Effects of particle type on the particle fluidization and distribution in a liquid–solid circulating fluidized bed boiler

A liquid-solid circulating fluidized bed boiler is designed and built for visualization research by applying the fluidized bed heat transfer and fouling prevention technology to the water side of the boiler. Four types of engineering plastic particles with different physical properties are selected as the solid working media. The effect of particle types on the fluidization and distribution of particles in the boiler is investigated under different feedwater flow rates and amount of added particles by using the charge couple device image measurement and acquisition system. The results show that all kinds of particles can't be normally fluidized and accumulate in the drum at low amount of added particles and feedwater flow rate. The particles with great density and low sphericity are more likely to accumulate. The average solid holdup in the riser tubes increases with the increase in feedwater flow rate and the amount of added particles. The non-uniform degree of particle distribution in the riser tubes generally decreases with the increase in feedwater flow rate and the amount of added particles. The particles with small density and settling velocity have high average solid holdup in the riser tubes under close sphericity. In generally,the smaller the density and settling velocity, the more uniform the particle distribution in the riser tubes.Three-dimensional diagrams of the non-uniform degree of particle distribution in the riser tubes of the boiler are established.

EXPERIMENTAL RESEARCH OF FLOW STRUCTURE IN A GAS-SOLID CIRCULATING FLUIDIZED BED RISER BY PIV

Particle Imaging Velocimetry （PIV） techniques were applied to investigate the particle motion and cluster properties in a gas-solid two-phase flow in a circulating fluidized bed riser. Visual images and micro-structure of various clusters were captured. After the boundary of clusters was determined by the gray level threshold method, clusters were classified by the distance between particles and the shape and position of clusters. In addition, the process of clusters forming and breaking up was described, and the sizes of clusters were also obtained. With the Minimum Quadric Difference （MQD） cross-correlation algorithm suitable for high-density particles, the axial velocities of the particles were obtained in the dilute phase section. The features of particle motion were revealed by investigating statistically the magnitude and distribution of particle axial velocity in the radial direction. At most radial cross-sections, there exists a parabola-shaped distribution of upward axial velocity of particles, namely, the magnitude of axial velocity in the core region is higher than that near the wall region of the riser.

Particle clustering(mesoscale structure)of high-flux gas-solid circulating fluidized bed

Particle clustering is an important dynamic phenomenon in circulating-fluidized-bed(CFBs)systems,and has been suggested as a key contributing factor to the non-uniform hydrodynamics of CFBs.Studies show that particle clusters can be affected by solids flux,in terms of frequency,duration,and solids holdup.To understand the characteristics of particle clusters under high-solids-flux conditions,experimental and modeling studies in high-solids-flux gas-solids CFBs were reviewed and summarized.Optical and electrical measurements and imaging methods were used to monitor the particle-clustering phenomenon in CFBs.Particles were found to cluster in high-flux CFBs,and were characterized by a denser cluster-solids holdup and a shorter time fraction,which was different from the behavior in low-flux CFBs.Particle properties affected particle clustering in high-flux CFBs significantly.In modeling work,Eulerian-Eulerian and Eulerian-Lagrangian methods were used to study the particle-cluster characteristics.Good results can be obtained by using the Eulerian-Eulerian method to simulate the CFB system,especially the high-flux CFBs,and by considering the effects of particle clusters.The Eulerian-Lagrangian method is used to obtain detailed cluster characteristics.Because of limits in computing power,no obvious results exist to model particle clusters under high-solids-flux conditions.Because high-solids-flux conditions are used extensively in industrial applications,further experimental and numerical investigations on the clustering behavior in HF/DCFBs are required.

Experimental Study on Gas-Solid Mass Transfer in Circulating Fluidized Beds

This study is devoted to gas-solid mass transfer behavior in heterogeneous two-phase flow. Experiments were carried out in a cold circulating fluidized bed of 3.0m in height and 72mm in diameter with naphthalene particles. Axial and radial distributions of sublimated naphthalene concentration in air were measured with an online concentration monitoring system HP GC-MS. Mass transfer coefficients were obtained under various operating condition, showing that heterogeneous flow structure strongly influences the axial and radial profiles of mass transfer coefficients. In the bottom dense region, mass transfer rate is high due to intensive dynamic behavior and higher relative slip velocity between gas and clusters. In the middle transition region and the upper diluter region, as a result of low mass transfer driving force and the influence of flow structure, mass transfer rate distribution becomes non-uniform. In conclusion, among the operating parameters influencing mass transfer coefficients, the superficial gas velocity is the most important factor and the solid circulation rate should be also taken into account.

