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Density of wet peat (i.e. mass of peat and water/total volume) and density of dry peat (i.e. mass of peat/total volume) vs. moisture content (MC) with experimental uncertainty.
Source publication
Smouldering combustion is the driving phenomenon of wildfires in peatlands, and is responsible for large amounts of carbon emissions and haze episodes world wide. Compared to flaming fires, smouldering is slow, low-temperature, flameless, and most persistent, yet it is poorly understood. Peat, as a typical organic soil, is a porous and charring nat...
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Tackling peatland wildfires, the largest fires on Earth in terms of fuel consumption, is an emerging combustion topic in the context of climate change and environmental protection. The understanding of the basic mechanisms of ignition of peat to initiate self-sustained smouldering is essential in the development of mitigation technologies, but it i...
Wildfires can be divided in two types, flaming or smouldering, depending on the dominant combustion processes. Both types are present in most wildfires, and despite being fundamentally different in chemical and physical terms, one transitions to the other. Traditionally, science has focused on flames, while smouldering is often misinterpreted. But...
Citations
... The wind speeds used in these tests were commensurate with velocities used in other small-scale experiments [42,51,52]. For example, Christensen et al. used 0.86 m/s in [42] and noted that a higher windspeed resulted in the movement of the fuel particles (peat) in the wind. ...
Over the last 20 years, all states within the US have required all cigarettes sold to be “fire safe” or “fire standards compliant” meaning that they must pass ASTM standard E2187. Though these cigarettes are designed to self-extinguish, there have been recent studies suggesting that these “fire safe” cigarettes (FSCs) can still ignite mattresses and other furnishings, but there has been no guidance for fire investigators whether FSCs can ignite natural fuels, such as duff and needles, that can be the source of a wildland fire. This work sets out to investigate whether FSCs can indeed be the ignition source of wildland fuels. Experiments were conducted by placing “fire safe” cigarettes burned a fixed length (1 cm) onto fuel beds of two surrogate fuel types placed at the outlet of a wind tunnel and under a halogen lamp to mimic a sunny day. The fuel beds consisted of either a bed of partially chopped pine needles or a layer of whole needles on top of a layer of peat. Five replicates with three wind speeds were tested. Mass loss rates of the fuel beds were recorded, and the experiments documented using both a visual and infrared camera. In nearly every case, smoldering ignition was seen that sustained propagation and spread well away from the cigarette, even when the cigarette appeared to self-extinguish. These results clearly indicate that “fire safe” cigarettes can indeed still start wildland fires, particularly in dry and windy conditions.
... In contrast, controlled laboratory experiments enable the isolation and investigation of specic peat combustion variables, such as moisture content, burn temperature and inorganic content, and their impact on re dynamics and emissions. 18,[42][43][44][45][46][47] Laboratory experiments have also provided frameworks for the peat smouldering progression which include ignition, growth, steady, and burn-out stages. 18 The combustion stages are determined through the monitoring of CO 2 and CO emissions, temperature proles, and MLR: the measurements show very little variation during the steady stage of the re. ...
Peat fires emit large quantities of particles and gases, which cause extensive haze events. Epidemiological studies have correlated wildfire smoke inhalation with increased morbidity and mortality. Despite this, uncertainties surrounding particle properties and their impact on human health and the climate remain. To expand on the limited understanding this laboratory study investigated the physicochemical characteristics of particles emitted from smouldering Irish peat. Properties investigated included number and mass emission factors (EFs), size distribution, morphology, and chemical composition. Fine particles with a diameter less than 2.5 μm (PM2.5), accounted for 91 ± 2% of the total particle mass and the associated mass EF was 12.52 ± 1.40 g kg⁻¹. Transmission electron microscopy imaging revealed irregular shaped metal particles, spherical sulfate particles, and carbonaceous particles with clusters of internal particles. Extracted particle-bound metals accounted for 3.1 ± 0.5% of the total particle mass, with 86% of the quantified metals residing in the fraction with a diameter less than 1 μm. Redox active and carcinogenic metals were detected in the particles, which have been correlated with adverse health effects if inhaled. This study improves the understanding of size-resolved particle characteristics relevant to near-source human exposure and will provide a basis for comparison to other controlled and natural peatland fires.
... Flaming combustion encompasses both surface fires and canopy fires; smoldering combustion mainly occurs in soil humus or peat layers and spreads on the surface and underground, also referred to as forest ground smoldering combustion [3,4]. Forest ground smoldering combustion is an incomplete burning process with low temperatures; it is flameless and slow-spreading and spreads on the surface and underground [5]. The detection and suppression of smoldering combustion present a significant challenge, so it can persist for months or even years. ...
