Chemical Model for Thermal Treatment of Sewage Sludge

Abstract

Sewage sludge is here studied as a valuable source for processing or energy conversation thanks to its high nutrition and energy content. However, various origins of the wastewater, different water cleaning technologies, and seasonal and regional dependencies lead to the high variability of the sewage sludge properties. In thermal treatment units, that is, incineration, gasification and pyrolysis, sewage sludge serves as feedstock or fuel, hence a proper characterization and a mathematical description of the sewage sludge are required to estimate product streams and to formulate numerical simulations and optimization methods. The presented work introduces a surrogate concept that allows replication of sewage sludge’s ultimate composition, moisture, and ash content. The surrogate approach aims to model the decomposition of any sewage sludge sample, opposite to the established determination of kinetic rates for individual samples. Based on chemical solid surrogate species and corresponding reaction mechanisms, the thermal decomposition path is described. Sewage sludge is represented by a combination of lignocellulosic species, proteins, sugars, lipids, and representative inorganic species. The devolatilization and heterogeneous reactions are formulated such that they can be used together with a detailed gas-phase model, including tar oxidation and emission models for nitrogen and sulfur oxides, recently proposed by the authors. The developed chemical model is applied using a zero-dimensional gasification reactor in order to model weight loss within the thermogravimetric analysis, pyrolysis, gasification and combustion conditions. Weight loss, the composition of product gases, and emission release (nitrogen and sulfur oxides) are captured well by the model. The flexible surrogate approach allows us to represent various sewage sludge samples.