(12)
`
`United States Patent
`Brun et al.
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`US 6,517,341 B1
`Feb. 11, 2003
`
`US006517341B1
`
`(54) METHOD TO PREVENT RECESSION LOSS
`()1? SILICA AND SILICON_C()NTAINING
`MATERIALS IN COMBUSTION GAS
`ENVIRONMENTS
`(75) Inventors: Milivoj Konstantin Brun, Ballston
`_
`.
`Mk9 NY (Us), Knshan La] Luthra’
`N1S1<ay1lI1a,NY(US)
`_
`_
`(73) Asslgneer General Electrlc Company,
`Niskayuna, NY (US)
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U50 154(9) by 0 days-
`
`(21) Appl. N0.: 09/259,364
`(22) Filed:
`Feb. 26, 1999
`
`Int. (:1.7 ............................ ..
`
`F23B 7/00
`(52) US. Cl. ........................ .. 431/2; 431/4; 427/255.18;
`_
`427/237; 44/320; 264/39; 110/343
`
`5,094,901 A * 3/1992 Gray ........................ .. 428/141
`5,298,587 A * 3/1994 Hu et al. .................. .. 427/489
`2
`2918; et a1
`mg
`7
`7
`5,435,948 A * 7/1995 Staffolani et a1. ........... .. 264/30
`2
`i/1997 Splegler et a1‘ """"""" " 44/320
`,
`,
`/1997 Fey ........................... .. 44/320
`5,677,060 A * 10/1997 Terentieva et a1. ........ .. 427/228
`5,686,028 A * 11/1997 Meynckens et a1. ........ .. 264/30
`5,688,295 A 11/1997 Yang ................... ..
`44/320
`5,767,192 A
`6/1998 Battice et a1.
`..... .. 44/320
`5,871,820 A * 2/1999 HasZ et al. .... ..
`.. 427/419.2
`5,914,189 A * 6/1999 HasZ et al. ............... .. 428/335
`L,
`_
`d
`( 1st Connnue on next page‘)
`FOREIGN PATENT DOCUMENTS
`
`AU
`EP
`EP
`EP
`GB
`GB
`
`* 7 1947 ............... .. 427 140
`124609
`42982
`/
`0048910
`5/1991
`0425825
`7/1992
`0493376
`* 5/1956
`748478
`2 110 200 A * 6/1983
`
`............... .. 427/140
`
`OTHER PUBLICATIONS
`“Paralinear Oxidation of CVD SiC in Water Vapor”, by E. J.
`
`
`
`
`
`of Search . . . . . . . . . . . . . . . . . . . . . . . .. 427/ igg’zggll’7z3js’szig’
`
`4,
`
`
`
`Ceranl~ SOC‘, Pending US. patent application Ser. No. 08/777,129, ?led
`
`255 39'3_ ’44 32'0 ’321 ‘35% 36'2 3’64 3'70’
`'
`’
`/ 585 £64 3’0_ 11’0 343’ 342’
`’
`/
`’
`/
`’
`
`Dec. 30, 1996, by A. SZWeda et al., entitled “Article and
`Method for Making Complex Shaped Preform and Silicon
`Carbide Composite by Melt In?ltration”.
`
`(56)
`
`References Cited
`U‘S PATENT DOCUMENTS
`
`2,811,467 A 10/1957 Hull 61 a1. ................ .. 427/333
`2,867,516 A * 1/1959 Pedersen ................... .. 44/361
`3,843,306 A 10/1974 Whittington et a1.
`431/8
`3,994,699 A 11/1976 Scott ................. ..
`. 44/320
`4,047,875 A
`9/1977 May et a1, _
`431/3
`4,061,473 A 12/1977 Norris ..... ..
`. 44/320
`4,131,433 A 12/1978 Scott - - - - - -
`- - - -- 44/320
`
`Primary Examiner—Henry Bennett
`Assistant Examiner—Josiah C. COCkS
`(74) Attorney, Agent, or Firm—Noreen C. Johnson;
`Christian G_ Cabou
`
`(57)
`
`ABSTRACT
`
`While silicon-containing ceramics or ceramic composites
`are prone to material loss in combustion gas environments,
`this invention introduces a method to prevent or greatly
`
`47466997 A * 8/1984 FY6590“ - - - - -
`2 * 1(9);
`im?’wert
`4,541,838 A
`9/1985 Zaweski et a1‘
`
`,
`
`,
`
`e ey e a. ................ ..
