Hay pellets — The influence of particle size reduction on their physical–mechanical quality and energy demand during production

The densification of hay into pellets will be a complex undertaking, strongly affected by its raw material characteristics as well as the technology used. The physical–mechanical properties of such pellets are most important and highly dependent on grinding, conditioning and the operation of the pellet press. This study tries to evaluate the most efficient way of producing high quality hay pellets by finding the lowest specific energy input while achieving the highest bulk density and durability values of the pellets in accordance with ISO 17225-6. Therefore, the performance of three different grinding techniques was researched: cutting, impact and hammer mill with different influencing parameters (mesh sizes of 2, 4, and 6mm, with or without additional aspiration). Afterwards, the densification of the ground material in a ring die press was investigated. Eventually, a hammer mill with a 4mm mesh was found to yield hay pellets with the required bulk density and durability while having the lowest specific energy input (Espec,total 157Wh/kg). •Optimal fiber length distribution of ground hay is produced by using a hammer mill.•High quality hay pellets were produced with a hammer mill and a 4mm sieve.•Hay as fibrous material is kept together mostly by interlocking bonds.•Optimal size distribution reduces the specific energy input of the pellet production.