Journal of the Korean Wood Science and Technology
The Korean Society of Wood Science & Technology
Original Article

Improving the Calorific Value of Nyamplung (Calophyllum inophyllum L.) Seed Shell Pellets by Torrefaction Treatment for Their Use as a Renewable Energy Resource

Johanes Pramana Gentur SUTAPA1,https://orcid.org/0009-0000-4689-1177, Geraldy KIANTA2, Budi LEKSONO3, Ahmad Harun HIDAYATULLAH4
1Department of Forest Product Technology, Faculty of Forestry, Universitas Gadjah Mada, Yogyakarta 55281, Indonesia
2Alumni of Faculty of Forestry, Universitas Gadjah Mada, Yogyakarta 55281, Indonesia
3Research Center for Applied Botany, National Research and Innovation Agency (BRIN), Jawa Barat 16911, Indonesia
4Alumni of Master of Forest Science Faculty of Forestry, Universitas Gadjah Mada, Yogyakarta 55281, Indonesia
Corresponding author: Johanes Pramana Gentur SUTAPA (e-mail: jpgentursutapa@ugm.ac.id)

Copyright 2024 The Korean Society of Wood Science & Technology. This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Received: Mar 20, 2024; Revised: Apr 08, 2024; Accepted: Apr 28, 2024

Published Online: Jul 25, 2024

ABSTRACT

Nyamplung (Calophyllum inophyllum L.) seeds, which account for 40% of the fruit, have been used as a raw material for biofuels, and the seed shells remaining after their extraction are wasted. In this study, we investigated the potential of waste Nyamplung seed shells in the form of pellets as a biomass energy resource. A completely randomized research design was implemented to evaluate the effects of torrefaction and heat treatment on the quality of produced pellets. Two observed treatments, namely, particle size (0.18–0.25, 0.25–0.43, and 0.43–0.84 mm) and torrefaction temperature (200°C, 225°C, and 250°C), were investigated. Our results showed that the calorific value of torrefied Nyamplung seed-shell pellets ranged from 4,245.60 to 4,528.00 cal/g, fulfilling the Indonesia Nasional Standard (≥ 4,000 cal/g). The quality of pellets were the best when produced from raw materials with a particle size of 0.18–0.25 mm and torrefaction temperature of 225°C. Thus, we concluded that waste Nyamplung seed shells are a good raw material for the production of pellets.

Keywords: Calophyllum inophyllum L; pellet; torrefaction temperature; particle size; calorific value

1. INTRODUCTION

Biomass densification through pelletizing has currently become a priority for various stakeholders, as pellets can be developed into a renewable energy resource (Jang, 2022). In recent years, the development of renewable energy has been prioritized worldwide owing to the intensification of fossil-fuel use, which has accelerated environmental problems and climate change (Connolly et al., 2011; Jung et al., 2015). Biomass is a potential renewable energy source, especially plant biomass. Biomass is a carbon-neutral material as the emissions produced by its utilization will be absorbed by other plants during their growth (Yokoyama and Matsumura, 2008). Indonesia is a tropical country with large plantations and agricultural resources; thus, biomass is a potential energy source, with an estimated value of 32.77 GW if solely converted for electricity (Primadita et al., 2020). Pelletizing is the densification of homogenously sized biomass materials into pellets of certain shapes and energy quantities (García-Maraver et al., 2011). Biomass in the form of pellets is also advantageous; for instance, they are easier to store and transport (Damayanti et al., 2017). Torrefaction is one of the processes used to improve the quality and appearance of pellets, especially to increase their energy density and hydrophobicity (Sutapa and Hidyatullah, 2023). Torrefaction is a mild thermochemical pre-treatment comprising the slow heating of biomass in an oxygen-free chamber with a temperature between 200°C and 300°C. Compare with original biomass, torrefied biomass is more hydrophobic, compactable, and grindable, has higher energy density, and has a lower oxygen to carbon ratio (O/C; Prins et al., 2006; Shankar Tumuluru et al., 2011; Tumuluru et al., 2021). Condensable and non-condensable volatiles in the pellets are also released during torrefaction (Yun et al., 2021). Another factor that affects the quality of a pellet is the particle size or dimensions of the raw material. Decreasing the particle size increases the density, durability, and calorific value (CV) of the pellets (Harun and Afzal, 2016; Hidyatullah et al., 2022).

