Patent Application
20120159952
1. Field of the Invention
The present invention relates to gas turbine components such as bucket/nozzles, combustors, and shrouds of gas turbines. The invention particularly relates to gas turbine components that excel in heat resistance and corrosion resistance and suitable for use in high temperature corrosive environments, and to a gas turbine employing such components.
2. Description of the Related Art
Operation temperature in gas turbines has become higher year by year with an aim of improving efficiency. To cope with the increase in the operation temperature, some of high temperature components or parts used in gas turbines are adapted to have their surfaces covered with thermal barrier coatings (hereinafter also referred to as TBC) formed of ceramics so as to moderate the component temperature. Although it depends on working conditions, application of TBCs lowers a substrate temperature by 50 to 100° C. in general.
JP-62-211387-A, etc. disclose a TBC that comprises, over a substrate by way of a MCrAlY alloy layer, a thermal barrier layer formed from partially stabilized zirconia with low heat conductivity and good heat resistance. M represents at least one of iron (Fe), Ni, and Co, Cr represents chromium, Al represents aluminum, and Y represents yttrium.
It has been a subject of TBCs used in gas turbines to improve heat resistance as to cope with increasing temperature of combustion gas. The TBCs are also demanded to be improved in resistance against particle erosion due to solid particles (oxides mainly) contained in combustion gas. JP-9-78258-A discloses a case where a dense protective layer is provided at the surface of a porous thermal barrier layer with high heat resistance to improve erosion resistance.
In gas turbines, relatively high grade fuels such as liquefied natural gas (LNG), kerosene, light oil, etc. have been used commonly in view of improving the combustibility and preventing corrosion. Fuels described above contain extremely small amounts of corrosion factors such as sulfur (S) and ash: 0.01 mass % or less S and substantially no ash. Thus, damage to the high temperature components can be mostly prevented.
However, due to rocketing of fuel cost and in terms of resource saving, low grade fuels such as heavy oils has been more used in recent years for gas turbines as well. Low grade fuels contain elements constituting corrosion factors such as sulfur, alkali metals, and vanadium. The elements react with each other or with oxygen (O) or sea salt particles (NaCl, etc.) in combustion air in a complicated manner, thereby forming compounds that cause high temperature corrosion. Such compounds bring about damage to the TBC, particularly, to zirconia layer portions.
The present invention is intended to provide a gas turbine component(s) having durability and reliability for use in a corrosive environment such as a turbine employing a low grade fuel, and a gas turbine equipped with the component(s).
A bucket/nozzle for a gas turbine according to the present invention comprises: a substrate made from an alloy containing Ni, Co, or Fe; a bonding layer made from an alloy disposed over the substrate, and; a thermal barrier coating disposed over the bonding layer; wherein the thermal barrier coating includes: a thermal barrier layer of porous ceramics; an environmental barrier layer containing silica; and an impregnation layer having a material containing silica in the pores of the porous ceramics.
The environmental barrier layer supplies a barrier effect against environment and improves corrosion resistance. In addition, a part of the environmental barrier layer is impregnated into the thermal barrier layer so that the adhesion between them is enhanced, which prevents damage to the thermal barrier layer. Consequently, durability and reliability are improved.
A key of the present invention resides in the employment of TBCs that has a corrosion resistant environmental barrier layer disposed on the surface of a thermal barrier layer. Specifically, over an alloy substrate mainly formed from Ni, Co, or Fe, a thermal barrier coating is disposed by way of a bonding layer. The thermal barrier coating comprises a porous thermal barrier layer formed from ceramics and a corrosion resistant environmental barrier layer. Further, an impregnation layer, which is a layer that part of the environmental barrier layer is impregnated into the thermal barrier layer, is present between the environmental barrier layer and the thermal barrier layer.
Specifically, the thermal barrier layer is made of a porous zirconia layer and the environmental barrier layer mainly made of silica, and part of the environmental barrier layer is impregnated into pores of the porous zirconia layer.
Since the environmental barrier layer inhibits contact between a molten salt and the thermal barrier layer, the thermal barrier layer can be prevented from being corroded by molten salts. The impregnation of the environmental barrier layer into the pores of the porous stabilized zirconia layer also enhances adhesion between the two layers. Further, the porous thermal barrier layer present below the impregnation layer yields a thermal stress moderating function.
