Supercritical Circulating Fluidized Bed Boiler Research Articles

Evaluation of effects of ammonia co-firing on the thermal performances of supercritical pulverized coal and circulating fluidized bed boilers

Ammonia is an effective fuel in reducing carbon dioxide due to a carbon-free fuel. Accordingly, the use of ammonia co-firing technology for carbon neutrality has emerged in the power generation industry. It is necessary to determine the optimal operating conditions by analyzing boiler performance during ammonia co-firing for optimal power plant operations. In this study, a process simulation was performed to determine the effect of ammonia co-firing on the thermal performances of supercritical pulverized coal (PC) and circulating fluidized bed (CFB) boilers. The target boilers were 870 and 550 MWe supercritical PC and CFB boilers, respectively. The process simulations were conducted using co-firing ratios of 5, 10, 15, 20, and 30%, as well as various coal grades and load conditions at an ammonia co-firing ratio of 20%. Although the amount of CO2 reduction during ammonia co-firing was confirmed, the radiation and convective heat transfer rates decreased due to changes in the flue gas composition and due to increased moisture loss. Accordingly, the plant efficiency decreased due to lower main and reheat steam temperatures. These findings can be used to establish optimal operating conditions for CO2 reduction and improve plant efficiency, which are operational trade-offs during ammonia co-firing.

Investigation on lateral dispersion characteristics of fuel particles in dense phase zone of a large-scale circulating fluidized bed boiler under combustion conditions

The dispersion characteristics of fuel particles in the dense phase zone in circulating fluidized bed (CFB) boilers have an important influence on bed temperature distribution and pollutant emissions. However, previous research in literature was mostly on small-scale apparatus, whose results could not be applied directly to large-scale CFB with multiple dispersion sources. To help solve this problem, we proposed a novel method to estimate the lateral dispersion coefficient (Dx) of fuel particles under partial coal cut-off condition in a 350 MW supercritical CFB boiler based on combustion and dispersion models. Meanwhile, we carried out experiments to obtain the Dx in the range of 0.1218–0.1406 m2/s. Numerical simulations were performed and the influence of operating conditions and furnace structure on fuel dispersion characteristics was investigated, the simulation value of Dx was validated against experimental data. Results revealed that the distribution of bed temperature caused by the fuel dispersion was mainly formed by char combustion. Because of the presence of intermediate water-cooled partition wall, the mixing and dispersion of fuel and bed material particles between the left and right sides of the furnace were hindered, increasing the non-uniformity of the bed temperature near furnace front wall.

Investigation on the internal and external solid flow rates in large-scale circulating fluidized bed with different layouts of suspended heating surfaces

Suspended Heating Surfaces (SHSs) in large-scale circulating fluidized bed (CFB) boilers have a significant impact on internal ( G s,i ) and external ( G s,o ) circulating solid flow rates that is important for heat transfer. In this paper, the distribution of G s,i and G s,o in a 650 × 680 × 3500 mm 3 CFB apparatus was simulated and verified by experimentations. On this foundation, the effects of SHSs structural layouts on G s,i and G s,o were investigated in a 350 MW supercritical CFB boiler. Results showed that G s,i increased by 65.8% as SHSs number increased from 0 to 20, whereas G s,o decreased by 17.8%, so G s,o / G s,i decreased correspondingly. Meanwhile, increasing SHSs height had a greater improvement on G s,i than width, which would be more conducive to the heat transfer near SHSs. Finally, empirical formulas of G s,o , G s,i and layout parameters of SHSs were developed, and the research results can provide meaningful reference for the operation optimization of large-scale CFB boilers. • Effects of suspended heating surface layout on circulating flow rates are studied. • The ratio variation of internal and external circulating flow rates is explored. • Internal circulating flow rate increases with increasing surface number and area. • Empirical formula of structural parameters on circulating flow rate is achieved.

