Valorization of Wood Residues from Vegetation Suppression during Wind Energy Plant Implementation and Its Potential for Renewable Phenolic Compounds through Flash Pyrolysis: A Case Study in Northeast Brazil’s Semi-Arid Region

Table 1 presents the physicochemical properties of three wood residues, encompassing proximate composition, heating values, lignocellulosic composition, bulk density, and the chemical composition of ash, which assume significance due to their close association with the bioenergy potential of the investigated lignocellulosic residues. In the literature, the suitability of solid biofuels for thermochemical conversion is typically recognized when their moisture content is below or close to 10.0 wt.% [33]. Acceptable moisture levels (below 10.9 wt.%) are observed in the three wood residues for their application in thermochemical conversion processes, as higher moisture values indicate the necessity of drying and present challenges for the ignition and combustion of biomass.

The proximate composition, as shown in Table 1, reveals that the AP wood residue exhibits a greater volatile matter content compared to that of the CP and CL wood residues. This observation suggests that the AP wood residue is characterized by a higher propensity for volatile product release during pyrolysis, thus favoring the production of renewable chemicals. Concerning fixed carbon, the CP and CL wood residues, with values ranging from 7.8 to 9.5 wt.%, align with the typical composition range of biomass fuels reported in the literature (66–85 wt.%) [34]. Concerning fixed carbon, the CP and CL wood residues, with values ranging from 7.8 to 9.5 wt.%, align with the typical composition range of biomass fuels reported in the literature (7–20 wt.%) [35]. A low ash content below 2.9 wt.% was observed in the three wood residues, indicative of their suitability for bioenergy applications. The presence of inorganic matter (ash) in wood residues is limited, thereby mitigating potential operational issues, such as corrosion and fouling, during the thermochemical conversion and ensuring adequate efficiency in bioenergy production. The inorganic content of the three wood residues aligns with the reported range for commercial lignocellulosic biomass fuels (0.6–9.8 wt.%) [34].

As indicated in Table 1, cellulose contents in CL and AP wood residues were higher and closely approximated at 46.26 wt.% and 42.64 wt.%, respectively, with the lower content of 39.03 wt.% observed in CP wood residue. The cellulose mass contents align well with those observed in hardwood biomasses (43–47 wt.%) [36]. The hemicellulose contents in AP and CL wood residues were closely approximated at lower levels, measuring 11.34 wt.% and 13.48 wt.%, respectively, while CP wood residue exhibited the highest content at 19.79 wt.%. The hemicellulose mass content in CP wood residue aligns closely with the range observed in hardwood biomasses (16.8–18.7 wt.%) [37]. Typical volatile reaction products from the thermal decomposition of cellulose and hemicellulose include short-chain oxygenated compounds, acetic acid, furan derivatives, ketones, and a small content of hydrocarbons [38,39,40]. Lignin contents ranged from 19.1 to 30.6 wt.%, with the highest observed in CL wood residue. The lignin content in the three wood residues closely matches the published range for wood species found in the Brazilian semi-arid region (23.7–32.8 wt.%), according to the literature [41]. High-lignin biomass is proposed to generate a condensable pyrolysis product (bio-oil) rich in phenolic compounds. Thus, wood residues from suppressed native species hold significant potential for producing valuable renewable chemicals (phenolic compounds) through flash pyrolysis.

Figure 1 illustrates the temperature-dependent mass loss and rate of mass loss profiles recorded during the thermal decomposition of three wood residues. The initial stage of mass loss, occurring below 100 °C, primarily involves the evaporation of inherent moisture, with an average mass loss below 10 wt.% observed in the three wood residues, consistent with the inherent moisture content indicated in the proximate composition (Table 1). The prevalent mass loss stage, occurring between 100 °C and 400 °C, is characterized by the predominant release of volatile matter, corresponding to the simultaneous devolatilization of hemicellulose and cellulose with a smaller contribution from lignin. This assertion is plausible, as the literature commonly attributes the temperature ranges of 240–325 °C and 325–400 °C to the thermal decomposition of hemicellulose and cellulose, respectively [42,43], while lignin undergoes continuous and gradual thermal decomposition over a broad temperature range spanning approximately 180–700 °C [44,45]. On average, the three wood residues exhibit an approximate 60 wt.% mass loss in this prevalent stage, aligning with the reported mass losses for the prevalent stage of wood species in the Brazilian semi-arid region [41]. At the final temperature of 900 °C, an average remnant mass below 10 wt.% was observed for three wood residues, consistent with the findings of proximate analysis (sum of fixed carbon and ash content).

