Solid circulation characteristics in an internally circulating fluidized bed with orifice-type draft tube

Effects of superficial gas velocities to a draft tube, to an annulus section and particle size on the solid circulation rate (G,) have been determined in an internally circulating fluidized bed (0.28 m I.D. × 2m high) with an orifice type draft tube. The solid circulation rate from the draft tube to an annulus section increases with increasing gas velocities to the draft tube(U _d ) and annulus section (U_a) and consequent increase in pressure drop across the orifice (ΔP_or). However, the values ofG _s decrease by 7–21% with increasing particle size from 86 to 288 μm. The pressure drop across the orifice increases with increasingU _d andU _a . However, ΔP_or decreases by 5–23% with increasing particle size. To predictG _s in an internally circulating fluidized bed, a correlation is proposed as a function of ΔP_or This paper is dedicated to Professor Dong Sup Doh on the occasion of his retirement Korea University.     

Solid circulation and gas bypassing characteristics in a square internally circulating fluidized bed with draft tube

The effects of gas velocities to draft tube (26.64–52.54 cm/s) and to annulus section (8.14–11.84 cm/s) on solid circulation rate and gas bypassing fractions were determined in a square internally circulating fluidized bed reactor with an orifice-type square draft tube. The solid circulation rate and gas bypassing fraction from the annulus section to the draft tube increase but gas bypassing fraction from the draft tube to the annulus section decreases with increasing gas velocity to the draft tube. With increasing gas velocity to the annulus section, the solid circulation rate and gas bypassing fraction from the draft tube to the annulus section increase but, gas bypassing fraction from the annulus section to the draft tube decreases. The solids circulation rate was correlated with the pressure drop across the orifice and the opening area ratio based on the orifice theory. The gas bypassing fraction was correlated with gas velocities to the fluidized and the moving beds. Based on the gas bypassing fraction data, the gas flow rates across the orifice were correlated with gas velocities to the fluidized and the moving beds, opening area ratio, particle size and solids height in the bed.     

Hydrodynamics of a spouted bed with an impermeable draft tube for binary particle systems

A spouted bed with an impermeable draft tube was employed to obtain fundamental data of binary mixtures of glass beads for both the operating conditions and the design factors. These data were compared with those for the coarser particle system only. From this view point, minimum spouting velocity, pressure drop, hold-up of solid particles within a draft tube, upward gas flow rate within the annulus and solids circulation rate were determined by changing the total gas flow rate and mass fraction of finer particles as operating parameters and by changing distance of entrainment zone and draft tube diameter as geometric parameters.     

Hydrodynamic characteristics of fine particles in the riser and standpipe of a circulating fluidized bed

The hydrodynamic properties in the riser and standpipe. and the cyclone efficiency have been determined in a circulating fluidized bed (CFB) unit consisting of a riser (0.05 m-IDX3.8 m high), a standpipe (0.068 m-IDX2.5 m high) as a primary cyclone/bubbling fluidized bed, and a secondary cyclone. Silica gel powder (mean diameter = 46 μm) was used as the bed material. The effects of gas velocity in the riser and initial solid loading on the solid circulation rate, and the solid holdups in the riser and standpipe have been determined. The effects of gas velocity in the standpipe on the efficiencies of primary and secondary cyclones have been also determined as functions of solid circulation rate and solid entrainment rate. The solid circulation rate increases with increases in the gas velocity in the riser and in the initial solid loading. The efficiencies of primary and secondary cyclones increase with an increase in the gas velocity in the riser. However, the efficiency of primary cyclone decreases and that of secondary cyclone increases slightly, with an increase in the gas velocity in the standpipe.     

Axial dispersion of gas in a circulating fluidized bed

An axial dispersion of gas in a circulating fluidized bed was investigated in a fluidized bed of 4.0 cm I.D. and 279 cm in height. The axial dispersion coefficient of gas was determined by the stimulus-response method of trace gas of CO₂. The employed particles were 0.069 mm and 0.147 mm silica-sand. The results showed that axial dispersion coefficients were increased with gas velocity and solid circulation rates as well as suspension density. The experimentally determined axial dispersion coefficients in this study were in the range of 1.0-3.5 m²/s.     

