Large-scale Circulating Fluidized Bed Research Articles

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.

Formation of agglomerates with core–shell structure in a large-scale CFB boiler

Cylindrical agglomerates with cross-sectional diameters over 200 mm are found in the bottom ash discharging chute of a 550 MWe large-scale circulating fluidized bed (CFB) boiler, which results in disfunction of the bottom ash discharging system due to the blockage of the bottom ash discharging chute. The agglomerates with core–shell structure are found to be significant to understand the mechanisms of the agglomeration, and agglomerates with similar structure are also found inside the furnace. The X-ray diffraction analysis on the core–shell structured agglomerate particle shows both the core and the shell comprise a significant amount of mullite; meanwhile, cristobalite is found in the shell, which is supposed to be formed at temperature higher than the bed temperature. The results of the transmission electron microscopy analysis show that mullite crystal in the shell is distributed inside the amorphous SiO2. Sintering experiments show that the formation temperature for mullite and cristobalite is ca. 1050 ℃, while the transformation temperature of quartz to amorphous silica is ca. 1150 ℃, which is much higher than the bed temperature. Thus, the agglomeration is supposed to be resulted from the high-temperature condition caused by the burning char inside the furnace, and the core–shell structure of the agglomerates is supposed to be formed due to the temperature gradient along the particle diameter of the burning char.

MP-PIC simulation on CO2 gasification of biomass in a pilot plant circulating fluidized bed gasifier

CO2 gasification of biomass offers a potential carbon-negative solution which combines biomass-to-energy conversion with carbon capture and utilization. In this study, biomass gasification with CO2 in a pilot plant circulating fluidized bed (CFB) gasifier is numerical investigated via a reactive multiphase particle-in-cell (MP-PIC) model. Gas-solid hydrodynamics and thermochemical characteristics under different percentages of CO2 in gasifying agent (mixed with air from 0 to 100 wt%) are comprehensively analyzed, in terms of the biomass conversion, bed temperature, gas species distributions, chemical reaction rates, product gas yields, lower heating value of product gas (LHVgas) and cold gas efficiency (CGE). Simulation results indicate that the CO2 percentage in gasifying agent has insignificant influence on gas–solid distribution, while it directly affects the char conversion and product gas compositions. CO2 gasification favors the CO production, increases the CO/H2 ratio in product gas and mitigates the CO2 emission. However, increasing the CO2 percentage in gasifying agent results in a decreased char conversion rate and reduced bed temperature, which restricts the use of pure CO2 as gasifying agent. The optimal CO2 percentage occurs to 60 wt% where the gasifier has the best thermochemical performance, i.e., the highest compositions of combustible gases, the largest LHVgas and CGE. This research sheds light on biomass gasification with CO2 in a large-scale CFB facility, and would be helpful for the design and scale-up of this technology.

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.

Thermal and flow calculations of platen superheater in large scale CFB boiler

Circulating Fluidized Bed (CFB) boilers are desirable technology to utilize solid fuels, operate within a wide load range, and promote the high flexibility in burning a wide range of coal and other solid fuels, keeping high efficiency and low gaseous pollution. The paper presents thermal and flow modeling results of steam superheater operated in the large-scale coal-fired CFB boiler, both in steady and transient states. The mathematical model of the superheater wall was developed using Control Volume-based Finite Element Method. The heat transfer coefficient as the sum of radiative and convective heat transfer coefficients was considered in the presented approach to modeling thermal processes in a circulating fluidized bed. The overall heat transfer coefficient was computed for the analyzed type of the boiler at full load and steady-state conditions using available measuring data and results obtained using Computational Fluid Dynamics (CFD) modeling. Depending on the modeling approach adopted in the CFD model, the heat transfer coefficient at full boiler load varies between 159.96 and 189.63 W/(m2·K). In the analyzed period of boiler start-up, results of steam outlet temperature (455 °C–485 °C), heat transfer coefficients on the flue-gas side (100 W/(m2∙K) and 120 W/(m2∙K)), and heat transfer coefficients on the steam-side (925–2008 W/(m2∙K)) vary with time. The methods presented in the paper can be applied to modeling transient flow and thermal processes in superheaters with complicated flow arrangements. The advantage of the proposed method is a short computing time compared to detailed and complex 3D models. The developed model of the superheater with the transfer coefficient determination methods can be used to perform calculations in the full range of boiler load, both during steady and transient boiler operation.

