## 3 Results and Discussion  
### The Effect of Airflow Rate on Biochar Surface Properties  
Biochars produced at airflow rates ranging from 8 to 20 L/min were found to be mostly basic (pH \(>\) 7.0). As a result, no acidic functional groups were detected on the surface by Boehm titration characterization [19]. Table 1 presents the results of the pH and the basic functional groups of the biochar when the airflow rate increased in the TLUD gasifier. In general, lower airflow rates resulted in biochar with lower pH. However, no statistical difference in pH was found when analyzing all the biochar samples. The basic functional group of the biochar increased with the airflow rate, which significantly increased from 0.0175 to 0.115 mmol g\({}^{-1}\). The increase in the basic functional group of the biochar was correlated with the peak temperature of the gasification layer that increased from 661 to 840 \({}^{\circ}\)C (R\({}^{2}\) = 0.78). With increasing airflow, the O/C atomic ratio of biochar was significantly reduced from 0.14 to 0.1 which induced carbonization due to aromatization in which polycyclic aromatic structures were formed leading to an increase in aromatic carbon concentration [27]. Similarly, [PERSON] [1] performed microwave pyrolysis experiments of corn stover by increasing the reaction temperature from 550 to 650 \({}^{\circ}\)C. The increase in temperature caused reduction in the O/C ratio of the biochar from 0.06 to 0.04 while the H/C ratio did not change. This suggested a more aromatic-based biochar due to the increase in carbon content and reduction of oxygen (O) [4]. As a result, the basic functional groups of biochar increased. carbon-oxygen groups of acidic characters such as lactone and anhydride can be formed on the surface of carbon materials after air oxidation. However, the temperatures used in both research work were relatively low, 250 \({}^{\circ}\)C and 377 \({}^{\circ}\)C, respectively. [PERSON] et al. [30] suggested that the air-oxidized chars can present carboxylic acid groups based on the assumption that the hydrolysis of carboxylic anhydride occurs in aqueous solutions. This carboxylic anhydride was formed at temperatures above 650 K and carboxylic acids were only promoted at lower temperatures. Since the lowest temperature reported in the present work was 614 \({}^{\circ}\)C, it was likely that no acidic functional groups were formed at higher temperatures when the fixed particle size was tested. In addition, the predominant alkaline nature of the produced biochar in the present study agrees with previously reported data [31]. [PERSON] [21] reported that the oxidation of black carbon at 420 \({}^{\circ}\)C did not present acidic functional groups because of the rapid carbonization of organic materials in the presence of oxygen. The BET surface area was also found to be positively correlated with the peak temperature (R\({}^{2}\) = 0.99) and with the basic functional group concentration of biochar (R\({}^{2}\) = 0.73). This suggests that the increase in peak temperature not only promotes the basic functionality [31], but also enhances the porosity of the carbon-rich material [32]. [PERSON] et al. [33] compared data of biochar produced at varying conditions, and found that the specific surface area of biochar can be enhanced by the micro-porosity. Similarly, it was found that higher reaction temperatures increased biochar porosity. The results of the surface charge of the biochar are also presented in Table 1. The CEC of biochar from wood chips increased from 3.43 to 6.33 cmole kg\({}^{-1}\) when the airflow rate increased from 8 to 20 L/min. This increase in the CEC of the biochar can be associated with the combustion temperature (R\({}^{2}\) = 0.55) that increased with increasing airflow rate. [PERSON] [34] carried out experiments to study the effect of carbon decomposition of oak wood biochar produced with slow pyrolysis.