Exploring the Potential of Waste Biomass from the Brazilian Legal Amazon in Bioproducts Production: a Comprehensive Analysis and Promising PerspectivesAbstract
The Legal Amazon Region has numerous lignocellulosic biomasses with unknown technological properties, contrasting with an urgent demand for energy transition towards a more sustainable economy. This study analyzed the biomasses of baru husk (BH), cupuaçu husk (CH), and pequi seed (PS) through slow pyrolysis at 650 °C to assess the suitability of the bio-oils and biochars as potential biorefinery bioproducts. The physicochemical characterizations showed that all residues are potential candidates for bioproduct production, with CH biomass obtaining the best results for biochar production due to its high carbon content (80.65%) and surface area of 298.0491 m2 g-1, while BH and PS are more suitable for bio-oil production due to their high volatile matter content (> 84%). The main compounds in the bio-oil identified by gas chromatography-mass spectrometry (GC-MS) were phenolic (48.5, 56.7, and 60.36% for BH, CH, and PS, respectively) and furanic (22, 12.17, and 10.28% for BH, CH, and PS, respectively). Finally, it is concluded that based on the physicochemical and morphological characteristics obtained in this study, the pyrolysis products have potential for future investigation in various processes such as adsorption, carbon sequestration, and obtaining valuable chemical compounds, contributing to the promotion of the bioeconomy through sustainable waste management.
Keywords: native fruits; lignocellulosic biomass; thermochemical process
Introduction
Contemporary global challenges, such as climate change and the depletion of fossil resources, highlight the urgent need to develop alternative economic models to the traditional linear post-Industrial Revolution approach. The 21st-century bioeconomy emerges as a promising approach, promoting the effective reuse of resources for a sustainable economy.1-3
Adopting multiple approaches adapted to the specificities of different ecosystems and socioeconomic contexts is more effective than a single model.4 Biological diversity offers significant potential, providing resources that can be transformed into high-value economic products.2,4-6
The valorization of these biological resources occurs through various pathways, including the use of plant residues in thermochemical conversion processes.7-10 These innovations focus on converting the carbon present in raw materials into biofuels and valuable chemicals, representing a significant advancement in the utilization of these resources.10-13
In the last two decades, research on pyrolysis has advanced considerably.7 This sustainable thermochemical process converts biomass into pyrolytic gas, bio-oil, and biochar at temperatures ranging from 280 to 1000 °C in the absence of oxygen.8,14-16
Bio-oil, a dark brown liquid with high oxygen content, has low calorific value, making its use as a fuel challenging.7,17 To improve its calorific value and extract valuable chemicals, techniques for fractionating bio-oil are employed, including physical, chemical, and catalytic methods.17,18 Biochar, a by product of pyrolysis, is a stable, carbon-rich, and low cost material with great adsorptive potential due to its highly porous structure and aromatic functional groups (COOH, OH, and NH2), as well as its affinity for metals.19-21
The characteristics of biomass influence the production of biochar with specific structures. Therefore, research22,23 is focused on the mediation of biomass feedstock and pyrolysis parameters, such as temperature, heating rate, and residence time. Additionally, treatments are applied to obtain biochar with preferred surface area and porosity, expanding its applications in various fields, such as metal adsorption in water, soil fertility enhancement, and clean energy production.24
In the investigation of new sources for energy use, lignocellulosic waste from native fruit species of the Legal Amazon region demonstrates significant potential for energy utilization. This region is a legal and administrative boundary established by Law No. 5.173/66,25 encompassing 100% of the Brazilian Amazon biome, as well as parts of the Cerrado and the Pantanal of Mato Grosso (Brazil). Fruits such as Baru (Dipteryx alata Vog.), Cupuaçu (Theobroma grandiflorum (Will. Ex Spreng.) K. Schum), and Pequi (Caryocar brasiliense Camb.) are commonly found in the diet of local populations; however, a considerable percentage is not properly utilized. In this regard, researchers26-33 have been investigating the physicochemical properties of the waste from these fruits, integrating processes and optimizing reaction conditions for the production of biorefinery bioproducts, such as furan compounds, levulinic acid, biochar, and bio-oil.
Considerable amounts of this waste lack proper disposal, making the search for alternatives increasingly attractive, not only to encourage technological and economic advancement but also to highlight the important role of these species in maintaining ecosystem balance. The main objective of this work was to characterize the bio-oils and biochars obtained through pyrolysis, analyzing the nature of the biomass, the physicochemical compositions, and the morphological characteristics of each product, as a promising possibility for the energy use of waste produced in the Legal Amazon.
