## 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 CO\({}_{2}\) 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 torrefraction of already-formed biopellets, which improves their fuel properties and obviates the need for binders during pelletizing. Torrefraction is a way to overcome the disadvantages of wood pellets.