## 2 Technologies for Purifying the Tail Gases from N2O and NOx
### Radial-Axial Flow Reactors  
Many methods of deNOx and deN2O catalyst beds arrangement in the reactor for purifying the tail gases from nitrogen oxides are described in the patent literature. These are mainly ThyssenKrupp Uhde and Casale solutions. Some of them concern the use of  
Figure 7: Radial flow reactor with deflectors for heterogenous catalytic processes (based on patent [91]): 1—solid plate constituting the bottom part of the basket, 2—deflectors, 3—perforated side wall of the basket, 4—gas-impermeable upper wall of the basket, A and B—deNOx (**a**) and deN2O (**b**) catalyst beds. two different catalyst bed configurations, forcing the gas stream to flow in both radial and axial directions. Figure 8a,b show two variants of the catalytic reactor in a longitudinal section with two catalyst beds. The black arrows indicate the direction of the gas flow. In the first case, the tail gases mixed with ammonia are introduced onto the deNOx catalyst bed, which forces it to flow in the axial direction (the upper and lower part of the catalytic basket is perforated, while the side walls are gas-impermeable). The tail-gas stream, purified at the first stage from NOx, flows to a space surrounded by the deN2O catalyst bed installed in the annular basket. The side walls of the basket are perforated, while the top and bottom parts are gas-impermeable. The gas stream flows radially through the deN2O catalyst bed from the central part of the reactor towards its side walls and is directed to the outlet. This type of reactor could be used for conducting the EnviNOx process according to variant II. In this case, tail gases mixed with ammonia would be fed to the reactor (the first gas purification stage--deNOx process). A hydrocarbon would be introduced into the tail gases upstream of the second bed--deN2O catalyst. The second variant of the catalytic reactor (Figure 8b) could be used in the I variant of the EnviNOx process. The tail gases initially flow along the reactor axis, hit the top cover plate and enter the annular space between the reactor and perforated side walls of the catalytic basket. The tail gases flow radially through the deN2O catalyst bed, on which direct catalytic N2O decomposition occurs. In the central collector, they are mixed with ammonia and directed axially to the deNOx catalyst bed, placed in a cylindrical basket. In Figure 8b, one of the methods of introducing ammonia to the space between the two catalyst beds is shown (pipe with holes). It can also be an ammonia injection grid. The flow of the tail gases in the axial-radial direction can also be realized by installing the annular baskets coaxially along the reactor diameter (Figure 9). Contrary to the solutions  
Figure 8: Reactors for conducting heterogeneous catalytic reaction with the dual-bed catalytic system (A—deNOx catalyst bed, B—deN2O catalyst bed), forcing the gas to flow both in the axial and radial direction: (**a**)—axial–radial flow, (**b**)—radial–axial flow (based on patent [35; 92]). presented in Figure 8a,b, both catalyst beds (deNO\({}_{\rm x}\) and deN\({}_{2}\)O) are installed in the annular catalytic baskets. The side walls of the catalytic baskets and the top plate are perforated, while the bottom walls are gas-impermeable. The first catalyst bed is in the outer basket and the upper part of the inner annular basket. The second catalyst bed is in contact with the first catalytic bed through the perforated side walls of the basket. However, in the upper part it is separated from the layer of the first catalyst bed by a solid baffle/plate. The purified gas stream flows through the first catalyst bed (deNO\({}_{\rm x}\)) in the axial and radial direction, while through the second catalyst bed (deN\({}_{2}\)O) only in the radial one. Then, it is collected in the central space of the reactor and directed to its outlet. The reducing agent (ammonia or ammonia + hydrocarbon) is mixed with the tail-gas stream before entering the first catalyst bed.