Conversion of Sewage Sludge to combined heat and power: Modeling and optimization

Combined Heat and Power (CHP) generation from Sewage Sludge (SS) offers two simultaneous advantages: greenhouse gas emission reduction and increase of renewable energy generation as promoted by the European Union Green Deal 2021. In this work, a numerical model has been developed via Aspen Plus for the evaluation of CHP generation potentiality from SS through gasification integrated with an internal combustion engine system. The model is applied to the case of Italy and eight other European countries for the first time. The gasification model has been developed based on the experimental data on syngas generation from SS in a bench-scale rotary kiln reactor under laboratory conditions available in the literature. Sensitivity analysis revealed optimal operating temperature and equivalence ratios for gasification were 900 °C and 0.2 respectively. The CHP generation potentiality of SS resulted to be 2.73 kWh per kg SS as dry solid. According to the statistical analysis used in the present study, SS generation will reach 680 kt per year as dry solid by 2030 based on the current sludge generation rate as well as improvement in the wastewater collection and treatment expected for the future in Italy. Within this time, the projected electrical and thermal energy generation rate per year can reach 714 GWh and 1142 GWh respectively. Electrical and thermal energy generation rates from sewage sludge have been estimated for eight EU countries in 2015 and compared with the Italian scenario, founding the highest one in Spain and the lowest in Luxembourg.