## 3 Results and Discussion
### The Effect of Biomass Compactness on Biochar Surface Chemistry  
Figure 3A shows the correlation between pH and biomass compactness (R\({}^{2}\) = 0.91). As the biomass compactness increased from 0 to 3 kg, the pH of the biochar decreased from 12.0 to 0.95. The reaction temperature was positively correlated with pH (R\({}^{2}\) = 0.69), Figure 3B. As a result, it was found that the carboxylic functional groups of the biochar increased from 0 to 0.016 mmol g\({}^{-1}\), while the formation of basic functional groups was surpassed from 0.115 to 0.073 mmol g\({}^{-1}\) (Table 5). The variation in individual functional groups of biochar was highly correlated with reaction temperature and pH. The carboxylic and basic functional groups showed R\({}^{2}\) of 0.91 and 0.95, respectively, as the reaction temperature increased. Similarly, pH was correlated with the carboxylic and basic functional groups with R\({}^{2}\) of 0.85 and 0.87, respectively. The increase in biomass compactness meant that more biomass was available per unit volume within the TLUD gasifier. Consequently, relatively less air was available for the incomplete oxidation reactions, which resulted in lower reaction temperatures that led to a decrease in BET surface area (from 405.3 to 191.8 m\({}^{2}\) g\({}^{-1}\)). The change in the proportion of biomass relative to the available oxygen can also be observed in the O/C ratio that was significantly reduced from 0.1 to 0.063 when the biomass was compacted from 0 to 1 kg. The reduction in the O/C ratio indicated that the reactions were driven in a less oxidative atmosphere. Moreover, further compacting the biomass (>2 kg) did not significantly affect the O/C ratio, which could be attributed to an insignificant change in the peak reaction temperature. The decrease in O/C ratio in biomass pyrolysis processes represents carbonization due to aromatization and dehydrogenation reactions that commonly occur due to changes in the reaction temperature [4]. Therefore, in the TLUD gasification process, the decrease in biochar O/C ratio might be further encouraged by the reduced oxygen relative to the amount of biomass. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Avg. Particle Size (mm)**} & \multicolumn{2}{c}{**Functional Groups**} & \multicolumn{2}{c}{**BET Surface Area**} & \multicolumn{2}{c}{**Atomic Ratios**} \\ \cline{2-6}  & **Carboxylic (mmol g\({}^{-1}\))** & **Basic (mmol g\({}^{-1}\))** & **(m\({}^{2}\) g\({}^{-1}\))** & **H/C** & **O/C** \\ \hline
2 & 0.012 \({}^{\text{a}}\) & 0.059 \({}^{\text{b}}\) & N/A \({}^{\text{*}}\) & 0.356 \({}^{\text{a}\text{b}}\) & 0.113 \({}^{\text{a}}\) \\
7 & 0 \({}^{\text{b}}\) & 0.115 \({}^{\text{a}}\) & 405.3 & 0.271 \({}^{\text{b}}\) & 0.100 \({}^{\text{a}}\) \\
17 & 0.011 \({}^{\text{a}\text{b}}\) & 0.026 \({}^{\text{c}}\) & 50.9 & 0.382 \({}^{\text{a}\text{b}}\) & 0.070 \({}^{\text{b}}\) \\
30 & 0.012 \({}^{\text{a}}\) & 0.012 \({}^{\text{c}}\) & 6.2 & 0.545 \({}^{\text{a}}\) & 0.088 \({}^{\text{a}\text{b}}\) \\ \hline \hline \end{tabular} N/A—Not Available. \({}^{\text{*}}\) BET surface area analysis was not performed due to high tar content in the biochar. Different superscripts represent significant differences in the order of a \textgreater{} b \textgreater{} c.  
\end{table} 
Table 4: Chemical properties of biochar from wood chips produced with varying particle sizes. BET: Brunauer-Emmett-Teller. In previous studies, TLUD gasification was demonstrated as a feasible alternative method for biochar production under varying reaction conditions and biomass physical properties [16; 19]. The yield of pine woodchips biochar varied from 16.2 to 27.1% when the airflow increased from 8 to 20 L/min [16]. Biomass particle size (2 to 30 mm), moisture content (10 to 22%), and compactness (0 to 3 kg) also showed significant impacts on biochar yield, which ranged from 12.2 to 21.8%, 9.9 to 12.0%, and 12.2 to 17.0%, respectively [19].