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Article

Quality of Pellets Obtained from Whole Trees Harvested from Plantations, Coppice Forests and Regular Thinnings

by
Francesco Latterini
1,
Vincenzo Civitarese
1,
Małgorzata Walkowiak
2,
Rodolfo Picchio
3,*,
Zbigniew Karaszewski
2,
Rachele Venanzi
3,
Mariusz Bembenek
4 and
Piotr S. Mederski
4
1
Consiglio per la Ricerca in Agricoltura e l’Analisi dell’Economia Agraria, Centro di Ricerca Ingegneria e Trasformazioni Agroalimentari, Via della Pascolare 16, 00015 Monterotondo, Italy
2
Łukasiewicz Research Network–Poznań Institute of Technology, Center of Wood Technology, Winiarska 1, 60-654 Poznan, Poland
3
Department of Agriculture and Forest Sciences, University of Tuscia, Via San Camillo de Lellis, 01100 Viterbo, Italy
4
Department of Forest Utilisation, Faculty of Forestry and Wood Technology, Poznań University of Life Sciences, Wojska Polskiego 71A, 60-625 Poznań, Poland
*
Author to whom correspondence should be addressed.
Forests 2022, 13(4), 502; https://doi.org/10.3390/f13040502
Submission received: 10 February 2022 / Revised: 9 March 2022 / Accepted: 21 March 2022 / Published: 23 March 2022
(This article belongs to the Section Wood Science and Forest Products)
Browse Figure
Versions Notes

Abstract

:
Woody biomass represents a sustainable type of fuel that is marketed directly as firewood or pre-treated by grinding, drying and compressing. Densified biomass fuels provide benefits in transport, storage and handling, and wood pellets in particular are one of the most commercially attractive products. With this in mind, the objective of the presented research was to evaluate the quality of various pellets obtained from different tree parts of five species (and various age): Eucalyptus spp., Fagus sylavtica L., Pinus eldarica Medw., Pinus radiata D.Don, and Robinia pseudoacacia L. In all the analysed cases, the wooden material was harvested according to the whole tree system (WTS), with their branches and leaves. The samples were chipped, refined by a shredder and dried. Pelletisation was finally carried out using a 4 kW Bianco line (Cuneo, Italy) pelletiser. Physical and chemical analyses were undertaken to determine the quality of the pellets as an energy source. Results of this classification showed that the best pellets, mostly due to better mechanical properties, were from plantation trees of Eucalyptus spp. (6- and 3-year-old) and from thinnings of Fagus sylavtica L. (70-year-old), whereas the least attractive types were made from 18-year-old, coppice Eucalyptus spp. and 3-year-old Robinia pseudoacacia L. Regarding the classification developed within this study, all the pellet types were found to be classifiable as none exceeded the toxic content limit. In terms of the influence of the species, management system and stem age on pellet quality, this study showed the presence of a link between cycle duration and pellet quality, with increasing stem age resulting in better pellet characteristics.

1. Introduction

Many factors have contributed to the steady rise in the use of renewable energy, such as the need to mitigate climate change [1,2,3], the global increase in demand for energy [4,5], the petroleum market and its availability, and the lower environmental impact of renewable fuels [6,7,8,9].
Woody biomass represents a sustainable type of fuel that is marketed directly as firewood or pre-treated by grinding, drying and compressing [10,11,12,13]. Densified biomass fuels provide benefits for transport, storage and handling [14,15,16], and wood pellets in particular [17,18,19,20,21] are one of the most commercially attractive products that can be placed on the market, with a price of over EUR 300 per ton in 2018 [22]. Moreover, pellets can be produced from several different feedstocks, which are also derived from the agriculture sector, such as bagasse, sorghum and soybean residues [23,24].
Europe is an important producer of pellets, but it is also a big consumer and importer [25,26,27]. After the United Kingdom, Finland and Sweden, other significant consumers are Italy, Denmark, Belgium and the Netherlands [28].
Increasing and improving the pellet chain, not only from agroforestry dedicated plants and orchards [29], but also from silviculture [30], may be an effective way to attain sustainable forest management (SFM) goals [31]. In particular, it is possible to obtain biomass for pellet production from forest operations, starting with the first commercial thinning [32,33,34], but also from the conversion of coppice forests to high stands.
The coppice itself can be a good source of wood for pellets, particularly in the Mediterranean zone [22]. However, coppice forests in the Mediterranean context have often been abandoned. In particular, this occurs in the protection of forests, which, however, need some management, such as periodic harvesting to support good forest health. However, the landscape conditions that characterise a protection forest, e.g., extremely steep terrain, sensitive soil, a remote location and the prescriptions of a specifically designed silviculture, tend to constrain harvesting and make it particularly difficult [35,36,37,38,39]. Some authors also indicate that pellet bioenergy systems have a low environmental impact globally but have a high impact on local areas [40].
Small-scale pellet production in rural zones can also contribute to the income of local farmers, and small and medium-sized forest enterprises [41,42,43]. To achieve this aim, it is important to evaluate the quality of the pellets produced from raw materials derived from forest or agroforest plants, without the need for pre-pelletisation steps such as bark or leaf removal.
High value pellets are generally produced at the industrial scale from pure sawdust [3,44]. In comparison, pellets manufactured from different feedstocks, with a high share of bark or leaves, can have lower quality [45,46].
In a comparison of the quality of poplar pellets produced from whole trees or debranched stems, Picchio et al. found that pelletised material with branches and leaves significantly decreased bulk density and ash content [22]. By comparison, detailed studies on this topic concerning the use of different tree species are missing in the current literature. The attention of the literature in the recent years has been paid mainly to pellet production from several types of agro-residue [3]. However, pellets from agricultural wastes have been identified as containing excessive heavy metals and ash, with poor mechanical properties [24].
With this in mind, the objective of the presented research was to evaluate differences in the qualitative characteristics of various pellets obtained from woody material (including branches and leaves) of forest and agroforest origin. Analyses were carried out on the different pellet samples in order to find out which tree species and which form of management resulted in the best pellet quality. In this study, a number of pellets were considered, obtained from wood biomass originating from various forest species grown under different forms of management.

