Microwave-assisted pyrolysis of waste LDPE: Unveiling the role of induced gas-solid thermal gradients on pyrolysis oil product distribution

Microwave-assisted heating plays an important role in minimizing the high energy consumption of traditional thermochemical recycling processes. Its application to plastic waste recycling routes enables a more efficient production of new renewable feedstocks for the petrochemical industry. This work presents the experimental assessment of low-density polyethylene (LDPE) waste recycling in a microwave-assisted pyrolysis (MAP) fixed bed reactor and the in-depth evaluation of the obtained pyrolysis oils in comparison with conventional plastic waste pyrolysis oils. The role of main process variables (temperature, nitrogen flow rate and susceptor-to-LDPE ratio) on the pyrolysis oil yield and composition was addressed. The maximum attained oil and wax yields were of 60–75 wt%. As a general trend, the microwave-assisted process led to a distinct chemo-selectivity compared to that of traditional LDPE pyrolysis, shifting the products distribution from saturated paraffins to olefinic species and longer molecular chains. Olefins content rose from 28 % under conventional pyrolysis to 75 % under MAP. The negligible content of aromatic compounds in MAP oils offers great potential to serve as a recycled feedstock for steam crackers. These findings are ascribed to gas-phase reactions quenching due to the microwave-induced thermal gradient between the selectively heated bed and the comparatively colder surrounding atmosphere. A statistical parametric analysis and regression model have been implemented to further evaluate the impact of each variable, and mechanistic considerations were considered on the experimental microwave-induced thermal gradients and fixed bed mass and heat transfer properties. By integrating real polyolefin waste streams as feedstocks for chemical building blocks through microwave-assisted pyrolysis, this study helps paving the way for a transition to circularity in petrochemical industries.