Analysis of the Pyrolytic Runaway Dynamics during Agricultural Waste Conversion

Potato plant waste pyrolysis is investigated by means of a cylindrical packed-bed reactor exposed along the lateral surface to heat fluxes of 20.6-33.2 kW/m2 (corresponding to heating temperatures, Ts, around 530-770 K). After an abrupt transition from torrefaction, a pyrolytic runaway regime is established (570 K <= Ts <= 620 K), characterized by temperature overshoots up to 300 K and actual conversion times only 4-15% of the preheating times. A uniform radial temperature profile is first associated with a trigger point positioned at the bed center and the propagation of a progressively thick thermal wave toward the lateral surface and then with one or more trigger points and the almost simultaneous conversion of the entire bed. Subsequently the trigger point moves to the lateral surface and a thermal wave propagates toward the bed center. The latter heating dynamics are also qualitatively preserved for Ts > 620 K when, in the presence of increasing radial gradients, a self-controlled pyrolysis regime is established with the reaction heat mainly exploited to preheat the bed.