Experimental and numerical investigation of weak, self-sustained conditions in engineered smouldering combustion

Abstract

Engineered smouldering applications require self-sustaining conditions where the fuel is consumed without any addition of energy beyond ignition. In practical applications, it is preferred to create scenarios that are robust (i.e., far from extinction). In this work, smouldering column experiments were conducted, spanning conditions from robust self-sustaining through weak self-sustaining and to extinction by systematically lowering the applied air flux and the fuel concentration. A previously developed onedimensional numerical model, which was validated under robust smouldering conditions, was used to simulate the experiments. Experiments showed front deformation in the weak and extinction cases due to multi-dimensional effects. This deformation resulted in a redistribution of the air flow, altering the global energy balance at the centre-line. The model was found to be unable to accurately reproduce these weak and extinction cases with the original validation parameters. A sensitivity analysis revealed that treating the heat of combustion as a fitting parameter, and letting it vary beyond its realistic maximum, enabled the model to create a condition of sufficient energy at the centre-line and was the best way to quantitatively simulate the weak and extinction cases for this specific scenario. This reveals that the model assumptions, eff Effective g Gas 0 Initial oxid Oxidation pyr Pyrolysis rad Radiation rem Removed s Solid/bitumen sp Sphere

Heat losses in applied smouldering systems: Sensitivity analysis via analytical modelling

“…This is often observed in robust smouldering within inert media (e.g., [42,43,46,47]). Furthermore, near-wall extinction, which is sometimes observed in laboratory experiments as a thin ring of char inside the reactor wall [41,48], was neglected.…”

Heat losses in a smouldering system: The key role of non-uniform air flux

“…However, if this buffer against extinction is relatively small, but the Damköhler number remains above the critical value, weak smouldering occurs. This happens, for example, due to low oxygen flux or low fuel concentration [13,30,35,43,46,66,74,[77][78][79]. Under such conditions, slight changes in ( ̇^=__ , can strongly affect weak smouldering characteristics, dropping peak temperatures and propagation velocities [66].…”

Scaling up self-sustained smouldering of sewage sludge for waste-to-energy

“…No mass loss curve in Fig. 6 extends completely to -1, as some sewage sludge remained unburned along the edges in all experiments (common to many smouldering systems, e.g., (Pironi, 2009;Pironi et al, 2009;Rashwan, 2015;Zanoni et al, 2020b)). However, Fig.…”

“…This suggests that larger reactors may be ideal for encouraging more robust self-sustained smouldering conditions. However, perimeter heat losses may also negatively influence the smouldering front by changing the distribution of air flux (i.e., temperature gradients in the cooling zone can lead to non-uniform air flux) (Rashwan et al, 2021a) and/or form an unreacted crust near the wall (i.e., temperature gradients in the reaction zone ca lead to non-uniform reactions) (Pironi et al, 2009;Rashwan et al, 2021a;Zanoni et al, 2020b). These potential negative effects -and the balance between the positive and negative effects at scaleare not well understood.…”