Evaluation of the environmental performance of novel anaerobic digestion biorefineries for the development of a sustainable circular bioeconomy in Ireland

The circular bioeconomy (CBE) is a novel paradigm that seeks the integration of circularity principles into the life cycle of renewable biological resources, to help keep humanity within safe operating conditions, respecting planetary boundaries. This doctoral thesis advances state-of-the-art environmental evaluation of anaerobic digestion (AD) biorefineries as core technological platforms for the development of a CBE. The work comprises a series of distinct yet related studies. Initially, a critical evaluation of 76 peer-reviewed studies on environmental life cycle assessment (LCA) of biorefineries was conducted to establish the state-of-the-art. Inconsistent approaches to model biogenic emissions, digestate management, and product substitution (i.e., avoiding production of equivalent products in other systems) were major methodological challenges. To overcome these limitations, a novel, comprehensive, and open-access LCI model, LCAD 2.0, was developed and applied. First, a systematic evaluation of 150 prospective AD-biorefinery configurations using cattle manure, food waste, and grass elucidated critical factors defining environmental performance and nutrient circularity, inter alia: type of feedstock, energy substitution potential, fugitive methane emissions, emissions during digestate application, and the application of carbon capture technologies. AD-biorefinery systems using organic waste as feedstocks, in particular those with high methane yield, such as food waste - offer the strongest potential for environmental mitigation, especially when considering avoided impacts from counterfactual waste management. Then, an integrated techno-economic and environmental assessment of a biomethane plant processing food waste at industrial scale in Ireland was undertaken. This revealed significant economic challenges linked to high volatility and market fluctuations after disruptive events such as the war in Ukraine and COVID-19. The calculated levelised cost of energy was 2.9 times higher than values previously defined in 2019 and 1.6 times the benchmark established in the National Biomethane Strategy, making policy incentives crucial for financial viability. On the other hand, although environmental mitigation potential is possible, with climate mitigation up to 222 kgCO2-eq/tonne of food waste digested, this mitigation potential is contingent upon the type of energy used in the plant, proper control of biogenic emissions, and efficient digestate application according to the calendar defined by the Nitrates Directive. Finally, six novel biorefinery configurations producing biofuel, biofertilisers, platform chemicals, and elemental carbon were assessed for environmental performance. All six evaluated configurations achieved environmental mitigation potential, with maximum climate mitigation between 340 and 542 kgCO2-eq/tonne food waste, though simpler configurations generally outperformed more complex ones due to energy-intensive purification and chemical consumption. This research contributes to the body of knowledge on AD-biorefinery sustainability, by applying comprehensive expanded boundary LCA to novel AD-biorefinery configurations. It presents LCA practitioners and researchers with a new tool to overcome methodological challenges linked to data collection, advancing robust environmental evaluation of prospective biorefineries needed to power the CBE.

Funder

Environmental Protection Agency (IE)

Publisher

University of Galway

Rights

CC BY-NC-ND

cbn

Collections

University of Galway Theses (PhD Theses)