Economic assessment of producing and selling biomethane into a regional market

Biogas is a promising renewable energy resource produced by using anaerobic digestion of organic substrates and it is mainly used to generate electricity by means of biogas engines. Other potential utilisations are growing, e.g. grid injection and biofuels production but, generally, a treatment aimed to increase its quality is mandatory and greater investments are generally required to produce upgraded biogas (biomethane). Despite the increasing of interest on these applications, a lack of information is still present from an economic point of view. The present paper performs an extended economic assessment of upgrading and selling biogas starting from a typical farm-based anaerobic digestion plant in Northern Ireland assumed as reference. Several economic indexes have been considered to assess the economic performance of the upgraded anaerobic digestion plant, namely net present value, pay-back period, profitability index and internal rate of return. Moreover, different scenarios in terms of fossil fuel prices have been analysed. The results show that producing and selling biomethane can be economically feasible if an adequate market is fostered. Optimum anaerobic digestion sizes between 26.9 and 64.4 tonne/y have been found, with correspondent net present value and pay-back period values in the range of £6.7–64.4M and 2.8–7.5 years, respectively, depending on the price scenarios analysed and the economic index chosen. Generally, adopting the net present value as objective function of the optimisation leads to greater anaerobic digestion size than the other indexes for any price scenarios considered. Finally, tougher market conditions (i.e. higher fossil fuel prices) lead to better economic performances of the upgraded anaerobic digestion configuration.