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Cruz, T., Maranon, A., Hernandez, C., Alvarez, O., Ayala-García, C., and Porras, A. (2024). “Exploring the potential of cashew nutshells:  A critical review of alternative applications,” BioResources 19(3), Page numbers to be added.

Abstract

The production of cashew nuts has been increasing globally, leading to a greater volume of waste materials that require proper management. Nevertheless, cashew nutshells (CNS), currently considered waste by most processors, offer a noteworthy opportunity for alternative applications owing to their distinct physical, chemical, and thermal properties.  This article reviews alternative applications for CNS that can leverage these properties, while evaluating research gaps. The potential uses are classified into three categories: material development, energy production, and substance absorption. In the materials segment, various examples are discussed where CNS serves as raw material to synthesize biopolymers, cementitious materials, and a broad range of composites. The energy production section discusses various processes that utilize CNS, including pyrolysis, gasification, and briquette production. The absorption section presents CNS and activated carbon derived from CNS as effective absorbents for liquid-phase and gas-phase applications. While this review highlights numerous research-level possibilities for CNS utilization, only a few of these options have been implemented within the industry. Consequently, further research is essential, particularly in CNS characterization, economic and environmental assessment, and real-life implementation, to broaden and enhance the integration of this biomass into applications that can contribute to the value of both its production and processing chain.

 


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Exploring the Potential of Cashew Nutshells: A Critical Review of Alternative Applications

Tatiana Cruz,a Alejandro Maranon,b Camilo Hernandez,c Oscar Alvarez,a Camilo Ayala-García,d and Alicia Porras a,*

The production of cashew nuts has been increasing globally, leading to a greater volume of waste materials that require proper management. Nevertheless, cashew nutshells (CNS), currently considered waste by most processors, offer a noteworthy opportunity for alternative applications owing to their distinct physical, chemical, and thermal properties. This article reviews alternative applications for CNS that can leverage these properties, while evaluating research gaps. The potential uses are classified into three categories: material development, energy production, and substance absorption. In the materials segment, various examples are discussed where CNS serves as raw material to synthesize biopolymers, cementitious materials, and a broad range of composites. The energy production section discusses various processes that utilize CNS, including pyrolysis, gasification, and briquette production. The absorption section presents CNS and activated carbon derived from CNS as effective absorbents for liquid-phase and gas-phase applications. While this review highlights numerous research-level possibilities for CNS utilization, only a few of these options have been implemented within the industry. Consequently, further research is essential, particularly in CNS characterization, economic and environmental assessment, and real-life implementation, to broaden and enhance the integration of this biomass into applications that can contribute to the value of both its production and processing chain.

DOI: 10.15376/biores.19.3.Cruz

Keywords: Cashew nutshells (CNS); Energy production; Substance adsorption; Materials development

Contact information: a: Grupo de Diseño de Productos y Procesos (GDPP), Department of Chemical and Food Engineering, Universidad de los Andes, CR 1 ESTE 19A-40, Bogotá, 111711, Colombia; b: Structural Integrity Research Group (GIE), Department of Mechanical Engineering, Universidad de los Andes, CR 1 ESTE 19A-40, Bogotá, 111711, Colombia; c: Sustainable Design in Mechanical Engineering Research Group (DSIM), Mechanical Engineering, Escuela Colombiana de Ingeniería Julio Garavito, AK 45 205-59, Bogotá, 111166, Colombia; d: Department of Design, Universidad de los Andes, CR 1 ESTE 19A 40, Bogotá, 111711, Colombia; *Corresponding author: n-porras@uniandes.edu.co

GRAPHICAL ABSTRACT

 

INTRODUCTION

The utilization of agro-industrial waste has recently emerged as an essential focus within diverse sectors concerned with sustainability. As global food demand, production, and consumption steadily increase, the amount of waste generated by the agro-industrial sector in the form of husks, peels, shells, and seeds inevitably accumulates in landfill sites (Kumar et al. 2022). This situation has exacerbated various environmental problems, including the increase in methane and carbon dioxide emissions, water pollution, and soil degradation (Mafakher et al. 2010; Shin et al. 2016).

Nevertheless, agro-industrial residues also offer promising opportunities for valorization through diverse applications. These applications vary depending on the residue type and can include energy production, biochemical and pharmaceutical applications, the creation of water absorbents, and the development of biopolymers and materials (Yaashikaa et al. 2022). A prime example of such residues is cashew nutshells (CNS), a lignocellulosic fiber derived from the cashew tree (Anacardium occidentale L.) that remains as waste after cashew kernel harvesting.

