Patent Application
20130025291
The invention relates to silicon-containing materials in combustion gas environments, and more particularly, relates to a method to reduce or prevent the material loss of silica and silicon-containing materials in high temperature combustion gas environments, such as encountered in industrial land-based turbines, aircraft engines, diesel engines and heat exchangers.
Silicon-based monolithic ceramics, such as silicon carbide or silicon nitride, and silicon-containing composites, including continuous-fiber reinforced ceramic composites, are attractive candidates for high temperature structural applications, such as component parts for gas turbines, aircraft engines, and heat exchangers. However, the environments characteristic of these applications often contain reactive species, such as water vapor, which at high temperatures has been shown to cause significant surface recession and mass loss in silicon-bearing materials. The water vapor reacts with the structural material at high temperatures to form volatile silicon-containing species, often resulting in unacceptably high recession rates.
Various methods to mitigate high-temperature degradation of silicon bearing materials have been developed. For example, U.S. Pat. No. 6,517,341 describes a method to reduce this degradation by injecting silicon into a combustion gas environment where silicon-bearing components are being used. The increased level of silicon in the environment suppresses the chemical reactions occurring at the surface of the silicon-bearing components. This method may result in somewhat increased particulate concentrations in the exhaust stream.
Other methods for mitigating recession of silicon-bearing materials include application of coatings. Environmental barrier coatings (EBC's) are applied to silicon-bearing materials and other material susceptible to attack by reactive species, such as high temperature water vapor; EBC's provide protection by prohibiting contact between the environment and the surface of the material. EBC's applied to silicon-bearing materials, for example, are designed to be relatively stable chemically in high-temperature, water vapor-containing environments.
One illustrative conventional EBC system, as described in U.S. Pat. No. 6,410,148, comprises a silicon or silica bond layer applied to a silicon-bearing substrate; an intermediate layer comprising mullite or a mullite-alkaline earth aluminosilicate mixture deposited over the bond layer; and a top layer comprising an alkaline earth aluminosilicate deposited over the intermediate layer. A second type of EBC comprises a topcoat that includes a rare earth silicate compound (see, for instance, U.S. Pat. Nos. 6,296,941; 6,312,763; and 6,759,151). These ceramic coating systems can be effective in mitigating degradation under certain circumstances, but under some conditions can be susceptible to cracking and spalling.
Therefore, there remains a need in the art for systems and related methods that can operate at high temperature while mitigating component degradation.
Embodiments of the present invention are provided to meet these and other needs. One embodiment is a system. The system comprises a plurality of components disposed to define a gas path. At least one component comprises a silicon-bearing substrate over which is disposed a coating, and the coating comprises a recession-resistant material exposed to the gas path. A silicon source is disposed in fluid communication with the gas path and is configured to be delivered to the gas path to maintain, in gas flowing in the gas path over the coating, a silicon mass concentration in the range from about 1.8×10−4 parts per million to about 1 part per million.
Another embodiment is a method. The method comprises directing a combustion gas flow within a gas path, the gas path defined by a plurality of components, wherein at least one component comprises a silicon-bearing substrate over which is disposed a coating, the coating comprising a recession-resistant material exposed to the gas path; and delivering a silicon-containing material to the gas flow from a silicon source disposed in fluid communication with the gas path to maintain within the gas flow over the coating, a silicon mass concentration in the range from about 1.8×10−4 parts per million to about 1 part per million.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about”, is not limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value. The singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise, and alternatives joined by “or” should not be construed as limiting lists or as mutually exclusive alternatives unless explicitly stated otherwise. Thus a description of “A or B” contemplates embodiments involving A, embodiments involving B, and embodiments involving both A and B together.
Turbines typically have more than one row (“stage”) of components to extract energy from the hot gas flowing through the gas path. As the gas passes through successive stages, the gas expands to a larger volume, and the gas temperature is reduced. The injection method of U.S. Pat. No. 6,517,341, referred to above, may result in increased silica or other particulate formation within the gas. These particulates may deposit on later stage components that are cooler than combustion components or early-stage turbomachinery components, thereby potentially reducing aerodynamic efficiency of such components by fouling.
The service life of an aluminosilicate-based EBC is limited by the volatilization/recession rate of the aluminosilicate material. To ensure adequate service life, e.g., 32,000 hours at a material surface temperature of 1300° C., the thickness of the material is generally large, such as 500-800 micrometers. As thickness increases, the probability of processing defects that lead to premature EBC cracking and spallation also increases. EBC's of this type have thus been restricted to applications in which the nominal material surface temperature is less than about 1300° C. EBC's comprising a rare earth silicate compound have adequate volatilization/recession resistance over a wide service temperature range, but may have a complex architecture that is susceptible to formation of defects upon processing and heat treatment.
