Energy and Economic Analyses of Integrated Biogas-Fed Energy Systems

The process, which includes production, collection, carriage, and transformation of biomass into renewable fuels and then into energy (both electrical and thermal), involves a large number of decisions to select the most efficient plant layout. In order to identify the optimal solutions, models, which simulate the whole process, represent a useful and practical tool. In this paper, the energy and economic analysis of the entire process from biomass to energy production is presented. Among the different transformation processes, the thermophilic batch anaerobic digestion is considered in this paper. The analyses performed allow the comparison of the results for different scenarios characterized by different types of biomass (ensiled corn and organic fraction of municipal solid wastes), yearly mass of biomass, anaerobic digestion process parameters (number of yearly batch cycles and number of batch digesters), and type of energy systems (micro gas turbine and internal combustion engine). The results are presented in terms of classical economic indices for the investment and of producible electric and thermal energy. With respect to the economic indices, micro gas turbines allow a higher profitability than internal combustion engines, mainly because internal combustion engines require a scrubbing system to remove hydrogen sulphide from biogas. The contrary occurs with the producible electric and thermal energy. With regard to the digested substance, even if the methane yield is lower for organic fraction of municipal solid wastes than for ensiled corn, the net present values for organic fraction of municipal solid wastes are always higher than those obtained by using ensiled corn, and they are always positive, since municipal waste digestion avoids their disposal costs. The efficiency of the cogeneration process, evaluated in terms of primary energy saving index, usually shows quite high values and confirm the good capability of these systems.