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SUMMARY Prediction of coke strength based on measurements of vitrinite reflectance and data from group‐maceral analyses requires information about the coking power of the different coal macerals in relation to rank. A range of bases are considered by different individuals to be important in prediction studies, e.g. the optimum ratio of reactive to inert constituents for each ‘V‐step’ (vitrinite oil reflectance step × 10); the proportion of vitrinite and clarite and rank; the amount of inertinite with no consideration given to an optimum ratio of reactives to inerts; the rank with little regard given to varying maceral composition. Entirely satisfactory results can be obtained by all investigators for coke‐strength predictions from certain coals under given coking conditions. For other coals, some of the determined values lie well outside the permissible limits of error. Reasons for these deviations may lie in an erroneous, or at least in an inadequate evaluation, of the different microscopical parameters. With this introduction, different predictive methods are discussed and tested. Limits of applicability are demonstrated. Possible improvements are suggested and questions, which have so far not been answered entirely satisfactorily, are posed. Above all, the underlying importance of light microscopy to a major industrial field is stressed.

Early work has shown that coke strength can be calculated from the results of maceral analyses and reflectance measurements, assuming that the coking conditions are constant. Juranek, Ritter, Mackowsky, and Wolff have contributed information on the influence of particle size, bulk density, and coal rank on coke formation. The results of these latter investigations are summarized, and the effect of varying the rate of heating between 0.5 and 300°C./min. is reported. Coals of different rank behaved differently as the rates of heating were varied. Hence, coke formation should not be described solely in terms of coal composition. Finally, an attempt is made to explain some anomalies in the coking behavior of coals, particularly expansion, in terms of differences in the chemical composition of the coals.Investigating the relationship between coal rank and maceral composition on the one hand, and coking properties on the other hand, has been one of the main topics of applied coal petrography for more than 30 years. From studies by Amossow and Eremin (I, 6) Berry (2), Harrison (7), and Schapiro, Gray, and Eusner (14,15), it was possible to calculate coke strength with considerable accuracy on the basis of known maceral composition and reflectance distributions. Paralleling these studies, Mackowsky and Echterhoff (5,10,11), Ritter and Juranek (12,13), and recently Brown and Taylor (3) have studied the transformation of coal to coke. Some results of these studies on coke formation are discussed.The first insight into processes occurring in the plastic zone resulted from research by Echterhoff and Mackowsky (5). Figure 1 represents an overall picture of the microscopic research findings. A plastograph curve has been 527

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