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Volume 113
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Assessment of the interplay between convective heat transfer and reaction kinetics during plastic pyrolysis (Open Access)
Feichi Zhang a *, Muhao Li a, Xiaoyue Ma a, Niklas Netsch a, Salar Tavakkol a *, Thorsten Zirwes b, Rui Zhang c, Dieter Stapf a
a Institute for Technical Chemistry, Karlsruhe Institute of Technology, Kaiserstr.12, 76131, Karlsruhe, Germany
b Institute for Reactive Flows, University of Stuttgart, Pfaffenwaldring 31, 70569, Stuttgart, Germany
c Nanjing University of Science and Technology, Xiaolingwei Street, 210094, Nanjing, China
10.1016/j.partic.2026.03.019
Volume 113, June 2026, Pages 180-188
Received 30 November 2025, Revised 17 February 2026, Accepted 8 March 2026, Available online 27 March 2026, Version of Record 4 April 2026.
E-mail: Feichi.Zhang@kit.edu; salar.tavakkol@kit.edu
Highlights

• Revealed the interplay between convective heating and reaction kinetics in plastic pyrolysis.

• Used the Pyrolysis number to delineate regimes governed by heating rate versus reaction rate.

• Built a predictive model for pyrolysis time to guide reactor design optimization.

• Pinpointed optimal heating conditions at a Pyrolysis number near unity.

• Quantified the influence of intra-particle conduction versus convection on the overall pyrolysis conversion.


Abstract

This work numerically investigates the pyrolysis of five common thermoplastics using a homogeneous, single-particle model (0D) to elucidate the interplay between convective heat transfer and reaction kinetics. The results reveal a fundamental competition between external heat supply and the endothermic cooling effect of the reaction, manifesting as a temperature plateau where heat input is balanced by the reaction enthalpy. We demonstrate that enhanced heat transfer—achieved via smaller particle sizes or higher Nusselt numbers—shifts the process toward higher reaction rates and temperatures. To quantify this behavior, we utilize the Pyrolysis number (Py), defined as the ratio of the characteristic chemical reaction time to the convective heat transfer time. A universal inverse correlation is identified between the dimensionless pyrolysis time and Py, valid across all investigated polymers and operating conditions. This correlation delineates two distinct operational regimes: reaction-limited control (Py > 1) and convective-heating limited control (Py < 1). These findings provide a predictive framework for optimizing heating rates and estimating residence times for complete conversion. Finally, comparison with particle-resolved (1D) simulations shows that neglecting intra-particle heat conduction causes faster heating and pyrolysis conversion, thereby underestimating the overall pyrolysis duration.

Graphical abstract
Keywords
Plastic pyrolysis; Chemical recycling; Pyrolysis reaction; Heat transfer; Pyrolysis number
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