Multi-scale analysis of the exothermic behavior of agricultural biomass pyrolysis

Potato crop residues are one of the few feedstocks which, packed in a bed heated for a narrow range of low temperatures, undergo pyrolytic runaway. This regime is assimilable to intrinsically fast pyrolysis, rapidly occurring in the presence of large temperature overshoots, potentially exploitable for autothermal conversion. To understand the origin of the exothermicity, the influences of the plant parts (foliage, branches, stem and top and bottom parts of the stem) are studied on the packed bed characteristics for a heating temperature of 605K. The magnitude of the temperature overshoot is maximum for the bottom part of the stem and tends to decrease for the top part and the branches (local and spatially averaged maxima roughly in the range 290-230K and 260-200K, respectively). These findings can be associated with a corresponding decrease in the cellulose/starch contents and thickness of the stem/branch wall. Hence the concentration, the reactivity and the intra-particle residence time of primary vapors are progressively reduced as well as the exothermic activity of secondary charring reactions. Foliage does not contribute significantly to the process exothermicity (maximum spatially averaged temperature overshoots around 20K). Micro-scale thermograms, dominated by primary decomposition, globally show mild, medium and high exothermicity for the foliage, branches and stem in the order (global reaction heats around -80, -300 and -420J/g, versus 270J/g for beech wood).