Roles of calcium oxide on the evolution of substituted polycyclic aromatic hydrocarbons released from sewage sludge pyrolysis

Substituted polycyclic aromatic hydrocarbons released from sewage sludge pyrolysis bring the concern of the clean utilization for the waste. Herein, the roles of calcium oxide on the release of these compounds, especially the heavy ones containing nitrogen and oxygen atoms were investigated. The results showed that calcium oxide significantly decreased light substituted polycyclic aromatic hydrocarbons in the volatiles from 450 to 850 °C and showed obvious inhibitory effects on the formation of heavy substituted polycyclic aromatic hydrocarbons at 850 °C. To understand the impact mechanisms, pyrolysis experiments of sludge model compounds lipid, protein and polysaccharides were conducted. It was found that calcium oxide could catalyze or react with all three components to decrease the content of light substituted polycyclic aromatic hydrocarbons. Heavy oxygen-containing aromatics in sludge derived volatiles mainly resulted from polysaccharides and protein, and they considerably decreased after mixing with calcium oxide. Oxygen-containing aromatics content in the lipid derived volatiles, however, would increase due to polymerization reactions. The decreasing release of nitrogen substituted polycyclic aromatic hydrocarbons from sludge could be attributed to the synergy effects of CaO and other minerals or the effects of CaO on the interaction between protein and other organics. •CaO promoted the release of substituted PAHs at 450 °C but decreased that above 650 °C.•Oxygen substituted PAHs mainly derived from polysaccharide and protein.•CaO decreased oxygen substituted PAHs from polysaccharide and protein.•CaO promoted the generation of heavy oxygen substituted PAHs from lipid.•CaO increased heavy nitrogen substituted PAHs from protein.