Coal sulfur forms

The types of sulfur in coal, as well as their distribution and reactivity, have a profound impact on the efficiency of desulfurization processes. For practicality, coal sulfur forms are commonly classified as sulfate sulfur, pyritic sulfur, and organic sulfur. Pyritic and organic sulfur account for almost all the sulfur in coals. Sulfate sulfur is usually much less than 0.1% in freshly mined coals, and increases as the coals are exposed to the atmosphere or "weather". 

The selectivity toward sulfur compound structure was differently induced for the CB1 culture, than that of the CB2 culture. In the first case DBT was used as the sole sulfur substrate, while in the second case, biphenyl sulfide was employed. One might deduce then, the application of the CB1 strain for oil and CB2 strain for coal and heavy oils. Plasmid characterization and strain catalytic evaluation results indicated that oxidation of DBT by CB 1 is not plasmid mediated and that inorganic sulfur forms cannot be oxidized by CB 1. 

Direct Determination and Quantification of Sulfur Forms in Heavy Petroleum and Coal... 

This work has demonstrated that organically bound sulfur forms can be distinguished and in some manner quantified directly in model compound mixtures, and in petroleum and coal. The use of third derivatives of the XANES spectra was the critical factor in allowing this analysis. The tentative quantitative identifications of sulfur forms appear to be consistent with the chemical behavior of the petroleum and coal samples. XANES and XPS analyses of the same samples show the same trends in relative levels of sulfide and thiophenic forms, but with significant numerical differences. This reflects the fact that use of both XPS and XANES methods for quantitative determinations of sulfur forms are in an early development stage. Work is currently in progress to resolve issues of thickness effects for XANES spectra and to define the possible interferences from pyritic sulfur in both approaches. In addition these techniques are being extended to other nonvolatile and solid hydrocarbon materials. 

Other important chemical and physical tests performed to characterize coal include (I) Heating value (Btu content) (2) sulfur forms (31 ash fusibility temperatures (4) ash analysis (5) trace elements (6) free swelling index and (7) hardgrove grindability. 

Sulfur in Coal. Sulfur occurs in coal in two forms (I) organic sulfur, which is chemically bonded to the coal, and (2) pyritic sulfur, which occurs... 

Pyritic sulfur can be removed by froth flotation, which takes advaniage of the differences of specific gravity of the two types of sulfur. To be effective, the coal must be pulverized into particles in the micron region. The process can be enhanced by adding limestone, catalyst, und soda ash to the coal dust. After treatment, the coal is formed into briquettes for ease of handling by convenlional conveyors. 

In the determination of sulfate, 2 to 5 g of the analysis sample is mixed with HC1 (2 volumes concentrated HC1 + 3 volumes of water), and the mixture is gently boiled for 30 minutes. After filtering and washing, the undissolved coal may be retained for the determination of pyrite sulfur, or it may be discarded and a fresh sample used for pyrite sulfur. Saturated bromine water is added to the filtrate to oxidize all sulfur forms to sulfate ions and ferrous ions to ferric ions. After boiling to remove excess bromine, the iron is precipitated with excess ammonia and filtered. This precipitate must be retained for the determination of nonpyrite iron if a fresh sample of coal was used for the determination of the pyrite iron. The sulfate is then precipitated with ISaCE, and the BaSC>4 is determined gravimetrically. 

Sulfur forms analytical percentage by weight of coal sulfate, pyrite, and organic sulfur. 

Sources of Sulfur in Coal and Oil. The major coal beds of eastern North America are of Pennsylvanian age. During that time, there was a constantly fluctuating sea level across flat lowlands over the North American interior. Coal was formed just before the onset of marine conditions, so that coal swamp forests occurred on broad lands along or near the sea shore. Thicker sections accumulated on the more rapidly subsiding Illinois and Forest City basins and in the Appalachia fireland basin (14). 

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. 

This work has demonstrated for the first time that organically bound sulfide and thiophenic sulfur forms can be distinguished and in some manner quantified directly in model compound mixtures and in petroleum asphaltenes and coal. The use of the third derivative XANES spectra was the critical factor in allowing this analysis. 

The tentative quantitative determinations of sulfur forms appear to be consistent with the chemical behavior of the asphaltene and coal samples. Further work is in progress to extend these techniques to other nonvolatile and solid hydrocarbon materials. 

It is generally accepted that there are several forms of mineral sulfur, other than sulfate and pyritic, present in coals. Mossbauer studies of inorganic sulfur forms in coal have shown the presence of pyrrhotite (FeS) (2), and the presence of other sulfides such as sphalerite (ZnS), and chalcopyrite (CuFeS2) have been reported by other workers (3). Several investigators have reported as much as 1% sphalerite in some northwestern Illinois coals (4-6). The presence of elemental sulfur has also been reported (7-9V The failure to account for these mineral sulfur forms results in high values for organic sulfur using the ASTM method. 

It is evident that detectable quantities of sulfur form in the pristine coals in less than two months. The rate of formation of sulfur appeared to be enhanced significantly when the coal was suspended above water in a closed desiccator. Clearly, a broad array of factors such as particle size, air currents and so forth will influence the rate of sulfur production, but there is no doubt that it is a facile process. 

Figure 2 shows the results of the pyrolysis experiments conducted with the Spanish lignite at 750-960°C at residence times of 0.52-0.72 sec. It is seen that under the pyrolysis conditions used, 60 - 70% of the sulfur in this coal appears in the gaseous products as H2S, COS, and CS2. As in the previous sulfur study (1), the principal sulfur gaseous product at all temperatures is H2S, with some CS2 formed at T >840°C. The CS2 is apparently formed at the expense of the H2S, by any of several reactions H2S may react with the carbon of the coal and/or the methane evolved in the pyrolysis of the coal to form CS2- A small amount of COS is detected at all temperatures trace amounts of SO2 are also detected. Moreover, the total sulfur yield appears to reach a maximum about 900°C. The decrease in sulfur volatilization as pyrolysis temperature is increased above 900°C is attributed to sulfur retention in the char due to the reaction of H2S with coke or char to form more stable thiophenic structures (2). GC/MS analysis of the tars (diluted to 10 ml) from the pyrolysis at 750 and 850°C did not reveal any sulfur-containing structures. Tars from the pyrolysis at 900 and 950°C, however, contain dibenzothiophene. 

Until recently, there have been only a few reports of aliphatic sulfur structures in coal (1,12). These results together with the experiments of Gorbaty et al. (11), give further support for the presence of labile (presumably aliphatic) sulfur moieties in high-organic sulfur-containing coals. In the previous sulfur study (1), one bituminous coal was analyzed for organic sulfur forms both by the... 

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. 

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