## 2 Technologies for Purifying the Tail Gases from N2O and NOx
### Axial Flow Reactor  
The gas stream flows through the catalyst bed parallel to the reactor axis in axial flow reactors. The catalyst bed is installed in a cylindrical basket or on a perforated plate placed at the bottom part of the reactor. This design allows more space inside the reactor for the catalyst bed installation. If a cylindrical basket is used, it is necessary to seal it around the reactor perimeter to avoid gas by-passing.  
The use of a seal is not required when the catalyst bed rests on a perforated plate and is in contact with the side walls of the reactor. Figure 3a,b show an exemplary interiors of the \(\mathrm{deNO_{x}}\)/\(\mathrm{deN_{2}O}\) reactor, with the construction that forces the axial flow of the tail-gas stream through the catalyst bed. The black arrow shows the direction of the purified gas flow schematically. A perforated plate, installed upstream of the catalyst bed (1), is used to improve gas distribution over the catalyst bed.  
To increase the gas purification efficiency in an axial flow reactor, without increasing the gas flow resistances, the diameter of the reactor should be increased. However, this requires using a thicker, heavier and more expensive reactor shell, which will be subject to faster exploitation. When designing a reactor with a larger diameter, the plant's production capacity and the efficiency of the media transport equipment should be considered.  
The advantages of the axial flow reactor are the easy loading and unloading of the catalyst and the simple design of the catalytic basket. With the same space inside the reactor, a larger catalyst bed volume can be installed in the cylindrical basket compared to the other solutions (described below). It is due to the absence of additional baffles and empty spaces, constituting the zone of the gas inflow onto the catalyst bed. Reactors with a design that forces the axial gas flow through the catalyst bed generate greater pressure drops compared with reactors in which the gas flows through the bed perpendicular to the reactor axis (radial or lateral flow). In addition, they are characterized by a higher linear gas flow velocity, which translates into a lower conversion of nitrogen oxides in relation to the radial or lateral flow reactor. In the axial flow reactor, the pressure drop and catalyst efficiency can be influenced by changing the form and size of the shaped catalyst body.  
In axial flow reactors, monolithic catalysts can be installed (e.g., CuO-NiO on a ceramic monolith based on alumina-silicates, BASF O4-85 catalyst based on vanadium oxide) without fear of too-high pressure drops. Their specific \"architecture\" enables high efficiencies of the tail gases purifying from nitrogen oxides at the low pressure drops [61; 65; 81; 82; 83; 84; 85].  
When using a dual-bed catalytic system, e.g., \(\text{deNO}_{\text{x}}/\text{deNO}_{\text{x}}\), the catalyst beds can be installed inside one catalytic basket and separated from each other by a perforated plate or gauze (mesh size should be smaller than the size of the shaped catalyst body). The \(\text{deNO}_{\text{x}}\) catalyst bed should be placed in the upper part of the catalytic basket, while the ammonia is mixed with the tail gases before entering the reactor. If the arrangement of the catalyst beds is changed, they should be separated from each other by a space, into which gaseous ammonia is introduced and mixed with the tail-gas stream. The gaseous ammonia pipeline would then be installed on the side wall of the reactor. Ammonia could be introduced into the gas stream using an ammonia injection grid.  
The construction of the catalytic reactor, operating in the axial flow mode, can be easily converted into the axial-radial or radial flow mode, without significantly modifying the reactor interior. Such modification, however, reduces the space available for the catalyst bed.