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Mine Site Greenhouse Gas Mitigation

The Optimisation and Application of Hybrid Chequer Bricks in Regenerative Thermal Oxidisers for VAM Abatement

Mine Site Greenhouse Gas Mitigation » Mine Site Greenhouse Gas Mitigation

Published: July 20Project Number: C28075

Get ReportAuthor: Mohammed Al Zuraiji, Jafar Zanganeh, Behdad Moghtaderi | University of Newcastle

The capture and use of ventilation air methane (VAM) is regarded as one the most effective means of mitigating fugitive methane emissions in underground coal mines. Methane is the most potent of the greenhouse gases which is emitted during the production and transportation of coal, natural gas and oil. A particular problem is fugitive emissions of methane from the ventilation systems of underground coal mines. In recent years, regenerative thermal oxidiser (RTO) and catalytic thermal oxidiser (CTO) technologies have been adopted to reduce the VAM emissions from underground coal mines. In these technologies, the fixed-bed mediums (i.e., chequer bricks) are considered to be the primary layer of protection to arrest any potential flames travelling from the abatement devices into the coal mine. Despite the important role that these chequer bricks play, from a safety point of view their performances, particularly against flashbacks from the RTO/CTOs into the mine, should be investigated and verified further.

The principal vision of this project was to develop methods for the optimisation and enhancement of the flame arresting properties of the chequer bricks. To fulfil this vision, the project aimed to develop a number of hybrid chequer brick configurations which combined several brick designs in order to examine the behaviours of the hybrid configurations under fire and explosion scenarios pertinent to RTO/CTO based VAM abatement systems. This small-scale pilot study examined the properties, configurations and flame mitigation performances of the newly designed chequer brick flame arresters.

To gain fundamental insights into the flame arrester chequer bricks developed for potential future use in RTOs and CTOs, the project undertook:

  • An examination of the mechanical and thermophysical properties of chequer bricks.
  • The design and manufacture of the most suitable and practical configurations of the hybrid brick flame arresters for testing, assessment and potential future use. These were achieved through comprehensive desktop and laboratory studies.
  • An assessment of the flame mitigation performances of a number of different configurations of the small-scale brick flame arresters were undertaken in methane flame propagation conditions. The testing was carried out in a small-scale propagation tube which was 5 m long and 75 mm in diameter.
  • The optimisation of the small-scale bricks and then the manufacture of different configurations of the large-scale flame arrester chequer bricks. The mitigation performance testing of the large-scale brick flame arresters was conducted in a pilot-scale detonation tube.

 

The key findings of the project:

 

Flame Arresting Performances of the Chequer-Bricks

The experimental assessment of the flame arresting properties of the chequer bricks indicated that there were some mixed and inconclusive results. The small-scale and large-scale brick flame arresters demonstrated successful flame mitigation performances, however while under the same conditions and for the same scenarios they failed to stop and mitigate the flames caused by different methane concentrations (e.g., 6.5%, 8%, 9.5% and 12.5%). Amongst the three different configurations of the small-scale brick flame arresters (i.e., Configurations A, B and C), the Configuration C flame arrester demonstrated a relatively better mitigation performance in comparison to the other configurations. All three configurations of the brick flame arresters demonstrated better flame mitigation performances for the stoichiometric methane concentrations. The failure rates of the arresters were less for the stoichiometric methane concentrations in comparison to the leaner and richer methane concentrations. This was mainly due to the stronger flame propagation pushback created by the reflected pressure wave for these concentrations. The pressure wave was reflected when the flame propagation wave hit the flame arrester and bounced back.

 

Impacts of the Size of the Flame Arrester (Length to Diameter (L/D) Ratio) on the Flame Mitigation

It was determined that the length to diameter (L/D) ratio had a significant impact on the flame mitigation performances of the brick flame arresters. Overall, the chequer bricks with larger L/D ratios demonstrated better flame mitigation performances. The experimental study for both the small-scale and large-scale brick configurations was carried out for L/D ratios of up to 8. It was found that for the small-scale flame arresters with an L/D ratio < 4, the flame could penetrate through the brick flame arresters and then accelerate in the form of a jet fire. It was identified that the frequency of the failures of the bricks was significantly reduced as the L/D ratio increased, however none of the L/D ratios could completely stop the propagation of the flames.

Material Selection and Fabrication

Although different fabrication methods for the brick flame arresters were tried at an early stage of the project, it was identified that the water jet cutting approach was the most reliable and precise method of producing the chequer bricks in any shape and configuration, and from different materials.

Impact of the Stabilised Combustion on the Failure of the Flame Arrester Bricks

A stabilised combustion refers to combustion on the upstream side of the flame arrester. In this study, it was found that a period of stabilised combustion seemed to have a large impact on the failure rate of the flame arrester. The impacts of the stabilised combustion on the flame arresters were more pronounced for those with an L/D ratio > 4. The duration of the period of stabilised combustion for the small-scale brick flame arresters was up to about 280 ms.

Dynamic Pressure Reductions Across the Brick Flame Arresters

The application of brick flame arresters was shown significantly increase reductions in the dynamic pressure. It was found that the pressure drops increased as the L/D ratio increased. The highest dynamic pressure reduction range was about 97%, which was for the Configuration C flame arrester with an L/D ratio of 8.

 

Influence of Coal Dust on the Failure of the Flame Arresters

It has been identified that fine coal dust particles enhance the severity of methane explosions. The assessment of the experimental performances of the large-scale brick flame arresters indicated that the application of coal dust particles mixed with the methane significantly increased their failure rate in stopping the propagation of flames.

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