Individual Fly Ash Research Articles

X-ray fluorescence tomography of individual waste fly ash particles

, Fluorescence tomography is a non-destructive, non-invasive technique that provides information about the internal spatial distribution of each element that emits a detectable fluorescence signal from the measured slice of the sample. The reconstruction problem for fluorescence tomography is much more difficult than it is for transmission tomography, due to the absorption of the photons within the excitation and detection paths. The present work presents a reconstruction technique that is based on the SART (Simultaneous Algebraic Reconstruction Technique) algorithm, which has been modified in order to take into account absorption corrections. This technique has been applied to the analysis of individual waste fly ash particles of about 8O 150 micron diameters, which have been scanned using an X-ray beam of V*H=2*5 μm 2 spot-size and placing an energy dispersive Si(Li) X-ray detector at 90 degrees to the incoming excitation beam. From previous scanning μ-SRXRF measurements it is clear that the elemental distribution within individual fly ash particles is highly inhomogeneous but no information could be obtained on the location of the different investigated elements (within/on the surface of the particle). On the other hand the location of toxic elements within individual fly ash particles affects the possible fate of these elements during e.g. fly-ash recycling. Thus the aim of this study was to investigate the 2D internal elemental distribution of the particles, with special attention to that of the toxic metals, such as Zn, Cd, Pb.

Direct determination of cadmium speciation in municipal solid waste fly ashes by synchrotron radiation induced mu-X-ray fluorescence and mu-X-ray absorption spectroscopy.

Cadmium is a toxic metal that causes environmental concern in connection with utilization and land filling of ash from combustion of municipal solid waste (MSW). Collecting information about the chemical associations of Cd in ash is fundamental since this affects its solubility and leachability from the ash material. In the work presented here, the content, distribution, and chemical forms of toxic metals especially of Cd on/in individual Municipal Solid Waste (MSW) fly ash particles have been investigated in situ by synchrotron radiation induced mu-X-ray fluorescence and absorption spectrometry. The use of an excitation energy of 27 keV made it possible to detect trace metals, such as Cd, present at ppm levels routinely. Changing the excitation energy in the vicinity of the absorption edge of Cd (26.71 keV), the absorption spectra of this element were measured for the first time in this high energy range in micron-sized spots of individual fly ash particles. The measurements indicated Cd to be preferably concentrated in some small areas ("hot-spots") with high concentration (up to 200 ppm) rather than in a homogeneous distribution or as a thin coating on the whole particle surface, making the surface-reaction the most probable mechanism of Cd enrichment during MSW combustion processes. Comparisons of XAS spectra of fly ashes and reference compounds showed that in the particles studied Cd is present in the oxidation state +2. Analyses of linear combinations of standard spectra allowed estimating the Cd presence within fly ash particles as an admixture of primarily CdSO4, CdO, and CdCl2 as well as an unidentified compound not included as a standard.

Crystallinity and selected properties of fly ash particles

The morphology, composition and crystallinity of both precipitator (solid) and cenosphere (hollow) fly ash particles of different sizes were studied with scanning electron microscopy (SEM), EDX, and X-ray diffraction (XRD). Bulk density, tap density and real density of both precipitator and cenosphere particles of different sizes as well as the wall thickness to diameter ratio of cenosphere particles were measured. The microhardness of individual fly ash particles embedded in the matrix of aluminum alloy was also measured. The crystalline to amorphous ratio weight percentage in fly ash particles, and the weight or volume fraction of each crystalline component varies with the particle size. The crystallinity of precipitator particles increased as the particle size increases, whereas the crystallinity of cenosphere decreased as the particle size increases. The elastic modulus of fly ash was estimated from the crystallinity of fly ash and the volume fraction of each component, using the rule of mixtures. The calculated upper limits for Young's modulus of precipitator particles were 126 GPa for particles in the size range 150–250 μm and 98 GPa for particles in the size range 5–10 μm. Young's modulus of cenosphere particles was estimated to be approximately in the range of 13–17 GPa in all particle size ranges. The hardness of the larger precipitator fly ash particles (120 μm) exhibited a wide scatter in the range of 160–400 kg mm−2, while the hardness of the smaller size precipitator particles (20 μm) were in a narrow range from 250 to 270 kg mm−2.

Quantitative trace element analysis of individual fly ash particles by means of X-ray microfluorescence.

A new quantification procedure was developed for the evaluation of X-ray microfluorescence (XRF) data sets obtained from individual particles, based on iterative Monte Carlo (MC) simulation. Combined with the high sensitivity of synchrotron radiation-induced XRF spectroscopy, the method was used to obtain quantitative information down to trace-level concentrations from micrometer-sized particulate matter. The detailed XRF simulation model was validated by comparison of calculated and experimental XRF spectra obtained for glass microsphere standards, resulting in uncertainties in the range of 3-10% for the calculated elemental sensitivities. The simulation model was applied for the quantitative analysis of X-ray tube and synchrotron radiation-induced scanning micro-XRF spectra of individual coal and wood fly ash particles originating from different Hungarian power plants. By measuring the same particles by both methods the major, minor, and trace element compositions of the particles were determined. The uncertainty of the MC based quantitative analysis scheme is estimated to be in the range of 5-30%.

