Advanced Ultra-supercritical Steam Research Articles

Oxidation behaviour of austenitic stainless steel 304HCu in Advanced Ultra Supercritical (AUSC) steam and the efficacy of shot-peening treatment

Effect of shot-peening on the oxidation behaviour of alloy 304HCu in controlled advanced ultra-supercritical (AUSC) steam at 650 ◦C, 31 MPa for 600 h was investigated. Non-shot peened surfaces form thick Fe-rich oxides, undergo internal oxidation, and suffer oxide spallation sooner than those reported in literature. Shot-peened surfaces form a thin, external chromia scale with Mn(Cr,Fe)2O4 and MnCr2O4 spinels growing above and below it respectively. Shot-peening enhances Cr and Mn diffusivity through grain refinement and increased dislocation density, which aids in resisting chromia scale breakdown, eliminating iron oxides, and preventing oxide spallation. Oxide growth mechanism in AUSC steam is discussed.

Oxidation behaviour of Ni-base superalloy 617 in simulated Advanced Ultra Supercritical (AUSC) steam

Alloy 617 was exposed in a simulated Advanced Ultra Supercritical (AUSC) steam oxidation test loop at 650–710 ℃, 31 MPa for 600 h. Detailed characterization of the oxide scales was carried out using Transmission Electron Microscopy coupled with Precession Electron Diffraction. The alloy 617 showed insignificant weight gains with super-parabolic kinetics. The oxide scale formed on the alloy consisted of a thin inner Cr2O3 layer and outer islands of (Mn,Ni,Fe)Cr2O4 spinel with negligible internal oxidation zone comprising Al2O3. The mechanistic aspect of AUSC steam oxidation is discussed.

The use of numerical simulation to determine the coal combustion efficiency in the A-USC boiler

In this paper, numerical simulation of coal combustion in the invert furnace of a boiler for a 500 MW power unit designed to operate on advanced ultra-supercritical steam parameters (A-USC) is performed. The use of A-USC technology makes it possible to reduce fuel costs for electricity production compared to steam turbine units of previous generations. Also it influences to reduce the level of emissions of harmful substances into the atmosphere. Modeling of the invert furnace at the nominal boiler load was carried out for the developed direct-flow burners and nozzles arrangement of burning coal. After numerical simulation the results were obtained and made it possible to analyze the furnace aerodynamics and the temperature distribution inside the furnace volume. The calculation of the indicators of efficiency and environmental friendliness was carried out and their values appeared smaller than regulatory permissions, which allows us to recommend the developed burners arrangement scheme for its use in invert furnaces. The furnace operation simulation at reduced loads of 70% and 50% was carried out and the results showed the reliability of the developed combustion scheme.

The Coal Dust Combustion Scheme for an Invert Furnace of an A-USC M-Shaped Boiler

The M-shaped boiler construction for the advanced ultrasupercritical steam parameters (A-USC) is proposed in this work. The boiler was designed to operate in a 500 MW unit on low volatile hard coal. This design allows reducing the pipelines length of the high cost steam pipelines made of nickel alloys. A downstream (invert) furnace is offered for this boiler type. The coal dust burning scheme design using the direct-flow burners and nozzles in a system of vertical and horizontal tangential torches and the solid ash removal are proposed. This approach was extensively used earlier on standard shaped boilers, and it was upgraded now for an invert furnace. The goals are achieved by conducting research on the physical furnace model and thermal furnace processes numerical simulation by computational fluid dynamics software. The most significant research results were as follows: the oxidizer stage supply was performed along the torch length and furnace height; the dynamic jet pressure on the furnace walls was excluded; a high degree of coal burnout was ensured due to the vortex furnace aerodynamics implementation; the uniform furnace section filling with air jets was performed; turbulent jets ejection was significantly higher than that for a flat submerged jet; chemical underburning loss did not exceed 0.1%; and unburned carbon loss was 0.8%. The carbon monoxide concentration at the furnace outlet in terms of air excess ratio equal to α=1.4 was 226 mg/nm3 . The nitrogen oxides concentration in the flue gases (normalized) was 424 mg/nm3 . The results significance obtained during the research is efficient solid fuel use with high technical and economic boiler performance.

