Minimum Ignition Energy of dust
Physical meaning and use in risk assessment of powder MIE
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| Section summary |
|---|
| 1. Definition of
MIE |
| 2. Determination
of MIE |
| 3. MIE typical values |
| 4. Use in risk assessment |
| 5. MIE of common dusts |
1. Definition of MIE
What is the MIE of a powder ?
The Minimum Ignition Energy MIE is very important for dust explosion risks analysis, it will show how easy a dust cloud can be ignited. The MIE is equal to the minimal energy needed to be brought by a spark to a dust cloud of a given concentration in order to ignite it following a specific experimental procedure [Laurent].
2. Experimental determination of MIE
How to measure the Minimal Ignition Energy of a dust cloud ?
The MIE is measured experimentally. The experiment consists in putting in suspension the powder of the material to be tested in a cloud that has a concentration in between the minimum explosion concentration and maximum explosion concentration (determining this concentration may need additional experiments) and then attempt to ignite it. It may be necessary to standardize the particle size distribution prior to testing the powder [Glor] : having dust less than 63 microns is mentioned in European norm and in different research papers. It is important as the MIE decreases with the particle size and even if the product has a size larger than 63 microns, bulk solids handling operations like pneumatic transport may lead to the generation of fine dusts [Glor]. The humidity, an important contributing factor in dust explosion, should also be standardized for the test [EN13821].
Sparks are then created thanks to electrodes. Depending on the energy involves, the sparks are able or not to trigger an explosion.
The MIE is the smallest energy observed on the electrodes that triggers an explosion of the powder at the concentration tested. Smaller energies do not trigger an explosion.
To be representative, such experiences must be carried out by reputable test institutes with a clear protocol (example : ASTM E2019 in US, IEC 1241-2-3 or EN 13821 in Europe). The test machine mentioned in the norm is also important as values may be slightly different from one apparatus to another (MIKE 3 and HARTMANN tube for example [Janes]). Many parameters can influence the result of the MIE measure, hence the need to get in touch with professional institutes.
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It must also be noted that MIE can be tested with and without inductance in the test apparatus [Janes]. Testing with inductance is conservative and should give the lowest possible value of MIE. However, if the process conditions are well defined and risks only concern static electricity sparks, it may be interesting to discuss with the test institute in order to check if the MIE without inductance would be more relevant. If other sources of ignitions are to be considered or if the process conditions are not well known, tests with inductance will give more safety margins.
3. MIE typical values
What are the values of MIE creating significant dust explosion risk ?
MIE typically range from 1 mJ to 1000 mJ [Janes]. The lower the MIE, the higher is the risk of explosion as a very small energy input can trigger a dust cloud explosion.
MIE < 3 mJ should be processed with specific measures, the dust is extremely sensitive to ignition. Some equipment suppliers even refuse to handle projects with such low MIE as they are extremely sensitive even to small sparks.
MIE = 3 mJ and above requires special attention and measures to process the powder taking into account such risks as static electricity, mechanical sparks...etc...
Each processor must carry out a dust explosion risk analysis in order to assess the risk related to a specific material in a specific process and take necessary precaution and mitigation measures.
What are the parameters influencing the MIE ?
The nature of the material is of course one of the parameters that is increasing or decreasing the MIE, some materials are simply easier to burn than others and have lower MIE. The humidity is also important, as well as the temperature. Finally, one key parameter is actually the size of the powder. Indeed, the MIE varies along the cube of the diameter of the dust :
MIE = f(d3)
This means that the smaller the particles are, the lower is the MIE and the higher will be the dust sensitive to an ignition source. This dependency is important to interpret and understand the variations of MIE that can happen in between testings of the same material : indeed, engineers dealing with explosions risks have probably remarked that different sources may give different MIE for a same material.
This is often due to the size of the particles that has been tested. Testing institutes will try to bring the powder below 63 microns for testing, but they will not be exactly at this value and the actual mean particle diameter of the sample tested will vary. As MIE = f(d3) this means that even small variations of particle size will have a strong impact on the MIE. It is therefore advised, when carrying tests or comparing tests to get the actual d50 and d90 of the sample tested so that results can be properly interpreted and the right MIE selected.
4. Use in risk assessment
How to assess dust explosion risks using the MIE ?
The MIE is a key data to consider for every source of ignition as the energy that the source of ignition will be able to generate will or will not be susceptible to trigger an explosion depending if it is lower or higher than the MIE.
It is necessary to calculate the energy triggered by the source of ignition during a risk assessment, for example for the following sources of ignition :
- Spark discharge
- Brush discharge
- Corona discharge
- Propagating brush discharge
- Cone discharges
- Mechanical sparks
- etc...
It is also VERY important to verify if the gas in the mixture dust / gas is only air or if any flammable gas or solvent is susceptible to be present. Indeed, a flammable gas can drastically decrease the MIE of the mixture and make situation extremely hazardous [Glor]. This should be determined in risks analysis.
5. MIE of common dusts
Please find below some MIE data given in the litterature.
WARNING : these are general values given without guarantee, a risk assessment and design must ALWAYS refer to the MSDS of the ACTUAL product used for tests carried out specifically on the ACTUAL material by a reputable institute.
Results from different sources can also differ as the test procedures are very sensitive many parameters (see above), it thus requires extra caution to check in which conditions the dusts were tested, and if necessary requires additional tests.
Table 1 : Minimum Ignition Energy MIE of common materials in mJ, in air
| Material | MIE mJ |
|---|---|
| Coffee |
85 [Mills] |
| Grain dust |
55 [Mills] |
| Sugar |
35 [Mills] |
| Wheat flour |
50 [Mills] |
| |
|
| Coal |
55 [Mills]
|
| Wood flour | 40 [Mills] |
| Nylon | 20 [Mills] |
| Polyethylene | 10 [Mills] |
| Polystyrene | 15 [Mills] |
| Aluminium | 15 [Mills] |
| Magnesium | 40 [Mills] |
One question that is often asked is : what is the MIE of a powder mixture ? Actually many industries are handling mixtures as final product (pharma, food processing) thus it is needed to assess the MIE of mix.
Calculating the MIE of a mixture is actually not easy, although some models have been proposed in the literature [Bisel]. It appears from this study that the MIE of the mixture seems to be governed by the lower MIE component until reaching min 50% in weight of an "inert" component (or at least a component having a higher MIE), and in some cases until 80% of the inert in the mixture. The study shows that actually this may be due to the % of fines : if the mixture has a proportion of particles of small size, they will govern the MIE of the mixture, and if the fines belong to the component with the lower MIE, then the resulting MIE will be low for the mixture.
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Sources
# [Laurent] Securite des procedes chimiques, Andre Laurent, Tec et Doc, 2003, page 241
# [Glor] Ignition hazard due to static electricity in particulate processes, Martin Glor, Powder Technology, 135-136 2003
# [EN13821] European norm EN13821:2002 Potentially explosive atmospheres — Explosion prevention
and protection — Determination of minimum ignition energy of dust/air mixtures
# [Janes] MIKE 3 versus HARTMANN apparatus : comparison of measured minimum ignition energy (MIE), Journal of Hazardous Materials, Elsevier, 2008, 152 (1), pp.32-39.
# [Mills] Pneumatic Conveying Design Guide, David Mills, Butterworth Heinemann, 2004, page 577