Development of novel two-interconnected fluidized bed system

Two-interconnected fluidized bed systems are widely used in various processes such as Fisher-Tropsch, hot gas desulfurization, CO₂ capture-regeneration with dry sorbent, chemical-looping combustion, sorption-enhanced steam methane reforming, chemical-looping hydrogen generation system, and so on. However, conventional two-interconnected fluidized bed systems are very complex, large, and difficult to operate because most of these systems require a riser and/or pneumatic transport line for solid conveying and loopseals or seal-pots for gas sealing, recirculation of solids to the riser, and maintaining of pressure balance. To solve these problems, a novel two-interconnected fluidized bed system has been developed. This system has two bubbling beds, solid injection nozzles, solid conveying lines, and downcomers. In this study, the effects of operating variables on solid circulation rate and gas leakage between two beds have been investigated in a cold mode two-interconnected fluidized bed system. The solid circulation rate increased as the hole diameter on the injection nozzle, the diameter of the injection nozzle, the solid height above the holes, and the number of holes on the injection nozzle increased. The gas leakage between the beds was negligible. Moreover, long-term operation of continuous solid circulation up to 60 hours was performed to check the feasibility of stable operation. The pressure drop profiles in the system loop were maintained steadily and solid circulation was smooth and stable.     

Continuous measurement of solids flow in a circulating fluidized bed

A modified impact probe for continuous measurement of solids circulation rate in a circulating fluidized bed has been developed based on a principle similar to a momentum probe. The response curves of solid flow from the probe have been characterized and calibrated in a test column (0.05 m-I.D.x0.80 m-high). The probe was validated in situ in the downcomer of a circulating fluidized bed (0.10 m-I.D.x4.80 m-high). The solid circulation rates obtained by the modified impact probe well agree with the measured solids circulation rate by the descent time method. Presented at the Int’/Sym. on Int’l Symp. on Chem. Eng. (Cheju, Feb. 8–10, 2001), dedicated to Prof. H. S. Chun on the occasion of his retirement from Korea University.     

The control of bed height and solids circulation rate in the standpipe of a cold flow circulating fluidized bed

Circulating fluidized beds (CFBs) are used widely in the chemical industry. Knowing or estimating the bed height in the standpipe and the solids circulation rate are essential for effective control of the system. This paper incorporates a 2-region model to calculate the bed height in the standpipe with a Kalman filter algorithm to estimate the solids circulation rate (SCR). Simulations of both the standpipe bed height and SCR were compared with experimental data and shown to give good agreement.

In addition, a neural network method was applied to model the entire cold flow CFB system and measured data sets were used to train the neurons of the network. Finally, a linear controller was applied to control both the bed height and solids circulation rate to desired set points. Simulations were performed for both positive and negative step inputs for both variables and satisfactory control was demonstrated using this controller in combination with the neutral network and Kalman estimator.     

Hydrodynamic behaviour of solid transport for a closed loop circulating fluidized bed with secondary air injection

The aim of this work was to study the mechanism of solid circulation in a Circulating Fluidized Bed pilot as a function of secondary air flow rate A rectangular column of 7 m height equipped with a U type siphon was used for this purpose The results obtained showed that the solid circulating phenomenon depends on different limiting steps like feeding step (dense bed), siphon circulating capacity and suspension saturation capacity.     

Co-combustion of coal and biomass in a circulating fluidized bed combustor

In this research, co-combustion of coal and rice husk was studied in a circulating fluidized bed combustor (CFBC). The effects of mixed fuel ratios, primary air and secondary air flow rates on temperature and gas concentration profiles along riser (0.1 m inside diameter and 3.0 m height) were studied. The average particle size of coal from Maetah used in this work was 1,128 mm and bed material was sand. The range of primary air flow rates was 480–920 l/min corresponding to U _g of 1.0–2.0 m/s for coal feed rate at 5.8 kg/h. The recirculation rate through L-valve was 100 kg/hr. It was found that the temperatures along the riser were rather steady at about 800–1,000 degrees Celsius. The introduction of secondary air improved combustion and temperature gradient at the bottom of the riser, particularly at a primary air flow rate below 1.5 m/s. Blending of coal with biomass, rice husk, did improve the combustion efficiency of coal itself even at low concentration of rice husk of 3.5 wt%. In addition, the presence of rice husk in the feed stocks reduced the emission of both NO _x and SO₂.     

