Nitrous oxide emissions from nitric acid production
Nitrous oxide (N2O) is naturally present in the atmosphere as part of the earth’s nitrogen cycle and originates from various natural sources. However, human activities such as agriculture (in particular the use of synthetic fertilisers), fossil fuel combustion, wastewater management, and industrial processes are increasing the amount of N2O in the atmosphere. According to the 6th Assessment Report of the Intergovernmental Panel on Climate Change from 2020, the concentration of N2O in the atmosphere has increased by 23 % since pre-industrial times, and N2O is the third largest contributor to climate change. The N2O concentration is the highest measured during the last 800.000 years . Agricultural activities, such as fertiliser use, represent the primary source of N2O emissions. Other sources are manure management, transportation and stationary combustion. The two industrial processes contributing most to N2O emissions are the production of nitric acid and of adipic acid. N2O emissions from adipic and nitric acid production contribute to about 0.2% of global emissions (roughly 100 MtCO2eq), which is equivalent to 24% of non-CO2 GHG emissions from key industrial processes. Besides, N2O is the most important ozone-depleting substance not covered by the Montreal Protocol.
Nitric Acid Production
The main industrial use for nitric acid (HNO3) as a raw material is the production of fertilisers, which accounts for around 75 – 80% of the acid produced annually. Here, HNO3 is neutralised using ammonia to form ammonium nitrate NH4NO3, which is predominantly used in agriculture as a high nitrogen fertiliser but is also often used in explosives and as an oxidising agent in rocket fuels.
The manufacture of nitric acid is likely to increase as ever-expanding food production will ensure continued strong demand for ammonium nitrate fertilisers. There is also growing demand for explosives in the mining and construction sectors.
Nitrous oxide is an undesired by-product of nitric acid production formed unintentionally during ammonia oxidation.
Typical N2O concentrations at the absorber outlet are between 300-1,500 ppm and a typical nitric acid plant emits 6-9 kg of N2O per tonne of HNO3 produced. This corresponds to around 1.7-2.9 tonnes of CO2 equivalents.
In many cases, the N2O is just vented into the atmosphere, where it functions as a GHG with a global warming potential of 273 on a 100-year time horizon. That means that N2O is 273 times more harmful to the climate than CO2. In addition, nitrous oxide molecules stay in the atmosphere for an average of 114 years before being removed by a sink or destroyed through chemical reactions. 
Abatement of N2O emissions from nitric acid production
N2O emissions from nitric acid production can be reduced relatively easily and at a low cost compared to other greenhouse gas (GHG) abatement options. Technical abatement costs usually range from as low as €0.50 to €5.00/tCO2eq, depending on the abatement technology employed and specific plant characteristics.
There are three main abatement technologies classified as primary, secondary and tertiary solutions. Primary and secondary technologies are applied in the ammonia oxidation stage of the nitric acid production process. Whereas tertiary technologies reduce N2O from the tail gas by installing a catalytic or thermal reactor downstream of the absorption stage at which the final product nitric acid is produced.
Among the primary technologies, the combination of an optimised oxidation reactor and high-efficiency primary catalysts can achieve up to 40% abatement rates. However, these options are not covered by the financial support of NACAG.
Secondary N2O abatement solutions consist of a catalyst located right underneath the primary gauzes. Its operation requires no additional heat or energy input. The very high-temperature levels inside the ammonia oxidation reactor are sufficient to ensure effective operation. The highest abatement efficiencies reached by these technologies are around 95 per cent.
Alternatively, tertiary N2O abatement technology is installed as a separate reactor, where the tail gas is treated before exiting the production process. A wide spectrum of configurations has been developed by different technology providers so that they can obtain high performances for a large range of temperatures of the tail gas, or in combination with NOx abatement, typically achieving more than 95 per cent of abatement rate.
Policy advice – The Nitric Acid Climate Action Group (nitricacidaction.org)
The NACAG Secretariat is providing advice to governments in terms of policy questions of nitrous oxide abatement.
 Climate Change 2021: The Physical Science Basis The Working Group I contribution to the Sixth Assessment Report addresses the most up-to-date physical understanding of the climate system and climate change, bringing together the latest advances in climate science. Summary for policymakers, p. 8, Summary for Policymakers (ipcc.ch)
 IPCC, 5th AR, Working Group III Report ”Climate Change 2014: Mitigation of Climate Change“, Technical Summary, p. 42 and (10) Industry, Table 10.4, p. 753
 Intergovernmental Panel on Climate Change: Climate Change 2022 Mitigation of Climate Action (2022)