Date of Award


Document Type

Doctoral Thesis

Degree Name

Doctor of Philosophy


Civil, Structural & Environmental Engineering

First Advisor

Dr. J.D. Murphy

Second Advisor

Dr. E. McKeogh


Ireland needs a biofuel industry- to aid compliance with the Kyoto Protocol, the Biofuels Directive and to secure its energ>' supply. The overall aim of this work is to assess the potential of CH4-enriched biogas as a transport fuel in Ireland. Biogas is produced during the anaerobic digestion of biomass. This work investigates biogas production from two different feedstocks; biodegradable municipal waste and agricultural crops. To assess the potential for biogas as a transport fuel the alternatives to biogas are investigated and compared. Thus digestion and composting of biodegradable municipal waste, and biogas and ethanol production from agricultural crops arc investigated on a technical, economic and environmental basis.

Ireland is facing a waste crisis; municipal waste production has grown at a significant rate over the last number of years; to compound this problem the Landfill Directive requires a reduction in the quantity of biodegradable municipal waste consigned to landfill. This thesis compares composting to anaerobic digestion with biogas utilisation as a transport fuel. The composting model is based on two weeks in-vessel composting followed by eight to ten weeks aerated static pile composting. The anaerobic digestion model is based on a dry digestion process, followed by water absorption for biogas upgrading, and biogas compression. At present biogas is not subjected to excise duty; if this situation remains then biogas is economically preferable to composting when treating more than 18,000 t/a (which equates to the waste produced by 80,000 people). However, if excise duty were applied to biogas, then composting is economically preferable when treating less than 48,000 t/a.

The area of land that is available for energy crops was examined. Crops for energy production can be grown on set-aside land and land previously under sugar beet without affecting food production. Currently there are three main crops in Ireland; wheat, barley and sugar beet. Biogas production from these crops is investigated for various rotations, however in addition to biogas these crops can also be used for ethanol production. Thus biogas from these crops is in direct competition with ethanol; consequently biogas and ethanol are compared. On a technical basis it was found that biogas produces more energy per hectare than ethanol. It was also found that a crop rotation of wheat, wheat and sugar beet is preferable in terms of energy production for both biogas and ethanol production. On an economic basis, biogas is preferable to ethanol when compared per unit of energy’. On an environmental basis ethanol saves more CO2 than biogas; this is due to present documented levels of leakage of methane at the biogas facilities. If biogas losses were reduced to between 3.4-4.4% then biogas saves more CO2 than ethanol.

Overall this work shows that biogas from biodegradable municipal waste and agricultural crops on 48,000 ha of land previously under sugar beet and set aside can replace up to 3.3% of petroleum fuels in 2010. Biogas on its own will not enable Ireland to comply with the Biofuels Directive but can aid Ireland if used in a combined approach with other renew able fuels and biogas from different sources.

F3123648NiamhPowerPhD2007.pdf (10651 kB)
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