A dynamic nonlinear model used for controller design of a 600 MW supercritical circulating fluidized bed boiler-turbine unit

• Energy storage characteristic and variable coal quality are considered in modeling. • Parameters in model change with different load condition to improve model accuracy. • The model is validated by a 600 MW supercritical once-through CFB unit. • Open-loop step responses show the different inertia and nonlinearity of CCS. • Transfer function matrix of CCS is derived from the dynamic nonlinear model. Large-scale circulating fluidized bed (CFB) power generation units with high steam pressure and temperature have been put into operation in order to obtain high cycle efficiency and less emissions. To ensure the flexible operation of such CFB units, it is necessary to build a model for the design of coordinated control system (CCS). However, few studies focus on modeling of such CFB units. Thus, this work develops a mechanistic dynamic nonlinear model of a large-scale supercritical CFB unit, which is suitable for CCS design because of its relatively low complexity. Firstly, model structure is derived from mass and energy conservation laws, combined with analysis of operational characteristics of a 600 MW supercritical CFB unit. Then, unknown parameters and nonlinear functions in model are identified based on running data by using regression analysis and optimization algorithm. Validation results show that the model has satisfactory accuracy and it can capture essential dynamic characteristics of the unit and nonlinearity of CCS. More importantly, the model accuracy is further improved by proposed variable dynamic parameters and correction of coal quality. After this, transfer function matrix of CCS is derived from dynamic nonlinear model, which can be applied to controller design and simulation analysis.

Prediction of attrition rate of coal ash for fluidized bed based on chemical composition with an artificial neural network model

Attrition of ash has a significant effect on the performance of a circulating fluidized bed (CFB) combustor, and the attrition rate coefficient Kaf is widely used to analyze the mass balance of the CFB combustor. In this study, a four-layer artificial neural network (ANN) model is developed to estimate the value of Kaf according to the chemical components of the ash based on a training database consisting of 40 sets of samples. An optimum structure comprising two hidden layers with ten neurons in each layer is adopted, and the mean square error for training and validation stages is reduced to 0.00175 and 0.13842, respectively. To verify the validity of the model, field tests are conducted in a large-scale CFB boiler by burning two types of coal with different blending ratios. The Kaf of the two kinds of coal is estimated using the trained ANN model. The ratio of the coal ash discharged via fly ash to the total coal ash decreases, while the Sauter mean diameter of the circulating material increases with an increase in the blending ratio of the coal with a smaller value of estimated Kaf; this exhibits good validity of the model.

Solids back-mixing in the transport zone of circulating fluidized bed boilers

This work investigates the back-mixing of solids in the transport zone of large-scale circulating fluidized bed (CFB) boilers, with the aims of identifying and evaluating the governing mechanisms and providing a mathematical description based on a solid theoretical background rather than on purely empirical correlations. In addition, transient Direct Numerical Simulation (DNS) modeling is used to identify the mechanism that drives migration of the solids from the dilute up-flow in the core region to the down-flow at the furnace walls. Previously published concentration and pressure profiles are collated and analyzed through modeling of the steady-state mass balance of the dispersed solids in the transport zone. The study shows that solids back-mixing at the furnace wall layers is limited (hence governed) by the core-to-wall layer mass transfer transport mechanism rather than by the lateral movement of solids within the core region. The latter is shown by the 3-dimensional (3D) mass balance model, and the transient DNS modeling indicates that this is due to a turbophoresis mechanism. We also show that the use of Pe-numbers to describe the lateral solids dispersion is not straightforward but rather depends on the unit scale, and that Pe-numbers < 26 are needed to yield the solids back-mixing rates measured in large-scale CFB boilers. Finally, we propose a mathematical expression for the core-to-wall layer mass transfer coefficient derived from a Sherwood number (Sh)-correlation fitted to measured values of the characteristic decay constant that result from the solids back-mixing. This expression shows better agreement with the large-scale measurements than do the expressions given in the literature.

