## 4 Discussion
### Compression, Mechanical Properties and Springback  
This study postulates that the length of the pellet, as well as springback, differ between the hemicelluloses. The pellet length that mannan and xylan reached under maximum pressure inside the die varies by only a few millimeters, as shown in Table 3. However, as soon as the pressure is released, mannan tends to springback more than xylan, and the variation in length for mannan increases much more than for xylan pellets. As Table 3 shows, the length increase is in the range of 15%-20% for mannan when the moisture content is 10%. When the moisture content is increased to 15%, the length increase was over 30%, (independent of the pressure force). For the xylan pellets, this length increase was only around 5% for all tests, except the test with 15% moisture content and at 14 kN that yielded a length increase of about 20%. This variation between xylan and mannan can explain the results of some earlier studies. [PERSON] and [PERSON] tested compression and springback properties of hardwood (hard maple) and softwood (white spruce) and found that the length expansion from comp pellets to green pellets was 16.28% for hardwood, while for softwood pellets, the value was 20.56% [13]. This is a variation that can be explained by the type of hemicelluloses in each sample. The study by [PERSON] and [PERSON] presented a table of the chemical compositions showing the amount of pentoses in hard maple as 17.1% and in white spruce as 12.1%. Maple has a high amount of xylan in the hemicellulose, while the main hemicellulose component in spruce is mannan [28; 39]. Thus, this variation between xylan and mannan can explain the difference between different types of biomass behaviors related to springback possibilities. However, more studies must be carried out to verify these results, and this study can conclude that it is important to include data on the hemicellulose components when studies correlated to springback in pellets are conducted. This study can verify that biomasses with a high content of flexible polysaccharides are more sensitive to the effect of having a low moisture content when the pressure is low than biomasses with a high amount of stiffer polysaccharides. As shown in Figure 7, the hardest pellet was produced from Avicel, followed by beech and then xylan. However, there is no big change in hardness at low moisture content for Avicel and spruce whenever the material is compressed to 5 or 14 kN, but the opposite is observed for xylan and beech wood at low moisture contents. In Figure 7c, it can be seen that Avicel has small variations between 5 and 14 kN, meaning that cellulose as biomass is not so dependent on high pressure, which is independent of moisture. At a 15% moisture content, pellet hardness decreases for Avicel, despite any variation between 5 and 14 kN. Also, in Figure 7c, it can be seen that for pectin, the variation in hardness at different pressures increases if the moisture is decreased. Pectin and xylan also showed a large deviation at 10%, (see Figure 7) both of these materials visually exhibited cracks in the pellets that resulted in a low level of hardness; these cracks were the reason behind the large deviations. However, those pellets that were not cracked were very strong, meaning that xylan and pectin have very good potential to act as strong bridges between fibers in a pellet and they generate strong bonds to make pellets with high hardness. This information is important, because it explains why biomasses with a high share of flexible polysaccharides, such as hardwood, are sometimes difficult to pelletize and why they are sensitive to moisture content. It is clear that moisture is related to the need for pressure during densification. This can be attributed to a more plastic behavior of the material. As presented in Figure 1, a compression curve can be consolidated through particle rearrangement, slippage, and particle deformation, and if the particles reach elastic deformation, it is reversible, but for plastic deformation, it is not.