Characterization of biomass pyrolysis tars produced in the relative absence of extraparticle secondary reactions

Product distributions from the atmospheric pressure pyrolysis of sugar-cane bagasse and silver birch were determined in a wire-mesh reactor as a function of temperature (300–900 °C) and heating rate (1–1000 K s−1), over a range of holding times between 0 and 100 s. Major groups of components present in the tars were identified by g.c.-m.s.. Above 500 °C when increasing the heating rate from 1 to 1000 K s−1 in experiments using holding times longer than several seconds, tar yields were observed to increase by about 10 wt% to nearly 54% w/w daf bagasse; total volatile yields increased by similar amounts. Number average molecular masses (MMn) of tars determined by vapour pressure osmometry (v.p.o.) were in the range 150–250 Da, those of silver birch tars being systematically larger than those of sugar-cane bagasse by 15–30 Da. For both sets of tars, MMn were observed to decrease with increasing temperature between 400 °C and 600 °C, but remained unchanged between 600 °C and 900 °C. The molecular masses were also observed to decline with increasing heating rate. Lignocellulosic substrates appear to cleave readily to give relatively small, compact and relatively stable molecules able to evaporate completely at temperatures around 500 °C. Extensive chemical modification of primary products appears to result from intraparticle secondary reactions, these reactions being intensified during pyrolysis at higher heating rates. Whilst the wire-mesh reactor configuration appears not to alter tar yields or structures subsequent to release from parent substrate particles, comparison with tars obtained at 600 °C in a ‘hot-rod’ (fixed-bed) reactor suggests that whilst overall yields may be adversely affected and the relative abundance of some major components altered, extraparticle secondary reactions do not substantially change the character of the tar at this temperature.