Effect of prolonged heating on the optical properties of vitrinite

Vitrinites ranging from sub-bituminous to anthracitic rank have been heated at 150 and 350 °C for residence times extending up to 32 weeks. Devolatilization vacuoles develop in all the vitrinites at 350 °C, apart from the anthracitic. Occasional small vacuoles formed in the vitrinites of coking rank at 150 °C are most probably related to gas evolution from liptinites. Slight rounding of the margins of vitrinite particles of coking rank at 350 °C indicates softening, while heating at the longest residence time has been sufficient to allow vitrinites of coking rank to undergo ‘active decomposition’. Mosaic structures have developed at temperatures as much as 125 °C below those normally attributed to the formation of such structures in short-term laboratory carbonization experiments on vitrinites of the same rank. Again, apart from the vitrinites of anthracitic rank, the reflectance of vitrinites shows a rise at 150 and 350 °C that would continue beyond the longest residence time used. At 350 °C a large part of the reflectance increase occurs during the first week of heating. The reflectance data and the derived refractive and absorptive indices indicate that with sufficiently long residence times, changes that are more fundamental than ‘molecular stripping’ must take place, probably involving condensation reactions that introduce increased coplanarity into the vitrinite molecules. The implication of these data to thermal metamorphism of coals and to the normal coalification process is briefly discussed.     

The behaviour of petrographic constituents of hard coals in coking process and structure of the cokes obtained

The behaviour in the coking process of the main petrographic constituents - vitrinites, exinites, micrinites and fusinites - separated from hard coals of different rank (from flame coal to anthracite), and the differences in structure of the obtained cokes were examined and discussed.     

Characterization of Canadian coals by nuclear magnetic resonance spectroscopy

Apparent aromaticities of a series of Canadian coals of different rank were estimated by solid state nuclear magnetic resonance spectroscopy. The aromaticities varied from 0.57 for a lignite up to 0.86 for a semi-anthracite coal. The aromaticities correlated well with fixed carbon and oxygen content of the coals as well as with the mean reflectance of the coals. Correlations were also established between aromaticities and the H/C and H_aru/C_ar ratios of the coals. Uncertainties in calculation of the hypothetical H_aru/C_ar ratios, from experimental data were pointed out.Structural parameters of the chars derived from the coals by pyrolysis at 535°C were, also, estimated. The H/C and H_aru/C_ar ratios of the chars were markedly lower than those of coals. This was complemented by higher apparent aromaticities of the chars compared with the coals.     

The variation of structural characteristics of macerals during pyrolysis

Vitrinite and inertinite were separated by DGC from Chinese Shenmu bituminous coal and the structural characteristics of the macerals, before and after pyrolysis, were analyzed by ultimate analysis, FTIR and ¹³C NMR. The results showed that vitrinite chars always had higher H and lower C content than inertinite char at the same pyrolysis temperature. The FTIR and ¹³C NMR indicated that vitrinite had more aliphatic C-H, hydrogen bonding and lower aromaticity. With increasing temperature, the aliphatic C-H decreased, aromatic C-H, aromaticity and H_ar/H_al ratio increased. At the same temperature, inertinite always had higher H_ar/H_al ratio than vitrinite, which is consistent with that inertinite had higher aromaticity than vitrinite. And the H_ar/H_al ratio was also related to the remainder volatile matter. With increasing H_ar/H_al ratio, the remainder volatile matter in vitrinite and inertinite decreased. The higher aromaticity and H_ar/H_al ratio and lower H content of the inertinite in all temperature range were correlated with its higher thermal stability and lower volatile yield than vitrinite.     

