Conference Publication Details
Mandatory Fields
Murphy J.D., Wall D. and O’Kiely P
Key Note Lecture European Grassland Federation, Iceland
Second generation biofuel: biomethane from co-digestion of grass and slurry
Optional Fields
A. Helgadottir, A. Hopkins
Akureyri, Iceland

Grass is an excellent source of biomethane; it is a high solid content feedstock (for anaerobic digestion) with a high specific methane capacity. However mono-digestion of grass can be somewhat problematic and unstable over time. This is mainly due to a deficiency of essential trace elements. Co-digestion with slurry is a more stable digestion process. This is serendipitous as beef and dairy farmers have slurry as a residue and typically operate grass-based farms. A model proposed for Ireland suggests 100 number, 75,000 t a-1 biodigesters each treating 14,000 t a-1of grass silage (ca. 4200 t dry solids (DS) a-1 from 350 ha of grass land) and 61,000 t a-1of slurry (8% DS). Each biodigester would produce of the order of 2,150,000 mn3 per annum of biomethane (equivalent to 2,150,000 l a-1of diesel) which is sufficient to fuel 2,000 cars travelling 15,000 km a-1. The proposed industry could generate 4% of expected energy in transport by 2020. As a second generation biofuel it qualifies for a double credit under the Renewable Energy Directive and thus satisfies 80% of the 2020 target of 10% renewable energy in transport. The distribution system for the gaseous biofuel is the existing natural gas grid system.

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