Ultra-supercritical Unit Research Articles

Study on Mechanical Properties of P92 Steel Welded Joint of the Main SteamPipe of Ultra Supercritical Unit After 50,000 Hours Service

Combined with a scanning electron microscope, this paper carried out a tensile test and a bending test to study the mechanical properties of P92 steel welded joint of the main stream pipe after the joint was in service for 50,000 hours. The results show that the tensile strength and hardness of the P92 steel weld metalare higher than those of the base metal after it was treated at a high temperaturefor 50,000 hours, but the plasticity is lower than that of the base metal. This is related to the precipitation and growth of Laves and M23 C6 phases in the steel during treatment; after the metal was treated at a high temperature for 50,000 hours, the number of Laves phases is larger and the size is bigger than that in the base metal. The Laves phases are precipitated rapidly along the austenite grain boundary and martensite lath boundaryand then distributed in the grain boundary, lath boundary, and plate strip boundary, reducing the strength and toughness of the matrix.

Review of oxide scale on high temperature heating surface in boiler

The problem of oxide scale has always been a problem that hazards to the safe and stable operation of ultra-supercritical units, and is also the focus of equipment manufacturers, research institutes and production units. Therefore, the correct understanding and constructive result of investigation on the oxide scale’s origin, damages and formation has great guiding significance for the long-term operation of the unit.

Flexibility Enhancement for Coordinate Control System of 1000MW Ultra-Supercritical Unit by Optimized Error-Based ADRC Approach

Flexibility Enhancement for Coordinate Control System of 1000MW Ultra-Supercritical Unit by Optimized Error-Based ADRC Approach

Research on the status and priority needs of developing countries to address climate change

Abstract Identifying what developing countries need to address climate change is of great significance for promoting North-South and South-South climate cooperation and implementing the Paris Agreement. In this study, based on questionnaires surveys of 143 representative government officials, experts, scholars, and industry engineers from developing countries who have been engaged in climate change, the current situation and priority requirements of developing countries in terms of policies and actions, technology, financing, capacity building, and international cooperation for addressing climate change were systematically analyzed. We found that 1. Most developing countries have already taken national general actions and sectoral and industry-level actions focus on renewable or clean energy, waste management and recycling, sustainable urban transport and forestry carbon sequestration. 2. The demands for climate change mitigation are mainly concentrated in technology and capital, and the priority areas are energy and electricity and waste management; for climate change adaptation, the demands differ significantly among regions. 3. Developing countries show high preferences for emission reduction and energy-saving technology, solar energy, wind energy and bio-energy, whereas a low preference for supercritical and ultra supercritical power units, nuclear energy and tidal energy. 4. Agroforestry, energy conservation and efficiency, renewable energy and water resources are priority areas for climate change financing. 5. Building institutional capacity, improving technology R&D networks and institutions, formulation and implementation ability of planning schemes, and data statistics and verification are priority areas needed for capacity building. To better address climate change, developing countries need to establish and improve information exchange channels or platforms and strengthen South-South climate cooperation while optimizing the allocation of climate change resources according to the priority needs.

Impacts of the coal resource tax on the electric power industry in China: A multi-regional comprehensive analysis

A coal resource tax is a common policy tool for promoting rational exploitation and utilization of resources. This study establishes a co-optimization model for both electricity and coal that not only depicts the power dispatched at the micro temporal scale but also considers the capacity expansion that occurs at the macro temporal scale and the spatial dispatch of coal. Based on this model, we investigate the impact of the coal resource tax on China’s electric power industry from four perspectives: economic efficiency, generation portfolio, inter-regional energy dispatch, and environmental benefits. The results show that the current tax rate has a relatively limited impact on the power system. Increasing the tax rate will inhibit the construction of supercritical and ultra-supercritical units and accelerate the diffusion of solar PV, small hydropower, and nuclear units. At the regional level, East China is most affected by the change in the resource tax, and other northern provinces are affected to varying degrees. The coal resource tax has little impact on inter-regional power grid investment, but it does affect the utilization efficiency of existing lines and reduces the volume of almost all coal transportation routes. Thus, it is necessary to consider the relationship between various policies and plans during decision-making.

