3.1. Energy Value of Yield
The energy value of Eko-Salix willow biomass yield was significantly differentiated by the type of soil site and by the varieties or clones and by their interactions as shown in
Table 1. The yield energy value for the 7-year rotation willow was 957 GJ ha
−1 (
Table 2). The energy value of the yield at Kocibórz was the significantly highest: 1119 GJ ha
−1. It was higher than at Obory and Leginy by 114 GJ ha
−1 and by 372 GJ ha
−1, respectively. The Ekotur variety had the significantly highest biomass energy value (1522 GJ ha
−1) among the varieties and clones in the study. This value was significantly higher than for the UWM 095 clone and for the Turbo (homogeneous group b and c) and Tur (homogeneous group d) varieties. However, the lowest biomass yield energy value was determined for the UWM 200 clone (541 GJ ha
−1, homogeneous group f).
The yield energy value as converted to a year of plantation use ranged from 74 to 288 GJ ha
−1 year
−1 as shown in
Figure 1. The Ekotur gave the highest yield energy value at all three sites, whereas this value was significantly different at each site. The highest energy value for this variety was determined at Kocibórz, where the plants grew on organic, peat−muck soil (average 288 GJ ha
−1 year
−1). It was lower by 64 GJ ha
−1 year
−1 at Obory (riparian, heavy complete humic alluvial soil) and was lower by 149 GJ ha
−1 year
−1 at Leginy (very heavy mineral clay soil). The UWM 095 clone biomass energy value was higher at Obory than at Kocibórz (homogeneous groups c and d, respectively) and was significantly lower at Leginy (homogeneous group g). The Tur variety gave a low biomass yield with a low energy value both at Obory and at Kocibórz (homogeneous group f) at both sites). The UWM 200 clone gave a biomass yield with a very low energy value (74–78 GJ ha
−1 year
−1) at all three experiment sites (homogeneous groups l and k).
It is noteworthy that the yield energy value obtained in the experiment from Eko-Salix willow grown on marginal soils and harvested in a 7-year-long rotation was highly varied, both by the soil site, variety and clone. Ekotur and UWM 095 had much higher yield energy value of wood compared to the other willow varieties or clones. Moreover, the high yield energy value for these two varieties was a consequence of much higher biomass yield compared to the yield of the other varieties and clones because the yield energy value was calculated based on the fresh willow biomass yield and its LHV. Furthermore, the yield differentiation between the willow varieties and clones had a genetic foundation as all clones were grown in a similar manner. Moreover, other studies, where SRC willow was grown in 2–4-year rotations, also showed that the yield energy value varied depending on factors such as species, variety or clone, harvest cycle, fertilisation and other agrotechnical factors. For example, the biomass energy value for
Salix viminalis harvested in a 2-year rotation in Canada ranged between 73–290 GJ ha
−1 year
−1 [
28], which was similar to the current results in the Eko-Salix system. Meanwhile, the average annual gross energy yields for 3-, 4- and 5-year rotation willow in Sweden (depending on the nitrogen fertilisation rate: N–high, N–medium, and N–zero) were: 185, 138 and 88 GJ ha
−1 year
−1, respectively [
32]. In a different study with willow production in a 3-year harvest rotation in Poland, the yield energy value ranged from 46 to 242 GJ ha
−1 for UWM 155 and UWM 006 clone, respectively [
33]. The biomass energy value in the cultivation of a 4-year rotation of willow on poor quality soil depended on the method of soil enrichment and ranged from 88 to 175 GJ ha
−1 year
−1, for a nonfertilisation site and for a site where lignin, mycorrhiza and mineral fertilisation were applied simultaneously, respectively [
27]. On the other hand, in the cultivation of willow in 4-year rotation on very good soil, the average yield energy value was much higher (243 GJ ha
−1 year
−1) [
29]. An equally high yield energy value was obtained in Italy in a study with poplar grown in different harvest rotations with mineral fertilisation and irrigation: 257 GJ ha
−1 year
−1 [
34] and 270 GJ ha
−1 year
−1 [
35]. Furthermore, very high yield energy value (450 GJ ha
−1 year
−1) was achieved in another study, in the first three-year harvest rotation for poplar with mineral fertilisation [
36]. A much lower yield energy value (70.9 GJ ha
−1 year
−1) was achieved in an extensive poplar cultivation in a 4-year harvest rotation [
4]. In another study, in which poplar was also obtained in a 4-year harvest rotation, the yield energy value ranged between 93–177 GJ ha
−1 year
−1 depending on the soil enrichment method [
27]. The poplar yield energy value was also high (ca. 190 GJ ha
−1 year
−1) in a 24-year-long plantation life cycle [
37]. Furthermore, black locust as the third SRC species achieved lower yield energy value between 29 and 95 GJ ha
−1 year
−1 in a 4-year rotation depending on the soil enrichment method [
27]. This index was much higher in the 6-year harvest rotation (190 GJ ha
−1 year
−1) [
38]. The yield energy value for other perennial herbaceous crops, e.g.,
Sida hemaphrodita varied and lay within the range of 79–226 GJ ha
−1 year
−1 [
20,
39,
40,
41].
