Acid-insoluble lignin was determined after a two-stage sulfuric
acid hydrolysis as recommended NREL/TP-510-42618 method [26].
Oil content was determined using Soxhlet extraction with n-
hexane, according to the ISO 659 standard [27].
The mechanical durability of the sample was determined ac-
cording to UNI EN 15210-1 [28] using a mechanical durability tester
(mod. Holmen Lignotester). The durability is expressed as the
percent ratio of the pellets retained on a sieve after tumbling in
reference to the mass before tumbling. The test was conducted on
500 g samples of pellets tumbled at 50 rpm for 500 rotations. A
3.15 mm mesh sieve was used to collect crushed pellets after
tumbling.
The bulk density was calculated according to UNI EN 15103 [29]
by the mass material contained in a standard container of 2 L vol-
ume for the milled material.