Emissions of SO_2, NO and N_2O in a circulating fluidized bed combustor during co-firing coal and biomass

This paper presents the experimental investigations of the emissions of SO2, NO and N20 in a bench scale circulating fluidized bed combustor for coal combustion and co-firing coal and biomass. The thermal capacity of the combustor is 30 kW. The setup is electrically heated during startup. The infuence of the excess air, the degree of the air staging, the biomass share and the feeding position of the fuels on the emissions of SO2, NO and N2O were studied. The results showed that an increase in the biomass shares resulted in an increase of the CO concentration in the flue gas, probably due to the high volatile content of the biomass. In co-firing, the emission of SO2 increased with increasing biomass share slightly, however, non-linear increase relationship between SO2 emission and fuel sulfur content was observed. Air staging significantly decreased the NO emission without raising the SO2 level. Although the change of the fuel feeding position from riser to downer resulted in a decrease in the NO emission level, no obvious change was observed for the SO2 level. Taking the coal feeding position R as a reference, the relative NO emission could significantly decrease during co-firing coal and biomass when feeding fuel at position D and keeping the first stage stoichiometry greater than 0.95. The possible mechanisms of the sulfur and nitrogen chemistry at these conditions were discussed and the ways of simultaneous reduction of SO2, NO and N2O were proposed.

Properties of Concrete Incorporating Bed Ash from Circulating Fluidized Bed Combustion and Ground Granulates Blast-furnace Slag

The properties of concrete incorporating circulating fluidized bed combustion （CFBC） bed ash and ground granulates blast-furnace slag （GGBS） were studied.Compressive strength,drying shrinkage,mercury intrusion porosimetry （MIP）,scanning electronic microscopy （SEM）,and X-ray diffraction （XRD） of concrete samples containing CFBC bed ash and GGBS were used.This work used initial surface absorption test （ISAT） and rapid chloride penetration test （RCPT） on concrete to measure the absorption and the ability of concrete to resist chloride ion characteristics for different concrete samples containing CFBC bed ash and GGBS.Open circuit potential （OCP）,direct current polarization resistance were obtained to evaluate rebar corrosion.The CFBC bed ash was X-ray amorphous and consist of SiO2,Al2O3 and CaO compounds.As the replacement of CFBC for sand increases,the rate of initial surface absorption （ISA） increases but compressive strength decreases.When the content of CFBC bed ash replacement for sand maintains constant,the replacement of GGBS for cement increases,compressive strength increases but the rate of ISA decreases.Chloride and corrosion resistance of rebar significantly improve by utilizing a proper amount of CFBC bed ash and GGBS in concrete.

Pressure Drop of Liquid–Solid Two-Phase Flow in the Vertical Tube Bundle of a Cold-Model Circulating Fluidized Bed Evaporator

A cold-model vertical multi-tube circulating fluidized bed evaporator was designed and built to conduct a visualization study on the pressure drop of a liquid–solid two-phase flow and the corresponding particle distribution.Water and polyformaldehyde particle(POM)were used as the liquid and solid phases,respectively.The effects of operating parameters such as the amount of added particles,circulating flow rate,and particle size were systematically investigated.The results showed that the addition of the particles increased the pressure drop in the vertical tube bundle.The maximum pressure drop ratios were 18.65%,21.15%,18.00%,and 21.15%within the experimental range of the amount of added particles for POM1,POM2,POM3,and POM4,respectively.The pressure drop ratio basically decreased with the increase in the circulating flow rate but fluctuated with the increase in the amount of added particles and particle size.The difference in pressure drop ratio decreased with the increase in the circulating flow rate.As the amount of added particles increased,the difference in pressure drop ratio fluctuated at low circulating flow rate but basically decreased at high circulating flow rate.The pressure drop in the vertical tube bundle accounted for about 70%of the overall pressure drop in the up-flow heating chamber and was the main component of the overall pressure within the experimental range.Three-dimensional phase diagrams were established to display the variation ranges of the pressure drop and pressure drop ratio in the vertical tube bundle corresponding to the operating parameters.The research results can provide some reference for the application of the fluidized bed heat transfer technology in the industry.