An example of a heat resistant component/part adapted to have the above configuration includes a substrate of an alloy mainly comprising Ni, Co, or Fe; an alloy layer formed from a MCrAlY alloy or a MCrAl alloy (M represents at least one of Fe, Ni, and Co) covering the substrate; a porous ceramic layer formed from partially stabilized zirconia covering the alloy layer; and a dense glass layer mainly composed of silica covering the ceramic layer. Part of the glass layer is impregnated into the pores in the upper portion of the porous ceramics to form an integrated portion with the glass filled.
The high temperature components in gas turbines provided with the TBCs can suppress the component temperature low by the thermal barrier effect of the TBC compared to components without the TBC. TBCs are often used in gas turbine parts that particularly require high temperature resistance (for example, bucket/nozzles, combustors, turbine shrouds, etc.). It is extremely effective to apply the TBC to high temperature parts of the gas turbine. At first, the corrosion damage to the TBC is to be described in details.
As low grade fuels, low sulfur A heavy oils (LSA heavy oil) containing relatively low sulfur and ash content have been used mainly for medium and small sized gas turbines to the present. There is also a great demand for use of even lower grade fuels, e.g. high sulfur A heavy oils or B, C heavy oils. The low grade fuels contain corrosive factors such as sulfur content, and alkali metals (Na, K) and vanadium (V) in the ash content. According to the specification in Japanese Industrial Standards (JIS) K2205 “heavy oil”, for example, the low sulfur A heavy oil (class 1, No. 1 heavy oil) contains 0.5% or less 5 and 0.05% or less ash, the high sulfur A heavy oil (class 1, No. 2 heavy oil) contains 2.0% or less 5 and 0.05% or less ash, the B heavy oil (class 2 heavy oil) contains 3.0% or less 5 and 0.05% or less ash, and the C heavy oil (class 3, No. 1 heavy oil) contains 3.5% or less 5 and 0.1% or less ash. In some heavy oils of lower grades, the contents for S and ash are not specified. High grade fuels used conventionally contain 0.01 mass % or less S and substantially no ash in general.
Corrosion factors contained in the low grade fuels react with each other or with oxygen (O) or sea salt particles (NaCl, etc.), etc. in combustion air in a complicated manner, thereby forming various compounds which cause high temperature corrosion. Compounds of relatively low melting point are particularly deposited and condensed in a molten state on the surface of high temperature components to bring about violent high temperature corrosion (so-called molten salt corrosion). Since heat resistant alloys used for high temperature components in the gas turbine corrodes violently due to the molten salt corrosion as well, the corrosion is a significant problem for the use of low grade fuels. In particular, protective oxide films of chromia (Cr2O3) or alumina (Al2O3) formed over the surface of the heat resistant alloy in order to contribute in oxidation resistance and corrosion resistance tend to dissolve into molten salts.
Partially stabilized zirconia in the TBC is also damaged by the molten salt corrosion in the same manner. In an environment where the employed fuel contains more than 0.5% S and more than 0.01% V, conventional TBCs especially tend to suffer damage in the partially stabilized zirconia used as the thermal barrier layer due to the molten salt corrosion. This results in occurrence of peeling in a short time. The damage caused by molten salt corrosion needs to be suppressed to provide a TBC possessing sufficient durability and reliability even for long time operation under corrosive environment of molten salt employing low grade fuels.
The present inventors have provided a thermal barrier layer comprising ceramics by way of a bonding layer and further, covered the surface thereof with an environmental barrier layer in respect to high temperature corrosion. Instances of these layers are, an alloy layer such as a MCrAlY layer or a MCrAl layer as the bonding layer, a porous ceramic layer such as a stabilized zirconia layer as the thermal barrier layer, and a silica-based layer such as a silica glass layer as the environmental barrier layer. The environmental barrier layer is formed such that a portion thereof is impregnated into the pores of the porous thermal barrier layer while the other portion remains on the surface. Thus, the environmental barrier layer inhibits contact between the molten salt and the thermal barrier layer and prevents molten salt corrosion of the thermal barrier layer. The partial impregnation of the environmental barrier layer into the pores of the thermal barrier layer also enhances adhesion between them. Further, the porous thermal barrier layer provides a thermal stress moderating function. Heat resistant components employing such TBCs excel in corrosion resistance and heat resistance.
