Which is the preferable transport fuel on a greenhouse gas basis; biomethane or ethanol?

Biomethane and ethanol are both biofuels which are generated from agricultural crops that can be utilised to meet the Biofuels Directive. In Ireland with the demise of the sugar industry 48,000 Ha of land is readily available for biofuel production, without unduly effecting food production. Which biofuel should dominate? This paper investigates biofuel production for three different crop rotations: wheat, barley and sugar beet; wheat, wheat and sugar beet; wheat only. A greenhouse gas balance is performed to determine under what conditions each biofuel is preferable. For both biofuels, the preferred crop on a weight basis is wheat, while on an area basis the preferred crop is sugar beet. Biomethane scenarios produce more gross energy than ethanol scenarios. Under the base assumption (7.41% biogas losses, and biomethane utilised in a converted petrol engine, such as a bi-fuel car, and thus underperforming on a km/MJ basis) ethanol generated more net greenhouse gas savings than biomethane. This was unexpected as biomethane produces twice the net energy per hectare as ethanol. If either biogas losses were reduced or biomethane was utilised in a vehicular engine optimised for biomethane (such as a bus powered solely on gaseous biofuel) then biomethane would generate significantly more net greenhouse gas savings than ethanol. It was found that if biogas losses were eliminated and the biomethane was used in a vehicle optimised for biomethane, then the net greenhouse gas savings are 2.4 times greater than those from ethanol generated from the same feedstock.