Minimum ignition energy of hybrid mixtures of combustible dusts and gases

• Minimum ignition energy of dust decreases upon addition of gas which is itself not ignitable.
• Model to predict the MIE of hybrid mixture was compared to experimental results.
• Minimum ignition energy of hybrid mixtures of different combination of dusts and gases were tested.
• The ignition sensitivity of combustible dust increase upon addition of a non explosible concentration of gas.

Mixtures of suspended combustible dust and flammable gas are usually encountered in various processes and systems where substances of different states of aggregate are handled. Knowing the lowest amount of energy needed to ignite such mixtures are critical to identify possibilities of accidental hazards in industry. Investigation of the minimum ignition energy (MIE) of a hybrid mixture of two flammable gases (methane and propane) and eight combustible dusts (wheat flour, starch, protein, polyethylene, peat, dextrin, wood coal and brown coal) were carried out in the modified Hartmann apparatus. The determination of the MIE of the dusts alone was in accordance with the European standard EN 50281. In the case of hybrid mixtures testing, this protocol had to be slightly modified, as hybrid mixtures are not included in the standard mentioned. The device used is limited to a lowest ignition energy of 4 mJ. Thus, the MIE of pure gases could not be as tested directly, as their values are all below that energy. The MIE values as well as the lower explosible limits (LEL) for gases were taken from the literature. To determine the MIE of hybrid mixtures at different concentrations of gas below the respective LEL were added to the pressurized air that used to generate the dust cloud in the MIE apparatus. The experimental results demonstrated a significant decrease of the MIE of the dusts and an increase in the likelihood of explosion when a small amount of gas that was below its LEL was mixed with the dust. For example, the MIE of polypropylene was observed to decrease from 116 to 5 mJ when only 1 vol.% of propane (below its LEL) was added. Moreover, an empirical model to predict the MIE of hybrid mixtures was presented and further compared with the experimental results were done.