## 4 Discussion  
### Effect of Solar Pyrolysis Temperature on Biochar Production  
The main factors influencing the biochar properties are feedstock types and pyrolysis operational conditions. A solar parabolic dish collector was proposed as a new approach to pyrolysis. The physicochemical parameters of raw sesame stalk (SF) and the corresponding biochar samples obtained by different pyrolysis conditions showed different variations compared to biochar produced under controlled conditions (SB-3) because of the nonuniform temperature during the pyrolysis. That means that the carbonization process by solar radiation of the used reactor was heterogeneous; at the same time, the used chamber design needs some modification to enhance the uniformity of radiation distribution. Further studies are needed to increase the pyrolysis process's residence time during exposure to solar energy to permit the uniform distribution of heat inside the chamber. However, the data revealed a gradient in ash and volatile contents from SB-1 to SB-3 due to the irregular temperature distribution in the solar pyrolysis reactor. However, the temperature around the inner sidewalls of the internal chamber was lower than in the upper part. [PERSON] et al. (2020) observed in their study of the thermochemical properties of sesame stalk pyrolyzed at different temperatures that the volatile gas content decreased from 33.45 to 27.12 with the rising temperature from 350-650 \({}^{\circ}\)C, while the ash percentage increased from 6.57% to 8.20% in the biochar produced at the same temperature [22]. Generally, many researchers have reported that the ash percentage was lower in feedstock compared to biochar and vice versa in the case of volatile percentage due to an increase in inorganic constituents' concentration [11; 22; 23]. All biochar types have higher pH than SF due to the thermal decomposition of easily degradable compounds during the pyrolysis process (carboxylic functional groups) and increasing of basic functional groups [23]. This means that rising pH occurs due to the decomposition of oxygen-rich functional groups [9]. Additionally, rising biochar pH is evidence of the higher ash content and the concentration of nonvolatile inorganic elements, which are affected by the pyrolysis temperature uniformity. [PERSON] et al. (2017) in their study showed that sugar cane bagasse and rice husk biochar produced at 450 \({}^{\circ}\)C had lower pH values (8.40 and 8.33, respectively) than those obtained at 550 \({}^{\circ}\)C (9.00 and 9.44, respectively). This difference could be due to the concentration of non-pyrolyzed inorganic elements [11]. This observation was confirmed by the chemical compositions of different biochar (Table 1), especially basic cations (Ca, Mg, K, and Na), which were higher in different biochar types than in sesame stalk feedstock [9]. All biochar types are generally characterized as non-saline material, whose values are less than the unity. These results agree with other researchers [9; 11; 20; 21]. The dissimilarity in CEC values between SB-1 and SB-2 could be ascribed to the nonuniform pyrolysis temperature distribution in the solar reactor. However, [PERSON] et al., 2022, reported that the difference trend in the CEC values of biochar depends on the biomass type, the ash content in feedstock, the cellulose and lignin contents, and the pyrolysis temperature [9]. For instance, [PERSON] et al., 2015, found that the huge variation in CEC values of biochar derived from diverse manure wastes was due to the difference in ash content, which was higher in chicken manure than pig manure [24]. It was reported that lignin-rich biomass produces a higher CEC than biomass with higher cellulosic content [11; 24]. In addition, the reduction of H and O values is due to the decomposition of the simple organic compound during the pyrolysis process and the release of hydrogen compounds such as light hydrocarbons and small-chain polymers [25]. Decreasing the O/C ratio increases the aromaticity and decreases the degradation rate [22].