Investigation of the fireside corrosion of a new wrought Ni-Fe-Cr-based alloy in advanced ultra-supercritical steam boilers

The gas-phase and coal ash-related fireside corrosion behavior of a new type of Ni-Fe-Cr based superalloy GH984G and two other contrast alloys were investigated for its application under advanced ultra-supercritical (A-USC) conditions. Results showed that the oxidation activation energy of the three alloys at 600–780 °C in the flue gas environment can be ranked from large to small: C-HRA-1>GH984G>Alloy 625. In the coal ash environment, GH984G and C-HRA-1 exhibited satisfactory corrosion resistance, while Alloy 625 had weaker corrosion resistance due to the presence of molybdenum. The corrosion mechanism of GH984G and the influence of constituent elements were elucidated.

Numerical simulation of fuel staged swirl combustion in the invert furnace of boiler on advanced ultra-supercritical steam parameters

The paper considers the schemes of Kuznetsky lean coal combustion for the M-shaped boiler. With such a boiler profile, it is possible to significantly reduce the length of main steamlines, which is especially important for the advanced ultra-supercritical parameters of the superheated steam. The furnace in this boiler unit is performed downward (invert). In this work, the aerodynamics of 6 combustion schemes was simulated by means of computational fluid dynamics. All considered schemes were designed on the basis of direct-flow burners and nozzles. For the most aerodynamically reasonable scheme the thermal processes in the boiler furnace firing Kuznetsky lean coal have been simulated by means of computational hydrodynamics. The simulation results showed a high efficiency of fuel burnout: loss due to unburned combustible equaled 0.1%, carbon-in-ash loss equaled 0.8%. Carbon monoxide concentration at the furnace outlet in conversion to excess air equal α = 1.4 amounted 226 mg/m3, the nitrogen oxides concentration in the flue gases (in conversion to normal conditions) equaled 424 mg/m3. It is appropriate to use the results obtained in this research in the development of new solid fuels combustion schemes.

Coupling chemical looping combustion of solid fuels with advanced steam cycles for CO2 capture: A process modelling study

Chemical looping combustion is a cost-competitive solution for producing low carbon electricity. In this paper, we investigate by means of a process modelling study, the coupling of chemical looping combustion of solid fuels with advanced steam-based power cycles, viz. supercritical, ultra-supercritical and advanced ultra-supercritical Rankine cycles. The energy and exergy efficiencies of the various chemical looping combustion power plant configurations are compared against the reference plants without carbon capture. Our models incorporate practical considerations for reactor design. With an upper operating temperature limit of 950 °C, the maximum efficiencies achievable by integrated gasification combined cycle chemical looping combustion (IGCC–CLC) and in situ gasification chemical looping combustion power plants (iG-CLC) are 41.3% and 41.5%, respectively. Overall, iG-CLC emerges as the most efficient CLC configuration. Comparing to an integrated gasification combined cycle without carbon capture, the energy efficiency penalties for capturing CO2 from iG-CLC coupled with subcritical, supercritical, ultra-supercritical or advanced ultra-supercritical steam cycles are 5.1%, 5.0%, 5.2% or 13.0%, respectively. The biomass-fired chemical looping combustion power plants also show low energy efficiency penalties (<2.5%) compared to the reference biomass power plants without CO2 capture. Our modelling results suggest that chemical looping combustion will remain an attractive carbon capture technology for solid fuel power plants, in a future when supercritical steam turbines become the norm.

