Search

Gaseous inhibitors are used in many industries for the explosion prevention of combustible dusts, mitigating the potential hazard to humans, properties and environments. This work experimentally and theoretically studied the inerting effect of gaseous inhibitors on the ignition process of dust clouds in O2/N2/CO2 atmospheres, with an emphasis on the role of the CO2/N2 ratio. 10 different combustible carbonaceous dusts were selected, including grain dust, biomass dust and coal dust. Experimental results showed that the inhibition effect of CO2/N2 is closely related to the ignition mechanism of dust clouds. Specifically, a higher ratio of CO2/N2 yields a stronger inhibition effect on the ignition process of dust samples with relatively low volatile matter contents predominated by heterogeneous ignition. In addition, two novel steady-state ignition mechanism models were developed to interpret the experimental observations. Maxwell-Stefan equations were used to describe the diffusivity in the ternary O2/N2/CO2 gas mixtures. The analytical results were in good agreement with the experimental data of the minimum ignition temperature of dust cloud (MITC) in oxygen-lean atmospheres. The mechanism modelling can be used to estimate the critical ignition temperature of all carbonaceous dust clouds with a wide range of volatile matter content under different inert atmospheres, which will provide a reference for the explosion hazard assessment of dust posed by a hot surface in the process industries.

Dust explosions have been often reported in many industries that handle or process combustible dusts, with great losses to life and property. This explosion risk could be amplified under some extreme conditions like high-temperature environments. In this work, we develop a 2D numerical method to investigate explosion characteristics of corn starch cloud with an emphasis on the roles of high-temperature environments. The particle tracking trajectories and devolatilization evolution at high-temperature environments are described. The dust explosion severity characteristics including maximum explosion pressure (Pex) and maximum rate of pressure rise (dp/dt)max, are also investigated. In addition, the characterized devolatilization and explosion time are applied to analyze dust explosion behavior and flame propagation. The devolatilization evolution of dust cloud and flame propagation modes are found to vary with ambient temperature. And three ignition modes are observed after ignition. The simulation also provides a satisfactory prediction for dust explosion behavior at high-temperature environments.

Smouldering fires and explosions arising from self-ignition of coal dust deposits represent a serious hazard for human beings, environment and industry. It is essential for plant operators to know the conditions (temperature, duration and quantities) at which storage will be safe. In this work, self-ignition behaviour of three bituminous coal dusts in large scales are theoretically studied, based on the experimental data obtained via a standardized hot-basket apparatus. A comprehensive 2-D transient model is developed, using a 2nd-order reaction kinetics considering both coal and oxygen consumptions, to investigate self-ignition parameters of coal dust accumulations. The numerical model shows a less conservative prediction compared with the steady-state methods. The computed self-ignition temperature and ignition delay time show a satisfaction agreement with lab-scale experimental results. In addition, the influences of ambient temperature and moisture content are analysed. The result shows that the moisture content delays the ignition and a small variation of the ambient temperature nearby the critical condition will lead to a large difference in the ignition delay time.

Hydrogen leakage of vehicles in the tunnel is a great threat to the safety operation of the tunnel and longitudinal ventilation strategies have always been utilized to control the fire and smoke movement of rail transit, electric and fossil-fueled vehicles in the engineering field. It is in doubt whether the longitudinal ventilation strategy could still help to reduce the jet fire hazard of transportation with H2 power in the tunnel, considering the rapid development of the hydrogen energy. In present work, a numerical research on effects of longitudinal ventilation strategies on hydrogen jet flames in the tunnel is conducted. The results illustrate that longitudinal ventilation could affect the flame characteristics of jet flames greatly in the tunnel. The critical ventilation velocity increases firstly with the increase of hydrogen leakage rates and then changes little after a critical value. The predicted theoretical model of pool fires could well predict the critical ventilation velocity for hydrogen jet fires. With the increase of longitudinal ventilation velocity, maximum ceiling temperatures are decreased greatly. According to the heat releases, jet speeds and ventilation velocities, three kinds of flame bending characteristics of hydrogen jet fire could be observed due to different effects of the inertial force. At last, the stable thermal stratification could also be destroyed by large ventilation velocities but the corresponding ventilation velocity is far larger than the critical ventilation one. With the increase of longitudinal ventilation velocities, the height of thermal layer is reduced firstly and then maintained at a constant value.

