Nitrous oxide (N2O)

Nitrous oxide, with the formula N2O, is a colourless nitrogen oxide with low water solubility. In the fertiliser industry, it forms chiefly during the catalytic oxidation of ammonia into nitric acid via the Ostwald process, an intermediate stage in the manufacture of nitrogen fertilisers such as ammonium nitrate. The gas is also produced in soils through microbial denitrification, particularly following application of nitrogenous fertilisers. Its atmospheric lifetime is roughly 120 years^[1].

Emissions from nitric acid production

N2O formation during ammonia oxidation depends closely on operating parameters. Elevated temperature promotes ammonia combustion yet lowers nitric oxide (NO) yield by raising diazote (N2) and nitrous oxide output^[2]. Plants typically run between 850 and 950 °C to achieve NO yields above 96 %; beyond 950 °C, catalyst losses through volatilisation increase^[2]. The ammonia/air ratio directly affects reaction temperature: a 1 % rise in ammonia proportion raises it by approximately 68 °C^[2]. N2O, unstable at these temperatures, partially decomposes into N2 and O2, a phenomenon encouraged by extended residence time and high temperature in the catalytic reactor^[2].

Emission levels associated with best available techniques (BAT) vary by installation type. For new units, emissions fall between 0.12 and 0.6 kg N2O per tonne of 100 % nitric acid, equivalent to 20 to 100 ppmv^[3]. For existing installations, the range extends from 0.12 to 1.85 kg/t, corresponding to 20–300 ppmv^[3]. Industry and one Member State challenged these levels for existing sites, arguing that N2O abatement techniques remained under development and that De-N2O catalyst lifetimes were still uncertain; they sought inclusion of a 2.5 kg/t value for these units^[3].

Abatement techniques

Several routes exist to curtail N2O emissions. Optimising the NH3/air ratio, pressure and temperature of the oxidation stage, together with monitoring catalyst performance, constitute first-line levers^[3][4]. Extending the reaction chamber in new units allows thermal decomposition of N2O^[3][4]. Catalytic abatement in the reactor or in post-combustion, alongside combined NOx/N2O treatment in tail gases, represent complementary solutions^[3][4].

One M/M-type plant, fitted with an improved Heraeus-type platinum catalyst, achieved levels of 500 to 1,000 ppm N2O over half-year campaigns, averaging roughly 800 ppm, representing a 30 to 50 % reduction^[5]. Alternative oxidising catalysts, platinum-free, can cut N2O by 80 to 90 %, but at the cost of lower NO yield and higher ammonia consumption^[5]. Dual-layer systems, combining platinum gauzes with a non-platinum oxide catalyst, reduce platinum usage by 40 to 50 % and losses of this metal by 15 to 30 %^[5].

European regulation of fertilising products

Regulation (EU) 2019 on fertilising products governs the use of denitrification inhibitors in Union fertilising products^[6]. Such an inhibitor must slow or block the conversion of nitrate (NO3–) into diazote (N2) without affecting nitrification, so as to limit N2O formation^[7][8]. To gain recognition, it must demonstrate at least a 20 % reduction in the nitrous oxide release rate in an in vitro test, measured 14 days after application at a 95 % confidence threshold^[7][8]. This requirement applies to products where at least 50 % of total nitrogen is present as ammonium (NH4+) and urea (CH4N2O)^[8].

Agricultural and climatic context

Agricultural N2O emissions constitute the predominant anthropogenic source of this gas, whose atmospheric abundance has risen by more than 20 % since pre-industrial times^[1]. Its current contribution to warming stands at roughly 0.1 °C and continues to grow^[1]. Nitrous oxide is moreover the most significant ozone-depleting substance currently emitted^[1]. In adipic and nitric acid plants, relatively low-cost abatement measures could virtually eliminate these emissions^[1].