Circulating Fluidized Bed Research Articles

Effects of tube configurations on the heat transfer and hydrodynamic behavior in a gas-solid fluidized bed

The study of surface-to-bed heat transfer and flow characteristics in a gas-solid fluidized bed with different immersed tube configurations were investigated using the Eulerian-Eulerian method coupled with the kinetic theory of granular flow (KTGF) computationally. Compared with cases of in-line configurations, the staggered configurations increased the heat transfer coefficients (HTC) of the center-immersed heating tube. Moreover, it was found that with an increase in the rows and columns of the configurations, the HTCs increased and decreased respectively. The surface-to-bed heat transfer process was investigated combined with the particle renewal mechanism, and it was demonstrated that the HTCs were significantly affected by the surrounding packet fraction distribution and the packet residence time on the immersed heating surface. The results could be quantitatively employed to aid the heating tube configuration in designing the external heat exchanger (EHE) of a circulating fluidized bed (CFB) boiler for the treatment of municipal solid waste (MSW).

Simulation of the L-valve in the circulating fluidized bed with a coarse-grained discrete particle method

Stable and controllable solid flow is essential in circulating fluidized bed (CFB) systems. The L-valve is a typical non-mechanical valve that can provide flexible solid feeding. The investigation of the solid circulation rate and the hydrodynamic characteristics of the L-valve is crucial to its design and operation. The gas-solid flow in the L-valve of a full-loop CFB is studied with the coarse-grained discrete particle method (EMMS-DPM). Good agreements on the solid circulation rate and the pressure drop through the L-valve are achieved between the simulated and experimental data. The solid circulation rate increases linearly with the aeration velocity until the stable particle circulation of the CFB is destroyed. The flow patterns in the horizontal section of L-valve are gas-solid slug flow above the stationary solid layer and the moving solid layer, respectively. The effects of L-valve geometric parameters on the solid flow characteristics are also investigated. The results indicate that reducing the diameter and length of the horizontal section of L-valve can improve the solid transport efficiency, especially at low aeration velocity. Besides, the solid conveying capacity and flow stability are improved when the sharp bend of L-valve is modified to be a gradual bend.

Distribution of PCDD/Fs and PCBs at different locations in a circulating fluidised bed municipal solid waste incinerator.

This study investigates a circulating fluidised bed (CFB) incineration plant to examine the concentrations and fingerprints of polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), and biphenyls (PCBs) at five locations downstream of the post-combustion zone. Sampling encompassed both flue gas and ash, spanning from the high-temperature superheater to the outlet of the baghouse filter, thus covering a wide range of flue gas temperatures. The analysis reveals a continuous increase in PCDD/F and PCB concentrations in the flue gas from the superheater to the inlet of the air pollution control system (APCS). The maximum concentrations observed were 75.8ng/Nm3 for PCDDs, 219ng/Nm3 for PCDFs, and 763ng/Nm3 for PCBs. These values represent 9.14, 11.5, and 6.37 times their respective concentrations at the outlet of the high-temperature superheater. Concurrently, the levels of PCDD/Fs and dioxin-like PCBs (dl-PCBs) in the ash steadily increased along the cooling path of the flue gas within the plant. Comparing dl-PCBs to the total amount of 209 PCB congeners, it was evident that dl-PCBs exhibited a trend more akin to that of PCDD/Fs. A robust linear correlation was observed between dl-PCBs and PCDD/Fs (R2 = 0.99, p < 0.001), surpassing that between PCBs and PCDD/Fs (R2 = 0.92, p < 0.01), suggesting that dl-PCBs share closer formation pathways with PCDD/Fs. Additionally, elemental composition analysis of fly ash samples aimed to explore potential links between fly ash characteristics and PCDD/F and PCB formation. The Cl/S ratio increased from 1.58 to 5.13 with decreasing flue gas temperature. Principal component analysis (PCA) was employed to visualise the concentrations of PCDD/Fs and PCBs in the flue gas alongside elemental contents in the fly ash. With the exception of PCBs in ash, all other PCDD/Fs and PCBs in fly ash exhibited positive correlations with both carbon (C) and chlorine (Cl). Furthermore, a positive relationship between C/Cl and PCDD/Fs-PCBs in fly ash implies that fly ash serves as the primary reaction surface for dioxin generation during low-temperature heterogeneous catalytic reactions.