Experimental
Samples
For this study, the fruits of Baru, Cupuaçu, and Pequi were sourced from local farmers’ markets in the city of Palmas, Tocantins, Brazil. The pulps and almonds were manually removed with the aid of a knife. The husks of Baru and Cupuaçu, and the stones of Pequi were then cleaned and air-dried for 48 h, followed by additional drying in an oven (SolidSteel, model SSDc-110L) at 50 °C for 24 h. After drying, the raw materials underwent a physical pre treatment, where they were ground using a Willey-type knife mill (22 mesh) (Fortinox, model Start FT 50). The resulting particles were sieved until they reached a size range of 180 to 850 μm and stored in airtight glass jars, following the ASTM D3173-8734 standard for future use.
Approximate chemical analysis
The approximate chemical analysis, or immediate analysis, of the samples was conducted following the standards of the American Society for Testing and Materials (ASTM). This included the determination of four main components: moisture content (ASTM D3173 87);34 volatile matter (ASTM D3175-07);35 ash content (ASTM D3174 04);36 and fixed carbon, calculated by subtracting the sum of percentages of moisture, volatile matter, and ash content from 100%. Each analysis was performed in triplicate to ensure the accuracy of the results.
Extractive
The extractive analysis was conducted using a Soxhlet extractor (Biomixer, model BX-500). Approximately 3 g of each biomass sample was subjected to extraction with 190 mL of ethanol 95% (v/v) for a period of 12 h, following the methodology of the National Renewable Energy Laboratory (NREL).37 After extraction, the cartridges containing the samples were removed and placed on Petri dishes for air drying for 48 h.
Determination of hemicellulose and cellulose
The proportion of hemicellulose in the samples was determined by the difference between neutral detergent fiber (NDF) and acid detergent fiber (ADF), according to the methodology of the Association of Official Analytical Chemists (AOAC).38
Acid hydrolysis
To determine the contents of structural carbohydrates and lignin, the samples underwent a two-stage acid hydrolysis process following the methodological procedure outlined by the National Renewable Energy Laboratory (NREL).39 In this process, 300 mg of each sample were mixed with 3 mL of sulfuric acid 72% (v/v) and maintained in a water bath (Fisatom, model 550) at 60 °C for 1 h, with agitation every 10 min. Subsequently, 84 mL of water were added, and the samples were transferred to an autoclave (Prismatec, model CS-A) where they were held at 120 °C for 1 h. After the reaction, the solutions were filtered through medium-porosity crucibles (10 to 15 μm) using a vacuum pump with a compressor (Limatec, model LT 65).
Lignin content
The lignin contents were determined following the procedure from the National Renewable Energy Laboratory (NREL).37 Acid-soluble lignin (ASL) was quantified using a ultraviolet-visible (UV-Vis) spectrophotometer (Hach, model DR5000) at a wavelength of 294 nm, using a H2SO4 4% (v/v) solution as a blank. The solids retained in the crucibles were dried at 105 °C for acid-insoluble residue (AIR) analysis, and subsequently calcined at 575 °C for 4 h to determine acid-insoluble ash (AIA). Klason lignin (KL), or insoluble lignin, was obtained by subtracting AIA from AIR. Total lignin (TL) was calculated as the sum of KL and ASL.
Slow pyrolysis
The thermal conversions of the samples were conducted with approximately 65 g of biomass in a fixed-bed reactor made of stainless steel, measuring 100 cm in length and 10 cm in outer diameter. The reactor was heated by a tilting two-chamber furnace (Flyever, model FE50RPN, line 05/50) and operated in batch mode. The stripping gas used in the reaction was steam heated to 133 °C in an autoclave. The procedure was carried out at a temperature of 650 °C, with an inert gas flow rate of 6 mL min-1 and a heating rate of 30 °C min-1. The pyrolysis process lasted for 15 min at 150 °C, followed by 30 min at 650 °C. After cooling, the solid material was recovered directly from the reactor, and the pyrolytic liquids were collected after steam condensation using a phase separation funnel.
Analysis of biochars
Elemental analysis
The carbon, hydrogen, and nitrogen contents in the biochars were determined using a PerkinElmer 2400 series II elemental analyzer. The biochar samples, with a particle size of 0.044 mm (Tyler/mesh 325), were combusted in a pure oxygen atmosphere. Each analysis was conducted using 0.5 g of sample.
