Fluidization Properties Research Articles

Solid fuels in chemical-looping combustion using a NiO-based oxygen carrier

The feasibility of using three different solid fuels in chemical-looping combustion (CLC) has been investigated using NiO as oxygen carrier. A laboratory fluidized-bed reactor system for solid fuel was used, simulating a chemical-looping combustion system by exposing the sample to alternating reducing and oxidizing conditions. In each reducing phase 0.2 g of fuel was added to the reactor containing 20 g oxygen carrier. The experiments were performed at 970 °C. Compared to previously published results with other oxygen carriers the reactivity of the used Ni-particles was considerably lower for the high-sulphur fuel and higher for the low-sulphur fuel. Much more unconverted CO was released and the fuel conversion was much slower for high-sulphur fuel such as petroleum coke, suggesting that the nickel-based oxygen carrier was deactivated by the presence of sulphur. The NiO particles also showed good reactivity with methane and a syngas mixture of 50% H 2 and 50% CO. For all experiments the oxygen carrier showed good fluidizing properties without any signs of agglomeration.

Use of Ores and Industrial Products As Oxygen Carriers in Chemical-Looping Combustion

A screening of Fe- and Mn-based ores and industrial products was made in order to identify suitable low-cost materials that could be used as oxygen carriers in chemical-looping combustion (CLC). A laboratory fluidized bed reactor system, simulating chemical-looping combustion by exposing the sample to alternating reducing and oxidizing conditions, was used. Fifteen grams of each material with a particle size of 125−180 μm was exposed to a flow of 450 mLn/min of either methane or syngas (50% CO, 50% H2) during reduction. During the oxidizing phase to a flow of 1000 mLn/min, 5% O2 in nitrogen was used. All materials had a high reactivity with syngas. Some materials such as the Mn-based Colormax and the Fe-based Glödskal had also a high reactivity with methane making them possible candidates for CLC with gaseous fuel. Some of the materials, especially the Mn-based ones, showed poor mechanical stability and poor fluidizing properties. Roughly half of the Fe-based materials, but only one of the Mn-based materials, had properties that could make them suitable as oxygen carriers in a CLC system for solid fuels.

Effects of Increase in Gas Volume on the Fluidization Properties in Fluidized Bed Reactors

There are many industrially useful reactions accompanied by the change of the gas volume due to the stoichiometry. Although it is important to consider the effects of the gas-volume changes for the accurate estimation of the reactor performance, the effects on the fluidization properties have not been experimentally studied. In this study, experimental simulation was carried out to study the effects of the increase in gas volume on the fluidization properties. The gas volume was increased by the evaporation of water impregnated in the pores of alumina particles at elevated temperature. The effects on the expansion of the emulsion phase, average bubble holdup, bubble size and pressure fluctuations were studied. The bubble growth, splitting and coalescence would become vigorous, whereas the emulsion phase expansion was not affected and the bubble size was slightly increased.

Direct Synthesis of a Fluidizable SAPO-34 Catalyst for a Fluidized Dimethyl Ether-to-Olefins Process

The dimethyl ether to olefins process is very important for converting coal to chemicals. A SAPO-34 molecular sieve with a particle size up to 50 μm has been synthesized with morpholine as the template. The cold model fluidization experiment indicates that the SAPO-34 catalyst with average size of 45–50 μm is of good fluidization properties. After high temperature steam treatment at 1073 K for 8 h, this large fluidizable catalyst showed good reactivity and selectivity for the dimethyl ether to olefins (DTO) reaction in a micro-reactor. X-ray diffraction characterization showed that the structure of the catalyst was maintained well. A fluidized reactor of 50 mm inner diameter was used to carry out the DTO reaction, which showed that the fluidizable SAPO-34 is a selective catalyst for light olefin production.

