Identification and quantification of polycyclic aromatic hydrocarbons generated during pyrolysis of sewage sludge: Effect of hydrothermal carbonization pretreatment

Abstract

Hydrothermal carbonization (HTC) is of increasing importance to improve fuel quality and dewaterability of sewage sludge (SS). In this study, the effect of HTC of SS on the formation of 16 priority polycyclic aromatic hydrocarbons (PAHs) during pyrolysis, the primary stage of combustion, were investigated. The results showed that HTC improved fuel quality of SS while the PAHs were concentrated in the resultant hydrochar. Hydrochar pyrolysis generated less tar (3.49–8.82%) than SS pyrolysis (4.73–14.83%) while the PAHs yields in hydrochar pyrolytic tar were higher than the yields of SS pyrolytic tar, ranging from 104.93 mg/kg to 1199.07 mg/kg in the temperature range of 500–800 °C. In addition, the proportions of highly toxic middle molecular weight (MMW) and high molecular weight (HMW) PAHs in hydrochar tar were much higher than that for SS. As for the pyrolytic char, the PAHs in hydrochar char were also far higher than that in SS pyrolytic char at the temperature higher than 650 °C. These differences resulted in more total PAHs emissions and higher toxic equivalent quantity (TEQ) from hydrochar pyrolysis than those from SS pyrolysis in the temperature range of 500–800 °C. This study indicated that more attention should be paid to control PAHs emission energy recovery from hydrothermally treated SS.