CFD simulation of a circulating fluidized bed carbon capture system using a solid-supported amine sorbent

Carbon capture, utilization, and sequestration (CCS) is one of the key technologies to reduce the emission of carbon dioxide, including that from exiting flue gas of fossil fuel-fired power plants. In this study, the two-fluid model approach was used to simulate the hydrodynamics of the entire circulating fluidized bed (CFB) loop of the integrated carbonation and regeneration sections using the mass and momentum equations and constitutive relations for both the gas and solid phases. Then, the verified models of the non-reactive simulation of the CFB loop were combined with species, energy equations, and carbonation regeneration rate of reactions for both phases to simulate the entire circulating fluidized bed carbon capture unit. The predicted extent of CO2 capture in this system showed the potential of continuous carbon capture using amine-based solid sorbents through the circulating fluidized bed CO2 capture unit.

Numerical study of hydrodynamics processes fluidized bed

Boiler units with a circulating fluidized bed (CFB) have become widespread in foreign countries, since the CFB technology has proven itself to be an effective way to burn various types of solid fuels. Traditionally, when creating domestic boilers, a normative method is used, but it does not take into account the peculiarities of the movement of the fluidized bed. The hydrodynamic characteristics of the layer have a significant impact on the design of the combustion chamber. To determine them, both mathematical and numerical modeling methods are used. As a result of mathematical calculation, the dependence of the fluidization mode on the air supply speed was revealed. Also, through numerical modeling, it was possible to establish the correspondence of the particle motion mode with the data from the mathematical model.

Modern design solutions in the design of boiler units, ovens and fireboxes burning solid fuel in a fluidified bed

Coal fuel has always played an important role in Russia’s fuel and energy balance due to its extensive reserves, availability in a number of regions and relatively low cost. Currently, coal flaring technology is widely used in domestic boiler plants, but it should be noted that this approach is more applicable for high-quality high-calorie coals with low ash content. In addition, flaring is sensitive to changes in the composition of the fuel and deviations from the specified values can lead to a decrease in the reliability of the boiler unit. There are also regions where there are deposits of low-grade coal that are not suitable for flaring. The problem of boiler sensitivity to the use of unspecified fuels, as well as the involvement of local low-grade fuels, can be solved by introducing circulating fluidized bed (CFB) combustion technology.

A method for measuring the solid circulation rates of CFB boilers based on the particle flow-around principle

The solid circulation rate (Gs) is a key parameter for the design and operation of a circulating fluidized bed (CFB) industrial equipment, while the detection and measurement of the Gs value in an actual industrial CFB equipment remains difficult. To fill this gap, a method for the Gs measurement based on the flow-around principle was proposed in this study. The mathematical model for the signal transformation was established along the path “detected strain value – impact force – particle velocity – Gs value”, and the corresponding measurement device was also developed. The performance of the Gs measurement device was quantitatively validated in cold-state experiments and semi-quantitatively verified in an industrial trial in a CFB boiler. The results of a theoretical analysis indicate that the detected strain linearly increased with the impact force received by the intrusive spherical obstacle, and the slope coefficient was determined as per the experiment. The correlation of the impact force to the particle flow velocity was proposed and fitted using the experimental data. The computational particle fluid dynamics method was applied to simulate the particle flow field in the standpipe of the CFB boiler to determine the installation location of the Gs measurement device. The lab-scale cold-state experiments showed that the relative standard uncertainty of the Gs measurement using the device built in the present study was less than 2.0 %. This device was then applied in an industrial trial in a 116 MWth CFB boiler and the results proved the feasibility and applicability of the proposed Gs measurement method.