The Role of Fines in the Modification of the Fluidization and Dispersion Mechanism Within Dry Powder Inhaler Formulations

To investigate the role of in situ generated fine excipient particles on the fluidization and aerosolization properties of dry powder inhaler (DPI) formulations. Carrier based DPI formulations were prepared under low and high shear blending. Powder rheometery was utilized to measure bulk powder properties in a consolidated and aerated state. Powder fluidization and aerosolization characteristics were related to bulk powder properties using high speed imaging and inertial impaction measurements. High shear blending of formulations resulted in the in situ generation of excipient fines, which corresponded to an increase in aerosolization efficiency. The generation of fines were shown to increase the tensile strength and free volume of the carrier, which resulted in a characteristic change in the fluidization properties, as observed by high speed imaging. The increase in minimum fluidization velocity and aerodynamic drag forces required to aerate the powder may provide the source of energy for the increase in fine particle re-suspension. The in situ generation of excipient fines affect bulk powder properties of DPI formulations, which directly affects fluidization and aerosolization behaviour of DPI formulations. The study suggests an alternative mode of action by which fines increase DPI formulation performance.

The fluidization pattern of density-segregating binary mixtures

This paper analyses the process of fluidization of mixtures of two spherical solids differing only in density and addresses the relationship between bed suspension and component segregation. It is shown that relating the progress of segregation to the excess gas velocity u– u mf, a parameter that should represent the intensity of bubbling, gives way to serious errors, due to the fact that extending to binary beds the concept of minimum fluidization velocity leads to overlook some important aspects of their behaviour. A clearer description of the fluidization properties of density-segregating mixtures is that based on the definition of the ‘initial’ and ‘final fluidization velocity’ of the binary bed. Such an alternative approach provides an effective representation of the actual behaviour of these systems, as it correctly accounts for all the variables which affect their phenomenology of fluidization.

Simulation and experimental studies on fluidization properties in a pressurized jetting fluidized bed

The influence of pressure on the bubble size and average bed voidage has been investigated experimentally and computationally in a circular three-dimensional cold-flow model of pressurized jetting fluidized bed of 0.2 m i.d. and 0.6 m in height with a central jet and a conical distributor, which roughly stands for the ash-agglomerating fluidized bed coal gasifier. The pressurized average bed voidage and bubble size in the jetting fluidized bed were investigated by using electrical capacitance tomography (ECT) technique. The time-averaged cross-sectional solids concentration distribution in the fluidized bed was recorded. The influence of pressure on the size of bubble and the average bed voidage in a pressurized fluidized bed was studied. Both experimental and theoretical results clearly indicate that there is, at the lower pressure, a small initial increase in bubble size decided by voidage and then a decrease with a further increase in pressure, which proves the conclusion of Cai et.al. [P. Cai, M. Schiavetti, G. De Michele, G.C. Grazzini, M. Miccio, Quantitative estimation of bubble size in PFBC, Powder Technology 80 (1994) 99–109]. At higher pressure, bubbles become smaller and smaller because of splitting. The average bed voidage increases gradually with the pressure at the same gas velocity. However, there is a disagreement between the experimental results and simulation results in the average bed voidage at the higher gas velocity, especially at the higher pressure. It suggests that the increase in density of gas with pressure may result in the drag increase and the drag model needs to be improved and revised at higher pressure.

The fluidization process of binary mixtures of solids: Development of the approach based on the fluidization velocity interval

The main characteristic of the fluidization process of particulate mixtures is the interaction between bed suspension and segregation–remixing dynamics of their components. On addressing the fluidization properties of binary beds of solids differing either in particle density or diameter, the paper shows how interpretations based on definition of a minimum fluidization velocity of the mixture can lead to erroneous conclusions about the influence played by system variables on their behaviour. An alternative method of investigation is followed which takes into consideration the existence of a finite velocity interval, bounded by the “initial” and “final fluidization velocity” of the mixture, along which the whole process of fluidization has place. The results of a wide series of experiments demonstrate that this alternative approach allows recognizing the independent variables of binary fluidization; they also highlight the differences of behaviour between density- and size-segregating beds.

