## 3 Results and Discussion
One of the most important parameters determining the calorific value of biomass it its moisture content. The air-dried mass of good quality biomass intended for direct combustion should contain 10-15% moisture. However, for the production of briquettes or pellets, the moisture content of biomass should not exceed 10% [49]. The high moisture content of the material obtained from some species is a disadvantage in the context of their energy use. According to [PERSON] et al. [56], plants whose above-ground parts dry out after the end of the growing season have a high content of dry mass without the need for additional drying, which also translates into a low moisture content of their biomass. In our own studies, the moisture content of reed canary grass biomass depended on the harvest date and the age of the plantation. The highest moisture content in the biomass was found in the summer harvest from the annual plantation (12.5%) and the lowest moisture content was obtained in the winter harvest from the six-year plantation (7.7%), which was statistically confirmed. Similar research results regarding the moisture content of reed canary grass biomass were obtained by [PERSON] et al. [56], who showed that the average moisture content of canary grass was 9.8%. However, different values for biomass moisture content were obtained by [PERSON] et al. [43] and [PERSON] [24], who obtained biomass from reed canary grass with moisture contents of 6.8% and 7.7%, respectively. The final effect of any combusted biomass is solid residues called ash. Depending on its ash content, the availability of energy from biomass may vary. It has been found that the correlation between the calorific value of biomass and that of its ash content is negative. Moreover, during the combustion process in boilers, ash creates so-called slag, which worsens the heating properties of such a boiler [61]. In our own studies, the ash content in the studied biomass also depended on the harvest date and plantation age. The highest ash content was found in the biomass obtained from the summer harvest of the annual plantation (75.00 g kg\({}^{-1}\) d.m. ), and the lowest value was found in the biomass obtained from the winter harvest of the six-year plantation (45.00 g kg\({}^{-1}\) d.m. ), which was confirmed statistically. It should also be added that the  
Figure 9: Sulfur (S, **(A)**) and chlorine (Cl, **(B)**) content in the biomass of reed canary grass (_P. arundinacea_) and its variability depending on the harvest date and plantation age. Means followed by same letter or symbol do not significantly differ (_p_ = 0.05, [PERSON]’s HSD). Sulphur: interaction years \(\times\) harvest time = 0.074, standard deviation = 0.026. Chlorine: interaction years \(\times\) harvest time = 0.1030, standard deviation = 0.0363.  
ash content in the biomass of reed canary grass harvested in the winter (irrespective of the plantation age) was at a similar level (45.00-47.00 g kg\({}^{-1}\) d.m.) to that of canary grass harvested in the summer, which was confirmed by statistical analysis (Figure 10A,B). These values were similar to the results obtained by [PERSON] et al. (2018). These authors report that, in their studies, the ash content in dry matter obtained from reed canary grass from a summer harvest was 8.51% and that from a spring harvest was 6.6%. Lower values for ash, which were 5.1% (summer harvest) and 2.3% (spring harvest), respectively, were obtained by [PERSON] et al. (2018). However, the studies conducted by [PERSON] et al. (2018) indicate that the ash content in dry matter obtained from reed canary grass may be higher and reach the value of 9.31%. The most important thermal and physical parameters of biomass intended for energy purposes are its calorific value and combustion heat. In our own studies, it was found that the calorific value and combustion heat depended on the harvest date and the age of the plantation.