Characterization of the Residue and Liquid Products Produced from Husks of Nuts from Carya cathayensis Sarg by Hydrothermal Carbonization

Abstract

Hydrothermal carbonization is considered to be an environment-friendly method to pretreat biomass. Therefore, hydrothermal carbonization of the husks of nuts from Carya cathayensis Sarg, at various temperatures and pHs were performed and the fuel properties of the obtained residue, water-soluble products (WSP) and acetone-soluble products (ASP) were characterized. It was found that the residue yield decreased with increasing hydrothermal carbonization temperature while the yield of total soluble products (WSP and ASP) was nearly the same. The residue and total soluble products yields had no large changes in the pH range of 4.0–10.0, but fewer residues and more total soluble products were obtained at pH 13.0. It was confirmed that hemicellulose was more reactive than cellulose and lignin during hydrothermal carbonization and that lignin could be effectively hydrolyzed/decomposed at pH 13.0. The higher heating values (HHVs) of residues were in the ranges of 22.0 to 28.2 MJ kg–1 at temperatures from 180 to 260 °C and pH from 4.0 to 10.0. The HHVs of these residues could be comparable with those of some commercial coals, such as lignite. However, the HHVs of residues decreased when the pH reached to 13.0. The WSP and ASP showed lower HHVs compared with heavy fuel oils, which meant the HHVs of WSP and ASP should be increased before being applied as bio-fuels.