Elutriation Of Fines Research Articles

Investigation into the elutriation of fines from binary mixtures via CFD simulation with a multi-scale drag model

In the coal-direct chemical looping combustion process, the separation of char and ash from oxygen carriers is essential for reducing the requirement of fresh oxygen carriers and avoiding the char into the air reactor. In this work, a three-dimensional simulation is conducted to investigate the solid-solid separation process in a fluidized bed reactor by means of the multi-fluid model, where the multi-scale drag model is implemented to describe the bubble effect in the bed. The elutriation process of fine particles from mixture is evaluated. The model prediction is compared with the experimental results. The results demonstrate that the dependence of the elutriation rate on fine particle diameter is not monotonous owing to the formation of agglomeration. Meanwhile, the increase of operating temperature promotes the non-uniform degree of interphase drag force and the elutriation rate of fine particles is reduced.

Effects of various parameters on the attrition of bed material in a recirculating fluidized bed with a draft tube

We performed an experimental study to investigate the effects of various parameters on the attrition of bed material and its size distribution with increasing operation time in a recirculating fluidized bed (RCFB). The studied parameters included superficial velocity of fluidizing air, bed inventory, and spacing between the jet top and draft tube bottom (spacer height). The bed material was prepared from Indian Standard (IS) Grade I sand from sieves with a size range of 2.20–1.00mm. Experiments were performed at ambient conditions, with the superficial air velocity ranging from 7.13–9.16m/s, a bed inventory of 7–10kg, spacing of 0.085 and 0.045m between the jet top and draft tube bottom, and an operating time of 40h. We investigated the influence of these parameters in terms of changes in the size distribution of particles, changes in the %-weight of particles of different size ranges, generation of particles with smaller diameters, the decrease of the downcomer bed height, variations in the coefficient of uniformity and coefficient of curvature, and material loss from entrainment of fines with increasing operation time. The mode of attrition was abrasion in all experiments. We found that with increasing operation time and other parameters (bed inventory, superficial air velocity, and spacer height) attrition of the bed material also increased. Generation and elutriation of fines were more pronounced at higher superficial air velocity, bed inventory, and spacer height.

Elutriation of fines from binary particle mixtures in bubbling fluidized bed cold model

The elutriation of fine particles from a binary mixture of particles with different densities and diameters has been investigated in bubbling fluidized bed (BFB). A series of experiments were conducted in a 10cm diameter, 170cm tall cylindrical bubbling fluidized bed under various operating conditions. Bed materials with different particle sizes, ranging from 93μm to 1000μm powder, and particle densities ranging from 7.9 to 2.45g/cm3 were used in these experiments. Various combinations of these solids were mixed and fluidized at several superficial gas velocities. Solid elutriation was measured by collecting the carryover solids exiting the column with a filter. Experimental data on the effects of particle density, particle size, and gas velocity on the elutriation of particles from bubbling fluidized bed of binary mixture are examined. Influence of weight fraction of fines in the binary mixture on coarse particles was also investigated and discussed. The results indicated that the elutriation rate constant increases with increasing superficial gas velocities and weight fraction of fines in the bed. A generalized correlation for the elutriation rate constant is proposed using the ratio of Ug/Ut, weight fraction of fines, and fines to coarse particle density ratio.

Investigation of a calcium manganite as oxygen carrier during 99 h of operation of chemical-looping combustion in a 10 kWth reactor unit

Chemical-looping with oxygen uncoupling is a technology for combustion with inherent carbon dioxide separation. A solid oxygen carrier circulates between the fuel reactor, where it provides oxygen for fuel oxidation, and the air reactor, where it is reoxidized. In this study a 10kWth pilot reactor was used to examine a calcium manganite based oxygen carrier in continuous operation with natural gas as fuel during 99h. The composition of the oxygen carrier can be described by the formula CaMn0.775Ti0.125Mg0.1O3-δ. The main part of the material forms a perovskite crystal structure which has oxygen releasing properties. The fuel conversion was generally above 95% and full conversion was reached for certain operating conditions. The elutriation of fines, defined as particles smaller than 45μm, decreased over time to eventually be below detection limit. That suggested a loss of fines of less than 0.011wt%/h, indicating a lifetime of over 9000h. A high fuel conversion with no thermodynamic limitation, good mechanical strength, low cost and very low toxicity shows that this calcium manganite material qualifies as a very promising oxygen carrier.

