Anomalies in Vicker's microhardness of subbituminous and high volatile bituminous coals

Vickers microhardness (MH) of coal is known to be strongly correlated with coal rank. To examine coal rank and other coal quality parameters, such as organic sulfur, that might influence MH, a suite of more than 300 samples from the Penn State Coal Quality database with vitrinite Rmax < 1.1 % were examined. The data set was narrowed down to 296 coals with moisture (as-received basis) < 20 %. As MH is a parameter measured on vitrinite, vitrinite Rmax was used as the rank parameter. The Eocene Big Dirty coal (Washington state) stood out as a high MH/high-moisture coal while Hanna and Green River basin coals (Wyoming) had low atomic H/C values and K Unita Basin (Utah) coals had high H/C. Organic S did not show a correlation with MH within discrete rank ranges. With respect to vitrinite Rmax vs. MH, the Big Dirty coal and some Illinois and Iowa coals lie on the high-MH/low-Rmax side and the Pennsylvanian Tioga (West Virginia) and the Indiana Brazil Formation coals, all dominated by dull lithotypes, lie on the low-MH/high-Rmax side of the main data trend. Overall, the quadratic regression of vitrinite Rmax vs. MH yields an R2 of 0.55, indicating a significant correlation at the 95 % level.