## 3 Results and Discussion
### Measurement of Mechanical Properties during Storage  
The average values of the destructive force Fmax of the analyzed pellets with a diameter of 6 mm, during the 90-day storage period, increased. The sunflower stalk pellet (P1) was characterized by the lowest value of destructive force at the set dates of mechanical properties measurement. For pellets made of tobacco stalks (P3), a higher value of destructive force was noted compared to pellets made of sunflower stalks (P1). During the storage of pellets, pellets made of Jerusalem artichoke stalks (P5) turned out to be the most resistant to external loads. The value of the destructive force for this type of fuel was 1 697.69 N. The addition of coniferous sawdust in the case of sunflower (P2) and tobacco (P4) stems  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{3}{c}{**F [N]**} \\ \cline{2-4}
**The Type** & \multicolumn{3}{c}{**Time of Storage [days]**} \\ \cline{2-4}
**of Pellets** & **30** & **60** & **90** \\ \hline P1 & 621.43 \({}^{a}\)\(\pm\) 141.12 & 645.47 \({}^{a}\)\(\pm\) 135.45 & 656.13 \({}^{a}\)\(\pm\) 112.96 \\ P2 & 672.12 \({}^{a}\)\(\pm\) 137.45 & 699.99 \({}^{a}\)\(\pm\) 149.78 & 704.49 \({}^{a}\)\(\pm\) 138.46 \\ P3 & 805.34 \({}^{a}\)\(\pm\) 199.45 & 845.64 \({}^{a}\)\(\pm\) 112.12 & 852.89 \({}^{a}\)\(\pm\) 121.14 \\ P4 & 827.46 \({}^{a}\)\(\pm\) 126.78 & 865.47 \({}^{a}\)\(\pm\) 114.69 & 870.71 \({}^{a}\)\(\pm\) 107.78 \\ P5 & 1650.12 \({}^{b}\)\(\pm\) 170.89 & 1680.52 \({}^{b}\)\(\pm\) 101.89 & 1697.69 \({}^{b}\)\(\pm\) 110.74 \\ P6 & 1267.09 \({}^{a}\)\(\pm\) 190.78 & 1285.36 \({}^{a}\)\(\pm\) 110.14 & 1292.23 \({}^{a}\)\(\pm\) 170.14 \\ \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 2: Average values of the destructive force of the tested samples of plant biomass (pellets) in the process of their uniaxial compression (in the horizontal axis) during storage.  
improved the damage resistance of the tested solid fuels in comparison with pellets made of the same raw materials but without the admixture ofiferous sawdust (P1 and P3). On the other hand, the addition of coniferous sawdust to Jerusalem artichoke stalks reduced the destructive force of this type of fuel (P6) in comparison with pellets made of 100% raw material stalks (P5).  
Pellets stored outdoors are exposed to the weather, even if they are placed under roofs [22]. The relative humidity present in the air increases the humidity of the pellets, which affects their durability and mechanical properties. Pellets then become less resistant to bending, and their abrasion and dust content increase [22; 23]. [PERSON] et al. [24] noted that storing rapeseed straw pellets in hermetrically sealed bags for 48 weeks in an unheated barn decreased their length and durability. [PERSON] et al. [25] conducted research on the impact of storage conditions in the atmospheric conditions of Finland on the quality of pellets. When storing wood pellets for a period of five months in mesh bags with a capacity of 0.5-1 L, both in the open air and under cover, they observed a decrease in the durability of these pellets.  
In addition, the quality of pellets is determined by the calorific value, ash content, volatile compounds, chemical composition, and exhaust gas emission. Tests determining the quality of solid fuels were performed similarly to mechanical tests 24 h after pellet production.