Miscanthus ×
giganteus is often mentioned as a species with the highest potential of yield energy value of all perennial grasses, which can reach 235–479 GJ ha
−1 year
−1 [
42,
43]. However, it was demonstrated in other studies that its yield energy value was much lower in the first years of cultivation and it reached 118 GJ ha
−1 year
−1 in the fourth year [
20]. On the other hand, the much higher yield energy value in the study cited above was achieved for another grass species,
Miscanthus sacchariflorus – 175 GJ ha
−1 year
−1. It must be stressed that such a large diversity of the yield energy value in the literature reports was mainly caused by the species and variety choice, soil quality, climatic conditions, fertilisation and agrotechnical measures, harvest cycle, etc. These factors affected the biomass yield and, in consequence, the yield energy value. Compared to these data, the yield energy value of Eko-Salix willow, achieved in the current experiment for the Ekotur variety and the UWM 095 clone should be regarded as high and satisfactory.
3.2. Thermophysical Properties and Elemental Composition of Biomass
The higher heating value determined in the willow dry matter obtained in the 7-year rotation in this experiment was 19.5 MJ kg
−1 d.m. on average, as shown in
Table 2. This value was slightly (although significantly) lower in the biomass obtained at Obory and Kocibórz (homogeneous group B) than at Leginy (homogeneous group A). The higher heating value determined in biomass of the Ekotur variety and the UWM 200 clone was the same (homogeneous group b) and significantly lower than in the other varieties in the study (all—homogeneous group a). On the other hand, the lower heating value of fresh Eko-Salix willow biomass at the three sites was not significantly different at 8.6 MJ kg
−1, as shown in
Table 1 and
Table 2. The value of this feature in the Tur variety (
Salix viminalis, homogeneous group a) was 9.2 MJ kg
−1 and it was higher by 1.2 MJ kg
−1 than in the UWM 095 and UWM 200 clones (both
Salix alba species, homogeneous group d). In a different study, the lower heating value for willow harvested in a 4-year rotation ranged from 8.3–8.5 GJ Mg
−1 [
27]. A similar value for willow harvested in the same rotation was found by Monedero et al. [
44]. The lower heating value for willow harvested in a 3-year rotation ranged from 7.7–9.3 GJ Mg
−1, depending on the clone [
45]. Moreover, the LHV depends on the moisture content for each biomass type. A very strong negative correlation between the biomass moisture content and LHV (−0.99) was also demonstrated in the current experiment as shown in
Table 3. Furthermore, the biomass moisture content depends on its type, plant species, harvest date and weather conditions during the harvest. Three groups are identified among perennial energy plants: (1) SRC, which give woody biomass and included willow; (2) herbaceous crops, which give semiwoody biomass; (3) grasses which give biomass as straw [
25]. The highest moisture content at harvest as determined in the studies cited above was observed in poplar, which is why the heating value for this species was lower and ranged from 6–7 GJ Mg
−1. In contrast, LHV for black locust was much higher and exceeded 10 GJ Mg
−1. Delaying the harvest of herbaceous crops and grasses from November to March had a significant, beneficial effect on the biomass moisture content decrease and, in consequence, it increased the biomass heating value to as much as 15 GJ Mg
−1. Such a high LHV was also achieved for cereal straw, which is production waste in grain production, and is obtained in summer after plants have naturally dried [
30]. On the other hand, woody biomass of agricultural origin (willow, poplar) or obtained from forests (pine, birch) can have high LHV, provided that their moisture content was reduced by natural or forced drying, e.g., during biomass storage [
30,
46,
47].