The TBC of the present invention is suitable for high temperature components used in gas turbines hence the TBC has excellent corrosion resistance and heat resistance in a severe molten salt corrosion environment compared to traditional ones. Improvement of the high temperature components in durability and reliability in gas turbine operations elongates exchanging or inspecting cycle of the high temperature components. As a result, fuel cost in gas turbine operations can be suppressed. In addition, by allowing use of inexpensive low grade fuels in gas turbine operations, it may reduce fuel cost for a gas turbine. High temperature components in a gas turbine that are operated under severe corrosive and high temperature environment are bucket/nozzles, shrouds, combustors, etc.
The high temperature part of the gas turbine according to the present invention has layers disposed over a heat resistant alloy substrate: the layers include a bonding layer, a thermal barrier layer, an impregnation layer, and an environmental barrier layer. The alloy substrate may be formed from various heat resistant alloys that are used for bucket/nozzles, shrouds, combustors or other parts in a gas turbine. Alloys comprising Ni, Co, or Fe as a main ingredient are suitable.
The bonding layer is constructed to have a thermal expansion coefficient between the alloy substrate and the thermal barrier layer as to improve adhesion between them. Further, the bonding layer is preferably made of an alloy having higher corrosion resistance and oxidation resistance than those of the substrate. Alloys that are generally known to be used as bonding layers of TBC can be applied as appropriate. For example, an alloy comprising Ni and Co substantially as a main ingredient with addition of Cr and Al is suitable. Substances such as Y, Hf, Ta, Si, Ce may be further added in a range from 0 to 10% by weight. In particular, an alloy referred to as MCrAlY (M represents one or a plurality of Ni, Fe, and Co) in common is preferably used. The bonding layer is formed most desirably by low pressure plasma spraying. Alternatively, high velocity gas spraying such as HVOF thermal spraying or HVAF thermal spraying, or cold spraying method, or atmospheric plasma spraying may also be applied. The thickness of the bonding layer is preferred to be within a range from 0.05 to 0.3 mm.
The thermal barrier layer is of porous ceramics and is preferably of ZrO2 ceramics. It is further preferred to be of a partially stabilized zirconia containing at least one member selected from Y2O3, MgO, CaO, CeO2, Sc2O3, Er2O3, Gd2O3, Yb2O3, Al2O3, SiO2, and La2O3. Yttria (Y2O3)-partially stabilized zirconia is extremely suitable. The thermal barrier layer made of porous stabilized zirconia can be formed by atmospheric plasma spraying or the like. The thermal barrier layer used in a high temperature component of a gas turbine is preferred to have thickness ranging from 0.1 to 1 mm. If the thickness is less than 0.1 mm, sufficient thermal barrier property as TBC cannot be obtained sometimes. If the thickness is more than 1 mm, heat resistance is lowered and the layer becomes more liable to suffer peeling. The porosity of the thermal barrier layer made of porous stabilized zirconia is preferably within a range from 10 to 30% in terms of area ratio of the pores in a cross sectional structure. If the porosity is less than 10%, the thermal stress moderating mechanism provided by the pores would not sufficiently function, resulting in heat resistance degradation. If the porosity is 30% or higher, the physical strength of the film is lowered which makes peeling more likely to occur.
The environmental barrier layer has a function of avoiding contact between molten salt and the thermal barrier layer, the bonding layer, and the heat resistant alloy substrate so as to prevent damage thereof due to corrosion. Accordingly, an environmental barrier layer to be employed should be such that does not react nor melt upon contact with molten salt but maintains its film state and suppresses passage of the molten salt. In specific, the porosity of the environmental barrier layer is preferably 5% or less. If the porosity is more than 5%, there would be too many open pores and molten salt becomes prone to penetrate and pass through the environmental barrier layer via the pores. The thickness of the environmental barrier layer is preferably within a range from 0.05 to 0.1 mm. If the thickness is less than 0.05 mm, sufficient barrier property cannot be obtained as the environmental barrier layer. If the thickness is 0.1 mm or more, cracking is more prone to occur due to residual stress and thermal stress applied upon film deposition. The cracking reduces the barrier property through the cracks and renders peeling of the environmental barrier layer liable. The environmental barrier layer is formed by using slurry adjusted in viscosity or by using a sol-gel method.
