Gas-pressurized torrefaction of biomass wastes: Self-promoted deoxygenation of rice straw at low temperature

Abstract Gas-pressurized (GP) torrefaction of biomass, proposed in our previous work, has a much higher oxygen removal efficiency compared to conventional torrefaction, but how the pressure promoted the deoxygenation remains unclear. In this work, the GP torrefactions under different partial pressures of volatiles (Pvol) and total pressures (Ptot) were conducted. The results revealed that the Pvol was the root cause rather than the Ptot of the acceleration of biomass decomposition and deoxygenation for the first time. This process can be regarded as a self-promoted deoxygenation process, and consisted of three stages. Stage 1 was the volatiles formation stage, where hemicellulose decomposed, volatiles formed and the Pvol increased. Stage 2 was self-promoted dehydration stage, where the hydroxyl in biomass was dramatically removed. Stage 3 was structure rearrangement stage, where the aromatic structure formed. The stage 2 and stage 3 were highly correlated with the Pvol, and resulted in the deep deoxygenation of biomass. This study should be not only suitable for the GP torrefaction, but can also be extended to pressurized pyrolysis and gasification of biomass.