Rasa coal

When energy alternatives are available, a compromise between cost and quaHty is often realized. Blending of coals can be used to achieve more desirable quaHties. For example, lignite from the former Yugoslavia has been blended with, and even substituted for, the highly caking Rasa coal used for coke production in the iron (qv) and steel (qv) industries. 

The details of the XANES experimental setup and data analyses have been described previously 3b,8). All model compounds used in this study were obtained from Aldrich Chemical Company and were used without further purification. The asphaltene samples were prepared from their respective petroleum residua by precipitation from n-heptane following the procedure of Corbett (9). A sample of Rasa coal was generously provided by Dr. Curt White of the Pittsburgh Energy Technology Center. 

Figure 2. XANES of Rasa Coal Top XANES spectrum Bottom 3rd Derivative of XANES spectrum.

X-ray absorption spectra were recorded at the Stanford Synchrotron Radiation Laboratory, which is funded by the Department of Energy, under contract DE-AC03-82ER-13000, Office of Basic Energy Sciences, Division of Chemical Sciences and the National Institutes of Health, Biotechnology Resource Program, Division of Research Resources. The writers wish to thank Dr. Curt White (PETC) for providing a sample of Rasa coal. 

The utility of sulfur K-edge X-ray absorption spectroscopy for the determination and quantification of sulfur forms in nonvolatile hydrocarbons has been investigated. X-ray Absorption Near Edge Structure (XANES) spectra were obtained for a selected group of model compounds, for several petroleum asphaltene samples and for Rasa coal. For the model compounds the sulfur XANES was found to vary widely from compound to compound, and to provide a fingerprint for the form of sulfur involved. The use of third derivatives of the spectra enabled discrimination of mixtures of sulfide and thiophenic model compounds, and allowed approximate quantification of the amount of each component in the mixtures, in the asphaltene samples and the coal. These results represent the first demonstration that nonvolatile sulfide and thiophenic sulfur forms can be distinguished and approximately quantified by direct measurement. 

Assuming that the composition of the sulfur forms in the asphaltene samples and the Rasa coal is approximated by the simple two component mixture of dibenzothiophene and dibenzylsulfide models, an estimate of the relative molar quantities of sulfide and thiophenic forms can be obtained as described above and from Figure 3. These approximate values are listed in Table II. 

Figure 4. Sulfur XANES spectra of asphaltenes and Rasa coal.

The left panel shows the absorption spectra, and the right panel the corresponding third derivatives, from top to bottom, 1, 2, 3 and Rasa coal. 

The analysis indicates that 30% of the organic sulfur is sulfide and 70% is thiophenic in the Rasa coal sample. Some confirmation of these values comes from the work of Kavcic (20), who showed that about 75% of the sulfur in this coal was not reactive toward methyl iodide this lack of reactivity was attributed to the sulfur being bound in ring structures. 

Rasa coal has generated substantial interest over the years because of its high organic sulfur content. Information concerning the nature and distribution of organic sulfur moieties in coal is desirable for the design and evaluation of coal desulfurization processes. A major impediment to the character-... 

Kreulen reported that Rasa coal was a humic coal and contained no sapropelites, waxes, or resins (4). He noted that Rasa coal exhibits dual character, i.e., it exhibits both low- and high-rank characteristics. Chemical tests indicated that a small amount of the sulfur present is in side chains, and a large amount occurs in ring structures. Using a statistical structural analysis method based on density, van Krevelen computed that 59% of the carbon in Rasa coal is aromatic (4). 

Kavcic treated several samples of Rasa coal, hydrogenated Rasa coal, and the residue left after acetophenone extraction of Rasa coal with methyl-... 

WHITE ETAL. Organosulfur Constituents in Rasa Coal... 

Kavcics results on the forms of organic sulfur were confirmed by Ignasiak et al. (7), who determined that about one third of the sulfur in Rasa coal is in thioether links. This result was determined using two independent techniques. Ignasiak et al. also concluded that mercaptanic sulfur was absent from Rasa coal because unspecified chemical tests failed to reveal its presence. 

