4.1. Evaluation of the Thermophysical Properties of Virginia Fanpetals Biomass
The average moisture content of Sida biomass during the 14-year experiment was 25.25% (
Figure 2A). This parameter differed significantly across years, ranging from 21.10% (harvest in January 2013) to 31.95% (harvest in January 2019). The type of propagating material and plant density had no significant effect on the moisture content of the biomass. In most cases, the moisture content of the biomass derived from the analyzed types of propagating material did not differ by more than one percentage point (pp), and the difference in moisture content reached 1.30–1.41 pp in three harvest seasons (2010, 2013, 2020). Only in 2019 was the moisture content of the biomass produced from rhizomes 3.77 pp higher than the moisture content of the biomass derived from seeds and 2.58 pp higher than the moisture content of the biomass produced from seedlings.
The results of the statistical analysis for 14 harvest seasons, three types of propagating material, two sowing/planting densities, and three replications (
Table 3) indicate that the moisture content of Sida biomass was generally low at harvest and that this parameter was characterized by low variation.
Biomass for the production of solid biofuels should have low moisture content at harvest. The moisture content of Virginia fanpetals biomass is lower than that of lignified biomass [
34]. Numerous studies have shown that delayed harvest decreases the moisture content of Virginia fanpetals shoots from more than 40% in the fall to around 20–25% in the winter [
35,
36,
37,
38]. According to Jablonowski et al. [
9], the moisture content of Sida biomass decreased to around 10% when dry shoots were left in the field until March (data not published). Weather conditions directly before harvest, in particular, relative humidity, significantly influence the moisture content of the biomass [
34].
The average ash content of Sida biomass was 2.78% DM, and It differed considerably across years (
Figure 2B). Ash content was significantly highest (4.47% DM) after the first growing season (2010) (
Figure 2B). The lowest ash contents were noted in 2013 (2.05% DM), 2014 (2.07% DM), and 2011 (2.28% DM). In the remaining harvest seasons, the values of this parameter formed two homogeneous groups in the range of 2.56–2.71% DM and 3.18–3.30% DM.
The ash content of Sida biomass produced from seeds was only somewhat higher than the ash content of the biomass obtained from rhizomes and seedlings, but the difference was statistically significant. No significant differences were noted between the ash content of the biomass derived from rhizomes and seedlings (
Figure 2B). These results could be attributed to the high ash content of one-year-old shoots propagated from seeds.
Ash content was significantly higher in biomass from young, one-year-old shoots, in particular, in plants grown from seeds and rhizomes, which contributed to variation in this parameter during the experiment. The ash content of the biomass derived from older plants was characterized by low variation (
Table 3). The ash content of Virginia fanpetals biomass was similar in other studies [
9,
24,
39,
40]. Dry Sida biomass contains more ash than wood biomass derived from willows and oaks [
39]. However, Sida biomass is less abundant in ash than other species of perennial herbaceous crops [
20,
41] and other sources of agricultural biomass, such as rapeseed straw [
42]. According to Stolarski et al. [
43,
44], ash content was lower in biomass derived from two-year-old and older plants than from one-year-old plants. The cited authors also noted that ash concentration in the biomass decreased when harvest was delayed [
34,
37].
In all harvest seasons, the HHV of Sida biomass was determined at 18.97 GJ Mg
−1 DM on average, and all experimental factors induced minor but statistically significant differences in this parameter (
Figure 2C). The HHV of the biomass was significantly highest in the ninth growing season (2018) (19.26 GJ Mg
−1 DM) and lowest in the first growing season (2010) (18.57 GJ Mg
−1 DM). Much smaller, but significant, differences in this parameter were observed between biomass samples derived from various propagating materials. The values of HHV were significantly higher in treatments where Virginia fanpetals was grown from seedlings (18.99 GJ Mg
−1 DM) and seeds (18.98 GJ Mg
−1 DM), compared to rhizomes (18.95 GJ Mg
−1 DM).
The HHV of the biomass was significantly higher in treatments with lower plant density than in those with higher plant density, although the difference was minimal at only 0.02 GJ Mg
−1 DM. The HHV was the least varied trait in the group of the analyzed parameters (
Table 3). The HHV was bound by a relatively strong negative correlation with ash content, a moderately strong negative correlation with nitrogen content, and weak, but significant, negative correlations with the moisture content and sulfur content of the biomass (
Table 4). A weak positive correlation was also noted between the HHV and the carbon content of the biomass.
