Effect of elevated temperature and silica sand particle size on minimum fluidization velocity in an atmospheric bubbling fluidized bed

The impact of temperature and particle size on minimum fluidizing velocity was studied and analyzed in a small pilot scale of bubbling fluidized bed reactor. This study was devoted to providing some data about fluidization to the literature under high temperature conditions. The experiments were carried out to evaluate the minimum fluidizing velocity over a vast range of temperature levels from 20 °C to 850 °C using silica sand with a particle size of 300–425 μm, 425–500 μm, 500–600 μm, and 600–710 μm. Furthermore, the variation in the minimum fluidized voidage was determined experimentally at the same conditions. The experimental data revealed that the U_mf directly varied with particle size and inversely with temperature, while ε_mf increases slightly with temperature based on the measurements of height at incipient fluidization. However, for all particle sizes used in this test, temperatures above 700 °C has a marginal effect on U_mf. The results were compared with many empirical equations, and it was found that the experimental result is still in an acceptable range of empirical equations used. In which, our findings are not well predicted by the widely accepted correlations reported in the literature. Therefore, a new predicted equation has been developed that also accounts for the affecting of mean particle size in addition to other parameters. A good mean relative deviation of 5.473% between the experimental data and the predicted values was estimated from the correlation of the effective dimensionless group. Furthermore, the experimental work revealed that the minimum fluidizing velocity was not affected by the height of the bed even at high temperature.     

Geldart A类颗粒节涌床气固流动特性的实验及模拟

针对气固节涌床,在实验基础上,基于欧拉?欧拉双流体模型结合颗粒动力学理论,考虑Geldart A类颗粒聚团对气固间曳力的影响,采用修正后的Gidaspow曳力模型对气固节涌床进行数值模拟。结果表明,通过与实验结果及经验公式进行对比,修正的模型可准确合理地模拟流化床内节涌特性。表观气速0.09 m/s≤Ug≤0.39 m/s时,床层内部压力脉动标准偏差随表观气速增加而增加,流型由鼓泡转变为节涌直至节涌程度最大,床内气固流动主要受轴对称栓运动特性影响,床内压降、床层膨胀比、气栓平均上升速度、最大轴对称栓长度随表观气速增加而增加,最大轴对称栓产生位置随表观气速增加而降低;Ug>0.39 m/s后,床内压力脉动标准偏差随表观气速增加而降低,节涌程度降低至向湍动流态化流型转变,床内气固流动主要受壁面栓运动特性影响,增加表观气速,节涌床内压降变化幅度较小,气栓平均上升速度增加幅度加大,床层膨胀比及最大轴对称栓长度降低,最大轴对称栓产生的位置略有升高。     

Slug characteristics of polymer particles in a gas-solid fluidized bed

The slug characteristics (frequency, rising velocity and length) have been determined by analyzing pressure fluctuations in a fluidized bed (0.38 m-I.D.x4.4m-high) of linear-low-density-polyethylene (LLDPE) and polypropylene (PP) particles. The slug characteristics of LLDPE and PP particles have been determined as a function of gas velocity (0.6-1.2 m/s) and the axial height (0.65–1.15 m) from the distributor. The rising velocity and vertical length of slug increase with increasing superficial gas velocity and the axial height of the bed. The slug shape of LLDPE particles is found to be the square-nose whereas that of PP particles is the round-nose. The slug frequency and its length have been correlated in terms of the excess fluidizing velocity, column diameter and bed height based on the data from the present and previous studies.     

Fluidization and slugging in large-particle systems

Square-nose slugging that occurs with large particles in relatively small-diameter fluidized beds shows certain similarities with the fluidization behaviour in a fluidized bed coal combustion system with closely packed heat-exchanger tubes. In the present investigation, square-nose slugging is studied in fluidized beds of 0.1 and 0.15 m I.D. with coarse sand and alumina particles, at ambient conditions. Recording of pressure fluctuations was used to analyse the fluidization behaviour. A remarkable change in the pressure fluctuation pattern occurs at the transition from normal fluidization to slugging: a more regular signal with a narrowed frequency spectrum is found.In the square-nose slugging regime, the pressure fluctuations seem to be caused by the disintegration of a rising solids slug, followed by the raining down of the particles. Experimental evidence for this mechanism was found in the behaviour of the magnitude of the pressure fluctuations as a function of operating variables.The frequency of square-nose slugging increases with approximately the square root of the bed diameter and appears to be independent of the type of particles used. The slug frequency decreases slightly with the gas velocity between about 0.8 and 1.8 m.s⁻¹, and is inversely proportional to the stationary bed height between 0.15 and 0.4 m.     

