Explosion severity of methane–coal dust hybrid mixtures in a ducted spherical vessel

    Explosion severity of methane–coal dust hybrid mixtures in a ducted spherical vessel Sazal K. Kundu, Jafar Zanganeh, Daniel Eschebach, Behdad Moghtaderi PII: DOI: Reference:

S0032-5910(17)30769-6 doi:10.1016/j.powtec.2017.09.041 PTEC 12846

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Powder Technology

Received date: Revised date: Accepted date:

30 January 2017 30 June 2017 21 September 2017

Please cite this article as: Sazal K. Kundu, Jafar Zanganeh, Daniel Eschebach, Behdad Moghtaderi, Explosion severity of methane–coal dust hybrid mixtures in a ducted spherical vessel, Powder Technology (2017), doi:10.1016/j.powtec.2017.09.041

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ACCEPTED MANUSCRIPT Explosion Severity of Methane–Coal Dust Hybrid Mixtures in a Ducted Spherical Vessel

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Sazal K. Kundu, Jafar Zanganeh*, Daniel Eschebach, Behdad Moghtaderi

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Priority Research Centre for Frontier Energy Technologies & Utilisation, Discipline of Chemical Engineering, School of Engineering, Faculty of Engineering & Built Environment, University of Newcastle, Callaghan, NSW 2308, Australia

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Abstract

This article reports an investigation on the explosion characteristics of methane–coal dust

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hybrid mixtures in a ducted spherical vessel. Methane–coal dust hybrid mixture explosion can occur in coal mines and spread into mine tunnels. While investigating the effects of

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methane addition to coal dust–air mixtures, the violence of coal dust explosions was found to

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increase significantly in the presence of methane. The energy of ignition was found to impact on the pressure rises in the vessel and in the duct. The experimental data and scientific

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analysis presented can assist in addressing ducted explosions originating from hybrid

Keywords

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mixtures in process industries such as coal mines.

Hybrid mixture explosion, hazards in process industry, coal mine accidents, explosions in process industry, explosion venting

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[email protected]

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ACCEPTED MANUSCRIPT 1. Introduction When a gas and a dust coexist in air and explode simultaneously, a complex physio-chemical

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process occurs that eventuates into several notorious hazards, including toxic gases, thermal

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radiation and naked flame [1]. Gas and dust, of non-explosive concentrations on their own,

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can form an explosive mixture. The mixtures of two states (gas and solid) of flammable matters, that have the potential to explode, are known as hybrid mixtures in explosion studies. The explosion characteristics of hybrid mixtures and their pure components are quite

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different [2].

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Since the industrial revolution, several hybrid mixture accidents have been documented. One of the most notable susceptible industrial activity zones for hybrid mixture explosions is in

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underground coal mines, where methane and coal dust coexist. A number of coal mine

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accidents have been described in literature [3-6] which have motivated research on understanding the explosion severities of methane [7], coal dust [8, 9] and methane–coal dust

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hybrid mixtures [10-13]. The most devastating coal mine accident in human history occurred in 1942 at the Benxihu Colliery in China, leaving 1,549 people lifeless [14]. The worst coal

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mine explosion in Australia occurred in 1902, which killed 96 people at Mount Kembla Mine [15]. Although progress has been made in understanding coal mine explosions, the loss of lives due to coal mine accidents has not been eliminated. In 2013, the reported death toll due to coal mine explosions, in China alone, was 1,049 [16]. Other industries that are sensitive to hybrid mixture explosions include the pharmaceutical and agricultural chemical industries [17, 18]. In 1997, a severe explosion of agricultural chemical killed three fire fighters and injured sixteen people at BPS Inc., West Helena, Arkansas, USA [19]. The explosion was caused by a dust-air hybrid mixture which was produced from the thermal decomposition of Azinphos methyl (AZM) pesticide. Industries that handle other organic dusts, such as wood dust, cornflour dust, wheat dust, coffee dust 2

ACCEPTED MANUSCRIPT and sugar dust, are also prone to hybrid explosions as the decomposition of organic dusts can generate flammable vapours.

