Jeffrey R. Price

Evaluation of CFCC liners with EBC after field testing in a gas turbine

Abstract Under the Ceramic Stationary Gas Turbine (CSGT) Program sponsored by the U.S. Department of Energy (DOE), a team led by Solar Turbines Incorporated has successfully designed engines, utilizing silicon carbide/silicon carbide (SiC/SiC) continuous fiber-reinforced ceramic composite (CFCC) combustor liners. Their potential for low NO x and CO emissions was demonstrated in eight field-engine tests for a total duration of more than 35,000 h. In the first four field tests, the durability of the liners was limited primarily by the long-term stability of SiC in the high steam environment of the gas turbine combustor. Consequently, the need for an environmental barrier coating (EBC) to meet the 30,000-h life goal was recognized. An EBC developed under the National Aeronautics and Space Administration high speed civil transport, enabling propulsion materials program was improved and optimized under the CSGT program and applied on the SiC/SiC liners by United Technologies Research Center (UTRC) from the fifth field test onwards. The evaluation of the EBC on SiC/SiC liners after the fifth field test with 13,937-h at Texaco, Bakersfield, CA, USA is presented in this paper.

Exposure of Ceramics and Ceramic Matrix Composites in Simulated and Actual Combustor Environments

A high-temperature, high-pressure, tube furnace has been used to evaluate the long term stability of different monolithic ceramic and ceramic matrix composite materials in a simulated combustor environment. All of the tests have been run at 150 psia, 1204 degrees C, and 15% steam in incremental 500 h runs. The major advantage of this system is the high sample throughput; >20 samples can be exposed in each tube at the same time under similar exposure conditions. Microstructural evaluations of the samples were conducted after each 500 h exposure to characterize the extent of surface damage, to calculate surface recession rates, and to determine degradation mechanisms for the different materials. The validity of this exposure rig for simulating real combustor environments was established by comparing materials exposed in the test rig and combustor liner materials exposed for similar times in an actual gas turbine combustor under commercial operating conditions.

EBC Protection of SiC/SiC Composites in the Gas Turbine Combustion Environment: Continuing Evaluation and Refurbishment Considerations

Silicon carbide fiber reinforced silicon carbide composites (SiC/SiC CMC’s) are attractive for use in gas turbine engines as combustor liner materials because the temperature capability allows for reduced cooling. This enables the engine to operate more efficiently and enables the design of very stringent emission goals for NOx and CO. It has been shown, however, that SiC/SiC CMC’s and other silica formers can degrade with time in the high steam environment of the gas turbine combustor due to accelerated oxidation and subsequent volatilization of the silica due to reaction with high pressure water (ref.s 1, 2, 3, & 4). As a result, an environmental barrier coating (EBC) is required in conjunction with the SiC/SiC CMC in order to meet long life goals. Under the U.S. Department of Energy (DOE) sponsored Solar Turbines Incorporated Ceramic Stationary Gas Turbine (CSGT) engine program (ref. 5), EBC systems developed under the HSCT EPM program and improved under the CSGT program have been applied to both SiC/SiC CMC coupons and SiC/SiC CMC combustion liners which have been evaluated in long term laboratory testing and in ground based turbine power generation. This paper discusses the continuing evaluation (see ref. 6) of EBC application to SiC/SiC CMC’s and the results from laboratory and engine test evaluations along with refurbishment considerations.Copyright

Evaluating Environmental Barrier Coatings on Ceramic Matrix Composites After Engine and Laboratory Exposures

SiC/SiC continuous fiber-reinforced ceramic matrix composite (CFCC) combustor liners having protective environmental barrier coatings (EBCs) applied to the liner working surfaces have been field-tested in a Solar Turbines’ Centaur 50S SoLoNOx engine at the Chevron, Bakersfield, CA engine test site. This latest engine test ran for a total of 13,937h. The EBCs significantly increased the lifetime of the in-service liners compared with uncoated CFCC liners used in previous field-tests. The engine test was concluded when a routine borescope inspection revealed the formation of a small hole in the inner liner. Extensive microstructural evaluation of both the inner and outer liners was conducted after removal from the engine. Post-test analysis indicated that numerous degradation mechanisms contributed to the EBC and CFCC damage observed on the liners, including EBC volatilization, sub-surface CFCC oxidation and recession, and processing defects which resulted in localized EBC spallation and accelerated CFCC oxidation. The characterization results obtained from these field-tested liners have been compared with the analyses of similarly-processed CFCC/EBCs that were laboratory-tested in a high-pressure, high temperature exposure facility (the ORNL “Keiser Rig”) for >6000h.Copyright

The Evaluation of CFCC Liners After Field Testing in a Gas Turbine — III

Under the Ceramic Stationary Gas Turbine (CSGT) Program and the Advanced Materials Program, sponsored by the U.S. Department of Energy (DOE), several silicon carbide/silicon carbide (SiC/SiC) combustor liners were field tested in a Solar Turbines Centaur 50S gas turbine, which accumulated approximately 40000 hours by the end of 2001. To date, five field tests were completed at Chevron, Bakersfield, CA, and one test at Malden Mills, Lawrence, MA. The evaluation of SiC/SiC liners with an environmental barrier coating (EBC) after the fifth field test at Bakersfield (13937 hours) and the first field test at Malden Mills (7238 hours) is presented in this paper. The work at Oak Ridge National Laboratory (ORNL) in support of the field tests was supported by DOE’s Continuous Fiber-Reinforced Ceramic Composite (CFCC) Program.Copyright