`
`- - - ~~ 44/320
`' 6%
`
`reduce the thickness loss by injecting directly an effective
`amount, generally in the part per million level, of silicon or
`s1l1con contammg compounds mto the combustion gases.
`
`-
`
`-
`
`_
`
`-
`
`-
`
`-
`
`-
`
`4,542,888 A * 9/1985 Robyn et a1. ............... .. 264/30
`5,015,540 A
`5/1991 Borom et al.
`
`45 Claims, 2 Drawing Sheets
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`\
`in 90% Water - 10% oxygen
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`

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`US 6,517,341 B1
`Page 2
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`US. PATENT DOCUMENTS
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`8/1999 Yang ......................... .. 44/320
`5,931,977 A
`8/1999 Wallace
`. 44/359
`5,944,858 A
`5,952,100 A * 9/1999 Corman et a1. ........... 428/384
`
`5,955,182 A * 9/1999 Yasuda et a1. ............ .. 428/217
`5,955,391 A * 9/1999 Kameda et a1.
`427/226
`6,045,877 A * 4/2000 Gleason et aL ___________ u 427/522
`
`* cited by examiner
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`U.S. Patent
`
`Feb. 11,2003
`
`Sheet 2 of2
`
`US 6,517,341 B1
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`
`US 6,517,341 B1
`
`1
`METHOD TO PREVENT RECESSION LOSS
`OF SILICA AND SILICON-CONTAINING
`MATERIALS IN COMBUSTION GAS
`ENVIRONMENTS
`
`This invention Was performed under a United States
`government contract With the Department of Energy, con
`tract number DE-FC02-92CE41000. The United States gov
`ernment may have certain rights in the invention.
`
`BACKGROUND OF THE INVENTION
`
`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 com
`bustion gas environments, such as encountered in industrial
`land-base turbines, aircraft engines, automobiles and heat
`exchangers.
`Silicon-based monolithic ceramics, such as silicon
`carbide, silicon nitride, and silicon-containing composites,
`including continuous-?ber reinforced ceramic composites,
`are attractive candidates for high temperature structural
`applications, such as component parts for gas turbines,
`aircraft engines, and heat exchangers. These silicon
`containing materials are particularly appealing because of
`their excellent high temperature properties and loWer den
`sity. For instance, in combustion gas environments, a per
`formance bene?t is obtained by replacing cooled metal
`components With uncooled or reduced cooling silicon
`containing ceramic components. Material substitution of hot
`gas path components With such ceramics yields higher
`output poWer, improved thermal ef?ciency and reduced NOx
`emissions. Depending on the siZe of the component part and
`the mechanical speci?cations that the component must meet
`in service, silicon-containing composite ceramics including
`continuous or discontinuous-?ber reinforced ceramic
`composites, such as silicon carbide ?ber reinforced silicon
`carbide or silicon-silicon carbide matrix composites, are
`sometimes selected over monolithic ceramics because of
`superior thermal and mechanical shock resistance, higher
`damage tolerance and strain-to-failure. Examples of discon
`tinuous ?ber reinforced composites include composites rein
`forced With silicon carbide Whiskers. Examples of mono
`lithic ceramics are silicon carbide, silicon nitride, and
`silicon-silicon carbide ceramics.
`Aprimary advantage then of silicon-containing ceramics
`or silicon-containing composites (herein, silicon-containing
`ceramics or composites) over metals is their superior high
`temperature durability Which enable higher turbine rotor
`inlet temperatures. In addition, they exhibit loW coef?cient
`of thermal expansion and loWer density in comparison to
`nickel-base superalloys. The relatively high thermal conduc
`tivity of silicon-containing composite systems is similar to
`nickel-based alloys at the use temperatures.
