Energy and exergy assessment of anaerobic digestion process for ammonia synthesis: Toward a sustainable water-energy-food nexus

Abstract

This study investigates a novel process that combines anaerobic digestion with ammonia synthesis, partial oxidation of biogas, water-gas shift reaction, and an advanced cryogenic air separation unit. Using Aspen Plus for process modeling, the system produces ammonia and digestate, contributing to the sustainability of the water-energy-food nexus, particularly benefiting agriculture and animal husbandry. Cow manure undergoes anaerobic digestion, producing digestate and biogas, which is refined to generate hydrogen for ammonia synthesis. The process achieves a cycle exergy efficiency of 62.73 % and an overall system energy efficiency of 67.04 %, yielding 8.4 kg/day of ammonia and 1961.20 kg/day of digestate. Sensitivity analysis shows that increasing anaerobic digestion temperature by 60 K boosts methane production by 86.83 %, while higher pressures decrease it by 27.28 %. In the partial oxidation reactor, raising the temperature increases hydrogen production by 4.19 %, with pressure reducing hydrogen output by 0.79 %. This integrated approach provides significant benefits for sustainable practices in agriculture and animal husbandry.