Surface area test - BET (Brunauer-Emmett-Teller)
The surface area and pore size distribution of the biochars were determined using a surface area system and porosimetry equipment (Micromeritics, model ASAP 2010). Samples of 0.5 g were analyzed to establish the surface area by the N2-BET method and pore size distribution. The pore diameter range used varied from 0.35 to 300 nm, and the surface area was evaluated in the range of 0.01 to 3.000 m2 g-1, with thermal treatment between 30 and 350 °C.
Thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTG)
Thermogravimetric analyses were conducted using a TGA Q5000 instrument (TA Instruments Inc.), calibrated with dihydrate calcium oxalate (CaC2O4.2H2O, 99.9%). Thermal characterization of the samples was performed at a heating rate of 10 °C min-1 up to 750 °C, using a sample mass ranging from 3 to 6 mg. The analysis atmosphere consisted of nitrogen with a flow rate of 25 mL min-1. Samples were analyzed without prior drying or isotherm for volatile interferents removal. Data were processed using TA Universal Analysis 2000 software, version 4.5 (TA Instruments Inc.).
Fourier transform infrared spectroscopy (FTIR)
The identification of functional groups on the surface of the biochars was performed using a FTIR spectrometer (Agilent, Cary 630 FTIR spectrometer). The analyses covered the range from 4.000 to 650 cm-1, with a resolution of 4 cm-1 and 32 scans per spectrum. Each spectrum represented the average of two observations, and adjustments of the average spectra were used for data analysis.
Scanning electron microscopy (SEM) analysis
A scanning electron microscope (Carl Zeiss, model Sigma 300 VP) equipped with a Gemini column, featuring a Schottky field emission gun (FEG) (tungsten filament coated with zirconium oxide) and an energy-dispersive X-ray spectrometry detector (Bruker, Quantax EDS) was used for the solid characterization of biomasses and biochars. To obtain images with a resolution of up to 1 nm, a secondary electron (SE) detector (operating in high vacuum) with an excitation voltage of up to 1 kV and in high vacuum mode (gun vacuum better than 10-9 mbar) was used. Energy-dispersive X-ray spectrometry was performed using a Quantax 200-Z10 system (Bruker) operating in variable pressure (VP) mode with an excitation voltage of up to 20 kV.
Analysis of bio-oil
Analysis by gas chromatography-mass spectrometry (GC MS)
The analyses were carried out with the aid of GC-MS QP2010 Plus equipment (PerkinElmer), equipped with a capillary column DB5MS 30 m by 0.25 wide by 0.25 µM film thickness with ultra pure helium carrier gas. The sample injection into the equipment occurred directly by introducing 1 µL of sample in 1:50 split sample mode with the dissolution of the oil sample in a 1/100 percentage by volume (v/v) in hexane. The system temperatures were as follows: injector, 155 °C; oven temperature programming starting at 45 °C for 3 min with subsequent increase to 150 °C, remaining for 5 min at a rate of 20 °C min-1, ending at 250 °C with a run time of 48 min, ionization source at 230 °C and the quadrupole analyzer at 150 °C.
Results and Discussion
Characterization of biomass
The raw materials analyzed were collected during the rainy season from a region spanning two biomes: the Cerrado-Amazon route.
The results of the chemical composition obtained in this study are presented in Table 1. The BH, CH, and PS biomasses showed ash contents of 2.74, 1.94, and 2.26%, and moisture contents of 0.35, 7.29, and 2.24, respectively. Rambo et al.29 separately analyzed the endocarp and mesocarp of baru, finding ash contents of 0.46 and 2.72%, and moisture contents of 6.81 and 10.70%, respectively. For cupuaçu husk, Borges et al.26 found moisture contents of 6.39% and ash contents of 3.35%, while Marasca et al.33 reported 3.22% ash content. In the case of pequi seed with almond, Rambo et al.,40 and Miranda et al.,28 obtained moisture contents of 28.56 and 7.20%, and ash contents of 1.22 and 2.86%, respectively.