To reveal and improve our understanding of the ignition behavior and mechanism, G-G furnace experiments of three typical organic dusts were performed to investigate the minimum ignition temperature (MIT) in O2/N2, O2/Ar and O2/CO2 atmospheres with oxygen mole fraction from 8.4% to 50%. The experimental results were presented in oxygen-lean and oxy-fuel atmospheres to evaluate the ignition sensitivity of dusts in different atmospheres. It was found that CO2 is the strongest in terms of lowing the ignition sensitivity of the three dusts, and the dust explosion risk increases significantly with increasing O2 mole fraction for the three dusts through a logarithmically and significantly reducing MIT. However, for different dusts, inert gases show different suppression effects. In addition, a modified steady-state homogeneous ignition model was proposed and successfully applied to oxygen-lean atmospheres, and in oxy-fuel atmospheres, this model has also been improved to estimate the ignition mechanism. This ignition mechanism model could be used to successfully predict the minimum ignition temperature of high volatile dust under different inert atmospheres controlled by homogeneous ignition, which will provide a reference for the ignition hazard assessment of dust on hot surfaces.

The self-ignition of coal dust deposits and its subsequent smoldering combustion pose a high fire hazard to oxy-fuel power systems which burn fuels using pure oxygen for the sake of carbon capture and storage. The increasing risk of explosion in the gas-phase and self-ignition in the solid-phase for an oxygen enhanced combustion environment has not been well studied yet. In this work, the heterogeneous reactions of a bituminous coal dust are investigated by using a novel hot-basket apparatus with an emphasis on the roles of O2 and diluent gas in chemisorption and smoldering. Experiments show that increasing O2 mole fraction accelerates both self-ignition and the following smoldering combustion. On the other hand, the presence of CO2 increases the ignition temperature and reduces the maximum smoldering temperature. However, the promotion in the fire and explosion risk by elevating O2 mole fraction is substantially stronger than the retardation effected by presence of CO2. The emission-gas measurements show that the CO to CO2 ratio increases significantly after self-ignition, and CH4 counts for 1–8% of the total carbon emission. This research may help improve the understanding of heterogeneous coal combustion and the fire safety in oxy-fuel power systems.

Self-ignition of coal dust deposits poses a higher risk of fires in oxygen-enriched oxy-fuel combustion systems. In this work, we develop a numerical method, using the commercial software COMSOL Multiphysics, to investigate self-ignition behaviour of coal dust accumulations with a main emphasis on the roles of oxygen, diluent gas and dust volume. A one-step 2nd-order reaction kinetic model considering both coal density and oxygen density is used to estimate reaction rate using the kinetic parameters from previously conducted hot-oven tests. This model is validated to predict the transient temperature and concentration profiles of South African coal dusts until ignition. The computed self-ignition temperatures of dust volumes show a good agreement with experimental results. In addition, it is found that the inhibiting effect of carbon dioxide is comparatively small and oxygen consumption increases dramatically after ignition. Parameter analysis shows that the heating value and kinetic parameters have a comparatively pronounced effect on self-ignition temperature. The model provides a satisfactory explanation for the dependence of self-ignition behaviour on gas atmospheres, thus helping to further understand the fire risk of self-ignition in oxy-fuel combustion systems.

Minimum ignition temperature of dust clouds (MITC) was studied experimentally and theoretically in different atmospheres. Three carbonaceous dusts were tested in both air and O2/CO2 atmospheres with CH4 mole fraction from 0% to 2%. Results showed that the ignition risk of the three dusts significantly increases (decrease of MITC by ~100 ℃) with increasing XO2 from 21% to 50%, but significantly decreases replacing N2 in air with CO2. The inhibition effect of CO2 on MITCs could be diminished by increasing XO2 or adding CH4. The addition of small amount of CH4 has different effects on the MITCs of different dust samples, following the opposite order of volatile matter content: anthracite>bituminous coal>starch. Two modified steady-state ignition models, considering the density of mixture gas and dust cloud, XO2 and its diffusivity, were developed to interpret the experimental observations. The analysis revealed that the global heterogeneous ignition model suits well for the hybrid mixtures of anthracite or bituminous coal dusts. In contrast, the proposed global homogeneous ignition model was found to be only valid for the pure starch dust, and the extra CH4 addition could strongly affect the ignition process of starch, particularly in O2/CO2 atmospheres with higher XO2.

Understanding the fundamental basis of coal dust layers ignition behavior is of interest for the mitigation of industrial fires and explosions. This paper develops a 2D numerical method to investigate the self-ignition of coal dust layers with an emphasis on the roles of the oxygen mole fraction and diluent gas. A global one-step 2nd-order reaction kinetic model considering both coal density and oxygen density is used to estimate the reaction rate of self-heating process until ignition. In addition, the overall heat transfer coefficient is evaluated based on a steady state method and the kinetic parameters are estimated by fitting experimental and numerical temperature evolution figures. This model has been found to be valid to predict the transient temperature and concentration profiles until ignition. The numerical results of the minimum ignition temperature of dust layers (MITL) are in close agreement with experimental observations. The ignition delay time and the most sensitive ignition position (MSIP) of dust layer are found to vary with varying ambient gas conditions, hot plate temperatures and ambient temperatures using the verified model. The model provides a satisfactory explanation for the dependence of ignition characteristics of coal dust layer in oxygen-enriched oxy-fuel atmospheres.