The ash content corresponds to minerals absorbed during biomass growth, forming the inorganic part post-burning, and its chemical composition is influenced by the soil during biomass growth. As an inherent characteristic, soils in Northeast Brazil’s semi-arid region exhibit considerable levels of nutrients, including calcium (Ca) and potassium (K) [46,47]. Hence, the high content of CaO and K₂O in the inorganic matter (ash) of three wood residues is justified, whereas elements like Fe₂O₃ and SiO₂ were observed in lower amounts. As indicated in Table 1, calcium is the prevalent mineral observed in the chemical composition of the ashes from AP and CP wood residues. In contrast, potassium is the prevalent mineral exhibited in the chemical composition of the ashes of CL wood residue. One plausible explanation is the more facile absorption and fixation of nutrient K from the soil by CL trees. The catalytic action mechanism of biomass ashes is elucidated in the literature, wherein alkaline and alkaline earth metals are found to form complexes with hydroxyl and/or phenolic groups inherent in the macromolecular structure of biomass [48,49]. Literature investigations have furnished evidence that the complexation of these cations within the cellulose structure facilitates scission in the polymeric chain, leading to the production of smaller compounds, such as furan derivatives and short-chain oxygenated compounds (C₁–C₄), instead of levoglucosan [50,51].

Favorable HHV values are demonstrated by wood residues derived from suppressed native species (Table 1), with the following order: AP (18.39 MJ kg⁻¹) > CL (17.99 MJ kg⁻¹) > CP (17.47 MJ kg⁻¹). The energy content of the three wood residues closely aligns with published values for other wood species from the Brazilian semi-arid region (17.9–20.5 MJ kg⁻¹) [41]. Competing equivalently with the reported values for commonly used lignocellulosic materials in commercial-scale solid biofuels, which typically have potential energy values ranging from 14.6 to 19.4 MJ kg⁻¹ [34], the herein demonstrated HHV values underscore the promising potential of wood residues derived from suppressed native species as a valuable feedstock for bioenergy applications.

Bulk density is practical in formulating an effective logistics strategy for biomass handling, storage, and transportation [52]. Higher bulk density values are anticipated to correlate with reduced storage, transportation, and handling costs. As indicated in Table 1, the studied wood residues exhibited the following bulk density order: AP (268.70 kg m⁻³) > CP (237.10 kg m⁻³) > CL (126.50 kg m⁻³). Bulk densities for typical lignocellulosic biomass utilized for bioenergy purposes, such as pinus wood, rice husk, and sugarcane bagasse, were reported as 176 kg m⁻³, 129 kg m⁻³, and 119 kg m⁻³, respectively [53,54,55]. These values were higher than or comparable to those observed for the wood residues characterized in this study. An additional crucial consideration related to handling and transportation is the energy density (LHV per unit volume), reflecting the available potential energy concerning the biomass volume. A high energy density has been demonstrated in AP and CP wood residues, with values of 4.92 and 4.14 GJ m⁻³, respectively, in contrast to CL wood residue, which exhibits a lower density of 2.28 GJ m⁻³. These values were observed to be competitive with those typical of lignocellulosic biomass used for bioenergy, such as rice husk (3.40 GJ m⁻³), coffee wastes (4.41 GJ m⁻³), sugarcane bagasse (1.80 GJ m⁻³), bamboo cellulose pulp (2.77 GJ m⁻³), and maize wastes (2.85 GJ m⁻³) [56]. Therefore, the wood residues derived from suppressed native species can be presumed to be promising feedstocks for bioenergy purposes.

While the bulk density of wood residues derived from suppressed native species is higher than or comparable to other well-known biomass residues, it is noted that these residues are not competitive with densified biomass in the form of pellets and briquettes (350–750 kg m⁻³) [57]. This disparity may pose a significant practical barrier to the large-scale application of these underexplored lignocellulosic residues for bioenergy purposes. One well-established approach is the utilization of densification processes to produce a highly densified solid biofuel that complies with standard specifications for solid fuels [58,59]. Through the conversion of wood residues in sawdust form into pellets or briquettes, an anticipated enhancement in bulk density (and consequently energy density) is expected to be achieved at a competitive level. Following densification, the anticipated outcome is the production of solid biofuel suitable as a competitive feedstock for industrial- or medium-sized domestic biomass burners serving bioenergy purposes. In this context, recommendations for future research involve conducting studies on densification processes, such as pelletization and briquetting, to broaden the potential applications of wood residues derived from suppressed native species in the field of bioenergy.