`The gas turbine component parts Where silicon-containing
`ceramics or silicon-containing composites are being consid
`ered include the shroud and the combustion liner. The
`shroud forms the turbine outer ?oWpath and creates a sealing
`surface over the rotor buckets. It is a primary element in the
`turbine tip clearance and roundness system and is segmented
`in larger machines. It serves as a heat shield and insulates the
`turbine casing from the hot gas stream temperature. As part
`of the How path, the shroud must have suf?cient oxidation/
`corrosion resistance and be structurally sufficient to meet
`design life requirements for the engine temperature, pressure
`and How environment.
`
`2
`The combustion liner contains the combustion reaction
`Zone and conveys the hot gases to the turbine inlet. In loW
`emissions combustors the ?ame temperature is minimiZed to
`limit production of thermal NOx. This is accomplished by
`putting most of the compressor air, except for turbine
`cooling air, through the premixers and minimiZing the
`amount of cooling or dilution air through the liner. Complex
`thermal gradients and elevated temperatures in liners can
`lead to excessive distortion in metals causing loss of sealing,
`restrictions in cooling air ?oW, and increases in hot side heat
`transfer. Silicon-containing composites offer loW cycle
`creep-fatigue resistance and very little deformation. As the
`case With the shroud, the combustion liner must have
`suf?cient oxidation/corrosion resistance. Additional pieces
`of turbine components comprise noZZles, vanes, blades,
`buckets and transition pieces.
`High oxidation resistance is imparted by formation of a
`protective silica (SiO2) ?lm on the silicon-containing
`ceramic or composite surface. The above proposed applica
`tions for the silicon-containing materials position them in
`direct contact With combustion gases, Which are the product
`of the combustion of liquid fuels or natural gas hydrogen or
`coal. For natural gas, liquid or coal fuels, the products of
`combustion contain up to about nineteen percent Water
`vapor by volume dependent on the fuel-to-air ratio. Even
`higher Water vapor levels are obtained for mixtures of
`natural gas and hydrogen or for pure hydrogen. In an
`environment containing Water vapor and oxygen, thermo
`dynamic calculations indicate the primary reactions Which
`occur for the oxidation of silicon (present for example, as
`silicon carbide) are:
`
`Hydrogen and carbon monoxide react together to form Water
`vapor and carbon dioxide. The silica ?lm formed on the
`silicon-containing ceramic or composite in an oxygen/Water
`vapor gas mixture may simultaneously volatiliZe by forming
`a silicon hydroxide or silicon oxyhydroxide species. For
`instance, some possible volatiliZation reactions are:
`
`(4)
`
`(5)
`
`The volatiliZation of silica results in material loss result
`ing in reduction of the thickness of the silicon-containing
`ceramic or composite materials. The observed rates of loss
`are of the order of a feW mils to tens of mils per thousand
`hours of operation in the combustion gas environment.
`Depending on the fuel used, such fuel as natural gas, the
`reaction is favored by high Water vapor content (up to about
`19% by volume), high pressures (generally up to 30—40
`ATM) and high temperatures (up to about 1200—1500° C.)
`found in many turbine, engine and heat exchanger applica
`tions. Thus, for long-term chemical durability of silicon
`containing ceramics or composites in combustion environ
`ments the volatility of the silica ?lm needs to be controlled
`during the lifetime of the component.
`
`10
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`15
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`25
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`35
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`45
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`55
`
`SUMMARY OF THE INVENTION
`
`65
`
`The above-identi?ed needs are satis?ed by this invention
`Which provides a method to reduce material loss of the
`silicon-containing ceramics and silicon-containing ceramic
`
`GE-1009.005
`
`

`
`US 6,517,341 B1
`
`3
`composites in a combustion gas environment comprising the
`step of injecting an effective amount of silicon into said
`combustion gas environment, Where the silicon is at least
`one of elemental silicon, a silicon-containing compound or
`mixtures thereof. The silicon or silicon-containing
`compound(s) can be added to the combustion gases, the fuel,
`the combustion air, directly to the combustor or mixtures
`thereof. Further, some Ways that the silicon or silicon
`containing compounds can be added to the combustion gas
`environment are as solid matter, a slurry liquid or
`suspension, a liquid, a liquid solution, an atomiZing spray a
`gaseous substance or a mixture of any of the above
`mentioned. An effective amount of silicon means an amount
`of silicon in the combustion gas environment that prevents
`or reduces the volatiliZation of the silica ?lm located on the
`silicon-containing ceramic or on the silicon-containing
`ceramic composite, and Which may further prevent the
`recession loss of the silicon-containing ceramic or ceramic
`composite. As a result of reducing the volatiliZation of the
`silica ?lm, the underlying silicon-containing ceramic or
`ceramic composite substantially maintains its operational
`thickness and does not suffer from attack and recession in
`the combustion environment. This invention is also useful
`When protective coatings are used on the components in
`combustion gas environments.
