Development of Advanced Ultra Supercritical Fossil Power Plants in Japan： ：Materials and High Temperature Corrosion Properties Yuji Fukuda 1
Kure Research Laboratory, Babcock-Hitachi K.K., Kure-shi, Hiroshima-ken, JAPAN
[email protected]，
Keywords: Fireside corrosion, Steam oxidation, A-USC Coal fired boiler, Ni-based alloy, Fe-Ni based alloy
Abstract. Japanese government project of the A-USC technology was started in 2008 August. 700°C class boiler, turbine and valve technologies, which include high temperature material technology, will be developed. This report provides the present state of the art and technical background of this development effort for A-USC in Japan, especially focusing the high temperature corrosion and the steam oxidation behavior of available and developmental materials for boiler. Introduction. Advanced ultra supercritical plants (A-USC), which target steam temperature is 700°C, are under development worldwide to improve efficiency and reduce CO2 emissions [1]-[4]. Material development and selection are critical to the success of these efforts. In the ‘80s and ‘90s, 600°C class USC technology was developed through project led by J-Power. Based on this effort, many 1,000MW coal fired USC plants with 600°C range have been launched successfully in Japan since 2000(Figure1 and 2)[5]. This steam condition increase was achieved by the development of high strength 9-12%Cr steels and the appropriate manufacturing technologies such as forming and welding, as well as highly sophisticated design. The strong need to reduce CO2 emission has recently provided an additional incentive to increase efficiency. Thus, target steam temperatures of efficient fossil power plants are in the 700°C range in the next 20 years. The main enabling technology for this development is the stronger high temperature steels, including Ni and Fe-Ni based alloys capable of operating under high stresses at ever increasing temperatures. In addition, resistance to fireside corrosion and steam side oxidation is required. For this reason, Japanese government project of the A-USC technology was started in 2008 August. 700°C class boiler, turbine and valve technologies, which include high temperature material technology, will be developed. This report provides the present state of the art and technical background of this development effort for A-USC in Japan, especially focusing the high temperature corrosion and the steam oxidation behavior of available and developmental materials for boiler.
Figure 1 Power Plant Steam Condition in Japan. Source Ref 4 and 5
Figure 2 Capacity of coal fired plant in Japan. Source Ref 5
USC technology development project in Japan. Boiler material and issues. Candidate materials of boiler component classified based on steam pressures and temperatures of superheater and reheater are shown in Table2. To achieve 700°C class USC plants, the development of heat resistant materials with higher creep rupture such as newly developed Ni base alloys, Fe-Ni base alloys is needed. In addition to the strength requirement, resistances to fireside corrosion and to steam side oxidation are required (Figure 4).
Figure 3 Steam condition and used materials of boiler
Figure 4
Main material issues of boiler
A-USC technology development project. A long term A-USC technology development project (2/3 of the budget is subsidized by the Japanese government) began in 2008. In the first half of the project, boiler, turbine and valve materials are being developed and verified [4], [5]. In the second half, boiler components and small turbine tests will be done to verify the reliability of each component (Table 1). Throughout the project, some candidate materials for boilers are being tested. Turbine rotor and casing materials are being developed and tested, as well. Table 2 shows the candidate boiler materials, which were prepared by Sumitomo Metals and test items. HR6W is a Ni-Fe based alloy and HR35, Alloy 617, Alloy263, Alloy740, and Alloy141 are Ni based alloys for use at temperatures higher than 650 °C. High boron 9Cr steel, low carbon 9Cr steel and SAVE 12AD are ferritic steels for use at temperatures lower than 650°C. These materials are being tested to verify the characteristics regarding creep rupture, fatigue, steam oxidation and fireside corrosion. Welding and bending tests have been conducted to check the manufacturability of the materials. 10 companies and institutes, National Institute for Materials Science (NIMS), ABB Bailey Japan, IHI, Sumitomo, Toshiba, Babcock-Hitachi, Hitachi, Fuji, MHI and Central Research Institute of Electric Power Industry (CRIEPI) are participating. Table 1 A-USC project schedule. Source Ref 4 and 5
Table 2 Candidate materials and testing items. Source Ref 4 and 5
Corrosion properties of candidate materials for A-USC Boiler SH/RH Fireside Corrosion. Superheater/reheater (SH/RH) fireside corrosion is not a major problem for Japan units using 600 °C steam because sulfur and chlorine content in used coal in Japan is not high. Historically, it has been a significant problem for units operating in the U.S. and the U.K. above 565°C [6], where high sulfur and chlorine coals were used. SH/RH fireside corrosion is normally caused or accelerated by molten salts consisting of sodium-potassium-iron tri-sulfates (Na,K)3Fe(SO4)3 in deposit ash and the corrosion rate is affected by coal properties (S, Cl, Na, K), chromium content in material to be used and gas/metal temperature. Figure 5 is a typical equiv.-corrosion loss map for 18Cr8Ni austenitic steel (Ka-SUS304J1HTB), which is commonly used for superheater tubing material of 600°C class USC boiler. The corrosion rate is affected by SO2 content (S content in coal) above 0.15% at around 650°C [7][8]. This indicates that SH/RH fireside corrosion for 700°C class A-USC boiler using high S coal will be a major problem. Therefore laboratory corrosion test of candidate materials such as Fe-Ni based and Ni based alloys for 700°C class A-USC boiler are carried out in the Japanese project. Figure 6 shows the relationship between weight loss of the materials and temperatures. The weight loss showed a bell-shaped curve with peaks near 700°C for all the materials, because sodium -potassium-iron tri-sulfates (Na, K)3Fe(SO4)3 exist in a molten state between 600°C and 750°C. Figure 7 shows relationship between the chromium content of various boiler tube materials and metal loss. While the effect of chromium content on corrosion is not much at low metal temperature condition, dependency of the chromium content at high temperature region is significant. Those corrosion data indicate that Fe-Ni based and Ni based alloy for 700°C A-USC has the same corrosion resistance to 25Cr-20Ni alloy, which has been used for 600°C USC boiler using high sulfur coal.
