Quality Enhancement of Torrefied Biopellets Prepared by Unused Forest Biomass and Wood Chip Residues in Pulp Mills

Abstract

The effects of torrefaction of the biopellets made from hardwood chip residue (HW), camellia oilseed cake (CO), and pruning remnants of the toothache tree (TA) and mulberry tree (MT) were evaluated. Torrefaction of the biopellets reduced the volatile matter content of biopellets by 18–58% and increased their heating value by 18–58% without negatively impacting durability or fines content. Torrefaction also reduced the initial ignition time of biopellets by 50–59% and prolonged their combustion duration by 15–24%. Regardless of the type of feedstock, all biopellets exhibited mass yields in the range of 60–80% and energy yields ranging from 80–95%. The novelty of this study lies in the application of torrefaction to already-formed biopellets, which enhances pellet quality without the need for binders, and the use of unused forest biomass and wood chip residue from pulp mills. The use of unused forest biomass and wood chip residue from pulp mills for biopellet production not only provides a sustainable and efficient method for waste utilization but also contributes to environmental conservation by reducing the reliance on fossil fuels. Overall, the torrefaction of biopellets represents a promising technology for producing high-quality solid biofuel from a variety of woody biomass feedstocks without compromising pelletizing efficiency.

1. Introduction

The United Nations Framework Convention on Climate Change (UNFCCC) has called for more use of biomass as a primary energy source [1,2]. Biomass is a renewable resource that can produce various energy products, including biofuels, biogas, and heat [3]. Biomass-based fuels and chemicals can help to reduce greenhouse gas emissions and address the growing global energy demand [4]. One of the most promising methods for utilizing biomass as fuel is to convert it into pellets [5,6,7,8]. Pellets are produced by compressing raw materials at a high density, which makes them easier to store and transport. They also have a higher energy density than biomass in its original form, making them a more efficient fuel [9]. In addition to their high energy density, pellets have many other advantages. Wood pellets are carbon-neutral, meaning that the CO₂ released when they are burned is reabsorbed by woody biomass that grows to replace them. They also emit fewer pollutants than fossil fuels, such as sulfur dioxide and nitrogen oxides.

Unused biomass is an essential source of raw material for making pellets. It is a renewable resource often available in large quantities, making it a cost-effective option for pellet production. Unused biomass can include various materials, such as wood residues, agricultural byproducts, and municipal solid waste. The European Union (EU) is committed to increasing the use of bioenergy as part of its efforts to reduce greenhouse gas emissions and achieve climate neutrality by 2050 [10]. In 2020, bioenergy represented over half of the EU’s renewable energy (56.8%) and supplied 12.4% of the EU’s total energy. The EU has set a target of increasing the share of bioenergy in its total energy consumption to about 22% by 2050. According to the USDA Foreign Agricultural Service’s Global Agricultural Information Network, the EU’s consumption of wood pellets, as one of the sources of bioenergy, is expected to increase from about 25 million tons in 2022 to 26 million tons in 2023 [11].

In 2020, biomass accounted for 19% of South Korea’s total renewable energy power generation, second only to solar power (52%). The use of biomass for power generation in South Korea is regulated by the Renewable Portfolio Standard (RPS), which requires power generation companies with facilities larger than 500 MW to supply a certain percentage of their electricity from new and renewable energy sources [12]. To meet the RPS target, power generation companies in South Korea have relied heavily on wood pellet-fired or co-fired power generation with coal. In 2021, about 95% of the wood pellets used for power generation in Korea were imported, amounting to about 2.88 million tons. The use of imported wood pellets has been criticized for its environmental impact, as it can lead to deforestation and the displacement of wildlife [13]. Additionally, the transportation of wood pellets can have a significant carbon footprint.

It is worth noting that the use of unused forest biomass in South Korea carries a meaningful incentive [14,15,16]. It qualifies for a Renewable Energy Certificate (REC) subsidy with a weight of 2.0 [17]. This implies that domestic power companies utilizing unused forest biomass receive double the REC weight compared to those using alternative fuels [18]. This not only encourages the adoption of sustainable energy sources but also incentivizes the responsible management of forest resources.

