Pyrolysis of Green Coconut Husk Pellets: Process Conditions for the Integrated Production of Biochar, High-Quality Bio-Oil, and Hydrogen-Rich Gas

Abstract

Green coconut husk is an abundant and underutilized agro-industrial residue in Brazil, contributing significantly to landfill overload. This study investigates the pyrolysis of pellets derived from this biomass as a technological alternative for its valorization, focusing on the integrated characterization of the three resulting products. Pellets were subjected to pyrolysis in a fixed-bed reactor under two distinct conditions: at 400 °C to maximize biochar production, and at 600 °C to enhance gas generation. The raw material and resulting solid, liquid, and gaseous fractions were characterized using physicochemical, thermal, morphological, and chromatographic analyses. Pyrolysis at 400 °C yielded biochar with high fixed carbon content (67.03%) and elevated heating value (27.80 MJ/kg), suitable for soil amendment and carbon sequestration. At 600 °C, the non-condensable gas exhibited a higher hydrogen concentration (35.84%) and an H₂/CO ratio of 1.84, favorable for chemical synthesis applications. Notably, palletization resulted in a significant bio-oil and gas yield even under 400 °C. The bio-oil underwent chemical upgrading, which significantly increased the phenolic content and raised its heating value to 20.40 MJ/kg. Additionally, combustion tests revealed that the gas produced emitted lower levels of NOx compared to natural gas.

1. Introduction

The global energy transition has become a critical priority considering the pressing challenges posed by climate change and the urgent need for sustainable alternatives to fossil fuels. Accordingly, developing and implementing solutions that reduce greenhouse gas emissions and promote the adoption of renewable energy sources are essential for mitigating environmental impacts and fostering a low-carbon economy [1].

Lignocellulosic biomass, obtained from agro-industrial residues, represents an abundant and strategic raw material for bioenergy production. Through thermochemical conversion processes, such as pyrolysis, this biomass can be transformed into value-added bioproducts [2].

Among lignocellulosic biomasses, green coconut husk stands out as a potential raw material for thermal conversion processes. Green coconut husk, which is usually improperly discarded, results in environmental problems such as soil and water contamination and landfill overloading. Its application as biomass can contribute to solid waste management [3]. Predominantly composed of cellulose, hemicellulose, and lignin, its structure provides favorable physical and chemical characteristics for transformation into value-added products [3]. Cellulose contains carbohydrate chains that favor the formation of bio-oil during the pyrolysis process. Hemicellulose has greater thermal instability and decomposes at lower temperatures, significantly contributing to the release of non-condensable gases [4]. Lastly, lignin, with its structure rich in aromatic carbon, is particularly suited for producing high-quality biochar [5,6].

The green coconut husk is a versatile biomass with potential applications in agriculture, construction, environmental recovery, and renewable energy systems [2,7]. Transforming this biomass into pellets, which are densified solid biofuels produced by compacting biomass, offers advantages such as higher energy density, improved thermal performance, and logistical and cost benefits, including ease of transport, storage, and handling [8].

Despite the recognized potential of pyrolysis as a technological route for utilizing green coconut husk, scientific literature still presents significant gaps that hinder the advancement of this application. There is a noticeable lack of studies focused on the pyrolysis of biomass in its pelletized form, which is suitable for industrial scale use due to the logistical and energetic advantages that pellets offer. Moreover, it is uncommon to find research that provides a comprehensive characterization of all three products generated by the process. Most studies focus on only one or two of these products, resulting in a partial and fragmented view of the true valorization potential of green coconut husk as a renewable biomass source.

Although previous studies have assessed aspects of coconut biomass pyrolysis, they remain fragmented. Romão et al. [9] evaluated the energy and economic potential of green coconut husk for producing biochar and bio-oil but overlooked both non-condensable gases and the pelletized form crucial for industrial scalability. Figueiredo et al. [10] investigated pyrolysis products in a rotary reactor using raw biomass, providing no insights into densification effects. Ekanayaka et al. [11] limited their analysis to biochar characterization alone, leaving the behavior of the liquid and gaseous fractions unexplored. By directly addressing these limitations, this work delivers an integrated assessment of pelletized green coconut husk pyrolysis, linking operational parameters to the simultaneous valorization of biochar, bio-oil, and hydrogen-rich gas.

It is hypothesized that the pyrolysis of green coconut husk pellets will yield biochar, bio-oil, and non-condensable gases with distinct physicochemical properties and potential industrial applications, particularly in renewable energy, sustainable agriculture, and waste valorization. A comprehensive characterization of these products will provide valuable insights into their potential, aligning them with the principles of circular economy and sustainable development [12].

In addition to the integrated characterization, the bio-oil obtained was subjected to a selective chemical upgrading process which significantly enhanced its phenolic content and energy density. To assess the impact of this transformation, a novel quality index was proposed, enabling a comparative evaluation of the treated bio-oils. These innovations contribute to the development of high-performance biofuels and chemical precursors derived from coconut biomass.

Furthermore, the study explores the dual application potential of the pyrolysis products, both as renewable energy sources and as strategic chemical intermediates, particularly phenolic compounds with industrial relevance.

This initiative aligns with several United Nations Sustainable Development Goals (SDGs): SDG 7 (Affordable and Clean Energy), by promoting the diversification of renewable energy sources; SDG 11 (Sustainable Cities and Communities), through improved solid waste management; SDG 12 (Responsible Consumption and Production), by converting waste into high-value products; and SDG 13 (Climate Action), by reducing greenhouse gas emissions and enabling carbon sequestration through biochar applications [12].

Considering this scenario, the present study aims to address the gaps in literature by conducting a comprehensive analysis of the pyrolysis of green coconut husk pellets. Biochar, bio-oil, and synthesis gas will be produced and characterized, with an evaluation of the impact of temperature (400 °C and 600 °C) on their properties. The goal is to provide a complete overview and generate critical data to support the development of new value chains based on one of Brazil’s most abundant agro-industrial residues.

