Logistical considerations in the development of on-farm anaerobic digestion systems in Ireland

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Ó Céileachair, Dónal
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University College Cork
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Ireland has 4 million ha of grassland, and over 6.56 million cattle, including over 1.4 million dairy cows. The potential resource of grass silage and animal slurry can be considered significant if utilised in anaerobic digestion for the production of biogas (and biomethane). On-farm anaerobic digestion (AD) can provide a renewable, sustainable fuel (in the form of biogas) that can decarbonise the energy system and reduce emissions from the agricultural sector. However, cattle in Ireland spend the majority of the year outdoors in pasture-based farming systems, leading to intermittent slurry collection by the farmer. Thus, there is an inherent seasonality with regards to feedstock availability. On-farm AD plants may also have no gas grid infrastructure access, and therefore may be required to facilitate a seasonal gas demand of a large energy user in their vicinity. Although the Irish biomethane industry is emerging, there are no significant virtual pipeline or biogas pipeline networks. The work herein assesses these logistical characteristics such as on-farm biomass resources, their location with respect to both gas grid infrastructure and large industry energy users, and how seasonal feedstock availability and seasonal gas demand can affect the operation of an on-farm AD plant. Furthermore, this work investigates the logistical considerations in the development of virtual pipelines and biogas pipelines serving on-farm AD plants, and how to optimise them. The findings from this work indicate that the total on-farm biomethane resource in Ireland is estimated to be 67 PJ a-1. Approximately 17% of this resource is more than 15 km away from gas grid infrastructure whilst also within 15 km of a large industry energy user. Seasonal slurry availability on an Irish dairy farm can lead to a 21% reduction in total biomethane production, whilst increasing the carbon intensity of the biomethane produced (as compared to year-round availability) by 11 g CO2 MJ-1. Seasonal gas demands may be facilitated without significant effect to biomethane sustainability. Liquid digestate must be recirculated during times of no slurry availability to keep the solids content of the digester low and thus ensure sufficient mixing takes place. In the delivery of the energy to the end user, virtual pipelines serving on-farm AD plants may reduce the total routing requirements and emissions by opting for larger biomethane haulage vehicles. The use of mobile-upgrading and compression units may be logistically unfeasible due to the slow upgrading speed of current technologies. Heuristics (practical methods to solve very complex problems) used in GIS software may be employed to quickly find optimal biogas pipeline layouts which either follow the shortest possible path (Steiner minimum spanning tree (MST)) or follow the road network. Commercial software was used to design and cost 20 Steiner MST and 20 road network biogas pipeline layouts. A road network pipeline layout connecting 10 AD plants to the biogas user costs 26% more than a Steiner MST would. Multi-criteria decision analysis revealed that the designed Steiner MSTs always outperformed road network layouts, except when pipeline material cost was of no importance. This work highlights the potential of on-farm anaerobic digestion to provide a source of sustainable, renewable fuel source for decentralised areas and industries in Ireland, whilst highlighting the logistical considerations and challenges associated with feedstock location, seasonal feedstock availability, seasonal gas demand, and the development of gas delivery methods such as virtual biomethane pipelines or biogas pipelines.
Biogas , Anaerobic digestion , Biomethane , On-farm , Silage , Slurry , Logistics , Virtual pipeline , Pipeline , Seasonality , Sustainability
Ó Céileachair, D. 2023. Logistical considerations in the development of on-farm anaerobic digestion systems in Ireland. PhD Thesis, University College Cork.