Nyamplung trees are found in the coastal areas of Indonesia and are estimated to cover 480,000 ha of land (Bustomi et al., 2012; Zalsabila et al., 2024). Nyamplung seeds, which account for 40% of the fruit, have been used as a raw material for biofuel production, leaving behind the seed shells as waste after their extraction. Waste Nyamplung seed shell can also be used as a biomass energy source (Kartika et al., 2017; Leksono et al., 2014, 2017, 2019). As a charcoal briquette, Nyamplung seed shells have a CV of 5,431.35 cal/g (Hazra and Sari, 2011). Senthil and Mohan (2015) conducted proximate and ultimate analyses of Nyamplung seed shells and found that the moisture, ash, volatile matter, and fixed carbon contents (FCCs) were 9.56%, 1.75%, 69.04%, and 19.65%, respectively. Based on ultimate analysis, Nyamplung seed shells contained carbon (76.46%), hydrogen (6.46%), oxygen (5.29%), mineral matter (1.93%), nitrogen (0.31%), and sulfur (trace amounts; Senthil and Mohan, 2015).

In this study, we aimed to evaluate the effects of particle size and torrefaction temperature on the quality of Nyamplung seed-shell pellets (NSPs). Pellet samples were produced using the single-pellet method. Lee and Kim (2020) stated that the single-pellet process is an appropriate, fast, and low-cost method to obtain samples for small-scale tests to examine the pellet quality. The quality of torrefied NSPs was evaluated by measuring the properties of produced pellets, in accordance with the Indonesian National Standard (SNI) for household purposes (Badan Standardisasi Nasional, 2018).

2. MATERIALS and METHODS

The raw materials, namely, Nyamplung seed shells, used in this study were obtained from Purworejo, Central Java, Indonesia, which is a vast Nyamplung plantation in Indonesia that has been developed as a home industry for Nyamplung/Tamanu oil production. A completely randomized research design was employed with two treatment factors, namely, particle sizes (0.18–0.25, 0.25–0.43, and 0.43–0.84 mm) and torrefaction temperature (200°C, 225°C, and 250°C), with five replications. Variant analysis was applied to all research data, followed by Tukey’s honestly significant difference test to determine the significant differences in the average values.

2.1. Densification for pellet formation

Nyamplung seed shells were air-dried until the moisture content (MC) reached ± 12% and grounded until their size was homogeneous. Grounded raw material was sieved to 0.18–0.25, 0.25–0.43, and 0.43–0.85 mm. The resulting powders were densified (pelletizing) using a single pelletizer with hydraulic power at a pressing rate of 68 mm/min until a maximum compressive load of 150 kg/cm2 was reached. The die hole had a diameter and height of 8 and 55 mm, respectively, with a cylindrical pusher with a length of 100 mm.

2.2. Torrefaction treatment

Torrefaction was conducted at temperatures of 200°C, 225°C, and 250°C for 15 min. Torrefaction was conducted in a furnace with the following specifications: Thermolyne FB 1410M-33, single setpoint with a capacity of 2.1 L, power consumption of 1,520 W, temperature range of 100°C to 1,100°C, temperature stability of ± 5.0 at 1,000°C, and electrical requirements of 240 V at 50/60 Hz. Pellets without torrefaction were used as controls. Furthermore, the yield of the torrefaction treatment of NSPs was evaluated based on the weight loss of the sample.

2.3. Evaluation of torrefied Nyamplung seed shell pellets

The quality of the torrefied samples was evaluated by examining the compressive strength (CS) according to ASTM (2001), MC according to ASTM (2007), volatile matter content (VMC) according to ASTM (2007), ash content (ASH) according to ASTM (2007), FCC according to ASTM (2007), and CV according to ASTM (2010).

2.3.1. Yield of torrefaction treatment

The yield was determined using torrefied pellets. The yield was calculated using the following Equation (1):

Yeild ( % ) = W 2 W 1 × 100 %
(1)

W1: pellet weight before torrefaction (g); W2: weight of the torrefied pellet (g).

2.3.2. Compressive strength evaluation

The CS was evaluated as follows: the NSPs were placed between two flat metal surfaces on a Universal Testing Machine, and a load pressure was applied to determine the maximum CS of the samples.

2.3.3. Proximate description of Nyamplung seed shell pellets

The proximate description analysis of the pellets provided the basic characteristics of the pellet according to the following parameters: MC, VMC, and ASH based on the ASTM D176-84 standard, and FCC based on the ASTM D1762-84 standard.

The Equations (2) to (5) for the proximate evaluation are as follows:

MC ( % ) = [ ( a b ) / a ] × 100 %
(2)

a = weight of the air-dried sample (g).

b = weight of the sample after drying at 105°C (g).

V M C ( % ) = [ ( b c ) / b ] × 100 %
(3)

c = weight of the sample after heating at 950°C (g).

ASH ( % ) = ( d / a ) × 100 %
(4)

d = weight of the sample after heating at 750°C for 6 h (g).

