Chemical characterization of volatile products of biomass pyrolysis under significant reaction-induced overheating

The gaseous and liquid products of the fixed-bed pyrolysis of agricultural residues (hazelnut shells, olive pomace, straw pellets) and wood (beech wood and softwood pellets) are chemically characterized and compared. The heating temperature is varied in the range 473-800 K but the process occurs in the presence of large thermal overshoots. These reach their maxima for heating temperatures below (residues, overshoots up to about 225 K) or above (wood, overshoots up to about 85 K) approximately 650 K. Though mainly originated from secondary decomposition, overheating also highly enhances the primary decomposition rates, globally leading to short volatile residence times. As a consequence, the product distribution is modified. In particular, at low heating temperatures, also following pyrolytic runaway, the residues produce the largest amounts of volatiles and the yields of several condensable organic compounds (acetic acid, propionic acid, formic acid, hydroxypropanone, levoglucosan, guiacols and syringols) reach their maxima, with even qualitative deviations from the usual trends observed in pyrolysis.