## 3 Results and Discussion  
### 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  
Figure 4: Experimental image of analyzing time for the initial ignition and combustion duration of biopellets: (**a**) pellet ignition by a potable gas torch; (**b**) ignited pellet with a flame.  
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 torrefraction. Torrefraction reduced the volatile matter (VM) content of the biopellets, regardless of the type of woody feedstock. This is because torrefraction drives off the volatile matter in the biomass, resulting in an increase in the fixed carbon (FC) content [46]. Therefore, the torrefiel biopellets with a higher FC content have a higher calorific value and produce less smoke than the non-torrefiel biopellets. Specifically, the VM reduction of the biopellets made from HW and MT feedstocks following torrefraction was notably substantial, ranging from 17% to 24%. This observation can be attributed to the extent of torrefraction 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 torrefiel biomass. For instance, [PERSON] et al. (2011) observed a significant decrease in volatile matter and an increase in fixed carbon content in torrefiel 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 torrefraction. After torrefraction, 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 torrefraction 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 torrefraction. Nitrogen and sulfur are both non-volatile elements, meaning that they do not evaporate at the temperatures used in torrefraction. As a result, their concentration in the biopellets remained relatively constant before and after torrefraction.  
Figure 5: Proximate analysis of the prepared pellets before and after torrefraction.  
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, [PERSON] and [PERSON] (2011) reported similar increases in carbon content and reductions in hydrogen and oxygen contents in torrefied pine and hardwood [49].