FCC ( % ) = 100 % [ VMC ( % ) + ASH ( % ) ]
(5)
2.3.4. Evaluation of the calorific value

The CV of the torrefied pellets was evaluated using an IKA C-200 bomb calorimeter in accordance with ASTM-D5865-10.

3. RESULTS and DISCUSSION

The results and appearance of the torrefied NSPs are shown in Fig. 1. The average CS values and proximate evaluation results for the torrefied NSPs are listed in Table 1.

wood-52-4-363-g1
Fig. 1. Appearance of torrefied NSPs. P1, P2, and P3: particle sizes of 0.43–0.84, 0.25–0.43, and 0.18–0.25 mm, respectively. T1, T2, and T3: torrefaction temperatures of 200°C, 225°C, and 250°C, respectively. NSPs: Nyamplung seed-shell pellets.
Download Original Figure
Table 1. Averages of the quality parameters of torrefied Nyamplung seed shells
Particle size (mm) Torrefaction temperature (°C) Compressive strength (N) Moisture content (%) Volatile matter (%) Ash content (%) Fixed carbon (%) Calorific value (cal/g)
0.43–0.84 200 201.34 7.77 79.95 2.58 17.47 4,263.40
225 99.63 10.32 79.01 4.21 16.78 4,245.60
250 55.66 8.68 79.19 4.41 16.78 4,307.60
Control 68.16 14.10 80.45 2.76 16.79 4,144.80
0.25–0.43 200 199.62 6.58 77.43 4.87 17.70 4,454.60
225 77.57 8.71 78.99 5.28 15.72 4,319.20
250 58.42 8.15 77.33 6.11 16.56 4,528.00
Control 118.45 14.71 80.51 3.06 16.43 4,233.00
0.18–0.25 200 169.48 5.70 78.12 2.46 19.42 4,416.60
225 73.33 7.59 79.37 4.03 16.60 4,372.20
250 50.55 7.95 77.95 5.50 16.55 4,426.80
Control 97.67 13.24 80.62 4.55 14.84 4,277.80
Download Excel Table
3.1. Physical appearance and yield of torrefied pellets

The torrefied NSPs exhibited slight deformation and color changes. The color became slightly darker, the shape of the torrefied pellet was slightly bent, and a few voids appeared on the entire surface. This color change is in accordance with the findings of Kim and Kim (2019), who reported that temperature treatment of P. tomentosa branches resulted in a color change from light brown to deep brown.

In this study, we showed that the torrefaction temperature had a significant effect on the yield of torrefied NSPs; the highest yield (91.39%) of torrefied NSPs was obtained at a torrefaction temperature of 225°C (Fig. 2). The decrease in the amount of torrefied biomass or mass loss, which is expressed in terms of the number of yields, is caused by the initial VMC that is easily removed with high-temperature treatment along with the torrefaction or activation process of biomass (Chandra et al., 2009; Kim, 2016; Sutapa and Hidyatullah, 2023).

wood-52-4-363-g2
Fig. 2. Effect of torrefaction treatment on the yield (%) of torrefied NSPs. a,b Groups of homogeneous subsets. HSD: honestly significant difference, NSPs: Nyamplung seed-shell pellets.
Download Original Figure
3.2. Compressive strength

The CS of a pellet depends on the inner forces of the mechanical interlocking of particles established during the densification process (Yang et al., 2019). Analysis of variance (ANOVA) indicated that the interaction between the particle size and torrefaction temperature had a significant effect on the CS of NSPs at the 1% precision level. The CS of the torrefied NSPs ranged from 50.55 to 201.34 N, which was lower than that of Calliandra wood pellets after torrefaction treatment at a temperature of 250°C for 30 min, namely, 328.70 N, as indicated by Sutapa and Hidyatullah (2023). In this study, the highest CS value (201.34 N) of the torrefied NSPs was observed in pellets derived from a particle size and torrefaction temperature of 0.43–0.85 mm and 200°C, respectively. This CS value was higher than that of the control pellets (68.16 N). Increasing the torrefaction temperature and decreasing the particle size resulted in a decrease in the CS of torrefied NSPs (Fig. 3). The CS of torrefied NSPs with torrefaction temperatures of 200°C–250°C decreased by 15.83%–26.40%. These findings are consistent with those of Stelte et al. (2013), who found that after torrefaction at a temperature of 250°C–300°C, wheat stem-powder pellets showed a decrease in CS. Increasing the torrefaction temperature resulted in a change in the chemical composition of the pellet in the form of cellulose and hemicellulose degradation, which caused a decrease in the radial CS of the pellets. This is because at temperatures above 250°C, cellulose begins losing its stability and degrades, and its polymerization number decreases (Stelte et al., 2013). However, a lower CS provides benefits during the crushing and grinding of pellets used in the praxis of co-combustion with coal as the energy required for grinding and crushing is lower for pellets with lower CS (Stelte et al., 2013; Sutapa and Hidyatullah, 2023).

