Compressed Biogas

Grass is an excellent energy crop due to long persistence of high yields accompanied by low energy inputs. Approximately 91% of Irish agricultural land is under grass. The national herd has decreased and will continue to do so. Cross compliance does not encourage the conversion of permanent pastureland to arable land; thus we have and will continue to have increased quantities of excess grassland. Therefore, grass must be considered a significant source of biomass. Current grass species and cultivation practices are favourable for anaerobic digestion (AD), which is a mature technology. Upgrading biogas to biomethane, injecting into the gas grid, leads to an effective bioenergy system complete with distribution to all major cities and 620,000 houses. The Renewable Energy Directive allows a double credit for biofuels derived from residues and lignocellulosic material (such as grass). It is shown that 100,000 ha of grass (2.3% of agricultural land) will allow compliance with the 10% renewable energy in transport target for 2010. Alternatively, this would substitute for 35% of residential gas consumption. Reactor design must take account of the specific feedstock or combinations of feedstock; the reactor must be suited to the feedstock. This is not technically difficult. Of significant concern in the sustainability of the biofuel produced is the parasitic energy demand of the process and the vehicle efficiency. Emission reductions are optimised by the use of green electricity and the use of biomass for thermal energy input. On a field-to-wheel basis, it is essential that the vehicle operating on biomethane has an equivalent efficiency (expressed as MJ/km) as the displaced fossil fuel. The Renewable Energy Directive requires an emission savings of 60% compared with the displaced fuel for new facilities constructed after 2017. This is readily achieved for grass biomethane through optimisation of the system. Allowing for carbon (C) sequestration in grassland of 0.6 t C ha/year will lead to emissions savings of 89%. This would suggest that grass biomethane is one of the most sustainable indigenous, non-residue-based transport biofuels. The economics of biomethane are shown to be difficult. There is a requirement for innovative policy and marketing of the industry. A compressed natural gas transport fuel market is an essential prerequisite to using biomethane as a transport fuel. Mandating a certain percentage of biomethane in natural gas sales is of benefit to biomethane as both a transport and a thermal biofuel. Government policy is required to support a biomethane industry. Further research is required in the following areas:
Bioresource mapping: This includes the creation of a Geographical Information System to highlight sources of the organic fraction of municipal solid waste (OFMSW), slurry, slaughter waste and areas of high-yielding silage production. The system would include distribution systems (natural gas grid, electricity grid) and demand nodes (e.g. transport fleets, district heating, new towns) to propose areas with significant potential for biomethane production.
Assessment of biomethane facilities: This includes full life-cycle analysis of different biomethane facilities, including co-digestion of slurries and grass silage, mono-digestion of OFMSW, and mono-digestion of slaughter wastes. The research should allow assessment of the cost of the produced biomethane.
Digester design: This basic research should assess optimal digester systems for different feedstocks.
Agricultural impact of AD: This research includes monitoring carbon sequestration in grasslands where silage is cut and digestate is applied. This should be compared with carbon sequestration on grazed pastures. The fertiliser value of different digestates needs to be assessed along with the emissions associated with application of digestate. The research should also assess the effect on biodiversity.