Minimum Ignition Temperature of dust

Physical meaning and use in risk assessment of powder MIT

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Section summary
Definition of MIT
Determination of MIT
Typical values
Use in risk assessment
MIT of common dusts

Bulk Solids Handling Safety
# Dust explosion physical properties # MIT - Minimum Ignition Temperature of dust

1. Definition of MIT

If a dust cloud enters in contact with a hot surface, it may auto-ignite and explode. The Minimum Ignition Temperature is the minimum temperature for which a hot surface will ignite a dust cloud [Laurent].

2. Experimental determination of MIT

The MIT is measured experimentally. The experiment consists in putting in suspension dust 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 submitting the dust cloud to a hot surface in an oven.

The MIT is the smallest temperature of the oven observed that triggers an explosion at the concentration tested.

To be representative, such experiences must be carried out following a clear protocol (example : ASTM E1491 in US, EN 50281-2-1 in Europe). The test machine mentioned in the norm is also important as values may be slightly different from one apparatus to another. Norms and literature are mentioning the Godbert Greenwald furnace and the BAM furnaces as apparatuses to perform the tests.

Those protocols should not be confused with the minimum ignition temperature of dust layers which corresponds to other conditions and physical meanings.

3. MIT typical range

MIT values are generally in between 150 and 700 degrees celsius, of course dependent of the nature of the material.

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.

4. Use in risk assessment

The MIT must be compared to the maximum temperatures that can be observed in a process. This can be within the process or even outside (motors for example). The process plant operators must make sure that dust is not entering in contact with hot spots. At design stage, the MIT of the dusts involved in the process can be used to specify equipment and make sure by design that temperature cannot reach the MIT of any component of the process.

Maximum admissible temperatures that can be put in contact with a powder cloud is given as 2/3 of TMI.

Tmax = 2/3*TMI [Stahl]

5. MIT of common dusts

Please find below some MIT 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.


Table 1 : Minimum Ignition Energy MIT of common materials

Material MIT °c
Aluminium
520 [Laurent]
640 [Mills]
Barley
450 [Polka]
Buckwheat
450 [Polka]
Carrots dried
580 [Polka]
Cereals
520 [Laurent]
Coal
600 [Laurent]
610 [Mills]
Coffee 410 [Mills]
Cornflakes 470 [Polka]
Epoxy powder
510 [Laurent]
Grain dust 430 [Mills]
Hop 460 [Polka]
Lemon balm 480 [Polka]
Magnesium 520 [Mills]
Milk powder
610 [Laurent]
Nettle
500 [Polka]
Oatmeal
480 [Polka]
Polyethylene 440 [Laurent]
390 [Mills]
Polystyrene 490 [Mills]
Rice flakes 430 [Polka]
Semolina 450 [Polka]
Senna fruit 520 [Polka]
Starch
350 [Laurent]
460 for corn starch [Polka]
Sugar 490 [Laurent]
350 [Mills]
Sulfur 280 [Laurent]
Sunflower hulls 460 [Polka]
Valerian 520 [Polka]
Wheat Flour 380 [Mills]
Wood flour
490 [Laurent]
430 [Mills]




Sources

# [Laurent] Securite des procedes chimiques, Andre Laurent, Tec et Doc, 2003, page 237
# [Mills] Pneumatic Conveying Design Guide, David Mills, Butterworth Heinemann, 2004, page 577
# [Stahl] The basics of dust explosion protection, manual edited by Stahl, page 25
# [Polka] Experimental analysis of minimal ignition temperatures of a dust layer and clouds on a heated surface of selected flammable dusts, Polka et al., Procedia Engineering 45, 414-423, 2012,




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