Single-particle Studies Research Articles

Char-nitrogen oxidation under fluidised bed combustion conditions: single particle studies

Oxidation of bituminous coal char-nitrogen to NO, N 2O, and N 2 has been studied by single particle modelling. The single particle model forms an essential part of the CFB NO x /N 2O emission tendency predictor under development. In a CFB a significant part of nitrogen oxide emissions originate from oxidation of char-nitrogen, and the char inventory in the reactor affects indirectly the fate of nitrogen components by numerous heterogeneous or heterogeneously catalysed reactions. In an earlier work a parametric study with the single particle char oxidation model was carried out, and showed a need for improvement of the model for more accurate prediction of nitrogen oxides in a CFB. In this work a re-estimation of the kinetic rate constants in the model has been carried out based on available experimental data under a wide condition range (750–950 °C, 0–21% O 2, 0–2% CO, 0–200 ppm NO, 0–100 ppm N 2O) and including measured data on CO/CO 2 ratio close to the particle surface. A new reaction has also been added in the mechanism: the char catalysed N 2O reduction by CO, which may lead to enhanced N 2 formation at FBC conditions. Further, an energy balance has been incorporated, and the description of external mass transfer has been improved in order to better account for its different rates in different parts in a CFB reactor. The new set of rate constants differs mostly from the original constants by a higher rate constant values for NO reduction by CO catalysed by char, and for N 2O reduction on the char. The temperature dependence for the ratio of the rate constants k 2/ k 1 describing the split of oxidised surface intermediate (–CNO) to N 2O and NO, respectively, is opposite. The new model and the original model lead to different predictions for nitrogen oxides. The conversion factors of char-N to NO and N 2O are lower than predicted by the original model. The predictions with the new model seem to be in better agreement with available experimental quantitative data. The new model gives also better fit for the burning times and the temperature effect on char reactivity. The most severe limitations in the model are assumed to be the omission of the HCN formation in the mechanism and the boundary layer reactions. The latter will lead to the fact that the model experiences more oxidising conditions than if the film layer reactions were included.

The effects of organic species on the hygroscopic behaviors of inorganic aerosols.

Water-soluble organic compounds have recently received much attention because of their ability to absorb water and alter the hygroscopic properties of inorganic aerosols. The effects of glycerol, succinic acid, malonic acid, citric acid, and glutaric acid on the water cycles (water activities during particle evaporation and growth), crystallization relative humidities (CRH), and deliquescence relative humidities (DRH) of sodium chloride (NaCl) and ammonium sulfate (AS) were studied using an electrodynamic balance (EDB). The growth factors of these inorganic and organic mixtures were lower than those of the pure inorganic species. The presence of all these organics in the mixed particle reduce the water absorption of NaCl but enhance that of AS relative to that of the pure inorganic salts. Glycerol and succinic acid did not affect the deliquescence properties of NaCl and AS, although succinic acid increased the CRH of NaCl and AS. Malonic acid and citric acid, behaving as nondeliquescent species in single particle studies, caused NaCl and AS particles to absorb a significant amount of water before deliquescence. Glutaric acid caused NaCl and AS to deliquesce gradually, spanning a wide range of relative humidity. The ZSR model was found to be useful in predicting the water activity of the mixtures and the growth ratios. However, the detailed crystallization and deliquescence behaviors of the organic/inorganic mixtures cannot be easily predicted from the hygroscopic properties of the individual components.

Laser diffractometer for single-particlescattering measurements

A laser diffractometer (polar nephelometer)constructed for single-particle studies is described. It takesadvantage of an array of optical fibres and a scanning disc toallow the use of a single photomultiplier tube for thedetection of scattered light. A background subtraction schememakes the instrument especially valuable in cases where background scattering exceeds the scattering from the observedparticle, for example for living cells in aqueous media. Thesystem can be easily integrated with particle trappingequipment such as laser tweezers or an electrodynamic balance.

