Optimal Design and Operation of a Biogas Fuelled Mcfc (molten Carbonate Fuel Cells) System Integrated with an Anaerobic Digester

In this paper, a 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 annual operation of an operating anaerobic digestion plant. The hybrid system produces electricity and supplies heat to the digester. Heat is necessary to keep correct operating temperature of 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 optimal system is then considered in off-design conditions caused by variations in biogas production and thermal request of the digester. 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. Key-words: Hybrid plant, Biogas, Anaerobic digestion, Optimal operation, Multi-objective optimization INTRODUCTION In the north west part of Italy there are various experiences of anaerobic digestion both for municipal solid wastes and for waste sludge from sewage treatment. Biogas produced in these installations is usually used for electricity generation or cogeneration in internal combustion engines (ICEs). High temperature fuel cell systems may be used instead of ICEs to achieve larger electrical efficiencies. In this paper, a hybrid system, obtained by integrating a molten carbonate fuel cell with a micro-turbine is considered. The objective of this work consists in the optimization of the design parameters as well as its simulation in possible off-design conditions caused by variations in biogas production. Possible strategies to improve the plant operation are also examined. The system is designed starting from a similar plant [1], fuelled with landfill gas and generating electricity and hydrogen. The size is selected on the basis of the maximum biogas production registered by monitoring annual operation of one of the two anaerobic digestion plants in Pinerolo. This production is about 16000 Nm 3 /day, which means about 1940 kW in the case of 24 hour operation. The system should also supply heat to the digester, in order to keep a correct operating temperature for bacteria (48°C-53°C). Maximum heat power registered during annual operation is about 400 kW. This heat flux is supplied through hot water, entering the digester at 80 °C and returning to the generator at about 60 °C. A schematic of the system is presented in figure 1. The plant is composed of two interconnected sections: the micro-turbine and the molten carbonate fuel cell. Compared to commercial micro-turbines, here three main design changes are required: 1) an air extraction downstream the air compressor (flow 13), which is used to feed the fuel cell cathode; 2) an injection of the gases exiting the fuel cell cathode in the combustion chamber (flow 15) or downstream the (flow 16) air pre-heater; 3) the installation of an evaporator, where exhausts are used to evaporate water required in the fuel cell section. In the micro-turbine, ambient air (flow 25) is compressed in the compressor (AC) and then split in two flows: flow 21, directed to gas turbine, and flow 13 directed to the fuel cell section. Flow 21 is pre-heated in the air pre-heater, using the exhausts exiting the gas turbine (flow 20), and then mixed with biogas (flow 14) and with the flow returning from the fuel cell section (flow 15) in the burner (CC). The combustion gas (flow 23) enters the turbine (GT) and expands producing mechanical work to run the compressor and produce electricity in the static generator (G). The enthalpy of the exhausts exiting the turbine is partially recovered in air pre-heating and then in the heat recovery evaporator, to produce steam necessary for the methane steam reforming. Before exiting the plant, exhausts (flow 18) are used in a heat exchanger to supply heat to the anaerobic digester.