Discussion on the autoignition probability of dust clouds

Several tests to determine the dust explosibility parameters were presented in the technical and scientific literature which can be used to assess the characteristics of a dust sample. Some parameters describe a countable variable, which is defined by a numerical value (e.g. P max, kst) while others define a threshold above (or below) which the explosion should not occur (MIE, LOC, autoignition temperature). These latter should be regarded as limit values at which the probability of occurrence of ignition (or explosion propagation in case LOC or LEL are considered) is low enough to be considered negligible. Technical standards describe the procedure to measure the different parameters. The Cloud Autoignition Temperature (AIT), according to the European standard CEI EN 50281 is the temperature at which 10 consecutive ignition attempts result negative (no ignition). Similar assumption is made by UNI 13821 as MIE is considered. Generally speaking, these technical standards do not state what is the confidence of the threshold value determined by standard procedures. The ignition probability at such threshold is supposed to be zero, but to date it has to be considered just as an assumption, since the relationship between the probability of occurrence of the studied phenomenon (ignition for example) and the independent variable (the temperature, in the case of AIT) is not yet well established. Nevertheless, it is known that ignition probability does not change abruptly from 0 to 1 as a step function but through a smooth slope over a temperature interval. This work aims to investigate the dependence of the cloud Autoignition probability of a Lycopodium cloud versus the temperature in the temperature interval close to AIT with a twofold purpose. The first is to describe the relationship Ignition probability vs temperature in the range where the probability varies from zero to one (we call such range probability escalation interval, PEI), providing an appropriate statistical model to describe this dependency. The second goal of the study is to investigate the reliability of the AIT value determined by the procedure defined by technical standards and to propose a new method of defining the AIT, based on statistically sound methodology. Experimental data can indeed be used to identify the threshold below which the ignition probability is lower than a suitably predefined level with prescribed confidence, providing a more solid ground on which the definition of the AIT could be based.