CO2 savings affect the break-even distance of feedstock supply and digestate placement in biogas production

Anaerobic digestion promotes a valorisation of residual and dedicated biomass of agricultural origin. The EU Directive 2009/28/CE mandates a minimum 35% saving in greenhouse gas (GHG) emission for biofuels and bioliquids. National legislations add further limits, such as a 70 km maximum distance in the supply of biomass for energy uses (Short chain support scheme in Italy). In order to assess if the substitution of fossil fuels with biogas complies with such obligations and which are the factors most influencing GHG savings, a simulation study was carried out on a digester fed by maize silage, as many plants built in the frame of the current incentives. A simplified balance was drawn between the CO2 emitted and saved by a 1000 kWel digester aimed for heat and electricity co-generation. The simulation output was the operational radius of feedstock supply and digestate disposal allowing a 35% CO2 saving in a standard scenario. A sensitivity analysis was also run by varying several traits concerning process efficiency, feedstock characteristics, logistic aspects, plus the amount of net thermal energy actually exploited and CH4 and N2O emissions from the stored digestate. The standard scenario involving a 35% CO2 saving allowed an operational radius of 192 km, almost three times as large as the maximum distance prescribed by the Short chain scheme. As an alternative, a 50% CO2 saving was possible at a radius of 70 km. Varying the process, feedstock and logistic traits by +/-10% determined variations in the break-even distance associated with a 35% CO2 saving, ranging from +/-20% (total solids; volatile solids; anaerobic biogasification potential; biogas CH4 content), to +/-15% (energy efficiency), to only +/-5% (CO2 emissions in the cropping phase; the carrying capacity of transport means). Actual heat utilisation and digestate GHG emissions, respectively set at 100% and 0% in the standard scenario, are traits with a high uncertainty in the normal practice. Therefore, a respective variation of -50% and +50% was adopted in the sensitivity analysis, determining in both cases a strong reduction (50%) of the break-even distance. In order to overcome the critical points represented by the amount of net thermal energy actually valorised and by GHG emissions from the stored digestate, special care is needed in planning biogas plants, in order to ensure a good utilisation of heat and recycle residual CH4 within the process.