This paper presents a procedure for exacly determination the conversion factor of fuel-bound nitrogen to nitrogen oxides in the combustion process of wet spruce wood in boiler grate furnaces with a heat input of 300 to 2,500 kW. They are referred to as fuel-bound nitrogen oxides, thermal nitrogen oxides, and immediate nitrogen oxides. The conversion rate of spruce wood fuel-bound nitrogen is determined based on an analysis of biofuel and emissions-technological measurements of nitrogen oxide concentrations in flue gases from the combustion process of biofuels. For combustion tests, wet spruce chips from 16 sites in Slovakia were used. The value of the conversion factor of spruce wood fuel-bound nitrogen is X_NOx = 65.4±8.3%. The calculated value of the conversion factor of spruce wood fuel-bound nitrogen to NO_x emissions can be considered fair value, both from the aspect of combustion conditions of spruce chips and made emissions-technology measurements and from the technical calculation itself. It is also declared by the following observations and statements.