Smolder Propagation Research Articles

Predicting extinction limits of concurrent smoldering spread by a reduced analytical model

Smoldering is a flameless combustion mode occurring on the surface of charring fuels, such as wood and cigarettes. Although the smoldering process is slow and has a low temperature compared to flaming, it is easy to be initiated by a weak heat source and persists under poor oxygen conditions. Extensive work has been done for flame extinction to develop scaling models to predict the limiting oxygen concentration (LOC), but limited work is available for the smoldering extinction. This study develops a reduced analytical model to predict the extinction limits of smoldering. The model simultaneously solves smoldering propagation rate, surface temperature, and surface oxygen mass fraction as part of the solutions. The extinction limit is determined as the critical condition where solutions satisfying all governing equations cease to exist. The model provides a qualitative description and captures the essential characteristics of a previous experiment. The smoldering rate decreases with increasing fuel diameter, and a larger-diameter fuel is easier to extinguish. The mechanisms of the extinction process are investigated, showing the dominant role of radiative heat loss in the smothering limit at low airflow velocities and convective heat loss near the blowoff limit at high airflow velocities. Further analysis of the effect of oxygen concentration shows an increasing trend of LOC with fuel diameter, and the smothering branch cannot be predicted without considering the heat loss through radiation from the solid surface. Novelty and Significance StatementThe novelty of this research is the prediction of smoldering propagation rates and extinction limits across various fuel diameters using a reduced analytical model. The model is modified to accommodate a thin fuel configuration and high airflow velocity condition, where convective heat and mass transfer play a more important role. The model's significance is that it not only provides essential insights into the mechanisms but also has the capability to simultaneously determine key parameters such as spread rate, reaction temperatures, and surface oxygen concentration as part of the solutions. Additionally, this study demonstrates the model's ability to reproduce the limit conditions, including smothering/blowoff limits and limiting oxygen concentration (LOC).

Modeling smothering limit of smoldering combustion: Oxygen supply, fuel density, and moisture content

Smoldering, characterized as a slow, low-temperature, and flameless reaction process, represents one of the most persistent types of combustion phenomena. While oxygen supply is one of the governing mechanisms controlling smoldering, the specific oxygen threshold or smothering limit remains inadequately investigated. Herein, we built a physics-based 1-D computational model integrating heat-and-mass transfer and 5-step heterogeneous chemistry to investigate the oxygen threshold or smothering limit of smoldering propagation in porous pine needle beds subjected to forced internal oxidizer flow. Simulation results revealed that, the required oxidizer flow velocity or oxygen supply rate increased as the oxygen concentration decreased, and the predicted limiting oxygen concentration (LOC) was about 3 %, agreeing well with the experimental observations and theoretical analysis. Moreover, the required airflow velocity was predicted to increase as the fuel density and environmental temperature decreased, or the moisture content increased, and the predicted maximum moisture content capable of supporting smoldering was about 110 %. At the smothering limit, the modeled minimum smoldering temperature and propagation rate were around 300 °C and 0.5 cm/h. This work helps deepen our understanding of the limiting conditions of smoldering combustion, thus improving the mitigation strategy of smoldering fire and the efficiency of applied smoldering systems. Novelty and significance statementsOxygen supply is one of the key mechanisms governing the smoldering propagation. In the literature, scattered studies have examined the limiting oxygen concentration (LOC) of smoldering under different conditions, leading to disparate results even for the same fuels. In our previous work, for the first time, we developed a tubular smoldering reactor capable of precisely controlling the flow of oxidizer, and successfully quantified the exact oxygen supply limit of smoldering combustion. However, no computational model was established specifically for the oxygen threshold of smoldering with forced internal oxygen supply.The novelty of this research is the establishment of the first-ever computational model to investigate the oxygen threshold or smothering limit of smoldering propagation in porous pine needle beds subjected to forced internal oxidizer flow. It is significant because (1) it contributes to a fundamental understanding of smoldering combustion; (2) it investigates the role of fuel properties and environmental conditions in smoldering dynamics and the oxygen supply limits; (3) it assists in optimizing the applied smoldering systems and enriching prevention strategies for smoldering fires.

