Co-pyrolysis of pinewood and HDPE: pyrolysis characteristics and kinetic behaviors study

Co-pyrolysis of lignocellulosic biomass and hydrogen-rich petroleum-based polyolefin plastics is a promising to way to improve bio-oil quality and alleviate the waste plastic pollution issues. In this study, co-pyrolysis of pinewood and HDPE was systematically investigated. The addition of HDPE decreased yield of char and gas while increased that of bio-oil, enhancing the selectivity to alcohols and hydrocarbons. The most obvious synergistic effect was observed at the HDPE mixing proportion of 0.25, at which hydrocarbon selectivity derived from co-pyrolysis experiments was 41.19% higher than the calculated weighted average values. As pyrolysis temperature increased from 500 degrees C to 700 degrees C, the yield of bio-oil from co-pyrolysis at the HDPE mixing proportion of 0.25 decreased from 69.11 wt.% to 50.33 wt.%, alkanes selectivity decreased from 27.41% to 3.67% and olefins selectivity increased from 14.96% to 47.12%. At 700 degrees C, aromatics started to produce with a selectivity of 15.50%. The surface morphologies of char were not significantly affected by the HDPE mixing proportion and pyrolysis temperature. The thermogravimetric analysis results revealed that the global co-pyrolysis process can be divided into two major degradation stages, based on which multi-step method was adopted to analyze the kinetics of the process. The average apparent activation energies of stage I and stage II were 167.73 kJ/mol and 274.74 kJ/mol, respectively. The results from this work provide a theoretical guide for further development of co-pyrolysis of pinewood and high-density polyethylene (HDPE).