THERMOCHEMICAL AND KINETIC ASPECTS OF REACTIONS RELEVANT IN WASTE GASIFICATION PROCESSES

The scarcity of the petroleum resources constitutes the driving force for the development of innovative methodologies for the environmentally sustainable production of fuels and energy. To solve this problem, it becomes necessary to harness the potential energy of wastes, whether industrial or municipal, through the use of valorization techniques, for the production of both energy and alternative fuels. Despite the incineration is the most applied technique when it comes to energy valorization, two more techniques, pyrolysis and gasification, can be used presenting some advantages when compared to the previous one. Pyrolysis is a process of irreversible chemical modification of compounds under the action of heat and in the absence of oxygen, an energy valorization technique, which causes thermal degradation. In the pyrolysis process, the reaction is endothermic and therefore requiring an external source of heat for the reaction to be initiated, always on an indirect way, neither oxygen nor water vapor are ever injected on the reactor. The presence of organic compounds on wastes, especially plastics, is considered an important source of energy. However, most of these plastics contain polyvinyl chloride (PVC). The presence of PVC in wastes causes recycling problems when it is considered a thermal valorization process for its treatment (Borgianni et al. 2002), preventing the use of those residues on these processes, which main goal is the energy recovery (Kamo et al.; Zevenhoven et al. 2002). On wastes treatments through thermal processes, such as pyrolysis, the presence of PVC in wastes containing high levels of chlorine, promotes serial pollution problems, mainly due to the formation of hydrochloric acid, chlorine gas and dioxins (Borgianni et al. 2002). For this type of wastes, the management hierarchy usually considered is the safe disposal on landfill, constituting a serious environmental issue (Kim 2001; Ma et al. 2002). On these context, pyrolysis is considered by several authors (Zevenhoven et al. 2002; Kim 2001; Ma et al. 2002; Jaksland et al. 2000; Qiao et al.; Qing-lei et al. 2007; Saeed et al. 2004), as the most promising technique for the energy recovery from wastes containing PVC, through the thermal degradation of the chlorine molecule. Thus, it is possible to break down this molecule, allowing the chlorine recovery as hydrochloric acid or chloride (Tanaka et al.), with potential economic gains. Considering thermogravimetric analysis, it is assumed that the degradation of PVC occurs between 200 and 400 oC (Zevenhoven et al. 2002). At 250 oC, the decomposition of PVC has already begun, with maximum at approximately 300 oC. At 350 oC the amount of chlorine present in PVC waste is already less than 0.1%, thus the PVC at 350 oC has already released 99.5% of chlorine in it (Zevenhoven et al. 2002). After the process of chlorine removal, there is still some residual chlorine on the waste (Tanaka et al.). The C-Cl bonds in the structure of PVC have a relatively lower binding energy than the C-C and C-H bonds. This justifies the fact that the bonds of chlorine are the first to be broken, thus starting the thermal degradation of PVC (Ma et al. 2002; Qing-lei et al. 2007).