The environmental barrier layer is preferably formed from a silica (SiO2) based material which is unreactive with molten salt. Assuming a case where a fuel containing sulfur is employed, although it is affected by other present compounds and atmospheric environments, alkali metal sulfates (e.g., Na2SO4, K2SO4) are the main compounds that tend to deposit and condense on the surface of the high temperature parts. Since alkali metal sulfates have the properties of acid salt, they react with amphoteric oxides such as alumina and chromia, and basic oxides such as zirconia and yttria, thereby dissolved to cause molten salt corrosion. Meanwhile, highly pure silica is an acidic oxide and is unreactive with molten salt, thus shows high durability against molten salt corrosion. The environmental barrier layer comprising silica may be supplied with additives other than silica. However, zirconia or alumina or other ingredients that are not resistant to sulfur or vanadium should be refrained from adding. If the purity of silica is too low, the impurities or other factors may lower corrosion resistance of the environmental barrier layer against molten salt corrosion. Quartz glass material with a silica purity of 90% or higher is preferably used to avoid this.
As a method of forming an environmental barrier layer mainly composed of silica, a sol-gel method using an alcohol solution of a metal alkoxide is applicable. Alternatively, solutions containing silica precursors such as colloidal silica solution or alkali metal silicate solution may also be used. After the solution is coated to form a precursor layer, the precursor layer is dried and baked, thus forming a silica film. It is also possible to use marketed silica coating agents. However, a final silica layer is required to have a corrosion resistance against molten salt within a range that allows for influences from additives and impurities.
The impregnation layer improves the adhesion between the environmental barrier layer and the porous thermal barrier layer to prevent peeling of the environmental barrier layer. The impregnation layer is constructed from porous ceramics that derive from the thermal barrier layer and silica compounds that derive from the environmental barrier layer. The thickness of the impregnation layer is about 10 to 20% of the thickness of the thermal barrier layer. If the impregnation layer is excessively thin, adhesion between the environmental barrier layer and the thermal barrier layer is insufficient. If the impregnation layer is excessively thick, the porosity of the thermal barrier layer is substantially decreased, whereby lowering the thermal barrier property and decreasing the stress moderating effect. The decrease in stress moderating effect produced by the pores may also render peeling liable to occur. The thickness of the impregnation layer is preferably within a range from about 0.01 to 0.1 mm.
When the environmental barrier layer is to be formed by using a solution containing a silica precursor, the viscosity of the solution is previously adjusted to a proper level. The solution coated over the surface of the porous thermal barrier layer can then partially penetrate into the pores of the thermal barrier layer by a capillary phenomenon. The impregnated solution solidifies and forms silica by way of drying and baking in the pores of the thermal barrier layer comprising porous stabilized zirconia, thus forming an impregnation layer. The thickness (depth) of the impregnation layer can be regulated by adjusting the viscosity of the solution as mentioned or by changing the pore size of the thermal barrier layer.
One example of the heat resistant part is to be described with reference to the schematic cross sectional view of
The heat resistant components according to the present invention have good durability against corrosive environment. Accordingly, they are suitable as high temperature parts used under a high temperature condition such as bucket/nozzles and combustors of gas turbines. Further, the heat resistant part is applicable not only to the gas turbine but also to other high temperature equipment such as air craft engines.
A gas turbine bucket of this embodiment is to be described with reference to
A bonding layer was formed on a substrate by low pressure plasma spraying of a CoNiCrAlY alloy (Co-32 wt % Ni-21 wt % Cr-8 wt % Al-0.5% Y) powder. The layer was further subjected to diffusion heat treatment in vacuum at conditions of 1121° C.×2 h+843° C.×24 h. The thickness of the bonding layer is about 200 μm.
