Diffusion Coatings for Corrosion-Resistant Components in Coal Gasification Systems
Advanced electric power generation systems use a coal gasifier to convert coal to a gas rich in fuels such as H{sub 2} and CO. The gas stream contains impurities such as H{sub 2}S and HCl, which attack metal components of the coal gas train, causing plant downtime and increasing the cost of power generation. Corrosion-resistant coatings would improve plant availability and decrease maintenance costs, thus allowing the environmentally superior integrated-gasification-combined-cycle (IGCC) plants to be more competitive with standard power-generation technologies. Heat-exchangers, particle filters, turbines, and other components in the IGCC system must withstand the highly sulfiding conditions of the high-temperature coal gas over an extended period of time. The performance of components degrades significantly with time unless expensive high alloy materials are used. Deposition of a suitable coating on a low cost alloy will improve is resistance to such sulfidation attack and decrease capital and operating costs. The alloys used in the gasifier service include austenitic and ferritic stainless steels, nickel-chromium-iron alloys, and expensive nickel-cobalt alloys. The Fe- and Ni-based high-temperature alloys are susceptible to sulfidation attack unless they are fortified with high levels of Cr, Al, and Si. To impart corrosion resistance, these elements need not be in the bulk of the alloy and need only be present at the surface layers. In this study, the use of corrosion-resistant coatings on low alloy steels was investigated for use as high-temperature components in IGCC systems. The coatings were deposited using SRI's fluidized-bed reactor chemical vapor deposition technique. Diffusion coatings of Cr and Al were deposited by this method on to dense and porous, low alloy stainless steel substrates. Bench-scale exposure tests at 900 C with a simulated coal gas stream containing 1.7% H{sub 2}S showed that the low alloy steels such SS405 and SS409 coated with {approx}20%Cr and Al each can be resistant to sulfidation attack for 500 h. However, exposure to an actual coal gasifier gas stream at the Wabash River gasifier facility for 1000 h in the temperature range 900 to 950 C indicated that Cr and Al present in the coating diffused further into the substrate decreasing the protective ability of these elements against attack by H{sub 2}S. Similarly, adherent multilayer coatings containing Si, Ti, Al, and Nb were also deposited with subsequent nitridation of these elements to increase the corrosion resistance. Both dense and porous SS409 or SS 410 alloy substrates were coated by using this method. Multilayer coatings containing Ti-Al-Si nitrides along with a diffusion barrier of Nb were deposited on SS410 and they were found also to be resistant to sulfidation attack in the bench scale tests at 900 C. However, they were corroded during exposure to the actual coal gasifier stream at the Wabash River gasifier facility for 1000 h. The Cr/Al coatings deposited inside a porous substrate was found to be resistant to sulfidation attack in the bench-scale simulated tests at 370 C. The long-term exposure test at the Wabash River gasifier facility at 370 C for 2100 h showed that only a minor sulfidation attack occurred inside the porous SS 409 alloy coupons that contained Cr and Al diffusion coatings. This attack can be prevented by improving the coating process to deposit uniform coatings at the interior of the porous structure. It is recommended that additional studies be initiated to optimize the FBR-CVD process to deposit diffusion coatings of the corrosion resistant elements such as Cr, Al, and Ti inside porous metal filters to increase their corrosion resistance. Long-term exposure tests using an actual gas stream from an operating gasifier need to be conducted to determine the suitability of the coatings for use in the gasifier environment.
- Research Organization:
- SRI International, Menlo Park, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- FC26-03NT41616
- OSTI ID:
- 926667
- Country of Publication:
- United States
- Language:
- English
Similar Records
Diffusion Coatings for Corrosion Resistant Components in Coal Gasification Systems
DIFFUSION COATINGS FOR CORROSION RESISTANT COMPONENTS IN COAL GASIFICATION SYSTEMS
Related Subjects
01 COAL, LIGNITE, AND PEAT
ALLOYS
CHEMICAL VAPOR DEPOSITION
COAL
COAL GAS
COAL GASIFICATION
COATINGS
CORROSION
CORROSION RESISTANCE
DIFFUSION BARRIERS
DIFFUSION COATING
DIFFUSION COATINGS
ELECTRIC POWER
FLUIDIZED BED REACTORS
HEAT EXCHANGERS
LOW ALLOY STEELS
OPERATING COST
POWER GENERATION
STAINLESS STEELS
SULFIDATION