## 4 Discussion  
This study covered various parameters that impact the reduction rate of NOx in the example stoker boiler WR-25. The first analyzed parameter was NSR. The urea concentration in the solution injected into the boiler was constant at 12.5%. CFD modeling shows that an increase in \(NSR\) contributes to reduction efficiency growth. The change between \(NSR\)  
Figure 19: NOx reduction as a function of NSR for different reagent concentrations. Figure 18: NOx reduction as a function of NSR for different nozzle designs in the injection lance. and 2 is significant (more than 2 times higher efficiency. The same trend was confirmed in the bench tests (Figure 14) for all analyzed boiler power levels. The increased amount of reagent contributes to injected mass flow ratio growth, so the penetration of the reagent in the horizontal cross-section of the chamber is deeper (Figure 11). Similar trends were confirmed in [23] (the OP-650 boiler, with a reagent concentration of 32%, NSR 0.5-3), where the highest-efficiency growth was also observed between \(NSR\) 1 and 2. A further doubling of \(NSR\) does not contribute to such significant efficiency changes. This can be explained by the fact that in the reagent-excess conditions, chemical component mixing becomes the limitation for NOx reduction. Typical \(NSR\) values found in the literature are in the range of 0.5-3 [16; 24], with an optimal value of approximately 1.5 [4; 25], which remains in agreement with the obtained results. The test results indicated decreased NOx reduction efficiency with increasing boiler power (Figure 15). The decrease in NOx reduction with increasing power is probably due to the insufficient residence time of the reagent under the required thermodynamic conditions in order to properly perform the reaction. These conditions include mixing and particle residence at the proper temperature range. If the reagent injection is directly into the flame, then reagent combustion occurs, and the reduction process does not take place. Thus, the installation works when the reagent is injected into the flue gas (but not the flame) at a specific temperature. In addition, at high power, the exhaust gases and the fuel supply increase. This results in a high flame filling of the chamber, making it more difficult to hit the right temperature range. The temperature window moves higher so that the reagent injection is implemented higher. In addition, the exhaust gas flow velocity through the chamber increases, so the reagent residence time in the respective temperature zone shortens. This mechanism results in a decrease in NOx reduction efficiency. Despite the aforementioned correlations, measurements taken show that some probes indicate significantly higher NOx-reduction values than the average determined by the trend line (Figure 14). This is probably due to the influence of the variable configuration of the injection lances used. The impact of three types of nozzle tips was investigated within the tests: one-hole, two-hole, and three-hole. The results presented in Figures 16 and 18 indicate that three-hole and two-hole nozzles allowed for several percentage points higher NOx reduction than single-hole nozzles. According to the authors, this results from the significantly higher injection angles achieved for the multi-hole nozzles, providing better reagent distribution inside the combustion chamber. The urea concentration in the solution also affects NOx reduction, which was confirmed in tests. A reagent with a concentration of 12% showed an improvement of more than 10% in plant efficiency compared to a 32.5% reagent. The reason for this phenomenon could be that when using a lower reagent concentration, a significantly higher amount of water is introduced into the boiler. This water takes longer to evaporate. As a result, the penetration of the reagent droplets improves, and the rate of mixing with the flue gas increases, so the reduction improves. Therefore, a reagent diluted to several percent of the urea concentration is recommended for target installations.