2. Materials and Methods

2.1. Materials

The green coconut husks were gathered in Fortaleza’s Paracuru municipality. Hexane, distilled water, dichloromethane, 1.75 M ethanolic KOH solution, and 6 M hydrochloric acid (HCl) were all Sigma brand (Sigma-Aldrich, St. Louis, MO, USA) reagents used in the experiments.

2.2. Biomass Pre-Treatment and Pellet Production

Green coconut husks were collected, in the municipality of Paracuru, in the state of Ceará and transported to the Energy Conversion and Innovation Laboratory (LCE+) at Ceará State University (UECE). The husks were manually opened and sun-dried at the laboratory for four days. Following the drying process the husks were shredded with a TRF 70 shredder (Trapp, Santa Catarina, Brazil).

The resulting mixture, composed of powder and fibers, was then processed into pellets using a pelletizer (Pellet Machine—SKI 200, Żeromskiego, Poland) at 80 °C, with moisture content adjusted to approximately 9–13% to ensure proper compaction. The resulting pellets were uniform in diameter, with 6 mm diameter and lengths of 20–40 mm.

2.3. Physicochemical Characterization

The proximate analysis of the biomass and biochar was conducted according to ASTM standards for moisture (ASTM E871-82 [13]), volatile matter (ASTM E872-82 [14]), ash (ASTM D1102-84 [15]), and for the biochar specifically (ASTM D5142 [16]). Fixed carbon was calculated by difference. Elemental analysis (C, N, H, O, S) followed the ASTM D5373 standard. The Higher Heating Value (HHV) was determined using a PARR 1341 calorimeter, following the ASTM D240 [17] standard. Approximately 1 g of the sample was combusted in a 2 cm diameter crucible under controlled oxygen conditions. This equipment was used to analyze both solid and liquid samples.

2.4. Thermogravimetric Analysis (TGA)

Thermogravimetric analysis (TGA) was carried out at the Federal University of Ceará. The measurements were performed on a Mettler Toledo TGA instrument (Mettler Toledo, Columbus, OH, USA) operated with STARE SW 13.00 software. Approximately 5 mg of the sample was placed in a 70 µL alumina ceramic crucible and heated from 30 °C to 800 °C under a nitrogen atmosphere (50 mL/min) at a constant heating rate of 10 °C/min.

2.5. Scanning Electron Microscopy (SEM)

SEM analysis was conducted using a VEGA3 TESCAN (Brno, Czech Republic) at Embrapa Tropical Agroindustry. Samples were mounted on stubs with double-sided carbon tape and coated with platinum using an Emitech K550 sputter coater (Quorum Technologies, Lewes, UK) at 50 mA for 360 s. Imaging was performed at 15 kV and 2000× magnification under controlled conditions.

2.6. Gas Chromatography Coupled with Mass Spectrometry (CG/MS)

The chemical analysis of the sample components was performed using a Shimadzu QP-2010 GC-MS (Kyoto, Japan) instrument under the following conditions: a fused silica capillary column coated with Rtx-5MS (5% diphenyl/95% dimethylpolysiloxane), with dimensions of 30 m × 0.25 mm × 0.25 µm df; initial temperature of 250 °C, in split mode (1:100); programmed column temperature as follows: 0–10 min at 140 °C, 10–25 min from 140 to 250 °C (7 °C/min), and 25–35 min at 250 °C. The detector temperature was set at 250 °C. Mass spectra were obtained at 70 eV using electron impact ionization. The sample was injected at a volume of 1 µL. Compound identification was based on their Kováts retention indexes, calculated by linear regression interpolation relative to the GC retention times of reference compounds, and by comparison of their mass spectra with those in the computer database (NIST) [18,19].

2.7. Pyrolysis

Pyrolysis of 1200 g of the pelleted biomass was conducted under two different conditions: 400 °C, 10 °C/min heating rate, 60 min residence time (Condition 1) and 600 °C, 15 °C/min heating rate, 30 min residence time (Condition 2). Both processes were carried out under a 500 mL/min inert nitrogen atmosphere. Figure 1 illustrates the operational flow of the pyrolysis process.

Figure 1. Pyrolysis Process Flowchart adapted [20]—(1) N₂ cylinder; (2) Reactor; (3) Electric heater; (4) Thermal insulation; (5) Ultra-thermostatic bath; (6) Condensers; (7) Gas scrubbers; (8) Gas meter; (9) Flare; (10) Gas analyzer; (11) Control panel; (12) Computer.

The unit consists of a fixed-bed reactor operating in batch mode, constructed from stainless steel with internal dimensions of 500 mm in height and 100 mm in diameter. The process was conducted at atmospheric pressure. The sample compartment is heated by a 4000 W ceramic electric resistance capable of reaching up to 800 °C. Temperature and heating rate control were performed using a Novus N1020 electronic controller (Novus, Crystal Lake, IL, USA), with monitoring carried out by ceramic type K thermocouples coated in stainless steel, connected to a data acquisition system (FieldLogger Multi-channel Data Logger—Novus, Crystal Lake, IL, USA), enabling real-time recording and visualization of temperatures and gas flow.

The pyrolysis vapors exited the reactor and were directed to a multi-stage condensation system to recover the bio-oil. The primary condensation system consisted of two identical cylindrical shell-and-tube towers (100 mm D × 400 mm H, each containing nine internal ducts), placed in series. Both condensers were cooled by a Solid Steel ultrathermostatic bath maintaining a constant fluid temperature of 10 °C. The subsequent non-condensable gases were passed through two 1000 mL gas scrubbers in series, containing water, to capture any remaining volatile compounds. The final non-condensable gases were quantified by a gasometer and directed to a flare.

