Experimental and theoretical study on the smoldering combustion of size-fractioned forest duff particles

•Particle size mainly affects pyrolysis process through heat transfer.•Particle size mainly affects fuel oxidation through both heat and mass transfer.•Particle size was found to positively affect the kinetic-controlled char oxidation.•Particle size was found to negatively affect the diffusion-controlled char oxidation.•Diverse properties of size-fractioned particles alter kinetics, heat and mass transfer. Smoldering combustion process has been suggested as a new method potentially useful in the treatment of biosolids. Natural forest duff (FD) often consists of organic fuel layers with varying particle sizes, yet the influence of the size-fractioned particles on the smoldering combustion dynamics in terms of the heat and mass transfer is not well understood. In this study, the oxidative pyrolysis and smoldering behavior of FD samples with four particle sizes (0 < d1 ≤ 0.425 mm, 0.425 < d2 ≤ 1 mm, 1 < d3 ≤ 2 mm, 2 < d4 ≤ 4 mm) were experimentally and theoretically investigated. Micro-scale thermo-gravimetric (TG) analysis, and a four-step kinetic model incorporating water evaporation, FD pyrolysis, FD oxidation and char oxidation showed that the activation energy of the FD pyrolysis and the ash content are negatively correlated, while the activation energy of the char oxidation increases from 87.26 kJ mol−1 to 119.22 kJ mol−1 with the particle size increasing from d1 to d4. Furthermore, the order of the combustion performance of the FD samples is shown as d1d3) of the combustion performance found in the TG tests. A simplified heat transfer analysis qualitatively revealed the beneficial effects of the size-fractioned particles on the smoldering spread rate, while a mass transfer analysis revealed the favorable and adverse influences of the particle size on the kinetic-controlled and diffusion-controlled char oxidation rate, respectively. These findings verify that particle sizes alter the FD physicochemical properties (e.g., specific surface area, bulk density, porosity, permeability, chemical components, and lower calorific value), which in return impact the chemical kinetics, heat and mass transfer process in smoldering combustion. This work provides new insights in