One-Reactor Scheme for NO and N2O Low Temperature Abatement from Tail Gas in Nitric Acid Production
Nitric acid production is one of the major sources of NO and N2O in the chemical industry. To reduce NO emission in nitric acid production, conventional de-NOX systems for selective catalytic reduction (SCR) NO by ammonia (UKL-7 and AK-72M plants) or non-selective by CH4 (AK-72 plant) are used in Russia. Concentration of N2O in tail gases depends not only on the primary platinum gauzes catalyst selectivity but on the de-NOX conditions as well, because N2O concentration may substantially increase in the case of SCR by ammonia. Emission of N2O from one UKL-7 plant was found to be more than 450 tons per year. High impact of N2O on the environment as greenhouse gas, and the international agreements and regulations require the development of efficient and economical systems for N2O abatement in nitric acid production.
Combination of de-NOX SCR unit with N2O decomposition unit in one reactor is one of attractive ways. To this purpose, EnviNOx® technology developed by Uhde is the exemplary embodiment of combined removal of N2O and NOX from tail gas at temperatures 330-600°C . A similar combined system was proposed  for Russian UKL-7 plant which was comprised of vanadia-alumina de-NOX catalyst working at 220-300°C as the primary bed, and Fe-zeolite de-N2O catalyst working at 350-400°C as the secondary bed.
This work is devoted to the development of de-N2O catalyst working at lower temperature 250-300°C, and modeling one-reactor scheme for combined processing of NOX and N2O in tail gases. Experimental data on the catalytic activity of different oxide systems for N2O decomposition at moderate temperatures are shown in Table 1. The oxide catalyst based on NixCo3-xO4 doped with Cs cations has the spinel-like structure and is the most active and promising system for N2O decomposition at 250-300°C.
For combined NOX+N2O abatement, it was proposed to use the commercial SCR V-Al catalyst AVK-10 at the 1st stage and the proprietary catalyst under this study at the 2nd stage. Modeling of N2O decomposition process in the 2nd bed was carried out under the following conditions that are typical for the 1st bed outlet: [N2O] 1500 ppm, [O2] 2.5%, [H2O] ≈3%, [NOX] 50 ppm, [NH3 ]/[NOX] ≈1.1; temperature 250°С, pressure 0.101 MPa, GHSV 7200 h-1.
Table 1 Catalytic activity of oxide catalysts for N2O decomposition
Catalyst T*,°С Reaction conditions
(feed gas composition, gas carrier, and GHSV) Refs.
Fe/ZSM-5 450 700 ppm N2O, 1000 ppm NO, 1100 ppm NH3, 4.5% vol. O2, 2% vol. H2O, in He; 7200 h–1 
Pb0.04CoOx 414 2000 ppm N2O, 500 ppm NO, 5% vol. O2, in He; 20000 h–1 
K - Co2.6Zn0.4O4 350 900-1050 ppm N2O, 400-530 ppm NO, 1100 ppm NH3, 1-1.3% vol. O2, 0.3-0.5 % vol. H2O, in He; 20000 h–1 
Cs - NixCo3-xO4 300 1500 ppm N2O, 1000 ppm NO, 1100 ppm NH3, 2.5% vol. O2, 3% vol. H2O, in He; 7200 h–1 This
* The lowest reaction temperature at which conversion of N2O is greater than 90%.
The loading of the commercial reactor 3.8m ID by the catalyst in the form of 3-5mm granules was predicted by simulation. At the bed height 0.4-0.5 m, temperature 250°C the conversion of N2O was no less than 98%, thus reducing N2O emission in tail gas to 30 ppm at the lowest.
According to the current results, a one-reactor approach for low-temperature combined abatement of nitrogen oxides at the UKL-7 plant can be realized on the existing SCR reactor in 2 consecutive adiabatic beds. Using the industrial vanadia catalyst for SCR of NOX by ammonia in the 1st bed and Ni-Co catalyst for N2O decomposition in the 2nd bed shall secure the expected conversion, respectively, NOX >97%, N2O >98%, thus meeting the mandatory emission levels.