Combined heat and power production based on sewage sludge gasification: An energy-efficient solution for wastewater treatment plants

The main operating costs of wastewater treatment plants are related to the energy consumption and the disposal of by-products. Energy recovery from sewage sludge may be a solution to face both these challenges, improving the sustainability of wastewater treatment plants and making them an example of a circular economy. In this work, the energy and economic analysis of an integrated combined heat and power system based on sewage sludge gasification is proposed. The whole process is simulated by using the commercial software Aspen Plus (R). A restricted equilibrium model is used to simulate the gasification of sewage sludge in an atmospheric fluidized bed reactor using air as a gasification agent. Syngas produced from gasification is used as a fuel in an internal combustion engine for combined heat and power production. Different solutions are compared: the internal combustion engine is supposed to be fuelled with syngas or with syngas and methane. In line with the pertinent literature on integrated biomass gasification-internal combustion engine systems, the engine is modeled by combining a compressor, a combustor and a turbine to simulate the four thermodynamic steps of an internal combustion engine. Electric and thermal energy produced by the system is used to supply a fraction of the demand for wastewater and sludge treatment. The energy analysis is carried out for a real wastewater treatment plant that serves 1.2 million of population equivalent, located in Southern Italy. The obtained results are used to carry out an energy and economic analysis, which aims at assessing the feasibility and environmental benefits of the proposed system over conventional technologies.