Oxidative pyrolysis of biosolid: Air concentration effects on biochar formation and kinetics

To investigate the potential of utilizing oxidative pyrolysis for treating a large volume of biosolid without high energy consumption, this study conducts the oxidative pyrolysis of biosolids and determines the pyrolysis kinetics and the biochar qualities produced from different oxidative conditions. The oxidative pyrolysis is performed at three heating rates (20, 25 and 30 °C/min) under varying air concentrations (1.25%, 12.5% and 25%) until reaching 700 °C. The kinetic triplet (kinetic model, activation energy, and frequency factor) is determined by utilizing the mass loss data obtained from the thermogravimetric analysis (TGA). Regardless of air concentration, the average activation energy for the evaporation of water and the decomposition of hydrated compounds remains similar at 107.54 kJ/mol. The activation energy and the frequency factor begin to decrease, starting from the oxidative pyrolysis stage at air concentrations of 12.5% and 25%. With increasing air concentration, the specific surface area of biosolid-derived biochar decreases from 24.23 m2/g to 14.19 m2/g. The ideal conditions for producing oxidised biochar without high energy consumption can be air concentrations below 12.5%. This study demonstrates the pyrolysis kinetics and thermal decomposition behaviour of biosolids at different air concentrations, which is required to establish energy efficient large-scale pyrolysis facilities.