The occurrence of forest ground smoldering combustion has been increasingly frequent in recent years, resulting in significant emissions that have a detrimental impact on the ecological environment. Current research on smoldering emissions, however, is relatively scarce and fragmented across the fields of ecology, environmental protection, medicine, and forestry. The present paper offers a comprehensive review of the research methodologies employed in emissions, emission quantification results, the correlation between emissions and fire behaviors, and the potential for identifying smoldering combustion through emissions. Due to various factors such as the research region, characteristics of the samples, and experimental methods, it becomes challenging to arrive at comprehensive and accurate conclusions regarding the carbon cycle, dynamics of smoke plumes, and toxicological hazards. The limited research on the correlation between emissions and fire behaviors hampers the development and application of smoldering combustion identification technology based on emissions. It is suggested that standardized and universal methods for studying emissions should be popularized among researchers. In addition, the research on the correlation between emission characteristics and combustion temperature, spread direction, and spread velocity needs to be further strengthened.
... A portion of the heat generated during surface fuel combustion is transferred to, and can ignite the thick layer of peat underground that can be up to 11 m deep, leading to uncontrolled and long-lasting smouldering peat fires (Usup et al. 2004;Page et al. 2011). These smouldering fires have a long residence time of heat, with peak temperatures typically ranging from 450 to 700°C, and can penetrate deeply into the ground, resulting in severe soil thermal damage, which can have lethal impacts on soil properties and local biological systems (Rein et al. 2008;Huang et al. 2016;Santoso et al. 2022). The extensive consumption of soil during peat fires not only involves the burning of ancient carbon (up to 10,000 years old), but also has the potential to cause long-term impacts on local vegetation, such as changes in flora species and incomplete vegetation recovery, in all types of peatland settings (Rein 2013;Kettridge et al. 2015;Hu et al. 2018a). ...
... On the other hand, peatland conversion and management practices have been shown to affect fire EFs (Smith et al. 2018). Furthermore, natural variations in peat physicochemical properties, such as moisture content, inorganic content, and bulk density, have been demonstrated to have significant impacts on fire dynamics (Huang et al. 2016;Huang and Rein 2017;Hu et al. 2019Hu et al. , 2020Cui 2022). ...
... In laboratory studies, soil properties (e.g. moisture content, inorganic content and bulk density) have been shown to affect the smouldering fire dynamics (Huang et al. 2016;Huang and Rein 2017;Christensen et al. 2019;Hu et al. 2019Hu et al. , 2020. In this 0 E I 2 E I 5 S S 1 S S 3 S S 5 S S 6 S S 8 S S 1 0 S S 1 2 S S 1 3 S S 1 5 S S 1 7 S S 1 9 S S 2 1 S S 2 2 S S 2 4 S S 2 6 S P 1 S P 3 S B 3 500 1000 1500 2000 2500 3000 ...
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.
... Apart from that, peat soil which is formed due to the accumulation of organic material makes it laden with 'fuel' which of course will spread fire very easily. This phenomenon is often called peat smouldering which will be very difficult to extinguish if a fire below the peat surface has occurred (Cochrane, 2015;Huang et al., 2016;Goldstein et al., 2020). Therefore, in the context of forest and land fires disaster mitigation, especially in peatlands, apart from the importance of looking at historical weather parameters, it is also necessary to observe the physical condition of the peat which is measured directly beneath the peat soil itself, such as humidity and temperature (Bonn et al., 2016;Wilkinson et al., 2018). ...
Background: In Indonesia, the persistent occurrence of forest and land fires highlights the critical importance of early detection in determining the success of mitigation efforts. Method: This study explores several key aspects related to peatland wetness and its impact on fire prevention. Firstly, it examines the relationship between rainfall and the humidity and temperature of peatlands. Secondly, the study investigates peatland wetness as an indicator of hotspot emergence. Thirdly, the study evaluates stakeholder perceptions regarding the use of peat wetness monitoring in determining the emergency status of forest and land fire disasters. Findings: The study's results indicate that rainfall significantly influences peatland humidity, which in turn reflects the level of peat humidity and temperature. It was also found that peatlands with a Dry-Moderate humidity category can be a reliable indicator of the emergence of fire spots. The consensus among stakeholders is that monitoring peatland humidity is very important for decision-making related to emergency status. Finally, this study proposes a forest and land fire mitigation concept based on peatland humidity. Conclusion: This approach aims to reduce the risk of such fires by utilizing monitoring results to enhance preparedness, taking into consideration the current state of peatland wetness. Overall, this research underscores the importance of integrating peatland wetness monitoring into forest and land fire mitigation strategies to improve early detection and reduce the risk of fires. Novelty/Originality of this study: A study of forest fires in Indonesia links peatland wetness to fire hotspots, providing a reliable indicator for early fire detection. This is an innovative approach to forest fire prevention.