Development and validation of a dynamic flowsheet model for a 350 MWe supercritical circulating fluidized bed boiler

The circulating fluidized bed (CFB) boiler is highly recommended in peak shaving for its wide load regulating range compared with the pulverized coal (PC) boiler. However, the research on dynamic flowsheet model of the CFB boiler is quite limited due to its complex flow regime. In this paper, both steady state and dynamic flowsheet model of a supercritical 350 MWe CFB boiler was developed based on Aspen Plus and Aspen Dynamics software coupled with comprehensively user defined combustion kinetics and heat transfer coefficient with FORTRAN subroutines. This model was then validated by the operating data of the 350 MWe supercritical CFB boiler. Based on this model, the dynamic characteristics of three typical step signal disturbances, coal flow rate, primary flow rate and secondary air flow rate, were further analyzed to provide theoretical references to dynamic operation. Also, the quantitative relation between the stabilization time and step signal amplitude was calculated. The calculated dynamic characteristics demonstrates that the heat distribution does not vary much after adding step signal disturbances of coal flow rate, primary air flow rate and secondary air flow rate. The step signal disturbance of secondary air flow rate has less influence to the CFB boiler system compared with that of primary air flow rate. The superheated steam temperature is more sensitive to step signal disturbances compared with reheated steam temperature. The higher step signal amplitude results in longer stabilization time. Above all, this dynamic flowsheet model can not only be used to conduct dynamic behavior prediction and analysis but also provide transfer functions to the further development of CFB automatic control system.

Experimental study and safety analysis on the heating surfaces in the 660 MW supercritical CFB boiler under sudden electricity failure

AbstractThe necessity of equipping with an emergency water supply system in a supercritical circulating fluidized bed (CFB) boiler has long been a controversial topic in the industry. In this study, the heating surface of a 660 MW supercritical CFB boiler at the Pingshuo Power Station of China Coal Group is taken as the research object. And a boiler electricity failure experiment was accomplished at the power plant, and the variations of thermal parameters during electricity failure were obtained from power plant distributed control system. By analyzing the physical process of the heating surfaces during electricity failure, a mathematical model of heat transfer to the heating surface was established and a calculation program in Fortran language was developed. In this study, the effect of different flow rates and operating times of the supply pumps on the safety of the heating surfaces were then investigated. The numerical simulation results show that an emergency supply pump with a flow rate of 250 t/h, activated within 120 s, ensures the safety of the heating surface. The results of this study have important implications for the selection of emergency supply pumps and the determination of flow rates and pressures for supercritical CFB boilers with external beds.

Coupled simulation and deep-learning prediction of combustion and heat transfer processes in supercritical CO2 CFB boiler

The supercritical CO2 (S-CO2) power cycle has a wide application prospect in coal-fired power generation field because it’s highly effective, compactly structured, and flexible of operation. To observe more accurate heat transfer and coal combustion characteristics in the circulating fluidized bed (CFB) with the distinctive S-CO2 boundary, a 3D computational fluid dynamics (CFD) simulation of the furnace-side combustion process treated by the multiphase particle-in-cell (MP-PIC) method was conducted in a 600 MW S-CO2 CFB boiler coupled with the heat transfer process on working fluid side based on the polynomial fitting calculation model. Furthermore, a novel method to predict simulation results via Radial Base Function (RBF) neural network was proposed to simplify the computational process, enhance the sample data fusion, and improve the prediction accuracy. Results show that staggered high-temperature fluid and high heat flux was a major concern in S-CO2 heating surface arrangement. The temperature rise of wall heaters was less than the conventional steam CFB, and the heat flux of spiral and vertical heat transfer tubes decreased along the tube. The predicted temperature distribution of tubes and cold walls was in a good agreement with the coupling simulation results, whose accuracy can meet the engineering requirements.

Modeling study on the dynamic characteristics in the full-loop of a 350 MW supercritical CFB boiler under load regulation

To improve the load regulation ability of the circulating fluidized bed (CFB) unit, investigating the hydrodynamic characteristics of the gas-solid flow in the CFB loop during such processes is of vital importance. In this work, based on the mass and pressure balance in the whole loop, a one-dimensional dynamic model on the gas-solid flow in the full-loop of an industrial CFB was established. In particular, the solid mass flow rate at the dust exit was correlated with the total inventory of the cyclone, and the re-circulating rate to the furnace was depicted by the pressure drop in the loop seal. The model was validated by the field test data of a 350 MW supercritical CFB boiler during a load increasing process. Subsequently, the impact of the load change rate and operating bed pressure on the hydrodynamic performance in the whole loop of the CFB boiler during the load reduction process were analyzed. The results show that the bed pressure in the furnace will fluctuate more violently when the load change rate increases, while the operating bed pressure has a weak effect on the load regulation process. These conclusions can provide theoretical guidance to the variable load operation of the CFB boiler.