2.8. Bio-Oil Treatment

The raw bio-oil was subjected to a chemical modification process using a 1.75 M ethanolic KOH solution, a method recognized for its efficiency in the selective upgrading of lignocellulosic bio-oils, especially those with high phenolic content. The choice of this method is based on its operational simplicity and selectivity toward oxygenated compounds, as discussed by Lachos-Perez et al. [21], who highlight the use of liquid solvents as an effective alternative for enriching valuable chemical fractions in lignocellulosic bio-oils.

A 10 mL sample of bio-oil was measured and mixed with 50 mL of the alkaline solution, then kept under constant stirring at 65 °C for 8 h. After treatment, the mixture was transferred to a separatory funnel, where water was added followed by hexane, resulting in the formation of distinct organic and aqueous phases. Next, 6 M aqueous HCl was added until the system reached pH 1, promoting further separation of compounds. The organic phase was washed with hexane and then with water until the pH stabilized at 6. Drying at 120 °C was employed to ensure complete removal of residual hexane. However, it is acknowledged that this condition, while effective for solvent elimination, also promotes the volatilization of light organic compounds present in the bio-oil. Therefore, the subsequent analysis predominantly reflects the semi-volatile and non-volatile fractions of the treated product.

The dried sample was diluted in dichloromethane and analyzed by gas chromatography–mass spectrometry (GC-MS), enabling the characterization of the oil profile with a focus on predominant phenolic compounds.

Finally, to quantify the selectivity and effectiveness of this upgrading treatment, a Quality Index was subsequently established based on the results of the GC-MS analysis. This index consists of a set of two key compound ratios calculated from the relative peak areas: the Phenol/Alcohol Ratio and the Aromatics/Oxygenates Ratio. These ratios were used to assess the enrichment of desirable, stable compounds relative to less stable, oxygenated species.

2.9. Non-Condensable Gas Analysis

The chemical composition and Lower Heating Value (LHV) of the non-condensable gases (NCG) were determined using a benchtop gas analyzer (Gasboard-3100, CUBIC-RUIYI, Wuhan, China). During each pyrolysis test, a 1 L sample of compressed natural gas (NCG) was collected in sampling bags for subsequent analysis. Gas samples were collected at the final pyrolysis temperatures (400 °C and 600 °C) and at preceding temperatures (300 °C for 400 °C; 500 °C for 600 °C).

The analyzer was calibrated before each analytical session using nitrogen as the control gas and oxygen as the reference gas. The gas composition was quantified on a volumetric basis (% vol), with values corrected for the absence of N₂ by mass balance. The equipment software automatically calculated the LHV of the gas mixture (MJ/m³), based on reference conditions of 25 °C and 101.3 kPa.

In parallel, the quantitative analysis of exhaust gases from flare combustion was carried out using a CHEMIST 902 IR3 portable combustion gas analyzer (Hignal Technology, Shanghai, China). The sampling probe was inserted directly into the flare exhaust stream. The analyzer monitored pollutant concentrations, exhaust levels, and gas temperature. To ensure a methodologically valid comparison with literature standards (EPA AP-42 [22]), all reported emission concentrations were corrected to a standard reference value of 3%.

Composition of the gas mixture and concentrations of combustion gases are presented and discussed in the Results section.

2.10. Statistical Analysis

All experiments and analyses were conducted in triplicate (n = 3). The results are presented as mean ± standard deviation. Differences between samples and treatments were evaluated using Analysis of Variance (ANOVA), followed by Tukey’s test (p < 0.05) for pairwise comparisons. In the tables, means in the same row followed by different letters indicate statistically significant differences among the samples.

3. Results and Discussion

The analysis were organized into three sequential sections, following the transformation pathway of the products: first, a detailed characterization of the solid materials (biomass, pellet, and biochar) is presented; followed by the composition of the liquid fraction (bio-oil) and the effect of its treatment are evaluated; and finally, the composition and potential use of the gaseous fraction are investigated.

The mass balance of the pyrolysis process is presented in Table 1. This table quantifies the yields of the solid, liquid, and gaseous fractions at both operating temperatures. The mass losses observed during the process are mainly attributed to the formation of gases and light tars that were not fully retained by the condensation system.

Table 1. Mass balance of the pyrolysis process at 400 °C and 600 °C, showing the yields of solid, liquid, and gaseous fractions.

Temperature (°C)	Biochar (%)	Bio-Oil (%)	NCG (%)	Losses (%)
400 °C	31.25 ± 0.03	29.08 ± 0.06	21.75 ± 0.1	17.92 ± 0.01
600 °C	28.47 ± 0.02	28.75 ± 0.03	29.11 ± 0.1	13.67 ± 0.02

3.1. Characterization of Raw Material and Solid Products

3.1.1. Physicochemical Properties and Energy Potential

The physicochemical and energy properties of solid materials are presented in Table 2.

Table 2. Physicochemical characterization of biomass and its solid products.

Parameter	Biomass	Pellet	Biochar 400 °C	Biochar 600 °C
Proximate Analysis
Wet basis (%)
Moisture	9.62 ± 0.03 b	12.42 ± 0.02 c	4.57 ± 0.01 a	9.40 ± 0.17 b
Volatile Material	71.54 ± 0.06 d	62.46 ± 0.01 c	14.05 ± 0.00 a	14.62 ± 0.07 b
Ash	2.74 ± 0.00 a	4.26 ± 0.01 b	14.36 ± 0.01 d	12.72 ± 0.07 c
Fixed Carbon	16.10 ± 0.20 a	20.95 ± 0.13 b	67.03 ± 0.01 d	63.45 ± 0.06 c
Dry basis (%)
Volatile Material	79.15 ± 0.02 d	71.32 ± 0.01 c	14.72 ± 0.01	16.14 ± 0.06 b
Ash	3.03 ± 0.03 a	4.86 ± 0.02 b	15.05 ± 0.01 d	14.04 ± 0.06 c
Fixed Carbon	17.81 ± 0.06 a	23.92 ± 0.02 b	70.24 ± 0.02 d	68.81 ± 0.07 c
Elemental Analysis (%)
Carbon	32.76 ± 0.04 a	42.16 ± 0.01 b	75.52 ± 0.01 d	65.42 ± 0.07 c
Hydrogen	5.24 ± 0.05 c	5.88 ± 0.05 d	2.27 ± 0.05 a	2.64 ± 0.07 b
Nitrogen	0.45 ± 0.00 a	0.52 ± 0.00 a	1.05 ± 0.00 c	0.94 ± 0.07 b
Oxygen	49.12 ± 0.00 d	34.64 ± 0.05 c	2.22 ± 0.05 a	8.91 ± 0.04 b
Sulfur	0.059 ± 0.00 a	0.101 ± 0.00 b	0.056 ± 0.05 a	0.073 ± 0.02 ab
HHV (MJ/Kg)	11.60 ± 0.21 a	16.72 ± 0.24 b	27.80 ± 0.09 d	26.20 ± 0.09 c

