Supercritical Once-through Boiler Research Articles

Advances in zero liquid discharge multigeneration plants: A novel approach for integrated power generation, supercritical desalination, and salt production

In light of the escalating worldwide demand for energy and water resources, the imperative for developing sustainable multigeneration systems has gained prominence in recent research and development. This study aims to investigate the feasibility and performance of a novel multigeneration plant operating at supercritical conditions to simultaneously generate freshwater, power, and dry salt as a by-product while eliminating liquid discharge. This approach offers a promising solution to address the interconnected challenges of energy and water sustainability. The theory contends that combining supercritical desalination with power generation increases energy efficiency and water production rates, surpassing conventional systems. Therefore, a model is developed employing a supercritical once-through boiler capable of supplying the necessary heat duty required for steam generation from the integrated desalination plant, producing power. Additionally, multiple heat exchangers and flash separators are implemented, enhancing freshwater productivity and obtaining dry salt, eliminating the need for brine waste disposal. A thorough sensitivity analysis is conducted utilizing a robust model developed on Aspen HYSYS to ascertain the optimal operational parameters of the proposed plant. The findings of this study underscore the potential of concurrent production of 1.65 million m3/year of freshwater, 302.5 GWh of electricity, and 64,845 ton/year of dry salt for 0.38 $/m3, 0.07 $/kWh, and 0.02 $/kg, respectively. In conclusion, this research highlights the prospects of revolutionary progress in multigeneration systems for sustainable development.

Virtual reference setting and its application in thermal power unit control

In thermal power units, superheater outlet temperature is a very important parameter, but because of its long delay time and large inertia, it is difficult to obtain good control effect by traditional control algorithm. In this paper, taking a supercritical once-through boiler as the research object, combined with the established superheated steam temperature object model, a two-degree-of-freedom superheater outlet temperature control system based on VRFT is designed, and compared with the conventional tuning method, it is proved that the virtual reference feedback tuning (VRFT) method has many advantages, such as simple design, less computation, no need to select initial parameters and good convergence, is an application-oriented direct control system design method.

Modeling of Steam Generation Process in the Supercritical Once-Though Boiler in the Thermal Power Plant with MATLAB Application

The paper establishes a simple and practical model for a water-wall system comprising supercritical once-through boilers. The mathematical model of the system is developed and built based on mass, energy and momentum balance equations. The system of equations gives the linear and differential algebraic equations expressed by pressure and temperature. The model is solved by a program written in the MATLAB application. The data in the literature are also provided for comparison, and good agreement with the model results. The MATLAB program could be applied for basic design of process in the supercritical once though boiler in the thermal power plants in Vietnam.

Application of Iterative Feedback Tuning Method in Superheated Steam Temperature Control System

Taking a 600 MW supercritical once-through boiler as the study object, this paper designed an IFT-based two-degree-of-freedom (DOF) PID superheated steam temperature control system. Besides, the IFT-PID controller was formed and compared with the conventional tuning method. The simulation results show that this method can be successfully applied to great-inertia and large-lag controlled objects, with evident superiority.

Compensation Design of Coordinated Control System for Supercritical Once-Through CHP Plants Based on Energy Analysis

To increase the flexibility of power generation, the supercritical once-through boiler combined heat and power (CHP) plant should improve its coordinated control system (CCS). The difficulty of CCS for supercritical once-through boiler unit is to determine the water-fuel ratio in the dynamic process. First, a CHP unit model is established and simplified into a three-input three-output object under pure condensing working conditions. Second, dynamic relative matrix gain method and simulation is used to analysis several once-through boiler coordinated control schemes commonly used in engineering in which the compensation link is designed on the basis of water-fuel ratio. Simulation results show that, the water following CCS scheme has better intermediate point temperature performance and worse throttle pressure performance than the fuel following CCS scheme. To improve pressure performance, the closed-loop characteristics of pressure response to fuel and feed water flow are analyzed and compared through wavelet analysis. Finally, a method for determining the inertia time constant in the dynamic compensation link by comparing the wavelet coefficients is proposed. Wavelet analysis show that the imbalance in the speed of energy changes to fuel and feed water flow is the essential cause of large fluctuations in intermediate point temperature and throttle pressure before turbine of the supercritical once-through boiler unit. Simulation results show that using the proposed dynamic compensation in water following CCS can effectively reduce fluctuations of the intermediate point temperature and fluctuations of throttle pressure before turbine when power load command changes.

