Smouldering combustion is the slow, low-temperature, flameless burning of porous fuels and the most important phenomenon of wildfires in peatlands. Smouldering fires propagate both horizontally and vertically through organic layers of the ground and can reach deep into the soil. In this work, we develop a one-dimensional computational model of reactive porous media in the open-source code Gpyro. We investigate the vertical in-depth spread of smouldering fires into peat columns 20cm deep with heterogeneous profiles of moisture content (MC), inert content (IC) and density. The model solves the species, momentum and energy conservation equations with five-step heterogeneous chemistry, to predict the transient profiles of temperature, species concentration, reaction rates and depth of burn from ignition to spread and to extinction. Modelling results reveal that smouldering combustion can spread over peat layers with very high MC (>250%) if the layer is thin and located below a thick, drier layer. It is shown that the critical moisture for extinction can be much higher than the previously reported critical MC for ignition (e.g. extinction MC up to 256% for low-IC peat, with critical ignition MC of 117%). The predicted critical MC values and depths of burn are compared with experimental measurements for field samples in the literature, showing good agreement. This study provides the physical understanding of the role of moisture in the ignition and extinction of smouldering peat fires, and explains for the first time the phenomenon of smouldering in very wet peat layers.
