The optimal production of biogas for use as a transport fuel in Ireland

The Biofuels Directive places an onus on EU member states to ensure that biofuels are placed on their markets. This paper investigates the use of CH4-enriched biogas as a fuel. A number of options, which produce CH4-enriched biogas, were analysed from technical, economic and environmental perspectives. Biogas may be produced at a centralised anaerobic digestion (CAD) facility, accepting agricultural slurries plus a portion (typically 20%) of municipal waste such as the organic fraction of municipal solid waste (OFMSW). Alternatively, OFMSW may be the sole feedstock utilised in the dry anaerobic combustion (DRANCO) process. Importation of green electricity to satisfy parasitic electrical demand and the burning of biogas to satisfy thermal demand was found to be more advantageous than using biogas in a combined heat and power (CHP) plant to cover parasitic electrical and thermal demand. The DRANCO process produced more fuel than the CAD process due to the lower thermal demand of the digester. The economics of the DRANCO process were superior to the CAD process due to the greater gate fee (it is assumed that no gate fee is obtained from agricultural slurry) and the greater quantity of biogas available for sale. The DRANCO process leads to greater greenhouse-gas savings than the CAD process due to the displacement of more OFMSW from landfill and more petrol from transport. The options when applied to Ireland could lead to a substitution of 1% of petrol and diesel and to a 12% reduction in greenhouse-gas emissions from the transport sector.