Downward spread of smouldering peat fire: the role of moisture, density and oxygen supply

Abstract

Abstract. Smouldering fires in peatland are different from the flames in wildland fires. Smouldering peat fire is slow, low-temperature and more persistent, releasing large amounts of smoke into the atmosphere. In this work, we experimentally and computationally investigate the vertical downward spread of smouldering fire in a column of 30 cm-tall moss peat under variable moisture content (MC) and bulk density. The measured downward spread rate decreases with depth and wet bulk density, and is ,1 cm h À1 equivalent to a carbon emission flux of 200 tonnes day À1 ha À1 . We observe that downward spread increases as MC increases substantially at least inside the range from 10 to 70%, which is not intuitive and goes against the trend observed for the horizontal spread in the same peat. We also conduct one-dimensional computational simulations to successfully reproduce the experimental observations. The analysis shows that the spread rate increases with MC and decreases with density because smouldering spread is controlled by the oxygen supply. The volume of the porous peat expands when absorbing water, which reduces the density of organic matter and decreases the heat release rate. This shows that the widely assumed conclusion that the spread rate of wildfire decreases with MC is not universal when applied to smouldering fires.

Computational study of the effects of density, fuel content, and moisture content on smoldering propagation of cellulose and hemicellulose mixtures

“…For validation, the bulk density of cellulose and hemicellulose were measured experimentally [12]. The bulk density of char and ash were calculated using the relations ρ char ≈ 0.25 × ρ cellulose [11] and ρ ash ≈ IC/100×10×ρ cellulose , where IC stands for inorganic content [15]. The IC for cellulose and hemicellulose is taken as 0.3% and 1.7% respectively [16].…”

“…For simulations with moisture content, the natural expansion process during water absorption is taken into account. To account for this process, we applied a correlation to calculate the dry bulk density (ρ dc ) and wet bulk density (ρ wc ): ρ dc = (170 + 40M C)/(1 + M C) and ρ wc = (170 + 40M C) = ρ dc (1 + M C), respectively [11]. This correlation was developed for peat, which has a porosity of around 0.91, close to that of cellulose at 0.88; in contrast, the porosity of hemicellulose is around 0.53.…”

“…Peat, for example, can have MC ranging from 10 to 300% depending on the weather conditions in a given region [10]. Huang and Rein [11] recently showed that downward propagation speed of smoldering increases with increasing moisture content for peat both experimentally and computationally for a range of moisture content from 0 to 70%. They attributed this increase in spread rate to enhancement of thermal conductivity and reduction in the density of fuel due to addition of water [11].…”

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

“…Smouldering peat fires propagate horizontally and vertically through deep layers, leading to the largest fires on Earth (in terms of mass of fuel consumed per unit surface) (Rein 2013;Huang and Rein 2017). They are frequently reported in tropical, temperate and boreal regions including North America, Siberia, the British Isles, the sub-Arctic, south-east Asia and southern Africa (for example, Botswana in 2000, Scotland in 2006, the USA in 2004, central Russia in 2010 and Indonesia in 2015) (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). Complex behaviours like the 'overhang' phenomenon, where smouldering spreads fastest beneath the free surface, is formed by the spread rate difference between the top and lower peat layers as well as the competition between oxygen supply and heat losses (Fig.…”