Gas-pressurized torrefaction of biomass wastes: The optimization of pressurization condition and the pyrolysis of torrefied biomass.

A novel gas-pressurized (GP) torrefaction with high oxygen removal efficiency at mild temperature was proposed in our previous work. However, the optimal condition of the GP torrefaction and subsequent pyrolysis of the torrefied biomass were not clear. In this work, the effect of pressure on the GP torrefaction and pyrolysis product properties of the torrefied biomass were studied in detail. The results show that the pressure increasing from 1.7 MPa to 5.0 MPa just slightly contributed to further oxygen removal, and 1.7 MPa was thus selected as the optimum pressure. The GP torrefaction significantly improved the product property of biomass pyrolysis compared to the conventional torrefaction (AP torrefaction). The acids content in bio-oil was reduced from 15-20% to less than 5%, and the calorific value of biogas increased to as high as 16.57-19.31 MJ/Nm3. Furthermore, an overall conversion mechanism of combined GP torrefaction and subsequent pyrolysis of biomass was proposed.

Gas-pressurized torrefaction of biomass wastes: Roles of pressure and secondary reactions.

A gas-pressurized (GP) torrefaction method, proposed in our resent work, can significantly promote the upgrading and oxygen removal of biomass wastes, compared to the traditional torrefaction (AP). However, the mechanism of the GP torrefaction process is not clear. In present work, semi-closed (SC) torrefaction, GP torrefaction, and AP torrefaction were conducted to reveal the roles of pressure and secondary reactions during GP torrefaction quantitatively. The results showed that the pressure significantly promoted the upgrading of biomass during GP torrefaction at 200 °C. The contribution of pressure on the oxygen removal of GP torrefaction at 200 °C was 63.87%. At relatively high temperature of around 250 °C, the promotions were caused by the synergistic effect of pressure and secondary reactions. The contribution of secondary reactions on the oxygen removal was 53.99%. Thus, the process of the GP torrefaction of biomass wastes was preliminarily understood.

Kinetic study of biomass pellet pyrolysis by using distributed activation energy model and Coats Redfern methods and their comparison.

In this study, the pyrolysis behavior and kinetics of raw biomasses and their pellets were studied by Coats Redfern and DAEM methods. The results demonstrated that the similar activation energies obtained by both methods confirmed accuracy of the kinetics calculation. The activation energy of the pellets was 132.49-232.44 kJ mol-1, slightly higher than those of raw biomasses, which was 120.58-210.55 kJ mol-1. The results from Coats Redfern method showed that the pyrolysis of all the samples were controlled by mass and heat diffusion. DAEM revealed that the activation energies of the pellets were higher than those of raw biomasses during hemicellulose and cellulose decomposition stages, and was opposite for the lignin decomposition stage. Physical structure characterization indicated that the pellets had smaller surface area and more compact surface than those of their raw biomasses. Hence, the mass and heat diffusion were suppressed and more cross-linking reactions occurred during pellets pyrolysis.