Semi-pilot scale sewage sludge pyrolysis and characterization of obtained fractions by thermal analysis

Abstract

Sewage sludge pyrolysis is an alternative to incineration and presents as advantages the generation of three fractions: gas (non-condensable volatiles), liquid (condensable volatiles) and solid (carbonaceous coke). In this context, the purpose of the present study is to quantify and to characterize by thermal analysis the fractions produced by sewage sludge pyrolysis in semi-pilot scale. The semi-pilot pyrolysis was performed in two steps, the first dynamic up to 500 °C, followed by an isothermal step at this temperature for 3 h. The solid and liquid fractions produced were analyzed by TG/DTG/DTA, and the volatiles were analyzed by thermogravimetry coupled to mass spectrometry (TG-MS). The influence of different pyrolysis conditions (dynamic and isothermal) was verified by differential thermal analysis (DTA) characterization and by the higher heating value (HHV) of the liquid fractions produced by sludge pyrolysis. In total, 60.1, 28.6 and 11.3 % of the sludge initial mass were obtained, respectively, as solid, liquid and gas pyrolytic fractions. The liquid fraction was composed of aqueous and oily phases. By TG-MS, the volatile compounds detected in pyrolysis were as follows: H2O, C3H6, CO2, NO2, C4H8, SO2, C6H6 and C7H8. The aliquots obtained at higher time periods by isothermal pyrolysis at 500 °C showed higher DTA exothermic peaks and higher HHV values as well. Sewage sludge isothermal pyrolysis at 500 °C produces a liquid fraction with HHV of 36.2 MJ kg−1, indicating that it can be used as renewable energy source. A good correlation exists between total DTA peak area of the oily fractions obtained from sewage sludge pyrolysis and respective HHV values, indicating that it can be used to estimate these values from corresponding DTA curves.