2. Materials and Methods

The analysis involved 13 different wood materials originating from various forest species and from forestry or agroforestry management systems (Table 1).
In all the analysed operations, the wooden material was harvested according to the whole tree system (WTS); therefore, in every case the trees were extracted together with their branches and leaves so that the pellets were produced from every part of the trees. An exception was in FS30CST, where coppicing was performed in winter and the pellets were only produced from the stems and branches but not the leaves. The samples were chipped by a Farmi Forest (Iisalmi, Finland) CH260 forestry chipper, refined by a BL-100 shredder (Cuneo, Italy) with a 6 mm grid and then subjected to a drying process. Pelletisation was finally carried out using a 4 kW Bianco line (Cuneo, Italy) pelletiser. For each pellet type, ca. 50 kg of pellets was selected for analysis.
However, commercial pellets CP1 and CP2 were obtained from the manufacturer, and they consisted of pure wood pellets without bark and leaves.
A list of the characteristics of the investigated pellets is given in Table 2.
From a macro sample of 30 kg of pellets for each investigated pellet type, 5 samples of 100 g each were used for analysis regarding each parameter, with the exception of pellet size, where 50 single pellets were used as a sample.
Ash content was determined according to EN ISO 18122 (2016) using a Lenton EF11/8B (Hope Valley, UK) muffle furnace. Samples of 1 g were heated to 250 °C for one hour and then to 550 °C for two hours, with the ash content measured as the weight loss of the samples after the heating process. For the fusion analysis (ash melting behaviour), according to EN ISO 21404 (2020), cylindrical ash samples of fine and homogeneous powder were placed in a Sylab SHV-IF 1500 (Metz, France) analyser, which identified by means of image analysis the temperature at which deformation occurred in the sample.
The heating value was determined according to EN ISO 18125 (2017) using an Anton Paar 6400 (Moline, IL, USA) calorimeter for higher heating value (HHV) and a logarithmic formula for the lower heating value (LHV). The pre-treatment of the samples involved two sequential shredding processes, with a Retsch SM 100 (Düsseldorf, Germany) knife mill and a Retsch ZM 200 (Düsseldorf, Germany) centrifuge mill, and the preparation of 1 g tablets using a Pellet Press 2810 (Düsseldorf, Germany).
After pre-treatment of the samples (500 mg each) consisting of acid attack (HNO3 and H2O2) using a microwave digester (Start D, Milestone, Sorisole, Italy), the heavy metal content was determined according to EN ISO 16968 (2015) using an Agilent 7700 ICP–MS (Santa Clara, CA, USA).
According to EN ISO 16948 (2015), ten capsules of 1 mg each were placed into a CHNS-O Costech ECS 4010 (Cernusco sul Naviglio, Italy) combustion oven to analyse the carbon (C), hydrogen (H), nitrogen (N), and oxygen (O) content.
The bulk density was evaluated in accordance with EN ISO 17828 (2016) by weighing a standard container of known weight and volume (0.005 m3, height-to-diameter ratio within 1.25 and 1.50), filled with pellets. The average pellet length and diameter were determined according to EN ISO 17829 (2016) by measuring 50 pellets selected randomly.
Mechanical durability was analysed by means of a mechanical durability tester (Andritz Sprout (Graz, Austria) rotation pellet testing apparatus) according to EN ISO 17831-1 (2016). After sieving (using a 3.15 mm sieve), the sample was subjected to a tumbling process in a durability tester. After this, the sample was re-sieved through the 3.15 mm sieve. The mechanical durability was expressed as the percentage weight of pellets that did not break down into grains smaller than 3.15 mm after the tumbling process.
The moisture content was determined, according to EN ISO 18134-1 (2015), as a percentage of the weight loss after processing in a drying oven (Memmert UFP800 (Schwabach, Germany)) at 105 ± 2 °C.
Shear resistance was determined using a MetroCom (Garbagna Novarese, Italy) MI10 machine for testing the shear resistance of the material. As this is not a regulated parameter, there is not a specific protocol for biomass fuels; therefore, this parameter was evaluated according to the protocol for the shear resistance of other materials.
Particle density was determined according to EN ISO 18847 (2017) using the metric/weight approach.
Statistical analyses were carried out using Statistica 7.0 Software (Statsoft, Tulsa, OK, USA). These consisted of ANOVA, MANOVA and Tukey’s HSD Test. The non-metric multidimensional scaling (nMDS) approach was used to analyse differences in the main characteristics of the tested pellet performance.
In addition, in order to have a clear link between research results and technical applications, various pellet types were categorised, firstly according to a simple classification developed within this study (S1) (consisting of three classes: M as “medium”, G as “good”, and O as “optimum”) and then according to EN ISO 17225-2 (S2). This classification was developed in view of the objective of the present study, which was focused on a rural value chain in which, starting from wooden material without any pre-treatment such as debarking, pellets can be produced for a short energy supply chain. Such a theoretical supply chain takes into consideration pellets burnt on commercial boilers; therefore, the reference standard considered is the one for commercial pellets. However, such a standard was developed with a view to evaluating commercial pellets generally produced by big manufacturers from relatively pure feedstock. Therefore, the classification developed for the present study may be suitable to evaluate pellet quality from unusual raw materials.

