Effect of Particle Geometry and Microstructure on Fast Pyrolysis of Beech Wood

The influence of particle geometry and microstructure in fast pyrolysis of beech wood has been investigated. Milled wood particles (<0.08-2.4 mm) and natural wood cylinders (2-14 mm) with different lengths (10-50 mm) and artificial wood cylinders (D-p = 0.5-14 mm) made of steel walls, filled with small milled wood particles (<0.08-0.140 mm), have been pyrolyzed in a fluidized bed at 500 degrees C. From the results of the experiments, the influence of particle geometry and microstructure on char, gas, and pyrolysis oil yield and pyrolysis oil composition has been derived. The product yields of large cylinders with diameters of 6-14 mm are primarily determined by the outer diameter and resulting heating rate. The microstructure of these cylinders, being either natural channels or randomly packed small milled wood particles, has turned out to be much less important. For the smaller milled wood particles, the microstructure does have a profound effect on the product yields. The smallest particles (<0.140 mm), which consist only out of cell wall material and have lost their typical wood channel structure, show a clearly higher oil yield and lower char yield. It is postulated that the high pyrolysis oil yield can be explained by larger mass transfer rates of pyrolysis products from these smallest particles, as compared to mass transfer from particles containing channels.