Determining the Conditions That Lead to the Self-Extinguished and Self-Sustained Smoldering Combustion of Wood

To improve our understanding of flaming, smoldering, or self-extinction in the burning of wood, it is necessary to quantify the conditions that lead to self-extinguished and self-sustained smoldering combustion. Experiments were performed in a cone calorimeter under an external irradiation of 10 to 25 kW/m2 to analyze the temperature and mass loss of self-extinguished and self-sustained smoldering. The smoldering front depth was the significant parameter used to capture the smoldering characteristic, and it was defined as the axial thickness that reaches the smoldering characteristic temperature. The critical smoldering front depth of self-extinguished smoldering was lower than 10–15 mm for 30 mm thick wood at 15.5 kW/m2 irradiation. This critical depth decreased with the increase in heat flux, from 26.5 ± 1.5 mm at 10 kW/m2 to 11 ± 1 mm at 25 kW/m2. A simple theoretical analysis is proposed to explain the smoldering thickness threshold of self-sustained smoldering propagation based on the local heat balance. The equation predicts that the critical depth decreases as the heat flux increases, from 23.9 mm at 8 kW/m2 to 7.3 mm at 25 kW/m2. The predicted critical depth and heating duration were consistent with the experimental results. This study proposes a feasible parameter to help understand the threshold of smoldering propagation and the development of biomass burners.

Numerical Modeling of Steady Smoldering Propagation in Slim Char Rods Characterized by Oxidation of Char Cone

ABSTRACT To clarify the mechanism on natural steady smoldering, propagation of smoldering in slim char rods was investigated theoretically and experimentally. A numerical model of smoldering propagation in char rods characterized by oxidation of char cone (the conical reaction zone) was developed based on energy conservation as well as oxygen transport through ash and flue gas. Shape and temperature field of char cone were calculated via Gauss–Seidel iteration, and sensitivity analysis of parameters was performed. Experiments on propagation velocity, shape and temperature field of char cone were also performed using char rods with different materials and diameters to validate the model. It is found that the predicted results agree well with the experimental results, indicating that the model is able to describe steady propagation of smoldering of char rods appropriately. The main parameters affecting the height and temperature of char cone are activated energy of fuels and ambient pressure, respectively, and the height of char cone is more sensitive than temperature to the parameters.

Modelling oxygen-limited and self-sustained smoldering propagation: Thermochemical treatment of food waste in an inert porous medium

Smoldering treatment is emerging as a valuable engineering tool for many processes, including food waste treatment. However, smoldering systems are currently not well-understood nor optimized. Therefore, numerical models provide invaluable insight into the process dynamics, which improves our understanding and supports the development of novel systems. These smoldering models couple heat, mass, and momentum transfer with pyrolysis and oxidation chemical reactions within porous media. While recent models have untangled many aspects of these systems, local oxygen transport rates from bulk flow to the fuel surface are still not well-resolved. In this work, local oxygen-transport equation was approximated by an analytic derivation based on the gas–solid oxygen non-equilibrium hypothesis. With the improved oxygen-transport equation, a 2D model with five-step reaction scheme for smoldering propagation of food waste in sand was developed. Kinetic parameters obtained from TG experiments were incorporated into the bed-scale smoldering propagation model. The developed model was validated with experimental data that stretched from robust to weak smoldering propagation. It was demonstrated that the developed model matches well with experiments. Furthermore, this model revealed: (i) the emergence of non-uniform gas flow in the reactor, (ii) the evolution of the kinetic- and oxygen-transport-limiting regimes, and (iii) valuable insight into the fundamental changes with smoldering robustness.

