Hydrothermal carbonization of bamboo in an oxalic acid solution: effects of acid concentration and retention time on the characteristics of products

Abstract

Bamboo was hydrothermally carbonized in a batch reactor using an oxalic acid solution at 190 °C for the production of hydrochar and aqueous products. The influences of oxalic acid concentration and retention time were examined, and the products obtained were characterized in terms of energy yield, chemical components and structural properties. It was found that the energy densification values at a high acid concentration of 0.8 mol L⁻¹ were comparable to those of most torrefied lignocelluloses. No hemicelluloses were detected in the hydrochar obtained under the conditions of highest severity (0.8 mol L⁻¹ acid concentration and 20 min), indicating that the lignocellulose hemicelluloses were completely removed. In addition, cellulose also showed some degradation; the highest degradation rate of cellulose was 56.69% at the highest carbonization severity. The lignin content showed an increasing trend in the hydrochar with increasing carbonization severity. The thermal stability of the hydrochar increased with the carbonization severity. The aqueous solution obtained had a high antioxidant capacity with a inhibition rate of 63.5% for 2,2-diphenyl-1-picrylhydrazyl, higher than that of a commercial antioxidant, butylated hydroxytoluene (46.4%). The present study indicated that the hydrothermal carbonization of bamboo produced a solid hydrochar with high potential for fuel applications and degraded chemicals with high potential as antioxidants.