Novel Catalytic Systems for Waste Tires Pyrolysis: Optimization of Gas Fraction

In this paper, catalyzed pyrolysis of scrap tires was studied in order to identify the influence of catalysts on gas composition during the main thermal range of the decomposition process. The aim is related to gas fraction optimization in terms of yield, composition, and distribution during the pyrolysis process. This is an original work using for the first time powder catalysts (MgO, Al₂O₃, CaCO₃, and zeolite ZSM-5) uniformly distributed on a single layer of oyster shells (OSs) particles. The catalyst/tires mass ratio was kept for all the tests at 1/30. Depending on used catalyst, pyrolysis products yields ranged from 39 to 42 wt.% for char, from 26 to 38 wt.% for oils, and from 16 to 30 wt.% for gas. Compared to the thermal pyrolysis, it was found that the liquid yield increases in the presence of MgO/OS, while the use of Al₂O₃/OS decreases it significantly. The gas yield grows in the presence of Al₂O₃/OS ranging from 24.6 wt.% (thermal pyrolysis) to 30.6 wt.%. On the other hand, ZSM-5/OS and CaCO₃/OS did not bring significant changes in products yield, but there are considerable influences on the evolution of gas composition during the tires decomposition. Also, two important advantages of using these new catalytic systems are identified. These relate to the formation of gaseous species throughout the waste decomposition, thus harmonizing the calorific value for the entire thermal range, and the disappearance of heavy molecules in liquid fractions, simplifying or canceling further upgrading processes.