Creep-Fatigue Behavior of a Newly Developed Ultra-Supercritical Steam Turbine Grade Nickel-Based Superalloy, HAYNES 282

Abstract Most engineering components used in gas/steam turbines are exposed to a range of complex loading conditions resulting from startup and shutdown procedures. These loading conditions involve superimposition of time-dependent creep on cyclic fatigue and can be simulated by properly designed high-temperature creep-fatigue tests. Creep-fatigue interaction is a function of duration and position of dwell in the loading waveform, and the material microstructure. The objective of this work is to investigate the creep-fatigue interaction response of a newly developed γ′-strengthened wrought nickel-based superalloy (HAYNES 282), which has a potential application in advanced ultra-supercritical steam turbines. Creep-fatigue tests are conducted at 760°C with strain dwell either at tensile peak or compressive peak or at both tensile and compressive peak positions for different dwell times of 100 and 1,000 s. The test results are analyzed with respect to evolutions of peak stress, stress amplitude, stress relaxation, hysteresis loop, inelastic strain energy density, and degree of softening. Degree of softening is found to increase with dwell position at tensile, compressive, and both peaks in that order. Tests with dwell at both tensile and compressive peak positions are found to be the most damaging, showing the least life. Between tensile dwell and compressive dwell tests, interestingly, those with compressive dwell show a significantly reduced life. Increasing dwell time aggravates the damaging effect manifold. The mechanism of fracture at the end of life is illustrated with fractographic characterization.

Techno-economic analysis of supercritical carbon dioxide cycle integrated with coal-fired power plant

Supercritical carbon dioxide (sCO2) cycles can achieve higher efficiencies than an equivalent steam Rankine cycle at higher turbine inlet temperatures (>550 °C) with a compact footprint (tenfold). sCO2 cycles are low-pressure ratio cycles (~4–7), therefore recuperation is necessary, which reduces the heat-addition temperature range. Integration of sCO2 cycles with the boiler requires careful management of low-temperature heat to achieve higher plant efficiency. This study analyses four novel sCO2 cycle configurations which capture the low-temperature heat in an efficient way and the performance is benchmarked against the state-of-the-art steam Rankine cycle. The process parameters (13–16 variables) of all the cycle configurations are optimised using a genetic algorithm for two different turbine inlet temperatures (620 °C and 760 °C) and their techno-economic performance are compared against the advanced ultra-supercritical steam Rankine cycle. A sCO2 power cycle can achieve a higher efficiency than a steam Rankine cycle by about 3–4% points, which is correspond to a plant level efficiency of 2–3% points, leading to cost of electricity (COE) reduction. Although the cycle efficiency has increased when increasing turbine inlet temperature from 620 °C to 760 °C, the COE does not notably reduce owing to the increased capital cost. A detailed sensitivity study is performed for variations in compressor and turbine isentropic efficiency, pressure drop, recuperator approach temperature and capacity factor. The Monte-Carlo analysis shows that the COE can be reduced up to 6–8% compared to steam Rankine cycle, however, the uncertainty of the sCO2 cycle cost functions can diminish this to 0–3% at 95% percentile cumulative probability.

Physical modeling of the M-shaped boiler furnace aerodynamics

In order to increase the thermodynamic efficiency of the Rankine cycle, in Russia, China, the USA, and EU countries, new-generation power units with advanced ultra-supercritical (A-USC) steam parameters are being developed. Using A-USC steam parameters (temperature – 700-760 °C, pressure – 35 MPa) will lead to the need to use expensive nickel steels. A single-shell design of the M-shaped boiler for burning Kuznetsk coal grade TP with solid slag removal (SSR) was proposed to reduce the total length of the main steam pipelines. A solid fuel combustion scheme with direct-flow burners, nozzles and system of vertical-horizontal tangential flames (VHTF) has been developed for this boiler, which will ensure: emissions of nitrogen oxides into the atmosphere at a level lower than standard; continuous operation of the boiler in the absence of slagging of the furnace walls and overheating of the pipe metal; a high degree of burnout of coal dust, with the provision of the standard value of the mechanical underburning of fuel; long minimum boiler load at a level not exceeding 40% of the nominal one according to the conditions of coal dust combustion stability.

