USC Boiler Research Articles

High-temperature corrosion resistance of Fe–Cr–Mo amorphous coating for water wall protection of USC boiler

High-temperature corrosion resistance of Fe–Cr–Mo amorphous coating for water wall protection of USC boiler

High-temperature corrosion behavior of S30432 in high-efficiency ultra-supercritical boiler burning low-alkali and high-sulfur coal

Abstract In this study, the resistance of S30432 to low-alkali coal ash corrosion in a high-efficiency USC boiler was investigated. Currently, S30432 is widely used in the high-temperature superheater and reheater of boilers burning low-alkali and high-sulfur coal. During the experiment, S30432 coated with low-alkali coal ash were fixed in a high-temperature tubular reactor, and hot gas at 650 °C and 700 °C containing SO2 passed over the specimens for 2000 h. Then the specimens were tested by X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy. It was found that S30432 specimens mainly underwent high-temperature oxidation. For the gas temperature of 700 °C and SO2 volume concentration of 0.35 %, the sulfidation reaction occurred and the weight change was only 1.08 mg cm−2. The results show that there is no obvious high-temperature corrosion after three years of operation of a 700 MW 620 °C boiler. The results are in contrast with the high-alkali coal ash corrosion resistance of S30432. It is concluded that S30432 can meet the requirements of a high-efficiency USC boiler burning low-alkali and high-sulfur coal.

Study on microstructure and mechanical behavior relationship for laser-welded dissimilar joint of P92 martensitic and 304L austenitic steel

A dissimilar laser beam welded joint of ferritic/martensitic P92 steel, and 304L austenitic stainless steel (ASS) has been examined in as-weld and post-weld heat treatment (PWHT) at 760 °C for 120 min for microstructure feature and mechanical aspects. The microstructure of the weld metal, interface, and heat-affected zone (HAZ) was characterized in detail, and it was observed that a great level of inhomogeneity exists in a dissimilar joint. In addition, the effect of the inhomogeneity in microstructure on mechanical properties, microhardness, and impact strength of the welded joint was discussed. The welded joint's mechanical properties met the USC boiler requirement for both as-weld and PWHT situations. The welded joint showed a decent combination of tensile strength and impact strength for both as-weld and PWHT conditions. The tensile results showed the failure from the 304L SS base metal, which ensured the acceptable strength of the weld metal for the laser welded joints. The part of the welded joint with relatively poor impact strength was the weld metal. Though, the Charpy impact toughness of the weld metal (89 J) was measured higher than the ASME standard value (>41 J) and EN 1599:1997 standard value (>47 J) in the as-weld situation. The impact strength of the weld fusion zone was 138 J in PWHT condition.

Analysis of the Explosion of Dissimilar Steel Joints in the 1050MW USC Boiler

The leakage of dissimilar steel welded joint near the third superheater occurred in a power plant boiler. Through the analysis of tube cutting sampling, it is considered that the welding quality of dissimilar steel joint is good, the dissimilar steel joint cannot bear the bending load, and the improper arrangement of weld joint position together cause the leakage. It is suggested that the welding joints of dissimilar steel should not be arranged in the position with complex structural stress in the design, and the inspection of anti-abrasion and anti-explosion blind area should be strengthened in the process of maintenance.

Perspectives for 700 °C ultra-supercritical power generation: Thermal safety of high-temperature heating surfaces

The advanced 700 °C ultra-supercritical (A-USC) power generation is the most important developing direction of power generation due to the highest efficiency and great potential for energy conservation. The heating surfaces will be more sensitive to metal temperature due to the narrow safety temperature allowance at higher steam parameter. The tube wall temperature is depended on heat transfer characteristics between flue gas and working fluid in tube. However, the tube wall temperature was usually calculated with decoupled method for predicting the energy flux between flue gas and working fluid. The accuracy of tube wall temperature by using this method is sufficient for 600 °C USC boiler design based on huge experience data of power plants. While for A-USC boiler, the accuracy of that was not sufficient due to the more sensitive of tube wall temperature on boiler safety operating. To address those issues, some perspectives are given: some experimental apparatus which combined combustion and heat transfer surfaces at 700 °C level of working fluid is needed to conduct heat transfer characteristics; the coupled heat transfer simulation method is essential for A-USC boiler design and operating; long-term testing verification of candidate surfaces is imperative for commercialization of A-USC technology.

