Torrefaction of walnut oil processing wastes by superheated steam: Effects on products characteristics

Walnut oil production waste (WOPW) is a by-product of walnut oil processing. The organic waste is rich in holocellulose and lignin, showing good potential to be converted by thermal process to valuable products. Superheated steam (SHS) torrefaction is a recently proposed thermal process enabling fast and unformal biomass heating, resulting in high-quality solid products as direct fuel. The potential of SHS to torrefy lipids and proteins (being rich in WOPW) is attractive for broader application of SHS torrefaction to upgrade more biomass wastes. SHS torrefaction was studied in this work to upgrade WOPW for solid products with different reaction temperatures (200, 250, 300 °C) and residence times (20, 40, 60 min). The lowest weight yield was 43.64 wt% under the severest treatment of 300 °C and 60 min, accompanied with the highest energy enhancement of 1.34 (reaching HHV of 27.03 MJ/kg). Response surface method is employed to reveal the effects of temperature and residence time. Residence time of 40 min under 300 °C was supposed to be an ideal condition to upgrade WOPW with HHV of 26.68 MJ/kg and in the range of coal from Van Krevelen diagram. Combustion indices (e.g., fuel ratio, combustion index, and volatile ignitability) indicated that the aforementioned torrefied WOPW had favourable properties as co-firing material. On the other hand, combustion behaviours analysis demonstrated that SHS torrefied WOPW could perform well as direct fuel. Aqueous effluent was also condensed and analyzed, where products from lipids and proteins were massively presented, giving an insight into the decomposition of those two constitutes undergoing SHS torrefaction.