## 3 Results and Discussion
### The Effect of Biomass Moisture Content on Biochar Surface Properties  
The pH of the biochar was significantly reduced from 12.0 to 7.43 when the moisture content of the biomass varied from 10 to 14% (Figure 1). However, further addition of moisture did not encourage the reduction of pH of the biochar. As a result, the variation in moisture content did not show acid functional groups in the biochar. Moreover, higher moisture content resulted in more basic functional groups that significantly increased from 0.115 to 0.15 mmol g\({}^{-1}\) (Table 3). The basic functional group exhibited a correlation with the reaction temperature (R\({}^{2}\) = 0.66) and pH (R\({}^{2}\) = 0.81). This effect can be associated with the fact that the O/C atomic ratio was also significantly reduced from 0.1 to 0.068 when the moisture content increased from 10 to 14%, which was also related to more aromatic compounds in the biochar [32]. The O/C atomic ratio was correlated with pH (R\({}^{2}\) = 0.87) that increased at higher O/C ratios. In addition, the H/C ratio was also positively correlated with pH (R\({}^{2}\) = 0.97). All of these indicate that the addition of moisture to the biomass promoted the reduction of the oxidation effect that resulted in lower pH levels. Thus, it caused an increase in the basic functional group of biochar. The BET surface area of biochar was also reduced from 405.3 m2 g\({}^{-1}\) to 352.8 m\({}^{2}\) g\({}^{-1}\) because of the increase in the moisture content of the biomass (R\({}^{2}\) = 0.84). This reduction can also be attributed to the inhibition of oxidation due to the excess of moisture.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{3}{*}{**Conversion**} & \multirow{3}{*}{**Biomass**} & \multirow{3}{*}{**Reaction**} & \multirow{3}{*}{**BET Surface**} & \multicolumn{3}{c}{**Surface Charge**} & \multirow{3}{*}{**pH**} & \multirow{3}{*}{**H/C**} & \multirow{3}{*}{**O/C**} & \multirow{3}{*}{**Ref.**} \\ \cline{4-4} \cline{6-9}  & & & & & & & & & **CEC** & & \\ \hline Pyrosis & Pine & 650 & 285 & 7 & 11 & 4.5 & N/A & N/A & [11] \\ Pyrosis & Oak & 650 & 184 & 8.6 & 18.4 & N/A & N/A & 0.3 & [15] \\ Pyrosis & Corn stover & 700 & N/A & 10 & 9.0 & 5 & 0.29 & N/A & [28] \\ Pyrosis & Red oak & 600 & N/A & 10 & 9.8 & 2.6 & 0.31 & N/A & [28] \\ TLUD & \multirow{3}{*}{Pine} & \multirow{3}{*}{661} & \multirow{3}{*}{56.0} & \multirow{3}{*}{10.39} & \multirow{3}{*}{3.43} & \multirow{3}{*}{1.38} & \multirow{3}{*}{0.27} & \multirow{3}{*}{0.14} & \multirow{3}{*}{**} \\ gasification & & & & & & & & & \\ TLUD & \multirow{3}{*}{Pine} & \multirow{3}{*}{840.50} & \multirow{3}{*}{405.3} & \multirow{3}{*}{12.54} & \multirow{3}{*}{6.33} & \multirow{3}{*}{1.59} & \multirow{3}{*}{0.27} & \multirow{3}{*}{0.10} & \multirow{3}{*}{**} \\ gasification & & & & & & & & & \\ \hline \hline \end{tabular} N/A—value not available in the reference; * results of this study at 8 L/min, particle size 7 mm, 10% moisture content, not compacted biomass; ** results of this study at 20 L/min, particle size 7 mm, 10% moisture content, not compacted biomass.  
\end{table}
Table 2: Properties of biochars produced under different conversion methods. BET: Brunauer-Emmett-Teller; CEC: Cation Exchange Capacity; AEC: anion exchange capacity; TLUD: Top-lit Updraft.  
Figure 1: The pH and reaction temperature of biochar produced under varying biomass moisture contents (airflow 20 L/min, particle size 7 mm, and biomass compactness 0 kg). Different letters represent significant different level at 10%.