Proximate analysis is presented on both a wet basis and dry basis for comparison. Elemental analysis and higher heating value (HHV) are reported on a dry basis. Means in the same row followed by the same letter do not differ significantly from each other according to Tukey’s test (p < 0.05).

The first transformation step, biomass pelletization, resulted in a significant improvement in the material’s properties. Statistical analysis (Table 2) confirms that the pellets exhibit significantly higher Fixed Carbon content and Higher Heating Value (HHV) (p < 0.05) compared to the raw biomass. Although pelletization is fundamentally a physical process, the intense heat generated by friction during compaction induces the onset of mild thermochemical reactions that promote the release of volatile oxygenated compounds and alter the biomass composition [23]. This phenomenon enriches the solid matrix, increasing the concentration of elements such as carbon and hydrogen and raising the fixed carbon content. This transformation leads to a direct increase in the energy value of the pellets, reflected in the higher heating value (HHV). The greater presence of these elements contributes to longer and more efficient combustion, making the material more suitable for applications requiring high thermal performance.

It was observed that the final moisture content of the pellets was higher than that of the raw biomass. This increase is attributed not only to the controlled addition of water during the raw material conditioning stage, a standard procedure to optimize particle agglomeration and ensure physical integrity, but also to the hygroscopic nature of the biomass, which can absorb moisture from the surrounding air during cooling and handling. Despite this increase, the moisture values remained within the range considered suitable for thermochemical conversion processes (typically 10–20%), ensuring the material’s energy viability for thermal applications [24].

The low oxygen content and reduced volatile matter in the biochar at 400 °C, along with the preserved volatiles in the pellets, indicate that pelletization favors the release of condensable compounds during pyrolysis at 400 °C, enabling a significant bio-oil yield even under these conditions. Interestingly, despite the slower heating rate, the H₂ content in the non-condensable gas at 400 °C was higher than the values reported in the literature [25]. This suggests that pelletization may enhance hydrogen-rich gas formation even under mild pyrolysis conditions. Although larger particle size typically reduces intra-particle heat transfer due to lower specific surface area, pelletization improves convective heat distribution across the reactor bed. The compact and uniform structure of pellets prevents agglomeration and channeling effects common in powdered biomass, allowing for more homogeneous heating. This thermal uniformity alters the decomposition pathways and favors secondary reactions that promote hydrogen release [26].

The results obtained by Ismail et al. [27] who analyzed raw biomass and pellets produced from Khaya senegalensis wood, showed that the raw biomass had a fixed carbon content of 8.5% and a HHV of 15.20 MJ/Kg, while the pellets resulting from densification reached 12% fixed carbon and 19.65 MJ/Kg HHV, with moisture content adjusted to 16%. These data highlight thermochemical enrichment, attributed to the higher concentration of fixed carbon in the pellets and the release of volatile oxygenated compounds during pelletization, a phenomenon that induces mild chemical changes which directly impact on the material’s energy density.

Thus, in addition to improving the physical properties of the pellets, such as density and structure, pelletization produces a chemically enriched and energetically denser precursor. The process not only enhances the material’s energy utilization but also improves its viability as a solid biofuel, underscoring its importance within the energy production chain [28].

The conversion of pellets into biochar, carried out under two distinct pyrolysis conditions (400 °C and 600 °C), resulted in physicochemical changes in the material’s structure, as shown in Table 2. The effectiveness of carbonization was demonstrated by the reduction in volatile matter content, accompanied by a significant increase in the fixed carbon fraction. At the elemental level, this transformation led to carbon enrichment and to the deoxygenation of the raw material, which is crucial for energy valorization.

When comparing the two thermal treatments, the data indicate that the process conducted at 400 °C produced biochar with superior properties. This material exhibited statistically higher levels of fixed carbon, elemental carbon, and higher heating value compared to the product obtained at 600 °C. The explanation for this difference lies in the distinct objectives and mechanisms of each condition. The process at 400 °C with a longer residence time is designed to enhance the quality of the biochar, favoring secondary reactions that form a stable and dense carbon structure. In contrast, the conditions at 600 °C, with higher heating rates and shorter residence time, primarily aim at gas production by rapidly extracting volatiles. For this reason, its solid byproduct is less carbonized [29].

Additionally, the unexpected reduction in fixed carbon and elemental carbon at 600 °C, may be attributed to catalytic effects from mineral constituents inherent to the biomass. Coconut husk is known to contain high levels of alkali metals, particularly potassium (K), which can catalyze secondary gasification reactions at elevated temperatures. Studies have shown that potassium compounds promote the thermal conversion of solid carbon into gaseous products, thereby reducing the carbon content of the resulting biochar [21,30].

Furthermore, the slight decrease in ash content at 600 °C suggests volatilization of alkali salts, a phenomenon that intensifies above 500–600 °C. This dual effect, carbon loss via gasification and ash loss via volatilization, leads to a relative increase in oxygen content, explaining the compositional shift observed in Table 2.

