Autothermal fast pyrolysis of birch bark with partial oxidation in a fluidized bed reactor

Fast pyrolysis of birch bark sawdust with partial (air) oxidation was studied in a bubbling fluidized bed reactor at reaction temperatures of 500 and 550 C. The bio-oil vapors were fractionated using a series of three condensers maintained at desired temperatures, providing a dry bio-oil with less than 1 wt.% water and over 93% of the total bio-oil energy. Oxygen feed was varied to study its effect on yield, composition, and energy recovery in the gas, char and oil products. The addition of oxygen to the pyrolysis process increased the production of gases such as CO and CO₂. It also changed the dry bio-oil properties, reducing its heating value, increasing its oxygen content, reducing its average molecular weight and its concentration of heavy bio-oil compounds, while increasing its viscosity and its phenolics concentration. The lower reaction temperature of 500 C was preferred for both dry bio-oil yield and quality. Autothermal operation of the pyrolysis process was achieved with an oxygen feed of 0.08 g per g of biomass at the reaction temperatures of 500 and 550 C. At these two temperatures, when compared with the standard pyrolysis with pure nitrogen, the yield of dry bio-oil was reduced by 22% and 31%, whereas the total energy content of the dry bio-oil was reduced by 25% and 34%, respectively.