Release and Transformation of Zn during Waste Tire Pyrolysis using Various Reactors

Waste tires contain abundant zinc (Zn), which could cause severe environmental issues during waste tire pyrolysis. This study presents a systematic investigation into Zn release and transformation behavior during the pyrolysis of waste tires at 400–900 °C using various reactors, including a fixed-bed reactor for slow pyrolysis (Type A), a drop-tube/fixed-bed reactor for fast pyrolysis with continuous feeding (Type B) or pulse feeding (Type C), and a fluidized-bed reactor for fast pyrolysis (Type D). In the Type A reactor, over 90% of Zn was retained in char during waste tire pyrolysis at 400–700 °C, but the Zn release dramatically increased from ∼9.8 to ∼47.0% as the pyrolysis temperature increased from 700 to 900 °C, possibly due to the reduction of ZnO to volatile Zn and the dissociation of ZnS. Compared to the Type A reactor, the high heating rates in the Type B, C, and D reactors caused rapid melting and volatile release, as proven by the micromorphology analysis, resulting in more Zn release at all temperatures. The highest Zn release of ∼67.6% was obtained in the Type D reactor at 900 °C. Using a multistep extraction process, our results clearly showed the transformation of acid-soluble to acid-insoluble Zn at temperatures higher than 700 °C (especially for the Type B and D reactors), likely due to the formation of zinc silicate. The Zn mass balance analysis showed that Zn species in the volatiles were mainly in the inorganic form, which could be easily captured by a bubbler system with diluted HNO3 solution.