## 2 Technologies for Purifying the Tail Gases from N2O and NOx
### Radial Flow Reactor  
The radial gas flow reactor has a more complex design than the axial flow one and the catalytic basket has an annular shape. The side walls of the basket are perforated, while  
Figure 3: Reactor for heterogeneous catalytic processes with a structure, that forces the axial gas flow through the catalyst bed. (**a**) Reactor interior with an empty catalytic basket: 1—perforated plate improving mixing of ammonia with the tail gases, 2—bottom perforated basket wall, 3—solid side wall of the basket. (**b**) Catalytic basket with a double catalyst/s bed: A—deNO\({}_{\text{x}}\) catalyst bed, B—deN\({}_{\text{2}}\)O catalyst bed. its lower part is gas-impermeable. After loading the catalyst into the basket, a solid top cover plate is installed, the side walls of which are usually several centimetres high. They protect the basket against gas bypassing in the event of catalyst bed settling (e.g., due to catalyst sintering or settling the bed under its weight). The gas stream entering the reactor first flows in the axial direction, and it hits the top cover plate of the catalytic basket. Then, it enters the annular space between the reactor wall and the side wall of the basket. It flows radially through the catalyst bed into the central tube, through which it exits the reactor. The gas flow through the catalyst bed should be mainly radial, preferably with no axial velocity component. The share of axial velocity components may cause an effect similar to \"back-mixing\", reducing the gas purification efficiency (mixing of gas streams with different degrees of reactants conversion). In radial flow reactors, two gas flow directions are possible: from the central part of the reactor towards its side walls or from the annular space between the basket and the reactor side wall to the central part of the reactor, through which the gas leaves it. Figure 4a,b show the interior of a radial reactor with a single empty annular catalytic basket and two annular catalytic baskets with NO\({}_{\text{x}}\) and N\({}_{\text{2}}\)O catalyst beds. The black arrows show one of the possible directions of the gas flow through the catalyst basket. The tail gases mixed with the reducing agent are fed to the reactor through the inlet. A large part of the gas stream hits the top cover plate of the catalytic basket and changes direction. Then, it enters the space between the reactor and basket side walls. The gas stream flows radially through the annular-shaped catalyst beds to the central pipe and exits the reactor at the outlet. The inner side wall of the outer annular basket is simultaneously the outer side wall of the inner annular basket. To ensure radial flow, the bottom part of the catalytic basket is gas-impermeable, and the upper part is closed by the top cover plate, while the side walls are perforated. Figure 4: Reactor for heterogeneous catalytic processes with a structure that forces radial gas flow through the catalyst bed. (**a**)—basket with one annular space, in which one catalyst bed can be installed: 1—perforated plate improving gas flow distribution and mixing, 2—solid top cover plate, 3—outer perforated side wall of the basket, 3a—perforated inner side wall of the basket, 4—solid bottom plate of the basket. (**b**)—dual-bed catalytic system: A—deNO\({}_{\text{x}}\) catalyst bed, B—deN\({}_{\text{2}}\)O catalyst bed. When designing radial flow reactors, certain conditions should be taken into account:
* It is necessary to ensure uniform gas distribution over the catalyst bed, along the entire length of the catalytic basket to achieve higher nitrogen oxide conversion (uneven gas distribution results in uneven exploitation of the catalyst bed and basket, which may lead to corrosion);
* Settling of the catalyst bed during operation may result in an empty space formation in the upper part of the bed and gas bypassing. In total, 5-20% excess of the catalyst bed should be used in relation to the amount required to achieve the assumed nitrogen oxides conversion, compensating for bed settlement.