Co-Depolymerization of Lignin and Plastic Waste Using Hydrothermal Liquefaction Process

Abstract

It is desirable to find a sustainable solution for increasing the use of underutilized lignin and plastic beyond incineration for energy. Processes like pyrolysis and combustion are associated with energy disadvantages as there is a need to dry the feedstock before processing. However, this shortcoming can be overcome with Hydrothermal liquefaction technology (HTL) due to its ability to upgrade wet materials to value-added products like high-quality fuel. In this research, the effects of HTL process conditions (temperature between 300 °C to 400 °C, pressure, and residence time) on the quality and quantity of co-depolymerization of polyolefin waste and lignin products are delineated. We further determine the effect of molecular weight of polyolefins on the quality of produced fuel intermediates under subcritical and supercritical water conditions. For HTL polyethylene, increasing the temperature from 300°C to 400°C resulted in higher oil yields. However, for HTL of lignin and polypropylene, oil yields decreased beyond certain temperatures. The maximum oil yields were 22% for lignin at 300°C for 30 minutes, 93% for low molecular weight polyethylene at 400°C for 30 minutes, 88% for low molecular weight polypropylene at 375°C for 1 hour, 86% for high molecular weight polyethylene at 375°C for 1 hour, and 83% for high molecular weight polypropylene at 400°C for 1 hour. GC-MS analysis revealed oxygenated compounds in the HTL of both low and high molecular weight polyethylene, with a decrease in these compounds at higher temperatures. Elemental analysis confirmed that higher temperatures reduced oxygenated aromatics in the HTL oil derived from polyethylene. Co-liquefaction of lignin and plastics improved some properties compared to lignin HTL alone. Under certain reaction conditions, the interactions between lignin and plastic intermediates appeared to enhance oil yield, suggesting a synergistic effect.

Keywords: Polyolefins, Moelcular Weight of Plastic, Polyethylene, Polypropylene