Energy and environmental balance of biogas for dual-fuel mobile applications

Considerable research is currently being devoted to seeking alternative fuels to comply with transportation needs while reducing the environmental impact of this sector. Within the transport activity sector, on road vehicles and agricultural machinery require around 2 Mtoe energy in France. The anaerobic digestion of farm waste could roughly cover these needs. This paper aims to study the environmental and energy interest of this short power supply path. An ideal biogas production system has been built up from the average characteristics of current rural biogas plants in France. Pollutant emissions, energy demands and production are assessed for various scenarios in order to produce methane for dual fuel engines. Life cycle assessment (LCA) is used to evaluate the environmental impact of dual fuel agricultural machines, compared to diesel engines. The energy balance is always in disfavour of biogas fuel, whereas LCA energy indicators indicate a benefit for biogas production. This gap is related to the way in which the input of biomass energy is handled: in conventional biofuel LCA, this energy is not taken into account. A carbon balance is then presented to discuss the impact of biogas on climate change. Dual fuel engines were found to be interesting for their small impact. We also show, however, how the biogenic carbon assumption and the choice of allocation for the avoided methane emissions of anaerobic digestion are crucial in quantifying CO₂ savings. Other environmental issues of biogas fuel were examined. Results indicate that are management and green electricity are the key points for a sustainable biogas fuel. It is concluded that biofuel environmental damage is reduced if energy needs during biofuel production are covered by the production process itself. As agricultural equipment is used during the biofuel production process, this implies that a high substitution rate should be used for this equipment.