Comparison of torrefaction and hydrothermal carbonization of high-moisture microalgal feedstock

Abstract

A comprehensive comparison is conducted to explore the conversion performance of torrefaction and hydrothermal carbonization of high-moisture microalga for producing solid biofuel with low expense and environmental impact. The results suggest that when their solid yields are close, the hydrochar has higher HHV (higher heating value) (21.10–25.39 MJ kg−1) and energy yield (72.48–92.29%) than the biochar. The graphitization degrees of the biochar and hydrochar are 1.298–3.107 and 2.193–3.049, respectively. Considering fuel grade, the biochar is in biomass and peat zones, but the hydrochar is only in the biomass zone. The biochar's elemental valence transformation and graphitization degree are better than hydrochar's. The pyrolysis and combustion characteristics of the hydrochar are better than those of the biochar when considering the thermodegradation data, leading to a lower expense and environmental impact. The results are conducive to efficiently figuring out a solution with a comparative advantage for microalgal thermochemical conversion. The hydrothermal carbonization conditions at 160 °C for 2 h are the best operation for microalgal solid biofuel production.