An argument for using biomethane generated from grass as a biofuel in Ireland

Document Type



Civil and Environmental Engineering

Publication Details

Biomass and Bioenergy


The Biofuels Directive proposes 5.75% of transport fuel (by energy) to be replaced by biofuel in the year 2010. This equates to 11.3 PJ in Ireland, which equates to 538 million litres of ethanol or 323 million litres of biodiesel. However, if using biodiesel produced through bioesterification of rapeseed oil, then 6.3% of Irish agricultural land is required to produce 5.75% of transport fuel. Furthermore this equates to 70% of arable land.

Using ethanol produced from wheat, 3.9% of Irish agricultural land is required to produce 5.75% of transport fuel. Ethanol produces less energy from a crop, than the energy in the biogas generated when the crop is digested. The ethanol production process uses up to 60% of the produced energy in the final ethanol product. It is shown for compressed biomethane generated from silage that the total parasitic demand of the process is of the order of 25%.

Grass/silage is a crop that Irish farmers are familiar with, over 90% of Irish agricultural land is under grass. Grass does not require rotation, it does not require annual ploughing (releasing NOx), and it sequesters carbon into the soil. Digesting silage, scrubbing the biogas to biomethane, and compressing and utilizing it as a transport fuel, is suggested to be the optimum biofuel for Ireland. The 2010 biofuels target can be met with 1.6% of agricultural land; this is four times less land than required using rapeseed. A conservative economic analysis would suggest a lower cost than ethanol produced from wheat.

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