Lignocellulosic and chitin-rich agroindustrial residues as biochar precursors: a screening study based on proximate analysis and analytical pyrolysis

1 Introduction

The current energy crisis, characterized by volatility in fossil fuel markets, sustained demand growth, and the need to reduce greenhouse gas emissions, has necessitated the search for technological pathways compatible with the decarbonization of the global energy system (Jie and Rabnawaz, 2024; Leung et al., 2026). Consequently, climate change has made it a priority to progressively replace conventional energy sources with renewable, sustainable, and technically scalable alternatives (Leung et al., 2026). In this context, biomass has gained strategic importance due to its renewable nature, wide availability, and ability to integrate into circular economy schemes through the utilization of agricultural, forestry, and industrial waste [3]. In particular, lignocellulosic biomass stands out as a raw material of interest due to its potential to be transformed into fuels, chemicals, and carbon-based materials with added value (Blay-Roger et al., 2024; Singh and Clough, 2024).

There are various thermochemical conversion pathways; pyrolysis is recognized as a promising technology for the energy recovery of biomass (Blay-Roger et al., 2024; Singh and Clough, 2024). This process involves the thermal decomposition of organic matter in the absence of oxygen, generating solid, liquid, and gaseous fractions whose distribution depends on variables such as temperature, heating rate, residence time, and the physicochemical properties of the substrate (Singh and Clough, 2024). The importance of pyrolysis lies not only in its ability to produce biochar, bio-oil, and combustible gases, but also in its versatility to direct the process’s selectivity toward products of energy or chemical interest.

However, this versatility also implies high analytical complexity, since the thermal response of biomass and the formation of products are strongly influenced by its structural composition, particularly by the relative proportions of hemicellulose, cellulose, and lignin (Ungureanu et al., 2025). Accordingly, thermodynamic and kinetic analysis constitute a central component in the study of biomass pyrolysis. Thermogravimetric analysis (TGA) has established itself as a benchmark for characterizing mass loss stages, identifying thermal degradation intervals, and estimating material stability under increasing temperatures (Mishra and Mohanty, 2018; Ungureanu et al., 2025).

Previous studies have shown that the main biopolymers in biomass exhibit distinct thermal behaviors; for example, hemicellulose tends to degrade at relatively lower temperatures, cellulose exhibits more pronounced decomposition within a narrow range, and lignin degrades more gradually over a broad temperature range (Yang et al., 2007). Furthermore, kinetic approaches applied to TGA data have enabled the estimation of activation energies and other relevant parameters to interpret material reactivity, compare different biomasses, and provide useful criteria for reactor design and optimization (Mishra and Mohanty, 2018; Várhegyi et al., 2010). For this reason, thermodynamic evaluation not only describes the system’s thermal behavior but also provides a quantitative basis for assessing the viability of the valorization process.

Along with thermal behavior, the characterization of the gaseous fraction and the volatile compounds generated during pyrolysis is of key importance. The composition of these products largely determines the energy potential of the gas obtained, the quality of the condensable vapors, and the possibility of recovering platform molecules of industrial interest (Akalin and Karagöz, 2014; Bridgwater, 2012). For gas characterization, gas chromatography coupled with mass spectrometry (GC-MS), particularly in its analytical pyrolysis configurations, has proven to be a rapid and robust technique for identifying compounds released during the thermal degradation of biomass (Akalin and Karagöz, 2014; Hao et al., 2025).

It has been reported that the evolution of species such as CO, CO₂, H₂, and CH₄, as well as phenols, aldehydes, ketones, acids, and other oxygenated compounds, is closely linked to the material’s chemical structure and the process’s operating conditions (Akalin and Karagöz, 2014; Yang et al., 2007). Thus, GC-MS characterization complements thermodynamic analysis by allowing correlation of mass-loss events and energy transformations with the molecular nature of the products formed. Despite significant progress in this field, the literature still has limitations that warrant further research. Primarily, a significant portion of the studies have approached pyrolysis from a perspective focused on overall yields or general descriptions of the process, without sufficiently integrating thermal analysis, kinetic modeling, and detailed characterization of volatile products within a single experimental framework (Hao et al., 2025; Yang et al., 2007; Zhang et al., 2015).

Furthermore, the available thermodynamic information remains inconsistent across different biomass types, making it difficult to extrapolate activation energies, degradation intervals, or kinetic parameters between feedstocks with different compositions and mineral contents (Mishra and Mohanty, 2018; Várhegyi et al., 2010; Zhang et al., 2023). Finally, although the GC-MS technique has been widely used, in many cases the characterization of gases and volatiles is reported only partially or without an explicit correlation with the decomposition stages observed in thermal analyses (Akalin and Karagöz, 2014; Hao et al., 2025). These results limit a comprehensive understanding of the process and reduce the ability to select operating conditions that maximize the energy and chemical utilization of biomass.

For these reasons, it is necessary to develop integrated studies that link biomass pyrolysis to the analysis of its thermodynamic behavior and to the identification of the gases and volatiles produced. An approach in this context links the thermal decomposition of the material to the formation of specific chemical species, strengthening the mechanistic interpretation of the process and providing relevant information for its scaling and optimization. In light of these considerations, the objective of this study is to pyrolyze different types of biomass, evaluate their thermodynamic behavior using thermal analysis techniques, and characterize the gases and volatile compounds generated during pyrolysis via GC-MS. In doing so, the study aims to help fill gaps identified in the literature and to provide useful experimental evidence for the sustainable recovery of biomass energy (Mehta, 2025; Mehta and Chelike, 2024).

2 Materials and methods

2.1 Sample preparation

The samples were prepared according to the methodology described by Garcia-Rollan et al. (2026), Mohd Noor et al. (2024). Five types of biomass were chosen based on their availability and relevance to the study: three woody biomasses (Alternanthera truxillensis, Byttneria beyrichiana, and Guazuma ulmifolia), cocoa husks (Theobroma cacao L.), and shrimp exoskeletons (Litopenaeus vannamei). Each sample was selected, dried at 110 °C to remove moisture, ground, and sieved using a 355 µm mesh to obtain a homogeneous particle size for the study.