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Process units of various geometries are employed in process industries handling organic

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dusts. Some examples of process units include storage hoppers, bag filters, blenders, dryers,

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cyclones and grinders. Explosions from these process units can spread easily to the connected pipelines [20]. The transmission of explosion violence from a vessel or process unit to the

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connected pipelines is known as a ducted explosion [21]. In coal mines, the mine area may be connected to a channel which can resemble the geometry of a ducted vessel. Therefore,

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understanding the explosion characteristics of gas–dust hybrid mixtures in a ducted vessel can assist in developing safety measures for process industries such as coal mines.

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Previous studies of various dusts in ducted vessels have suggested that the transition from

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deflagration to detonation (DDT) may occur in a duct, depending on various parameters,

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including the vessel’s volume, the concentration of the dust, the length to diameter (L/D) ratio of the duct, the explosivity of the dust and the energy of the ignition source [22, 23]. A similar phenomenon is expected for hybrid explosive mixtures. Therefore, understanding the

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explosion characteristics of hybrid explosive mixtures in ducted vessels is critical in addressing the hazards produced in those systems. Previous studies revealed explosion characteristics of methane-coal dust hybrid mixture explosions in a closed vessel [11, 24]. Methane-coal dust flame propagation was also studied in a straight tube [10, 25]. However, to the extent of our knowledge, the study of explosions in methane–coal dust hybrid mixtures has not been conducted before in a ducted vessel. Previous ducted vessel explosion studies were focused mainly on gas explosion [21, 26-28]. The present study was devoted to developing the explosion characteristics of methane–coal dust hybrid mixtures in a ducted vessel. A spherical vessel of 20 litres volume, connected 3

ACCEPTED MANUSCRIPT with a 2813 mm long duct, was employed for this purpose. A unique approach was applied in characterising the explosion behaviours of methane–coal dust hybrid mixtures, where a

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spherical vessel was employed as vessels of a spherical geometry theoretically produce

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maximum explosion pressures, compared to vessels of other geometries [29, 30]. With the

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aim of correlating the experimental data with a real world scenario, a wafer check valve, comprised of a flexible disc, was employed in the experimental investigations and the flexible disc was fully opened at the time of the explosion. This approach provided an opportunity to

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understand the explosion characteristics of methane–coal dust hybrid mixtures in the absence

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of any membrane or disc that are often employed in the investigations of ducted explosions. The effects of the ignition energy, duct length and concentration of methane on explosion

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parameters such as the peak explosion pressure and rate of pressure rise have been analysed

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and reported in this article.

2. Experimental Procedure and Technique The investigations were carried out by employing a 20 litre explosion sphere apparatus

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(ANKO Trading Limited, see Figure 1). A closed circuit water cooling system was included with the test vessel, where the water was driven by a pump and cooled in a heat exchanger. The pressures were measured by piezoelectric pressure transducers (Kistler 701A) integrated in the vessel. With the aim of achieving the goals of the present investigation, the 20 litre sphere apparatus was transformed to a ducted vessel by connecting a duct to its viewing port (see Figure 2 [28]). This transformation involved a connection between the vessel and a duct, where the vessel was connected with a connector and a valve, and the duct was attached at the other side of the valve. The lengths of the connector, valve and the first and second portions of the

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ACCEPTED MANUSCRIPT duct were 70, 203, 1000 and 1540 mm, respectively. The internal diameter of the valve was 50 mm, while the connector and both portions of the duct had an identical internal diameter

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of 42.72 mm. This enumerates an L/D value of 65.2 for the entire duct. In the semi-confined

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experimental investigations, the three configurations of valve only (273 mm), valve and short

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tube (1273 mm) and valve and both tubes (2813 mm) were employed. In the valve only investigations, the duct length was very small, and this configuration was considered as a

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vented vessel configuration.

Five pressure transducers (model – ATM.ECO/Ex, manufacturer – STS Sensor Technik

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Sirnach AG, 0-10 bar) and five photodiode sensors (model – DET10A, manufacturer – Thorlabs, 200-1100 nm) were employed along the length of the duct. Photodiode sensors

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were employed with viewing ports which narrowed down the viewing angle (the half angle

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was approximately 10°). This narrow sensing photodiode architecture of the explosion apparatus assisted in capturing light that mainly travels in a perpendicular direction from the

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duct axis. The employed photodiode sensors had a response time of 1 ns. The data acquisition system had a data collection capability at a rate of 10 MS s-1.

