Effect of transition metals on hydrothermal liquefaction (HTL) of agricultural residues

Abstract

Hydrothermal liquefaction (HTL) of two agricultural residues, empty fruit bunch (EFB) and coconut shell (CCNS) was performed with an autoclave reactor within a diverse temperature range (240–330 °C) in the presence of various transition metal chlorides (ZnCl2, CuCl2 and NiCl2) to investigate its effects on the properties of HTL products: HTL oil, hydrochar, and water soluble fraction (WSF). The highest yield of HTL oil from EFB was 24.2 wt% at 300 °C, which was higher than that from CCNS (14.0 wt%) at the same temperature. The yield of gas showed an inversely proportional correlation to the yield of HTL oil. At 240 °C the yield of hydrochar from EFB was higher than 50% due to the incomplete degradation of cellulose, and it steadily decreased with increases in temperature. Water content of HTL oil ranged from 2.0% to 5.6% for EFB, 3.3% to 9.6% for CCNS, respectively. Total acid number (TAN) value of oil for both biomasses increased with a rising temperature up to 300 ºC and then decreased at 330 ºC. Gas chromatography-mass spectrometry (GC/MS) analysis revealed that HTL oils from EFB and CCNS at 300 °C had the largest amounts of chemical compounds, such as furfural and cyclic ketones derived from holocellulose and phenol, guaiacol, and syringol derived from lignin. In the presence of transition metals, the yield of HTL oil decreased with an increased transition metal dose of up to 10.0%, while changes in the amount of gas, WSF, and hydrochar were different by according to the kind of feedstock or metals. Water content of HTL oil rose by the increasing amount of the metals, while the TAN value was varied and chemical compounds decreased. γ-Valerolactone (GVL) and levulinic acid were found, derived from acidic hydrothermal decomposition of cellulose. Unlike other metals, relatively small amounts of GVL and large amounts of levulinic acid were detected, especially in the presence of CuCl2. It is suggested that CuCl2 inactivates the conversion of levulinic acid to GVL under subcritical condition.