Understanding the role of water evaporation and condensation in applied smoldering systems

Accurately resolving the way energy is stored and transferred is critical to applied smoldering systems in porous media. Water evaporation and condensation are important mechanisms in the energy conservation of smoldering. This is particularly important when significant water presence can affect the potential for self-sustained smoldering as it occurs in a broad range of practical applications. This paper describes a one-dimensional smoldering model considering water phase change. The model was calibrated to a series of wet smoldering experiments of GAC varying initial water saturation from 5 to 20%. The results show that introducing simple calibration constants on the thermal properties of a wet porous medium can accurately predict the transient and spatial resolution of the progression of evaporation, condensation, and smoldering reactions. A sensitivity analysis suggests that wet smoldering could be improved by increasing fuel concentration and injected airflow, which provides practical insights into wet smoldering applications. In addition, the model was applied to establish five characteristic zones for the wet smoldering front: evaporation, condensation, pre-heating, smoldering, and cooling. The evaporation and condensation zones challenge smoldering propagation due to the high water saturation. The pre-heating zone represents the region separating the smoldering front and evaporation zone, which serves as an important dry buffer and requires a minimum thickness (4 mm) to avoid extinction.

Variations of smoldering spread characteristics of Shanxi anthracite during forward smoldering propagation: Effect of ventilation rate

In view of the increasing risk of smoldering fire in coal mines, and to study the effect of ventilation rate on smoldering propagation deeply, the forward smoldering propagation process of Shanxi anthracite (SXA) is studied for nine different ventilation rates (10.0–30.0 L/min). Both simulations and micro-structure testing were performed. The results show that there are five distinct combustion stages (t0-t5) for the coal samples in the temperature range between 25.0 and 800.0℃. With increasing ventilation rates (33.3 to 100.0 mL/min), both the characteristic temperature and the activation energy of the studied coal at each stage decrease first before they increase. The heat release, however, occurs in reverse order (all samples reach a turning point at 75.0 mL/min). The critical ventilation rate range for a unit mass of SXA coal to sustain smoldering propagation was 0.18–0.27 m3/h kg−1. With increasing ventilation (between 10.0 and 30.0 L/min), key parameters such as average peak smoldering-temperature, average peak smoldering-temperature rise rate, spread rate of the average moisture-evaporation front, and average peak-temperature front also increase before they decrease, reaching their maxima at 22.5 L/min. The smolder reaction occurs in the following sequence: The moisture evaporation occurs first, which is followed by the pyrolysis reaction and the char-oxidation reaction. Then, the temperature gradually decreases after reaching its maximum.

Modelling oxygen-limited and self-sustained smoldering propagation: Underground coal fires driven by thermal buoyancy

Modelling oxygen-limited and self-sustained smoldering propagation is of significance for prevention of fire hazards and optimization of applied systems. However, two issues remain unsolved in the conventional models: (a) adjustment of TG-scale kinetic parameters applied to bed-scale propagation, and (b) decoupling oxidative reaction and oxygen transport in multi-scale porous media. In this work, an analytic expression of oxidative reaction rates limited by oxygen transport is derived from the conservation equations of oxygen species transport in gas and solid. Then, both oxidative reaction rates controlled by the kinetic reaction (Arrhenius equation) and oxygen transport (analytic expression) are integrated into the conservation equations of mass, energy, and oxygen species transport. In this model, five-step reaction scheme is considered and their kinetic parameters obtained from TG experiments are employed without any adjustment. Along with the Darcy air flow driven by thermal buoyancy, this model is applied to predict oxygen-limited and self-sustained smoldering propagation of underground coal fires. The proposed model is compared with laboratory experiments and the conventional model. Results show that the proposed model well predicts the oxygen-limited and self-sustained smoldering propagations of underground bituminous and anthracite coal fires. The predictability of the proposed model is better than the conventional model in spite of great effort to modify kinetic parameters best fitting with experimental data. It is validated that the proposed model addresses the two puzzled issues in the conventional model with respect to buoyancy-driven, oxygen-limited, and self-sustained smoldering propagation of underground coal fires. This work may help to improve models of self-sustained propagation of other smoldering fires and applied smoldering systems.