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The piezoresistive pressure transducers had a response time of less than 1 ms. The accuracy of these piezoresistive pressure transducers are ±0.25% of Full Span Value. The full span value of those pressure sensors were 10 barg. This provides the maximum tolerance value of ±0.025 barg. These pressure sensors were calibrated introducing an elevated pressure in the ducted vessel where the open end of the duct was closed by a blind flange. The distance of the sensors from the sphere and the locations of the sensors around the periphery of the duct are presented in Table 1. The valve employed in the investigation was a wafer check valve and this valve included a rubber coated flexible disc for its closure mechanism. The valve was sourced from Process

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ACCEPTED MANUSCRIPT System Private Limited (Model number: FCVC-050). The flexible disc functioned in the closure of the flow-path, while keeping the vessel in vacuum.

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After completing start-up of the instruments and conducting safety checks, coal dust was

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manually poured into the dust chamber. Methane was then introduced into the system through

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a modified line by evacuating the sphere to a desired pressure level (see Table 2). While the lower flammability limit (LEL) of methane is 4.6±0.3% [3], explosion can occur at a

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concentration of methane lower than LEL in the presence of coal dust, as discussed previously. Therefore, the lowest value of methane concentration considered in the

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investigation was 3.5%, as described in Table 2.

When the vessel pressure reached – 0.60 barg, the transportation of coal dust from the dust

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chamber to the sphere was initiated. The pressure unit ‘barg’ represents gauge pressure in the

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unit of ‘bar’ [31]. In order to transport coal dust to the sphere, the dust chamber was initially

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pressurised to 20 barg. In the next stage, dust chamber containing coal dust and pressurised air was released into the sphere via opening a fast acting valve. The pressurised stream of

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coal dust and air then mixed with methane, producing a uniform hybrid mixture. The time delay introduced into the control system from discharge of coal dust to initiation of ignition is known as injection to ignition time delay [32]. A 60 ms injection to ignition time delay was applied for the experiments. This delay is purposeful and reduces the turbulence to a level that does not impact much on the pressure transducer reading; however, the uniformity of the mixture is maintained after the elapse of the injection to ignition time delay. After 60 ms elapsed from release of coal dust containing pressurised air into the sphere, the system’s pressure equilibrated to atmospheric pressure (1 atm), the flexible disc of the check valve (mounted upside down to allow gravity assisted opening) opened just prior to the time the igniter was initiated. A small variation was found in the opening of the check valve and 6

ACCEPTED MANUSCRIPT the tolerances of pressure and time values were 40 mbar and 1 ms respectively; this means that the check valve opened 1 ms to zero ms earlier than the initiation of igniter and at 973 to

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1013 mbara pressure. Here ‘mbara’ represents absolute pressure in the unit of ‘mbar’[31].

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The experiments were carried out with compressed air (Coregas). The methane gas (Coregas)

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employed in the tests was of 99.95 % purity. Pyrotechnic igniters of 1, 5 and 10 kJ (Simex Control) were employed in the investigations and they were employed at the centre of the

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explosion vessel. The pressure rise values due to igniters were deducted in the reported explosion pressure data.

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The coal dust sample was sourced from a New South Wales thermal coal mine in Australia. The particle size distributions and proximate analysis of the coal dust samples are presented

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in Table 3. The experiments were conducted outdoor in Newcastle, Australia. The

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temperature variation from one day to other was minimum (average temperature was 22 °C

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[33]). With the aim of reducing the effect of temperature, experiments were conducted in the morning and in the afternoon avoiding the mid-day time.

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The pressure and light sensing data for the duct were collected from the photodiode sensors and the piezoresistive pressure transducers employing an ADLINK data acquisition system comprised of a PXI express chassis (PXES – 2590(G)), a data acquisition card (PXIe – 3975) and six digitizer cards (PXIe – 9816). The software employed in the data acquisition process was ANKO Dvmsoft (version 2.0.1.0). The pressure data for the vessel was collected from the dynamic pressure transducers employing ANKO Explosion Plotter (version 1.0.0.0) software. Experiments were repeated for each concentration. The data presented in the manuscript represent average values of two experiments. Error bars, if included, represent the deviation from the average values of the experimental data points.

