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

•The effect of sewage sludge treatment on its pyrolysis and combustion was studied.•Evolution profiles of CO2, H2O, H2 and N, S, Cl compounds were determined by TG-MS.•Fe2O3 from FeCl3 and CaO modify thermogravimetric and gas profiles and kinetics.•Fe2O3 promotes organic matter oxidation, more oxidized compounds being emitted.•During pyrolysis, CaO and Fe2O3 promote H2 formation and char gasification.

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.