The cashew tree (Anacardium occidentale L.) is native to South America and was introduced to India and Mozambique in the 16th century (Tola and Mazengia 2019; Malik and Bhadauria 2020; Orduz-Rodríguez and Rodríguez-Polanco 2022). Since then, cashew

farming has spread to several parts of Asia and Africa. Although the cashew fruit consist of a visually striking yellow or red peduncle, the actual fruit is the cashew nut (Oliveira et al. 2020), which contains an edible kernel (Malik and Bhadauria 2020) within a distinctive kidney-shaped grayish shell, or pericarp (Orduz-Rodríguez and Rodríguez-Polanco 2022). The cashew nutshell (CNS) comprises the following layers: the epicarp, an external leathery layer; the mesocarp, a spongy layer that has a honeycomb structure containing a caustic and flammable liquid called cashew nutshell liquid (CNSL) (Oliveira et al. 2020); the endocarp, the innermost and hardest layer; and the testa, a thin and papery coat adhered to the kernel. Both the endocarp and the testa protect the kernel from contact with the CNSL (Orduz-Rodríguez and Rodríguez-Polanco 2022) (Fig. 1).

Fig. 1. Cashew nut parts

The main traded products are raw cashew nuts, cashew kernels, and CNSL (Malik and Bhadauria 2020), while by-products including CNS, CNS cake (CNS after CNSL extraction), testa, and the pseudo-fruit are often wasted (Sawadogo et al. 2018). Ivory Coast, India, Cambodia, Vietnam, and Tanzania were the top cashew producers in the 2022/23 period, during which global production of raw nuts reached 5 million tons (International Nut and Dried Fruit Council 2023). Other countries in the Americas, Asia, and Africa also contribute to cashew production to a lesser extent (Nair 2021). The demand for cashews is rising due to their nutritional properties, flavor, and versatility (Dendena and Corsi 2014). Global raw cashew nut production is expected to increase as cultivated hectares expand and crop management improves, leading to higher yields (Oliveira et al. 2020; Nair 2021; Orduz-Rodríguez and Rodríguez-Polanco 2022).

However, cashew production generates significant waste and by-products, with less than 30% of the nut being edible (Van Hoof et al. 2020). Around 70% of the nut is discarded, resulting in approximately 3.5 million tons of CNS waste globally in 2022/23 (International Nut and Dried Fruit Council 2023). The current management of these residues presents environmental and economic challenges. Some plants incinerate CNS residues for energy generation, emitting harmful vapors (Oliveira Galvão et al. 2014). In other cases, this waste is left on the ground due to the high transportation cost for disposal, resulting in soil acidification and fire hazards (Dendena and Corsi 2014; Sawadogo et al. 2018; Nair 2021). Therefore, it becomes crucial to find effective and profitable solutions for CNS waste management in the context of the growth of cashew production, global concerns regarding sustainability, circular economy, and the Sustainable Development Goals.

Table 1. Some Properties of CNS

Current research on cashew nut processing mainly concentrates on kernel and pseudo-fruit production and applications (Oliveira et al. 2020; Chen et al. 2023), CNSL extraction and valorization (Kyei et al. 2023), as well as testa utilization (Sruthi and Naidu 2023). However, CNS management receives less attention despite its promising physical, chemical, and thermal properties (Table 1). Further investigation into these properties is warranted. While Table 1 showcases extensive studies on proximate and ultimate analysis, and thermal behaviors, only three studies have explored CNS chemical composition while writing this article. The results from these studies also exhibit significant disparity (Ocheja et al. 2015; Paternina Reyes et al. 2023; de Paiva et al. 2024).

To the authors’ knowledge, no comprehensive review article has been dedicated solely to CNS. Therefore, this article aims to address this gap by reviewing alternative applications of cashew nutshells (CNS) waste that could enhance the value chain of cashew nut production and mitigate its environmental impact. This review categorizes CNS applications into three groups: material development, energy production, and substance adsorption. This review explores the properties of CNS relevant to each application, compares findings from various studies, discusses potential opportunities and challenges, and proposes future research directions.

EXPERIMENTAL

Bibliometric Analysis

A comprehensive literature research was conducted using three online databases: Scopus, Web of Science, and Google Scholar. The search terms employed were: (cashew) AND (nutshell OR nut-shell OR “nut shell”) AND (character* OR property*) in the title, abstract, or keywords of the articles. The search included articles published up to December 2023 with no restrictions on country, journal ranking, or publication type to ensure a thorough review of all relevant sources. This resulted in the identification of 1091 articles.

