Formation of PAHs during the pyrolysis of dry sewage sludge

The formation of the 16 polycyclic-aromatic hydrocarbons (PAHs), characterized by USEPA as priority pollutants, was studied during the pyrolysis of dry sewage sludge in a wide range of operation conditions, i.e., reaction temperature (250–1250 °C), sample mass (0.1–4 g), gas residence time (2–16 s) and gas flow (50–1000 mL/min). The pyrolysis was conducted in a tubular reactor with well-controlled temperature profile and carrier gas flow. The yield of PAHs strongly increased with temperature, reaching a maximum value at 950 °C, while the toxicity equivalent (TEQ) became highest at 1150 °C. The sample mass also influenced upon both the PAHs concentration and the TEQ, with the lowest output found for 1 g sample; rising residence time of the gas flow inflates the PAHs formed and their TEQ up to 10 s, and then they decreased slightly. Raising the gas flow from 50 to 100 mL/min moderately decreases the concentration of PAHs, yet sharp declines their TEQ. To evaluate any influences of the experimental parameters the fractional distribution of the total PAH amount and the TEQ of the 16 PAHs, as well as their subtotal grouped by ring number, were separately displayed. The present results may be valuable for assessing sewage sludge pyrolysis as thermal treatment method and also for studying the formation and destruction mechanism of PAHs during sludge pyrolysis.