3. Results

3.1. Physical Parameters

Among the various pellet types, most variables showed the presence of statistically significant differences in physical parameters (Table 3 and Table 4).
The average diameter of the cylinders was over 6 mm, with a minimum of 5.99 mm for FS30CST and a maximum of 6.64 mm for CP2, and a length range between 8.72 mm for RP3SRF and 18.83 mm for ES6SRF.
Bulk density, which is one of the principal parameters of pellet quality, varied from 0.438 g/cm3 for RP3SRF to 0.640 g/cm3 for ES6SRF, which showed the best value for this variable.
In terms of moisture, PE18TH had the highest value, whereas the lowest value was found in ES18C, although all the samples showed values under 10%.
For shear resistance, ES6SRF performed the best with 7.13 N/mm2. In contrast, the lowest value was shown by PR18TH with 3.87 N/mm2, but PE18TH and RP3SRF also yielded very low values. In terms of particle density, the higher value was achieved by FS70TH with 1.39 g/cm3, with the lowest shown by PE18TH with only 1.26 g/cm3.
Mechanical durability is, along with bulk density, one of the most important physical parameters of pellets. This indicates the percentage of pellets that can disintegrate due to mechanical damage, e.g., during transport. A low amount of pellet dust in the end-user’s store room is one of the most important product features. Mechanical durability ranged from 94.95% for RP6SRF to 98.17% of ES3SRF.
Another key parameter, and perhaps the most significant when characterising a substance as fuel, is the heating value, which is given by the amount of energy produced by the combustion of a unitary mass of fuel. The higher heating value (HHV) detected for various pellet types ranged between 17.656 for ES3SRF and 20.302 MJ/kg for PE40TH, with statistically significant differences. Regarding the lower heating value (LHV), the results were in line with the HHV, with PE40TH having the highest result (17.415 MJ/kg) and ES3SRF the lowest (14.29 MJ/kg).
In order to define pellet quality, it is also important to know the ash content. A high ash content can cause problems in the combustion of biomass, because it produces slag, encrustations and corrosion in the combustion device, with an inevitable reduction in the performance of the plant itself [47]. In this study, the ash content ranged from 0.64% for FS70TH to 1.3% for RP3SRF.
For most parameters, with the exception of the ash melting point, ANOVA, MANOVA, and Tukey’s HSD test showed statistically significant differences (p < 0.05) among the various pellet types.

3.2. Chemical Parameters

Most chemical parameters showed the presence of statistically significant differences among the various pellet types (Table 5 and Table 6).
As mentioned above, the heavy metal content did not show statistically significant differences (p > 0.05) among the various pellet types. However, for other variables, there were statistically significant differences (p < 0.05).
Regarding carbon content, which considerably influences the heating value, the best performances were shown by the pine pellet types, in particular PE18TH and PR50TH, with 50.7 and 50.33, respectively. The lowest carbon content was shown by ES18C. ES6SRF had the highest oxygen content (45.88%), whereas PE18TH had the lowest (42.00%). In terms of hydrogen, the highest value was in PR18TH, whereas ES3SRF had the lowest value. FS30CST showed the highest percentage of nitrogen (0.65%) and ES6SRF had the lowest content (0.22%). Finally, in terms of pH, all the types showed a sub-acid value, which ranged from 4.2 (PR18TH and PR50TH) to 4.9 (RP3SRF).
Principal non-metric multidimensional scaling (nMDS) tests produced a two-dimensional ranking (Figure 1) that provided a significantly greater reduction in statistical stress than expected by chance (α = 0.05). The nMDS diagram of the main pellet characteristics matrix (Figure 1) was created to discover any linear correlations between the pellet production system/species and the characteristics of the thirteen types studied. For the pellet characteristics, the principal components, “Coordinate 1” and “Coordinate 2”, explained 75% and 22% of the total variance, respectively. The twenty-three variables analysed showed the maximum correlation with the ordination axes (Figure 1). Almost all the variables studied (18) illustrated the pellet characteristics on the weighted scale of axis 1 (Figure 1). The pellet characterisation along axis 2 was dominated mainly by ash melting point, shear resistance, moisture, and Cu and Ni content, but only partially by pellet length and durability (Figure 1). According to the nMDS results (Figure 1), the best features were for ES6SRF, FS70TH, FS30CST, and PR50TH, but good results were also found for PE18TH, RP6SRF, and PE40TH. In terms of the influence of species, treatment, and stem age on pellet quality, this study showed the presence of a link between cycle duration and pellet quality, with increasing stem age resulting in better pellet characteristics.

