Natural gas is one of the cleanest organic fuels, nevertheless, it produces pollutants in the combustion process. The major pollutants of the combustion in the atmosphere are carbon monoxide CO, nitrogen oxides NO, NO2, sulphur dioxide SO2 and hydrocarbons. The current article deals with the problematic of the high NOx formation rates under the high power operation. The object under research was a middle-pressure natural gas boiler with a power of 35 MW. The reason for the high NOx formation rate was eliminated due to primary deNOx methods.