Effect of wet torrefaction and densification molding on energy properties of solid biofuels with Japanese cedar

Abstract

One of effective paths to achieve net zero energy-related CO₂ emissions by 2050 is to shift away from coal. A reforming technology that combines torrefaction treatment with densification molding is considered to be the promising production method for solid biofuels to replace coal and coal coke. In the study, an optimum combination condition of wet torrefaction (WT) and densification molding is investigated to improve energy properties of solid biofuels such as higher heating value (HHV) and char yield (CY). HHV of wet torrefied samples is correlated with mass ratio of biomass to water (B/W) as well as solid mass yield (SMY), and HHV for B/W =1/20 is 3 to 4% lower than that for B/W =1/5 for the same SMY. From the proposed model to estimate HHV of wet torrefied sample, it is found that the change in fixed carbon content due to B/W results in the change in HHV. Particle density of densified biofuel with wet torrefied sample (WTB-fuel) in the SMY range above 0.6 is equal or higher than that with raw sample. But the advantage of densification molding of wet torrefied sample is not observed in the SMY range lower 0.6. For the same WT condition, the increase in CY due to densification molding is closely related to the suppression of volatile matter generation, which is confirmed by the increase in activation energy. From the investigation results on both effects of wet torrefaction and densification molding on improvement of char yield of WTB-fuel, it is found that enhancement factor of fixed carbon (EFC) for any molding temperature reaches a maximum at SMY of around 0.8, and the maximum EFC is obtained at molding temperature of 200 ℃. Therefore, it is concluded that the optimum combination conditions to produce WTB-fuel are wet torrefaction in the SMY range between 0.7 and 0.8 at higher B/W and densification molding at molding temperature of 200 ℃.