4. Discussion

4.1. Pellet Quality

The overall data obtained highlighted that ES6SRF, FS70TH, FS30CST, and PR50TH reached a high quality (Figure 1). A good level of quality performance was achieved by three types: PE18TH, RP6SRF, and PE40TH. In these cases, the parameter values were remarkable. Medium quality was attained by the last six investigated pellet types: ES18C, CP1, CP2, RP3SRF, ES3SRF, and PR18TH.
In comparison with these findings, this research group developed a technical classification (Table S1) in order to provide an overview of the effectiveness of these pellets in domestic plants. The overall classification did not take into consideration the heavy metal content, because all pellet types reached the “O” class for these parameters.
In accordance with the analytical results, the pellet types ES6SRF, FS30CST, FS70TH, and PR50TH reached high quality. This means that they were tractable in simple energetic chains with no specialised structures and showed good pelletisation efficiency. A good level of quality performance, not completely confirmed by analytical results, was achieved by five types: ES18C, PE40TH, RP6SRF, CP1, and CP2. In these cases, the parameter values were remarkably high, but at the same time seemed to be less tractable in less complex pelletisation systems.
Furthermore, in this case, partially in accordance with the analytical results, the medium quality level was attained by the last four investigated pellet types: RP3SRF, ES3SRF, PE18TH, and PR18TH. These types may be therefore treatable only in industrial plants and showed an overall quality that is not suitable for small domestic plants.
Finally, as a further check of the results obtained with respect to the official technical classification (Table S2), the final classification was undertaken according to EN-ISO 17225-2, which identified three classes, A1, A2, and B, with A1 as the best and B as the worst. For some parameters, the EN ISO 17225-2 standard did not distinguish different classes, but it indicated whether the pellet reached the minimum standard, marked as 1, or did not reach that standard, marked as 0 (Table S2). For example, moisture had to be <10% for classes A1, A2, and B.
Before discussing the results, it is important to underline that practically none of the types, apart from the pine pellets and RP6SRF, reached the minimum LHV level for classification, which is 16.5 MJ/kg.
The pellet ES6SRF did not reach the standard for the lower heating value only. Other Eucalyptus pellets, ES3SRF and ES18C, showed significant shortfalls in durability, LHV, and bulk density level.
These results are in line with previous studies [48]. For the other parameters, ES3SRF fell between classes A1 and A2, but was closer to A1. By comparison, ES18C was between A2 and B, but still closer to the A2 class.
The pine pellets, PR18TH, PE18TH, PE40TH, and PR50TH, showed many similarities, although there was a slight difference in terms of classification. They all showed deficits in durability and bulk density level. It is important to underline that only these pine pellets reached the minimum standard for LHV due to their high resin, lignin, and C content. Pellets PE18TH and PE40TH were classified as class A2, whereas PR18TH and PR50TH were classified as class B.
The beech pellets, FS30CST and FS70TH, reached the A1 or A2 class requirements for the majority of the parameters. In these cases, the main deficits were in HHV, mechanical durability, and bulk density, with the same explanations given above for other pellet types. Finally, the commercial pellets, CP1 and CP2, showed the same deficits as the other pellet types in terms of bulk density, HHV, and mechanical durability level, demonstrating that these standards were also reachable with difficulty in terms of industrial production.
After the final classification, it is possible to assert that the best pellets were ES6SRF, followed by FS70TH and ES3SRF, whereas the worst types can be identified as ES18C and RP3SRF. According to the technical regulation for pellet quality 17225-2, standard reference values of diameter, length, moisture, heavy metals content, and ash melting point were achieved by all the pellet types. Therefore, in order to give a quality ranking, it is necessary to focus on the remaining five parameters, i.e., LHV, bulk density, durability, ash content, and N.
ES6SRF and FS70TH both achieved standard values for four of the above-mentioned parameters, but the former achieved the A1 class for three variables, in comparison to two A1 scores for FS70TH. PE18TH, PE40TH, PR18TH, PR50TH, RP3SRF, RP6SRF, and ES3SRF achieved the standard values for three of five variables, but ES3SRF was the only one to achieve the A1 class for N content.
Finally, ES18C, RP3SRF, CP1, and CP2 achieved the target values for only two of five parameters. However, ES18C and RP3SRF showed higher ash content in comparison to the commercial pellets, i.e., B vs. A2.
The results obtained yield some interesting suggestions for forest management when pellet production is, or can be, seen as one of the products targets. Firstly, the two best types, i.e., ES6SRF and FS70TH, derive from two markedly different management concepts, the first from an artificial plantation and the second from a natural forest. Starting from the beech stand, the high quality of the obtained pellets is certainly interesting for the development of a rural value chain. However, it is crucial to highlight that beech trees of that age can also provide high quality timber [49,50,51]. Therefore, as suggested in the New European Forest Strategy [52], the development of a wood pellet value chain from beech forests, but also other species, should be based on low value material, i.e., branches and tree tops [53,54]. The situation is different for ES6SRF—the results highlighted that increasing the length of the management cycle from short (3 years in this case) to longer, but still short (6 years in this case), rotation forestry led to a substantial improvement in pellet quality. In the present study, the main cause of this difference can be attributed to the lower ratio between bark and wood shown by the six-year-old trees in comparison to three-year-old ones, which may have led to an increase in pellet quality, mostly in terms of mechanical characteristics such as bulk density.
Focusing instead on the worst types, i.e., ES18C and RP3SRF, a possible explanation for the results obtained is the difference between the two pellet types. On the one hand, RP3SRF showed poor pellet quality, as expected when dealing with short rotation forestry trees, as a consequence of both the above-mentioned high bark/wood ratio and the harvesting system for such plants. Indeed, SRF plantations are generally harvested in the form of wood chips from modified self-propelled forage harvesters, systems which can easily lead to the entrance of foreign matter within the harvester (soil and little rocks), lowering the quality of the material obtained and mostly resulting in increased ash content [55]. Regarding ES18C, the explanation is more complex since an increase in pellet quality should be expected with increasing tree age, as a consequence of a lower bark/wood ratio. Instead, the overall quality of ES18C is much lower than that of ES6SRF. This may be related to the physiology of Eucalyptus spp. Indeed, these trees with increasing age develop thicker leaves [56]. Therefore, the ratio between leaf biomass and wood biomass increases with increasing age. This may be the reason for the poor mechanical properties of the ES18C pellets, which showed lower bulk density and durability in comparison to the Eucalyptus pellets from younger trees.

