Cypress Wood and Bark Residues Chemical Characterization and Utilization as Fuel Pellets Feedstock

Abstract

In order to meet the growing demand for raw material of solid biofuels, it is imperative to find alternative materials of low cost, underutilized so far. In this study, wood and bark material of two common cypress species (Mediterranean and Arizona cypress) were chemically characterized through gravimetric and spectroscopic (FTIR) analyses, to assess their potential to be used as raw materials in the production of fuel pellets. Low bark concentrations (0%, 2%, and 7%) were applied, and the mixtures were densified in a flat-die pellet press. The produced pellets were examined in terms of thermal, physical, hygroscopic, and mechanical properties, using standard ISO17225 thresholds as benchmark. The results revealed that the effect of bark presence in low content οn pellet properties and quality was positive. The ash content of both wood species is adequately low for biofuels production, whereas their bark cannot be purely used as feedstock due to the high ash content. By using low bark contents (2% or 7%), the ash content of pellets was kept adequately low to be categorized in the highest quality classes (A1 and A2: for residential applications), while the produced pellets demonstrated improved dimensional stability, mechanical durability, and slightly improved calorific value. The moisture content, dimensions, and bulk density of all the produced pellet categories fulfilled the standard requirements. Even though the pellets of 2% bark share presented much lower ash contents, only the pellets of 7% bark share were proven to have considerably improved mechanical durability, suitable for residential use. The chemical composition of raw materials (especially the extractives and holocellulose) plays a major role in the mechanical durability of pellets.

1. Introduction

In recent years, the fuel pellet market has been one of the fastest growing markets of solid biofuels in the energy sector (in 2021, wood pellet consumption in the EU was 23.1 million metric tons—MMT) [1,2], and it is expected to continue rising (EU demand is expected to further grow to 24.3 MMT) due to the current socioeconomical conditions, the commitments to EU, and international goals concerning climate change mitigation [3].

More specifically, pellets constitute a standardized product of high consistency, resulting from the mechanical compression of wood material and its densification by passing it through the holes of a die [4]. During pelletization, the high pressure and temperature developed provide adequate particle adhesion and hence cohesiveness and mechanical strength to the final material of pellets [5,6]. Lignin and extractives appear to act as natural binders of particles during the densification process due to their polymerization [7], and their action is influenced by the feedstock material moisture content (MC) and temperature during pelletization [8]. High quality of pellets is required to maintain the respective heating devices service-life, enhance the control of combustion process, and ensure high efficiency and low emissions. More specifically, pellets should not break and crumble easily, as this would have an impact on storage, regulation of heating systems, and could possibly interrupt the automated fuel feeding mechanisms [4]. The optimum MC of the raw material during the densification/pelletization process appears to be 8%–15% [9], in order to achieve an accepted level of particles consistency. The ash content is a crucial factor, which as a by-product of combustion remains at the bottom or floats in the air, contributing to slagging and corrosion phenomena [10]. Other significant factors are the bulk density and the calorific value of the biofuel, which combined determine the energy density of pellets [9]. Additionally, the MC of wood pellets may negatively affect their performance as fuel, and this is influenced, among other factors, by the chemical composition of the feedstock material, the pelletization process, and the conditions to which the pellets are exposed.

Industries of pulp, paper, particleboard, fiberboard, and other wood-based products are also highly interested in the same feedstock material (sawmill by-products, logging waste, etc.), increasing the demand for wood chips and sawdust. To meet the immensely growing needs of already existing and emerging fuel pellets industries for raw material, there is an imperative need to find alternative low-value and low-cost feedstock materials that have not been extensively utilized so far [11,12].

Recently, there has been a growing interest in utilizing tree bark in various directions, such as its incorporation as a filler for adhesives, as a substitute for wood particles or fibers in particleboard or fiberboard production, respectively, and as a basic material for the production of decorative, sound-absorbing, and thermally insulating panels, pure or embedded in various polymeric matrices [13,14], Nevertheless, research on the production of densified biofuels made of bark or wood–bark mixtures seems to be still at an early stage [15,16]. The limited existing literature reveals that the presence of bark in pellets leads to a higher ash content and probably lower calorific value (because of the higher ash content), higher hydrophobicity (due to higher contents of hydrophobic extractives) [17], and higher loss of volatiles during drying [18]. Pure (100%) bark pellets may cause the ash layer settled on the burning chamber wall to ignite [7], though this phenomenon does not appear when burning pellets of wood–bark mixtures. Previous studies revealed that particularly high pellet quality can be achieved by using the mixture of 90% wood with 10% of bark, since such a mixture corresponds to reasonably low ash content (around 0.7%) and improves other critical properties, such as a high dimensional stability and hydrophobicity [14,19,20,21]. Lehtikangas [22] demonstrated as well an excellent resistance of bark pellets to relative humidity fluctuations, along with higher dimensional stability and density of individual pellets compared to those of pure wood. The high extracts content of bark (mainly phenolic substances) also provides antibacterial and antifungal protection to the material, as well as to the derived product of pellets [23]. Additionally, Kamperidou et al. [24] reported that the incorporation of bark of some fast-growing broadleaf species, such as black locust, poplar, and sagebrush, significantly increased the calorific value of the final biofuel and that using appropriate wood–bark ratios could help maintain a low ash content.