Fireside Corrosion Behaviors of Super304H and HR3C in Coal Ash/Gas Environment with Different SO2 Contents at 650 °C

The corrosion behaviors of Super304H and HR3C used for USC boiler applications were investigated in simulated coal ash/gas environments with 0.1 and 1.5% of SO2 at 650 °C for 500 h. The results indicated that the increase in SO2 accelerated the corrosion rate and the spalling tendency of the corrosion layer in both tested alloys. Fe2O3, Cr2O3 and FeCr2O4 main peaks were revealed by XRD on Super304H, but on HR3C only the Cr2O3 peak showed a high intensity. The SO2 content did not affect the corrosion product composition of any of the alloys, but accelerated the inner sulfidation and the spallation on Super304H. No obvious internal sulfidation was observed on HR3C in either SO2 content. Based on the experimental results, the alloy corrosion mechanism and the influence of sulfur content on the corrosion process were discussed.

USC 보일러 입구 온도 분포 변화에 따른 내부 열교환기 표면 온도 및 스팀 온도 분포 예측

USC 보일러 입구 온도 분포 변화에 따른 내부 열교환기 표면 온도 및 스팀 온도 분포 예측

Numerical investigation of oxy-fuel combustion in 700 °C-ultra-supercritical boiler

The combination of the advanced ultra-supercritical (AUSC) and oxy-fuel combustion technologies is considered the most feasible and promising choice for carbon capture and storage. The high efficiency of AUSC boilers compensates for the cost incurred in controlling CO2 emissions. In this study, a new improved model was proposed for gas-radiative properties as a polynomial function of the continuous molar ratio of H2O to CO2 and temperature, which was compiled as user-defined functions in C language. The modified radiative model was employed in the computational fluid dynamics analysis to simulate a tangentially fired 700°C-USC boiler with a capacity of 300MW under the oxy-fuel condition. The results show that the improved model has a good accuracy for gas-radiative properties compared to other classical models. In the oxy-fuel cases, the heat transfer obtained in the simulation is approximately 4–11% lower than that in the thermodynamic calculation because of the char gasification. The radiative capability of the flue gases in the oxy-fuel case is much greater than that in the air–fuel case with the increase in the molar fraction of oxygen. The heat transfer via the water wall is approximately 4–8% higher than that via air, when the molar fraction of oxygen increases from 26% to 29%. The increase in the heat absorption helps in better arrangement of the heat-exchanger surfaces in the furnace of the AUSC boiler.

Assessment of High Temperature Creep Characteristics for P92 Steel Weldment in USC Boiler Header

Assessment of High Temperature Creep Characteristics for P92 Steel Weldment in USC Boiler Header

ICOPE-15-C162 Research on the superheater header material for 700℃ USC boiler

ICOPE-15-C162 Research on the superheater header material for 700℃ USC boiler

Creep Strength of Dissimilar Welded Joints Using High B-9Cr Steel for Advanced USC Boiler

The commercialization of a 973 K (700 °C) class pulverized coal power system, advanced ultra-supercritical (A-USC) pressure power generation, is the target of an ongoing research project initiated in Japan in 2008. In the A-USC boiler, Ni or Ni-Fe base alloys are used for high-temperature parts at 923 K to 973 K (650 °C to 700 °C), and advanced high-Cr ferritic steels are planned to be used at temperatures lower than 923 K (650 °C). In the dissimilar welds between Ni base alloys and high-Cr ferritic steels, Type IV failure in the heat-affected zone (HAZ) is a concern. Thus, the high B-9Cr steel developed at the National Institute for Materials Science, which has improved creep strength in weldments, is a candidate material for the Japanese A-USC boiler. In the present study, creep tests were conducted on the dissimilar welded joints between Ni base alloys and high B-9Cr steels. Microstructures and creep damage in the dissimilar welded joints were investigated. In the HAZ of the high B-9Cr steels, fine-grained microstructures were not formed and the grain size of the base metal was retained. Consequently, the creep rupture life of the dissimilar welded joints using high B-9Cr steel was 5 to 10 times longer than that of the conventional 9Cr steel welded joints at 923 K (650 °C).

