Supercritical Unit Research Articles

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.

Analysis of high energy consumption of 1000MW ultra supercritical unit

A 1000MW ultra supercritical unit heat consumption rate was higher than design value 324.81 kJ/kW h, in this paper, by comparing the conventional test and assessment test, the result of the analysis of conventional test data at the same time, found out the factors that affect the unit heat consumption, low efficiency of unit mainly include cylinder and putting-in-service proactively reheat steam consumption reduction of warm water, small machine increase, feed water by-pass leakage, system other internal leakage, etc., application of equivalent enthalpy drop method, calculated the influence of the influence factors on the heat consumption of value.

Optimization of an eco-friendly dyeing process in both laboratory scale and pilot scale supercritical carbon dioxide unit for polypropylene fabrics with special new disperse dyes

A new synthetic route was developed to create azo based dispersed dyes which are derived from 3-(3-Chloropheney)-2-((4-substituted) diazenyl)-3-oxopropanenitrile and 3-substituted 5-amino-4-arylazopyrazoles. The new dyestuff was used to dye unmodified polypropylene fabrics under both aqueous and supercritical carbon dioxide mediums. A pilot scale experiment was performed on the dyestuff 3a to prove the potentiality of the dyestuff toward polypropylene fabric. The new dyes were characterized by MS, IR and NMR spectral data and found to be in good agreement with the proposed structures. The colour uptake expressed as colour strength (K/S) was assessed and the values obtained showed that the dyed polypropylene fabrics using scCO2 method was significantly better than the aqueous one. The colour fastness properties of all the dyed fabrics were evaluated and exhibited excellent results. Moreover, it is also a green approach for using industrial emissions of CO2 instead of water in the textile dyeing process.

Failure Analysis on High Temperature Reheater Tube of T23 Steel in a 660MW Thermal Power Plant Boiler

The cause of the failure of high temperature reheater tube of T23 Steel in 600MW supercritical unit of a power plant was analysed by visual examination, chemical analysis, microstructural examination, and mechanical property test (at room temperature). The results showed that the cause of reheater tube failure was a long-term overheating over 570 °C (in the furnace), which led to its mechanical properties to decrease, resulting in the tube burst with the longitudinal crack ruptures.

A L1-LEMPC hierarchical control structure for economic load-tracking of super-critical power plants

The super-critical thermal power plants are undertaking more and more responsibilities for the balance of the power grid intermittency of the renewable energies. However, the frequent wide-range load regulation may deteriorate the operational efficiency of the power plant. To this end, a hierarchical control structure with two layers is proposed in this paper. An economic model predictive controller using a locally linearized model of the plant (LEMPC) is employed in the upper layer to realize an optimal load tracking. A L1 adaptive controller in the lower layer forces the plant to track the optimal trajectory by estimating and compensating the lumped uncertainty between the real plant and the linear model. The tracking performance is theoretically proved to reach the desired transient process. The proposed hierarchical control architecture is validated through simulations on a simplified 1000MW super-critical boiler-turbine unit model with comparison to the other two conventional real-time optimization control approaches. The results show that the proposed L1-LEMPC control system produces better load tracking performance than the other two conventional control strategies with higher operational economy efficiency. Moreover, under the environment of severe external disturbance and parameter perturbation, the proposed control system still maintains satisfactory performance.

Novel configuration of supercritical water gasification and chemical looping for highly-efficient hydrogen production from microalgae

This study proposes a novel system to efficiently produce hydrogen from microalgae, based on supercritical water gasification and syngas chemical looping, and its conversion to methylcyclohexane. The process consists of a gasifier, a syngas chemical looping reactor, and a methylcyclohexane synthesis reactor as the main units. Microalgae are converted to syngas in the supercritical water gasification reactor. Thereafter, the produced syngas is introduced into the syngas chemical looping module to produce pure hydrogen and a separated carbon dioxide stream. The hydrogen is then reacted with toluene through the hydrogenation reaction to produce methylcyclohexane as a hydrogen carrier. The heat released from the methylcyclohexane synthesis module and chemical looping combustor is utilized to sustain the thermal balance of the supercritical water gasification unit. The system performance is observed under different feed moisture contents, operating temperatures in the supercritical water gasification unit, and operating pressures in the syngas chemical looping unit. A steady-state process simulation of Aspen Plus software is used for this purpose. The proposed integrated system exhibits of approximately 13.7%, 45.3%, and 59.1% for power generation efficiency, hydrogen production efficiency, and total energy efficiency, which demonstrates an efficient process of hydrogen production. The preliminary economic assessment shows that more than half of the operating cost accounts for microalgae production. This indicates the microalgae feedstock is one of the critical cost drivers in the microalgae-to-hydrogen production system.

