Supercritical Power Unit Research Articles

A real-time phase transition modeling of supercritical steam cycle and load variation rate enhancement of thermal power plants under deep peak shaving

The ambitious green revolution to renewable energy sources in global power grids necessitates massive integration of solar and wind energy, which involves intermittent and unpredictable challenges. Thermal power plants are crucial in stabilizing the grid and addressing these challenges through flexibility reformation including deep peak shaving and frequent load variations since the unsteady state energy transfer and thermal dynamics during combustion and heat transformation in thermodynamic processes vary significantly. These conditions lead to issues such as furnace instability and latent heat of phase transition. This study introduces a novel approach to modeling phase transitions of supercritical steam cycle, and investigatesthe length, position, temperature, and energy transfer of the working medium and components under normal and low operational states. Conducting and analyzing the thermal feasible region associating the security of components and working medium this study establishes a control strategy for dynamic heat transfer to reduce component degradation effects andenhance load variation rate under flexible operations. Simulation model of a supercritical power unit based on the proposed method demonstrates an accuracy of 97.94%. Results from the optimal approach in maximizing load variation rate show the effectiveness and achieve 1.2%p.e./min most under transition process.

Environmental, technological, and economic analysis of supercritical coal-fired power system.

Developing countries primarily rely on fossil-based energy sources to meet their energy demands. The use of fossil fuels has several adverse environmental repercussions that damage the biosphere both directly and indirectly. Among fossil fuels, coal brings about the heaviest environmental externalities, yet its abundance makes its use widespread, particular in countries having significant power generation deficits, such as Pakistan. This study presents an environmental, technological, and economic analysis of a supercritical coal-based power unit located in Pakistan and used for electricity generation. For environmental assessment, the CML-1A baseline method in OpenLCA software was used, and eight midpoint impact indicators were selected. The functional unit chosen was 1 MWh of generated electricity. The results indicated that the category of ozone layer depletion has the least impact, whereas global warming potential has the highest impact score. Except for photochemical oxidation and human toxicity, the plant operational stage dominated most of the selected impact categories. The current paper also reveals that the removal efficiency of CO2 and other pollutants is higher in supercritical compared to subcritical plants. Moreover, the economic feasibility of supercritical plant is compared with chemical looping combustion (CLC)-based supercritical coal-fired power plant, and results shows that CLC-based coal-fired power plant is a more competitive and environmentally friendly option. The utilization of a scientific cleaner energy-management system in real-time, as exemplified in this study, may facilitate the development of a optimalpolicy framework that encouragesfor the adoption of cleaner coal power generation in developing countries, ultimately resulting in improved energy sustainability. Furthermore, this paper also presents some policy implications which could be helpful for policymakers, researchers, and industrialists to improve the sustainability of energy in emerging economies.

Thermodynamic analysis of the air separation unit and CO2 purification unit in the semi-closed supercritical CO2 cycle with nearly zero emission

The natural gas-fired semi-closed supercritical CO2 cycle is an efficient oxyfuel combustion power generation technology featuring nearly zero emissions. This study investigated the interaction of the semi-closed supercritical CO2 power unit with the air separation unit (ASU), which supplies O2 for combustion and adiabatic compression heat for the heat recovery process in the power unit, and the CO2 purification unit (CPU), which refines the CO2 in the exhaust gas to 99.97% for enhanced oil recovery (EOR) applications. The detailed thermodynamic models of the power unit, ASU, and CPU have been constructed to explore the impact of variations in oxygen purity and excess oxygen coefficient on the overall thermodynamic performance of the system. The results demonstrate that the net electric efficiency peaks at an oxygen purity of 98%, and a one percent decrease in oxygen purity reduces the net electrical efficiency by approximately 0.36% to 0.97% points. Moreover, the net electric efficiency decreased by 0.10% points when the excess oxygen coefficient increased from 3.00% to 13.67%. These findings may offer valuable guidance for developing oxyfuel combustion power generation technologies.