However, there are some challenges associated with the use of biomass pellets. One challenge is that biomass has a high moisture content, which can make it difficult to store and transport [19,20]. Another challenge is that the quality of biomass can vary depending on the source, which can affect the performance of the pellets [21,22]. Our study addresses these challenges by exploring the torrefaction of already-formed biopellets, which improves their fuel properties and obviates the need for binders during pelletizing.

Torrefaction is a way to overcome the disadvantages of wood pellets. Torrefaction is a thermal treatment process that is used to improve the properties of biomass materials, such as wood pellets. The process involves heating the biomass material under mild conditions (200–300 °C) in the absence of oxygen. This results in several changes in the material, including reduced moisture content (from 10–20% to 5–8%), increased energy density (by up to 30%), increased hydrophobicity (making the material less susceptible to moisture), increased grindability (making the material easier to process), and increased resistance to biological degradation (making the material more stable) [23,24,25]. However, when pellets are formed after the torrefaction of lignocellulosic feedstock, pelletizing efficiency is very poor. Therefore, when manufacturing biopellets using torrefied woody feedstocks, it is necessary to consider torrefaction treatment for already-manufactured pellets to improve the pelletizing efficiency [26,27,28,29].

The types of unused lignocellulosic biomass used for torrefaction in this study were oilseed cake of Camellia japonica, pruning remnants from toothache (Sapium japonicum) and mulberry (Morus alba) trees, and hardwood chip rejects from a pulp mill. Oilseed cake from Camellia japonica is a byproduct of the extraction of camellia oil. It is a solid, brown material that contains about 25% oil [30]. The oilseed cake is collected from the Korea Camellia Research Institute in Tongyeong, Gyeongnam Province, Korea. This research center produces 12 to 15 tons of camellia oil per year, which generates about 3 to 4 tons of oilseed cake. Toothache and mulberry trees are pruned annually to facilitate the harvest of high-quality fruits [31,32]. This results in the generation of a substantial volume of pruning remnants. The pruning process involves cutting off last year’s old branches and new shoots, leaving only the tree’s crown intact. The cultivation area of mulberry trees for mulberry production continued to increase, increasing from 744 ha in 2007 to 1751 ha in 2013. There is a notable trend of expanding the cultivation of toothache trees. This growth can be attributed to the anticipation that cultivating these trees on abandoned agricultural land will emerge as a new source of income for rural and mountainous communities.

Within the framework of this study, we embarked on an exploration of the application of torrefaction as a means to enhance the quality of biopellets manufactured from these neglected forest biomass byproducts. Our findings revealed that torrefaction wielded a significant influence in augmenting both the energy density and hydrophobic properties of the resulting pellets. The results of this study suggest that torrefaction is a promising technology to further improve the quality of unused biomass pellets. This advancement may serve as a means to mitigate certain challenges associated with biomass pellet utilization, thereby rendering them a more viable and sustainable option for renewable energy production.

2. Materials and Methods

2.1. Materials

Hardwood chip rejects (HRs), a mixture of tiny fragments of Korean mixed oak species and Vietnamese Acacia (Acacia mangium and Acacia auriculiformis), were supplied by Moorim P&P Co., Ltd. in Ulsan, Korea. Camellia oilseed cake (Camellia japonica, CO), a byproduct of oil extraction from Camellia japonica, was provided by the Korea Camellia Research Institute Co., Ltd. in Tongyeong, Republic of Korea. Pruned remnants of toothache trees (Zanthoxylum schinifolium, TA) aged 5–10 years and mulberry trees (Morus alba, MT) were taken from the research forest of Gyeongsang National University in Sancheong, Republic of Korea.

Table 1. Physical–chemical characteristics of raw materials used for torrefaction.