Computational Particle Fluid Dynamics Simulation on Combustion Characteristics of Blended Fuels of Coal, Biomass, and Oil Sludge in a 130 t h−1 Circulating Fluidized Bed Boiler

In this study, the co-combustion of coal and biomass, and the tri-combustion of coal, biomass, and oil sludge in a 130 t h−1 circulating fluidized bed (CFB) boiler are investigated via the computational particle fluid dynamics (CPFD) approach. Furthermore, the effect of biomass feeding position is also comprehensively evaluated. The results show that for the co-combustion of coal and biomass, the O2 mole fraction at the furnace outlet rises from 0.0541 to 0.0640 as the biomass blending ratio enhances from 40% to 100%, while the CO2 mole fraction reduces from 0.1357 to 0.1267. The mole fraction of NOx and SO2 at the furnace outlet decreases from 4.5867 × 10−5 to 3.9096 × 10−5 and 2.8253 × 10−4 to 4.6635 × 10−5, respectively. For the tri-combustion of three fuels, the average NOx mole fraction initially grows quickly and then declines gradually, ranging from 4.1173 × 10−5 to 4.2556 × 10−5. The mole fraction of SO2 at the furnace outlet increases from 3.5176 × 10−4 to 4.7043 × 10−4 when the ratio of oil sludge rises from 10% to 20%. The uniformity of temperature and gas components distribution is “new inlet &gt; secondary air inlet &gt; feed inlet”. As for the three inlet positions, the mole fractions of NOx at the furnace outlet are between 3.9096 × 10−5 and 5.1537 × 10−5, while those for SO2 are between 2.5978 × 10−4 and 2.5278 × 10−4.

Numerical simulation on gas-solid flow during circulating fluidized roasting of bauxite by a computational particle fluid dynamics method

A full-cycle numerical simulation of a circulating fluidized bed (CFB) by the use of the computational particle fluid dynamics (CPFD) method has been developed. The effects of the presence or absence of the secondary air, different secondary air positions, and different secondary air ratios on the gas–solid flow characteristics were explored. The results show that the presence of the secondary air makes a core-annular structure of the velocity distribution of particles in the fluidized bed, which enhances the uniformity of particles’ distribution and the stability of fluidization. The position and the ratio of the secondary air have a significant impact on the particle distribution, particle flow rate, and gas flow rate in the fluidized bed. When the secondary air position and ratio are optimal, the particles, particle flow rate, and air flow rate in the CFB are evenly distributed, the gas–solid flow state is good, and the CFB can operate stably.

Inhibiting agglomeration of bed particles in CFB burning high-alkali fuel: Experiment, mechanisms and criteria for recirculating bottom ash or selecting alternative bed materials

Bed material agglomeration during circulating fluidised bed (CFB) combustion of high-alkali fuels not only affects stable fluidisation of the solid particles, but also requires extra expenditure to refresh or substitute the bed materials. This work therefore verified experimentally the role of three different bed materials in initiating or mitigating particle agglomeration, led to a proposal of criteria for selecting effective bed materials or recirculating bottom ash. Using XRF, SEM-EDX, XRD analysis and FactSage calculations, results show that the formation of low melting-point K/Na/Ca silicates up to 41.4% at the combustion temperature was responsible for agglomerate formation. However, this may be mitigated by using alternative bed materials rich in either aluminosilicate or alkali-resistant constituents, to form high melting-point akermanite, forsterite and merwinite meanwhile inhibit the slag-liquid phase formation. FactSage calculation also confirmed that a decrease in SiO2 and an increase in Al2O3, CaO and MgO would minimise liquid phase formation. Consequently, two indices including ((K2O + Na2O) × SiO2)1/2/Al2O3 and (K2O + Na2O)/(CaO + MgO), representing the likelihood of alkali silicates formation over the inhibiting effect of Al2O3, CaO and MgO, were proposed and also supported by the available literature data, which provides a reference for selecting alternative bed materials or recirculating bottom ash in CFB.

An Improved Data-Driven Integral Sliding-Mode Control and Its Automation Application

Circulating fluidized bed (CFB) boilers are widely used in industrial production due to their high combustion efficiency, low pollutant emissions and wide load-adjustment range. However, the water-level-control system of a CFB boiler exhibits time-varying behavior and nonlinearity, which affect the control performance of the industrial system. This paper proposes a novel data-driven adaptive integral sliding-mode control (ISMC) method for the CFB control system with external disturbances. Firstly, the scheme designs a discrete ISMC law based on the full-format dynamic linearization (FFDL) data model, which is equivalent to a nonlinear system. Furthermore, a new reaching law is proposed to quickly drive the system state onto the sliding-mode surface. The improved ISMC control scheme only utilizes the input–output data during the design process and does not require model information. After theoretically verifying the stability of the method proposed in this paper, it is further applied in MIMO systems. Finally, the control and practical effects of this method are evaluated by using the DHX25-1.25 CFB boiler installed in the special-equipment testing center. The experimental results show that, compared with the traditional sliding-mode control (SMC) and model-free adaptive-control (MFAC) methods, the improved control method can quickly track the given signal and exhibit resistance to noise interference. Furthermore, it can rapidly respond to changes in the working conditions of the CFB system.