Research on Fault Diagnosis of Thermal Power System of Thermal Power Unit under Different Loads

Aiming at the problem of difficulty in acquiring fault data under different loads, a domain-adversarial-based fault diagnosis method for convolutional neural networks is established by combining convolutional neural networks with adversarial domain adaptive methods. The method uses convolutional neural networks to extract the fault features of the source and target domain samples, and simultaneously implements joint adjustment of the distributions of the domain-level and class-level features, aligns the feature distributions of the source and target domains by minimizing the difference in the distributions of the two domains through adversarial learning of the domains and aligns the feature distributions of each class of faults by making the distributions of the same kind of features closer through the loss of the central discriminant. With the help of a 600MW supercritical thermal power unit full-working condition simulation system, experiments are carried out under 100%, 90%, 80% and 70% of the rated load, and the results show that the method has effectiveness and superiority in the fault diagnosis of thermal system under different loads.

CO2 capture from flue gases in a temperature swing moving bed

This paper presents a test stand for the capture of CO2 from flue gases arising due to firing pulverised hard coal. The stand, financed from the 2014–2021 Norway Grants, is installed at a Polish power plant. The innovation of the proposed CO2 capture method, developed by the Norwegian partner in the project (SINTEF Industry), lies in the use of activated carbon in the process of temperature swing adsorption in a moving bed. The paper also presents preliminary results of numerical simulations performed using the General PROcess Modelling System (gPROMS) software. The simulations concerned the operation of a supercritical power unit combined with a system for capturing CO2 from flue gases. Transient operation of the system was analysed, assuming rapid changes in the power unit load. Special attention was paid to the CO2 capture process energy consumption at an increase in load by 5% of the power unit nominal capacity in 30 s. It is found that the proposed CO2 capture method “keeps up” with such rapid load changes at the method energy consumption smaller than 2 MJ/kg CO2.

Repowering a Coal Power Plant Steam Cycle Using Modular Light-Water Reactor Technology

This article presents the results of a techno-economic analysis of repowering a coal-fired power plant’s steam turbine system to instead accept heat produced by a pressurized water reactor-type small modular nuclear system (PWR SMR). This type of repowering presents a challenge due to the significantly lower steam pressure and temperature produced by the nuclear system. A 460 MW supercritical power unit with steam parameters of 28 MPa/560 °C/580 °C, operated in the Łagisza Power Plant in Poland, was selected for the analysis. After repowering, the turbine system would be fed with saturated steam from the steam generators of the SMRs at a pressure of 7 MPa and a temperature of 285 °C. In total, four options for repowering were analyzed. In all cases, the existing high-pressure section of the turbine was disconnected, and the existing low-pressure stages of the turbine, as well as all auxiliary and outward components (feedwater heaters, pumps, generator, condenser, condenser cooling, etc.), are re-used in their existing configurations, except for a feedwater-heater pump that needs to be replaced. In three cases, the existing intermediate pressure turbine section acts as the high-pressure stage of the repowered system. These cases include repowering without an additional reheater (case A), with an added single-stage reheater (B) and with an added two-stage reheater (C). In the fourth case (D), the existing intermediate pressure section was replaced by a new high-pressure turbine stage suited to the SMR live steam conditions. While all four repowering options are technically possible and may represent an economic advantage compared to a complete greenfield SMR installation, option D with a new high-pressure stage is clearly the best option available, with significant cost savings, leading to a lower levelized cost of electricity (LCOE) and a higher net present value (NPV) and net present value ratio (NPVR) than the greenfield case and all other repowering. For relatively new coal power plants with equipment in good condition, this type of repowering may present a cost optimal near-term pathway.

