Direct determination of organic sulphur in raw and chemically desulphurized coals

An electron beam technique has been developed for the direct determination of organic sulphur in coal, using a scanning electron microscope coupled with an energy-dispersive X-ray analysis system with quantitative data reduction capabilities. The direct measurement of organic sulphur is performed on a number of particles of the maceral vitrinite, previously located within the mounted coal sample by reflected light microscopy. The particles are first examined by SEM at ≈5000 × magnification to allow proper identification of most mineral inclusions and to avoid these areas during analysis. The X-rays generated from an area 3 μm in diameter are monitored quantitatively by EDX for sulphur and iron, as well as for calcium, aluminium and silicon. After ensuring that no other elements, indicating mineral matter, are present at the point of analysis, the sulphur value obtained is taken as a measure of the organic sulphur in coal. The results obtained by this technique also allow estimation of the intra- and inter-particle variation in the organic sulphur level. Statistical methods can be used to construct confidence bands about the mean sulphur levels and estimate standard deviations within and between particles. This permits evaluation of the precision of the analysis on a sample-by-sample basis and provides a measure of the heterogeneity of the coal. The SEM-EDX method has been applied to a large number of raw coals and coals chemically desulphurized by the Ames, PETC and Ledgemont oxydesulphurization processes, as well as the molten caustic (Gravimelt) process of TRW Systems Inc. The results of this direct technique agree fairly well with those of the ASTM procedure, for both raw and chemically processed coals, although for the latter the SEM values often show less desulphurization than the ASTM values.     

Study of sulphur in Assam coals by X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) has been used to study the nature and content of sulphur contained in coal samples from Assam. In each case two sulphur 2p (S 2p) peaks were observed. The 169–171 eV 2p peak corresponds to sulphate, whilst the 164–165 eV peak is assigned to S in iron sulphide compounds such as pyrite, marcasite, etc. and also organic sulphur. The complete absence of a 169–171 eV S peak from a coal sample from which both pyritic and sulphate sulphur were removed by hydrodesulphurization strongly indicates that the remaining 164.7 eV S 2p peak corresponds to the total organic sulphur present in the coal.     

Analysis of sub-micron mineral matter in coal via scanning transmission electron microscopy

The fine-scale mineral matter in three US coals has been analysed via scanning transmission electron microscopy (STEM). The samples observed were a North Dakota lignite, a Kentucky bituminous, and a Pennsylvania semi-anthracite. Specific mineral types, differing among the three coals examined, appear to predominate at this fine size scale (particles ? 200 nm in diameter). Fe-rich and Ba-rich minerals in the lignite, a Ti-rich mineral in the bituminous and Ca-rich and Ti-rich minerals in the semianthracite were the predominant species found. The inherent mineral content in the observed organic background also differed from coal to coal. The distributions of mineral species in the size range ? 200 nm reported herein do not reflect the distributions in the larger size ranges obtained by more macroscopic techniques.     

Determination of organic sulphur in raw and chemically cleaned coals by scanning electron microscopy and energy dispersive X-ray spectrometry

Scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDS) is a useful method to evaluate chemical cleaning processes and to obtain values of the organic sulphur remaining in cleaned coals. Pressed pellets can be used when the cleaning process has made the coal too heterogeneous to analyse by the conventional epoxy mounting method. The SEM-EDS analysis time is the same for both methods, whereas the preparation of pellets is much easier than that of epoxy mounts. The overall analysis time should therefore be decreased by using the pellet technique.     

Direct measurement of organic sulphur in coal

A method of direct determination of the organic sulphur content of coal has been developed using the transmission electron microscope. An electron beam is focussed on a thinned section of coal and X-rays emitted from the elements in the volume irradiated by the electrons are measured by X-ray emission spectroscopic methods. Both the characteristic K_α line of sulphur and the bremsstrahlung from the material in the irradiated volume are detected by a solid-state detector. An analytical method has been developed which uses these two measurements for a direct determination of the organic sulphur concentration. Minerals can easily be avoided since they are plainly seen by conventional electron microscope techniques. Several measurements of the organic sulphur concentration in coals have been made; they agree with the conventional chemical values reported for these coals. Of most importance, though, is the use of this technique to detect spatial variations of sulphur over small distances within the coal maceral.     

Studies of elemental distributions within discrete coal macerals: Use of secondary ion mass spectrometry and X-ray photoelectron spectroscopy

Secondary ion mass spectrometry has been used to measure qualitatively the distributions of many trace and minor elements within some well-characterized coal macerals: vitrinite, fusinite and exinite. Complementary information was obtained by using X-ray photoelectron spectroscopy. Within the vitrinite specimens studied, a widely-dispersed largely inorganic aluminosilicate phase was identified, which contained a number of other trace elements. By contrast, within the organic-rich phase of this macerai, titanium was detected in a very even distribution; this suggests that some titanium may be bonded in an organic complex. The fusinite was shown to contain elevated concentrations of calcium, magnesium and fluorine, dispersed over a large portion of the maceral. The high fluorine content could be identified as an organo-fluoro compound using XPS. Exinite was found to contain lower concentrations of most elements, compared to other macerals. The organic composition of this maceral was significantly different from that of surrounding macerals; this could be seen from the enhanced emission of the C₂H₂-secondary ion and images of this ion thus clearly outlined the position of such macerals.     

