Biomass potassium impregnation effect on oxidative torrefaction

Oxidative torrefaction, along with non-oxidative torrefaction, is a mild pyrolysis process usually carried out between 200 and 300 °C aiming to increase energy content, the hydrophobicity, and grindability of biomass. Pursuing advances in the torrefaction treatment for energy cost-saving, torrefaction can be performed with flue gases containing a limited amount of oxygen. In these conditions, torrefaction has a higher reaction rate, thereby shortening the torrefaction duration. Moreover, it has been shown potassium catalysis the component's conversion mechanisms during thermal degradation. Therefore, the present study evaluated the combined effect of potassium impregnation and flue gas atmosphere on the torrefaction process. Demineralized woody biomass (Amapaí – Brosimum potabileDucke) samples were impregnated with di.erent concentrations of K2CO3(0.003M, 0.006M and 0.009M). Oxidative torrefaction was conducted at 275 °C in a laboratory fixed-bed reactor under a flue gas atmosphereup to an anhydrous weight loss (AWL) of 25%, and condensable species were recovered and analyzed by GC-MS. Isothermal TGA at 275 °C was also performed. The combined impact of oxidative atmosphere and potassium impregnation was evaluated by the material reaction kinetics (TG and DTG) and the released condensable. The synergistic behavior of the oxidizing atmosphere and biomass potassium concentration were explored by three indexes, namely the torrefaction severity index (TSI), the catalytic effect area (CEA), and the catalytic index (CI). The potassium addition anticipated the oxidation to early torrefaction times and the oxidative atmosphere enhanced the potassium catalytic effect of increasing the maximum reaction rates during torrefaction. The TSI values enable a dimensionless parameter to calculate the CI. A strong linear correlation (R2 > 0.90) between CI and K% was evidenced. CI values were 0–0.17. The increase in furfuryl alcohol production with increasing K contents was promoted in the presence of oxygen. Under oxidative conditions, vanillin and syringaldehyde yields decreased with increasing K contents, whereas negligible effects were detected in nitrogen. Results suggest that the residence time and the production of targeted condensable compounds could be adjusted according to the biomass potassium content and torrefaction atmosphere.