Rise velocities of slugs and voids in slugging and turbulent fluidized beds

Using the cross-correlation function of two pressure fluctuation signals, the rise velocities of slugs and voids and the slug frequency in the slugging and turbulent fluidized beds of glass beads(d_p= 0.362 mm) have been determined in a 0.1 m-lD x 3.0 m high Plexiglas column The slug rise velocity in the slugging flow regime increases with an increase in gas velocity, while the void velocity remains almost constant with the variation of gas velocity in the turbulent flow regime. The slug frequencies are found to be insensitive to the increase of gas velocity in the slugging flow regime within the frequency range of 0.47–0.64 Hz. The data of the present and previous studies on the slug rise velocity in the slugging flow regime have been correlated as $$U_s = 1.73 \times 10^{ - 2} (\frac{{\bar d_p }}{{D_t }})^{0.093} (\frac{{\rho _S }}{{\rho _g }})^{ - 0.616} (U_g - U_{mf} ) + 0.35(gD_t )^{1/2} $$      

Characteristics of slug flow in a fluidized bed of polyethylene particles

The slug flow behavior of polyethylene particles was examined in a fluidized bed of 7 cm ID and 50 cm in height. The employed polymer particles were high density polyethylene (HDPE) with the average particle size of 603 μm. The slugging flow of polyethylene particles was analyzed from the measured pressure drop signals by classical statistical methods such as absolute average deviation, probability density function, power spectrum, auto-cor-relation, and cross-correlation. The results show that in spite of high dielectric constant of polymer particles, the slugging phenomena such as incipient slugging velocity, slugging frequency and slugging rise velocity were very similar to the Geldart B type non-polymeric particles. It was observed that slug frequencies decreased with gas velocity and the limiting slug frequency was observed for the gas velocities in this study.     

Heat transfer to horizontal tubes immersed in a fluidized-bed combustor

Experiments are carried out to measure the heat transfer rates to water-cooled horizontal tubes (D_T = 25.4 mm) immersed in an atmospheric fluidized-bed combustor burning North Dakota lignite. Silica sand, with average diameter ranging from 0.544 mm to 2.335 mm, is used as bed material. The tests are conducted at average bed temperatures ranging from 587 to 1205 K. For bed temperatures less than 950 K, the bed is heated by a propane gas heater. The superficial velocity is varied from 0.73 to 2.58 m/s. Thus, the effect of bed temperature (T_B = 587 – 1205 K), particle size (d̄_p = 0.544 – 2.335 mm), and fluidizing velocity (U = 0.73 – 2.58 m/s) on the heat transfer rate to horizontal tube immersed in a fluidized-bed combustor (0.45 m × 0.45 m) is investigated. Among the existing correlations, those correlations proposed by Glicksman and Decker, Zabrodsky et al., Catipovic et al., Grewal, and Bansal et al. are found to predict the present data quite well, when the contribution due to radiation is included. The radiative heat transfer is estimated as the difference between the heat transfer to oxidized boiler tube and gold-plated tube. The relative contribution of radiation is found to be 13% for a bed of sand particles (d̄_p = 0.9 mm) operating at 1087 K.     

Behaviour of gas-fluidized beds of fine powders part I. Homogeneous expansion

The minimum bubbling velocity has been correlated from the results of experiments on 48 gas/solid systems and is a function of the density and viscosity of the fluidizing gas, the mean sieve size of the powder and the fraction of fines less than 45 μm. An improved equation is presented to predict U_mb/U_mf; this parameter also correlates well with the maximum non-bubbling bed expansion ratio H_mb/H_mf which is used to calculate the maximum dense phase voidage ?_mb. An equation based on the Carman-Kozeny theory can be used to predict bed voidages between incipient fluidization and bubbling. An accurate value of particle density is essential in these equations and a simple comparative method has been used to determine particle densities of fine porous powders.     