Cypress constitutes a gymnosperm coniferous resinous tree of short rotation and of low water and soil quality demands [25]. It is a highly adaptable species to various environments, from sub-alpine ecosystems to torrent beds, and is resistant to extreme events, frost, drought, and fire [26]. The cypress species provides hard, biologically highly durable (due to the oleoresin/Cypressene it contains), homogenous, dimensionally stable, aromatic wood of medium weight and high quality [27]. It is readily dried and easily mechanically processed, while its mechanical strength is moderate. These properties provide cypress wood with incomparable advantages for various applications, such as carpentry, furniture, railway sleepers, poles, floors, building constructions, pulp, boxes, and lathes [26,27] Since this species is being intensively utilized in a great range of applications, wood and bark residual biomass, coming from its logging and mechanical processing, is also being produced in the form of small-sized particles or powder. According to the literature review, a great lack of information has been identified concerning the potential utilization of such residual material of cypress wood and bark in the production of solid biofuels. Another interesting characteristic that constitutes an incentive for the thorough utilization of the cypress species is that it regenerates naturally with very little human involvement, even without an organized planting or reforestation scheme (from cut stumps, etc.). Further research is undoubtedly necessary to assess the suitability of this species, wood in combination with bark, as a viable source of feedstock for solid biofuels and energy production.

Therefore, in the current study, the wood and bark of two commonly found cypress species (Mediterranean and Arizona) were chemically characterized through conventional gravimetric chemical analyses, as well as Fourier-transform infrared spectroscopy (FTIR), in order to assess, for the first time according to the literature, the potential for mixtures of such materials to be valorized as feedstock materials for pellets production. The effect of low bark presence (contents of 0%, 2%, and 7%) mixed with wood material on the produced pellets’ properties was thoroughly investigated, and the quality of the produced pellets was examined in terms of their physical, thermal, hygroscopic, and mechanical properties, considering the ISO 17225 standard thresholds as benchmark.

2. Materials and Methods

Two trunks of Mediterranean cypress (Cupressus sempervirens L., mean age of 35 years) and two of Arizona cypress (Cupressus arizonica Greene, mean age of 27 years) were obtained from the campus of AUTh University (Finikas, Thessaloniki, Greece). The trunks were placed in the laboratory, cut into disks, and 10 disks from different trunk heights (1–2.5 m) were kept from each of the trunks to be used as raw material. The bark was separated from wood, and all the materials were conditioned at normal climate (65% relative humidity, 20 °C) till constant weight and then crushed using a chipper and a “Willey”-type rotating-blade mill (Thomas Scientific, Swedesboro, NJ, USA) equipped with a 40-mesh sieve. A sieve shaker (Endecotts, London, UK) was used for the fractional separation of the particles in terms of their size and the formation of the optimum granulometry of the material, both wood and bark, to be pelletized (0–0.5 mm, 10%; 0.5–1 mm, 85%; 1–2 mm, 5%). The raw material intended to be used in the pelletization process was selected by applying the well-established “coning and quartering” method. In parallel, samples randomly obtained from the ground materials were used for the determination of MC according to EN ISO 18134-1 (

Table 1. Equilibrium moisture content (EMC) values of different studied raw materials.

Material	EMC
C. arizonica Wood—CA	9.35 (0.50) *
C. sempervirens Wood—CS	8.88 (0.79)
C. arizonica Bark—CAB	8.92 (0.35)
C. sempervirens Bark—CSB	10.38 (0.35)

* Standard deviation values in parentheses.

) and the conduction of chemical analyses of the materials, described in the following. The equilibrium MC of the raw materials after grinding is presented in Table 1.

2.1. Chemical Characterization Using Analytical Methods

Gravimetric determination of extractives soluble in ethanol–toluene mixture was performed according to ASTM D1107-21. A “Willey”-type rotating-blade mill (Thomas 31 Scientific, Swedesboro, NJ, USA) equipped with a 40-mesh sieve was used for the preparation of the wood or bark samples. For the extractions, 4 typical 250 mL Soxhlet extractors were used. For the implementation of this process, a laboratory drying oven, a glass desiccator loaded with silica gel, and an analytical balance were also used. Five wood or bark powder samples were used for each type of material extraction, and the mean and standard deviation values were calculated. Dry, extracts-free wood of dimensions between 180 and 250 μm was used.

Klason lignin was determined according to ASTM D1106-96. Holocellulose content was determined by a process previously described in detail [28,29]. An amount of 10 mL of NaClO₂ (25%) was applied to 2 g wood powder and heated in a water bath set at 70 °C for 5 h after adjusting pH to 4, to induce oxidation of lignin. Afterwards, the sample was rinsed with cold water to remove Cl₂, dried, and weighed. The above process was repeated until constant mass was reached. The remaining part at the end of the process is the holocellulose of the initial wood powder sample and consists of cellulose and hemicelluloses. For each material category, 3 replications were performed and the mean and standard deviation values were calculated.

The ash contents of the materials were determined according to ASTM D1102-2001. Samples of at least 1 g were weighed to the nearest 0.1 mg in dry, clean, and preweighed porcelain crucibles and then transferred to a cold muffle furnace (Heraeus MR 170, Leipzig, Germany) with a ventilation rate of about 5 changes per minute. The samples were heated to 250 °C within 50 min, and the temperature was kept constant for 60 min. In the next step, the temperature was increased to 580 °C within 60 min and was maintained at that level for 3 h. Afterwards, the crucibles were transferred to an empty desiccator without lid for 5 min, followed by 15 min with closed lid, and then weighed. Four replicates were used to calculate the mean and standard deviation values.

