Delineating and explaining the limits of self-sustained smouldering combustion

Abstract

Self-sustained, forward smouldering combustion is both a major fire hazard that resists engineering control and an applied technology for destroying organic contaminants and wastes. In both contexts, success depends on understanding the threshold between self-sustaining and extinction conditions as well as the system's sensitivity to parameters that drive it in the desired direction. In this work, a previously validated one-dimensional numerical model was employed to simulate a wide range of bitumen-contaminated sand scenarios, quantifying the complex interplay between chemical reactions and heat transfer processes evolving in space and time during smouldering. It was confirmed that the traditional, local (smoulder front) energy balance becomes negative when the reaction is extinct. However, the work reveals that a global (bed) energy balance always becomes negative earlier, predicting extinction conditions despite active smouldering. Progress towards extinction looks similar for all cases, regardless if caused by low air flux, low fuel concentration, very low oxygen content, low fuel energy content, or high heat losses. Moreover, the cause is always similar: the energy gained by oxidation is exceeded by global heat losses, which are significant and neglected in the local energy analysis. Smouldering robustness was shown to be quantified by the degree to which the global energy balance exceeds zero. Robustness was promoted most effectively by increases in injected air flux, fuel concentration, and fuel energy content. These parameters also were the dominant influence on the peak temperature, since that was shown to be dependent on the local net energy rate. However, the front velocity (i.e., mass destruction rate) was shown to depend on the rate that energy convectively exits the front, which was controlled primarily by the air flux. Taken as a whole, these results provide a new way of understanding the balance of energy components that dictate the behavior of smouldering systems and provide novel insights into manipulating them, towards extinction or robust conditions, for a wide range of applications.