2.2 Moisture analysis

The moisture content of the samples was determined using the methodology described by Cortés-Herrera et al. (2021), Czaja et al. (2020). The MB23 thermogravimetric balance (OHAUS Corporation, Parsippany, NJ, USA) used for this purpose measures mass loss as a sample is heated, which provides an estimate of how much water is present. Samples were dried until they reached a constant weight, meaning the mass loss measured by the instrument was less than 1 mg in 60 s. Drying was performed at the standard temperature of 105 °C. This approach enabled later analyses to be based on a dry sample and reduced variability from leftover water.

2.3 Ash analysis

We determined the ash content of the samples using the methodology described by Kag et al. (2024), Salem et al. (2022). We employed the gravimetric ashing method, following established procedures for plant matrices and similar organic wastes. We weighed a 0.5 g sample into a pre-tared crucible and incinerated it in a model 2200851 muffle furnace (SNOL, Utena, Lithuania) at 650 °C for 3 h. After calcination, we cooled the crucible in a desiccator to room temperature and recorded the mass of the inorganic residue. We calculated the ash content based on the mass of the final residue relative to the initial mass of the sample, expressed as a percentage on a dry basis.

2.4 Thermogravimetric analysis

The pyrolysis of the samples was evaluated according to the methodology described by (Almusafir and Smith, 2024; Apaydın Varol and Mutlu, 2023; Fischer et al., 2024; Gonnella et al., 2022; Tumuluru et al., 2021; Yang and Chen, 2020). For this purpose, thermogravimetric analysis (TGA) was performed using a TGA/DSC 1 STAR System (Mettler Toledo, Greifensee, Switzerland) under an inert nitrogen (N₂) atmosphere, following a two-stage heating protocol.

In the first stage, the samples were heated from 25 °C to 300 °C at 20 °C/min, with a N₂ flow rate of 30 mL/min. This temperature range was selected to cover the drying and mild pyrolysis zones, considering that free and bound water are removed below 150 °C–200 °C, while torrefaction or mild pyrolysis occurs between 200 °C and 300 °C in an inert atmosphere. The moderate heating rate minimized thermal lag during the initial stage, as higher rates can shift the peaks of the thermogravimetric derivative toward higher temperatures and intensify the effects of heat and mass transfer during devolatilization.

In the second stage, the temperature was increased from 300 °C to 900 °C at 60 °C/min, with a N₂ flow rate of 10 mL/min. This stage was designed to evaluate the main decomposition region and the conversion of the carbonaceous residue, since the greatest mass loss typically occurs between 200 °C and 600 °C, while lignin decomposition can extend above 600 °C and reach approximately 835 °C, depending on the material’s composition. The nitrogen flow was maintained during both stages to ensure a non-oxidizing atmosphere and promote the removal of volatile compounds, a condition that is methodologically essential in thermogravimetric pyrolysis analysis.

A programmed cooling ramp was not utilized in this methodology. Upon reaching the final temperature of 900 °C, the analysis was concluded. The equipment then cooled naturally in an inert nitrogen atmosphere to prevent oxidation of the carbonaceous residue.

2.5 GC-MS analysis

The characterization of pyrolysis gases was performed according to the methodology described by (Wang et al., 2025; Xue et al., 2020). For this purpose, gas chromatography coupled with mass spectrometry (GC-MS) was employed, using an Agilent 5977E MSD system equipped with a 30 m × 300 μm × 0.25 μm Agilent 19091J-413 HP-5 column.

Chromatographic separation resulted from the differential partitioning of analytes between mobile and stationary phases. Compound resolution depended primarily on volatility and affinity for the low-polarity phase. The HP-5 column was chosen for its ability to analyze complex hydrocarbon mixtures and volatile compounds produced during pyrolysis. Its 5%-phenyl-methylpolysiloxane phase provided broad analytical applicability and robust separation of aliphatic and aromatic species.

Analyses were performed using helium as the carrier gas, with a flow rate of 1.5 mL min^-1 and a column head pressure of 1.9952 PSI. Compound identification was performed by comparing the obtained mass spectra with the NIST spectral library, following established methodologies for bio-oils and vapors derived from biomass pyrolysis.

3 Results

3.1 Biomass pyrolysis and thermogravimetry

Thermogravimetric analysis (Figure 1) revealed a multistage mass loss pattern across the five matrices. Initially, a slight decrease occurred at low temperatures, followed by an accelerated thermal degradation stage, and finally a gradual loss associated with the remaining carbon or mineral fraction. This behavior aligns with previously reported TGA profiles for lignocellulosic matrices, where the initial loss is primarily due to moisture and light volatile compounds. The steepest slope corresponds to the degradation of hemicellulose and cellulose, while lignin, owing to its complex aromatic structure, degrades over a broader temperature range and contributes to the solid residue at elevated temperatures (Apaydın Varol and Mutlu, 2023; El-Sayed et al., 2023).

For Alternanthera truxillensis, the mass decreased from approximately 38–39 mg to 10–11 mg after heating, representing a total loss of about 72%–73%. The most pronounced mass loss occurred between 260 °C and 370 °C, where the curve dropped sharply from around 35 mg to 16–17 mg. Between 400 and 900 °C, mass loss was slow and progressive, indicating the persistence of a thermostable residual fraction. Guazuma ulmifolia exhibited an initial mass of approximately 34–35 mg and a final residue of about 9–10 mg. The primary mass loss occurred between 300 °C and 405 °C, with the steepest decline observed around 330 °C–380 °C. Compared to A. truxillensis, the onset of accelerated degradation shifted to higher temperatures, suggesting differences in structural composition or the proportion of less labile components. After 450 °C, the curve indicated a gradual loss up to 900 °C.