BAM furnace apparatus tests were conducted to investigate the minimum ignition temperature of coal dusts (MITC) in O2/CO2 atmospheres with an O2 mole fraction from 20 to 50%. Three coal dusts: Indonesian Sebuku coal, Pittsburgh No.8 coal and South African coal were tested. Experimental results showed that the dust explosion risk increases significantly with increasing O2 mole fraction by reducing the minimum ignition temperature for the three tested coal dust clouds dramatically (even by 100°C). Compared with conventional combustion, the inhibiting effect of CO2 was found to be comparatively large in dust clouds, particularly for the coal dusts with high volatile content. The retardation effect of the moisture content on the ignition of dust clouds was also found to be pronounced. In addition, a modified steady-state mathematical model based on heterogeneous reaction was proposed to interpret the observed experimental phenomena and to estimate the ignition mechanism of coal dust clouds under minimum ignition temperature conditions. The analysis revealed that heterogeneous ignition dominates the ignition mechanism for sub-/bituminous coal dusts under minimum ignition temperature conditions, but the decrease of coal maturity facilitates homogeneous ignition. These results improve our understanding of the ignition behaviour and the explosion risk of coal dust clouds in oxy-fuel combustion atmospheres.

Experimental study on premixed fuel–air mixture venting explosion process with various gas volume concentrations and vent sizes was conducted in a 20 L cylinder vessel. The results showed that both overpressure dynamic process and flame evolution process could be divided into 4 stages, including 2 positive and 2 negative overpressure peaks. Specifically, the 1st and 2nd positive overpressure peaks were respectively induced by flame venting and external explosion, while the 1st and 2nd negative overpressure peaks were respectively caused by expansion wave after the external explosion and external large-scale vortex. Both the overpressure and flame parameters were related to initial gas volume concentrations. As the gas volume concentration increased from 1.00% to 2.99%, both the positive overpressure peaks, flame velocity and flame propagation distance presented a first increasing and then decreasing trend, but the negative overpressure peaks and the time corresponding to each overpressure peak presented a first decreasing and then increasing trend. Moreover, as the vent coefficient Kv increased from 1.16 to 10.44, the maximum positive overpressure peaks increased from 2.32 to 60.54 mbar, and the maximum axial flame velocity increased from 9.21 m·s−1 to 53.85 m·s−1. However, as the Kv increased, the minimum negative overpressure peak and the maximum radial flame velocity changed non-monotonically.

The overpressure peaks and flame propagation characteristics of hydrocarbon fuel-air mixtures vented deflagration in a 20-L cylindrical vessel with a slight static activation overpressure (PST = 2.5 kPa) and five vent opening ratio were studied by a series of experiments. The experiments focused on the effect of vent opening ratio on the overpressure peaks and flame propagation characteristics of hydrocarbon fuel-air mixture vented deflagration. The internal overpressure-time profiles and high-speed photographs of flame propagation processes were obtained. The results showed that three overpressure peaks were distinguished in the internal overpressure-time profiles, caused by the burst vent cover (pburst), the acceleration of burnt gas (pfv), and the fierce external deflagration of vented unburned fuel (pext), respectively. The changing of the vent opening ratio had almost no effect on the value of pburst and (dpburst/dt). With increasing vent opening ratio, the values of pfv, pext, (dpfv/dt) and (dpext/dt) showed a decreasing trend while the values of pburst and (dpburst/dt) were nearly constant. The flame presented a hemispherical shape before the vent cover ruptured then developed as a mushroom shape after accelerated to external field. There were three flame speed peaks during flame propagation process, resulted from venting flow acceleration, external deflagration, and axial heat flux formed by internal combustion. With the increase of vent opening ratio, all of the maximum flame speed, external average flame speed, maximum flame distance and external flame duration showed a downward trend, excepting for the internal average flame speed almost remained constant.

