The article shows a solution to the problem of reducing nitrogen oxide (NOx) emissions to a concentration of 150 mg/m3, on an example of 261 MWe fluidized bed boiler. It is a lignite-fired boiler. To reduce emissions a selective noncatalytic reduction (SNCR) method was used. Among the others optimization of urea injection nozzles’ position through a series of on-site experiments after previous 3D simulations were carried out. The result of the experiments was to reduce the number of injection nozzles, determine their optimal position and determine urea streams and injection angles. Major and interesting findings and actual contribution of this manuscript to the field is: dependence of nitrogen oxide NOx emissions on about 100 different variables determined by the correlation with nitrogen oxide emissions’ analysis. Individual variables such as temperature, O2 in exhaust gases, air-to-coal ratio, mass flow and pressure of urea are not sufficient to describe mathematically the NOx capture process. Technical novelty of the SNCR system consists of the urea stream control systems, which can maintain either a constant urea concentration or a constant urea pressure. The NOx reduction systems used so far do not use pressure control. It is experimentally shown that reaction between NOx and reagent can be achieved at lower temperature equal to 700°C in some parts of boiler, which is lower temperature than reported in the literature.