Microstructural Evaluation of Welded Nickel-Based Superalloy Inconel 740H After Creep Testing

Inconel 740H® alloy is considered a leading candidate for advanced ultra-supercritical steam power generation at temperatures up to 750°C and pressures up to 35 MPa. Stable alloy microstructures for extremely long-term use are needed; however, extended exposure to high temperatures and stresses can lead to microstructural instabilities when the material is joined with other alloys, thereby decreasing component lifetime. It is, therefore, critical to understand the microstructural evolution of weldments of this alloy during high-temperature and high-pressure exposures. This research specifically aims to evaluate the effects of stress and joining processes on the lifetime of Inconel 740H welded with Thermanit 263 filler metal (based on Alloy 263™). Electron microscopy techniques were used to evaluate the evolution of $$ \gamma^{\prime } $$-precipitates, grain boundary phases, grain sizes and boundary types, weldment interfaces, and other relevant microstructural characteristics resulting from long-term exposure at elevated temperatures (650–850°C) and external stresses (53–440 MPa) during creep testing. The creep strength of Inconel 740H weldments with Alloy 263 (740H/263) as a filler metal was less than that of Inconel 740H/740H base metal because of the microstructural instability of Alloy 263 at the subject creep temperatures.

Development of Test Protocol for Accelerated Creep and Transient Thermo-mechanical Testing of AUSC Steam Turbine Rotor in High-Temperature Spin Test Rig

Rotor constitutes one of the critical components of steam turbine. Steam turbine rotor undergoes severe thermal transients during start up and shut down. When steam turbine starts up, temperature of rotor surface gradually rises with the incoming steam temperature. During this process temperature rise of central part lags behind that of rotor surface, resulting in extreme compressive stresses developed within the rotor. Similarly, the rotor surface is subjected to tensile stresses during the cooling down in the shutdown process. After a certain number of cycles, the surface or core of rotor may generate cracks due to low cycle fatigue (LCF) phenomena. Steam turbine unit for advanced ultra-super-critical (AUSC) power plant works under very high pressure and temperature, constituting a severe working environment. For steam turbine rotor, Alloy 617 M is planned to be used for the first time. Hence, it is required to test the rotor in rotating condition in a high-temperature spin rest rig before putting into actual operation. To test the AUSC turbine rotor in a high-temperature test rig, it is required to simulate the radial temperature gradient to achieve the required thermal stress in the rotor. No standard test protocol exists for the accelerated LCF testing for the rotor. A test protocol is developed for Accelerated Creep and Transient Thermo-mechanical testing of the AUSC steam turbine rotor at high temperature taking the guideline from the existing ASME standard.

Behaviour of rarified steam at very high temperature an orientation-averaged interaction potential approach towards its accurate description

We propose an orientation-averaged intermolecular potential model for the accurate computation of relevant thermodynamic and transport properties of steam at conditions typically encountered in extreme environments such as rocky exoplanetary atmospheres, aircraft and rocket engines, high temperature steam reforming and electrochemical reactors, as well as advanced ultra-supercritical steam power generators. We assess the reliability of the potential model to describe accurately the temperature dependence of relevant thermodynamic properties of rarified steam up to and beyond 3000K. Moreover, we discuss some fundamental issues including (i) the conditions at which an orientation-averaged intermolecular potential for a molecular fluid becomes an accurate representation of the fluid behaviour in extreme environments, (ii) the temperature range of validity of the approximations underlying an orientation averaging approach and, (iii) the methodology of force-field parameterisation to obtain an optimised representation, as well as the concomitant modelling of the thermodynamics and transport properties of rarified fluids. Finally, we illustrate how the simultaneous accurate description of the temperature dependence of the fluid’s collision integrals and second virial coefficient is crucial to the successful modelling of rarified steam in extreme environments.

Reducing steam transport pipe temperatures in power plants

A cycle analysis has been applied to a model of a advanced ultra-supercritical steam plant with novel steam pipes. The transfer pipes proposed incorporate internal thermal coatings and are externally jacketed to enable cooling. This enables higher temperature working steam, while keeping the pipe wall temperature below the acceptable limit for more conventional steel alloys and avoiding the need to use higher cost austenitic stainless steels and nickel base alloys. The baseline design had a superheat temperature of 700C∘ and a reheat temperature of 720C∘. A thermal coating thickness of 2.8 mm is sufficient to keep the wall temperatures of the steam transfer pipe after the supercritical boiler below 600C∘. For the transfer pipe located after the reheater a thicker coating or less ambitious reheat temperature is required to achieve acceptable pipe wall temperatures. Whereas subcritical plant has a calculated cycle efficiency of 42.1%, the elevated temperature and pressure in a customary ultra-supercritical steam boost cycle efficiency to 52.2%. Modifying this design with a thermal barrier lowers the cycle efficiency to 51.4%, still appreciably better than for subcritical plant. Alternative plant cooling arrangements might improve pipe temperatures but have minimal impact on overall cycle efficiency.