Effect of temperature on gas bypassing and solids circulation rate in an internally circulating fluidized bed

The effect of bed temperature in the annulus region on gas bypassing from annulus to riser and solids circulation rate has been determined in an internally circulating fluidized bed. The bed voidage in the annulus region increases with increasing temperature. The average fractions of gas bypassing of annulus gas are 59.4% and 87% at room temperature and high temperature (400°C to 800°C), respectively. Solids circulation rate increases with increasing annulus gas velocity, but is nearly independent of riser velocity. Solids circulation rate increases with increasing temperature.     

含内构件的循环流化床的动态压力特性

针对含内构件的循环流化床,以石英砂为物料,使用动态压力传感器测量了含内构件的流化床中气固两相流的动态压力,分析了床内的瞬时压力特性. 结果表明,在进出口总压降中,文丘里压降最大,占主床压降的60%以上. 表观气速和固体颗粒循环流率共同影响循环流化床内的压力特性. 压力瞬时波动功率谱分析表明,压力波动对应一个主频,表观气速越小、颗粒循环流率越大时,压力波动越大,且循环流化床底部压力波动比上部大. 加入内构件能有效引导气流,使流动更均匀.     

Gas backmixing in the dense region of a circulating fluidized bed

The gas backmixing characteristics in a circulating fluidized bed (0.1 m-IDx5.3-m high) have been determined. The gas backmixing coefficient (D_ba) from the axial dispersion model in a low velocity fluidization region increases with increasing gas velocity. The effect of gas velocity onD _ba in the bubbling bed is more pronounced compared to that in the Circulating Fluidized Bed (CFB). In the dense region of a CFB, the two-phase model is proposed to calculate D_bc from the two-phase model and mass transfer coefficient (k) between the crowd phase and dispersed phase. The gas backmixing coefficient and the mass transfer coefficient between the two phases increase with increasing the ratio of average particle to gas velocities (U_p/U_g).     

Temperature fluctuations and heat transfer in a circulating fluidized bed

Characteristics of temperature fluctuations and heat transfer coefficient have been investigated in the riser of a circulating fluidized bed (0.102 m ID and 4.0 m in height). Effects of gas velocity and solid circulation rate on the temperature fluctuations, suspension density and heat transfer coefficient between the immersed heater and the bed have been considered in the riser. To analyze the characteristics of temperature fluctuations at the wall of the riser, the phase space portrait and Kolmogorov entropy of the fluctuations have been obtained, and the relation between the temperature fluctuations and the heat transfer coefficient has been examined. It has been found that the heat transfer system becomes more complicated and irregular with decreasing gas velocity and increasing solid circulation rate or suspension density in the riser. The heat transfer coefficient and Kolmogorov entropy of the temperature fluctuations have decreased with increasing the superficial gas velocity, while they have increased with increasing the solid circulation rate or suspension density in the bed. The heat transfer coefficient has been well correlated in terms of the Kolmogorov entropy, suspension density as well as operating variables in the riser. This paper is dedicated to Professor Dong Sup Doh on the occasion of his retirement from Korea University.     

Powder coating efficiency of small particles and their agglomeration in circulating fluidized bed

The coating efficiency of fluidizing small particles and their agglomeration were investigated to evaluate the possibility of powder coating by the use of a circulating fluidized bed. Glass beads, whose mean diameter was 43 Μm, and silica powder of 1 Μm were used as a core and a coating material. Polyvinyl alcohol was used as a binder and its solution was supplied together with silica powder from a spray nozzle equipped in the circulating fluidized bed. Glass beads of 43 Μm, which had been impossible to coat in a conventional fluidized bed coater, were successfully coated with silica powder in a circulating fluidized bed, and agglomeration among core particles was prevented. From this result, it was confirmed that a circulating fluidized bed performs excellently as a coater, especially for fine core particles, so a circulating fluidized bed coater has bright prospects for particle coating.     

