Influence of FeCl3 and lime added to sludge on sludge-coal pyrolysis

Copyrolysis of sewage sludge-coal blends at different ratios (0:100, 10:90, 50:50 and 100:0) was investigated using a simultaneous thermogravimetry-mass spectrometry analyser. During copyrolysis three thermal decomposition stages were identified between 180 and 800 °C. From 180 °C to 385 °C, the process is dominated by the sludge pyrolysis. From 385 °C to 560 °C, the coal is pyrolysed with a lower fraction of the sludge. In the last stage, the coal pyrolysis occurs together with carbonate decomposition. In the operational conditions, copyrolysis occurs with some interactions, which are principally due to the fact that inorganic matter from sludge (mainly lime and FeCl3) affects some secondary reactions. The composition of pyrolysis gas (H2, CO2, H2O, light hydrocarbons, CH3COOH, chlorinated hydrocarbons and HCl) depends on both the temperature and the influence of inorganic products added to sludge in the wastewater treatment plant. The addition of FeCl3 with lime affects the process in two ways: 1) an increase of H2 is produced at 488 °C due to lime action on water-gas shift reaction, and 2) an increase of HCl and chlorinated hydrocarbons at 470 °C is also produced. The kinetic parameters were determined by using the global reaction model for each one of two first consecutive reaction stages.