Evaluation of sorption-enhanced reforming over catalyst-sorbent bi-functional particles in an internally circulating fluidized bed

An internally circulating fluidized bed (ICFB) has been applied in various industrial processes owing to its potential in the reduction of heat loss and compact size. In this work, the sorption-enhanced reforming process in the ICFB is investigated. The dense discrete phase model (DDPM) is employed to evaluate the performance of catalyst-sorbent bi-functional particles, considering the particle size distribution. The results demonstrate that the utilization of bi-functional particles can significantly increase hydrogen production. The impacts of operating parameters including solid loading and regenerator velocity on solid circulation rate and gas leakage are examined. It is found that the gas leakage between reactors is increased by 46.6% when the regenerator gas velocity varies from 1.8 m/s to 2.4 m/s so as to weaken the hydrogen yield.     

Studies on the effects of physical properties of feed samples on the performance of fluidized bed dryer

The physical properties of grains are analysed at different levels of moisture contents using a fluidized bed dryer. The performance of fluidized bed dryer measured in terms of its efficiency is determined by using different grains as feed samples in the present work. The effects of various system parameters on the moisture content of the samples in turn on the dryer performance are studied. Again, effects of moisture content on different properties of sample, namely, grain volume, surface area, sphericity, bulk density, true density and porosity are studied. These properties of feed sample which determine the flow characteristics of the sample in a fluidized bed dryer are found to affect the dryer efficiency, in turn, the drying quality. An attempt is made to correlate the performance of the dryer against these physical properties of the materials. Thus, the efficiencies of the dryer calculated through the developed correlation and as per the literature are compared with each other. The comparison results show good agreement, thereby implying that the proposed correlation can be used for estimating the dryer performance over a wide range of parameters. With this study, the dryer can be designed properly in a cost-efficient manner.     

Simulation of co-gasification of coal and wood in a dual fluidized bed system

A comprehensive three-dimensional (3D) model based on multiphase particle-in-cell (MP-PIC) approach is developed to simulate the co-gasification process of coal and wood in a dual fluidized bed (DFB) system. The pyrolysis of coal and wood, formation and conversion of tar and gaseous pollutants, as well as gasification and combustion reactions are integrated with dense gas-solid flow and heat transfer. The pilot DFB system comprises a bubbling bed gasifier and a fast fluidized bed combustor connected by loop seals. The model correctly predicts profiles of pressure and temperature, the yield and components of the product gas. The gas composition distributions, allocations of particle mass, and solid residence time inside the reactors are explored. The effects of various fuel blend ratios and particle sizes on gasification performances are also investigated. The results show that increasing coal ratio accelerates the steam gasification due to higher char content, which results in the increment of H₂ and CO concentrations. The tar content in the product gas continues to decline, while the emissions of NH₃ and H₂S increase. The size variation of feedstock is not enough to dramatically affect product gas components. The tar content and product gas yield appear a slight upward trend with the smaller size. The variations of NH₃ and H₂S concentrations are consistent with those of bed temperature.     

Elimination of stagnant particles from a N-valve with side aeration in circulating fluidized bed

The existence of stagnant particle layer in the conventional non-mechanical valves limits their utilization in CFB with the feedstock of caking particles. A new N-valve consisted of a fluidized weir chamber with bottom aeration and a moving-bed angled standpipe with side aeration was developed to eliminate the stagnant particle layer and reach high solids circulation rate G_s in CFBs. The particle flow behavior and its control in the N-valve were studied experimentally. By combining the bottom aeration for weir chamber and the side aeration for angled standpipe the G_s over 270 kg/(m² s) was achieved, and the stagnant particle layer completely disappeared. The G_s increased with increasing the side aeration gas flow rate Q_sa, and this loosing gas flow was optimally injected from the bend between the downcomer and the angled standpipe. At a constant but enough high Q_sa, the increase in the bottom aeration gas flow rate Q_ba elevated G_s linearly.     

Evaluation of direct quadrature method of moment for the internally circulating fluidized bed simulation with ultrafine particles

In the framework of the Euler-Euler gas–solid two-fluid model, the particle population balance equation is solved by the direct quadrature method of moment. The dynamic process of ultrafine particle movement and aggregation in an internally circulating fluidized bed is simulated. The distribution of the concentration and velocity of the agglomerates in the flow process is given, and the changes of the moments in the bed are shown. The effects of different breakage coefficients and inlet gas rates on the concentration distribution of agglomerates are compared. The results show that the particle size decreases with the increase of breakage coefficient, and the time required to reach steady fluidization state increases; the higher the inlet velocity, the better the effect of circulating particles in the bed. When there is a certain gas velocity difference between the two sides, the effect of circulating particles in the bed is better.     