The LHV of the biomass was significantly correlated with the year of harvest and the interactions between the year and the remaining experimental factors. The LHV was most significantly differentiated by year (75.3%) (
Table 5); it was highest after the fourth growing season (2013) and lowest after the tenth growing season (2019) (
Figure 2D). This parameter was not differentiated by the type of propagating material or plant density.
The LHV was strongly negatively correlated with the moisture content of the biomass (
Table 4). The LHV was also bound by a weak, but significant, negative correlation with the concentrations of ash, nitrogen, and hydrogen in Sida biomass. The LHV was characterized by very low variation during the entire 14-year experiment (
Table 3).
Sida biomass is characterized by a high energy value compared to other types of agricultural biomass. In the present study, the energy value of Virginia fanpetals biomass approximated the higher range of HHV values and the lower range of LHV values reported by other authors [
24,
34,
35,
37]. These parameters were similar to the values noted in wood biomass [
34,
39]. In other studies, the HHV and LHV of Virginia fanpetals biomass varied across years, and minor differences resulting from the harvest date (fall, winter, early spring) were noted [
9,
34,
35,
37,
43,
44]. The HHV and LHV of the biomass were generally higher when Virginia fanpetals was harvested in the spring rather than in the fall [
38], but these trends varied [
34,
37]. Szyszlak et al. [
45] observed that the HHV was affected by the thickness of Virginia fanpetals shoots and suggested that the value of this parameter could be influenced by plant density.
4.2. Evaluation of the Elemental Composition of Virginia Fanpetals Biomass
The elemental composition of Virginia fanpetals biomass (concentrations of carbon, hydrogen, sulfur, and nitrogen) differed significantly across years (harvest seasons) (
Table 6). Carbon, sulfur, and nitrogen contents were also significantly influenced by the interactions between the year and the main experimental factors. Hydrogen, sulfur, and nitrogen contents were significantly differentiated by the type of propagating material, whereas plant density significantly affected the sulfur and nitrogen contents of the biomass (
Table 6).
The average carbon content of Sida biomass was 49.20% DM. Carbon concentration was highest in the biomass harvested in the tenth growing season (2019) and lowest in the biomass harvested in the first growing season (
Figure 3A). In these years, carbon concentration was significantly differentiated by the type of propagating material and plant density. Carbon content was the least varied trait in the elemental analysis (
Table 3).
Carbon is the main component of solid fuels, and its concentration increases the HHV of fuel [
37]. In the present study, the carbon content of Sida biomass was bound by a weak, but significant, correlation with the HHV. Carbon concentration was similar to that noted by Stolarski et al. [
42] and Šurić et al. [
38], and it was higher than that reported by Šiaudinis et al. [
43]. In the work of Bilandžija et al. [
37], carbon concentration was much lower when Sida biomass was harvested in January (32.22% DM) than in November and March (46.79% and 50.08% DM, respectively). In other studies, carbon concentration was higher when the biomass was harvested in early spring (March) than in the fall (November) [
35,
38]. Sida biomass is less abundant in carbon than woody plants. In the group of herbaceous plants, the carbon content of Virginia fanpetals biomass is similar to that noted in
H. salicifolius,
H. tuberosus, and
S. pectinata and higher than that reported in
M. sinensis,
A. donax, and
S perfoliatum [
35]. Ţîţei [
20] demonstrated that Sida biomass was more abundant in carbon than eight other perennial energy crops.
The average hydrogen concentration in Sida biomass was 5.98% DM. Hydrogen content was highest in the biomass harvested in the 13th growing season (2022) and, similar to carbon content, lowest in the biomass harvested in the first growing season (2010) (
Figure 3B). In these years, hydrogen content was also differentiated by the type of propagating material and plant density. Hydrogen concentration in the biomass was minimally higher in plants propagated from seeds than from seedlings. This difference reached only 0.03 pp, but it was statistically significant.
Similar to carbon, hydrogen is also one of the most important components of solid fuels. The hydrogen content of Sida biomass approximates 6% [
37,
41,
43]. Virginia fanpetals biomass is more abundant in hydrogen than other herbaceous plants, and its hydrogen content is only somewhat lower than that noted in many species of woody plants [
20,
35]. In a study by Stolarski et al. [
35], hydrogen concentration in the biomass was much higher when Virginia fanpetals was harvested in March than in November or January. Other authors did not report such clear differences in the hydrogen content of the biomass across harvest seasons [
37,
38].