The average moisture content determined in the biomass of 7-year old Eko-Salix willows was 49.8% as shown in
Table 4. The values of this feature for biomass obtained at the three sites in the study did not differ significantly, as displayed in
Table 1. The biomass moisture content for the UWM 095 and UWM 200 clones did not differ (homogeneous group a) and was significantly higher than that determined in the biomass of the Ekotur (homogeneous group b), Turbo and UWM 046 (c) and Tur (d). In other studies, the moisture content of willow biomass harvested in 3- and 4-year rotations exceeded 50% [
44,
45]. As mentioned above, in the discussion of LHV, among the SRC species, lower moisture content compared to willow was observed in black locust (ca. 40%), and higher moisture content was observed in poplar (ca. 60%) [
25]. High moisture content (40–60%) was determined in the fresh biomass of forest trees or production residues (e.g., sawdust) produced in the processing of fresh wood. The moisture content in sawdust, shavings, etc. produced in the processing of naturally seasoned or thermally dried wood can be below 10%. Low moisture content, usually below 15–20%, was also determined for the straw of cereals and oily crops harvested in summer [
30] and biomass of herbaceous crops and grasses obtained just before the start of a new growing season (March–April) [
25].
Shown in
Table 1, the mean ash content in the biomass of 7-year old willow obtained at the three sites in the study did not differ significantly and was 1.3% d.m., as displayed in
Table 4. A beneficial, significantly lower ash content was found in the biomass of the Tur and Ekotur varieties (homogeneous group c) than in the biomass of the Turbo variety (homogeneous group a). A higher ash content (1.9% d.m.) was found in the biomass of 4-year rotation willow [
44]. An even higher ash content in willow biomass (more than 3%) was found in another study [
48]. Nevertheless, the ash content in woody SRC biomass was usually lower (1–2% d.m.) compared to the ash content in biomass of perennial herbaceous crops (semiwoody biomass, 2–4% d.m.) and grasses (straw, 2–4% d.m.) [
25]. An even higher ash content, often exceeding 5% d.m., was determined in the straw of cereals and oily crops. The lowest ash content was determined in clean, debarked wood (below 0.5% d.m.), and the value did not exceed 1% d.m. in non-debarked wood obtained from a forest [
30]. The average ash content in residue biomass from palm kernel shell (PKS) in the studies cited above was 2.35% d.m., and other data show that the ash content in PKS ranged from 1.3–10.8% d.m. [
49]. Due to mineral contaminations of biomass, such as sand, clay, etc., which may be present at various stages of the logistic process, the ash content in solid biofuels received by the end recipient may be higher than given above.
The average content of volatile matter as determined in the current study in the willow biomass was 79.7% d.m., as shown in
Table 4. A slightly lower volatile matter content was found in biomass at Leginy (homogeneous group B) than at Obory and Kocibórz, where the content was similar (homogeneous group A). The value of this feature in the UWM 046 clone biomass was slightly lower (homogeneous group B) than in the other varieties and clones, whose values did not differ (homogeneous group a). A higher volatile matter content (83.6% d.m.) was found in the biomass of a 4-year willow rotation [
45]. Furthermore, the fixed carbon content, as determined in biomass obtained at Leginy in this study, was slightly higher (homogeneous group A) than at the other two sites, where the values were similar (homogeneous group B). The highest fixed carbon content was found in biomass of the Turbo variety (19.8% d.m.). The same fixed carbon content was found in the biomass of a 3-year willow rotation [
46]. The fixed carbon content in the current study was significantly positively correlated with LHV and with the sulphur content, as shown in
Table 3.