Solids flow characteristics in loop-seal of a circulating fluidized bed

The hydrodynamics of solids (FCC) recycle in a loop-seal (0.08 m) at the bottom of the downcomer (0.08 m-I.D.x4.0 m-high) in a circulating fluidized bed (0.1 m-I.D.x 5.3 m-high) have been determined. Solid flow rate through the loop-seal increases linearly with increasing aeration rate. At the same aeration rate, the maximum solid flow rate can be obtained at a loop-seal height-to-diameter ratio of 2.5. The effects of solid inventory, solid circulation rate and gas velocity on pressure balance around the CFB have been determined. At a given gas velocity and solid circulation rate, pressure drops across the downcomer and loop-seal increase linearly with increasing solids inventory in the bed. At a constant solid inventory, pressure drops across the riser and the downcomer increase with increasing solid circulation rate but decrease with increasing gas velocity in the riser. The obtained solid flow rate has been correlated with pressure drop across the loop-seal.     

循环床锅炉炉膛压降及轴向固体浓度的计算

本文提出了循环流化床锅炉炉膛的轴向压力及固体浓度分布的计算式。当给出表观气、循环固体流率以及颗粒性质时，可准确地预测炉膛压降与轴向固体浓度分布，为炉膛设计提供了计算依据。     

An analysis of pressure drop fluctuation in a circulating fluidized bed

The characteristics of pressure drop fluctuation in a 5.0 cm I.D.×250cm high circulating fluidized bed with fine polymer particles of PE and PVC were investigated. The measurements of time series of the pressure drop were carried out along the three different axial locations. To determine the effects of coarse particles and relative humidity of air on the flow behavior of polymer powders-air suspension in the riser, we employed deterministic chaos analysis of the Hurst exponent, correlation dimension and phase space trajectories as well as classical methods such as standard deviation, probability density function of pressure drop fluctuation. From a statistical and chaos analysis of pressure fluctuations, the upper dilute region was found to be much more homogenous flow compared to that in the bottom dense region at the same operating conditions. It was also found that the addition of coarse particles and higher humidity of air reduced the pressure fluctuations, thus enhancing flow stability in the riser. The analysis of pressure fluctuations by statistical and chaos theory gave qualitative and the quantitative information of flow behavior in the circulating fluidized bed.     

Wear of water walls in a commercial circulating fluidized bed combustor with two gas exits

The tube thickness profiles of water walls, including wing walls, were measured in a large commercial circulating fluidized bed furnace (200 tonnes steam/h, 4.97 m × 9.90 m × 28.98 m high) by means of an ultrasonic thickness gauge in order to investigate tube wastage patterns. Wastage of all water walls, including wing walls, was significant in the transition region just above the refractory lining around the circumference of the combustor, especially at the center of the sidewalls and both sides of the front and rear walls. The lateral wear profile seemed to be influenced mainly by secondary flow, stronger toward the center than in the corners, and somewhat by the wing walls and two gas exits. Wastage of some tubes was also found to be appreciable around the gas exit. The lateral profiles of tube thickness at the gas exit level and other findings indicate asymmetric flows between the two exits.     

Parametric studies on catalytic pyrolysis of coal-biomass mixture in a circulating fluidized bed

Pyrolysis is an efficient way of thermally converting biomass into fuel gas, liquid product and char. In this research, pyrolysis experiments were carried out in a circulating fluidized bed reactor with a riser diameter of 25 mm and height 1.65 m. The biomass used was corn cobs. The experiments were conducted systematically using two level factorial design with temperature ranging from 650 to 850 degree Celsius, corn cobs and catalyst contents in feed ranging from 0 to 100%, and from 1 to 5 wt%, respectively, and Ni loaded on catalyst ranging from 5 to 9 wt%. The results showed that when temperature and catalyst contents in feed and Ni loaded on catalyst increased, the percent of hydrogen and carbon monoxide increased. The amount of corn cobs was found to have an effect only on the composition of hydrogen. Carbon dioxide was also observed to increase slightly. On the other hand, the percent of methane was considerably decreased. The optimum conditions were 850 degree Celsius, corn cob content in feed of 100%, catalyst content in feed of 5% and Ni loaded on catalyst of 9%. At this condition the percentages of hydrogen and carbon monoxide were 52.0 and 18.0, respectively.