Combined Hydrothermal Liquefaction of Polyurethane and Lignocellulosic Biomass for Improved Carbon Recovery

Due to the high versatility of polyurethane (PUR), its share among synthetic polymers to manufacture consumer goods is increasing. This study proposes, tests, and validates through pilot processing a highly efficient method for the conversion of PUR to an oil phase concentrated in carbon using hydrothermal liquefaction (HTL) aided by lignocellulosic biomass. Hot liquid water mixed with PUR residues and a lignocellulosic material (two species of Miscanthus ) were treated at subcritical water temperatures, generating an oil phase rich in hydrocarbons. A high-synergistic effect in the coliquefaction was observed, leading to carbon and chemical energy recovery to the oil of 71 and 75%, respectively. Pilot plant processing, using optimized process parameters, yielded a total process efficiency accounting for heating utilities of 61% and resulting in a 3.2 ratio of energy return over investment. By using spectroscopic and high-resolution mass spectrometry analysis, the oil revealed a high content of nitrogen hetero aromatic and polyol compounds. The high synergy observed for the coliquefaction of PUR and Miscanthus is attributed to recombination of synthetic and biological materials, specifically due to highly active nitrogen-containing intermediate compounds that recombine with lignocellulosic-derived molecules. During continuous processing, around 58% of the nitrogen contained in the feed is transferred to the oil produced, resulting in a 4 wt % nitrogen concentration in the oil. The results show that HTL can be an efficient method for the mass, carbon, and energy recovery of PUR aided by biomass.