Naresh Shah

Mineral behavior during coal combustion 1. Pyrite transformations

Abstract The physical and chemical transformation of excluded crystalline illite particles and of illite grains included within a carbon matrix were examined in a laboratory scale reactor. Scanning electron microscopy was used to determine the particle morphology, and energy dispersive X-ray analysis, Mossbauer spectroscopy, and XAFS expectroscopy were used to monitor the chemical changes. At temperatures above 1400 K, illite lost its crystalline structure and was transformed to a glass. Melting, pore generation, and cenosphere formation were observed. For both included and excluded illite particles, neither segregation of volatile components at the particle surface, nor vaporization of potassium species, was observed during combustion. Combustion of synthetic chars containing illite inclusions demonstrated coalescence of these inclusions to form larger ash agglomerates. Comparison of these results with ash particle compositional data obtained from the combustion of a bituminous coal containing illite showed intermediate compositions indicating interaction between the molten illite and quartz, kaolinite, and pyrite. Deposition experiments revealed a distinct temperature range above which the transformed illite particles had sufficiently low viscosity to deform and stick upon impaction.

Mode of occurrence of arsenic in four US coals

An integrated analytical approach has been used to determine the mode of occurrence of arsenic in samples of four widely used US coals: the Pittsburgh, Illinois #6, Elkhorn/Hazard, and Wyodak. Results from selective leaching, X-ray absorption fine structure (XAFS) spectroscopy, and electron microprobe analysis show that pyrite is the principal source of arsenic in the three bituminous coals, but the concentration of As in pyrite varies widely. The Wyodak sample contains very little pyrite; its arsenic appears to be primarily associated with organics, as As3+, or as arsenate. Significant (10–40%) fractions of arsenate, derived from pyrite oxidation, are also present in the three bituminous coal samples. This information is essential in developing predictive models for arsenic behavior during coal combustion and in other environmental settings.

Speciation of arsenic and chromium in coal and combustion ash by XAFS spectroscopy

Abstract While there are a variety of methods to determine the concentration of trace elements in coal and ash, there have been few attempts to determine the speciation of these elements. In this paper, it is demonstrated that X-ray absorption fine structure (XAFS) spectroscopy is capable of providing speciation information at realistic concentration levels of 10–100 ppm, provided a solid-state multielement germanium detector is used. The initial studies have concentrated on arsenic and chromium. For arsenic, two principal forms of occurrence are observed in coal: As contained in pyrite and As(V) in arsenate (AsO4−3). In one coal (Pittsburgh #8, DECS-12), the As is present as arsenopyrite. The As in pyrite is readily oxidized under ambient conditions to the arsenate form. In combustion ashes, all arsenic is in the form of arsenate, with at least two forms of arsenate present; As in aluminosilicate slag and calcium orthoarsenate are possibilities. Chromium in coal and in ash is observed to be present predominantly (> 95%) in the Cr+3 state. Chromium oxyhydroxide is the standard compound whose XAFS spectrum most closely resembles that of the chromium in coal, while the chromium in ash may be incorporated into the aluminosilicate slag phase.

Distribution of trace elements in selected pulverized coals as a function of particle size and density

Abstract Trace elements in coal have diverse modes of occurrence that will greatly influence their behavior in many coal utilization processes. Mode of occurrence is important in determining the partitioning during coal cleaning by conventional processes, the susceptibility to oxidation upon exposure to air, as well as the changes in physical properties upon heating. In this study, three complementary methods were used to determine the concentrations and chemical states of trace elements in pulverized samples of four US coals: Pittsburgh, Illinois No. 6, Elkhorn and Hazard, and Wyodak coals. Neutron Activation Analysis (NAA) was used to measure the absolute concentration of elements in the parent coals and in the size- and density-fractionated samples. Chemical leaching and X-ray absorption fine structure (XAFS) spectroscopy were used to provide information on the form of occurrence of an element in the parent coals. The composition differences between size-segregated coal samples of different density mainly reflect the large density difference between minerals, especially pyrite, and the organic portion of the coal. The heavy density fractions are therefore enriched in pyrite and the elements associated with pyrite, as also shown by the leaching and XAFS methods. Nearly all the As is associated with pyrite in the three bituminous coals studied. The sub-bituminous coal has a very low content of pyrite and arsenic; in this coal arsenic appears to be primarily organically associated. Selenium is mainly associated with pyrite in the bituminous coal samples. In two bituminous coal samples, zinc is mostly in the form of ZnS or associated with pyrite, whereas it appears to be associated with other minerals in the other two coals. Zinc is also the only trace element studied that is significantly more concentrated in the smaller (45 to 63 μm) coal particles.