The estimated moisture content for these residues was considered ideal, as pyrolysis typically requires dry biomass with a moisture content below 10% to ensure process quality and facilitate heat transfer, as well as the processing and storage of resulting products.41 Moreover, a low ash content is equally beneficial in pyrolysis processes because it reduces the likelihood of ash accumulation and fouling, as well as minimizes corrosion on furnace surfaces and also contributes to the production of biochar with better micropore surface areas.24,40
The analyzed biomasses showed high volatile matter contents (84.81% for BH, 79.73% for CH, and 86.18% for PS), indicating ease of decomposition during the process. These results are similar to those found by Rambo et al.,29 for mesocarp (80.60%) and endocarp (87.51%) of baru; Borges et al.,26 for cupuaçu husk (80.64%); finally Rambo et al.,40 and Miranda et al.28 for pequi seed (70.33 and 83.01%, respectively). Volatile matter content plays a crucial role in energy generation through combustion, implying better bio-oil and gas yields.10,26
The amount of carbohydrates, especially holocellulose (hemicellulose + cellulose), is crucial for obtaining bioproducts, with at least 25% ideal in their composition.40 In this study, the three analyzed biomasses showed a significant amount of holocellulose, with percentages exceeding 30%. Cupuaçu husk stands out with 54.12%, a result similar to that found by Marasca et al.,33 for the same biomass (59.6%). Subsequently, the pequi seed, with 47.81%, was higher than that found by Miranda et al.,28 and by Rambo et al.,40 in their analyses with the pequi seed with kernel (22.28 and 42%, respectively). Lastly, baru husk presented 34.76%. Rambo et al.29 found 25.43 and 54.28% for endocarp and exocarp of baru, respectively.
Biomasses rich in extractives tend to have a higher volatile matter content, which results in a superior calorific value, facilitates lignin decomposition, allows for a greater release of these volatile compounds during heating, and consequently reduces its fixed carbon content.42 In this study, the highest extractive content was observed in the PS biomass (28.35%), a result similar to the 28% found by Rambo et al.,40 but significantly lower than the 40.73% found by Miranda et al.28 This difference is possibly related to the regional characteristics of the biomass used by Miranda et al.,28 as discussed in their work. For BH, an extractive content of 11.84% was obtained, similar to the 11.30% found by Rambo et al.29 for the baru mesocarp. Meanwhile, the CH biomass exhibited a content of 3.75%, a result similar to previous studies26,33 (5.89 and 5.95%) for the same biomass.
The fixed carbon contents found for BH (12.45%) and CH (18.32%) are similar to data found in the literature.26,29 However, the PS, with 11.57%, showed a lower value compared to the data from Rambo et al.,40 and Miranda et al.,28 who found values of 26.80 and 15.77%, respectively. This difference in fixed carbon contents may be related to the higher volatile matter content observed in this study, suggesting that the higher holocellulose content found may have contributed to an increased volatilization process during the heating of the biomass. As a result, there is a reduction in the remaining fixed carbon content, which may impair biochar yield in pyrolysis processes.10
Elevated concentrations of lignin suggest excellent utility as a source of thermal energy and in pyrolysis processes.43,44 The lignin concentrations obtained in this study (37.76% for BH, 37.17% for CH, and 52.19% for PS) were significant and higher than those reported in the literature26,28,29,40 for the same biomass types. The presence of high lignin content may favor the production of biochar with greater surface area and porosity.41,45,46
Analysis of products obtained by pyrolysis
Biochars
The biochar yields for the evaluated biomass were 17.63% for baru husk (BBH), 13.68% for cupuaçu husk (BCH), and 13.11% for pequi seed (BPS). These yields are notably lower than those reported in the literature,47,48 which, although involving similar biomass, typically employ pyrolysis conditions with temperatures and residence times different from those used in this work (650 °C with a heating rate of 30 °C min-1). For example, Rambo et al.,29 subjected baru husk to pyrolysis at 500 °C for 30 min after acid hydrolysis, obtaining yields of 48.5 and 47.9% from the mesocarp and endocarp of baru, respectively. Lisbôa et al.,49 reported a yield of 32.36% for cupuaçu husk pyrolysis at 300 °C, which dropped to 17.73% at 500 °C. With pequi seed (including the almond), Miranda et al.28 achieved biochar yields of 35.73% at 430 °C with a 7 h retention time and 42.26% with a 3.5 h retention time.
The literature15,16 indicates that the yields of carbonaceous materials are influenced by the composition of the raw material, pyrolysis temperature, residence time, and heating rate. Specifically, biochar yield tends to decrease at high temperatures due to the removal of water and the decomposition of complex components such as cellulose and hemicellulose, favoring the production of gases.13,47,48 However, higher temperatures can improve biochar quality by increasing the carbon content due to greater release of volatile compounds and consequent carbon deposition.50 In general, these biochars produced at high temperatures are suitable for use in pollutant adsorption processes in water, as they possess greater surface area, enhanced hydrophobic characteristics, and increased microporosity.51
According to the literature,15,16 the yields of carbonaceous materials depend on the composition of the raw material used, the pyrolysis temperature, the residence time, and the heating rate. Pyrolysis at lower temperatures tends to produce a higher quantity of liquid products. On the other hand, higher temperatures, such as those used in this study (650 °C with a heating rate of 30 °C min-1), favor the generation of gaseous products.15,16,47,48
The chemical and elemental composition analyses of the biochars (Table 2) revealed a significant increase in carbon content in BBH (47.6%), BCH (45.8%), and BPS (33%) compared to their original biomass.