2.2. Chemical Characterization Using Infrared Spectroscopy (FTIR)

Samples of powder material were prepared as mentioned above and sieved twice to attain the appropriate particle size (fraction between grid openings of 40–60 mesh). To prepare the samples for FTIR analysis, KBr was mixed with wood/bark powder at a ratio of 200/1 mg and each sample was ball-milled for 20 s to a very fine powder (Micro Dismembrator, Sartotius, Göttingen, Germany) [30]. Afterwards, it was placed in a standardized pill press of a 13 mm diameter die. A force of about 10 t was applied under vacuum conditions (75 kN cm⁻² for 10 min/sample). A Thermo Scientific Nicolet IR200 FTIR infrared spectrometer (Thermo Fischer Scientific, Waltham, MA, USA) working in the range 500–4000 cm⁻¹ was used. Each measurement was an average of 32 scans. The alignment of the light equipment and the background spectra were collected before all the tests (every 15 min) to avoid noise in the created graphs. Spectral analysis was performed using OMNIC software version 9.2. The resulting spectra were averaged, baseline-corrected (using the linear algorithm method available in OMNIC software), and smoothed (7 pt.) before analysis using the adjacent averaging method.

2.3. Pellet Production

All materials were conditioned in a lab air-conditioning chamber at 65 ± 2% rh (20 ± 3 °C) till constant weight. The desired granulometry (0–0.5 mm, 10%; 0.5–1 mm, 85%; 1–2 mm, 5%) was achieved using test sieves adjusted to an Endecotts sieve shaker (Endecotts, London, UK) as mentioned above. Wood was mixed with bark material at different ratios (bark content of 0%, 2%, and 7% by volume of the material) in order to keep the ash content in quite low levels (Figure 1), complying with the standard requirements of fuel pellets production concerning low ash contents [9,31]. The mixtures were properly homogenized manually for approximately 10 min/mixture, to ensure a subsequent easy and stable flow of the raw material in the pelletizer die.

Pellets were produced (Figure 2) using a laboratory-scale flat-die pelletizing press (Pellet-Presse PP 225 23001, Laizhou Chengda Machinery Co., Ltd., Weifang, China) with a maximum output of 150 kg/h (Figure 3A) and die temperature of approximately 90 °C. During pelletizing, the material was pressed through the flat die by pan grinder rollers of 110 × 45 × 110. The die has a total of 198 holes with diameter of 6 mm and press-channel length of 40 mm. Initially, the press was run until steady state conditions were obtained. Approximately 6 kg of pellets were produced from each of the materials mixtures. The energy consumption of the pelleting process unfortunately could not be accurately recorded in the current work; therefore, it was impossible to depict the potential difference in energy consumption of bark and the studied mixtures of wood:bark compared to pure wood pelletization.

2.4. Characterization of Pellets

The produced pellets were conditioned under stable conditions (65 ± 2% rh; 20 ± 3 °C) till constant weight, and their MC was determined according to EN ISO 18134-1.

For each pellet category, a representative sample of 50 pellets was obtained from the produced material, and the pellets length and diameter were measured, according to ISO 17829, using a Mitutoyo digital caliper with a resolution of 0.01 mm (Mitutoyo 500-196-30). The pellet-sampling process was implemented using a two-dimensional rectangular sampling grid with points spaced 40 mm apart that had been placed over each of these created subunit areas. Pellets found at each intersection of the grid were included in the samples to be characterized. Particular attention was paid to ensure the representativeness of the samples.

For the calculation of density of individual pellets, 15 randomly chosen pellets from each material category were weighed, their edges were flattened using sandpaper, and their dimensions (length and diameter) were measured (MC: 2.7%–3.87%). The density results were expressed in kilograms per cubic metre (kg/m³).

2.5. Mechanical Strength

The mechanical durability is considered one of the most important parameters for assessing pellet quality, since it is the optimal indication of the risk of pellet deterioration and fines production during transport and handling. The mechanical durability of pellets was determined following the methodology of ISO 17831-1. Two representative samples of pellets of 500 ± 10 g were placed in the containers of the durability testing device (Figure 3B). These containers rotate for a specific duration of time (10 min; 500 circles), causing friction and disintegration of pellets. Using a sieve of 3.15 mm hole diameter, the detached, fine-grained material was separated from the pellets and measured to determine the intact part of pellets after the mechanical durability test process. Two measurements were performed for each pellet material category.

2.6. Bulk Density

The measurement of the produced pellets bulk density was based on the process described in ISO 17828. Two standardized containers (based on the standard requirements; Figure 3C) were filled to the brim with pellets, condensed with a fixed exposure to mild vibration, each time similarly performed. The large container for measuring bulk density was 360 mm in diameter and 491 mm in height, and the small container was 167 mm in diameter and 228 mm high. The pellets found on the top of the container were leveled using a spatula. The bulk density was determined through the net weight per standard volume and was indicated by the specified MC of the material as the standard requires. At least two measurements of bulk density were carried out for each pellet category.

2.7. Hygroscopic Properties-Dimensional Stability

The dimensions (length and diameter) and weight of at least 10 pellets randomly selected for each of the studied pellet categories were measured, and then they were placed in a conditioning chamber to be exposed to stable climate conditions of high humidity (100% rh; 20 °C). During the exposure period to such a high-humidity atmosphere, each pellet sample was removed individually from the chamber at 2-day intervals and weighed quickly on a digital balance (to the nearest 0.01 g), and their dimensions were also recorded using a digital caliper (Mitutoyo 500-196-30) of 0.0005”/0.01 mm resolution. After the period of 1 week, pellets were examined again in terms of weight and dimensions, in order to determine the absorption and swelling rate (in diameter and length) values of the pellet samples.

