Techno-environmental assessment of small-scale Haber-Bosch and plasma-assisted ammonia supply chains

Abstract

Haber-Bosch (HB) process, the main method for ammonia (NH3) production, contributes to near 2% of the global carbon emissions because the hydrogen input is obtained from fossil sources. NH3 production is concentrated in a few countries, adding emissions due to global distribution. Distributed plants next to farmers and fed by renewable energy can reduce these impacts, as well as NH3 storage, shortage risks, and price volatility. Distributed plants cannot reach low NH3 production costs as centralised plants, but they can be promoted by the environmental benefits of its products lifecycles. Therefore, life cycle assessments of NH3 production pathways and specific modelling for NH3 transport in Australia were performed, from cradle-to-site, to identify the influence of storage, transport, and energy sources in their environmental profiles. The carbon footprint of centralised production was up to 2.96 kg.CO2-eq/kg.NH3, from which 29.3% corresponded to transport. Local production demonstrated substantial avoided transport impacts and that CO2-eq can reach reductions over 100% when including co-product credits such as oxygen and carbon black. Local plants using electrolysers to supply mini-HB loops obtained rates of 0.12, −0.52, and −1.57 kg.CO2-eq/kg.NH3 using electricity from solar, wind, and biogas (other than manure) sources, respectively. The alternative using high temperature plasma reactor instead of electrolyser obtained its best rate of −0.65 kg.CO2-eq/kg using biogas different from manure. At farm electrolyser-based plants using novel non-thermal plasma reactors, considering potential energy yields and simplified NH3 separation technology, could reach a rate of −1.07 kg.CO2-eq/kg.NH3, using solar energy. Among the assessed pathways, the most notable impact was on freshwater eutrophication in the electrolyser-based plants generating reductions up to 290%, due to oxygen credits. Despite these results, the use of solar energy raises concerns on land use and terrestrial ecotoxicity due to the area needed for solar farms and the manufacture of their components.