Enhancing the production and utilisation of renewable energy is among the most important goals of the European Union.
Solid biomass currently represents the main source of renewable energy in Europe; however, to achieve the target on energy independence, its use should continue to expand.
Nonetheless, there are several concerns which persist regarding the implementation of efficient biomass supply chains, mostly as a consequence of the fact that biomass is characterised by a low bulk density, and it is generally scattered on the territory.
One of the most commonly applied solutions to overcome the problem of the low bulk density of biomass is pelletisation.
The great advantages of pellets in comparison to other forms of solid biomass are reflected in the impressive growth of pellet production, which has been occurring in the last years, mostly in Central and Eastern Europe.
Mediterranean countries, particularly Italy, heavily rely on extensive pellet imports to satisfy the requests of the internal market.
Pellets are primarily produced from the residues of softwood species, predominantly sawdust.
Softwood sawdust is the perfect substrate for pelletisation, considering that it is an unprocessed residue, and that even small traces of pollutants can be removed by debarking and washing the logs before sawing.
However, countries that have a forestry system which is less reliant on softwood may face some challenges in producing high-quality pellets. The growing demand for wood pellets and the limited supply of sawmill residues have sparked a great deal of interest in exploring the use of alternative sources.
Just as any other biomass supply chains, pellet production can be done on a small scale or industrial basis.
To minimise transportation expenses and environmental issues, the industrial manufacturing of pellets necessitates large quantities of readily available feedstocks concentrated in a small geographical area.
However, recent research has shown how crucial it is to start small-scale pellet manufacture using local raw materials from one or a small number of producers. Small-scale pellet production in rural locations can further help local small agriculture and forestry businesses by increasing their income.
It is important to underline that high-value pellets are frequently produced on an industrial scale from pure sawdust, and other processing stages including pre-treatment and the inclusion of binders are also used.
In contrast, pellets made from different feedstocks that contain a significant amount of bark or leaves could result in worse quality.
Small farmers, owners of sawmills, or other forest enterprises that produce pellets on a small scale cannot afford to further invest time, energy, and financial resources into pre-treating the biomass.
Therefore, in the case of small-scale pellet production in the Mediterranean area, the use of a poor-quality feedstock such as hardwood with the presence of bark and leaves is often the only affordable solution.
The main issues regarding fuel quality related to the production of pellet from such kind of raw materials are associated to decreased bulk density and heating value, as well as increased ash content.
At the international level, pellet quality is assessed according to the standard EN ISO 17225. This standard was released in 2014 and replaced the previous EN 14961.
Within EN ISO 17225, the standard EN ISO 17225-1 regards the general quality requirements for pellets, while EN ISO 17225-2 is about wood pellets for both residential and commercial usage, and EN ISO 17225-6 regulates non-woody pellets.
Essentially, the standard assigns a quality class to the pellet based on various parameters. For instance, considering pellets for residential use, the top-quality class is A1, followed by A2 and B. The standard provides some threshold constant values for the various parameters, i.e., to be considered a class A1 pellets, the ash content should be ≤0.7 percent.
This approach is valid for commercial purposes but has some drawbacks concerning the scientific evaluation of pellet quality. The commonly applied statistical analysis techniques, such as the analysis of variance (ANOVA), are constructed to compare different data distributions among others and not one distribution and a constant, as it is in the case of pellet quality standard.
Hence, typically in scientific papers addressing pellet quality, there exists either a non-statistical comparison between the average values of the examined pellets and the standard value, or a statistical comparison among the data of various pellet types.
In both cases, however, the differences between the investigated pellet values and the values reported by the standard are not analysed through an inferential statistic approach.
Aim & Pellet Types
In this study, we tried to address the above-mentioned issues by proposing an alternative statistical approach to evaluate the quality of pellets.
Our aim was to develop an approach for evaluating pellet quality that elucidates whether the difference between the values of a pellet type and the standard requirements is statistically significant.
We applied this method to investigate the effects of different species and stem age on the quality of pellet produced from broadleaved species typical of the Central and Southern Italy forestry context, namely Turkey oak (Quercus cerris L.), European beech (Fagus sylvatica L.), poplar (Populus nigra L. x Populus x Generosa A.Henry–Populus x Euroamericana Moench.)) and eucalyptus (clone Eucalyptus camaldulensis x C. bicostata), in an attempt to shed light on the potential combinations of species and management system, in terms of stem age, that can lead to the satisfaction of the quality standards for A1 class, according EN ISO 17225, concerning bulk density, lower heating value, and ash content.
