*2.2. Hardness Test*

The hardness (also compressive resistance) is defined as the resistance to fracture of the pellets and is supposed to help characterize the pellets regarding their stability during e.g., storage or handling [23]. The pellet hardness values were measured using a Pellet Hardness Tester with display and PC interface (Amandus Kahl GmbH, Reinbek, Germany) which was designed for single pellets with Ø = 3–14 mm. In this test, the pellet rests in a flat position on an anvil and is stressed by a plunger with a flat tip with a diameter of 5 mm (see Figure S1).

In order to achieve a representative value, the following procedure was applied: At first 20 pellets were randomly selected and the average length (Lavg) was determined (data shown in Section 3.1). Based on the Lavg another 20 pellets in the range of Lavg ± 2 mm were selected. These 20 pellets were the actual pellets being individually placed on the anvil of the Hardness Tester. Each pellet was exposed to the plunger pressing into the pellets with a speed of 5 mm/min and the increase in force (given in N) was recorded (with 100 values per second). The actual hardness value is the maximum value of N, which occurs right before the pellet fractures and a sudden decline in N is observed. From each set of 20 hardness values the min and max value was removed to smooth the distribution and an adjusted mean value was calculated (see results in Section 3.2).