Research on active disturbance rejection control of the main steam temperature of supercritical thermal power unit based on improved particle swarm optimization

The stability of the boiler’s main steam temperature is very important for the safe operation of the unit. At present, the mainstream PID control scheme is simple to use and convenient to set, but it cannot accomplish a acceptable ascendancy effect in the face of the main steam temperature control structure; with a long time delay, large inertia. In view of this situation, first of all, this paper introduces the linear active disturbance rejection control (LADRC) technology in a steam temperature control structure. The main loop adopts the linear active disturbance rejection controller and the subloop adopts the P-controller. Then, the particle swarm optimization algorithm is improved and used to modify the parameter of the LADRC, so that the LADRC system adjusted according to the theory can better complete the control task. Finally, a simulation to control the temperature is given, and the results show that the LADRC control system can track quickly and stably; in addition, as against with this traditional PID control strategy, it can achieve strong anti-interference performance.

Assessment of the Hot-Cracking Susceptibility of Welded Joints of the 7CrMoVTiB10-10 Bainitic Steel Used in Heat Exchangers

Bainitic steel containing approx. 2.25% Cr and 0.6 Mo with micro-additions of V, Ti, and B, designated as 7CrMoVTiB10-10 (T/P24), is one of the new construction materials used in new supercritical power units. The weldability of 7CrMoVTiB10-10 is defined as the hot-cracking susceptibility, it should be stated that the hot cracking in the welded joints of 7CrMoVTi10-10 is determined by phenomena occurring in the high-temperature brittleness range (HTBR). In this work, the HTBR is established for both the base material and welding conditions, taking into account the critical temperature-strain intensity (CST) and the critical strain speed (CSS). The HTBR for 7CrMoVTi10-10 is 122 °C wide and covers temperatures from 1394 °C to 1516 °C. Under imposed deformation conditions (typical during welding), the HTBR extends to a width of 293 °C towards lower temperatures, i.e., from 1516 °C to 1223 °C. The CSS = 0.83 1/s, the CST = 0.003 1/°C, and the Rf index = 0.12, which can be adopted as the criteria for the susceptibility of 7CrMoVTiB 10-10 to hot cracking under imposed deformation conditions. The hot cracking in 7CrMoVTiB10-10 occurs as a result of the loss of cohesion by the thin liquid layer crystallising between the growing weld crystals. Such cracks appear when the CSS or the CST is exceeded within the HTBR.

Offline reinforcement learning control for electricity and heat coordination in a supercritical CHP unit

With a high proportion of renewable energy generation connected to the power grid, the combined heat and power (CHP) units need to have flexible operation and control capabilities over a wide range of variable load conditions. In China, the proportion of supercritical combined heat and power (S-CHP) units gradually increases due to their high thermal efficiency. In this paper, we propose a data-driven environment modeling method and an offline reinforcement learning-based electricity–heat coordinated control approach for the wide and flexible load adjustment capabilities of the S-CHP unit. First of all, a modeling method based on the multiple multilayer perceptron (MLP) ensemble is proposed to address the possible over-fitting produced by a single MLP. Then, the state and reward are set considering the dynamic characteristics of the S-CHP unit. Moreover, policy training is implemented through a soft actor-critic algorithm with a maximum entropy model to ensure more robust search capabilities under variable load conditions and targets. The simulation results show that the generalization ability of the environment in the multiple MLP ensemble mode is more substantial than that in the single MLP mode. In addition, when the electric load command is between 267 MW and 325 MW, the offline reinforcement learning can significantly reduce the integral of absolute error matrices of the output parameters, demonstrating that the proposed strategy can achieve electricity–heat coordinated control under variable load conditions.