Reaction Kinetics of Freeze-Granulated NiO/MgAl2O4 Oxygen Carrier Particles for Chemical-Looping Combustion

The kinetics of reduction and oxidation of Ni based oxygen carrier particles with CH4 and O2 have been investigated. The kinetic parameters were obtained from reactivity data using a thermogravimetric analyzer (TGA), where the freeze-granulated particles were tested using different reactant gas concentrations, temperatures, and particles sizes. The particles showed high reactivity during both reduction and oxidation at temperatures above 900 °C. The shrinking-core model for spherical grain geometry of the reacting particle with chemical reaction control was used to determine the kinetic parameters during both the reduction and oxidation reactions. The reaction order found was 0.4 and 1 for CH4 and O2, respectively, while the activation energies found were 114 and 40 kJ/mol for reduction and oxidation reactions, respectively. The reactivity data and kinetic parameters were used to estimate the solid inventory needed in a chemical-looping combustion (CLC) system. The total solid inventory varies with the solid conversion at the inlet of the fuel and air reactor, and for the investigated particles, the minimum solid inventory was 22 kg/MWf. It was found that to operate fuel and air reactors of a CLC system at 950 and 1000 °C, respectively, using an NiO/MgAl2O4 oxygen carrier with a 50% active NiO content, the conversion variation between the two reactors should not exceed 0.18. The recirculation rate between the air and fuel reactors needed was 4.15 kg/(s MWf). The high reactivity of the NiO/MgAl2O4 both with methane and oxygen found in this work, together with the good fluidizing properties found in earlier studies, suggests that this is an excellent oxygen carrier for a CLC system.

Effects of magnetic field on fluidization properties of magnetic pearls

An experimental study of the influence of external magnetic field on the fluidization behavior of magnetic pearls was carried out. Magnetic pearls are a magnetic form of iron oxide that mainly consists of Fe 2O 3 which are recovered from a high-volume power plant fly ash from pulverized coal combustion. Due to its abundance, low price and particular physical and chemical properties, magnetic pearls can be used as a heavy medium for minerals or solid waste dry separation based on density difference. This paper introduces the properties of magnetic pearls and compares the performance of magnetic pearls fluidised bed operation with or without an external magnetic field. Experimental results show that an external magnetic field significantly improves the fluidization performance of magnetic pearls such as uniformity and stability.

Separation of fine binary mixtures under vibration in a gas-solid fluidized bed with dense medium

The coke-slag mixture discharged from water-air generators not only causes a serious environmental problem, but also is a potential resource. In order to obtain a high economic return, separation of fine binary mixtures in a vibrating fluidized bed with dense media was experimentally investigated. The effects of volume ratio (mono-component volume to dense medium volume or binary mixture volume to dense medium volume), the fluidization velocity and vibration parameters on separation efficiency are discussed. The experimental results show that vibration can prevent the coalescence and growth of bubbles and improve fluidization properties of particles and the stability of bed density. The major factors associated with separating efficiency are discussed. Based on the experiments, the ratio of the volume of particles to bed volume and the vibration conditions are also reported. It was found that the separation efficiency for a fine coke-slag mixture was greater than 70% and close to 80% under optimal conditions.

Self-diffusion of particles in gas-driven granular layers with periodic flow modulation

We study self-diffusion of particles in gas-driven granular layers by high-speed fluorescent video microscopy. We show that periodic flow modulation results in an enhancement of the particle's diffusion. The diffusion enhancement, which in turn is an indication of more efficient fluidization of the granular layer, is associated with the onset of disordered subharmonic patterns. Our measurements provide a sensitive characterization method of the fluidization properties of particulate-gas systems.

Quantitative measurement of segregation phenomena in a binary-mixture fluidized bed by neutron radiography

Owing to a wide variety in fluidization properties of particles, segregation phenomena are hardly avoided in industrial fluidized-bed facilities. For the stability and controllability in the operation of such fluidized-beds, understanding of the relevant mechanism of particulate segregation is indispensable. Many factors, such as the difference in particle size, density, shape, and also arrangement of distributors, can be pointed out as the dominant causes for the segregation. Thus, any marking of particles for quantitative measurement may severely influence the segregation characteristic to be measured. But neutron radiography can be used for the quantitative measurement without disturbing the segregation characteristic. For estimating the segregation phenomena, evaluations of the particle fraction and the void fraction are required. For this purpose, two types of radiographies, i.e., X-ray and neutron radiographies were used in this investigation. Experimental results demonstrated the high performance of this method in investigating of segregation phenomena.

