The International Organization for Standardization (ISO) has created global pellet quality standards. To name a few, EN ISO 17225-1 covers general quality requirements, EN ISO 17225-2 covers graded wood pellets for residential and commercial usage, and EN ISO 17225-6 covers graded non-woody pellets. The Austrian standard NORM M 7135, the Swedish standard SS 187120, the German standards DIN 51731 and DIN EN 15270, the Italian standard CTIR04/05, and the French recommendation ITEBE represent just a few of the European nations that have previously developed laws and standards for pellet quality certification [53]. The EN ISO 17225 set of ISO fuel specification standards, which took the place of EN 14961, was released in May 2014.

The usage of pellets for both industrial and non-industrial purposes is covered by the graded wood pellet standard (EN ISO 17225-2). Use of fuels in smaller appliances, such as those found in homes, small commercial establishments, and government structures, is referred to as non-industrial use [54]. The best quality class according to this guideline is A1, which refers to virgin wood and chemically undisturbed wood residue low in ash and nitrogen. Pellets classified as A2 have slightly higher nitrogen and ash contents. Property class B comes last. Chemically processed industrial wood byproducts and residue fall under this category [54]. A classification of pellets for industrial usage is also reported in ISO 17225-2. Three alternative quality classes (I1, I2, and I3) are provided by this classification, and they have significantly stricter requirements than classes A1, A2, and B for pellets intended for household use. The introduction of this standard at the European level marked a significant development for the industry by guaranteeing better product transparency throughout products’ entire evolution and enabling greater conformity with global markets.

Standards for non-woody pellets (ISO 17225-6) cover pellets formed from mixtures and blends, such as biomass from herbaceous plants, fruits, or aquatic life. Two classification tables are provided by this standard: one for pellets made of straw, miscanthus, and reed canary grass and the other for biomass and blends of herbaceous and fruit materials. Non-woody pellets typically have higher levels of ash, chlorine, nitrogen, and sulfur [54], as well as a lower heating values (LHVs). Since the required standard is less stringent and they can have lower quality levels than wood pellets, it would be preferable to pay more attention to clearly communicating qualitative differences and usage suggestions. Given the high level of dynamism in this industry, it is critical that robust biofuel standards continue to be developed.

A summary of the requirements of ISO 17225-2 is given in Table 1.

Basically, pellets are mostly already produced from forest residues; in particular, with coniferous sawdust derived from sawmills. However, there has been great interest in recent years in producing pellets from alternative species, both from natural stands and from dedicated plantations, and from other types of forest residues, such as bark, cones, or material from low-diameter wood like branches [55,56].

In a recent study examining the production of pellets by adding cones and bark to spruce sawdust, it was found that the addition of these residues lowered the overall quality of the produced pellets, mostly in terms of ash melting behavior, nitrogen content, and ash content [57]. However, the major part of the produced mixtures reached the quality standards for at least the domestic B class, apart from ash melting behavior, for which the melting point was too low [57]. Terzopoulou et al. [58] confirmed that a low percentage (<7%) of cypress bark should be kept in feedstock for pellet production to achieve satisfactory quality. In another trial that investigated the possibilities of using stone pine (Pinus pinea L.) bark, medium branches, thin branches, and needles to produce pellets, the authors revealed that it was not possible to achieve high enough quality by only using these alternative feedstocks [59]. Higher quality could be achieved by mixing them with stone pine debarked wood in a certain ratio that varied based on the type of residue (about 15% for bark, 30% for medium branches, and less than 15% for needles and thin branches), which could yield the highest-quality pellets [59]. The authors recommended using the thick wood (trunk plus thick branches), as well as a portion of the medium branches and bark. It would be more practical to leave the needles and thinnest branches in the forest for their incorporation into the soil due to their high nutrient concentration and poor quality for energetic uses [59].