Both in reviews such as that by Tin et al. [31] and in experimental studies like that of Maaoui et al. [8], it is confirmed that pyrolysis at lower temperatures improves the quality and yield of solid biochar, especially when longer residence times are employed. In contrast, pyrolysis at higher temperatures prioritizes the production of volatile, with less control over the structure and energy density of the solid residue.

Qurat-ul-Ain et al. [28] observed a decrease in heating value from 20.39 MJ/kg to 17.85 MJ/kg when the pyrolysis temperature was raised from 300 °C to 600 °C. Similarly, Maaoui et al. [8] attributed this trend to the increase in ash content, which lowers the energy value of the biochar.

Therefore, the analysis demonstrates that the choice of processing conditions is important for the quality of the biochar. Producing a solid biofuel with higher carbon purity and energy density, treatment at 400 °C has proven to be the most effective route.

3.1.2. Thermal Stability Analysis (TGA/DTG)

To complement the data from the proximate analysis and visualize the impact of volatile matter removal on material stability, thermogravimetric analysis (TGA/DTGA) was conducted. This technique allows observation of the degradation profile of each sample, providing visual confirmation of the effectiveness of carbonization. Figure 2 presents the TGA and DTGA curves for the raw material and the produced biochar.

Figure 2. TGA/DTGA profiles of the analyzed materials: (A) original pellet; (B) biochar obtained by pyrolysis at 400 °C; (C) biochar obtained by pyrolysis at 600 °C.

The initial weight loss observed at approximately 100 °C in all samples is associated with moisture evaporation. The thermogravimetric analysis of the pellet (Figure 2A) reveals two distinct stages of weight loss. The first occurs between 30–80 °C and is attributed to its initial moisture content. The second, more substantial loss begins around 200 °C, consistent with the pellet’s high volatile matter content. The DTGA curve reveals the sequential decomposition of the main lignocellulosic components. The first stage, identified by a shoulder between 220 °C and 320 °C, corresponds to the degradation of hemicellulose, which has a structure with lower thermal stability. Next, an intense peak is observed around 380 °C, characteristic of cellulose decomposition, whose structure promotes rapid volatile release. Lignin, in turn, undergoes slow and continuous degradation, evidenced by the extended tail of the DTGA curve beyond 400 °C, reflecting its structural complexity and higher thermal resistance [32].

The conversion of pellets into biochar through pyrolysis represented a fundamental transformation in the material’s thermal stability, as shown in Figure 2A alongside Figure 2B,C. The low volatile matter content and high fixed carbon content of both biochar samples, quantified in Table 1, are reflected in their highly stable TGA profiles, which exhibit minimal mass loss throughout the entire heating range. The absence of major decomposition peaks in the DTGA curves of the biochar samples (Figure 2B,C) provides visual evidence of the successful removal of the volatile fraction during the process. When comparing the two biochar samples, both demonstrate the high thermal stability expected of a carbon-rich material [33].

Thus, the TGA/DTGA analysis not only confirms the effectiveness of the pyrolysis process in removing the volatile fraction but also highlights the significant increase in fixed carbon content in the biochar samples. This structural enrichment resulted in highly stable thermal profiles, with minimal mass loss throughout the entire heating range, reflecting the enhanced residual thermal resistance of the carbonized materials [34].

3.1.3. Chemical and Morphological Evolution Analysis

Figure 3 presents the Van Krevelen diagram for the raw material and the produced biochar samples, clearly illustrating the trajectory of the thermochemical transformation. The point corresponding to the pellets is in the upper right corner of the graph, a typical position for raw biomass with high H/C and O/C ratios. After pyrolysis, a pronounced shift of the biochar points toward the origin of the graph is observed, indicating intense deoxygenation and dehydrogenation of the material.

Figure 3. Van Krevelen Diagram for the biomass, pellets, and the biochar samples produced at 400 °C and 600 °C.

This shift confirms the effectiveness of the carbonization process, in line with the literature, which associates the reduction in H/C and O/C ratios with increased aromaticity and carbon stability. The comparative analysis between the two biochar reveals the most significant conclusion: the 400 °C biochar is positioned lower and further to the left than the 600 °C biochar. This provides direct visual evidence that pyrolysis at 400 °C was more efficient in removing hydrogen and oxygen relative to carbon, resulting in a product with a higher degree of carbonization and aromaticity compared to the pyrolysis process at 600 °C [35]. Thus, the Van Krevelen diagram synthesizes the elemental and thermal analysis data, conclusively illustrating the advantage of the 400 °C process for producing a more stable biochar.

Scanning Electron Microscopy (SEM) analysis was employed to investigate and compare the morphological changes in the biomass after being subjected to two distinct pyrolysis conditions. The micrographs (Figure 4) reveal the structural differences between the precursor pellet and the resulting biochar.

Figure 4. Micrographs obtained by Scanning Electron Microscopy (SEM) of the pellets (A,B), biochar produced at 400 °C (C,D), and biochar produced at 600 °C (E,F).

The starting material, the pellet (Figure 4A,B), exhibits a characteristic morphology of fibrous biomass particles that have been physically compacted. This structure consists of an entangled network of elongated fibers with varying diameters, forming a porous matrix with voids between the interwoven particles. Following conversion, the pyrolysis conditions revealed the final structure of the material. The biochar produced at 400 °C (Figure 4C,D) exhibits a solid and consolidated morphology, with a relatively smooth and non-porous surface at the microscale. This result is characteristic of a controlled carbonization process, in which the original biomass structure collapses and fuses. In contrast, the biochar obtained at 600 °C (Figure 4E,F) displays a highly porous and open morphology, with well-defined channels and pores, resulting from the rapid release of volatiles during the process.

