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Enhancing the alternative use and circularity potential of grass through an anaerobic digestion-based biorefinery approach
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Date
2025-09-30
Authors
Shinde, Rajas Dattatray
Journal Title
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Publisher
University College Cork
Published Version
Abstract
A predominantly livestock-based farming system makes agriculture the largest source of greenhouse gas emissions in Ireland. The sector contributes around 38% of total emissions, primarily attributed to methane produced by ruminants. Combined with increasing socio-economic pressures, this makes agriculture a particularly challenging sector for achieving Ireland’s climate-neutrality goals. There is an urgent need to develop farm diversification pathways that can enhance farm viability while reducing emissions. Over 90% of Ireland’s agricultural area is under grassland to support livestock, and given this abundant grass resource, there is growing interest in diversifying its use beyond forage.
This thesis investigates the potential of an anaerobic digestion (AD)-based biorefinery approach to unlock alternative uses for grass. A two-phase AD system, combining a leach bed reactor (LBR) and an upflow anaerobic sludge blanket (UASB) reactor, was experimentally validated for grass biorefinery applications. Such systems enable the extraction of intermediates of the AD process, namely volatile fatty acids (VFAs), alongside biogas production. VFAs are high-value green chemicals used as building blocks in food, pharmaceutical, cosmetics, and biofuel industries. The LBR-UASB system yielded approximately 23 kg of VFAs and 75 kWh of biogas per tonne of silage. A novel demand-driven operational strategy was developed where biogas production was aligned with peak electricity demand hours, while VFAs were prioritised at other times. This dynamic AD-biorefinery approach represents a key innovation, demonstrating the potential to complement variable renewables and provide on-site demand management through dispatchable biogas, while enhancing revenue and resilience to evolving market conditions.
Building on the experimental results, a techno-economic assessment was conducted for a conceptual biorefinery model, processing 35,000-tonne fresh weight of silage per year (20% dry matter), producing polyhydroxyalkanoate (PHA) biopolymer from VFAs, fibre insulation from residual silage, and utilising biogas produced on demand to meet internal energy needs during peak hours. The capital and annual operational costs were estimated at approximately €370 and €114 per tonne of silage, respectively. Recognising the central role of farmers in providing feedstock, a bio-economic model was integrated to evaluate the opportunity cost of diverting land from livestock to silage production for a biorefinery. The minimum silage price required to compensate farmers for both production costs and the foregone livestock income was estimated at €72/t fresh weight. The biorefinery achieved a 20-year net present value (NPV) of –€4 million, indicating it was not financially viable under current conditions. Sensitivity analysis identified fibre yield, silage dry matter content, and feedstock cost as key profitability drivers. Scenario analysis identified plausible pathways for improvement. A combination of 20% capital subsidy, 11% reduction in annual operational costs, and a 10% improvement in the silage dry matter content could achieve a 20-year NPV of €3.5 million and a discounted payback period of 8 years.
A Delphi survey involving expert stakeholders across the Irish bioeconomy identified key enablers and barriers to developing grass biorefinery value chains. Stakeholders emphasised rapid deployment of basic AD infrastructure as a foundation for more complex cascading biorefineries such as the one proposed in this study. They also highlighted the need for comprehensive sustainability assessments, public engagement, and coherent policy frameworks to de-risk investment.
Integrating experimental validation, techno-economic assessment, and stakeholder insights, this thesis provides a comprehensive evaluation of a dynamic AD-based biorefinery model for enhancing alternative uses of grass. The findings demonstrate that with modest policy support and continued process improvements, such systems could provide Irish farmers with a viable farm diversification pathway while contributing to Ireland’s climate and circular bioeconomy objectives. The integrated methodological framework developed offers a blueprint for the development of grass biorefineries in Ireland and beyond.
Description
Keywords
Anaerobic digestion , Biorefinery , Biogas on-demand , Volatile fatty acids , Techno-economic assessment , Circular bioeconomy
Citation
Shinde, R. D. 2025. Enhancing the alternative use and circularity potential of grass through an anaerobic digestion-based biorefinery approach. PhD Thesis, University College Cork.