3.2.1. Physical Properties

The size of pellets greatly influences feeding and combustion processes. In general, shorter pellets promote a more continuous inflow, as solid obstructions are more unlikely to occur. Longer pellets can be more easily broken during storage, handling, and processing but can also provoke bridging problems. Thinner pellets allow a more uniform and e fficient combustion rate.

As shown in Table 1, the norm [24] sets the diameter limit at die diameter ± 1 mm and a length range between 3.15 and 40 mm. Table 5 shows the dimensions of RS and RH pellets. Die diameter was 6 mm in all cases.

**Table 5.** Averaged dimensions of RS and rice husk (RH) pellets.


Visual observations of pellets showed di fferences in size (RS—Figure 6a; RH—Figure 6b). RH pellets showed lower length values than RS pellets and a more fragile behavior during its manipulation.

**Figure 6.** RS (**a**) and RH (**b**) pellets.

In any case, the dimensions of both pellets were within the range established by the norm. A slight increase in the diameter indicated a pellet expansion after its release from the die, an e ffect also shown in other materials [25]. RH pellets had the lowest *L*/*d* ratio and both materials presented values similar to other biomass pellets (e.g. black poplar = 3.64; holm oaks = 2.18; and leaves of olive trees = 1.76 [12]).

Bulk density (ρ*b*) greatly influences storage and transport processes as well as combustion efficiencies. Particle density (ρ*p*) might a ffect combustion e fficiency as highly packed materials might prevent oxygen to access within the particles. The norm [24] suggests a value ρ*b* > 600 kg/m3. No guidelines are established for ρ*p* with a recommended value of 1200 kg/m3.

Porosity (<sup>ε</sup>*0*) influences heat and mass transfer phenomena and thus the combustion parameters such as burning rate, conversion e fficiency, and emissions [26]. Table 6 shows the densities and porosities of RS and RH pellets.


**Table 6.** Averaged density and porosity of RS and RH pellets.

RS was just within the range established by the norm and above the recommended particle density value. On the contrary, RH fell out of the optimal ranges presenting lower values than those recommended in all cases. These results are in accordance with other studies in which RS pellets presented higher density values than RH pellets [11].

In summary and regarding physical properties of pellets, RS was considered as a suitable material according to the standards whereas RH presented length values close to the minimum permitted and a density out of the recommended range.
