DSS068: Fundamental Differences Between Gas and Dust Flames

In this episode of the DustSafetyScience Podcast, we’re talking about the fundamental differences between gas and dust flames. This is the fourth installment in the Fundamental Science Series. The previous installments were:

As we review the importance of considering the differences between gas and dust flames, we talk about:

The five major differences between gas and dust flames
How these differences impact our understanding of dust explosions today

Why is it Important to Understand the Differences Between Gas and Dust Flames?

Most of our theoretical understanding of dust explosions and dust flames can be attributed to research that has been done on gas flames. Theories based on gas flames and combustion have been modified to determine the impact of dust explosions. However, dust flames are much more complicated than gas flames, and it is important to keep these differences in mind.

What are Five of the Major Differences Between Dust and Gas Flames?

Below is an overview of five major differences between dust and gas flames.

1. The Reaction Zone

When an explosion occurs, fuel is reacting and releasing heat, which is visible in the form of the flame itself. In the end, there are burned products.

With gas flame, the reaction zone is thin, being no more than a millimetre, and it is comparatively easy to understand what is happening. Fuel is undergoing chemical reactions and creating burned products. A lot of research also exists on the different fuels and molecules that could be involved. For methane, there could be 300 or 400 different molecules.

Dust is more complicated. You have the fuel, reaction zone, and combustion products, but the combustion mechanism has several steps. The particle must be heated up to get to the point where it can react. It may be a surface reaction or a gas-based reaction. It may release volatiles, and these react. The reaction zone is thicker and more difficult to understand. Not only are chemical processes taking place, but there are also physical processes such as mass transport, energy transport, and other different phenomena.

2. Number of Variables

With gas, you have the fuel, such as methane. You have the equivalence ratio, which is a measure of the concentration. You have the temperature, and you have the pressure. Those four variables will identify the conditions you need to know in order to estimate what the flame speed is going to be and how fast the flame is going to propagate.

The number of variables for a dust flame is significantly higher. They are reviewed in detail below.

Fuel: There are the fuel and its physical properties and conductivity, which in turn affect the overall combustion process. There are flame standoffs: when a flame forms close to the particle surface, a standoff occurs. If it involves a big enough distance, it will overlap with adjacent particles. When this happens, a different infrastructure occurs in the burning dust. These features are not present in gas flames.
Particle Size: Particle size is another variable. If you have methane, you can measure its laminar burning velocity and get a textbook value. But for particles, you have the fuel itself and the particle size. If it is aluminum, for example, you have to take into account the particle size and distribution.
Moisture Content: Moisture content also has a much larger effect on dust flames than on gas flames. The moisture in the dust generally has to be driven out before reaction can occur.

3. Suspension

Dust settles out of suspension which cause a lot of issues for testing in a laboratory setting. Gas is injected with a syringe or injected with a tube into a box, automatically fills the box. Since dust settles out it is much harder to get a uniform concentration to test.

4. Radiation

Because dust particles are a solid mass that heats up, they eject more radiation and absorb more radiation than gases. Radiation ejection causes the flame to slow down, but because that particle is ejecting radiation and the particles upstream in the unburned cloud are absorbing radiation, those particles can preheat.

The bigger the flame, the more this process enhances the burning velocity. The bigger the enclosure housing the explosion is, the more chance that this radiation can preheat ahead of the flame. A flame may start off at 30 cms per second, then quickens to a meter a second. It changes with the size of the vessel.

In [Affiliate] Dust Explosions In The Process Industries, Dr. Rolf Eckhoff presented an interesting test. A cloud of dust was ejected and ignited to create a flame or explosion. They put a glass wall beside the cloud and then ejected another cloud beside the wall so the two clouds would not physically touch. The heat can’t transmit very fast through the glass plate. The only thing that can be transmitted is radiation.

They tested different metal dusts on both sides of the glass: the ignited side and the not-ignited side and found that radiation can only ignite from some materials to others. With aluminum on both sides, the cloud on the non-ignited side of the glass will ignite because of radiation, but iron might be different.

5. Heat Loss Due to High Dust Loading

This difference complicates the understanding of how the flame will act because there is a heating delay and then a reaction that does not exist to the same extent in gas flames. You can load a lot more fuel into the flame, and this excess dust has a big impact on the flame. And as of today, we don’t have a strong understanding of how these fuel-rich flames behave in terms of understanding their flame propagation characteristics.

How Do These Differences Impact Our Understanding Of Dust Explosions?

Fundamental testing is very difficult with dust, which makes it hard to understand key parameters like laminar burning velocity. The reason why this is important goes back to the fact that gas theory is being used a lot in the understanding of dust flames.

The problem is that gas theory is based on having a laminar burning velocity. Researchers at McGill University in Montreal have measured the laminar burning velocity of a number of dusts, but the apparatus is complex and in some cases extremely expensive.

Furthermore, you can’t just test ‘aluminum’ like you do methane. You need to test aluminum at 10, 20, 50, and even 100 microns. In an actual dust environment, there is a particle size distribution of all those different mixtures. These varying sizes make it hard to fundamentally test combustible dust and understand the flame propagation effects, which is why we have to use a lot of experimental testing for each processing operation.

Dispersion also affects everything. The fact that dust settles out of suspension and most gases fill volumes has had a direct experimental impact. It’s hard to get the dust into a state of suspension. You either use an air blast in a cup or have a tube, shake the dust in the top, and let it fall down. This makes it very hard to get a uniform concentration of material to test

The closed fan system is the best way to get a repeatable, homogeneous suspension, but another problem is it can’t be done under laminar conditions. You can only do it under turbulent conditions. It is also difficult to measure what the level of turbulence is.

The airblast in a cup is basically what we do in the 20-litre chamber, but it is hard to get the same type of dispersion each time. The result is experimental scatter that makes it challenging to create overarching theories for how dust behaves.

Concluding Thoughts

The fact that combustible dust is difficult to ignite actually makes it a larger hazard in industrial settings. This allows it to build up over time and to extremely hazardous conditions before an explosion happens. The outcome can be a large explosion that causes destruction of the entire facility.

You wouldn’t stand in a room that you know is filled with methane gas and light a lighter. You also wouldn’t stay in a room that had a lot of liquid fuel on the ground, like gasoline, and light a lighter. But because sawdust just sits there, it does not have the same scare appeal as liquids or gaseous fuels. People are much more likely to allow ignition source to be present. Understanding the seriousness of the hazard can bring about life-saving changes.

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.