2.8. Calorific Value

The calorific value corresponds to the absolute value of the specific energy of combustion in calories per unit mass of a solid biofuel burned in oxygen (in a calorimetric bomb) under specified conditions. Here, the lower heating value or net calorific value was determined in an isoperibol bomb calorimeter (Parr 1261, Parr Instrument Company, Moline, IL, USA), according to the method described in EN ISO 18125. Sample pellets with a mass of 1.0 g ± 0.1 g, with a diameter of 13 mm, were produced in the shape of pills using a hydraulic pellet press (custom-made, Thessaloniki, Greece) applying a load of approximately 7 t for 1 min. The pellets were weighed to the nearest 0.01 g in a crucible and then placed inside a Paar 1108 oxygen combustion bomb. The net calorific value measurements were conducted in 6 replicates for each material and for each collection period. Prior to the measurements, the calorimeter was calibrated and validated with 6 individual calibration routes using benzoic acid pellets. The net calorific values were expressed in megajoules per kilogram (MJ/kg).

2.9. Statistical Analysis

The statistical package SPSS Statistics was used to determine the statistically significant differences between the mean values of the properties of the different materials studied through one-way ANOVA analysis, using Bonferoni and Tamhane method. A multiple linear regression analysis (f-test) was performed to describe the influence of ash, lignin, holocellulose, and extractives, as well as the species and nature (wood/bark) of feedstock material, and the effect of these variables on the mechanical durability of the produced pellets (considered as one of the most crucial properties). Through the multilinear regression analysis, the significance (p < 0.05) and the degree (%) of influence of the abovementioned independent variables on the dependent variable (mechanical durability) was investigated, to assess correlations among the properties and the potential for quality prediction or improvement of pellets.

3. Results

3.1. Raw Materials Chemical Characterization

According to the results of chemical analyses (Figure 4), the highest content of extracts was found in CAΒ (bark biomass of C. arizonica), while CS (C. sempervirens wood) presented the lowest content. In both species, the bark material marked a significantly higher extractive content than that of the respective wood, as it was expected. C. arizonica (CA) was found to have higher extracts content concerning both the wood and bark, compared to the respective materials of the species of C. sempervirens. Santos et al. [32] reported lower extracts content values, 3.3% and 2.5% for wood of C. sempervirens and C. arizonica, respectively, while Okino et al. [33] reported the extracts content of 4.1% for C. sempervirens. The effect of extractives is fundamental for the extended utilization of wood, especially of those species that have not been extensively used by the forest-based industries so far, especially in the production of pulp and paper, among other applications, where the extracts usually negatively affect the production, as well as increase the costs and inhibit the delignification reaction [34,35], as well as the production and performance of biofuels [28]. Higher extractives content means higher mass losses during combustion, which, depending on the burning system used, may not be converted into energy. Nevertheless, higher extractives content could favor the pelletization process of biomass particles during the production of densified solid biofuels.

According to one-way ANOVA, the Levene’s test showed that the null hypothesis referring to the fact that the error variance of the dependent variable (extracts content) is the same between groups was accepted (sixth condition for successful ANOVA), recording a significance level of p = 0.207 > 0.05. It was also revealed that a statistically significant effect of the species and nature (wood/bark) of the material was detected on the level of extracts content. The factor “wood–bark” significantly affected the variance of the extracts content by 99.2%, while the factor of different “species” showed the significant effect of 98.4%. The interaction between the two factors (species and nature) significantly affected the extractives content by 95.3%. Since there are more than two groups to compare, post hoc comparisons were performed to determine the statistically significant differences between all possible combinations of pairs of groups. According to Tukey HSD test, significant differences were revealed between CA and CS and between CAΒ and CSΒ.

The highest lignin content of the studied materials was found in CS, which differed significantly from the rest of the materials. As regards the lignin content in bark, the highest value was found in CAΒ. Santos et al. [32] reported lignin content of 28.48% and 36.08% for C. sempervirens and C. arizonica, respectively. As regards a similar cypress species, that of C. glauca, Okino et al. [33] found lignin content (Klason) of 33.5%, while Hafizoglu and Usta [36] reported lower values of Klason lignin for C. sempervirens in sapwood, 33.3%, in heartwood, 31.6%, inner bark, 16.2%, and outer bark, 38.2%. Two-way ANOVA test showed that the factor “wood–bark” significantly influenced the variance of the lignin content by 99.2%, while the factor of different “species” showed a statistically significant effect on lignin of 79.9%. Additionally, the interaction between the two factors significantly influenced the lignin content by 98.2%.

The highest percentage of holocellulose as regards wood and bark was found in C. sempervirens, with the bark to mark an even higher content. The recorded values of holocellulose and lignin contents agreed with those found by Foelkel and Zvinakevicius [37], who evaluated the quality of cypress biomass for cellulose kraft production. In relevant studies of the chemical properties of cypress species, a higher holocellulose content was found in C. sempervirens, as in the study of Okino et al. [33], who reported a holocellulose content of 71.8%, while Hafizoglu and Usta [36] reported for C. sempervirens a holocellulose content of 38.2% in sapwood, 49.2% in heartwood, 33% in the inner bark, and 28.6% in the outer bark.

A statistically significant effect of the factors “species” and “nature” of materials (wood/bark) was found on the level of holocellulose content. More specifically, the analysis revealed that the “wood–bark” factor significantly affected the variance of holocellulose content by 98.3%, while the “species” factor showed a statistically significant effect on holocellulose content of 99.5%. The interaction between the two factors significantly influenced the holocellulose content by 98.2%.

The bark ash content was found to be much higher than the respective values of wood, with the two bark ash contents of the two different species to be almost equal. However, the highest value was recorded in CSΒ material. Hafizoglu and Usta [36] reported similar results for ash content in C. sempervirens: sapwood, 0.5%, heartwood, 0.4%, inner bark, 7.5%, outer bark, 5.1%. The low ash contents of cypress wood species suggest a high potential for use in the production of high-quality solid biofuels (pellets, briquettes, chips, etc.). Specifically, according to ISO 18122, the threshold value of ash content related to the best quality class of pellets (ENplus A1) is 0.7%, for the second class (ENplus A2) it is ≤1.2%, and for the third class (ENplus B) it is ≤2%, concerning residential uses. Therefore, the studied wood materials undoubtedly meet the requirements for low ash content and could be classified in the best quality class ENplus A1 for residential use (≤0.7%). On the contrary, the bark material of both species could not be used alone as raw material for the production of solid biofuels (pellets), neither for residential nor for industrial applications, due to the high ash content. However, wood and bark could be utilized combined in mixtures to save wood material and probably promote some other properties of solid biofuels, such as mechanical strength or dimensional stability.

