Weathered coal

This study shows that limited oxidation at 373 K or weathering (ambient) of coal feedstock reduces the thermoplastic properties of a coal This is manifest as a transformation in char type, from cenospheres to inertoids, at high rates of heating (10 -10 K s" ) in an Entrained Flow Reactor at 1273 K. The specific types of char are related to the chemical structure of the coal and an inverse relationship exists between the occurrence of cenospheric chars and the atomic 0/C ratio of the oxidised or weathered coal from which they are derived... 

Figure 2. The Atomic 0/C ratios for the oxidised and weathered coals against the log of time.

Figure 8. The changes in the level of unbumt carbon within the char combustion product derived from the weathered coal series A and B.

The reduction in thermoplasticity of the metaplast is possibly the result of the promotion of aromadsation/char condensation reaaions at die expense of production of stable volatile species during coal pyrolysis. In this way, the generation of specific char types at 1273 K in the EFR can be related to the proportion of elemental oxygen within the original coal stmcture of the oxidised or weathered coal particle below an atomic 0/C ratio of =0.15 the predominant char is that of the cenosphere variety due to the maintenance of nunaplast fluidity during pyrolysis. 

A. Fresh coal. B. Weathered coal, and C. Difference spectrum (B-A). 

Bhupendra Mazumdar. The author s results show that a sample of weathered coal was found to contain appreciably more of metal complexes than the unweathered coal. I would be glad to know if Mr. Zubovic can explain this phenomenon. Has he studied any correlation between extra metal retention and additional acidic groups (in particular COOH groups) usually found in weathered coals ... 

Mr. Zubovic. We have not made any additional studies of these weathered samples. I do not know if there is any correlation of the elements with a larger number of acidic groups in this coal. I do believe that the additional amounts of elements found in the weathered coals are not held there as chelates but rather as simple complexes such as are formed in ion exchange reactions. 

In a coal desulfurization study, Narayan et al. (50) were able to extract an appreciable amount of elemental sulfur (36% of total sulfur) with perchloroethylene at 120 °C from weathered coal, but not from fresh coal. Hackley et al. (51) determined the isotopic composition of elemental sulfur extracted by perchloroethylene and obtained results consistent with the interpretation that the elemental sulfur originates from the oxidation of pyrite. 

It is worth noting that XANES and EXAFS spectra obtained from a severely weathered bituminous coal were nearly identical to those obtained from lignite samples. This indicates that in the weathered coal calcium is also present in a dispersed form in which it is bonded to carboxyl groups in the macerals. A more detailed report of this work is given elsewhere (17). 

In this study no other sulfide-containing minerals, except the ones mentioned above, were detected in fresh coals by using Mossbauer spectroscopy. Pyrrhotite was detected in some heavy, weathered coal (Figure 3). The presence of Fe. S was determined by temperature-dependent measurements and the analysis of low-temperature ashes, both by X-ray diffraction and Mossbauer spectroscopy. Other minerals, like spharelite, chalcopyrite, and arsenopyrite, were not detectable in these experiments. Some of the latter minerals have been identified by using scanning electron microscopy, but their presence in the coal was too small to make their contribution to the Mossbauer spectrum significant. 

Other Minerals. In this work only the minerals mentioned above were detected using Mossbauer spectroscopy. However, in heavily weathered coals and coal refuse the presence of iron oxides (hematite and, to a lesser extent, magnetite) was observed. The Mossbauer parameters for the major iron oxides are given in Table IV. 

The sulfur minerals include pyrites with some marcasite. Marcasite has the same chemical composition as pyrites but a different mineralogical structure. Sulfur is also present as organic matter and occasionally as sulfate. The latter usually occurs in weathered coal such as in outcrops. The amount of sulfate sulfur in coal is generally less than 0.01 percent. 

Kocan RM, Matta MB, Salazar SM. 1996. Toxicity of weathered coal tar for chortnose sturgeon (Acipenser brevirostrum) embryos and larvae. Arch Environ Contain Toxicol 31 161-165. 

Goethite, orFeOOH 0.37 Var 0 - 390 Common in weathered coals, from pyrite oxidation. 

Finally, since coal quality can be affected by oxidation or weathering (Joseph and Mahajan, 1991), the question is raised about the effects of oxidation and weathering on combustion and whether oxidized or weathered coal could maintain a self-sustaining flame in an industrial boiler. 

F.B. Waanders, E. Vinken, A. Mans, A.M. Mulaba-Bafubiandi, Iron minerals in coal, weathered coal and coal ash—SEM and Mossbauer results, Hyperfme Interact. 2003, 148-149, 21-29. 

Figure 41 Surface-enhanced Raman spectra of the fulvic acid from weathered coal. The circuit is open. (A) pH 11 (B) pH 9 (C) pH 7 (D) pH 5 (E) pH 3 (F) pH 1 [121]. (Reproduced from Journal of Environmental Science and Health, Part A, A34, Wang, T, et al., Fourier transform surface-enhanced Raman spectra of fulvic acid from weathered coat adsorbed on gold electrodes, pp. 749-765. Copyright 1999, with permission from Marcel Dekker, Inc.)...

Crelling et al. (ref. 13) quantified effects of additions of weathered coals during co-carbonizations with fresh coals. A decrease in coke stability and an increase in coke reactivity and breeze content were observed as the proportion of weathered coal in the blend was increased. An increase of reactivity and a decrease of strength was aiso observed by Pis et al. (ref. 14) in cokes from low temperature oxidised coals. Pre-... 

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