Petrographic coal analysis

Subsequent to the petrographical coal analysis, both a chemical and a chemo-physical investigation were carried out. Figure 7 shows the chemical and physical properties of the investigated brown coal lithotypes. 

Coal petrography (Chapter 4) has become widely used for predicting coke quality based on coal analysis and has led to a system for predicting coke stability based on petrographic entities and reflectance of coal (Schapiro and Ciray, 1960). Thus, an optimum blend of coals could be selected to produce desired coke quality. 

Seam correlations, measurements of rank and geologic history, interpretation of petroleum (qv) formation with coal deposits, prediction of coke properties, and detection of coal oxidation can be deterrnined from petrographic analysis. Constituents of seams can be observed over considerable distances, permitting the correlation of seam profiles in coal basins. Measurements of vitrinite reflectance within a seam permit mapping of variations in thermal and tectonic histories. Figure 2 indicates the relationship of vitrinite reflectance to maximum temperatures and effective heating time in the seam (11,15). 

A variety of instmmental techniques may be used to determine mineral content. Typically the coal sample is prepared by low temperature ashing to remove the organic material. Then one or more of the techniques of x-ray diffraction, infrared spectroscopy, differential thermal analysis, electron microscopy, and petrographic analysis may be employed (7). 

England, B.M. Mikka, R.A. Bagnall, B.J. Petrographic Characterization of Coal Using Automatic Image Analysis, J. Microscopy. 1979, 116, 329-336. 

Because SEM-AIA is often used to explain behavior under specific processing conditions, samples are prepared in the same size in which they are received. Coal samples with their included mineral matter are prepared for image analysis by mixing samples. of the dry coal with polyethylene powder (as a diluent) and molten carnauba wax in a volume ratio of 1 2 2. Pellets are then cut along the cylindrical axis to expose a vertical cross section of coal and mineral matter and polished using standard petrographic procedures. The surfaces are coated with 150 A of carbon to provide a conductive surface for SEM examination. 

Berry, W.F. Cole, D.L. Preparation and Polishing of Coal and Coke for Petrographic Analysis. Bituminous Coal Research Inc., Pennsylvania, 1965. 

Fluidity data were determined by the Gieseler apparatus on the 16 size fractions of the medium volatile coal. The values obtained were relatively low, ranging from 8 to 81 dial divisions per minute. An attempt was made to correlate these results with petrographic data. Some of the possible relations examined are shown in Figure 4. A plot of the maceral vitrinite against fluidity shows considerable scatter as does a plot of the ash content vs. fluidity. The ash content was determined by proximate analysis (see Table I) and is included here for comparison. 

All coals submitted to BCR for petrographic analysis are routinely checked for oxidation by comparing as-received analytical results with the established fresh coal R -volatile matter curve shown in Figure 1. Coals falling within reasonable limits of this correlation line (within Sy = 2.3%) are accepted as unoxidized all others are considered oxidized. 

Briquets prepared at BCR for petrographic analysis are maintained in a desiccator for an extended period prior to microscopic analysis. Specific high moisture coals have been analyzed for reflectance at regular intervals following their insertion in the desiccating atmosphere under these conditions of continued desiccation, reflectance remained relatively constant. 

Oxidized coal coal whose properties have been modified fundamentally as a result of chemisorptions of oxygen in the air or oxygen dissolved in ground-water. Chemisorption is a surface phenomenon rarely detectable by chemical analysis but usually detectable by petrographic examination. It reduces the affinity of coal surfaces for oil and seriously impairs coking, caking, and agglutinating properties. 

The most recent petrographic method used to characterize coal macerals is quantitative fluorescence analysis. In this method the macerals are excited by incident ultraviolet light and the spectrum of the resulting fluorescent light is used to characterize the macerals. This technique has led to the discovery of new macerals (18), the quantitative discrimination between certain macerals in a given coal (19), and the correlation of the fluorescence properties of macerals to the rank, and technological properties of coal (20-22). 