`In this invention the term ‘material loss’ means that
`material loss of the composite occurs through reaction of
`silicon-containing material With a gas atmosphere, particu
`larly Water vapor, at high temperatures. In a fully dense
`material, the material loss results in reduction of the thick
`ness of the component. If the material is porous, there Will
`be material loss also taking place Within the open porosity,
`in addition to thickness reduction similar to that of a dense
`material. Thus porous material is going to experience higher
`material loss than a fully dense material.
`Yet another embodiment of the invention is a method to
`maintain long-term chemical durability of silicon-containing
`ceramics including intermetallics, or silicon-containing
`composites in combustion gas environments comprising
`mixing parts per million of silicon or silicon-containing
`compounds in combustion gases during operation of a
`turbine engine or heat exchanger.
`Another embodiment of the invention is a silicon
`containing ceramic or ceramic composite component in a
`combustion gas environment of at least 500° C. having a
`chemically stable silicon oxide ?lm on a surface of said
`ceramic or ceramic composite component.
`
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`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1. Depicts a graph shoWing the Weight loss of a
`silicon carbide sample exposed to a steam environment.
`FIG. 2. Depicts a graph shoWing the control of Weight loss
`in a silicon carbide sample in a silicon-saturated steam
`environment.
`
`50
`
`55
`
`DESCRIPTION OF THE INVENTION
`While silicon-containing ceramics or ceramic composites
`are prone to thickness loss (also sometimes referred to as
`recession loss or material loss) in combustion gas
`environments, this invention introduces a method to prevent
`or greatly reduce the material loss by injecting an effective
`amount, generally in the part per million level, of silicon or
`silicon-containing compounds into the combustion gases.
`An example of an effective amount is a silicon level of about
`0.01 to about 10.0 parts per million or about 0.009 to 0.6
`parts per million by Weight of the combustion gases. The
`
`60
`
`65
`
`4
`silicon or silicon-containing compounds can be added
`directly to the combustion fuel, to the combustion air,
`directly to the combustor, directly to the combustion gases,
`or all of the above. The silicon or silicon-containing com
`pounds can be in the solid, liquid, or gaseous state, provided
`that the compounds volatiliZe in the combustion gases. Thus
`an important aspect of the present invention is the preven
`tion or alleviation of material loss by preventing the reaction
`of the silicon-containing ceramic or ceramic composite
`components With the combustion gases.
`The silicon-containing ceramic or ceramic composite can
`be a silicon-based ceramic or ceramic composite Where the
`largest percentage of the material composition by Weight is
`silicon. LikeWise, the silicon-containing ceramic composite
`can be a continuous ?ber reinforced ceramic composite,
`sometimes referred to as CFCC. Examples of silicon
`containing ceramics are silicon carbide (SiC), silicon nitride
`(Si3N4), silicon-silicon carbide, and molybdenum disilicide.
`Examples of silicon-containing ceramic composites are
`silicon-silicon carbide composites (Si/SiC) and silicon
`carbide-silicon carbide composites (SiC/SiC), to mention a
`feW. An example of a silicon-containing continuous ?ber
`ceramic composite is a silicon-silicon carbide composite
`With silicon carbide-containing ?bers and any of the above
`mentioned ceramics or composites that contain ?bers are
`CFCC. By “silicon carbide-containing ?ber” is meant a ?ber
`having a composition that contains silicon carbide, and
`preferably contains substantially silicon carbide. For
`instance, the ?ber may have a silicon carbide core sur
`rounded With carbon, or in the reverse, the ?ber may have a
`carbon core surrounded by or encapsulated With silicon
`carbide. These examples are given for demonstration of the
`term “silicon carbide-containing ?ber” and are not limited to
`this speci?c combination. Other ?ber compositions are
`contemplated, so long as they contain silicon carbide.