We considered four species largely present in Central and Southern Italy. For each species, we focused on different management regimes (except for turkey oak) characterised by different stem age at harvesting. As a result, we obtained ten pellet types. The investigated pellet types are reported as the following table:
SpeciesManagementStem AgeAcronym
BeechCoppice30beech30
BeechHigh forest70beech70
Turkey oakCoppice20oak20
Turkey oakCoppice25oak25
EucalyptusShort Rotation Coppice3euc3
EucalyptusMedium Rotation Coppice6euc6
EucalyptusCoppice18euc18
PoplarShort Rotation Coppice3poplar3
PoplarMedium Rotation Coppice6poplar6
PoplarCoppice9poplarg
Preparation & Quality Assessment
All the investigated types were derived from harvesting interventions carried out with the whole tree system, therefore all the base materials for preparing the pellet contained a certain percentage of bark, branches, and leaves.
Following chipping, the biomass was refined using a BL-100 shredder fitted with a grid with 6 mm holes.
Following reduction, we placed the various biomass types in bins and occasionally exposed them to the sun to speed up the dehydration process until they reached the appropriate moisture level (about 10%).
We then proceeded to pelletise the biomass. Pelletisation was carried out at two different sites with two different machines, a four kW Bianco Line (Cuneo, Italy) and a 5.5 kW Ceccato (San Giorgio delle Pertiche, Italy).
It is worth highlighting that the methods and procedures for producing the pellets were exactly the same in both the sites, only the machine was different.
In detail, we reported the technical data for the two pelletizers as follows: maximum production capacity of 60–70 kg/h; six mm flat die model for the production of pellets with a length of 30 mm (±5 mm).
Before production, the pelletisers were warmed up to an optimal temperature of 70 deg C. Once the optimum temperature was reached, indicated by the outflow of durable pellets, the production of samples started. The production cycles were monitored by maintaining a variable temperature range between 60 and 100 deg C.
For the evaluation of pellet quality, we focused on those parameters that generally represent the major challenges for producing high-quality pellets from feedstocks other than softwood sawdust, namely bulk density, lower heating value, and ash content.
For all the parameters, we referred to the specific standards for their determination. Bulk density was assessed directly after the pellet production with a 0.005 cubic metre container, filled and weighed with a 0.01 precision scale, and 50 replications per pellet type were carried out.
Lower heating value was assessed by a calorimeter Anton Paar 6400 (Moline, IL, USA), and 50 measurements per pellet type were carried out. Ash content was assessed using a muffle furnace (Lenton EF11 8B, Hope Valley, UK) at 250 deg C for one hour and at 550 deg C for two hours; also, in this case, 50 measurements per pellet type were performed.
The proposed statistical approach is based on the calculation and further summarisation of the effect size, namely the standardised mean difference expressed as Cohen’s d.
Te Linear Mixed-Effects Models (LMMs) fitted by Restricted Maximum Likelihood (REML) was used to calculate the 95 percent confidence intervals for the effect size and the influence on species, stem age, and pellet type.
A mixed-effects approach was used considering that the production of pellets with two different machines could represent a source of data nesting. We therefore indicated in the model the machine used for pellet production (Bianco Line or Ceccato) as a random effect. We fitted an
LMM for every single independent variable (bulk density, ash content and lower heating value), indicating the species and stem age as fixed effects with the lme4 package in R 4.3.1 software.
Then, we further fitted LMM for every variable indicating the pellet type as fixed effect. We used the packages ggeffects and emmeans o calculate and visualise the marginal response and marginal means representing the mean values of each experimental treatment, assuming a constant level of all other predictors and no random effect (global estimate).
Tukey’s test was applied as a post hoc analysis to compare the effect sizes for species and pellet type.
This approach allowed us to simultaneously evaluate both the presence of statistically significant differences among the experimental treatments, and the significance of the differences of each treatment in comparison to the threshold value of the standard.
It was necessary to verify whether the confidence intervals calculated by REML intersect with the 0 line. If the 95 percent confidence intervals for the effect size intersect the 0 line, it indicates that the difference from the standard is not significant, as the potential range of standardised mean difference includes 0.
Conversely, if the confidence intervals do not intersect the 0 line, the difference is statistically significant.
This method of interpreting results mirrors the approach used in any meta-analysis based on standardised mean differences.
Proof of the Research
This paper aimed to achieve two primary objectives as follows: (1) introducing an alternative statistical approach for evaluating pellet quality and (2) assessing the possibility of small-scale pellet production from material derived from different broadleaved species found in the Mediterranean forestry.
Regarding our primary objective, we have validated the effectiveness of the proposed approach. The calculation and summarisation of the effect size were highly suitable to assess pellet quality.
This methodology was the most effective as the focus is on the comparison of the value of a given pellet type with the reference standard, rather than the comparison among different pellet types, as typically performed with ANOVA.
Effect size based on Cohen’s calculation gave an immediate idea of how much the obtained value differs from the standard, and the calculation and visualisation of the confidence intervals represents an immediate and clear method to assess the statistical differences.
We therefore recommend the application of this approach in studies conducted on pellet quality. Furthermore, this approach is not only limited to pellets, but to all those cases in which there is the need of comparing the obtained value with a constant threshold, as is the case for wood chips quality or wood-based panels mechanical performance.
