Correlations between the compressive strength of the hydrochar pellets and the chemical components: Evolution and densification mechanism

Abstract

Hydrochars of cotton stalk (CS) at the hydrothermal temperature of 180−280 °C and the retention time of 0−120 min were used for the purpose of developing fuel pellets. Subsequently, the physical properties and combustion characteristics of the hydrochar pellets were evaluated to assess the solid biofuel production potential. Then, the hydrochar samples were analyzed by FTIR, TGA, XRD, and SEM, aiming to investigate the evolution mechanism of the chemical compositions (hemicellulose, cellulose, and lignin) with the increasing hydrothermal temperatures and their effects on the compressive strength of the hydrochar pellets. The results indicated that the hydrochar pellets exhibited higher fixed carbon contents, elevated heating values, enhanced physical properties, and more stable combustion characteristics than the raw biomass pellets. Moreover, the skeleton structure of the crystalline cellulose was the critical factor affecting the compressive strength of the hydrochar pellets. In parallel, lignin was observed to be beneficial for fabricating solid bridges and also exhibited a substantial influence on the bonding performance. However, the crystalline cellulose completely decomposed into amorphous carbon, and the high carbonization degree of the lignin caused it to lose its bonding capacity, which led to a sharp decrease in the compressive strength of the hydrochar pellets pretreated at 280 °C to 2.9 MPa, lower than the CS pellets (4.2 MPa). Finally, the pictorial depiction of the chemical components’ structural characteristics exhibited in the hydrothermal carbonization process was displayed along with their influences on the hydrochar densification process.