A dynamic nonlinear model for a once-through boiler-turbine unit in low load

Under the background of serious wind and solar energy curtailment in China, flexibility modification of coal-fired supercritical units is selected as an effective technical path to provide space for renewable energy into the grid. Modeling of coordinated control system in low load has yet to be conducted to date. In this work, a simplified dynamic model for coal-fired once-through boiler-turbine unit aimed at low load working condition is built up. First, the major model structure is established mainly based on mass and energy balance laws as well as thermodynamic equilibrium of important parameters combined with the analysis of operation characteristics. Then, the unknown parameters in the model are estimated by regression analysis, statistical analysis, and nonlinear optimization algorithm. Comparison results between measurement and simulation results show that the model has acceptable accuracy and, importantly, also captures the essential dynamic characteristics of units.

Model establishment and performance evaluation of a modified regenerative system for a 660 MW supercritical unit running at the IPT-setting mode

Abstract: Improving denitrification efficiency of supercritical unit for peak load regulation by changing feed water temperature has become a focus of thermal power engineering under wide load. However, the effect mechanism of feed water temperature on the performance of supercritical unit is indistinct absolutely at the intermediate point temperature (IPT)-setting mode, which obstructs the performance optimization and the excavation of energy saving of supercritical unit with a change of feed water temperature aims at improving denitrification efficiency of supercritical unit. For the purpose, a closed cycle theory in which thermal power unit operating parameters interact and form a closed influence mechanism was defined and proposed, with such a core ideology, the performance analysis and assessment models under IPT-setting mode is constructed on the differential theory, energy balance and thermodynamics method, and the accuracy of established model is verified. These developed models are applied to assess the operational performance and to identify energy saving potential of supercritical unit for peak load regulation. Subsequently, a typical 660 MW ultra-supercritical coal fired power plant is selected as a reference case. Not only a quantitative analysis of effect of increasing feed water temperature adding HP heaters on supercritical unit performance is performed, but the effect of reducing feed water temperature stopping HP heaters on supercritical unit performance is implemented as well at the IPT-setting mode. Finally, the influence mechanism of feed water temperature on boiler thermal efficiency, steam turbine thermal consumption rate, standard coal consumption rate, power generation, denitrification efficiency, NOx production, steam temperature and other operating parameters of supercritical unit is absolutely revealed on IPT-setting mode. The investigation results show that the sensitivity of feed water temperature to supercritical unit performance varies greatly in different range of feed water temperature, especially when the feed water temperature is heated to the near the pseudo critical temperature, a dramatic change would take place in operation parameters of supercritical unit. Such a viewpoint is presented rigorously that increasing feed water temperature does not always achieve energy saving for supercritical units, also contrary to the existing viewpoint.

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.

Generation IV supercritical water-cooled nuclear reactors: Realistic prospects and research program

Existing conditions make possible obtaining information that being discussed openly by wide scientific community could help outlining or even establishing the expediency of a particular area of present and future research. Use link http://www.sciencedirect.com to learn about the topics or areas that most attract researchers from different countries. The Generation IV International Forum (GIF-IV) established in January 2000 has set a goal to improve the new generation of nuclear technologies in the following areas: stability, safety and reliability, economic competitiveness, proliferation resistance and physical protection. The purpose of the present publication is to prepare a discussion of one of the directions of development of fourth-generation NPPs, the groundwork for which has already been laid in thermal power engineering in various countries. The number of papers published annually on this topic is the largest among other similar topics dedicated to nuclear power plants of the fourth generation. Judging from the operating experience of existing nuclear power plants using water as a coolant, it can be ascertained that the tendency of building water-cooled nuclear power plants will remain during the next 30 to 50 years. During the present stage the task in the development of alternative types of reactors will be limited to demonstration of their performance and acceptability for future power engineering and the society. The project of supercritical water-cooled reactor is based on the operating experience of VVER, PWR, BWR reactors (more than 14,000 reactor-years); many years of experience accumulated in operating fossil thermal power plants (more than 400 power units; 20,000 years of operation of power units) using supercritical (25 MPa, 540°C) and super-supercritical (35–37 MPa, 620–700°C) water steam. In Russia more than 140 supercritical pressure units are currently in operation. Numerical calculation and design of supercritical water-cooled reactor (similarly to BR-10 reactor) will allow not only training personnel for future development of this technology, but will also help revealing the most difficult points requiring experimental confirmation with application of independent test facilities, as well as formulating the plan of first priority experimental studies. Knowledge accumulated over the last 10 years in the world allows the following: further specifying the already developed concept; developing a plan of specific priority studies; compiling task order for designing small-power pilot VVER SKP-30 reactor (30 MW-th). The scope of problems that are to be solved to substantiate VVER-SCP reactor and commence designing an experimental reactor with thermal capacity of 30 MW is the same as that in developing any type of nuclear reactor: physics of the reactor core; material related matters (primarily concerned with the reactor pressure vessel, fuel, and fuel rod cladding); thermal hydraulics of rod bundles in the near- and supercritical areas; water chemistry at supercritical pressure; corrosion of materials, development of safety systems. Research must be carried out both in static conditions and under irradiation. The absence in Russia during the extended time period of approved program with allocation of appropriate funding and preservation of the existing status during the coming two or three years will lead to the situation when Russia will be hopelessly lagging behind in the development of SCWR technology.

