3.1. Biomass Yield and its Energy Value
Both the poplar biomass yield and its energy value were significantly differentiated by the clone, harvest rotation and the interaction of these attributes (
Table 1). The significantly largest fresh and dry biomass yields were obtained from the UWM 2 clone—89.5 Mg ha
−1 and 40.0 Mg ha
−1 from the two 4-year rotations, respectively (
Table 2). These figures for the UWM 3 and UWM 1 clones were lower by ca. 30% and 40%, respectively, and the yield from the UWM 4 clone was more than twice smaller than that of UWM 2. The average yields of fresh (87.4 Mg ha
−1) and dry biomass (40.6 Mg ha
−1 DM—dry matter) of poplar in the second 4-year rotation were significantly higher (by 260% and 278%, respectively) compared to the average yield from the first harvest rotation. The significantly largest average yields of fresh (119.7 Mg ha
−1) and dry biomass (54.5 Mg ha
−1 DM) were obtained from the UWM 2 clone in the second 4-year harvest rotation. Expressed per 1 year of the plantation use, they were 29.9 Mg ha
−1 year
−1 and 13.6 Mg ha
−1 year
−1 DM, respectively. However, the average dry biomass yield for the four poplar clones under study was lower in the second rotation: 10.1 Mg ha
−1 year
−1 DM, and it was very low in the first rotation: merely 3.6 Mg ha
−1 year
−1 DM
In a different study conducted in Poland, the yield of poplar
Populus nigra ×
P. Maximowiczii Henry cv. Max-5 in a 4-year harvest rotation was 8.2 Mg ha
−1 year
−1 DM [
11]. The experiment cited above demonstrates the significant effect of different methods of soil enrichment on poplar yield; it was the highest on a plot in which lignin was applied in combination with mineral fertilisers (10.5 Mg ha
−1 year
−1 DM). The poplar yield on a control plot, with no soil enrichment, was lower by 48%. A higher yield from a poplar plantation was obtained in Italy [
32]. A clone of
Populus deltoides L. in a 2-year harvest rotation gave a yield of 11.7 Mg ha
−1 year
−1 DM. Moreover, extending the harvest rotation to three and four years had a significant positive effect on the poplar yield, causing it to increase to 15.0 and 18.4 Mg ha
−1 year
−1 DM. A similarly high yield of poplar in a 4-year harvest rotation (18.0 Mg ha
−1 year
−1 DM) was found for the clone
Populus maximowiczii × P. nigra (NM6) in Canada [
20]. A very high yield of six genotypes of poplar in three consecutive two-year harvest rotations was obtained in Italy by Sabatti et al. [
33]. In that experiment, poplar was grown at an agricultural site with highly productive soil and a large amount of nitrogen fertilisation. Biomass production differed significantly among rotations starting from 16 Mg ha
−1 year
−1 DM in the first, peaking at 20 Mg ha
−1 year
−1 DM in the second, and decreasing to 17 Mg ha
−1 year
−1 DM in the third rotation. However, other authors have reported that seven clones of poplar of the
Populus ×
canadensis and seven of the
Populus deltoides species grown in Italy did not give such a high yield [
34]. It was found after nine years of poplar cultivation and four harvest rotations that the average yield of biomass of the
Populus deltoides species was 9.7 Mg ha
−1 year
−1 DM, and it was lower for clones of the
Populus x
canadensis species—5.6 Mg ha
−1 year
−1 DM. It was shown in the study cited above that the average yield for 14 poplar clones was 8.34 Mg ha
−1 year
−1 DM. The yield of poplar obtained in other studies also varied (1.6–28 Mg ha
−1 year
−1 DM) depending on the climatic conditions, the type of soil, species and clone, harvest rotation, age of the plantation, level of fertilisation and other agricultural procedures [
35,
36,
37,
38,
39]. It was also demonstrated that the annual yield of poplar biomass increased with the age of plants up to a maximum between the third and fourth growing seasons [
40]. In a study in Belgium, the average dry biomass yield of poplar established on degraded land and maintained as a low-energy input system during the fourth rotation was 4.3±3.4 Mg ha
−1 year
−1 DM across all clones, but the most productive clones yielded up to 10.5 Mg ha
−1 year
−1 DM [
41]. Therefore, it can be claimed that the yield was very similar to that obtained in the current study.
