Sulfur coal macerals

Direct Characterization Techniques. The in situ analysis of elemental composition of coals by ion microprobe was first demonstrated by Dutcher t al. (85). Raymond (86) has applied this technique to examine the variation in composition of coal macerals which has been especially effective for looking at sulfur distribution. An example of the organic sulfur distribution for two bituminous coals is shown in Table II which is taken from reference (86). Note that the liptinites contain the... 

Table II. Distribution of Organic Sulfur in Bituminous Coal Macerals (86)...

Characterization of Organic Sulfur Compounds in Coals and Coal Macerals... 

Although the selective oxidation of coal has been extensively studied (31-33, surprisingly little has been reported about sulfur species in the oxidation products. Even less is known about the distribution of organic sulfur species between different coal macerals despite the fact that this information is important for the development of any future desulfurization technology. 

Spatial Variation of Organic Sulfur. The excellent spatial resolution of focused electron beams offers the possibility of examining variation of organic sulfur within macerals. The electron microprobe and scanning electron microscope allow resolution of a few microns (14-16). The transmission electron microscope allows even better resolution (less than 1 pm) because the thin foils and powders produce less electron scattering. We have used this capability to measure the distribution of S in a number of coals. 

The size and distribution of pores and the size, distribution, and identity of minerals in coal specimens from an eastern Kentucky splint coal and the Illinois No. 6 coal seam were determined by means of transmission electron microscopy (TEM) and analytical electron microscopy (AEM). The observed porosity varies with the macerals such that the finest pores (<2-5 nm) are located in vitrinite, with a broad range of coarser porosity (40-500 nm) associated with the macerals exinite and inertinite. Elemental analyses, for elements of atomic number 11 or greater, in conjunction with selected area diffraction (SAD) experiments served to identify the source of the titanium observed in the granular material as the mineral rutile. Only sulfur could be de-tected in the other coal macerals. Dark-field microscopy is introduced as a means for determining the domain size of the coal macerals. This method should prove useful in the determination of the molecular structure of coal. 

Inorganic elanents can be included in coal as minerals or as elanents incorporated in the organic structure (Chapters 3, 7, and 10). The most common example of the latter is the incorporation of sulfur into macerals as organic sulfur. Minerals can be incorporated into the peat during deposition, result from epigenetic processes, or be the consequence of metamorphic changes within the coal. 

H/C = atomic hydrogen-to-carbon ratio V = vitrinite content of coal VM volatile matter St = total sulfur TRM = total reactive macerals The adequacies of these reactivity correlations, expressed as a percentage of the total variation in the data set explained by the model, were 80.0%, 79.2%, and 47.5% respectively. A later paper in the series (21) concentrated on the development of reactivity correlations for a set of 26 high volatile bituminous coals with high sulfur contents, and extended the models previously developed in include analyses of the liquefaction products and coal structural features. These structural features included the usual... 

Peter H. Given Whereas Tschamler and Fuks, and Peover studied more or less pure vitrinites, Mazumdar apparently worked with whole coals. Moreover, Indian coals, being from Gondwanaland strata, are most probably of very different petrographic composition compared with European and North American coals (rich in exinites and inert macerals See p. 284). Quite apart from the question whether sulfur dehydrogenation really is free of side reactions, there may well be a spread of data at any level or rank because of petrographic differences. 

Figure 5. Distribution of sulfur compounds determined from Py-MS for five maceral samples. The vitrinite and sporinite are from the Brazil Block Seam coal.

This paper summarizes our understanding of the geochemistry of sulfur in coal in the following areas 1) abundance of sulfur in coals of major coal basins in the U.S., 2) distribution of sulfur in coal lithotypes and macerals, 3) characteristics and geochemical significance of sulfur-containing organic compounds,... 

In view of these shortcomings we have combined the need to characterize organic forms of sulfur with the recent progress obtained in the separation of coal into its single maceral fractions (34). This affords an opportunity to compare the sulfur chemistries of individual macerals with that of their parent coals. 

Determination of the Effects of Separation Processes on Organic Sulfur Forms in Model Compounds. Three substituted dibenzothiophenes were subjected to the coal preparation and maceral separation processes. The model compounds used are shown below. 

Treatment of Model Compounds. As anticipated, each of the sulfur-containing model compounds were recovered quantitatively after their exposure to the micronization, acid treatment and floatation processes used in coal and maceral preparation. In all cases the proton and carbon-13 NMR spectra, the FTIR spectra and the melting points of the recovered materials matched those of the starting materials, indicating that these sulfur species remain unchanged during the processing. 

Although many of the oxidation products are common to all of the samples analysed, their distribution varies considerably from sample to sample. In addition, there are some oxidation products that appear exclusively in the FID traces of some samples. For instance, there are compounds in sporinite and inertinite samples which do not appear in the FID trace obtained for their parent floated coal. The absence of these compounds in the FID traces of the floated coals is explained by the presence of the more abundant maceral vitrinite, the oxidation products of which either swamp or dilute those from the lesser macerals, making their detection very difficult. Here we see how maceral separation is important for the characterization, not only of the individual macerals themselves, but of the whole coal. Observation of sulfur constituents that are unique to minor macerals components may be difficult to detect during the analysis of a whole coal, but are easily observed during analysis of individual macerals. 

Once again we see sulfur compounds present in some macerals but not in others, and indeed sulfur compounds in macerals which appear to be absent in the floated (unfractionated) coal. For instance, sulfur compounds III and IV contribute significantly to the FPD traces of oxidized Herrin No.6 floated and vitrinite samples and the Indiana No.5 sporinite sample, but are very weak or apparently absent in the remaining samples. Also sulfur compound number VII appears in the Herrin No.6 inertinite sample but no other. We attribute the fact that certain sulfur compounds appear in the sporinite and inertinite chromatograms but not in the chromatograms of their parent floated coals to the presence of a large excess of vitrinite in the floated coal samples. 

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