Performance evaluation of a supercritical circulating fluidized bed boiler for power generation under flexible operation

It is well known that the Łagisza power plant in Poland is the world’s first supercritical circulating fluidized bed (CFB) boiler, whose commercial operation started on June 2009. It has attracted a great deal of interest and operational data are publicly available, therefore it has been chosen as the object of the present study aimed at assessing load and fuel flexibility of supercritical CFB plants. First, the thermal cycle was modelled, by means of the commercial code Thermoflex®, at nominal and part load conditions for validation purposes. After having verified the validity of the applied modelling and simulation tool, the advantage of having supercritical steam combined with CFB boiler over subcritical steam and pulverized coal (PC) boiler, respectively, was quantified in terms of electric efficiency. As a next step, the designed fuel, i.e. locally mined hard coal, was replaced with biomass: 100% biomass firing was taken into account in the case of subcritical CFB boiler whereas the maximum share of biomass with coal was set at 50% with supercritical CFB boiler, consistently with the guidelines provided by the world leading manufacturers of CFB units. A broad range of biomass types was tested to conceive mixtures of fuel capable of preserving quite high performance, despite the energy consumption in pretreatment. However, the overall efficiency penalty, due to biomass co-firing, was found to potentially undermine the benefit of supercritical steam conditions compared to conventional subcritical power cycles. Indeed, the use of low-quality biomass in thermal power generation based on steam Rankine cycle may frustrate efforts to push the steam cycle boundaries.

A comprehensive understanding of the non-uniform characteristics and regulation mechanism of six external loops in a 600 MW supercritical CFB boiler

As the carrier of material circulation, the uniformity of multi-parallel external loops directly affects the operation of large-scale circulating fluidized bed (CFB) boilers. Some non-uniform phenomena have been found, but the non-uniform characteristics and the absorptive capacity in full process of external loops under changeable operating parameters were seldom considered. To solve this, comprehensive cold/thermal field tests were conducted on the external loops of the first 600 MW supercritical CFB boiler. Operating characteristics of the loops and the fluidization uniformity of fluidized bed heat exchangers (FBHEs) under various cases were fully demonstrated. The influence of air-coal distribution on the operating non-uniformity of the external loops were analyzed, and the absorptive capacity to thermal deviations were quantified. The regulation mechanisms of cyclones and FBHEs were clarified, and the key reason of thermal deviations in FBHEs was also figured out. Eventually, a new coal feeding method to maintain heat balance in dense phase zone was proposed. The results of this paper fully reflected the operating characteristic of the full process in external loops and the response characteristics to air-coal adjustments, which could provide the most direct guidance for academic research as well as optimal design and operation of large-scale CFB boilers.

Study on thermal load test of water wall of 350MW supercritical circulating fluidized bed boiler

The thermal load distribution of water wall is the key parameter for the structural design of water walls in supercritical and ultra-supercritical circulating fluidized bed boiler (CFB). In this paper, the temperature distribution of the water wall of a 350MW supercritical CFB boiler under typical working conditions was measured by installing 118 temperature measuring points at the back-fire water wall tube and adjacent fins. The deviation in the heat load of water wall as a function of furnace height, width and depth was obtained as well. The experimental results show that temperature of the working medium in water-wall tube increases with increasing furnace height. The thermal deviation of the steam temperature in water wall tube in the direction of furnace width and depth is large at the initial boiler start-up period, the transition period from dry to wet state, and the low load condition. Despite these, the heat load distribution of water wall at the same furnace height remains the same. The results provide a reference for the structural design and operation optimization of supercritical and ultra-supercritical CFB boiler.

