Greenhouse gas emissions of an agro-biogas energy system: Estimation under the Renewable Energy Directive

Agro-biogas from energy crops and by-products is a renewable energy carrier that can potentially contribute to climate change mitigation. In this context, application of the methodology defined by the Renewable Energy Directive 2009/28/EC (RED) was performed in order to estimate the 100-year Global Warming Potential (GWP100) associated with an agro-biogas supply chain (SC) in Southern Italy. Doing so enabled calculation of Greenhouse Gas (GHG) emission saving in order to verify if it is at least equal to 35% compared to the fossil fuel reference system, as specified by the RED. For the assessment, an attributional Life Cycle Assessment (LCA) approach (International Organization for Standardization (ISO), 2006a,b) was integrated with the RED methodology applied following the guidelines reported in COM(2010)11 and updated by SWD(2014)259 and Report EUR 27215 EN (2015). Moreover, primary data were collected with secondary data extrapolated from the Ecoinvent database system. Results showed that the GWP100 associated with electricity production through the biogas plant investigated was equal to 111.58 g CO2eq MJe-1 and so a 40.01% GHG-emission saving was recorded compared to the RED reference. The highest contribution comes from biomass production and, in particular, from crop cultivation due to production of ammonium nitrate in the overall amount used for crop cultivation. Based upon the findings of the study, the GHG saving calculated slightly exceeds the related minimum proposed by the RED: therefore, improvements are needed anyway. In particular, the authors documented that through replacement of ammonium nitrate with urea the GHG-emission saving would increase to almost 68%, thus largely satisfying the RED limit. In addition, the study highlighted that conservation practices, such as NT, can significantly enable reduction of the GHG-emissions coming from agricultural activities. Therefore, those practices should be increasingly adopted for cultivation of energy crops, because the latter significantly contribute to biogas production yield enhancement.