Characterization of Fine Particulate Matter Produced by Combustion of Residual Fuel Oil

ABSTRACT Combustion experiments were carried out on four different residual fuel oils in a 732-kW boiler. PM emission samples were separated aerodynamically by a cyclone into fractions that were nominally less than and greater than 2.5 |j.m in diameter. However, examination of several of the samples by computer-controlled scanning electron microscopy (CCSEM) revealed that part of the PM2.5 fraction consists of carbonaceous cenospheres and vesicular particles that range up to 10 |j.m in diameter. X-ray absorption fine structure (XAFS) spectroscopy data were obtained at the S, V, Ni, Fe, Cu, Zn, and As K-edges and at the Pb L-edge. Deconvolution of the X-ray absorption near edge structure (XANES) region of the S spectra established that the dominant molecular forms of S present were sulfate (26-84% of total S) and thiophene (13-39% of total S). Sulfate was greater in the PM2.5 samples than in the PM25+ samples. Inorganic sulfides and elemental sulfur were present in lower percentages. The Ni XANES spectra from all of the samples agreed fairly well with that of NiSO4, while most of the V spectra closely resembled that of vanadyl sulfate (VO•SO4•xH2O). The other metals investigated (i.e., Fe, Cu, Zn, and Pb) also were present predominantly as sulfates. Arsenic was present as an arsen-ate (As+5). X-ray diffraction patterns of the PM2.5 fraction exhibit sharp lines due to sulfate compounds (Zn, V, Ni, Ca, etc.) superimposed on broad peaks due to amorphous carbons. All of the samples contain a significant organic component, with the loss on ignition (LOI) ranging from 64 to 87% for the PM2.5 fraction and from 88 to 97% for the PM2.5+ fraction. Based on 13C nuclear magnetic resonance (NMR) analysis, the carbon is predominantly condensed in graphitic structures. Aliphatic structure was detected in only one of seven samples examined.

Mode of occurrence of chromium in four US coals

The mode of occurrence of chromium in three US bituminous coals and one US subbituminous coal has been examined using both X-ray absorption fine structure (XAFS) spectroscopy and a selective leaching protocol supplemented by scanning electron microscopy (SEM) and electron microprobe measurements. A synthesis of results from both methods indicates that chromium occurs principally in two forms in the bituminous coals: the major occurrence of chromium is associated with the macerals and is not readily leached by any reagent, whereas a second, lesser occurrence, which is leachable in hydrofluoric acid (HF), is associated with the clay mineral, illite. The former occurrence is believed to be a small particle oxyhydroxide phase (CrO(OH)). One coal also contained a small fraction (<5%) of the chromium in the form of a chromian magnetite, and the leaching protocol indicated the possibility of a similar small fraction of chromium in sulfide form in all three coals. There was little agreement between the two techniques on the mode of occurrence of chromium in the subbituminous coal; however, only a limited number of subbituminous coals have been analyzed by either technique. The chromium in all four coals was trivalent as no evidence was found for the Cr6+ oxidation state in any coal.