Failure Analysis of Dissimilar Steel Welded Joints in a 3033t/h USC Boiler

In this work, macroscopic and microscopic examinations on the failure specimen were conducted on T91/HR3C dissimilar steel welded joints for the high temperature reheater in a 3033t/h USC boiler. The cracking is along the circumferential direction of tube, initiated in the CGHAZ (course grain HAZ) and terminated at the FGHAZ (fine grain HAZ) of the outer wall. The high hardness and unstability microstructure in the HAZ are responsible for the cracking failure. Post welding heat treatment (PWHT) is suggested for improving the ductility and reducing the residual stress for the welded joints. The delta ferrite with lower hardness was observed near the fusion line of T91 side. However, it was not related to the failure.

Development of Nickel Alloys Based on Alloy 617 for Components in 700 °C Power Plants

Abstract High Temperature Nickel based alloy 617 (A 617) is considered as a candidate material for the 700 °C power plants due to its combination of creep strength and good fabricability. The alloy has been investigated in numerous publicly sponsored and privately financed programmes with regard to its use in USC boilers. Based on the gained experience, the material has been tailored to fit the specific purpose of USC boilers. The first development step resulted in a modified version of Alloy 617 labeled Alloy 617 B, with creep strength at 700 °C about 25% higher than that listed by ASME and VdTUV. A second step focused on the definition of welding procedures and heat treatment, demonstrating that crack formation in large welded components can be avoided by suitable post-weld heat treatment. This paper will briefly describe the effect of boron and heat-treatment on the mechanical behaviour of Alloy 617 within the scope of the German USC boiler programmes.

Managing Steam-Side Oxidation and Exfoliation in USC Boiler Tubes

Abstract The growth and exfoliation of thermally grown oxides, or scales, in steam power plants is a complex phenomenon that depends on alloy composition, microstructure (including surface condition), temperature, pressure, and plant operation. This article discusses the factors that contribute to oxide scale buildup in ultrasupercritical boiler tubes and ongoing efforts to mitigate problems due to steam-side oxidation and exfoliation in high-efficiency power plants.

OS7-1 700℃超級先進超々臨界圧ボイラの開発(OS7 高温・高効率,循環型社会における動力エネルギー技術)

From 1980's to 1990's, USC technology had been worked on as a national project and achieved 600℃-class USC plants in Japan. So far, total 20 units are operated under 600℃-class temperature condition in Japan. Basically the 700℃-class A-USC boiler can be developed by advancing existing coal fired USC boiler technology, even though there are some issues to be solved about boiler material, structure and manufacturing. As boiler candidate materials, Ni based and Ni-Fe based alloy materials are considered to adopt high temperature region for 700℃-class A-USC boiler.

Steam oxidation resistance of thermal sprayed coatings for USC boiler applications

Plasma sprayed coatings of 50Ni–50Cr as undercoat coupled with an Al topcoat were investigated with the aim of filling the pores in the Ni–Cr coating with the Al topcoat at high temperatures. For comparison purposes, a thick 50Ni–50Cr coating (∼450 μm) with sealant according to BS 5493 was also applied. The results showed that both the two layer coating of Ni−Cr+Al and the thick coating with the sealant had excellent resistance against steam oxidation at 650°C for up to 3000 h of testing. It is interesting to note that the two layer coating showed a similar performance, despite the lower coating thickness compared to the thick coating. Both Ni and Cr diffused into the Al coating structure. Diffusion of Al into the Ni–Cr system (coating) was slower than that into the Fe–Cr system (substrate).

Steam oxidation resistance of two-layered Ni–Cr and Al APS coating for USC boiler applications

Thermal spray of Ni–Cr and Al coatings was attempted on modified 9Cr–1Mo steel, to evaluate their steam oxidation resistance. Atmospheric plasma spray (APS) coatings of 50Ni–50Cr as undercoat and Al topcoat were attempted with the aim that the pores produced by 50Ni–50Cr undercoat can be filled with Al topcoat during the steam oxidation. The steam oxidized samples evinced the Ni and Cr diffusion towards the Al coating structure and changed the topcoat in to the Ni–Al intermetallics. Though the two-layered coating exhibited an excellent performance against the steam oxidation for the base steel substrate till 3000 h of test, the top layers of the coating underwent significant internal oxidation.