Does Your Dust Explosion Have a Hybrid Problem?

From the first experiments in 1885 by Engler (see Chapter 1.3.9 in Eckhoff [1]) mixtures of combustible dust and flammable gas demonstrated complex flammability phenomena that are still not well understood today. In the words of Dufaud et al. (2009, [2]) accidents involving hybrid mixtures are "neither scarce nor minor" and include severe explosions in Courrie`res, France (1906, 1099 Fatalities), Benxihu, China (1942, 1549 Fatalities) and Nova Scotia, Canada (1992, 26 Fatalities).

Hybrid explosions represent an enhanced hazard to various industries processing combustible dust and flammable gas. Ignition and burning characteristics are significantly different than either of the individual fuels (e.g., see the video here). In many scenarios explosion severity and ignition sensitivity are markedly increased.

This post briefly reviews some important aspects of hybrid explosions. Specific industries that may suffer from these hazards are reviewed, followed by the conflicting state of knowledge for explosion severity and flammability limits. The information given here is based on the literature summary from the post "12 Journal Article Investigating Explosion of Hybrid Mixtures".

Effected Industries

Mining

The mining industry is a commonly cited sector suffering from dust and hybrid explosion hazards. Combustible dust (e.g., coal) and flammable gas (e.g., coalbed methane) are naturally present during mining operations and routinely released. Ignition sources in the mine can ignite an initial gas or hybrid explosion, followed by a secondary coal dust explosion propagating through the mine shafts.

Pharmaceutical

The pharmaceutical industry has received much attention in the hybrid explosion literature (e.g., see [2, 3, 4, 5, 6]). Typical operations such as spray drying can involve combustible dust such as Niacin (Vitamin B), magnesium stearate (inactive or filler ingredient), and antibiotics (active ingredient). These samples are often mixed with flammable solvents in liquid or gaseous form [6].

Nuclear

The nuclear industry has identified specific hazard scenarios that involve hybrid mixtures. These include Loss-Of-Vacuum (LOVA) or Loss-Of-Coolant (LOCA) situations that may lead to hydrogen release in the vicinity of Beryllium or Tungsten dust (Denkevits and Hoess, 2015 [7]). These explosions could have devastating effects with rates of pressure rise in excess of 2000 bar/s.

General Processing

All general processing industries that involve combustible dust and flammable gas should be aware of hybrid explosions. These mixtures may occur directly as a natural part of the process operation (e.g., cork stopper manufacturing as described by Pilao et al., 2006 [8]) or may occur due to upset conditions such as the mining and nuclear scenarios listed above.

Explosion Severity

Low Gas Concentrations

The majority of work in this area (see all papers in the review) demonstrate significant increases in maximum overpressure and rate of pressure rise with small amounts of gas added to combustible dust. It is important to note that these increases occur even when the the gas is below its lower flammability limit.

"More than Additive Effects"

There is disagreement in the literature on the "worst-case scenario" for hybrid mixtures. Some research suggests that the gas alone near it's stoichiometric concentration has the highest rate of pressure rise [4, 5]. Others suggest "more than additive effects" exist [2, 3, 9], and that the worst case scenario involves mixtures of dust and gas.

This lack of knowledge poses two specific problems for industry. The first is the range of scenarios to protect against (e.g., timing triggers on a 150 bar/s dust explosion may be significantly different than a 1000 bar/s gas explosion). The second problem is definition of a worst-case scenario to protect against.

It is not currently understood how elemental composition, turbulence, and combustion phenomena can be combined to understand explosion severity of hybrid mixtures. Guidelines on performing hazard analysis for these mixtures can be found in Dastidar et al. (2005, [10]).

Explosion Limits

Minimum Explosible Concentration (MEC)

Similarly to explosion severity, the majority of studies find that explosions occur at concentrations below both the dust MEC and the gas lower flammability limit. Under hybrid conditions, the minimum explosible characteristics of the fuels individually cannot be used.

Comparison to Gas-Gas Mixtures

Two competing theories for explosion limits have been proposed: Le Chatelier's Law and Bartknecht’s relation. Le Chatelier's Law treats that dust-gas mixture as a gas-gas mixture and assumes linear mixing of flammability limits (see the discussion of Mashuga and Crowl [11] for assumptions of Le Chatelier's Law).

Bartknecht’s relation assumes that flammability limits are wider than Le Chatelier's Law [12] and is said to apply for gases with low burning velocity. This relation predicts that less fuel is required to have an explosion than linear mixing.

