Pathways to a renewable gas industry in Ireland

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O'Shea, Richard
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University College Cork
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The use of renewable gas produced via the anaerobic digestion of biodegradable material has been mooted as a source of renewable energy in Ireland. The production of renewable gas in power to gas systems could also allow for the storage of significant quantities of excess renewable electricity in the form of methane gas, while demand driven biogas systems could act as a source of controllable and dispatchable renewable electricity. This work aims to assess the scale of these resource in Ireland. The total theoretical resource of biomethane which could be produced via the anaerobic digestion of waste streams was found to be 12.5PJ equivalent to 6-7% of final energy consumption in transportation and final energy consumption in heat production. Most of this potential resource arose from cattle slurry and was concentrated in the southern and north-eastern regions of Ireland. Initial biomethane plants processing waste streams should use source separated household organic waste and should locate in regions where this resource is highest. Biomethane plants processing waste streams could produce 3.4-3.8 PJ of energy. The total theoretical resource of biomethane associated with grass silage was found to be 128.4PJ, equivalent to 64% of energy consumption in transport and 72% of energy thermal energy consumption. The majority of the potential grass silage resource is located in western regions of Ireland. Biomethane plants processing grass silage and cattle slurry could provide 12.2PJ of energy. Plant scale, feedstock type, feedstock mixture, gate fees, feedstock price, and incentive value strongly influenced the quantity of biomethane that could be produced. The use of decentralised anaerobic digestion systems can reduce the energy consumption and greenhouse gas emissions associated with the anaerobic digestion of wet feedstocks such as pig slurry by 21-22% and 18-19% respectively compared to a centralised anaerobic digestion system. This could increase the greenhouse gas emissions savings of biogas, allowing it to meet future stringent sustainability criteria. Advanced sources of renewable gas such as microalgae (used in anaerobic digestion) and power to gas systems (converting excess renewable electricity into methane gas using biogenic sources of CO2) could theoretically provide 1.8PJ and 1.4PJ of renewable gas respectively. These systems are technically less advanced, however, power to gas systems present an interesting opportunity for energy storage. Feeding regimes for a demand driven biogas system to generate electricity at times of high demand, and biomethane outside of these periods were developed using lab scale trials and could inform the operation of full scale plants.
Renewable energy , Renewable gas , Biomethane , Biogas
O'Shea, R. 2017. Pathways to a renewable gas industry in Ireland. PhD Thesis, University College Cork.