wood-52-4-363-g3
Fig. 3. Compressive strength of torrefied NSPs (N). a–f Groups of homogeneous subsets. HSD: honestly significant difference, NSPs: Nyamplung seed-shell pellets.
Download Original Figure
3.3. Moisture content

The ANOVA results showed that the interaction between particle size and torrefaction temperature had a significant effect on the MC of torrefied NSPs at the 1% precision level. The MC of torrefied NSPs ranged from 5.70% to 10.32%. In this study, the lowest MC value of torrefied NSPs (5.70%) was observed in pellets with a particle size of 0.43–0.84 mm and torrefied at a temperature of 250°C. This value is lower than that of control pellets with a particle size of 0.43–0.84 mm, namely, 14.10%. In this study, increasing the torrefaction temperature and decreasing the particle size significantly decreased MC (Fig. 4). The MC of NSPs with particle sizes of 0.43–0.84 and 0.25–0.43 mm torrefied at temperatures of 200°C and 250°C decreased by 26.64% and 26.45%, respectively. This phenomenon is consistent with the biomass drying stages; namely, the non-reactive drying (50°C–150°C), reactive drying (150°C–200°C), and destructive drying (200°C–300°C) phases. During heating at temperatures from 50°C to 150°C, water evaporates, and chemical changes occur as the result of the decrease of biomass weight and water content (Shankar Tumuluru et al., 2011). This result of NSPs is consistent with that of pellets from Southern Yellow Pine sawdust; namely, after torrefaction at a temperature of 250°C, the MC of these pellets decreased by 6.22%–1.45% (Manouchehrinejad and Mani, 2018). The MC of torrefied NSPs ranged from 5.70% to 10.32%, thereby fulfilling SNI 8675:2018 (≤ 12%).

wood-52-4-363-g4
Fig. 4. Moisture content of torrefied NSPs (%). a–d Groups of homogeneous subsets. HSD: honestly significant difference, NSPs: Nyamplung seed-shell pellets.
Download Original Figure
3.4. Volatile matter content

The component that evaporates at high temperatures in the absence of air is known as VMC (excluding water vapor). We observed a correlation between volatile compounds and quantity of smoke released during combustion. The bound carbon content of pellets is correlated with the amount of VMC in their components (Cahyani et al., 2023; Speight, 2015). In several countries, such as Korea, Japan, and the European Union, VMC from wood pellets is regulated by atmospheric environmental standards as it contributes to the emission number (Yang and Han, 2018). ANOVA indicated that the interaction between particle size and torrefaction temperature had a significant effect on the VMC of torrefied NSPs with 1% precision. The VMC of the torrefied NSPs ranged from 77.33% to 79.95% (Fig. 5). The lowest VMC of torrefied NSPs (77.33%) were observed in pellets with a particle size of 0.18–0.25 mm and torrefied at a temperature of 225°C; this value is lower than that of control pellets (4.55%) with a particle size of 0.18–0.25 mm. The VMC of torrefied NSPs with a particle size of 0.18–0.25 mm and a torrefaction temperature of 250°C was 77.95%. This value is higher than that of pellets derived from Merbau waste (76.57%), which was torrefied at 250°C for 15 min (Sutapa and Prasetyadi, 2023), and lower than that of Calliandra wood pellets (78.68%) torrefied at a temperature of 250°C for 60 min (Sutapa and Hidyatullah, 2023). In this study, increasing the torrefaction temperature and decreasing the particle size decreased the VMC of the torrefied NSPs (Fig. 5). The VMC of NSPs with particle sizes of and 0.43–0.84 and 0.18–0.25 mm and torrefied at temperatures of 200°C to 250°C decreased by 2.28% and 1.57%, respectively. This decrease was caused by the evaporation of water vapor and the decrease in the VMC materials during torrefaction. The internal process during torrefaction changes from endothermic, which forms volatile substances, to exothermic, which forms ash at a temperature of ± 270°C (Ju et al., 2020; Yun et al., 2021). The high VMC of wood pellets ranges from 70% to 86% of the dry weight; therefore, they are more reactive when burned compared with coal, whose VMC is only 35% (van Loo and Koppejan, 2008). The VMC of torrefied NSPs ranged from 77.33% to 79.95%, and these values are in accordance with the SNI quality standard (≤ 80%).