Byttneria berychiana showed the lowest residual mass among the samples, with final values of 7–8 mg, corresponding to approximately 23%–25% of the original mass. Consequently, the total mass loss was among the highest observed. The main degradation stage occurred between 250 °C and 365 °C, with a marked reduction in mass over a narrow temperature range. This behavior suggests a higher proportion of thermolabile fractions or a lower presence of components that promote carbonaceous residue formation. Recent studies indicate that lignocellulosic composition and ash content directly influence the final residue and thermal parameters derived from TGA (Paredes et al., 2024).

The thermogravimetric curve for Litopenaeus vannamei displayed a thermal pattern distinct from those observed in the plant matrices. The main stage of mass loss for Litopenaeus vannamei occurred between 320 °C and 430 °C, which is at higher temperatures than those observed in B. berychiana and Theobroma cacao L. This shift likely results from the presence of structural biopolymers of animal origin, particularly chitin and protein-mineral fractions. For chitin extracted from white shrimp, major degradation has been reported between 260 °C and 400 °C, with a maximum degradation temperature near 380.7 °C, coinciding with the interval of greatest loss observed in the L. vannamei curve.

For Theobroma cacao L., mass loss began gradually at relatively low temperatures, with a sustained decrease between 180 °C and 380 °C. Unlike samples with abrupt transitions, this matrix exhibited broader degradation, possibly reflecting the overlap of organic components with varying thermal stabilities. The final residue was close to 30%, among the highest in the set, indicating greater retention of carbonaceous or mineral material at 900 °C.

3.2 Moisture content

Figure 2 shows that Alternanthera truxillensis had the highest moisture content, with two replicates at 66%–67% and a third at 51%. Cocoa husks followed, ranging from 53% to 64%. Byttneria beyrichiana showed intermediate values at 51%, 53%, and 60%. Guazuma ulmifolia was the driest and most consistent, with all measurements near 4%. The ranking was A. truxillensis ≈ cocoa husk > B. beyrichiana >>> Guazuma ulmifolia, with a difference of over 60 percentage points between the highest and lowest values. For pyrolysis gas production, biomasses with moisture above 50% would require significant pre-drying, as much of the process heat would be used to evaporate water, reducing decomposition temperature, thermal efficiency, and the yield of combustible gas. Only G. ulmifolia would be suitable for direct reactor feeding, supporting better energy recovery or upgrading to synthesis gas (Inayat et al., 2022; Lin et al., 2015; Shi et al., 2020).

3.3 Ash content

Figure 3 illustrates that ash content was highest in two distinct samples. In the low range (less than 10% dry weight), Litopenaeus vannamei exhibited the lowest value (2.87%), followed by Theobroma cacao (3.09%) and Guazuma ulmifolia (3.80%). In contrast, Bytneria beyrichiana (54.30%) and Alternanthera truxillensis (61.30%) constituted a group with very high mineral content (greater than 50%). These values exceed the threshold for classifying biomass as high-ash (greater than 10%) and surpass average values reported for agricultural residues and woody biomass in thermochemical processes (Esteves et al., 2023; Puri et al., 2024).

Agroindustrial residues from T. cacao and L. vannamei exhibited lower ash contents than those reported for cocoa shells (6.7%–11.38%) and shrimp shell waste (approximately 17%), respectively. This indicates a relatively high combustible fraction and greater suitability as energy feedstock (Londoño-Larrea et al., 2022; Saha et al., 2023). Similarly, G. ulmifolia remained within the low-ash range characteristic of tropical woody biomass and tree foliage, which typically ranges from 3% to 4% in tropical woods and 4.3%–11.8% in tropical tree and shrub foliage (Ángeles-Mayorga et al., 2022; Esteves et al., 2023).

Conversely, the extreme values for A. truxillensis and B. beyrichiana indicate significant dilution of the organic fraction, which is associated with lower biochar energy yield and a greater propensity for operational challenges. Additionally, their high mineral content may enhance catalytic activity during pyrolysis, particularly through the influence of alkali and alkaline earth metals on vapor cracking and product distribution (Esteves et al., 2023; Puri et al., 2024).

3.4 Pyrolysis-gas chromatography/mass spectrometry

Table 1 presents a volatile profile characterized by low-molecular-weight oxygenated compounds, predominantly carbonyls and unsaturated cyclic structures. Among dicarbonyls and aliphatic ketones, 2,3-pentanedione was detected at 2.119 min with a relative area of 11.25% and a NIST probability of 63.60%. 2-Propanone, 1-(acetyloxy)-, appeared at 4.345 min with 8.66% and a 54.60% probability. This compound distribution aligns with the fragmentation of simple sugars and the hemicellulose fraction, where cleavage, ring-opening, and dehydration pathways generate light carbonyls during pyrolysis. In the cyclopentenone fraction, 2-cyclopenten-1-one, 2-methyl- was identified at 5.126 min (5.40%; 82.70%) as the most reliable signal in this group. Additionally, 2-cyclopenten-1-one, 3-methyl- was recorded at 6.561 min (5.13%; 23.10%), and two signals assigned to 2-cyclopenten-1-one, 2,3-dimethyl- appeared at 7.388 min (2.01%; 14.10%) and 8.496 min (4.33%; 19.40%).

TABLE 1. Chemical compounds detected by GC-MS in Theobroma cacao L.

As their probabilities were below 50%, these three assignments are considered tentative. Collectively, these cyclic ketones are associated with cellulose degradation and the secondary cyclization of fragments derived from polysaccharide chains, a mechanism described for the formation of cyclic furans, lactones, and enones during carbohydrate pyrolysis (Ghalibaf et al., 2018; Wang et al., 2020). Furanic and lactonic compounds constituted another significant portion of the chromatogram. 1-(2-Furanyl)ethanone was detected at 5.209 min with a relative area of 2.95% and a probability of 53.10%.