Studies focusing on CNS as a raw material for various applications were included during the screening process. Articles solely discussing CNSL or written in languages other than English, Spanish, or Portuguese were excluded. Finally, only studies with a well-described methodology for CNS utilization were considered after a full-text review. Additionally, the reference lists of the selected articles were examined to identify further relevant sources. In the end, 108 publications were included in this review article.

The compiled data were categorized based on application area, processing methods, CNS states (ground, ash, etc.), and investigated properties. This categorization ensured a complete analysis of the available literature and facilitated including high-quality and relevant studies in the review.

Based on the gathered information and the most frequently co-occurring topics (refer to Fig. 2a), this revision proposes three main categories for the alternative uses of CNS (refer to Fig. 2b), corresponding to the clusters depicted on the map. The green cluster, centered on the term “Cashew nutshells”, regroups terms related to activated carbon applications, which also connect to the three proposed categories. The red cluster associates the keywords related to energy production, while the yellow cluster presents terms linked to material development, such as starch, cellulose, and mechanical properties. Finally, the blue cluster presents terms related to substance adsorption. An additional trend observed among all the depicted keywords is a focus on agro-industrial waste, waste management, and sustainability.

The bibliometric analysis reveals a rising trend in research on this topic, with the highest number of publications occurring in 2018. Furthermore, among the reviewed articles, India had the highest number of publications, followed by Brazil and Nigeria. Interestingly, while the most cited articles focused on substance adsorption, the most published papers address energy production; this bibliometric analysis strengthens the authors’ claim of novelty, as no prior comprehensive study of this scope has been documented.

a)

b)

Fig. 2. a) Co-occurrence map of cashew nutshells related topics. Made with VOSViewer. b) Main cashew nutshell alternative applications

MATERIALS DERIVED FROM CASHEW NUTSHELLS

Construction Industry

The construction industry relies heavily on materials, particularly cement composites (Tantri et al. 2021). Unfortunately, cement production contributes significantly to CO2 emissions and incurs high energy costs (Thirumurugan et al. 2018; Oyebisi et al. 2019, 2021; Mendu and Pannem 2021). To address this problem, researchers have explored CNS-derived supplementary cementitious materials (SCMs) pretreated at different temperatures such as CNS ash (CNSA) (Lima and Rossignolo 2010; Pandi and Ganesan 2015; Thirumurugan et al. 2018; Oyebisi et al. 2019, 2021; Tantri et al. 2021, 2022), uncalcined CNS (burned below 400 °C), and CNS powder (CNSP) (Pavithra et al. 2020).

Among the studies, examination of CNSA particles below 10 µm using SEM revealed a variety of sizes ranging from 2 to 10 µm. These particles exhibited lamellar features with overlapping layers. Concrete samples containing 15% CNSA also showed similar characteristics (Mendu and Pannem 2021). Field emission scanning electron microscopy (FE-SEM) analysis of CNSA revealed a dense structure with flat medium to small-sized particles. Some voids were observed, likely due to burned-out and unburnt carbon (Tantri et al. 2022).

CNSA exhibits promise as a pozzolanic material and a potential SCM due to its oxide composition, allowing partial cement replacement in mortar and concrete applications. Studies indicated that a 15% replacement with CNSA is ideal for structural purposes, while a 20% substitution is suitable for non-load-bearing applications, resulting in denser and stronger concrete (Thirumurugan et al. 2018; Oyebisi et al. 2019; Mendu and Pannem 2021; Tantri et al. 2021, 2022). Additional research suggests that up to 25% CNSA substitution reduces water absorption, improves curing, minimizes pores, and enhances strength (Pandi and Ganesan 2015; Oyebisi et al. 2019). In contrast, uncalcined CNS is inadequate as a cement substitute due to its weak mechanical properties. Calcining CNS biomass increases its effectiveness as a substitute, allowing for a higher replacement ratio than uncalcined CNS. Specifically, studies suggest that 15 to 20% substitution with CNSA is possible, compared to only 8 to 10% with raw or uncalcined CNS (Tantri et al. 2021, 2022). Additionally, research on CNSP suggests that a maximum substitution percentage of 8% in cement is possible without compromising properties when combined with chicken feathers (Pavithra et al. 2020).

