Ignition and explosion characteristics of coal dust and methane mixture under conditions pertinent to ventilation air methane abatement

Al-Zuraiji, Mohammed Jabbar Ajrash

Title: Ignition and explosion characteristics of coal dust and methane mixture under conditions pertinent to ventilation air methane abatement
Creator: Al-Zuraiji, Mohammed Jabbar Ajrash
Relation: University of Newcastle Research Higher Degree Thesis
Resource Type: thesis
Date: 2020
Description: Research Doctorate - Doctor of Philosophy (PhD)
Description: Fire and explosion caused by combustible dusts and flammable gases is a major safety concern in the extractive industries, such as petroleum and coal mining. Combustible coal dust and methane gas can be exploded in the presence of an ignition source with sufficient ignition energy. These accidents can claim many lives and cause considerable property damage. The indirect costs of these types of accidents, however, are substantially higher and cannot be expressed in dollars. To avoid fires and explosions from occurring in coal mines as well as to reduce the consequences of accidental explosions, an indepth understanding of this phenomena is critical. To understand the most important fire and explosion driving components, as well as to develop the most suitable counter measures, a comprehensive experimental work was carried out at the University of Newcastle, Australia. The aim of this research was to determine the ignition and explosion characteristics of coal dust clouds and methane mixtures in Ventilation Air Methane (VAM) Capture Ducts. The objectives of this work include investigating the impact of environmental conditions and ignition source energy on the thermal, fire and explosion properties of coal dust and methane gas, which are often in the VAM stream. The first study focused on the Minimum Auto Ignition Temperature (MAIT) for coal dust particles deposited in layer form inside the VAM safety capture duct. This study was conducted by employing a flat top furnace (made by the ANKO company) in accordance with the ASTM E2021 and IEC 1241 standards. The MAIT for coal dust layer outcomes indicated that the humidity and the hot environment have slight influences on the MAIT. However, the coal dust particle sizes and packing have a significant impact on the MAIT. The multi-ignition phenomenon of the dust layer is attributed to the moisture content in the dust bed. The influences of environmental conditions and the particles’ compositions on the MAIT of dust clouds were investigated according to the potential coal dust particles that were likely to be present in a VAM capture duct stream. The goal was achieved by employing an ANKO MAIT dust cloud apparatus (ASTM E1491). It was revealed that 15 g.m⁻³ of coal dust cloud is able to auto ignite when it is in cloud form, and the flame of combustion travels and reaches the end of the furnace. The moisture content, which was up to 4.21 %, had no influence on the MAIT of the dust cloud. For particle sizes between 0-74μm it was found that for a coal dust concentration above 300 g.m⁻³ the volatile matter content has a profound impact. However, for coal dust concentrations below 100 g.m⁻³ the mean particle size (D₅₀) has a pronounced impact on the volatile matter due to the reaction on the surface which drives the ignition process. The third investigation focused on the methane’s lower flammability limit and the lower explosion limit of a hybrid methane-coal dust explosion. Two apparatus were employed for this aim. These were the FL-Range apparatus (ASTM E681) and the 20L explosion chamber (ASTM E1226). The outcome showed that the Lower Flammable Limit (LFL) of methane could be reduced to only 2.9 % as the ambient temperature increased to 144°C , however, the pressure rise was in the range of 8-11 mbar at all the ambient temperature ranges (25°C to 150°C). The humidity condition of up to 80 % RH did not have any significant effect on the LFL of the methane. Additionally, it was found that even 0.75 - 1.25 % of methane could reduce the Minimum Explosion Concentration (MEC)of the coal dust cloud, according to the ignition energies (1, 5 and 10 kJ). Also, the influence of methane played an important role in the deflagration index (K_st) of the coal dust cloud. A large scale detonation tube (11 sections, 30 m total length, 0.5 m diameter) was also employed to investigate the methane flame deflagration at varied reactive section lengths of up to 30 m. The maximum pressure wave velocity was 312 m.s⁻¹ at stoichiometric methane concentration; however, the maximum flame velocity was 170 m. s⁻¹ (in the 25 m reactive section length). The worst expected damage from the explosions of 9.5 % and 7.5 % methane concentrations at 6 m, 12 m and 25 m reactive section lengths were 1 - 99 % of fatalities among exposed populations, due to direct blast effects. The detonation tube was further used to investigate the initial ignition energy influences on the methane flame deflagration. The lower flame velocities were 25.56 m.s⁻¹ and 28.8 m.s⁻¹ respectively for 7.5 % and 9.5 % methane concentrations at the beginning of the detonation tube, using a 1 kJ initial ignition energy, and the maximum flame velocities were 134 m.s-1 and 179 m.s-1 respectively for 7.5 % and 9.5 % methane concentrations when using a 10 kJ initial ignition source at 26 m distance. The pressure wave velocity at the end of the detonation tube doubled when the initial ignition energy was increased by 5 times. It was revealed that increasing the initial ignition energy by 5 and 10 times results in increasing the stagnation pressure by 35 % and 58 % respectively for stoichiometric methane concentrations. Also, it was found that the initial ignition source increase not only boosts the pressure rise at the section of the ignition, but also boosts the pressure rise along the detonation tube, the maximum pressure rise of a 9.5 % methane concentration increasing by 56 % as the initial ignition source was increased 10 times. The influence of the coal dust cloud concentration on the flame deflagration in the detonation tube was thoroughly investigated. The stagnation pressure and the dynamic pressure significantly increased to about 3.6 times when adding 30 g.m⁻³ in hybrid form with a 5 % methane concentration. Also, the flame travelling distance for 5 % methane increased by 14 m as 30 g.m⁻³ coal dust concentrations were introduced. In contrast, the 10 g.m⁻³ coal dust in a hybrid form with a 7.5 % methane concentration significantly increased the stagnation pressure by about 2.4 times. The flame travelled for a 12.5 m distance for a 5 % methane concentration, however, the flame travelled for 20.5 m for the hybrid 5 % methane/10 g.m⁻³ coal dust concentration. Introducing a 30 g.m⁻³ coal dust concentration as a hybrid form with a 5 % methane concentration extended the flame travelling distance to the end of the detonation tube. The flame velocity accelerated by about 1.4 times when a 30 g.m⁻³ coal dust concentration and a 5 % methane concentration was introduced. Moreover, the average flame velocity along the detonation tube accelerated by about 1.15 times as a 10 g.m⁻³ coal dust concentration was introduced. The investigations presented in this thesis addressed the potential hazards and the consequences of methane-coal dust in the VAM capture duct conditions. These findings give a better understanding of the ignition, explosion, and flame deflagration of flammable gases and combustible dust in the process industries. These findings will also assist in future work on designing effective mitigation and alarm systems for flame deflagration in chemical plants that deal with flammable gases and diluted combustible dust.
Subject: ventilation air methane; fire; pressure relieving; detonation tube; flame propagation; mine safey; fire countermeasure; explosion countermeasure; explosion; coal dust ignition; minimum ignition energy; auto ignition temperature; hybrid explosion; methane explosion; methane; flame deflagration
Identifier: http://hdl.handle.net/1959.13/1412553
Identifier: uon:36498
Language: eng
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