Torrefied biomass as feed for fast pyrolysis: An experimental study and chain analysis

A torrefaction pre-treatment could enhance the fast pyrolysis process to produce bio-oil by decreasing the required energy for grinding biomass particles and by improving bio-oil characteristics so they resemble more those of fossil fuels. To evaluate this hypothesis, this work compares fast pyrolysis experiments of raw and torrefied woody biomass feedstocks, using a 500 g/h entrained down flow reactor. The feedstocks used were hardwood (ash wood), softwood (spruce) and mixed waste wood. These feedstocks were torrefied at various temperatures between 250 °C and 300 °C by means of two torrefaction processes: a directly heated moving bed, and the Torbed® process. The effect of pelletizing was also analyzed for the hardwood feedstock, comparing torrefied chips and torrefied pellets. The obtained bio-oils from experiments with torrefied feedstock had overall improved oxygen and heating value properties compared to bio-oils from raw feedstock. Hardwood pellets torrefied at 265 °C with a residence time of 45 min produced the oils with the highest quality with respect to oxygen mass fraction (decreased from 45.7% to 37.2%) and higher heating value (increased from 19.1 MJ/kg to 23.1 MJ/kg), compared to bio-oil produced from raw hardwood feedstock. However, these properties came at a severe loss of oil yield, decreasing from 44% for raw feedstock to an average of 31% for torrefied feedstock. Nonetheless, a chain analysis shows that a torrefaction pre-treatment could be more attractive on energy basis compared to a conventional fast pyrolysis process with a deoxygenation upgrading step.