Experimental investigation of the minimum auto-ignition temperature (MAIT) of the coal dust layer in a hot and humid environment
Introduction
Dust explosions and ignition hazards are major safety concerns in a majority of industries which have dusty environments, such as silos and food factories, and in the coal mining and petrochemicals industries. Accidental fires and explosions caused by the ignition of combustible dust and gas lead to massive property damage and loss of life around the world every year. In 2005 alone, 13 explosions were reported in agricultural factories in the United States [1], [2]. Between 2000 and 2012, five catastrophic explosions occurred in the United States mining industry resulting in 350 deaths and over 1000 people being injured. The total of the financial assets damages to the mines was in excess of $560 million. In addition, each mine was out of operation for a couple of months, significantly impacting on profits, and in some cases the lost production is likely to exceed the total cost of the assets damage [3]. The indirect costs of these accidents, such as the trauma imposed on families due to the loss of loved ones, however, cannot be expressed in financial terms. Hence, to prevent these accidents from happening it is vital to have a better understanding of the driving mechanisms which result in fire and explosion events.
There are numerous studies in the open literature in the field of fires and explosions in the industrial processing industries. The majority of these studies have been conducted on combustible metal dusts, such as aluminum powder [4], [5], [6] and cereal dusts, such as wheat dust in silos [7], [8], [9]. Underground coal mines are classified as highly hazardous areas from the point of view of fires and explosions. Despite the application of highly restrictive safety rules and continuous monitoring, fire and explosion accident avoidance is not always successful. This is mainly due to the variability in the characteristics of underground coal mines. The coal dust explosion severity depends on the coal type, the coal particle properties and the environmental conditions [10], [11], [12], [13], [14], [15]. Despite the large number of studies on fires and explosions in coal mines [3], [6], [7], [8], [9], [11], [12], [16], [17], there remain shortcomings in the knowledge, such as around the impacts of the properties of coal dust (such as C/H, volatiles) on the initiation of explosions under certain conditions. Sapko et al. [16] examined the variations of coal particle size in the ventilation air samples from eleven coal mines in the USA. They observed that approximately 60–77% (±14) of the particles were below 212 µm and that approximately 26–39% (±11) of the particles were below 75 µm [16]. Based on the study by Sapko et al., it can be concluded that the majority of the coal dust particles that are emitted from the mines’ shaft ventilation air are below 212 µm in size. Chen [17] conducted research to examine the components present in Ventilation Air Methane (VAM). He examined four VAM systems in Australia, including in the Hunter Valley, NSW, and the Bowen Basin, QLD. He noted that the relative humidity in the VAM systems were above 75% and in some cases reached 100%.
For a fire and/or explosion to occur in a coal mine, in addition to having a proper mixture of air and fuel, an ignition source with adequate energy is also necessary. The ignition may come from different sources such as electrical sparks, mechanical impact, open fires, hot surfaces and coal dust auto-ignition. Coal dust is a potential source of ignition due to the exothermic oxidation of coal, which has the potential to exceed the auto-ignition temperature. As such, it is important to investigate the possibility of coal dust as an ignition source as well as the environmental properties which may impact on this behavior.