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ACCEPTED MANUSCRIPT 3. Results and Discussion

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3.1. The effects of variations of methane concentrations

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Hybrid mixtures exhibit unique explosion characteristics compared to their pure components.

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The explosion violence of coal dust particles significantly increases in the presence of methane, and this phenomenon is reflected in Figure 3. Investigations carried out at 5 kJ

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ignition energies were plotted in this figure to explore the effect of variation of methane concentrations on explosion pressure. The effects of ignition energies on explosion pressure

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were also examined and reported in the later section of the article. As can be seen in Figure 3, in the absence of methane, an explosion with coal dust concentration of 100 g m-3 produced a

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reduced peak explosion pressure of lower than 1 barg in the ducted spherical vessel. The term

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reduced peak explosion pressure is usually used to describe the highest pressure rise inside a vessel for vented and ducted systems. The addition of methane increases the reduced peak

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pressure and, depending on the particle size of the coal dust and its chemical composition, the values of reduced peak pressure reach their highest values and then reduce.

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For methane–air mixtures, the highest explosion pressure is produced at the stoichiometric condition (9.5 % methane in air) [3]. The presence of coal dust made a difference in Figure 3 and, as illustrated in the figure, a coal dust concentration of 100 g m-3 produced a unique hybrid characteristic with a methane concentration of 6.5 %. It yielded the highest reduced peak explosion pressure for the coal dust – air system. For methane-air mixtures, the maximum explosion pressure is generated at stoichiometric or near stoichiometric conditions. Figure 4 illustrates explosion pressure profiles for methane-air mixtures [34-37].

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ACCEPTED MANUSCRIPT Mechanistically, coal dust particles can participate in explosion phenomena by two pathways (Figure 5 [38, 39]). In heterogeneous combustion pathways, coal dust particles directly

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participate in the explosion event. In homogeneous combustion, the coal dust particles are

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required to follow a few steps including heating and devolatilisation. The reactions of volatile

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gases then lead to explosion.

The ignition energy of methane is lower than coal dust particles. Therefore, methane gas

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initially participates in the explosion phenomena. The burning of methane gas therefore provides energy to coal dust particles to participate in the explosion event.

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With the introduction of coal dust particles, the chemistry of methane-air explosion phenomena changes. In presence of coal dust particles, the maximum explosion pressure

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produces much lower than stoichiometric methane-air mixture. As found in the experimental

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investigation, 6.5 % methane and 100 g m-3 coal dust produced an optimum hybrid mixture

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which generated the maximum explosion pressure. While the pressure rise in the vessel of a ducted vessel is termed as reduced explosion

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pressure, there is no reduced pressure phenomenon involved in the duct portion of this system. Thus the pressure-time profile developed at the duct of a ducted vessel is explosion pressure profile. The explosion pressures recorded by the various pressure transducers in the present study are disclosed in Figure 6. The pressure trends at the various locations on the duct (Figure 6) were very similar to the pressure trends of the vessel (Figure 3). The difference in peak explosion pressures at the first two sensors (PT1 and PT2) was very minimal, and the initial portion of the duct experienced the highest explosion pressure compared to the rest of the duct. Downstream of the duct, the peak explosion decreased and the value of this pressure drop was approximately 0.8 bar for the explosion tests of 100 g m-3 coal dust and 6.5 % methane hybrid mixtures.