All experimental factors and their interactions induced small, but significant, differences in the sulfur content of Sida biomass. Sulfur concentration was most significantly differentiated by year. During the 14-year experiment, the sulfur content of the biomass ranged from 0.035% DM in the second growing season to 0.058% DM in the sixth growing season. Sulfur concentration in the biomass harvested in treatments where Virginia fanpetals was grown from seeds was 0.004 pp higher in comparison to treatments where plants were grown from seedlings and 0.09 pp higher in comparison to treatments where plants were grown from rhizomes. Sulfur concentration was highest in the seventh harvest season in treatments where seedlings were planted at a density of 60,000 seedlings ha
−1 (0.084% DM) and lowest in the second growing season in treatments where rhizomes were planted at a density of 60,000 rhizomes ha
−1 (0.029% DM) (
Figure 3C). Similar observations were made by other researchers [
20,
41,
46]. Significantly higher sulfur concentration in Sida biomass was reported only by Bilandžija et al. [
37]. In other studies, the biomass of
S. hemaphrodita was characterized by the significantly lowest sulfur content (0.031% DM on average) in the analyzed group of 26 genotypes of perennial energy crops, including woody and perennial plant species [
35,
47]. Stolarski et al. [
35] and Bilandžija et al. [
37] found that sulfur content decreased when harvest was delayed from fall to early spring.
The average nitrogen content of Sida biomass was 0.41% DM, and similar to sulfur content, it was differentiated by all examined factors and their interactions, in particular, the year, which contributed most to the observed variation. Nitrogen concentration was highest in the biomass harvested after the first growing season (0.736% DM) and lowest in the biomass harvested after the fifth (0.254% DM), third (0.258% DM), and second (0.269% DM) growing season, and the values noted in the fifth, third, and second harvest season formed a homogeneous group. Nitrogen concentration was highest in the biomass harvested from treatments where Virginia fanpetals was grown from seeds, and it was 0.038 pp higher in comparison to treatments where plants were propagated from seedlings and 0.059 pp higher in comparison to treatments where plants were grown from rhizomes. An analysis of the interactions between all experimental factors and years revealed that the nitrogen content was highest in the biomass harvested after the first growing season from treatments where plants were grown from seeds sown at 1.5 kg ha
−1 (1.140% DM) and lowest in the biomass harvested after the fifth growing season (2013) from treatments where plants were grown from seeds sown at 4.5 kg ha
−1 (0.240% DM) (
Figure 3D). The lowest nitrogen values belonged to a homogeneous group of values (0.240–0.380% DM) that were noted across years in 39 treatments with different types of propagating material and plant densities. Nitrogen concentration was bound by a strong positive correlation with ash content and a moderate positive correlation with sulfur concentration. Weak, but significant, negative correlations were found between nitrogen concentration and the carbon and hydrogen content of Sida biomass.
The nitrogen content of Sida biomass noted in this study approximated the values reported by other authors [
20,
37,
38,
42,
48]. Studies examining other species of perennial energy crops revealed that Virginia fanpetals biomass was characterized by the lowest nitrogen concentration [
20,
35,
48]. In this respect, Sida biomass meets the solid biofuel requirements stipulated in Standard DIN EN ISO 17225-7:2014-09 [
9]. Stolarski et al. [
35] and Šurić et al. [
38] found that Virginia fanpetals biomass was less abundant in nitrogen when harvested in early spring (March) than in late fall (November).
The elemental composition of Sida biomass is determined by various factors, including environmental conditions, fertilization, plantation age, plant growth stage, and others. When Virginia fanpetals is grown for solid biofuel, its biomass consists mostly of dry stems. The quality of such raw material remains relatively stable. According to Siaudinis [
43], the crude fiber content (strongly correlated with the HHV) is lower when Virginia fanpetals plants still have leaves and flowers. The content of cellulose and lignin in stems stabilizes already during flowering and remains relatively unchanged until the end of December, whereas it increases in leaves over the same period [
9]. Therefore, the proportion of plant parts other than stems (leaves, seeds) in the biomass harvested at the end of the growing season may also contribute to variability in its elemental composition. Sida biomass harvested in winter is characterized by lower concentrations of nitrogen and sulfur, higher carbon content, and higher LHV than in Sida biomass harvested at the end of the growing season.