Agricultural small anaerobic digestion plants : combining economic and environmental assessment

In the most recent decade, anaerobic digestion (AD) has been widely studied as an interesting and profitable solution to produce renewable energy. Focusing on AD plants fed with agricultural feedstock, the most investigated ones were big plants fed with cereal silages. However, the spreading of these plants has reduced after the variation of the subsidy framework, and it has been counterbalanced by the increase of small plants (< 300 kW electrical power) fed almost only with animal slurries. Therefore, in this study were analysed two small AD plants in order to evaluate their environmental and economic performances. Life Cycle Assessment is adopted to assess the environmental side, while Net Present Value, payback period and Internal Rate of Return are adopted for the economic evaluation. Of the two small plants, the one totally fed with slurries showed both the best results on all the studied environmental impact categories, and the most interesting economic results, while the plant digesting pig slurry and maize silage was less beneficial. Moreover, the cost for reducing Greenhouse Gases (GHG) emissions (expressed as tons CO2 eq) was quantified for the two small plants in comparison with two other plants of 999 kW, resulting that small plants fed with slurries allow reducing GHG with lower subsidies respect to big plants that co-digest energy crops, even if these last plants produce more electricity.