A statistically significant effect of the different “species” and different nature (“wood–bark”) of material (wood/bark) on the ash content showed that all the categories differed significantly between one another in terms of ash content. The factor “wood-bark” significantly affected the variance of the ash content by 99.9%, while the factor of different “species” showed a statistically significant effect on ash content of 98.2%. The interaction between the two factors significantly influenced the ash content by 98.1%.

FTIR spectra are characteristic of the chemical composition of samples, and bands can be tentatively assigned to specific molecular bonds or functional groups. These spectra provide a quick overview of the ratio of lignin, carbohydrates, proteins, lipids, and aromatic, and other, compounds (Figure 5). Due to the complexity of biological samples and the overlap of absorption bands, their assignment to specific molecular bonds or even to specific chemical compounds is not always clear. Moreover, the band assignment of IR spectra for lignin and holocellulose in different hardwood and softwood species is quite complicated because of the presence of many broad and overlapping bands corresponding to overlapping and combination bands of fundamental vibrations. There are no single clear peaks, but multiple wavenumbers associated with the overtones of each chemical component [38].

Nevertheless, the FTIR spectra showed the same basic structure: a strong broad OH stretch at 3300–3600 cm⁻¹, CH stretch at 2800–3000 cm⁻¹, and several distinct peaks in the fingerprint region between 450 cm⁻¹ and 1800 cm⁻¹. Most of these bands originate from both carbohydrates (cellulose and hemicelluloses) and lignin. Figure 5 presents a typical FTIR spectrum concerning CA, CS, CAB, and CSB within the fingerprint region. Comparing the FTIR spectra of the materials, there is a clear difference between bark (CAΒ and CSΒ) and wood (CA and CS). However, the CS spectrum demonstrates characteristic peaks at 1607 cm⁻¹ and 771 cm⁻¹, which are not apparent in the spectra of CA, CSB, and CAB. There are also differences detected in the peaks between 1300 and 1200 cm⁻¹ and a lower intensity of the peak at 1740–1745 cm⁻¹. The spectrum of CS presents differences from the spectrum of CA, since there are two local peaks in a bimodal distribution at 3450–3420 cm⁻¹ and 730–709 cm⁻¹, while only one is found at 3380 cm⁻¹ in CA. The CA spectrum shows a characteristic peak at 618 cm⁻¹ that is not intense in CA spectra. The spectrum of CS also shows an offset compared to the spectrum of CA. Comparing the spectra of the four different materials (wood and bark of the two species), clear differences can be seen between wood and bark, as well as between the different species (CA and CS).

Concerning the region just above and below 3000 cm⁻¹, the absorption is observed in the 3015 cm⁻¹ (CS), 3013 cm⁻¹ (CA), and 3010 cm⁻¹ (CAB) peaks, indicating that it concerns aromatic compounds or a double bond [39]. The absorptions at 1650 cm⁻¹ for CS, 1652 cm⁻¹ for CA, and 1620 cm⁻¹ for CAB show that it concerns C=C double bonds (1680–1620 cm⁻¹). The absorptions at 1511 cm⁻¹ for CA, 1506 cm⁻¹ for CS, and 1511 cm⁻¹ for CSB and CAB indicate the presence of a saturated aromatic compound. There are also absorptions at 2931 cm⁻¹ for CAB, 2925 cm⁻¹ for CSB, 2928 cm⁻¹ for CA, and 2930 cm⁻¹ for CS, indicating a saturated aliphatic compound. The region of 1780–1680 cm⁻¹ reveals the presence of a C=O bond. There are peaks at 1744 cm⁻¹ for CS and 1741 cm⁻¹ for CA, while there are two absorptions in the 1300–1000 cm⁻¹ range, from which the presence of ester is apparent. An absorption in the range of 3500–3100 cm⁻¹ reveals whether the compounds are alcohols or amines, which are also absorbed in the range 1250 cm⁻¹ to 970 cm⁻¹. There is a double absorption at 3420–3450 cm⁻¹ for CS and 3340–3420 cm⁻¹ for CAB and CSB. In this range, the materials absorb the hydroxyl of alcohols and the N–H of amines. The double and moderate absorption strength indicates a primary amine. This is also confirmed by the absorptions, which range from 1650 to 1550 cm⁻¹. In the sample of CA, there is an absorbance of 3380 cm⁻¹, which indicates alcohol and bonds from OH based on the intensity. No absorption is observed in the range of 2260–2100 cm⁻¹, so there is not a triple bond.

FTIR spectra are characteristic of the chemical composition of the studied materials, and the bands can be tentatively assigned to specific molecular bonds or functional groups. The range from 1530 to 1490 cm⁻¹ can be tentatively assigned to lignin [40]. Generally, the findings of FTIR spectral analyses were in line with the wet chemical analysis results that proceeded.

Table 2. Interpretation of the most significant and apparent findings of the FTIR spectra of the studied materials (CS: Cupressus sempervirens wood, CA: Cupressus arizonica wood, CAB: Cupressus arizonica bark, CSB: Cupressus sempervirens bark).

