Study of sewage sludge pyrolysis mechanism and mathematical modeling

The pyrolysis of sewage sludge, with and without ZnCl2 activation, was monitored using TGA-FTIR, with the results utilized in a proposed decomposition mechanism. For Ehe pyrolysis of sludge without ZnCl2 activation, three global stages of mass loss occurred and are proposed as dehydration, initial depolymerization, and deep degradation. For the pyrolysis of sludge with ZnCl2 activation, apart from these three stages, an additional mass loss stage could be also observed at high temperature. It is suggested this results from the evaporation of ZnCl2. To make a comparison between these two cases, it was found that the pyrolysis of sludge with ZnCl2 activation had a longer dehydration phase and a shorter degradation phase. Based on the speculated mechanism, a kinetic model of four consecutive competitive reactions was adopted to simulate the pyrolysis of sewage sludge with and without ZnCl2 activation. Globally, the proposed model was able to fit the experimental data very well for both cases, with the correlation coefficients above 0.99.