Ultra-supercritical Circulating Fluidized Bed Research Articles

Thermal–Hydraulic Calculation and Analysis on Water Wall System of a 700 MWe Ultra-Supercritical CFB Boiler

Ultra-supercritical circulating fluidized bed (CFB) boiler combustion technology has the advantages of environmental protection and good commercialization. As one of the key aspects in ultra-supercritical CFB combustion technology, the water wall system directly determines whether the CFB boiler can safely and effectively transform heat energy. This paper studies the thermal–hydraulic characteristics of the designed water wall system of a 700 MWe ultra-supercritical CFB boiler, the largest one that will be built next year. At four loads of the water wall system, hydrodynamic parameters are calculated based on the successfully verified mathematical models. The results show that the hydrodynamic characteristics of the system are good. The temperature distribution of the working fluid and metal of the water wall system is acceptable and safe at each load, which can support the technical development and improvement of the 700 MWe ultra-supercritical CFB boiler.

Safety Analysis on the Heating Surfaces in the 660 MW Ultra-Supercritical CFB Boiler under Sudden Electricity Failure

Ultra-supercritical circulating fluidized bed (CFB) boilers are taking up an increasing proportion of the CFB boiler fleet in China, making the safety concern about the heating surfaces in this type of boilers under sudden electricity failure draw more and more attention from the industry. For the time being, however, few studies have made efforts to resolve this concern. Given this, the physical process in a 660 MW ultra-supercritical boiler during the electricity failure accident was precited with a comprehensive model composed of mass and energy conservation equations in this work. The tube temperature of the boiler components with the highest safety risk, i.e., the water wall and a superheater, was obtained to evaluate the safety of the heating surfaces. The results revealed that the tube temperature (about 516 °C and 544 °C) would be obviously lower than the maximum permissible temperature of the tube material (600 °C and 630 °C) even when electricity could be restored at the power plant, indicating that the heating surfaces in the 660 MW ultra-supercritical CFB boilers would generally be recognized to be safe under sudden electricity failure.

Discussion on the Feasibility of Deep Peak Regulation for Ultra-Supercritical Circulating Fluidized Bed Boiler

In order to meet the flexibility operation needs of coal-fired units under the goal of carbon peak and carbon neutralization, it is imperative for circulating fluidized bed (CFB) units to participate in deep peak regulation. By systematically summarizing deep peak regulation operation practice of existing SC and subcritical-parameter-levels CFB units, the feasibility of deep peak regulation technology of an ultra-supercritical (USC) CFB unit under development and being built is analyzed and demonstrated; meanwhile, the deep peak regulation capacity of the boiler is also predicted. The results show that by analyzing the structural characteristics and design performance of the USC-CFB boiler, for technical problems such as stable combustion under low load, hydrodynamic safety, denitration performance under wide load, and rapid boiler load change rate existing in deep peak regulation, technical measures were implemented by selecting advanced boiler furnace type, adopting good design technology of the secondary rising water wall and uniformity design of bed temperature and bed pressure, strengthening the reducing atmosphere inside the furnace, improving the performance of wear-resistant refractory materials, quickly controlling the furnace bed material stock under variable load, optimizing the control strategy of CFB unit, and so on. The boiler achieved good operation characteristics and good deep peak regulation performance, and the pollutant emissions can steadily achieve ultra-low emission standards. When the USC-CFB unit participates in deep peak regulation, the minimum stable combustion load of the boiler can reach 20~30% BMCR, and a boiler load change rate under 30% BMCR or above could reach 1.5~2% BMCR/min, while that below 30% BMCR could reach 1% BMCR/min. The research results can provide references for the deep peak regulation of in-service supercritical (SC) CFB units and design optimization of similar USC-CFB units.