It is for these reasons that we have initiated this correlative study of peat petrography and peat industrial-chemical (coal quality) properties. Note that the information reported herein represents preliminary results based on a limited number of different types of peats that were analyzed for only a few coal quality tests (i.e., proximate analysis, ultimate analysis, and BTU content). Future studies will involve measurement of other petrographic parameters and include other industrial analyses (such as, gas and liquid yields, physical properties, organic chemical yields, and so forth). 

Although the characterization of coal macerals on the basis of their fluorescence spectra is a recent innovation, it has already proven to be an excellent fingerprinting tool for the various macerals. In some cases, it is even more sensitive than normal petrographic analysis. The initial results of fluorescence spectral studies show that the various fluorescent macerals in single coals can be statistically discriminated on the basis of their spectral parameters and that even varieties of a single maceral type can be distinguished. Although the spectra obtained at this time are rather broad and not suitable for chemical structure analysis, the potential for structural analysis exists and may be realized with improvements in instrumentation. 

Part of this data in Table II is a series of British maceral concentrates. The Woolley Wheatly Lime sample is 93% fusinite while the Teversal Dunsil concentrate is 80% semifusinite with 13% fusinite. The Aldwarke Silkstone sample contains 43% semifu-sinite and 43% fusinite. The petrographic analysis of PSOC-2 reveals nearly equivalent amounts of fusinite, semifusinite, micrinite, and macrinite (6.8, 8.1, 7.5 and 8.5% respectively in the whole coal) while PSOC-858 contains primarily semifusinite as the inertinite. The differences in faH values for these iner-tinite samples are greater than the experimental error and these differences suggest that NMR techniques may be useful in characterizing the chemical structural differences between inertinite macerals. 

Petrographic analysis of the separated macerals, density determinations, and elemental analyses were performed at Argonne National Laboratories. The ash content of these samples is less than 1%. The oxygen levels reported here are obtained by difference. Computer correlations of the resulting parameters were done using the Statistical Analysis System on the VS/CMS system at the ER E-Linden site. Linear-regression analyses are also performed with that system. In the correlation plots which follow, samples will be identified by coal rank and maceral group. 

This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, U.S. DOE under contract W-31-109-ENG-38. The authors thank K.L. Stock for the maceral separations, G.R. Dyrkacz for the petrographic analysis and helpful discussion, and VI. Spackman, Penn. State University for choosing and providing the coal samples. 

Table II. Petrographic Analysis of Maceral Concentrates from PSOC-828 and PSOC-1103 Coals.

Given, et. al. (2) have reported the molecular parameter analysis of asphaltenes obtained from coals of variable petrographic compositions based on proton NMR analysis. However,... 

Alginites were obtained from selected Permo-Carboniferous torbanites (i.e. algal-rich coals) which, upon petrographic analysis, were found to be rich in the required component. No concentration steps were required for samples used in this work as small blocks were selected in which the initial alginite concentrations were adequate (see Table I). 

Powell T. G., Boreham C. J., Smyth M., Russel N., and Cook A. C. (1991) Petroleum source rock assessment in nonmarine sequences pyrolysis and petrographic analysis of Australian coals and carbonaceous shales. Org. Geochem. 17, 375-394. 

It is interesting that a bituminous coal (Sample 4) gave organic acids qualitatively similar to those of lignite coal see Figure Id). Major identified compounds were p-hydroxybenzoic acid and two isomers of hydroxybenzenedicarboxylic acid, benzene di- and tricarboxylic acids. No ortho or meta isomer of hydroxybenzoic acid was detected. We have found that solvent-extractable hydrocarbons obtained from this raw coal consist mainly of n-alkanes (Cjj to 3 ). This is quite different from other results which showed that aromatic hydrocarbons were the major solvent-extractable material of several bituminous and anthracite coals (21). Indeed, petrographic analysis shows that this coal has a high content of sporinite (14.3 wt %) and a low content of vitrinite (30.2 wt %) (33). 

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