Focusing on pellet production from broadleaf species, Quercus spp. pellets produced in various trials across the world with different species reached generally satisfactory results. Carrillo-Parra et al. [60] produced pellets from three Mexican oak species and achieved satisfactory heating values and low ash content, as well as good mechanical durability, which made it possible to achieve the quality standards for domestic use. Pellets from the same Mexican oak species could be further improved by adding up to 20% coniferous sawdust from debarked stems [61]. Regarding European oak species, pellets produced from residues from urban green area management of Quercus ilex L. reached quality standard A2 for the heating value and also the minimal requirement for bulk density [62]. Quercus robur L. pellets showed satisfactory heating values and ash content when pelletizing feedstock with low initial moisture content [63]. In contrast, in another study, Quercus robur L. did not achieve the quality standard for the heating value, bulk density, and mechanical durability as a consequence of low lignin content when compared to coniferous oak wood [64]. Results for beech pellets are very similar to those reported for oak ones, and different studies highlighted satisfactory results [50,63,65], even if Stolarski et al. highlighted excessively low heating values, bulk density, and mechanical durability [64]. Pellets produced from poplar and birch showed generally high ash content, making them suitable only for industrial applications [64,66]. Similar results were reported for eucalyptus bark pellets, which showed a high level of fines [67].

Concerning pellet production from short-rotation coppice plantations, the literature shows that pellets obtained from this form of management are generally suitable only for industrial uses; they reach only B class for domestic applications [65,68]. The main problem is related to the low diameter of the shoots, which results in a high bark/wood ratio [63]. Increasing the rotation cycle to medium-rotation coppicing (MRC; about six to seven years for rotation; Figure 1) generally leads to an increase in the quality of the pellets obtained. This has been confirmed for both poplar and eucalyptus MRC plantations compared to SRC ones [50,69]. However it is worth highlighting that increasing the rotation time leads to higher dimensions for the stems to be harvested, thus no longer allowing for single-passage harvesting but requiring double-passage harvesting carried out with machinery specifically developed for forest management (chainsaw, feller-buncher, harvester, forwarder, cable skidder) [70,71,72,73]. Apart from low bulk density and mechanical durability, some authors have also found excessive nitrogen, sulfur, and chlorine as a consequence of the fertilization activity carried out in SRC plants [74]; however, this strongly depends on the plantation management adopted and on the specific characteristics of the growing site.

Finally, it is worth highlighting that, in view of small-scale pellet production, low mechanical durability and bulk density can also be related to the usage of low-cost, small-scale pelletizers that do not have the capacity to reach the compressive force and working performance of industrial machinery for pellet production [75].

In summary, it is obvious that the best-quality pellets are produced from pure coniferous sawdust derived from debarked stems. However, different types of forest biomass can be used for pellet production, even if there are some technical limitations. The characteristics that are mainly affected when producing pellets from alternative feedstocks are mechanical durability and bulk density, considering that bark, cones, leaves, needles, and broadleaf wood all have lower lignin content than coniferous sawdust. Lignin is the main factor in the binding process in pellet production [67]; therefore, it is understandable that materials with lower lignin content produce pellets with worse mechanical properties. Sawdust from broadleaf species, particularly oak species, generally contains 13–16% lignin, while in pine sawdust, the share of lignin can reach 26.3% [60].

Lignin also has a high heating value [76,77], and this also explains why some of the pellets produced from alternative feedstocks show lower heating values as compared to coniferous pellets. This is also explainable considering that coniferous wood also contains resin, which can increase the heating value [63]. However, low bulk density and mechanical durability may not be an insurmountable obstacle in developing a small-scale pellet supply chain. Bulk density is a parameter related to transport cost, and keeping a short distance between the pellet production site and the plant, approximately within a radius of 10 km [22], can limit the impact of the low bulk density of the produced pellets. A pellet with low mechanical durability may be less resistant to abrasion stress during handling, transport, and storage because of the mechanical load but may also show higher moisture absorption [78]. This could increase the risk of fire and explosions [79]. However, limiting transport distance and storage time could considerably decrease this risk. On the other hand, pellets with high ash content can represent a major problem from the perspective of small-scale production and utilization, leading to a substantial increase in maintenance costs [80]. Considering the above, wooden material from oak and beech trees seems to be a more interesting feedstock for small-scale pellet production than coniferous residues and SRC biomass, given that the first show low mechanical properties but low ash content while the latter has high ash content and low mechanical properties.