## 2 Technologies for Purifying the Tail Gases from N2O and NOx
### Radial Flow Reactor
It consists of a three-stage tail-gas purification from the nitrogen oxides in one reactor, including NOx reduction with ammonia on the first and third catalytic beds and N2O decomposition on the second catalytic bed. This solution may be advantageous if iron-modified zeolite catalysts are used in deN2O process, for which the co-catalytic effect of the NOx presence on N2O decomposition has been found. Figure 5 shows an exemplary arrangement of deNOx and deN2O catalyst beds in a radial reactor, according to the [PERSON] solution. The direction of the tail-gas flow across the reactor is shown by black arrows. The reducing agent is introduced into the tail-gas stream upstream of the first and third catalyst beds. On the first deNOx catalyst bed, the NOx content in the gas stream is reduced to 50-200 ppm. The presence of these oxides upstream of the second catalyst bed increases the efficiency of N2O decomposition (the following reaction occurs: NO + N2O \(\rightarrow\) NO2 + N2). The tail-gas stream leaving the deN2O catalyst bed is collected in a central collector, to which another portion of the reducing agent, necessary to reduce NOx concentration to 25 ppm, is introduced. The use of the Casale solution allows for reducing NOx (vanadia-titania or Fe-zeolite catalyst) and N2O (Fe-zeolite catalyst) emissions to the levels of <25 ppm and <30 ppm (efficiency >95%), respectively, with ammonia slip no greater than 2 ppm. Its advantage is the more efficient use of the reducing agent (ammonia) and the possibility of using part of the NOx amount in the N2O decomposition process. The disadvantage is the complex design of the reactor, which hinders the operation of loading and unloading catalysts to the baskets. Additionally, the ammonia injection/dosing, and the control and measurement system are more complex. It is necessary to install two separate ammonia injection zones, use an additional measurement point of NH3 and NOx concentration in the tail-gas stream  
Figure 5: Radial reactor with triple-bed catalytic system, with catalytic beds arranged co-axially along the reactor’s diameter and axis: A—deNOx catalyst bed, B—deN2O catalyst bed (based on patent [56]). (between the second and third catalyst beds), as well as another other device to control the amount of ammonia introduced upstream of the third catalyst bed [56; 90]. The radial reactor, with a construction similar to the Casale solution, can also be used in the EnviNO\({}_{\mathrm{x}}\) technology. Figure 6a,b show an exemplary arrangement of deNO\({}_{\mathrm{x}}\) and deN\({}_{\mathrm{2}}\)O catalyst beds for the first and second variants of the EnviNO\({}_{\mathrm{x}}\) processes. In the first case, a mixture of ammonia and tail gases are introduced into a catalytic reactor and directed to the deNO\({}_{\mathrm{x}}\) catalyst bed. After reducing the NO\({}_{\mathrm{x}}\), the gas flows through the deN\({}_{\mathrm{2}}\)O catalyst bed, where direct catalytic N\({}_{\mathrm{2}}\)O decomposition occurs. The use of the deNO\({}_{\mathrm{x}}\) process in the first stage of tail-gas purification is advantageous. As a result of this process, the temperature of the purified gas increases before it enters the deN\({}_{\mathrm{2}}\)O catalyst bed. For the second variant of the EnviNO\({}_{\mathrm{x}}\) process, the deN\({}_{\mathrm{2}}\)O catalyst bed is installed in the upper annular catalytic basket, while the deNO\({}_{\mathrm{x}}\) catalyst is installed downstream of the deN\({}_{\mathrm{2}}\)O catalyst bed. In this case, it is necessary to use two types of reducing agents, so there are two possibilities. Hydrocarbon and tail-gas mixtures are introduced into the reactor to reduce N\({}_{\mathrm{2}}\)O on the first catalyst bed, while ammonia (for NO\({}_{\mathrm{x}}\) reduction) is introduced into the space between the deN\({}_{\mathrm{2}}\)O and deNO\({}_{\mathrm{x}}\) catalyst beds (Figure 6b). Another possibility is to introduce a mixture of tail gases with both reducing agents. Figure 6 shows one possible way of injecting ammonia into the gas stream.