The effect of temperature on the catalytic conversion of Kraft lignin using near-critical water

• The conversion of lignin in near-critical water was investigated at 290–370 °C.
• ZrO2/K2CO3 was used as catalytic system and phenol as char suppressing agent.
• The lignin-oil has higher HHV and lower contents of oxygen and sulphur than lignin.
• The main 1-ring aromatics are anisoles, alkylphenols, guaiacols and catechols.
• The yield of 1-ring aromatics increases remarkably with an increase in temperature.

The catalytic conversion of suspended LignoBoost Kraft lignin was performed in near-critical water using ZrO2/K2CO3 as the catalytic system and phenol as the co-solvent and char suppressing agent. The reaction temperature was varied from 290 to 370 °C and its effect on the process was investigated in a continuous flow (1 kg/h). The yields of water-soluble organics (WSO), bio-oil and char (dry lignin basis) were in the ranges of 5–11%, 69–87% and 16–22%, respectively. The bio-oil, being partially deoxygenated, exhibited higher carbon content and heat value, but lower sulphur content than lignin. The main 1-ring aromatics (in WSO and diethylether-soluble bio-oil) were anisoles, alkylphenols, catechols and guaiacols. The results show that increasing temperature increases the yield of 1-ring aromatics remarkably, while it increases the formation of char moderately. An increase in the yields of anisoles, alkylphenols and catechols, together with a decrease in the yield of guaiacols, was also observed.