Influence of the elemental composition of individual fly ash particles on the efficiency of the electrostatic precipitators

The Oxford Scanning Proton Microprobe has been used to determine the elemental composition of individual fly ash particles in the 1–5 μm geometrical size range, collected before and after a pilot electrostatic precipitator (3 MWe equivalent power) of a power plant burning South African coal (Amcoal). The electrostatic precipitator was working at high collection efficiency (99.8%) obtained by operating under semi-pulsed energization mode. Al, Si, P, S, K, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, Ga and Sr have been always detected in particles collected simultaneously both before and after the electrostatic precipitator. In some particles also V, Ge, As and Zr resulted above the detection limits. Both at the ESP inlet and outlet one hundred particles have been analysed. According to major element concentrations, particles can be grouped into seven classes showing some preferential associations of minor and trace elements with specific matrix compositions. Most particles (about 80%) belong to classes 1 and 2 characterized by the same aluminosilicate nature with higher concentrations of several minor (P, Ca, Ti, Fe) and trace (S, Mn, Ni, Zn, Ga) elements in class 2. Within the same particle size range, the electrostatic precipitator seems to be selective with respect to particle composition: class 2 particles show a higher penetration. This result is discussed in terms of the particle composition effects on the fly ash electrical properties, which are relevant parameters for the electrostatic precipitation process.

Macroscopic and microscopic approach to the study of enrichment phenomena in the electrostatic precipitation of coal fly ash particles

Ordinary PIXE analysis of fly ash samples collected at the inlet and outlet of the electrostatic precipitators of coal-fired power plants, shows the occurrence of enrichment phenomena for several elements. This is usually attributed to the volatile nature of compounds associated with these elements and to the decreasing electrostatic precipitator efficiency versus particle size. A microscopic approach to the study of enrichment phenomena, based on the analysis of hundreds of individual fly ash particles collected at the inlet and outlet of the electrostatic precipitators of coal-fired power plants, indicates the occurrence of an additional mechanism, besides that shown by ordinary PIXE analysis. In particular, the enriched trace element concentrations at the outlet of the electrostatic precipitators can be related also to the preferential penetration of particles with higher concentrations of trace elements because of their influence on the electrical properties of the particle themselves.

The microscopic nature of coal fly ash particles investigated by means of nuclear microscopy

Individual fly ash particles collected at the outlet of a pilot electrostatic precipitator of a coalfired power plant have been analysed by a proton microprobe. Major and trace element concentrations have been obtained for about one hundred particles in the 0.5–5 μm size range. As a result of the combustion of the different components of the mineral matter in the parent coal, particles can be grouped in seven classes showing characteristic matrix compositions and different trace element concentrations. The relative abundances of these groups seem to have a size dependence. Trace elements are found to be non-uniformly distributed over all particles: a size dependence as well as some preferential association with the particle matrix have been found. Enriched concentrations of potentially toxic elements exist on smaller, most respirable particles examined in this study. Possible reasons for this are suggested.

Chemical composition and size distributions for fly ashes

Abstract Inter-particle variations in particle diameter and chemical composition in coal fly ashes are significant, and can be quantitatively studied using currently available microanalytical techniques. One such technique, computer controlled scanning electron microscopy (CCSEM), is now routinely used to determine chemical compositions and diameters of individual fly ash particles, using statistically significant sample sizes. This paper proposes simple mathematical formulae for expressing the inter-particle variations. Additionally, these formulae can be used for economically storing the large amount of data generated by CCSEM. Two apparent limitations of the CCSEM technique are discussed — one relating to the measurement of size distribution, and the other to determining the iron oxide distribution in the ash. These limitations are imposed by statistical considerations, and by artifacts of sample preparation techniques.

Trace element characterization of coal fly ash particles

XRF, PIXE, RBS and STIM were used for a study of trace element distributions in Greek lignites and their ashes. Samples of fly ash collected from electrostatic precipitators have been separated into seven fractional sizes ranging from 300 μm to less than 25 μm. Trace element concentrations show variation as function of grain size. In addition, the results of analyses of 50 individual fly ash particles are presented.

Quantitative PIXE analysis of single fly ash particles by a proton microbeam

Methods of quantitative PIXE analysis of individual particles in the diameter range between 5 and 10 μm are presented. Two different approaches to the analysis are considered. In the first case, an analysis is performed with a proton mubeam centered at the middle of the particle. In the second case, the beam is scanned over the whole particle. Absorption correction methods were developed for both cases. Advantages and drawbacks are discussed on the basis of analysis of individual fly ash particles.