This increase in carbon content is associated with the use of higher pyrolysis temperatures, as observed by Babu et al.52 Comparing the carbon content of the BBH in this study with the data from Rambo et al.32 at a temperature of 500 °C (30.29 and 37.81%), this increase is evident. In addition to increasing carbon content, higher pyrolysis temperatures also enhance the specific surface area, microporosity, and promote the development of oxygen-oriented functional groups in biochar, improving its effectiveness in carbon sequestration and water adsorption processes.13,15,53
The biochars showed higher ash contents than their corresponding biomasses, differing from results by Rambo et al.29 for mesocarp (0.57%) and endocarp (0.27%) baru biochars. This increase in ash content indicates the formation and accumulation of mineral elements on the biochar surface during pyrolysis.13 However, even with this increase, the values obtained in this study are relatively low (< 8.8%), suggesting a positive correlation where lower ash contents are associated with higher carbon contents in biochars.29
The biochars of BCH and BPS showed a high moisture content (> 2%). Besides these biomasses already having a higher moisture content in the initial characterization, the high moisture may have been influenced by the reactor used in this study, which features a mechanism that hinders the exit of vapors formed during the reaction.54 This may have contributed to the condensation of liquids along the reactor extension. When the vapors are prevented from exiting quickly, they can condense again and return to the solid product, thus increasing the moisture content in the biochars obtained. Pedroza et al.54 and Paz et al.55 also found high moisture content using the same equipment and similar experimental conditions.
Finally, as for the fixed carbon (FC), the content increased substantially compared to the original biomass. This result can be associated with the presence of high molecular weight and aromatic compounds, favoring the use of this material in adsorption processes.15
Surface area analysis - BET
Characterization of the internal structure of biochar is essential and can be achieved through the analysis of its pore distribution. The porosity of biochar is inherently linked to its surface area, with micropores playing a particularly crucial role in contributing to this area.24 The significance of micropores lies in their ability to provide a larger surface area available for adsorptive interactions, which is vital in many practical applications such as contaminant adsorption. Table 3 provides detailed results of the porosity of the studied biochars, including measurements of surface area and distribution of different pore sizes.
According to the International Union of Pure and Applied Chemistry (IUPAC),56 pores in solid materials are classified into three categories: micropores (< 2 nm), mesopores (2-50 nm), and macropores (> 50 nm). In this study, the biochars obtained showed satisfactory BET surface area values, all exceeding 200 m2 g-1, with a significant area of micropores, above 180 m2 g-1. Notably, the biochar derived from cupuaçu husks stood out in terms of surface area and micropore volume.
The surface area and porosity of the produced biochar can be influenced by the biomass compositions.24 In this study, it was observed that the higher ash content in the BH and PS biomasses may have contributed to the lower micropore surface area found in their respective biochars (BBH and BPS). In contrast, the CH biomass, with its higher organic carbon content, may have favored the production of a greater micropore surface area.45
It was observed that the PS biomass, despite having the highest lignin content compared to BH and CH, resulted in a biochar with a lower surface area. According to the literature,45,46 although a high lignin content is typically associated with increased surface area and porosity of biochar, pyrolysis at high temperatures, such as that used in this study (650 °C), can reverse this trend. The degradation of the complex lignin structure at elevated temperatures may lead to a reduction of these properties compared to other biomasses.24
In similar studies using lignocellulosic biomass, Pires et al.57 reported a surface area value of 357.27 m2 g-1 from babaçu husk, obtained at 500 °C for 30 min, while Leal et al.58 achieved a higher value of 553.7 m2 g-1 using açai seeds at 500 and 550 °C, also for 30 min. It is important to highlight that both the products obtained in this study and the results from comparative studies were generated exclusively through physical activation. Commercially used carbons have larger surface areas; however, they undergo chemical activation processes, which produce activated carbon with higher solid yield, higher carbon content, as well as a more developed specific surface area and porosity.59 The potential for optimization may be achieved through chemical activation in future investigations.
Thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTG)
In Figure 1, overlaid thermogravimetric analysis (TGA) curves of biomass samples (Figures 1a and 1c) and biochar (Figures 1b and 1d) are presented. The X-axis indicates the temperature variation (°C), while the Y-axis shows the mass variation (%).