Mechanical,expansion and rheological properties of circulating fluidized bed fly ash based ecological cement:A critical review

Circulating fluidized bed fly ash(CFBFA)is a solid waste product from circulating fluidized bed(CFB)boilers in power plants,and the storage of CFBFA is increasingly become an environmental problem.Previous scholars have made contributions to improve the resource utilization of CFBFA.Especially,ecological cement is prepared by CFBFA,which is more conducive to its large-scale utilization.In recent years,a lot of effort has been paid to improve the properties of ecological cement containing CFBFA.In this work,the physicochemical properties of CFBFA are introduced,and recent research progress on the mechanical,expansion,and rheological properties of CFBFA based ecological cement(CEC)is extensively reviewed.The problem of over-expansion of f-CaO is summarized,which limits the scale application of CFBFA in ecological cement.Hence,the challenge for f-CaO in CFBFA to compensate for cement volume shrinkage is proposed,which is beneficial to the utilization of CFBFA in ecological cement,and the reduction of CO_(2) emissions from the cement industry.In addition,the environmental performance,durability,and economy of CEC should be valued in future research,especially the environmental performance,because the CFBFA contains heavy metals,such as Cr,As,which may pollute groundwater.

Synergetic NO reduction by biomass pyrolysis products simulating their reburning in circulating fluidized bed decoupling combustion

The present work investigated the synergetic effect of pyrolysis-derived char,tar and gas(py-gas)on NO reduction,which may occur in circulating fluidized-bed decoupling combustion(CFBDC)system treating N-rich fuel.Experiments were carried out in a lab-scale drop-tube reactor for NO reduction by some binary mixtures of reagents including char/py-gas,tar/py-gas and tar/char.At a specified total mass rate of0.15 g·min^-1 for NO-reduction reagent,the char/py-gas(binary reagent)enabled the best synergetic NO reduction in comparison with the others.There existed effective interactions between char and some species in py-gas(i.e.,H2,CxHy)during NO reduction by pyrolysis products,meanwhile the tar/py-gas or tar/char mixture only caused a positive effect when tar proportion was necessarily lowered to about 26%.On the other hand,the synergetic effects were not improved for all tested binary reagents by increasing the reaction temperature and residence time.

Synthesis gas production using oxygen storage materials as oxygen carrier over circulating fluidized bed

A novel process for synthesis gas production over Circulating Fluidized Bed （CFB） using oxygen storage materials as oxygen carder was reported. First, oxygen in the air was chemically fixed and converted to lattice oxygen of oxygen storage materials over regenerator, and then methane was selectively oxidized to synthesis gas with lattice oxygen of oxygen storage materials over riser reactor. The results from simulation reaction of CFB by sequential redox reaction on a fixed bed reactor using lanthanum-based perovskite LaFeO3 and La0.8Sr0.2Fe0.9CO0.1O3 oxides prepared by sol-gel, suggested that the depleted oxygen species could be regenerated, and methane could be oxidized to synthesis gas by lattice oxygen with high selectivity. The partial oxidation of methane to synthesis gas over CFB using lattice oxygen of the oxygen storage materials instead of gaseous oxygen should be possibly applicable.

Optimization of the Internal Circulating Fluidized Bed Using Computational Fluid Dynamics Technology

The computational fluid dynamics(CFD)technology is analyzed and calculated utilizing the turbulence model and multiphase flow model to explore the performance of internal circulating fluidized beds(ICFB)based on CFD.The three-dimensional simulation method can study the hydrodynamic properties of the ICFB,and the performance of the fluidized bed is optimized.The fluidization performance of the ICFB is improved through the experimental study of the cross-shaped baffle.Then,through the cross-shaped baffle and funnel-shaped baffle placement,the fluidized bed reaches a coupled optimization.The results show that CFD simulation technology can effectively improve the mass transfer efficiency and performance of sewage treatment.The base gap crossshaped baffle can improve the hydraulic conditions of the fluidized bed and reduce the system energy consumption.The cross-shaped baffle and funnel-shaped baffle can perfect the performance of the reactor and effectively strengthen the treatment in the intense aerobic process of industrial sewage.