The hazards of a combustible dust deposited on a hot surface was first referred to by Plamer [18], whose experimental work was carried out using a heated steel plate on which dust was set in conical heaps. Coal dust and dusts from other materials were used. It was observed that the minimum ignition temperature of coal dust varied with the depth of the bed and the composition of the dust, while no effect of packing density was noted. Nagy [19] examined the auto-ignition temperature for Pittsburgh coal dust layers. He used a typical laboratory muffle furnace to examine the variations in auto-ignition temperatures with particle size, bed depth and packing pressure for dust particle sizes ranging from 74 µm to 210 µm. He observed that the minimum auto-ignition temperature increased from 160 °C to 230 °C as the particle sizes increased from 74 µm to 212 µm. However, the minimum auto-ignition temperature (MAIT) increased as the bed depth decreased from 25.4 mm to 3.175 mm. Bowes and Townshen [20] used the same apparatus designed by Plamer et al. and observed that the particle size had a minimal effect on the MAIT while only the depth of the dust layer had a significant impact. They used coal dust samples with particles size diameters of less than 840 µm, and the coal dust particles were packed inside 5, 10 and 20 mm stainless steel rings. The minimum auto-ignition temperatures observed were 235 °C, 205 °C and 173 °C, respectively, for each thickness of steel ring. Miron [21] applied the Notational Academy of Sciences (NAS) procedure to determine if there was any correlation between the MAIT and the particle size. He examined explosive dusts such as coal dust, metal dust and oil shale for this purpose. He observed that there is a direct correlation between the MAIT and the particle size. The MAIT is higher for larger particles and a thinner dust layer [21]. These agree with the results presented by Bowes et al. [20]. Miron et al. also observed that the volatile content and chemical composition of each coal sample had a significant impact on its MAIT. Reddy [22] performed experiments on two types of coal samples: Princes coal from Cape Berton and Pittsburgh coal. The experiments were performed following the ASTM E2021 standard, with coal dust layer thicknesses of 5, 10 and 15 mm. He observed that the exothermic reactions (self-heating) gradually disappeared as the thickness of the coal dust layer increased. The first exothermic reaction was attributed to the release of volatile organic compounds and volatile matter combustion, while the second exothermic reaction refers to the char combustion. Moreover, despite the Prince and Pittsburg coals being of the same rank and the same size range (below 75 µm), there was a noticeable difference of 20 °C in terms of the MAIT. Park [23] conducted a series of experiments using samples from the Pittsburgh coal seam to measure the MAIT for different bed thicknesses of 6.4, 12.7, 19.2 and 25.4 mm, in order to develop a mathematical equation to predict the critical point of the ignition of the dust layer. The ignition for the bed thicknesses mentioned took place after approximately 2500, 4000, 6000 and 8000 seconds, respectively. Park, in an earlier study [24], examined the effects of the air flow on the test procedure for the ASTM E2021 standard and recommended that the air flow over the coal bed surface be minimized. He concluded that 33 cm/s air flow over the test bench was enough to increase the MAIT for Pittsburgh coal from 220 °C to 230 °C.
The effects of the relative humidity of the air on the MAIT of dust layers are as yet unclear. Morios [21] indicated that a small quantity of humidity increases the heating, while a large quantity (over 5%) retards the heating of the coal dust. The drying and wetting of coal dust also accelerates the heating process [25].
In underground coal mines, layers of coal dust can accumulate on a hot surface, especially if the surface is enclosed. The accumulation of the coal dust may cause compression, leading to an increase in the density of the dust layer. This has the potential to change the MAIT of the compacted coal dust layers. Nagy and Verakis [15] tested layers of three different materials (including Pittsburgh coal) to determine how the density affects the MAIT.
In the case of the Pittsburgh coal dust, increasing the density of the dust layer did not present a noticeable impact on the MAIT. Without testing at different layer thicknesses, it cannot be assumed that this behavior is synonymous with all thicknesses. In the testing of beech sawdust, Bowes and Townshend [20] found that only thin layers were affected by density changes. Other studies [26], [27], [28], [29], [30], [31] addressed the effects of weathering on the spontaneous ignition of coal dust. In a study by Kucuk [26], the effect of moisture on the spontaneous ignition of a coal dust layer was investigated. A water concentration by mass of 25% was added to a coal dust sample followed by air drying (at 90 °C) until all the added moisture was evaporated. It was concluded that the real effect of humidity on the minimum ignition temperature of coal cannot be determined as the temperature impacts on the active functional groups and reduces the oxygen by enhancing the oxidation process [32], [33]. Beamish [34] established a relationship between the spontaneous ignition of coal dust with the self-heating rate, called “R70”, which was first introduced by Humphreys [35]. Beamish [36] investigated the effects of moisture on the self-heating of eleven coal dust samples. He concluded that moisture defers the self-heating for a specific time, depending on the amount of moisture in the coal sample. In addition, he observed that the moisture trapped in the coal sample prolonged the evaporation stage. As such, the entire process, which includes the intense oxidation and self-ignition, was extended. Despite the importance of the relative humidity effects and the coal dust moisture content on the self-heating of coal dust layers, there are not many studies to address this shortcoming in the knowledge. As such, a comprehensive study was conducted at the University of Newcastle, Australia, to examine these effects further. The aims of this study are to: (1) investigate the effects of relative humidity on the MAIT of coal dust layers; (2) determine the MAIT deposited in the proposed VAM capture duct; (3) examine the correlation between the MAIT and the physical and chemical properties of the coal dust; and (4) examine the effect of the packing density on the MAIT of the coal dust layers inside the proposed VAM capture duct.
