Comparison of PAHs behaviors in the pyrolysis products of oily sludge with different ash contents

Due to the presence of petroleum hydrocarbons, the production of polycyclic aromatic hydrocarbons (PAHs) is inevitable after oily sludge pyrolysis. The dispensation of PAHs in pyrolysis products was studied. The results indicated that PAHs were mainly enriched in liquid products, accounting for 97.97–99.88 %. Besides, the percentage of low-ring PAHs in products was the largest, subsequent to middle-ring PAHs. In the pyrolysis products of oily sludge with distinguishing ash contents, the impressions of pyrolysis temperature and residence time on the behaviors of PAHs were also compared. When pyrolysis temperature or residence time were variables, there were some similar characteristics in PAHs behaviors of oily sludge with different ash contents. The total concentration of PAHs in liquid products was positively correlated with pyrolysis temperature and residence time. The equal alteration was also exhibited in the concentration of high-ring PAHs in gas products with an increase in pyrolysis temperature. The formation of low-ring PAHs in liquid and gas products was improved by extended residence time. However, there were also differences in the evolution of PAHs in products of low-ash oily sludge (LOS) and high-ash oily sludge (HOS). The concentrations of low-ring and middle-ring PAHs in LOS gas products were positively correlated with pyrolysis temperature. Antithetically, the formation of low-ring and middle-ring PAHs in HOS gas products was inhibited by elevated pyrolysis temperature. Interestingly, the low-ring PAHs were converted into high-ring PAHs in HOS gas products with increasing pyrolysis temperature. As the residence time prolonged, the total concentrations of PAHs in LOS solid and gas products enlarged first and then reduced. Nevertheless, those of HOS were improved by continued residence time. •The behaviors of PAHs were compared during pyrolysis of oily sludge with different ash contents.•PAHs were mainly distributed in liquid products after pyrolysis.•The higher the pyrolysis temperature, the larger the concentration of PAHs in liquid products.•Low-ring PAHs were converted into high-ring PAHs in HOS gas products with a rise in pyrolysis temperature.