Characterization of coals by automated optical image analysis 1. Vitrinite reflectance

An increasing awareness of the importance of petrographic characterization of coals to efficient selection of coals for pulverized coal combustion and metallurgical coking, has highlighted the need to improve slow and subjective optical microscopic procedures. Automated image-analysis procedures to measure vitrinite random reflectance are examined here in some detail, giving particular attention to inertinite-rich coals. It is shown by consideration of intra- and inter-particle reflectance variance that a given accuracy for vitrinite mean random reflectance (R_v) can be achieved by selection of an appropriate surface sampling procedure. The repeatability of R_v for single coals is similar to that for manual microscopy, but the reproducibility, as established by an international interchange exercise, is not yet good enough to specify a standard procedure. In some coals even vitrinite sub-macerals can be distinguished; however, caution is required when extending this method to vitrinite reflectance distributions of blends containing different rank coals or of heat-altered coals.