Butyrolactone appeared at 5.297 min with 6.77% and a 63.10% probability. Both compounds are consistent with dehydration, rearrangement, and cyclization of hexose and pentose units derived from cellulose and hemicellulose. In the phenolic fraction, phenol was observed at 7.030 min (7.69%; 57.80%), and 2-methoxyphenol at 9.856 min (8.06%; 60.30%). The simultaneous detection of phenol and guaiacol indicates a substantial lignin contribution to the bio-oil, corresponding to guaiacyl unit depolymerization and β-O-4 bond cleavage, followed by demethoxylation and secondary cracking. From a valorization perspective, guaiacol is particularly relevant as a precursor for vanillin and other high-value aromatics. The phenolic fraction as a whole serves as a chemical marker of lignin’s role in the condensate (D’Arrigo et al., 2024; Lu and Gu, 2022; Zhou et al., 2022).

The most abundant compound was 3-cyclopentene-1-acetaldehyde, two-oxo-, detected at 3.694 min with a relative area of 32.70%. Due to its NIST probability of 47.50%, it is reported as a tentative compound. Despite this spectral limitation, the strong chromatographic peak suggests it is a principal component of the volatile profile of Theobroma cacao L. bio-oil, likely originating from the formation of cyclic oxygenated intermediates during hemicellulose or cellulose degradation. Its abundance aligns with the carbohydrate fraction’s propensity to generate cyclic ketones, furans, and other condensable oxygenated compounds. In contrast, cyclobutene, 2-propenylidene-, was detected at 2.770 min with a relative area of 5.04% and a probability of 22.70%, rendering its identification tentative and precluding definitive structural assignment. Similarly, the signals for 2-cyclopenten-1-one, 2,3-dimethyl- at 7.388 and 8.496 min remain tentative, consistent with the presence of isomers or co-elution artifacts in a chromatographic region where other cyclopentenones coexist (Ghalibaf et al., 2018; Lu and Gu, 2022).

Overall, the distribution of dicarbonyls, cyclic ketones, furanics, lactones, and phenols indicates a chemically reactive, highly oxygenated bio-oil. This pattern is valuable for inferring the relative contributions of polysaccharides and lignin in cocoa biomass. The resulting profile serves as a compositional fingerprint for quality monitoring, fractionation, and upgrading strategies. The detection of guaiacol highlights the condensate’s potential as a source of aromatic precursors, while the cyclopentenone fraction is relevant as a set of synthesis intermediates. Notably, cellulose-derived 3-methylcyclopent-2-enone-type compounds have been proposed as platforms for subsequent deoxygenation pathways and the production of advanced chemicals or fuels (Liu et al., 2021; Martins-Vieira et al., 2023).

As shown in Table 2, the GC-MS profile of the volatiles released during the pyrolysis of the exoskeleton of Litopenaeus vannamei was dominated by phenols. There was also a significant fraction of nitrogen-containing heterocycles and several low-molecular-weight oxygen-containing compounds. Among the nitrogen-containing heterocycles, pyridine (RT 2.588 min; 7.42%; NIST probability 70.60%) was identified with confidence. However, 2-methylpyridine (RT 3.509 min; 3.52%; 35.90%) and 1H-pyrrole, 1-ethyl- (RT 3.316 min; 1.78%; 21.80%) were considered tentative. This distribution was consistent with N-acetyl group cleavage, deamination, and cyclization/aromatization pathways of amine intermediates from chitin and the protein fraction. The simultaneous presence of pyridines and pyrroles distinguished this condensate from plant biomass bio-oils. This suggested a higher nitrogen content and a potential need for upgrading for energy or chemical applications (Al-Kwradi and Altarawneh, 2024; Duan et al., 2024; Sebestyén et al., 2020).

TABLE 2. Chemical compounds detected by GC-MS in Litopenaeus vannamei.

Non-phenolic aromatics included toluene (RT 2.712 min; 5.40%; 33.40%), bicyclo [4.2.0]octa-1,3,5-triene (RT 4.734 min; 7.46%; 27.40%), and benzaldehyde (TR 6.339 min; 1.02%; 36.90%). All were reported as tentative. For the bicyclo compound, the assignment allowed for the possibility of an aromatic isomer or a thermal rearrangement artifact. These signals were consistent with the decomposition of aromatic amino acids from the protein matrix. This decomposition likely occurred via decarboxylation, deamination, and side-chain fragmentation pathways, which generate aromatic hydrocarbons and aromatic aldehydes at high pyrolysis temperatures (Al-Kwradi and Altarawneh, 2024; Kibet et al., 2013).

The phenolic family was the dominant fraction in the chromatogram. Phenol (TR 6.994 min; 20.23%; 69.90%) was the major compound and had the most robust spectral assignment among the predominant peaks. Its abundance was consistent with the pyrolysis of tyrosine and, to a lesser extent, phenylalanine. It was also consistent with the breakdown of organic domains, in which chitin and structural proteins coexisted. Phenol, 3-methyl- (RT 9.551 min; 12.14%; 34.70%) was the second most abundant component, although its identification was tentative. p-cresol (RT 8.954 min; 3.71%; 7.96%) was treated only as a tentative signal of very low reliability. Phenol, 2-methoxy- or guaiacol (RT 9.841 min; 4.50%; 35.90%) was also tentative.

Formic acid, 2,6-dimethoxyphenyl ester (RT 11.230 min; 12.98%; 18.80%) showed high chromatographic response, but its assignment was uncertain. The appearance of guaiacol and this dimethoxyphenyl species suggested the presence of methoxylated aromatic substructures in the residual organic fraction. However, their origin in chitinous biomass was less direct than in lignocellulosic matrices. This did not allow for a conclusive structural assignment based on the available spectral data (Kirkok et al., 2019).