Machine Learning Aided Numerical and Experimental Investigation of Hydrodynamic Performance in the Circulating Fluidized Bed Boiler

Abstract This study attempts to illustrate the benefits of integrating the concepts of machine learning algorithms with the field of thermo-fluidic applications. The current work is aimed at identifying effective or significant hydrodynamic input parameters, which are capable of deriving full benefit of fluidization that could yield a better circulating fluidized bed (CFB) furnace design using the Apriori algorithm. For this, historical datasets from the literature are collected and pretreated based on the design under observation. Association analysis performed by this Apriori algorithm measures the comparative strength of parameters under consideration. Also, this algorithm is capable of identifying the right combinations of parameters that can produce maximum fluidization performance. The end results suggested by this Apriori algorithm are validated using the computational fluid dynamics package. For this, the transient behavior of a scaled-down (1:20) reactor model of a real-time industrial CFB boiler is simulated using ansys fluent 18.0. In specific, the effects of fluidizing velocities, inventory heights of the bed, and particle sizes recommended by the Apriori algorithm are investigated. Here, the effects are assessed in terms of volume fraction distribution and axial velocity profile distribution profiles. From the results of simulations, it was clearly found that 2 m/s inlet velocity produced good circulating fluidized bed patterns on a bed inventory height of 0.5 m for a mean particle size of 200 µm. The results obtained from the simulations are once again validated visually against snapshots obtained during real-time laboratory fluidization experimental runs. Among all the cases of comparisons, the best agreement is demonstrated by Apriori algorithm compared to the numerical and the experimentally obtained results. Also, it is found that the manual time taken to identify the right combinations of parameters is drastically reduced by this method compared to conventional optimization algorithm and trial error methods.

Fragility Analysis of the Main Building–Coal Conveyor Trestle Interaction System of a Thermal Power Plant

Thermal power plants play a crucial role in the power system as critical lifeline infrastructure. In order to meet the production process requirements, the main building of a thermal power plant is often connected to a coal conveyor trestle. This study focuses on investigating the seismic interaction between the common three-row reinforced concrete frame-bent main building and the steel trestle in a circulating fluidized bed (CFB) unit. The objective is to assess the influence of the trestle on the main building and understand the failure mode of the trestle structure. The seismic interaction is analyzed through fragility analysis based on Incremental Dynamic Analysis (IDA). The results indicate that the trestle has minimal influence on the main building, except during the large deformation stage. The study identifies the failure mode of the coal conveyor trestle as excessive relative displacement along the longitudinal direction at the connection points, leading to collisions or falls. A seismic demand model based on longitudinal relative displacement is developed to obtain the fragility curve for the trestle structure. These findings offer valuable insights for assessing the seismic performance of thermal power plants.

Study on the emission characteristics of VOCs under the condition of biomass blending combustion

