Organic sulfur, distribution

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 Organic Sulfur Distribution in Various Coals(I)...

Thiophene (C4H4S) is representative of the organic sulfur compounds that are hydrogenated in the commercial hyditodesulfurization of petroleum naphtha. Estimate both the combined and effective diflfusivities for thiophene in hydrogen at 660 °K and 3.04 MPa in a catalyst with a BET surface area of 168 m2/g, a porosity of 0.40, and an apparent pellet density of 1.40 g/cm3. A narrow pore sized distribution... 

It has been suggested that organic sulfur is uniformly distributed throughout a coal sample, on the basis of solvent extraction work (30). For example, the sulfur content of extracts produced with various solvents is about the same m one... 

The sulfur distribution from the coal conversion plant is shown in Figure 4. The wt% of sulfur remaining in the ash depends on several factors among which are the relative distribution of organic and inorganic sulfur in the coal and the chemical composition of the ash. High alkaline ashes will capture sulfur as sulphide or sulfate. 

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

Application of these methods showed that the distribution of types of organic sulfur compound(s) (OSC) in crude oils is variable. In other words, the ratio of thermally labile to more resistant sulfur has a large range in crude oils. Geochemically, the large range is expected and understandable because of variations in source, maturity and other alteration processes. 

Studies of sulfur-containing organic compounds in coal provide information on the origin of organic sulfur and the transformation of sulfur compounds during coal maturation. The distribution and types of organic sulfur compounds in coal have profound implications for desulfurization processes. 

As already assumed the intramolecular incorporation of sulfur into the organic matter is not a random process (20).The distribution of the sulfur compounds will depend on the chemical capacity of the molecules present during early diagenesis in each kind of organic matter to accept an inorganic sulfur atoms. These two different types of organic matter enable the intramolecular incorporation of sulfur in different molecules, and consequently result in different distribution patterns of organic sulfur compounds. 

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". 

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-... 

Distribution of Organic-Sulfur-Containing Structures in High Organic Sulfur Coals... 

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. 

Solid solutions in metallic alloys are normally compositionally very uniform random variations of 5% would be unusual. Also, a two phase layer normally is found between two solid solutions. The sulfur distribution in coal seems not to behave this way. Apparently, the distribution pattern established at some early stage of coal formation is frozen-in and the organic sulfur is bound so tightly to its hydrocarbon sites that it cannot diffuse until the temperature of the coal is raised to 400°C or above (18). 

Figure 4. Distribution of organic sulfur within volumes of about 1 im as a function of sulfur content. Illinois 6.

Organic Sulfur in the Vicinity of a Pvrite Particle. Distribution of organic sulfur is not uniform through a coal, even within a particular maceral. But the variations about the average value in a particular maceral are small (fractionally) and seemingly random. But the question may be asked as to the "constancy of the organic sulfur content in the immediate vicinity of a pyrite particle. 

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