4.2. Comparison with Other Similar Studies

The results obtained were compared with other similar studies on wood pellet characterisation from the physical and chemical point of view. Previous literature studies focused on different types of pellets, including black locust and beech pellets, regarding ash content, bulk density, and HHV [57].
Pellet types in the present study performed better than those reported in literature concerning ash content, with a reduction of about 45% for black locust and 35% for beech in comparison to the literature data [57]. HHV values reported in the present study were also higher than those reported in literature, with an increase of about 9% for black locust and 12% for beech. In contrast, Giungato et al. [57] revealed higher bulk density both for black locust and beech.
Concerning black locust, different values reported by Giungato et al. [57] are probably related to variations in forest management systems. In their study, there were few details on the management system (growing and harvesting) for black locust, but it was simply reported that the black locust wood chips were obtained from a local mountainous area. Therefore, it can be speculated that the wooden material was collected from a black locust coppice forest. This would explain the higher ash content, which may be related to a different harvesting system in harsher terrain conditions [58], in which skidding causes logs to be contaminated with soil, finally increasing the ash content. A different management system may again explain the higher bulk density; in this case, due to a different rotation age, which in natural black locust coppice is generally higher than 10 years. Regarding beech, the material from Giungato et al. [57] was a commercial pellet; therefore, in this case, too, the higher bulk density presented in the literature can be explained by the different raw material used for pellet production, which in this case was probably just wood without leaves and bark.
Focusing on pine species, Garcia et al. [59] found values similar to those reported in the present study concerning HHV. By comparison, the results of the Brazilian study reported lower ash content and higher bulk density and durability.
The lower value of bulk density obtained in the present study, and the less attractive properties regarding ash and lower pellet durability, are probably related to the kind of feedstock for pellet production. Indeed, Garcia et al. [59] used pure pine sawdust for pellets, without leaves and bark, which were instead incorporated in the pellets produced in the present study.
The results of another study carried out in Spain concerning pine pellets [60] also revealed higher bulk density and lower ash content in comparison with the results of the present manuscript. By comparison, results similar to those of the present study were obtained concerning heating values and chemical characteristics such as carbon content and hydrogen content. However, a substantially lower N content of 0.09% was reported in the Spanish study [60], probably as a consequence of different environmental conditions in which biomass was collected.
Focusing on Eucalyptus pellets, the obtained results are in line with those reported for commercial Eucalyptus pellets concerning ash content, HHV, and bulk density, and even higher concerning mechanical durability [61].
Another interesting comparison for Eucalyptus pellets can be made with a recent study carried out in Chile [44]. This study analysed the quality of pellets obtained from a 5-year rotation Eucalyptus plant; that is, a similar management system to that investigated in this study but in a different part of the world. The results obtained in the present study are in line with the literature data from Pegoretti Leite de Souza et al. concerning mechanical durability and HHV, but a substantially higher bulk density (increased by about 8%) was reported in the Chilean study [44].
Recently, Civitarese et al. [22] analysed various poplar (Populus × euroamericana AF-6 clone) pellet characteristics for LHV, ash content, ash melting point, and heavy metal content. In particular, they studied the differences among 3-, 6- and 9-year-old poplar pellets obtained both from the whole tree and from the stem only. A recent study showed that poplar harvesting from SRF can be a substantial source of biomass [34], which can also be used for pellets.
Comparing the cited results with those found in this study, it can be noted that Civitarese et al. [22] found higher heating values ranging from 17.55 MJ/kg for 9-year-old trees, stem only, to 17.85 MJ/kg for 3-year-old trees, stem only. These values were considerably higher than those obtained in the presented study, with values ranging from 14.290 MJ/kg for ES3SRF to 17.415 MJ/kg for PE40TH.
Regarding ash content, the study of Civitarese et al. [22] showed higher values ranging from 1.82% for 6-year-old trees, whole tree, to 2.78% for 3-year-old tress, whole tree, compared with this study’s range of 0.64% for FS70TH to 1.3% for RP3SRF.
Regarding ash melting point, the Italian study found values ranging from 1404.6 °C for 6-year-old trees, whole tree, to 1479.8 °C for 9-year-old trees, whole tree. Italian values were higher than those shown in the presented study. In fact, ash melting point values ranged from 1377.3 °C for FS30CST to 1480.8 °C for ES3SRF.
In contrast, the Civitarese et al. [22] results for heavy metal content were similar to those found in this study.
Finally, it can be said that the results of the present study were comparable with other similar studies, with the exceptions of durability and bulk density. This was probably due to the feedstock used, because leaves and bark negatively affected these parameters. Mechanical properties are further highlighted by the low values of shear resistance for the majority of the investigated types.
Species, and therefore the wood material characteristics, seemed to be the most relevant factor. In fact, pellets from the same species, or similar, in the case of Pinus eldarica and Pinus radiata, showed similar characteristics.
Nevertheless, the management system and, consequently, stem age also seemed to have an important influence. In particular, this study’s results showed that the pellet qualities tended to improve as stem age increased. Moreover, this link between wood age/management system and pellet quality appeared to be more pronounced when dealing with young trees.
In fact, ES6SRF (overall class “O”) and RP6SRF (overall class “G”) showed obviously better qualities than ES3SRF (overall class “G”) and RP3SRF (overall class “M”). Therefore, with regard to dedicated plants, 6-year SRF was found to be the best management system for good pellet quality standards. Wood harvested from thinning interventions at 18 years of age provided a lower quality pellet than 40–50-year-old materials. PE18TH and PR18TH were classified as “M”, whereas PE40TH and PR50TH were classified as class “G” and “O”, respectively. This was not confirmed for beech pellets, because FS30CST and FS70TH were in the same class of “O”. Thus, the link between stem age/management and pellet qualities seemed to lose significance with increasing management cycle age.

5. Conclusions

The analysed pellet samples should not be classified according to EN 17225-2 due primarily to their lower LHV and bulk density values. However, these deficits were also found in commercial pellet samples. According to the proposed classification, the most attractive pellets were obtained from ES6SRF and ES3SRF.
Regarding the classification developed within this study, all the pellet types were found to be suitable for the proposed classification, because none of them exceeded the limits of toxic content. According to this classification, the best types were ES6SRF, FS70TH, and FS30CST.
In terms of the influence of species, management system, and stem age on the pellet qualities, this study showed the presence of a link between cycle duration and pellet qualities, because an increase in stem age resulted in better pellet characteristics. This link seemed to be particularly strong in short rotation systems, for example migrating from 3 years of SRF to 6 years of SRF, and weaker in the long term. In fact, the beech pellets from coppice showed very similar qualities to those from high stand thinning.
The results obtained in this study show the importance of understanding the efficiency level and the capability of managing specific systems, including small-scale scenarios, both for self-consumption and for short supply chain commerce. Overall, the virgin wood material without any additives showed possible deficiencies but also undoubted strength in terms of, for example, pollutant and ash melting point levels, two parameters which are important in small heating systems. The results are very promising for the proposed innovative production of pellets.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/f13040502/s1, Table S1: Pellet type classification according to standard developed * within this study for physical parameters: “M”: medium, “G”: good, and “O”: optimum; Table S2: Classification of various pellet types according to EN ISO 17225–2. Symbols “1” or “0” in columns indicate whether the pellet reached the minimum standard in classification for selected parameter, for which the EN ISO 17225-2 classification doesn’t distinguish different classes. The classification according EN-ISO 17225-2 identified three classes: A1, A2 and B, with A1 as the best pellet quality class and B as the worst.

