Diego Cavallero**, Luca Marmo**, Flavio Parozzi*, Edoardo Malgarida* VGR – V EDIZIONE – 2006 Convegno sulla Valutazione e Gestione del Rischio negli insediamenti Civili ed Industriali Pisa 17-19 Ottobre 2006 **Politecnico di Torino *CESI RICERCA The research investigates about the hazard of formation of explosible coal dust clouds inside thermoelectric plants. The study was addressed to experimentally measure two key-parameters for risk analyses: the potential resuspension rate of dust layers and the minimum explosible concentration of the clouds that can be generated. Representative samples of coal dust were collected from components and floors of typical power plants and characterized in terms of density, burnable fraction, morphology and size distribution. A small- scale experimental facility was set-up to perform ad-hoc tests to quantify the fraction that can be resuspended in airborne form due to an air jet. In this apparatus, the samples were subjected to fully turbulent gas flows, with velocities ranging from 0 to more than 50 m/s. Over 80 experimental tests were carried out, obtaining good repeatability and continuity of the results. A high speed video camera was also used to observe the fluid-particle interaction and the behaviour of the particle entrainment inside the bulk flow. The analysis of the results indicated that the mobilization of coal dust is detectable at air speed higher than 6-7 m/s. The experiments clearly showed that the mixing of air and particles is not instantaneous, as the dust particles roll and jump before being injected into the gas bulk. This behaviour can create a flow zone, situated near the surface of dust deposits, having a wide range of particle concentration, that can fall within the explosibility range. As regards the explosibility, the attention was focused on an experimental campaign devoted to measure the Minimum Explosible Concentration of the same coal dusts, as a function of the most relevant factors (i.e. size distribution, moisture content, initial temperature). For the collected coal dusts, the measurements gave a Minimum Explosible Concentration ranging from 40 g/m 3 to 70 g/m 3 , with some influence of particle size and moisture content.