Fast pyrolysis in a novel wire-mesh reactor: Decomposition of pine wood and model compounds

In fast pyrolysis, biomass decomposition processes are followed by vapor phase reactions. Experimental results were obtained in a unique wire-mesh reactor using pine wood, KCl impregnated pine wood and several model compounds (cellulose, xylan, lignin, levoglucosan, glucose). The wire-mesh reactor was typically operated at a set temperature of 500 degrees C, high heating rates (up to 7000 degrees C/s) under vacuum and with liquid nitrogen cooling to ensure a short vapor residence time (<15-25 ms) and a low vapor phase temperature. In this way good suppression of vapor phase reactions and reactions inside the decomposing particle were achieved as shown by the (almost) complete recovery of glucose and levoglucosan after pyrolysis. The influence of temperature (250-700 degrees C) and heating rate (50-7000 degrees C/s) was studied on product yields and compositions (GC, SEC, NMR, FTIR, HPLC). A high oil yield of 84 wt% and a low char yield of 5 wt% was observed for pine wood around 500 degrees C. Above this temperature a slight drop in oil yield was observed, but the oil yield was still 60 wt% at 700 degrees C. Also high oil yields and low char were obtained during the pyrolysis of cellulose (oil/char: 95/0 wt%) and lignin (oil/char: 78/12 wt%). Our data were interpreted with a physical-chemical mechanism. The basic idea behind this mechanism is that the amount and composition of the "decomposing biomass", which can be in the solid or liquid state under the pyrolysis conditions, is changing in time due to competition between (cross)-linking reactions, scission reactions and mass transfer of decay fragments out of the decomposing particle. Observations such as the twice as large char yield for K-impregnated pine wood (10 wt%) compared to untreated pine wood (5 wt%) and the 20 wt% increase in char yield from lignin when decreasing the heating rate from 6000 to 60 degrees C/s could be explained by this mechanism. (C) 2012 Elsevier B.V. All rights reserved.