Evolution in Microstructure and Mechanical Properties of Inconel 783 Alloy Bolts after Long Term High-Temperature Aging at 700 °C

This paper described systematically the changes in microstructure and mechanical properties of Inconel 783 alloy after a considerably long time (equivalently 55,000 h, about 76.4 months) of thermal exposure. Based on the Inconel 783 alloy bolts of an intermediate pressure main stop valve used in a 1000 MW ultra-supercritical unit, the evolution of microstructures and mechanical properties were studied after 700 °C aging temperature with different aging times (1000 h, 3000 h and 20,000 h, corresponding to about 1.4 months, 4.2 months and 27.8 months, respectively), using an optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray diffractometer (XRD), a universal tensile testing machine and impact testing machine. The results indicated that the bolts aged for 1000 h in two temperatures, showing the second needle β phase, of which the quantity and size obviously increased with aging time. Meanwhile, the characteristics in quantity and shape of the primary β phase changed obviously with the aging time, which transformed to strip the Ni5Al3 and Laves-Nb-rich brittle phase in the matrix after aging for 20,000 h. The size of the γ’ phase grew bigger with aging time, and orientation distributions have been observed obviously at 3000 h aging in 700 °C. Compared with the 650 °C aging temperature, the coarsening of γ’ precipitates and second needle β, the orientation distributions of γ’ were more obvious at the 700 °C aging temperature with aging time, which resulted in the rapid decline in yield strength and tensile strength and obvious increase in the brittleness for Inconel 783 alloy bolts.

Hidden danger and Optimization for the DEH Logic of STC-Siemens Ultra Supercritical Unit

With the wide application of STC-Siemens ultra supercritical unit technology, the structure design and control logic of the turbine electro-hydraulic control system (DEH) are also popularized. However, due to the defects of Siemens DEH logic design and the unavoidable mistakes of STC auto control center in the process of translating Siemens DEH logic, some hidden dangers of this type of unit are gradually exposed, which affects the reliability and safety of the unit. It is mentioned in this paper that due to the hidden trouble of ATT test logic, during the process of ATT test, 12% command signal of regulating valve setting is sent out, and the unit abnormal shutdown; When the unit is put into the AGC mode to track the AGC command, the logic hidden danger causes triggering of the quick closing condition C20 of the regulating valve (the deviation between the flow instruction converted by the valve feedback and the actual flow instruction is greater than 40%), and the unit abnormal shutdown. In order to solve these problems, the optimization suggestions of DEH logic are put forward, in order to improve the reliability of unit operation. It provides a reference for this type of unit.

Prediction of Carbon Content in Fly Ash in Ultra Supercritical Generation Unit with Long Short-term Memory Neural Network

The accurate measurement of carbon content in fly ash is the basis of improving the thermal conversion efficiency of boiler and reducing the coal consumption of power generation. Based on the operation data of 1000MW Ultra-supercritical Unit in a thermal power plant, the input parameters of the model were selected by correlation analysis. The Long Short-Term Memory neural network (LSTM) was used to establish a prediction model with the boiler operation parameters as the input and the carbon content of the boiler fly ash as the output. According to the actual operation data of the unit, the prediction performance of the model was trained and tested. The simulation results show that the average prediction error of the LSTM model is 2.8136%, and 86% of the total samples are data with error less than 4.5%, which means it has high accuracy and strong generalization ability.

Analysis on the microstructure and properties of P92 steel after long-term service at high temperature

SA335-P92 steel is a kind of heat-resistant steel with martensitic lath as matrix and multi alloy composite strengthening, which has excellent high temperature performance. It is used for high temperature components of high parameter units in China for nearly 20 years. Many research institutions have done a lot of research on the properties and characteristics of high temperature aging and creep durability in the laboratory, but there are few researches on the micro-characteristics and properties of P92 steel after long-term service. In this paper, the studies are carried on the P92 steel of high temperature reheat steam pipeline taken from the first group of ultra-supercritical units in China. The microstructure characteristics and properties of the material are systematically studied by means of test methods such as micro metallography, scanning electron microscopy, EDS and mechanical properties. It is concluded that although the P92 steel after long-term service still remains the microstructure of martensitic lath, but the toughness greatly reduces caused by large amounts of large scale precipitates at the original austenite grain boundaries and lath boundaries.

Effect of aging at 650 °C on microstructure and mechanical properties of GH783 alloy bolt

Taking the GH783 alloy bolt of intermediate pressure main stop valve used in 1000 MW ultra-supercritical unit as the research object, the optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffractometer (XRD) and mechanical performance tests were carried out to study the microstructure transformation and mechanical properties of GH783 alloy bolts at different aging stages under 650 °C aging environment. The results showed that the secondary needle β phase began to appear in the bolts aged for 1000 h, and the density of secondary β-phases in bolts aged for 3000 h significantly increased. When the aging time reached up to 20,000 h, the secondary β phase coarsened significantly. At the same time, as the aging time prolonged, the morphology and quantity of the primary β phase changed significantly. In terms of mechanical properties, the tensile strength and impact toughness of the bolts gradually decreased. This was due to the growing γ' phase, the formation of Ni5Al3 phase and the Nb-rich Laves phase in the structure during the aging process.