wood-52-4-363-g5
Fig. 5. Volatile matter content of torrefied NSPs (%). a–d Groups of homogeneous subsets. HSD: honestly significant difference, NSPs: Nyamplung seed-shell pellets.
Download Original Figure
3.5. Ash content

The material remaining in the pellets after burning contains ash, and its primary constituents are silica, calcium, potassium, and magnesium. The amount of ash present affects the thermal energy generated; the greater the amount of ash, the lesser is the generated energy (Cahyani et al., 2023; Poddar et al., 2014). Furthermore, pellets with higher ASHs are not recommended for industrial use (Wistara et al., 2017). The ANOVA results indicated that the interaction between particle size and torrefaction temperature had a significant effect on the ASH of torrefied NSPs at the 1% precision level. The ASH of torrefied NSPs ranged from 2.47% to 6.11%. In this study, the lowest ASH (2.47%) of torrefied NSPs was observed in pellets with a particle size of 0.43–0.84 mm and torrefied at a temperature of 250°C; this value is lower than that of control pellets with a particle size of 0.43–0.84 mm (2.76%). In this study, increasing the torrefaction temperature and decreasing the particle size increased ASH (Fig. 6). This result is consistent with that of Cahyanti et al. (2020), who found that increasing the torrefaction temperature increased the ASH. Other studies on the torrefaction of rice husks at temperatures of 210°C, 240°C, 270°C, and 300°C indicated that the ASH increased from an initial value of 15% before torrefaction to 16.20%, 16.90%, 20.20%, and 23.40%, respectively (Chen et al., 2018). These results are similar to the previous results on pellets derived from mixed raw materials of fir, pine, and spruce, which comprised two different particle sizes of 1.00–2.00 and 0.25–0.50 mm, and both materials were treated with the same torrefaction temperature of 240°C; the results indicated increasing ASHs of 0.40% and 0.52%, respectively (Wang et al., 2017).

wood-52-4-363-g6
Fig. 6. Ash content of torrefied NSPs (%). a–d Groups of homogeneous subsets. HSD: honestly significant difference, NSPs: Nyamplung seed-shell pellets.
Download Original Figure

According to Manouchehrinejad and Mani (2018), smaller particles are more easily penetrated by heat during heating processes, such as torrefaction. Pellets with small particle sizes require shorter heating times; thus, they experience low mass losses, including ash, as non-combustible materials. The average ASH of torrefied NSPs was 4.38%, which already fulfilled SNI 8675:2018 (≤ 5%).

3.6. Fixed carbon content

The FCC of the torrefied NSPs ranged from 15.72% to 19.42%. ANOVA showed that the interaction between particle size and torrefaction temperature significantly influenced the FCC of the torrefied NSPs at a 5% precision level. In this study, the highest FCC of torrefied NSPs (19.42%) was observed in pellets with a particle size of 0.43–0.84 mm and torrefied at a temperature of 250°C. The FCC of torrefied NSPs with a particle size of 0.43–0.84 mm and torrefaction temperatures of 200°C–225°C and 250°C increased by 1.31% and 11.61%, respectively (Fig. 7). The FCC of torrefied pellets increased owing to the decrease in VMC and MC (Aytenew et al., 2018). Furthermore, previous studies have found that as the torrefaction process reduced the O/C and H/C ratios, the carbon content of Larix kaempferi wood chips increased with increasing temperature and torrefaction duration (Lee et al., 2015). The increase in FCC of NSPs is consistent with the findings of Sutapa and Hidyatullah (2023), who stated that the FCC of Calliandra wood pellets increased when the torrefaction temperatures were increased to 250°C and 300°C. The FCC results for the torrefied NSPs fulfilled the SNI standard (minimum of 14%).

wood-52-4-363-g7
Fig. 7. Fixed carbon content of torrefied NSPs (%). a–c Groups of homogeneous subsets. HSD: honestly significant difference, NSPs: Nyamplung seed-shell pellets.
Download Original Figure
3.7. Calorific value