Similarly, in the construction industry, achieving soils with high bearing capacity is crucial, often requiring soil stabilization. In this context, James et al. (2022) utilized CNSA combined with lime to enhance the strength of a highly expansive soil, replacing 0.5% of CNSA with 5.5% of lime, which led to a significant increase in the compressive strength of the soil. Likewise, Bhat et al. (2019) incorporated CNSA and glass industrial waste into a lateritic soil, but only marginal improvements in soil properties were observed. Although the CNSA enhanced soil cohesion and reduced the friction angle, decreasing the soil’s maximum dry density. Further research is needed to determine the optimal percentage of CNSA for soil stabilization, as well as to evaluate its long-term effects and cost-effectiveness in comparison to other stabilizers.

In conclusion, CNSA is presented as a promising partial replacement for cement in construction applications and lime in soil stabilization. However, further research is necessary to assess its viability fully. When considering CNSA as a cement substitute, it is crucial to investigate its thermal properties, long-term durability, cost-effectiveness, and CO2 emissions compared to traditional materials (Jannat et al. 2021). Additionally, exploring the impact of CNSA on other critical soil properties, such as permeability, swelling potential, or even mineral contribution to the soil matrix, would be valuable.

Biopolymers

To counter environmental challenges posed by non-biodegradable polymers from petrochemicals, researchers are developing biopolymers from plant-based materials (Yuliana et al. 2012; Harini et al. 2018; Minh et al. 2019; Bamgbola et al. 2020). Utilizing polysaccharides, proteins, lipids, and polyesters derived from renewable agricultural sources, biodegradable plastics can be produced (Harini et al. 2018). Starch and cellulose are especially crucial in this biopolymer research, with starch standing out as a renewable, biodegradable, and non-toxic alternative to fossil fuels in the polymer industry.

CNSc starch was isolated through wet milling (Yuliana et al. 2012), resulting in a starch with 85.0% starch content, along with protein, fiber, ash, and impurities in the form of polymeric resins. The CNSc starch had 75.4% amylopectin and 24.6% amylose, which influence solubility and swelling, which are crucial for industrial applications. Despite high amylopectin content, CNSc starch exhibited low swelling due to the identified polymeric resins. CNSc starch showed twice the solubility of maize starch, high crystallinity, and the presence of agglomerated resins. These characteristics make it a promising renewable material in the polymer industry. However, its thermal processing temperature should not exceed 174 °C.

Additionally, CNSc was utilized to obtain pure cellulose (Bamgbola et al. 2020) through a modified acid hydrolysis (HNO3) method, followed by alkali (NaOH) treatment and bleaching (NaOCl). SEM images revealed that the isolated cellulose presented a porous structure resulting from lignin degradation during alkali treatment. Successful removal of lignin was confirmed by micrographs from morphological characterization and FTIR analysis. TGA analysis supported these findings, revealing that obtained cellulose decomposed in a single stage between 300 and 400 °C. X-ray diffraction analysis the extracted cellulose had a crystallinity index of 77.7%, and energy-dispersive X-ray (EDX) spectra confirmed the presence of essential elements in the cellulose fibers. These cellulose fibers hold potential applications in industries such as binders, reinforcements, and packaging.

In contrast, Minh et al. (2019) developed a phenolic resin using CNSc as the main component. Liquefaction of high lignin content biomasses is a process wherein formaldehyde can be substituted with biomass, in the present of specific catalysts, to produce phenolic resins in a more environmentally friendly manner (Alma et al. 1998). CNSc’s high lignocellulosic content makes it a desirable feedstock for liquefaction and transformation into valuable chemicals and products (Minh et al. 2019). The liquefaction process involved heating grounded CNSc and phenol with sulfuric acid. The 2:1 phenol to CNSc ratio yielded the highest amount of phenolic resin. Gel permeation chromatography indicated the resin was an oligomer based on its polydispersity. Further characterization using FTIR analysis and nuclear magnetic resonance (NMR) analysis confirmed the success of liquefaction process, revealing functional groups and chemical structure typical of a phenolic compound. This innovative work paves the way for further research on synthesizing CNS-derived resins through liquefaction, while more comprehensive characterizations in future investigations are recommended.

In conclusion, CNS is a valuable source of starch, cellulose, and lignin essential components of biopolymers. Starch-based biopolymers serve as eco-friendly substitutes for petroleum-based plastics in various applications like packaging, cutlery, bags, and films, offering biodegradability and reduced environmental impact (Bertolini 2009; Yuliana et al. 2012). Plant-based biopolymers are also used in adhesives, coatings, paints, and fabric production (Bertolini 2009; Harini et al. 2018). In agriculture, biopolymers find applications in soil stabilization, seed coating, fertilizers, and biodegradable pots, providing sustainable alternatives to conventional materials (Majeed et al. 2016).