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ACCEPTED MANUSCRIPT 3.2. The effects of variations of the coal dust concentrations The effects of the coal dust concentration on a particular methane–air system (9.5 % methane

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in air) were investigated and are presented in Figure 7. Interestingly, the reduced peak

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pressure of the vessel was observed to be reduced with the introduction of coal dust in a

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stoichiometric methane–air mixture, and the value of this parameter continually decreased with the increase of the coal dust concentration. From Figure 3, it can be certain that coal dust

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participates in the combustion process. With the increase of coal dust in the initial mixture, the air/fuel ratio decreases. This leads to incomplete combustion of coal dust particles. As the

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concentration of the coal dust particles increased in the hybrid mixture of methane–coal dust– air system, the number of unburned coal dust particles increased. The unburned coal dust

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particles functioned as a heat sink [40], and as the number of unburned particles increased,

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the heat absorption increased. The higher heat absorption in the methane–coal dust hybrid mixture with high coal dust concentration therefore reduced the higher reduced peak

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pressure, as reflected in Figure 7.

The pressure profile in the duct was very similar to that of the vessel. As disclosed in Figure

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8, the peak explosion pressures at every pressure transducer reduced with the increased coal dust concentrations in 9.5 % methane concentration in air. However, the decrease of the peak pressure rise in the duct was lower than that in the vessel. In the vessel, the reduced peak pressure decreased 0.8 bar when 100 g m-3 coal dust was added in stoichiometric methane-air mixtures. In contrast, the peak explosion pressure decreased, up to 0.6 bar, when 100 g m-3 coal dust was added in stoichiometric methane-air mixtures. This suggests that the coal dust particles continually combust as they move along the tube. Additionally, the mechanism of the gas–dust burning process can be understood from this observation as it suggests that the gas molecules combust initially, and then the dust particles. The lower values of the reduced peak pressure in the vessel suggest that a smaller amount of dust particles were burned in the 10

ACCEPTED MANUSCRIPT vessel. As the burning materials moved along the duct, the number of dust particles burned increased and released energy. Therefore, the value of the peak explosion pressure increased

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with the increase of coal dust concentration with a fixed concentration of methane in air.

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3.3. Pressure and flame histories

Figure 9 illustrates the light intensity profiles recorded at the various photodiode sensors.

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These profiles essentially provide an understanding of the burning characteristics of the methane – coal dust hybrid mixtures along the length of the duct. The first photodiode sensor

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(PD1) experienced the flame earlier than any other sensor. Additionally, the amount of unburned flammable materials reduced along the length of the duct. These two factors lead to

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a gradual reduction of the flame’s duration in the photodiode sensors, and the recorded flame

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durations from the first to the fifth sensors were 68, 49, 44, 19 and 12 ms, respectively.

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The third photodiode sensor, located at 1350 mm from the vessel-duct connection point, was exposed to the highest light intensity. This was because the highest burning rate in the duct

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was in the vicinity of the third photodiode sensor. This finding suggests that the level of turbulence increased up to 1350 mm of the duct. As the fourth and fifth photodiode sensors also showed light signals, the unburned flammable materials remained at least up to the fifth sensor. The continuous burning of flammable materials up to the fifth sensor suggests that a violent turbulence was maintained up to the end of the duct. All the pressure transducers showed steep pressure rises (see Figure 10). These pressure transducers represent the pressure characteristics along the length of the duct, and this finding suggests that the pressure rises at any point on the duct occurred sharply. Once the peak value of the explosion pressure was attained, it reduced at a relatively gradual rate. This suggests

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ACCEPTED MANUSCRIPT that a high level of turbulence is produced in the duct, and is maintained up to the end of the duct.

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The pressure and light sensing curves obtained at the fourth and fifth sensors are interesting

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and, as illustrated in Figures 9 and 10, the spans of the light intensity are shorter than the

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durations of the pressure signals in those locations (fourth sensors – 19 ms versus 37 ms; fifth sensors – 12 ms versus 32 ms). Combustion waves decay as the amount of the flammable

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materials in the flow stream reduce, while the pressure waves, once generated, maintain a great pressure level in confined and semi-confined spaces. The amount of combustible

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materials in the flow stream reduced at the locations of the fourth and the fifth sensors; however, the levels of confinement remained very high in these locations. Therefore,

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durations of the light sensing curves were found to be much lower in those locations as

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compared to durations of the pressure curves.