Limiting oxygen concentration and supply rate of smoldering propagation

Smoldering combustion of porous fuel is a slow, low-temperature, and flameless process that is sustained by heterogenous oxidations. The influence of oxygen supply on smoldering propagation is of vital significance but still not fully understood. This work explores the smoldering propagation on peat soil under an internal oxidizer flow with velocity up to 14.7 mm/s and oxygen concentration of 2 - 21%. After ignition in the middle of the fuel bed, by decreasing the internal airflow velocity, the smoldering propagation changes from bidirectional (forward + opposed) to unidirectional (opposed). Further reducing the airflow velocity to 0.3 mm/s, no propagation occurs with a minimum smoldering temperature of about 300 °C. As the internal flow velocity increases, the limiting oxygen concentration for smoldering decreases and approach to a minimum value that is below 2%. The minimum oxygen supply rate for smoldering approaches a constant of 0.08 ± 0.01 g/m2·s, when the oxygen concentration is larger than 10%. Further reducing the oxygen concentration below 10%, the required minimum rate of oxygen supply increases significantly, because of the enhanced convection cooling by a larger internal flow velocity. This work quantifies the minimum oxygen supply for the smoldering combustion and advances the fundamental understanding of the persistence of smoldering propagation in underground peat layers.

Fast moving smoldering fronts in thin liquid fuel films dispersed on inert porous media

An experimental study of smoldering in thin layers of pyrolysis oil on an inert substrate of calcium silicate boards has been conducted. The smoldering fronts propagated with velocities up to 0.4mm/s, which is significantly faster than other types of smoldering without forced air flow. The smoldering propagation velocity increased both with thinner layer of pyrolysis oil and higher initial fuel temperature. Our experimental observations are in close correspondence with a theoretical model based on energy balance, where the availability of oxygen at the substrate surface is the main limiting mechanism. Both the front peak temperature and the front width are correlated to the initial fuel temperature. We also report observations of instability phenomena, including double fronts propagating in close succession. The temperature dynamics were observed by high resolution infrared thermography. The findings contribute to the understanding of the governing mechanisms of smoldering combustion of liquids on inert media, and in particular the fast smoldering fronts that have been observed along the surface of porous insulating materials in experiments with smoldering cotton.

A computational study on the quenching and near-limit propagation of smoldering combustion

Smoldering is the slow, low-temperature, and flameless burning of porous fuel and one of the most persistent types of combustion phenomena. The influence of cooling on the smoldering propagation and quenching is of practical significance but still poorly understood. In this work, a physics-based 2-D computational model that integrates heat and mass transfer and heterogeneous chemistry is built to investigate the limiting quenching conditions of in-depth smoldering propagation in a typical biomass sample. Simulation results predict that the smoldering quenching occurs as the sample width decreases or the wall-cooling coefficient increases, agreeing well with experiments. The modelled minimum smoldering temperature is about 350 °C, and the minimum propagation rate is around 0.5 cm/h. Further analysis demonstrates that either the smoldering temperature or propagation rate increases with the sample width and eventually approaches it maximum value. Finally, the influences of the ambient temperature and oxygen level on the smoldering quenching distance are explored. This is the first time to use a comprehensive physics-based model to predict the quenching behavior of smoldering, which provides a deeper understanding of the persistence and extinction limit of smoldering fire phenomena.

