## 3 Results and Discussion  
### Measurement of Mechanical Properties  
Mechanical parameters determine the quality of pellets, which are very important from the point of view of being used as fuel for automatic boilers. Appropriate values of mechanical parameters determine the resistance of biomass to disintegration during transport and affect its burning time. This paper presents the results of testing selected mechanical properties of six types of pellets (static tests), which allowed us to determine the quality of the tested biomass. The tests were carried out 24 h after the production of pellets (Table 1). In order to determine the impact of pellet storage time, testing of mechanical properties were carried out on the 30 th, 60 th, and 90 th day of their storage (Table 2). For the analyzed pellets with a diameter of 6 mm, the values of the destructive force F\({}_{\text{max}}\) were varied. The lowest value of destructive force was characteristic for the sunflower stalk pellet (P1). In turn, the pellet made of tobacco stalks (P3) was characterized by a slightly higher value of this parameter compared to the pellet of sunflower stalks (P1). On the other hand, pellets made of Jerusalem artichoke stalks (P5) were the most resistant to damage. The value of the destructive force for this type of fuel was 1 591.45 N. Pellets with an admixture of coniferous sawdust, in the case of sunflower (P2) and tobacco (P4) stalks, were characterized by a slightly higher resistance to damage compared to pellets made of the same raw materials (P1 and P3), but without the admixture of coniferous sawdust. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**The Type of** & **F\({}_{\text{max}}\)** & **W to F\({}_{\text{max}}\)** & **Relative Deformation** \\
**Pellets** & **[N]** & **(mJ)** & **\(\boldsymbol{\mathsf{E}}\)** \\ \hline P1 & 601.53 \({}^{\text{a}}\)\(\pm\) 258.85 & 147.13 \({}^{\text{a}}\)\(\pm\) 65.65 & 0.18 \({}^{\text{a}}\)\(\pm\) 0.08 \\ P2 & 633.17 \({}^{\text{a}}\)\(\pm\) 106.08 & 214.67 \({}^{\text{b}}\)\(\pm\) 73.95 & 0.19 \({}^{\text{a}}\)\(\pm\) 0.06 \\ P3 & 784.45 \({}^{\text{a}}\)\(\pm\) 161.15 & 317.49 \({}^{\text{a}}\)\(\pm\) 80.77 & 0.05 \({}^{\text{a}}\)\(\pm\) 0.02 \\ P4 & 788.71 \({}^{\text{a}}\)\(\pm\) 172.09 & 347.87 \({}^{\text{a}}\)\(\pm\) 73.63 & 0.06 \({}^{\text{a}}\)\(\pm\) 0.02 \\ P5 & 1591.45 \({}^{\text{b}}\)\(\pm\) 240.68 & 489.53 \({}^{\text{b}}\)\(\pm\) 57.87 & 0.07 \({}^{\text{a}}\)\(\pm\) 0.01 \\ P6 & 1119.95 \({}^{\text{a}}\)\(\pm\) 199.44 & 429.11 \({}^{\text{a}}\)\(\pm\) 71.85 & 0.09 \({}^{\text{a}}\)\(\pm\) 0.02 \\ \hline \hline \end{tabular} Statistically significant differences marked by different letters (\(p\leq\) 0.05). The data were analyzed separately for each type of materials.  
\end{table} 
Table 1: Average values of selected mechanical properties of tested samples of plant biomass (pellets) in the process of their uniaxial compression (in the horizontal axis). However, in the case of topimabnur stems, the addition of sawdust reduced the destructive force (P6) compared to pellets made of 100% raw material stems (P5). When analyzing the values of energy needed to destroy the pellet (W to Fmax), it was observed that they corresponded to the values of the destructive force parameter. The sunflower stalk pellet had the lowest energy value until destruction (P1- 147.13 mJ), and the topimabur stalk pellet had the highest value (P5- 489.53). The addition of coniferous sawdust in the case of the sunflower stalk pellet (P2) and tobacco stalk pellet (P4) slightly increased the value of the tested parameter. On the other hand, for the pellet made of Jerusalem artichoke stems with the addition of sawdust (P6), a decrease in the value of energy needed to destroy this pellet was noted in relation to the pellet produced from 100% raw material stalks (P5). The relative deformation values of the tested pellets, up to the moment of destruction, were diversified. The highest values of this parameter were recorded for pellets made of sunflower stalks in 100% (P1) and 50% (P2).