2.4.1. Experimental Setup

The experimental set-up to estimate the SGE consisted of the following equipment: (i) mill; (ii) feeding unit; and (iii) power measuring device.

Three common laboratory mill types for grinding of biomass where chosen each imposing a different kind of stress on the material (see pictures of the mill interior in Figure S2). In the cutting mill (CM: Pulverisette 19, FRITSCH GmbH, Idar-Oberstein, Germany), the material is predominantly cut by the blades, while, in the hammer mill (HM: CHM 230, NETZSCH GmbH, Selb, Germany), the flexible hammer elements crush the material. The impact mill (IM: Rekord A, Jehmlich GmbH, Nossen, Germany) has neither cutting elements nor flexible elements, and the grinding is caused by the specific geometry of a disc (grinding ring) rotating in the chamber. For each mill an outlet sieve with mesh size Ø = 1 mm is used. For tests with HM and IM a round-hole sieve is used and for the CM a sieve with trapezoidal meshes is used. All mills are constructed for grinding soft to medium-hard or fibrous biogenic feedstock. The technical specifications are summarized in Table 1.


manufacturer)

 and realized feeding rates


In order to feed the pellets at a uniform mass flow into the mill a screw feeder (GLD 87, Gericke AG, Regensdorf, Switzerland) was used. On average the feeder was operated with 16.2 (HM) and 16.7 Hz (IM, and CM) realizing a comparable pellet mass flow between the different sets of experiments. However, due to the difference in bulk density, length and diameter individual feeding rates were estimated for the different pellet types, which could vary between 10 and 21.2 Hz (see Table 1).

A clamp meter (345 PQ, Fluke Corporation, Everett, WA, USA) was used as the power measuring device and set to record one value per second.