Dynamic Characteristics Analysis of a 660 MW Ultra-Supercritical Circulating Fluidized Bed Boiler

The 660 MW ultra-supercritical circulating fluidized bed (CFB) boiler, which is the maximum capacity and largest scale boiler in the world has entered construction stage in China. This study established a full-scale dynamic simulation model of the 660 MW ultra-supercritical at 100% boiler maximum continuous rating (BMCR) condition. The model consists of an air-flue gas system, a water-steam system, and an ash circulation system. The “core-annulus” of the gas-solid two-phase flow structure and “six-equation” model of water-steam two-phase flow were applied to simulate the behaviors of the gas-solid phase and water-steam system, respectively. The model was calibrated and verified at 100% BMCR condition, and the steady-state simulation results presented a high accuracy compared with the designed parameters. A dynamic simulation of three typical conditions were carried out as well, including a 5% feed water decrease, 5% air decrease, and 5% coal decrease, respectively. The results showed that the dynamic simulation model established in this study can simulate the dynamic behaviors of the 660 MW ultra-supercritical CFB boiler reasonably.

Study on the Uniformity of Secondary Air of a 660 MW Ultra-Supercritical CFB Boiler

Based on the field test of a 600 MW supercritical circulating-fluidized bed boiler, this paper optimizes the secondary air pipe layout scheme of a 660 MW ultra-supercritical circulating fluidized bed boiler with a similar furnace structure and carried out a numerical simulation on its air distribution uniformity. The secondary air box of the 660 MW ultra-supercritical circulating-fluidized bed boiler adopts a variable section design, and the secondary air branch pipe adopts a separate air inlet mode. The experimental results showed that the oxygen concentration was uniform near the rear wall, but all exhibited a “decrease–increase–decrease” profile along the horizontal line, which indicated that the trajectory of the secondary air jet was first in the shape of bending downward and then upward. To achieve a more uniform secondary air distribution for supercritical CFB boilers, further optimization of the layout of the bellows and branch pipes should be considered. Numerical results showed that the deviation rate of the internal and lower secondary air reached 17%, and there was optimization space for the secondary air branch pipe layout of the boiler. Its uniformity can be increased by adding valves and other measures. The velocity deviation of the secondary air of the 660 MW ultra-supercritical circulating-fluidized bed boiler can be within 3% by means of separate air supply and pipe diameter change, and better uniform air supply can be achieved to reduce the speed deviation by adding valves and adjusting the combustion.

Effect of bed particle size on the gas-particle hydrodynamics and wall erosion characteristics in a 550 MWe USC CFB boiler using CPFD simulation

To reduce wall erosion in 550 MWe ultra-supercritical (USC) circulating fluidized bed (CFB) boiler, the effect of bed particle size with gas-particle hydrodynamics characteristics is investigated here through computational particle fluid dynamics (CPFD). Most studies on erosion until now have dealt with pilot scale system, but the present study aims to provide valuable information on erosion at full scale in the CFB boiler. The results showed that particle size significantly affected the gas-solid flow in the furnace. Larger particles were difficult to be entrained by the gas stream, which increased the internal circulating rate instead of reducing the external circulation rate. Furthermore, erosion was also influenced by both particle size and circulation rate but did not linearly increase with the particle size. In the cyclone, the erosion rate is highest, which gradually decreases with increasing particle size due to external circulation rate, but the erosion tendency of furnace and other devices are rather highest with the medium-sized particles. As the hydrodynamics and erosion of a USC CFB boiler were studied through CPFD simulation for the first time, the findings of this study may provide considerable insight into and guidance for efficient boiler operation and prevention of shutdown due to erosion.

Experimental and numerical investigation on heat transfer in the vertical upward flow water wall of a 660 MW ultra-supercritical CFB boiler

Heat transfer in the vertical upward water wall of a 660 MW ultra-supercritical circulating fluidized bed (CFB) boiler was experimentally and numerically studied. A new variable turbulent Prandtl (Prt) model was used in the numerical simulation. The effects of specific heat ratio, buoyancy parameter, and acceleration parameter on the heat transfer of supercritical water were investigated through experimental data. Results show that the independence between the heat transfer coefficient and these dimensionless parameters was weak with the majority of them, only buoyancy parameter proposed by Jackson showed a strong independence. Additional parameters are required if specific heat ratio and acceleration parameter are used to predict the supercritical water heat transfer. A new variable Prt model was proposed as a function of the turbulent viscosity ratio and the molecular Prandtl number on the basis of the analyses of the effect of Prt on the supercritical water heat transfer. Experimental data of supercritical water were used to validate the accuracy of this Prt model. The results indicated that this Prt model provided accurate heat transfer predictions, especially for the heat transfer deterioration (HTD) cases. The variable Prt model demonstrated a significant effect on the radial velocity distribution, thereby leading to an ‘‘M” shape velocity profile. The zero velocity gradients limit the turbulent mixing, thereby leading to a significant reduction in the turbulent kinetic energy and HTD. The safe heat transfer characteristics of a 660 MW ultra-supercritical CFB boiler water wall were analyzed on the basis of the experimental research and simulation with the new Prt model. The results show that the water wall adopts a smooth tube can ensure the nonexistence of film boiling and overtemperature, and the water wall heat transfer is safe.