Failure analysis of a high temperature superheater pipe

The failure causes of a superheater pipe of a supercritical once-through boiler were analyzed based on the chemical composition analysis, tensile test, hardness and microstructure analysis. The results show that the microstructure changing from martensite to austenite due to the high temperature exceeded Ac1 causes the sharply decrease in the strength of superheater pipe.

Thermal Stress and Cyclic Stress Analysis of a Vertical Water-Cooled Wall at a Utility Boiler under Flexible Operation

Supercritical once-through utility boilers are increasingly common in flexible operations in China. In this study, the tube temperature changes at a vertical water-cooled wall are analyzed during a fluctuating flexible operation. There are considerable differences in the temperatures of the parallel tubes at the minimum load, and the resulting thermal stress distributions at a front water-cooled wall are established using structural calculation software ANSYS 17.1, USA. A wide thermal stress distribution occurs among the parallel tubes, and the local cyclic stress amplitudes under flexible operation are higher than those under cold, warm, hot, or load-following operations. Because of the water wall expansion structure at the furnace, the higher tube temperature areas suffer from compressive stress, while the lower tube temperature areas suffer from tensile stress. During flexible operation, combustion uniformity and a two-phase flow distribution can improve the safety of vertical water-cooled wall operation. The minimum load of the utility boiler should be set as a limitation, and the tube temperature is an important parameter affecting the thermal and cyclic stresses.

Quantitative Effect Model of Setting No.0 High Pressure Heater on Exhaust Flue Gas Temperature of Supercritical Once-through Boiler

Quantitative Effect Model of Setting No.0 High Pressure Heater on Exhaust Flue Gas Temperature of Supercritical Once-through Boiler

Study of flue-gas temperature difference in supercritical once-through boiler

The 600 MW coal-fired once-through Boilers with opposed firing at a power plant are found to experience marked temperature variation and even overtemperature on the wall of the heating surface as a result of flue-gas temperature (FGT) variation in the boiler. In this study, operational adjustments were made to the pulverizing, combustion, and secondary air box systems in these boilers, in order to solve problems in internal combustion. The adjustments were found to reduce FGT difference and optimize the boiler’ combustion conditions. The results of this study can provide a reference for optimization of coal-fired boiler of the same type in similar conditions.

Dynamic simulation of the water-steam flow in a supercritical once-through boiler

Once-through steam generator systems with supercritical fluids are applied to achieve large capacity and high efficiency in power generation systems. A supercritical fluid is considered as a single-phase substance, but properties of the fluid sharply change near the pseudo point, where specific heat of the fluid reaches its maximum at the corresponding pressure. Therefore, properties calculation and correlations of the heat transfer coefficient and pressure drop have to be evaluated carefully to predict the dynamic behavior. In this study, a 1-dimensional discretized dynamic model was developed for the dynamic simulation of the once-through boiler with supercritical fluids. The mass, momentum and the energy balance were solved in each cell. Validation of the model was checked against the experimental data for the single tube, and the dynamic behavior was simulated for the case of the simple step disturbance. After checking the single tube, this model was applied to the water-steam flow system of the commercial 500 MWe once-through boiler. The steady state values were compared with the design data. To simulate the dynamic behavior of the once-through boiler system, the feed water flow rate and the heat duty of the evaporator were changed as the disturbance. Finally, the operation simulation of the water-steam flow of the once-through boiler was performed for the case of dynamic step-down from the supercritical pressure to the subcritical pressure.