When working with CNS, it is crucial to consider the biomass composition percentage to isolate the appropriate components. The environmental impact and cost of isolation procedures should also be considered, aiming to maximize biomass utilization (Alma et al. 1998; Bertolini 2009). Utilizing CNS as a low-cost and readily available raw material allows for the mitigation of environmental and economic implications, enhancing its overall benefits (Yuliana et al. 2012).

Composites

Various forms of CNS have been used in composites as fillers, reinforcements, and matrix constituents for structural materials, coatings, and packaging. For example, Harini et al. (2018) synthesized a composite using CNSc starch to create bio-thermoplastic films. Walnut shell cellulose served as reinforcement, with pomegranate antioxidants and antimicrobial compounds added for intelligent packaging purposes. The composite showed improved oxygen transfer rates with higher CNSc starch concentration, and mechanical properties increased with CNSc starch content, which are relevant features for packaging. Moisture retention decreased with higher CNSc starch concentration, and solubility of CNS starch films ranged from 40% to 48%. The addition of pomegranate peel extract to the best-performing cellulose-reinforced film showed favorable properties for intelligent packaging.

Researchers explored the use of polymeric matrices to create composites with CNS. Gomes et al. (2018) combined CNSP with a recycled high-density polyethylene (rrHDPE) matrix to develop Wood Plastic Composites. Different CNS concentrations were investigated, and the composite properties were evaluated. TGA and DSC analyses revealed a two-stage degradation with decreasing temperature as CNSP percentage increased, hindering crystalline structure formation, and resulting in lower crystallinity. Due to residual CNSL, the fluidity of the composite increased, and FTIR analysis showed deterioration of the composite interface. SEM images exhibited voids after processing, and tensile tests indicated decreased elastic modulus and increased elongation at fracture. Potential causes for the poor performance included incomplete CNSL extraction, low resin absorption by the fiber, inadequate control of CNSP particle size, and void formation during processing. The author suggested that CNS-reinforced polymer composites could be suitable for less demanding applications, but complete CNSL extraction is essential for structural use.

Consequently, some researchers have explored the use of CNSA as an alternative to address the negative effects of residual CNSL. For instance, Saravanan et al. (2017) developed an environmentally friendly composite by combining epoxy resin, CNSA, rice husk (RH), and sawdust (SD). The composite consisted of varying proportions of ashes: CNSA (0 to 40%), RH (20 to 30%), and SD (0 to 40%), although the initial state of the CNSA was unclear. The composite was prepared by blending predetermined amounts of biomass ash with an epoxy resin and hardener using the hand-lay-up technique to form rectangular specimens and casting cylindrical samples. Mechanical properties, including tensile strength, torsion, hardness (Rockwell), and impact strength (Izod), were measured. Among all tested mechanical properties, the second-best performing sample was the one with 40% CNSA, coming after the sample that incorporated ashes from all three biomasses.

Similarly, Sundarakannan et al. (2019) developed unsaturated polyester resin composites reinforced with CNS-derived biochar after pyrolysis. The impact of pyrolysis carbonization time and the percentage of biochar in the composite was studied. The biochar treated for 3 h showed higher crystallinity (28.4%) compared to 1-h treatment (26.4%).

Mechanical tests revealed that the composite with 10% biochar treated for 3 h exhibited the highest tensile, flexural, impact, and hardness strengths, showing improvements compared to the unfilled matrix. Samples with 15% biochar showed the highest flexural strength with a 40% increase. The enhanced mechanical properties were attributed to the increased crystallinity of the 3-hour-treated biochar, facilitating better resin penetration and generating a stronger interface, crack resistance, and improved impact energy absorption. SEM images confirmed that the best-performing sample had a smooth surface; other samples showed voids and microcracks. The study further supports the benefits of using CNSA in composite materials.

Another example of CNS composites is shown in the work of Mari and Villena (2016), where ground CNSc was combined with wood residues to create particle boards. Adhesive type and CNS/wood ratio influenced strength and dimensional stability. These two properties were negatively affected by CNSc replacement due to uneven particle geometry and residual CNSL. However, boards exhibited reduced flammability compared to pure wood, taking longer to ignite and extinguishing in a shorter time, resulting in less damage to the wood board area. Based on the results, CNSc is suitable for less demanding applications with cost-saving benefits.

