Coal assay

In some instances the information gained from the proximate analysis coal sample requires being supplemented by data that provides a more detailed examination of 

During the first period of heating, observe the temperatures at which the following events occur (1) appearance of oil vapor, (2) evolution of gas as distinct from displacement due to expansion, and (3) further evolution of gas, at the onset of which the rate of discharge of water increases suddenly. These three temperatures are usually clearly defined and are characteristic of the sample being assayed. 

In this assay, the tar vapors are cracked over crushed silica brick and the increased yield of gas is accommodated in a larger receiver of some 10 L capacity. Owing to the possibility of tar fog remaining uncondensed, the condenser in this case is followed by a U-tube packed with absorbent cotton wool, and both are immersed 

Since the high-temperature assay is used mainly for the examination of gasmaking coals, the range of coke types normally encountered is considerably smaller than that met with in low-temperature assays, and as a consequence, no standard classification of coke types has been drawn up, nor is such a classification so necessary in this case. Nevertheless, a full description of the coke should always be made. 

In the agglutinating value test, the caking properties of the coal are progressively destroyed by the addition of sand of specified characteristics until the coal-sand mixture, on carbonization under controlled conditions, just fails to satisfy one or both of two conditions. 

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As an extension of the proximate analysis or coal assay, it is worthy of note that new methods continued to be developed. For example, thermogravimetric analysis has been extended to cover determinations of volatile matter (as well as determination of moisture and ash) in coal and coke. These constituents can be measured by pyrolyzing the samples in oxygen and air, and the weight loss at prescribed temperatures was measured by using a thermobalance. 

Table 10. Average Yields and Range of Yields of Fischer Assay of Various Coals ...

Properties. A high volatile western Kentucky bituminous coal, the tar yield of which by Fischer assay was ca 16%, gave a tar yield of ca 26% at a pyrolysis temperature of 537°C (146—148). Tar yield peaked at ca 35% at 577°C and dropped off to 22% at 617°C. The char heating value is essentially equal to that of the starting coal, and the tar has a lower hydrogen content than other pyrolysis tars. The product char is not suitable for direct combustion because of its 2.6% sulfur content. 

Properties. Results for the operation using subbituminous coal from the Wyodad mine near Gillette, Wyoming, are shown in Table 13. Char yields decreased with increasing temperature, and oil yields increased. The Fischer assay laboratory method closely approximated the yields and product assays that were obtained with the TOSCOAL process. 

Eor the Gray-King coke-type assay test (91,92) coal is heated in a retort tube to 600°C and the product coke is compared to a series of standard cokes. Eor a strongly swelling coal, enough anthracite or electrode carbon is added to the coal to suppress the swelling. This method is primarily used in Europe. 

Process development on fluidized-bed pyrolysis was also carried out by the ConsoHdation Coal Co., culminating in operation of a 32 t/d pilot plant (35). The CONSOL pyrolysis process incorporated a novel stirred carbonizer as the pyrolysis reactor, which made operation of the system feasible even using strongly agglomerating eastern U.S. biturninous coals. This allowed the process to bypass the normal pre-oxidation step that is often used with caking coals, and resulted in a nearly 50% increase in tar yield. Use of a sweep gas to rapidly remove volatiles from the pyrolysis reactor gave overall tar yields of nearly 25% for a coal that had Eischer assay tar yields of only 15%. 

Other large-scale coal pyrolysis process developments were carried out by the Tosco Corp., with its TOSCO AT, process (36). Essentially a direct copy of Tosco s rotating kiln technology that was developed for pyrolysis of oil shale, this slow heating scheme achieved tar yields at maximum temperatures of 482—521°C that were essentially identical to those obtained by a Eischer assay. 

The Fischer assay is an arbitrary but precise analytical tool for determining the yield of produces from low-temperature carbonization. A known weight of coal is heated at a controlled rate in the absence of air to 773 K (932°F), and the produces are collecled and weighed. Table 27-3 gives the approximate yields of products for various ranks of coal. 

TABLE 27-3 Fischer-Assay Yields from Various Ranks of Coal (As-Received Basis)... 

Sulfur and phosphorus content Coals should assay low in these harmful impurities as they have adverse effects on the properties of the metal. Moreover, gases produced from sulfur- and phosphorusbearing coals are corrosive to equipment and pollute the atmosphere. 

Estimates also were made for 65 coke plants in 12 states. Coke ovens produce benzene as a by-product, but not all of it can be recovered. It has been estimated that benzene contributes about two-thirds of one percent of the coal gas generated. Potential points of emissions from one type of coke battery are illustrated in Figure 7. Emissions from coke ovens were derived from estimated emission factors (based on coke oven product assays and benzene yields) and coal charging rates. 

Figure 4. Dependence of tar yield, determined by low-temperature Gray-King carbonization assay, n atomic hydrogen-to-carbon ratio for a wide range of Australian coals. Tar yield = 50.4 X H/C — 25.9 correlation coefficient, 0.95.

Demarini DM, Brimer PA, Hsie AW. 1984. Cytotoxicity and mutagenicity of coal oils in the CHO/HGPRT assay. Environ Mutagen 6(4) 517-527. 

Coal is our major source of fossil fuel, being about 87% of the total. Oil shale is 9%, petroleum is 2.5%, and natural gas is 1.5%. These estimates are based on the probable yield of liquid fuel. They are, at best, only very approximate (1, 9, 11). Thus, if all of the known oil-shale deposits in the United States were included, the probable yield of liquid fuel would be about 50% of the total since the 9% estimate excludes shale assaying less than 10 gallons per ton. Similarly, in estimating the probable yield of liquid fuel from coal, somewhat arbitrary assumptions must be made concerning the feasibility of mining thin seams. 

Copper, Chromium, Manganese, and Nickel. The analytical method for determining copper, chromium, manganese, and nickel involves digesting the coal with nitric and perchloric acids, fusing the residue with lithium metaborate, and determining the combined digestion and leach solutions by atomic absorption spectrophotometry. Since there is no standard material to analyze for the construction of calibration curves, the standard additions method is used for the assay. While this method increases the time required for analysis, it helps to eliminate the effect of the matrix. 

Precise and accurate trace element assays on coal are extremely difficult and require constant vigilance to avoid errors that can be introduced both in the laboratory and in the coal-handling procedures. In the future this program will extend the present studies to coals from different parts of the country, study additional trace elements in coal such as arsenic, selenium, beryllium, and others, and apply the developed techniques to more power plants. 

The composition of trace element emissions during coal combustion is described by Bolton et al. in Chapter 13. The actual quantities are somewhat variable depending upon the coal source, the combustion process, the pollution abatement equipment, and the assay itself. Much less is known about important local concentrations of emissions in and around the source, their chemical and physical characteristics, and their fate in the environment. 

Coal Rank Gray-King Assay Volume of gas... 

Coal Conditions of Treatment Gray-Kina Assay Volume of... 

It appears that the discrepancy in the results reported above is caused chiefly by the difference in the method of conducting the coal-sulfur reaction. If coal and sulfur are mixed intimately (in 2 1 proportion) and heated in a Gray-King assay retort from room temperature to 360°C. for one hour and maintained at this temperature for another hour (as was done by Mazumdar et al. (17, 18) in their initial studies on coal dehydrogenation), followed by... 

Coals classified by class and by group are further subdivided into subgroups, defined by reference to coking properties. The coking properties are determined by either the Gray-King coke type of assay or the Audibert-Amu dilatometer test (ISO methods). These tests express the behavior of a coal when heated slowly, as in carbonization. 

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