Raw Materials	Cellulose (%)	Hemicellulose (%)	Lignin (%)	Extractive (%)	Ash (%)
Hardwood chip rejects (HR)	44.3 ± 0.04	25.7 ± 0.09	23.1 ± 0.03	4.4 ± 0.01	2.5 ± 0.01
Camellia oilseed cake (CO)	24.8 ± 0.02	32.9 ± 0.08	29.7 ± 0.04	10.7 ± 0.05	1.9 ± 0.01
Toothache tree (TA)	45.5 ± 0.05	26.3 ± 0.07	21.1 ± 0.02	5.4 ± 0.02	1.7 ± 0.01
Mulberry tree (MT)	44.7 ± 0.07	24.8 ± 0.06	21.9 ± 0.04	6.7 ± 0.02	1.9 ± 0.02

summarizes the chemical and physical characteristics of the raw materials.

2.2. Pelletization

The woody biomass was dried to achieve below 5% moisture content at room temperature. Then, it was ground using a laboratory blender (WB-1, Kastech, Tokyo, Japan) to produce uniform particles falling with a size range of 1 to 1.5 mm, as illustrated in Table 1. The ground woody biomass was adjusted to a moisture content (Figure 1), achieving levels between 12% and 15%. Finally, it was pelletized into 32–35 mm long pieces using a pelletizer (Duko, Daejeon, Republic of Korea) equipped with a flat die with a diameter of 8 mm (refer to Figure 2). This pelletizer has the capacity to produce up to 100 kg per hour. During pellet preparation, the pelletizing temperature was kept at around 85 °C.

Following a 10-day stabilization period in a controlled environment chamber at 20 °C with a relative humidity of 55%, an assessment of the quality of the prepared pellets was conducted.

2.3. Torrefaction

When torrefaction was performed using the ground particles of the woody biomass in Table 1, the pelletization yield was not good, so pellets were manufactured and then torrefied to analyze the quality characteristics. The torrefaction process of the prepared pellets was conducted using a laboratory biomass torrefier (Duko, Daejeon, Republic of Korea). To ensure an anaerobic environment within the torrefaction chamber, an inert gas, nitrogen, was continuously introduced at a rate of 0.05 L/min throughout the torrefaction process. The temperature gradually increased at a rate of 10 °C/min until it reached the target torrefaction temperature of 250 °C. Once the reactor reached 250 °C, it was held at that temperature for a duration of 10 min. Upon completion of the designated reaction time, the torrefied pellets were transferred to an anaerobic container to prevent oxygen exposure and halt any further pyrolysis reaction.

Figure 3 shows the images of the pellets before and after torrefaction using the different raw materials. Even under the same torrefaction conditions, the surface color and cracking of the pellets varied depending on the type of woody biomass.

2.4. Proximate Analysis and Ultimate Analysis

The ultimate analysis of the pellets before and after torrefaction was carried out using Micro Elemental Analyzer (Flash 2000, Thermo Fisher Scientific, Cambridge, UK) following ISO 16948 [33] and 16994 [34].

The proximate analysis of the pellets, both before and after torrefaction, was performed according to the ASTM methods recommended by Chandrasekaran et al. [35] and Poddar et al. [36]. Moisture (MC), ash, and volatile matter contents (VM) were determined using ISO 18134-1 [37], ISO 18122 [38], and ASTM D 3175-20 [39], respectively. Finally, the fixed carbon content (FC) of each sample was calculated as the difference between 100 and the sum of the measured moisture content, volatile matter, and ash content.

2.5. Quality Analysis of the Biopellets

Durability testing of the prepared pellets was conducted following the guidelines outlined in ISO 17831-1 [40] using a laboratory-scale durability tester (KOS1, Gimhae, Republic of Korea). Bulk density was assessed according to ISO 17828 [41]. Fines contents of biopellets were determined based on ISO/DIS 5370 [42]. The fines content is defined as the percentage in weight of material below 3.15 mm (100 mesh) in size.