Experimental and predicted approaches for biomass gasification with enriched air-steam in a fluidised bed.

Thermo-chemical gasification of sawdust refuse-derived fuel was performed on a bench-scale fluidised bed gasifier with enriched air and steam as fluidising and oxidising agents. Dolomite as a natural mineral catalyst was used as bed material to reform tars and hydrocarbons. A series of experiments were carried out under typical operating conditions for gasification, as reported in the article. A modified equilibrium model, based on equilibrium constants, was developed to predict the gasification process. The sensitivity analysis of operating parameters, such as the fluidisation velocity, oxygen percentage of the enriched air and steam to biomass ratios on the produced gas composition, lower heating value, carbon conversion and cold gas efficiency was investigated. The results showed that the predicted syngas composition was in better agreement with the experimental data compared with the original equilibrium model. The higher fluidisation velocity enhanced gas-solid mixing, heat and mass transfers, and carbon fines elutriation, simultaneously. With the increase of oxygen percentage from 21% to 45%, the lower heating value of syngas increased from 5.52 MJ m(-3) to 7.75 MJ m(-3) and cold gas efficiency from 49.09% to 61.39%. The introduction of steam improved gas quality, but a higher steam to biomass ratio could decrease carbon conversion and gasification efficiency owing to a low steam temperature. The optimal value of steam to biomass ratio in this work was 1.0.

Combined oxides of iron, manganese and silica as oxygen carriers for chemical-looping combustion

Spray-dried particles with the chemical compositions of Fe0.66Mn1.33SiO3 and FeMnSiO3 have been examined as oxygen carrier materials for chemical-looping combustion. The experiments were carried out in a fluidized-bed reactor system designed for a thermal power of 300W. Both materials were able to release gas phase oxygen in inert atmosphere at temperatures between 800 and 950°C, and with approximately equal oxygen concentrations. Fe0.66Mn1.33SiO3 provided higher conversion of natural gas as compared to FeMnSiO3 and the fuel conversion increased with temperature for both materials. During natural gas operation with Fe0.66Mn1.33SiO3 the conversion reached 100% at around 950°C with a fuel reactor inventory of 235kg/MW. The fuel conversion improved when the solid inventory was increased; this improvement could especially be observed for FeMnSiO3 as the fuel conversion was lower for this material. Fe0.66Mn1.33SiO3 also provided higher fuel conversion than FeMnSiO3 when syngas was used as fuel. The fuel conversion increased with temperature for both materials and full conversion was reached above 800°C with a fuel reactor inventory of 225kg/MW for Fe0.66Mn1.33SiO3, while FeMnSiO3 was incapable of providing full conversion. A rather large elutriation of fines and a significant change in particle size distribution could be observed during operation for both materials.

Fluidized bed calcium looping cycles for CO2 capture under oxy-firing calcination conditions: Part 1. Assessment of six limestones

Six limestones were tested for CO2 capture during calcium looping cycles in a lab-scale fluidized bed apparatus. Batch tests were carried out under alternating calcination–carbonation conditions representative of a process with calcination in an oxy-firing environment (T=940°C, 70% CO2). The effect of the presence of SO2 was also studied at two concentration levels: 1500ppm SO2 during both carbonation and calcination, simulating CO2 capture from uncontrolled flue gas and regeneration in an oxy-fired calciner burning high-sulfur coal; 75ppm SO2 during carbonation and 750ppm SO2 during calcination, simulating CO2 capture from already desulfurized flue gas and regeneration in an oxy-fired calciner burning medium-sulfur coal. Limestone attrition processes were characterized during the tests by measuring the changes of the sorbent particle size distribution and the fines elutriation rate along conversion over repeated cycles.Results showed that for all the limestones the CO2 capture capacity decreased with the number of cycles reaching a very low asymptotic value. The combination of high bed temperature and high CO2 concentration during the calcination stage significantly enhanced particle sintering. Moderate attrition rates were experienced by the sorbent particles mostly during the first cycle. In the subsequent cycles the attrition rate progressively declined, due to the concurrent chemical–thermal treatment making the sorbent surface increasingly hard. The presence of a high SO2 concentration significantly depressed the sorbent CO2 capture capacity, because of the buildup of a compact CaSO4 layer on the particle surface. Under a lower SO2 concentration level, the effect on CO2 capture capacity was less important. In both conditions, limestone attrition was only influenced to a limited extent by SO2. The slight variations of the sorbent particle size distribution indicated that very low particle fragmentation occurred over the repeated cycles.

