Comparative Evaluation of Hydrothermal Carbonization and Low Temperature Pyrolysis of Eucommia ulmoides Oliver for the Production of Solid Biofuel

This study evaluates the feasibility of two thermal pretreatments including hydrothermal carbonization (HTC) and low temperature pyrolysis (LTP) on the production of Eucommia ulmoides biochar. The waste wood of Eucommia ulmoides Oliver was pretreated and characterized for fuel applications. The results confirm that both LTP and HTC are promising processes for improving fuel properties. However, for the same char yield, the required temperature for HTC is lower than LTP, as the char yields of H(200) and L(300) were quite close (66.50% vs. 66.74%). The surface morphology is significantly different between the pyrolytic carbon and the hydrochar. In addition, it was found that the H/C and O/C ratios of H(300) were 0.82 and 0.21, respectively, and the H/C and O/C ratios of L(340) were 0.77 and 0.22, respectively. They were similar to that of sub-bituminous. Moreover, under the same reaction temperature, hydrochar showed better grindability, hydrophobicity, and reduction in inorganic content. Comparing the integrated combustion characteristic index (S), LTP process had the better performance within the lower temperature under 220 °C while HTC process performed better at temperature higher than 300 °C. The results reveal that HTC has the potential to produce solid carbonized products with better fuel quality.