Frank E. Huggins

Investigation of the high-temperature behaviour of coal ash in reducing and oxidizing atmospheres

The high-temperature behaviour of ashes from a suite of coals exhibiting a wide range of mineralogies has been investigated. Phase analysis of ash samples quenched from various temperatures under either a reducing (60% CO/40% CO2) or an oxidizing (air) atmosphere was performed by Mossbauer spectroscopy, scanning electron microscopy (SEM)/automatic image analysis (AIA), and X-ray diffraction (XRD). It was found that significant partial melting of the ashes occurred at temperatures as low as 200–400 °C below the initial deformation temperature (IDT) defined by the ASTM ash cone fusion test. Melting was greatly accelerated under reducing conditions, for which the percentage of melted ash increased rapidly between 900 and 1100 °C, saturating at temperatures above ≈ 1200 °C. The observation of such phases as wustite (FeO), fayalite (Fe2SiO4), hercynite (FeAl2O4), and ferrous glass in samples quenched from 900 to 1200 °C indicates that ash melting in a reducing atmosphere is usually controlled by the iron-rich corner of the FeO-Al2O3-SiO2 phase diagram. Ashes rich in CaS are an exception to this rule, for large quantities of iron sulphide are formed and the melting behaviour is controlled in part by the FeO-FeS phase diagram. Under oxidizing conditions, potassium appears to be the most important low-temperature fluxing element, as the percentage of glass in samples quenched from temperatures below 1100 to 1200 °C was proportional to the amount of the potassium-bearing mineral illite contained in the coal. Above 1200 °C in air, calcium and, to a lesser extent, iron became effective as fluxing elements; melting accelerated between 1200 and 1400 °C, and was near completion between 1400 and 1500 °C for most ashes. To retard ash melting, it is generally concluded that aluminium is the most desirable constituent of ash, whereas the most undesirable constituents are iron, calcium, and potassium.

Mossbauer studies of coal and coke: quantitative phase identification and direct determination of pyritic and iron sulphide sulphur content

Abstract Mossbauer spectroscopy has been used to analyse the included minerals and mineral-derived phases of a large suite of coal, coke, and other coal-related samples. Scanning electron microscopy, chemical analysis, and X-ray diffraction were used as supplementary techniques. Most of the samples studied were bituminous coals, and the dominant Fe-bearing minerals observed were pyrite, carbonate phases (siderite, ankerite), and ferrous silicates (principally illite). Pyrite was present in all coals investigated and was the dominant Fe-bearing phase in approximately 70% of the samples. Absorption peaks from ferrous Fe contained in illite and other clay minerals were observed for 90% of the coals, and were dominant for approximately 20% of the samples. The carbonates siderite and ankerite were present in approximately half of the coals, and siderite was the dominant Fe-bearing phase in four samples. In weathered coals and coal refuse, Fe sulphates, oxides and oxyhydroxides were also observed. Mossbauer data obtained from numerous coals before and after coking established that the following transformations occur during high-temperature carbonization: 1. (1) pyrite → troilite ( FeS ) + pyrrhotite ( Fe 1 − x S ); 2. (2) illite + other silicates → ferrous glass ; 3. (3) siderite → metallic Fe . A new method was developed for making direct, quantitative determinations of the amount of pyritic sulphur in coal, which involves comparing the observed Mossbauer mass absorption coefficient for pyrite in coal to a standard calibration curve. Preliminary data for coke samples show that the same technique can be applied to determine the amount of sulphur contained in the Fe sulphide phases, troilite and pyrrhotite.

Observations on low-temperature oxidation of minerals in bituminous coals

Mineral matter in three naturally weathered coals from Pennsylvania strip mines and in two laboratory-oxidized coals has been characterized by 57Fe Mossbauer spectroscopy, scanning electron microscopy and other techniques to determine mineralogical trasnformations that occur in coals during weathering. Pyrite was found to be the most readily oxidized mineral, forming a variety of iron sulfates initially and geethite eventually. The iron sulfates formed were different in the two laboratory-oxidized coals, despite identical oxidation treatments. Calcite disappeared from one calcite-rich coal with increasing oxidation, but was not replaced by an equivalent amount of gypsum. A severely weathered strip-mine coal was enriched in calcium, which was dispersed through the oxidized macerals. Extended X-ray absorption fine-structure spectroscopy indicated that this dispersed calcium was most likely present as salts of carboxylic acids. Siderite was suprisingly resistant to oxidation at room temperature. Less direct evidence indicates that clay minerals also take part in the alteration to some extent. The coals oxidized in the laboratory showed alteration behavior that differed in a number of respects from that of the strip-mine coals. For example, iron sulfates were much less common in the latter coals; also, the formation of geothite appeared to be controlled to a large extent by the pyrite particle size in the strip-mine coals, but not in the laboratory-oxidized coals. The oxidation of an individual pyrite grain is not only a function of general conditions (temperature, humidity, oxygen partial pressure), but also the immediate local (< 1 mm) chemistry, as a variety of iron sulfates were observed in the coals, often in close proximity. Also, assemblages of gypsum and goethite were observed in otherwise slightly oxidized coal, which indicates that the alteration of pyrite and calcite, when in close contact, proceeds most rapidly.