Overlaid TGA curves of biomass (a) and biochar (b); and overlaid DTG curves of biomass (c) and biochar (d). BH: baru husk; CH: cupuaçu husk; PS: pequi; BBH: baru husk biochar; BCH: cupuaçu husk biochar; BPS: pequi seed biochar.
From the thermogravimetric analysis, it was possible to verify that the thermal decomposition of BH samples occurs in two decomposition stages (Figure 1c), while in CH and PS biomasses, it occurs in three stages. The initial accentuation up to 100 °C refers to the loss of water and low molar mass volatile organic compounds, more evident in the CH biomass with higher moisture content.10,28 The next decomposition stage refers to the presence of organic extractives, which occurs first in the range between 130 and 150 °C and a second time occurring in the range between 250 and 550 °C, temperatures similar to the decomposition of cellulose (> 300 °C), hemicellulose (190-320 °C), and lignin (> 250 °C).31,60
In this second stage, it is essential to emphasize that the initial carbonization temperature starts for the three biomasses, with a greater mass loss peak recorded for PS, a difference that may be associated with the higher content of extractives found in this biomass. Still in the second stage, in the TGA graph (Figure 1a), it is possible to observe the decomposition of the holocellulose of the three biomasses with the greatest mass loss recorded for PS. At temperatures above 400 °C, the gradual mass decrease indicates the decomposition of lignin pyrolysis, showing a greater weight loss for PS, corroborating its high lignin content.61
The third stage is evidenced in the DTG graph (Figure 1c) for the BH and PS biomasses and may be associated with the second decomposition event of the extractive compounds found more prominently in these two biomasses.62 Therefore, when analyzing the mass losses of the biomasses, PS loses more mass than BH and CH. This greater removal of components results in a material richer in fixed carbon as evidenced in Table 2, giving the generated biochar more chemical stability, meaning a biochar that is less reactive and less susceptible to degradation.63
For the biochar samples (Figures 1b and 1d), it was possible to verify that at temperatures between 40 and 47 °C, there was a mass loss of 4 to 7% more pronounced for BH and CH, which may be related to the higher composition of volatile compounds present in these samples, in addition to the moisture content. No other additional significant decomposition peak occurred, only a mass loss of 9 and 13% was verified up to 750 °C. A study proposed by Malucelli et al.64 observed that, under a nitrogen flow, the mass loss of biochar is much less evident, presenting a high residual mass when compared to the use of airflow for TGA analyses, a fact that corroborates the results obtained for both samples, presenting high residual mass at the end of the analysis (> 80%) when using nitrogen flow.
Functional groups by Fourier transform infrared spectroscopy (FTIR)
Figure 2 presents the FTIR spectra of biomass samples (Figure 2a) and biochars (Figure 2b), covering the wavelength range from 4,000 to 650 cm-1.
Infrared spectra for (a) biomass and (b) biochars. BH: baru husk; CH: cupuaçu husk; PS: pequi seed; BBH: baru husk biochar; BCH: cupuaçu husk biochar; BPS: pequi seed biochar.
Broad peaks in the region of 3,358 to 3,357 cm-1 correspond to the O-H vibration of cellulose, hemicelluloses, or lignin, as well as glycosidic bonds. The bands from 2,925 to 2,851 cm-1 are attributed to CH2 vibrations with axial deformation and CH3 bonds, possibly related to lignin.62 In the range of 1,600 to 1,750 cm-1, stretching of carbonyl groups is observed, primarily from ketones and esters, associated with components such as waxes, fatty acids, lipid esters, and high molecular weight aldehydes.65 Between 1,000 and 1,275 cm-1, the peak signals are linked to alcohols, ethers, carboxylic acids, and esters. The peaks between 1,500 and 1,600 cm-1 are associated with C-C and C-O stretches in the aromatic ring, suggesting the presence of lignin. Finally, the aliphatic C-O-C and alcohol C-O stretches, located between 1,043 and 1,225 cm-1, reflect the presence of oxygenated functional groups in.20
In general, the structure of the chemical groups in the studied raw materials is similar because they are lignocellulosic biomasses. However, differences can be observed, such as intense bands in the PS spectrum related to CH2 and CH3 vibrations (2,925 to 2,851 cm-1), indicating a significant presence of lignin in the biomass. Additionally, PS exhibited peak intensity in the stretching of carbonyl groups (1,600 to 1,750 cm-1). These prominent peaks stood out because this biomass is rich in lipids, which comprise the extractives, as previously mentioned in the chemical characterization. Furthermore, all three biomasses showed medium intensity in the spectral band of oxygenated functional groups (1,043 to 1,225 cm-1), highlighting the presence of cellulose found in the biomasses, with a particular emphasis on the CH biomass.
