Influence of sewage sludge treatment on pyrolysis and combustion of dry sludge

Abstract

The influence of sewage sludge treatment on its pyrolysis and combustion and the N, S and Cl emissions was studied by thermogravimetry-mass spectrometry in the range 25–800 °C. Two types of sludge were used (L and F). L underwent anaerobic digestion and was treated with organic polyelectrolyte, while F was treated with FeCl3 and lime. The treatment with inorganic additives had a notable influence on the pyrolysis and combustion processes. During F pyrolysis, the most significant effects of CaO/CaCO3 from lime and Fe2O3 from FeCl3 were: 1) Fe2O3 promoted oxidation reactions, and char gasification at around 600 °C; 2) CaO promoted H2 formation at around 480 °C due to its action on water–shift reaction, and the partial SO2 retention by forming CaSO4. During combustion, Fe2O3 exhibited a high catalytic activity on oxidation reactions. Thus, an intense devolatilisation was produced in the range 200–377 °C, this being accompanied by a high formation of CO2, NO2 and Cl2. Also, the formation of SO2/SO3 was promoted, which were retained by CaCO3/CaO at temperatures higher than 377 °C. The nth-order reaction model describes adequately the global reaction that occurs in each stage, except for the second stage of L combustion that was described by the geometrical contraction model.