Free radicals in coals and coal conversion. 3. Investigation of the free radicals of selected macerals upon pyrolysis

The nature and concentration of free radicals in natural and pyrolysed samples of several high-purity resinites, vitrinites and fusinites have been determined by electron spin resonance (e.s.r.) spectrometry. Careful measurements of the g-values, linewidths and integrated intensities have been made. The natural concentration of free radicals in the various macerals was measured and found to vary by approximately 5 orders of magnitude. Of the three macerals, the free radical concentration of vitrinites was found to have the greatest dependence upon pyrolysis temperature. The g-values measured offer insight into the chemical nature of the free radicals and how they are altered upon pyrolysis. Interpretation of the data is based upon earlier work on pure macerals and coal pyrolysis. The implications of the nature and behaviour of free radicals in macerals in coal-conversion processes are discussed.     

Chemistry of some maceral concentrates from British coals: Provenance of samples and basic compositional data

A number of concentrates of vitrinite, liptinite and inertinite groups of macerals, prepared and sealed under nitrogen by Fenton and Smith, have been made available to us. They are being studied in a number of institutions using various techniques. The set comprises seven vitrinites, four liptinites and eight inertinites, the last-named including some reputed semifusinite concentrates. Identifications of seams and sample history are provided here, together with chemical analyses and re-analysis of petrographic compositions using reflected light methods. The vitrinites cover a range of rank represented by carbon contents from 81.5 to 91.4% dmmf (hvB to Iv). The liptinites consist very largely of sporinite. Dilatation tests indicate that little if any weathering has occurred during 20 years of storage of the vitrinites.     

Macerals in bituminous coals and the coking process: Part 1: Effect of basic coal properties on the process of thermal degradation

Bituminous coals produced in the Ostrava—Karviná coal basin show considerable variation in their maceral composition and rank. In order to explain the significance of the petrographic composition, structure and chemical parameters in coal degradation processes, a large number of coal samples was investigated. From the relationships obtained, the behaviour of macerals during these processes is discussed. It is found that isometamorphic vitrinites are characterized by a distinctive behaviour affected to a first approximation by the atomic ratios C/H and C/O. Inertinite, despite its occasional significantly higher volatile matter, does not yield an appreciable amount of fused material.     

Fourier transform infrared photoacoustic spectroscopy of raw coal

Photoacoustic Fourier transform infrared (PA-FT-i.r.) spectrometry was employed to examine raw coal containing 13–64 wt%, daf volatile matter (VM). Sample preparation for i.r. examination was the same as for ASTM proximate analysis. Technological and spectroscopic information on representative samples were obtained. PA scan time is longer than for conventional FT-i.r. operation; however sample preparation is faster and easier. The time-consuming KBr pellet technique, possibly affecting coal structure, is avoided. The i.r. absorption in the OH, NH, CH_ar and CH_x,al regions may allow the determination of VM for both high- and low-rank coal. Some differences between present PA and diffuse reflectance spectra of raw coal from the literature are discussed.     

13C Nuclear magnetic resonance characterization of coal macerals by magic-angle spinning

¹³C cross-polarization and magic-angle spinning (CP/MAS) nuclear magnetic resonance (n.m.r.) experiments are reported on two fusinites, three vitrinites, one micrinite, one sample of attrite, one exinite, one resinite and one lignite. With one exception, the maceral spectra show no more fine structure than the CP/MAS n.m.r. results for whole coals. The maceral spectra are discussed in relation to each other and to the corresponding spectra of whole coals.     

Cd(II) sorption onto chemically modified Australian coals

Cadmium (II) adsorption onto chemically modified Australian coals has been studied as a function of p[H⁺] at 25 °C. The low rank Collie and Loy Yang coals and the bituminous Norwich Park and Mount Arthur coals were modified with hydrogen peroxide. While both treated bituminous coals showed little affinity for Cd(II) the adsorption properties of the two low rank coals were drastically modified by treatment with hydrogen peroxide. NICA-Donnan model calculations for the treated Collie coal indicated an increase in functionality (ca. 5-fold) which was reflected in the important increase in Cd(II) adsorption obtained from batch adsorption experiments. On the other hand, the modified Loy Yang coal underwent a ca. 6.25 decrease in surface functionality and decomposition of the coal structure itself. These changes were reflected in the DRIFT spectra in bands characteristic of the oxygen-containing functional groups. The Cd(II) adsorption capacity of this modified coal was consequently substantially decreased. The two low rank coals were also treated with ammonia at p[H⁺] 7.5 and 9.5. This treatment did not, however, induce any substantial change in the adsorption of Cd(II). DRIFT spectra showed that the predominant functional groups present in both Collie and Loy Yang coal remained essentially unchanged after treatment with aqueous ammonia.     

