Effective feeding of lignin to pyrolysis units using molten salts in combination with a twin-screw extruder

Thermochemical conversions of waste lignocellulosic biomasses such as kraft lignin are highly relevant for the production of biobased chemicals and fuels. Of the many available thermochemical technologies, pyrolysis and (pressured) hydropyrolysis are promising pathways to produce liquids from biomass. However, pressurization and continuous feeding of solids into pyrolysis reactors operated at elevated temperatures and pressures is a practical challenge. In this study, we report the use of a molten salt (ZnCl₂:NaCl:KCl with a molar composition of 60:20:20) in combination with a twin-screw extruder to pressurize and transport a molten salt-lignin mixture. The effect of different operating parameters such as the residence time (determined by residence time distribution (RTD) analysis) in the extruder and the mass ratio of lignin to salt was studied in detail at a fixed operating temperature of 230 °C. The mass of recovered lignin was up to 92 % at optimized conditions (35 s residence time, lignin to salt ratio of 1 to 10), the remainder being char. It was found that lower residence times and lower amounts of lignin in the feed have a positive effect on the amount of recoverable lignin. The extrusion process also affects the molecular structure of the lignin. 2D-NMR HSQC analysis of the modified lignin before and after processing showed a strong reduction in the intensity of peaks in the oxygenated aliphatic region, indicating demethoxylation during the extrusion process, supported by elemental analyses. The findings may be used to feed lignin effectively to pyrolysis or hydropyrolysis units.