Distribution, migration, and removal of N-containing products during polyurethane pyrolysis: A review

Due to the wide applications of polyurethane (PU), production is constantly increasing, accounting for 8% of produced plastics. PU has been regarded as the 6th most used polymer in the world. Improper disposal of waste PU will result in serious environmental consequences. The pyrolysis of polymers is one of the most commonly used disposal methods, but PU pyrolysis easily produces toxic and harmful nitrogen-containing substances due to its high nitrogen content. This paper reviews the decomposition pathways, kinetic characteristics, and migration of N-element by product distribution during PU pyrolysis. PU ester bonds break to produce isocyanates and alcohols or decarboxylate to produce primary amines, which are then further decomposed to MDI, MAI, and MDA. The nitrogenous products, including NH3, HCN, and benzene derivatives, are released by the breakage of C-C and C-N bonds. The N-element migration mechanism is concluded. Meanwhile, this paper reviews the removal of gaseous pollution from PU pyrolysis and discusses the removal mechanism in depth. Among the catalysts for pollutant removal, CaO has the most superior catalytic performance and can convert fuel-N to N2 by adsorption and dehydrogenation reactions. At the end of the review, new challenges for the utilization and high-quality recycling of PU are presented.