Study of Ti in solvent-refined coal by X-ray absorption spectroscopy

The chemical environment of titanium in solvent-refined coal (SRC-I and SRC-II) has been studied using X-ray absorption spectroscopy. Evidence that organic complexes of Ti are produced in the liquefaction process from mineral forms of Ti present in the feed coal is presented, based on the pre-edge structure of the X-ray absorption spectra.     

X-ray structural parameter for coal

It has been shown that the interatomic distance of ≈0.43 nm in coal is predominantly due to turbostratic-graphitic order. The height of the corresponding peak in the G(r) function which is the Fourier transform of the properly corrected and normalized X-ray diffractogram, has been investigated as a function of the concentration of volatile matter and the heat treatment of the coal. The results clearly show that this new parameter which is based on the whole diffraction pattern, is a useful aid in the study of coal, in particular when the structural changes within one coal are investigated.     

Determination of specific heats of coal powders by differential scanning calorimetry

Specific heats (C_p) of bituminous and subbituminous coals were investigated in the temperature range 300–360K by differential scanning calorimetry (DSC). To establish the validity of the procedure, specific heats of glass beads and graphites in powdered and bulk form were determined. Good agreement was obtained with the values for the specific heats of glass and graphites in the literature, and it was established that the specific heats were not dependent on the degree of diminution of these materials. Specific heats of coal samples were found to depend upon mesh size, temperature, rank, moisture content and whether the coal powder was wet- or dry-screened. However, there were only minor differences in C_p between bituminous and subbituminous coals.     

Characterization of coal and coal blends by automatic image analysis: Use of the Leitz texture analysis system

An analytical procedure is described for the microscopic determination of rank and maceral-group composition of seam coal and composition of coal blend using the image analyser TAS (Leitz). The procedure runs automatically and reduces the time of analysis by a factor of 0.1. The results agree with those obtained by conventional methods (reflectance measurements and point-count analysis).     

FTIR analaysis of coal. 1. techniques and determination of hydroxyl concentrations

The preparation of coal samples for the determination of hydroxyl content by FTIR is described. It is concluded that the FTIR method provides good quality infrared spectra on opaque materials such as coal and char. If sample preparation procedures are followed carefully, quantitative spectra of coals can be reproduced within ±5%. Beer's law is followed in the range 0.3–1.3 mg of coal/cm². The broad absorbance in the range 2000–3600 cm^?1 appears when using several different preparation techniques and is caused by hydrogen-bonded OH. The use of quantitative FTIR specta offers a simple method of estimating the hydroxyl content of a coal. Reasonable results have been obtained despite the problem of interference by water. The KBr pellets were dried for 48 h at 105 °C and the absorbance was reduced at 3200 cm^?1 and the problem of scattering was minimized by using a rectilinear baseline. With these procedures the hydroxyl oxygen concentrations in the coal were determined to an accuracy of ±10%.     

Determination of organic oxygen content of coals

Fast neutron activation analysis has been used to determine organic oxygen (O_org) content of coals by subtracting the oxygen content of the mineral matter from the total oxygen content of the coals. Mineral matter was isolated by low temperature ashing in an oxygen plasma. Optimum ashing conditions produce minimal changes in chemical composition of mineral matter; these changes were taken into account when calculating the O_org content. The O_org contents of whole coals are substantially different — in some cases by as much as 47% — from the ASTM oxygen contents and those calculated on a dry, mineral matter free basis from the ultimate analysis data. Excellent agreement between the FNAA O_org contents of the whole and demineralized coals lends support to the reliability of this experimental approach.     

Comparison of microwave and radiofrequency low-temperature ashing of coal using Fe Mössbauer spectroscopy of sulphur forms

Low-temperature ashing of coal was carried out by the normal radiofrequency method and a new microwave technique to compare the effects of the two procedures on the Fe-S compounds in coal. Results from ⁵⁷Fe Mössbauer spectroscopy and chemical analyses indicate that the microwave technique, like the normal LTA procedure, effectively ashes the coal without the oxidation of pyritic sulphur. Dehydration of iron sulphates and oxidation of Fe²⁺ to Fe³⁺ in sulphates occurs in both ashing procedures under the conditions tested, the sequence being FeS0₄.4H₂O → FeS0₄.H₂0→Fe³⁺. Dehydration is caused by sample heating rather than by reaction with the activated oxygen. The combination of the Mössbauer method and chemical analysis has allowed the determination of the recoilless fraction ratio f(sulphate)/f(pyrite) = 0.55 ±0.15, which puts the Mössbauer results on a more quantitative basis than before. This is thought to be the first quantitative work on coal and coal products that has accounted for differences in f-values for different Fe compounds.     