Analysis and Evaluation of Multi-Energy Cascade Utilization System for Ultra-Supercritical Units

To address the large temperature difference in the air heater (AH) inlet of a traditional exhaust heat utilization system and energy grade mismatch problems during the heat and mass transfer processes, this study proposed a new multi-level waste heat cascade utilization system. Based on a principle of “temperature-to-port and cascade utilization”, this system uses the boiler side high-temperature flue gas and low-temperature air, and the turbine side high-temperature feed water and low-temperature condensate water, to conduct cross heat exchange according to the energy grade matching principle. Combined with a typical 1000 MW coal-fired unit, the heat transfer characteristics and energy-saving benefits of the new system were analyzed. The results showed that the new system has excellent performance: the heat rate decreased by 91 kJ/kWh, coal consumption decreased by 3.3 g/kWh, and power generation efficiency increased to 49.39%.

Effect of the microstructure evolution on the high-temperature strength of P92 heat-resistant steel for different service times

Grade 92 heat-resistant steel is an important material for ultra-supercritical units. The degradation of the P92 properties affects the safety of structural components during long-term service. In this paper, the microstructural evolution of P92 depending on different service times was quantitatively studied using scanning electron microscopy and transmission electron microscopy. The effect of each strengthening mechanism on the strength of P92 is discussed. The results show that the dimensions of M23C6 and Laves phases increase with increasing service time, but the growth rate of the Laves phase is higher than that of M23C6 and the size of MX carbonitrides remains almost unchanged. The width of the martensite lath increases from the initial 350 nm–750 nm after 70,000 h of service. The dislocation density significantly decreases and then stabilizes with increasing service time. The high- and room temperature tensile properties worsen and then stabilize during long-term service. The thermal yield stress of P92 during service was calculated. The results show that the high-temperature strength of P92 steel is mainly affected by lath and dislocation hardening during the initial service time; as the service time increases, the strengthening mechanism transforms into lath and precipitate hardening.

Modeling and Control of Supercritical and Ultra-Supercritical Power Plants: A Review

This paper presents a critical review of the research conducted for modeling and controlling supercritical power plants. Thermal power plants are classified according to the boiler pressure to supercritical and subcritical. The modeling concepts and control strategies of supercritical generation units are far more complex than those of subcritical. On the other hand, supercritical generation technologies are more efficient and much cleaner than subcritical generation units. From a deep technical analysis of the literature, there is no review that is dedicated to models-based control of supercritical power plants and most previous reviews are found to be too general to modeling-based control of fossil fuelled energy sources. This review reports the advancements on modeling and control of supercritical and ultra-supercritical plants as cleaner generation technologies. The various published achievements for modeling supercritical and ultra-supercritical units have been reviewed. The control strategies that fulfill the practical load demand requirements while keeping optimum efficiencies are also reviewed. Finally, expected future directions are reported as recommendations to overcome future challenges. The paper can be used as a brief educational directory to the postgraduate students or future researchers in the field.

Data-driven model identification of boiler-turbine coupled process in 1000 MW ultra-supercritical unit by improved bird swarm algorithm

Ultra-supercritical units with high steam temperature and pressure have been widely employed in thermal power plants due to their high efficiency and environmental friendliness. To ensure efficient unit operation, it is necessary to develop a model for the boiler-turbine coupled process of the unit. The present models may be complicated or non-transparent, preventing their practical application. In this work, the model structure is a transfer function matrix by dynamic analysis, and the model parameters are obtained by the proposed data-driven multivariable model parameters intelligent identification scheme with the cloud adaptive chaotic bird swarm algorithm combining sheep optimization and lion swarm optimization. By employing operational data from 1000 MW ultra-supercritical unit, extensive experimental results were given to show that the model identified was reasonable and accurate by comparison tests between identification outputs and process outputs around 800 MW operation condition. The developed model can provide reference for further control strategy synthesis and performance optimization.

Modeling of an ultra-supercritical boiler-turbine system with stacked denoising auto-encoder and long short-term memory network

The ultra-supercritical (USC) coal fired boiler-turbine unit is an advanced power generation system with low emissions and high efficiency. It is also a typical multivariable nonlinear system with great inertia. Generally, building an accurate analytic model using the conventional system identification methods are quite difficult. However, the big data generated by the monitoring system can reflect the USC unit's operation status and reveal the internal mechanism, if appropriate data analysis methods are developed. A deep neural network (DNN) is proposed in this paper to model a 1000 MW USC unit. In this DNN, stacked denoising auto-encoder is adopted to obtain the intrinsic features from the input data, while the long short-term memory network is in charge of outputting the expected normal behaviors of USC system along the time axis. Furthermore, to guarantee the convergence of this network, a reasonable intensity of added noise is identified via Lyapunov stability method. The DNN model is compared with the traditional multi-layer perception network, the stacked denoising auto-encoder, and two other random neural networks, to show the advantages in forecasting the dynamic behavior of USC unit.

