duration of 2000 hours. The testing procedure involved periodically removing the samples from the pressure vessel, typically at intervals of around 600 hours, and weighing them by using a four-decimal-place Gibertini E42-B balance to assess the weight gain / loss relative to the initial value. After weighing, the samples were reinserted into the vessel. This approach allowed for the examination of the corrosion process over time for both bulk samples and coated samples subjected to the test. It is pointed out again that the mass gain determined based on the sample's weight variation is a parameter commonly used in literature to assess corrosion levels. In particular, the following samples of alloys were analysed:• Sample 1 : UNS N07725 (forged),• Sample 2: UNS N06625 (cast),• Sample 3 : UNSN07718 (forged),• Sample 4: a Ni-Cr-Co cast alloy (cast),• Sample 5: UNS N07001 (forged),• Sample 6: the same Ni-Cr-Co cast alloy of sample 4 coated with AlCrO by PVD and• Sample 7: the same Ni-Cr-Co cast alloy of sample 4 coated with CrNO by PVD.In particular, the table below shows the measured values (expressed in weight concentration) of Cobalt, Chromium, Nickel and of the other main components of said alloys.TABLE 1

[0033] The results of the test are summarized in Figure 2, which shows the mass gain in mg / cm2calculated from the weight values measured for the different samples.

[0034] From the results shown in said Figure, it is evident that the best performing alloys were the samples 4 and 5. Both alloys are characterized by a significant amount of Co in the alloy composition. Specifically, the reaction rate of cobalt in sulfur-rich atmospheres is considerably lower than that of Ni and Fe. It is worth noting that the sample 4 has a higher content of Co and Cr compared to sample 5 (the lower mass gain of sample 4 demonstrates its superior performance over sample 5). In summary, both sample 4 and sample5 gave good results. In particular, sample 4 gave the best results with reference to uncoated alloys. As for the coatings, coated with AlCrO by PVD coating provided superior performance compared to CrNO by PVD.

[0035] Furthermore, SEM-EDS analyses were conducted on some samples to verify the formation of corrosion products and identify their composition (see Figure 3 and Figure 4). In the case of sample 2, a thick layer of corrosion products containing Ni, S, and O was observed, covering the surface quite uniformly. The SEM-EDS cross-section revealed the presence of two layers: an inner oxidized layer enriched in chromium and with presence of Sulphur, topped by an outer layer made of coarse sulphides nodules (rich in S and Ni). Sample 3 alloy showed substantially same corrosion products structure. However, the sample exhibited a more localized corrosion phenomena, with alternance of significantly affected areas (with inner layer and sulphides nodules on top) and zone of lower interaction. With increasing exposure time, affected area surface extension increased leading at the end of experiment to comparable weight gain to sample 2. In case of the sample 1 alloy, SEM analysis revealed again same structure and composition of corrosion product, showing however a more compact inner zone with limited thickness and a reduced presence of external sulphides, confirming the reduced reaction kinetic compared to other Ni-based alloys as per weight gain results. For the sample 4 alloy, the corrosion products formed a very thin interaction layer characterized by Cr, Ni, Co and O, with only low (negligible) levels of S (thus suggesting the good resistance of the alloy to the interaction with this element). Similar observations applied to the sample 5 alloy, confirming its similarities with the sample 4 alloy, and the beneficial effect of higher Co contents. SEM analysis of the AlCrO coating cross-section showed a continuous, adherent coating of constant thickness. Since no interaction was visible at coating / substrate interface, the coating appeared to perform as a protective barrier. Limited weight gain observed, could possibly be justified by Sulphurspecies deposition within the coating external region due to atmosphere decomposition during test.

[0036] Moreover, further high-temperature corrosion tests were carried out under the same conditions as defined in par.

[0032] in order to analyse the following samples:• Sample 8: UNS R30006 (cast),• Sample 9: UNS R30605 (forged).

[0037] The performance of said samples after 2000 hours of exposure is consistent with the one observed with reference to the sample 4 (see Figure 5). We note that samples 8 and 9 are characterized by a significant amount of Cobalt in the alloy composition and a low amount of Nickel (i.e., samples 8 and 9 do not represent a Nickel based alloy, since the amount of said compoundin said samples is lower than 50%).

[0038] Furthermore, micrographic analyses in cross section were conducted on samples 8 and 9 (see Figure 6). For both samples, the corrosion products formed a very thin interaction layer with only low (negligible) levels of S (thus suggesting the good resistance of the alloy to the interaction with this element, consistently with what was observed with sample 4).

Claims

CLAIMS1. Use of a composition comprising or consisting of Carbon Dioxide and Sulphur Dioxide as working fluid for a supercritical and / or transcritical power cycle wherein said supercritical / transcritical power cycle is performed in an energy generation system whose at least one component configured to be in contact with the working fluid during the supercritical / transcritical power cycle consists of or comprises an alloy comprising Cobalt, wherein the weight concentration of Cobalt with respect to the total weight of the alloy is equal to or higher than 8%.

2. Use according to claim 1, wherein the molar fraction of Sulphur Dioxide in said composition varies from 5 and 35%, more preferably from 10 to 25%, even more preferably equal to 20%.

3. Use according to any previous claims, wherein the weight concentration of Cobalt with respect to the total weight of the alloy is equal to or higher than 12%.

4. Use according to any previous claims, wherein the alloy further comprises Nickel and / or Chromium, more preferably said alloy is a Nickel-chromium- cobalt based alloy.

5. Use according to any previous claims, wherein the alloy is present in form of weld overlay.

6. Use according to any of claims 1-4, wherein the surface of said at least component further comprises a coating which comprises Cobalt and / or Chromium, wherein said coating is selected from a Co-Cr based coating, a Cr-N based coating, a Cr-N-0 based coating, an Al-Cr-0 based coating and a MCrAlY coating.

7. Turbomachinery component configured to be in contact during a supercritical and / or transcritical power cycle with a working fluid comprising or consisting of Carbon Dioxide and Sulphur Dioxide, wherein said turbomachinery component consists of or comprises an alloy whichcomprises Cobalt, wherein the weight concentration of Cobalt with respect to the total weight of the alloy is equal to or higher than 8%.

8. Turbomachinery component according to claim 7, wherein the weight concentration of Cobalt with respect to the total weight of the alloy is equal to or higher than 12%.

9. Turbomachinery component according to any of claims 7-8, wherein the alloy further comprises Nickel and / or Chromium, more preferably said alloy is a Nickel-chromium-cobalt based alloy.

10. Turbomachinery component according to any of claims 7-9, wherein the alloy is present in form of weld overlay.

11. Turbomachinery component according to any of claims 7-10, wherein the surface of said turbomachinery component further comprises a coating which comprises Cobalt and / or Chromium, more preferably said coating is selected from a Co-Cr based coating, a Cr-N based coating, a Cr-N-0 based coating, an Al-Cr-0 based coating and a MCrAlY coating.

12. Turbomachinery component according to any of claims 7-11, wherein said component is an expander or an expansion turbine.

13. Energy generation system configured to perform a supercritical and / or transcritical power cycle which comprises at least a turbomachinery component according to any of claims 7-12.

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