CS		CA		CAB		CSB
Wavenumber cm−1	Assignment	Wavenumber cm−1	Assignment	Wavenumber cm−1	Assignment	Wavenumber cm−1	Assignment
3450–3420	O–H stretching (hydrogen-bonded)	3380	O–H stretching (hydrogen-bonded)	3420–3340	O–H stretching (hydrogen-bonded)	3428–3341	O–H stretching (hydrogen-bonded)
2930	C–H stretching vibration in methyl and methylene groups	2930	C–H stretching vibration in methyl and methylene groups	2940	C–H stretching vibration in methyl and methylene groups	2919	C–H stretching vibration in methyl and methylene groups
1730	C=O stretching band in the lignin	1738	C=O stretching vibration in non-conjugated ketones and free aldehyde present in lignin and hemicellulose
1619	C=C of the lignin molecules	1619	C=C of the lignin molecules	1620	C=C of the lignin molecules	1620	C=C of the lignin molecules
1650	Absorbed O-H and conjugated ν(C=O) Lignin	1658	C=O stretching in conjugated q-substituent aromatic ketenes
1510	aromatic C=C deformation	1507	aromatic C=C deformation
1465	Hemicellulose Xylan	1460–1418	C-H deformation in lignin and carbohydrates	1448	Cellulose	1431	CH2 stretching vibrations related to the structure of cellulose
1420	C-H deformation in lignin and carbohydrates, δ scissoring (CH2) and (CH3) of lignin			1371	Lignin, δ (CH)—δs (CH3), Symmetric C–H bending vibrations from methoxy group	1378	Lignin, δ (CH)—δs (CH3)
1380	Lignin δ(CH)—δs (CH3)	1372	Lignin & C-H deformation in cellulose and hemicellulose	1323	C-H vibration in cellulose and C-O vibration in syringyl derivates	1317	C-H vibration in cellulose and C-O vibration in syringyl derivates
1268	Lignin, ν(C-O) lignin	1266	Lignin, ν(C-O) lignin and mannosan	1262	Hemicellulose Mannosan
1057	Lignin	1050	Hemicellulose Xylan νs (C-O-C)	1062	Hemicellulose Mannosan, ν(C-O)	1060	Cellulose C–O stretching vibrations: acetyl alkoxy bond stretching vibration
895	Cellulose, v(C-C)Glucose ring stretching, C1–H deformation; C–H stretching out of plane of ring due to b-linkage	897	Hemicellulose Xylan, C-H deformation in cellulose	885		889
814	Hemicellulose Mannosan δ (C-C); Ν (mannosan)	815	Hemicellulose Mannosan δ (C-C); Ν (mannosan)	779		779

summarizes the most significant findings revealed by these FTIR spectra.

3.2. Pellets Characterization

3.2.1. Moisture Content and Pellet Density

MC is one of the most crucial factors, affecting the efficiency, stability, and completeness of combustion of a solid biofuel. Additionally, pellets of high MC are subject to deterioration due to microbial decomposition, resulting in significant mass losses. The mean MC values of the studied pellet samples were found to be quite low, ranging from 2.7% to 3.87% (

Table 3. Mean values of pellet moisture content (%) and density of pellet mass (kg/m³).

Pellet Categories	MC (%)	Pellet Density (kg/m3)
C. arizonica—0% Bark—(CA_0)	3.75 (0.23) *	1175.16 (50.85)
C. arizonica—2% Bark—(CA_2)	3.87 (0.19)	1251.75 (60.37)
C. arizonica—7% Bark—(CA_7)	3.70 (0.22)	1269.63 (114.77)
C. sempervirens—0% Bark—(CS_0)	3.84 (0.28)	1258.94 (65.81)
C. sempervirens—2% Bark—(CS_2)	2.81 (0.22)	1340.39 (47.48)
C. sempervirens—7% Bark—(CS_7)	2.70 (0.17)	1204.89 (52.09)

* Standard deviation values in parentheses.

), recording a statistically significant MC loss compared to the initial MC of the raw materials before the pelletization process. Therefore, all the pellet samples recorded an MC lower than 10%, meeting the requirements of ISO 18134 for residential use, regarding A1, A2, and B quality classes. The CA_2 pellets (C. arizonica wood with 2% bark) presented the highest MC value, while the CS_7 (C. sempervirens wood with 7% bark) pellets presented the lowest one. Especially in the case of C. sempervirens, the CS_7 and CS_2 pellet samples revealed significantly lower mean MC values, indicating the positive effect of bark presence in pellet formulation. The density of pellets mass presents an increasing trend with the increase of bark participation in the mixture going from 0% to 7%, concerning both studied cypress species. This reveals that the different chemical composition of bark compared to wood, as well as the performance of bark under pelletization process conditions, affects the efficiency of densification and the final density of pellets. However, this increasing trend of density with the increase of bark incorporation does not correspond to statistical significant differences between the density values of pellets of pure wood and those of 2% or 7% bark material. Lehtikangas [22] also recorded higher density in pellets that contained bark, compared to those of pure wood. Many published data in the literature reveal similar values of pellet densities as those recorded in the current study.

3.2.2. Dimensions

The dimensions of pellets, both diameter and length, are critical in terms of combustion. Pellets of lower diameter allow for a more uniform combustion rate than those of high diameter, especially in small furnaces [41]. The length of pellets affects the fuel feeding properties; the shorter the pellets, the easier it is to arrange a continuous flow. The standard ISO 17829, dealing with the determination of length and diameter of pellets, refers that they should be in the range of 3.15–40 mm and 6 ± 1 mm, respectively, to be classified in quality classes of residential applications (A1, A2, and B). As it is observed (Figure 6), the mean length values of pellet ranged from 25.75 mm to 38.55 mm and were all within the dimensional length limits set by the corresponding standard. More specifically, the pellet sample CS_0 (pellets of pure C. sempervirens wood) presented the lowest mean length value and the CA_2 pellet samples presented the highest length value.

