Yield, Composition, and Property of Biochar Obtained from the Two-Step Pyrolysis of Rice Husk Impregnated with Boric Acid

Adding catalysts such as boric acid into biomass is a flexible method to achieve high biochar yield with low carbon emission levels. This work is directed towards investigating the effects of boric acid content on biochar yield, composition, and property through a two-step pyrolysis of rice husk. The results showed that biochar yields, compositions, and properties were associated with the amounts of boric acid impregnated. Yield, C content, and calorific value of biochar were improved by adding 2.0 wt % of boric acid into rice husk, and those values were 46.4 wt %, 59.3 wt %, and 22.6 MJ/kg, respectively. However, the fixed-carbon content of biochar was around 55.4 wt % and was barely affected by the changes in the amounts of boric acid absorbed. Therefore, the optimized boric acid content of rice husk was 2.0 wt % when biochar was applied to the fields of carbon storage and solid fuel. Meanwhile, the specific surface area of biochar continuously decreased with increasing boric acid content of rice husk, indicating that the boric acid had a negative influence on the pore structure of biochar. Thus, the addition of boric acid is not recommended when preparing a biochar to be used as an absorbent.