Typical Damage Prediction and Reliability Analysis of Superheater Tubes in Power Station Boilers Based on Multisource Data Analysis

The superheater and re-heater piping components in supercritical thermal power units are prone to creep and fatigue failure fracture after extensive use due to the high pressure and temperature environment. Therefore, safety assessment for superheaters and re-heaters in such an environment is critical. However, the actual service operation data is frequently insufficient, resulting in low accuracy of the safety assessment. Based on such problems, in order to address the issues of susceptibility of superheater and re-heater piping components to creep, inaccurate fatigue failure fracture, and creep–fatigue coupling rupture in a safety assessment, their remaining life prediction and reliability, as well as the lack of actual service operation data, multisource heterogeneous data generated from actual service of power plants combined with deep learning technology was used in this paper. As such, three real-time operating conditions’ data (temperature, pressure, and stress amplitude) during equipment operation are predicted by training a deep learning architecture long short-term memory (LSTM) neural network suitable for processing time-series data and a backpropagation through time (BPTT) algorithm is used to optimize the model and compared with the actual physical model. Damage assessment and life prediction of final superheater tubes of power station boilers are carried out. The Weibull distribution model is used to obtain the trend of cumulative failure risk change and assess and predict the safety condition of the overall system of pressurized components of power station boilers.

Modern heat-resistant 11%Cr martensitic steel for power industry

Purpose: The purpose of the investigations was to characterise and describe the modern creep-resistant Thor 115 steel for supercritical power units. Design/methodology/approach: The investigations were performed based on the knowledge and experience of the author, the preliminary results of his own research and the available literature on the subject-matter of the considerations presented in the paper. Findings: Modern steel Thor 115 was subjected to overall analysis in terms of its use as a potential heat-resistant structural material for power plant components. Based on the preliminary results of own research and the available literature data, it has been shown that the analysed steel may be a structural material of full value if the assumed creep resistance in the service temperature range of 600-650ºC is confirmed. Research limitations/implications: The comprehensive analysis of degradation of microstructure of the steel after ageing (and/or creep) requires TEM examinations. Finding the correlation between the creep and ageing conditions and changes in the microstructure of the steel. Practical implications: The investigations carried out as part of the paper and the considerations on the subject-matter of the analysed steel may be the basis for the development of a database of material characteristics for steels, alloys and welded joints. Originality/value: The analysis of chemical composition, heat treatment and mechanical properties and the investigations of microstructure of Thor 115 steel are presented.

Creep deformation and rupture behavior of 10Cr-3Co-2W heat-resistant steel weldments in ultra supercritical power units

Creep deformation and rupture behavior of a martensitic heat resistant steel (10Cr-3Co-2W) welded joints prepared by vacuum electron beam welding for supercritical power generation unit are investigated employing traditional uniaxial tensile creep, 898 K (625 ℃) to 948 K (675 ℃) and 105 MPa to 250 MPa, and room temperature nanoindentation on different welding micro-zones. The rupture location shifts from base metal (ductile rupture) for the shortest creep time of 74 h, to heat affected zone (brittle Type IV rupture) for the rest crept specimens. Accordingly, a softening phenomenon is observed in fine grain heat affected zone in crept specimens for longer creep times, where voids, precipitates and reduced dislocation density are found and further discussed. The extrapolating creep rupture time under various creep temperature and stress combinations and the 100,000 h creep rupture strength for different temperatures are predicted employing the Larson-Miller parameter method, which satisfies the industry service condition. Compared with tensile creep the nanoindentation creep yields faster creep rates and higher stress exponents; and the largest strain rate sensitivity value (m) in weld metal implies that weld metal has the best creep resistance while heat affected zone and base metal are more likely to appear creep rupture.

Flexibility oriented adaptive modeling of combined heat and power plant under various heat-power coupling conditions