Embodiments of the present invention addresses the disadvantages noted above, taking advantage of synergistic cooperation between a recession-resistant EBC and an environment that suppresses recession of the coating material.
Referring to
System 10 further comprises a silicon source 60 disposed in fluid communication with gas path 30. Source 60 is configured, such as through sufficient volume, position, composition, and metering, where appropriate, to maintain a silicon mass concentration in the range from about 1.8×10−4 parts per million to about 1 part per million within gas flowing in gas path 30. This silicon mass concentration is significantly lower than that described in U.S. Pat. No. 6,517,341.
One advantage of system 10 over conventional protection systems is its potential for use of coatings that are much thinner than those conventionally applied in the art. Due to the higher silicon activity of the gas flowing over coating 50, recession rates may be significantly lower than those expected in the absence of silicon injection. With lower recession rates, thinner coatings may be applied to achieve long lifetimes. In one embodiment, coating 50 has a thickness up to about 250 micrometers. In certain embodiments, the thickness is even less, such as up to about 125 micrometers, and in particular embodiments the coating thickness is up to about 50 micrometers. The use of such thin coatings may mitigate risks of processing defects noted above for conventionally designed coatings.
Another advantage of system 10 is its use of very low silicon injection concentrations. Due to the presence of recession-resistant coating 50, silicon concentration levels may be up to a factor of 50 lower than those described in U.S. Pat. No. 6,517,341. This use of lower silicon levels may reduce costs attributable to consumption of injected source material. It also may reduce the risk of fouling of latter-stage components because the amount of particulate formation may be significantly lower in the present system.
The advantages noted above are synergistic in that the combination of thin coating and low injected silicon level provides advantages beyond what either of the two could provide on its own.
Coating 50 comprises a recession resistant material, meaning a material that is resistant to reaction with water vapor at high temperatures, such as those temperatures nominally used in turbomachinery and other high-temperature applications. In some embodiments, coating 50 comprises a silicate. One example of a silicate is an aluminosilicate. In some embodiments, the recession-resistant material comprises an alkaline-earth aluminosilicate. In particular embodiments, the alkaline earth aluminosilicate comprises an alkaline-earth element such as barium or strontium. Such embodiments include those in which the recession-resistant material comprises barium strontium aluminosilicate. Another example of a silicate is a rare-earth disilicate. As used herein, a rare earth disilicate may include any of the lanthanide series elements, yttrium or scandium. In particular embodiments, the recession-resistant material includes yttrium disilicate, ytterbium disilicate, or ytterbium doped yttrium disilicate.
Coating 50 may be a single layer, or in some embodiments may include multiple layers. It will be appreciated that recession-resistant material need not reside in an outermost layer of coating 50 to be exposed to gas in gas path 30; thus in some embodiments, recession-resistant material may be disposed beneath an outer layer but may still be exposed, via cracks, pores, etc., in the outer layer(s), to a portion of the gas flowing within gas path 30 during operation of system 10. It will also be appreciated that, where coating 50 includes multiple layers, recession-resistant material may be present in more than one layer. In some embodiments, as shown in
More complicated coating arrangements may also be used. As an example of the possibilities, a particular embodiment is illustrated in
Coating 50 (including component layers, as appropriate) may be fabricated by various techniques. Examples of commonly used and well-known techniques for fabricating coatings of the type described herein include plasma spraying, chemical vapor deposition, physical vapor deposition, and slurry coating techniques.
Silicon source 60 provides silicon in some form to flow path 30 during the operation of system 10. Source 60 is disposed in fluid communication with gas path 30 to enable injection of silicon-bearing material from source into gas path 30. Source 60 thus may be connected to gas path 30 directly. In some embodiments, as shown in
In one embodiment, source 60 comprises elemental silicon or a silicon-containing compound, and source 60 may comprise a solid, liquid, or gas. It is generally desirable to add the silicon or silicon-containing compounds from source 60 in a form that leads to rapid volatilization of silicon as silicon hydroxide. The silicon compounds can be added to liquid fuel as organic compounds in a solution or as a slurry that can be emulsified. Examples of organic compounds that can be used for addition to the fuel are siloxanes, such as, but not limited to, octamethylcyclotetrasiloxane and hexamethyldisiloxane. Both of these compounds are low viscosity liquids with good stability in the presence of water vapor in ambient air.
