• biogas plant
• cogeneration
• inverted Brayton cycle
• numerical simulation
• ORC

In this thesis, a thermodynamic analysis under design conditions of the performance of a biogas-fuelled mGT based on inverted Brayton cycle is investigated. The case study is an existing biogas plant near Pisa which performs co-digestion of a mixture of civil sewage and organic waste. A preliminary study of potential biomethane production had been made. A remodelling of the cogeneration system has been considered in this work. The current plant consists of a gas turbine operating on the conventional Brayton cycle. This study aims to compare this system with one operating on the inverted Brayton cycle. The upgrading system reduces the size of the cogenerator and at the same time requires a large amount of heat for the separation of the carbon dioxide. Different configurations of the cogenerator for two sizes of the upgrading system have been considered. A steady-state simulation was performed by using the daily average conditions for the one-year long operation of the plant. The first comparison between the performance of an mGT in the case of traditional and inverted Brayton cycle shows that lower electricity production is achieved in the second case.
The opportunity to increase efficiency through the employment of an ORC as a bottoming cycle has been considered for both systems. The combination of the two cycles, as well as the selection of the organic fluid, depends on the thermal load of the plant.
The results show that the integration of an ORC produces better results in the case of the Brayton cycle for all plant sizes considered.
An increase in IBC performance alone and with ORC are obtained when a given percentage of flue gas is recirculated into the combustion chamber.