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 and sinks. However, human activities such as agriculture, fossil fuel combustion, wastewater management, and industrial processes are increasing the amount of N2O in the atmosphere. According to the 5th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) from 2013, the concentration of N2O in the atmosphere has increased by a factor of 1.2 since pre-industrial times and N2O has become the third largest contributor to climate change.[1] 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 that 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 greenhouse gas (GHG) emissions from key industrial processes.[2]

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 continue, 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 the oxidation of ammonia.

In many cases the N2O is just vented into the atmosphere, where it functions as a GHG with a global warming potential (GWP) of 265 on a 100-year time horizon. That means that N2O is 265 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.[3]

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 GHG abatement options. Technical abatement costs range from €0.90 to €3.20/tCO2eq depending on the abatement technology employed and specific plant characteristics.

There are two main abatement techniques: secondary controls reduce the N2O directly after it is formed in the oxidation reactor; and tertiary controls reduce N2O by installing a catalytic reactor either upstream or downstream of the tail gas expansion unit following the absorption stage at which the final product nitric acid is produced.

The most common secondary N2O abatement catalyst technology consists of a base metal catalyst made of cylindrical pellets. 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. Abatement efficiency can exceed 98 percent, but is usually lower, varying from plant to plant depending on different factors.

Alternatively, tertiary N2O abatement technology can be installed at a point much further downstream, after the absorption column of the plant and before the tail gas stack. Tertiary catalyst units are installed as a separate reactor, and typically reduce N2O by around 95-98 percent.

 

[1] IPCC, Assessment Report Climate Change 2013: The Physical Science Basis, WG1, Technical Summary, p. 52 and 54

[2] 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

[3] EPA, 2016, https://www.epa.gov/ghgemissions/overview-greenhouse-gases#N2O%20references