The high and low heating values were determined using an oxygen combustion bomb calorimeter (Parr-6400, Parr Instrument Company, Moline, IL, USA) at the High-Tech Materials Analysis Core Facility in Gyeongsang National University according to ISO 18125 [43].

2.6. Mass Yield and Energy Yield

During torrefaction, volatile substances and water are gradually expelled from solid fuels, noticeably altering both mass and calorific value. The mass yield and energy yield after torrefaction of the prepared pellets were calculated using Equations (1) and (2):

$$\mathrm{Mass} \mathrm{yield} (\%)={\displaystyle \frac{Mass of biopallet after torrefaction}{Mass of biopallet}}\times 100$$

(1)

$$\mathrm{Energy} \mathrm{yield} (\%)={\displaystyle \frac{Mass yield\times High heating value of torrefied biopallet}{High heating value of biopallet}}\times 100$$

(2)

2.7. Ignition and Combustion

Experiments to evaluate the ignition and combustion duration of the prepared biopellets were conducted in a controlled environment; specifically, in a dark room isolated from external light and air current. The initial ignition time was compared by measuring the time it took for a flame to form in the pellet using a portable gas torch (GT-3000; Prince, Matsudo, Japan) at a position 30 cm away from the specimen (refer to Figure 4). The combustion duration of the pellet was measured as the time between initial ignition and flame extinction.

2.8. TGA (Thermogravimetric Analysis)

The thermal decomposition characteristics of the prepared pellets were investigated using a thermogravimetric analyzer (Q600, TA Instruments, New Castle, DE, USA) at the High-Tech Materials Analysis Core Facility in Gyeongsang National University according to ISO 11358-1 [44]. The temperature increase was carried out at 10 °C/min, and nitrogen was injected as an inert gas at 50 mL/min to investigate thermal decomposition characteristics up to 800 °C.

2.9. SEM Observation

A Field-Emission Scanning Electron Microscope (FE-SEM, JSM-7610F, JEOL, Tokyo, Japan) and cross-section polisher (IB-09020CP, JEOL, Japan) were employed to observe the surface structure and cross-section of the biopellets

3. Results and Discussion

3.1. Ultimate and Proximate Analysis of Biopellets before and after Torrefaction

The percentages of volatile material, fixed carbon, and ash in the prepared pellets were determined through proximate analysis both before and after torrefaction. Volatile matter in solid biofuels refers to the fraction of combustible substances present in the fuel that can be easily vaporized or converted into gas when the fuel is heated [45]. Fixed carbon in solid biofuels is the portion of the fuel that remains after the volatile matter has been driven off by heating in the absence of air. It is composed of the more complex and less volatile organic compounds in the fuel, such as cellulose, hemicellulose, and lignin.

Figure 5 shows the proximate analysis of biopellets before and after torrefaction. Torrefaction reduced the volatile matter (VM) content of the biopellets, regardless of the type of woody feedstock. This is because torrefaction drives off the volatile matter in the biomass, resulting in an increase in the fixed carbon (FC) content [46]. Therefore, the torrefied biopellets with a higher FC content have a higher calorific value and produce less smoke than the non-torrefied biopellets. Specifically, the VM reduction of the biopellets made from HW and MT feedstocks following torrefaction was notably substantial, ranging from 17% to 24%. This observation can be attributed to the extent of torrefaction within the biopellets themselves, suggesting that the variation in VM content may depend on the uniformity of thermal decomposition throughout the pellets.

Similar trends in the reduction in volatile matter content and increase in fixed carbon content have been reported in previous studies on torrefied biomass. For instance, Wannapeera et al. (2011) observed a significant decrease in volatile matter and an increase in fixed carbon content in torrefied wood and agricultural residues, indicating enhanced fuel properties [47].