Investigation of fluidized bed behaviour using electrical capacitance tomography

The temporal and cross-sectional distributions of particles in a 127 mm diameter fluidized bed have been obtained using a new generation, high-speed electrical capacitance tomography. Two planes of eight electrodes were used and mounted at 160 and 660 mm from the gas distributor which was a 3 mm thick porous plastic plate (maximum pore size of 50-70 μ m). 3 mm diameter, nearly-spherical polyethylene granules made up the bed. Experiments at sampling frequencies of 200-2000 cross-sections per second and gas superficial velocities from just below the minimum fluidization to 83% above minimum fluidization velocities were used. The time series of the cross-sectional average void fractions have been examined both directly and in amplitude and frequency space. The last two used probability density functions and power spectral densities. The information gathered shows that the fluidized bed was operating in the slugging mode, which is not surprising given the size of the particles. It has been found that an increase in the excess gas velocity above the minimum fluidization velocity resulted in an increase in the mean void fraction, an increase in the length and velocity of the slug bubbles as well as the bed height, and a slight decrease in the slug frequency. The results are presented in a level of detail suitable for comparison with later numerical simulation.     

Void characteristics in turbulent fluidized beds

Void properties (size, rising velocity) in the turbulent flow regime have been determined in a 0.1 m-ID X 3.0 m high Plexiglas column of glass beads (d_p = 0.362, mm) by using an optical fiber probe system. The bubble size increases with an increase in gas velocity in the slugging flow regime but it sharply decreases in the turbulent flow regime. The mean amplitude of pressure fluctuations is linearly related to the bubble or void size in the bed. The void rising velocity is almost constant in the turbulent flow regime. Uniform condition of the bed structure in the turbulent flow regime can be determined from the void distribution coefficient in the bed. In addition, the bed condition in the turbulent How regime has been evaluated from the variations of the void velocity coefficient and the propulsive power of a rising void with gas velocity.     

THE EFFECT OF TEMPERATURE ON FLUIDIZED BED BEHAVIOUR

Most published correlations for the minimum fluidizing gas velocity have been derived from tests under ambient conditions and increasing discrepancy is found in their application over wider ranges of operating conditions. Up to 1000°C the Ergun equation is reliable but it requires a knowledge of the particle shape factor and bed voidage for its application. Bed voidage is found to vary with temperature for laminar gas flow conditions.

Paralleling changes in gas flow conditions with operating temperature are changes in bed-to-surface heat transfer coefficients. There is a distinct transition from the interphase gas convective to the particle convective component of heat transfer being the dominant mechanism as the operating temperature increases and Re_mf reduces through 12,5 at Ar ～ 26000. This is thought to be a consequence of change in bed bubbling behavior.     

Mass loss and apparent kinetics of a thermally thick wood particle devolatilizing in a bubbling fluidized bed combustor

This work presents the results of experiments conducted to determine the mass loss characteristics of a cylindrical wood particle undergoing devolatilization under oxidation conditions in a bubbling fluidized bed combustor. Cylindrical wood particles having five different sizes ranging from 10 to 30 mm and aspect ratio (l/d = 1) have been used for the study. Experiments were conducted in a lab scale bubbling fluidized bed combustor having silica sand as the inert bed material and air as the fluidizing medium. Total devolatilization time and mass of wood/char at different stages of devolatilization have been measured. Studies have been carried out at three different bed temperatures (T_bed = 750, 850 and 950 °C), two inert bed material sizes (mean size d_p = 375 and 550 μm) and two fluidizing velocities (u = 5u_mf and u = 10u_mf). Devolatilization time is most influenced by the initial wood size and bed temperature. Most of the mass is lost during the first half of the devolatilization process. There was no clear influence of the fluidization velocity and bed particle size on the various parameters studied. The apparent kinetics estimated from the measured mass history show that the activation energy varied narrowly between 15 and 27 kJ/mol and the pre-exponential factor from 0.11 and 0.45 s⁻¹ for the wood sizes considered.     