The CV can be increased by increasing the temperature and lengthening the torrefaction period (Lee and Kim, 2020; Matali et al., 2016; Sutapa and Hidyatullah, 2023). The ANOVA results indicated that the torrefaction temperature significantly affected the CV at the 5% precision level. The CV of torrefied NSPs ranged from 4,245.6 to 4,528.00 cal/g. In this study, the highest CV of torrefied NSPs (4,528.00 cal/g) was observed in pellets with a particle size 0.18–0.25 mm and torrefied at a temperature of 225°C; this value was higher than that of control pellets with a particle size 0.18–0.25 mm (4,277.80 cal/g). Our results indicated that an increase in torrefaction temperature from 200°C to 225°C and 250°C increased the CV by 3.79% and 3.11%, respectively. The CV values of this study are in line with the results of torrefied Calliandra wood pellets and torrefied palm fronds obtained by Sutapa and Hidyatullah (2023) and Lau et al. (2018), respectively; both studies reported an increase in the CV resulting from the increase in heating temperature. The CV of palm fronds increased by 29.58% at a heating temperature of 300°C for 30 min. Fig. 8 shows that the highest CV (4,433.93 cal/g) was obtained at a torrefaction temperature of 225°C, which is higher than the CV of NSPs torrefied at a temperature of 250°C. Therefore, this result implies that the limitations of the torrefaction temperature must be considered to obtain NSPs with good CV, because such pellets will lead to lower energy consumption and greater profitability from an economic perspective. The CV of torrefied NSPs ranged from 4,245.6 to 4,528.00 cal/g, thereby meeting the SNI standard (≥ 4,000 cal/g).

wood-52-4-363-g8
Fig. 8. Calorific value of torrefied NSPs (cal/g). a,b Groups of homogeneous subsets. HSD: honestly significant difference, NSPs: Nyamplung seed-shell pellets.
Download Original Figure

4. CONCLUSIONS

Our results indicated that Nyamplung seed shells are a promising material for pellet production. Our results showed that the interaction between the particle size and torrefaction temperature significantly affected the CS and proximate values of NSPs. Increasing the torrefaction temperature and decreasing the particle size resulted in a decrease in the CS, MC, and VMC of the torrefied NSPs and an increase in ASH and FCC. The CV of torrefied NSPs increased with increasing torrefaction temperature but decreased after a temperature of 225°C. The optimal parameters for producing NSPs are as follows: particle size of 0.18–0.25 mm and torrefaction temperature of 225°C. These parameters will result in a product with a high CV of 4,528.00 cal/g; this value fulfils the SNI standard (≥ 4,000 cal/g).

CONFLICT of INTEREST

No potential conflict of interest relevant to this article was reported.

ACKNOWLEDGMENT

We sincerely thank the Faculty of Forestry of Universitas Gadjah Mada, especially the Laboratory of Bioenergy and Biomaterial Conversion, for supporting our study.

REFERENCES

1.

American Society for Testing and Materials [ASTM]. 2001. Standard Test Method for Single Pellet Crush Strength of Formed Catalyst Shapes. ASTM D4179-01. ASTM International, West Conshohocken, PA, USA.

2.

American Society for Testing and Materials [ASTM]. 2007. Standard Test Method for Chemical Analysis of Wood Charcoal. ASTM D1762-84. ASTM International, West Conshohocken, PA, USA.

3.

American Society for Testing and Materials [ASTM]. 2010. Standard Test Method for Gross Calorivic Value of Coal and Coke by the Adiabatic Bomb Calorimeter. ASTM D5865-10. ASTM International, West Conshohocken, PA, USA.

4.

Aytenew, G., NIgus, G., Bedewi, B. 2018. Improvement of the energy density of rice husk using dry and chemical treated torrefaction. Journal of Advanced Chemical Engineering 8(1): 1000185.

5.

Badan Standardisasi Nasional. 2018. Pelet Biomassa Untuk Energy. SNI 8675:2018. Badan Standardisasi Nasional, Jakarta, Indonesia.

6.

Bustomi, S. 2012. Yamplung (Calophyllum inophyllum L.): Sumber Energi Biofuel Yang Potensial. Badan Penelitian dan Pengembangan Kehutanan, Jakarta, Indonesia.

7.

Cahyani, N., Yunianti, A.D., Suhasman, Pangestu, K.T.P., Pari, G. 2023. Characteristics of bio pellets from spent coffee grounds and pinewood charcoal based on composition and grinding method. Journal of the Korean Wood Science and Technology 51(1): 23-37.

8.

Cahyanti, M.N., Doddapaneni, T.R.K.C., Kikas, T. 2020. Biomass torrefaction: An overview on process parameters, economic and environmental aspects and recent advancements. Bioresource Technology 301: 122737.

9.

Chandra, T.C., Mirna, M.M., Sunarso, J., Sudaryanto, Y., Ismadji, S. 2009. Activated carbon from durian shell: Preparation and characterization. Journal of the Taiwan Institute of Chemical Engineers 40(4): 457-462.

10.

Chen, D., Gao, A., Ma, Z., Fei, D., Chang, Y., Shen, C. 2018. In-depth study of rice husk torrefaction: Characterization of solid, liquid and gaseous products, oxygen migration and energy yield. Bioresource Technology 253: 148-153.

