Process simulation and techno-economic analysis of 400 t/d pilot plant for municipal sewage sludge drying and combustion

To achieve harmless treatment of sludge, the thermal conversion represented by combustion is considered the most promising treatment technology because it reduces sludge volume and recovers energy. This study develops a process of handling 400 tons of sludge per day by integrating sludge drying and combustion, and accompanied by energy analysis and economic evaluation. The proposed system provides the energy required for the drying process by recovering waste heat from the combustion section, thereby investigating the feasibility of achieving energy self-balance. The impact of variations in carbon and moisture content on the economic performance of the process by adjusting the moisture content of sludge with three different carbon levels has been explored. The findings indicate a strong correlation between achieving energy self-balance in the process and the moisture and carbon content of the sludge. Sludge 1, with a carbon content of 16.99 %, achieves energy self-balance at a moisture content of 60 %, while sludge 2, with a carbon content of 13.82 %, achieves energy self-balance at a moisture content of 50 %. Conversely, sludge 3, which contains a carbon content of 5.49 %, struggled to achieve self-balance even at 50 % moisture content. Despite the potential benefits of high-carbon sludge, the substantial investments required for the dryer and boiler diminish its economic attractiveness. Net present value (NPV) and internal rate of return (IRR) indicate that reducing moisture content and carbon content can enhance the process's economic efficiency. Results indicate that annual operating cost (AOC) predominantly drives economic indicator fluctuations with varying moisture content. In contrast, total capital cost (TCI) and annual depreciation cost (ADC) exert greater influence on economic indicators under changes in carbon content.