*2.3. Modified Hardgrove Index*

The standard procedure is described in ISO 5074 [18]. A specialized ball mill apparatus (BM) is calibrated using reference coals with known HGI values. A defined mass of coal (50 g) with a specific particle size range (1.18 and 0.6 mm) is then ground and sieved, and the mass fraction of the particles passing through the 75-μm sieve is determined. From this value the HGI is calculated. The lower the HGI value the harder and less grindable the material is.

Bridgeman et al. [16] recognized that this method needs to be adapted for biomass because the volumes of biomass and coals differ significantly receiving the same grinding energy in the mill [24,25] and results will thus not be comparable. Instead of using a defined mass the modified HGI (mHGI) requires a defined volume (50 cm3) of the substrate to be ground. Thus, the torrefied biomass pellets were pre-crushed and sieved according to ISO 5074 and treated following the method laid out by Bridgeman et al. [16] with an initial calibration of the mill using four standard coals with known HGI values. Mean values of the duplicates were calculated and are used for further discussion in this study.
