Effect of inerts on ignition sensitivity of dusts

Abstract

This work studies the influence of added inert dust on the ignition sensitivity of the combustible/inert dust mixture. Different types of combustible dust of various sizes, such as coal, powdered sugar, lycopodium, anthraquinone, rice and coffee husks, beech wood flour or ground coffee were mixed with alumina and kieselguhr before being tested. Parameters measured were 1) minimum ignition temperature (MIT) in 5 mm layer, 2) minimum ignition temperature in cloud (Godbert-Greenwald oven) and 3) minimum ignition energy (MIE) (in a Kuhner-Mike 3 apparatus). It appears from this study that the MIT in 5 mm layer is strongly influenced by an inert fraction >= 60 % (weight). There is also a significant increase in MIT in cloud form when the fraction of inert dust exceeds 60 % (weight). This threshold is a function of the bulk density of the inert substance selected for these tests. However, in cases where the density of the inert dust is different from that of the combustible dust, there appears to be a separation of the two constituents of the mixture after its pneumatic dispersion during the test. As a consequence, the influence of inert dust is less pronounced in the case of MIT for dust clouds than for the MIT of a dust layer. A similar phenomenon is observed for the MIE of dust clouds.