The elemental composition of a 7-year Eko-Salix willow rotation biomass was significantly differentiated by the soil site and the varieties or clones and their interactions, as displayed in
Table 1. The average carbon content in the biomass was 51.1% d.m., shown in
Table 5. A slightly higher content of this element was determined in the biomass obtained at Obory (homogeneous group A) than in the biomass obtained at the other two sites (similar values, homogeneous group B). The carbon content determined in the biomass of the Tur variety was the highest (homogeneous group a), and the significantly lowest content of this element was found in biomass of the Ekotur variety and the UWM 095, UWM 200 clones (homogeneous group c). The carbon content in willow biomass was significantly and positively correlated with the hydrogen content and with the LHV, as shown in
Table 3.
The hydrogen content in biomass at the three sites ranged from 5.8% to 6.0% d.m. (homogeneous group B and A), as displayed in
Table 5. A lower content of the element was found in biomass of the UWM 095 and UWM 200 clones (homogeneous group b) than in the other varieties and clones (similar values, homogeneous group a). The average sulphur content in biomass obtained from the 7-year rotation willow in this experiment was 0.039% d.m. The highest element content was found in the biomass at Leginy (homogeneous group A); it was significantly higher than at Obory (homogeneous group B) or Kocibórz (homogeneous group C). The sulphur content in the biomass of the clones and varieties in the study differed significantly and ranged from 0.029% d.m. in UWM 200 to 0.052% d.m. in the Turbo variety. The nitrogen content in the willow biomass at Leginy was significantly lower (homogeneous group C) than at Obory (homogeneous group B) and Kocibórz (homogeneous group A). The nitrogen content in the biomass of UWM 046 (0.37% d.m.) was significantly higher than in the Ekotur variety (0.28% d.m.). The sulphur content was significantly negatively correlated with the volatile matter content and was significantly positively correlated with ash and fixed carbon content, HHV and LHV, as shown in
Table 3.
The elemental composition of biomass depends (among others) on the species, rotation length and harvest date as well as on the soil chemical composition. Therefore, for example, in a different study [
44], willow biomass harvested in a 4-year rotation contained less carbon, more hydrogen and nitrogen and similar sulphur content compared to the current study. In another study, willow biomass was also found to contain less carbon and more nitrogen [
45,
48]. Considering the elemental composition of different biomass types, it should be noted that the carbon content in cereal or rape straw is usually lower (below 50% d.m.) compared to woody biomass, which usually contains over 50% d.m. of carbon. Moreover, the carbon content in old wood, especially that obtained from forests, can exceed 54% d.m., whereas it ranges from 50–52% d.m. in younger wood from energy SRC crops [
30]. Moreover, the studies cited above show that woody biomass of forest origin is a fuel of higher quality than other solid fuels due to its low content of sulphur and chlorine (below 0.02% d.m.) and nitrogen (below 0.2% d.m.). Much higher content of sulphur, chlorine and nitrogen was determined in wheat straw—0.13, 0.20 and 1.2% d.m., respectively. Even larger amounts of these undesirable elements were found in rape straw (0.32, 0.42 and 1.3% d.m., respectively). Therefore, it can be claimed that the willow biomass obtained in the current experiment in a 7-year harvest rotation was a fuel of much better quality in terms of its elemental composition compared to straw. Moreover, the content of these elements in willow wood was slightly higher, but also more similar to their content in forest wood.
The findings of this study can be applied in commercial production of Eko-Salix willow biomass, especially on soil where ploughing before setting up a plantation is difficult or impossible. However, one should emphasise that it is a challenge in setting up an Eko-Salix plantation to prepare long cuttings (more than 2 m long) from two- or three-year-old shoots of properly selected willow clones or varieties. Therefore, in order to set up an Eko-Salix plantation, one should identify the origin of the planting material and agree on the time and form of its preparation. Harvesting willow in a 7-year rotation is also a challenge because of a large shoot diameter, as well as the plants’ height and weight. Manual harvest is possible in small-area plantations. On the other hand, harvesters with cutting heads, used for obtaining forest wood, can be used in large plantations. However, soil must be frozen in winter or the soil load-bearing capacity in winter must be sufficient so that it is possible for heavy equipment to enter the plantation. It should also be noted that in the future, one must examine and analyse the intensity of growing of new shoots out of stumps after the willow harvest and determine the productivity and biomass quality from successive rotations. It will be an interesting issue and a challenge to analyse and determine the number of harvests and the total biomass yield energy value for the whole use period for Eko-Salix plantations.