Author Contributions

Conceptualization, F.L., V.C., R.P. and P.S.M.; Data curation, F.L., V.C., M.W., R.P. and R.V.; Formal analysis, F.L., V.C., R.P. and R.V.; Investigation, F.L., V.C. and R.V.; Methodology, F.L., V.C., M.W., R.P., Z.K., R.V. and M.B.; Supervision, R.P. and P.S.M.; Validation, V.C. and R.P.; Writing—original draft, F.L., V.C., M.W., R.P., Z.K., M.B. and P.S.M.; Writing—review and editing, F.L., V.C., R.P., R.V., M.B. and P.S.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data available on request from the corresponding author, due to privacy restrictions.

Acknowledgments

The study was carried out within the activities of the AGROENER project (Energy from agriculture: sustainable innovations for the bio-economy, financing MiPAAF D.D. n. 26329 of 4 April 2016, http://agroener.crea.gov.it/ accessed on 1 February 2022). This work was also supported by the Italian Ministry for Education, University and Research (MIUR) (Law 232/2016, Italian University Departments of excellence) UNITUS-DAFNE WP3. The publication was co-financed within the framework of the Polish Ministry of Science and Higher Education’s programme: “Regional Initiative Excellence” in the years 2019–2022, Project No. 005/RID/2018/19.

Conflicts of Interest

The authors declare no conflict of interest.