There is significant disagreement in the review post on explosion limits of hybrid mixtures. Out of the posts reviewed and referenced 25 dust/gas combinations were analyzed. From these 6 demonstrate agreement with Le Chatelier's law, 6 demonstrate wider flammability limits as predicted by Bartknecht’s relation, and 13 demonstrate flammability limits that are narrower than predicted by both theories (more fuel is required than linear mixing to have an explosible mixture).

Of note in this area are the recent works of Jiang et al. (2014, [13]) and Jiang et al. (2015, [14]), which provide a relation for narrower flammability limits; However, this relation does not appear to explain the other data found elsewhere in the literature.

Take-Aways

This brief review demonstrates the difficulties in understanding and dealing with hybrid mixtures in industry. In addition to uncertainty with explosion severity and flammability limits, experimental testing is complicated due to the number of tests required. Other explosion parameters such as Minimum Ignition Energy are also heavily impacted by the presence of the gas and should be considered in a full hazard analysis.

If anyone is involved in research or working in this area, I would love to hear about your work. Please post a comment below, or you can reach me through linked in or my personal email, [email protected]. More posts on gas, dust, and hybrid explosion can be found from my website or blog post keywords.

Original Post

12 Journal Article Investigating Explosion of Hybrid Mixtures

References

[1] R. E. Eckhoff, Dust explosions in the process industries, thrid edition, Gulf Professional Publishing, 2003. 

[2] O. Dufaud, L. Perrin, and M. Traoré, “Dust/vapour explosions: Hybrid behaviours?,” Journal of loss prevention in the process industries, vol. 21, pp. 481-484, 2008. 

[3] O. Dufaud, L. Perrin, S. Traoré, S. Chazelet, and D. Thomas, “Explosion of vapour/dust hybrid mixtures: A particular class,” Powder technology, vol. 190, pp. 269-273, 2009

[4] A. Garcia-Agreda, A. Di Benedetto, P. Russo, E. Salzano, and R. Sanchirico, “Dust/gas mixtures explosion regimes,” Powder technology, vol. 205, pp. 81-86, 2011.

[5] R. Sanchirico, A. Di Benedetto, A. Garcia-Agreda, and P. Russo, “Study of the severity of hybrid mixture explosions and comparison to pure dust-air and vapour-air explosions,” Journal of loss prevention in the process industries, vol. 24, pp. 648-655, 2011. 

[6] N. Hossain, P. Amyotte, M. Abuswer, A. Dastidar, F. Khan, R. Eckhoff, and Y. Chunmiao, “Influence of liquid and vapourized solvents on explosibility of pharmaceutical excipient dusts,” Process safety progress, vol. 33, pp. 374-379, 2014. 

[7] A. Denkevits and B. Hoess, “Hybrid H2/Al dust explosions in Siwek sphere,” Journal of loss prevention in the process industries, vol. 36, pp. 509-521, 2015. 

[8] R. Pil?o, E. Ramalho, and C. Pinho, “Explosibility of cork dust in methane/air mixtures,” Journal of loss prevention in the process industries, vol. 19, pp. 17-23, 2006.

[9] Y. Khalil, “Experimental investigation of the complex deflagration phenomena of hybrid mixtures of activated carbon dust/hydrogen/air,” Journal of loss prevention in the process industries, vol. 26, pp. 1027-1038, 2013. 

[10] A. G. Dastidar, B. Nalda-Reyes, and J. C. Dahn, “Evalutation of dust and hybrid mixture explosion potential in process plants,” Process safety progress, vol. 24, pp. 294-298, 2005. 

[11] C. V. Mashuga and D. A. Crowl, “Derivation of Le Chatelier’s mixing rule for flammable limits,” Process safety progress, vol. 19, pp. 112-117, 2000. 

[12] W. Bartknecht, Dust explosions: course, prevention, protection, Springer-Verlag Berlin and Heidelberg GmbH & Co. K, 1989. 

[13] J. Jiang, Y. Liu, and S. Mannan, “A correlation of the lower flammability limit for hybrid mixtures,” Journal of loss prevention in the process industries, vol. 32, pp. 120-126, 2014. 

[14] J. Jiang, Y. Liu, C. Mashuga, and S. Mannan, “Validation of a new formula for predicting the lower flammability limit of hybrid mixtures,” Journal of loss prevention in the process industries, vol. 35, pp. 52-58, 2015.