Figure 6 shows the ultimate analysis of the prepared biopellets both before and after the process of torrefaction. After torrefaction, the biopellets exhibited a notable increase in their elemental carbon content, with a range of 20% to 62%. Concurrently, there was a reduction in the oxygen and hydrogen content by 23% to 53% and 11% to 22%, respectively. This transformation can be attributed to the decomposition and volatilization of oxygen-containing compounds, such as cellulose, hemicellulose, and lignin, found within the woody biomass during the torrefaction process. These compounds are released as volatile matter, including water and carbon dioxide. Consequently, this phenomenon leads to a significant alteration in the H/C (hydrogen-to-carbon) and O/C (oxygen-to-carbon) ratios of the remaining residue, causing the fuel properties to shift in a manner reminiscent of coal, as depicted in the Van Krevelen diagram [48]. The nitrogen and sulfur contents of biopellets did not show a big change before and after torrefaction. Nitrogen and sulfur are both non-volatile elements, meaning that they do not evaporate at the temperatures used in torrefaction. As a result, their concentration in the biopellets remained relatively constant before and after torrefaction.

The changes in elemental composition and the resulting shifts in the Van Krevelen diagram align with findings from previous studies on torrefied biomass. For example, Phanphanich and Mani (2011) reported similar increases in carbon content and reductions in hydrogen and oxygen contents in torrefied pine and hardwood [49].

3.2. Quality Analysis of Biopellets before and after Torrefaction

Figure 7 shows the graph comparing the durability of the biopellets before and after torrefaction. Because torrefaction was performed on the premanufactured pellets, there was no remarkable difference in pellet durability before and after torrefaction, regardless of the type of woody biomass feedstock. The excellent durability of the biopellets before and after torrefaction resulted in minimal fine particle generation. The fines content of the biopellets was also measured. The fines content is the percentage of particles that pass through a 3.15 mm mesh. A high fines content is undesirable because it can increase the risk of dust explosions and pose a health hazard when inhaled. Regardless of the types of lignocellulosic biomass before and after torrefaction of the biopellets, very few fines were generated, with a fines content of less than 0.06%. This suggests that, even if the biopellets themselves are torrefied, there is no negative effect on the fines content. Our findings on pellet durability and fines content are consistent with previous studies that reported minimal changes in these properties following torrefaction. For instance, Li et al. (2012) found that torrefied pellets retained their mechanical strength and had low fines content, similar to our observations [50].

In conclusion, the torrefaction of biopellets could be considered an ideal technology for producing energy-intensive renewable fuel without any negative effects on the durability or fines content of the biopellets.

Figure 8 compares the bulk density of the biopellets before and after torrefaction. Bulk density is an important parameter for fuel deliveries on a volume basis, and together with the net calorific value, it determines the energy density. Torrefaction reduced the bulk density of the biopellets by removing volatile substances and other extraneous components, regardless of the type of woody feedstock used. This reduced the mass of the biopellets without remarkably changing their volume. As a result, torrefied biopellets can have a higher energy density, facilitating more efficient energy transport and cost-effective storage.

These results are in line with previous research by Bates and Ghoniem (2012), who reported a reduction in bulk density and an increase in energy density for torrefied biomass pellets [51].

Figure 9 is a graph comparing the heating values of the biopellets prepared from various woody biomass before and after torrefaction. Torrefaction treatment increased the heating values of the biopellets by 18–58%, with TA and MT exhibiting the highest increase of 53–58%. HW and CO increased by 48% and 18%, respectively. Nevertheless, the rate of increase in the heating value of the torrefied biopellets varied depending on the type of woody biomass, but the resulting heating values for these biopellets consistently fell within the range of 22 to 24 MJ/kg, indicating a highly favorable heating value. It was finally confirmed that the torrefaction treatment of biopellets made from lignocellulosic biomass was a very useful technology in improving fuel efficiency. The observed increases in heating values are comparable to those reported by Chen et al. (2015), who found that torrefaction significantly enhanced the calorific value of biomass pellets, making them competitive with coal [52].