Mass transfer around freely moving active particles in the dense phase of a gas fluidized bed of inert particles

The mass transfer coefficient around freely moving active particles under bubbling/slugging fluidized bed conditions was measured in a lab-scale reactor. The technique used for the measurements consisted in the oxidation reaction of carbon monoxide at over one or few Pt catalyst spheres immersed in an inert bed of sand. It was shown that this technique is simple and accurate, and allows to overcome most of the difficulties and uncertainties associated with other available techniques. The experimental campaign was carried out by varying the fluidization velocity (0.15-0.90 m/s), the active particle size (1.0-10.0 mm) and the inert particle size (0.1-1.4 mm). Results were analyzed in terms of the particle Sherwood number. Experimental data showed that Sh is not influenced by the fluidization velocity and by a change of regime from bubbling to slugging, whereas it increases with a square root dependence with the minimum fluidization velocity and with the active particle size. These results strongly suggest that the active particles only reside in the dense phase and never enter the bubble/slug phase. Data were excellently fitted by a Frössling-type correlation:

Sh=2.0·ε_mf+K·(Re_mf/ε_mf)^1/2·Sc^1/3     

Fate of solids fed pneumatically through a jet into a fluidized bed

Solid tracer particles were fed pneumatically through a jet into a fluidized bed to simulate the feeding of solids via a pneumatic transport line into a fluidized-bed reactor operating in the slugging-bed mode. The fluidized bed was defluidized instantaneously at different times after the initiation of the tracer particle injection. The bed was then sampled layer by layer to provide the radial and axial concentration profiles of the tracer. Regular and high-speed movies (1,000 frames per second) were taken to study the operation of the fluidized bed and the phenomena of the gas-solid two-phase jet. Experimental results on solid mixing, jet constriction and slugging frequencies, slugging bed height, slug length, jet penetration, and jet half-angle at three nominal jet velocities of 52, 37, and 25 m/s and corresponding solids loadings are presented. Additional experimental results on jet constriction and slugging frequencies, and slug volume (axial slug size) obtained for a wider range of jet velocities confirm the hydrodynamic trends observed during the tracer particle injection experiments. The results indicate that solids mixing increases, and well-mixed conditions are reached earlier, with an increase in jet injection velocity. The obtained mixing times were correlated successfully in terms of the excess gas velocity. The experimental data on jet penetration and slug motion were satisfactorily correlated by modified versions of existing theoretical relations. The modifications included the effect of the injected solids on jet penetration and jet half-angle and also the effect of our semicircular column geometry and single wall-slug configuration on the observed slug motion.     

Rice husk co-firing with coal in a short-combustion-chamber fluidized-bed combustor (SFBC)

This study encompassed the characteristics and performance of co-firing rice husk, a by-product of rice-milling process, with coal in a short-combustion-chamber fluidized-bed combustor (SFBC). Bed phenomena investigated in a cold-flow model combustor showed that with the different mixes of materials, the anticipated offshoot of combustion, the minimum fluidizing velocity (U_mf) was 0.4-0.8 m/s. In concord with axial temperature profiles, axial gas concentration profiles implied that a recirculating ring was able to circumscribe CO within the short-main chamber. The formation, decomposition, and eventual maturity of NO_x characterized the NO_x evolution, inferred from concentration profiles. The impacts of fluidizing velocity and blending ratio on gas emissions and combustion efficiency (E_c) are described. The fluidizing velocity had consequential effect on gas emissions, except NO_x. Surprisingly, NO_x did not hinge much on increased N-content of the mixtures with coal. As expected, increased SO₂ was relevant to increased coal mass. Increased fluidizing velocity adversely affected E_c while increased coal fraction enhanced E_c, mostly >97%.     

Slugging characteristics of group d particles in fluidized beds

Pressure fluctuations within a bed of Type D particles (coarse sand, long grain rice, red spring wheat, 10 mm glass beads, pearl barley and whole peas) were measured at various bed heights. Analysis of the pressure fluctuation profiles leads to correlations that predict the minimum slugging velocity and slugging frequency, within experimental ranges. Results reveal that past correlations have little success in predicting the slugging characteristics for large particles. Particle shape plays an important role on the type of slug formed and therefore on the values of slugging frequency and minimum slugging velocity. Large particles with low sphericity formed wall slugs with high slugging frequencies and required the greatest amount of excess air to achieve slugging.     