`For instance, other core materials Which may be envel
`oped by silicon carbide include carbon and tungsten. The
`?brous material can be amorphous, crystalline, or a mixture
`thereof. The crystalline material may be single crystal or
`polycrystalline. Examples of silicon carbide-containing
`?brous materials are silicon carbide, Si—C—O, Si—C—
`O—N, Si—C—O-Metal, and Si—C—O-Metal Where the
`Metal component can vary but frequently is titanium or
`Zirconium or aluminum. There are processes knoWn in the
`art Which use organic precursors to produce silicon carbide
`containing ?bers Which may introduce a Wide variety of
`elements into the ?bers.
`Additionally, a continuous ?ber reinforced silicon
`containing ceramic composite may comprise a silicon
`silicon carbide composite or a silicon carbide-silicon carbide
`composite With carbon-containing or silicon-containing
`?bers or silicon and carbon containing ?bers With or Without
`coatings on said ?bers. Acceptable coatings for such ?bers
`Would be, but not limited to, nitrides, borides, carbides,
`oxides, silicides, or other similar ceramic refractory mate
`rial. Representative of ceramic carbide coatings are carbides
`of boron, chromium, hafnium, niobium, silicon, tantalum,
`titanium, vanadium, Zirconium, and mixtures thereof. Rep
`resentative of the ceramic nitrides useful in the present
`process is the nitride of boron, hafnium, niobium, silicon,
`tantalum, titanium, vanadium, Zirconium, and mixtures
`thereof. Examples of ceramic borides are the borides of
`hafnium, niobium, tantalum, titanium, vanadium, Zirconium,
`and mixtures thereof. Examples of oxide coatings are oxides
`of aluminum, yttrium, titanium, Zirconium, beryllium,
`silicon, and the rare earths. The thickness of the coating can
`be about 0.1 to about 4.0 micrometers thick. A preferred
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`GE-1009.006
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`

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`US 6,517,341 B1
`
`5
`thickness is about 0.3—1.0 micrometers. Some other
`examples of coatings for ?bers are selected from the group
`consisting of boron nitride, silicon doped boron nitride,
`silicon nitride, silicon carbide, carbon and mixtures thereof.
`The ?brous material may have more than one coating. An
`additional coating should be Wettable With silicon and be
`about 500 Angstroms to about 3 micrometers. Representa
`tive of useful silicon-Wettable materials is elemental carbon,
`metal carbide, a metal coating Which later reacts With molten
`silicon to form a silicide, a metal nitride such as silicon
`nitride, and a metal silicide. Elemental carbon is preferred
`and is usually deposited on the underlying coating in the
`form of pyrolytic carbon. Generally, the metal carbide is a
`carbide of silicon, tantalum, titanium, or tungsten. The metal
`nitride may be a nitride of boron, hafnium, niobium, silicon,
`tantalum, titanium, vanadium, Zirconium, and mixtures
`thereof. Generally, the metal silicide is a silicide of
`chromium, molybdenum, tantalum, titanium, tungsten, and
`Zirconium. The metal Which later reacts With molten silicon
`to form a silicide must have a melting point higher than the
`melting point of silicon and preferably higher than about
`1450° C. Usually, the metal and silicide thereof are solid in
`the present process. Representative of such metals is
`chromium, molybdenum, tantalum, titanium, and tungsten.
`The silicon-containing ceramics or composites are made
`by methods knoWn in the art. To illustrate this point, a
`silicon-silicon carbide composite can be made by melt
`in?ltration techniques, as described in Us. Pat. Nos. 5,015,
`540; 5,330,854; and 5,336,350, incorporated herein by ref
`erence. Also, the continuous ?ber silicon-containing ceramic
`composites can be made by various methods knoWn in the
`art, such as taught in Us. Pat. No. 6,024,898 (Ser. No.
`08/777,129), incorporated herein by reference.
`This invention is applicable to any silicon-containing
`material that is exposed to combustion gas environments.
