Closing the loop in agriculture: life cycle assessment from manure to hydrogen and biofertilizer

Abstract

Dairy sector accounts for 14–20 % of global greenhouse gas (GHG) emissions where the major emissions come from manure-management and fertilizers consumption. To address these hotspots, a closed-loop system is developed to valorise livestock manure for simultaneous hydrogen and fertilizer production, thereby fulfilling on-farm energy and nutrient needs. The system, modelled using SuperPro Designer®, consists of anaerobic digestion (AD) for biogas generation, followed by steam methane reforming and a water-gas shift reaction for hydrogen production. The digestate, enriched in nitrogen, phosphorus, and potassium, serves as a biofertilizer, reducing reliance on synthetic alternatives. Simulation results demonstrate that the system can meet the farm's energy demands and supply up to 32 % of its fertilizer requirements. Life Cycle Impact Assessment (LCA) was conducted using primary data from both the farm and the simulation model, employing an economic allocation approach within a cradle-to-gate boundary. The functional unit was defined as 1 kg of fat and protein corrected milk (FPCM). Two scenarios were evaluated: a conventional intensive dairy farm (Scenario-1) and the same farm with the proposed system (Scenario-2). Scenario-2 resulted in a significant shift in economic value towards hydrogen (63.01 %) and achieved a 66 % reduction in global warming potential per kg of FPCM without compromising milk yield. These results highlight the potential of integrated manure valorisation systems to promote circularity and reduce the environmental footprint of dairy farming. However, there is a need to study the feasibility and scalability of this approach for different farms, climatic-regions and geographical constraints.