Coalification paths of exinite,vitrinite and inertite

Coalification is a burial metamorphic process fundamental to understanding the origin and nature of coals and petroleum. Accurate assessment of Coalification requires removal of the variation in coal properties owing to differences in coal type. This is best achieved by assessing the coal rank of the maceral groups exinite, vitrinite and inertinite (and ultimately the macerals within each group) in terms of maximum reflectance. The mean maximum reflectances of exinite (R?_E max), vitrinite (R?_v max) and inertinite (R?_I max) are highly correlated over the rank range, soft brown coals to low-volatile bituminous coals. A single exinite Coalification break is identified at 0.4 to 0.5% R?_V max. Thereafter, exinite coalification occurs at an increasing rate (R?_E max relative to R?_V max) over the rank range 0.5–2.0% R?_V max, crossing the vitrinite Coalification path at 1.55% R?_V max. A phase of rapid inertinite Coalification (0.7–2.0% R?_I max) occurs between 0.2 and 0.9% R?_V max. Since exinite is a major land-plant source of hydrocarbons, and R?_E max is a sensitive indicator of organic maturation over the range 0.2–1.1% R?_E max, which corresponds to the main phase of oil generation, R?_E max can be a most useful indicator of petroleum generation. The inertinite lnkohlungsprung represents a dramatic compositional change which should be taken into account when considering the origin of inertinite, its utilization properties and the general processes of rank change and organic maturation.     

Programmed-temperature pyrolysis of vitrinites of various rank

Nine vitrinites separated from coals of various rank (69–93% carbon, daf) were studied by programmed-temperature pyrolysis. Samples were heated at 11°C/min in a stream of helium which swept the pyrolysis products into a flame-ionization detector. With the experimental system used this responds only to hydrocarbons below C₁₀. For all the vitrinites the curve of detector response as a function of temperature showed a single maximum and the temperature of the peak maximum was found to be located at higher temperatures for the higher-rank samples. The temperature of the peak maximum was not very sensitive to rank between 70% and about 90% carbon but varied rapidly outside this range. This suggests that the mechanism which generates low-molecular-weight hydrocarbons from coal changes above approximately 90% carbon.     

Low temperature oxidation of coals of different rank and different sulphur content

Five coals of different rank and of different content of sulphur were subjected to oxidation by peroxyacetic acid (PAA), 5% nitric acid, by oxygen in 0.5N Na₂CO₃ aqueous solution and the air oxidation for 7 days at 125 °C. The processes of oxidation were carried out for coal samples demineralised by the Radmacher method, and additionally for the pyrite-free coal samples. Proximate, elemental and spectral analyses of the initial coals and the products obtained from them were made. The most effective oxidising agents were 5% HNO₃ and PAA. As a result of oxidation, a significant part of the organic components of coal is converted into acid soluble products. Depending on the oxidising agent, the loss of sulphur in the solid oxidation products was different and the highest for the coal samples oxidised with HNO₃ and PAA. Formation of oxidised sulphur species (SO and -SO₂) was detected by IR spectroscopy. FTIR data also provide useful information on evolution of the molecular structure of different rank coals on oxidation, in particular in the carbonyl and aliphatic ranges (1800-1500 and 3500-2800 cm⁻¹).     

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 Mössbauer 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 Mössbauer 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.     

