**4. Conclusions**

The proposed method for assessing the specific grinding energy of torrefied biomass pellets is a promising tool that could allow further characterization of torrefied pellets beyond the existing physical-mechanical and chemical parameters already being measured. The method itself is simple enough to be conducted by specialized laboratories and pellet producers alike without grea<sup>t</sup> investment cost or training. Even though the impact mill seems to have the most promising features and results, further investigations could be necessary to decide whether the results obtained by this mill type represent the characteristics most relevant to plant operators. To increase the impact of the newly introduced parameter (SGE), it would also be necessary for plant operators to acquire first-hand experience on how the specific grinding energy values translate into adjustment of their mills on-site. Therefore, if the specific grinding energy is to be used as a quality parameter, it might also be useful to complement the single value information with either a particle size distribution curve or the distribution width as part of the characterization.

**Supplementary Materials:** The following are available online at www.mdpi.com/1996-1073/9/10/794/s1. Figure S1: Picture of the anvil and plunger used during the hardness test, Figure S2: View of the grinding chamber of each of the three mills used, Figure S3: Typical power curve when recoding the power consumption during grinding, Figure S4: Cumulative particle size distribution obtained when using a ball mill on three selected torrefied biomass pellets and four standardized HGI coals, Table S1: Characteristic values of the particle size distributions obtained by the different mills for the different biomass types.

**Acknowledgments:** The authors would like to thank the managers of the SECTOR-Project for their organizational skills and the European Union for their funding (The SECTOR-Project has received funding from the European Union's Seventh Programme for research, technological development and demonstration under gran<sup>t</sup> agreemen<sup>t</sup> n◦ 282826GA). Furthermore, the authors would like to thank the laboratory staff at the Deutsches Biomasseforschungszentrum who did a grea<sup>t</sup> load of the practical work for this study.

**Author Contributions:** Diana Leistner (D.L.), Nadja Weller (N.W.) and Leilani Darvell (L.I.D.) conceived and designed the experiments while Jan Hari Arti Khalsa (J.H.A.K.) and D.L. did most of the data analyses. J.H.A.K. did most of the writing with input from D.L., N.W. and L.I.D. Ben Dooley (B.D.) and L.I.D. were responsible for the HGI measurements while all other parameters were measured under supervision of D.L. The torrefied biomass pellets were a courtesy of the SECTOR-Project with numerous researchers being involved in the production. These researchers are not included in the list of authors ye<sup>t</sup> their contribution is greatly appreciated.

**Conflicts of Interest:** The authors declare no conflict of interest.