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Forests 13 00502 g001 550
Figure 1. nMDS analysis of main pellet characteristics in terms of possible differences among thirteen pellet types studied (light green: CP1; dark green: CP2; red: ES18C; blue: ES3SRF; violet: ES6SRF; dark violet: FS30CST; dark blue: FS70TH; light blue: PE18TH; olive green: PE40TH; dark red: PR18TH; teal: PR50TH; yellow: RP3SRF; light grey: RP6SRF). The diagram in the right corner shows the detail of the biplot analysis for the relationship among studied coordinates and characteristics.
Figure 1. nMDS analysis of main pellet characteristics in terms of possible differences among thirteen pellet types studied (light green: CP1; dark green: CP2; red: ES18C; blue: ES3SRF; violet: ES6SRF; dark violet: FS30CST; dark blue: FS70TH; light blue: PE18TH; olive green: PE40TH; dark red: PR18TH; teal: PR50TH; yellow: RP3SRF; light grey: RP6SRF). The diagram in the right corner shows the detail of the biplot analysis for the relationship among studied coordinates and characteristics.
Forests 13 00502 g001
Table
Table 1. Different types of wood material used for pellet production.
Table 1. Different types of wood material used for pellet production.
AcronymSpeciesAgeManagement
CP1Commercial pellet 1 (Picea abies (L.) Karst)--
CP2Commercial pellet 2 (Pinus nigra Arn.)--
ES18CEucalyptus spp.18 yearsCoppice
ES3SRFEucalyptus spp.3 yearsShort rotation forestry
ES6SRFEucalyptus spp.6 yearsShort rotation forestry
FS30CSTFagus sylavtica L.30 yearsCoppice with standards
FS70THFagus sylavtica L.70 yearsThinning in even-aged high forest
PE18THPinus eldarica Medw18 yearsThinning in even-aged high forest
PE40THPinus eldarica Medw40 yearsThinning in even-aged high forest
PR18THPinus radiata D.Don18 yearsThinning in even-aged high forest
PR50THPinus radiata D.Don50 yearsThinning in even-aged high forest
RP3SRFRobinia pseudoacacia L.3 yearsShort rotation forestry
RP6SRFRobinia pseudoacacia L.6 yearsShort rotation forestry
Table
Table 2. List of investigated parameters and influence of single parameter on pellet quality.
Table 2. List of investigated parameters and influence of single parameter on pellet quality.
ParameterInfluence on Pellet Quality (+, − or //)
Bulk density [g/cm3]+
Moisture [%]−
Pellet size (length and diameter) [mm]+
Shear resistance [N/mm2]+
Particle density [g/cm3]+
Mechanical durability [%]+
Heavy metal content [mg/kg]−
Carbon content [%]+
Oxygen content [%]−
Nitrogen content [%]−
Hydrogen content [%]−
Higher Heating Value [MJ/kg]+
Lower Heating Value [MJ/kg]+
Ash content [%]−
Ash melting point [°C]+
pH//
The symbol “+” indicates that a high value of the parameter had a positive influence on pellet quality, whereas the symbol “−” indicates that a high value of the parameter had a negative influence on pellet quality. Symbol “//” indicates that the parameter’s influence on pellet quality was not related to the value in itself but to a particular characteristic of it.
Table
Table 3. Average ± standard deviation for some physical parameters of various pellet types. It can be seen that most parameters show statistically significant differences. The highest values are highlighted in Bold, while the lowest values are in Bold-Italic. Different letters show statistically different groups (at a significance level of 5%) following Tukey’s test for physical parameters. (d.m.: dry matter).
Table 3. Average ± standard deviation for some physical parameters of various pellet types. It can be seen that most parameters show statistically significant differences. The highest values are highlighted in Bold, while the lowest values are in Bold-Italic. Different letters show statistically different groups (at a significance level of 5%) following Tukey’s test for physical parameters. (d.m.: dry matter).
Pellet TypeBulk Densityd.m. (g/cm3)Moisture
(%)		Length (mm)Diameter (mm)Shear Resistance (N/mm2)
CP10.583 ± 0.024 a7.08 ± 0.06 a17.02 ± 5.59 a6.59 ± 0.20 a6.52 ± 0.39 a
CP20.587 ± 0.011 a7.25 ± 0.04 a14.01 ± 5.01 b6.64 ± 0.09 a6.54 ± 0.45 a
ES18C0.560 ± 0.024 a4.81 ± 0.19 b9.04 ± 3.17 c6.04 ± 0.10 a,b5.55 ± 0.19 b
ES3SRF0.566 ± 0.014 a5.20 ± 0.05 b11.68 ± 4.71 d6.06 ± 0.17 a,b6.65 ± 0.24 a
ES6SRF0.640 ± 0.027 b7.51 ± 0.04 a18.83 ± 7.21 a6.08 ± 0.06 a,b7.13 ± 0.64 a
FS30CST0.592 ± 0.017 a7.24 ± 0.08 a11.84 ± 3.80 d5.99 ± 0.27 a,b6.67 ± 0.35 a
FS70TH0.601 ± 0.011 a,b7.02 ± 0.06 a12.08 ± 2.50 d6.02 ± 0.32 a,b6.99 ± 0.22 a
PE18TH0.528 ± 0.024 a,c8.34 ± 0.28 c13.26 ± 3.61 b,d6.29 ± 0.21 a4.33 ± 0.28 c
PE40TH0.565 ± 0.031 a8.21 ± 0.12 c13.01 ± 2.55 d6.31 ± 0.25 a4.41 ± 0.18 c
PR18TH0.521 ± 0.027 a,c7.88 ± 0.01 a,c11.29 ± 3.34 d6.24 ± 0.24 a3.87 ± 0.44 c
PR50TH0.585 ± 0.033 a7.67 ± 0.02 a12.02 ± 1.64 d6.23 ± 0.25 a3.95 ± 0.23 c
RP3SRF0.438 ± 0.013 c7.95 ± 0.09 a,c8.72 ± 2.78 c6.29 ± 0.39 a3.93 ± 0.16 c
RP6SRF0.557 ± 0.028 a7.09 ± 0.08 a9.67 ± 1.67 c,d6.32 ± 0.41 a4.04 ± 0.11 c
ANOVA/MANOVA p value<0.001<0.001<0.001<0.001<0.001
Table
Table 4. Average ± standard deviation for some physical and physico-chemical parameters of various pellet types (most of the parameters show statistically significant differences). Upper values are highlighted in Bold, while lower values are in Bold-Italic. Different letters show statistically significant differences between groups (at a significance level of 5%) following Tukey’s test for physical parameters. (d.m.: dry matter; d.b.: dry basis).
Table 4. Average ± standard deviation for some physical and physico-chemical parameters of various pellet types (most of the parameters show statistically significant differences). Upper values are highlighted in Bold, while lower values are in Bold-Italic. Different letters show statistically significant differences between groups (at a significance level of 5%) following Tukey’s test for physical parameters. (d.m.: dry matter; d.b.: dry basis).