1-Dimensional modelling of in-situ desulphurization performance of a 550 MWe ultra-supercritical CFB boiler

In the present work, the in-situ desulphurization performance of the world first 550 MWe ultra-supercritical (USC) circulating fluidized bed (CFB) boiler was investigated mainly through simulation method. A pseudo steady-state 1-dimensional CFB model has been developed that integrates and simultaneously predicts the particle balance, combustion, lime sulphation and some other aspects. The specially designed integrated recycle heat exchanger (INTREX) and bottom ash self-circulation system of this boiler are also considered. The predicted median diameters of fly ash, bottom ash and circulating ash are 16 μm, 238 μm and 142 μm, respectively, which agree well with the field test data. Both test and modelling results reveal that the when the fine limestone with a median diameter of 50 μm is applied, the in-situ desulphurization efficiency can reach up to 95% and the final SO2 emission is only around 20 ppm, while no any back-end FGD system is applied. A sensitivity analysis is carried out using the model to examine the effects of cyclone efficiency and limestone particle size on the in-situ desulphurization performance. It indicates that by improving the cyclone efficiency, the SO2 emission in the outlet of furnace significantly decreases. Besides, appropriately reducing the limestone size can obtain higher desulphurization efficiency. The model presented in this paper can be used to further study the operation characteristics of USC CFB boilers, also including in-situ desulphurization, which is important for accurate prediction and cost-effective control of SO2 emission.

Model research on NOX emission of an ultra-supercritical circulating fluidized bed boiler

It requires that coal-fired generator units to meet ultra-low emission standards by 2020 in policy of environmental protection in China. Compared with pulverized coal boiler, circulating fluidized bed (CFB) boiler technology has the natural advantage of low NOX emission, which has the ability to directly realize ultra-low NOX emission inside the furnace. To accurately obtain NOX emission characteristics in an ultra-supercritical CFB boiler under construction, model research and experimental verification of NOX emission prediction were carried out in this paper. The results indicate that three models could all predict NOX emission in the CFB boiler, and the deviation among the predicted values calculated by these models can be controlled within 15%. When an ultra-supercritical CFB boiler burns existing engineering coal, NOX emission would be higher than 50 mg/Nm3, which requires structural improvement and operation parameters optimization to ensure that NOX emission inside the furnace of the CFB boiler could directly reach the ultra-low emission standard.

A comprehensive mass balance model of a 550 MWe ultra-supercritical CFB boiler with internal circulation

The world’s first 550 MWe ultra-supercritical circulating fluidized bed (CFB) boiler designed with integrated recycle heat exchangers (INTREX) commenced operation in 2016. One of the distinguishing features of the INTREX is the internal circulation opening (ICO) that allows the particles in the furnace to flow into the INTREX to enhance the bed-to-tube heat transfer, while the heat transfer is strongly affected by both the mass flow rate of internal circulation m˙ic and particle size in the INTREX, i.e., the mass balance of the INTREX. In the present study, we established a comprehensive one-dimensional model to investigate the mass balance of the boiler including the internal circulation. The coefficient of rebound ηICO was adopted to simulate particle behavior during internal circulation. The experimentally acquired parameters of ash formation and attrition of the coal ash were adopted in the model. The size of INTREX ash obtained from the field test on the 550 MWe boiler was used to determine the value of ηICO. The model results were verified with field tests, and they showed reasonable validity. The model results of the linear correlation between m˙ic and bed pressure displayed good agreement with those from the field tests.