Comparative sensitivity of various analytical techniques to the low-temperature oxidation of coal

A number of analytical techniques were used to investigate the low-temperature oxidation of a high-volatile and a low-volatile bituminous coal. Two oxidation treatments were used: stockpiling at a size of < 3 mm out of doors, and treating in moist air at < 420 μm in a laboratory retort maintained at 50 °C, 65% r.h. 57Fe Mossbauer spectroscopy and computer-controlled scanning electron microscopy (CCSEM) were used to investigate the oxidation of included minerals; diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and electron spectroscopy for chemical analysis (ESCA) were used to examine maceral oxidation; and the alteration of several technological properties (Gieseler plasticity, free swelling index and coke reactivity) was also monitored. Gieseler fluidity was by far the most sensitive of these measurements to the early stages of oxidation, while the Mossbauer measurement of the oxidation of pyrite to sulphates or oxyhydroxides was the second most sensitive indicator of oxidation for the coals investigated. DRIFT and ESCA exhibited significant features derived from oxygen functional groups in macerals only after the oxidation was severe enough to have nearly destroyed the plasticity. However, band ratio techniques appear promising as a method of enhancing the sensitivity of DRIFT to oxidation.

Investigation of the structural forms of potassium in coke by electron microscopy and X-ray absorption spectroscopy

Abstract An investigation of the chemical and structural forms of potassium in coal, coke, and potassium-enriched coke has been made. The principal analytical techniques used were X-ray absorption spectroscopy, computer controlled scanning electron microscopy, and scanning transmission electron microscopy. The potassium in the bituminous coals investigated was contained in the clay mineral illite, which was transformed to a predominantly amorphous or glassy potassium aluminosilicate during coking at temperatures of 1050–1100 °C. In cokes artificially enriched in potassium and annealed at 1260 °C under argon, two principal forms of potassium were identified: potassium in K -enriched aluminosilicates and silicates, and potassium dispersed throughout the carbonized macerals and bonded to carbon. Although the aluminosilicates and quartz act as effective potassium receptors, approximately half of the added potassium was dispersed through and bonded to the carbonized macerals. Both potassium forms were predominantly amorphous in structure. Analysis of the electronic structure and local atomic environment by X-ray absorption spectroscopy showed that these amorphous phases exhibited some similarities to chemically similar crystalline phases such as leucite (KAlSi 2 O 6 ) and intercalated potassium-graphite (KC 8 ). X-ray absorption spectroscopy measurements on samples extracted from a blast furnace experiencing high alkali levels indicated that the forms of potassium in the blast-furnace coke were qualitatively similar to those in the laboratory prepared cokes, and suggested a possible mechanism for alkali attack on refractories.

Investigation of alkali and alkaline earth coal gasification catalysts by EXAFS spectroscopy

Abstract EXAFS spectroscopy and several supplementary techniques have been used to investigate a variety of coal and polymer chars containing alkali and alkaline earth catalysts, including Ca, K and Rb. In-situ EXAFS measurements were performed on Rb-loaded chars during gasification. At 450 °C in 90% N 2 -10% O 2 , the Rb XANES exhibited a doublet structure similar to that of Rb 2 O and Rb 2 CO 3 . EXAFS and other techniques show that the Ca in lignites, both naturally occurring and ion-exchanged, is essentially molecularly dispersed and bonded to carboxyl groups. Calcite-like features are induced in both the near-edge structure and radial structure functions of chars with increasing pyrolysis time and temperature. The structure of K is found to differ significantly in polymer, lignite and bituminous coal chars.

NICKEL SPECIATION OF URBAN PARTICULATE MATTER

A four-step sequential Ni extraction method, summarized in Table AB-1, was evaluated for identifying and quantifying the Ni species occurring in urban total suspended particulate (TSP) matter and fine particulate matter (<10 {micro}m [PM{sub 10}] and <2.5 {micro}m [PM{sub 2.5}] in aerodynamic diameter). The extraction method was originally developed for quantifying soluble, sulfidic, elemental, and oxidic forms of Ni that may occur in industrial atmospheres. X-ray diffraction (XRD) and x-ray absorption fine structure (XAFS) spectroscopy were used to evaluate the Ni species selectivity of the extraction method. Uncertainties in the chemical speciation of Ni in urban PM{sub 10} and PM{sub 2.5} greatly affect inhalation health risk estimates, primarily because of the large variability in acute, chronic, and cancer-causing effects for different Ni compounds.

Rapid Mössbauer-Based Methods for Applications in the Coal and Steel Industries

Conventional Mossbauer spectroscopy is not suited for routine quality- control applications in industry because of the long sample turn-around times and relatively sophisticated nature of the technique and its interpretation. Moreover, because quality-control applications tend to involve repetitive determinations of the same quantity, the full power of the technique is generally not necessary. It appears that Mossbauer spectroscopy is perhaps 1overqualified1 for industrial applications on the shop floor. As a result, simpler and more rapid Mossbauer techniques have been proposed and developed for such applications.

Investigation of Topics of Interest in Steelmaking Technology by X-Ray Absorption Spectroscopy

The capability of X-ray absorption spectroscopy (XAS) to determine the electronic state and local atomic environment of dilute elements in complex materials is well suited to many problems in steelmaking. Dilute constituents (∿0.01 to 1.0%) often control the bulk properties not only of steels, but also of the raw materials, fuels, and by-products of steelmaking. In this paper, we present two examples of the application of XAS to such problems. In one case, the dilute elements (V and Nb in microalloyed steel) have a beneficial effect, while in the other (K in coke), the effects are harmful.