The flexible operation of thermal power plant has become a research hotspot for its safety and validity in eliminating power grid fluctuation from large-scale integration of renewable energy. Heat-power decoupling is effective technique to promote flexibility of combined heat and power plant. To address the insufficient dynamics investigation of combined heat and power plant under different heat-power decoupling strategies, an adaptive modeling approach is proposed. This method systematically fuses the mechanism analysis and data-driven fuzzy modeling to prepare for the controller design for rapid and deep load regulation in flexible operation. Firstly, the model structures with unknown parameters are determined by mechanism analyses to reflect the effects of various decoupling techniques on the characteristics of plant. Secondly, in view of the on-site data under wide load operation conditions, corresponding model structure is adaptively matched, and a new Takagi–Sugeno fuzzy modeling scheme depend on biquantum pigeon optimization algorithm is constructed for model parameters acquisition. The effectiveness of the proposed approach is illustrated based on a 350 MW supercritical combined heat and power unit. Simulation results illustrate that the adopted method can rapidly characterize dynamics of plant under different heat-power coupling conditions. All the model identification and verification errors in wide load operation conditions are less than 3.1% plant output, which reveals high modeling precision more than 97%. Thus, the established model provides solid foundation on highly matched controller for ensuring operational flexibility.

The influence of the temperature on the mechanical properties of Sanicro 25 steel

Austenitic steel 22Cr25NiWCoCu (Sanicro 25) is one of the newest and most promising steels for use in supercritical and ultra-supercritical power units. High resistance of Sanicro 25 steel to corrosion and oxidation in the steam atmosphere at up to 700 °C is ensured by chromium content at 21.5-23.5 wt%. Also, higher chromium content provides less mass decrement during high-temperature corrosion at 700 °C. Currently, the Sanicro 25 steel is characterised by the highest creep strength among the commercially used creep-resistant stainless steels. In this work, the study of precipitation processes and microstructure stability in the delivery state and after the ageing process in time up to 20,000 h at 750 °C was performed. Identification of secondary phases was performed using electron diffraction on thin films using transmission electron microscopy and X-ray phase composition analysis. Long-term ageing depending on time and temperature showed that there are four secondary phases in Sanicro 25: M23C6, MX, NbCrN, σ-phase and Cu_rich. The precipitation and increase in the size of secondary phase particles during ageing results in a decrease in matrix saturation with alloying elements, which leads to a reduction in hardening by solid solution mechanism. These processes and effects result in changes in mechanical properties. The alloy overageing effect observed for the ageing temperature of 750 °C results in obtaining strength properties similar to those of the steel in the as-received condition.

Combustion Adjustment in Initial Startup Stage of 660MW Supercritical Fossil Power Unit

In order to better improve economical efficiency, the newly built supercritical units mostly adopt more fuel-saving ignition mode, but in the initial startup stage, the parameters of unit and boiler are not compatible. This paper carries out study and analysis on parameters in the initial startup stage, and optimizes all parameters by adjusting startup feed-water quantity, fuel quantity, air volume and ratio for several times. With favorable effect, this paper may be used as good reference for other similar units.

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.

Parametric optimization of supercritical power plants using gradient methods

Gradient nonlinear optimization methods are among the most effective approaches to determine the optimal parameters of thermal power plants. These methods allow us to consistently optimize a large number of thermodynamic and design parameters and mass flow rates of actuation fluids and of heat-transfer fluids. This paper proposes an original gradient method to optimize the parameters of a 660 MW supercritical coal-fired power unit. The specific feature of the method is that it can start from a point where there is no solution of the combined equation defining the plant. The exact solution of the nonlinear combined equation is achieved only at the optimal point, consequently the calculation time is reduced. The method provides better convergence and greater accuracy of solving the nonlinear combined equation defining the processes in the power unit. Optimization is carried out according to the criterion of maximum efficiency, minimum specific plant investments and minimum cost of electricity at different fuel prices. The austenitic and nickel alloys are considered for superheater and reheater tubes of a steam generator. Co-optimization of the thermodynamic and design parameters of the power unit by the criterion of the minimum cost of electricity allowed us to get original solutions that combine the high temperature (615–622 °C) and low pressure (16.7–18.1 MPa) of live steam.