The silicon-containing compounds can be added into the air used for combustion, which may provide rapid volatilization, such as by injecting silicon-bearing material from source 60 downstream of a compressor (not shown) and just before the combustor 400. They can also be added directly into the combustion gases exiting from combustor 400. The silicon-containing compounds of source 60 can be in the form of organic compounds, which would volatilize readily, or in the form of slurries of fine particulate silicon-containing compounds, such as, but not limited to, silicon oxide, silicon, silicon carbide, silicon nitride, silicon boride, and mixtures thereof. Additional examples of materials useful for source 60 include siloxanes, silanes, silica, silicones, silicon carbides, silicon nitrides, silicon oxides, silicates, and sand. Tetramethylsilane, with a boiling point of 26.5 degrees C., can be added directly to natural gas fuel. The siloxanes mentioned above, octamethylcyclotetrasiloxane and hexamethyldisiloxane, have higher boiling points then tetramethylsilane; these may be injected as liquids into air input 410. The technique would also include premixed prevaporized concepts, where prevaporization of the fuel and silicon-containing material then undergoes premixing of the vaporized fuel/silicon-containing material with compressed air before combustion occurs. In addition, colloidal silica, or other silicon-bearing particulate, dispersed in water can be injected directly into the air input 410.
The level of silicon needed by injection or mixing into the combustion gases is an amount to form a sufficient concentration of silicon hydroxide products, such as Si(OH)4 , so as to significantly reduce or eliminate the thermodynamic driving force for volatilization of silica present in the recession-resistant material of coating 50. The silicon concentration in the gas flowing over coating 50 needed to alleviate or prevent the loss of material increases with the pressure, temperature, water vapor concentration, and proximity to stoichiometric combustion. Details and examples of calculations of this type are illustrated in U.S. Pat. No. 6,517,341, which is incorporated by reference herein. Because the activity of silica is much lower in the recession-resistant materials of coating 50 than in uncoated parts as described in U.S. Pat. No. 6,517,341, the resultant levels of silicon needed to mitigate recession are significantly lower.
One embodiment of the present invention is a method. The method comprises directing a gas flow, such as a combustion gas flow, along a gas path, such as gas path 30 described above and depicted in
In certain embodiments, the gas flow is directed in part by accepting an air input 410 (
It will be appreciated that various methods of maintaining the desired silicon concentration in the gas flow are possible and depend in part on the manner in which the silicon is to be injected into the gas flow. For instance, where a silicon-containing material is to be added to fuel, the concentration in the fuel may depend in part on the expected fuel flow rate; where the material is to be injected into air input 410, the rate of injection may depend in part on the concentration of silicon in the source 60 and the rate of air injection into combustor 400. Calculations and apparatus to determine and control the appropriate flow rates and concentrations are readily available to those skilled in the art.
The following example is provided to further illustrate embodiments of the present invention and should not be construed as limiting the scope as described herein.
A hot gas path component of a gas turbine engine comprises a ceramic matrix composite (CMC) comprising silicon carbide at a typical operating temperature of 1315 C (2400 F). Under a particular set of operating conditions, in the absence of a protective coating comprising recession-resistant material, the CMC can be protected by Si injection of a suitable Si-bearing organic species at a concentration of approximately 20 parts per million by weight (wppm) Si in the fuel or approximately 0.6 wppm Si in the air according to U.S. Pat. No. 6,517,431. As the Si-bearing gas is cooled by passing through the engine, it becomes supersaturated in Si and cooler portions of the engine may experience deposition of SiO2 with attendant fouling. On the other hand, the CMC can be protected by an environmental barrier coating with a barrier layer comprising an aluminosilicate according to U.S. Pat. No. 6,410,148. In order to prevent loss of this layer by reaction with the water vapor-bearing combustion gas within the anticipated service life, the barrier layer may have to be 500-800 micrometers thick. Such a thick coating runs the risk of cracking or spallation with loss of the protective function during the service lifetime of the part. The aluminosilicate barrier layer can be protected by a Si injection level of approximately 0.5 wppm in the fuel or 0.017 wppm in the air. At this low Si injection level, subsequent cooling of the combustion gas as it passes through the engine will not cause SiO2 deposition on airfoils or other regions where such deposition would be deleterious. In addition, since the aluminosilicate barrier layer is completely or largely protected from water vapor reaction by the injected Si, it can be thinner, e.g., 125 micrometers or even 50 micrometers thick, and the risk of cracking and spallation owing to processing or in-service defects is substantially mitigated.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