Reducing the Superheating of Extraction Stream on Advanced-Ultra Super Critical Power Plants with Regenerative Turbines in South Korea: An Economic Analysis

In this study, an advanced-ultra super critical (A-USC) simulation model was developed using the Performance Evaluation of power system efficiencies (PEPSE) software and data collected from a 500 MW ultra-supercritical (USC) coal-fired power plant in South Korea. Using the operational USC and a typical A-USC power plant steam conditions, the model analyzed the impacts of adding an additional feedwater heater (FWH) and reheater to the baseline single reheater (SR) and 8 FWH case. Through the process of introducing reheat and/or regenerative cycles, the authors found: (1) A-USC steam conditions offers an approximate 4% power plant efficiency increase in comparison to the baseline USC steam conditions and; (2) power plant efficiencies increase approximately 1.5% when a 9th FWH and double reheater are added, however; (3) this also results in an approximate 64 °C increase in the superheating of extraction stream. This excessive rise in the superheating of extraction steam was found to cause overall energy loss, reducing the overall efficiency of the power plant. Therefore, it was surmised that if the increase in the superheat degree of extraction steam from the improved steam cycle, which can effectively reduce, the efficiency of the power plant could be further improved. To determine the efficiency variations based on the reduction of the superheat degree of extraction steam, the authors applied a regenerative turbine (RT) to the model. Introducing the RT to the A-USC DR and 9 FWH was found to decrease from the average extraction steam temperature from 221 °C to 108 °C and result in an increase in power plant efficiency of approximately 0.3% to 49.5%. An economic analysis was also performed to assess the fiscal feasibility of adding an RT. Assuming the initial investment to be USD 1409 million, implementing an RT equated to a net present value increase of approximately USD 33 million as compared to that of similar life (30 years of durability) expectancy of A-USC without using an RT. The findings of this study have the potential to improve South Korea’s energy policy, reducing the superheat degree of extraction steam that rises excessively during A-USC steam condition optimization. While this study is focused on South Korea, said findings are also generalizable to worldwide energy policies, serving as an effective method to not only increase system efficiencies, but enhance the economic feasibility as well.

Development of heat-resistant Fe-based alloys for A-USC steam boiler using ultra-high purity (UHP) technology

The design of ultra-high purity (UHP) Fe-based alloys for advanced ultra-supercritical (A-USC) technology is suggested in this work. The creep testing at 700 °C and a stress level of 150 MPa have been performed in air. The addition of small amount of reactive-elements, W, Nb, Zr and Sc, has been undertaken into model alloys with the intention to promote the precipitation strengthening. Optimizing the chemical composition coupled by pre-straining enabled improving the creep resistance; hence a creep-rupture life of 750 h was reached. The oxidation resistance in supercritical water is remarkably enhanced by small addition of Sc.

Effect of stress and atmosphere on grain boundary oxidation of Ni-based superalloy for 700°C A-USC steam turbine