Among the compounds associated with the polysaccharide fraction, levoglucosenone (TR 10.523 min; 6.55%; 64.50%) showed probable identification. It served as a marker of dehydration and reorganization of the chitin hexosamine skeleton with a pyranose ring. Its presence, along with nitrogen-containing heterocycles, reinforced the chitinous nature of the condensate compared to plant biomass profiles. Butyrolactone (RT 5.267 min; 3.19%; 41.30%) was considered tentative. 2-Cyclopenten-1-one, 2-methyl- (RT 5.098 min; 2.24%; 55.70%) had a probable identification. 2-Cyclopenten-1-one, 2,3-dimethyl- (RT 8.464 min; 2.85%; 16.30%) was uncertain. Altogether, these oxygenated compounds were consistent with the mechanisms of glycosidic cleavage, dehydration, and rearrangement of carbonyl intermediates described for polysaccharides and the pyrolysis of chitin/N-acetylglucosamine.

Finally, 11-Hexadecyn-1-ol (RT 6.537 min; 2.59%; 9.08%) and 1,2-Epoxy-5,9-cyclododecadiene (RT 5.998 min; 2.43%; 15.20%) were classified as minor signals with uncertain assignment, owing to their very low NIST probabilities and ambiguous spectral features. The former may correspond to a derivative of residual lipids or a chromatographic artifact. The latter did not allow for a mechanistic interpretation supported by the available spectral evidence. In summary, these results defined a bio-oil characterized by phenols, nitrogen-containing heterocycles, and markers of polysaccharide degradation. This was consistent with the chitin-protein nature of the exoskeleton of L. vannamei (Sebestyén et al., 2020).

Tables 3–5 demonstrate that the fraction of non-phenolic aromatics distinctly separated A. truxillensis from the two Malvaceae species. In A. truxillensis, toluene was the predominant compound (20.595%; probable identification), followed by benzene (8.503%; probable), 1,3-dimethylbenzene (3.470%; probable), and two p-xylene signals (8.251% and 7.430%; both tentative). Additional compounds included several alkylbenzenes, indene, and other aromatic signals with uncertain assignments. The two p-xylene signals and those of 1-ethyl-3-methylbenzene (TR 6.369 and 6.831 min) were not combined, as their presence was consistent with isomerism or incomplete chromatographic separation. In contrast, B. beyrichiana and G. ulmifolia did not exhibit BTEX or similar non-oxygenated aromatics, indicating a lower contribution of aromatic extractives or a reduced extent of secondary deoxygenation and aromatization reactions under the tested conditions. The signals attributed to mesitylene in A. truxillensis had probabilities below 14% and therefore did not support mechanistic interpretation (Venegas-Vásconez et al., 2024).

TABLE 3. Chemical compounds detected by GC-MS in Alternanthera truxillensis.

TABLE 4. Chemical compounds detected by GC-MS in Bytneria beyrichiana.

TABLE 5. Chemical compounds detected by GC-MS in Guazuma ulmifolia.

Furanic and other oxygenated compounds of carbohydrate origin exhibited an inverse distribution. Guazuma ulmifolia displayed a profile dominated by furfural, with a main peak at 3.687 min (31.328%; probable identification) and a tentative peak at 3.068 min (7.477%), which were not summed. The exclusive and reliable presence of 2-furancarboxaldehyde, 5-methyl (2.233%; 74.50%) further indicated a significant contribution from hemicellulosic pentoses, particularly xylans. In G. ulmifolia, two signals of 2-propanone, 1-hydroxy- were observed: one reliable at 1.201 min (8.909%; 86.0%) and another tentative at 1.936 min (14.863%), consistent with cellulose depolymerization and dehydration pathways.

All three biomasses contained 2-furanmethanol, though in varying amounts: minor in A. truxillensis (0.869%), higher in B. beyrichiana (5.527%), and also elevated in G. ulmifolia (3.503%). Byttneria beyrichiana also contained 1-(2-furanyl)-ethanone (3.960%; probable), while 2-butanone, 1-(acetyloxy)- was present in B. beyrichiana (8.868%) and G. ulmifolia (7.143%), but absent in A. truxillensis (Sun et al., 2023; Venegas-Vásconez et al., 2024).

The cyclopentenone family was both more diverse and more abundant in B. beyrichiana. This species exhibited unsubstituted 2-cyclopenten-1-one as the second most abundant compound (18.910%; tentative), resulting in a profile co-dominated by this cyclic ketone and guaiacol. It also contained 2-cyclopenten-1-one, 2-methyl- in a reliable proportion (6.505%) and 2-cyclopenten-1-one, 3-methyl- (6.060%; uncertain assignment). In A. truxillensis, 2-methylcyclopentenone was present at much lower levels (1.704%), while in G. ulmifolia it appeared as two probable low-abundance signals (1.430% and 1.382%), which were not summed.

The signal attributed to 2-cyclopenten-1-one, 2-hydroxy-3-methyl- was detected in all three species, but with very low or uncertain probabilities (11.7%–17.8%) and was therefore reported only as a compositional coincidence. The dimethylated signals from A. truxillensis had probabilities below 10% and were considered highly uncertain. This overall pattern suggests greater accessibility of the cellulosic fraction in B. beyrichiana, as lactone, furan, and cyclopentenone derivatives are characteristic products of structural polysaccharide pyrolysis (Sun et al., 2022; Sun et al., 2023).

Butyrolactone was detected in all three biomasses, but with significant differences in abundance. Byttneria beyrichiana exhibited the highest and most reliable value (13.069%; 76.70%), making it the third most abundant component in this species. Guazuma ulmifolia showed a probable identification at 3.983%, while A. truxillensis had an even lower probable signal (1.762%). The predominance of butyrolactone in B. beyrichiana supports a bias toward oxygenated volatiles derived from polysaccharides and aligns with lactone formation pathways from low-molecular-weight oxygenated intermediates.