Single-Molecule and Single-Nanoparticle SERS: Examining the Roles of Surface Active Sites and Chemical Enhancement

Recent research in several groups has identified a new class of metal colloidal nanoparticles that is able to enhance the efficiencies of surface-enhanced Raman scattering (SERS) by as much as 1014−1015 fold. This enormous enhancement allows single-molecule detection and spectroscopy at room temperature. Previous single-molecule and single-particle studies have yielded important insights into the mechanism of electromagnetic field enhancement, but little is known about the contributions of surface active sites and chemical enhancement. Here we report a direct examination of chemical enhancement by using an integrated flow injection and ultrasensitive optical imaging/spectroscopy system. A key feature is that colloidal silver nanoparticles are immobilized on a glass surface inside a microflow device and that single-particle SERS signals are observed in real time while the immobilized particles are treated by chemical reagents in the flow cell. In situ surface plasmon scattering studies of spatially isolated particles indicate that their electromagnetic properties do not change after chemical treatment. Thus, the observed SERS spectral changes should primarily come from chemical enhancement at surface active sites. Our experimental data reveal that three halide ions (Cl-, Br-, and I-) have a substantial activating effect, whereas other ions such as citrate, sulfate, and fluoride have little or no effect on single-particle SERS. A “quenching” effect is observed for thiosulfate ions, which completely destroys the SERS activity. The chemical enhancement factors are estimated to be about 102−103 for rhodamine 6G molecules adsorbed on single Ag nanoparticles

Labeling phosphorylated LHCII with microspheres for tracking studies and force measurements

We report a method to selectively label phosphorylated, membrane proteins with microscopic particles. This technology is particularly useful in single particle studies. In such studies, the particles may serve to visualize protein diffusion and/or as ‘handles’ to study the force of interaction between the labeled protein and the membrane matrix. In the latter kind of experiments, forces can be applied and measured by calibrated optical tweezers. Optical tweezers were used in this work to test the strength of the particle labeling. Labeling a single protein with a particle produces a long-lived, distinct tag and is particularly useful for proteins in photosynthetic membranes, which contain endogenous fluorophores that would render single fluorescent proteins difficult to detect.

Single-particle study of protein assembly.

A study of protein assembly in solution with single-particle imaging and reconstruction techniques using cryoelectron microscopy is reported. The human glutamine synthetase enzyme, important in brain metabolism, and previously assumed to be assembled into a homogeneous quaternary structure, is found to be heterogeneous, with three oligomeric states that co-exist at room temperature. This result corrects an old structural and kinetic model determined by ensemble averaging techniques that assumed a homogeneous system. Unexpectedly fast protein dissociation kinetics results from a stabilized transition state.

Near-infrared Raman spectroscopy of single particles

Raman spectroscopy using non-invasive near-infrared (NIR) laser light has become a powerful tool for the microscopic analysis of organic and biological materials. A Raman tweezers microscope (RTM) was developed by combining NIR Raman spectroscopy with the laser trapping technique, which enables us to expand the scope of single particle studies. Recent results obtained using the RTM for single droplets and polymer spheres in a micrometer range are reported.

A numerical study of two coaxial axi-symmetric particles undergoing steady equal rotation in the slip regime

The problem of two axi-symmetric particles (separated by a certain distance) that rotate about their common axis of symmetry in an infinite viscous fluid with slip boundary conditions at their surfaces have been studied numerically. Aerosol particles are usually nonspherical with the exception of liquid droplets in certain cases, and the shape of particles has a significant impact on frictional drag (for particle translation) and torque (for particle rotation), and, hence, on Brownian motion, and the deposition, sampling and coagulation of particles. The effects of the rotation of particles prior to their collision and coagulation have usually been ignored in favor of simpler calculations. The study of two-particle systems should give more information about the interaction between particles that cannot be understood from the study of single particles alone. In this work, the Laplace equation (resulting from the steady Stokes equation) with slip boundary conditions is converted into a Fredholm integral equation of the second kind via the use of Green's function. The integral equations are then solved by the singularity subtraction method. The local stresses are calculated at each nodal point and the torques are then calculated from the summation of the local stresses. Explicit numerical results for the local stresses and torques are reported for three systems of two axi-symmetric particles, i.e. sphere-sphere, sphere-spheroid, and spheroid-spheroid. While the formulation of the problem is quite general, the results reported here have been limited to calculations for systems in which both particles have identical angular velocities. The numerical method is, however, valid for arbitrary axi-symmetric particles and its modification to systems containing other shaped particles or differing angular velocities is straightforward. Numerical results of the torques for each system studied show in every case that the presence of slip results in a reduction in the torques. As a consequence, the impact of the slip on the torques and local stresses is substantial and cannot be ignored. The distance between the centers of the particles (the separation distance) also plays an important role in determining the values of the torques and local stresses. In the systems that have been studied here, as the particles get further apart, the torques on both particles increase.

