Pahs emission characteristics and the mitigation in the valuable products from used mineral oil thermolysis recycling

Abstract

Pyrolysis of used mineral oil (UMO) to produce various valuable products is a flexible and efficient way for UMO recycling. Along with the preferred pyrolytic products, toxic polycyclic aromatic hydrocarbons (PAHs) may also be generated. To mitigate the PAHs emission during UMO pyrolysis, an understanding of emission characteristics and formation mechanism of PAHs is needed. In this work, four used minerals (engine, hydraulic, turbine, transformer) oil and three base oil were pyrolyzed at 600 °C at different heating modes (slow: 10 °C min−1, medium: ∼1000 °C min−1, fast: greater than10000 °C min−1) under N2/CO2 atmosphere. The results show that a higher heating rate can lead to more gas but less oil production. At different heating modes, PAHs emission factors change markedly and are in the range of 34.63–1492.85 ug g−1, among which more than 92 % of PAHs are enriched into pyrolytic oil. Naphthalene, phenanthrene and pyrene are the first three PAHs species in the gaseous product. The mechanism of evaporation and chemical evolution control PAHs formation for the SP process and FP process, respectively. PAHs formation is related to both the naphthene and aromatic contents of UMO, and the contribution of aromatics is about fourfold of the naphthenes. In the CO2 atmosphere, more liquid product and ∼ 33 % less PAHs formation can be realised due to the depressed dehydrogenation reaction.