Gray-King carbonization assays

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.

The use of coal as a popular fuel in Britain, from the time of the industrial revolution, has led to the development of a system of classification in which code numbers are used to denote the different types of coal. The system relies heavily on the coke-forming characteristics of the various coals as well as on the types of coke produced by a standard of coking test (the Gray-King carbonization assay). The system also employs the amounts of volatile matter produced in percentage by the various coals (Table 2.11). 

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. 

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. 

The Gray-King and Fischer assays determine the proportions of coke or char (carbonaceous solids), tar (organic liqnids), liquor (ammonia-rich solutions), and gas produced when the coal is carbonized (heated in the absence of air) under laboratory conditions. Hence, they provide a basis for estimating the yields of coke and coke by-products obtained from the coal in an industrial coke oven or oil shale-processing plant. 

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