Exploiting synergies between sustainable ammonia and nitric acid production: A techno-economic assessment

Abstract

The decarbonization of the fertilizer industry is an important step towards a climate-neutral society. Nitric acid produced via the Ostwald process is a key precursor to nitrogen-based fertilizers, with the production of ammonia representing an important intermediate step. This work focuses on the search for synergies between the sustainable production of ammonia feedstocks – hydrogen and nitrogen – and nitric acid production in the Ostwald process: Oxygen a by-product of sustainable hydrogen and nitrogen production was considered for injection in the Ostwald process at 4 points. A techno-economic assessment was performed to identify the technical and economic potential of this air enrichment. In addition, the safety implications of applying a higher oxygen concentration in the ammonia combustion step of the Ostwald process was evaluated based on explosion limit simulations. The techno-economic assessment was based on a mono-pressure nitric acid plant with a plant capacity of 700t/d of pure nitric acid at a concentration of 60wt% in water. The investigations of injecting oxygen at four different points of the plant show that daily nitric acid production can be increased by 0.31% and final NOx concentration of the absorption column can be reduced by up to 43.6% by enriching the main air pipeline with oxygen. Optimal oxygen injection could reduce the capital costs by 0.41M€. Nevertheless, the operating costs dominated the final cash flow analysis with ammonia costs accounting for ca. 60% of the total operating costs. The CO2 price would need to increase to at least 233€/t to compensate for the higher green ammonia price and equalize the minimum selling price. As the oxygen injection at the main air line comes with safety risks due to the ammonia explosion limits, we evaluated the effect of O2-enrichment on the explosion limits of NH3-O2-N2 mixtures. The foreseen oxygen enrichment reduced the lower explosion limit only marginally. For a medium pressure plant at 5.8bar the lower explosion limit is reduced to 10.3% for the highest considered oxygen injection case, which is close to the applied ammonia concentration.