Laboratory investigation of the ignition and spread of smouldering in peat samples of different origins and the associated emissions

Tackling peatland wildfires, the largest fires on Earth in terms of fuel consumption, is an emerging combustion topic in the context of climate change. The understanding of fundamental smouldering dynamics and their application to peatlands are essential to mitigation methodologies’ development, but not yet fully understood in the literature. Most of the previous laboratory smouldering studies used horticultural peat which has a great advantage in controlling the influential factors, but has lower bulk density, and is not representative of some higher bulk density peat found in the field. In this thesis, a series of laboratory experiments were conducted to investigate the critical ignition conditions and governing fire spread parameters of smouldering in peat of various origins, and to quantify the associated emissions. To better understand natural variations, field samplings were conducted in Sumatra, Indonesia and Flow Country, Scotland. In addition, five types of horticultural peat were studied. The results show high bulk density peat from long-term drained peatlands experiences more extensive burning in terms of the amount of carbon and particle emitted, while newly drained peat with low bulk density is more vulnerable to fire in terms of easier ignition and faster fire spread. Evidence was found in this thesis that the heat sink density and the organic density, not only control horizontal spread and in-depth spread, but also determine ignition probability. Furthermore, as smouldering is multidimensional, a critical angle of spread direction (65˚ relative to horizontal plane) above which smouldering cannot self-sustain was found. By studying the emissions of different types of peat, the modified combustion efficiency (MCE) range for smouldering was broadened to 0.74 – 0.88, and found to be significantly dependent on the fuel composition. This thesis provides a better understanding of how smouldering wildfires start and spread in different types of peat and the associated emissions, thus contributing to prevention and mitigation.Open Acces

GAMBUT field measurement of emissions from a tropical peatland fire experiment: from ignition to spread to suppression

Background Accurate quantification of emissions from peatland wildfire is crucial for understanding their feedback to the atmospheric and Earth system. However, current knowledge on this topic is limited to a few laboratory and field studies, which report substantial variability in terms of the fire emission factors (EFs). Aims We aim to understand how emissions vary across the life cycle of a peatland fire. Methods In August/September 2018, we conducted the largest and longest to-date field-scale experimental burn on a tropical peatland in Sumatra, Indonesia. Field measurements of gas emissions from the fire experiment were conducted using an open-path Fourier transform infrared spectroscopy to retrieve mole fractions of 11 gas species. Key results For the first time, we calculated and reported EFs from 40 measurement sessions conducted over 2 weeks of burning, encompassing different fire stages (e.g. ignition, smouldering spread, and suppression) and weather events (e.g. rainfall). Our findings provide field evidence to indicate that EFs vary significantly among fire stages and weather events. We also observed that the heterogeneous physicochemical properties of peatland site (e.g. moisture content) influenced the EFs. We also found that modified combustion efficiency was highly sensitive to complex field variables and could introduce large uncertainties when determining the regimes of a peat fire. Conclusions and implications Further studies to investigate peat fire emissions are needed, and more comprehensive mapping of peatland heterogeneity and land use for emissions inventories, accounting for spatial and temporal variability in EFs since the initiation of a fire event is required

GAMBUT field experiment of peatland wildfires in Sumatra: from ignition to spread and suppression

Peat wildfires can burn over large areas of peatland, releasing ancient carbon and toxic gases into the atmosphere over prolonged periods. These emissions cause haze episodes of pollution and accelerate climate change. Peat wildfires are characterised by smouldering - the flameless, most persistent type of combustion. Mitigation strategies are needed in arctic, boreal, and tropical areas but are hindered by incomplete scientific understanding of smouldering. Here, we present GAMBUT, the largest and longest to-date field experiment of peat wildfires, conducted in a degraded peatland of Sumatra. Temperature, emission and spread of peat fire were continuously measured over 4-10 days and nights, and three major rainfalls. Measurements of temperature in the soil provide field experimental evidence of lethal fire severity to the biological system of the peat up to 30 cm depth. We report that the temperature of the deep smouldering is 13% hotter than shallow layer during daytime. During night-time, both deep and shallow smouldering had the same level of temperature. The experiment was terminated by suppression with water. Comparison of rainfall with suppression confirms the existence of a critical water column height below which extinction is not possible. GAMBUT provides a unique understanding of peat wildfires at field conditions that can contribute to mitigation strategies