Flow visualization of segregation process in a fluidized-bed by neutron radiography

Owing to a wide variety in fluidization properties of particles, segregation phenomena are hardly avoided in industrial fluidized-bed facilities. For the stability and controllability in operation of such fluidized-beds, understanding of the relevant mechanism of particulate segregation is indispensable. This paper describes an experimental observation of the particulate segregation in a binary mixture system visualized successfully by using a neutron radiography (NR). The NR gives real-time information of the segregation process, and this information will be very useful in understanding the segregation mechanism.

Multi-mode combustion facility for thermal treatment studies of wastes and biomass

This article describes newly built Multi-Mode Combustion Facility (MCF) used for investigating thermal destruction of industrial wastes and combustion of biomass. A flexible, refractory-lined combustion chamber consists of individual sections of various heights and diameter of 0.5 m. The MCF can be used either as a fluidized bed combustor (FBC) to study the combustion of solid residues or as a single-burner furnace (SBF) to study cofiring of biomass and natural gas. The facility is designed such that the outer wall temperature should not exceed 327 K with the use of water-cooling system and refractory materials. The inner temperature of each section is independent of the rest of the sections and controlled individually. This arrangement allows for the combustion process to be carried out in a multizone manner called low–high–low (LHL) temperature approach. The LHL approach means that the waste/biomass is initially fed into a low temperature zone (<1060 K) and then subjected to the high temperature treatment (∼1500 K) that is followed by another low temperature zone (<1160 K). The LHL setup allows for heavy metals encapsulation and immobilization within the fly ash particles. The facility has 25 openings for sampling of solids and gases at different stages of the combustion process, as well as in situ observation. Experiments reported in this article were performed in the bubbling FBC mode with the purpose of testing the leachability of heavy metals (Cd, Cr, and Pb) from fly ash generated during two different combustion approaches: (1) multi-zone LHL treatment, and (2) no-LHL. Baseline fluidization properties of different bed materials were tested. Axial profiles of temperature and gas concentration (CO2, NO, and NOx) were compared. The results show that the leachability of the heavy metals (Cd, Cr, and Pb) contained in the LHL-generated ash particles was negligible (0.14, 0.061, and 1.55 ppm, respectively), while the leachability data from the no-LHL technique were 30.7, 14.3, and 0.647 ppm, respectively. It was concluded that the MCF facility is easy to operate, flexible, and useful for studies of various waste-to-energy options. Our results also show an improvement in heavy metals leachability when using the LHL combustion technique.

Influence of pressure on fluidization properties

An experimental study of the influence of pressure on fluidization phenomena was carried out in a bubbling fluidized bed over the range of operating absolute pressures 101–2100 kPa with Geldart A and B bed materials in an apparatus 15 cm in diameter. Bed voidage data was gathered using electrical capacitance tomography (ECT) and the resulting time series were analysed to give average bed voidage, average absolute deviation, average cycle frequency and Kolmogorov entropy. Bed collapse tests were also performed to establish the influence of operating pressure on dense phase voidage. For the Geldart B powder U mf was found to decrease slightly with pressure whilst voidage at U mf was unaffected and no change in behavior from Geldart B to A was observed. Analysis of the ECT results for the Geldart B powder showed no influence of pressure on cycle frequency and average absolute deviation, but an increase in mean bed voidage with pressure. For the Geldart A powder, no influence of pressure on U mf, U mb and ε mf was observed, though ε mb was found to increase slightly with pressure. Analysis of the ECT results for the Geldart A powder revealed a decrease in average bed voidage, cycle frequency and average absolute deviation with pressure. The trend in Kolmogorov entropy within increasing pressure was not entirely revealed over the range of velocities used, but results suggest a decrease.