Attrition of lignite char during fluidized bed gasification

Fragmentation and attrition phenomena occurring to lignite char particles during fluidized bed gasification by CO2 have been studied in a lab-scale apparatus. The influence of bed temperature, fluidizing velocity and inlet CO2 concentration on attrition and carbon conversion was investigated. Secondary fragmentation and attrition by abrasion of char particles during gasification were significant, suggesting a gasification-assisted attrition enhancement effect. This mechanism, associated to the low reactivity of the generated fines, made the loss of carbon by fines elutriation during char gasification more significant than that typically found under combustion conditions. Up to 10% of the carbon initially fed was lost in the elutriated fines. The carbon loss by elutriation increased with the fluidization velocity, and when the bed temperature and CO2 concentration decreased. The effect of these last two variables was connected to their influence on the char gasification rate.

Effect of fines hold-up in the freeboard on elutriation from a fluidized bed

Elutriation of fines within a bubbling fluidized bed of a two-particle system was investigated. The coarse particles were glass beads with a mean size of 545 μm. The fine particles with mean sizes of 81, 97, 115, 137 and 163 μm were used, respectively. Fines elutriated from the fluidized-bed column were weighted continually by an electronic balance (OHAUS TP4KD). Weight of fines hold-up in the freeboard was measured by the method of the pressure drop. The first-order equation was used to determine the elutriation rate of fines. Weight of fines hold-up in the freeboard was removed from the bed weight. The results show that the elutriation rate constant increases with superficial gas velocity. Also it was found that the elutriation rate constant decreases with increasing fine particle size and the freeboard height. In this study, the elutriation rate constant is lower than that from the first-order equation which does not consider fines hold-up in the freeboard indicating that the previous researchers overestimate the elutriation rate constant. Moreover, a generalized correlation was proposed correlating with superficial gas velocity, fine particle size and the freeboard height.

Depositional processes in a kimberlite crater: the Upper Cretaceous Orapa South Pipe (Botswana)

AbstractThe Orapa A/K1 Diamond Mine, Botswana, exposes the crater facies of a bilobate kimberlite pipe of Upper Cretaceous age. The South Crater consists of layered volcaniclastic deposits which unconformably cross‐cut massive volcaniclastic kimberlite of diatreme facies in the North Pipe. Based on the depositional structure, grain‐size, sorting and composition of kimberlite in the South Crater, six units are distinguished in the ∼70 m thick stratiform crater‐fill sequence and talus slope deposits close to the crater wall, which represents a multistage infill of the volcanic crater. Monolithic basalt breccias (Unit 1) near the base of the crater‐fill are interpreted as rock‐fall avalanche deposits, generated by the sector collapse of the crater walls. These deposits are overlain by a basal imbricated lithic breccia and upper massive sub‐unit (Unit 2), interpreted as the deposits of a pyroclastic flow that entered the South Crater from another source. Vertical degassing structures within the massive sub‐unit show evidence for elutriation of fines and probably were formed after emplacement by fluidization due to air entrainment. Units 3 and 5 are thinly stratified deposits, characterized by diffuse bedding, reverse and normal grading, coarse lenticular beds, mudstone beds, small‐scale scour channels and load casts. These units are attributed to rapidly emplaced sheet floods on the crater floor. Units 3 and 5 are directly overlain by poorly sorted volcaniclastic kimberlite (Units 4 and 6) rich in basalt boulders, attributed to debris flows formed by the collapse of crater walls. Unit 7 comprises medium sandstones to cobble conglomerates representing talus fans, which were active throughout the deposition of Units 1 to 6. The study demonstrates that much of the material infilling the South Crater is derived externally after eruption, including primary pyroclastic flow deposits probably from another kimberlite pipe. These findings have important implications for predicting diamond grade. Results may also aid the interpretation of crater sequences of ultra‐basic, basaltic and intermediate volcanoes, together with the deposits of topographic basins in sub‐aerial settings.

