## 2 Technologies for Purifying the Tail Gases from N2O and NOx
These are mainly different variants of EnviNO\({}_{\rm x}\) (Tyssen Krupp/UHDE) process, consisting of:  
* Variant I--catalytic decomposition of N\({}_{2}\)O and selective catalytic reduction of NOx with ammonia in the tail gases stream, which is used in the plants with the tail gases temperature in the range of 425-520 \({}^{\circ}\)C [27; 43; 53; 54]: \[2{\rm N}_{2}{\rm O}\to 2{\rm N}_{2}+{\rm O}_{2}\] (11) \[6{\rm NO}_{2}+8{\rm NH}_{3}\to 7{\rm N}_{2}+12\;{\rm H}_{2}{\rm O}\] (12) \[4{\rm NO}+{\rm O}_{2}+4{\rm NH}_{3}\to 4{\rm N}_{2}+6{\rm H}_{2}{\rm O}\] (13)* Variant II--selective catalytic reduction of NO\({}_{\rm x}\) with ammonia (reactions 12 and 13) and selective catalytic reduction of N\({}_{2}\)O with hydrocarbons (reaction 14, e.g., methane) in the tail gases stream, most commonly used in the plants with the tail gases temperature \(<\)425 \({}^{\circ}\)C (guaranteed operating temperature range 340-520 \({}^{\circ}\)C) [27; 53; 54]: \[(3{\rm n}+1)\;{\rm N}_{2}{\rm O}+{\rm C}_{\rm n}{\rm H}_{2{\rm n}+2}\to(3{\rm n }+1)\;{\rm N}_{2}+{\rm n}\;{\rm CO}_{2}+({\rm n}+1)\;{\rm H}_{2}{\rm O}\] (14)  
The EnviNO\({}_{\rm x}\) technology allows N\({}_{2}\)O removal with 98-99% efficiency and for reducing NO\({}_{\rm x}\) emission to \(<\)10 ppm. Above 45 EnviNO\({}_{\rm x}\) units operate in nitric acid plants worldwide [55]. The temperature range in which the tail gases purifying from nitrogen oxides is carried out is 300-650 \({}^{\circ}\)C.  
Casale S. A. additionally proposed a solution consisting of three-stage purifying of the tail gases from NO\({}_{\rm x}\) and N\({}_{2}\)O. NO\({}_{\rm x}\) is reduced with ammonia on a first and third bed of a zeolite catalyst modified with iron or copper ions. On the second bed of iron-modified zeolite catalyst, N\({}_{2}\)O is removed via a catalytic decomposition. The efficiency of this solution in purifying the tail gases from NO\({}_{\rm x}\) is above 95% [56]. The efficiency of the SCR process is verified based on the maximum NO\({}_{\rm x}\) and N\({}_{2}\)O conversion (reduction or decomposition), that can be achieved on the catalyst bed, and ammonia slip (translating into ammonia consumption index). The conversion degree is determined by measuring the concentration of NO\({}_{\rm x}\) and N\({}_{2}\)O at the reactor inlet and outlet during the tail gases' purifying process. In addition, the pressure drop across the catalyst bed is also measured, especially in the case of high-pressure plants. Its value translates into the amount of energy recovery in the expansion turbine. The main parameters that affect the catalyst efficiency in the deNO\({}_{\rm x}\) (SCR) reactor are GHSV (Gas Hourly Space Velocity), NH\({}_{3}\)/NO\({}_{\rm x}\) molar ratio and operating temperature. GHSV determines the residence time of the tail gases components in the reaction zone (expressed as the ratio of the gas flow rate and the catalyst bed volume). The value of this parameter increases with the increase in the tail gases flow rate, which may translate into a reduction in the nitrogen oxides conversion on a given catalyst bed. For the plants with high gas flow rates, the deNO\({}_{\rm x}\) and deN\({}_{2}\)O process efficiency can be improved by lowering the linear gas velocity, using a catalytic basket of appropriate design. In the deNO\({}_{\rm x}\) process, controlling the NH\({}_{3}\)/NO\({}_{\rm x}\) molar ratio is necessary to minimize NH\({}_{3}\) slip and reduce the ammonia consumption index while maintaining a high NO\({}_{\rm x}\) conversion. At the operating temperature lower than the optimum for a given catalyst type, the concentration of a reducing agent in the tail gases stream has little effect on the NO\({}_{\rm x}\) reduction. It may contribute to the increase in ammonia slip. The efficiency of the deNO\({}_{\rm x}\) process increases with increasing NH\({}_{3}\)/NO\({}_{\rm x}\) molar ratio at the temperatures close to the optimum temperature for a given catalyst.