Finally, studies were conducted on natural rubber filled with CNSP as a renewable and cost-effective additive (Mamza et al. 2016; Okele et al. 2016, 2018). Natural rubber requires vulcanization to enhance its mechanical properties, often using fillers such as carbon black derived from depletable and costly petrochemicals. However, researchers attempted to replace it with agricultural residues including CNSP. Findings revealed that carbon black exhibited better compatibility with rubber, while CNSP contained reinforcing particles of silicon dioxide and silicon carbide (Okele et al. 2016). CNSP composites had comparable rheological properties but longer curing times due to their acidic nature, with lower viscosity and torques than carbon black composites (Mamza et al. 2016). Neutralizing the acidic nature of CNSP and enhancing particle size and surface area could improve CNSP composite performance. Thus, in the last study (Okele et al. 2018), semi-nano CNSc powder (25%) mixed with carbon black (5%) improved hardness, abrasion resistance, and tensile strength, surpassing the solo carbon black sample. Therefore, semi-nano CNSc powder combined with carbon black presents a potential low-cost and eco-friendly reinforcing filler for natural rubber.

Overall, CNS has shown promise as a renewable and cost-effective component in composite materials, offering opportunities to enhance mechanical, physical, and chemical properties while reducing costs in specific applications. Table 2 illustrates the changes in mechanical properties resulting from CNS inclusion in composites. It is important to note that the impact on properties can vary, with enhancements observed in some cases, while decreases occur in others. Therefore, a comprehensive evaluation should consider additional aspects of composite processing, such as cost and other desired properties. For example, while CNS inclusion may improve flammability resistance, there might be a trade-off with a decrease in mechanical properties (Mari and Villena 2016). Further research and optimization are necessary to fully understand the potential of CNS, particularly in addressing residual CNSL, and to facilitate its expanded utilization across diverse industries.

Table 2. Mechanical Properties Comparison between Composites Made with CNS and Other Biomasses

Supercapacitors

CNSAC has been used in electrochemical double-layer capacitors (EDLCs) due to its combination of high surface area, superior porosity, excellent chemical stability, and increased electrical conductivity (Pulikkottil et al. 2022). Merin et al. (2021) successfully produced CNSAC through chemical activation with varying KOH ratios, followed by carbonization under argon. SEM revealed an extensive, 3D honeycomb-like porous structure in the CNSAC, ideal for electrolyte storage and ion transport. This porous morphology was attributed to the intercalation and removal of potassium during KOH activation and carbonization. X-ray diffraction (XRD) and FTIR analyses confirmed the presence of a graphitic carbon structure, contributing to its electrochemical activity. The high surface area of CNSAC provides numerous interfaces for charge storage, making it a desirable electrode material for supercapacitors. Interestingly, the pore size increases with higher KOH activation ratios, although this effect becomes less predominant beyond a 1:2 activation ratio. The reported specific surface area ranged from 772 to 1150 m²/g, with pore size distribution between 0.47 and 0.56 cm³/g. CNSAC with a 2:1 activating agent ratio showed higher capacitance (214 F/g), faster charge transfer, and 98% capacitance retention after 1000 cycles, making it a promising alternative to market products for supercapacitor applications.

In general, CNSAC exhibits favorable properties for electrochemical applications, particularly due to its high porosity and surface area. This material sustained high capacitance even after 1000 cycles, suggesting its potential for long-term performance in real-world devices. However, a comprehensive evaluation of the complete device incorporating supercapacitors made of CNSAC is necessary. Future research should focus on assessing key parameters like energy density, power density, cycle efficiency, and long-term stability. This area lacks in-depth investigation, presenting a significant opportunity for further exploration of CNSAC. Additionally, exploring environmentally friendly activation methods, such as physical activation, could minimize the use of hazardous substances and may be a valuable avenue for sustainable production of CNSAC.

Other Materials Applications

More et al. (2018) studied the use of silane-treated ash from CNS, groundnut shells, tamarind shells, rice husks, and sugarcane bagasse to enhance epoxy-amine coatings on metal sheets. Ashless epoxy resin was compared with amine-modified ash resins at various percentages, and ash effects on the chemical and mechanical properties were investigated. Characterization results for CNSA showed successful introduction of amine groups on its surface. The coating exhibited good chemical resistance to acid, alkali, and solvents without wrinkling, softening, or blistering. However, scratch hardness and anticorrosive effects were enhanced with different CNSA percentages. The study concluded that the excellent interaction between the epoxy resin and the CNSA improved the coating’s properties, making it a promising additive for improving coating performance.