... Further increasing the wind velocity (which may intensify heating but shorten the heating duration because the firebrand can burn out faster; Salehizadeh et al. 2021) still cannot achieve ignition. In the literature, peat fire thresholds in terms of moisture contents were widely investigated (Frandsen 1997(Frandsen , 2011Rein et al. 2008;Huang et al. 2016b;Huang and Rein 2017;Hu et al. 2019;Lin et al. 2019;Palamba et al. 2020). For example, self-sustaining peat fire may not be initiated by a coil heater at 100 W for 30 min if the peat moisture content is higher than ~160% (Rein et al. 2008;Huang et al. 2016b;Huang and Rein 2017;Hu et al. 2019). ...
... In the literature, peat fire thresholds in terms of moisture contents were widely investigated (Frandsen 1997(Frandsen , 2011Rein et al. 2008;Huang et al. 2016b;Huang and Rein 2017;Hu et al. 2019;Lin et al. 2019;Palamba et al. 2020). For example, self-sustaining peat fire may not be initiated by a coil heater at 100 W for 30 min if the peat moisture content is higher than ~160% (Rein et al. 2008;Huang et al. 2016b;Huang and Rein 2017;Hu et al. 2019). Frandsen (1997) found a critical moisture content of around 110% to achieve self-sustaining smouldering peat fire using a similar ignition protocol. ...
Background
Wildfires represent a significant threat to peatlands globally, but whether peat fires can be initiated by a lofted firebrand is still unknown.
Aims
We investigated the ignition threshold of peat fires by a glowing firebrand through laboratory-scale experiments.
Methods
The oven-dried weight (ODW) moisture content (MC) of peat samples varied from 5% ODW to 100% ODW, and external wind (ν) with velocities up to 1 m/s was provided in a wind tunnel.
Key results and conclusions
When MC < 35%, ignition is always achieved, regardless of wind velocity. However, if MC is between 35 and 85%, an external wind (increasing with peat moisture) is required to increase the reaction rate of the firebrand and thus heating to the peat sample. Further increasing the MC to be higher than 85%, no ignition could be achieved by a single laboratory firebrand. Finally, derived from the experimental results, a 90% ignition probability curve was produced by a logistic regression model.
Implications
This work indicates the importance of maintaining a high moisture content of peat to prevent ignition by firebrands and helps us better understand the progression of large peat fires.
... Forest fires encompass surface fires, canopy fires, and sub-surface fires. Even though sub-surface fires occur less frequently compared to canopy or surface fires, their impact is substantial [2]. Subsurface fires typically occur within the humus or peat layer [3]. ...
... This may have been due to the insufficient smoldering in the surface layers. Therefore, there would have been a period for an overhanging phenomenon, which was consistent with Huang et al. [2]. This also reflects the hidden danger of sub-surface fires. ...
... The peak temperature of sub-surface fire smoldering indicated a strong positive correlation with slope, horizontal distance, and vertical depth. Huang et al. [2] also found that as smoldering spread progressed, the peak temperature increased. Steep slope angles, high humus content in the upper soil layer, large collapse area, introducing a large amount of oxygen to deep layers, and high oxygen content could make smoldering more severe [15], resulting in high peak temperatures. ...
In recent years, the influence of extreme weather patterns has led to an alarming increase in the frequency and severity of sub-surface forest fires in boreal forests. The Ledum palustre-Larix gmelinii forests of the Daxing’an Mountains of China have emerged as a hotspot for sub-surface fires, and terrain slope has been recognized as a pivotal factor shaping forest fire behavior. The present study was conducted to (1) study the effect of terrain slope on the smoldering temperature and spread rate using simulated smoldering experiments and (2) establish occurrence probability prediction model of the sub-surface fires’ smoldering with different slopes based on the random forest model. The results showed that all the temperatures with different slopes were high, and the highest temperature was 947.91 °C. The spread rates in the horizontal direction were higher than those in the vertical direction, and the difference increased as the slope increased. The influence of slope on the peak temperature was greater than that of spread rate. The peak temperature was extremely positively correlated with the slope, horizontal distance and vertical depth. The spread rate was extremely positively correlated with the slope. The spread rate in the vertical direction was strongly positively correlated with the depth, but was strongly negatively correlated with the horizontal distance; the horizontal spread rate was opposite. The prediction equations for smoldering peak temperature and spread rate were established based on slope, horizontal distance, and vertical depth, and the model had a good fit (p < 0.01). Using random forest model, we established the occurrence prediction models for different slopes based on horizontal distance, vertical depth, and combustion time. The models had a good fit (AUC > 0.9) and high prediction accuracy (accuracy > 80%). The study proved the effect of slope on the characteristics of sub-surface fire smoldering, explained the variation in peak temperature and spread rate between different slopes, and established the occurrence prediction model based on the random forest model. The selected models had a good fit, and prediction accuracy met the requirement of the sub-surface fire prediction.
