Coal oxidative degradation with

It seems likely that aromatic amines which are found in liquefaction products have been produced by a combination of thermolysis and hydrogenation. There is no evidence for aromatic amines in coals from either selective oxidation degradations (22) or from direct X-ray Photoelectron Spectroscopy measurements (23). Oxidations would produce very stable nitroaromatics which are not seen. Another possible structure for this formula is phenoxazine(Vb). Such a molecule would not survive high temperature combined with long reaction times. Although annelated thiophene with a pyrrole(VI) would appear to be a likely structure in coal, there is no evidence for its existence in any of the coal derived materials. 

Figure 2. Thermal oxidative degradation of untreated Indiana Minshall Seam (HvCb) coal with and without a diluent...

Figure 4. Thermal oxidative degradation of untreated and oxydesulfurized Montana Rosebud Seam (Sub B) coal with diluent...

We recognized at least two potential difficulties with this oxidative degradation procedure (1) Mass transfer can serve to broaden the observed sulfur dioxide evolution as compared with the undiluted coal. (2) At the low flow rates utilized, variation in the exit flow rate as the degradation proceeds could cause a broad sulfur dioxide evolution to appear as peaks. Results of the two experiments described below indicate to our satisfaction that these potential problems, if present, do not interfere with our interpretation. 

This picture has been broadly validated, and in some respects refined, by other work in which coal was depolymerized by acid-catalyzed transalkylation [as by interaction of coal with phenol and BF3 (23-27)] or by similar, less clearly defined, phenolation reactions (28-31), or selectively degraded by specific oxidants, such as dichromates (32-34), hypohalites (35-38), or peroxy-acids (39-43). But these studies have also revealed some previously unsuspected features. Buffer-controlled oxidation with Na2Cr207 (34) and KMn04 (44) have indicated an occasionally significant presence of straight-chain (up to C21) and branched-chain (up to C8) aliphatic compounds in coal. Oxidation with performic acid (41-43) has yielded substituted compounds that are clearly related to the microbial or chemical degradation products of lignin or flavonoids. And when applied to supposedly very similar coals, virtually identical depolymerization or oxidation procedures often furnished distinctly different product slates (45). 

However, the diversity of the oxidants renders the oxidation of coal very complex because the experimental parameters can vary widely (Speight, 1987). The diversity in the structural types in coal (which vary not only with rank but also within the same rank) causes many problems associated with studies of the oxidative degradation of coal. For example, optimal conditions of time, temperature, and ratio of oxidant to coal can only be determined when several experiments are performed for each oxidant. Furthermore, the presence of the mineral matter must also be considered to be an integral part of coal oxidation since mineral constituents may change the chemistry of the oxidation. If coal is pretreated with hydrochloric acid to remove mineral matter prior to oxidation, the actual oxidation reaction may be more facile. 

With respect to coal emissions, mercury, for example, was found to exist principally (as much as 96% ) in the elemental form (30). Previously, it was argued by some that ultraviolet radiation transformed it to the less toxic mercuric oxide (30). Sunlight tends to degrade mercurial compounds to the elemental form (47). Beryllium emissions from coal combustion may be in the nontoxic elemental form (46), but this is not known for certain. Fluoride, which is generally assumed to be 100% volatized (19), may be trapped with lime in particulates (33), but this also is questionable. Highly toxic nickel carbonyl (48) and arsine (49) emissions have not been reported to date, although the former is a distinct possibility (50). 

If it is also recalled that alkali soluble material (humic acid) builds up much more slowly than acidity (and always markedly dependent on T and [O]), and that the distinctly acidic parent coal is effectively insoluble in alkali, it becomes evident that acidity and alkali solubility are not necessarily covariant, and that accepted definitions of humic acid are, chemically speaking entirely arbitrary. Under the conditions of this study the oxidation appears to involve two simultaneous but seemingly unrelated reactions which result in the development of acidity and in molecular (skeletal) breakdown, respectively, and this suggests that alkali solubility is mainly a consequence of degradation which is only coincidentally connected with the formation of acidic functional groups. Figure 20 illustrates this concept qualitatively and leads to the inference that the wide spread in molecular weights of humic acids reported... 

Further developments of the work include a more accurate study of the mechanisms of desulfurization processes using instrumental improvements. This will enable an easy quantitation of gas yield and a thermochemical approach of elemental processes. We also have been using model polymers to better study the interactions of pyrite and sulfur with the organic matrix during coal pyrolysis, oxidation and combustion (34 and to examine more accurately the specific role of organic sulfur in thermal degradation processes. 

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