Interactions of beech wood–polyethylene mixtures during co-pyrolysis

To investigate the interactions between beech wood (BW) and polyethylene (PE) during co-pyrolysis, they were pyrolyzed both individually and as their mixtures from ambient temperature to 650 °C at a heating rate of 10 °C min⁻¹ using a thermogravimetric analyzer and a tube reactor. In the presence of PE, the production of levoglucosan (LG) and methoxyphenols with unsaturated alkyl side chains from BW was enhanced, resulting in maximum 1.65 and 1.55 times higher yields comparing with their expected yields from BW:PE (w:w) = 40:60 mixture, respectively. It was due to the stabilization of the radical species of those by H-abstraction from the PE pyrolysates in the vapor phase. According to this, CO, CO₂, C₂-C₃ compounds, and 5-membered ring compounds, which are LG decomposition products, showed 0.86, 0.85, 0.58, and 0.68 times smaller yields comparing with their expected yields. Methoxyphenols with saturated alkyl side chains, phenols, and catechols, also showed 0.77, 0.66, and 0.87 times smaller yields because they are mainly derived from methoxyphenols with unsaturated alkyl side chains. On the other hand, it was suggested that hydrogen radicals produced during the polyaromatization of BW stabilized PE radical species, resulting in products from PE with higher carbon numbers. Thus, the present work reveals that the interactions between BW and PE mainly involve H-exchange to stabilize radical species. These findings increase our understanding of the operative mechanism in the co-pyrolysis of lignocellulosic biomass and synthetic polymers, which will be useful in future for achieving pyrolysis reaction control and recovering desirable products from difficult-to-separate initial mixtures.