`Examples of components or parts used in turbine engines are
`combustion liners, vanes and blades, noZZles, buckets, tran
`sition pieces, shrouds, and retro?t ceramic vanes.
`It is desirable to add the silicon or silicon-containing
`compounds in a form that leads to rapid volatiliZation of
`silicon as silicon hydroxide. If rapid volatiliZation does not
`occur, then greater levels of silicon than those demonstrated
`in Tables 1 and 2 Would be needed to prevent recession or
`thickness loss of the silica ?lm and the underlying silicon
`containing ceramic or composite. The silicon compounds
`can be added to the liquid fuel as organic compounds in a
`solution or as a slurry that can be emulsi?ed. Examples of
`organic compounds that can be used for addition to the fuel
`are siloxanes, such as, but not limited to, octamethylcy
`clotetrasiloxane {Si4O4(CH3)8} and hexamethyidisiloxane
`{Si2O(CH3)6}. Both of these compounds are loW viscosity
`liquids With good stability toWard Water vapor in air.
`The silicon-containing compounds can also be added into
`the air used for combustion. They can be added doWnstream
`of the compressor and just before the combustor. They can
`also be added directly into the combustion gases but it Would
`be preferable to add them into the air used for combustion
`Which Will alloW rapid volatiliZation. The silicon-containing
`compounds can be in the form of organic compounds, Which
`Would volatiliZe readily, or in the form of slurries of ?ne
`particulate silicon-containing compounds, such as, but not
`limited to, silicon oxide, silicon, silicon carbide, silicon
`nitride, silicon boride, and mixtures thereof. Additionally,
`the silicon-containing compound is selected from the group
`consisting of siloxanes, silanes, silica, silicones, silicon
`carbides, silicon nitrides, silicon oxides, silicates, sand and
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`mixtures there of. Tetramethylsilane {Si(CH3)4} With a
`boiling point of 26.5 ° C., can be added directly to the natural
`gas fuel. The siloxanes mentioned above, octamethylcy
`clotetrasiloxane {Si4O4(CH3)8} and hexamethyldisiloxane
`{Si2O(CH3)6}, have higher boiling points then
`tetrametylsilane, so they could be injected as liquids into the
`compressed air just ahead of pre-mixers. 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 the compressed air before combus
`tion occurs. In addition, colloidal silica dispersed in Water
`can be injected directly into the air stream before the
`combustor. The colloidal silica is present in the dispersion in
`an amount up to 60 Weight percent, and preferably about 40
`Weight percent.
`The silicon introduced into the combustion gases Will be
`exhausted from the turbine or other combustion gas envi
`ronment in the form of ?ne silica or other compounds Which
`could form by reaction of the silica With other impurities
`present in the fuel or air. High purity liquid fuels typically
`contain a feW parts per million of impurities. Many liquid
`fuels used in industrial gas turbines contain tens to several
`hundreds parts per million of impurities Which ultimately go
`into the exhaust gases. By using the method of this invention
`and injecting the combustion gases With silicon or silicon
`containing compounds, the particulate level in the exhaust
`gases Would increase for high purity fuels, such as used in
`aircraft engines, and Would be negligible change for impure
`or dirty fuels used in turbines.
`The level of silicon needed by injection or mixing into the
`combustion gases is an amount to form a suf?cient concen
`tration of silicon hydroxide products, such as Si(OH)4 so as
`to signi?cantly reduce or eliminate the thermodynamic force
`for volatiliZation of the silica (silicon oxide) ?lm, located on
`silicon-containing ceramic or ceramic composite compo
`nents. The silicon levels needed to alleviate or prevent the
`thickness loss of the silica ?lm and the component increase
`With the stoichiometric ratio, pressure, and temperature. The
`higher the Water vapor level in the combustion gases, the
`higher the silicon level that is needed to be injected into the
`combustion gases. For instance, by calculating the
`hydrogen/carbon ratio (H/C) in the fuel, one can calculate
`the Water vapor level in the combustion gases and the
`amount of silicon needed to convert to silicon hydroxide or
`silicon oxyhydroxide products. The natural gas fuel With the
`H/C atomic ratio of about 4.0 Would require a higher silicon
`injection level compared to liquid fuels With a H/C atomic
`ratios of about 1.7 to about 2.0. The fuel to air ratio of about
`one, Which is a stoichiometric mixture, corresponds to the
`ratio that Will burn With no excess air. A ratio beloW one
`indicates an excess of air While a ratio above one indicates
`insufficient air for combustion. The method outlined can be
`used for air to fuel ratios both beloW and above one. Table
`1 demonstrates the results for a stoichiometric ratio up to one
`using silicon doping levels under varying turbine operating
`conditions. The silicon levels are given both for fuel and air.
