Combined Heat and Power Generation from Mechanically Dewatered Sewage Sludge: Numerical Modelling

In the present study, a system for Combined Heat and Power (CHP) generation from mechanically dewatered Sewage Sludge (SS) is numerically analyzed through Aspen Plus software. The proposed system is composed of three consecutive processes: drying, gasification, and energy generation through an internal combustion engine. The gasification model is calibrated by applying a restricted chemical equilibrium approach and validated for four experimental outcomes available in the literature. Optimum gasification temperature (900 °C) is identified at an ideal equivalence ratio which is the ratio between actual air fed to the reactor to the stoichiometric air required for complete combustion of 0.2 predicted in a previous study through a sensitivity analysis. The CHP generation potentiality is assessed, finding 0.89 kWh/kg SS as Dry Solid (DS) of electrical and 1.67 kWh/kg of SS as DS of thermal energy. This allows supplying around 50 % of electrical energy required to run the wastewater treatment plant and is sufficient to complete the thermal drying of mechanically dewatered SS.