Numerical investigation of downward smoldering combustion in an organic soil column

Abstract

Smoldering combustion of natural organic layers like peatlands leads to the largest fires on Earth. Smoldering wildlfires may propagate slowly for several months, consuming organic matter and threatening to release ancient carbon stored deep in the soil. It has become a topic of global interest linked to ecosystem perturbations, carbon sequestration and climate change. Although experimental studies have revealed the main factors affecting ignition and spread of natural smoldering, there is little modeling work explaining these experiments. This study aims at bridging the gap by developing a numerical model of smoldering combustion spreading downwards through a column of organic soil. Two spreading fronts are considered, drying and combustion, fed by oxygen diffusion through the top ash layer. The model predicts the transient evolution of the temperature profiles in a vertical column, and the smoldering rate for organic soils with different properties. The role of moisture content, inorganic content, bulk density and heat of smoldering combustion in the critical conditions for self-sustained propagation is investigated. The simulation results show that the critical moisture content increases linearly with heat of smoldering combustion, decreases linearly with the inorganic content and decreases with organic bulk density. These results are consistent with previous experimental observations, indicating the model is capable for guiding further experimental studies.