Experimental Investigations of Coal Dust-Inertant Mixture Explosion Behaviors - Summary

Experimental Investigations of Coal Dust-Inertant Mixture Explosion Behaviors – Summary

1-Sentence-Summary: The effect of ignition energy, dust concentration, and calorific value on dust inerting requirements is explored for calcium bicarbonate and monoammonium phosphate.

Authors: A. Kuai, W. Huang, J. Yuan, B. Du, Z. Li, and Y. Wu

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Experimental Investigations of Coal Dust-Inertant Mixture Explosion Behaviors - Summary

The authors perform explosion testing of coal dust and inerting dust mixtures in a 20-L chamber. Two coal samples with different calorific values (20 kJ/g and 31 kJ/g,, respectively) are used. Both samples are sieved between 75 and 125 µm.

The explosions are initiated using pyrotechnic ignitors with energies of 1, 2, 5, and 10 kJ. Furthermore, the standard ignition delay time of 60 ms is used. Lastly, each test is repeated three times and the average result is reported.

Explosion time traces, maximum pressure and rate of pressure rise verses concentration, minimum inerting concentrations (MIC), and minimum explosible concentration (MEC) are reported. Calcium bicarbonate and monoammonium phosphate are used at different concentrations to inert the dust explosions.

Three of the main findings from this paper are:

  1. The ignitor strength must be increased between 5 to 10 kJ before MIC and MEC are independent of ignition energy
  2. Both the calorific value and concentration of the coal dust increase the amount of inertant required.
  3. Much more calcium bicarbonate is required than monoammonium phosphate to inert coal dust explosions.

The following sections outline the main findings in more detail. The interested reader is encouraged to view the complete article at the link provided below.

Finding #1: Ignition energy of 5 to 10 kJ is recommended for MIC and MEC testing.

The authors perform MIC and MEC testing with 1, 2, 5, and 10 kJ chemical ignitors. The high calorific value coal and calcium bicarbonate are used for these tests.

Following the work of Hertzberg et al., 1981, Cashdollar and Chatrathi, 1993, and Going et al., 2000, the authors suggest that “realistic” values of MEC and MIC should be independent of ignition energy. From a safety standpoint this is providing a measure of the potential flammability or explosibility of the fuel, independent of ignition conditions.

The experimental results show that the effective MEC decreases from 100 g/m3 to 50 g/m3 moving from 1 kJ to 5 kJ ignition energy. However, moving to 10 kJ causes the MEC to only decrease to 45 g/m3, suggesting the the asymptotic limit is almost reached.

The MIC shows a same trend as MEC with a larger increase between 1 and 5 kJ, and then levelling off at 10 kJ. The authors propose that at least 5 kJ ignition energy should be used for MEC and MIC testing.

Finding #2: Inertant requirements increase with coal calorific value and concentration

The experimental results also demonstrate that more inertant is required for the higher calorific dust. The amount of inertant required is 2 to 3 times higher for this sample.

The amount of inertant required increases on a percentage basis as the coal dust concentration increases. At the worst case coal concentration (approximately 350 g/m3) 80% inertant by mass is required.

Finding #3: Monoammonium phosphate is a better inertant than calcium bicarbonate on a per mass basis.

The experimental results show that monoammonium phosphate is much better at inerting the explosion than calcium bicarbonate. For the high calorific coal at 250 g/m3 only 30% by mass of monoammonium phosphate is required compared to 70% of calcium bicarbonate.

The main inertant mechanism of calcium bicarbonate is thermal inhibition. In other words, the inertant acts as a heat sink reducing flame temperature and quenching the explosion. The authors propose that monoammonium phosphate has several features in addition to thermal inhibition. These additional features include:

  • A low thermal decomposition temperature (explored by Chatrathi and Going, 2000 and Abbasi and Abbasi, 2007)
  • Production of free ammonia which can distinguish the flame chemically
  • Production of phosphorus pentoxide which can coat the burning coal particles

My Personal Take-Aways From
“Experimental Investigations of Coal Dust-Inertant Mixture Explosion Behaviors”

The findings presented in this paper are useful to the mining industry, as well as other industries protecting from dust explosion using solid inertants. As the authors discussed, high strength chemical ignitors are required before the MEC and MIC become independent of ignition energy.

One difficulty with using monoammonium phosphate instead of calcium bicarbonate is the cost. A cost/benefit type of analysis for industry would be a useful contribution to this area, as calcium bicarbonate and similar compounds (e.g., limestone) are relatively inexpensive.

A second issue that needs to be considered, is the physical amount of calcium bicarbonate required. If 80% by weight is needed to suppress the explosion, the inertant is required in a 4:1 ratio to the coal dust. Depending on how much fuel is present this could be an obstructive amount, and a more efficient inertant may be required.

Full Citation:

  • A. Kuai, W. Huang, J. Yuan, B. Du, Z. Li, and Y. Wu, “Experimental investigations of coal dust-inertant mixture explosion behaviors,” Procedia engineering, vol. 26, pp. 1337-1345, 2011.
    [Bibtex]
    @ARTICLE{Kuai2011,
    title={Experimental Investigations of Coal Dust-Inertant Mixture Explosion Behaviors},
    author={Kuai, A. and Huang, W. and Yuan, J. and Du, B. and Li, Z. and Wu, Y.},
    journal={Procedia Engineering},
    volume={26},
    pages={1337--1345},
    year={2011},
    link={},
    summary={},
    }

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