The minimum ignition temperature (MIT) is the lowest temperature of a hot surface that will cause a dust cloud, rather than a dust layer, to ignite and propagate flame. The test follows BS EN 50281-2-1:1999 (Methods of determining minimum ignition temperatures).
Approximately 0.1 g of combustible dust is placed in a dust holder at the top of a temperature controlled furnace with an open bottom. The dust is dispersed by compressed air downwards past the hot surface of the furnace to see if ignition occurs and flames are produced. If the dust does not ignite, the furnace temperature is increased and the test repeated until ignition of the dust occurs.
Once ignition has been established, the mass of the dust sample and injection pressure are varied to find the most vigorous explosive flame discharge. The temperature of the furnace is then reduced incrementally until flame propagation is no longer observed. At this temperature, the dust mass and injection pressure are varied to confirm that no ignition is found over ten consecutive tests. The minimum ignition temperature (MIT) is the lowest temperature of the furnace at which flame is observed minus 20 oC for furnace temperatures over 300 oC or minus 10 oC for furnace temperatures under 300 oC.
For items of plant such as driers, testing the minimum ignition temperature is important to prevent a dust explosion occurring through contact with a hot surface. The internal temperature is generally limited to two thirds of the MIT when measured in degrees Centigrade. For example, a dust with a MIT of 450 oC would require a maximum operating temperature in the drying process of 300 oC, giving a safety margin of 150 oC.
|Example test results |
Minimum ignition temperature (oC)
||no ignition up to 850|
|Sewage sludge dust