`The required silicon levels in combustion gases Will be
`essentially similar to those in air because the air/fuel Weight
`ratios are very high. The silicon levels needed to prevent the
`recession loss are expected to be maximum for a stoichio
`metric ratio of about 1 but could be easily determined by
`proper experimentation by one skilled in the art. At higher
`stoichiometric ratios, the Water vapor content Would again
`decrease Which Would require optimiZation of additions of
`silicon or silicon compounds to be de?ned by test condi
`tions.
`
`GE-1009.007
`
`

`
`US 6,517,341 B1
`
`7
`
`TABLE 1
`
`Silicon doping levels needed to alleviate silica and silicon-containing
`ceramics or ceramic composite surface recession loss under different
`turbine operating condition
`
`Fuel
`
`Stoichiometric
`
`Air/Fuel Water
`Ratio
`Level in
`(By
`gasses Pressure Temp.
`
`Silicon
`Levels Required
`ppm
`
`Type
`
`Ratio (Phi)
`
`Weight) (Vol %) (atm)
`
`(0 C.)
`
`In Fuel
`
`In Air
`
`Natural
`Gas
`
`Liquid
`Fuel
`
`0.250
`0.250
`0.250
`0.250
`0.325
`0.325
`0.325
`0.325
`0.400
`0.400
`0.400
`0.400
`0.250
`0.250
`0.250
`0.250
`0.250
`0.250
`0.250
`0.250
`0.325
`0.325
`0.325
`0.325
`0.400
`0.400
`0.400
`0.400
`
`68.7
`68.7
`68.7
`68.7
`52.8
`52.8
`52.8
`52.8
`42.9
`42.9
`42.9
`42.9
`68.7
`68.7
`68.7
`68.7
`57.1
`57.1
`57.1
`57.1
`44.0
`44.0
`44.0
`44.0
`35.7
`35.7
`35.7
`35.7
`
`5.1
`5.1
`5.1
`5.1
`6.6
`6.6
`6.6
`6.6
`8.1
`8.1
`8.1
`8.1
`5.1
`5.1
`5.1
`5.1
`3.1
`3.1
`3.1
`3.1
`4.0
`4.0
`4.0
`4.0
`4.9
`4.9
`4.9
`4.9
`
`15
`15
`15
`15
`15
`15
`15
`15
`15
`15
`15
`15
`1
`10
`20
`30
`15
`15
`15
`15
`15
`15
`15
`15
`15
`15
`15
`15
`
`1100
`1200
`1300
`1400
`1100
`1200
`1300
`1400
`1100
`1200
`1300
`1400
`1200
`1200
`1200
`1200
`1100
`1200
`1300
`1400
`1100
`1200
`1300
`1400
`1100
`1200
`1300
`1400
`
`6.5
`9.1
`12.3
`15.9
`8.4
`11.8
`15.8
`20.5
`10.3
`14.4
`19.3
`25.1
`0.6
`6.1
`12.2
`18.3
`2.0
`2.7
`3.7
`4.8
`2.5
`3.5
`4.8
`6.2
`3.1
`4.3
`5.8
`7.5
`
`.10
`0.13
`0.18
`0.23
`0.16
`0.22
`0.30
`0.38
`0.24
`0.34
`0.45
`0.58
`0.01
`0.09
`0.18
`0.27
`0.03
`0.05
`0.06
`0.08
`0.06
`0.08
`0.11
`0.14
`0.09
`0.12
`0.16
`0.21
`
`Bene?ts to protect the silica and silicon-containing ceram
`ics and composites can also be obtained at lower silicon
`levels than those shown in Table 1. Some reduction will
`occur at silicon levels below those in Table 1. Table 2 show
`the effect of the silicon level on the reduction of the
`recession loss rate. Table 2 demonstrates that the recession
`loss problem can be fully prevented theoretically. However,
`in practice a 100% reduction of recession loss may not be
`achievable. Also, higher silicon levels, up to a factor of
`about 5 to about 10, than those shown in Tables 1 and 2
`might be needed because of the slow rate of volatiliZation of
`silicon additives and because of different operating condi
`tions of turbines than those shown in Tables 1 and 2.