3.4. The effects of ignition energies on explosion pressure and flame speed

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Pyrotechnic igniters with 1, 5 and 10 kJ ignition energies were employed in the experimental investigations. The ratings of these igniters correspond to the amount of ignition powder charged into them. The higher the ignition powder charged, the higher the rated energies of these igniters. As a pyrotechnic igniter with a high rating contains a high amount of ignition powder, it produces a high flame area. The overall burning rate is proportional to the flame area produced [41] and, therefore, the explosion pressure increased with the increase of the rated value of the ignition energies of the igniters. Table 4 discloses the reduced peak explosion pressures in the vessel and the peak explosion pressures along the length of the duct at various ignition energies. The data presented in this table was collected from the investigations with the hybrid mixtures of 9.5 % methane 12

ACCEPTED MANUSCRIPT concentration and 100 g m-3 coal dust in a ducted vessel employing a duct length of 2.813 m. As can be seen in this table, the explosion pressures, in general, increased with the ignition

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energies.

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In the duct, the first two pressure transducers (PT1 and PT2) experienced very similar peak

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explosion pressures and the values of this parameter from those two pressure transducers were within a range of 0.1 bar. When the ignition energies of 1, 5 and 10 kJ were applied for

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the investigations of the 9.5% methane concentration and 100 g m-3 coal dust mixtures, the peak explosion pressures at the first pressure transducer were observed at 119.9, 105.1 and

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97.3 ms, respectively. Employing the data, the estimated values of explosion times were 32.9, 18.1 and 10.3 ms. The explosion time is the time interval to detect the peak explosion

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pressure where the time is counted from the ignition time [42]. This suggests that the peak

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explosion pressure attained in the duct was earlier for the higher ignition energies.

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Figure 11 presents the pressure profiles along the length of the duct at 119.9, 105.1 and 97.3 ms when the ignition energies of 1, 5 and 10 kJ were applied. The diameter of the duct was employed in the estimation of dimensionless length in the x-axis of this figure. It can be

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considered that these pressure profiles illustrate the pressure characteristics along the length of the duct, when the duct experienced the highest explosion pressures. A gradual pressure decrease was observed along the length of the duct for all the ignition energies applied; however, higher pressures were observed at all points of the duct at higher ignition energies. In the present study, the flame speed was observed to be dependent on the introduced igniter’s energy (Figure 12). In the explosion studies of the 9.5% methane and 100 g m-3 coal dust hybrid mixtures with the 1, 5 and 10 kJ igniters, the lowest flame speed was yielded by a 1 kJ igniter, with a value of 203 m s-1. This flame speed value relates to a high level of Reynolds numbers indicating the presence of turbulence along the length of the duct. With

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ACCEPTED MANUSCRIPT the increase of ignition energy, the flame became atrocious, and the highest flame speed was

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found to be 395 m s-1 at 10 kJ.

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3.5. The effects of the presence of the duct

The presence of a duct transforms a vessel from its confined to a semi-confined state and, therefore, the pressure rise in a vessel due to an explosion is reduced in ducted vessels

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compared to confined vessels. The length of the duct plays an important role in the

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confinement and impacts on the explosion pressure and rate of pressure rise. As the length of the duct increases, the confinement increases. These phenomena are reflected in Figure 13

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and, as can be seen in this figure, the highest explosion pressure was found in the confined

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vessel. These pressure profiles were developed by deducting the explosion pressure profiles of igniters from the experimentally obtained pressure profiles for methane-air mixtures from

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the ignition point (87 ms in the figure). As illustrated in Figure 13, the reduced peak explosion pressure in the vented explosion test was the lowest when compared to the

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confined vessel or ducted vessel experiments. The reduced peak explosion pressure increased from the vented to the ducted configuration, and between the two ducted configurations the value of this parameter increased slightly with the increase of the duct length. The effects of the confinement due to the presence of the duct were further studied by varying the ignition energies, and are presented in Table 5. While the ignition energy played an important role for each configuration, the patterns of the peak pressure values, from higher to lower confinement, were similar for each of the ignition energies. Similar to the 5 kJ investigation, as presented in Figure 13, the highest peak explosion pressures were observed for the confined configuration, while the lowest reduced peak explosion pressures were found

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ACCEPTED MANUSCRIPT in the vented configuration and the increase of the duct length increased the reduced peak pressures in the 1 and 10 kJ experiments.