Mass flow sedimentology within the HYC Zn?Pb?Ag deposit, Northern Territory, Australia: evidence for syn-sedimentary ore genesis

Laterally continuous mass-flow deposits are an important feature of the HYC stratiform sediment-hosted Zn–Pb–Ag deposit, which reveal more about the HYC mineralising system than has been previously recognised. Mass flow deposits are interbedded with sheet-like mineralised lenses in a carbonaceous dolomitic siltstone host rock. Sedimentological processes of mass-flow deposit emplacement are proposed that constrain stratiform mineralisation to the top metre of the sediment pile, based on mass-flow geometry and detailed clast petrology. Four distinct sedimentary facies are identified within the mass-flow units: framework-supported polymictic boulder breccia; matrix-supported pebble breccia; and gravel-rich and sand-rich graded turbidite beds. The boulder breccias are weakly reverse graded and show rapid lateral transition into the other facies, all of which are distal manifestations of the same sedimentary events. The flow geometry and relationships between these facies are interpreted to reflect mass-flow initiation as clast-rich debris flows, with transformation via the elutriation of fines into a subsequent turbulent flow from which the turbidite and matrix-supported breccia facies were deposited. All the mass-flow facies contain clasts of the common and minor components of the in-situ laminated base-metal mineralised siltstone. Texturally these are identical to their in-situ counterparts, and are clearly distinct from other sulphidic clasts that are of unequivocal replacement origin. In the boulder breccias, intraclasts may be the dominant clast type and the matrix may contain abundant fine-grained sphalerite and pyrite. Dark coloured sphaleritic and pyritic breccia matrices are distinct from pale carbonate-siliclastic matrices, are associated with high abundance of sulphidic clasts, and systematically occupy the lower part of breccia units. Consequently, clasts that resemble in-situ ore facies are confirmed as genuine intraclasts that were incorporated into erosive mass flows prior to complete consolidation. Disaggregation and assimilation of sulphidic sediment in the flow contributed to the sulphide component of the dark breccia matrices. The presence of laminated sulphidic intraclasts in the mass-flow facies constrains mineralisation at HYC to the uppermost part of the seafloor sediment pile, where this material was susceptible to erosion by incoming clast-rich mass flows.

Fluidized bed combustion of tyre derived fuel

Mechanistic aspects of the fluidized bed combustion of tyre derived fuel (TDF) have been analyzed both experimentally and theoretically. The time-resolved rates of carbon fines elutriation during the fluidized bed combustion of TDF have been measured during batch experiments in a bench scale reactor at 850 °C, under different operating conditions. Experimental results indicated that both gas superficial velocity and oxygen partial pressure exert influence upon the overall fixed carbon combustion efficiency. The efficiency increases slightly with the oxygen concentration and significantly if the gas superficial velocity decreases. Experimental data are further analyzed in the framework of a fluidized bed combustor model especially suited for high-volatile solid fuels feedings. The model takes into account phenomena that assume particular importance with this kind of fuels, namely fuel particle fragmentation in the bed and combustibles segregation and postcombustion above the bed. Experimental and model results indicate that the efficiency of the fluidized bed combustion of TDF is controlled by the competition between combustion and entrainment of char fines and volatile matter released in the early stage of fuel conversion.

Tephra fallout in the eruption of Soufrière Hills Volcano, Montserrat

Abstract Four mechanisms caused tephra fallout at Soufrière Hills Volcano, Montserrat, during the 1995-1999 period: explosive activity (mainly of Vulcanian type), dome collapses, ash-venting and phreatic explosions. The first two mechanisms contributed most of the tephra-fallout deposits (minimum total dense-rock equivalent volume of 23 x 10 6 m 3 ), which vary from massive to layered and represent the amalgamation of the deposits from a large numbers of events. The volume of co-pyroclastic-flow fallout tephra is in the range 4-16° of the associated pyroclastic flow deposits. Dome-collapse fallout tephra is characterized by ash particles generated by fragmentation in the pyroclastic flows and by elutriation of fines. Vulcanian fallout tephra is coarser grained, as it is formed by magma fragmentation in the conduit and by elutriation from the fountain-collapse flows and initial surges. Vulcanian fallout tephra is typically polymodal, whereas dome-collapse fallout tephra is predominantly unimodal. Polymodality is attributed to: overlapping of fallout tephra of different types, premature fallout of fine particles, multiple tephra-fallout sources, and differences in density and grain-size distribution of different components. During both dome collapses and explosions, ash fell as aggregates of various sizes and types. Accretionary lapilli grain size is independent of their diameter and is characterized by multiple subpopulations with a main mode at 5ø. Satellite data indicate that very fine ash can stay in a volcanic cloud for several hours and show that exponential thinning rates observed in proximal areas cannot apply in distal areas.

