# A Thermogravimetric Analysis of Biomass Conversion to Biochar: Experimental and Kinetic Modeling
## 3 Results and Discussion
### Single-Step Kinetic Model  
Usually, when the biomass decomposition is supposed to follow a single global reaction scheme, a single-step kinetic model can be used to describe the decomposition of biomass residues. Given our results, the conversion regime for the Starink single-step kinetic model was chosen in the conversion range 0.1-0.7 both for apple and potato peel. The values of \(R^{2}\) coefficients as well as slopes and intercepts for all linear regressions are given in Table 4. The apparent activation energies \(E_{\alpha}\) are plotted with respect to the conversion (\(\alpha\)) in Figure 5. The average value of \(E_{\alpha}\) for apple peel was about 200 kJ \(\cdot\cdot\) mol\({}^{-1}\) and as for potato peel 130 kJ \(\cdot\) mol\({}^{-1}\). Given that the first value of activation energy is higher, this could be due to the complexity of the biomass structure and the way different bonds are arranged in their structure. Also, it may contain some minerals that lead to an increase in the activation energy [58]. Values of the same magnitude for the apparent activation energies have been obtained in other studies published in the literature [68; 69] for different bi  
Figure 4: Isoconversional Starink plots for apple peel (**A**) and potato peel (**B**) thermal decomposition. Multistep Kinetic Model  
In order to identify the decomposition mechanism on each region evidenced in Figure 3, corresponding to the conversion ranges given in Table 5, Coats-Redfren analysis was carried out considering the theoretical forms of \(g(\alpha)\) given in Table 3. The left-hand side of Equation (10) was plotted with respect to \(\frac{1}{\Gamma}\), and the adequacy of degradation mechanism was emphasized based on the R\({}^{2}\) coefficient in each region. For the apple peel thermal decomposition, the degradation mechanism for Stage I for conversion values below 0.1 shows 3D diffusion. This diffusion mechanism is expected, because it is associated with the presence of low volatile compounds in the structure of the bioresidues [70]. For Stage II, taking place for conversion in the range 0.1 to 0.6, the decomposition mechanism corresponds to the theoretical Avrami-Erofeev (A\({}_{4}\)) model, which is a nucleation model where the nuclei growth describes the biomass degradation process, for which, according to the study of [PERSON], [PERSON], and [PERSON] [58] there is some limitation to the nuclei growth. For Stages III and IV, at conversion values above 0.6 the mechanism corresponds to third-order reaction F\({}_{3}\), probable to the internal rearrangement of chemical structure rich in carbon [71]. The validation of the models proposed for each stage can be carried out based on the correlation coefficients in Table 4 as well as by the parity diagrams given in Figure 6 for apple peel decomposition and Figure 7 for potato peel decomposition stages. \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{\(\alpha\)} & \multicolumn{3}{c}{**Apple Peel**} & \multicolumn{3}{c}{**Potato Peel**} \\ \cline{2-7}  & **Slope** & **Intercept** & **R\({}^{2}\)** & **Slope** & **Intercept** & **R\({}^{2}\)** \\ \hline
0.1 & \(-4.17\times 10^{4}\) & 123.54 & 0.996 & \(-7.42\times 10^{3}\) & 15.98 & 0.972 \\ \hline
0.2 & \(-1.90\times 10^{4}\) & 48.91 & 0.987 & \(-9.49\times 10^{3}\) & 22.08 & 0.934 \\ \hline
0.3 & \(-1.44\times 10^{4}\) & 33.59 & 0.981 & \(-7.03\times 10^{3}\) & 13.61 & 0.973 \\ \hline
0.4 & \(-1.76\times 10^{4}\) & 41.42 & 0.975 & \(-1.57\times 10^{4}\) & 38.59 & 0.972 \\ \hline
0.5 & \(-2.49\times 10^{4}\) & 58.60 & 0.925 & \(-2.56\times 10^{4}\) & 63.94 & 0.957 \\ \hline
0.6 & \(-2.94\times 10^{4}\) & 66.10 & 0.848 & \(-2.19\times 10^{4}\) & 48.52 & 0.968 \\ \hline
0.7 & \(-3.18\times 10^{4}\) & 67.00 & 0.800 & \(-1.91\times 10^{4}\) & 36.85 & 0.997 \\ \hline \end{tabular}
\end{table} 
Table 4: Starink method results. Figure 5: Apparent activation energies for different conversion values (apple peel—(A); potato peel—(B)).