These distinct morphologies imply equally distinct application potentials for each type of biochar. The 400 °C biochar, with its dense and consolidated structure, suggests greater stability and recalcitrance. This characteristic is ideal for long-term applications, such as soil carbon sequestration, where resistance to decomposition is essential [36]. In contrast, the biochar produced at 600 °C, with its highly porous and open morphology, presents a distinct set of advantages. This porous structure is desirable for physical adsorption processes, such as gas capture, or as a support material for catalysts. When applied to soil, this high porosity can contribute to the improvement of physical properties, such as aeration and water retention capacity [8].

3.2. Characterization of the Liquid Fraction (Bio-Oil)

The analysis of the liquid fraction (bio-oil) assessed the quality of the pure product generated under each pyrolysis condition (400 °C and 600 °C), and subsequently, the impact of an upgrading treatment on its properties. Table 3 presents the distribution of chemical families and the lower heating value (LHV) for the bio-oil in both its pure and treated states. Figure 5 illustrates the distribution of chemical compound families in the pure and treated bio-oils.

Table 3. Chemical composition, compound ratios, and Lower Heating Value of the pure and treated bio-oils obtained from the pyrolysis of green coconut husk pellets at 400 °C and 600 °C.

Family (Area %)	Pure 400 °C	Treated 400 °C	Pure 600 °C	Treated 600 °C
Alcohol	32.08	11.97	31.67	24.99
Aldehyde and Furans	7.84	11.86	9.03	10.49
Aromatic Hydrocarbons	2.27	6.51	15.94	15.57
Ethers	7.18	5.29	12.52	13.89
Phenol	38.44	54.85	14.6	26.57
Phenol/Alcohol Ratio	1.20	4.58	0.46	1.06
Aromatics/Oxygenates	0.86	2.11	0.57	0.85
LHV (MJ/kg)	15.80 ± 0.16 a	18.40 ± 0.21 a*	16.80 ± 0.18 b	20.40 ± 0.22 b*

Means followed by different letters (a, b) differ significantly from each other (temperature effect, p < 0.05). The asterisk (*) indicates that the value for the ‘Treated Bio-oil’ is statistically higher than that of the ‘Raw Bio-oil’ (treatment effect, p < 0.05).

Figure 5. Assessment of the Bio-oil Quality Index, which is represented by its two key components: the Phenol/Alcohol ratio and the Aromatics/Oxygenates ratio.

The pure bio-oil at both temperatures is rich in oxygenated compounds. The upgrading treatment applied to the bio-oils induced a significant and beneficial chemical transformation. For both temperatures, the treatment resulted in a reduction in the proportion of alcohol and a marked increase in the concentration of phenols. This transformation is clearly quantified by the rise in the phenol-to-alcohol ratio, which increased significantly for the 400 °C oil and more than doubled for the 600 °C oil, indicating an effective conversion.

This effective conversion is attributed to a combination of two main mechanisms. First, physical separation occurs. The alcohol added during the treatment acts as a solvent. It physically extracts oxygenated compounds out of the bio-oil phase. This physical separation alone improves the properties of the final product by removing these undesired components. Second, simultaneous chemical reactions occur. The primary reaction is Esterification, where the added alcohol reacts with the corrosive acids in the crude bio-oil, converting them into more stable and energy-dense esters (R-COOH + EtOH → R-COOEt + H₂O).

The treatment also affected other compound families in distinct ways depending on the pyrolysis temperature. For instance, the aromatic hydrocarbons increased significantly in the 400 °C bio-oil, which correlates with the sharp reduction in alcohol. This suggests that dehydration and aromatization reactions converted a portion of the alcohol into stable aromatic structures. In contrast, the 600 °C bio-oil already contained a high fraction of aromatic hydrocarbons formed during severe pyrolysis, which remained nearly unchanged after treatment due to their thermal stability. Regarding ethers, the 400 °C bio-oil showed a reduction accompanied by a substantial increase in phenols, indicating cleavage of ether bonds in lignin-derived compounds. Conversely, the 600 °C bio-oil exhibited a slight increase in ethers and a simultaneous decrease in alcohol, suggesting that condensation reactions may have occurred, forming new ether linkages from residual alcohols under the treatment conditions [21].

The direct connection between this change in chemical composition and the fuel’s energy quality is evident. The energy upgrading of bio-oil is closely linked to deoxygenation. The reduction in the alcohol family and the enrichment of compound families with greater stability and carbon content, such as phenols, resulted in a significant increase in the heating value of both products.

To quantify the effect of the treatment on the overall fuel quality, a Quality Index was calculated for each sample (Figure 5). The index demonstrates that the treatment enhanced the quality of both bio-oils. This improvement is a direct reflection of the observed chemical transformation, particularly the conversion of alcohol into phenols, which resulted in a more deoxygenated and energy-dense product.

The 400 °C bio-oil sample achieved the highest value in the quality index, reflecting the most selective conversion among compound families. In contrast, the 600 °C sample reached the highest lower heating value, indicating superior overall energy performance due to a more favorable distribution of energy-relevant compounds.

The analysis of the liquid fraction demonstrates that the upgrading treatment was highly effective in enhancing the quality of the bio-oil produced through both pyrolysis routes. The chemical conversion, which favored the formation of phenols over alcohols, was the key mechanism driving fuel deoxygenation. This transformation resulted in a substantial increase in heating value, establishing the bio-oil as a high-performance liquid biofuel.

According to Omar et al. [37], bio-oil obtained from pyrolysis of softwood, specifically pine sawdust, a widely used lignocellulosic biomass in thermochemical conversion research, was investigated. The authors highlight that pure bio-oil exhibits instability and a high content of oxygenated compounds, which limits its direct application as fuel. After undergoing treatment, a significant reduction in oxygenated compounds was observed, notably with an increase in the proportion of more stable phenolic compounds and a substantial enhancement in the lower heating value, rising from 17.51 MJ/kg to as much as 28.85 MJ/kg.

