Computational study of critical moisture and depth of burn in peat fires

Abstract

Abstract. 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 20 cm 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.

Experimental study of the effect of water spray on the spread of smoldering in Indonesian peat fires

“…The smoldering reaction reaches a temperature of 628 ± 7°C for Papuan peat and 686.9 ± 11°C for South Sumatran peat. These temperatures are similar to previous research which have been conducted with a similar setup [5,16,20,21]. The total time required for smoldering combustion of South Sumatran peat is less than Papuan peat.…”

“…After the igniter activated, the smoldering front moves with high spread rate and decreases while the drying process occurs. This trend is similar to the stages from the heating of water until the char oxidation dominates the smoldering spread [21].…”

“…In contrast with the vertical spread rate, the value of the spread rate in the area near the igniter will be higher due to the radiant heating from the igniter. Huang et al [21] have pointed out that there were four different stages across the smoldering of peat. After the igniter activated, the smoldering front moves with high spread rate and decreases while the drying process occurs.…”

Review of emissions from smouldering peat fires and their contribution to regional haze episodes

“…For these two reasons -easy ignition and difficult suppressionsmouldering fires present the most persistent combustion phenomena on Earth (Rein 2013). Moisture content is the most important property governing the ignition and the spread of smouldering peat fires (Rein 2015;Archibald et al 2017), while oxygen supply and heat transfer are the two mechanisms controlling its spread rate (Huang and Rein 2015;Rein 2015). Real smouldering fires are a multidimensional phenomenon, which includes two main components: in-depth vertical and surface lateral spread (Rein 2013;Huang and Rein 2017).…”

Smoldering of porous media: numerical model and comparison of calculations with experiment

“…Interesting experimental results of smoldering combustion in some porous materials (cellulose, polyurethane, wood char powder) under free convection were obtained in [5][6][7][8]. In [9][10][11][12][13][14] numerical studies of smoldering combustion under free convection were carried out for various porous materials. At the same time, some important aspects of the smoldering in porous objects under natural conditions remain unstudied.…”