A Study in the Swirling Fluidizing Pattern.

An innovative swirling fluidizing pattern generated by a multi-horizontal nozzle distributor was developed to minimize the axial momentum of fluidizing air and to provide momentum in both the radial and tangential directions. Experiments in pressure fluctuations and fines elutriation in the fluidized-bed system were conducted in 0.29-m I.D. and 0.19-m I.D. fluidized-bed cold models respectively to investigate the influence of the fluidizing pattern. The performances of the “swirling fluidizing pattern” and the conventional “axial fluidizing pattern” caused by a perforated plate were compared. In pressure fluctuation experiments, the pressure signals within the bed were converted into the frequency spectra using a Fast Fourier Transform (FFT) to proceed the spectral analysis. Effects of superficial air velocity, static bed height and distributor open-area ratio were taken into account. At a low nozzle air-discharge velocity, a stable concentration of fines within the bed would be attained. The first-order elutriation rate equation cannot describe the elutriation behavior of fines under a swirling fluidizing pattern, except at an extremely high air-discharge velocity. The swirling fluidizing pattern proved to produce a remarkable improvement in both the fluidization quality and elutriation reduction.

Comparison of the pyrolysis and gasification of biomass: effect of reacting gas atmosphere and pressure on Eucalyptus wood

The present study was undertaken to clarify the purpose of using reactive gases during the thermochemical processing of biomass. Conversions observed during pyrolysis (He) have been compared directly with gasification in H 2, CO 2 and steam-helium mixtures. Experiments have been carried out at pressures between 1 and 20 bar, in a fixed-bed ‘hot-rod’ reactor, operating at a relatively low heating rate (10°C s −1). Conversions in helium were between 85 and 88%, leaving little scope for improvement by the use of reactive gases. Up to 95% has been achieved elsewhere by heating at faster rates. As the use of steam or hydrogen also tends to enhance heat transfer to particles, the use of reactive gases may still be justified when particles are heated rapidly to lower reactor temperatures, and/or when residence times at peak temperature are very short. In pilot or industrial scale fluidised-bed operation, loss of fuel through elutriation of fines is usually suppressed by a combination of increased fuel particle size and the use of reactive gases. The present study shows that comparable or higher conversions can be achieved by operating above 800°C in an inert gas, provided elutriation is suppressed by design options other than increasing particle size, which tends to produce unreactive chars. The use of reactive gases such as steam would appear to partially compensate for possibly non-optimal choice of fluidising parameters.

Elutriation of fines from gas fluidized beds: mechanisms of elutriation and effect of freeboard geometry

Experimental work on the entrainment of Geldart group A and C powders was conducted in an 8.2 cm diameter column, fluidized batchwise with air at superficial velocities between 0.1 and 0.8 m/s. The effect of the freeboard geometry was also studied. The entrainment rate at low air velocities is strongly reduced by interparticle adhesion forces. For larger group A particles, gravitational forces dominate. At higher air velocities the hydrodynamic forces overrule adhesion effects. Experimental results are used (i) to define the critical particle size at which interparticle adhesion forces become significant, (ii) to check the applicability of established correlations for the elutriation rate constant K i ∗ in estimating the entrainment rates, (iii) to develop a mdel that relates entrainment to a monolayer of particles carried in the roof and/ or wake zone of erupting bubbles and (iv) to predict the effect of the freeboard diameter on the amount of powder carried over from the fluidized bed.

