(32ds) Influence of Inert Dust Dilution on the Ignition Sensitivity of Combustible Dust

The risk associated with dust explosions still exists in industries that process or handle combustible dust. This explosion risk could sometimes be prevented or mitigated by applying the principle of inherent safety (moderation). This is achieved by adding an inert material to a highly combustible material to decrease the ignition sensitivity of the combustible dust. Mixing an inert dust or a less flammable material with a combustible dust can be regarded as a direct application of moderation, as it allows the use of the hazardous material in a less hazardous form. The principle of influencing the explosibility of combustible dust, that is, decreasing the ignition sensitivity or explosion severity by mixing the combustible dust with an inert dust, is not new to process industries. Some industrial applications of such mixtures are: in the plastics or textile industries, mineral loadings are used to obtain thermally stable polymers or to improve the mechanical properties of the fibers, membranes, or coatings. In the case of solid inerting, for instance, when rock dusts are mixed with coal in the mining industries or when flame-retardants are added to foams or textiles, the main objective is to prevent the materials from igniting. However, the addition of an inert dust can have significant adverse effects on the properties of the final product, not to mention the possible adverse processing and handling issues. In order to further validate this principle of moderation, detailed experimental investigation of the influence of adding an inert dust on the minimum ignition energy (MIE) and the minimum ignition temperature (MIT) of the combustible/inert dust mixtures has been performed using three inert materials (magnesium oxide, ammonium sulphate and sand) and six combustible dusts (brown coal, lycopodium, toner, niacin, corn starch and high-density polyethylene). The experimental investigation was done in two laboratory scale equipment, the Hartmann apparatus and the Godbert-Greenwald furnace for the MIE and the MIT, respectively. Generally, increasing the concentration of the inert material increases the MIE as well as the MIT until a threshold is reached where no ignition is obtained. A direct correlation between these research findings and industrial applications, as well as case scenarios, will be presented. For instance, adding a small amount of inert material could move the MIE to two orders of magnitude. For example, Niacin dust with an MIE of 10 mJ (highly sensitive) could increase to 150 mJ (moderately sensitive) when 20 % of Magnesium oxide is added. Hence, the test results prove that the sensitivity of highly combustible dust could be altered (decreased) when mixed with an inert solid. In addition to adding inert dust to highly combustible dust, increasing the moisture content as well as the particle size could also decrease the sensitivity of combustible dust.