Circulation model for describing mixing of solid particles in a fluidized bed

We present experimental response curves for the pulsed introduction of labeled (heated) particles into a fluidized system with a noncireulating solid phase, and compare these curves with those calculated by using the circulation and diffusion models.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 46, No. 5, pp. 820–824, May, 1984.     

Verification of optimal models for 2D-full loop simulation of circulating fluidized bed

To verify the optimal models for a two-dimensional (2D) full loop simulation of a circulating fluidized bed (CFB), different turbulent models and drag models are studied according to relevant pressure profile, voidage distribution and particle collision energy. With regard to a laminar model and turbulent models including Standard k-ε, RNG k-ε and Realizable k-ε, the experimental data reveals that the RNG k-ε model is the best at predicting pressure, voidage, axial solid velocity and granular temperature. Besides, through the comparison of four drag models, it is found that the Gidaspow model can achieve a higher accuracy of prediction. Therefore, it can be concluded that the combination of the RNG and Gidaspaw models is suitable for the 2D full loop simulation of a CFB, and therefore potential models for the prediction of flow characteristics.     

CFD-DEM study on the mixing characteristics of binary particle systems in a fluidized bed of refuse-derived fuel

The fluidization of quartz in the fluidized bed has great influence on the combustion and gasification of refuse-derived fuel (RDF). The combined computational fluid dynamics (CFD) and discrete element method (DEM) approach was used to explore the gas-solid hydrodynamics and mixing characteristics in a three-dimensional fluidized bed. All numerical analyses were performed referring to the experiments (Goldschmidt, Beetstra, and Kuipers 2004 Goldschmidt, M. J. V., R. Beetstra, and J. A. M. Kuipers. 2004. Hydrodynamic modelling of dense gas-fluidised beds: Comparison and validation of 3D discrete particle and continuum models. Powder Technology 142 (1):23–47. doi:10.1016/j.powtec.2004.02.020[Crossref], [Web of Science ®] , [Google Scholar]). The simulation results indicated that the quartz volume fraction agrees well with the experimental data. Furthermore, the cylinder-shaped RDF particles can mix well with the quartz particles as they were added from upside. For binary systems, it is necessary to investigate solid flow characteristics as well as pressure drops and examine the influence of superficial gas velocity on the solid mixing. Two main parameters are discussed: mixing degree and the time required to reach the steady state. It is also found that inlet gas velocity and particle properties (particle density ratio, shape and size) are significant factors on particle mixing in a fluidized bed.     

Experimental study on fluidization characteristics of vinegar residue in a vibrated fluidized bed

As a by-product in the vinegar brewing process, vinegar residue always has a high moisture content, which is detrimental to the storage and recycle process. The vibrated fluidized bed can be used to dry the vinegar residue. In present work, inert particles were added to a vibrated fluidized bed to improve the fluidization of vinegar residue. Experimental studies were carried out to investigate the fluidization behaviors of the binary mixtures. Flow pattern maps indicated that there was an upper limit to the vinegar residue mass concentration c_w at which stable fluidization could be achieved. The minimum fluidization velocity u_mf of the binary mixture increased as the vinegar residue mass concentration c_w increased and decreased with the increase of the vibration intensity Λ. As increasing vibration intensity Λ or decreasing vinegar residue mass concentration c_w, the drying rate of vinegar residue increased.     

Mathematical models of drying processes in a fluidized bed

Mathematical models are described for drying processes in a fluidized bed with various idealizations. These may be applied to calculations on drying equipment involving a fluidized bed and to the development of an automatic process control system using computing techniques.     

Experimental study of fluidization characteristics of Geldart-D particles in pressurized bubbling fluidized bed

The pressurized bubbling fluidized bed shows great advantage in retreating municipal solid waste because it could effectively capture CO₂ and enhance the reaction rate of the process of combustion and gasification. In the present work, fluidization characteristics of Geldart-D particles at elevated pressure were experimentally investigated, such as flow pattern, pressure drop, minimum fluidization gas velocity. At the same fluidization gas velocity, as elevating operating pressure, the fluidization of Geldart-D particles became more intense, the bubbles got larger, the standard deviation and the power density of dominant frequency of the pressure drop signal increased. While, under the same fluidization number, as increasing operating pressure, the fluidization of Geldart-D particles became smoother, the bubble size decreased, both the standard deviation and the power density of dominant frequency of the pressure drop signal decreased. It seems that, under elevated pressure, the fluidization behavior of Geldart-D particles would transition to that of Geldart-B particles. Finally, the minimum fluidization velocity of the Geldart-D particles was found decreased with the increase of the operating pressure. A new correlation for the prediction of the minimum fluidization velocity of Geldart-D particles at elevated pressure was also formulated based on the present experimental results.     

