# Study of Cocoa Pod Husks Thermal Decomposition
## 3 Results and Discussion
### Thermal Decomposition of CPH  
The study of the thermal decomposition of CPH was carried out using the TGA and the method described previously. Figure 1 shows the TG and DTG curves obtained using different atmospheres and heating rates. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Variable** & **Unit** & **Value** \\ \hline H & \% & 6.2 \(\pm\) 0.2 \\ O* & \% & 41.6 \(\pm\) 1.0 \\ S & \% & 0.20 \(\pm\) 0.04 \\ Empirite formula & - & CH\({}_{1.79\pm 0.03}\)O\({}_{0.75}\) \\ H/C molar ratio & & 1.79 \(\pm\) 0.07 \\ O/C molar ratio & & 0.75 \(\pm\) 0.03 \\ HHV ** & MJ kg\({}^{-1}\) & 17.3 \(\pm\) 0.4 \\ Hemicellulose *** & \% & 21.2 \(\pm\) 0.4 \\ Cellulose *** & \% & 23.2 \(\pm\) 0.5 \\ Lignin *** & \% & 15.0 \(\pm\) 0.3 \\ pH & - & 5.7 \(\pm\) 0.1 \\ Electrical conductivity & mS & 6.7 \(\pm\) 0.2 \\ \hline \end{tabular}  
* Obtained by difference; ** Higher heating value obtained from elemental analysis and Gaur—Reed equation [23]; *** Approximation obtained from deconvolution of DTG.  
\end{table} 
Table 1: _Cont._  
Figure 1: Thermal decomposition of cocoa shells in TG using different atmospheres and heating rates. (**a**,**c**) inert atmosphere; (**b**,**d**) oxidizing atmosphere. DTG curves (**e**) inert atmosphere; (**f**) oxidizing atmosphere. The mass loss curves as a function of time (Figure 1a,b) shift to the left (lower times) by increasing the heating rate and, as was expected, the opposite behavior is observed in the mass loss curves as a function of temperature (Figure 1c,d), that shift slightly to the right (higher temperatures). Although this displacement is not very large, it is noticeable, especially under air atmosphere. This type of behavior has been previously observed in other biomass kinetic studies and has been mathematically interpreted using kinetic equations [24; 25; 26]. Thus, it has been shown that only considering the kinetics of the process, without taking into account possible changes in mechanisms or poor heat transfer, the position of the maximum peak of DTG can be expressed as a function of the heating rate [27; 28], so that with a higher heating rate, the maximum peak is located at a higher temperature. Physically, it can be visualized by considering that at low heating rates the sample remains for longer times at each temperature interval, and consequently it has more chances for reacting; the peaks thus appear at lower temperatures as compared with higher heating rates. Furthermore, the magnitude of this displacement depends on the kinetic parameters of the process: the higher the activation energy and the lower the pre-exponential factor, the greater the displacement magnitude [29]. The differences between the curves obtained in an inert atmosphere (Figure 1e) and an oxidizing atmosphere (Figure 1f) are seen around 400 \({}^{\circ}\)C, where the oxidation and combustion reactions of the biomass begin, giving rise to a greater weight loss. In the case of the oxidizing atmosphere, two decomposition zones are clearly distinguished, one more centred around 280 \({}^{\circ}\)C (more related to pyrolysis) and the other around 450 \({}^{\circ}\)C (more related to combustion). A sharp peak around 480 \({}^{\circ}\)C is also observed, influenced by the heating rate (it does not appear at 5 \({}^{\circ}\)C min\({}^{-1}\)) and which has already been observed in the decomposition of other biomasses in an oxidizing atmosphere. This peak would correspond to the reaction of the fraction of the carbonaceous residue that is being formed in the decomposition process and that is more accessible to oxygen (for example char arranged in cracks, as superficial sheets or as fine particles) and that reacts very quickly, while the rest of the residue reacts more gradually. A fourth reaction (appreciable above 600 \({}^{\circ}\)C) can be related to the decomposition and reaction of the inorganic fraction of the biomass.