Improvement of hydrochar/biochar pellets prepared from cotton stalk by hydrothermal pretreatment process

Hydrothermal (HT) pretreatment can change the internal components of biomass to efficiently improve the comprehensive utilization value of the resource. In this paper, cotton stalk (CS) was hydrothermally pretreated firstly at 180–280 °C and densified into hydrochar pellet and then pyrolyzed at 400 °C to prepare biochar pellet. A variety of the characterization analyses (SEM, XRD, etc.) were conducted to investigate the effect of the evolution behavior of the main components (cellulose, hemicellulose and lignin) in CS during HT process on the physicochemical property of the resulted biofuel pellets. The results show that HT process improves significantly the quality of the resulted biomass. As HT temperature increasing from 180 °C to 280 °C, the carbon content of the hydrochar increases from 46.91% to 67.26%, while the oxygen content decreases from 46.61% to 26.0%. Meanwhile, the alkali metal content of CS decreases significantly during HT process. HT process promotes the significant decompositions of the hemicellulose below the process temperature of 230 °C and cellulose below 280 °C and the aromatization growth of the resulted CS. Crystalline cellulose is the major contributor to the mechanical properties of the hydrochar and biochar pellets. Compared with the hydrochar pellet, the biochar pellet has the much lower properties due to the decomposition of the crystalline cellulose during the pyrolysis process. The combustion characteristic of the hydrochar decreases significantly due to the decreasing cellulose content with HT intensity, while the correlation between them is severely weakened by the pyrolysis process. HT process significantly improves HHV of the resulted hydrochar pellet and the lignin is responsible for the increasing HHV. However, HHV of the resulted biochar pellet shows a slowly downward trend with the increasing HT temperature. The hydrochar and biochar pellets with the maximum energy density both were prepared from CS pretreated at 230 °C and HT process improves the energy density of the resulted hydrochar pellet and biochar pellet to different certain. The apparent density has a crucial effect on the quality of the biofuel pellet. Thus, the quality of the biochar pellet can be improved greatly by HT process.