Multi-fluid modelling of hydrodynamics in a dual circulating fluidized bed

In this work, a multi-fluid model based on the Eulerian-Eulerian framework is used to study the gas-solid hydrodynamics, such as solid distribution, particle motion and solid velocity, in a three-dimensional (3D) dual circulating fluidized bed (DCFB). The influence of four different drag force models, including two classic models, i.e. Gidaspow, EMMS drag model and two recent drag models, i.e. Rong and Tang drag model, on hydrodynamics in DCFB are assessed. Numerical results show that the characteristics of solid distribution and velocity in different sections are distinct. For qualitative analysis, all the drag models can predict a reasonable radial solid distribution and pressure distribution, but only the EMMS, Rong and Tang drag model can capture the phenomenon of dense solid concentration in the low part. For quantitative analysis, the solid circulating rate predicted by the EMMS drag model is the closest to the experimental value while the Gidaspow drag model shows the most significant deviation. The overall assessments confirm that the drag model selection has a significant influence on the simulations of gas-solid flow in DCFBs. This study sheds lights on the design and optimization of fluidized bed apparatuses.     

Novel method of air distributor design for enhancing bed stability and reducing impurities in gas–solid fluidized bed system

Coal plays a key role in the economic development of China. It is greatly significant to improve the efficient use of coal through high-efficiency dry separation. In this paper, a porous sponge was used to optimize the air distributor, and its fluidization characteristics were studied. Response surface methodology was used to study the collaborative optimization effects of gas velocity, separation time, and bed height on the fine coal separation. Gas velocity was the main factor which affects separation efficiency. When separation is in operation parameters, the yield and ash content were 65.48 and 10.89% as to the clean coal product and 9.24 and 80.47% as to the gangue product, respectively. Furthermore, the yield and ash content of middlings were 25.29 and 20.32%, respectively. The probable error, E, values were between 0.085 and 0.100?g/cm³. Using XRF and FTIR analysis, it was observed that the harmful elements and impurities in coal were reduced during the separation process.     

Sulfation and attrition of calcium sorbent in a bubbling fluidized bed

A bubbling fluidized bed reactor was used as a desulfurization apparatus in this study. The height of the bed was 2.5 m, and the inner diameter was 9 cm. The bed materials were calcium sorbent and silica sand. The effects of the operating parameters of the flue gas desulfurization including relative humidity, temperature, superficial gas velocity, and the particle size of calcium sorbent on SO₂ removal efficiency and calcium sorbent conversion and attrition rate in the fluidized bed were investigated. It was found that the temperature effect in our system was negligible from 40 to 65°C. A higher relative humidity had a higher calcium conversion and a higher sulfur dioxide removal efficiency. Moreover, a smaller particle size of calcium sorbent had a lower calcium conversion in the cyclone but a higher sulfur dioxide removal efficiency. A lower superficial gas velocity resulted in a higher sulfur dioxide removal efficiency and a higher calcium conversion, thus, the total volume of the flue gas treated was maximum near the minimum fluidization velocity. Finally, an attrition rate model proposed in this study could predict the elutriation rate satisfactorily.     

Investigating the fluidization of disk-like particles in a fluidized bed using CFD-DEM simulation

Fluidization of monodispersed disk-like particles with different aspect ratios in the fluidized bed is simulated by CFD-DEM, with disk-like particles being modeled by the super-ellipsoids. The relatively comprehensive investigations are performed in order to understand the fluidization behaviors of disk-like particles and to evaluate how the aspect ratio influences the fluidization. The results obtained demonstrate that disk-like particles with a larger aspect ratio possess stronger particle movement and more apparent fluidization. Comparisons between spherical particles and disk-like particles elucidate their differences in the fluidization behavior. Particle orientation is also investigated in this paper due to its important influence on the fluidization. Particles possess different preferred orientations in the static bed and in the fluidization state, and a reduced aspect ratio can drive particles to be in the preferred orientation. The existence of the walls will prompt particles to align their cross sections to be parallel to the plane of the walls.     