Behavior of basic elements during coal combustion

Abstract X-ray absorption fine structure (XAFS) spectroscopy, Mossbauer spectroscopy and computer-controlled scanning electron microscopy (CCSEM) have been used to investigate the reactions of Ca, Fe and alkalies in combustion systems. Ca may either transform to a CaO fume that reacts with SO 2 to form CaSO 4 , or may react with clays, quartz and other minerals to form slag droplets, or flyash. Similarly, pyrite may devolatilize and oxidize exothermically to form molten or partially molten iron sulfide-iron oxide mixtures, or may react with other minerals to become part of the slag. Alkalies in lignites (principally Na) volatize and may react with either SO 2 to form sulfates or with clay minerals (principally kaolinite) to form aluminosilicate slag droplets. K in bituminous coal is contained in illite which melts and becomes part of the slag phase. The calcium and alkali sulfates and the iron-rich species are observed to be concentrated in the initial layers of deposits, while the complex aluminosilicate slag droplets collect to form an outer glassy layer.

XAFS spectroscopic characterization of elements in combustion ash and fine particulate matter

X-ray absorption fine structure (XAFS) spectroscopy is a powerful non-destructive, direct technique for determining the speciation of environmentally important elements in products derived from combustion of fossil fuels. Such information is potentially important (i) for assessing the threat to human health posed by specific forms and oxidation states of such elements in combustion products (ash) or in combustion-derived airborne particulate matter (PM), and (ii) for possible source identification and apportionment in PM investigations. The specific examples discussed include the speciation of various elements classified as hazardous air pollutants (HAPs) in ash products from combustion of coal (As and Cr), residual oil (Ni and Cr), and biomass (Cd and Zn) and in airborne PM collected on a PM10 filter (S, Cl, Cr and As). Chromium and arsenic, which could exist in these materials in different oxidation states, were typically found predominantly in less toxic oxidation states, Cr(III) and As(V). All metal species (Cr, Ni, Cd, Zn, As) were shown to be present in the combustion ashes in predominantly oxidic environments (i.e., oxides, sulfates, arsenates, etc.). Most of the sulfur in the PM10 filter sample was present as sulfate, but minor organosulfur forms (thiophene) were also identified. For comparison with the data obtained for elements on the PM10 filter, XAFS data are also presented for the corresponding elements in two National Institute of Standards and Technology (NIST) particulate matter Standard Reference Materials (SRMs): Urban PM (SRM 1648), and Diesel PM (SRM 1650).

Size dependence of magnetic parameters and surface disorder in magnetite nanoparticles

Magnetic properties of oleic acid/oleylamine coated magnetite nanoparticles of average diameter D=4, 6, 8, 10, and 12nm are reported. The samples were characterized by transmission electron microscopy and x-ray diffraction (XRD) with XRD showing increasing disorder with decreasing D. Magnetization M vs temperature T data show the blocking temperature TB decreasing with decreasing D from TB=38K for 12nm to TB=6.5K for D=4nm. The saturation magnetization Ms at 2K also decreases from Ms=62emu∕g for 12nm to Ms=17emu∕g for 4nm but the coercivity at 2K increases with a decrease in D. It is shown that Ms fits Ms=92(1−2d∕D)3 with d=0.68nm for D>4nm and d=0.86 for D=4nm. This equation is derived assuming a core-shell model with shell of thickness d consisting of disordered spins not contributing to Ms.

Characterization of ultrafine coal fly ash particles by energy-filtered TEM

In this study, energy‐filtered transmission electron microscopy is demonstrated to be a valuable tool for characterizing ultrafine coal fly ash particles, especially those particles encapsulated in or associated with carbon. By examining a series of elemental maps (K‐edge maps of C and O, and L‐edge maps of Si, Al, Ti and Fe) recorded using the three‐window method, considerable numbers of titanium and iron species with sizes from several nanometres to submicrometre were shown to be present, typically as oxides dispersed in the carbonaceous matrix. Crystalline phases, such as rutile and iron‐rich oxide spinel, were also identified from electron diffraction patterns and high‐resolution TEM images. Information about these ultrafine coal fly ash particles regarding their size, morphology, elemental composition and distribution, and crystalline phases, which has not been available previously in conventional ash studies, should be useful in toxicological studies and related environmental fields.