An Experimental Study on the Characteristics of NOx Distributions at the SNCR Inlets of a Large-Scale CFB Boiler

The unknown NOx distributions inside large-scale CFB (circulating fluidized bed) boilers have always hindered the economy of the SNCR (selective non-catalytic reduction) process. In this study, field tests were carried out on a typical 300 MW CFB boiler, where multi-level 316 L-made probe and Ecom-J2KN/Testo 350 analyzers were used to perform detailed two-dimensional distributions of flue gas composition at SNCR inlets for the first time. The penetration depth inside the horizontal flue pass was up to 7 m. The NOx distributions were analyzed in detail combining with the auxiliary test in the dilute phase zone. Key results show that the average O2 concentrations in #A and #C regions were 6.52% and 0.95%, respectively. The vertical NOx distributions of #A and #C SNCR inlets were similar, showing a trend of first increasing and then decreasing with peak value all appeared at 5 m depth, while the NOx distribution of #B SNCR inlet was basically increasing. Some local areas with extremely high NOx concentration (over 2000 mg/m3) were observed near the inclined edge of SNCR inlets, which has never been reported before. Based on this, the optimization of urea injections was conducted, which could save 15.7% of the urea solution consumption while ensuring ultra-low emission of NOx.

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.

Effects of particle size on bed-to-surface heat transfer in bubbling fluidized bed heat exchangers of 550 MWe ultra-supercritical CFB boiler

Fluidized bed heat exchangers (FBHEs) are widely used as result of the scale-up in the capacity of circulating fluidized bed (CFB) boilers. In this study, the effect of particle size on the bed-to-surface heat transfer in the bubbling FBHE of a 550 MWe ultra-supercritical CFB boiler is investigated. The bed-to-surface heat transfer coefficient (hw) is experimentally obtained via field test; the results show that hw increases with decreasing particle sizes. A packet renewal model is proposed using the correlation of hydrodynamic parameters based on the experimental data of particles whose sizes approximate those of particles in the FBHE. The predicted hw value exhibits good agreement with that obtained via field test to within ±18.5%; however, it fails to predict the effect of particle size on hw. A single particle model is also adopted to evaluate hw. The derived results agree well with field test results, the predicted hw increases with decreasing particle sizes. In this work, it is observed that a finer particle size, which can be achieved by improving the cyclone efficiency, can enhance heat transfer in the FBHE. The results of this study are anticipated to offer engineering guidance for the design and operation of large-scale CFB boilers.

Experimental Investigations on Lateral Dispersion Coefficients of Fuel Particles in Large-Scale Circulating Fluidized Bed Boilers with Different Coal Feeding Modes

The dispersion characteristics of fuel particles over the bed have always been a key consideration in the design of fluidized beds. However, the lateral fuel dispersion coefficients (Dsr) reported in the previous literature widely ranged from 10−4 to 10−1 m2/s, which were hard to be directly applied in large-scale CFB (circulating fluidized bed) boilers with wider bed width (close to 30 m) and variable coal feeding modes. To solve this problem, field tests were conducted on the first 600 MW CFB boiler in this paper, and the distributions of flue gas composition and flue gas temperature were obtained near the coal feeding port under four working conditions. The dispersion characteristics of fuel particles were analyzed in detail combined with the dispersion model. In addition, a new method based on bed heat balance was also proposed to estimate Dsr and applied in a 300 MW CFB boiler with a different coal feeding mode. The results revealed that Dsr through the local heat balance method ranged from 0.10–0.35 m2/s, which were 9–18% higher than the values through the dispersion model. Compared with the coal feeding port immersed in the bed, Dsr of the fuel particles fed into the splash zone increased by about 34.7%. Dsr under the coal feeding mode of a screw feeder with pressurized air ranged from 0.12–0.16 m2/s, which were lower than Dsr under the coal feeding method of importing fuel into circulating ash. The method and results in this paper could expand the applicability of fuel dispersion, and provide the most direct guidance for the design and optimization of coal feeding ports in large-scale CFB boilers.

