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Front. Mech. Eng. | doi: 10.3389/fmech.2018.00018

The role of heat transfer limitations in polymer pyrolysis at the microscale

  • 1Imperial College London, United Kingdom

Thermal degradation of synthetic and natural polymers is an important process in many fields of engineering such as fire safety, thermal recycling, and biomass power generation. The kinetics of thermal degradation is usually studied by thermogravimetric analysis (TGA), which is based on measuring the mass loss rate of a microscale sample and the temperature of the surrounding fluid during controlled heating. The literature is rich in TGA measurements, which are commonly assumed to be governed solely by kinetics. Heat and mass transfer effects, however, can occur when the sample is insufficiently small in size. Only a few studies quantify a threshold for this maximum size. These thresholds vary due to different input parameters and formulations of the used model. Here, we aim to systematically analyse the role of heat transfer in TGA experiments, quantify the uncertainty of current models, and provide a novel threshold for maximum sample size. We focused on the natural polymer cellulose, a surrogate for biomass, and split the problem into heat transfer within the sample (intraparticle) and between the sample and the fluid (interparticle). Using dimensional analysis we derived two upper bound thresholds for the initial sample mass as a function of heating rate above these thresholds heat transfer effects are significant. One threshold is calculated based on interparticle heat transfer and depends on flow conditions, material and fluid properties. The other is calculated based on intraparticle heat transfer and only depends on material properties. Both thresholds were validated with experiments and previous studies from the literature. Comparing both thresholds shows that the maximum sample mass in a TGA is always limited by interparticle heat transfer. These results enable the selection of appropriate samples masses and heating condition in TGA experiments, which in turn will lead to a better understanding of the pyrolysis chemistry of polymers.

Keywords: Pyrolysis, Thermograv imetric analysis (TGA), Thermal lag, heat transfer, Kinetics, polymer, Cellulose

Received: 12 Aug 2018; Accepted: 26 Oct 2018.

Edited by:

David B. Go, University of Notre Dame, United States

Reviewed by:

Kyle E. Niemeyer, Oregon State University, United States
Michael J. Gollner, University of Maryland, College Park, United States
Wei Tang, National Institute of Standards and Technology, United States  

Copyright: © 2018 Richter and Rein. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

* Correspondence: Prof. Guillermo Rein, Imperial College London, London, United Kingdom, g.rein@imperial.ac.uk