# Valorization of Waste Tires by Pyrolysis and Activation Processes
## 3 Results and Discussion  
### Feedstock Characterization  
Table 1 shows the results of the proximal and elemental analyses conducted on the waste tire powder and acid-treated waste tire powder. The waste tire powder results are in good agreement with those reported by other authors [35; 36]. A slight increase in moisture is observed after H\({}_{2}\)SO\({}_{4}\) activation, which may be associated with an increased surface area and its consequent absorption properties. Volatile matter and ash content decreased with the acid treatment, which leads to a higher fixed carbon content, which could indicate a carbonization of the material. Regarding the elemental analysis, both materials have a high carbon content. The presence of sulfur is attributed to the vulcanization process performed during tire manufacturing, and, although it is present in both materials, the acid-treated waste tire powder has a higher content of sulfur, probably owing to the type of acid used during activation. The higher heating value of the waste tire powder was 36.62 \(\pm\) 0.18 MJ/kg and the lower heating value was 34.79 \(\pm\) 0.18 MJ/kg. These results are similar to those found in other investigations [37; 38], which are highly dependent on the type and manufacturer of the waste tire [34], mainly by the variations of natural rubber/synthetic rubber composition. Figure 3 shows the thermogravimetric analysis conducted on both materials. During the waste tire powder decomposition, three mass losses were observed. Up to 250 \({}^{\circ}\)C, the loss of moisture and additives occurs; between 250 and 480 \({}^{\circ}\)C, the natural rubber and styrene-butadiene rubber decomposition reaches almost 50% of the initial mass; and, after 450 \({}^{\circ}\)C, the butadiene rubber decomposes [39]. However, acid-treated waste tire powder loses mass at a lower temperature (\(\sim\)150 \({}^{\circ}\)C). This is because of desulfonation reactions where H\({}_{2}\)SO\({}_{3}\) is eliminated, causing the formation of unsaturated groups in the hydrocarbon, which leads to a better char-forming material [26]. The derivative thermograms showed a decrease on the intensity peak associated to the decomposition of rubbers on the acid-treated waste tire, which is consistent with the decrease of the volatile matter content occurred on this sample. Finally, after 700 \({}^{\circ}\)C, the degradation of the waste tire powder is diminished, while the acid-treated waste tire powder required a higher temperature. It is reported that waste tire pyrolysis is complete around 600 \({}^{\circ}\)C [38]. However, we observed \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Waste Tire Powder** & **Acid-Treated Waste Tire Powder** \\ \hline Moisture (wt\%) & 1.12 \(\pm\) 0.09 & 1.65 \(\pm\) 0.19 \\ Ash (wt\%) & 6.55 \(\pm\) 0.23 & 3.94 \(\pm\) 0.07 \\ Volatile matter (wt\%) & 67.16 \(\pm\) 0.04 & 47.04 \(\pm\) 0.58 \\ Fixed carbon a (wt\%) & 25.16 \(\pm\) 0.18 & 47.37 \(\pm\) 0.46 \\ Nitrogen (wt\%) & 0.32 \(\pm\) 0.01 & 0.26 \(\pm\) 0.25 \\ Carbon (wt\%) & 81.52 \(\pm\) 0.08 & 71.63 \(\pm\) 0.28 \\ Hydrogen (wt\%) & 7.34 \(\pm\) 0.09 & 4.02 \(\pm\) 0.12 \\ Sulfur (wt\%) & 1.76 \(\pm\) 0.09 & 6.93 \(\pm\) 0.19 \\ Oxygen and other a (wt\%) & 9.06 \(\pm\) 0.13 & 17.16 \(\pm\) 0.56 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Proximal and elemental analyses of the waste tire powder and acid-treated waste tire powder. that the decomposition of waste tires ended after 700 \({}^{\circ}\)C, which can be attributed to the high heating rate used. Using a heating rate of 10 \({}^{\circ}\)C/min, [PERSON] and [PERSON] [40] reported that pyrolysis is completed around 500 \({}^{\circ}\)C; however, [PERSON] and [PERSON] [41] analyzed the pyrolysis process at different heating rates, concluding that all samples tested required higher temperatures to complete the thermal degradation when higher heating rates were used, demonstrating that increasing the heating rate results in shifting the loss of mass towards higher temperatures.