700°C class A-USC (Advanced Ultra Super Critical) thermal power plant is one of the solutions to reduce carbon dioxide (CO2) emission. In order to meet the design requirement, which is capable to be operated under 700°C steam condition, the development of Ni-based superalloys is essential. Fundamental research and demonstration of mock-up plants have been driven by national project in Japan, US, Europe and China, to confirm the reliability of Ni-based superalloys. One issue of superalloys reported by others is the stress accelerated grain boundary oxidation (SAGBO), leading to the deterioration of creep rupture properties during service. We newly manufactured the test apparatus which enables to creep test under high temperature steam environment, and evaluated the grain boundary oxidation behavior of Ni-based rotor material, TOS1X-2, under various stresses and atmosphere. Microstructure observation after creep tested at 750°C for 2,500h, found that thin layer of Cr-rich and Al-rich oxides form at all stress and atmosphere condition. Detail studies revealed that the Al-rich oxide were formed at fine, recrystallized grain boundary, which is driven by initially induced strain by machining. The quantitative analysis of microstructure showed that steam atmosphere slightly accelerates the grain boundary oxidation. On the contrary, effect of applied stress on oxidation depth was not observed. The grain boundary oxidation observed in this study was very small, suggesting that TOS1X-2 exhibits good oxidation and creep rupture properties in 700°C A-USC operating condition.

Residual stress driven cracking in superalloy weldments

Superalloy weldments are normally given post weld heat treatments to homogenize the weld metal microstructure, relieve residual stress, and precipitate strengthening phases. The relationship between microstructure and post weld heat treatment is easily studied; it is less straightforward to study the effects of post weld heat treatment on residual stress relaxation. Using a self-restrained testing procedure, a relatively simple approach was used to investigate the effects of microstructure and post-weld heat treatment on cracking during residual stress relaxation. Candidate superalloys for Advanced Ultra Supercritical steam plants were studied. It was found that cracking due to residual stress relaxation is primarily dependent on grain size, and in cases of intermediate grain size, intragranular precipitation is a controlling factor. These results are in agreement with traditional stress relaxation cracking theories.

Creep deformation and rupture mechanism of an advanced wrought Ni[sbnd]Fe-based superalloy for 700 °C class A-USC steam turbine rotor application

The 700 °C class Advanced Ultra-Super-Critical (A-USC) technology with higher thermal efficiency is being developed for next-generation coal-fired power plant in recent years to reduce environmental impacts of energy consumption. A new wrought NiFe-based superalloy with excellent high creep strength and low cost has been developed recently and evaluated as a candidate material for the 700 °C class A-USC steam turbine rotor application. Creep tests were conducted on the new NiFe-based alloy at 700 °C/200 MPa, 700 °C/250 MPa and 725 °C/150 MPa using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The creep-rupture testing results show the new NiFe-based alloy has better creep-rupture strength than the existing commercially available NiFe-based and even some Ni-based alloys. The focus of this study is on understanding the relative important causes of better creep resistance and reduced rupture life in grain interiors and grain boundaries. Detailed creep deformation behaviors and related creep-rupture mechanisms were analyzed and discussed to explore the potential approaches for performance improvements. The results presented are helpful in providing a new approach to design and develop novel high performance alloys for A-USC steam turbine rotor application.

Gradient-free methods applied to optimisation of advanced ultra-supercritical power plant

Thermo-economic analysis and optimisation of thermal systems have become a key solution in providing a better system configuration on the design stage and in modification of already-working installation. Nowadays, there are many commercial software packages allowing one to perform 0-dimensional (0D) analysis of any power plant system. Some of these programs are equipped with their own optimisation tools; however, as is often the case, most of them are not able to carry out a multi-variable optimisation with a large number of decision variables. This paper presents application of the integrated package containing the IPSEpro and MATLAB software for numerical optimisation of advanced ultra-supercritical steam power plant. Searching for new technology makes it necessary to analyse the structure of the thermal cycle. This can be obtained by combining the numerical modelling with the multivariable optimisation of such an object. For that purpose a new concept of 900 MW advanced ultra-supercritical power unit was examined. To increase the efficiency and flexibility of calculations three different non-gradient methods were introduced into the code, i.e., Nelder–Mead, Hooke–Jeeves and Rosenbrock. It was shown that all of the three considered methods can find their usefulness in the optimisation process of power plant thermal cycle giving the rise in the efficiency by about 0.34 percentage points. The additional significant advantage of the Rosenbrock method is the lowest cost of computation, almost independent of the number of decision variables considered. It was shown that the gained efficiency improvement was accompanied with serious intervention into the thermal cycle configuration (in particular into operating pressures) which can lead to necessity of modifying the major power plant components.