On the Flexible Operation of Supercritical Circulating Fluidized Bed: Burning Carbon Based Decentralized Active Disturbance Rejection Control

Supercritical circulating fluidized bed (CFB) is one of the prominent clean coal technologies owing to the advantages of high efficiency, fuel flexibility, and low cost of emission control. The fast and flexible load-tracking performance of the supercritical CFB boiler-turbine unit presents a promising prospect in facilitating the sustainability of the power systems. However, features such as large inertia, strong nonlinearity, and multivariable coupling make it a challenging task to harmonize the boiler’s slow dynamics with the turbine’s fast dynamics. To improve the operational flexibility of the supercritical CFB unit, a burning carbon based decentralized active disturbance rejection control is proposed. Since burning carbon in the furnace responds faster than throttle steam pressure when the fuel flow rate changes, it is utilized to compensate the dynamics of the corresponding loop. The parameters of the controllers are tuned by optimizing the weighted integrated absolute error index of each loop via genetic algorithm. Simulations of the proposed method on a 600 MW supercritical CFB unit verify the merits of load following and disturbance rejection in terms of less settling time and overshoot.

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.

Research on energy distribution characteristics of supercritical circulating fluidized bed boiler

Combining the typical supercritical CFB boilers with H-shaped and M-shaped arrangement, the energy consumption indicators were analyzed and compared. And the lower energy consumption CFB technology used in energy-saving technology transformation or design was studied. The energy distribution characteristics of supercritical CFB boiler units were further researched through the experiment, which proposes the optimization direction of the lower energy consumption CFB technology to reduce the service plant rate that can be applied for (ultra-)supercritical CFB boilers.

Microstructure and mechanical properties of HR6W alloy dedicated for manufacturing of pressure elements in supercritical and ultrasupercritical power units

The article presents the results of research on the microstructure and selected mechanical properties of HR6W nickel-base alloy. The test alloy was subjected to isothermal ageing at 700°C and for up to 10000h. The tests of the HR6W microstructure were performed using the scanning electron microscopy (SEM) and the transmission electron microscopy (TEM). The performed microstructural tests of the HR6W alloy showed that in the as-received condition it was characterised by the structure of nickel austenite with numerous primary precipitates of NbC and TiN. Ageing of the investigated alloy contributed to the precipitation of numerous particles of varying morphologies inside the grains and at the grain boundaries, as well as at the boundaries of twins - they were the secondary precipitates of M23C6 and Laves phase. The number of the particles precipitated at the boundaries was so large that they formed the so-called continuous grid of precipitates. Inside the grains, the presence of compound complexes of precipitates was observed. These complexes consisted of the TiN particles, as well as the M23C6 carbides and Laves phase nucleating on them. The tests of the mechanical properties of HR6W alloy showed that in the as-received condition the alloy showed high plastic properties, with relatively low strength properties - in particular, the yield strength. Ageing of the HR6W alloy, as a result of precipitation of numerous particles in the matrix, through the strengthening with the precipitation mechanism, resulted in a considerable growth of the strength properties - inter alia the yield strength by over 60%, with the reduction of the plastic properties - elongation decreased by around 40%. Similar growth in the test alloy was observed for hardness.

Analysis and Treatment of 660 MW Ultra Supercritical Units Vibration

This paper analyses the excessive vibration of the bearing caused by the high-medium pressure rotor bending of the steam turbine, provides a basis for the weighted and balanced bending scheme of the high-medium pressure rotors of the 660MW ultra-supercritical unit and also provides reference for the same type of unit.