Application of fluorescence microscopy to investigation of coal hydrogenation residues

The application of incident-light fluorescence mictoscopy to the study of coal hydrogenation residues has to date received little attention. However, not only does this technique permit the clear observation of very low reflectance material that is difficult to observe by incident white-light microscopy, but it also provides a guide to the relative reactivity of particles during the hydrogenation process.     

Determination of aromatic and aliphatic CH groups in coal by FT-i.r.: 1. Studies of coal extracts

A set of coal extracts have been characterized by FT-i.r. and proton n.m.r. spectroscopy. The n.m.r. measurements of aliphatic and aromatic hydrogen allow a direct calibration of the absorption coefficients of appropriate i.r. bands. This allows methods for calculating these coefficients based on relating i.r.-band intensities to elemental hydrogen to be tested directly. Reasonable agreement was obtained.     

Some aspects of the analysis of coal by X-ray fluorescence spectroscopy

Bituminous coal samples from 84 distinct sources were analysed by conventional British Standards (BS) methods for phosphorus, sulphur, chlorine, ash and the ash-forming elements (calcium, silicon, aluminium, iron, sodium, magnesium, potassium, titanium and manganese). In most cases four determinations were made per element per source. Samples were crushed to ?212 μm (72 BS mesh) and pressed into aluminium cups at 20 t in^?2 (309 MPa) without binder or backing. Two pellets from each sample were analysed by wave-length dispersive X-ray fluorescence techniques, using a spectrometer equipped with a rhodium rube; the X-ray count took 60 s. The X-ray results were calibrated against the conventional results using multiple regression. The accuracy obtained was comparable with routine ‘wet chemical’ techniques. The X-ray technique is suitable for the routine determination of large numbers of samples.     

Mössbauer studies of iron-bearing minerals in coal and coal ash

⁵⁷Fe Mössbauer spectroscopy has been used to characterize the iron-bearing minerals present in some British Columbia coals. The minerals observed in a range of samples include pyrite/marcasite, siderite, jarosite, rozenite or melanterite, and Fe²⁺ -bearing clay minerals such as illite. The presence of jarosite is confirmed through the magnetic hyperfine interaction observed at 4.2 K. The absence of magnetic ordering at 4.2 K for the ferrous sulphate component indicates the presence of rozenite, FeS0₄ · 4H₂0, or melanterite, FeS0₄ · 7H₂0, rather than szomolnokite, FeS0₄ · H₂0 or FeS0₄. The effects of ashing on two of the coals is also studied.     

Effectiveness of SO2 sorbents by thermogravimetry-combustion with coal

A new experimental method is described for non-isothermal thermogravimetry (TG) involving combustion of mixtures of sieved coal with sieved calcium-containing sorbents. This rapid TG method utilizes a baseline for TG combustion of coal alone, derives an equation that gives a semi-quantitative measure (±10% repeatability) of the coal's reactive sulphur retained by the sorbent, the extent of retention of S0₂ generated in situ during combustion varying with different sorbents. The method permits direct variation in separate experiments of the calcium-to-sulphur ratio during combustion and relative ranking of different sorbents by retention of reactive sulphur in combustion. Relative rankings are presented for three pre-calcined natural stones (two limestones and one dolomite); these results correlate with relative rankings from another TG method reported in the literature. It is suggested that this new method is useful for pre-screening the effectiveness of S0₂ sorbents considered for use in fluidizedbed combustion of coal.     

Proton dynamic polarization and proton-carbon cross-polarization in coal

The n.m.r. proton signal in coal can be enhanced by irradiating near the larmor frequency of the free electrons present in the coal. Using proton-carbon cross-polarization, this enhancement is transferred to the ¹³C signal. This decreases the measuring time of a ¹³C spectrum with several orders of magnitude.     

Further studies of coal oxidation

The low-temperature oxidation of coal has been investigated by Fourier transform infrared spectroscopy (FT-i.r.). Samples were oxidized at 140 and 60 °C for intervals of a few hours to days. These FT-i.r. results were correlated to changes in the thermoplastic properties of the coal measured by a Gieseler plastometer. The loss of Gieseler fluidity as a function of oxidation time corresponds to loss of aliphatic CH groups. Curve-resolving methods were used to distinguish between various C = 0 functional groups. At both oxidation temperatures there was an overall increase in carbonyl and carboxyl groups, but in different relative proportions depending on the oxidation temperatures.