A Comprehensive Three-Dimensional Analysis of a Large-Scale Multi-Fuel CFB Boiler Burning Coal and Syngas. Part 1. The CFD Model of a Large-Scale Multi-Fuel CFB Combustion.

The paper is focused on the idea of multi-fuel combustion in a large-scale circulating fluidized bed (CFB) boiler. The article discusses the concept of simultaneous coal and syngas combustion. A comprehensive three-dimensional computational fluid dynamics (CFD) model is developed, which allows us to describe complex phenomena that occur in the combustion chamber of the CFB boiler burning coal and syngas produced from coal sludge.

A Comprehensive, Three-Dimensional Analysis of a Large-Scale, Multi-Fuel, CFB Boiler Burning Coal and Syngas. Part 2. Numerical Simulations of Coal and Syngas Co-Combustion.

This paper presents the results of numerical computations for a large-scale OFz-425 CFB (circulating fluidized bed) boiler utilizing coal and syngas. Four different operating scenarios are considered, including the reference variant, corresponding to the conventional, mono-combustion of bituminous coal, and three tests involving replacement of secondary air and part of the coal stream with syngas fed by start-up burners. Pressure, gas velocity, temperature, and carbon dioxide distribution in the combustion chamber are discussed in the paper. The results indicate that the syngas supply leads to an increase in local temperature and carbon dioxide concentrations. The proposed concept is not advisable as it may lead to frequent emergency stops of the CFB boiler.

Development and technical progress in large-scale circulating fluidized bed boiler in China

Circulating fluidized bed (CFB) boilers has realized the clean and efficient utilization of inferior coal like gangue and coal slime, high sulfur coal, anthracite, petroleum coke, oil shale and other resources. As a country with the largest amount of CFB boilers and the largest installed capacity in the world, China has 440 100–600 MWe CFB boilers with a total capacity of 82.29 GWe, including 227 units of 135 MWe, 95 units of 300 MWe, and 24 supercritical units. The statistics of typical 100–300 MWe CFB boilers showed that the average number of unplanned shut-down was only 0.37 times per year, among which the 135 MWe was 0.26 times per year and 300 MWe was 0.46 times per year. The auxiliary power ratio of some 300 MWe CFB boilers based on flow-pattern reconstruction can be reduced to about 4%, which is closed to the same level of pulverized coal (PC) boilers. This paper summarizes the development process and application status of China’s large-scale CFB boilers, analyzes the characteristics and technical performance of the iconic units, and introduces solutions to the problems such as water wall wear and bottom ash cooling.

Numerical study of coal gasification in a dual-CFB Plant based on the generalized drag model QC-EMMS

The gas-solid fluidization and coal gasification reactions in a newly-designed dual circulating fluidized bed (CFB) plant were analyzed by full-loop numerical simulation to provide support for industrial applications. The generalized drag model, QC-EMMS, was used in the Eulerian-Eulerian approach to model the entire coal gasification process in a 60 m high dual-CFB pilot plant with a capacity of 100 tons coal per day. The predicted pressures, temperatures, and syngas compositions are more consistent with field test data compared with the results predicted by using the conventional Gidaspow drag model. The unmeasured solid mass circulation rates were accurately predicted by the QC-EMMS model. The characteristics of gas-solid fluidization and coal gasification were estimated by analyzing the flow and reaction details in the plant. These results show that the QC-EMMS drag model has advantages in simulating the complicated gas-solid flows and reactions in large-scale CFB plants.

