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

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

“…Figure 4 shows that flow divergences due to non-uniform air flux lowered the effective axial convection along the reactor centreline. This is because the centreline was the hottest region in the reactor and had the greatest resistance to flow (with the highest air densities and lowest air viscosities) (Lutsenko, 2018;Rashwan et al, 2021a;Zanoni et al, 2021). This is shown by comparing the black and grey curves in Fig.…”

“…All low MC experiments (< 70% MC) dried the sludge to the desired MC and were used to better understand the sensitivity to MC. Additional details on the established experimental methods are available in (Pironi et al, 2011;Pironi et al, 2009;Rashwan et al, 2016;Rashwan et al, 2021a;Switzer et al, 2009;Switzer et al, 2014), and the experimental setups, sewage sludge properties, and methodologies used are further detailed in the Supplementary Materials, Section S.1.…”

“…The experiments used varying fuel fractions, moisture content, and air fluxes, and therefore fostered a broad spectrum of robustness behaviour. The methodology from (Rashwan et al, 2021a) was used to analyze the heat transfer within the cooling zone affected by non-uniform air flux, where a summary of that methodology is included in the Supplementary Materials, Section S.2. Section S.2 also identifies the characteristic heat transfer time scales, which are discussed later in Section 3.2.…”

“…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.…”

Understanding, controlling and optimising the cooling of waste thermal treatment beds including STARx Hottpads

“…to endothermic reactions like pyrolysis) and the front progresses without any external energy input. As such, smouldering is a highly energy-efficient process ( Rashwan et al, 2021a , 2021b ; Torero et al, 2020 ; Yermán et al, 2015 ). Smouldering is also self-terminating as the reaction extinguishes upon fuel depletion ( Crielaard et al, 2019 ; Switzer et al, 2014 ; Wang et al, 2021 ).…”