In order to understand the emission characteristics of volatile organic compounds (VOCs) in the flue gas under the mixed combustion of biomass, the study on the emission characteristics of VOCs in the flue gas was carried out on a 58 MW circulating fluidized bed (CFB) unit. The results show that the co-firing of biomass can significantly reduce the emissions of VOCs and NOx and SO2. Changes in blended fuel particle size and combustion temperature reduce VOCs emissions. The most obvious change in the emission reduction of VOCs is reflected in the increase of the biomass mixing ratio from 20 % to 30 %. Biomass contains less S and N elements is the reason for the reduction of NOx and SO2 emissions. The emission of pollutants such as VOCs was the lowest when the biomass blending ratio was 40 %. Based on the actual operation of the power plant, 30 % is the optimal mixing ratio. The analysis showed that the amount of VOCs components had a strong positive correlation with the proportion of biomass in the fuel. The emission of VOCs under the condition of biomass blending has different characteristics from coal-fired boilers and biomass boilers. Under the two different mixing ratios, benzene series accounted for the largest proportion of VOCs emissions, reaching 44.38 % (20 %) and 33.75 % (40 %), respectively. The emission of benzene series is dominated by benzene and toluene, the emission of alkanes is dominated by n-hexane, and the emission of esters is dominated by ethyl acetate. The ozone formation potential (OFP) was analyzed by the maximum incremental reactivity method. The contribution of ozone generation potential at 20 % and 40 % mixing ratios was mainly from benzene series, which contributed 69.88 % and 70.24 %, respectively, and alkanes. contribution can also account for 25.76 % and 17.75 %.

A CFD study on ultrasound-enhanced CFB riser with calcium oxide and activated coal for CO2 capture application

This numerical study explores techniques for optimizing fluidization in circulating fluidized beds (CFB), which are of significant industrial interest. Various intrusive and non-intrusive approaches have been investigated to mitigate issues such as cluster formation, core-annulus profiles, and solid back-mixing. Specifically, this research focuses on non-intrusive methods, employing ultrasonic waves to enhance solids distribution within a CFB riser by Computational Fluid Dynamics (CFD) simulations. Particulate phases, including calcium oxide and activated carbon particles, were studied alongside a gas phase of air with a 10 % carbon dioxide concentration, commonly encountered in carbon capture processes. A radial ultrasonic device featuring 20 transducers at 20 kHz and 40 kHz was affixed to the CFB to disperse particle flow. Numerical experiments within a 3-D CFD domain employed the URANS-k-ε-KTGF-EMMS mathematical model. Findings reveal that ultrasonic waves significantly influence particle motion, modifying riser flow profiles and enhancing solids distribution. While ultrasounds increase pressure drop within the transducers' region, they decrease it above, reshaping the flow profile for both particle types. Distinct outcomes are observed for calcium oxide and coal particles regarding the radial Péclet number and solids volume fraction dispersion coefficient, suggesting that ultrasounds effectively improve coal particle distribution along the riser's height.

Energy efficiency analysis and optimization of a pressurized oxy-fuel circulating fluidized bed combustion system

The commercial application of oxy-fuel combustion technology is limited by energy deduction which is caused by addition of air separation unit (ASU) and carbon dioxide compression and purification unit (CPU). Pressurized circulating fluidized bed (CFB) oxy-fuel combustion technology is one option to improve the net electric generation efficiency. In this research, a series of simulations and optimizations were carried out to analyze the energy efficiency of a pressurized oxy-fuel combustion system. The boiler structure and heat exchange surface arrangement under different pressure were calculated. The cross-sectional area of the furnace and tail flue were both reduced up to 10 % of the original size and the floor space of the boiler was considered to be the lowest at 1.1 MPa. The energy efficiency and exergy analysis were used to explore the influence of pressure on the net efficiency and the optimization potential of system. The net electric generation efficiency reached a maximum value of 24.16 % at 1.1 MPa, which is 5.4 % higher than that at atmospheric pressure. Finally, four parameters, i.e. preheat temperature, oxygen concentration, ASU outlet oxygen purity and CPU compressor outlet pressure, were optimized to improve the system net efficiency to 26.67 %, 29.04 %, 25.87 % and 24.36 %, respectively.

Considering solid shear stress in MP-PIC simulation of a CFB riser

To enable successful multi-phase particle-in-cell (MP-PIC) simulations of circulating fluidized beds (CFBs), both the interphase drag force and interparticle collisions need to be carefully modeled. Particle collisions are usually represented by the solid stress consisting of the normal and shear components, in which the normal stress was found to have a leading role in the numerical stability of MP-PIC simulations, whereas the impact of the shear stress has seldom been reported. In this work, the effects of the solid shear stress are investigated by using two-dimensional simulations of the moderately dense laboratory-scale CFB riser with in-house MP-PIC code implemented on MFIX open-source platform. Cases including only normal solid stress and those considering both normal and shear solid stresses are simulated. The results of solids flux, axial and radial solids profiles are compared with available experimental data. The results show that the solid shear stress plays a minor role on the accuracy of simulation, and the increase of PPP (number particles per parcel) leads to a lower accuracy of simulation.