Analyzing the biochar spectra (Figure 2b), it is observed that pyrolysis at high temperatures results in the removal of oxygenand hydrogen-rich functionalities compared to the raw materials. This is due to the release of gases such as CO2, CH4, and H2 from the thermal decomposition of cellulose, hemicellulose, and lignin at temperatures above 400 °C.66
As the pyrolysis temperature increases, there is a significant reduction in certain spectral features such as -OH groups (3,300 to 3,625 cm-1), aromatic C-C ring stretches (1,600 cm-1), aliphatic C-H stretches (2,925 cm-1), and symmetric C-O-C stretches (1,043 cm-1). In contrast, there is an increase in the intensity of peaks corresponding to aromatic C-C stretches (1,000 to 720 cm-1). These changes are attributed to the thermal degradation of lignocellulosic components such as cellulose and lignin, resulting in a more aromatic and stable structure in biochars.65
These findings are consistent with previous studies investigating the pyrolysis of lignocellulosic biomass. Studies by Nanda et al.,65 Rambo et al.,29,40 and Lisbôa et al.,49 have shown similar patterns of chemical transformation, confirming that pyrolysis promotes the removal of oxygenated groups and the formation of aromatic structures.
Thus, the infrared spectra enabled the identification of the main functional groups present in the studied biomasses, confirming their characteristic structural and non-structural components of lignocellulosic biomass. The analysis demonstrated that these biomasses have complex structures, which explains the various reactions that occur during pyrolysis and aids in understanding the presence or absence of functional groups on the surface of the obtained charcoal, as they may be inherent to the original material or formed during activation reactions.
Scanning electron microscopy (SEM) analysis
The physical morphology of the biomasses and their corresponding biochars was investigated using scanning electron microscopy (SEM), as depicted in Figures 3a-3f.
SEM images of (a) baru husk (BH), (b) cupuaçu husk (CH), (c) pequi seed (PS), (d) biochar from baru husk (BBH), (e) biochar from cupuaçu husk (BCH), and (f) biochar from pequi seed (BPS).
Before the thermochemical conversion process (Figures 3a-3c), a rigid and closed structure without pores can be observed on the surface of the biomass.67 After pyrolysis at 650 °C (Figures 3d-3f), the lignocellulosic structure and lignin of the raw materials decompose, forming pores on the surface.20 The opening of these micropores occurs due to the rapid release of volatile compounds during high-temperature pyrolysis.68 The presence of tubular structures derived from vascular vessels in the raw biomass gives biochar produced from lignocellulosic biomass higher microporosity compared to other biochar sources.24 This effect is evident in the biomasses analyzed in this study, where biochars with higher surface area, BCH (Figure 3e) and BBH (Figure 3f), exhibit more tubular structures.
The pequi seed biomass (Figure 3a) displays morphological characteristics similar to those observed in pequi seeds as studied by Martins et al.31 These authors suggest that structures with a more “rock-like” appearance, without pores, hinder the release of volatiles during pyrolysis, which can result in smaller surface areas. Similar structures under comparable reaction conditions to those of this study were also observed by Niazi et al.,69 in Japanese oak, by Chormare et al.,70 in casuarina, and by Altıkat et al.,48 in atriplex biomass.
Bio-oils
Due to the complexity of the bio-oil composition, resulting from the decomposition of the hemicellulose, cellulose, and lignin fractions present in the samples, the compounds identified by the GC-MS technique were categorized into groups to facilitate the discussion (Figure 4), and the components with an area percentage greater than 1% were presented in Table 4.
Chemical compounds of bio-oils from baru husk (BH), cupuaçu husk (CH), pequi seed (PS) determined by chromatography (GC-MS)
Products obtained from the pyrolysis process of each biomass. BH: baru husk; CH: cupuaçu husk; PS: pequi seed.
Phenols were the predominant compounds in the three bio-oils obtained, representing 44.51, 47.01, and 60.16% of the total area for BH, CH, and PS, respectively. The study conducted by Marasca et al.33 with pyrolysis of cupuaçu peel, at a temperature of 450 °C, also revealed a higher presence of phenols (73.9%) in the bio-oil obtained. Phenols are substances formed by the secondary metabolism of plants and contribute to important chemical structures like lignin, a polymer significantly present in the chemical composition of the biomasses used in this study (> 30%).71 The PS bio-oil, for example, stands out in phenol content due to the high lignin content in its composition compared to the other biomasses (52.19%).