... When the oven-dried peat was exposed to air, it can rapidly absorb ambient moisture and achieve a new equilibrium moisture content (MC p ) of ~10%. In order to achieve other desired moisture contents, the oven-dried samples were further mixed with water and stored in sealed boxes for at least 1 week for homogenisation, as in our previous work (Huang et al. 2016;Lin et al. 2019). The dry bulk density of peat (ρ p ) was measured to be ~145 kg/m 3 , a value that was kept for samples of different moisture contents. ...
Background
Smouldering wildfires in peatlands are one of the largest and longest-lasting fire phenomena on Earth, but it is unclear whether such underground peat fires can resurface to the ground and ignite a flame on the litter layer.
Methods
This work conducted a laboratory experiment by putting a 5-cm thick litter layer (banyan tree leaves with a density of 27–53 kg/m³) onto a 10-cm thick peat sample (moisture content of 10–100%).
Key results and conclusions
Tests confirmed that a smouldering peat fire, ignited at the bottom, can propagate upwards and resurface to ignite a flaming wildfire on the surface litter layer. The propensity of litter to be flaming ignited decreased with increasing peat moisture content and litter layer density. We found the threshold of such surface flaming as a function of temperature and temperature increase rate at the interface between peat and litter. Finally, large field experiments successfully reproduced and validated the laboratory observations.
Implications
This work reveals an important wildfire ignition phenomenon that has received little attention but may cause new spot fires, accelerate fire progression and exacerbate its hazards.
... The same reactor geometry has been verified and widely used in Refs. [16,17]. A 20 cm long cylindrical heater with a diameter of 1.6 cm was mounted on one side of the reactor, 5.0 ± 0.1 cm from the surface. ...
... The heating rod was buried, at the interface of the two layers, to (1) maximise the heat transfer to the sample and to minimise energy lost through the free surface; (2) promote a uniform ignition and initial smouldering front of two sizes simultaneously to investigate the spread front inside the dual-layered fuel bed. Following the same ignition protocol used in [16,17], duff was ignited by applying 100 W of power to the coil for 30 min. ...
... Then the samples were dried in an oven at 90 • C for 48 h. Following the same moisture controlling protocol used in [16,17], water was added to the dried samples to obtain a moisture content (dry base) of 15.03 ± 0.88 % that is equal to the original natural duff moisture content as determined from the drying process. The moisture was controlled so that only the influence of dual duff layer was investigated. ...
... Peat samples deposited in an open-top reactor with internal dimensions of 20 × 20 × 10 cm (built from mineral fibre boards) were ignited using a helical ignition coil (length 18 cm, diameter 1 cm) mounted on one side board (5 cm from the top surface) following the reactor and ignitor design of previous research (Huang et al. 2016;Hu et al. 2019a). At the ignition stage, the ignition coil had a constant power supply of 100 W, and it was turned off when the sample had 10% mass loss . ...
Background
Smouldering wildfires emit large amounts of carbon, toxic gases and particulate matter (PM), posing health and environmental hazards. It is challenging to conduct field measurements on wildfire emissions, and available instruments are limited by high cost and low mobility.
Aim
Here, we contribute to solving this challenge by studying three commercial low-cost and portable air quality analysers (KANE101, SDS011 and FLOW) and comparing them with research-grade instruments (FTIR, PM Cascade Impactor and DustTrak).
Methods
A series of laboratory experiments on peat smouldering were conducted including the stages of ignition, spread and burnout to provide conditions of emission measurements near the source.
Key results
The gas analyser KANE101 accurately measured CO2 and allowed calculation of modified combustion efficiency (MCE). The FLOW air pollution sensor was found unsuitable for PM measurements near fire sources because of its narrow range. FLOW captured the variation of volatile organic compounds (VOCs), but measurements did not correlate well with NO2 measurements. The SDS011 PM sensor responded well in measuring PM10 in this study.
Conclusions
KANE101 and SDS011 can be used in the field after calibration to measure CO2/CO and PM.
Implications
This work provides a better understanding of how low-cost and portable emission sensors can be of use for wildfire measurements in the field.