Agglomerate behavior in a recirculating fluidized bed with sheds: Effect of sheds

A Radioactive Particle Tracking (RPT) technique was used to study the effects of the internal baffles in the stripping section of the Fluid Coker?, called sheds, have on the behavior of wet agglomerates that are formed when residual oil is injected into the Coker. Vapor emitted by reacting wet agglomerates below the sheds rises and causes shed fouling. The release of vapor from agglomerates can be estimated by combining the RPT results with a coking reaction model. The study found that the sheds reduce the time agglomerates spend in the shed zone, which in turn reduces the amount of organic vapor that reaches the sheds, but at the same time increase the wetness of the agglomerates that exit to the recirculation line, which results in the loss of valuable liquid. The research also found that the best type of shed, from the point of view of agglomerate motion, is the mesh-shed. Finally, experimental data indicate that reducing the cross sectional area of the sheds from 50% to 30% increases the time that the agglomerates spend above the shed zone, and thus reduces the flow of vapor emitted below the sheds.     

Particle dynamics in a multi-staged fluidized bed: Particle transport behavior on micro-scale by discrete particle modelling

This work studies the particle exchange rates in horizontal fluidized beds equipped with different weir designs between compartments. These particle exchange rates provide information on the axial dispersion of the solid material within the process. For this purpose discrete particle modelling (DPM) was used to determine the particle exchange on microscopic level. This method uses a coupled CFD-DEM approach to observe particle dynamics in a fluid field. The model was validated against exchange rates in a lab-scale setup as determined by Particle Tracking Velocimetry (PTV) with very good quantitative agreement, showing the suitability of the method for the evaluation of weir designs. Simulations were performed for different weir designs and under variation of the hold-up mass, the feed rate and gas velocity to predict their transport behavior in a pilot-scale 3D horizontal fluidized bed. The results indicate that the solids transport behavior is strongly dependent on the used weir design and the main driving force for the particle transport that can be influenced by the process conditions. The installation of weirs between two compartments induces a transport resistance, while the base type without the installation of a weir between the two chambers represents the fastest possibility for mixing the particles of a two-compartment system. It has been observed that the general trend shows higher particle recirculation rates for the overflow weir and base configuration (no weir), whereas the underflow and sideflow weir applications improve the solids transport through the horizontal fluidized bed.     

A study of gas and solid mixing behaviors in a three partitioned fluidized bed

A previously unknown partitioned fluidized bed gasifier (PtFBG) has been developed for improving coal gasification performance. The basic concept of the PtFBG is a fluidized bed divided into two parts, a gasifier and a combustor, by a partitioned wall. Char is burnt in the combustor and the generated heat is supplied to the gasifier along with the bed materials. During that time, highly concentrated CO₂ is inevitably generated in the combustor. Therefore, vigorous solid mixing is an essential precondition as well as minimizing horizontal gas mixing. In this study, gas and solid mixing behaviors were verified in a cold model three partitioned fluidized bed (3-PtFB). Glass beads with an average diameter of 150 μm and a particle density of 2500 kg/m³ were used as bed materials. For the gas mixing experiments, CO₂ and N₂ were introduced into the beds through each distributor. Then, outlet gas flow rates and concentrations were measured by gas flow meters and an IR gas analyzer respectively. The calculated gas exchange ratios ranged from 3% to 10% with varying gas flow rates. For the solid mixing experiments, 1000 μm polypropylene particles with a density of 883 kg/m³ were continuously fed into the reactor. Then, the polypropylene particles were distributed to the entire beds evenly. Solid mixing behaviors were very analogous to liquid mixing behaviors in a continuous stirred tank reactor (CSTR).     

Granular temperature with discrete element method simulation in a bubbling fluidized bed

The Discrete Element Method (DEM) plays an important role in understanding and modeling the kinetic characteristics in granular systems. A soft-sphere method with a linear spring–dashpot model was used in the simulation of a bubbling fluidized bed. The time-averaged granular temperature and vertical particle velocity at different heights were numerically studied and compared to experimental measurements of Müller. The influence of a velocity-dependent coefficient of restitution and three drag models were also investigated in this work. Good agreement was found between the DEM simulation and Müller’s experiment, especially using the DiFelice drag model. The variable coefficient of restitution, with a sufficiently high yielding relative velocity, gives a granular temperature that is a little lower compared to that of a constant coefficient of restitution, while it predicts a more intense velocity fluctuation, with a lower yielding relative velocity. By comparing the granular temperature in the vertical direction and in the transverse direction, a strong anisotropy is found in the bed.