Large-scale Circulating Fluidized Bed Boilers Research Articles

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.

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.

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.

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.

Integrated control and process design in CFB boiler design and control - application possibilities

Integrated control and process design (ICPD) practices focus on an interaction between process and control design. The paper investigates ICPD design in circulating fluidized bed (CFB) power plants, which face increasing load change, efficiency and emission requirements. The state of ICPD research is examined and a classification of its methodologies is provided. The applicability of ICPD to large-scale CFB boilers is discussed for the first time based on this classification. Two ICPD case studies with a simple steam path mass storage model are presented for an industrial CFB boiler, with the aim of illustrating possibilities and challenges related to boiler ICPD. The steam mass storage amounts of the boiler superheating and evaporation sections are modified based on the dynamic relative gain array and closed-loop process optimization to generate processes with improved constant pressure mode output power setpoint tracking performance.

Design of large scale oxy-fuel fluidized bed boilers: Constant thermal power and constant furnace size scenarios

This work investigates the design of large scale oxy-fuel circulating fluidized bed (CFB) boilers in two industrial pathways of constant thermal power scenario and constant furnace size scenario using a new version of a model validated with 100 kWth and 4 MWth oxy-fuel CFB boilers. This work suggests that the major approach for elevating the circulating solids flux is to utilize more efficient cyclones with better efficiency which leads to higher share of fines and nanoparticles in the CFB loop while the amount of aerosols is reduced. Maximum heat flux from furnace walls is 2.4 times and CO peak is 2.2 times in constant furnace size scenario compared to constant thermal power scenario respectively, showing more intense combustion in the prior scenario. Based on the results found in this work, if the industry is to aim to operate the large-scale oxy-fuel CFB boilers at high O2 concentrations, the utilization of the constant thermal power scenario is much safer for the industry compared to constant furnace size scenario. This work shows that the aspect ratio of future large-scale oxy-fuel CFB furnaces are larger than the current air-fired ones leading the oxy-CFB furnace geometry to approach the fluid catalytic cracking fluidized beds.

Study on heat transfer characteristics of the high temperature reheater tube panel in a 300 MW CFB boiler with fluidized bed heat exchanger

On the basis of measurements on the fluidized bed heat exchanger (FBHE) of a 300 MW lean-coal-fired circulating fluidized bed (CFB) boiler under different thermal loads and numerical calculation for steam mass flow rate distribution inside the high temperature reheater (HTR) tube panels, the ash-side heat transfer coefficient distribution along the ash movement direction inside the FBHE was obtained in this paper. A cell model with energy and material balances was also developed for the FBHE. The measured and calculated results indicate that the post-combustion phenomenon exists in the FBHE. The ash-flow movement inside the FBHE has an important impact on the bed-to-tube heat transferring process. Specifically, along the ash moving direction, steam temperature at the outlet of corresponding tube panel decreases gradually, as well as the local ash-side heat transfer coefficient. These results can provide referential materials for design and operation of the FBHE in large-scale CFB boilers.

Modelling and experimental investigation of the full-loop gas–solid flow in a circulating fluidized bed with six cyclone separators

In the literature, there are few reports on the full-loop gas–solid flow in a circulating fluidized bed (CFB) with large scale and complex cyclone arrangement. In this paper, a new approach based on computational particle fluid dynamic (CPFD) method combined with electrical capacitance tomography (ECT) is used to investigate the hydrodynamic behavior of gas–solid flow in a CFB with six cyclone separators in order to improve the design and performance of a large scale CFB boiler. The full-loop CFB system for the simulation includes the CFB riser, cyclone, standpipe and U-loop. Two types of cyclone arrangement, i.e. axis and point based symmetric arrangement, are used for the CPFD simulation and ECT measurements. To validate the CPFD simulation, ECT is applied to measure the solids concentration in the standpipe with eight electrodes mounted on the outside of the standpipe. Key parameters including pressure, solids recirculation flux and velocity profile along different positions based on the CPFD simulation are analyzed and compared with experimental results. The CPFD simulation shows that the gas–solid flow is non-uniform among the six parallel cyclones. The solids concentration of four cyclones at the corner of the riser is higher than that of the others. The location of cyclone as well as the inlet angle of the cyclone needs to be optimized. The study shows that the presented approach based on CPFD simulation and ECT measurements can be used to optimize the arrangement of cyclone separators in a supercritical pressure circulating fluidized bed system.

Modeling of particle transport and combustion phenomena in a large-scale circulating fluidized bed boiler using a hybrid Euler–Lagrange approach

The constantly developing fluidized combustion technology has become competitive with a conventional pulverized coal (PC) combustion. Circulating fluidized bed (CFB) boilers can be a good alternative to PC boilers due to their robustness and lower sensitivity to the fuel quality. However, appropriate engineering tools that can be used to model and optimize the construction and operating parameters of a CFB boiler still require development. This paper presents the application of a relatively novel hybrid Euler–Lagrange approach to model the dense gas–solid flow combined with a combustion process in a large-scale industrial CFB boiler. In this work, this complex flow has been resolved by applying the ANSYS FLUENT 14.0 commercial computational fluid dynamics (CFD) code. To accurately resolve the multiphase flow, the original CFD code has been extended by additional user-defined functions. These functions were used to control the boiler mass load, particle recirculation process (simplified boiler geometry), and interphase hydrodynamic properties. This work was split into two parts. In the first part, which is referred to as pseudo combustion, the combustion process was not directly simulated. Instead, the effect of the chemical reactions was simulated by modifying the density of the continuous phase so that it corresponded to the mean temperature and composition of the flue gases. In this stage, the particle transport was simulated using the standard Euler–Euler and novel hybrid Euler–Lagrange approaches. The obtained results were compared against measured data, and both models were compared to each other. In the second part, the numerical model was enhanced by including the chemistry and physics of combustion. To the best of the authors’ knowledge, the use of the hybrid Euler–Lagrange approach to model combustion is a new engineering application of this model. In this work, the combustion process was modeled for air-fuel combustion. The simulation results were compared with experimental data. The performed numerical simulations showed the applicability of the hybrid dense discrete phase model approach to model the combustion process in large-scale industrial CFB boilers.

Investigation on Heat Transfer Performance of External Heat Exchanger for Circulating Fluidized Bed Boiler

A heat transfer model of external heat exchanger (EHE) in large-scale circulating fluidized bed (CFB) boiler was suggested based on experiment data and theoretical analysis. And the model parameters were also provided. With this model, the heat transfer performance of the EHE in an actual operating 300MW CFB boiler was predicted. The comparison between the model predictive results and the actual results showed that the relative error is less than 7%. This indicated that the model established in this paper is reliable and can provide a significant reference in EHE of large-scale CFB boiler design.

Heat transfer characteristics of fluidized bed heat exchanger in a 300 MW CFB boiler

In order to investigate the heat transfer characteristics of fluidized bed heat exchanger (FBHE), a series of experiments was carried out in a commercial 300MW circulating fluidized bed (CFB) boiler with FBHE. The parameters of steam, solids and air in FBHE were measured at different boiler loads, based on which the absorbed heat and heat transfer coefficient were calculated. Further study indicates that when the calculated results are applied to the design of large-scale CFB boilers, the bed side heat transfer coefficient in FBHE can be simplified as the function of solids temperature and flow. Therefore, the empirical model of heat transfer coefficient at bed side is put forward. The deviation between calculated results and measured values is acceptable in engineering application. This model provides strong support for the FBHE design in 600MW supercritical CFB boilers.