In another study, Rajendran et al. (2022) explored CNSP as an eco-friendly alternative in sound-emitting pyrotechnic formulations, aiming to reduce the reliance on sulfur usage in fireworks. Sound-emitting pyrotechnic formulations were developed by incorporating CNSP as a replacement for sulfur. The study compared the acoustic properties, sound intensity, thermal performance, safety (impact and friction sensitivity), and eco-friendliness of the CNS-based pyrotechnics with conventional sulfur-containing compositions. The findings indicated that 5% CNSP showed the best performance across all properties. SEM images revealed a flake-shaped structure of CNSP at micrometer scales, with measurements aligned with the average particle size data (43 µm) of CNSP. By using CNS in pyrotechnic compositions, the study suggests a potential pathway to reduce the environmental impact of fireworks, specifically lowering SO2 gas emissions and making use of CNS residues, while still maintaining their desired sound-emitting properties.

In conclusion, CNS demonstrates great potential for diverse material developments, thanks to its components including starch, cellulose, and lignin. These components make CNS a versatile and renewable resource suitable for various applications, including biopolymers, composites, cementitious materials, rubber additives, activated carbon, coatings, and adhesives. Despite its advantages, challenges exist, such as complex and energy-intensive processes to component extraction and processing, composition variability, limited advanced processing technologies, and performance limitations compared to petrochemical-derived materials. Nevertheless, CNS offers benefits such as abundance, renewability, cost-effectiveness, versatility, and eco-friendliness. These advantages lead to waste reduction, lower production costs, expanded material applications, and reduced environmental impact. To fully utilize CNS’s potential in different industries, further research, and technological advancements are necessary to overcome challenges and enhance its overall viability.

ENERGY PRODUCTION FROM CASHEW NUTSHELLS

While petroleum derivatives still dominate global energy production, exploring clean and renewable alternatives is becoming increasingly crucial. In this context, biomass derived from industrial and agricultural waste has emerged as a promising alternative in replacing fossil fuels (dos Santos et al. 2022). Agricultural crop processing generates substantial amounts of biomass waste that can serve as an energy source, addressing power supply challenges for agriculture-dependent economies (Chungcharoen and Srisang 2020). One such example is cashew nut processing waste, consisting of the CNS, CNS cake (CNSc), and CNSL, which can be valorized as fuel.

CNS by-products present an opportunity to convert process waste into energy, especially benefiting energy-intensive stages such as roasting and drying during cashew processing (Sawadogo et al. 2018). Currently, CNS are burned in boilers for energy; however, this method generates smoke and exposes individuals to anacardic acid, found in the CNSL, which is irritating and potentially carcinogenic (Sawadogo et al. 2018). Thus, it is essential to study energy production methods involving CNS that have minimal negative effects on the environment and human health. In the following sections, three applications (briquette fabrication, pyrolysis, and gasification) that utilize CNS as a sustainable energy source are reviewed.

Briquettes

Biomass exhibits promise as a sustainable energy source, yet certain forms require treatment to enhance their overall efficiency. In general, thermomechanical treatments, drying, roasting, and densification improve the suitability of biomass for power generation (dos Santos et al. 2022). Biomass briquettes, compacted cylinders made through densification, offer an effective alternative. Densification is a simple process, and the resulting briquettes can meet heating and cooking needs in rural areas (Sawadogo et al. 2018).

Briquettes have many advantages over charcoal, lignite, and firewood. Their higher energy density, calorific power, and combustion efficiency make them easier to store, transport, and incinerate (Chungcharoen and Srisang 2020; dos Santos et al. 2022). Furthermore, briquettes offer environmental benefits and superior quality compared to coal, as they are derived from renewable resources. Their use can reduce deforestation and provide a way to dispose of agro-industrial wastes (Sawadogo et al. 2018).

Before densification, biomass waste undergoes various treatments depending on the waste type and manufacturing process. These treatments typically involve cleaning, grinding, and pressing (densification). Biomass densification may also require biowaste carbonization and binder addition (Sawadogo et al. 2018). Carbonization techniques influence briquette properties, with carbonized biomass briquettes exhibiting higher thermal efficiency and lower pollutant emissions (Chungcharoen and Srisang 2020). Furthermore, biomass and binder content, particle size, and pressing technique all impact briquette quality (Sawadogo et al. 2018; Chungcharoen and Srisang 2020). While high binder percentages can improve cohesion, they can also decrease calorific value (CV), which is usually determined with use of a bomb calorimeter (Sawadogo et al. 2018).