A single lump, approximately 2 kg, of Rasa coal was obtained from M. Eckert-Maksic of the Rudjer Boskovic Institute of Zagreb, Yugoslavia. The lump was randomly selected, and no special storage conditions... 

Petrographic studies were conducted using the air-dried pulverized coal mixed with a nonreactive epoxy binder. Air drying was performed at ambient temperature. The ASTM method specifies minus 20 mesh (minus 850um). The Rasa coal sample was finer. The mixture, which contained about 18 to 25 percent... 

A Leitz Ortholux microscope was used to determine the maceral content of Rasa coal. The microscope was fitted with an oil- immersion 60X fluoride objective and 10X high eyepoint oculars to give an effective magnification of 720 diameters. A point count system of analysis was used for the maceral determination. 

Four points were identified per field, and a total of 1000 counts were determined for Rasa coal. This system employs a point count stage and an ocular graticule. The volume percent of macerals was calculated according to ASTM D2799-86 (14). 

The mean maximum reflectance of vitrinite was determined in oil (ca. 1.517 index of refraction) and in 546-nm wavelength green light. A photomultiplier photometer was used to measure vitrinite reflectance. Two glass standards were used to calibrate the equipment. A Leitz MPV2 microscope photometer was used. The reflectance values were determined as designated in ASTM D2798-85 (2 5). Twenty-five vitrinite reflectance values were determined for the Rasa coal. 

The low-voltage, high-resolution mass spectra (LVHRMS) were obtained on a Kratos MS-50 high-resolution mass spectrometer interfaced to a Kratos DS-55 data system. The Rasa coal extract was vaporized directly into the ion source by a direct-insertion probe. Spectra were recorded at probe temperatures of 100°C, 200OC, 300°C, and 350°C. The instrument was scanned from 700 to 60 amu at a scan... 

The results of the proximate and ultimate analysis of Rasa coal appear in Table I. The results of the forms of sulfur analysis and carbon aromaticity are also included in Table I. The total sulfur on an as-received basis was 10.77 weight percent, and includes 0.02 weight percent sulfate, 0.30 weight percent pyrite, and 10.45 weight percent organic sulfur. Organic sulfur is calculated as the difference... 

The carbon aromaticity of Rasa coal was 0.65, which is not significantly different from van... 

The major elements present in Rasa coal ash were determined. The results are in Table II. The major elements present in the ash, Ca and Mg, are consistent with the X-ray diffraction findings of calcite and dolomite. The weight percent values in Table II do not and should not add to 100%. The sum of all the values reaches 100% when the weight percent of the minor elements are included. 

Rasa coal is exceptional because of its high sulfur content, most of which appears to be organic sulfur. The high organic sulfur content is characteristic of marine-influenced bituminous coals. Rasa coal appears to have been formed in a high-pH marine environment where bacteria thrived. This hypothesis is supported by the presence of substantial amounts of both calcite and dolomite in Rasa coal. The calcium- and magnesium-rich environment where Rasa coal formed is expected to have been alkaline. The... 

An alkaline marine environment high in H2S and HS" is favorable for incorporation of sulfur into organic matter HS" is an aggressive nucleophile. Such conditions also would have been favorable for the formation of polysulfides and elemental sulfur. All of these species, either alone or in combination, are expected to have played a role in the incorporation of sulfur into the vegetable debris that ultimately formed Rasa coal. Many of these species are known to be reactive with hydrocarbons at mild temperatures. Elemental sulfur reacts with hydrocarbons to form organosulfur compounds, including thiophenes at mild temperatures (28,29). Polysulfides react with conjugated ene carbonyls at room temperature to form thiophenes and other sulfur heterocycles (30). ... 

The elemental analysis of the Rasa coal extract appears in Table III, along with the values for the coal. The elemental analyses of the coal and of the extract are similar. This is consistent with the extract being representative of the organic matter in the coal. The nitrogen content of the extract is... 

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