Products distribution and hazardous elements migration during pyrolysis of oily sludge from the oil refining process

Oily sludge is a hazardous waste due to the enrichment of nitrogen, sulfur, PAHs, and heavy metals. In this work, an oily sludge from oil refining factory was pyrolyzed at various temperatures of 250-850 °C in a fixed bed reactor focusing on product distribution and migration of hazardous compounds of PAHs, sulfur, nitrogen-containing compounds, and heavy metals. The mechanism of PAHs formation and migration of nitrogen, sulfur, heavy metals were elucidated by comprehensive analysis of the solid, liquid, and gas products. The distribution and risk analysis of heavy metals were also conducted. The pyrolytic products distribution was markedly affected by pyrolysis temperatures. A maximum oil yield was observed at 500 °C, which can further crack into gas due to secondary reaction. The pyrolytic gas was enriched in the order of CO₂ > CO > CH₄ > H₂. At lower temperatures, CO₂ was largely generated due to the elimination of oxygen-containing functional groups, while H₂ was mainly formed above 450 °C due to the recombination reaction. Higher temperatures promoted more N-/S-containing compounds into tar and gas phases. The N-/S-containing compounds mainly included NH₃, HCN, H₂S, SO₂, COS in the gas phase and amines, indoles, pyridines, nitriles, thiophenes in liquid phase. PAHs with 2-ring to 5-ring were mainly generated due to the secondary reaction at higher temperatures. Moreover, Pyrolysis caused the accumulation of heavy metals in chars. Cd presented a high potential risk while the other heavy metals in chars presented a low risk.