`
`35
`
`40
`
`45
`
`to suspend the sample from a platinum wire. The sample was
`held in a vertical tube furnace with an alumina muffle. The
`atmosphere was provided by ?ushing the tube with a mix
`ture of 90% steam and 10% oxygen. The weight of the
`sample was recorded prior to the experiment. During the
`exposure, the sample was periodically taken out of the
`furnace and weighed. Results of the experiment are shown
`in FIG. 1. It can be seen that the sample is continually
`loosing weight. When silicon carbide is oxidized, the speci
`men gains weight. The weight loss is due to the reaction of
`the silica ?lm with water which results in the removal of the
`?lm. The weight loss due to reaction of silica with water
`overwhelms the weight gain from the oxidation.
`
`TABLE 2
`
`EXAMPLE 2
`
`Effect of silicon level in combustion air on reduction in surface
`recession rate in silicon-containing ceramics"
`
`Silicon Level
`
`0.00
`
`0.05
`
`0.10
`
`0.15
`
`0.20 (ppm)
`in air
`
`55
`
`% Reduction
`
`0.0
`
`22
`
`45
`
`67
`
`90
`
`*Pressure = 15 atm, Phi = 0.325 (stoichiometric ratio of fuel to air), Tem
`perature = 12000 C.
`
`60
`
`To further demonstrate the invention, and in no way
`limiting the invention, the following examples are presented.
`
`EXAMPLE 1
`
`Apiece of sintered silicon carbide was heat treated in a
`steam environment at 1200° C. The sample was 1 inch by
`0.5 inches by 0.1 inch and a small hole was drilled at one end
`
`65
`
`Another silicon carbide sample, similar in every respect to
`example 1, was heat treated at the same temperature in the
`same atmosphere. However, the gas was saturated with
`silicon prior to contacting the sample. Saturation of the gas
`with silicon was obtained by passing gas through a silica
`sponge held at the same temperature as the sample. The
`results of the exposure in silicon-saturated steam are shown
`in FIG. 2. The silicon carbide sample is showing continuous
`weight gain. The weight gain is due to oxidation of silicon
`carbide.
`These examples show that saturation of the steam with
`silicon effectively prevents reaction of silica and water that
`results in mass loss of silicon carbide.
`What is claimed:
`1. A method to reduce material loss of silicon-containing
`ceramics and silicon-containing ceramic composites in a
`
`GE-1009.008
`
`

`
`US 6,517,341 B1
`
`combustion gas environment comprising the step of inject
`ing an effective amount of silicon into said combustion gas
`environment, Where the silicon is at least one of elemental
`silicon, a silicon-containing compound or mixtures thereof
`Where an effective amount of silicon is an amount of silicon
`injected in the combustion gas environment that prevents or
`reduces volatiliZation of a silica ?lm located on the silicon
`containing ceramic and silicon-containing ceramic compos
`ites.
`2. A method according to claim 1 Where the silicon
`containing ceramic is selected from the group consisting of
`silicon carbide, silicon nitride, silicon-silicon carbide,
`molybdenum silicide and mixtures thereof.
`3. A method according to claim 2 Where the silicon
`containing ceramic is silicon carbide.
`4. A method according to claim 2 Where the silicon
`containing ceramic is silicon nitride.
`5. A method according to claim 1 Where the silicon
`containing ceramic comprises silicon as the predominant
`component.
`6. A method according to claim 2 Where the silicon
`containing ceramic composite is a continuous ?ber rein
`forced ceramic composite.
`7. A method according to claim 6 Where the ?ber is
`selected from the group consisting of carbon, silicon
`carbide, silicon carbide-containing material, and mixtures
`thereof.
`8. A method according to claim 6 Where the ?ber has at
`least one coati