EBC Protection of SiC/SiC Composites in the Gas Turbine Combustion Environment

Silicon carbide fiber reinforced silicon carbide composites (SiC/SiC) are attractive for use in gas turbine engines as combustor liner materials, in part, because the temperature capability allows for reduced cooling. This enables the engine to operate more efficiently and to meet very stringent emission goals for NOx and CO. It has been shown, however, that SiC/SiC and other silica formers can degrade with time in the high steam environment of the gas turbine combustor due to accelerated oxidation and subsequent volatilization of the silica due to reaction with high pressure water (ref.s 1 & 2). As a result, an environmental barrier coating (EBC) is required in conjunction with the SiC composite in order to meet long life goals. Under the U.S. Department of Energy (DOE) sponsored Solar Turbines Incorporated Ceramic Stationary Gas Turbine (CSGT) engine program (ref. 3), EBC systems developed under the HSCT EPM program (NASA contract NAS3-23685) were applied to both SiC/SiC composite coupons and SiC/SiC combustion liners which were then evaluated in long term laboratory testing and in ground based turbine power generation, respectively. This paper discusses the application of the EBC’s to SiC/SiC composites and the results from laboratory and engine test evaluations.Copyright

Material Characterization of Candidate Silicon Based Ceramics for Stationary Gas Turbine Applications

The Ceramic Stationary Gas Turbine (CSGT) Program is evaluating the potential of using monolithic and composite ceramics in the hot section of industrial gas turbines. Solar Turbine’s Centaur 50 engine is being used as the test bed for ceramic components. The first stage blade, first stage nozzle and the combustor have been selected to develop designs with retrofit potential, which will result in improved performance and lowered emissions. As part of this DOE sponsored initiative a design and life prediction database under relevant conditions is being generated. This paper covers experiments conducted to date on the evaluation of monolithic silicon based ceramics. Mechanical property characterizations have included dynamic fatigue testing of tensile as well as flexural specimens at the temperatures representative of the blade root, the blade airfoil and the nozzle airfoil. Data from subcomponent testing of blade attachment concepts are also included.Copyright

Microstructural and Mechanical Characterization of a Hybrid Oxide CMC Combustor Liner After 25,000-Hour Engine Test

A hybrid oxide ceramic matrix composite (CMC) outer combustor liner was tested in a Solar Turbines Incorporated Centaur® 50S engine between 2003 and 2006, accumulating >25,000 hours of field exposure. The hybrid CMC liner, which was ∼76 cm in diameter, had an alumina matrix with a Nextel 720 fiber-reinforcement (A/N720). The CMC, produced by ATK-COI Ceramics, Inc., was coated with a ceramic insulation layer known as FGI (Friable Graded Insulation) developed by Siemens Energy Incorporated. Post-test microstructural and mechanical evaluation was conducted on the field-exposed liner at Oak Ridge National Laboratory (ORNL) to determine the types of surface and structural damage that occurred to the combustor liner during engine exposure to elevated temperatures (>1200°C), thermal cycling (stop-start cycles), and combustion gases (especially water vapor). In this study, numerous sections were cut from the liner for mechanical and microstructural characterization that exhibited varying amounts of FGI and/or CMC degradation. In this way, damage accumulation was assessed (1) within the CMC and FGI layers, both on the gas-path surface and below the surface and (2) as a function of liner position (fore-to-aft) in the engine. The amount and type of damage observed was directly related to the starting CMC and FGI microstructures. The tensile strength of the hybrid liner after field exposure was found to be 19 MPa. The FGI layer remained well bonded to the CMC and the fracture surface of the CMC exhibited scissor-like features, which is typical of composites with ±45° fiber architecture. The stress acting on the CMC at failure was 53 MPa.Copyright

Advanced Materials for Gas Turbine Combustion Systems: Program Summary

Solar Turbines Incorporated, under cooperative agreement number DEFC02-00CH11049, is improving the durability of advanced combustion systems while reducing life cycle costs. This project is part of the Advanced Materials in Advanced Industrial Gas Turbines program in DOE’s Office of Distributed Energy. The targeted development engine is the Mercury 50 gas turbine under development by Solar Turbines Incorporated under the DOE Advanced Turbine Systems (ATS) program (DOE contract number DE-FC21-95MC31173). The ultimate goal of the program is to demonstrate a fully integrated Mercury 50 combustion system, modified with advanced materials technologies, at a host site for 4,000 hours. The program focuses on a dual path development route to define an optimum mix of technologies for the Mercury 50 and future Solar gas turbine products. For linear and injector development, multiple concepts including high thermal resistance thermal barrier coatings (TBC), oxide dispersion strengthened (ODS) alloys, continuous fiber ceramic composites (CFCC), and monolithic ceramics are being evaluated before down selection to the most promising candidate materials for field evaluation. Preliminary component and sub-scale testing is being conducted to determine material properties and demonstrate proof-of-concept. Full-scale rig and engine testing will validate engine performance prior to field evaluation at a host site. Field testing of CFCC combustor liners in Centaur 50 engines at two field test sites is also being conducted under the Advanced Materials Program. This paper is a status review of the program, detailing the current progress.Copyright

Ceramic Stationary Gas Turbine Program: Monolithic Ceramic Component Development Summary

In the Ceramic Stationary Gas Turbine (CSGT) development program, sponsored by the U.S. Department of Energy (DOE) Offices of Industrial Technologies and Power Technologies (OIT/OPT), monolithic silicon nitride and silicon carbide ceramics were evaluated for application as structural materials for hot section components in an industrial gas turbine, the Solar Centaur 50S. First generation blades of GN10, NT164, and SN253 silicon nitrides, second generation blades of AS800 and SN281 silicon nitrides, and first generation SN-88 silicon nitride nozzles were evaluated in rigs and test engines. Hexoloy SA silicon carbide combustor liner tiles were tested in a subscale rig. The selection and evaluation of monolithic materials over the duration of the CSGT program (1992 to 2000) will be reviewed.© 2001 ASME