Phenols and methoxyphenols further highlighted differences in lignin contribution among the species. Guaiacol (phenol, 2-methoxy-) was present in all three biomasses, with the highest abundance in B. beyrichiana (16.098%; probable), followed by G. ulmifolia (5.679%; probable) and A. truxillensis (2.462%; probable). Simple phenol exhibited a similar trend, with 8.223% in B. beyrichiana, 2.498% in G. ulmifolia, and 1.493% in A. truxillensis. The elevated guaiacol content in B. beyrichiana suggests a greater contribution from guaiacyl units, as G-rich lignins typically release guaiacol and alkylguaiacols during pyrolysis. Alternanthera truxillensis was characterized by 3-methylphenol and p-cresol, both tentative and of low confidence, while G. ulmifolia contained mequinol (tentative) and creosol (highly uncertain), the latter being cautiously compatible with secondary methoxyphenols from lignin degradation.

Aliphatic hydrocarbons and miscellaneous compounds further differentiated A. truxillensis from the other species. Only A. truxillensis exhibited a C7–C12 series of alkanes and alkenes, including hexane, heptane, octane, nonane, decane, undecane, 1-octene, 1-decene, and 2-undecene, all with generally low probabilities. This pattern suggests a greater contribution from lipid extracts and waxes, though caution is warranted due to low spectral reliability. Byttneria beyrichiana displayed butanoic anhydride (5.897%; tentative) as an exclusive signal, while G. ulmifolia presented cyclopentanol (2.855%; uncertain assignment), 1,6-heptadien-4-ol, and 11-hexadecin-1-ol, the latter two with highly uncertain identification.

In A. truxillensis, the signal assigned to “Boldenone” (0.532%; 14.10%) was considered an unidentified compound with a long retention time and low reliability, possibly corresponding to a polycyclic aromatic species or another high-molecular-weight product, with no evidence supporting its identification as a steroid (Sun et al., 2023; Venegas-Vásconez et al., 2024). In summary, A. truxillensis exhibited the highest non-oxygenated aromatic content and a distinctive profile dominated by toluene and other BTEX compounds, distinguishing it from B. beyrichiana and G. ulmifolia. Byttneria beyrichiana showed the highest production of cyclopentenones, butyrolactone, and guaiacol, indicating a G-type cellulose–lignin fraction that is more susceptible to thermal degradation. Guazuma ulmifolia was characterized by furfural and hydroxyacetone, with a clear predominance of furan and carbonyl products derived from cellulose and pentosan hemicellulose.

These compositional differences resulted in distinct bio-oil qualities: A. truxillensis produced a potentially less oxygenated fraction, more suitable for aromatic valorization, while B. beyrichiana and especially G. ulmifolia generated more oxygenated mixtures, which have lower intrinsic stability but greater potential as sources of platform compounds such as guaiacol, furfural, and lactones. Due to the limited published pyrolytic characterization of these three species, this comparative analysis was necessarily restricted to related lignocellulosic biomass rather than broad taxonomic extrapolations (Carrasco Díaz et al., 2024; Gan et al., 2024; Pan et al., 2024; Rosado et al., 2021).

4 Discussion

Combined proximate analysis and Py-GC/MS screening demonstrated that these five residues represent at least three distinct thermochemical categories. While all are amenable to pyrolysis, wet lignocellulosic residues require conditioning, ash in mineral-rich herbaceous feedstocks likely influences secondary cracking, and the nitrogen-bearing marine residue exhibits unique volatile chemistry. This differentiation is critical for valorizing non-conventional biomass, as reactor operability, energy demand, condensate quality, and upgrading severity are influenced as much by feed conditioning and composition as by pyrolysis temperature (Rasaq et al., 2024; Talwar et al., 2025).

Moisture analysis distinguished G. ulmifolia from the other biomasses; its low water content renders it uniquely compatible with direct use in conventional pyrolysis units. In contrast, T. cacao, A. truxillensis, and, to a lesser extent, B. beyrichiana require pre-drying to prevent parasitic heat loss and reduced thermal efficiency. In fast pyrolysis, elevated feed moisture dilutes the organic condensate, decreases heating value, and shifts the output toward combustible gases and less stable bio-oil.

Consequently, high-moisture materials are best regarded as conditionally suitable rather than immediately reactor-ready. Ash content further reinforces these distinctions. Theobroma cacao, L. vannamei, and G. ulmifolia possess comparatively low ash, which favors conversion, whereas A. truxillensis and B. beyrichiana are ash-rich, indicating stronger mineral influence over devolatilization and vapor-phase reactions. Literature reviews indicate that alkali and alkaline-earth species can promote cracking, char formation, deoxygenation, and tar reforming, often reducing liquid yield but occasionally resulting in beneficial selectivity shifts. Within this context, the high ash burdens of A. truxillensis and B. beyrichiana are not only operational challenges but may also account for aspects of their distinctive pyrograms (Maj et al., 2025; Rasaq et al., 2024; Talwar et al., 2025; Ungureanu et al., 2025).

The results indicated significant differences in the proximate composition of the samples. Alternanthera truxillensis (51.0%–66.7%), Byttneria beyrichiana (50.6%–59.6%), and cocoa husks (52.8%–64.0%) exhibited high moisture contents, while Guazuma ulmifolia demonstrated low values (3.7%–3.9%), consistent with previously dehydrated biomass. The moisture content of cocoa husks was lower than the 84.62% ± 1.97% reported for fresh material but substantially higher than the 6.72%–16.10% range reported for dried shells, indicating partial dehydration prior to analysis (Londoño-Larrea et al., 2022; Vergara-Mendoza et al., 2022). For ash content, Theobroma cacao (3.09%) and G. ulmifolia (3.80%) were classified as low-ash biomass (<5%); however, cacao exhibited lower values than the 6.7%–13.5% range documented for dried cacao pods (Puri et al., 2024). Similarly, the ash content of Litopenaeus vannamei (2.87%) was considerably lower than the 17.0% reported for dry shrimp shell waste (Saha et al., 2023). In contrast, A. truxillensis (61.30%) and B. beyrichiana (54.30%) greatly exceeded the 0.3%–16.0% range described for agricultural residues. This discrepancy may result from a high accumulation of inorganic components, the presence of adhered mineral particles, or differences in sample preparation and calculation methods (Puri et al., 2024).

