Feasibility analysis of a biogas-fuelled trigeneration plant operating with a mGT

Recently, biogas production from anaerobic digestion of waste organic fraction is attracting more and more interest as a possible alternative to fossil fuels in both power generation and automotive applications. In the context of power generation, many plants already exist where biogas is used as a fuel for a thermal engine. In many cases, the focus of the plant is the production of electric power. Waste heat recovery is limited to the maintenance of digester operating conditions and the surplus is generally dissipated. Thanks to a lower maintenance burden, micro gas turbines are increasingly replacing internal combustion engines as thermal engines especially in small size plants. In those cases, the amount of heat not used in the process could be a significant component in the plant energy balance. This study is focused on the possible co-generative uses of this waste heat. As a case study, the bio-digester plant located in Viareggio (Italy) was considered. The anaerobic digester operates in co-digestion of sewage sludge and municipal bio-waste and is equipped with a 600 kWe Capstone micro gas turbine. The biogas produced is used to operate the micro-turbine and the exhaust gas heat is used to keep a constant temperature in the anaerobic digester. The opportunity to use the heat surplus (about 1/3) to cover part of the thermal and cooling request of the plant managing building has been analyzed thermodynamically and economically. Simulations have been carried in transient conditions with AMESim by considering one reference year and plant data. Hourly-discretized ambient temperature and radiation data have been considered in the heat transfer model of the anaerobic digester and in the computation of the air temperature at gas turbine inlet. Results highlighted that the tri-generation system improved the recovery efficiency in the range of 10-20%. Two smaller absorption chillers performed better than a larger unit, even if from an economic point of view the two solutions provided almost the same profitability.