In the study by Figueiredo et al. [10] using coconut husk, the bio-oil also exhibited a profile dominated by oxygenated compounds, with phenols representing the major chemical family (~51.9%) and including high-value compounds. The lower heating value (LHV) of 11.17 MJ kg⁻¹ found for coconut husk bio-oil is notably below the average expected for lignocellulosic biomass-derived bio-oils, which typically lies around 16 MJ kg⁻¹. In contrast, the pure bio-oils obtained in the present study were already aligned with this benchmark, presenting heating values of 15.80 MJ kg⁻¹ (400 °C) and 16.80 MJ kg⁻¹ (600 °C). These values were significantly enhanced by the upgrading treatment, reaching 18.40 MJ kg⁻¹ and 20.40 MJ kg⁻¹, respectively. This demonstrates the effectiveness of the treatment not only in meeting but in surpassing the standard energy quality of pure bio-oils, resulting in a higher-performance liquid biofuel.

The processed bio-oils’ quality indicates that they have the potential to be used as a fuel precursor as well as a source of chemical products. Given the high concentration of phenols, which are important intermediates in the manufacturing of resins, adhesives, and plastics, the chemical upgrading option is the most obvious. The rise in lower heating value (LHV) is a significant improvement as a fuel precursor. However, obstacles to direct application are the corrosiveness and instability of the residual oxygenated compounds. To convert it into a drop-in fuel that works with the current infrastructure, additional refinement procedures like hydrogenolysis are needed to get rid of any remaining oxygen. Consequently, the findings show that the process places bio-oil as a link in the chain of manufacturing for sustainable chemicals and fuels [38].

3.3. Analysis of the Gaseous Fraction (NCG) and Its Potential Applications

The characterization of pyrolysis products extends to the gaseous fraction (NCG), a mixture of permanent gases composed primarily of hydrogen (H₂), carbon monoxide (CO), carbon dioxide (CO₂), and methane (CH₄), as shown in Table 4. The yield and composition of this mixture are strongly influenced by temperature and process conditions, as presented in Figure 6.

Table 4. Characterization of NCG Produced at Different Temperatures.

Parameter	NCG (400 °C)	NCG (600 °C)
Composition (% vol)
H2	21.58 ± 0.77 a	35.84 ± 4.97 b
CO2	41.61 ± 1.02 b	30.00 ± 4.45 a
CH4	14.45 ± 0.36 b	12.49 ± 0.30 a
CO	19.89 ± 0.39 a	19.58 ± 0.47 a
C2H4	2.46 ± 0.13 ab	2.08 ± 0.35 a
Quality Indicators
H2/CO ratio	1.08	1.84
LHV (MJ/m3)	10.64 ± 0.49 a	10.78 ± 1.22 a

Means followed by the same letters do not differ significantly from each other (temperature effect, p < 0.05).

Figure 6. Volumetric composition of the non-condensable gases produced during pyrolysis as a function of process temperature.

Greater gas production at higher temperatures, as observed in the 600 °C process, is a direct consequence of secondary cracking reactions, in which the complex vapors from primary pyrolysis are thermally converted into simpler and more stable gaseous molecules, thereby increasing the overall yield of the fraction.

The analysis of the NCG composition shows that the process conditions directly influenced the generation of its components. The hydrogen (H₂) production, as shown in Figure 6, was significantly higher in the 600 °C process.

In addition to the temperature effect, the gas yield data presented in Table 1 suggests a potential synergistic effect of pelletization on non-condensable gas (NCG) production. At 400 °C, the NCG yield reached 21.75%, which is relatively high for moderate pyrolysis temperatures. This result may be attributed to the physical structure of the pellets, which enhances heat transfer and promotes uniform thermal decomposition. Pelletization reduces porosity and increases bulk density, allowing for more consistent exposure of biomass to heat, which can favor early devolatilization and secondary reactions even at lower temperatures. This effect helps explain the substantial hydrogen concentration observed at 400 °C, reinforcing the hypothesis that pelletized biomass contributes to improved gas-phase reactivity under milder thermal conditions [39].

Hydrogen concentration is one of the most important parameters in evaluating the quality of the gaseous fraction, as H₂ is a clean-burning fuel whose only combustion product is water. Its value also extends to its role as an essential feedstock in the chemical industry. Chemically, the increase in H₂ production at higher temperatures results from secondary reactions occurring in the vapor phase. At 600 °C, the complex vapors from pyrolysis undergo thermal cracking and reforming reactions, which break down hydrocarbon molecules and promote the conversion of compounds into H₂. Therefore, although pyrolysis at 600 °C is less effective for biochar production, it has proven to be a more promising route for generating hydrogen-enriched synthesis gas, aligning with the goals of advanced gaseous biofuel production [40].

It is important to note that the standard deviation of the hydrogen concentration at 600 °C was rather significant, suggesting that the composition of the gas varied amongst repetitions. This could be explained by the fact that secondary cracking and reforming reactions are more sensitive to minute changes in temperature and vapor residence time, which are more noticeable at higher temperatures [25].

Moreover, the analysis of the lower heating value (LHV) of the gaseous fraction revealed a particularly important point. Despite significant changes in composition and the notable increase in hydrogen concentration at 600 °C, the energy value of the gas mixture did not show a substantial rise compared to the gas produced at 400 °C. This phenomenon is explained by the energy balance among the main combustible gases. The heating value of methane (CH₄) is higher than that of hydrogen (H₂) and carbon monoxide (CO) on a per-volume basis. The same high-temperature conditions that favored H₂ production through thermal cracking also promoted the breakdown of part of the methane, which is more energy-rich. Thus, the process effectively replaces a small amount of high-energy gas with a larger quantity of gases with moderate heating values. This balance, combined with the presence of inert CO₂, explains why the total LHV of the gas mixture remained relatively constant, even with a composition chemically richer in hydrogen [31].