Within the lignocellulosic subset, T. cacao produced a mixed oxygenated and aromatic pattern, consistent with significant carbohydrate participation and a secondary contribution from guaiacyl-type lignin. The prevalence of diketones, acetylated ketones, cyclopentenones, furanones, and butyrolactones aligns with established fragmentation chemistry for cellulose and hemicellulose. Depolymerization, dehydration, ring contraction, and retro-aldol pathways generate carbonyl-rich volatiles and furanic intermediates. The presence of phenol and guaiacol indicates that lignin cleavage contributed to the aromatic oxygenate fraction, though this fraction was not dominant enough to obscure the carbohydrate-derived signature.

Consequently, cacao shell bio-oil is chemically attractive for fractionation toward oxygenated platform molecules, although substantial upgrading remains necessary for fuel-oriented applications. The most abundant peak in this matrix was only tentatively assigned and had low library confidence, so mechanistic interpretation should rely on the reproducible family-level pattern rather than a single structure. Overall, the cacao pyrogram is more consistent with a carbohydrate-rich lignocellulosic matrix containing guaiacyl lignin than with a lignin-dominated residue (Brown et al., 2024; Dong et al., 2020; Singh-Morgan et al., 2022; Wang et al., 2023).

A.truxillensis, B. beyrichiana, and G. ulmifolia further illustrate how different plant matrices redistribute carbon among hydrocarbon, oxygenate, and phenolic pools. The pyrogram of A. truxillensis was the most hydrocarbon-rich, featuring a pronounced BTEX fraction and a contiguous series of aliphatic hydrocarbons. Although Py-GC/MS does not directly resolve precursor macromolecules, this distribution is compatible with a greater contribution from extractives and lipid-like constituents, as well as enhanced secondary deoxygenation and aromatization promoted by the mineral-rich matrix.

In contrast, B. beyrichiana exhibited the most oxygenated profile, co-dominated by cyclopentenone-, lactone-, and guaiacol-type products, suggesting simultaneous carbohydrate ring fragmentation and guaiacyl lignin cleavage with limited deoxygenation. Guazuma ulmifolia represented another end-member, with furfural and hydroxyacetone serving as diagnostic products of pentosan and cellulose decomposition. The weaker phenolic contribution indicated a smaller effective lignin imprint in the analytical vapors, consistent with current mechanistic understanding.

Hemicellulose decomposes at lower thermal severity to furanic and carbonyl compounds, cellulose yields anhydrosugars and light oxygenates, and lignin broadens the product spectrum toward phenolics and methoxyphenols. From a valorization perspective, A. truxillensis is comparatively more attractive for hydrocarbon-oriented upgrading, while B. beyrichiana and G. ulmifolia are better suited for the recovery of oxygenated specialty chemicals or stabilized oxygenate streams (Brown et al., 2024; Lin et al., 2025; Talwar et al., 2025; Ungureanu et al., 2025). The shrimp exoskeleton functioned as a chemically distinct thermochemical feedstock. Its volatile fingerprint was not a simple marine analogue of lignocellulosic biomass; rather, it exhibited a chitin-protein pattern in which phenols coexisted with nitrogen-containing heterocycles and an anhydrosugar-like carbonyl, tentatively assigned as levoglucosenone.

Although this compound is reported with probable rather than definitive confidence, it is consistent with dehydration and rearrangement reactions on a hexosamine-based polymer backbone. The presence of pyridine and tentatively assigned pyrrolic species is also consistent with the intrinsic nitrogen content of chitin-derived materials and aligns with the known tendency of chitin and chitosan thermolysis to generate nitrogen-containing aromatics, char-promoting intermediates, and ammonia-releasing pathways.

The dominance of phenolic products is particularly significant, as it indicates that crustacean waste does not evolve an exclusively nitrogenous vapor phase. Secondary aromatization and cracking can still yield a substantial oxygenated aromatic fraction, even in the absence of plant lignin. This makes L. vannamei especially promising for differentiated valorization: its pyrolysis stream may be relevant for energy recovery, nitrogen-functional carbons, nitrogen-containing chemicals, or hybrid upgrading strategies that utilize both the condensable phase and the solid residue.

The low ash content, relative to many crustacean residues, further enhances its conversion potential, since excessive mineral matter is not expected to dominate the analytical chemistry as strongly as in the two ash-rich terrestrial biomasses (Alpaslan Takan and Özsin, 2025; Dziuba et al., 2025; Ilnicka and Lukaszewicz, 2018). Several analytical constraints limit the degree of mechanistic certainty that can be asserted. First, Py-GC/MS peak areas are semi-quantitative descriptors of analytical vapor composition under a specific heating program and do not directly correspond to reactor-scale product yields. Second, compound assignment quality varied across peaks, with several identifications remaining tentative due to low NIST similarity scores. This is particularly significant for isolated dominant peaks, as over-interpreting a single uncertain structure can distort the interpretation of the entire matrix.

Third, family-level interpretation is more robust than single-compound analysis in complex biomasses, especially when secondary reactions, co-elution, and matrix effects are present. Therefore, the current data support conclusions about the relative dominance of oxygenates, phenolics, hydrocarbons, and nitrogen-heterocycles with greater confidence than a full molecular reconstruction of each feedstock. Nevertheless, analytical pyrolysis remains highly informative for screening residue classes and anticipating which upgrading route—hydrodeoxygenation, catalytic aromatization, nitrogen management, or selective condensation—will be most demanding (Dong et al., 2020; Rasaq et al., 2024).