Fast Pyrolysis Biochar Flammability behavior for Handling and Storage

Biochar is a fairly new material in the research arena with limited information on safety aspects related to transportation, storage, disposal or field application methods. The objective of this research was to assess the flammability characteristics of fast pyrolysis biochars with test methods EPA 1030 and ASTM 4982. Results indicated that biochar is a non-flammable substance when tested with EPA 1030 ignitability of solids. However, when tested with ASTM D4982, a fast screening method, biochars showed potential risks of flammability. However, the addition of 20-50% moisture reduced any flammability concern.
 Fast pyrolysis biochar was more prone to be flammable than traditional charcoal and slow pyrolysis biochar tested in this study. Still, fast pyrolysis biochars presented lower flammability potential (ASTM 4982) in comparison to its precursor biomass. The flammability propagation measured with EPA 1030, had high correlations with oxygen content and surface area of the fast pyrolysis biochar. The combustion reaction of fast pyrolysis biochar is a flameless combustion process, with a slow burning rate, and most commonly exhibiting a hot ember smoldering propagation front.
 This paper illustrates the necessity of performing recurring tests due to biochar’s intrinsic variability stemming from the different modes of production and feedstock used.

Smoldering propagation and blow-off on consolidated fuel under external airflow

Propagation of smoldering combustion and its blow-off phenomena are of practical importance in evaluating the fire dynamics of solid fuels, but the scientific understanding is still limited. In this work, we quantify the smoldering propagation rates on consolidated biomass and the blow-off limits under concurrent and opposed external airflows up to 50 m/s. The incense cylinders with different diameters (1.5–5.0 mm) and densities (720–1,100 kg/m3) are tested. As the airflow velocity increases, the smoldering propagation rate first increases to its maximum value (Oxygen-limited Regime) and subsequently remains stable (Thermal Regime), regardless of the airflow direction. Afterward, it slightly decreases (Chemical Regime) until blow-off, and the blow-off of opposed smoldering is easier, similar to the pattern of flame spread. The blow-off airflow velocity (13–46 m/s) of smoldering combustion is around ten times larger than that of flaming combustion, and it decreases as the fuel diameter or density increases. This work advances the fundamental understanding of the smoldering propagation, blow-off, and its persistence; thus, helping guide the fire suppression strategies of smoldering.

Self-Sustaining Smoldering Characteristics of Corn Straw Powder Stacks.

Biomass fuels are expected to play an important role in future energy consumption. Meanwhile, the fire and explosion of biomass are the have-to-face problems in its production, storage, and application process. This work aims to reveal the influence of stacking and ventilation parameters on the smoldering propagation process and provide guidance for the safe storage of biomass pellets. The effects of stacking density and air flux on the smoldering propagation process were studied experimentally, the variations of bed temperature with these two parameters were analyzed using the numerical simulation technique, and the conditions of self-sustaining smoldering were determined by the local energy analysis method. The results showed that the peak smoldering temperature of corn straw powder was between 500 and 520 °C, and the smoldering propagation velocity was between 10 and 30 mm/h. When the stacking density was changed from 56.89 to 99.56 kg/m3, the peak smoldering temperature change rate was about 2% and the smoldering propagation velocity decrease amplitude was up to 30%. Meanwhile, when the air flux was in the range of 0.2–0.8 m3/h, the small ones had little effect on the peak smoldering temperature, while the large ones helped the peak smoldering temperature reach 560 °C. Finally, the local energy analysis showed that the net heating rate was positive with energy accumulation in the system, the smoldering was self-sustaining, and the smoldering front propagated from the bottom to top. These results provide data support to facilitate the safe storage of biomass pellets.

Smoldering fire propagation in corn grain: an experimental study

Smoldering fire exhibits low-velocity combustion rates and is a threat commonly presented in agriculture grain storages. Some physical and chemical properties of the grain, including particle size, porosity, moisture, and the environmental conditions influence the propagation of the smoldering process. Many works evaluate the conditions to develop smoldering combustion based on particulate size and granular materials. However, only a few studies focusing on smoldering propagation in cornflour and granular grains. In this work, different particle diameters (181.6 ​μm, 776.4 ​μm and 1411 ​μm) and moisture contents (0% and 15%) of corn are examined to understand the corresponding impact on the smoldering propagation. To achieve this, samples of corn were placed inside a cylindrical reactor with thermocouples distributed along the apparatus’ central axis and heated by a hot plate, then the smoldering reaction with a rising heating transfer profile was analyzed. Our results demonstrate how changing storage conditions can profoundly influence the smoldering propagation within a pile of corn.