Pellet TypeParticle Densityd.m. (g/cm3)Mechanical Durability (%)HHVd.m. (MJ/kg)LHVd.m. (MJ/kg)Ash Contentd.b. (%)Ash Melting Point (FT) (°C)
CP11.31 ± 0.04 a96.47 ± 0.35 a18.650 ± 0.450 a15.750 ± 0.650 a0.83 ± 0.10 a1427.856 ± 42.452
CP21.31 ± 0.03 a97.20 ± 0.87 b17.907 ± 0.866 b14.698 ± 0.852 a,b0.76 ± 0.03 a1413.914 ± 44.035
ES18C1.30 ± 0.02 a96.67 ± 0.38 a18.549 ± 0.525 a15.419 ± 0.593 a1.21 ± 0.08 b1428.929 ± 41.96
ES3SRF1.34 ± 0.01 a98.17 ± 0.31 b17.656 ± 0.190 b14.290 ± 0.329 b1.11 ± 0.13 b1480.771 ± 13.552
ES6SRF1.36 ± 0.04 a98.10 ± 0.50 b17.867 ± 0.208 b14.670 ± 0.271 a,b1.51 ± 0.05 c1414.607 ± 50.782
FS30CST1.32 ± 0.03 a97.13 ± 0.85 b19.107 ± 0.146 a15.707 ± 0.461 a0.70 ± 0.05 a1377.298 ± 26.746
FS70TH1.39 ± 0.05 a97.87 ± 0.54 b19.801 ± 0.201 a,c16.202 ± 0.314 a,c0.64 ± 0.04 a1395.104 ± 12.721
PE18TH1.26 ± 0.01 a,b95.53 ± 0.49 a20.235 ± 0.415 c17.175 ± 0.375 c0.97 ± 0.4 a,b1414.209 ± 18.743
PE40TH1.32 ± 0.03 a96.95 ± 0.29 a,b20.302 ± 0.245 c17.415 ± 0.301 c0.95 ± 0.3 a,b1421.120 ± 14.722
PR18TH1.27 ± 0.03 a,b95.50 ± 0.70 a20.135 ± 0.323 c16.767 ± 0.196 c0.86 ± 0.42 a1438.443 ± 6.138
PR50TH1.30 ± 0.04 a96.25 ± 0.67 a,b20.286 ± 0.133 c16.998 ± 0.106 c0.82 ± 0.24 a1409.127 ± 4.218
RP3SRF1.30 ± 0.02 a95.37 ± 0.74 a,c19.190 ± 0.645 a16.053 ± 0.384 c1.30 ± 0.13 b,c1429.437 ± 10.069
RP6SRF1.35 ± 0.04 a94.95 ± 0.67 a,c19.678 ± 0.565 a,c16.705 ± 0.318 c1.00 ± 0.10 a,b1401.401 ± 8.706
ANOVA/MANOVA p value<0.001<0.001<0.001<0.001<0.01>0.05
Table
Table 5. Average ± standard deviation for chemical composition of various pellet types. The upper value is highlighted in Bold, the lower value in Bold-Italic.
Table 5. Average ± standard deviation for chemical composition of various pellet types. The upper value is highlighted in Bold, the lower value in Bold-Italic.
Pellet TypeAs (mg/kg)Cd (mg/kg)Cr (mg/kg)Cu (mg/kg)Pb (mg/kg)Ni (mg/kg)
CP10.006 ± 0.0020.199 ± 0.0020.245 ± 0.0060.931 ± 0.0140.074 ± 0.0061.687 ± 0.014
CP20.005 ± 0.0020.202 ± 0.0040.246 ± 0.0080.955 ± 0.0190.076 ± 0.0031.694 ± 0.012
ES18C0.005 ± 0.0020.202 ± 0.0040.246 ± 0.0080.955 ± 0.0190.076 ± 0.0031.694 ± 0.012
ES3SRF0.008 ± 0.0020.203 ± 0.0020.242 ± 0.0080.959 ± 0.0180.069 ± 0.0011.695 ± 0.006
ES6SRF0.007 ± 0.0010.202 ± 0.0030.242 ± 0.0060.926 ± 0.0470.075 ± 0.0041.691 ± 0.004
FS30CST0.009 ± 0.0020.200 ± 0.0020.252 ± 0.0020.927 ± 0.0370.072 ± 0.0061.679 ± 0.002
FS70TH0.009 ± 0.0010.202 ± 0.0030.250 ± 0.0030.930 ± 0.0410.070 ± 0.0051.685 ± 0.003
PE18TH0.009 ± 0.0010.203 ± 0.0030.254 ± 0.0060.919 ± 0.020.069 ± 0.0031.684 ± 0.005
PE40TH0.009 ± 0.0030.202 ± 0.0020.252 ± 0.0080.911 ± 0.020.070 ± 0.0031.686 ± 0.006
PR18TH0.009 ± 0.0020.200 ± 0.0020.252 ± 0.0020.927 ± 0.0370.072 ± 0.0061.679 ± 0.002
PR50TH0.009 ± 0.0060.202 ± 0.0050.250 ± 0.0040.922 ± 0.0230.070 ± 0.0041.682 ± 0.005
RP3SRF0.009 ± 0.0010.204 ± 0.0020.253 ± 0.0020.953 ± 0.0180.079 ± 0.0021.705 ± 0.003
RP6SRF0.009 ± 0.0030.203 ± 0.0030.255 ± 0.0060.948 ± 0.0210.077 ± 0.0051.701 ± 0.010
ANOVA/MANOVA p value>0.05>0.05>0.05>0.05>0.05>0.05
Table
Table 6. Average ± standard deviation for chemical composition of various pellet types. The upper value is highlighted in Bold, the lower value in Bold-Italic. Different letters show statistically significant differences (at a significance level of 5%) between groups, following Tukey’s test for physical parameters.
Table 6. Average ± standard deviation for chemical composition of various pellet types. The upper value is highlighted in Bold, the lower value in Bold-Italic. Different letters show statistically significant differences (at a significance level of 5%) between groups, following Tukey’s test for physical parameters.
Pellet TypeZn (mg/kg)C (%)O (%)H (%)N (%)pH
CP114.875 ± 0.53549.81 ± 0.39 a42.85 ± 1.45 a5.86 ± 0.34 a0.50 ± 0.10 a4.30 ± 0.17 a
CP214.687 ± 0.31749.09 ± 0.43 a43.83 ± 0.29 a,b6.02 ± 0.46 a0.33 ± 0.11 b4.37 ± 0.21 a
ES18C14.687 ± 0.31748.43 ± 0.54 b45.53 ± 0.29 c5.29 ± 0.27 b0.41 ± 0.25 a,b4.70 ± 0.17 b
ES3SRF14.576 ± 0.37948.90 ± 0.26 a,b45.57 ± 0.41 c5.18 ± 0.19 b0.25 ± 0.04 b4.23 ± 0.15 a
ES6SRF14.813 ± 0.40048.70 ± 0.50 a,b45.88 ± 0.36 c5.03 ± 0.15 b0.22 ± 0.07 b4.53 ± 0.06 a,b
FS30CST14.891 ± 0.51248.65 ± 0.35 a,b45.45 ± 0.35 c5.20 ± 0.70 b0.65 ± 0.05 c4.47 ± 0.15 a,b
FS70TH14.886 ± 0.47148.97 ± 0.13 a,b44.85 ± 0.21 c,b5.22 ± 0.41 b0.63 ± 0.04 c4.51 ± 0.36 a,b
PE18TH14.788 ± 0.19950.70 ± 0.72 c42.00 ± 0.50 a6.17 ± 0.12 a0.33 ± 0.32 b4.47 ± 0.15 a,b
PE40TH14.795 ± 0.18750.11 ± 0.54 a,c42.22 ± 0.47 a6.01 ± 0.10 a0.31 ± 0.18 b4.42 ± 0.11 a
PR18TH14.891 ± 0.51250.25 ± 0.55 c42.65 ± 0.55 a6.40 ± 0.40 c,a0.57 ± 0.25 a4.20 ± 0.10 a
PR50TH14.878 ± 0.25050.33 ± 0.21 c42.05 ± 0.64 a6.28 ± 0.37 a0.55 ± 0.12 a4.20 ± 0.24 a
RP3SRF14.834 ± 0.42550.07 ± 0.57 a,c43.47 ± 0.10 a5.27 ± 0.40 b0.47 ± 0.06 a,b4.90 ± 0.10 b
RP6SRF14.801 ± 0.24550.18 ± 0.41 c43.68 ± 0.21 a,b5.29 ± 0.24 b0.48 ± 0.05 a4.79 ± 0.71 b
ANOVA/MANOVA p value>0.05<0.001<0.001<0.01<0.01<0.001
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Latterini, F.; Civitarese, V.; Walkowiak, M.; Picchio, R.; Karaszewski, Z.; Venanzi, R.; Bembenek, M.; Mederski, P.S. Quality of Pellets Obtained from Whole Trees Harvested from Plantations, Coppice Forests and Regular Thinnings. Forests 2022, 13, 502. https://doi.org/10.3390/f13040502

AMA Style

Latterini F, Civitarese V, Walkowiak M, Picchio R, Karaszewski Z, Venanzi R, Bembenek M, Mederski PS. Quality of Pellets Obtained from Whole Trees Harvested from Plantations, Coppice Forests and Regular Thinnings. Forests. 2022; 13(4):502. https://doi.org/10.3390/f13040502

Chicago/Turabian Style

Latterini, Francesco, Vincenzo Civitarese, Małgorzata Walkowiak, Rodolfo Picchio, Zbigniew Karaszewski, Rachele Venanzi, Mariusz Bembenek, and Piotr S. Mederski. 2022. "Quality of Pellets Obtained from Whole Trees Harvested from Plantations, Coppice Forests and Regular Thinnings" Forests 13, no. 4: 502. https://doi.org/10.3390/f13040502

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