Effect of Fast Pyrolysis Conditions on the Biomass Solid Residues at High Temperatures (1000-1400°C)

Fast pyrolysis of wood and straw was conducted in a drop tube furnace (DTF) and compared with the experimental work on the wire-mesh reactor (WMR) to study the influence of temperature (1000-1400°C), biomass origin (softwood, hardwood, grass) and heating rate (1000°C/s, 10^4 °C/s) on the char yield and morphology. Scanning electron microscopy (SEM/EDS), elementary analysis, CAMSIZER XT, ash compositional analysis were applied to characterize the effect of operational conditions on the solid and gas products. Char yield from fast pyrolysis in the DFT setup was 2 to 6 % (daf) lower than in the WMR apparatus. During fast pyrolysis pinewood underwent drastic morphological transformations, whereas beechwood and straw samples retained the original porous structure of the parental fuel with the slight melting on the surface. Particle size of Danish wheat straw char decreases in a half-width with respect to the parental fuel, whereas alfalfa straw char particle size remained unaltered with the higher temperatures. In this study, the retained shape of beechwood and herbaceous biomass samples is related to the presence of extractives and formation of silicates. Soot yield from herbaceous fuels occurs lower than from wood samples. During fast pyrolysis, additionally to the soot and char particles, spherical solid residues of particle size (60-300 nm) were obtained. Based on the previous studies, these smooth spherical particles are associated with the formation of condensed tars under fast heating rates and at very high temperatures.