Start-Up and dynamic processes simulation of supercritical once-through boiler

The rapid expansion of the electricity demand causes the gap between the peak load and valley load increasing in China. Since the coal-fired supercritical power plant needs to be started up frequently, the operations of power plant become more flexible. During the start-up process, the operation parameters are off-designed, the multi-transient phenomenon is inevitable and the intensive emission always appears during varying working condition, it is difficult to ensure the operator of the real power plant to start up the boiler following the design curve. In this work, a new dynamic model for simulating start-up dynamic process of a 600MW supercritical boiler is introduced in this paper. The six-equation solution of two-phase flow was applied to modeling the evaporator, and the process from ignition to the load at 30% Boiler Maximum Continuous Rating (BMCR) was simulated. The results demonstrate that the model is reasonable and dynamic behavior can be observed clearly. Also, these prove that the control strategy is advisable. Moreover, the fuel consumption of the start-up process can offer a reference to operator of the real power plant. The validated model is of high relevance for further optimizing the boiler start-up process.

Modeling of ash deposition in a pulverized-coal boiler by direct simulation Monte Carlo method

In this paper, a mathematical model of the fly ash deposition in a 660MW supercritical once-through boiler was developed. The mechanism of particle-particle and particle-wall collisions has been considered. Particle collision is modeled by a direct simulation Monte Carlo (DSMC) method. Three submodels (impaction, thermophoresis, and sticking) were included in the mathematical model. In the ash deposition model, the critical velocity model was used to calculate the ash sticking rate during initial deposition, and the viscosity model was used for the outer layer deposition. An ash deposition probe was inserted into the peephole and ash deposits were collected. The morphology of these samples was analyzed using SEM. The deposit thickness and the distributions of adhesion particle size were obtained by mathematical model. The mean deposit thickness calculated by the DSMC method agreed with the value measured by the ash deposition probe with the relative error within 3.79%. The simulation results show that the deposition inner layer consists of fine particles less than 10μm in size and sticky fine particles were selectively captured during the initial deposition stage. The DSMC method may be a practical alternative for studies of ash deposit formation.

A dynamic model used for controller design of a coal fired once-through boiler-turbine unit

Supercritical OTB (once-through boiler) units with high steam temperature and pressure have been widely used in modern power plants due to their high cycle efficiency and less emissions. To ensure the effective operation of such power generation systems, it is necessary to build a model for the design of the overall control system. There are already detailed models of once-through boilers; however, their complexity prevents them from being applied in the controller design. This study describes a lumped parameter dynamic model that has a relatively low complexity while faithfully capturing the essential overall plant dynamics. The model structure was derived by fundamental physical laws utilizing reasonable simplifications and data analysis to avoid the phase transition position problem. Parameter identification for the model structure was completed using operational data from a 1000 MW ultra-supercritical OTB. The model was determined to be reasonable by comparison tests between computed data and measured data for both steady and dynamic states. The simplified model is verified to have appropriate fidelity in control system design to achieve effective and economic operation of the unit.

Thermal-hydraulic calculation and analysis on water wall system of 600 MW supercritical CFB boiler

Thermal-hydraulic performance of water wall system is a key operating target for supercritical once-through boiler. In this paper, the water wall system of a 600 MW supercritical circulating fluidized bed (CFB) boiler with complex furnace structure is treated as a simplified series-parallel flow network, which consists of parallel flow loops, pressure nodes and connecting tubes. Based on the mass, momentum and energy conservation of these components, a complex but accurate mathematical model for predicting the thermal-hydraulic characteristics of boiler heating surface is developed, which introduces numerous empirical correlations for heat transfer and flow resistance. The model was iteratively solved using the quasi-Newton method, and the thermal-hydraulic parameters of the water wall system at different operating loads, including mass flux distribution, total pressure drops, outlet vapor temperature and metal temperature profiles are obtained. The results exhibit good flow distribution characteristics and low mass flux deviations in the water wall system, and the total pressure drop is far lower than that in conventional supercritical once-through boilers at different operating loads. It is also found that the outlet vapor temperatures and the temperature differences in the water wall system are all in a permissible range and the metal temperatures meet the boiler operating requirement completely. It implies the design of water wall system in supercritical CFB boiler is successful.

Development and Application of a New Type of Separators for Supercritical and Ultra-Supercritical Once-Through Boilers

The design of start-up separators for domestic once-through boilers is analyzed, and start-up separators of foreign-made boilers are reviewed. A design procedure and engineering solutions for a new type of start-up separators with top steam outlet and drain header are presented.

