Effect of fast pyrolysis conditions on biomass solid residues at high temperatures

• Char yields prepared at intermediate (103 °C/s) and fast (104 °C/s) heating rates were compared.
• Low fuel alkali content, high temperatures and fast heating rates lead to low biomass char yields.
• Higher yields of soot were detected for wood than for herbaceous biomass.
• Low fuel alkali content and extractives removal lead to stronger fluidity of biomass chars.
• Pinewood char obtained the most spherical shape, whereas beechwood and herbaceous char particles retained a cylindrical shape.

Fast pyrolysis of wood and straw was conducted in a drop tube furnace (DTF) and compared with corresponding data from a wire mesh reactor (WMR) to study the influence of temperature (1000–1400)°C, biomass origin (pinewood, beechwood, wheat straw, alfalfa straw), and heating rate (103 °C/s, 104 °C/s) on the char yield and morphology. Scanning electron microscopy (SEM), elemental analysis, and ash compositional analysis were applied to characterize the effect of operational conditions on the solid residues (char, soot) and gaseous products. The char yield from fast pyrolysis in the DTF setup was 3 to 7% (daf) points lower than in the WMR. During fast pyrolysis pinewood underwent drastic morphological transformations, whereas beechwood and straw samples retained the original porous structure of the parental fuel with slight melting on the surface. The particle size of Danish wheat straw char decreased in its half-width with respect to the parental fuel, whereas the alfalfa straw char particle size remained unaltered at higher temperatures. Soot particles in a range from 60 to 300 nm were obtained during fast pyrolysis. The soot yield from herbaceous fuels was lower than from wood samples, possibly due to differences in the content of lignin and resin acids.