The ﬁring and co-ﬁring of biomass in pulverized coal ﬁred power plants around the world is expected to increase in the coming years. Torrefaction may prove to be a suitable way of upgrading biomass for such an application. For transport and storage purposes, the torreﬁed biomass will tend to be in pellet form. Whilst standard methods for the assessment of the milling characteristics of coal exist, this is not the case for torreﬁed materials—whether in pellet form or not. The grindability of the fuel directly impacts the overall efﬁciency of the combustion process and as such it is an important parameter. In the present study, the grindability of different torreﬁed biomass pellets was tested in three different laboratory mill types; cutting mill (CM), hammer mill (HM) and impact mill (IM). The speciﬁc grinding energy (SGE) required for a deﬁned mass throughput of pellets in each mill was measured and results were compared to other pellet characterization methods (e.g., durability, and hardness) as well as the modiﬁed Hardgrove Index. Seven different torreﬁed biomass pellets including willow, pine, beech, poplar, spruce, forest residue and straw were used as feedstock. On average, the particle-size distribution width (across all feedstock) was narrowest for the IM (0.41 mm), followed by the HM (0.51 mm) and widest for the CM (0.62 mm). Regarding the SGE, the IM consumed on average 8.23 Wh/kg while CM and HM consumed 5.15 and 5.24 Wh/kg, respectively. From the three mills compared in this study, the IM seems better ﬁt for being used in a standardized method that could be developed in the future, e.g., as an ISO standard.
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