MODELING FINES ELUTRIATION FROM A SPOUTED BED OF PULP FIBERS

A model was developed to describe the elutriation of fines from a spouted bed of pulp fibers, which is a new method to separate fines from fibers. During spouting the pulp fibers are aggregated in fiber floes, which we model as coarse porous spheres. In general, we can distinguish three liquid regions inside and around a coarse particle in a shear flow: bulk liquid, liquid passing through the sphere, and liquid circulating inside it. A system of three ordinary differential equations describing the balance of fines in these regions was solved numerically. A source term was included to account for the release of fines during the dynamic equilibrium between formation and break-up of fiber floes. Comparison between theory and experiment showed that a simplified two-region model, which contains two adjustable parameters, is adequate to describe the elutriation data.

The fate of fixed carbon during the fluidized-bed combustion of a coal and two waste-derived fuels

The fate of fixed carbon during fluidized-bed combustion of a bituminous coal and two alternative fuels. a refuse-derived fuel (RDF) and a tyre-derived fuel (TDF), was investigated. A simple model was developed based on the assumption that fixed carbon present in the bed could be lumped into a coarse particles phase and a fine carbon phase. The model is based on a network of paths representing the generation of coarse and fine char particles from the parent fuel by primary fragmentation, the fine particle production by comminution of coarse char, the combustion of the coarse and of the fine char particles, as well as the elutriation of fines. Results of computations of carbon conversion were in good agreement with those measured in batchwise experiments with each of the three fuels. Conversion of the coal takes place mainly via fuel devolatilization to coarse char which further reacts to gaseous products. Conversion of TDF occurs via the generation, upon devolatilization, of amultitude of fines which eventually undergo combustion and elutriation. The phenomenology associated with RDF fluidized-bed combustion is intermediate between those of coal and TDF. The resulting framework for the evaluation of fixed carbon balance helps in identifying key factors in the conversion of such widely different fuels.

Modeling of sulphur retention in atmospheric fluidized bed combustors. Sensitivity analysis and simulation

AbstractFor the design, simulation and optimization of sulphur retention in atmospheric fluidized bed coal combustors, a mathematical model is needed that would be able to predict the behaviour of the combustor in a wide range of operating conditions. In this work, a sensitivity analysis of the sulphur retention predictions of the different hypotheses, equations and parameters, which define the different submodels and phenomena occurring in the combustor, has been carried out. It has been found that the hypotheses related to the gas flow, devolatilization type and sulphur distribution in the pyrolysis products imply an important division among models. The greatest effect on sulphur retention predictions is exercised by the parameters defining the fines elutriation and sorbent sulphation capacity. However, those corresponding to the bed hydrodynamics (minimum fluidization velocity and bed expansion) do not have a significant effect on the sulphur retention predictions.The sulphur retentions obtained in the combustion of high sulphur lignites with eight different limestones were used for model validation. A good fit of the experimental sulphur retentions was found, without using any adjustable parameter. Finally, a simulation of the process was made. The great effect of the bed height, air velocity and the particle size distribution of the limestone must be pointed out, as well as the effect of its reactivity through the maximum conversion attainable by each particle size.

Decoupling of pyroclastic currents: hazards assessments

Gravitational settling of fragments, gas elutriation and boundary layer processes cause concentration values to change within different parts of sediment gravity flows during movement. These processes result in flow transformations, density stratification and decoupling of single currents into separate sediment gravity flows. Density can change by an increase in solids concentration in basal zones of currents by downward fall of particles, or by incorporation of particles from the substrate, thereby leaving their upper parts more dilute. In some flows, upward-streaming gases and elutriation of fines cause dilute turbulent currents to develop at the top of a dense current. Increasing concentration dampens turbulent action, transforming the turbulent to the nonturbulent state, commonly at the current's base. Conversely, dilute parts of currents become or remain turbulent. Currents consisting of turbulent and nonturbulent parts can decouple by shearing past one another because of differing flow properties, with each part taking separate paths. The manner by which a pyroclastic flow moves affects its predicted runout distance, its distribution as continuous sheets or in isolated basins, and whether it can cross barriers such as high mountain ranges or water bodies. During the eruption of Unzen volcano on June 3, 1991, decoupling of a pyroclastic surge from the top of a pyroclastic flow caused the deaths of Harry Glicken, Maurice Krafft, Katia Krafft and 40 Japanese citizens. Mapping volcanic hazard zones requires that consideration be given to decoupling processes to help prevent similar tragedies in the future.