Construction and Validation of an Online Subcritical Adsorption-based Device for Assisting CO2 Recycling during a Supercritical Fluid Extraction Process

Background:An efficient process for extracting food ingredients from plant materials should demand the use of a reduced volume of extraction solvent to obtain a final product that is free of solvent and reduces both the processing time and the costs. In some cases, achieving a new efficient process requires the modification, reconfiguration or renewal of elements that are part of a processing unit.Objective:The goal of this work is to describe the development of a modification of a commercial supercritical fluid extraction pilot unit designed to assist CO2recycling based on subcritical adsorption on an adsorbent material. In addition to the construction and validation of the system, a cost survey was performed to estimate the cost of the homemade device developed to allow effective CO2recycling.Methods:The developed device was tested using cotton and Celite®as model adsorption materials and annatto seeds (Bixa orellana L.) as a model plant material. A 0.65 L adsorption column was installed with a set of connections and valves that complemented the unit’s recycle system. The validation procedure consisted of defatting annatto seeds.Results:The proposed online subcritical adsorption-based device was technically validated using cotton as an adsorbent material. The cost survey showed that an estimated total cost of USD 5731.36 would be required to install the developed device in a Supercritical Fluid Extraction (SFE) unit similar to the one coupled here (5 L).Conclusion:The proposed device was demonstrated to be very promising for application in the pseudocontinuous SFE, recirculating the same amount of CO2mass exceeding the S/F values by 14 times, when compared to a process without a CO2recycling system.

Influences of tube wall on the heat transfer and flow instability of various supercritical pressure fluids in a vertical tube

The subcritical steam generators in the current high temperature gas-cooled reactor pebble-bed module, HTR-PM, in China can be replaced by supercritical steam generators to better coordinate the reactors and the supercritical steam turbine unit to improve the thermal efficiency with no fluid phase change at supercritical pressures. This study then numerically analyzes the heat transfer and flow instabilities of supercritical pressure fluid flow in a vertical tube with zero or finite-thickness walls. The wall thickness has negligible effect on the heat transfer at supercritical pressure water at steady state. However, for transient calculations, the flow rate oscillates intensely beyond a critical heat flux with the zero-thickness wall, while the flows in tubes with finite-thickness walls are very different. The wall properties including density, specific heat and thermal conductivity are varied to analyze why the oscillations are suppressed by the finite-thickness wall. The flow and heat transfer characteristics and the heat stored in the wall compared with the total power are presented during the heating process at various moments. The flow instabilities are also analyzed for water at various pressures and CO2 to analyze the effects of the fluid properties.

Improved Model of an Intermediate Point Enthalpy Control System for Enhancing Boiler Efficiency

Control quality of an once-through boiler’s water-fuel ratio (WFR) and main-steam temperature are heavily influenced by the control quality of the once-through boiler’s intermediate point enthalpy (IPE), and it is also related to the economic and stable operation of the a once-through boiler. In order to control the IPE in a better way and to increase boiler efficiency, an improved model of IPE control system was built in this paper, matlab/simulink is used to build the IPE control system model based on a 600 MW supercritical unit, and the mechanism model of the control object is built in the same time. The feedforward of the feed-water temperature is brought to this model to increase the control rate. The control method of amendments to the amount of coal and the control method of amendments to the amount of feed-water are combined by the means of fuzzy control to solve the problem of the contradiction of the responding speed of the IPE and the separation interface’s stability of the steam-water separator. The simulation results show that the improved control method has better control effect and higher boiler efficiency was obtained as well.

Influence of Ambient Velocity on the Performance of SCAL Indirect Air-Cooled Towers

In order to explore the influence mechanisms of ambient wind speed on the performance of indirect air-cooled system, SCAL indirect air-cooled towers of a 600MW supercritical unit was taken as an example, and the corresponding calculation model was established. By the ways of CFD numerical simulation and field test, the variations along with ambient velocity of airflow field inside and outside the tower were simulated and the thermal performance of those towers under different ambient velocity was studied. The results showed that, the outlet resistance of air-cooled tower increased in pace with ambient velocity. Both temperature field inside the tower and pressure field inside and outside the tower were influenced by the ambient velocity to a large extent, while flow trajectory of flue gas inside the tower was basically similar. Moreover, the trends and changing laws of performance parameters from the level of segments and the tower were achieved. The conclusions would be of some reference significance for the optimal operation and design of indirect air-cooled system.