3.3. Mass and Energy Yield of the Biopellets by Torrefaction

The mass and energy yields of torrefied biomass are important factors to consider when designing and operating torrefaction reactors and when evaluating the potential of torrefied biomass as a fuel for different applications. The mass yield of the torrefied biomass is typically between 50% and 90%, depending on the torrefaction conditions (temperature, time, and atmosphere). Energy yield is typically between 70% and 95%, depending on the torrefaction conditions and the type of woody biomass. Figure 10 shows the mass yield and energy yield of different woody biomass following torrefaction. HW and CO have mass yields in the 60–70% range and energy yields in the 80–90% range. TA and MT have mass yields of about 60% and energy yields of over 95%. This suggests that TA and MT could be used to produce a high-quality torrefied biomass fuel with excellent combustion properties. TA and MT are both high in cellulose content, as indicated in Table 1. This particular component is relatively susceptible to breakdown during torrefaction, contributing to a lower mass yield and a higher energy yield than HW and CO. Similar findings have been reported by van der Stelt et al. (2011), who observed that the mass and energy yields of torrefied biomass are highly dependent on the feedstock type and torrefaction conditions [28].

3.4. Ignition and Combustion of the Biopellets before and after Torrefaction

Figure 11 compares the ignition characteristics and combustion duration of biopellets before and after torrefaction. Torrefaction of the biopellets reduced the initial ignition time by 50–59% and prolonged the combustion duration by 15–24%, regardless of the types of woody raw materials. This can be attributed to the significant removal of moisture and volatile compounds during torrefaction. The absence of these components facilitates easier ignition at lower temperatures, as less energy is required to overcome the latent heat of vaporization and decomposition of volatile compounds. Additionally, the increase in fixed carbon content in the biopellets due to torrefaction contributes to a slower and more uniform combustion rate, as fixed carbon serves as a steady source of energy during the combustion process, influencing its kinetics. These improvements in ignition and combustion properties are consistent with the results reported by Bridgeman et al. (2008), who found that torrefied biomass ignites more easily and burns more uniformly than raw biomass [53].

3.5. TGA Analysis of the Biopellets before and after Torrefaction

Figure 12 compares the thermal decomposition and the corresponding weight loss of the prepared biopellets as a function of temperature before and after torrefaction. Regardless of the types of lignocellulosic raw materials used, the rate of weight loss of the torrefied biopellets was slower than that of the non-torrefied ones. The lower mass loss rate in TGA analysis of biopellets after torrefaction results from torrefaction’s removal of moisture and volatile matter from the biopellets. This explains why the non-torrefied biopellets begin to lose weight before reaching 300 °C, whereas the torrefied biopellets do not commence weight loss until after surpassing 300 °C. Moisture and volatile matter are easily vaporized at relatively low temperatures, while fixed carbon is resistant to decomposition at high temperatures. It is widely recognized that moisture and volatile matter are the two primary components that contribute to the mass loss of biopellets during thermal decomposition. In addition, torrefaction increases the fixed carbon content of biopellets. Fixed carbon is a non-volatile component that does not decompose during thermal decomposition. Therefore, the increase in the fixed carbon content also contributes to the lower mass loss rate of the torrefied biopellets. Notably, MT, with the highest fixed carbon content of approximately 35% after torrefaction, displayed the slowest thermal decomposition characteristics, retaining 60% of its weight even at temperatures as high as 800 °C. This can be attributed to MT’s elevated fixed carbon content, rendering it more resistant to decomposition at elevated temperatures.

Our TGA results are in agreement with those of Chen et al. (2012), who found that torrefied biomass exhibited slower thermal decomposition rates and higher residual mass compared to non-torrefied biomass [54].

