Pyrolysis behaviors of thermally-thick sewage sludge particle with high moisture content: An experimental and modelling study

Abstract

Pyrolysis of sewage sludge with high moisture content and large particle size is commonly involved in industry. The large particle size will lead to high intraparticle temperature gradient, and as a result, simultaneous occurrence of drying, devolatilization and char gasification with complicated interaction are involved during pyrolysis process. In this work, the pyrolysis behaviors of large sewage sludge particle are studied both experimentally and numerically. Effects of external temperature, particle size and moisture content were investigated on a thermogravimetry, and the carbon contents inside particles were also measured to evaluate the extent of gasification. Meanwhile, a numerical model is developed to further analyze the influence of these factors and the interaction between drying, devolatilization and char gasification through the detailed intra-particle information. Front reaction method is applied for drying process while volume reaction method is adopted for devolatilization and gasification. Coordinate transformation technique and Chebyshev spectral method are implemented to resolve the intraparticle gradients with high computational efficiency and robustness. The results have shown that the large particle size together with the heat consumption of drying led to remarkable intraparticle temperature gradient. Consequently, drying and devolatilization coexisted almost throughout the pyrolysis process, as clearly indicated by the modelling results. Moreover, the compensation effect of drying and devolatilization was experimentally observed and numerically analyzed, leading to the overlapping weight-loss curves for different initial moisture contents. In addition, auto-gasification caused by moisture outflow was observed, which showed a more significant consumption of the char at the outer-layer when increasing initial moisture content.