The energy value of poplar yield (calculated as the product of fresh biomass yield and its LHV) harvested in two consecutive 4-year rotations was 462 GJ ha
−1, with a high standard deviation of 270 GJ ha
−1, which showed a high differentiation of this parameter between clones and harvest rotations (
Table 2). The significantly largest yield energy value was obtained from the UWM 2 clone—672 GJ ha
−1 on average from the two 4-year rotations (
Table 2). The mean value of this attribute for the UWM 3, UWM 1 and UWM 4 clones was lower by ca. 31%, 40% and 54%, respectively. The mean biomass yield energy value for the second 4-year rotation was higher by as much as 290% compared to the value from the first harvest rotation. The significantly highest yield energy value was found in the UWM 2 clone in the second harvest rotation: 230.6 GJ ha
−1 year
−1 (
Figure 1). The energy value of UWM 1 and UWM 3 clones in the second rotation was nearly 170 GJ ha
−1 year
−1 (homogeneous group b). Meanwhile, the attribute value for the UWM 4 clone was approx. 120 GJ ha
−1 year
−1 (homogeneous group c). Furthermore, in the first rotation, only the yield energy value of the UWM 2 clone exceeded 100 GJ ha
−1 year
−1 (homogeneous group c), and the other clones were in the last (homogeneous group d). Different results of yield energy value resulted mainly from different yields and biomass LHV, both between the clones and the rotations, since the yield energy value was calculated based on these two parameters. In the first rotation, the yield and biomass LHV were significantly lower than those obtained in the second rotation. The higher biomass yield in the second rotation resulted from the fact that poplar plants developed a considerably more developed root system compared with the plants of the first rotation when the root system underwent systematic expansion. It should be concluded that the differences in yield energy value between the studied clones were of genetic background since poplars were cultivated in analogous climatic and soil conditions.
The highest yield energy value for
Populus nigra ×
P. Maximowiczii Henry cv. Max-5 (177 GJ ha
−1 year
−1) in the authors’ other study was achieved on soil enriched with lignin and mineral fertilisers. The value of the attribute with other soil enrichment combinations was lower by 11–48% [
11]. A similar energy value of poplar yield of 188 GJ ha
−1 year
−1 was achieved in its production in a two-year harvest cycle [
42]. In other studies conducted in Italy, the energy value of poplar biomass grown in different harvest cycles, with mineral fertilisation and irrigation, was much higher—257 GJ ha
−1 year
−1 [
43] and 270 GJ ha
−1 year
−1 [
15]. Nassi o Di Nasso et al. [
44] analysed the effect of harvest rotation cycles (annual, biannual, triennial) on the energy balance of a 12-year-old short-rotation coppice poplar with mineral fertilization and obtained an even higher energy value of biomass of 450 GJ ha
−1 year
−1 after the first triennial cutting cycle. However, the energy value of the yield decreased to 350, 289 and 127 GJ ha
−1 year
−1 in the second, third and last triennial cutting cycle, respectively, which resulted in an average of 304 GJ ha
−1 year
−1 during the whole period of the plantation use. On the other hand, the energy value of poplar yield obtained in extensive cultivation in a 4-year harvest rotation was much lower (70.9 GJ ha
−1 year
−1) [
45]. This was confirmed by a study conducted by Dillen et al. [
41], which showed that the energy value of the yield of poplar grown on degraded land was about 92 GJ ha
−1 year
−1. However, when poplar was grown in Sweden (long harvest cycle), the gross energy yields over the plantation live cycle (24 years) were 4710 and 4430 GJ ha
−1, for fertilized and unfertilized poplar, respectively, which corresponded to 196 and 185 GJ ha
−1 year
−1, respectively [
26]. Therefore, the mean yield energy value obtained in the current study in north-eastern Poland for the UWM 2 clone (168 GJ ha
−1 year
−1) and, especially, the much higher value in the second 4-year harvest rotation (231 GJ ha
−1 year
−1), should be seen as high and satisfactory.
