## 1 Introduction  
The introduction and utilization of new and renewable energy sources is increasing in modern society. Biomass will play an important role in energy supply in future energy scenarios because of the use of fossil fuels and severe global warming [1]. Of all the renewable resources, biomass is considered to be the most utilized worldwide, including in both under-developed and developed countries [2]. However, the cultivation, harvesting, drying, storage, transportation, and conversion (e.g., palletization and co-firing) of biomass require careful assessment in any fuel procurement strategy. Recently, the development and use of large biomass sources have been hampered because biomass has been restricted for use as a solid fuel. These restrictions are primarily related to the physical and chemical properties of biomass. Compared to coal, biomass materials have a low volume density, abrasiveness, calorie content, and high moisture content. These restrictions have had a significant impact on the energy conversion efficiency of biomass materials. In addition, the entire biomass-to-energy value chain has been significantly affected by the storage, handling, and transportation of costly biomass [3; 4; 5]. Among the many pretreatment technologies available to overcome the restrictions on biomass, torrefraction is typically performed at 200-300 \({}^{\circ}\)C in an inert atmosphere; it is also an easy process to improve the quality of biomass feedstock. This process is usually performed at low temperatures (<200 \({}^{\circ}\)C) to remove water and preserve trees. However, at 200-300 \({}^{\circ}\)C, the weak structural component of biomass is partially decomposed; therefore, torrefraction is also referred to as mild pyrolysis. Torrefraction preserves most of the mass (>50%) and almost all the energy (approximately 90%) in raw biomass. It also improves biomass grinding by reducing energy consumption. In addition, it improves the flow characteristics and hydrophobicity of biomass, and prevents it from degrading. Thus, torrefraction is considered one of the most promising preprocessing technologies and has been widely studied as a potential component of the biomass burning and gasification process [6; 7; 8; 9]. By torrefraction technology, the energy density and bulk density of biomass can be increased, and the cost of transportation and storage can be reduced [10]. [PERSON] et al. indicated that torrefraction could significantly improve the physical density, energy density, and bulk density of biomass feedstock to effectively utilize storage space and reduce transportation costs. In addition, the biomass becomes brittle after torrefraction due to the breakdown of filaments in biomass by the release of gaseous and volatile products; consequently, the total pore volume and surface area of torrefied fuels is higher than that of the parent biomass [11]. The detailed mechanism of the evolution of pore structures has been explained by [PERSON] as follows: When the temperature is relatively low (approximately 230 \({}^{\circ}\)C), the specific surface area and pore diameter of the torrefied fuel change little compared with raw biomass. At approximately 250 \({}^{\circ}\)C, the pores are enlarged and more open pores are generated due to the increased speed of volatilization of gaseous products. At the same time, volatile tar in the semi-precipitated state may plug some pores to form new pores. This effect complicates the pore structure, thereby decreasing the average pore size and increasing the specific surface area. When the temperature reached approximately 270 and 290 \({}^{\circ}\)C, some pores are closed and restructured, resulting in an increased average pore size and reduced specific surface area [12]. Moreover, the density and porosity according to feedstocks such as woody and herbaceous may be different because the pretreatment of biomass is heavily dependent on the degradation of the constituents in the biomass [13].