A Kinetic Model of Carbon Burnout in Pulverized Coal Combustion

The degree of carbon burnout is an important operating characteristic of full-scale suspension-fired coal combustion systems. Prediction of carbon loss requires special char combustion kinetics valid through the very high conversions targeted in industry (typically >99.5%), and valid for a wide range of particle temperature histories occurring in full-scale furnaces. The present paper presents high-temperature kinetic data for five coal chars in the form of time-resolved burning profiles that include the late stages of combustion. The paper then describes the development and validation of the Carbon Burnout Kinetic Model (CBK), a coal-general kinetics package that is specifically designed to predict the total extent of carbon burnout and ultimate fly ash carbon content for prescribed temperature/oxygen histories typical of pulverized coal combustion systems. The model combines the single-film treatment of char oxidation with quantitative descriptions of thermal annealing, statistical kinetics, statistical densities, and ash inhibition in the late stages of combustion. In agreement with experimental observations, the CBK model predicts (1) low reactivities for unburned carbon residues extracted from commercial ash samples, (2) reactivity loss in the late stages of laboratory combustion, (3) the observed sensitivity of char reactivity to high-temperature heat treatment on second and subsecond time scales, and (4) the global reaction inhibition by mineral matter in the late stages of combustion observed in single-particle imaging studies. The model ascribes these various char deactivation phenomena to the combined effects of thermal annealing, ash inhibition, and the preferential consumption of more reactive particles (statistical kinetics), the relative contributions of which vary greatly with combustion conditions.

97/00551 Devolatilization studies of single coal particles at high heating rates (carbon dioxide laser pyrolysis, heat transfer)

97/00551 Devolatilization studies of single coal particles at high heating rates (carbon dioxide laser pyrolysis, heat transfer)

97/00018 Devolatilization studies of single coal particles at high heating rates (carbon dioxide laser pyrolysis, heat transfer)

97/00018 Devolatilization studies of single coal particles at high heating rates (carbon dioxide laser pyrolysis, heat transfer)

Erratum: Optothermal study of small single particles [Rev. Sci. Instrum. 66, 4716 (1995)

First Page

Shop problems: Plating and anodizing

It is only recently that nuclear microscopy has been applied in different fields of environmental science, for example studies of single airborne particles and analysis of concentration depth profiles in shellfish, tree rings, ancient ice, etc. A review of a number of environmental applications is given with the emphasis on analysis of single aerosol particles. The results are briefly compared with the results of other microanalytical techniques such as electron microprobe and laser microanalysis. Possibilities for optimization of experimental setup and also some problems still existing in quantitative analysis of nuclear microscopy images are discussed.

Nucleation of Magnetization Reversal in Individual Nanosized Nickel Wires.

