Structure of Laminar Premixed Carbon-Methane-Air Flames and Ultrafine Coal Combustion - Summary

Structure of Laminar Premixed Carbon-Methane-Air Flames and Ultrafine Coal Combustion – Summary

1-Sentence-Summary: A mathematical model for flame propagation in carbon dust/methane gas mixtures is compared to experimental burner results and extended to simulate coal dust flames.

Authors: D. Bradley, Z. Chen, S. El-Sherif, S. El-Din Habik, G. John, and G. Dixon-Lewis

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Structure of Laminar Premixed Carbon-Methane-Air Flames and Ultrafine Coal Combustion - Summary

The authors of this work experimentally investigate hybrid flames of methane and carbon dust in a reduced-pressure, flat-flame burner. Investigating the flames at 0.16 atm pressure widens the reaction zone and allows for instrumentation. Temperature, gas velocity, and species concentrations are reported.

The authors compare the experimental results to a mathematical model of one-dimensional flame propagation. The model includes a four step reaction mechanism, surface reactions, and radiation effects. The methane gas concentrations are varied at 0.6, 0.7, 1.07, and 1.2 equivalence ratios. The carbon dust has 4 µm particle diameter.

The mathematical model and experimental results compare well, validating the governing equations and solution method. The model is used to explore the impact of radiation and intermediate species on hybrid flame propagation. Lastly, the flame structure of coal dust at 144 g/m3 concentration is analyzed using the computational model.

Three of the main findings from this paper are:

  1. Intermediate radicals such as OH, CO, H, and O, play an important role in gas-phase and surface reactions for hybrid flames.
  2. At the conditions tested radiation has a measurable but small influence on flame structure and temperature.
  3. Burning velocity for fine coal dust is not sensitive to devolatilization rate.

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: Intermediate radicals play an important role in hybrid flame propagation

Graphite dust enhances burning velocity of fuel-lean methane.The maximum concentrations of H, OH, O, and CO increase with additional graphite under these conditions. This enhanced radical pool increases overall combustion rate in addition to the carbon reaction.

Graphite dust lowers burning velocity for fuel-rich methane. The maximum concentration of H, OH, O, and CO decrease with additional graphite under these conditions. This is due to competition between surface and gas-phase reactions and due to the lower flame temperature from heat loss to the dust.

Finding #2: Radiation has a limited effect on hybrid flame propagation under the conditions tested

The effect of radiation is explored for methane alone, methane/carbon mixtures, and coal dust flames. In all cases radiation demonstrated measurable changes in temperature and species profiles. However, the overall effect had limited impact on burning velocity and maximum species concentrations. The authors caution that these results could change if the background settings are higher temperature or the dust concentration is increased.

Finding #3: Devolatilization rate plays a limited role in burning velocity of ultrafine coal dust

The final simulation demonstrates flame structure, carbon consumption rate, and heat release rate for coal dust. The coal is 40% volatile matter and has a total concentration is 144 g/m3.

The coal burning velocity is predicted as 20 cm/s. This value is similar to the experimental value of 19 cm/s measured by Smoot et al., 1977. The current authors found that varying the devolatilization rate over four orders of magnitude only had a 5% decrease in burning velocity.

My Personal Take-Aways From
“Structure of Laminar Premixed Carbon-Methane-Air Flames and Ultrafine Coal Combustion”

This paper provides an extensive analysis of the flame structure for hybrid carbon/methane and coal dust flames at reduced pressure. The results provide many insights into the roles of intermediate reactions, radiation, and devolatilization. Extending the analysis to ambient pressure will provide useful information for dust and hybrid explosion in industrial conditions. This paper is recommended reading for anyone studying in this area.

In addition to illustrating flame structure for hybrid and dust-only flames, the current paper gives many useful references that build on mathematical model development. These references are also recommended for anyone working in this area and include: Smoot et al., 1977,
Bradley, 1988, Bradley, 1985, Bradley, 1989, and Dixon-Lewis et al., 1991.

Full Citation:

  • D. Bradley, Z. Chen, S. El-Sherif, S. El-Din Habik, G. John, and G. Dixon-Lewis, “Structure of laminar premixed carbon-methane-air flames and ultrafine coal combustion,” Combustion and flame, vol. 96, pp. 80-96, 1994.
    [Bibtex]
    @ARTICLE{Bradley1994,
    title={Structure of Laminar Premixed Carbon-Methane-Air Flames and Ultrafine Coal Combustion},
    author={Bradley, D. and Chen, Z. and El-Sherif, S. and El-Din Habik, S. and John, G. and Dixon-Lewis, G.},
    journal={Combustion and Flame},
    volume={96},
    pages={80--96},
    year={1994},
    link={http://www.sciencedirect.com/science/article/pii/0010218094901600},
    summary={http://www.mydustexplosionresearch.com/structure-laminar-premixed-carbon-methane-air-flames-ultrafine-coal-combustion},
    }

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