Strength and thermal behavior of low weight foam geopolymer using circulating fluidized bed combustion fly ash

A comparative study of the influence of elevated temperature on foam geopolymer using circulating fluidized bed combustion fly ash(CFA) was reported. Foam geopoymers were prepared with different amounts of foam agent and different Si O2/Al2O3 molar ratios of 3.1, 3.4, and 3.8. The mechanical, thermo-physical properties and microstructure of the foam geopolymers before and after exposure to elevated temperature of 800, 1000, and 1200 °C were investigated. The specimen with Si O2/Al2O3 molar ratio of 3.8 exhibits the highest compressive strength, better microstructure and dimension stability before and after firing. Carnegeite, nepheline, and zeolite crystalline phases appearing after exposure may contribute to the good post-exposure strength. Low weight foam geopolymer using CFA can increase strength and maintain higher stability as high as 1000 °C.

Developments in the understanding of gas–solid contact efficiency in the circulating fluidized bed riser reactor:A review

In the last several decades, circulating fluidized bed reactors have been studied in many aspects including hydrodynamics, heat and mass transfer and gas–solid two phase contacting. However, despite the abundance of review papers on hydrodynamics, there is no summary paper on gas–solid contact efficiency to date, especially on high density circulating fluidized beds(CFBs). This paper gives an introduction to, and a review of the measurement of contact efficiency in circulating fluidized bed riser. Firstly, the popular testing method of contact efficiency including the method of heating transfer experiment and hot model reaction are discussed, then previous published papers are reviewed based on the discussed methods. Some key results of the experimental work are described and discussed. Gas–solid contact efficiency is affected by the operating conditions as well as the particle size distribution. The result of the contact efficiency shows that the CFB riser is far away from an ideal plug flow reactor due to the characteristics of hydrodynamics in the riser. Lacunae in the available literature have been delineated and recommendations have been made for further work.

Numerical simulations and comparative analysis of two- and three-dimensional circulating fluidized bed reactors for CO2 capture

Carbon dioxide(CO2),the main gas emitted from fossil burning,is the primary contributor to global warming.Circulating fluidized bed reactor(CFBR)is proved as an energy-efficient method for post-combustion CO2 capture.The numerical simulation by computational fluid dynamics(CFD)is believed as a promising tool to study CO2 adsorption process in CFBR.Although three-dimensional(3D)simulations were proved to have better predicting performance with the experimental results,two-dimensional(2D)simulations have been widely reported for qualitative and quantitative studies on gas-solid behavior in CFBR for its higher computational efficiency recently.However,the discrepancies between 2D and 3D simulations have rarely been evaluated by detailed study.Considering that the differences between the 2D and 3D simulations will vary substantially with the changes of independent operating conditions,it is beneficial to lower computational costs to clarify the effects of dimensionality on the numerical CO2 adsorption runs under various operating conditions.In this work,the comparative analysis for CO2 adsorption in 2D and 3D simulations was conducted to enlighten the effects of dimensionality on the hydrodynamics and reaction behaviors,in which the separation rate,species distribution and hydrodynamic characteristics were comparatively studied for both model frames.With both accuracy and computational costs considered,the viable suggestions were provided in selecting appropriate model frame for the studies on optimization of operating conditions,which directly affect the capture and energy efficiencies of cyclic CO2 capture process in CFBR.

Hydrothermal synthesis of zeolitic material from circulating fluidized bed combustion fly ash for the highly efficient removal of lead from aqueous solution

The utilization of coal fly ash derived from circulating fluidized bed combustion(CFBFA)still faces great challenges because of its unique characteristics.In this study,a zeolitic material with Na-P1 zeolite as the main phase was successfully synthesized via a hydrothermal method by using CFBFA as the raw material.The effects of hydrothermal temperature,time,and added CTAB amount on the characterizations of synthesized materials were investigated by XRD,SEM,and XPS.The properties of the optimal zeolitic material and its adsorption performance for Pb^(2+)in aqueous solution were evaluated.The influences of pH,initial concentration,dosage,and temperature on Pb^(2+)adsorption were also examined.Results revealed the following optimal parameters for the synthesis of zeolitic material:NaOH concentration of 2 mol·L^(-1),solid-to-liquid ratio of 1:10 g·ml^(-1),hydrothermal temperature of 110℃,hydrothermal time of 9 h,and CTAB amount of 1 g(per 100 ml solution).The adsorption capacities of the zeolitic material reached 329.67,424.69,and 542.22 mg·g^(-1) when the pH values of aqueous solution were 5,6,and 7,respectively.The Pb^(2+)removal efficiency can reach more than 99%in aqueous solution with the initial concentrations of 100-300 mg·L^(-1) under pH 6 and suitable adsorbent dosage.The adsorption and kinetics of Pb^(2+)on the zeolitic material can be described by Langmuir isotherm and pseudo-second-order kinetic models,respectively.The ion exchange between Pb^(2+)and Na^(+)and chemisorption are the main adsorption mechanism.All these findings imply that the synthesis of low-cost adsorbent for Pb^(2+)removal from weak acid and neutral aqueous solution provides a highly effective method to utilize CFBFA.

