## 1 Introduction
The efficiency of nitrogen oxides absorption in water is never 100%. Therefore, NO and NO\({}_{2}\) are always present in the so-called tail gases stream, leaving the absorption column. In this area of the nitric acid plant, they are treated as pollutants. Their amount depends on the size of the absorption column and the conditions in which the absorption process is carried out, i.e., temperature, pressure and gas flow rate. The content of nitrogen oxides in the tail gases stream is usually 2-3 wt. % in relation to the amount of nitric acid produced. Thus, without the technology of NO\({}_{\rm x}\) removal, their concentration downstream of the absorption column may reach even 3000 ppm for low-pressure plants [4,9,10,11]. NO\({}_{\rm x}\) and N\({}_{2}\)O emissions are regulated by the Directive of the European Parliament on industrial emissions 2010/75/EU. Under the guidelines contained in the Official Journal of the European Union L248/3 [12] and the BAT document (Best Available Techniques Reference Document) [13], the current emission limits are 300 ppm N\({}_{2}\)O and 90 ppm NO\({}_{\rm x}\) for the existing nitric acid plants, while 100 ppm N\({}_{2}\)O and 75 ppm NO\({}_{\rm x}\) for the new ones [9]. Due to the European Union's pursuit of a \"zero-emission\" economy, these limits are expected to be tightened in the future. There are two aspects in favour of reducing N\({}_{2}\)O emissions: environmental and economic. According to the \"European Green Deal\" communication presented by the European Commission in 2019, by 2030, CO\({}_{2}\) equivalent emissions from industrial plants should be reduced by at least 42% in the so-called non-ETS countries (the current level is 30%) and by at least 52% in the EU-ETS countries (the current level is 43%), in relation to the emissions recorded in 2005. This involves the need to reduce greenhouse gas emissions, even in the plants that have been considered low-emission. The second aspect in favour of lowering N2O emission is related to the possibility of CO2 eq emission allowances trading. Currently, the reduction in nitrous oxide emissions to a level lower than that resulting from the established emission standards gives a profit of several dozen EUR/day for each additional 1 ppm of N2O reduced [14; 15; 16]. Due to the tightening legal requirements regarding N2O and NOx emission standards and the economic aspect, scientists and engineers put much effort into improving the technology for removing these harmful gaseous pollutants from nitric acid plants. Most of the work is focused on developing and testing the catalysts for N2O decomposition/reduction (deN2O) and NOx reduction. Only a few scientific and technical studies concern the design of catalytic baskets or reactors for the deNOx/deN2O processes (Section 2). The industrial experience of Lukasiewicz-INS shows that the design and technical condition of the catalytic basket, supporting a catalyst bed, significantly influences the catalyst efficiency. The article presents solutions in the field of technology for removing N2O and NOx from the tail gases in nitric acid plants, focusing mainly on the design of the catalytic baskets, installed inside deNOx/deN2O reactors.