3.2. Thermophysical Properties and Elemental Composition of Biomass
The poplar biomass moisture content was significantly differentiated by the clone and harvest rotation, while the fixed carbon content was only significantly differentiated by the harvest rotation (
Table 1). The average poplar biomass moisture content in the experiment was 54.8% wb (
Table 3). The UWM 3 and UWM 2 clones contained significantly more moisture compared to the other two clones. Moreover, the moisture content in the first 4-year harvest rotation (56.2%) was significantly (by 2.8 p.p.) higher compared to the mean moisture content in the second harvest rotation. The average content of fixed carbon and volatile matter in 4-year poplar shoots was 18.6% DM and 79.4% DM, respectively. In another study by the authors, the fixed carbon and volatile matter content in 4-year old shoots of
Populus nigra ×
P. Maximowiczii Henry cv. Max-5 was 20.8% d.m and 77.4% DM, respectively, and the moisture content was 55.7% [
46]. Other studies have also confirmed that the moisture content in poplar upon harvest is high and exceeds 50%; sometimes even exceeding 60% [
33,
47,
48,
49,
50].
The ash content in poplar biomass was significantly differentiated by harvest rotations and the interactions between the harvest rotation and the clone and was 1.4% DM on average (
Table 1 and
Table 4). The ash content in 4-year old poplar trees throughout the experiment ranged from 1.0 to 1.8% DM. On the other hand, HHV was significantly differentiated by the harvest rotation, and LHV was significantly differentiated by the clone and harvest rotation (
Table 1). LHV of poplar biomass as measured in this experiment was 7.5 MJ kg
−1 (
Table 4). The LHV values of UWM 4 and UWM 1 clones (homogeneous group A) were significantly higher compared to the other two clones (homogeneous group B). The LHV of poplar obtained in the second 4-year harvest rotation was significantly higher compared to the first harvest rotation. In another study, the average ash content in 4-year old shoots of
Populus nigra ×
P. Maximowiczii Henry cv. Max-5 was slightly higher (1.8% DM), and the LHV was very similar (7.5 MJ kg
−1) [
46]. Different LHV results in our study were determined by different moisture content and biomass HHV between the rotations since LHV was calculated based on these two parameters. In the second rotation, biomass moisture content was lower while HHV was higher, which resulted in a higher LHV when compared with those values obtained in the first rotation. Other studies have also shown that the ash content in poplar biomass may vary (0.98–3.12% DM) depending on the cultivar/clone, harvest rotation and other factors [
33,
47,
48,
49].
An assessment of the content of selected elements found that only the harvest rotation affected the C and Cl content (
Table 1). Significantly higher carbon content (average 51.5% DM) and lower chlorine content (average 0.009% DM) were found in the second 4-year harvest rotation (
Table 5 and
Table 6). The mean content of C, H, S, N and Cl in 4-year old poplar shoots was 51.1, 6.0, 0.028, 0.41 and 0.013% DM, respectively. A significant positive correlation was found between the S and N content (0.91) and between the ash content and N content (0.67) and S content (0.63) (
Table 7). Moreover, a positive correlation was found between the fixed carbon content and the S, N and Cl content. The C, H, S and N content as determined in the authors’ earlier examinations of 4-year old shoots of
Populus nigra ×
P. Maximowiczii Henry cv. Max-5 was similar: 51.0, 5.8, 0.024, 0.44% DM [
46]. The C, H, S and N content in other studies was also similar: 50.3, 6.1, 0.03 and 0.42% DM [
50]. A slightly higher content of C, H, S (51.8, 6.4, 0.04% DM, respectively) and a lower content of N (0.16% DM) in biomass of
Populus x
euroamericana (Dode) Guinier (AF2) was found by Monedero et al. 2017 [
49]. Considering the above, it could be concluded that poplar biomass may serve as fuel with potentially low SO
2 and particulate emission due to the low content of sulphur and ash. Moreover, the type of biomass conversion technology and equipment (e.g., boilers) used are of great importance here as well. It should be stressed that burning poplar biomass in small boilers with manual fuel feeding (old type) and stove, like any other solid fuel, may pose problems with black carbon, polycyclic aromatic hydrocarbons and volatile organic compounds emissions. Therefore, the use of modern automatic boilers for home heating and fluidal boilers in power plants and heat power plants is recommended. Such combustion technologies ensure low emissions of atmospheric pollutants [
51,
52,
53]. Combustion of the studied poplar biomass should not cause any significant corrosion of boiler elements since this biomass contains low amounts of chlorine and the S/Cl ratio based on the current study averaged 2.2. It is accepted that high-temperature corrosion occurs intensively when the fuel chloride contents are over 0.2%, and the S/Cl ratio is below 2.2 [
54,
55].