Bed-to-wall heat transfer characteristics in a circulating fluidized bed

The bed-to-wall heat transfer coefficients were measured in a circulating fluidized bed of FCC particles (d_p = 65 μm). The effects of gas velocity (1.0–4.0 m/s), solid circulation rate (10–50 kg/m²s) and particle suspension density (15–100 kg/m³) on the bed-to-wall heat transfer coefficient have been determined in a circulating fluidized bed (0.1 m-ID x 5.3 rn-high). The heat transfer coefficient strongly depends on particle suspension density, solid circulation rate, and gas velocity. The axial variation of heat transfer coefficients is a strong function of the axial solid holdup profile in the riser. The obtained heat transfer coefficient in terms of Nusselt number has been correlated with the pertinent dimensionless groups     

Heat transfer in a high temperature fluidized bed

The heat transfer characteristics between the bed and immersed tube in a high temperature fluidized bed (7.5 cm I.D.×70 cm H) were investigated with sand and iron ore particles. The heat transfer coefficients were measured at operating temperatures of 200–600°C and gas velocities of 1–10 U_mf. The bed emissivity measured by the radiation probe was found to be 0.8–0.9. The experimentally obtained radiative heat transfer coefficient was in the range of 30–80 W/m²K for the operating temperature of 400–800°C and the contribution of radiation to total heat transfer was about 13% and 18% for the operating temperatures of 400°C and 600°C, respectively.     

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.     

Coal combustion characteristics in a pressurized fluidized bed

The characteristics of emission and heat transfer coefficient in a pressurized fluidized bed combustor are investigated. The pressure of the combustor is fixed at 6 atm. and the combustion temperatures are set to 850, 900, and 950 °C. The gas velocities are 0.9, 1.1, and 1.3 m/s and the excess air ratios are 5, 10, and 20%. The desulfurization experiment is performed with limestone and dolomite and Ca/S mole ratios are 1,2, and 4. The coal used in the experiment is Cumnock coal from Australia. All experiments are executed at 2 m bed height. In this study, the combustion efficiency is higher than 99.8% through the experiments. The heat transfer coefficient affected by gas velocity, bed temperature and coal feed rate is between 550-800 W/m² °C, which is higher than those of AFBC and CFBC. CO concentration with increasing freeboard temperature decreases from 100 ppm to 20 ppm. NO_x concentration in flue gas is in the range of 5-130 ppm and increases with increasing excess air ratio. N₂O concentration in flue gas decreases from 90 to 10 ppm when the bed temperature increases from 850 to 950 °C.     

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.     

Enhancement of mass transfer in the fluidized bed electrode reactors

With the view of developing the fluidized bed electrode system, mass transfer coefficient, overpotential distribution, and copper degradation have been observed in this investigation. Particles whose diameters were one of 327, 388, 510, 548, 750, and 960 μm were fluidized by the 1,000 ppm copper sulfate electrolyte. This study used two types of the experimental reactor. One had 5x5.5 cm bed-dimension with various thickness in a rectangular side-by-side configuration; the other 3.2 cm bed-diameter with various height in a cylindrical flow-through configuration. Mass transfer coefficient increased with increasing particle diameter, and the optimum fluidization was obtained at the condition of bed porosity near 0.65. For processing a large fluidized bed reactor, the expansion of bed height at a distance between electrodes was found to be more effective than the enlargement of bed thickness between electrodes. By replacing a three-dimensional current-feeder with a plane feeder, degradation and residual concentration of copper ion in a batch recycling mode could be achieved to be higher than 99% and less than 5 mg/L, respectively.     

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.     

Radiative heat transfer in circulating fluidized bed furnaces

Three methods of estimating the effective emissivity of a gas-particle suspension are compared and the radiative heat transfer coefficient of an isothermal suspension is defined. Heat flux measurements obtained from circulating fluidized bed combustors are examined. Radiation from a particle suspension with core temperature dominates the radiative heat transfer in the upper part of the furnace, where the particle density is low and no substantial particle boundary layers are formed. Over the lower parts of the heat transfer surfaces, where significant thermal and particle boundary layers are present, the radiative heat flux is dominated by emission from the relatively low temperature particle layer in the vicinity of the heat receiving surface.     

Temperature fluctuations and heat transfer in a fluidized-bed combustor of waste oil

Temperature fluctuations and heat transfer characteristics were investigated in a fluidized-bed combustor of 0.102 m ID and 2.5 m in height, which was designed for waste oil combustion. Effects of excess air (A_E), injection height (H_I) and feeding rate of waste oil (Q_F) on the mean bed temperature (T_B), Kolmogorov entropy (K₂) of phase space portraits and heat transfer coefficient (U₀) in the fluidized-bed combustor were determined. T_B increased, but K₂ and U₀ decreased with increasingA _E . K₂ had a local minimum, but T_B and U₀ had a maximum at H_I of 0.4 m. T_B increased, but K₂ had a minimum and U₀ had a maximum with increasing Q_F in the combustor. T_B, K₂ and U₀ obtained at the optimum operating condition (A_E=40%, H_I=0.4 m, Q_F=30 g/min) were about 855 ‡C, 22 bits/s and 382 W/m²K, respectively.     

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.     

膏状物料在惰性粒子流化床中的干燥试验研究

利用4种物料在惰性粒子流化床干燥器中进行性能试验,得出了在试验条件下的容积传热系数、干燥强度及热效率。     

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。     

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.     

液状物料在惰性粒子流化床干燥器中的试验研究

利用两种物料在惰性粒子流化床干燥器中进行性能试验，得出了在试验条件下的容积传热系数，蒸发强度及热效率，并作了经济性评价。     

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.     

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.     

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.     

Computational fluid dynamics of high density circulating fluidized bed riser: Study of modeling parameters

CFD modeling of air and fluid catalytic cracking (FCC) particles in the riser of a high density circulating fluidized bed (HDCFB) has been performed. The implementation of correct inlet conditions was found to be critical for the successful simulation of the hydrodynamics. The simulated profiles of gas and solid velocity and volume fraction were overall in good agreement with experimental data reported in the literature. However, due to the difficulties in accurate modeling of the solid segregation toward the wall, the solid volume fraction was under predicted near the walls. The effect of modeling parameters including different drag models, wall restitution coefficient values, and solid slip conditions have been evaluated. While the wall restitution coefficient did not exhibit a significant effect on the riser hydrodynamics, the appropriate slip condition aided in predicting the solid segregation toward the wall.     

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.     

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).