Modelling oxygen-limited and self-sustained smoldering propagation: Thermochemical treatment of food waste in an inert porous medium

• Published in: Chemical engineering journal (Lausanne, Switzerland : 1996) Switzerland : 1996), 2023-07, Vol.468, p.143539, Article 143539
• Main Authors: Song, Zeyang, Zanoni, Marco A.B., Rashwan, Tarek L.
• Format: Article
• Language: English
• Subjects: Food waste; Low moisture content; Reaction-transport coupling; Smoldering reactor; Smoldering treatment; Thermochemical wave
• ISSN: 1385-8947
• DOI: 10.1016/j.cej.2023.143539

•Modelling of food-waste smoldering propagation in sand was developed and validated.•Proposed model advanced in determining kinetic-/oxygen-transport-limiting regimes.•Robust/weak smoldering was dominated by oxygen-transport-/kinetic-limiting regimes.•Non-uniform flow in the reactor was mainly caused by temperature difference.•Multi-direction propagation might take place for weak smoldering propagation. Smoldering treatment is emerging as a valuable engineering tool for many processes, including food waste treatment. However, smoldering systems are currently not well-understood nor optimized. Therefore, numerical models provide invaluable insight into the process dynamics, which improves our understanding and supports the development of novel systems. These smoldering models couple heat, mass, and momentum transfer with pyrolysis and oxidation chemical reactions within porous media. While recent models have untangled many aspects of these systems, local oxygen transport rates from bulk flow to the fuel surface are still not well-resolved. In this work, local oxygen-transport equation was approximated by an analytic derivation based on the gas–solid oxygen non-equilibrium hypothesis. With the improved oxygen-transport equation, a 2D model with five-step reaction scheme for smoldering propagation of food waste in sand was developed. Kinetic parameters obtained from TG experiments were incorporated into the bed-scale smoldering propagation model. The developed model was validated with experimental data that stretched from robust to weak smoldering propagation. It was demonstrated that the developed model matches well with experiments. Furthermore, this model revealed: (i) the emergence of non-uniform gas flow in the reactor, (ii) the evolution of the kinetic- and oxygen-transport-limiting regimes, and (iii) valuable insight into the fundamental changes with smoldering robustness.