Development and Validation of a Dynamic Flowsheet Model for a 350 MWe Supercritical Circulating Fluidized Bed Boiler

The circulating fluidized bed (CFB) boiler is highly recommended in peak shaving for its wide load regulating range compared with the pulverized coal (PC) boiler. However, the research on dynamic flowsheet model of the CFB boiler is quite limited due to its complex flow regime. In this paper, both steady state and d ynamic flowsheet model of a supercritical 350 MWe CFB boiler was developed based on Aspen Plus and Aspen Dynamics software coupled with comprehensively user defined combustion kinetics and heat transfer coefficient with FORTRAN subroutines. This model was then validated by the operating data of the 350 MWe supercritical CFB boiler. Based on this model, the dynamic characteristics of three typical step signal disturbances, coal flow rate, primary flow rate and secondary air flow rate, were further analyzed to provide theoretical references to dynamic operation. Also, the quantitative relation between the stabilization time and step signal amplitude was calculated. Above all, this dynamic flowsheet model can not only be used to conduct dynamic behavior prediction and analysis but also provide transfer functions to the further development of CFB automatic control system.

Investigation on the heat transfer coefficient and uniformity in a polygonal furnace of a 350 MW supercritical CFB boiler

The first international 350 MW supercritical circulating fluidized bed (CFB) boiler with a polygonal furnace was put into operation successfully in February 2018. The temperature profiles of the furnace water walls on four different heights, the temperature profiles near the outlets of the furnace water walls and dual-side water walls were measured under boiler loads of 50%, 75% and 100% BMCR, respectively. The coefficient and uniformity of heat transfer were investigated according to the test results. The results indicate that the temperature deviation of the furnace water walls on each layer at four different heights is no more than 6 °C, which illustrates the reliable hydrodynamic safety of the water walls of the polygonal furnace. Under boiler load of 100% BMCR, the temperature deviations of the front, rear, left and right water walls near the outlets of water walls are no more than 37 °C, and the temperature deviations of the 11 dual-side water walls are no more than 31.1 °C. As the boiler load increase from 50% BMCR to 100% BMCR, the average heat transfer coefficient of water walls increases from 98.1 W/(m2·oC) to 154.2 W/(m2·oC).

A note on large-size supercritical CFB technology development

Circulating fluidized bed (CFB) technology has been continuously developing with an increasing number of large-capacity and supercritical CFB boilers being put into operation. This article reviews the current developments in large capacity CFB boilers in China and presents a note on the most recent development trends. It includes discussions on (1) gas-solid hydrodynamics in large-size CFB furnaces, (2) heat transfer on suspended surfaces, (3) combustion atmosphere and emissions, (4) 3-D CFD numerical simulation for large-scale supercritical CFB boilers, (5) integrated energy saving technologies, (6) optimal operation of an integrated desulfurization system, (7) erosion and protection of heat transfer surfaces, (8) problems in operation and their solutions, (9) combustion of coal with high alkali content.

Safety analysis on the water wall in the 350 MW supercritical CFB boiler under sudden electricity failure

The necessity of setting up the emergency water supply system in the supercritical circulating fluidized bed (CFB) boiler is still in controversy in the industry. Based on the research methods in the previous literature and the operating experience, this paper proposed three possible emergency strategies for sudden electricity failure. And by analyzing the physical process in the furnace under sudden electricity failure condition, a mathematical model consisted of the mass and energy conservation equation was built, and the accuracy of this model was validated by the experiment data from a supercritical CO2 experimental system. With the help of the model, this paper can quantitatively predict the dynamic process of the working fluid concerning temperature, pressure, and mass in the water wall in the 350 MW supercritical CFB boiler under the above three emergency strategies. In addition, an estimation of the pipe-broken time for the water wall under such condition was given. Results showed that the longest pipe-broken time was about 52 mins considering the pressure effect. Thus this paper can draw a preliminary conclusion that setting up an emergency water supply system was not a necessity in the 350 MW supercritical CFB boiler.

