Feedstock flexible numerical analysis of sewage sludge gasification

Sewage sludge properties vary with the origin and treatment method of the wastewater. When converting the sewage sludge thermally for material and energy recovery, these properties affect the composition of the products and the efficiency of the process. Rotary kiln gasifiers can handle these variations in the raw materials. For further development, process integration, optimization of the process, upscaling, and industrial application, it is helpful to understand and describe the impact of the sewage sludge properties on operating parameters and product quality. This work analyses the impact of sewage sludge properties numerically using detailed chemistry and the surrogate approach. Sewage sludge is described using woody biomass components, sugars, lipids, proteins, inorganic species, moisture, and ash. The stochastic reactor model (SRM) is used to model the gasification process. In contrast to ideal reactors, stochastic reactors allow for resolving inhomogeneity. Hence, the predicted gas composition results from local temperature and available oxygen. For the analysis, temperature, airflow rate, and fuel properties, i.e., the amount and composition of volatiles, including fuel-bonded nitrogen and sulfur, and the moisture level are varied. Their results are analysed regarding the produced gas composition, emission precursor formation, and cold gas efficiency of the process. The highest efficiencies are found for temperatures of 1223 K and rich conditions. High concentrations of methane and hydrogen accompany this maximum. However, the producer gas's highest heating value is found at low temperatures thanks to the presence of small hydrocarbons. Furthermore, a high volatile amount and a dry feedstock favour methane and carbon monoxide formation. The hydrogen concentration is found to be sensitive to the moisture content and is the highest at 30%. The cold gas efficiency is predicted to depend strongly on the feedstock and varies for the same operating conditions up to 60%. With increasing fuel-bonded nitrogen and sulfur content, the concentrations of NO and SO2 formation and their precursors increase, respectively. The released ammonia leads to significantly reducing emitted NO through the ThermalDeNOx mechanism. Overall, the generated maps provide a detailed insight in species formation and allows for the prediction of process parameters sensitive to the sewage sludge properties.