Regarding broadleaved pellet quality, we confirmed that bulk density, ash content, and heating value are the most troublesome parameters to produce high-quality pellet from broadleaves species.
The previous literature largely confirmed that the thresholds indicated by the standard for these variables can be particularly challenging to be achieved.
None of the investigated pellet typologies showed fully satisfactory results for all the variables. For instance, beech pellets showed proper bulk density and satisfactory ash content, but the resulting heating value to be significantly lower than the standard’s threshold.
However, oak pellets showed satisfactory heating values, whereas bulk density was low and ash content was too high to be classified as an A1 pellet.
These results are related to the intrinsic characteristics of the raw materials. Broadleaved wood has lower lignin content than coniferous wood and, considering that lignin is the major binding component for pellet production, this can obviously affect the quality of the pellets.
Furthermore, the presence of material other than sawdust, as it happens when producing pellet from the whole tree, is another critical aspect that can decrease pellet quality.
However, based on the results obtained a possible solution could be blending raw materials from different species. Additional dedicated studies are needed to confirm this.
Based on our findings, it could be interesting to blend beech (high bulk density and low ash content) with turkey oak (high heating value). These species are widely distributed along the Italian Apennine.
Often, they are located in stands very close to each other and with a certain degree of admixture, so this kind of blend could represent an interesting solution.
Eucalyptus and poplar pellets showed excessive shortcomings, making them less preferable as suitable raw materials for small-scale pellet production. These facts confirm that producing high-quality pellet from short and medium rotation forestry is particularly challenging.
Our results showed the significant influence on pellet quality generated by not only the species but also of the stem age, which is a direct consequence of the forest management systems applied, as highlighted in Latterini et al.
Surprisingly, we did not find a significant influence on pellet bulk density, but only on ash content.
In particular, the increasing age of the raw material led to an improvement in ash content. This is probably related to an increase in the wood/bark ratio which is generally observed with increasing diameter of the stem.
Increased ash content is often observed with an increase in the amount of bark. The influence of bark on heating value is instead less straightforward and depends on the species intrinsic features; in beech, for instance, bark has a lower heating value than wood.
In summary, we applied an innovative statistical approach to evaluate the quality of pellets in relation to the official standards. The applied approach was revealed to be effective and easy to interpret.
We recommend that this approach be applied in the sector of biomass quality evaluation. We further confirmed the generally known shortcomings which often occur when producing pellet from broadleaves species,
in terms of high ash content, low heating value, and low bulk density. However, we found that increasing the stem age is beneficial for ash content, suggesting that material retrievable from thinning in high forests could be a better feedstock than material from coppice.
We further showed that beech pellets demonstrate promising results concerning bulk density and ash content, and that the shortcomings related to low heating value could be addressed by blending with turkey oak biomass.
This blend can compensate for the characteristics of turkey oak biomass as it was satisfactory only for the heating value. Thus, we recommend that future trials in the topic should investigate quality of pellets produced by blending beech and oak in the framework of small-scale pellet production.
Among the possible limitations of our study, it is possible to mention that this was a preliminary trial, based on a limited number of species and focused on pellet produced with small-scale machinery.
Future studies should involve more species and more management systems in order to develop models which are even more reliable. Furthermore, modelling the influence of bark percentage on pellet quality can represent an interesting research topic.
Most importantly, we obtained promising results concerning beech pellets, but it is important to highlight that the pellets produced from beech were also those with the highest age among the investigated ones. This aspect had for sure an influence on the positive performance of beech pellets.
However, it is also true that forest management in beech forests in the Mediterranean context consists mainly in conversion from coppice to high forests or in thinning in high forests. Thus, the logged stands always have an age of higher than 30 years, and our samples were representative of the beech wood which could be retrieved and used for pellet production in the study context.
Future Investigation
To assess the pellets’ quality in comparison to the mandated standards, we used a novel statistical technique.
We recommend the application of this approach in the field of biomass quality evaluation given that it has exhibited efficiency, and it is simple to interpret. This approach can be recommended for further application in the sector of biomass quality evaluation to compare different pellet types.
We also validated the well-known drawbacks that frequently arise when pelleting broadleaf species, which are associated with a high ash content, low heating value, and low bulk density.
We discovered that raising the stem age improves the ash content. This suggests that material which has been recovered through thinning in high forests might provide a better feedstock than material from shorter rotation management systems.
Additionally, we demonstrated that beech pellets had encouraging results in terms of bulk density and ash content. We suggest that the poor heating value of the beech pellets could be addressed by combining them with turkey oak biomass, which instead only showed a good heating value.
Furthermore, using the wooden materials retrieved from thinning in older stands can provide pellets with higher quality.
Therefore, future studies on the subject of small-scale pellet manufacturing in the Mediterranean region should focus on investigating the effects of blending among different broadleaved species on the final pellets’ quality.