Devolatilization of coals of northeastern India in inert atmosphere and in air under fluidized bed conditions

Devolatilization of five coals having volatile matter in the range of 31 to 41% was studied in argon and in air under fluidized bed conditions. The diameter of the coal particles varied between 4 and 9.5 mm. The variation of devolatilization time with particle diameter was expressed by the correlation, t_v = Ad_vⁿ. The superficial gas velocity was found to have a significant effect on the rate of devolatilization. The devolatilization rate increased with the increase in the oxygen concentration in the fluidizing gas. The correlations developed in this study fitted the mass versus time profiles of the coal particles satisfactorily. The same correlations were found to be appropriate for predicting devolatilization of a batch of coal particles. The correlations developed in the present study will be useful for the design of fluidized bed combustors.     

Particle segregation in a fluidized bed

Particle segregation data at low gas velocity, near the minimum fluidizing velocity, are presented for particles of different size (150 to 2800 μm) and density (720 to 2800 kg/m³) in a fluidized bed. Phase diagrams similar to those of liquid-solid systems are developed to present the results. The data were collected in a 100 mm diameter unit operating at pressures up to 660 kPa and a 114 mm diameter unit at atmospheric pressure. The particle segregation data were obtained to provide design and operating criteria for a two-stage. i.e., an agglomerating combustor/gasifier and a devolatilizer/desulfurizer, fluidized bed coal gasification process to produce low heating-value gas (890-1350 K-cal/m³) for power generation. The data indicate that the criteria for particle separation can be achieved if the operating gas velocity is close to the minimum fluidizing velocity of the dense particles, that is, agglomerated ash, and spent dolomite.     

CARBON COMBUSTION RATES AND TEMPERATURES IN SHALLOW FLUIDIZED BEDS

Abstract

The mechanism of combustion of carbon in shallow fluidized beds at temperatures 750-1000°C is studied by measuring burning rates and temperatures of spherical carbon particles ranging from 2 mm to 12 mm diameter directly in an experimental fluidized bed. Among variables investigated were inert particle size, superficial fluidizing velocity, temperature, the influence of neighbouring active particles and oxygen concentration in the fluidizing gas.

Under the experimental conditions explored, combustion was mainly kinetically controlled, so that with carbon particles larger than about 4 mm, burning rates are significantly higher than those predicted by combustion models which assume combustion to be controlled by the rate at which oxygen diffuses through a stagnant particulate phase surrounding the burning particle. The higher burning rate seems to arise because the greater mobility of particles in the bed causes the restriction to oxygen flow to the carbon surface offered by the particulate phase to be reduced and has important consequences for combustor design.

Measured carbon particle temperatures were influenced considerably by bed operating conditions ranging from 15 to 215°C higher than bed temperature.

Measured burning rates of carbon particles were found to be reduced significantly when other active particles were present in the bed. This sensitivity of burning rate to changes in active particle concentration in the bed was shown to be increasingly important once the concentration of carbon in the bed exceeded about 1%

Increasing the bed inert particle size, superficial fluidizing velocity, oxygen concentration in the fluidizing gas and bed temperature resulted in higher burning rates. The implication of these findings on combustor design are discussed.     

Favorable vibrated fluidization conditions for cohesive fine particles

The present study was performed to clarify the operational range for vibro-fluidization of fine cohesive particles (glass beads, d_p = 6 μm). Decreasing and increasing gas velocity methods were examined to clarify the favorable vibro-fluidization region. The upper limit of the gas velocity for intermittent channel breakage was higher in the case of the increasing gas velocity method than the decreasing gas velocity method. This was because the changes in the bed flow pattern from a favorable (intermittent channel breakage) to an unfavorable fluidization state (stable channels) were moderate in the case of the increasing gas velocity method. In the increasing gas velocity method, two kinds of cross-points were obtained from the relationship between the gas velocity and the bed pressure drop. At one of the gas velocities at these cross-points, the bed void fraction reached its maximum. In the present study, the above-mentioned gas velocity was defined as the upper limit of gas velocity for favorable vibro-fluidization, u_chu. A favorable vibro-fluidization region was determined by combining u_chu with u_chl, which is the lower limit of gas velocity for intermittent channel breakage obtained in a previous study. The value of u_chu was found to have a maximum corresponding to a certain vibration strength.