Effect of pyrolysis temperature on the composition of the oils obtained from sewage sludge

Sewage sludge was pyrolysed in a quartz reactor at 350, 450, 550 and 950 degrees C. The pyrolysis oils from the sewage sludge were characterized in detail by means of gas chromatographymass spectrometry (GC-MS). Changes in the composition of the oils related to the process conditions were assessed by normalizing the areas of the peaks. It was demonstrated that, as the temperature of pyrolysis increased from 350 to 950 degrees C, the concentration of mono-aromatic hydrocarbons in the oils also increased. Conversely, phenol and its alkyl derivatives showed a strong decrease in their concentration as temperature rose. Polycyclic aromatic hydrocarbons (PAHs) with two to three rings passed through a maximum at a pyrolysis temperature of 450 degrees C. PAHs with 4-5 rings also presented a major increase as temperature increased up to 450 degrees C, the concentration at 950 degrees C being slightly higher than that at 450 degrees C. Quantification of the main compounds showed that sewage sludge pyrolysis oils contain significant quantities of potentially high-value hydrocarbons such as mono-aromatic hydrocarbons and phenolic compounds. The oils also contain substantial concentrations of PAHs, even at the lowest temperature of 350 degrees C. The pathway to PAH formation is believed to be via the Diels-Alder reaction and also via secondary reactions of oxygenated compounds such as phenols. (C) 2009 Elsevier Ltd. All rights reserved.