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Several phenomena occur during a dust explosion and they can impact the overall kinetics: external convection, internal particle heating, pyrolysis reaction and gaseous-phase combustion. According to the particle size distribution, the powder nature, the external temperature and heating rate, any of these steps can be rate-limiting and must therefore be detailed in order to serve as a basis for the explosion modelling. But under which conditions the pyrolysis reaction can be considered as the rate-limiting step of the global phenomenon? For this study, a Godbert-Greenwald furnace was modified to ensure its airtightness and cellulose samples were dispersed through the heated chamber by argon pulses. At the outlet, pyrolysis products were collected and analysed. These tests led to the formulation of a representative gas composition produced by the pyrolysis of cellulose. This representative model gas was then used for explosions tests. The hybrid mixtures of cellulose and pyrolysis gas showed a peculiar behaviour: as the dust-to-gas ratio increased, maximum rate of pressure rise initially increased, then it decreased, while the maximum explosion pressure did not vary significantly. The powder boosting effect on the rate of pressure rise was thus observed. Therefore, the study of the radiative heat transfer mode by adding biochar enlightened a major dependency of the flame propagation on the radiation. In fact, small amounts of char (higher emissivity than cellulose) were enough to significantly increase the rate of pressure rise, with a small influence on the maximum pressure. The pyrolysis step must be considered as a major rate-limiting step in a dust explosion phenomenon, but other ones may play an important role, such as the radiative heat transfer to the particle and the particle heating step. In such a way, these phenomena seemed to control and to determine the flammability and explosibility characteristic of a powder. |