Thermodynamic analysis of an improved flue gas pre-dried lignite-fired power system integrated with water recovery and drying exhaust gas recirculation

Lignite is a domestic strategic reserve of low rank coals in many countries owing to its abundant resource and competitive price. Combustion for power generation is still an imperative approach to lignite utilization. However, the high moisture content invariably results in the low efficiency of lignite-direct-fired power plants. The present work focuses on an improved concept of the flue gas pre-dried lignite-fired power system (FPLPS), which integrates fan mill drying, deep water recovery and drying exhaust gas recirculation. A thermodynamic analysis model was developed to investigate the energy and water saving potentials of the FPLPS with various design parameters. Different configuration alternatives of FPLPS design were provided and compared in terms of energy and water conservation. Results indicated that flue gas pre-drying and waste heat recovery into the boiler contributed to 1.47%-pts plant thermal efficiency improvement (Δηtot) in a 660 MW supercritical power unit. Drying exhaust gas recirculation could be implemented for flame temperature reduction and emission control given effective removal and recovery of the moisture in lignite. The Δηtot in this case was determined as 0.87%-pts. Moreover, the Δηtot could be enhanced to 1.09%-pts with utilization of waste heat from the water recovery process into the steam cycle. The plant water saving rate of the improved FPLPS ranged from 0.22 to 0.32 t·MW h−1. Overall, an environmentally-friendly, energy-efficient and water-saving lignite power generation could be accomplished through the improved FPLPS.

Study on the movement and deposition of particles in supercritical water

The movement and deposition of fine particles in supercritical water is closely related to the safe operation of supercritical water reactors and supercritical thermal power units. When deposition of corrosion particles occurs, it will affect the heat exchange of the pipe wall and cause the wall temperature to rise rapidly. It will seriously threaten the safe and stable operation of supercritical water reactors or supercritical thermal power units. In this paper, the ANASYS numerical software was used to simulate the movement of fine particles in supercritical water. The effects of parameters such as heating power, inlet temperature, inlet velocity, average particle diameter, and asymmetry of heating on the influence of fine particles in the supercritical water were investigated. The results show that with the increase of inlet temperature, the peak value of particle deposition concentration gradually approaches the inlet section of the pipe. With the increase of inlet velocity, the peak value of particle concentration gradually moves to the outlet of the pipe. The heating of the pipe has a certain inhibitory effect on the concentration and deposition of particles. The simulation result of the particles deposition has been compared and verification with the existing experimental data. The verification results show that the particle deposition concentration distribution characteristics obtained by the simulation and the experimental data are consistent. It facilitates the mechanism analysis of the movement and deposition characteristics of the fine particles in the supercritical water. It is conducive to the safe design of supercritical water reactors.

Simulation of rapid increase in the steam mass flow rate at a supercritical power boiler outlet

The paper presents selected results of simulations of a rapid increase in the supercritical power boiler thermal load. The simulations concerned the possibility of obtaining the required increase in the live steam mass flow rate at the boiler outlet in a set period. This condition must be met to achieve the required increase in the power unit load, of the order of a few dozen MW in a few dozen seconds. For this purpose, it is necessary to decrease the boiler pressure and increase the fired fuel mass flow rate at the same time. The pressure reduction is carried out by opening, at a constant rate, the control valves installed on the pipeline at the turbine inlet. Assuming that the rate is known, the unknown parameters are the values of the target pressure reduction and the time of the pressure decrease process duration. The values vary depending on the power unit initial load. After the pressure decrease process is stopped, the required increase in the power unit load (the increase in the live steam mass flow rate) should be maintained only by firing more fuel. The required values of the target reduction in pressure within a specific range of the power unit load can only be determined by developing a precise mathematical model of the entire boiler and performing simulation computations. The obtained results are of fundamental practical importance because they are used by the boiler manufacturer to generate the power unit modified sliding curves. The results presented in the paper relate to a supercritical power unit now being constructed in one of the Polish power plants. An in-house mathematical model was developed for the power unit boiler. The model comprises all heating surfaces of the boiler and enables simulation of the boiler operation in conditions of rapid changes in loads.