The mechanisms of magnetization reversal in small magnetic particles have been much discussed in the last decades and prompted intense research activities, motivated in particular by applications in magnetic recording technology [1]. However, experiments were performed, in general, on large assemblies of particles, and the dispersion of morphologies, compositions, orientations, and separations of the magnetic entities limited the interpretation of the results. Furthermore, interactions between particles were difficult to take into account. Single particle studies were possible only in few cases [2]. Recently, insights into the magnetic properties of individual and isolated particles were obtained with the help of near field magnetic force microscopy [3], electron Lorentz microscopy or holography [4], and micro-SQUID (superconducting quantum interference device) magnetometry [5]. It is now possible to make a clear link between experiments performed on nanometer-sized single objects (particles, wires, etc.) and the numerical calculations based on the Brown micromagnetic equations [6]. We report the first study of isolated nanoscale wires with diameters smaller than 100 nm, for which singledomain states could be expected. The cylindrical geometry, with its large shape anisotropy, is well suited for comparison with theory. We obtained unique insight into the process of magnetization reversal by measuring histograms of the switching field as a function of the orientation of the wires in the applied field, their diameter, and the temperature. Furthermore, we measured the probability of switching as a function of the applied field and the temperature. Our studies reveal that the magnetization reversal proceeds by a distortion of the magnetization followed by a nucleation and a propagation process. The observed behavior illustrates the fundamental importance of the study of single, isolated magnetic particles in comparing models and experiments. We developed planar microbridge dc SQUID [7], made of Nb (thickness 20 nm), which were shown to be able to detect 10 4 mB [8]. The SQUID is made of a thin (20 nm) Nb layer in order to prevent flux trapping. The experimental setup allows measurements of hysteresis loops in magnetic fields of up to 0.5 T and temperatures below 6 K, with a time resolution of 100 ms. Ni wires were produced by electrochemically filling the pores of commercially available nanoporous tracketched polycarbonate membranes of thicknesses of 6 to 10 mm [9]. The pore size was chosen in the range of 30 to 100 nm [10]. In order to place one wire on the SQUID detector, we dissolved the membrane in chloroform and put a drop on a chip of some hundreds of SQUID’s. Magnetization measurements were performed on SQUID’s with a single isolated wire. Scanning electron microscopy (SEM) (Fig. 1) was used to determine the position and morphology of the wire. The surface roughness was around 5 nm, corresponding to our SEM resolution.

Interaction effects from reversal studies of single particles

Interaction effects within particulate recording tapes have been investigated by comparing single particle behaviour with measured tape bulk properties. The particle systems were audio γ-Fe 2O 3 and 8 mm MP video tapes plus CrO 2 hand spreads. Interactions in the γ-Fe 2O 3 and CrO 2 systems were negative and consistent with mean field behaviour. However, the MP particle interactions were in the opposite sense, in contradiction to the Henkel plots. A model for this behaviour involving the creation of chains is suggested.

Optothermal study of small single particles

The suitability of the thermal wave detection technique to examine nonspherical particulate contaminants on products is presented. The thermal wave signal decay curves are analyzed by using two existing decay models. The goodness of fit of the models to the experimental data is also discussed.

The Development of a Thermal Treatment Assessment Procedure for Soils Contaminated with Hydrocarbons

Hydrocarbon evolution from a montmorillonite clay-soil was studied at three scales and at temperatures ranging from 300 to 650 o C in order to develop a thermal treatment assessment procedure for contaminated soils in rotary kilns. The two components of the assessment procedure included experimental evaluation of the soil at bench- or pilot-scale in conjunction with data fitting using an appropriate mathematical desorption model. The results of the experiments have been embodied in the model which can be used for scaling and correlative purposes. Performing experiments at three scales permitted a clear delineation of intraparticle and interparticle effects and it also allowed the testing of the model's ability to correctly scale results. In addition to reactor size, the key parameters examined included hydrocarbon volatility, temperature, moisture levels, and soil particle size. The three experimental facilities included a 0.6-by-0.6 m, natural gas-fired, batch, pilot-scale rotary kiln, a 6 mm outside diameter, single particle reactor (SPR), and a 10-by-10 cm bench-scale rotary reactor (BSRR). Porous soil particles ranging in diameter from 0.4 to 7 mm were tested in the kiln and the SPR. The single particle studies showed decreasing desorption times with decreasing radius in agreement with an intraparticle, diffusion-limited process. The kiln results showed no effect of particle size over the range of sizes studied suggesting that interparticle mass transfer resistances were controlling in a bed of particles. The hydrocarbons included toluene, naphthalene, and n-hexadecane and their concentrations were each roughly 0.05 weight percent, on a dry basis. The ease with which they were removed from the soil decreased in the order given. Increasing the reactor temperature increased the desorption rates for all three compounds with the adsorption isotherm constants showing temperature dependencies that agreed with the van't Hoff equation. Desorption rates showed no dependence on the presence or absence of other hydrocarbons. Moisture, at concentrations ranging from 5 to 9 weight percent, had a large effect on the desorption rate of all compounds studied. The rate of toluene evolution in the kiln was accelerated while the rate of hexadecane evolution was slowed. Intermediate results were obtained with naphthalene. The rate of total hydrocarbon desorption from dry soils in the kiln was well correlated by a penetration model which used a distribution of heats of desorption. The desorption rate was first order with respect to individual and total hydrocarbon concentrations

