Novel insights into mass transfer-controlled radical-mediated co-pyrolysis of lignin with typical plastics

Co-pyrolysis of lignin with representative waste plastics, including polypropylene (PP), acrylonitrile butadiene styrene (ABS), polystyrene (PS), and polyethylene terephthalate (PET), was investigated, focusing on radical-mediated reactions, with radical detection via electron paramagnetic resonance (EPR). During the co-pyrolysis of lignin with PP, favorable synergistic effects were observed, resulting in a significant 17 % enhancement in phenolic compound yields within the fixed-bed reactor, whereas the presence of other plastics led to exacerbated cross-linking carbonization in contrast. Furthermore, TOF-MS and FT-ICR-MS analyses corroborated that the bio-oil derived from lignin/PP exhibited a higher phenolic content and lower carbon number distribution. Additionally, the radical concentrations in bio-oil of lignin/PP were determined to be the lowest (1.07 × 1013 spins/µL), providing compelling evidence of the strong interaction between lignin and PP. Interestingly, EPR spectroscopy detected the resultant lowest spin concentrations (1.28 × 1019 spins/g) in lignin/PP char, in contrast to lignin in isolation or within other plastics, which strongly indicating that hydrogen-rich radicals originating from PP were responsible for inducing more effective quenching reactions with lignin intermediates. Moreover, the lignin/PP char demonstrated the highest carbon content (80.30 %) and the lowest ID/IG ratio (1.98), further affirming a highly effective interaction between lignin and PP. The perspective drawn from this study is that the abundant hydrogen source of PP can provide hydrogen-rich radicals to quench lignin intermediates and improve phenolic yield. This intriguing work provides a fresh perspective to evaluate the effect of plastic origin on lignin co-pyrolysis based on radical theory.