3.6. Morphological Comparison of Biopellets before and after Torrefaction

Figure 13 presents the SEM images of bio-pellets, showcasing their morphological characteristics both before and after the torrefaction process. Initially, the bio-pellets exhibited relatively smooth surfaces with minimal cracks. However, post-torrefaction, a significant increase in surface cracking was observed, attributed to the thermal decomposition that occurs during the process. This increase in cracking suggests that the structural integrity of the bio-pellets is notably compromised by the torrefaction process. The thermal decomposition induces thermal stresses, which in turn lead to the breakdown of the internal matrix of the pellets. This structural alteration could adversely affect the mechanical properties and handling of the biopellets, indicating a potential trade-off between achieving desired torrefaction characteristics and maintaining the physical robustness of the final product. To address these challenges, further research may be necessary to optimize torrefaction conditions, with the aim of minimizing structural damage while preserving the beneficial properties of the biopellets. By refining the process parameters, it may be possible to enhance the durability and usability of bio-pellets, balancing the benefits of torrefaction with the need for structural integrity.

3.7. Importance of Torrefied Biopellets

Figure 14 illustrates the key benefits and applications of torrefied biopellets, highlighting their importance in the context of sustainable energy solutions based on our study.

Torrefied biopellets exhibit a significantly higher energy density compared to non-torrefied pellets. This is due to the increased fixed carbon content and reduced volatile matter, leading to enhanced calorific value and more efficient energy production. The torrefaction process reduces moisture and volatile compounds, resulting in biopellets that ignite more easily and burn more uniformly. This leads to lower ignition times and prolonged combustion duration, making them a more efficient and reliable fuel source. The higher fixed carbon content and lower volatile matter result in cleaner combustion with reduced smoke and particulate emissions. This makes torrefied biopellets an environmentally friendly alternative to traditional fossil fuels. Despite the removal of volatile substances, torrefied biopellets maintain excellent durability, with minimal fine particle generation. This reduces the risk of dust explosions and health hazards, ensuring safe handling and storage. The higher energy density and improved combustion efficiency of torrefied biopellets translate to cost savings in transportation and storage. Their use can also reduce dependency on fossil fuels, contributing to energy security and economic resilience. Torrefied biopellets can be used in power plants as a direct substitute for coal, reducing carbon emissions and enhancing the sustainability of energy production. Industries requiring high-temperature processes can benefit from the consistent and high energy output of torrefied biopellets.

In conclusion, the torrefaction of biopellets from unused forest biomass and wood chip residues offers a promising pathway to produce high-quality, energy-dense renewable fuel. This technology not only enhances the fuel properties of biopellets but also contributes to environmental sustainability and economic viability.

4. Conclusions

The torrefaction of lignocellulosic biomass feedstocks prior to pelletizing reduces pelletizing efficiency, necessitating the addition of binders. However, the torrefaction of already-formed biopellets enhances pellet quality and obviates the need for binders during pelletizing. This research demonstrated that torrefaction significantly improves the fuel properties of biopellets made from various woody biomass feedstocks, including hardwood chip residue (HW), camellia oilseed cake (CO), and pruning remnants of the toothache tree (TA) and mulberry tree (MT). The effects of torrefaction varied depending on the type of woody biomass feedstock used. Specifically, torrefaction resulted in a substantial reduction in volatile matter content and an increase in heating value, enhancing the overall energy density of the biopellets. The mass yield of torrefied biopellets ranged between 60% and 70%, while the energy yield was between 80% and 95%. Additionally, torrefaction reduced the initial ignition time of the biopellets by 50–59% and prolonged their combustion duration by 15–24%, making them easier to ignite and burn more uniformly. Torrefied biopellets also exhibited a slower mass loss rate during thermal decomposition compared to non-torrefied biopellets, primarily due to the removal of moisture and volatile matter. This indicates enhanced stability and durability during combustion. Moreover, the use of unused forest biomass and wood chip residues from pulp mills for biopellet production not only provides a sustainable and efficient method for waste utilization but also contributes to environmental conservation by reducing reliance on fossil fuels.

In summary, this research highlights the advantages of torrefaction as a promising technology for producing high-quality, energy-dense solid biofuel from a variety of woody biomass feedstocks. The enhanced fuel properties, improved combustion efficiency, and sustainable utilization of waste biomass underscore the potential of torrefied biopellets as a viable alternative to traditional fossil fuels.