Assessment of the devolatilization behavior of fuel pellets in fluidized bed

The volatile release or devolatilization is an early stage occurring upon fuel particle feeding in a combustor. For coarse fuel particles, as in the case of fluidized bed (FB) combustion, it is mainly controlled by the thermal properties of the bed and fuel. The direct observation of the particle behavior in fluidized bed is a simple and effective technique for getting information on the devolatilization time and the number of volatile bubbles issuing from the fuel particle. The paper reports on experiments of devolatilization of different fuel particles of similar size, with particular concern on pellets composed by wood and wood/coal. The experimental technique was mainly based on the visual observation of the bed surface, the data-acquisition by a video-camera and the post-process elaboration of the frame sequences. Devolatilization times of dozens of seconds were measured for pelletized fuels, longer than those of normal wood particles. Two kinds of events were observed during devolatilization: i) the eruption of a “hot bubble” produced by bursting a submerged fuel-rich bubble, ii) the generation of “flames” at the bed surface. The number of such events was counted, obtaining values up to 100 per single particle under more critical conditions. A correlation between the number of monitored events and a dimensionless variable - function of FB and fuel properties - was proposed, providing a reasonable dependence on most relevant variables.