The Potential for Grass Biomethane as a Biofuel

Climate Change Research Programme - CCRP Report 11

Summary: This report summarises work done by Dr. Jerry Murphy and his team in the Environmental Research Institute at University College Cork. The team has successfully modified standard technologies used to generate methane from waste and used them to generate methane from grass. The team has also undertaken analysis of the barriers to the development of this technology in Ireland.

Published: 2013

ISBN: 978-1-84095-427-2

Pages: 68

Filesize: 2,199 KB

Format: xlsm


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.