Weathering of coal

A map of the different coal fields of South Africa with (a) representing the Waterberg field, (b) the Free StateA/ereeniging field, (c) the Mpumalanga/Witbank field, and (d) the Kwazulu-Natal/ Ulundi field. (Adapted from Ref. 7.) 

TABLE 30.2 Room-Temperature Mossbauer Parameters of Various Coal Samples  

5 = isomer shift relative to a-iron A = quadrupole splitting H = magnetic hyperfine field. 

Mossbauer spectrum after exposure time of 2 days. 

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Furthermore, artificial weathering of coal under carefully controlled conditions in the laboratory reveals a rapid loss of thermally extractable mobile... 

Zevenbergen, C., Bradley, J. P van Reeuwijk, L. P., Shyam, A. K., Hjelmar, O. Comans, R. N. 19996. Clay formation and metal fixation during weathering of coal fly ash. Environmental Science and Technology, 33, 3405-3409. 

The weathering of coal samples can produce anomalous minor-element concentrations. 

Other uses of forms of sulfur data are the inclusion of the pyrite sulfur value in the formula for the estimation of oxygen by difference and as a possible means of predicting the extent of weathering of coal. The sulfate concentration increases upon weathering, so the sulfate sulfur value could be used as an indication of the extent of weathering of coal. 

Plastic properties are sensitive to the oxidation or weathering of coals. Maximum fluidity is lessened, and extensive oxidation may destroy the fluidity of coal completely. Samples should be tested as soon as possible after they are collected and should be stored under water or in a nonoxidizing atmosphere if there is to be a delay before they are tested. Proper packing around the stirrer in the plastometer is an important step in the measurement of plastic properties. Some coals may not pack easily due to their weathered condition or to the size consist of the sample. An excess of very fine coal makes the test sample hard to pack. 

About 2.5 million tons (2.3 million tonnes) of coal arc burned daily in U.S. power plants. This is equivalent to roughly 21,000 railcars in transit, so it is apparent that coorditiatiiig production and cotistimp-tioii is no easy task. Accidents, rail strikes, natural disasters (e.g., floods that take out bridges and rail lines) and severe weather (e.g., deep river freezes that halt barge traffic) can all severely disrupt deliveries for utility customers dependent on a reliable coal supply for base load plants. Nonetheless, to reduce costs U.S. utilities have significantly reduced typical inventory levels over time. Wliereas a coal inventory of ninety days of supply was once typical, inventories now frequently run in the range of thirty to forty-five days. 

Atmospheric exposure trials, carried out in Cambridge, established the fact that when rusty specimens were painted in the summer, their condition, after some years exposure, was very much better than that of similar specimens painted in the winter It was found that steel weathered in Cambridge carried spots of ferrous sulphate, deeply imbedded in the rust, and that the quantity of ferrous sulphate/unit area was very much greater in the winter than in the summer this seasonal variation was attributed to the increased sulphur dioxide pollution of the atmosphere in the winter, caused by the combustion of coal in open grates. It was concluded that there was a causal relationship between the quantity of ferrous sulphate and the effective life of the paint. It was suggested that these soluble deposits of ferrous sulphate short-circuit the resistance of the paint film and, since paint films are very permeable to water and oxygen, the ferrous sulphate will become oxidised and hydrolysed with the production of voluminous rust, which will rupture the film at numerous points, thus giving rise to the characteristic type of failure seen on painted rusty surfaces. 

External moisture that is not bonded to a particle of coal or other fuel. External moisture will ice in cold weather and lump fuel together in a solid mass. Where this occurs, thaw sheds may be used to heat the mass before further transport or use. 

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... 

However, the question whether the combustion of oxidised or weathered pulverised coal would maintain a self-sustaining flame in a full-size industrial boiler at maximum load cannot be addressed within the present study. The inhibition of volatile matter release, due to the promotion of char condensation reactions during coal pyrolysis, suggests that this may not be the case, a point pertaining especially to the more severely oxidised coals (i.e. 373 K for 112 days). Clearly there is need for ture work within this area of pulverised coal combustion. 

Influence of weathering, grain size on softening and fusion behaviour of coals... 

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