Figure 5 SH/RH fireside corrosion map of ka-SUS304J1HTB
Figure 6
Source Ref 7
Relationship between corrosion weight loss and temperature. Source Ref 8
Figure 7
Relationship between corrosion weight loss and Cr content. Source Ref 8
Steam oxidation. Another material issue for high temperature applications is steam-side oxidation of tubes, headers and piping. When the thickness of the steam oxidation scale grows significantly, it causes the hindrance of heat transfer. This situation results in many difficulties, i.e. a serious increase in outside tube metal temperature, the clogging or blockage of tubes and erosion damage to turbine blades by exfoliated oxide scale. Steam oxidation resistance of austenitic steels strongly depends on Cr contents and grain size. To improve the steam oxidation properties of 18Cr8Ni austenitic steels, a shot-blasting technique is selected [7][9]. The shot blasting on the inner tube surface introduces a cold-worked layer in the vicinity of inner surface and Cr diffusion is accelerated along the slip bands at high temperatures. It results in a Cr-rich oxide layer in the early stage of boiler operation, which can become a strong barrier against further scale formation. Thus, the shot-blasting technique enables to be applied to 600°C class USC boilers. Therefore steam oxidation properties of candidate materials such as Fe-Ni based and Ni based alloys for 700°C class A-USC boiler are evaluated compared with 25Cr-20Ni and shot blasted 18Cr8Ni austenitic steels in the project. The test will be conducted for a total of 10,000 hours. Figure 8 shows the comparison total oxide scale thickness of tested candidate alloys, which have been exposed to 700, 750 and 800C steam for 3,000 hours. The thickness of the scale on the Ni based alloys is considerably smaller than the scale on the 25Cr steel, Ka-SUS310J1TB (HR3C) and Ka- SUS304J1HTB shot blasted which have been commonly used in 600°C class USC boiler tube.
Figure 8
Comparison of total scale thickness for candidate alloys of A-USC boiler Source Ref 4 & 5
Figure 9 shows the cross section views of tested alloys exposed 3,000 hours at 750°C. As internal oxidation is slightly observed in Ni-based alloys, it is recommended that effect of internal oxidation on long-term creep life will be clarified for future studies.
Figure 9 Cross section views of candidate alloys after 3,000 hours exposed at 750C Source Ref 4 & 5
Conclusion. A-USC is one of the remarkable technologies being developed to reduce CO2 emissions from fossil fuel power plants and was chosen by Japan’s ‘Cool Earth-Innovative Energy Technology Program which was launched in 2008 to contribute to substantial CO2 emissions reductions. Major Japanese manufacturers of boilers and steam turbines and some institutes are cooperating in the project to develop the technology efficiently and quickly. Fireside corrosion and steam oxidation are one of important issues to realize the A-USC. In the project laboratory corrosion tests have been carried to study the corrosion resistance of many candidate materials. As laboratory test cannot simulate actual plant condition completely, long-term in-plant test are suggested for next study. References [1] R.Viswanathan, and R.Purgert, Paper No.Creep2007-26826, 8th International Conference on Creep and Fatigue at Elevated Temperatures, San Antonio, Texas (July 2007). [2] H.Tschaffon, Paper No. 2, 34th MPA-SEMINAR and VGB-Symposium on Materials and Components Behaviour in Energy & Plant Technology, Stuttgart, Germany (October 2008). [3] R.Blum, Paper No.1, 8th International NIMS-MPA-IfW-Workshop on Advances in High Temperature Materials for High Efficiency Power Plants, Tsukuba, Japan (March 2010). [4] M.Fukuda, Paper No.2, 8th International NIMS-MPA-IfW-Workshop on Advances in High Temperature Materials for High Efficiency Power Plants, Tsukuba, Japan (March 2010). [5] M.Fukuda, 9th Liège Conference (September 2010) [6] R.B. Dooley, Boiler Tube Failures: Theory and Practice, ISBN 0-8033-5058-9,EPRI (1995) [7] Y. Fukuda and M. Shimizu, Material Science Forum Vols.522-523,August 2006,p 189 [8] M. Shimizu, G. Bao and Y. Fukuda, Paper No. 09250, CORROSION 2009, Atlanta, GA (March 2009) [9] T. Sato, Y. Fukuda, K. Tamura and K. Mitsuhata, Proc. of 4th International Conf. on Advances in Materials Tech. for Fossil Power Plants, EPRI, Hilton Head Island (2004) p182-195
