Feasibility analysis of bio-methane production in a biogas plant: A case study

A feasibility analysis, to assess the suitability of converting the biogas produced in an existing anaerobic digestion plant to bio-methane, was carried out. The case study plant was equipped with a micro-gas turbine co-generator. Several upgrading systems of different sizes were considered, to determine the most suitable configuration from a thermodynamic and economic point of view. For this purpose, a model of the whole plant that included digesters, a micro-gas turbine, a sludge line, heat transfer loops, and heat exchangers was developed. A steady-state simulation was performed by using the daily average conditions for the one-year long operation of the plant. The results highlighted that the feasibility depended on the amount of bio-methane produced, as this affected the performance of the cogeneration system and the balance between the costs and revenues. When large amounts of biogas are upgraded to bio-methane, the heat provided by the micro-gas turbine during the winter season is not sufficient to keep the digesters at the desired temperature and, therefore, natural gas integration is necessary. In addition, by increasing the upgrading unit size, the amount of electric energy purchased by the grid increases accordingly. An economic analysis showed that the optimal upgrading system size was strongly dependent on the bio-methane selling price.