Then, over the bonding layer, a zirconia layer comprising porous ceramics with a thickness of about 0.5 mm and a porosity of about 20% was formed by atmospheric plasma spraying of a yttria-partially stabilized zirconia (ZrO2-8 wt % Y2O3) powder. The surface of the zirconia layer was covered by dipping method with a solution containing silica precursor that is a mixture of a marketed ceramic adhesive/filler and water. In this embodiment, “Aron ceramic C” (trade name) produced by TOAGOSEI CO., LTD. was applied as the marketed agent. Water was added in order to condition the viscosity of the solution: in this case the viscosity was conditioned to about 3 Pa·sec at a room temperature. After coating, the test bucket was naturally dried at a room temperature for 24 hours. The test bucket was then heated in an electric furnace at conditions of 90° C.×1 h and 150° C.×1 h. Finally, an environmental barrier layer mainly composed of silica was formed on the surface of the porous zirconia layer. From observation of a cross section of a small-sized test piece which was prefabricated in a similar manner, the thicknesses of the environmental barrier layer and the impregnation layer were found to be about 0.1 mm and about 0.05 mm, respectively.
Durability of the gas turbine bucket of this embodiment was tested by using an actual machine. The actual test was performed in a gas turbine that used special A heavy oil as a fuel and in which damage due to molten salt corrosion had been observed. For comparison, a bucket provided with a bonding layer and a thermal barrier layer but not having an environmental barrier layer (bucket provided with existent TBC) was also manufactured in the same method as that for the bucket of this embodiment as described above. The bucket for comparison was put to the test simultaneously. After operation for two years, the tested buckets were observed. No damage was confirmed in the TBC of the gas turbine bucket of this embodiment and it remained intact. On the other hand, in the existent bucket of the comparative example, local peeling in the thermal barrier layer was observed. The thermal barrier layer was physically ripped off and sampled from the peeled portion in order to analyze the cross section of the thermal barrier layer. As a result of the analysis, S and V were detected. It can therefore be considered that peeling was caused by molten salt corrosion. According to these results, it was confirmed that the gas turbine bucket of this embodiment excels in durability compared with the existent gas turbine bucket. It was also confirmed that the bucket of this embodiment possesses sufficient durability and reliability even for a long time operation under corrosive environment of molten salt employing low grade fuels.
The bucket of this embodiment is suitable for applying as a first stage bucket which tends to have high temperature. Not to mention, the bucket can be used also as a second or subsequent stage bucket.
A gas turbine nozzle of this embodiment is to be described with reference to
Then, over the bonding layer, a zirconia layer comprising porous ceramics having a thickness of about 0.3 mm and a porosity of about 20% was formed by atmospheric plasma spraying of a yttria-partially stabilized zirconia (ZrO2-8 wt % Y2O3) powder. The surface of the zirconia layer was covered by dipping method with a solution containing silica precursor that is a mixture of a marketed ceramic adhesive/filler and water. In this embodiment, “Aron ceramic C” (trade name) produced by TOAGOSEI CO., LTD. was applied as the marketed agent. Water was added in order to condition the viscosity of the solution: in this case the viscosity was conditioned to about 3 Pa·sec at a room temperature. After coating, the test nozzle was naturally dried at a room temperature for 24 hours. The test nozzle was then heated in an electric furnace at conditions of 90° C.×1 h and 150° C.×1 h. Finally, an environmental barrier layer mainly comprising silica was formed on the surface of the porous zirconia layer. From observation of a cross section of a small-sized test piece prefabricated in the same manner, the thicknesses of the environmental barrier layer and the impregnation layer were found to be about 0.1 mm and about 0.05 mm, respectively.
Durability of the gas turbine nozzle of this embodiment was tested by using an actual machine. The actual test was performed in a gas turbine that used low sulfur A heavy oil as a fuel and in which damage due to molten salt corrosion had been observed. For comparison, a nozzle provided with a bonding layer and a thermal barrier layer but not having an environmental barrier layer (nozzle provided with existent TBC) was also manufactured in the same method as that for the nozzle of this embodiment as described above. The nozzle for comparison was put to the test simultaneously. After operation for two years, the tested nozzles were observed. No damage was confirmed in the TBC of the gas turbine nozzle of this embodiment and it remained intact.