Section snippets
The experimental setup
The experimental apparatus used to determine the MAIT of the coal dust layers is shown in Fig. 1. It consists of a flat surface furnace, data logger, computer program, and some accessories. The hot plate furnace was comprised of a circular aluminum plate of 25.4 mm thickness and 203 mm diameter. A circular stainless steel ring of 5 mm height and 50.8 mm diameter was used to keep the coal dust particles at the center of the furnace plate. Three K-type thermocouples were used to continuously measure
The effects of particle size on the MAIT
Fig. 3 shows the variation of the MAITs with particle size for the coal dust samples below 212 µm from Mine B. The coal dust layer thickness for these sets of experiments was 5 mm.
As observed, the MAITs significantly increased with increasing particle size. The MAIT for coal dust particle sizes below 74 µm was approximately 235 °C, while the MAIT increased to 250 °C and 270 °C for the 74–125 and 125–212 µm size fractions, respectively. The MAITs reduced by about 13% as the particle sizes decreased
Conclusion
- •
The MAITs for coal dust layers significantly increases with increasing particle sizes. The MAITs changed for particle sizes both above and below 74 µm. For particle sizes in the range of 74 µm to 212 µm, the MAITs varied between 15 °C to 35 °C. The effect of the particle sizes on the time needed to ignite also varied from 1500 s, for the finest particles, to 2400 s for particles below 212 µm.
- •
The effect of the particle sizes on the severity of the ignition indicates that for coal beds consisting of
Acknowledgments
The authors wish to acknowledge the financial support provided to them by the Australian Coal Association and Low Emission Technology (ACALET) (G1201029 2), Australian Department of Industry (G1400523), the University of Newcastle (Australia). In addition, special gratitude is also given to the Higher Committee for Education Development (HCED) and the Midland Refineries Company (MRC) of the Iraq Government for sponsoring a postgraduate candidate working on this project.
References (45)
- et al.
Dust explosions-cases, causes, consequences, and control
J. Hazard. Mater.
(2007) - et al.
A review on understanding explosions from methane-air mixture
J. Loss Prev. Process Ind.
(2016) - et al.
Explosion characteristics of nano-aluminum powder-air mixtures in 20L spherical vessels
Powder Technol.
(2011) - et al.
Risk assessment of the ignitability and explosivity of aluminum nanopowders
Process. Saf. Environ. Prot.
(2012) - et al.
The effect of particle size polydispersity on the explosibility characteristics of aluminum dust
Powder Technol.
(2014) - et al.
Determination of the risk of self-ignition of coals and biomass materials
J. Hazard. Mater.
(2012) - et al.
Methane-coal dust hybrid fuel explosion properties in a large scale cylindrical explosion chamber
J. Loss Prev. Process Ind.
(2016) - et al.
Effects of ignition energy on fire and explosion characteristics of dilute hybrid fuel in ventilation air methane
J. Loss Prev. Process Ind.
(2016) Analysis of underground fires in Polish hard coal mines
J. China Univ. Min. Technol.
(2008)- et al.
Coal Dust Particle Size Survey of U.S. Mines
J. Loss Prev. Process Ind.
(2007)
Characteristics of coal mine ventilation air flows
J. Environ. Manag.
The ignition of dust layers on a hot surface
Combust. Flame
Effect of inerts on layer ignition temperatures of coal dust
Combust. Flame
A means to estimate thermal and kinetic parameters of coal dust layer from hot surface ignition tests
J. Hazard. Mater.
The role of sorption of water vapour in the spontaneous heating of coal
Fuel
The role of desorption of moisture from coal in its spontaneous heating
Fuel
A petrographic examination of coal oxidation
Int. J. Coal Geol.
Adiabatic testing procedures for determining the self-heating propensity of coal and sample ageing effects
Thermochim. Acta
Effect of moisture content on the R70 self-heating rate of Callide coal
Int. J. Coal Geol.
Smoldering combustion of dust layer on hot surface
J. Loss Prev. Process Ind.
The effect of moisture condensation on the spontaneous combustibility of coal
Fuel
Coal oxidation at low temperatures: oxygen consumption, oxidation products, reaction mechanism and kinetic modelling
Prog. Energy Combust. Sci.
Cited by (26)
Coal mine explosions in India: Management failure, safety lapses and mitigative measures
2023, Extractive Industries and SocietyExperimental study of the characteristics of explosions generated by methane mixtures, as a function of the type of atmosphere and environmental conditions
2022, Journal of Loss Prevention in the Process Industries