2.4.2. Procedure

**Table 1.** Technical

specification

 of the mills (as given by the

Prior to the actual measurements the following preparatory measures were taken: the fines of the pellets were screened with a 3.15 mm round-hole sieve (according to EN 15149-1); a 2.5 kg sample (with moisture as received) was prepared for the measurement; the feeder was adjusted to the previously estimated setting; the power measuring device was installed directly at the power supply line of the mill; and the mill was equipped with 1 mm mesh.

A full recording cycle (see Figure S3) was comprised of: a mill idling period of at least 2 min, during which the idle power consumption of the mill running empty was recorded; the actual milling period; and a second idle period of at least 2 min after no more material fell through the sieve.

The ground material was weighed to check on whether grinding was complete. The whole procedure was repeated at least once for each pellet type.

### 2.4.3. Calculating the Specific Grinding Energy

The determination of SGE involves the measurement of the idling power before grinding (PI1), the total active power during milling process (Ptotal), the idling power after the grinding (PI2), the actual duration of the grinding (tG) and the mass of material that was ground (mG). Preliminary tests showed that differences between PI1 and PI2 might occur; hence, both values are necessary. In order to make the measurement comparable across all samples it was decided to define tG as the time when Ptotal is at a stable plateau. Even though minimal grinding takes place before and after that (grey areas in Figure S3), it was found to be the simplest and most representative way of reading the Ptotal from the power curve.

As Ptotal is the sum of the power applied for grinding and the idle power the actual power needed just for the grinding (PG) is calculated by subtracting the averaged idle powers of PI1 and PI2 (PI) from Ptotal.

The average idle power PI (in W):

$$\mathbf{P\_{I}} = \frac{\mathbf{P\_{I1}} + \mathbf{P\_{I2}}}{2} \tag{2}$$

The power needed for grinding the pellets PG (in W):

$$\mathbf{P\_G} = \mathbf{P\_{total}} - \mathbf{P\_I} \tag{3}$$

By multiplying PG with tG (in h) the grinding energy (EG, in Wh) is calculated:

$$\mathbf{E}\_{\mathbf{G}} = \mathbf{P}\_{\mathbf{G}} \cdot \mathbf{t}\_{\mathbf{G}} \tag{4}$$

Finally, SGE (in Wh/kg) is defined as:

$$\text{SGE} = \frac{\text{E}\_{\text{G}}}{\text{m}\_{\text{G}}} \tag{5}$$
