Electricity Production from Anaerobic Digestion of Animal Slurries in a Farm Scale Plants

Currently, in Italy, more than 1200 agricultural biogas plants are running, mainly in the northern regions. At the end of 2012, 1.65% of the Italian electric consumption was produced from agricultural biogas plants. The public incentives framework for electricity production from biogas has been updated by the D.M. of 6 July 2012: the highest value is granted to AD plants with electrical power < 300 kW and mainly fed with by-products. Specific economic bonuses are provided for heat valorisation and nitrogen reduction in the digestate.Nevertheless, with regard to agricultural AD plants, remarkable environmental impacts could be due to biomass production as well as to digestate management. On the other hand, benefits could arise from electricity and heat generation from renewable sources and mineral fertilisation substitution.The aim of this study is to assess the environmental profile of electricity production from four different AD plant mainly fed with animal slurry and electrical power lower than 300 kW. Using LCA method, the environmental performance of electricity production from biogas has been evaluated using 1 kWh of electricity as functional unit.All processes involved as well as the processes avoided by the biogas production system (e.g. electricity and thermal energy production) were included in the system boundaries and therefore evaluated. The most critical stages (environmental hotspots) were identified and discussed; mitigation strategies of the environmental burdens where evaluated too. Moreover, the outcomes of the environmental assessment were compared with the environmental impact related to the Italian electricity mix.The achieved results show that the electricity produced from the AD plant has better environmental performances than electricity produced from fossil fuels in particular for impact categories such as global warming and fossil depletion. Moreover, the recovery and the valorisation of surplus heat reduces significantly the environmental burdens. Definitely, livestock slurries are a good feedstock for AD plants from an environmental point of view thanks to credits provided by a suitable waste management as well as to the absence of environmental burdens related to their production. The logistic aspects of the biomass supply must be carefully evaluated because the transportation of the feedstock over long distances can offset the environmental benefits due to the replacement of energy generation from fossil fuels.