Pyrolysis of Co-hydrothermally pretreated LDPE and straw for enhanced bio-oil recovery from agricultural plastic waste

Abstract

Agricultural plastics improve crop quality, reduce water use, extend growing cycles, and slow the spread of disease and infestations. Despite such benefits, agricultural plastic waste (APW) is poorly managed. To valorize APW, we propose a two-step process in which APW is pre-hydrothermally treated and then pyrolyzed. Hydrothermal processing (HTP) reduces the elemental oxygen and volatile matter present in raw lignocellulosic biomass. Consequently, we hypothesized that co-HTP of low-density polyethylene (LDPE) and straw would produce a bio-oil with fewer oxygenated compounds upon pyrolysis of the hydrochar than pyrolysis of the raw residues. LDPE, straw and a 4:1 LDPE-to-straw mixture were hydrothermally treated at 250 °C and 300 °C. The hydrochars were then pyrolyzed at 450 °C or 650 °C. Co-HTP produced a pyrolysis bio-oil primarily composed of aliphatic compounds and reduced the number of compounds with oxygen functional groups present. Additionally, co-HTP reduced the devolatilization of straw to a single step at 450 °C, reducing the energy required for the pyrolysis process.