Coal microlithotype response to froth flotation in selected Western Kentucky coals

Seven coals representing four western Kentucky coal seams and the coal rank range from high volatile C to high volatile A bituminous were selected for bench-scale froth flotation processing. Each coal was represented by two splits of the run-of-mine coal: a 12.7 mm × 0 fraction (crushed to 28 mesh × 0) and a 28 mesh × 0 fraction. The original 28 mesh × 0 split was found to be higher in the inertinite macerals fusinite and semifusinite than the coarser fraction. The separation of the inertinite macerals, expressed as the microlithotype inertite, from the vitrinite-rich vitrite and clarite microlithotypes proved to be markedly rank dependent. In the higher rank coals vitrite and clarite were concentrated in the clean coal while inertite was concentrated in the clean product in the high volatile C coals. Whereas in gravity-based coal cleaning only the finest pyrite usually remains in the clean coal, in this study no consistent trend in pyrite-size or pyrite-quantity partitioning was noted.     

Characterisation of an Egyptian coal by Mossbauer and FT-IR spectroscopy

Six coal samples collected from Maghara Coal Mine, North Sinai, Egypt, at different depths (52-87 m) were characterised by Mossbauer (MS) and Fourier Transform Infrared (FT-IR) spectroscopy. The moisture and ash content and the ultimate analysis are given. The iron minerals were studied by MS. Jarosite (ferric sulphate) and pyrite have been found in the sample taken at 52 m. However, ferrous sulphate and pyrite are present in the other samples. On the other hand, several structural parameters such as H_ar/H_al and H_ar/C_ar were calculated from FT-IR spectra as a function of depth. These parameters give a quantitative determination of the aromaticity, which was found in the range between 0.46 and 0.59, similar to that reported in the literature for subbituminous coals.     

Correlation of gasification rates of various coals measured by a rapid heating method in a steam atmosphere at relatively low temperatures

Twenty-five kinds of coals (carbon content on dry ash-free basis, C[%], ranges from 65.0 to 92.8%) were pyrolysed and gasified simultaneously by use of a rapid heating method (heating rate ≈ 1600 K min⁻¹) in steam at temperatures between 750 and 850 °C to clarify the factors which control the gasification rates of various coals. The relationships were examined in detail between the reactivity of each coal, represented by the initial gasification rate − r_cm0, and various properties such as pore surface area of char, ultimate and proximate analyses of coal, reflectance of coal, contents of metals in char, and the amount of oxygen trapped by char. For gasification at 800 °C, the relation between − r_cm0 and the carbon content C[%] changed abruptly at C ≈ 75–80%. For higher rank coals (C > 75–80%), − r_cm0 was rather small and was well correlated by C[%]. On the other hand, the plot of − r_cm0versus C % scattered largely for the lower rank coals (C < 75–80%). For these coals, the rate of CO₂ formation was much greater than that of CO formation, and was almost proportional to − r_cm0. The CO₂ formation reaction is known to be catalysed by alkali or alkaline earth metals such as Na, K and Ca. Then the reactivities of lower rank coals were supposed to be controlled mainly by the catalytic effect of the minerals in the coal.     

Carbon-13 CP/MAS spectroscopy of coal macerals

¹³C n.m.r. spectra of kerogen concentrates isolated from several different subbituminous to high-volatile bituminous coal macerals have been obtained by a combined cross polarization/magic-angle spinning technique. The samples comprise three vitrinites, two sporinites, two alginites and one fusinite, all of Upper Paleozoic age. It is shown that this technique can be used to differentiate the maceral types by providing characteristic spectral fingerprints. Aromaticities decrease in the order fusinite vitrinite sporinite alginite, as expected with the rank range studied. Furthermore, fine spectral details provide general information on the nature and distribution of discrete structural moieties and their variations with both type and rank.     

Coal flash pyrolysis: 4. Polymethylene moieties in coal macerals

An analysis for polymethylene moieties in purified coal macerals has shown the (CH₂)_n (wheren?5) content, by a liquefaction—¹ H n.m.r. method, to be highest in exinite macerals and lower in vitrinite and inertinite macerals. From any particular coal, the (CH₂)_n content of the vitrinite was higher than the inertinite but lower than the exinite. Pyrolysis of these macerals gave ethylene yields proportional to the (CH₂)_n content.