Investigation of gas-solid flow and temperature distribution uniformity of 350 MW supercritical CFB boiler with polygonal furnace

Based on the non-uniform gas-solid flow of large-scale circulating fluidized bed (CFB) boilers, an innovative polygon furnace technology was put forward. Combined with cold test, numerical calculation and boiler tests, the effects of three polygonal furnace structures on the uniformity of gas-solid flow and particle concentration distribution were investigated. The research results show the furnace adopts the chamfered-corner structure, the circulating flow rates of three circulation loops are greatly increased, there is no obvious particle agglomeration phenomenon in the chamfered corners center of the furnace, and the non-uniformity of the gas-solid flow and particle concentration distribution can be effectively improved. The maximum relative deviation of three cyclones inlet and outlet flue gas temperatures for 350 MW supercritical CFB boiler is less than 6.0%, and the temperature distribution of the furnace and three cyclones is relatively uniform, the technology demonstration of the first 350 MW supercritical CFB boiler with polygonal furnace is successful.

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.

Investigation on the operating characteristics of a 350 MWe supercritical CFB boiler with a polygonal furnace

A 350 MWe supercritical circulating fluidized bed (CFB) boiler with an optimized cyclone separator arrangement and an innovative polygonal furnace was successfully demonstrated. The boiler exhibited excellent uniformity in terms of bed material fluidization, bed temperature distribution, flue gas temperature distribution, and circulating ash temperature distribution. The standard deviation of the cross-sectional bed temperatures in the lower part of the furnace ranged between 12 and 20 °C under four different boiler loads. The bed temperature distribution on the rear wall was more uniform than that on the front wall. The flue gas temperature uniformity among three cyclone separator entrances was better than that among their exits. The flue gas temperature deviation between the middle cyclone separator entrance and exit was higher than that of the other two cyclone separators. The optimized cyclone separator arrangement improved the circulating ash temperature uniformity, and the circulating ash temperature deviation among three loop seals was 12–25 °C. During a continuous 168-hour trial operation, the average emission concentrations of NOx and SO2 were 17.1 mg/m3 and 22.1 mg/m3 (at 6% O2), respectively, which were lower than the Chinese ultra-low emission limits.

Research on heat absorption characteristics of high temperature heating surfaces of supercritical CFB boilers

Based on the field test and calculation, heat absorption characteristics and steam temperature deviation profile of high temperature heating surface of supercritical circulating fluidized bed (CFB) boilers with different heating surface arrangement were studied. The results indicate that heat absorption shares of the two supercritical CFB boielrs are almost the same in different water stage, and heat absorption share of EHE is about 17.6% in the boiler arranged with EHE. There exits steam temperature deviation at the same tube panel of high temperature superheaters and high temperature reheaters inside the furnace, and deviation among high temperature reheater tube panel is larger. Rational control of enthalpy increment of high temperature tube panels inside the furnace has effect on reduction of steam temperature deviation.

Ash formation characteristics of two Indonesian coals and the change of ash properties with particle size

It is difficult to maintain enough high circulating ash rate in circulating fluidized bed (CFB) boilers if they burn the Indonesian coals due to their low ash content. Hence, the ash formation data for these coals is of significance to assessing mass balance in CFB boilers. Two kinds of Indonesian coal that were burned in the 550 MW supercritical CFB boilers in Korea, Kideco and KCH, were tested with static combustion and cold sieving method to study their ash formation characteristics. Results showed that the average primary ash particle size of these two coals is quite fine overall and nearly no gangue exists. Besides, their attrition rate constant Kaf is much higher than that of some other bituminous or lean coals, and with the increase of particle size, the Kaf decays exponentially for Kideco, while it initially increases and then decreases for KCH. In addition, the physical and chemical properties of these two Indonesian coals' ash obviously change in particle size, including density, pore structure, chemical composition, etc. The larger ash particles are composed of some hard materials, such as quartz (SiO2) and mullite (Al6Si2O13). While some other materials with lower hardness value, like hematite (Fe2O3) and gehlenite (Ca2Al2SiO7), become major mineral components of smaller and softer ash particles. These results indicate that the difference of chemical compositions results in the discrepancy of attrition capability for different size ash particles.

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.