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.

Experimental analysis of phase segregation in gas-solid circulating fluidized bed riser with direct image calibration

As a widely-used reactor, the circulating fluidized bed (CFB) riser aims to offer an efficient space for the gas-particle contact. However, particles in the CFB riser aggregate to form clusters, which significantly influences the contact efficiency. However, cluster properties in the literature deviate up to one order of magnitude, even at similar operating conditions. This circumstance hinders the understanding of fluidization and, to a large extent, attributes to subjective methods to identify the cluster boundary. In this study, cold-flow experiments were conducted in a large-scale CFB (7.6 m high) and the flow behavior in the transparent rectangular riser was well recorded using a high-speed camera. With direct image calibration, the grayscale in images was converted to solids holdup pixel by pixel, then solids holdup gradient was computed to explore the cluster boundary. With the verification using optical fiber probe, an abrupt increase of solids holdup is found to exist around clusters, evidencing the phase segregation. Then, the cluster boundary is determined objectively to be where solids holdup increases most abruptly, and clusters are characterized systematically across the riser in terms of solids holdup, projected width, projected length, equivalent diameter and circularity. Collectively, this study provides an objective and effective method to qualify clusters based on solids holdup distribution, so as to achieve a reliable characterization of clusters.

Capturing the instantaneous flow structure in gas-solid circulating fluidized bed using high-speed imaging and fiber optic sensing

Knowing the instantaneous flow structure in a gas-solid circulating fluidized bed (CFB) is extremely crucial for the understanding of fast fluidization and the development of numerical models. Up to now, numerous studies have been reported on the instantaneous flow structure, but essential information, such as phase classification, solids holdup mapping, macro-scale fluctuation and much more, remains insufficient for a thorough understanding. In this work, the instantaneous flow structure was captured in large-scale CFBs using high-speed imaging and fiber optic sensing. The instantaneous solids holdup mapping across the riser is first computed from images with a verified calibration between solids holdup and grayscale. Based on the solids holdup characteristics, the gas-solid flow in a CFB riser is classified into distinctive phases. Macroscopically, there is a trough phase having continuous low solids holdup and a crest phase having continuous high solids holdup, causing the mean solids holdup across the riser to fluctuate significantly. Within the trough phase, there are trough clusters (particle aggregations) surrounded by dispersed particles (dilute atmosphere). Within the crest phase, there are crest clusters (particle aggregations) surrounded by coalesced particles (dense atmosphere). The above flow structure is further verified with the instantaneous solids holdup obtained using fiber optic sensing in terms of solids holdup characteristics and phase dimensions. Collectively, this study deepens the understanding of fast fluidization, prepares for further phase characterization, and contributes to the development of numerical modeling.

A material-property-dependent sub-grid drag model for coarse-grained simulation of 3D large-scale CFB risers

Developing, verifying and validating sub-grid methods is of crucial significance for enabling accurate coarse-grained two-fluid modeling of rapid gas-fluidized flows. However, very few studies in the literature so far have been focused on how the sub-grid modification depends on material properties. As an extension of previous sub-grid efforts, this fundamental investigation attempts to derive a material-property-dependent drag modification based on generated data from an initially homogeneous state of periodic gas-particle suspensions. The newly constituted model is then verified by highly-resolved two-fluid simulation results. We further validate the accuracy of model predictions via systematic assessments to experimental results that encompass a wide variety of material properties in five three-dimensional large-scale circulating fluidized bed (CFB) risers. Besides, we introduce a deviation index (DI) to quantify the predictive capability of the extended model. Computational results demonstrate an essential dependence of drag correction on material properties as an additional factor. Hydrodynamic validation predictions achieve satisfactory accordance with experiments. The current model is potential to serve as a more general tool for efficiently reducing the number of pilot-scale tests and effectively designing and controlling industrial process devices.