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Comparison of Methods for the Determination of Organic Oxygen in Coals

“Comparison of methods for the determination of organic oxygen in coals” FUEL 65 (1986): 1563-1570.

Ehmann, W. D., D. W. Koppenaal, C. E. Hamrin, Jr., W. C. Jones, M.N. Prasad, and W.-Z Tian.

Coals distributed widely in rank and geographical origin have been analysed for organically-bound oxygen by several conventional and fast neutron activation analysis (FNAA) techniques. Pyrolysis of demineralized coal with measurement of evolved CO2 by coulometry was found to be the most reliable of the conventional methods. Direct FNAA determinations of organic oxygen by analyses of demineralized coal (DMC) samples yielded data in excellent agreement with pyrolysis, as did values computed using a modified ‘by difference’ calculation. The convergence of data from these totally independent approaches, suggests a measure of true organic oxygen levels has been achieved. A ‘difference method’ based on FNAA determinations of total dry coal oxygen and inorganic oxygen in low temperature ash yielded organic oxygen data that were typically lower than pyrolysis values, possibly due to oxidation of organic sulphur in the ashing process and/or the presence of non-extractable mineral oxygen in the DMC used in the pyrolysis method. A third FNAA ‘difference method’ based on simultaneous determinations of both total oxygen and silicon contents of dry whole coals, followed by estimation of the inorganic oxygen contents based on the silicon contents, was found to be rapid and adaptable to on-stream analysis. However, observed mineralogy-dependent deviations from a simple inorganic oxygen-silicon relationship suggest that the latter technique would be most successful when applied to coals with similar mineralogy.

The occurrence of organically-bound oxygen is second only to carbon in most coals lignite through bituminous rank. Consideration of atomic O/C ratios for such coals indicates that an oxygen atom is bound to every 3-15 carbon atoms, depending on rank. The role of the oxygen in influencing the ‘primary structure’ of coal is therefore evident. Oxygen also possesses an electro-negative characteristic that manifests itself through extensive hydrogen bonding among the various molecular units of coal (i.e., the ‘secondary structure’). Oxygen thus exhibits a dual bonding role composed of intra- and inter-molecular associations that is unique in scope among the major constituents of coal.

The secondary structural effects have been implicated in the inhibition of coal-to-liquid conversion 1,2. Various investigators have examined oxygen functional group distribution among liquefaction products 3-5, yield dependence on oxygen distribution and removal 6,7, and the dependence of low temperature solubilization reactions on oxygen content 8,9. Theses studies indicate that oxygen and its compounds have an important, albeit partial, influence on the liquefaction behavior of coals.

The presence of oxygen is also an important factor in determining the surface properties of coal. Such properties are crucial to the wet beneficiation or preparation of coal. The polar nature of oxygen controls the hydrophilic character of the coal surface, thereby affecting the wettability of the material. Reports relevant to this property are available; one deals with relationship between oxygen group content and fluotation response 10, another with oxidation effects on coal hydrophobicity11.

Oxygen has not been an element easily determined in coal, either in total or as its functional moieties. The difficulties associated with its determination have been recognized by the coal science community; the discussions of given and co-workers have been especially enlightening 12-15. Total oxygen in organic materials has traditionally been determined using the approaches of Schütze16, Unterzaucher 17, or some variant of these methods 18-20. An international standard based on these methods exists for the determination of oxygen in hard coal;21 a similar standard is also available for other ‘organic materials’22.  Difficulties encountered when using these methods of coal have been reported to arise from coal moisture, oxygen-containing mineral constituents (carbonates, suphates, and hydration water), and the possible presence of reducible metal oxides. And the possible presence of reducible metal oxides. Other high temperature pyrolysis procedures have been applied to coal23, notably the work of Kinson and Belcher24, who used an RF-heating furnace to evolve all (organic and inorganic oxygen.

The Unterzaucher-Schütze procedures have never been widely applied to the analysis of oxygen in coal, probably due to the perception of their tedious analysis routines and the recognized need for judicious corrections13. Most coal scientists in the U.S.  still rely on the error-prone oxygen ‘by difference’ procedure, a mistake that is only promulgated by the persistence of the American standard in existence24.

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