Insight into the kinetics and thermodynamic analyses of co-pyrolysis using advanced isoconversional method and thermogravimetric analysis: A multi-model study of optimization for enhanced fuel properties

Abstract

Waste management is quite challenging and it has become a major concern across the globe. The most common wastes are in the form of polymers or plastics that are composed of a mixture of various commodity plastics like PE, PP, PVC, HDPE, LDPE, etc. Pyrolysis is found to be a promising technology for managing wastes as well as converting them into valuable hydrocarbons. The present study investigated to discern the synergism in thermal degradation behavior and to identify the interaction between polymer molecules in co-pyrolysis. The advanced isoconversional model was employed for trustworthy estimation of activation energy as the thermal effects on a reaction deviate the temperature of a sample from the set heating value. The results were also compared with the model-free methods to understand the synergism, reactivity, and thermodynamic system with the progress of the conversion. The activation energy was found to be lowest when the polymers were used in a certain proportion and the synergy effect was more significant when PP content was > 40 % in the mixture. The positive synergetic effect could be attributed to the intermolecular hydrogen transfer from a less stable polymer to a free radical de-propagating chain of the other polymer during thermal degradation. The complex governing thermochemical reaction mechanism was determined using the Criado master plot and the analysis revealed that with increasing PP content in a binary mixture the reaction mechanism shifted from R2 to R3. The high regression coefficient value (R2 > 0.99) in the regenerated TGA profiles signified a good agreement between the theoretical and experimental results. While thermodynamic analysis showed that the process was endothermic and non-spontaneous in nature. Moreover, the radical reaction during co-pyrolysis of polymers enhances intermolecular hydrogen transfer resulting in the formation of iso-alkane and iso-alkene along with secondary radicals, which undergo β-scission to form alkenes/dienes and short primary radicals. As a result, the enhanced intermolecular hydrogen transfer phenomenon initiates the co-pyrolysis reaction at a relatively lower activation energy compared to individual polymer molecules.