On the other hand, in the existent nozzle of the comparative example, local peeling in the thermal barrier layer was observed. The thermal barrier layer was physically ripped off and sampled from the peeled portion in order to analyze the cross section of the thermal barrier layer. As a result of the analysis, S and V were detected. It can therefore be considered that peeling was caused by molten salt corrosion. According to these results, it was confirmed that the gas turbine nozzle of this embodiment excels in durability compared with the existent gas turbine nozzle. It was also confirmed that the nozzle of this embodiment possesses sufficient durability and reliability even for a long time operation under corrosive environment of molten salt employing low grade fuels.
The nozzle of this embodiment is suitable for applying as a first stage nozzle which tends to have high temperature. Not to mention, the nozzle can be used also as a second or subsequent stage nozzle.
This embodiment is such that applies both the described bucket 46 and nozzle 45 in the first and second embodiments as the first stage bucket/nozzles, positioned at a high temperature side in the rotational portion of a gas turbine. In addition, in this embodiment, other parts were also provided with a ceramic coating layer that has an environmental barrier layer by a method basically according to that described for the second embodiment: the parts are, inner circumferential surface of the combustor liner 42 and the transition piece 43 which serves as a combustion gas channel, and the surface of the combustion gas channel of the turbine shroud 47 at the first stage.
Specifically, a bonding layer was formed on the combustion gas passage surfaces of the parts by atmospheric plasma spraying of a CoNiCrAlY alloy (Co-32 wt % Ni-21 wt % Cr-8 wt % Al-0.5 wt % Y) powder. The thickness of the bonding layer is about 200 μm. Then, over the bonding layer, a zirconia layer comprising porous ceramics with a thickness of about 0.3 mm and a porosity of about 20% was formed by atmospheric plasma spraying of a yttria-partially stabilized zirconia (ZrO2-8 wt % Y2O3) powder. The surface of the zirconia layer was covered by dipping method with a solution that is a mixture of a marketed ceramic adhesive/filler and water containing silica precursor. “Aron ceramic C” (trade name) produced by TOAGOSEI CO., LTD. was applied as the marketed agent. Water was added in order to condition the viscosity of the solution: in this case the viscosity was conditioned to about 3 Pa·sec at a room temperature. After coating, the parts were naturally dried at a room temperature for 24 hours. The parts were then heated in an electric furnace at conditions of 90° C.×1 h and 150° C.×1 h. Finally, an environmental barrier layer mainly composed of silica was formed on the surface of the porous zirconia layer.
In this embodiment, the ceramic coating layer including the environmental barrier layer of the present invention is applied to the first stage nozzle, the first stage bucket, and the first stage shroud in the turbine 44 configured with three stages. However, the present invention may also be provided to the subsequent second stage and third stage. The present invention may also be applied to all or selected stages of turbines having other numbers of stages, for example, two or four stages.
The gas turbine of this embodiment having the constitution as above was operated by using low sulfur A heavy oil as the fuel. After operation for two years, each of the parts was observed. Damage due to molten salt corrosion was scarcely observed in the TBC of this embodiment for each part of the gas turbine and they remained intact. On the other hand, in the gas turbine using the existent TBC not having the environmental barrier layer of the invention, local peeling due to molten salt corrosion was confirmed in the heat barrier layer of the TBC in a number of components after operation for one year. Thus, repair or replacement of those parts were necessary. The thermal barrier layer was physically ripped off and sampled from the peeled portion of TBC in order to analyze the cross section of the thermal barrier layer. As a result, S and V were detected.
The result described above indicates the fact that the great corrosion resistance of the high temperature parts allows the gas turbine of this embodiment to operate with high reliability while employing an inexpensive low grade fuel. The gas turbine excels in economical efficiency and operation stability and is suitable for power generation plants. It also has sufficient durability and reliability for long time operation under corrosive environment of molten salt employing a low grade fuel. Further, even when the turbine uses low grade fuels, e.g. high sulfur A heavy oils or B, C heavy oils, containing more corrosive ingredients than the low sulfur A heavy oil used in this embodiment, the high temperature parts can be expected to show high durability.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010-291548 | Dec 2010 | JP | national |