Figure 2 depicts the distribution of volatile reaction products released from three wood biomasses when subjected to flash pyrolysis, categorized by their chemical class. The volatile reaction products, subjected to analysis through the GC/MS system, were classified into primary categories determined by their organic functional groups. These categories include lighter oxygenated compounds (C₁–C₄), acetic acid, furan derivatives, phenolic compounds, cyclic ketones, other oxygenated cyclic compounds, alkanes, and unidentified compounds (with similarity scores below 85%). Table 2 displays the compounds identified in the volatile reaction products resulting from the flash pyrolysis of the three wood biomasses, including their molecular formula, molecular weight, and relative concentration, in which a predominance of oxygenated products is evident.

The volatile reaction products, including short-chain oxygenated compounds (C₁–C₄), acetic acid, furan derivatives, cyclic ketones, other oxygenated cyclic compounds, and alkanes, observed from the three wood residues during flash pyrolysis, can be attributed to the thermal decomposition of cellulose and hemicellulose. Literature investigations yielded evidence that supports this hypothesis [38,39,40]. As illustrated in Figure 2, the class of short-chain oxygenated compounds (C₁–C₄) constituted the second-largest proportion of the volatile reaction products, exhibiting the following order: CP (20.98%) > CL (20.59%) > AP (18.19%). A unique hydrocarbon, 2,7-dimethyl-octane, was identified in the volatile pyrolysis products of wood residues from all three suppressed species, with its highest relative concentration (3.0%) observed in the AP wood residue. The occurrence of 1,2,4–trimethoxybenzene, an oxygenated cyclic compound, in the volatile pyrolysis products derived from CP and CL wood residues, exhibits its maximum relative concentration (2.4%) in the CP wood residue and is commonly ascribed to the thermal decomposition of lignin [39].

As depicted in Figure 2, the acetic acid class constituted the third-largest proportion of the volatile reaction products, with the highest concentration observed in CP wood residue, accounting for approximately 19.0% of all volatile reaction products. This is a significant yield in comparison with acetic acid yields obtained from the pyrolysis of typical lignocellulosic biomass previously used for bio-oil production, including cherry wood (11.5%), olive wood (9.1%), hazelnut shell (12.4%), corn cobs (15.1%), almond shells (15.4%), and corn stalks (14.7%) [60]. Considering these findings and the potential applications of volatile reaction products into the condensable form (bio-oil) from the flash pyrolysis of CP wood residue, the extraction of compounds like acetic acid can be conceivable. Acetic acid, a valuable industrial chemical, is mainly utilized as a solvent owing to its exceptional solubility and miscibility. Furthermore, it is a crucial chemical raw material widely used in agriculture, medicine, textiles, adhesives, cosmetics, food, and other fields [61]. Another significant application lies in its role as a raw material for synthesizing cellulose acetate/polyvinyl acetate polymers and its importance as an acidity regulator in various chemical processes [60,61].

Furan derivatives, including 3–furaldehyde and 2–furan methanol, were identified among the volatile reaction products from the three wood residues, with the highest content observed in CP wood residue, constituting approximately 9.5% of all volatile reaction products. Furan derivatives serve as an important platform chemical employed in the chemical industry for various applications, including use as a solvent, alcohol, resin precursor, or intermediate in producing fragrances, C vitamins, and herbicides [40]. The main products from the thermal decomposition of cellulose, levoglucosan, and its derivatives were not detected among the volatile reaction products of the three wood residues. One possible explanation is that the alkaline metals, including K⁺ and Ca²⁺ present in biomass ash, act as natural catalysts, altering the cellulose depolymerization pathway and leading to the formation of products such as furan derivatives and short-chain oxygenated compounds (C₁–C₄) instead of levoglucosan. Literature investigations provided corroborative evidence for this assertion [50,51]. Also, the release of furan derivatives may be associated with secondary reactions involving levoglucosan, potentially catalyzed by alkali metals present in biomass ash [40].

Phenolic compounds, which primarily originate from the thermal decomposition of lignin [40,62], constituted the prevalent constituents among the resulting volatile reaction products from the analytical flash pyrolysis of each of the three lignocellulosic biomasses, as demonstrated in Table 2. As expected, the differences in the lignocellulosic chemical composition led to distinct relative concentrations of phenolic compounds: CL and AP wood residues, with higher lignin percentages in their compositions, exhibited the highest relative phenolic compound concentrations (40%–42%), whereas CP, with the lowest lignin content, displayed the lowest relative phenolic compound concentration (27%). It is well-known that the elevated molecular weight and viscosity of bio-oils are primarily attributed to lignin-derived products [62]. Also, phenolic compounds, classified as aromatic oxygenated compounds, may adversely impact bio-oil when intended for use as fuel due to oxidation reactions, imparting a degree of instability [63]. Despite the drawbacks associated with the presence of phenolic compounds, recent studies have emphasized that phenolic-rich bio-oil demonstrates insecticidal properties, a positive attribute [23,64].

