# Valorization of Waste Tires by Pyrolysis and Activation Processes
## 3 Results and Discussion
### Gas Fraction Characterization  
The gas fraction is mainly composed of H\({}_{2}\), CO, CO\({}_{2}\), CH\({}_{4}\), C\({}_{2}\)H\({}_{6}\), C\({}_{2}\)H\({}_{4}\), and H\({}_{2}\)S. The changes in the composition of the evolved gas from pyrolysis with respect to time/temperature are presented in Figure 9. Most of the gas was produced after reaching a temperature of 300 \({}^{\circ}\)C. During pyrolysis, G1 and G3 presented the same behavior and composition, while the results of G2 and G4 were very similar to each other, indicating that the chemical activation caused the differences presented in the gas fraction. Some of the differences caused by the chemical activation were the generation of H\({}_{2}\)S at higher temperatures and the presence of H\({}_{2}\), which were only observed with the acid-treated waste tire powder (G2 and G4). When temperature was maintained at 850 \({}^{\circ}\)C, only low concentrations of CO and CO\({}_{2}\) were generated in all processes. Then, during physical activation (Processes 3 and 4), only CO was generated via the Boudouard reaction, in which the solid fraction (carbon) reacts with the injected CO\({}_{2}\) to produce CO. The CO concentration was higher in processes involving the acid-treated tire powder. Of relevance is the observed increase in H\({}_{2}\)S in the gas fraction, which is depicted in Figure 9b,d. H\({}_{2}\)SO\({}_{4}\) is used during chemical activation; therefore, the gas fraction from the chemically activated waste tires presented a slight increase in H\({}_{2}\)S, which coincides with [PERSON] and [PERSON] [34], who reported that, at higher temperatures, the sulfur tends to be distributed in the gas fraction. However, H\({}_{2}\)S is typically found in the pyrolytic gas from waste tires, owing to the decomposition of the sulfur links of the vulcanized rubber [53]. In addition, it has been reported that the thermal decomposition of H\({}_{2}\)SO\({}_{4}\)--adsorbed during the chemical activation--produces SO\({}_{2}\). [54] In addition, the presence of H\({}_{2}\)SO\({}_{3}\) could be also presented in the gas phase [55]. Therefore, further studies carefully analyzing the sulfur distribution on the different pyrolytic fractions when using acid-treated waste tire powder are required. As the presence of sulfur represents an obstacle to the utilization of the pyrolytic products, techniques for their removal should also be considered. Figure 8: Liquid fraction qualitative characterization classified by aliphatic, aromatics, heterocyclic aromatics, compounds with heteroatoms, and unknown compounds. Figure 9: Composition of the evolved gas with respect to time/temperature from (**a**) G1, (**b**) G2, (**c**) G3, and (**d**) G4. Notes: H\({}_{2}\) and CH\({}_{4}\) concentrations in (**b**,**d**) were divided by four for comparison purposes with values obtained in (**a**,**c**); the remaining % in volume corresponds to injected gases (N\({}_{2}\) and CO\({}_{2}\)). The differences in the total % in volume from each gas detected between gases from waste tire pyrolysis and chemically activated tire pyrolysis are presented in Figure 10. As all experiments were performed in a N\({}_{2}\) atmosphere, data were analyzed on an N\({}_{2}\)-free basis. Although H\({}_{2}\)S is slightly higher in acid-treated waste tire powder, its proportion with respect to the total volume is much lower because a greater amount of other gases was generated. In the case of CO\({}_{2}\), it is desirable to keep its percentage at a low level, as it has no energy value. This also occurs in the processes with the chemically activated waste tire powder. Another compound of interest is CH\({}_{4}\). In this case, it is desirable to observe an increase that indicates a greater energy capacity of the gas, which is also observed in the processes using chemically activated waste tire powder.