## 1 Introduction  
The continuous development of technologies for removing harmful components of industrial off-gasses is a consequence of increasingly stringent emission standards and society's growing awareness about the need to protect the environment for future generations. The term \"hazardous gases\" refers to substances harmful to human health, the environment and construction materials. In the case of nitric acid plants, these gases are nitrogen oxides, N\({}_{2}\)O and NO\({}_{x}\) (NO, NO\({}_{2}\)). Nitrous oxide (N\({}_{2}\)O) is non-toxic to living organisms, but it is a greenhouse gas with a GWP index (Global Warming Potential) 265-300 times greater than CO\({}_{2}\)[1]. Due to the high efficiency in absorbing infrared radiation, reducing its emission, even to a small extent, results in a significant reduction in CO\({}_{2}\) equivalent emission. Accumulation of N\({}_{2}\)O is favoured by its long lifetime in the atmosphere, estimated at 116 years [2]. Nitrous oxide also indirectly contributes to the destruction of the ozone layer in the stratosphere, where it is photolyzed and reacts with oxygen to NO\({}_{x}\), which reacts then with O\({}_{3}\)[2, 3]. In nitric acid plants, nitrous oxide (N\({}_{2}\)O) is formed as a by-product in the process of ammonia oxidation to NO, carried out in the presence of Pt-Rh-Pd catalytic gauzes package [4]:  
\[2\text{NH}_{3}+2\text{O}_{2}\text{ }^{\text{\tiny{(6)}}}\text{ N}_{2}\text{O}+3\text{H}_{2}\text{O}+736\text{ kJ}/\text{mol} \tag{1}\]  
Moreover, it can also be formed downstream of the catalytic gauzes package in the reaction of unreacted ammonia with NO (Equation (2)), as well as in the process of selective catalytic reduction of NO\({}_{\rm x}\) (deNO\({}_{\rm x}\)), carried out in SCR reactors, installed downstream of the absorption column (Equations (1) and (3)):  
\[2{\rm NH}_{3}+8{\rm NO}\to 5{\rm N}_{2}{\rm O}+3{\rm H}_{2}{\rm O}+945.7\ {\rm kJ} \tag{2}\]  
\[4{\rm NO}+4{\rm NH}_{3}+3{\rm O}_{2}\to 4{\rm N}_{2}{\rm O}+6{\rm H}_{2}{\rm O}+24 03.3\ {\rm kJ} \tag{3}\]  
Nitrous oxide is chemically inert and stable at temperatures <700 \({}^{\circ}\)C. Therefore, without using technology for its removal, it does not undergo any changes in the entire technological line of nitric acid production and is emitted into the atmosphere with the tail gases stream at a concentration of 500-2000 ppm (depending on the plant type). The nitrous oxide emission can reach a level of \(\sim\)5 kg/t HNO\({}_{3}\), \(\sim\)7 kg/t HNO\({}_{3}\) and \(\sim\)12 kg/t HNO\({}_{3}\) for low-, medium- and high-pressure plants, respectively [5]. Nitrogen oxides, referred to as NO\({}_{\rm x}\), are not classified as greenhouse gases, although they are included in the overall radiation balance [6]. Unlike N\({}_{2}\)O, they have a significant harmful effect on human health. They cause respiratory and cardiovascular diseases, and sometimes cancer or even death. They also harm the environment, contributing to the formation of acid rain and the destruction of the ozone layer in the stratosphere. In addition, they react with other air pollutants, creating the so-called photochemical smog [7,8]. Nitric oxide (NO) is the main product formed in the process of ammonia oxidation, carried out in the presence of Pt-Rh-Pd catalytic gauzes package [4]:  
\[4{\rm NH}_{3}+5{\rm O}_{2}\to 4{\rm NO}+6{\rm H}_{2}{\rm O}+904\ {\rm kJ}/{\rm mol} \tag{4}\]  
In the technological line of the plant between the ammonia burner and the absorption column, NO partially oxidizes to NO\({}_{2}\):  
\[2{\rm NO}+{\rm O}_{2}\leftrightarrow 2{\rm NO}_{2} \tag{5}\]  
In this area of the plant, the presence of NO and NO\({}_{2}\) in the gas stream (so-called nitrous gases) is desirable for the production of nitric acid:  
\[3{\rm NO}_{2}+{\rm H}_{2}{\rm O}\to 2{\rm HNO}_{3}+{\rm NO} \tag{6}\]  
Reaction (6) occurs in the liquid phase on the sieve trays (plates) of the absorption column. Additionally, reaction (5) occurs inside the column between the plates in the gas phase.