In comparison to charcoal, lignite, and firewood, briquettes offer distinct advantages, including higher energy density, convenient storage, and cleaner incineration (Chungcharoen and Srisang 2020; dos Santos et al. 2022). Furthermore, briquettes present environmental benefits, as they are derived from renewable resources, contributing to reduced deforestation and the efficient management of agro-industrial waste (Sawadogo et al. 2018).

Several studies have explored the potential of CNS as a raw material for briquette production. For instance, cylindrical briquettes with CNS, cassava starch, as a binder, and water were developed. The aim was to identify optimal processing parameters. The analysis revealed that a starch content of 12.9%, water content of 50%, and a drying period of 7 days yielded the most favorable results. The study found a positive correlation between binder content and the compressive strength and durability of the briquettes. However, an increase in both binder and moisture content led to a decrease in the overall calorific value of the briquette. Conversely, extending the drying duration significantly reduced moisture content, leading to improved compressive strength and water resistance of the briquettes. Interestingly, the moisture content did not appear to have a substantial impact on the durability and shatter resistance of the briquettes (Ajith Kumar and Ramesh 2022). Similarly, Arulkumar et al. (2019) conducted experiments using a mix of 80% sawdust and 20% other biomasses, such as rice straw, tamarind shell, neem leaf, and CNS. Among these, 20% CNS exhibited the highest CV (4.28 MJ/kg) indicating a potential as a high-energy fuel source.

Mohod et al. (2008) prepared briquettes using CNSc, sawdust, cow dung, and waste wheat flour. The goal was to determine the optimal composition for maximizing CV and durability. The results indicated that while increasing the CNSc content resulted in a higher CV, it also negatively affected durability due to the lower compaction capacity of the biomass. The optimal composition was 55% CNSc, 25% sawdust, 10% cow dung, and 10% waste wheat flour. Briquettes produced using this formula demonstrated minimal energy consumption during production and low water absorption. Furthermore, a test measuring shatter and durability revealed good shock and impact resistance, good handling and transportation properties, a favorable energy density ratio and degree of densification, and a strong calorific value (21 MJ/kg). Additionally, a water boiling test confirmed that these briquettes burned entirely with a consistent flame, leaving minimal ash residue. In contrast, Santos et al. (2022) produced pellets without binders using densification instead of briquetting. CNS pellets were compared with coconut shell pellets. The CNS pellets exhibited a lower combustion temperature but released more combustion gases than the coconut shell pellets.

To address the emissions problems, some authors proposed to pretreat the biomass before forming the briquettes. For example, Tuates et al. (2020) produced briquettes from three forms of CNSc: untreated CNSc, CNSc treated with hexane, and carbonized CNSc. The briquettes were then assessed for their physical and mechanical properties, including density, shatter resistance and compressive strength. The hexane treated CNSc briquettes with 10% binder exhibited the best overall physical and mechanical properties. However, the carbonized CNSc briquettes produced less smoke during burning. Similarly, other studies thermally treated CNS and mixed it with cassava/tapioca starch as binder to produce briquettes. For instance, Sawadogo et al. (2018) determined the optimal percentage of components and evaluated briquettes’ mechanical, physical, and thermal properties. CV for these briquettes was 25.7 MJ/kg (higher than wood, similar to charcoal), with good mechanical properties (compressive strength index 383 kPa, impact resistance index 61.1), and thermal efficiency (33.9%). However, Ifa et al. (2020) used the remaining carbonized CNS from pyrolysis and found a CV of 29.5 MJ/kg. An economic analysis revealed a 37% cost savings in CNS briquette production compared to liquefied petroleum gas. In a study by Chungcharoen and Srisang (2020), combining 65% CNS with carbonized areca nuts resulted in briquettes that emitted lower greenhouse gases than charcoal combustion, achieving a CV of up to 21 MJ/kg, making it suitable for cooking applications. Additionally, Huko et al. (2015) utilized carbonized CNS and mango seeds, using banana peel as a binder, to produce briquettes and evaluated the impact of particle size on their properties. Density, moisture content, ash content, durability index, compressive strength, and carbon monoxide emissions decreased with larger particle size, while the CV remained constant.

Table 3. Calorific Value (CV) of Briquettes and Other Fuels