According to the literature, phenolic compounds are classified into four categories based on aromatic substituent groups: the guaiacol type (G–), the phenol type (H–), the syringol type (S–), and the catechol type (C–) [65,66]. The distribution of phenolic products, categorized by their aromatic substituent groups and resulting from the flash pyrolysis of three wood residues, is illustrated in Figure 3. Hence, it is inferred that the volatile products derived from the thermal decomposition of lignin primarily consisted of monoaromatic phenolic compounds, predominantly associated with hydroxyl and methoxyl groups. The G–type phenols were the predominant group, in contrast to the H– and S–types, with the highest proportion provided by CP wood residue. The predominant G–type phenolic compounds, delineated by their relative concentrations from the three wood residues, include 2–methoxy–5–methylphenol, 2–methoxy–4–vinylphenol, and 2–methoxy–6–(2–propenyl)–phenol. Within the G–phenolic compounds, 2–methoxy–4–vinylphenol emerged with the highest relative concentration. This phenolic compound holds industrial significance, as it serves as a flavoring agent in the food industry and finds applications in synthesizing resins and pharmaceuticals [67]. In contrast, phenol and 4–methylphenol emerged as the sole products among the H–type phenolic compounds, while 2,6–dimethoxyphenol stood out as the exclusive product among the S–type phenolic compounds. The formation of S–type phenolic compounds (containing two –OCH₃) is attributed to the cleavage of syringyl groups within the lignin macromolecule [39].

The prevalence of G–type phenolic compounds (methoxy groups indicates) indicates the predominance of the guaiacyl unit in the alkali lignin. G–type phenolic compounds can be produced through the direct cleavage of the β–O–4 bond, which possesses the lowest dissociation energy among all linkage bonds [65]. Cenostigma pyramidale belongs to the family Fabaceae, Commiphora leptophloeos belongs to the family Burseraceae, and Aspidosperma pyrifolium belongs to the family Apocynaceae; all are classified as Angiospermae families [68,69]. The term “hardwood’ is frequently employed to denote the extensive category of angiosperm trees, and this distinction is crucial as the characteristics of phenolic compounds depend on whether a “hardwood” or “softwood” species is subjected to pyrolysis [70]. While guaiacols and syringols form hardwood lignin in comparable proportions [70,71], a predominance of G–type phenolic compounds was observed in the flash pyrolysis of three wood residues, as opposed to S–type phenolic compounds. One possible explanation for the observed phenomenon is the demethoxylation reactions, which progressively convert S–type phenolic compounds into G–type phenolic compounds through O–CH₃ bond homolysis. This decomposition leads to the production of phenol, cresol, and catechol at temperatures above 400 °C [72,73]. Literature investigations yielded similar observations to those found in this case study [74], corroborating the results obtained.

In light of the encouraging outcomes observed in this case study, the transition of flash pyrolysis applied to the studied wood residues to a commercial scale is recommended, driven by its potential capacity to generate bio-oil rich in valuable phenolic compounds. The flash pyrolysis of wood residues derived from suppressing native species can be viewed as an alternative approach to replace fossil-based counterparts in producing phenolic compounds. This is particularly relevant for sustainability and aligns with potential applications in green chemistry scenarios. In recent literature, phenolic compounds have been emphasized for their value in various industrial applications. For example, 2,6–dimethoxyphenol is used in the production of synthetic smoke flavorings, and 2–methoxy–4–vinylphenol is recognized as an aromatic substance frequently employed as a flavoring agent, contributing to the natural aroma of buckwheat [75]. Furthermore, producing bio-oil rich in phenolic compounds allows for its subsequent use in producing phenol-formaldehyde resins, thereby diminishing reliance on petroleum-derived phenols in resin manufacturing [76]. Other relevant industrial applications for phenolic compounds include their use as additives in the fertilizing and pharmaceutical industries, as flavoring agents in the food industry, and their utility in preparing dyes, explosives, lubricants, pesticides, and plastics [76,77,78]. In summary, the valorization of wood residues resulting from suppressing native species through flash pyrolysis can reduce the environmental impacts associated with wind energy plant installation. This valorization route yields phenolic compounds with substantial applicative potential, contributing to the bioeconomy and concurrently aligning with Sustainable Development Goals (SDGs) 7, 9, 13, and 15 targets.