A Water-Wall Model of Supercritical Once-Through Boilers Using Lumped Parameter Method

This paper establishes a compact and practical model for a water-wall system comprising supercritical once-through boilers, which can be used for automatic control or simple analysis of the entire boiler-turbine system. Input and output variables of the water-wall system are defined, and balance equations are applied using a lumped parameter method. For practical purposes, the dynamic equations are developed with respect to pressure and temperature instead of density and internal energy. A comparison with results obtained using APESS, a practical thermal power plant simulator developed by Doosan Heavy Industries and Construction, is presented with respect to steady state and transient responses.

The Partition Period of Thermodynamic Calculation and the Numerical Simulation for Lignite Blended Supercritical Boiler

The purpose of this paper is to study the impact of furnace temperature and load after blending in lignite, based on CFD software FLUENT-6.3,this paper choose the appropriate geometry model and the physical and mathematical models, and numerical simulation of the different conditions 600MW supercritical once-through boiler blending lignite furnace combustion process is curried out. And through a 600MW supercritical coal-fired boiler furnace lignite blended performed sections thermodynamic calculation under different conditions, worked out the furnace flue gas temperature, CO, CO2concentration distribute trend and radiant heat each section surface heat load conditions. The specific amount were blended with 5%, 10%, 15%, 20% were not dried lignite and dried lignite 20% after five conditions. And obtained a conclusion is the temperature and radiation heating surface flue gas heat load in the overall trend under the various conditions.

Modelling and dynamic simulation of a supercritical, oxy combustion circulating fluidized bed power plant concept—Firing mode switching case

Dynamic process simulation provides a tool to evaluate operational issues of a new process concept before the plant construction. This paper studies a carbon capture and storage (CCS) capable power plant concept with a model including a supercritical once-through CFB boiler with gas and water steam sides, a turbine island, an interface from the air separation unit (ASU) and the control system to manage typical operational transients. Switching between the air and oxy firing modes is one of the key operations in oxy combustion processes. The selected mode switching strategy uses simultaneous linear ramps for the mass flows of the primary and secondary air, oxygen, and recirculated flue gas. The results show that the firing mode can be successfully switched within 25–37min. The flue gas path difference between the air-firing and oxy-firing modes due to the flue gas recirculation causes significant differences in dynamic behaviour. The simulations emphasize importance of good control and coordination of the gas flows. Feedback control of the flue gas and/or oxidants O2 content during the mode switching is suggested to improve robustness against disturbances, for example, in oxygen delivery, flow measurements, fuel feeding and combustion.

Positive Flow Response Characteristic in Vertical Tube Furnace of Supercritical Once-Through Boiler

Positive flow response characteristic in vertical tube furnace of supercritical once-through boiler provides a feasible and effective way to alleviate the thermal maldistribution to some extent. In the present paper, theoretical derivation and simulations were performed to investigate the mechanism and forming conditions of positive flow response characteristic systematically. Two types of transition criterions were built up to judge positive or negative flow response characteristic. Parametric study was carried out to analyze the influencing factors. Both the operating conditions (mass flux, pressure, temperature or quality, heat flux) and the geometric parameters (inside diameter, length, pitch and type of tube) were investigated. In addition, thermal-hydraulic simulation of a 600 MW supercritical W-flame boiler has been carried out to take all these factors into account simultaneously. The conclusions drawn from this study will be of help to the design of large capacity supercritical once-through boilers as well as the refurbishment of existing supercritical or subcritical boilers.

Simulation Investigation on the Dynamic Characteristics of a 600MW Supercritical Lignite-fired Pulverized Boiler

The real-time dynamic simulation model of a 600MW lignite-fired pulverized boiler, which is a supercritical once-through boiler with fan mill three drying media direct fired system, was established. The step reduction of hot flue gas flow rate was simulated. The dynamic responses of live and reheat steam temperatures were obtained and analyzed. Results show that the response of steam temperature is an oscillation process. The superheat and reheat steam overtemperature values are influenced by the boiler load, the hot flue gas extracting percent and the inertia time of pulverizing system.