## 2 Technologies for Purifying the Tail Gases from N2O and NOx  
According to the BAT guidelines [13], the best available technologies for nitrous oxide emission abatement in nitric acid plants are a high-temperature catalytic N2O decomposition in the nitrous gases stream (HT-deN2O), low-temperature decomposition (LT-deN2O) and catalytic reduction of N2O in the tail gases stream. The first of these processes is carried out in an ammonia oxidation reactor, where achieving a profound reduction in N2O emissions, exceeding 95% of the initial value, is technologically very difficult. A low-temperature N2O decomposition or reduction technology is a more practical and effective solution. The implementation of the N2O removal process at the end of the technological line of nitric acid production (downstream of the absorption column) for a given type of catalyst allows for achieving a nearly 100% reduction in N2O emission, by selecting the appropriate catalyst load, as well as the design of the catalytic basket and deN2O reactor [17]. The process of catalytic N2O reduction is mainly carried out in the presence of zeolites (e.g., ZSM-5, BEA, MFI, FAU or FER) modified with iron or copper ions (in the presence of Fe-FER a simultaneous reduction of NOx and N2O is possible) [18]. Hydrogen, or more often hydrocarbons (e.g., methane, propane), are used as a reducing agent:  
\[\mathrm{CH_{4}+4N_{2}O\to CO_{2}+2H_{2}O+4N_{2}\ (T=350\ ^{\circ}C)} \tag{7}\]  
N2O reduction with ammonia is also possible in the presence of iron-modified zeolites. However, this process is non-selective in the presence of NOx and requires an operating temperature >440 \({}^{\circ}\)C to achieve 95% N2O reduction [17; 19; 20]. In a low-temperature catalytic N2O decomposition process, catalysts based on transition metals (Cu, Co, Fe, Ni) on various supports, i.e., ZnO, CeO2, Al2O3, TiO2, ZrO2, hydrodtictic, perovskite and zeolite, can be used. Among the transition metal oxides, spinel-based catalysts, e.g., Fe3O4 and Co3O4 are preferred. A particular difficulty in using catalysts based on transition metal oxides may be the inhibitory effect of other tail gases components (i.e., NOx, O2 or H2O) on the deN2O process. Purifying the tail gases at a temperature >350 \({}^{\circ}\)C can reduce the adverse effect of the water presence on the activity of these spinels. The problem with NOx influence can be solved by using a reactor with a deNOx/deN2O dual-bed catalytic system, in which the gas stream is first purified of NOx. In the process of low-temperature N2O decomposition, zeolite catalysts modified with copper and iron ions are also used, the most popular of which is Fe-ZSM-5. These catalysts can perform a dual function of catalyzing the N2O decomposition and NOx reduction. A commercial catalyst for N2O decomposition is, for example, Clariant EnviCat\(\otimes\)N2O in the form of trillobes, ensuring over 99% efficiency of N2O removal from the tail gases stream [21; 22; 23; 24; 25]. One of the best available techniques for reducing NOx emissions from nitric acid plants is the selective catalytic reduction of NOx with ammonia, carried out between the absorption column and the expansion turbine. Such a location enables the deNO\({}_{\rm x}\) process to be carried out under high pressure, which allows for using a smaller volume of the catalyst bed than installing the reactor downstream of the expansion turbine. In such cases, preheating the tail gases to the operating temperature of the catalyst is often not required. However, in the case of tail gases purifying upstream of the expansion turbine, apart from the operating temperature, selecting a suitable form of the catalyst is also crucial. It should ensure low-pressure drops across the catalyst bed, not causing significant energy losses at the gas inlet of the expansion turbine [13; 26; 27].