Heat Transfer in a Liquid-Solid Circulating Fluidized Bed Reactor with Low Surface Tension Media

Heat transfer characteristics between the immersed heater and the bed content were studied in the riser of a liquid-solid circulating fluidized bed, whose diameter and height were 0.102 m (ID) and 2.5 m, respectively. Effects of liquid velocity, particle size, surface tension of liquid phase and solid circulation rate on the overall heat transfer coefficient were examined. The heat transfer coefficient increased with increasing particle size or solid circulation rate due to the higher potential of particles to contact with the heater surface and promote turbulence near the heater surface. The value of heat transfer coefficient increased gradually with increase in the surface tension of liquid phase, due to the slight increase of solid holdup. The heat transfer coefficient increased with the liquid velocity even in the higher range, due to the solid circulation prevented the decrease in solid holdup, in contrast to that in the conventional liquid-solid fluidized beds. The values of heat transfer coefficient were well correlated in terms of dimensionless groups as well as operating variables.

Boiling Heat Transfer in Circulating Fluidized Beds

A model is proposed to predict boiling heat transfer coefficient in a three-phase circulating fluidized bed (CFB), which is a new type of evaporation boiling means for enhancing heat transfer and preventing fouling. To verify the model, experiments are conducted in a stainless steel column with 39mm ID and 2.0m height, in which the heat transfer coefficient is measured for different superficial velocities, steam pressures, particle concentrations and materials of particle. As the steam pressure and particle concentrations increase, the heat transfer coefficient in the bed increases. The heat transfer coefficient increases with the liquid velocity but it exhibits a local minimum. The heat transfer coefficient is correlated with cluster renewed model and two-mechanism method. The prediction of the model is in good agreement with experimental data.

Lateral Solids Mixing in the Dense Zone of a Circulating Fluidized Bed

Lateral solid mixing was investigated experimentally in the dense zone of a 900mm×100mm×5.2m rectangular circulating fluidized bed riser.Using heated tracer injection,the lateral solid dispersion was determined by measuring the temperature response at different lateral positions. Furthermore, a one-dimensional dispersion model,which describes the solid mixing in the dense zone,is presented.The experimental results were used to determine the lateral particle dispersion coefficient under various operating conditions. A correlation of dispersion coefficient with bed height, gas velocity,and particle size is also proposed.

PHASE HOLDUP AND LIQUID DISPERSION IN GAS-LIQUID-SOLID CIRCULATING FLUIDIZED BEDS

Axial and radial profiles of gas and solids holdups have been studied in agas-liquid-solid circulating fluidized bed at 140mm i.d..Experimental results indicate that the axialand radial profiles of gas and solids holdups are more uniform than those in a conventionalfluidized bed.Axial and radial liquid dispersion coefficients in the gas-liquid-solid circulating fluidizedbed are investigated for the first time.It is found that axial and radial liquid dispersioncoefficients increases with increaes in gas velocity and solids holdup.The liquid velocity has littleinfluence on the axial liquid dispersion coefficient,but would adversely affect the redial liquiddispersion coefficient.It can be concluded that the gas-liquid-solid circulating fluidized bed hasadvantages such as better interphase contact and lower liquid dispersion along the axial directionover the expanded bed.

Compatibility of Fly Ash and Slag from Circulating Fluidized Bed Boiler for Concrete Beam

The applications of fly ash and slag from circulating fluidized bed boiler were studied as mineral admixture and aggregate for steel reinforced concrete beam.The results show that the concrete beam with fly ash and slag from circulating fluidized bed boilers has a similar ultimate cracking load coefficient as the ordinary cement concrete and a higher bending moment limit.Under the same load,it has a smaller deformation than the ordinary concrete.