DSS065: Effect Of Particle Size On Dust Deflagration

In this episode of the DustSafetyScience Podcast, we’re talking about the effect of particle size on dust deflagration. This particular episode is part of the Fundamental Science Series that we’ve been running throughout 2019 and into 2020. The previous installments were:

Topics covered include:

Why the rate of dust deflagration matters.
Why the particle size affects the rate of dust deflagration
The impact on different dust deflagration parameters like KST, minimum explosible concentration (MEC) minimum ignition energy (MIE)
Open challenges in understanding the impact of particle size on dust deflagration

Why Does The Rate Of Dust Deflagration Matter?

Rate of dust deflagration refers to the flame speed or propagation rate. If you have an ignition point inside of a dust cloud, how fast does that flame move through the cloud? This is important because it determines how rapidly the maximum pressure is reached in a vessel or building containing the explosion and, ultimately, how violent the explosion is.

How quickly a dust deflagration occurs determines its KST value. This parameter is used to design protection systems and determine the overall effect of that dust explosion.

Why Does Particle Size Impact The Dust Cloud Deflagration Rate?

There is a process involved for a dust particle to react and contribute to that flame propagation. The particle must be heated, and it takes a set amount of time to heat up to a value in which it can react. Then, after it reacts, it takes heat or energy out of the system because it is still being heated.

One of the complicating factors is this reaction step, as it is different for different materials. With a high-volatile coal, there is a homogeneous reaction. The volatiles will be released from the coal particle after it heats up enough. Then they react and combust, which is what leads to the next particle heating up and ultimately leads to flame propagation. In contrast, an iron particle reacts heterogeneously. The reaction happens at the surface between oxygen molecules in the air and the solid iron, resulting in iron oxide.

These steps are inversely proportional to the surface area of the particle. The higher the surface area, the faster that particle can heat up. This is why decreasing its size generally increases the combustion of a single particle.

Material porosity also affects the surface area, which is why things like activated carbon burn so fast. When carbon is activated, low volume pores are put into it to enhance reaction. This step also enhances the combustion reaction if that particle is being heated, resulting in a faster and more violent explosion when dust of that material is involved.

What Is The Impact On Dust Deflagration Parameters?

KST is the maximum rate of pressure rise determined through standardized testing. In his textbook, [Affiliate] Dust Explosions In The Process Industries, Dr. Rolf Eckhoff reports that KST is linearly related to the specific surface area for aluminum dust. As the particle size decreases, the specific surface area and maximum rate of pressure rise and the KST increase.

In terms of the effect on MEC (minimum explosible concentration), Hertzberg and Cashdollar did a report, Introduction to Dust Explosions, in 1987. They looked at the MEC of low-volatile coal, high-volatile coal, and polyethylene, and found that it decreases with decreasing particle size, but only up to a point.

The polyethylene decreased to around 100 micron, and then the MEC stayed constant. For high-volatile coal, it was lower, and for low-volatile coal, it was even less. This research highlighted two effects:

As particle size decreases beyond a certain point, the MEC stays constant.
As particle size decreases, the particles are more likely to lump together (agglomeration) and act like a single larger particle. Their surface area is not exposed anymore and only the outer area would be reacting.

The last deflagration parameter covered in this episode is the MIE, or minimum ignition energy, which was explored by Wolfgang Bartknecht in a 1987 report called Staub Explosion and referenced by Dr. Rolf Eckhoff.

In his text he found a cubic relationship between MIE and the particle diameter. In other words, MIE was proportional to the particle diameter cubed. For every halving or doubling of the particle diameter, the MIE went up or down by almost a factor of 10.

What Are Some Open Challenges In Understanding The Effect Of Particle Size On Dust Cloud Deflagration?

There are a couple of things that impact understanding of the effect of particle size on dust cloud deflagration.

Particle Limit

It would be natural to assume that because KST is linearly related to specific surface area, decreasing the particle size to very small would increase the rate and violence of an explosion, but this doesn’t seem to be the case for most dusts. Once a nanometer scale or less is reached, there is little effect on pressure rise and deflagration rate as the particle size decreases in 20-L testing. This effect can be attributed to agglomeration of the small particles.

Particle Size Distribution

If anyone takes a scoop of dust out of their dust collector, there will be varying particle sizes. 20% might be less than 10 microns and 50% might be less than 60 microns. In terms of how this particle size distribution impact parameters like KST, like MEC and like MIE, this continues to be an open-ended question.

Some say that a fixed percentage can be used if it is 10% of one type and 50% of another: they can be added together. Others say that the fines really do all the work and just leave the bigger particles behind.

The answer is probably a combination of the two. From his work, Dr. Chris Cloney suggests that the fines tend to do most of the legwork for combustion. They react and burn quickly, and then leave the larger particles behind in the oxygen-starved remnants. However, these larger particles are also sucking energy out of the system, so they have an impact on that flame propagation as well.

Effective Aspect Ratio

Most particles are not spherical. Things like textiles may have very long fibers. There are also metal flakes and plastic ellipsoids. When the shape is not a perfect sphere, the impact of the radial change is going to be much higher on the heating of those particles than longitudinal changes. With a thread, as the radius decreases, there will be a higher burning velocity. It won’t have as large an effect as shorter threads.

Conclusion

This episode was more fundamental than a lot of the subjects discussed on the podcast, but we got a lot of great feedback with the previous Fundamental Science Series episodes. People really like the opportunity to understand the science behind the dust explosions that we are committed to preventing.

If you have questions about the contents of this or any other podcast episode, you can go to our ‘Questions from the Community’ page and submit a text message or video recording. We will then bring someone on to answer these questions in a future episode.