Beyond the heating value for direct combustion, a key indicator for assessing the quality of the gaseous fraction as NCG is the H₂/CO ratio (Table 3). This parameter is crucial, as it determines the viability of the gas as a feedstock to produce liquid biofuels and high-value chemical products. The analysis of the gas composition revealed that the 600 °C process produced a gas with a higher H₂/CO ratio compared to the 400 °C process.

A higher H₂/CO ratio is highly desirable in industrial applications. Ratios close to 2, for instance, are ideal for processes such as methanol synthesis or Fischer–Tropsch synthesis to produce synthetic gasoline and diesel. A hydrogen-rich gas is also a more advantageous precursor for the production of purified hydrogen for fuel cells [31].

The comparison with the study by Garcia et al. [7], which analyzed different biomass types, highlights the superiority of lignocellulosic biomass in producing hydrogen-rich gas. Pyrolysis of pine sawdust at 600 °C yielded 19.8% H₂, whereas microalgae produced only 12.3%. Additionally, the gas derived from pine exhibited the lowest content of inert CO₂.

Pyrolysis at 600 °C of green coconut husk pellets proved promising to produce a high-quality gaseous fraction. The thermal cracking and reforming reactions, intensified by the elevated temperature, were key to generating a gas with high hydrogen (H₂) concentration and a favorable H₂/CO ratio. This hydrogen enrichment positions the resulting gas not only as a fuel with potential for cleaner and more efficient combustion, but also as a strategic intermediate for the synthesis of renewable chemical products. Therefore, the results are significant in validating a technological route capable of converting biomass waste into energy-rich products with desirable characteristics for diversifying the energy matrix.

In addition to its energy potential, the combustion characteristics of the NCG were analyzed to assess its environmental impact. Table 5 presents detailed data from the flare combustion, including exhaust O₂ levels, temperatures, and both raw and corrected emission values.

Table 5. Emission Analysis of Combustion Gases (NCG).

Gas	Unit	NCG (400 °C)	NCG (600 °C)
O2	%vol	17.88 ± 0.23	17.33 ± 0.61
CO	ppm	35.02 ± 0.63	24.00 ± 0.52
NO	ppm	17.63 ± 1.15	17.38 ± 1.52
NO2	ppm	0 ± 0	0.33 ± 0.57
SO2	ppm	0.33 ± 0.58	3.66 ± 4.04
CO2	%vol	3.72 ± 0.08	4.07 ± 1.09
CH4	%vol	0 ± 0	0 ± 0
NOx	ppm	17.33 ± 1.15	18.00 ± 1.00
COREF	ppm	84.00 ± 1.63	50.00 ± 1.0
Tgas	°C	261.90 ± 3.33	337.13 ± 5.64

A comparative summary of the main pollutant emissions (CO and NOx) relative to natural gas is presented in Figure 7.

Figure 7. CO and NOx emission profile for assessing the environmental performance of NCG compared to Natural Gas as a reference fuel.

The evaluation of pollutants emitted during the combustion of non-condensable gases (NCG) obtained from pyrolysis at 400 °C and 600 °C reveals environmentally competitive performance compared to conventional natural gas. This comparison can be further explored based on the emission factors published by the U.S. Environmental Protection Agency [22] in the AP-42—Compilation of Air Pollutant Emission Factors. According to AP-42, Chapter 1.4 (Natural Gas Combustion), typical emission values for industrial boilers using natural gas are approximately 84 ppm for carbon monoxide (CO), equivalent to 0.037 lb/MMBtu, and 100 ppm for nitrogen oxides (NOx), equivalent to 0.1 lb/MMBtu.

Regarding carbon monoxide (CO), the NCG produced at 600 °C exhibits lower emissions than conventional natural gas, indicating more efficient combustion and reduced formation of toxic byproducts. Meanwhile, the NCG obtained at 400 °C maintains an emission profile comparable to natural gas, demonstrating that even under less intense thermal conditions, pyrolysis-derived gas can meet the required quality standards.

Regarding nitrogen oxides (NOx), both NCG samples exhibit emissions significantly lower than the typical values for natural gas, representing a clear environmental advantage. Considering Brazilian CONAMA Resolution No. 436/2011 [41], which sets the maximum NOx emission limit at 200 mg/Nm³ for new equipment using gaseous fuels, the values obtained for the NCGs are well below this threshold, reinforcing their viability as alternative fuels for thermal and industrial applications.

Thus, the non-condensable gases generated through pyrolysis demonstrate environmental performance equal or superior to that of natural gas, particularly regarding NOx emissions. The NCG produced at 600 °C stands out further by exhibiting lower CO emissions, consolidating its potential as a promising alternative for partial or complete replacement of fossil fuels, with clear environmental benefits.

4. Conclusions

This study demonstrates that pyrolysis of pelletized green coconut husk is a versatile route for the full valorization of this abundant agro-industrial residue. Combining pellet densification with comprehensive three-product characterization, it offers practical insights for Brazil’s transition to a circular, low-carbon economy. The integrated analysis of solid, liquid, and gaseous fractions revealed that process temperature is a key control parameter for tailoring outputs. At 400 °C, the process produced high-quality biochar (67.03% fixed carbon, 27.80 MJ kg⁻¹ HHV), suitable for carbon sequestration and soil improvement. At 600 °C, it favored hydrogen-rich gas generation (35.84%, H₂/CO = 1.84) with good combustion performance and lower NOx emissions than natural gas.

Pelletization enhanced gas-phase reactivity, increasing non-condensable gas yield even at 400 °C due to improved thermal contact and uniform heat distribution that promoted secondary reactions. Upgrading treatments markedly improved bio-oil quality at both temperatures, promoting deoxygenation mainly via alcohol-to-phenol conversion, which raised phenol/alcohol ratios and heating values. These improvements boost their potential as liquid biofuels and precursors for renewable phenolic chemicals.

Overall, the results close a knowledge gap and demonstrate that green coconut husk can be transformed from an environmental liability into a valuable feedstock for renewable energy, carbon management, and sustainable chemical production.