11.

Connolly, D., Lund, H., Mathiesen, B.V., Leahy, M. 2011. The first step towards a 100% renewable energy-system for Ireland. Applied Energy 88: 502-507.

12.

Damayanti, R., Lusiana, N., Prasetyo, J. 2017. Studi pengaruh ukuran partikel dan penambahan perekat tapioka terhadap karakteristik biopelet dari kulit coklat (Theobroma cacao L.) sebagai bahan bakar alternatif terbarukan. Jurnal Industri Teknologi Pertanian 11(1): 51-60.

13.

García-Maraver, A., Popov, V., Zamorano, M. 2011. A review of European standards for pellet quality. Renewable Energy 36: 3537-3540.

14.

Harun, N.Y., Afzal, M.T. 2016. Effect of particle size on mechanical properties of pellets made from biomass blends. Procedia Engineering 148: 93-99.

15.

Hazra, F., Sari, N. 2011. To biomassa tempurung buah nyamplung (Calophyllum spp.) untuk pembuatan briket arang sebagai bahan bakar alternatif. Jurnal Sains Terapan 1(1): 8-13.

16.

Hidyatullah, A.H., Sutapa, J.P.G., Listyanto, T. 2022. Pengaruh ukuran partikel bahan baku terhadap kualitas pelet ranting kaliandra (Calliandra calothyrsus) dari limbah pakan ternak kambing. Jurnal Ilmu dan Teknologi Kayu Tropis 20(1): 31-39.

17.

Jang, E.S. 2022. Experimental investigation of the sound absorption capability of wood pellets as an eco-friendly material. Journal of the Korean Wood Science and Technology 50(2): 126-133.

18.

Ju, Y.M., Lee, H.W., Kim, A., Jeong, H., Chea, K.S., Lee, J., Ahn, B.J., Lee, S.M. 2020. Characteristics of carbonized biomass produced in a manufacturing process of wood charcoal briquettes using an open hearth kiln. Journal of the Korean Wood Science and Technology 48(2): 181-195.

19.

Jung, S.J., Kim, S.H., Chung, I.M. 2015. Comparison of lignin, cellulose, and hemicellulose contents for biofuels utilization among 4 types of lignocellulosic crops. Biomass and Bioenergy 83: 322-327.

20.

Kartika, I.A., Sari, D.D.K., Pahan, A.F., Suparno, O., Ariono, D. 2017. Ekstraksi minyak dan resin nyamplung dengan campuran pelarut heksan-etanol. Jurnal Teknologi Industri Pertanian 27(2): 161-171.

21.

Kim, A., Kim, N.H. 2019. Effect of heat treatment and particle size on the crystalline properties of wood cellulose. Journal of the Korean Wood Science and Technology 47(3): 299-310.

22.

Kim, Y.S. 2016. Research trend of the heat-treatment of wood for improvement of dimensional stability and resistance to biological degradation. Journal of the Korean Wood Science and Technology 44(3): 457-476.

23.

Lau, H.S., Ng, H.K., Gan, S., Jourabchi, S.A. 2018. Torrefaction of oil palm fronds for co-firing in coal power plants. Energy Procedia 144: 75-81.

24.

Lee, H.W., Kim, S.B. 2020. Study on the estimation of proper compression ratios for Korean domestic wood species by single pellet press. Journal of the Korean Wood Science and Technology 48(4): 450-457.

25.

Lee, J., Ahn, B.J., Kim, E.J. 2015. Effects of the torrefaction process on the fuel characteristics Larix kaempferi C. Journal of the Korean Wood Science and Technology 43(2): 196-205.

26.

Leksono, B., Hasnah, T.M., Windyarini, E. 2017. Conservation and zero waste concept of biodiesel industry based on Calophyllum inophyllum plantation. In: Yogyakarta, Indonesia, Proceedings of IUFRO - INAFOR Joint International Conference 2017, pp. 163-174.

27.

Leksono, B., Windyarini, E., Hasnah, T., Rahman, S.A., Baral, B. 2019. Calophyllum inophyllum for green energy and landscape restoration: Plant growth, biofuel content, associate waste utilization and agroforestry prospect. In: Phuket, Thailand, Proceedings of 2018 International Conference and Utility Exhibition on Green Energy for Sustainable Development (ICUE), pp. 1-7.

28.

Leksono, B., Windyarini, E., Hasnah, T.M. 2014. Budidaya Tanaman Nyamplung (Calophyllum inophyllum L.) untuk Bioenergi dan Prospek Pemanfaatan Lainnya. IPB Press, Jakarta, Indonesia.

29.