Effect of particle granularity on smoldering fire in wood chips made from wood waste: An experimental study

SummaryFires in wood waste storages cause financial losses, are difficult to extinguish, and emit large amounts of fire effluents. The mechanisms related to fires in wood chip piles are not well elucidated. To find suitable preventive measures for handling such fires in wood waste, a better understanding of the physical properties of wood waste is needed. The present study investigates how granularity affects mechanisms of smoldering fire and transition to flaming in wood chip piles. Eighteen experiments with samples inside a top‐ventilated, vertical cylinder were conducted. Heating from underneath the cylinder induced auto‐ignition and smoldering fire, and temperatures and mass loss of the sample were measured. The results showed that granularity significantly affects the smoldering fire dynamics. Material containing larger wood chips (length 4‐100 mm) demonstrated more irregular temperature development, higher temperatures, faster combustion, and higher mass losses than material of smaller wood chips (length <4 mm). The larger wood chips also underwent transition to flaming fires. Flaming fires were not observed for small wood chips, which instead demonstrated prolonged and steady smoldering propagation. The differences are assumed to be partly due to the different bulk densities of the samples of large and small wood chips affecting the ventilation conditions. Increased knowledge about these combustion processes and transition to flaming is vital to develop risk‐reducing measures when storing wood chips made from wood waste in piles.

STARx Hottpad for smoldering treatment of waste oil sludge: Proof of concept and sensitivity to key design parameters.

Growing stockpiles of waste oil sludge (WOS) are an outstanding problem worldwide. Self-sustaining Treatment for Active Remediation applied ex situ (STARx) is a treatment technology based on smoldering combustion. Pilot-scale experiments for the STARx Hottpad prove this new concept for the mobile treatment of WOS mixed intentionally with sand or contaminated soil. The experiments also allowed for the calibration and validation of a smoldering propagation numerical model. The model was used to systematically explore the sensitivity of Hottpad performance to system design, operational parameters, and environmental factors. Pilot-scale (~1.5 m width) simulations investigated sensitivity to injected air flux, WOS saturation, heterogeneity of intrinsic permeability, and heterogeneity of WOS saturation. Results reveal that Hottpad design is predicted to be successful for WOS treatment across a wide range of scenarios. The operator can control the rate of WOS destruction and extent of treatment by increasing the air flux injected into the bed. The potential for smoldering channeling to develop was demonstrated for the first time. Under certain conditions, such as WOS saturations of 80%, high heterogeneity of WOS saturations, or moderate to high heterogeneity of soil permeability, smoldering channeling was predicted to accelerate to the point that remedial performance was degraded. Field-scale simulations (~10 m width) predicted successful treatment, with WOS destruction rates an order of magnitude higher than the pilot-scale and treatment times increasing only linearly with bed height. This work is a key step toward the design and effective operation of field STARx Hottpad systems for eliminating WOS.

Differences in smoldering characteristics of coal piles with different smoldering propagation directions

Smoldering combustion frequently occurs in the mined-out gobs of coal mines. This can act as an ignition source, triggering gas explosions and resulting in serious accidents. In this study, a smoldering test device was constructed, in which horizontal, vertical, forward and reverse smoldering combustion experiments were performed to explore the smoldering characteristics (dynamic development process, smoldering rate and temperature, etc.) of a coal pile. It is found that owing to the evaporation moisture present in the coal and its re-condensation on the coal ahead of the smoldering front, a constant temperature of approximately 70–90 °C is maintained during the spread of forward smoldering. In the initial stage of reverse smoldering, both the pyrolysis and oxidation fronts exist and propagate backward; meanwhile, in the later stage, the intensity of the endothermic pyrolysis reaction is greatly reduced, resulting that the maximum smoldering temperature of reverse smoldering is slightly higher than that of the forward smoldering. The vertical smoldering temperature is 1.5–2 times that of horizontal smoldering, owing to the poorer oxygen supply in the horizontal spread. The propagation of the oxidation front is extremely slow, and is in the order of 0.1 mm/min; this makes it difficult to characterize and detect the smoldering fire, especially in inaccessible mined-out gobs with complex environments. This study provides useful insights into the identification of smoldering fire and assesses its development stage.

