Energy, exergy, economic, and environmental (4E) analysis of a hybrid sludge-biomass power system via integrated hydrocharization and co-gasification

Conventional sewage sludge (SS) treatment risks resource wastage and secondary pollution, while standalone biomass gasification exhibits limited efficiency. Therefore, an integrated waste-to-energy system combining hydrocharization (HTC) of SS with biomass co-gasification has been developed based on the energy cascade principle. This novel integrated system combines HTC of SS with biomass co-gasification for the first time, simultaneously achieving improved system performance and full-component utilization of waste. In the integrated system, sewage sludge is converted to hydrochar through heat treatment, drying and dewatering, and then co-gasified with biomass. The resulting syngas drives a gas turbine for primary power generation, while waste heat recovery generates steam through steam turbines for secondary power generation. Thermodynamic analysis reveals superior performance, with net thermal and exergy efficiencies of 58.09 % and 56.79 %, respectively. Economic evaluation demonstrates viability and risk tolerance, with a 5.62-year dynamic payback period, net present value of 432,443.77k$, levelized electricity cost of 66.88$/MWh, and 29.58 % internal rate of return. The results of the environmental analysis show a greenhouse gas house emission intensity of 562.46 kg CO2/MWh, highlighting the decarbonization potential. Sensitivity analysis further identifies key operational parameters for optimization. The study concludes that this integrated approach offers a technically robust, economically competitive, and environmentally sustainable solution for simultaneous sludge disposal and clean energy generation.