Effect of catalysts on distribution of polycyclic-aromatic hydrocarbon (PAHs) in bio-oils from the pyrolysis of dewatered sewage sludge at high and low temperatures

Abstract

This study was aimed at investigating the effect of four types of catalysts on the distribution characteristics of 16 polycyclic-aromatic hydrocarbon (PAHs) in bio-oils from the pyrolysis of dry sewage sludge. The pyrolysis experiments of sewage sludge were conducted in a tubular reactor at the high and low temperatures of 850 °C and 450 °C, respectively. CaO, KCl, Na2CO3, and Fe2O3were selected as catalysts for the catalyzed pyrolysis. In the non-catalyzed sludge pyrolysis, PAHs concentrations in bio-oil increased with temperature, reaching a maximum value of 15.25 μg/g at 850 °C. This value was 4.5 times higher than PAHs concentration from the non-catalyzed pyrolysis at 450 °C. With the presence of catalysts at 850 °C, an evident reduction of PAHs concentration was observed for all the bio-oil samples. The added catalysts proved to effectively inhibit PAHs formation in bio-oil and reduce the toxic equivalent quantity (TEQ) of PAHs in the experiment at high temperature. The lowest observed PAHs concentration in bio-oil was 4.23 μg/g, obtained with the use of KCl catalyst. However, an opposite trend was observed for catalyzed pyrolysis at 450 °C. The added catalysts promoted PAHs formation, up to the concentration of 8.89 μg/g with the use of CaO catalyst. Catalyst loading also influenced PAHs concentration and TEQ, with the lowest output found at 20% loading for all catalysts. Bio-oil yields, mass ratio of 16 PAHs concentrated into bio-oil, the TEQ of 16 PAHs and their subtotal group classified by ring number were separately examined to analyze their effects on fractional distribution of the total PAH amount. Overall, the presence of the selected catalysts was found to have the ability to inhibit PAHs production at higher pyrolysis temperature and to promote PAHs production at lower pyrolysis temperature.