The diameter values of the produced pellets were all proven to meet the requirement of the standard ISO 1782, which states that the diameter of pellets should be 6 ± 1 mm to be classified into quality classes for residential applications (A1, A2, and B). Specifically, the mean diameters of pellets were measured to be 5.89–6.48 mm, with most of them over 6 mm (diameter of the die).

3.2.3. Mechanical Durability

For the mechanical durability of pellets, the standard ISO 17831-1 defines ≥98% as the lowest threshold value for the best quality class, A1, and 97.5% for A2 and B quality classes, respectively. Only CA_7 and CS_7 pellets were classified in A2 quality classes (Figure 7) concerning their mechanical durability. All the other pellet samples presented mechanical durability lower than 97.5%. The CA_0 pellet sample revealed the statistically significant and, in parallel, lowest mean value of mechanical durability, while the CA_7 pellet sample revealed the highest value. C. sempervirens seems to provide higher mechanical durability, compared to C. arizonica, even without the addition of bark material, probably due to the higher content of lignin and holocellulose and the lower content of extractives, compared to the Arizona cypress. The higher mechanical durability of C. sempervirens pellets, compared to the respective mixtures of C. arizonica, is in compliance with the slightly higher density of C. sempervirens pellets.

It is evident that the increase in bark content has a beneficial effect on the mechanical durability of pellets concerning both species studied. This is an essential finding of the research towards bark utilization in solid biofuels. Moreover, it could be stated that, as was observed previously (Figure 4), the high extracts content found for C. arizonica (wood/bark) seems to decrease the mechanical durability of pellets. On the other hand, the pellets of C. sempervirens (wood/bark) raw material, characterized by lower extracts, presented a higher mean value of mechanical durability. The negative effect of extractives on the pellet strength is most likely related to a surface layer of extractives that seems to prevent the optimal bonding between the raw materials particles [42]. In addition, C. sempervirens raw material presented higher lignin content than that of C. arizonica, as can been seen in Figure 4, which results in better agglutination between the particles and the natural durability [43], acting as a natural bonding agent during the pelletization process.

3.2.4. Bulk Density

High bulk density of pellets is a significant parameter of solid biofuels as well, since it is related to high energy density and great mass to be transported or stored in a container or fixed-volume silo. In this sense, it can reduce transport, handling, and storage costs [9,44]. According to ISO 17828, the bulk density of pellets of residential use should be in the range of 600–750 kg/m³. Therefore, according to Figure 8, all the studied pellet categories met the requirements of the standard for residential use. The CS_2 pellet category recorded the statistically significant and lowest mean bulk density value among the different pellets. The CS_7 pellet category presented the highest mean value of bulk density among the samples, which corresponded to a statistically significant difference from the respective pellets without bark (CS_0). Therefore, bulk density seems to be significantly increased in pellets of 7% bark concerning both species.

3.2.5. Hygroscopic Properties

According to Figure 9, it is clear that the absorption rate, as well as the swelling percentage in terms of length and diameter of the studied pellets when they were exposed to an environment of high humidity (100% rh; 20 °C), decreased by increasing the presence of bark material in the pellets feedstock material from 0% to 7%. This improvement complies, as well, with the slightly higher density of pellets, which could play a significant role in swelling and absorption rate, and could probably, with the higher extracts content of bark, change the hydrophobicity of pellets. Therefore, the bark material seems to mitigate the hygroscopic nature of the feedstock, improving in this way also the dimensional stability of pellets. Similar results have been reported by Lehtikangas [17] for bark pellets of Norway spruce and Scots pine mixture, which demonstrated a very good form stability during water immersion, compared to the respective pellets of the same species made out of sawdust and logging residues. Specifically, even though the water absorption was higher in bark pellets than in sawdust pellets, the extractive compounds of bark demonstrated a capacity to keep the pellets intact despite the water absorption (increasing the dimensional stability). The pellets dimensional stability could be associated to their mechanical durability and its quality maintenance over time. The different cypress species studied in the current work did not present any statistically significant difference concerning their hygroscopic properties values and performance in regard to the absorption and swelling rate (in length and diameter).

3.2.6. Calorific Value

The net calorific values of the materials of wood and bark of both cypress species, as well as the calorific values of the mixtures of wood:bark studied in this experimental work, are presented in

Table 4. Net calorific values of the two cypress species (Cupressus sempervirens wood—CS and Cupressus arizonica—CA), concerning wood and bark materials separately. The calorific value of the studied mixtures of wood:bark are also presented (2% and 7%).

Species	C. sempervirens				C. arizonica
Net calorific value (MJ/kg)	Pure Wood CS_0	CS_2	CS_7	Pure Bark	Pure Wood CA_0	CA_2	CA_7	Pure Bark
	19.60 (0.87)	19.61 (1.1)	19.65 (1.7)	20.3 (1.7)	19.20 (1.3)	19.21 (1.6)	19.25 (2.1)	19.9 (0.94)

. It is observed that there are no statistically significant differences among the calorific values of the studied materials and all of them (19.2–20.3 MJ/kg) could be considered acceptable for the production of pellets according to the respective standard requirements. C. sempervirens presented slightly higher calorific values compared to C. arizonica, a fact which is probably attributed to the higher lignin content of C. sempervirens. In both cases, the bark material exhibited slightly higher net calorific value compared to the pure wood of the same species. Therefore, the presence of bark in fuel pellets feedstock does not seem to cause any trouble concerning the calorific value of the mixed material (wood:bark). Similar calorific values of cypress species were recorded by Afungchui [45] (from 20.6 to 20.9 MJ/kg), who studied cypress material harvested all the year round and from different regions of Cameroon. The specific data reveal that cypress wood in combination with cypress bark can be a viable source of feedstock for solid biofuels and energy production.