Collectively, the five residues support a differentiated rather than uniform valorization strategy. Guazuma ulmifolia is the most immediately processable feedstock, combining very low moisture with a carbohydrate-leaning vapor signature. Theobroma cacao and B. beyrichiana produce oxygen-rich condensates that may be more suitable for chemical recovery or staged upgrading than for direct fuel substitution. Alternanthera truxillensis yields the most hydrocarbon-like volatile pool, but its high ash fraction indicates that mineral management will be essential if maximizing liquid yield is a priority. Litopenaeus vannamei emerges as the most distinctive candidate for a parallel chitin biorefinery concept, where both the volatile fraction and nitrogen-rich solid fraction are considered target products. This distribution of behaviors underscores the importance of screening non-conventional agroindustrial residues at the interface of proximate analysis and molecular pyrolysis, rather than ranking them solely by bulk biomass labels. Their optimal thermochemical application depends on the combined influence of moisture, ash, and precursor biopolymers on the initial generation of pyrolysis vapors (Brown et al., 2024; Ji et al., 2024; Maj et al., 2025; Ungureanu et al., 2025).

Comparison of the assessed biomasses demonstrates significant differences relative to recent studies on agro-industrial waste, lignocellulosic matrices, and chitinous biomass. For T. cacao, the observed low ash content was lower than values reported for cocoa shells by (Londoño-Larrea et al., 2022), who identified a matrix with a higher mineral fraction and thermal decomposition associated with organic acids, ketones, and phenolic derivatives. The profile obtained in the present study, characterized by carbonyls, cyclopentenones, furans, butyrolactone, phenol, and guaiacol, partially aligns with existing literature on cocoa residues. However, the lower ash content indicates a more favorable combustible fraction and reduced risk of scaling during thermochemical conversion.

For L. vannamei, the presence of pyridine, pyrroles, phenols, and levoglucosenone clearly distinguishes this matrix from plant biomass (Meng et al., 2025). Reported that pyrolysis of shrimp waste produces compounds associated with–CH, –OH, and–NH groups, as well as hydrocarbon products at elevated temperatures. The higher proportion of phenols and nitrogen-containing heterocycles in the condensate indicates a significant contribution from chitin and proteins, consistent with deacetylation, deamination, and aromatization pathways of nitrogen-containing intermediates. Among the native plant species, G. ulmifolia most closely resembled low-mineral-content woody biomasses, due to its low moisture and ash content. The predominance of furfural and hydroxyacetone aligns with the degradation of pentosan hemicelluloses and cellulose, as described in recent studies on lignocellulosic pyrolysis (Sun et al., 2023; Venegas-Vásconez et al., 2024).

In contrast, B. beyrichiana exhibited considerable accumulation of guaiacol, butyrolactone, and cyclopentenones, indicating a greater contribution of guaiacyl lignin and thermally accessible carbohydrates. Alternanthera truxillensis deviated from this pattern, displaying a high proportion of BTEX and non-oxygenated aromatics, which more closely resembles processes involving secondary aromatization or advanced deoxygenation than conventional lignocellulosic pyrolysis. This difference, together with its high ash content, suggests chemical potential for aromatics but lower suitability as a direct fuel.

5 Conclusion

The thermochemical recovery potential of non-conventional waste materials was assessed by integrating proximate analysis with semi-quantitative volatile profiling using Py-GC/MS. Classification of these residues solely as biomass-derived, non-mineral fractions did not adequately predict their conversion behavior. Rather, the dominant biopolymer composition, mineral content, and volatile product distribution determined their suitability for specific thermochemical valorization pathways. The following conclusions are presented:

The analyzed materials did not form a homogeneous feedstock category but were instead grouped into three principal thermochemical behavior patterns. Lignocellulosic residues demonstrated potential for pyrolytic conversion, although pretreatment may be necessary depending on moisture content, ash content, and structural composition. Herbaceous wastes exhibited greater mineral interference, which influenced vapor formation, product selectivity, and process control. In contrast, nitrogenous marine residues displayed a distinct chemotype, characterized by chitin-derived compounds and nitrogen-containing products, indicating an alternative valorization route relative to conventional lignocellulosic biomass.

Among the residues studied, G. ulmifolia demonstrated the highest operational compatibility with standard pyrolysis due to its low moisture content, favorable fixed carbon fraction, and predominance of carbohydrate-derived products. In contrast, T. cacao and B. beyrichiana produced condensates rich in oxygenated compounds, such as cyclopentenones, lactones, and guaiacol derivatives. These findings indicate their potential as sources of platform molecules or as candidates for selective fractionation strategies. Collectively, the results suggest that oxygenated condensates from these residues may be more suitable for chemical recovery than for exclusive energy production.

Alternanthera truxillensis generated the most hydrocarbon-rich volatile fraction, including BTEX compounds; however, its high ash content may constrain process control and reduce the efficiency of liquid product recovery. In comparison, L. vannamei exhibited a distinctive volatile profile dominated by phenols, nitrogen-containing heterocycles, and levoglucosenone-type carbonyls, supporting its potential integration into chitin-based biorefinery schemes. In summary, the combined use of proximate analysis and Py-GC/MS screening provided an efficient approach for identifying viable value-added pathways and for informing subsequent bench-scale pyrolysis, catalytic upgrading, and fractionation studies.

Generative AI statement

The author(s) declared that generative AI was not used in the creation of this manuscript.

Any alternative text (alt text) provided alongside figures in this article has been generated by Frontiers with the support of artificial intelligence and reasonable efforts have been made to ensure accuracy, including review by the authors wherever possible. If you identify any issues, please contact us.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Footnotes

Edited by: Abrar Inayat, University of Sharjah, United Arab Emirates

Reviewed by: Prerna Mehta, GD Rungta Educational Society, India

Gabriela Pompeu, Embrapa Meio Ambiente, Brazil