3.2.3. Mechanical Properties

Axial and diametral compressive tests were performed on RS and RH samples at the Laboratorio di Ingegneria dei Materiali (DICCA, UNIGE). Ten specimens were tested except for the RH in the axial compression test where only one specimen was tested due to its extreme fragility. The load at fracture was obtained from the stress–strain curve (as shown in Figure 7a for RS and Figure 7b for RH) from which the main mechanical properties were calculated.

**Figure 7.** Axial compressive stress-strain curves for RS pellets (**a**) and RH pellets (**b**).

The mean values for axial compression strength (<sup>σ</sup>AC, MPa), deformation at break (<sup>ε</sup>, %), and diametral compression strength (<sup>σ</sup>DC, MPa) for RS and RH pellets are listed in Table 8. The high values of standard deviation were attributed to the heterogeneous nature of the biomass samples.


**Table 8.** Averaged results of axial and diametral compression tests for RS and RH pellets.

RS pellets presented higher mechanical resistance in both directions in comparison to RH pellets. This fact indicated a stronger bonding as a result of a more e fficient densification process. Mechanical strength varies significantly with direction of load and feedstock. Compressive stress was higher in the axial orientation for RS pellets whereas RH presented higher values in the diametral orientation. Because the diametral compression is an indirect measure of the tensile properties, a higher value of σDC with respect to σAC is probably due to the prevalent orientation of the fibers along the longitudinal direction of cylinder. Mechanical strength of pellets is highly related to their physical and chemical properties. The higher moisture content, lower density, and lower length of pellets resulted in lower mechanical strength values. This might be attributed to the lower adhesion forces between particles, in accordance with other works [11]. Compression strength values were consistent with other pellet values like eucalyptus pellets (<sup>σ</sup>*AC* = 5.5 MPa; σ*DC* = 9.9 MPa), wood pellets (<sup>σ</sup>*AC* = 4.1 MPa; σ*DC* = 9.5 MPa), or sunflower pellets (<sup>σ</sup>*AC* = 7.5 MPa; σ*DC* = 6.5 MPa) [31].

Following these results, it is concluded that RS pellets present an adequate mechanical strength whereas RH pellets were too fragile and not resistant to mechanical forces. This fact could lead to increased dust emissions and higher risks of fire and explosion and so they were not considered as a valid feedstock.

Durability is one of the most important factors defining the quality of pellets. Low values can lead to problems such as blocking in the feeding system, dust emissions, and higher risk of explosions during handling and storage [14]. The norm [24] recommends a value over 97.5%. The calculated durability values of RS and RH pellets were 99.8% and 91.8%, respectively. Durability of RS pellets was higher than RH pellets probably due to their di fferent physical characteristics. RS is more flexible, and particles can be in closer contact than RH particles during palletization favoring a correct densification. The durability of RS pellets was well over the limit value according to the standards. On the contrary, the durability of RH pellets was slightly lower than the recommended value and so failed to accomplish the established requirements.

Overall, commercial RS pellets showed adequate properties according to the quality standards. However, the low HHV values suggested that mixing with other materials could improve their calorific value. On the contrary, commercial RH pellets fell out of the standards in several tests such as recommended size or durability. Di fferent operational conditions during pellet production or mixing of rice husk with other biomass could permit using these residues for pellet production to take advantage of the discussed densification processes.

In summary, an adequate densification of biomass could reduce costs and operational di fficulties with handling, transportation, storage, and utilization of low bulk density materials. However, the feedstock and the operational parameters for pellets production need to be carefully examined to determine the technical suitability of the produced materials and potential required improvements. In addition to these technical considerations, further economic and environmental studies will be performed in the context of LIFE LIBERNITRATE to evaluate the global impact on the project.
