In this paper, a biogas fuelled hybrid system, obtained by integrating a molten carbonate fuel cell with a micro-turbine is considered. The size of the plant is selected on the basis of the maximum biogas production registered by monitoring the annual operation of an anaerobic digestion plant. The system produces electricity and supplies heat to the digester. Heat is necessary to keep correct operating temperature of the bacteria. A model of the system components is built and the plant optimization is performed. Design parameters are the fuel cell temperature, pressure ratio, inlet turbine temperature, reforming temperature, recirculation percentage, size of the two subsystems. Two competing objective functions are considered: the energy efficiency and the unit cost of electricity. The Pareto front shows that efficiencies close to 50% are obtained, with unit costs comparable with market prices of electricity. The off-design conditions caused by variations in biogas production and thermal request of the digester are also considered. Experimental data from the digester are used to investigate these variations. The optimal operation is selected depending on the daily heat request and biogas production. Possible economic and energy benefits that can be achieved by adding natural gas are also investigated. (C) 2012 Elsevier Ltd. All rights reserved.