Godbert-Greenwald furnace was used to investigate the minimum ignition temperature of dust clouds (MITC) in air with the presence of flammable gas which is lower than its lower explosion limit (LEL). Three flammable gases (CH4, H2 and CO) and three carbonaceous dusts (anthracite coal, bituminous coal and sweet potato starch) were tested. Results showed that all flammable gases have distinct effects on the MITC of the dust samples and volatile matter content of dust plays an important role during the ignition process. Specifically, the MITC of anthracite coal dust decreased from 610 °C to 560 °C, 580 °C and 570 °C with 3% CH4, 3% CO and 2.5% H2, respectively. Moreover, a heterogeneous ignition mechanism model was proposed to verify the equally global ignition characteristic between hybrid anthracite coal-CxHy mixture and bituminous coal. All three gases had an ignorable effect on the MITC of starch dust considering the experimental error. The presence of CO and H2 slightly promoted the ignition of bituminous coal dust, but the addition of CH4 showed a distinct concentration effect on the MITC of bituminous coal: the MITC decreased with 1% CH4 while increased with 2% and 3% CH4. This negative-effect of flammable gases at such low concentrations on ignition temperature of bituminous coal dusts was found for the first time. Furthermore, the presence of the 2nd flammable gas had a smaller effect on the MITC of dust samples with a higher volatile content, resulted from the competition of heterogeneous and homogeneous ignition mechanisms.

A 20-L spherical explosion chamber was used to investigate the minimum explosion concentration (MEC) of coal dusts with small amount of flammable gas which is lower than its lower explosion limit (LEL). Two dust samples (anthracite coal and bituminous coal) and three flammable gases (CH4, H2 and CO) were tested. Two methods respectively based on overpressure and combustion duration time were used to determine the MEC of the hybrid mixtures. Experimental results show that the explosion of hybrid mixtures occurs when both dust and gas concentrations are lower than the LEL or MEC of the single substances. As flammable gas concentration increases, either explosion pressure (Pex) and explosion pressure rise (dp/dt)ex) increase or the MEC decreases for all the hybrid gas-dust mixtures as a general trend, showing a strong concentration effect. At the same concentration of coal dusts, the addition of CH4 poses a higher explosion risk than the other two flammable gases. Moreover, it was found that the results of MEC determined by both methods agree each other well, suggesting that both methods are valid to determine MEC of hybrid mixture in the synergic explosion region. The distribution of experimental data in the explosion regimes shows that the restricted areas defined by empirical formulas are insufficient from safety considerations.

Experiments were conducted in a 907.5-L fuel tank to investigate the effects of concentration, temperature, ignition energy, and humidity on the explosion overpressure transients of gasoline-air mixture explosions. The results show that the overpressure-time profiles can be roughly divided into four stages: incubation stage, slow acceleration stage, abrupt acceleration stage, descending stage. During the explosion process, intense overpressure oscillation occurred in the fuel tank. Specifically, both the maximum overpressure (pmax) and the maximum rate of overpressure rise ((dp/dt)max) increased firstly and then decreased as the gasoline concentration increased, and the correction between overpressure parameters and gasoline concentration is approximately cubic polynomial. The peak values of pmax and (dp/dt)max were obtained at the concentration of 1.71%.The influence of temperature on pmax and (dp/dt)max was relatively complex, which may result from the vessel volume and the initial gas concentration. Moreover, with the increase of ignition energy, pmax and (dp/dt)max according to a linear increase trend and a logarithmical increase trend, respectively. With the increase of initial humidity, both pmax and (dp/dt)max showed a linear decrease trend. These results are useful for understanding the explosion characteristics of fuel-air mixtures under various initial conditions in medium-scale fuel tanks.

For the oxy-coal combustion, the accumulation of coal dust in the system has a fire risk of self-ignition. Therefore, understanding the ignition dynamics of coal dust deposits in oxygen-enriched environment is essential for the prevention of fire and dust explosion. In this work, both hot-oven and hot-plate tests were conducted to study the self-ignition behaviour of coal dusts in O2/CO2 ambient with O2 mole fraction from 21% to 50%. Three coal dusts: Indonesian Sebuku coal, Pittsburgh No. 8 coal and South African coal were tested with different sizes. Experimental results revealed that the self-ignition risk increased significantly with the increasing O2 mole fraction: reducing both the critical ignition temperature (10 °C in hot-oven test and 40 °C in hot-plate test) and the ignition delay time. Comparatively, the inhibiting effect of CO2 was found to be small for self-ignition. In addition, a modified Frank-Kamenetzkii analysis was proposed to explain all measured critical ignition temperatures, and the genetic algorithm was used to determine kinetic parameters of the one-step global reaction. The analysis showed that as the coal maturity/rank increased, both the self-ignition risk and the sensitivity to oxidation decreased, along with the decreasing apparent activation energy and pre-exponential factor. Such trend did not change with the ambient oxygen condition for all three coal dusts. These results improve our understanding of the self-ignition behaviour and the fire risk of coal dust in the oxy-fuel combustion system.