Experimental investigation of three-dimensional gas-solid flow in CFB cyclone separator using electrical capacitance volume tomography

The anisotropic three-dimensional flow field inside cyclone separators makes it challenging to capture the gas-solid flow characteristics, and the internal behavior of the cyclone separator remains not fully understood. The gas-solid flow characteristics inside a cyclone separator of the circulating fluidized bed (CFB) are investigated using electrical capacitance volume tomography (ECVT). The solid fraction, average velocity, and frequency characteristics of particles were extracted from ECVT images. The experimental results show that the number of turns for the dust strands spirals gradually increases as the inlet gas velocity Uf increases. The axial velocity of the particles in the cylindrical part of the cyclone separator is always greater than in the conical part. In addition, the difference in inlet gas velocity and particle size results in different particle flow characteristics in the CFB riser. It leads to the differences in the particle circulation rate and flow pattern in the cyclone separator.

Estimating Optimal Cost, Insulation Layer Thickness, and Structural Layer Thickness of Different Composite Insulation External Walls Using Computational Methods

All the modern gadgets and space conditioning in buildings consume lots of energy. Energy consumption can be optimized using Composite Insulation External Walls (CIEW) built from mortar plaster and structural and insulation layers. This study aimed to improve the overall performance of CIEW by optimizing the structural and insulation layer thickness. The objective was to minimize the Life Cycle Cost (LCC) and maximize the Life Cycle Savings (LCS) of CIEW. The nonlinear Least Squares Estimation (LSE) optimization technique for optimizing LCC and LCS of CIEW was used in the study. The study considered three insulation materials—Extruded Polystyrene (XPS), Rock Wool (RW), and Glass Wool (GW)—across three heat sources, including Circulating Fluidized Bed (CFB), Grate-Fired Boiler (GFB), and Air-Source Heat Pump (ASHP). The Life Cycle Cost Analysis (LCCA) methodology suggested by Huang using a traditional optimization technique was used as a basis for mathematical formulations and result comparison. The payback period of CIEW with optimal structural and insulation layer thickness was computed. The findings revealed that applying the LSE method enabled greater economic efficiency than the LCCA method, with an up to 9.12% increase in LCS value and an up to 7.41% decrease in LCC value. The research also revealed significant correlations between insulation and structural layer thicknesses and economic parameters.

Experimental Studies on Preheating Combustion Characteristics of Low-Rank Coal with Different Particle Sizes and Kinetic Simulation of Nitrogen Oxide

Low-rank coal, accounting for 45% of the global coal reserves, is easier to use in terms of realizing ignition and stable combustion due to its relatively high levels of volatile content. But the problem of low-rank coal combustion is that its NO formation is in the range of 300–600 mg/m3, which makes the emission’s meeting of the environmental regulation quite difficult or uneconomic. Preheating combustion was a prospective combustion technology which involved preheating in a circulating fluidized bed (CFB) first and then combustion in a combustor for preheated fuel. With three particle sizes (0–0.355 mm, 0–0.5 mm, and 0–1 mm), some experiments were carried out in a 30 kW test rig. The results showed that, in the CFB preheating, a particle size of 0–1 mm had the highest coal-gas heating value due to a long residence time. The release of species in the CFB preheating always followed the order H &gt; N &gt; C &gt; S. For preheated fuel combustion, a particle size of 0–0.355 mm showed the fastest combustion velocity, with the highest temperature point near the nozzle. For all three particle sizes, the combustion of preheated fuel showed a uniform temperature distribution with a small temperature difference. The lowest NO emission was 105 mg/m3 for the particle size of 0–0.5 mm. A GRI-Mech 2.11 mechanism was used to simulate the formation of NO with different influencing factors, such as temperature, oxygen concentration, and secondary-air ratio. There was a good agreement between the experimental data and the simulation’s results. The simulation showed that the NO formation could be further decreased with an optimal secondary-air ratio. This investigation provides support for the basic understanding of preheating-combustion technology and potential industrial applications in the future.