Potential energy recovery by integrating an ORC in a biogas plant

An increasing interest is devoted to biogas plants as they might play a key role in the reduction of current fossil fuel consumption for power production. The main component of the plant is the anaerobic digester where the organic fraction of waste products is converted in a gas with high concentration of methane and carbon dioxide. This biogas is converted in power and heat in a cogeneration unit that may consist in a micro gas turbine or an internal combustion engine. A portion of the heat is used to keep the digester at a constant temperature as requested by the anaerobic digestion, the reaming is generally dissipated. An Organic Rankine Cycle (ORC) can be adopted to increase thermal energy recovery and increase the power produced by the plant. This study focuses on the potential improvements of an anaerobic digestion plant when a commercial (ORC) is adopted as an additional thermal user to reduce the amount of dissipated heat and increase the power production. The study is based on an existing biogas plant operating in the town of Viareggio (Italy) which will be equipped with a 600kWe micro gas turbine. The integration of the two systems was studied in detail by investigating the plant modifications, which are necessary to reach high values of thermal energy recovery. A reference and a modified solution were simulated in AMESim by considering a yearlong period with actual ambient conditions. Three different commercial size for ORC (30, 40 and 50 kW) were considered to determine the most suitable from the thermodynamic and economic point of view. A sensitivity analysis on the sludge regenerator, which recovers part of the heat from the digestate to preheat the sludge entering in the plant, was carried out showing a strong increase in the ORC performance with the size of the sludge regenerator. The larger was the ORC size, the larger should be the sludge regenerator to increase system efficiency and avoid winter shutdown. The economic analysis showed that profitability index was maximum for the 30 kW ORC, due to the inefficiency of a larger size during winter, while the most remunerable solution was the 40 kW configuration, due to the larger amount of energy produced during the year.