Suspension Combustion of Wood: Influence of Pyrolysis Conditions on Char Yield, Morphology, and Reactivity

Abstract

Chars from pine and beech wood were produced by fast pyrolysis in an entrained flow reactor and by slow pyrolysis in a thermogravimetric analyzer. The influence of pyrolysis temperature, heating rate and particle size on char yield and morphology was investigated. The applied pyrolysis temperature varied in the range 673−1673 K for slow pyrolysis and between 873 and 1573 K for fast pyrolysis. The chars were oxidized in a thermogravimetric analyzer and the mass loss data were used to determine char oxidation reactivity. Char yield from fast pyrolysis (104−105 K/s) was as low as 1 to 6% on a dry ash free basis, whereas it was about 15−17% for slow pyrolysis (10−20 K/min); char yield decreased as pyrolysis temperature increased. During fast pyrolysis wood particles underwent melting, yet to different extents for the two investigated fuels: pine wood produced chars of porous spherical particles, whereas beech sawdust chars showed a somewhat less drastic change of morphology with respect to the parent fuel. Char produced by low heating rate pyrolysis fully retained the original fibrous structure of wood. Fast pyrolysis chars were significantly more reactive than slow pyrolysis chars; moreover, char oxidation reactivity decreased as pyrolysis temperature increased. The amount and composition of the ash forming matter of the wood fuels seems to play an important role in determining the differences in char yield, morphology and reactivity.