Characterization of organic monolayers on individual microparticles using microprobe mass spectrometry

Polycyclic organic matter (POM) represents a class of compounds of environmental significance. Many of these compounds have been shown to exhibit mutagenic characteristics. POM is formed during the combustion of fossil fuels such as coal. Emitted particulate matter such as coal flyash contains surface associated POM that undergoes chemical transformations due to photolysis and exposure to gaseous pollutants. Past studies of these compounds on flyash have relied on wet chemical techniques in which POM is extracted from large quantities of flyash and then analyzed by chromatography. This technique is hindered by several problems such as the extensive heterogeneity in flyash composition. Individual flyash particles of different composition will influence POM adsorption and reactivity. Studies of the different types of particles by the wet techniques require that the particles be physically separated (density, size, magnetic properties). Microprobe secondary ion mass spectrometry (SIMS) is a technique that can potentially overcome these obstacles by permitting the in situ analysis of POM on single particle surfaces.

Proton microprobe analysis of the trace element distribution in fly ash particles

The spatial distribution of trace elements in individual coal fly ash particles is an important factor in assessing their environmental impact. The Oxford Scanning Proton Microprobe (SPM) has been used to determine the distribution of elements in fly ash particles with sub-micrometre spatial resolution. PIXE was used to detect the minor and trace elements and simultaneous proton backscattering analysis was used to determine the matrix composition and thickness. Both areal maps and radial line scans of elemental concentrations were obtained. Results are presented and discussed.

Investigations on the Composition of Individual Fly Ash and CFB-Ash Particles

ABSTRACTThe individual particle compositions of three circulating fluidized bed combustion (CFB) ashes and one Type C fly ash were studied to determine differences in composition existing between the individual particles within a single ash. The methods used for the investigation included optical microscopy, XRD and XRF analysis. It was found that all the ashes studied consisted of several types of particles that differed in morphology, color and composition.

Characterization of North American Lignite Fly Ashes I. Chemical Variation

ABSTRACTThe chemical variation of central North American lignite fly ash can be compared using the major and minor oxide analyses and the distribution pattern of the inter-grain chemistries of the individual fly ash particles. AAS and electron microprobe chemical analysis of nineteen fly ashes have been used to illustrate the variations in bulk sample and individual grain chemistry. This information documents the chemical variability of the lignite fly ashes by geographic region. There is a direct relationship between grains rich in SiO2 and Al2O3 and in the Na2O content of the grains in lignite derived fly ashes. There is also a direct relationship between grains rich in CaP and the MgO and SO3 content of the grains in lignite derived fly ashes.

Characterization of individual fly ash particles emitted from coal- and oil-fired power plants

Characterization of individual fly ash particles emitted from coal- and oil-fired power plants

Characterization of Crystalline Phases in Fly Ash by Microfocus Raman Spectroscopy

AbstractThe laser Raman microprobe was used to interrogate individual fly ash particles as small as 1 μm diameter and record “fingerprint” Raman spectra from both crystalline and non-crystalline components.of the fly ash. When compared to reference patterns of known crystalline phases, the Raman spectra can be used to identify crystalline phases and can give some structural information on other phases. Furthermore, because this method characterizes fly ash particles on an individual basis, correlations to both color and morphology of the particles can be made.

A comparative study of in-plume and in-stack collected individual coal fly ash particles

Various analytical techniques have been used to study the surface and bulk composition of individual fly ash particles which have been collected both within a power plant smokestack and within the plume of that smokestack. A sampling system has been employed which collects the fly ash on dry soft metal substrates (e.g. indium) which is suitable for analysis under clean, ultra high vacuum conditions. Morphology and quasi-bulk composition of individual particles is obtained through the use of a scanning electron microscope (S.E.M.) equipped with an energy dispersive X-ray fluorescence attachment (E.D.X.). After S.E.M. studies, the samples are transferred to a scanning Auger electron spectroscopy system (A.E.S.) for surface analysis and ion beam depth profiling. Results of these experiments show a wide chemical and physical disparity between different particles even in the limited sample size range studied (5–30 μm). There is no evidence for a surface concentration (> 1 %) of high atomic number elements for either the particles collected from within the smokestack or those collected from the plume. A statistically significant difference between the ratios of aluminum to iron has been observed for the in-stack particles as compared with those collected within the plume.

Coordinated Scanning Electron Microscopy and Light Microscopy for the Evaluation of Chemical Element Heterogeneity Among Individual Fly Ash Particles

Fly ash, a major air pollutant associated with coal combustion, has been shown by bulk analysis technique to contain many elements toxic to the lung system and the exposure of lung cells to the foreign elements contained in fly ash can be strongly influenced by the distribution of these elements among individual fly ash particles. Light microscopy has been utilized by Fisher et al. (I) to identify 14 classes of particles on the basis of color, internal structure and opacity. The present study was undertaken in an attempt to evaluate the segregation of chemical elements among the various types of fly ash particles identified by light microscopy and to verify directly the suggested elemental composition of certain of the classes.