## 1 Introduction  
The effectiveness of SNCR (Selective Non-Catalytic Reduction) technology depends on various process parameters, such as the type of reagent, reaction temperature, contact time between the reagent and flue gases, chemical composition of the flue gases, and degree of mixing between the reagent and the flue gases [1; 2; 3]. The literature contains numerous studies presenting selected issues related to the development of SNCR technology dedicated to power boilers. For instance, publications [3_A_] considered the impact of modifying the reagent used in the system with chemical additives on increasing the efficiency of the process in a biomass boiler. In [5], the possibilities of optimizing the SNCR process in a grate-fired boiler dedicated to burning challenging fuels, such as sewage sludge and waste, were analyzed. Meanwhile, in [6], the authors conducted several studies related to a coal-fired grate boiler. However, most available studies present the results of computational and simulation analyses. Data from field tests, which provide verified knowledge about the actual performance of installations, are rarely presented. Moreover, the wide variety of sizes and designs of combustion chambers (even within a single boiler type), as well as the combustion technologies used, means that when implementing SNCR, process optimization requires a tailored approach to design. The implementation of SNCR technology in different types of boilers necessitates performingmodel calculations to adapt the technology to the actual geometric and operational parameters of the boilers, as well as selecting or designing nozzles and lances that ensure proper reagent distribution within the combustion chamber [7]. In light of the above information, it should be stated that the correct design of installations for reducing nitrogen oxide concentrations in flue gases requires solving several technological challenges (research problems). The most important of these include determining:  
* The method of reagent distribution, specifically the design and operational parameters of the reagent injection lances;
* The levels at which the reagent is introduced into the boiler furnace chamber;
* The number of installed lances and their spacing on the boiler walls;
* The molar ratio of the reagent to the incoming NOx;
* The concentration of the reagent solution. The key parameters of the injection nozzles, influencing the system's efficient operation, are as follows: the mean droplet diameter, the diameter distribution, and the injection velocity. The typical mean diameters (Sauter mean diameter, SMD [8]) are in the range of 50-150 \(\upmu\)m [7], while the typical injection velocity is 150-200 m/s. These parameters can be ensured using two-flow nozzles (liquid and pressurized gas as an atomizing medium [8]). Such nozzles are characterized by an additional parameter: the Air to Liquid Ratio (ALR), taking values of 0.1-2 [7]. Detail parameters are highly dependent on the installation type. The set of selected parameters of installations available in the literature are summarized in Table 1.  
In addition to establishing the design and operational parameters of the injection lances, one of the fundamental technological challenges in designing SNCR installations is their proper placement on the walls of the furnace chamber. In this regard, the first priority is to identify the optimal levels for reagent injection. As an initial condition for determining the desired levels for placing injection lances, the necessity of introducing the reagent into flue gases with temperatures within the so-called temperature window is assumed. Therefore, the search for the desired locations of the injection lances largely involves identifying areas with flue gas temperatures of a specific value. This condition must be met not only for the boiler's nominal load but also for any other load under which the analyzed boiler operates under normal conditions. As the load changes, the flue gas temperature distribution in the boiler's furnace chamber varies widely.