Ignition of Single Coal Particles in High-Temperature Oxidizers with Various Oxygen Concentrations - Summary

Ignition of Single Coal Particles in High-Temperature Oxidizers with Various Oxygen Concentrations – Summary

1-Sentence-Summary: Ignition behavior of large coal pellets is characterized into three categories based on ambient temperature and oxygen concentration.

Authors: A. Ponzio, S. Senthoorselvan, W. Yang, W. Blasiak, and O. Eriksson

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Ignition of Single Coal Particles in High-Temperature Oxidizers with Various Oxygen Concentrations - Summary

The current authors investigate ignition of single coal pellets place in a heated furnace. The pellets have a 15 mm diameter and are 35 mm long. Proximate analysis of the coal shows 19.4% volatile matter and 6.7% moisture content.

95 experiments are completed in total. Furnace temperatures are varied between 873 and 1273 K, and oxygen concentration is varied from 0 to 100% by volume. High-speed images of the flame, ignition delay time, temperature of the pellet, and mass loss at ignition are reported. The authors also develop a model for ignition delay time of the large pellets studied.

Three of the main findings from this paper are:

  1. Ignition is broken into three categories based on ambient temperature and oxygen concentration: glowing surface ignition (char oxidation), flaming ignition (homogeneous coal oxidation), and sparking ignition (heterogeneous coal oxidation).
  2. The first category does not appear to be applicable to flame propagation of smaller coal particles.
  3. Both homogeneous and heterogeneous coal oxidation regimes may be important to consider for coal dust flame propagation.

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 behavior of the coal pellets can be classified into three categories

The authors categorize coal pellet ignition into three categories based on the high-speed images. At low ambient temperatures and low oxygen concentration, the first sign of combustion is the glowing particle surface. No flame is visible under these conditions and ignition occurs due heterogeneous oxidation of the char component of the coal.

At higher ambient temperatures in the range of 1000 to 1300 K, a homogeneous gas-phase ignition occurs. Here devolatilization is faster than char oxidation, and a diffusion flame around the coal pellet is seen. This regime is seen for oxygen concentrations lower than 20% by volume, up to temperatures of 1300 K.

The third ignition category is labeled as “sparking ignition”. At high temperatures or increased oxygen concentration, heterogeneous reaction of non-volatilized coal occurs at the particle surface. Here the oxygen molecules directly attack the coal matrix before devolatilization occurs.

Finding #2: Char oxidation ignition does not appear to be applicable to pulverized coal dust flames

From the experimental data and model developed by these authors, ignition by heterogeneous reaction of the char occurs at temperatures lower than 950 K. This temperature is significantly lower than those found during flame propagation. Consequently, it seems likely that this mechanism does not cause pulverized coal ignition during flame propagation. Note that char combustion may play a role in the later time flame dynamics.

Finding #3: Both homogeneous and heterogeneous coal oxidation may influence pulverized coal dust flames

The conditions separating homogeneous and heterogeneous coal reaction fall around 20% oxygen at 1300 K. Furthermore, the amount of oxygen required for sparking ignition decreases as the flame temperature increases past 1400 K (the highest temperature reviewed in this work).

These operating conditions seem to be more realistic for flame propagation than the low temperatures required for char ignition to be dominant. These results suggest that both homogeneous reaction and direct heterogeneous ignition of the coal dust may be important to consider for flame propagation.

It is important to note that these results may change for small particle sizes or under flame propagation conditions instead of in a furnace heater. However, these results are interesting and may provide insight into important mechanisms for smaller dust particles, especially at differing oxygen concentrations.

My Personal Take-Aways From
“Ignition of Single Coal Particles in High-Temperature Oxidizers with Various Oxygen Concentrations”

Although this work is not directly focused on dust flames, its findings may be relevant to understanding the important processes involved. Studies using larger particles or pellets are useful as the ignition and flame process can be instrumented and recorded more easily. Readers interested in ignition of coal dust particles and flame propagation are encouraged to read other papers referenced in this work, such as the review paper of Essenhigh et al., 1989 and the articles of Gururajan et al., 1990, Chen et al., 1996, and Faundez et al., 2005.

Full Citation:

  • A. Ponzio, S. Senthoorselvan, W. Yang, W. Blasiak, and O. Eriksson, “Ignition of single coal particles in high-temperature oxidizers with various oxygen concentrations,” Fuel, vol. 87, pp. 974-987, 2008.
    [Bibtex]
    @ARTICLE{Ponzio2008,
    title={Ignition of single coal particles in high-temperature oxidizers with various oxygen concentrations},
    author={Ponzio, A. and Senthoorselvan, S. and Yang, W. and Blasiak, W. and Eriksson, O.},
    journal={Fuel},
    volume={87},
    pages={974--987},
    year={2008},
    link={http://www.sciencedirect.com/science/article/pii/S001623610700292X},
    summary={http://www.mydustexplosionresearch.com/ignition-of-single-coal-particles-high-temperature-oxidizers-various-oxygen-concentrations}
    }

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