Chemical Structure and Properties of Heat Treated Coal in the Early State of Carbonization (IV)

Helium Density of Heat Treated Coal

Abstract

The true densities of Japanese heat treated coals are measured with helium and these results are compared with those obtained from foreign heat treated coals, Japanese original coals and foreign original coals. The density is measured with the same apparatus used for original coal, as reported previously. For this experiment, five sample coals of different rank are used, which have been heat treated at various temperatures from 200°C to 500°C.
In Japanese coals, the change in density with heat treatment temperature is sum marized as follows. For the four coals except Tempoku coal, below 350°C there is no appreciable chang in density, which however begins to increase at 350°C and shows rapid increase from 400°C to 450°C. Between 450°C and 500°C, the increase in density becomes milder. For Tempoku coal, no appreciable change is observed below 400°C and density begins to increase at 450°C.
The change in density of heat treated coal with C% is very different from that of original coal with coalification. When the reciprocal of density is plotted against the hydrogen content, Japanese heat treated coals and foreign heat treated coals are found to lie on the same curve, and the end of the curve (H%=O) coincides with the point of graphite. Moreover, the curve nearly agrees with that of original coals.
In addition, according to van Krevelen's graphical densimetric method, the aromatic carbon fraction (f_a) is calculated from the measured density and the relation between f_a and heat treatment temperature is obtained. For Tempoku coal which is the lowest in coal rank among the samples, f_a increases gradually with the elevation of heat treatment temperature from 200°C. Yubari coal and Hashima coal which are situated at higher coal rank, do not indicate any appreciable change in f_a below 400°C, but begin to show increase at 400°C. Taiheiyo coal and Bibai coal located in the middle of coal rank, exhibit an intermediate property.
The value of f_a is plotted against H/C atomic ratio and it is found that the points of Japanese heat treated coals and foreign heat treated coals lie on the same curve. Besides, the curve nearly agrees with that of the original coal.
In conclusion, the density and f_a of heat treated coal are greatly affected by its hydrogen content and by making use of this relation, density and f_a of heat treated coal can be determined approximately.
The aromatic carbon fraction of heat treated coal increases with its heat treatment temperature. If the increase of f_a is ascribed to the distillation of the low molecular substances containing much aliphatic structure, f_a of each heat treatment temperature can be calculated. As a result, for the four samples except Tempoku coal, the calculated value agrees approximately with the experimental value.