Manouchehrinejad, M., Mani, S. 2018. Torrefaction after pelletization (TAP): Analysis of torrefied pellet quality and co-products. Biomass and Bioenergy 118: 93-104.

30.

Matali, S., Rahman, N.A., Idris, S.S., Yaacob, N., Alias, A.B. 2016. Lignocellulosic biomass solid fuel properties enhancement via torrefaction. Procedia Engineering 148: 671-678.

31.

Poddar, S., Kamruzzaman, M., Sujan, S.M.A., Hossain, M., Jamal, M.S., Gafur, M.A., Khanam, M. 2014. Effect of compression pressure on lignocellulosic biomass pellet to improve fuel properties: Higher heating value. Fuel 131: 43-48.

32.

Primadita, D.S., Kumara, I.N.S., Ariastina, W.G. 2020. A review on biomass for electricity generation in Indonesia. Journal of Electrical, Electronics and Informatics 4(1): 1-9.

33.

Prins, M.J., Ptasinski, K.J., Janssen, F.J.J.G. 2006. More efficient biomass gasification via torrefaction. Energy 31(15): 3458-3470.

34.

Senthil, R., Mohan, K. 2015. Comparison of yield and fuel properties of thermal and catalytic calophyllum seed shell pyrolitic oil. In: Viluppuram, India, International Conference on Recent Advancement in Mechanical Engineering & Technology, pp. 119-126.

35.

Shankar Tumuluru, J., Sokhansanj, S., Hess, J.R., Wright, C.T., Boardman, R.D. 2011. REVIEW: A review on biomass torrefaction process and product properties for energy applications. Industrial Biotechnology 7(5): 384-401.

36.

Speight, J.G. 2015. Assessing Fuels for Gasification: Analytical and Quality Control Techniques for Coal. In: Gasification for Synthetic Fuel Production: Fundamentals, Processes and Applications, Ed. by Luque, R. and Speight, J.G. Woodhead, Sawston, UK.

37.

Stelte, W., Nielsen, N.P.K., Hansen, H.O., Dahl, J., Shang, L., Sanadi, A.R. 2013. Reprint of: Pelletizing properties of torrefied wheat straw. Biomass and Bioenergy 53: 105-112.

38.

Sutapa, J.P.G., Hidyatullah, A.H. 2023. Torrefaction for improving quality of pellets derived from calliandra wood. Journal of the Korean Wood Science and Technology 51(5): 381-391.

39.

Sutapa, J.P.G., Prasetyadi, G.V. 2023. Torrefaction for upgrading the quality of merbau wood waste pellets. Environmental Research, Engineering and Management 79(3): 52-59.

40.

Tumuluru, J.S., Ghiasi, B., Soelberg, N.R., Sokhansanj, S. 2021. Biomass torrefaction process, product properties, reactor types, and moving bed reactor design concepts. Frontiers in Energy Research 9: 728140.

41.

van Loo, S., Koppejan, J. 2008. The Handbook of Biomass Combustion and Co-firing. Earthscan/James & James, London, UK.

42.

Wang, Z., Lim, C.J., Grace, J.R., Li, H., Parise, M.R. 2017. Effects of temperature and particle size on biomass torrefaction in a slot-rectangular spouted bed reactor. Bioresource Technology 244: 281-288.

43.

Wistara, N.J., Rohmatullah, M.A., Febrianto, F., Pari, G., Lee, S.H., Kim, N.H. 2017. Effect of bark content and densification temperature on the properties of oil palm trunk-based pellets. Journal of the Korean Wood Science and Technology 45(6): 671-681.

44.

Yang, I., Han, G.S. 2018. Comparison of domestic and overseas allowable standards related to emissions from wood pellet combustion. Journal of the Korean Wood Science and Technology 46(5): 553-564.

45.

Yang, I., Jeong, H., Lee, J.J., Lee, S.M. 2019. Relationship between lignin content and the durability of wood pellets fabricated using Larix kaempferi C. sawdust. Journal of the Korean Wood Science and Technology 47(1): 110-123.

46.

Yokoyama, S., Matsumura, Y. 2008. The Asian biomass handbook: A guide for biomass production and utilization. The Japan Institute of Energy 1: 61-62.

47.

Yun, H., Wang, Z., Wang, R., Bi, X., Chen, W.H. 2021. Identification of suitable biomass torrefaction operation envelops for auto-thermal operation. Frontiers in Energy Research 9: 636938.

48.

Zalsabila, A., Syafii, W., Priadi, T., Syahidah. 2024. Anti-termite activity of tamanu bark extract (Calophyllum inophyllum L.). Journal of the Korean Wood Science and Technology 52(2): 134-144.