Self-Ignition Behaviour of Growing Dust Layers

Despite all precautions in the dust-processing industry there are often hazardous situations leading to damage to property and persons. In drying processes self-ignition respectively smoldering fires of dust accumulations are often considered as leading causes for such incidents. Several well-established experimental test methods exist to analyse the self-ignition behaviour. These procedures have specific advantages and disadvantages, such as testing time, energy consumption and especially their different capability of scale-up from laboratory to industrial scale. Within the scope of a safety assessment of drying procedures these test methods often lead to a temperature level at which heat production exceeds the natural heat losses of the bulk and lead to spontaneous combustion. For the assessment of deposits in dryers etc. the described methods can be directly applied in the majority of cases, as well as to process conditions, e.g. a dust deposit with a defined thickness at different operating temperatures concerning their thermal hazards (double right arrow ignition source: glowing particle). But one point will be unidentified in this point of view: changes in the physical properties of dust accumulations over time and mainly for non-uniform layer thickness. These changes, however, remain unknown and are mostly ignored as possible source of spontaneous combustion. These parameters often represent the decisive issue to trigger a run-away reaction, especially in case of spray dryers with variable dust accumulations on the inner wall. To take this effect into account the conventional test procedures were adapted to simulate experimentally the layer increase during the drying process depending on the operating temperature and time.