Notes on the relationship between microlithotype composition and Hardgrove grindability index for rank suites of Eastern Kentucky (Central Appalachian) coals

Hardgrove grindability index (HGI) has been a standard test in the coal and coal-fired power generation industries since the 1930s. Previous studies have demonstrated the relationship between HGI and coal rank and the maceral and mineral composition. In particular, within the high volatile bituminous rank range, HGI increases with an increase in coal rank and, for any specific rank, decreases with an increase in the liptinite content. Fundamentally, the HGI test is approximately at the scale of coal microlithotypes, the microscopic assemblages of macerals. In this study, for two relatively narrow rank ranges, each spanning 0.05% R_max, we examined the relationship between HGI and several maceral and microlithotype ratios for Pennsylvanian eastern Kentucky coals. While some relationships do show statistically significant trends, not all were as well defined as might have been expected.