Dynamic simulation study of the steam temperature in a ultra-supercritical circulating fluidized bed boiler system

A dynamic simulation of a modern ultra-supercritical circulating fluidized bed (USC-CFB) boiler system was performed using a physics-based model, where the boiler system was composed of an integrated system of heat exchanger (HEX) blocks. In each of the discretized elements of the HEX blocks, supercritical steam flow interacted with the solid–gas flow via heat transfer, and conservation laws, and physical phenomena of the CFB, and the supercritical steam flow was modeled to obtain the flow properties of each material side. By utilizing this model, the dynamic behavior of the main steam temperature, which was selected as the representative performance parameter for achieving safe and fast load changes, was simulated in response to changes applied to the main operating parameters such as the feedwater and fuel flow rates. The dynamic characteristics of the USC-CFB boiler were demonstrated by presenting the temperature responses of the steam and the circulating solid–gas after a step change in the feedwater and fuel flow rates. In the case of a load change, the dynamic response of the steam temperature was quantitatively presented by showing the transient overshoot and undershoot behavior, depending on the selection of the ramp speed when changing the feedwater and fuel flow rates. The steam temperature was also shown to be controlled by manipulating the inlet solid mass flow rate into the final superheater section of the external HEX. The comprehensive set of plant performance data that was generated from the model simulation can be utilized in setting up the operation strategies and/or in determining the control parameters for achieving stable steam temperature behavior during a load change.

Operation characteristics of a bubbling fluidized bed heat exchanger with internal solid circulation for a 550-MWe ultra-supercritical CFB boiler

The world first 550-MWe ultra-supercritical circulating fluidized bed (CFB) boiler is designed with integrated recycle heat exchangers (INTREX) and internal solid circulation. In this research, in order to investigate the heat transfer behavior and the significance of the internal circulation, the operation characteristics of the four INTREXes as final superheaters including heat transfer coefficients and ash flow rates under different load conditions are experimentally studied. The calculation of ash circulating rate shows that as the boiler load decreases, the furnace bed temperature and the circulating ash temperature in the loop seal decrease, while the flow rate of the internal circulation and its proportion to the total solid flux increase as the boiler load decreases. In particular, under 60% load condition, the internal flow rate is found to be higher than that of external flow rate for the two INTREXes located in front of the middle sections of the furnace. This indicates that the internal circulation is of more significance in the low-load condition than in the high-load condition. The key factor determining the internal-circulation flow rate is found to be the bed pressure drop in the furnace. The phenomenon of non-uniform distribution of solid flux among INTREXes is also discussed.

Predictions of NOx/N2O emissions from an ultra-supercritical CFB boiler using a 2-D comprehensive CFD combustion model

NOx and N2O emissions from an ultra-supercritical circulating fluidized bed (CFB) boiler were predicted using a two dimensional (2-D) comprehensive computational fluid dynamics (CFD) combustion model. This model was developed from a three dimensional model for a supercritical CFB boiler previously constructed by our group. Based on an analysis of the NOx and N2O conversion processes in a CFB boiler, the primary formation and destruction reactions were introduced into the 2-D model and coupled. The resulting model was validated using data from the Baima 600 MW supercritical CFB boiler, and then applied to a 660MW ultra-supercritical CFB boiler. The effects of excess air, the secondary air (SA) to (primary air (PA) plus SA) ratio and the SA injection height on NOx and N2O emissions were investigated. The results show that a higher excess air volume increases both NOx and N2O emissions, while increasing the SA/(PA+SA) ratio somewhat reduces both the NOx and N2O concentrations. On the basis of the results of this work, optimal locations for SA injection ports so as to lower NOx and N2O emissions are recommended.