Effects of fuel content and density on the smoldering characteristics of cellulose and hemicellulose

Smoldering combustion in wildland fires poses hazards for both ecosystems and humans by destroying biomass, transitioning to flaming combustion, and releasing significant quantities of pollution. Understanding the parameters that control smoldering is necessary to help predict and potentially mitigate these hazards. A challenge in identifying these parameters is the wide variety of biomasses which occur in nature. The objective of this study is to identify the effects of density and fuel concentration on the smoldering characteristics of cellulose and hemicellulose mixtures. These fuels were considered because they are some of the major organic constituents within biomass. To this end, downward smoldering propagation velocities were measured for 50%, 75%, and 100% cellulose content at densities varying from 170 to 400 kg/m3. The horizontal smoldering propagation velocities and temperature distributions were also determined for loosely packed samples ranging from 100% to 0% cellulose (with residual hemicellulose). Additionally, horizontal smoldering propagation velocities were determined for systematically varied ratios of cellulose (50–100%) and densities (200–400 kg/m3). The fuel was burned in an insulated reactor box. An infrared camera measured the horizontal propagation velocity, and thermocouples measured the downward propagation. A one-dimensional reactive porous media model with reduced chemistry was used to identify key processes causing the observed sensitivities. At constant packing density, the propagation velocity increased as cellulose content decreased because of decreased heat release with increased cellulose content and the earlier onset of hemicellulose pyrolysis. The propagation velocity decreased with respect to packing density when the fuel content was constant because of reduced oxygen diffusion. The propagation velocity increased with cellulose content when the fuel was loosely packed because of the decreasing density.

Effects of Bulk Densities and Inlet Airflow Velocities on Forward Smoldering Propagation Properties of Flexible Polyurethane Foam

Abstract Smoldering processes of flexible polyurethane foam (FPUF) with densities of 20.0, 30.0 and 35.0 kg/m3 were simulated in a self-designed mesoscale generator with dimensions of 60.0 cm × 25.0 cm × 25.0 cm. Characteristic parameters like smoldering establishment time, average smoldering rate, smoldering temperature, and high temperature duration were proposed and analyzed at five different air flow rates of 1.0, 2.0, 3.0, 5.0 and 7.0 L/min. Scanning Electron Microscopy (SEM) was used to investigate the morphological evolution of bulk materials before establishment and propagation of smoldering. Air was forced in the direction of approximately one-dimensional forward propagation. Results show that FPUF (20.0 kg/m3) has the shortest average smoldering establishment time of 1828.0 s at five airflow rates, FPUF (30.0 kg/m3) has the longest average smoldering establishment time of 2646.0 s. The average smoldering rate of FPUF (35.0 kg/m3) is higher than that of FPUF (30.0 kg/m). Smoldering of FPUF (20.0 kg/m3) cannot be self-sustained, due to largest porosity and insufficient heat accumulation. The average smoldering temperature and duration of high temperature values of FPUF (30.0 kg/m3) are higher than those of FPUF (35.0 kg/m3), and the highest average smoldering temperature of FPUF (30.0 kg/m3) is 410.0 ℃. Since smoldering is derived from chemical reactions of oxygen transported and pyrolyzed products, the bulk material with low density shows higher porosity and contains more oxygen, it burns more fully and thus smoldering establishment time is less. In the process of smoldering, the oxidation and exothermic effects of porous carbon-based chars and oxygen is the main source of energy for the self-sustaining propagation of smoldering. The porosity of FPUF with high density (35.0 kg/m3) is lower, the burning produces a porous carbonate residue and limits the heat loss on the surface, so the smoldering rate of FPUF (35.0 kg/m3) is faster.