Analysis of cold flow fluidization test results for various biomass fuels

A systematic theoretical and experimental study was conducted to obtain hydrodynamic properties such as particle size diameter, bulk density, fluidizing velocity, etc. for locally available biomass residue fuels in Malaysia like rice husk, sawdust, peanut shell, coconut shell, palm fiber as well as coal and bottom ash. The tests were carried out in a cold flow fluidization bed chamber of internal diameter 60 mm with air as fluidizing medium. The height of the chamber could be raised up to 630 mm by five separate cylindrical rings. Bed-pressure drop was measured as a function of superficial air velocity over a range of bed heights for each individual type of particle. The data were used to determine minimum fluidization velocity, which could be used to compare with theoretical values. The particle size of biomass residue fuel was classified according to Gildart's distribution diagram. The results show that Gildart's particle size ( B) for sawdust, coal bottom ash, coconut shell have good fluidizing properties compared to rice husk, type (D) or palm fiber, type (A). The bulk density and voidage are found to be main factors contributing to fluidizing quality of the bed.

Experimental Analysis of the Fluidization Process of Binary Mixtures of Solids

The simultaneous treatment of dissimilar solids is encountered in a number of processes that exploit fluidization technology (granulation, combustion, pyrolysis, etc.), but a satisfactory description of multicomponent fluidization dynamics is far from being achieved even for the relatively simple case of binary fluidization.This paper discusses the fluidization properties of two-component beds of solids differing either in particle density or diameter. It is shown that although the minimum fluidization velocity of the mixture can be calculated by fully theoretical equations, it has very little meaning if referred to a two-component particle system. Experiments on both kinds of mixtures demonstrate that the binary fluidization process occurs within a characteristic velocity range whose boundaries coincide with the “initial” and the “final fluidization velocity” of the particle mixture. Substitution of the conventional concept of umf by these parameters allows us to recognize that the fluidization dynamics of any binary mixture is determined by the initial arrangement of the fixed bed, as well as by the system composition and size or density ratio of its components.

Anaerobic digestion of wine distillery wastewater in down-flow fluidized bed

In down-flow fluidization, particles with a specific density smaller than the liquid are fluidized downward by a concurrent flow of liquid. This paper describes the application of the down-flow (or inverse) fluidization technology for the anaerobic digestion of red wine distillery wastewater. The carrier employed was ground perlite, an expanded volcanic rock. Before starting-up the reactor, physical and fluidization properties of the carrier material were determined. 0.968 mm perlite particles were found to have a specific density of 280 kg m −3 and a minimum fluidization velocity of 2.3 m h −1. Once the down-flow anaerobic fluidized bed system reached the steady-state, organic load was increased stepwise by reducing HRT, from 3.3–1.3 days, while maintaining constant the feed TOC concentration. The system achieved 85% TOC removal, at an organic loading rate of 4.5 kg TOC m 3 d −1. It was found that the main advantages of this system are: low energy requirement, because of the low fluidization velocities required; there is no need of a settling device, because solids accumulate at the bottom of the reactor so they can be easily drawn out, and particles with high-biomass content, whose specific density have become larger than 1000 kg m −3 can be easily recovered.

Fluidity and Flow Properties of Fine Powders

ABSTRACT Fluidization and/or flow properties of many fine powders (d50 < 50 μm), including pharmaceutical powders, toners, powder paints, and ceramic powders, are of critical importance. Particles in this range behave as cohesive powder because of the relatively large inter-particle forces (electrostatic, van der Waals', and liquid bridge forces), compared to the hydrodynamic force exerted on the particles by the fluid flowing around the particles. Flow additives, mechanical agitation, and other forces such as acoustic and electromagnetic, are often applied for good fluidization and uniform dilute phase flow. In this paper, we present a brief discussion and experimental data on fluidization properties, fluidity, and flow behavior of several fine powders as functions of particle size distribution, relative humidity, relative concentration of flow additives, and the frequency and amplitude of mechanical agitation. Electrostatic charging, dependent upon the chemical composition and electrical conductivity of the particles, and its influence upon the flow properties are also presented.