3.2.7. Ash Content

Based on ISO 18122, the threshold value of ash content is 0.7% (dry basis)for the best quality class, A1, ≤1.2% for the second class, A2, and ≤2% for the third class, B, as regards the residential applications. According to the results, all the produced pellet categories met the requirements for a low percentage of ash for residential applications (≤2%) (Figure 10). Even though it was expected that the addition of bark in the mixture would increase the total ash content of the feedstock material, it seems that by using 0% or only 2% of bark material in pellet feedstock of cypress species wood, the pellets can be categorized in the best quality class of A1 concerning the ash content criterion. C. arizonica pellets of 7% bark content recorded much lower ash content than that of C. sempervirens, allowing the first one to be categorized in quality class A2. C. sempervirens pellets of 7% bark content demonstrated the highest ash content value among the studied mixtures; however, they were categorized in quality class B (as regards its ash content), which can also be used in residential applications.

The results of the multiple linear regression analysis of the examined properties revealed that there is a statistically significant (sig. = 0.000) correlation between the pellets’ mechanical durability and the chemical composition of the mixture materials in terms of extractives, lignin, holocellulose, and ash content values (considered to be the independent variables) (

Table 5. Degree (Pearson correlation coefficients) and significance of correlations among the characteristics of the pellet samples. In the second half of the table, the significant correlation values (<0.05) are depicted in bold.

Correlation Coefficients		Mech. Durab.	Ash	Lignin	Holocellulose	Extractives	Wood–Bark
Pearson Correl.	Mech.Durability	1.000	0.665	0.645	0.829	0.906	−0.794
	Ash	0.665	1.000	0.687	0.315	0.375	−0.229
	Lignin	0.645	0.687	1.000	0.128	0.296	−0.077
	Holocellulose	0.829	0.315	0.128	1.000	0.978	−0.986
	Extractives	0.906	0.375	0.296	0.978	1.000	−0.974
	Wood_Bark	−0.794	−0.229	−0.077	−0.986	−0.974	1.000
Sig.	Mech.Durability	-	0.009	0.012	0.000	0.000	0.001
	Ash	0.009	-	0.007	0.159	0.115	0.237
	Lignin	0.012	0.007	-	0.345	0.175	0.407
	Holocellulose	0.000	0.159	0.345	-	0.000	0.000
	Extractives	0.000	0.115	0.175	0.000	-	0.000
	Wood_Bark	0.001	0.237	0.407	0.000	0.000	-

Correlation is significant at the 0.05 level.

). It is widely accepted that lignin tends to act as an adhesive, binding the different particles together when the temperature of the material rises to its plasticization level [46]. It has been discussed in the literature that higher lignin content leads to more mechanically durable pellets with increased abrasion resistance [47], and the results of the current work are also in agreement with that. Other properties examined, such as bulk density and MC of pellets, were not found to be significantly correlated to the pellets’ mechanical durability. Higher extractives and holocellulose content seem to be correlated with higher mechanical durability of pellets. As shown in Table 4, when Pearson’s r is close to 1, a strong relationship between the variables is revealed. As can be observed, there was a strong relation of mechanical durability (MD) with extracts and holocellulose with r = 0.906 and r = 0.829, respectively. Therefore, the MD of pellets is found to be correlated in a positive way with the extractives and holocellulose content of pellets, affecting its variability by 90.6% and 82.9%. Furthermore, the MD of pellets is found to be correlated in a positive way to the lignin and ash content of pellets, influencing its variability by 64.5% and 66.5%, respectively. Moreover, the factor of material nature (wood/bark) seems to influence the MD by 79.4%, indicating that as the bark share increases in pellets feedstock material, the pellets’ MD increases as well.

4. Conclusions

According to the findings of the current research work, concerning the ash content, the wood of both examined cypress species (C. sempervirens and C. arizonica) is considered suitable for use in biofuels production, while the respective bark material is not suitable to be purely used by itself as biofuels feedstock. Using a bark share of 7% in pellets material could contribute to the production of denser, more dimensionally stable, mechanically durable pellets, of slightly improved calorific values, maintaining in parallel the ash content at such a low level that the pellets could be used in residential applications and be categorized in the highest pellets quality classes (A1 or A2). The requirements of the respective standards of pellets production were also met in regard to the bulk density, MC, and dimensions of pellets (length and diameter). The lower bark share of 2% in pellet feedstock demonstrated even lower ash content than that of the respective 7%, though it did not improve adequately the pellets’ mechanical durability in order to be categorized in quality classes for residential use. The high content of extractives and holocellulose of the raw materials seems to be strongly correlated to the mechanical durability of the produced wood–bark pellets. Given the lack of information concerning the biomass utilization of cypress species and taking into consideration the findings of the current work, it could be claimed that the effect of bark introduction in fuel pellets of cypress species biomass should be further studied in future. The investigation of the optimal pelletization conditions of cypress wood, bark, and the studied mixture ratios (probably higher MC means different granulometry), as well as the elemental analysis and the assessment of the ash melting point of cypress wood and bark, is considered important for the further utilization of these types of forest biomass. Additionally, the potential of mixing biomass of different forest species (wood and bark) with cypress biomass in the production of qualitative pellets is proposed to be thoroughly examined, keeping the bark presence at low levels (as also proposed in the present research). Such efforts would contribute to the introduction of alternative feedstock sources for the production of solid biofuels, the improvement of the pellets’ qualitative characteristics, and the thorough utilization of waste biomass (bark) in solid biofuels and energy production.