By: Dr. Ashok Ghose Dastidar, PhD, MBA, Vice President, Dust & Flammability Testing and Consulting Services, Fauske & Associates, LLC
While dust explosions can lead to large and devastating losses in industry, they are not the only form of loss related to the combustion of powders. Often overlooked is the loss from dust/powder fires of related to a phenomenon called self-heating. Under special circumstances, the heat from the general environment may be enough to cause material to spontaneously auto react. This auto reaction results in the smoldering combustion of the powder/dust pile without an external ignition source such as open flame, electric/electrostatic spark or burning embers. The smoldering combustion can then lead to the burning of the powder/dust material with visible flames or even result in a dust explosion.
The self-heating potential of powders/dusts can be evaluated using several complex methods, but this can be very time consuming and may result in costly testing of materials that don’t have any self-heating potential at practical temperatures. Many times, a quick screening test is all you really need to assess a material's self-heating potential. You can then screen out materials that auto-ignite at unlikely temperatures in a given environment and focus on materials that have some self-heating potential; thereby saving time and expense.
One such screening tool is the Grewer Oven. In this test, six 8-ml wire-mesh baskets are concurrently placed in a heated air stream. With this arrangement, five test samples and a reference material can be studied. The reference sample is typically graphite. Each of the six baskets is affixed with a thermocouple to monitor the internal temperature. The temperature of the oven and air stream can then be ramped from ambient temperatures to 300°-400°C at a rate of 1°C/min.
Figure 1 illustrates some typical results. In this figure the temperature history of Lycopodium and Soy Flour is plotted and compared with graphite. The Lycopodium starts to self heat at approximately 180°C while the Soy Flour starts to self-heat at 200°C. This indicates that Lycopodium and soy flour do have self-heat potential at temperatures commonly found in industrial settings. Further thermal hazards analysis is warranted for these materials with studies tailored to the specific temperature environment at a given facility.
The Grewer Oven method may not be as quick or as simple as a Differential Scanning Calorimetry (DSC) test (a more modern scientific technique), but it has one advantage in that the sample is constantly exposed to a heated air-stream. In the DSC test the sample is in a sealed pan where the potential of oxygen starvation of the self-heating reaction exists. Once starved of oxygen the fire potential of the material may be masked and the hazard risk may be dismissed prematurely.
For more information on characterizing and assessing the dust explosion and fire hazards at your facility, please feel free to contact me at firstname.lastname@example.org or (630) 887-5249 www.fauske.com