Influence of polyethylene chain structure on microwave-assisted co-pyrolysis of polyethylene and microalgae: Nitrogen reduction and oxygen increase effects in bio-oil

To upgrade microalgae pyrolysis oil, the effects of polyethylene (PE) chain structure and microalgae component on microwave-assisted co-pyrolysis of PE and microalgae were investigated using high-density polyethylene (HD), low-density polyethylene (LD), and linear low-density polyethylene (LLD), as well as microalgae rich in proteins (M), lipids (ML), and carbohydrates (MC). Co-pyrolysis with PE promoted bio-oil production but hindered gas generation. Moreover, the interactions within MC-HD and M-LLD exhibited a stronger promoting effect on bio-oil production compared to the other co-pyrolysis groups. Interestingly, co-pyrolysis with HD (M-HD, MC-HD, and ML-HD) resulted in a significant reduction in nitrogen content (0.85–1.68 wt%) and an increase in oxygen content (33.75–47.40 wt%) in the bio-oils, contrasting with the nitrogen (3.42–9.92 wt%) and oxygen (14.41–23.08 wt%) levels observed in the bio-oils from individual pyrolysis of microalgae, attributable to the formation of long-chain alcohols and esters. Moreover, the bio-oils from LD and LLD co-pyrolysis (M-LD, M-LLD) had slightly higher nitrogen content (2.27 and 2.60 wt%) but much lower oxygen content (19.37 and 17.96 wt%) compared to the bio-oils from HD co-pyrolysis. In general, the long main chains of PE contributed to reducing nitrogen content while increasing oxygen content of the bio-oils, and the branched chains could alleviate the increase in oxygen content. This study provides new insights for the high-value utilization and eco-friendly utilization of microalgae and PE.