Some of the impurities (Br, Cl, Fuel-N, and S) in fuels can reduce the radicals (O, H, and OH) formed in the flames on combustion, thus lowering the radical mole fraction. The variation in the radical mole fraction affects the NO x emissions. In this study, the radical reducing effects caused by these impurities were modeled based on measurements of the radical mole fractions. Linear relationships were obtained between the mole fractions of the impurities in the flames and the reciprocals of the radical mole fractions. The slope of this straight line was defined as the radical reducing effect coefficient (α). The value of α for each radical was determined for fuels containing HBr, CH3Br, HCl, CCl4, CH3NH2, NH3, H2S, and CS2 impurities. The OH radical was the most susceptible to the reducing effects of the fuel-N impurity. However, the calculated radical mole fractions (CH4–NH3 flame) by detailed chemical kinetics did not agree with the experimental results, and the O atom radicals were, in fact, most susceptible to the fuel-N impurities. The calculations overestimated the OH mole fraction and fuel-N conversion to NO x . The α value calculated for the OH radical was 19.4 times smaller than that obtained experimentally. The contributions of OH radicals to fuel-NO x generation for methane–ammonia and hydrogen–ammonia cocombustion were investigated by using a simplified reaction scheme for fuel-NO x . If the OH radical was a controlling factor in fuel-NO x generation, the difference in NO x conversion between experiment and calculation could be explained. On the basis of the results, the reaction scheme was extended to the solid fuel combustion of coal and biomass. The NO x conversion for the cocombustion of coal and ammonia was lower than that for the methane–ammonia cocombustion. The effect of Cl impurities on biomass combustion was also investigated. If the OH radical mole fraction was lowered by the presence of Cl in the biomass fuel, the NO x conversion tended to be low. However, if the OH mole fraction was too low to decompose ammonia in the flame, the NO x conversion tended to be high.