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Figure 14 presents the rates of pressure rise (dP/dt) in the vessel for the various

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configurations of the explosion unit. These rates of pressure profiles were constructed by

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deducting the rates of pressure profiles of igniters from the experimentally obtained rates of pressure profiles for methane-air mixtures. As can be seen, the maximum rates of pressure

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rise ((dP/dt)max) were found to be 485, 375, 350 and 330 bar s-1 for the vessels of vented, 1.273 m duct, 2.813 m duct and closed configurations, respectively.

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The reactivity of an explosive mixture can be understood from the rate of pressure rise data and, therefore, this is an important parameter in explosion studies [27, 43]. The rate of

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pressure rise mechanism in vented and ducted vessels is quite different from that in confined

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vessels. In vented and ducted systems, highly non-uniform flow fields usually develop. In

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contrast, the flow velocity decays to zero at the wall in closed vessels [41]. The non-uniform flow fields that develop in vented and ducted systems maintains high flow velocities and turbulences. A high level of turbulence induces a high burning rate and, therefore, the rates of

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pressure rise are higher in vented and ducted vessels when compared to closed vessels. In ducted vessels, the length of the duct influences the turbulence produced. The shorter the length, the lower the confinement is, and the higher the turbulence becomes. As the level of turbulence reduces with increased duct length, the rate of pressure rise was found to be higher in the vented vessel than in the ducted vessels.

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ACCEPTED MANUSCRIPT 4. Conclusion The severity of methane – coal dust hybrid mixture explosions has been investigated and

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presented in this article. The article explored primarily two aspects of explosion phenomena –

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a comparison of the explosion behaviours of hybrid mixtures with their pure components, and

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variations of explosion characteristics between ducted explosions and confined explosions. The pressure rises were observed to decrease when coal dust particles were added in a

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methane-air system with a fixed methane concentration. In this case, the unburned coal particles functioned as a heat sink, and reduced the pressure rises. In contrast, the pressure

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rises, both in the vessel and in the duct, were found to increase significantly with progressively higher methane concentrations added to a fixed concentration of coal dust in

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air.

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In the presence of the duct, the violence of the explosion in the vessel transmits to the duct

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and flame propagated through the duct occurred in a deflagration state. The pressure rises in the ducted vessel was lower than in the confined vessel. The duct length played an important role, and it was found that the higher the duct length, the higher the pressure rise was.

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Therefore, long tunnels in the real world of coal mines are susceptible to higher pressures rise if any explosion occurs. A zero duct length or a vented vessel, therefore, produces the lowest pressure rises in the vessel when vented, ducted and confined vessels are compared. In a ducted vessel, the initial portion of the duct is susceptible to high explosion pressures, compared to the rest of the duct, and attention needs to particularly be paid to the initial portion of the duct if it is intentionally employed as a safety feature, as previously mentioned. In addition to the above phenomena, the elevation of hazards due to ignition energy increment was also explored. It was found that peak explosion pressure, both in the vessel and in the duct, increased with the increase of ignition energy.

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ACCEPTED MANUSCRIPT Acknowledgements This work was supported by the Department of Resources Energy and Tourism, Australia

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(grant number – G1201029) and ACA Low Emissions Technologies Ltd., Australia (grant

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number – G1400523).

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Venting of gas explosion through relief ducts: Interaction between internal and external explosions, J. Hazard. Mater., 155 (2008) 358-368.

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[28] S. Kundu, J. Zanganeh, B. Moghtaderi, Explosion characteristics of methane-air mixtures in a spherical vessel connected with a duct, Process Saf. Environ. Protect., Accepted 19 Jun 2017, DOI: 10.1016/j.psep.2017.06.014.

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Promotion in the Process Industries, Cannes (F), 1986.

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ACCEPTED MANUSCRIPT Figure Captions: Figure 1: Ducted spherical vessel apparatus employed in the experimental investigations.

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Figure 2: Experimental setup employed in the investigations: 1) 20 litre sphere, 2) connector, 3) valve, 4) first portion of the duct, 5) second portion of the duct, 6) photodiodes (open markers), 7) pressure transducers (closed markers), 8) flexible disc that closes when the vessel is in vacuum and opens as soon as the vessel’s pressure increased from vacuum to atmospheric pressure [28].