Thermal-hydraulic calculation and analysis on evaporator system of a 660 MWe ultra-supercritical CFB boiler

Ultra-supercritical circulating fluidized bed (CFB) boilers have great commercial potential in large-scale clean utilization of low-quality fuels. Evaporator system is one of the keys in the combination of ultra-supercritical steam cycle with CFB combustion technology. Aiming at examining the thermal-hydraulic characteristic of the evaporator system of the conceptual design of a 660 MWe ultra-supercritical CFB boiler, a mathematical model was developed in this paper. A semi-empirical bed-to-wall heat flux model and a set of empirical correlations available for heat transfer and hydraulic resistance calculation were embedded in the thermal-hydraulic model. By iteratively solving the model, the thermal-hydraulic parameters of the evaporator system at different operating loads, including total pressure drops, mass flux distribution, and metal temperature were obtained. The results exhibit good flow distribution characteristic and low temperature deviations in the evaporator system. The outlet fluid temperature deviation and metal temperature in the evaporator system are both permissible, implying that the conceptual design of the evaporator system of the 660 MWe ultra-supercritical CFB boiler is acceptable.

Investigation on heat exchange feasibility of internal solids circulation for an ultra-supercritical CFB boiler

A novel ultra-supercritical circulating fluidized bed (CFB) boiler technology based on recycling the high-temperature internal solids in the dense-phase zone to heat final superheater and reheater was investigated in this paper. The merit of this technology was that it can guarantee the superheated and reheated steam outlet temperatures especially under the low load condition. The flow characteristics of internal solids circulation was investigated on a cold test rig using the circulating solids from the Baima's 600 MW supercritical CFB boiler and the valid internal solids circulation rate was measured by optical fiber probes. The effects of fluidizing air velocity, partition height and static main bed height on the valid internal solids circulation rate were studied. Moreover, the valid internal solids circulation rate of thermal state was measured on the Baima's 600 MW supercritical CFB boiler. By the cold and thermal tests, an empirical formula which could predict the valid internal solids circulation rate was summarized and refined. In addition, the heat exchange ability of internal solids circulation was analyzed and the needed internal solids circulation rate of actual boiler was calculated. Besides, practicable schemes of this technology were proposed in the end of the paper. Based on the above analysis, it was proved that the novel technology using the internal solids circulation was feasible to guarantee the temperatures of final superheated and reheated steam in an ultra-supercriticalCFB boiler.

Development of a supercritical and an ultra-supercritical circulating fluidized bed boiler

The supercritical circulating fluidized bed (CFB) boiler, which combines the advantages of CFB combustion with low cost emission control and supercritical steam cycle with high efficiency of coal energy, is believed to be the future of CFB combustion technology. It is also of greatest importance for low rank coal utilization in China. Different from the supercritical pulverized coal boiler that has been developed more than 50 years, the supercritical CFB boiler is still a new one which requires further investigation.Without any precedentor engineering reference, Chinese researchers have conducted fundamental research, development, design of the supercritical CFB boilers independently. The design theory and key technology for supercritical CFB boiler were proposed. Key components and novel structures were invented. The first 600 MWe supercritical CFB boiler and its auxiliaries were successfully developed and demonstrated in Baima Power Plant, Shenhua Group as well as the simulator, control technology, installation technology, commissioning technology, system integration and operation technology. Compared with the 460 MWe supercritical CFB in Poland, developed in the same period and the only other supercritical one of commercial running in the word beside Baima, the 600 MWe one in Baima has a better performance. Besides, supercritical CFB boilers of 350 MWe have been developed and widely commercialized in China. In this paper, the updated progress of 660 MWe ultra-supercritical CFB boilers under development is introduced.

Heat transfer and frictional resistance characteristics of the water wall tube of an ultra-supercritical CFB boiler

This paper presents an investigation of the flow and heat transfer characteristics of the water wall tube of an ultra-supercritical circulating fluidized bed (CFB) boiler for a better boiler design. The operating pressure ranges from 10.5 to 32MPa. The six-headed inner rifled tube used in the test has a diameter and wall thickness of 35mm and 5.67mm, respectively. Results show that the vapor quality during heat-transfer deterioration (vapor quality >0.8) increases with increased mass flux or decreased heat flux in the subcritical pressure regions. The effects of the pressure and heat flux on the inner-wall temperature in the supercritical pressure region are remarkable in the high-enthalpy region (enthalpy >2400kJkg−1). The effects of the heat flux and pressure on frictional pressure drop are more dominant than mass flux in both the subcritical and supercritical pressure regions. The optimized tube is found to exhibit good performance under ultra-supercritical parameters.