Pyrolysis of pellets prepared from pure and blended biomass feedstocks: Characterization and analysis of pellets quality

Recently, pyrolysis integrated with pelletization has received great attention. In the current study, various physical, mechanical, chemical and thermal characteristics of furfural residue pellets (FRPs), sewage sludge pellets (SSPs), furfural residue-sewage sludge pellets (FRSSPs) were studied at different pyrolysis temperature (200–850 °C) and residence time (15–45 min). The mass and energy yields of pellets dropped drastically with increasing pyrolysis temperature. Higher heating values (HHV) of pyrolysed furfural residue pellets (PYFRPs) and furfural residue-sewage sludge pellets (PYFRSSPs) increased from 22.22 to 26.76 MJ/kg, and 20.59–23.72 MJ/kg, respectively with increasing temperature from 200 to 850 °C. However, the HHV of pyrolysed sewage sludge pellets (PYSSPs) decreased with increasing temperature. The lowest pellet density and strength were found at 850 °C and residence time of 45 min. The pellets remained dimensionally stable during the water immersion test and after a storage time of 2 weeks, the relaxed densities and strength remained unchanged. The volumetric energy density of PYFRPs (30.35 kJ/m³) was found to be higher than PYSSPs (8.92 kJ/m³) and PYFRSSPs (23.85 kJ/m³) at the same process conditions. Thermal treatment enhanced the hydrophobicity of pellets. The lowest moisture uptake of 1.14 wt%, 1.8 wt%, and 1.58 wt% for the corresponding PYFRPs, PYSSPs and PYFRSSPs were observed at pyrolysis temperature of 300 °C. The maximum methylene blue and iodine adsorption values (i.e. 96 and 244.8 mg/g), (191 and 314.7 mg/g) and (121.6 and 263 mg/g) for PYFRPs, PYSSPs and PYFRSSPs were found, respectively, at temperature of ≥ 450 °C. Pyrolysis at higher temperature significantly reduced the heavy metals in PYFRSSPs and PYSSPs.