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Figure 3: Reduced peak explosion pressure of the vessel as a function of methane concentration when a duct length of 2813 mm and a coal dust concentration of 100 g m-3 were employed with variable concentrations of methane and an ignition energy of 5 kJ.

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Figure 4: LEL, UEL and maximum explosion pressure ratios for methane-air mixtures ignited at 20 °C and 100 kPa (open marker – successful, covered marker – unsuccessful) [34]. Figure 5: Illustration of the coal dust explosion pathways [38, 39].

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Figure 6: Peak explosion pressures at various locations in the duct as a function of methane concentration for a duct length of 2813 mm, coal dust concentration of 100 g m-3 and an ignition energy of 5 kJ.

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Figure 7: Reduced peak explosion pressure of the vessel as a function of coal dust concentration for a duct length of 2813 mm, a methane concentration of 9.5% and an ignition energy of 5 kJ.

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Figure 8: Peak explosion pressures at various locations in the duct as a function of coal dust concentration for a duct length of 2813 mm, a methane concentration of 9.5% and an ignition energy of 5 kJ.

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Figure 9: Light intensity profiles at various locations of the duct as a function of time as recorded by photodiode sensors for a hybrid mixture of 6.5 % methane and 100 g m -3 coal dust with an ignition energy of 5 kJ. Figure 10: Explosion pressure profiles at various locations on the duct as a function of time as recorded by the pressure transducers for a hybrid mixture of 9.5 % methane and 100 g m -3 coal dust with an ignition energy of 5 kJ. Figure 11: Explosion pressure profiles along the length of the duct, when the duct experienced the maximum pressure. Figure 12: Flame speeds along the length of the duct. Figure 13: Pressure profiles of the vessel as a function of time for 9.5% methane and 100 g m-3 coal dust mixture when an ignition energy of 5 kJ was applied. Figure 14: Rates of pressure rise (dP/dt) in the vessel as a function of time for 9.5% methane and 100 g m-3 coal dust mixture when an ignition energy of 5 kJ was applied.

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ACCEPTED MANUSCRIPT Table 1: Locations of the sensors included in the duct. Distance from the end

Pressure

Photodiode

designation

of the sphere (mm)

transducer (PT)

sensor (PD)

350

90°

270°

PT2 and PD2

850

90°

270°

PT3 and PD3

1350

90°

270°

PT4 and PD4

1850

90°

270°

PT5 and PD5

2350

90°

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PT1 and PD1

Designation of angles

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ACCEPTED MANUSCRIPT Table 2: Operational conditions to inject methane into the sphere. vacuum – 63.5

6.5

– 66.6

9.5

– 69.6

12.5

– 72.7

15.5

– 75.7

– 60 – 60 – 60 – 60 – 60

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pressure Vacuum pressure after methane injection (kPag)

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methane Initial (kPag)

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Particle size distribution 65.85

Mositure (wt%)

D50 (μm)

21.8

Ash (wt%)

D10 (μm)

2.91

Volatile matter (wt%)

27.5

Fixed carbon (wt%)

47.8

4.1 20.6

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D90 (μm)

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ACCEPTED MANUSCRIPT Table 4: Effects of ignition energies on the pressure rises (in barg) in the vessel and at various points on the duct for methane-coal dust hybrid mixtures (duct length = 2813 mm). PT1

PT2

PT3

3.2

2.3

2.4

2.2

5 kJ

3.9

2.9

3.0

2.7

10 kJ

4.4

3.2

3.2

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PT5

2.2

2.1

2.6

2.3

2.9

2.7

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5 kJ

4.8

3.9

3.8

10 kJ

5.0

4.4

4.1

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Confined vessel

Vented vessel 2.6 3.7 3.8

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Highlights

The addition of methane in a coal dust-air system increased the violence of an explosion.



Unburned coal dust functions as a heat sink and decreased the pressure rises.



Flame propagation in the duct was occurred in a deflagration state.

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An increase of duct length increased the confinedness and the pressure rises in the vessel.

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