Phenols give bio-oil characteristics such as corrosive potential and acidity, requiring additional treatment to make it suitable for use as fuel; however, they have significant commercial value when directed towards the chemical extraction route.18 They are known for their antioxidant, antimicrobial, anti-inflammatory, and antiproliferative properties and can be used as raw materials in the food, textile, and pharmaceutical industries.18,72
Furanoids constituted 29.56% (BH), 17.36% (CH), and 10.09% (PS). Based on Table 4, we can highlight within the furan group the significant presence of furfural in BH (17.34%) and CH (9.94%), furfuryl alcohol (2-furanmethanol) in BH (8.52%), and 2,3-dihydrobenzofuran in PS (6.23%). Hydrocarbons constituted 6.49% of BH, 19.53% of CH, and 21.35% of PS. The two main pathways for obtaining these compounds are cellulose and hemicellulose via depolymerization and decomposition.18,73 In the case of BH, with a cellulose content of 27.5%, it is likely that this cellulose was depolymerized mainly to form furan compounds. On the other hand, for CH and PS, with cellulose contents of 44.68 and 43.50% respectively, the depolymerization may have favored the formation of a higher proportion of hydrocarbons.
Additionally, another significant group of components was identified by gas chromatography, adding 13.06% (BH), 10.30% (CH), and 7.03% (PS) of ketones and esters to the composition of the studied bio-oils. These compounds were possibly formed through the cellulose ring-cleavage pathway and/or rearrangements (decarbonylation or dehydration) of hemicellulose.18,73 These results suggest that the initial lignocellulosic composition of the biomasses can substantially influence the profile of the pyrolytic products generated.
Conclusions
This study reported the physicochemical and thermochemical characteristics of the shells of baru (BH), cupuaçu (CH), and pequi seed (PS) and their thermal behavior during slow pyrolysis at 650 °C. The aim was to demonstrate the promising potential of the residual biomass produced in communities within the Legal Amazon for energy use.
The observed trends in biochar yield (17.63% for BH, 13.68% for CH, and 13.11% for PS), mass loss, morphology, and chemical content indicate a complex relationship between the compositions of the three biomass types and their behavior during pyrolysis. This knowledge is important for designing efficient pyrolysis processes that incorporate biomass suitable for sustainable energy generation.
The physicochemical characterization showed that, overall, all residues are potential candidates for biochar production, as their parameters of low moisture, low ash content, high carbon content, high lignin content, and thermochemical stability meet the necessary requirements for obtaining good carbonaceous materials. Among the results, it is suggested that the biochar from cupuaçu shell stands out as the most promising for adsorption processes and carbon sequestration due to its high carbon content (80.65%), larger surface area (298.0491 m2 g-1), and more well-defined tubular morphological structure.
The production of bio-oil from the biomass aligns with documented findings in the literature under similar reaction conditions. In the context of this study, the efficiency of BH and PS is emphasized due to their high volatile matter contents (84.81 and 86.18%, respectively). Among the various chemical compounds identified in the bio-oil, most are classified as phenols [44.51% (BH), 47.01% (CH), and 60.16% (PS)], followed by furans [29.56% (BH), 17.36% (CH), and 10.09% (PS)], hydrocarbons [6.49% (BH), 19.53% (CH), and 21.35% (PS)], and ketones and esters [13.06% (BH), 10.30% (CH), and 7.03% (PS)], making them attractive for applications in chemical industries.
Future investigations are recommended to optimize the methodologies used and the bioproducts obtained; however, the present work provides relevant data that can serve as a starting point for a fair energy transition in the region, promoting sustainable development and the conservation of native species.
Acknowledgments
The authors would like to express their gratitude to the Universidade Federal do Tocantins (UFT) for the support provided. This publication was funded with resources from Process 88881.895613/2023-01 CAPES/DS (PROAP 2024) PPG-BIONORTE, by the Pró Reitoria de Pesquisa (PROPESQ) of UFT through public notice number 019/2023, by the Postgraduate Program PPGCiamb through the translation and/or publication of scientific articles notice number 21/2023 and by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), funding code 001.
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Edited by
-
Editor handled this article:
Rodrigo A. A. Muñoz (Associate)
Publication Dates
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Publication in this collection
02 Dec 2024 -
Date of issue
2025
History
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Received
10 July 2024 -
Published
24 Oct 2024