Further development of the electrodynamic chamber for studying single-particle oxidation and nonuniform shrinkage of char particles

The aims of this study are (1) to present the further development of some of the characterization methods used with the electrodynamic chamber (EDC) for high-temperature studies of single particles, and (2) to present in a qualitative manner new results on char oxidation under kinetically controlled conditions. The following methods were either developed or improved for higher accuracy: (1) Shadowgraphy for size and shape measurements. (2) Two-dimensional Mie scattering for sizing and determining optical properties. (3) Drag force measurements (by forced convection) for the determination of the density. (4) Optical pyrometry using wide-band detectors in the visible and infrared regions for temperature determination. In this study, synthetic char particles, spherically shaped (Spherocarb), as well as spherical polystyrene particles were oxidized in atmospheric air. The particles were suspended in the center of the EDC and heated by a CO 2 laser beam. The particles were characterized prior to reaction by the above methods. Mass and size changes as well as the particle temperature were measured as a function of time during the oxidation process. The particles were consumed in the following manner: First they were consumed uniformly, following previous shrinkage observations. At about 60% conversion nonuniform shrinkage was observed, mostly preferential shrinkage from the top of the particle. At 70% conversion a clear cut on the top of the particle was monitored. At 80% conversion preferential consumption from the other side of the particle was established, showing a clear disk configuration. At 95% conversion a hole in the center of the disk was developed. The explanation of this preferential consumption is not yet clear; however, it is possible that the nonsymmetrical boundary layer around the particle, caused by free convection, generated a temperature difference where high temperature is always at the top of the particle. It is due to the capability of the EDC that we were able to observe this unique phenomenon using its ability to suspend a particle at the center of the chamber motionless.

Group Ignition of a Cloud of Coal Particles

Ignition of an isolated single coal particle is known to occur either heterogeneously or homogeneously. While single-particle studies may be useful for dilute coal sprays, their application to burners is limited since ignition occurs in the vicinity of the burners where the spray is dense. Rather than considering an isolated particle, one must consider a collection of particles in order to determine the change in ignition characteristics resulting from particle interactions. Thus, group combustion models have been developed essentially to predict the ignition and combustion characteristics of a larger number of interacting drops/particles. This paper presents results of the ignition characteristics of a spherical cloud of uniformly distributed coal particles in quiescent surroundings using a simple group combustion model. For the conditions studied, the results are as follows: (1) Ignition is heterogeneous if the cloud is dilute and homogeneous if the cloud is dense under the same ambient conditions; (2) there is a minimum ignition time for a given set of initial conditions corresponding to a certain cloud denseness; (3) ignition time is less sensitive to the denseness of the cloud at higher ambient temperatures; and (4) decreased proximate volatile matter can result in either increased or decreased ignition time depending on the cloud denseness (ignition mode). Qualitative comparisons to experimental data are given; however, these comparisons should be approached with caution since the experimental conditions and geometries may be vastly different than those used in the numerical study presented here.

Resistivity, thermopower and single-particle tunneling studies on some zinc-doped yttrium barium copper oxide superconductors

Abstract Resistance and thermopower measurements have been made on a series of compounds, YBa2Cu3- x Zn x O7-y, with x = 0.025, 0.05, 0.1, 0.15 and 0.2. The superconducting transition temperature decreases as the zinc concentration increases. In a range of temperatures below TM , the mid point of the transition, the resistance shows an exponential temperature dependence fitting the phenomenological formula proposed by Ausloos et al. From the plot of logarithm of resistivity vs. (TM ‒ T) 1/2/T, one deduces a value of the average dimension of the Josephson junction to be a few tens of A, suggesting the microtwin boundaries to be the location of the junctions. The thermopower shows a peak always just above Tc . This conclusively shows that phonon drag is not the cause of the peak. The temperature dependence of the thermopower appears to resemble closely the earlier observations of Srinivasan et al. on yttrium barium copper oxide. Single-particle tunneling measurements carried out for two concentrations, x = ...