The high-temperature retrofit research and application of subcritical units

Under the pressure of reducing CO2 and pollutants emissions, The Chinese Government requires that all the existing coal-fired power units shall achieve an average net efficiency higher than 39.6% (LHV) by 2020. Considering the fact that the typical annual average net efficiency level of subcritical units is about 37%, some are even lower than 35%, it seems to be overambitious to achieve this target. The high temperature retrofit of subcritical units, however, find a technically and economically feasible solution for this problem, mainly on 4 aspects: 1) By improving the main and reheat steam temperatures from 538 °C to 600 °C, both the cycle efficiency and turbine internal efficiency are greatly improved. 2) By keeping the main and reheat steam pressures unchanged, combined with innovative boiler heating surfaces retrofit, the pressure parts of the boiler and feedwater system are utilized as much as possible thus the 600 °C steam temperatures are achieved with reasonable cost. 3) Combined with other energy-saving technologies, like flue gas heat recovery technology and generalized regeneration technologies, the unit efficiency is further improved under rated load, and even larger under part load. 4) By applying special startup technology and other efficiency preservation technologies, the solid particle erosion problem is suppressed thus the unit can operate efficiently in long term. Now the high temperature retrofit is being conducted in a typical 320 MW subcritical unit in Xuzhou, China. The target net efficiency under rated condition is higher than 42.36%, much better than those of all existing supercritical units and even some ultra-supercritical units. This project is expected to be a demonstration, setting a benchmark for the high efficiency retrofit of subcritical units.

Research and Application of Multiobjective Robust H2 / H ∞ Fuzzy Control Algorithms for Double Reheat Steam System

In order to ensure the tracking and anti-interference performance of the second reheat steam system of the ultra supercritical unit, a multi-objective robust H 2 / H ∞ fuzzy tracking control scheme is proposed. T-S model is used to describe the reheat steam system, and the problems of state observation and control error are considered in the design of control strategy. The control design problem is transformed into a multi-objective constraint problem. The design process and robustness of the method are illustrated by a simulation example.

Deep-Neural-Network-Based Economic Model Predictive Control for Ultrasupercritical Power Plant

The dynamic economic optimization of the ultrasupercritical (USC) boiler-turbine unit has become an important task in modern power plants. Economic model predictive control (EMPC) has recently developed to be a promising method for realizing the dynamic economy. This EMPC essentially requires a highly reliable model for USC dynamic prediction which could reflect the internal mechanism of USC with big data feature. This article constitutes a deep-neural-network-based EMPC for the USC unit. Deep belief network (DBN) is used to model the USC unit with mathematical structure. To overcome the nonlinearity and time delay existing in the pulverized channel, an augmented model with predictor embedded is also incorporated into the EMPC design. The auxiliary controller and stability region have been constituted to guarantee closed-loop stability. Simulation results on a 1000-MW USC unit fully demonstrate the effectiveness of the proposed DBN-based EMPC.

Cause Analysis of Type IV Cracking of Main Steam Pipe of Ultra Supercritical Unit

In order to explore the cause of cracking of the main steam pipe of an ultra-supercritical unit, A series of corresponding physical and chemical tests for crack failure pipelines. Including chemical composition analysis, room temperature tensile test, high temperature tensile test, impact test, bending test of HAZ(Heat Affected Zone)and base metal, it’s also including metallographic observation and section scanning electron microscopy analysis of crack tip, overall force analysis of pipeline, etc. The results show: Failure crack has obvious characteristics of crack type IV. for example, a large number of creep holes appear in the fine-grained region of the HAZ, Creep holes accumulate, grow up, and form microcracks. In addition, the stress analysis shows that the outer arc side of the pipe is subjected to large tensile stress. In conclusion, the pipe cracking failure is caused by the combination of tensile stress and type IV crack.

Research Progress on High-temperature Oxidation of Austenitic Stainless Steel for Ultra-superCritical Boilers

In this paper, four typical austenitic heat-resistant steels (TP304H steel, TP347H steel, Super304H steel and HR3C steel) are taken as examples to review the research progress of high-temperature oxidation of austenitic stainless steel for ultra-supercritical unit boilers, and the mechanism and protective measures of high-temperature oxidation of austenitic stainless steel are discussed, so as to provide references for material improvement and safe use of unit boilers.